Protective Reley C Warrington

Protective Bela"s THEIR THEORY AND PRACTICE

VOLUME ONE

by

A. R. van C. WARRINGTON A.C.G.I., B.Sc.(Lond.), Fellow I.E.E.E., C.Eng., Fellow I.E.E. The English Electric Company Limited Stafford

1968

CHAPMAN & HALL LTD 11 NEW FETTER LANE EC4

Author's Prelaee

is an attempt to pass on to others the knowledge gained TbyHISthevolume author in 33 years of experience in the protective relay field, in the U.S.A. and Europe. It offers the student a new general theory of relay operation and brings the user up to date on modem design technique. It has been written primarily for protection engineers, but with an endeavour to make it easily understandable to students to whom this subject may be new. The second volume deals with static (solid-state) relays and provides additional information on c.t's., p.t's., fault incidence, transients and sources of relay error. A. R. van C. WARRINGTON.

ACKNOWLEDGEMENTS

WISH to thank the English Electric Company for their permission to I publish this book and for providing typing and drawing office facilities. I am greatly indebted, for editorial and technical advice, to Dr. Adamson and to my colleagues of the English Electric Company, and to those who coped with the heavy task of reproducing the manuscript and drawings. A. R. van C. W.

ix

Contents 1. PURPOSE OF PROTECTIVE RELAYS AND RELAYING

page 1

Causes of Faults. Definitions. Functions of Protective Relays. Application to a Power System. 2.

RELAY DESIGN AND CONSTRUCTION Characteristics. Choice of Measuring Units. Construction of Measuring Units. Construction of Timing Units. Details of Design. Cases. Panel Mounting. Operation Indicators. Finishes.

24

3.

THE MAIN CHARACTERISTICS OF PROTECTIVE RELAYS Phase and Amplitude Comparators. Relay Characteristics. General Equation for Characteristics. Inversion Chart. Resonance. Appendix.

99

4.

OVERCURRENT PROTECTION Time-Current Characteristics. Application. Limits of Error. Ratings. Directional Overcurrent Protection. A.C. Tripping. Schemes for Radial Feeders. Construction. Application. Problem.

141

5.

DISTANCE RELAYS General Principles. Special Characteristics. Limitations. Application to Lines. Settings. Multi-terminal Lines. Construction. A.C. Potential Supply. Simultaneous Ground Faults. Auto-reclosing Zero Sequence Compensation.

191

6.

SWITCHED AND POLYPHASE DISTANCE RELAYS Reduction of Measuring Units. Automatic Switching Schemes. Polyphase Distance Relays. Phase and Amplitude Comparators. Analysis of Polyphase Comparators.

273

7.

DIRECTIONAL PILOT RELAYING Basic Principles. Pilot Wire Schemes. Carrier Channel Schemes. Carrier Signal Checking. Future Trends.

298

8. A.C. PILOT RELAYING Pilot Wire Schemes. Phase and Amplitude Comparators. Effect of Load Current. Multi-terminal Lines. Pilot Wire Limitations. Pilot Supervision. Phase Comparison Carrier.

317

9. PROTECTION OF A.C. MACHINES Generator Protection. Stator Faults. Rotor Faults. Miscellaneous Faults. Motor Protection. Faults. Unbalanced Conditions. Power Station Auxiliaries. Current Differential Relaying.

347

10.

POWER TRANSFORMER PROTECTION Types of Faults. Gas Relays. Differential Relays. Magnetising Inrush. Minimising of Effects. Relay Solutions. Grounding Transformers. Generator Transformer Units. Transformer Feeders.

380

11.

BUS-ZONE PROTECTION General Principles. Current Differential Protection. Voltage Differential. Frame Leakage Protection. Directional Comparison. Back-up. Supervision.

413

xi

Contents 12. BACK-UP PROTECTION

page 429

Basic Principles. Precautions for Reliability Remote Back-up. Local Back-up. Relay Back-up. Breaker Back-up. A.C. Supplies. D.C. Supply. 13. MAINTENANCE AND TESTING OF RELAYS

440

Commissioning. Periodic Maintenance. Transfer to Test Circuit. Tools. Safety Measures. Mechanical Tests. Electrical Tests. Manufacture Tests. 14. MISCELLANEOUS Static Relays. Future of Electromagnetic Relays. D.C. Protection Relays. Protection Engineering as a Career.

468

REFERENCES

473

INDEX

481

List of Slim bois area; amperes susceptance; magnetic flux density C capacitance D discrimination factor; diameter E e.m.f. (usually at power source) F force G conductance H magnetising force I current J angular moment of inertia K a constant L self inductance M mutual inductance; numeric ratio or constant N number of turns; numeric ratio or constant o origin of a graph P point on a graph; general constant Q steady state amplitude of charge q; general constant R resistance; ratio S spacing or displacement T temperature V voltage W power X reactance Y admittance Z impedance A and B are also used as unspecified quantities or ratios, real or complex. A B

~3)

a

a /120 operator ( _!+j

b

susceptance per mile capacitance per mile diameter instantaneous value of potential difference frequency conductance per mile; gravitational constant height instantaneous value of current; unit vector a /900 operator a constant

c d

e

f

g

h

J k

0

xiii

List of Symbols

I length m mass; unspecified number n an unspecified number p in-phase component q quadrature component or electric charge r resistance per mile s modulus of attenuation t time v velocity x unknown quantity or reactance/mile y admittance per mile z impedance per mile ex

an angle an angle ')I attenuation factor (complex) ~ an increment e base of Naperian logarithms " efficiency () characteristic angle A. an angle (L penneability or prefix micro 1t radians in 1800 p resistivity [Jt reluctance (1 conductivity () characteristic angle; angle between system voltage vectors q, magnetic flux cjJ phase angle, generally the angle by which the current lags the voltage in a protected circuit '" an angle OJ frequency in radians/sec; ohms L summation n ohms ex is also used as the complex ratio of two currents and p their inverse ratio. cjJ and G on circuit diagrams refer to phase and ground relays respectively.

P

List of Subscripts A, B, C the terminals of a protected line

a, b, c the three phases d

e

J: F

difference, direct axis general suffix fault xiv

List of Symbols g,G h, i,j I, L

m n

o

p

q r R

s, S t

res max

min 1 2

[600

160°

IVI

ground general suffixes line magnetising neutral; nominal zero sequence; a basic value in phase component; primary; polarising quadrature; quadrature axis replica; restraint relay; relay (to distinguish in the case of a secondary quantity); also suffixes denoting restraining signals; receiving end source; secondary; sending end suffix denoting quantity variable with time residual maximum minimum positive sequence; suffix denoting a derived relaying quantity negative sequence; suffix denoting a derived relaying quantity lagged 60° advanced 60° scalar value of V

Abbreviations B.S.S. British Standard Specification C.E.G.B. Central Electricity Generating Board of Great Britain

1 Purpose of Proteetive Belays alUl Belaying Causes of Faults-Definitions-Functions of Protective RelaysApplication to a Power System capital investment involved in a power system for the generation, THE transmission and distribution of electrical power is so great that the proper precautions must be taken to ensure that the equipment not only operates as nearly as possible to peak efficiency, but also that it is protected from accidents. The normal path of the electric current is from the power source through copper (or aluminium) conductors in the generators, transformers and transmission lines to the load and it is confined to this path by insulation. The insulation, however, may be broken 'down, either by the effect of temperature and age or by a physical accident, so that the current then follows an abnormal path generally known as a short-circuit or fault. Whenever this occurs the destructive capabilities of the enormous energy of the power system may cause expensive damage to the equipment, severe drop in voltage and loss of revenue due to interruption of service. Such faults may be made infrequent by good design of the power apparatus and lines and the provision of protective devices, such as surge diverters and ground fault neutralisers, but a certain number will occur inevitably due to lightning and unforeseen accidental conditions. The purpose of protective relays and relaying systems is to operate the correct circuit breakers so as to disconnect only the faulty equipment from the system as quickly as possible, thus minimising the trouble and damage caused by faults when they do occur. It would be ideal if protection could anticipate and prevent faults but this is obviously impossible except where the original cause of a fault creates some effect which can operate a protective relay. So far only one type of relay falls within this category; this is the gas detector relay, used to protect transformers, which operates when the oil level in the conservator pipe of a transformer is lowered by the accumulation of gas caused by a poor connection or by an incipient breakdown of insulation. With all other equipment it is only possible to mitigate the effects of a shortcircuit by disconnecting the equipment as quickly as possible, so that the destructive effects of the energy into the fault may be minimised.

1.2

Protective Relays

1.1. CAUSES OF FAULTS

Insulation is usually either air or a high resistivity material which may also be used as a mechanical support. Air insulation can be accidentally short-circuited by birds, rodents, snakes, kite-strings, tree limbs, etc., or reduced in insulation strength by ionisation due to lightning or a fire. Organic insulation can deteriorate due to heat or ageing, or can be broken down by overvoltage due to lightning, switching surges, etc. Porcelain insulators can be bridged by moisture with dirt or salt and can become cracked. In all these cases the initial lowering of insulation resistance causes a small current to be diverted which hastens deterioration or ionisation, causing this current further to increase in a progressive manner until a power arc occurs. Furthermore, heavy faults, if not quickly interrupted, may heat conductors sufficiently to cause deterioration of other insulation which was previously in a healthy state. Line and apparatus insulation may be subjected to transient overvoltages whenever current is started or stopped. These surges are a component of the 'recovery' voltages and are analogous to 'water hammer' when a hydraulic valve is suddenly closed. The most severe switching surges occur when current which lags or leads the applied voltage by 90 degrees (such as short circuit current or line charging current) is interrupted. During unloaded line dropping on a grounded system, the line voltage may go to crest line-toneutral voltage on the first interruption; three times this value on the first restrike; five times this value on the second restrike; and so on as the arc restrikes on succeeding half cycles. The magnitude of these switching surges is appreciably greater for systems that are not solidly grounded. Thus system insulation may be subjected to serious overvoltages with breaker recovery voltages that are still higher when line charging current is interrupted. 1.2. PROGRESS IN THE DEVELOPMENT OF RELAYS AND PROTECTIVE SCHEMES

In the very eady days of the electrical industry a power system usually consisted of a small generator supplying a local load and it was possible for the station attendant, in an emergency, to open a switch manually and even swat out the arc with a duster! Since these historic times the sizes of power systems have increased enormously, the rate of increase for most countries lying between a doubling and a quadrupling per decade. Furthermore, industrialised countries and an increasing number of under-developed ones have integrated their systems on a national basis and numerous cases of substantial international connections are in evidence. With increases in the sizes of a generating plant and inter-connection, great demands have been made on the ingenuity of the designers of automatic switchgear. Such apparatus must operate to interrupt very high arc energies in a small fraction of a second if the equipment is to avoid destruction. Fuses were the first automatic devices to be employed to isolate the faulted equipment quickly. They were very effective and are still widely used in 2

Purpose of Protective Relays and Relaying

1.2

distribution circuits, but suffer from the disadvantage of requiring replacement before the power supply can be restored. This inconvenience was overcome by the automatic circuit-breaker with a built-in overload or undervoltage trip magnet. The final step was to divorce the selective function from the breaker and to incorporate it in separate protective relays, whose contacts controlled the trip coil of the breaker. The first attempts to design relays which would operate in response to short-circuit conditions involved attracted armature devices, with or without a definite time-delay provided by a dash pot mechanism. As power systems increased in size and complexity it was necessary to employ more precise relay mechanism and to obtain selectivity on an inverse time-current basis, i.e. the relay speed increasing with the current magnitude so that, since the current is greatest in the faulted section, that section will be isolated by its relays before those in the sound sections can operate. The only device then available which had this required accuracy was the induction disc watthour meter which was turned into a relay by substituting contacts for the indicating register. This resulted in the inverse time-overcurrent relay which is still in use today, although in an improved form (fig. 1.1). As the requirements for sensitivity and selectivity increased, a trend emerged towards the use of high-speed differential type relays on the maio

FIG.

l.Ia. Modern induction disc relay

3

1.2

Protective Relays

FIG.

l.lb. Induction vane differential relay

transmission system, time-overcurrent relays being retained only for distribution systems and for back-up purposes (reserve protection) on the main system. Differential relays compare electrical quantities derived from each end of the protected system (e.g. a transmission line 10 miles long) and operation takes place if the ratio, phase angle or algebraic sum of the derived quantities depart by a predetermined amount from some initially set value, for example, unity in the case of a differential relay measuring numerical ratio. The induction disc inverse time relay was introduced in the early 1920s and the high-speed differential type in the late 19.20s. Initially, the differential type also employed the induction disc principle but with short contact travel and a lighter armature; the desire for high speed led to the balanced beam unit but this was gradually displaced by the induction cup, which was a faster version of the induction disc unit, its inertia having been reduced by forming the disc into a narrow cup and its torque increased by better utilisation of the available flux in a 4-pole magnetic structure (fig. 1.2) similar to that of an induction motor. Greater sensitivity and accuracy have been achieved, particularly since the 1939-45 War, by the use of polarised d.c. relays energised through rectifier bridges. Detailed explanations of the principles of the various types of relays will be given in later chapters; more complete historical accounts of the progress 4

Purpose of Protective Relays and Relaying

FIG.

1.3

1.2. Induction cup unit

in the design of protective relays over the last 35 years can be found elsewhere (1). In the present chapter it is proposed to outline briefly the nature, function and mode of operation of relays. 1.3. DEFINITIONS

Throughout the two volumes some terms will be used which are peculiar to protective relays; these will be explained as they are brought into use. The following terms, however, are common to all relays and protective schemes and will be defined before proceeding further. The word 'normal' refers to the healthy or unfaulted condition of the protected circuit but, when used in connection with relay contacts, it implies that the relay is not energised. For instance, a 'normally open' contact is one which is open when the relay is not energised; it is also referred to as a circuitclosing contact, a make contact or an 'a' contact. Only in the case of relays designed to operate on an excess condition, such as overcurrent or overvoltage, would 'normal' position of the contact correspond to normal operating conditions. For instance, in an undervoltage relay, a normally open contact is one which is open when the relay is not energised whereas, under normal conditions of full voltage, this contact would be closed. A 'normally closed' contact is one which is closed when the relay is not energised. It is also known as a circuit-opening contact, a break contact or a 'b' contact. Most relays have a resetting means such as a spring or gravity. Most protective relays have a normally open contact which is closed when the relay trips; the following definitions refer to such relays and exclude undervoltage, undercurrent, etc., relays. 5

1.3

Protective Relays

All diagrams will show relays in the de-energised position and will be drawn so that, when the relay operates, its contacts move upwards (as if against gravity). Where it is inconvenient to draw the relay with its contacts moving upwards they will be drawn moving to the left. Relays are shown with their coils and contacts together where convenient. In schematic diagrams of complicated circuits, the contacts may be separated from the coils but they will be identified by a similar letter or number. 1.3.1. Glossary of Common Relay Terms

Operating Force or Torque Restraining Force or Torque Pick-up (Level)

That which tends to close the contacts of the relay.

That which opposes the operating force or torque and tends to prevent the closure of the relay contacts. The value of current or voltage, etc., which is the threshold above which the relay will close its contacts. Drop-out or Reset The value of current or voltage, etc., which is the (Level) threshold below which the relay will open its contacts and return to normal position or state. Characteristic (of a The locus of the pick-up or reset when drawn on a relay in the graph. In some relays the two curves are coincident and become the locus of balance or zero torque. steady-state) One which is energised by the contacts of the main Reinforcing Relay relay and, with its contacts in parallel with those of the main relay, relieves them of their current carrying duty. The seal-in contacts are usually heavier than those of the main relay. Seal-in Relay Similar to a reinforcing relay except connected to stay until its coil circuit is interrupted by a switch on the circuit breaker. A relay which operates, usually after a slight delay, if Back-up Relay the normal relay does not operate to trip its circuit breaker. Primary Relays Those which are connected directly in the protected circuit. Those which are connected to the protected circuit Secondary Relays through current transformers (c.t's) and potential transformers (p.t's). The ability of the relay to discriminate between a fault Selectivity in the protected section and normal conditions or a fault elsewhere on the system. The accuracy with which the relay can repeat its elecConsistency trical or time characteristics. An oscillation between groups of. synchronous a.c. Power Swing machines caused by an abrupt change in load conditions. 6

Purpose of Protective Relays and Relaying

1.3

A visual device, usually spring or gravity operated, for indicating the operation of a relay. Instantaneous Relay One which has no intentional time delay and operates in less than 0·1 second. Time Delay Relay One which is designed with a delaying means. Unit A self-contained relay unit which, in conjunction with one or more other relay units in a relay case, constitutes a complete protective relay. Element A part of a relay unit, such as an electromagnet or damping magnet or an induction disc. Power Consumption The power absorbed by the circuits of the relay, (Burden) expressed in volt-amperes if alternating current (a.c.) and in watts if direct current (d.c.), at the rated current or voltage. Operating time The time which elapses from the moment when the actuating quantity attains a value equal to the pick-up value until the relay operates its contacts. Reach The remote limit of the zone of protection provided by the relay; used mostly in connection with distance relays to indicate how far along a line the tripping zone of the relay extends. Errors in relay measurement resulting in wrong operaOverreach,' underreach tion or failure to operate, respectively. Blocking Preventing the protective relay from tripping, either due to its own characteristic or to an additional relay. Tapped Line One which has one or more tapped lines connected to it for supplying loads. Multi-terminal One with three or more terminals which can be power Line sources. Flag or Target

An interesting reference is the I.E.C. document on definitions for the technical committee No. 41 on protective relays. 1.3.2. Vectors

An electrical vector (phasor) has magnitude and phase relation. Phase relation means the time in a cycle relative to a reference moment. Current vectors are standardised in meaning but there is some divergence in the interpretation of potential vectors. In this book Va or Van will be taken as the voltage of the phase a conductor relative to the neutral and Vab will be taken as the voltage of the phase a conductor relative to the phase b conductor, i.e. Vab = Van - Vbn . The arrowhead will be drawn at the end of the vector corresponding to the conductor under consideration (fig. 1.3a). Voltage drop (1Z) will be drawn with the arrowhead in the other direction. 7

1.3

Protective Relays

Actually it does not matter whether this or the reverse concept of potential vectors is used because the same result is obtained as long as the method chosen is used consistently. E is used for e.m.f. and V for terminal voltage. In mathematical equations Vmax and I max are peak values and V and I are r.m.s values. Moduli are shown thus IVI. Currents are shown in vector diagrams with a white or hollow arrowhead. Potentials are shown with black or solid arrowheads. Figure 1.3a shows the relative potentials in a three-phase four-wire system. v. I.

t Zero or

reference

>----~.

(a) NormClI pha.se-to-neutra.1 potentia,is FIG.

'I, (b) La.gging loo.d currents

Vo. (c) Pha.se-to -pha.se delta. potentia.ls

]- --[--

1.3a-<:. Vectors representing balanced conditions in a four-wire system

FIG.

-

~ II t

-

1.3d. Subtractive polarity in a c.t. or p.t.

c.t. and p.t. polarity will be assumed to be subtractive, i.e. with the polarity marks at the same end of the primary and secondary windings, the polarity will be such that the direction of the currents in the circuit will be the same with the transformer removed (fig. 1.3b) and the circuit completed by the dotted lines. Normal phase rotation of vectors is taken as anti-clockwise with positive sequence vectors coming up in the order a, b, c and negative sequence a, c, b. 1.3.3. Vector Operators (141)

The operator 'j' moves a vector 90 degrees forward (anti-clockwise) without changing its magnitude. The operator 'a' moves a vector forward 120 degrees without changing its magnitude. The use of the operator 'a' considerably simplifies the vector algebra of a three-phase system, especially when dealing with symmetrical components. Fig. 1.4 shows how 'a' can be used for 30 degree phase angle shifts. The following table gives the relations between functions of a, j, 88 and phase shift. 8

1.3

Purpose of Protective Relays and Relaying TABLE

1.1

Vectorial Operators Function of Operator

Equivalent Operator

Exponential Equivalent

Algebraic Equivalent

a

.2n

a

elT

a2

a2

. V3 -t+J2 V-t-j-l-

e

.2" -13

eO

a3

.2"

1 a

a2

e-IT

l+a

- a2

el3

."

e - I3

1+ a2

-a

al

-a2

el3

a- l

-a

e - 13

I- a

V3 a- l

1- a2

V3 a l

v3e l 6

a - a2

V3 at

elz

.n

.n

v3e - 16

."

.n

e-

."Z

V3 at

a2 + a

- 1

[;irr

a2 + 1

-a

[; - 1:3

a+ 1

- a2

[;13

I

.n

.n

• Sn

vt -J . -3 2 v-

J6ijO

V3130°

It +

V3130°

vj-f

V3190°

- j V3

V3'190°

-1

1180°

V3 t-j-y

160°

. v:3 t+J-y

160°

v-1 t +J. 2 3

V31150°

-V3 at

v3 -1'6

a - a2

1 -at V3

1 ." V3 e - l 2'

V. -3 -1 t - J 2 1 j V3

-1

- e irr

-1

1 a + a2

.Sn

9

J6ijO

j V3

1

-

~

v. 3 -JT v1t -J. 2 3 t

v3 e l '6

a2

11200

~

-V3 a- l

a-I

11200

t+j~ 2

t+j~ 2

.n

a

-

V. 3 -J-y V-

.n

a2

t

-

1120·

100

1

-

Trigonometric Equivalent

v31150° 1 _ V3 19O°

1180·

1.4

Protective Relays

FIG.

1.4. Simple functions of the operator 'a'

t.4. THE NATURE OF A RELAY

Protective relays have been called sentinels and electric brains. From the economic point of view, relays are akin to insurance; they protect the power utility from financial loss due to damage to equipment (fig. 1.5a). From the underwriters' point of view they prevent accidents to personnel and minimise damage to equipment. From the customers' point of view good service depends more upon adequate relaying than upon any other equipment. The cost of this protection is between 1 and 2 % of the total cost of the power system, i.e. equivalent to an insurance premium costing about 0·1 %per year, assuming 15 years before replacement due to obsolescence (3). In the dictionary, four definitions of relays will be found which deal with foot races, post coaches, etc., but none even remotely fits this application. A protective relay is a device which responds to abnormal conditions on an electrical power system to contrpl a circuit breaker, so as to isolate the faulty section of the system with the minimum interruption to service. To do this, relays must be able to decide promptly which circuit breakers are to trip in order to isolate only the faulted section(s). These relays must be designed, therefore, to be responsive to electrical quantities which are different during normal and abnormal conditions.

10

Purpose of Protective Relays and Relaying

1.4

The basic electrical quantities which may change in the transition from healthy to faulty conditions are current, voltage, direction, power factor (phase angle) and frequency. It is generally necessary to provide relays responsive to more than one of these quantities because, for instance, the current in a fault during minimum generation conditions may be less than the normal load current during maximum generation. As another example, the power

FIG.

1.5a. Protective relays can prevent this

factor measured by the relay may be as low during a power swing as during a fault. Sometimes all of the above quantities may have to be used to obtain selectivity; furthermore, in the case of an a.c. railway, several heavy trains starting up together may present current, voltage and power factor so similar to that of a fault that an additional function is necessary, the rate-of-rise of current, which is instantaneous for a fault but incremental or slower for normal service conditions. Whereas the main requirement of -instrumentation is sustained accuracy, the most important requisite of protective relays is reliability since they may supervise a circuit for years before a fault occurs; if a fault then happens, the relay must respond instantly and correctly. For this reason the designers should always attempt to use simple constructions and simple connections of relays. In spite of good intentions in this respect, there is a tendency to extend the operation of relay schemes by adding additional features until complexity 11

1.5

Protective Relays

results .and then it becomes necessary to re-design. In other words, a graph of the progress of relay engineering as regards complexity tends to follow a sawtooth shape. For example, a simple way to protect a circuit is to compare the current entering the circuit with the current leaving it by means of a relay in which torques corresponding to the two currents are opposed so that, if either exceeds the other, it indicates diversion of the current through a shortcircuit and hence warrants relay operation. This simple principle soon becomes complicated because of transient magnetic conditions, such as the inrush of exciting current to a power transformer, which appear on one side of the circuit only and would cause relay operation if discriminatory blocking features were not added. Such a blocking feature, called harmonic restraint, sometimes has to be unblocked because harmonics may appear during fault conditions which demand tripping. Where possible, a principle is chosen to avoid such complications. 1.5. FUNCTION AND MODE OF OPERATION OF A RELAY

From the foregoing it can be seen that protective relays do the work of an untiring supervisor, continuously measuring the electrical quantities of the protected circuit and ready to disconnect the circuit immediately when the value of one of those quantities becomes abnormal. Actually, no human being could approach the constant alertness of a relay, nor its speed of action, nor its reliability and accuracy. For example, a reactance type distance relay for a transmission line disconnects the line from the bus if a fault occurs within its protecting zone and not if a fault occurs outside that zone. To do this it measures the reactance of the line between itself and the fault, i.e. it measures the current, voltage and phase angle, and computes

WI sin tP correctly to

within ± 2 %and closes its contacts (or not, depending upon the location of the fault), and a modem relay (fig. 1.2) will do this in an overall time of 20-40 milliseconds. Three men reading meters and a fourth with a slide rule could do the same thing within ± 5 %in about a minute, which is 3,000 times as slow and less than half the accuracy. In order to keep the size and cost of relays to reasonable values, the enormous currents and voltages of the actual primary circuit are reduced to relatively small values by current transformers (c.t's) and potential transformers (p.t's). The p.t's are often referred to as v.t.s, voltage transformers, in British practice. The relays measure these secondary electrical quantities and operate when the magnitude of one of them is abnormal or when the ratio between two of them is abnormal. In electromagnetic relays, the measurements are made by means of electromagnets which exert force on an armature carrying contacts; static circuits using semi-conductors, thermionic and cold-cathode tubes or magnetic amplifiers may also be used although not all of these are equally attractive. 12

1.6

Purpose of Protective Relays and Relaying

All protective relays have two positions, the normal position, usually with their contact circuit open, and the fault position usually with their contact circuit closed. A relay is changed to the fault position when a fault occurs by the preponderance of abnormal operating quantities (such as overcurrent) over normal restraining quantities (such as voltage or through-fault current). Fig. l.Sb shows schematically the basic connections of a relay to the trip coil of the circuit breaker which controls the power supply to the protected circuit. When the relay contacts close, the high L/ R ratio of the trip coil delays a ____~~------~s~~ ~t~ io~ n ~b~ u .~.----------~

b --~-+----------------------~rPotential tran,formus

(P.T:.)

>0."1"

;;: d ~I

I

al'

~

O-+-----.J T

Current trMsforme r

(C.T.)

S"conda.r~

potentIa l bus

FIG.

1.5b. Basic connections of a protective relay

the build-up of current so that a fast breaker is tripped before the current reaches its steady value. For this reason, and because the duration of the trip coil current is only a few cycles, the relay contacts need have a continuous rating of only 5 amperes and yet operate a 30 ampere trip coil SO times without needing maintenance. After the breaker has tripped, its auxiliary switch (marked a in fig. l.Sb) opens the highly inductive trip coil circuit and the relay can reset when deenergised by the opening of the breaker. It is important however that the relay contacts do not chatter while the trip current is flowing, otherwise they will be badly burned. This is ensured either by non-bounce design or by the use of a magnetic hold-in coil on the relay or by a separate relay, known as a seal-in relay, which is discussed in section 1.10.2 of this chapter. 1.6. IMPORTANT REQUIREMENTS

The primary requirements for relays are reliability and selectivity. How these are achieved in the relays themselves is explained in a later chapter, but the first step is to make these conditions possible by locating the relays in the correct places. Referring to fig. 1.6, it will be seen that, in order to have complete protection, the zones of protection given by each relay must overlap so as to leave no unprotected areas. Furthermore, fig. 1.8 shows that there must 13

1.6

Protective Relays

be a first and second line of defence to cope with the possibility of failure of the relay or the circuit breaker at anyone location. This is important because, even with the greatest care in manufacture and installation, it is never possible entirely to eliminate the possibility of a mistake or a defect in a mechanism such as a trip coil and linkage which has been overlooked in maintenance.

~ [SQJ1-~""-.'~-' [~l ~

r--+i=

I

u,,'_

1

-

I

=J

I

t---+-++---' r-I I II

~--f I

FIG.

I

I

I

I

L,n. ,.'o.y zone

1.6a. Zones of protection overlapping to avoid blind spots

G.nero.tor tro.nslormrr ..1------, r.lo.y

Bus zonr rrlo.y

To line

relo.y FIG.

To lin.

r.lo.y

1.6b. Locations of c.ts to provide overlapping zones of protection

Without back-up protection, a short-circuit in a line or piece of equipment would not clear at all and might result in the destruction of the equipment. Other important properties of a relay or relay schemes are sensitivity, speed and positive action, these being matters of design. Sensitivity gives high performance with low cost c.t's and p.t's. Speed minimises damage and risk of instability because both are functions of time. Positive action. eliminates the risk of contact burning, wrong tripping, or failure to trip. 14

Purpose of Protective Relays and Relaying

1.8

1.7. ECONOMICS OF RELAYING

The cost of protective relays is generally extremely small (t to 2 %) compared with the cost of the equipment protected; this is particularly true in the case of generators, transformers and high tension lines. In spite of this there is a tendency to treat protection not as a small percentage insurance charge but as separate item and then to pick the cheapest relay or relay scheme. Considering the saving in repair cost afforded by high-grade, high-speed relaying compared with cheaper slow-speed arrangements, it astonishes many engineers that the best protection is not always chosen; the cost of one major repair to a generator for instance would be many times the cost of the best protective relay schemes. Similarly, the cost of one day's loss of production in a copper mine or oil refinery may exceed the cost of adequate relaying. On the other hand, unnecessarily expensive and complex protectitm schemes are sometimes used for important lines so that the likelihood of an outage due to trouble in the protective gear may be comparable with the likelihood of an actual fault. It is the duty of the Application Engineer to choose the most economical scheme which will give complete protection and isolate faults selectively in the shortest possible time. Good electrical service, i.e. continuity of supply, depends to a great extent on adequate protection. This is sometimes difficult to achieve because of the tendency of system planners to relegate the relaying considerations until the system arrangement has been decided on and the equipment ordered; this often creates conditions that make it almost impossible to find anything on which the relay can base its selective measurement. In most cases, if the Relay Engineer had been invited to attend some of the meetings, he could have suggested some minor modifications for the layout which would have provided much better protection with negligible increase in cost or loss of flexibility. In short, Protection must be Considered before the Power System Layout is Finalised. 1.8. MAIN AND BACK·UP PROTECTION (106)

Fig. 1.7 shows the basic elements of an electric power system. Electric power is usually generated at voltages between 11 kV and 33 kV since this gives the most economical balance between the cost of copper, the cost of insulation and the cost of mechanical strength to resist centrifugal force. The voltage at the generator terminals is stepped up to a higher voltage, such as 132 kV, the precise value chosen being the one to give minimum cost and running losses dependent on the line length, power to be transmitted, etc. At the load, there is a further transformation down to a voltage of a few kV suitable for distribution, and again to a still lower voltage (usually 110 to 440 volts) for the ultimate consumers, i.e. industrial and residential loads. In order to isolate any of this equipment in case of trouble, each item must be separated from the others on each side of it by a circuit breaker. The relays themselves must be connected to trip only the breakers next to the 15

1.8

Protective Relays

01 unit per mile a.t 132 KV

Alterna.tor 1 unit GenerMion

0 '1 unit pcr MW including

boilus etc.

Ca.ble 3'3 KV

Hotel., houses etc.

lYRica.l cost 01 500 MW sy',tom ~g co.lol build Ing 55 units lor genera.t ion 18 unIt. lor tra.nsmission 27 units for distribution 100 unih tota.l (1 unit. £100,000 in 1960)

FIG.

1.7. Cost of electrical equipment

protected unit, and, as previously stated, the zone of protection of each relay must overlap the zones of the adjacent relays (fig. 1.6a) to ensure that there are no dead spots. Fig. 1.6b shows how these results can be achieved by the proper location of each C.t. These relays are the main relays. In addition to this first line of defence there must be a second line of defence provided by back-up relays, which will clear the fault if the primary relays for some reason fail to operate (fig. l.H). This subject wIll be dealt with in detaIl in Chapter 12, but a broad explanation is helpful at this point. Bus B

BusC

Nci!lhbouring~:t-~~:--i

Circuit

,--

~----------~I~~~~~==~~~-------­ I

I Fa.at locaJ protection a.t B

Rela.y A

Rela.y I B

x = Brea.ker

I FIG.

1.8. Back-up relays

There are three kinds of back-up relays: (a) those which trip the same breaker if the main relay fails (Relay

Back-up); (b) those which open the next nearest breakers on the same bus in case one of the local breakers fails to open (Breaker Back-up), or in case there is a failure of the local secondary current or potential supplies or the a.c. wiring; (c) those which operate from a neighbouring station so as to back-up both 16

Purpose of Protective Relays and Relaying

1.9

relays and breakers and their supplies (Remote Back-up) in case of the failure of any local supply including the battery, or in case a circuit breaker or relay fails to function. Relay back-up means literally the duplication of the main relays and their c.t's and p.t's, etc., but usually a compromise is employed resulting in the addition of a simple relay such as a time-overcurrent relay. The best relay back-up is a device using an entirely different principle, such as the gas detector relay in a transformer. Breaker back-up is necessary when a feeder breaker fails to trip on a fault (fig. 1.9) because the feeder fault then becomes virtually a busbar fault. It F

B

@H

~ .

____ _

-----FIG.

1.9. Failure of breaker to clear a fault

usually consists of a time-delay relay operated by the main relays and connected to trip all the other breakers on the bus if the proper breaker has not tripped within a half second after its trip coil was energised. Remote back-up is provided by a relay at the next station in the direction towards the source (fig. 1.6) which trips in a delayed time if the breaker in the faulted section is not tripped. It usually consists of an inverse time-current relay or by the second and third zones of a distance relay. This i~ the most widely used form of back-up protection. 1.9. RECLOSING (4) (70)

In cases where continuity of service cannot be maintained by quickly isolating the faulted circuit from the system, automatic reclosing relays are used to reconnect the circuit so that, if the fault is a transient one, the system is returned to normal operation. Automatic reclosing is used mostly on overhead transmission and distribution lines because there is statistical evidence that 90 % of the faults on such lines are caused by lightning or by objects passing near or through the lines (birds, vines, tree branches, etc.). These conditions result in arcing faults which can be extinguished by opening the circuit breakers to de-energise the line. Reclosing immediately the fault arc has been interrupted is hence a practical means of minimising the interruption to service, especially at unattended stations. Where there is only one transmission line between an important load and its power source, single pole switching is used, i.e. interrupting and reclosing only the faulted phase so that power is never completely cut off. The combination of high-speed tripping and high-speed reclosing is nearly equivalent (as far as disturbance to the rest of the power system is concerned) to the ideal condition of eliminating faults. B 17

1.10

Protective Relays

On high voltage lines where most faults are caused by lightning and where contact with trees, etc., is unlikely, a single instantaneous reclosure is used. Tests on high-voltage systems have shown that reclosure in 12 cycles is practical, the period depending upon the time necessary to dissipate the ionised air at the fault. Fast reclosing limits the phase separation of synchronous machines while the breaker is open and hence reduces the power oscillation which follows reclosure. On low voltage systems the fault may be caused by physical objects, such as tree branches and vines, which may require one or more reclosures to burn them clear. The usual procedure has been to reclose three times at intervals of between 15 to 120 seconds. If the breaker reopens after the third reclosure, the relay equipment locks it open, and it becomes necessary to reclose by hand. Four automatic tripouts of the breaker in succession must certainly indicate permanent damage on the feeder, such as a broken wire, a wire down on a tower or on the ground or other trouble which should be cleared before again energising the circuit. This will not be considered in detail because it is outside the subject of protection. 1.10. OTHER RELAY FUNCTIONS

Relays of the same types as those used for protection are also used for control and regulation. For instance, a voltage relay with both normally open and normally closed contacts can be used for progressive tap changing to keep the voltage between desired limits. The same principle may also be used for control of other quantities such as frequency or reactive kVA. It is probable in the future that protection and automatic control of power systems will be done together and that eventually power systems will be entirely automatic and both controlled and protected by static equipment. 1.10.1. Circuit Breaker Control

Fig. 1.11 shows a typical scheme which is known in the U.S.A. as the X-V scheme. It is 'trip-free', i.e. it permits the circuit breaker to be tripped by the protective relay even if the manual push-button C is held closed after the breaker is closed on a faulted circuit. It also prevents 'pumping', i.e. alternate tripping and closing if the closing button is held closed during a fault. The manual push-button switch C energises the contactor X which in turn energises the breaker closing coil CC. When the breaker has closed, the breaker auxiliary switch 'a' closes and 'b' opens, so that the release contactor Y short-circuits the coil of contactor X and the closing coil is de-energised. If the breaker is tripped by the protective relay PR it cannot reclose until C is opened and Y resets. 1.10.2. Seal-in and Flag Arrangements

Operation indicators have a flag or target normally hidden by a shield which is released when the relay is operated, exposing the flag. Alternatively, 18

1.10

Purpose of Protective Relays and Relaying

/ r(a)

Remote a.la.rm

,!~-

+0

Protective Auxil ia.ry Trip' rela.y (P. R.) switch cOil MECHANICAL FLAG Shunt or series sea.l-in rela.y required where trip coil current excuds 5 a.mps (one for 3 pha.ses)

+o---~

P.R.

(b)

Aux.sw.

SHUNT REINFORCING WITH FLAG Trip circuit sea.ls in until rela.y resets

Sea.l-in a.nd /Ia.g (c)

Trip' coli

Aux. sw.

Trip coil

SERIES SEAL-IN WITH FLAG Trip circuit sea.led In unttl brea.ker opens FIG.

1.10. Operation indicators and seal-in relays

+~)-~~llp-.R.--~------~JI-

Y. T.C.

R

y

I

b

x

C.C.

FIG. 1.11. The X-Y scheme for circuit-breaker control C.C. = closing coil; T.C. = trip coil; X = closing contactor; a, b= breaker auxiliary switches; Y = release relay for X; C = closing switch (manual); T = tripping switch (manual); P.R. = protective relay All contacts are shown in the de-energised position

the shield may be stationary and the flag may appear from behind it. The release of the flag latch may be done mechanically by movement of the armature of the relay or electrically by a solenoid when the relay contacts close. It is restored manually by a station attendant after the relay operation has been recorded, usually by a push rod in the relay case or cover. There are a number of points in favour of an electrical operation indicator. It assists standardisation because mechanical ones cannot be used with delicate or high-speed relays nor with relays having two units both of 19

1.11

Protective Relays

which have to operate to cause tripping; the electrical operation indicator simplifies relay application problems because it can be used with all types of relays. It is also preferable to the mechanical operation indicator because it indicates that the relay has caused the trip coil to be energised whereas the mechanical operation indicator merely indicates movement of the relay armature: finally, it is very difficult to release the mechanical operation indicator at exactly the same moment that the contacts meet and thus there is a risk of the one happening without the other. The series electrical operation indicator is shown on the left side of the relay in fig. 1.1. Fig. 1.10 shows typical connections of series and shunt sealin relays. The electrical operation indicator is often combined with an auxiliary contactor to provide extra contacts for other functions such as remote alarm. One pair of its contacts is connected across the main relay contacts so as to by-pass the trip current from them and thus minimises maintenance on the contacts of the protective relay, which are sometimes critically adjusted for precise performance. Maintenance on the seal-in relay contacts is easier because it is a simple 'go' or 'no-go' device. The seal-in feature prevents the protective relay contacts from interrupting the trip-coil current if the relay resets before the breaker auxiliary switch interrupts it. 1.11. DUALITY IN THE ANALYSIS OF RELAY CHARACTERISTICS

In both volumes, frequent use is made of the concept of duality as an economical and facile aid to analysis. The topic is mentioned at this stage because it has been largely neglected hitherto in relay engineering. The concept of duality is based upon geometry, vector algebra and tensor analysis, but a knowledge of the latter is not necessary to understand duality in relays. Examples of duality in relay circuits and power system analysis are: mesh current open-circuited terminals constant potential (Thevenin's theorem) series resistance inductance capacitance shunt resistance inductance capacitance relay circuits based on blocking relay circuits based on balanced voltage amplitude comparison the fuse

nodal potential short-circuited terminals constant current (Norton's theorem) shunt conductance capacitance inductance series conductance capacitance inductance relay circuits based on tripping relay circuits based on circulating current phase comparison the surge diverter (lightning arrestor)

20

Purpose of Protective Relays and Relaying

1.12

Not all of these dual relationships are obvious but they will be explained in the ensuing chapters. A very common case of duality in relay engineering is that the inverse of a circular characteristic passing through the origin of an impedance diagram is a straight line not passing through the origin on an admittance diagram. 1.12. CLASSIFICATION OF RELAY SCHEMES

A protective relay scheme consists of one relay or a group of relays which protect a section of line or piece of equipment against faults (9). The most common schemes are the following: 1.12.1. Time-Overcurrent Relaying

This scheme is used on most low voltage distribution networks. It takes advantage of the fact that, when one section of a network develops a fault, current flows into it via the remaining healthy sections so that the faulty section has the most current. If the overcurrent relays are provided with damping (fig. l.la) their operating time will be inversely proportional to the current magnitude and the relay nearest the fault will work fastest because it has the most current and hence will open its breaker and clear the fault before any of the more remote relays can do so. An alternative to inverse time-current relays is definite time relays. Because their time is fixed, irrespective of current magnitude, such relays have to be graded in time. This is practical on radial lines or loops but the inverse relay is preferable for complex networks. 1.12.2. Directional Relays

In certain equipment, such as generators, power will always flow outwards except if the generator has developed a fault or has lost its driving source, so that it is motoring and drawing power from the network. Such a condition is detected by a directional relay which closes its contacts for power (or a component ofkVA at a suitable angle) flowing in an abnormal direction (fig. 1.2). Directional relays are also used to control time-overcurrent relays where the power sources are so located that as much current passes through the relay for an external fault as for an internal fault in the circuit it is protecting. Such relays work on the product of the circuit current and potential. If the product is positive, let us say, the torque closes the relay contacts; if negative, it holds them open. Thus the relay can be arranged to trip only when the current flows out from the bus. Consequently, by connecting a directional relay in series with each overcurrent relay only the relays at the two ends of the faulty section will operate, thus isolating the fault without disturbing the other lines. Overcurrent and directional relays are discussed in detail in Chapter 4. 21

1.13

Protective Relays

1.12.3. Distance Protection

Where time delay is undesirable distance relays are often used. For a line section of given impedance ZL the current flowing through the section to a fault will produce a voltage V = IZL • Hence, if the relay compares V with 1 and is arranged to trip when V < IZ, it in effect measures Z =

-r

Since Z is

proportional to the length of line (5) the relay can be set to trip only for faults within the protected section of line (fig. 1.2). Selectivity is much easier to obtain with distance relays than with overcurrent relays because their reach is unaffected by current variation due to changes in generating conditions and system switching. 1.12.4. Unit Protection

The most positive method of protecting a circuit is to arrange relays to compare the currents entering and leaving it (fig. 1.1 b), which should be the same under normal conditions and during an external fault. Any difference current must be flowing into a fault within the protected circuit. When this system is applied to electrical equipment it is called differential current protection. When it is applied to lines or cables it is called pilot differential protection because pilot wires or an equivalent link or channel is required to bring the current to the relay from the remote end of the line. Since unit protection operates only for faults within the protected circuit, back-up protection must be provided which is inherent in time-current and distance schemes. 1.12.5. Balanced Current Protection

Parallel circuits of the same impedance should normally carry the equal currents. A fault in one circuit will increase the current in that circuit and operate a relay that compares the two currents. In the case of two parallel lines this is called 'current balance' or 'balanced current protection'. In the case of a generator with split windings it is called transverse differential current protection. 1.13. PRACTICAL APPLICATION OF RELAY SCHEMES

These schemes, their variations and methods of application are discussed in detail in later chapters. Chapter 2 describes the various types of relays that can be used to produce these characteristics, and Chapter 3 their operating characteristics. 1.13.1. Current Transformers

In order that a relay may be selective it must measure electrical quantities in the protected circuit accurately, and this means that c.t's and p.t's must· maintain their ratio over a wide range of conditions. The ability of a c.t. to do this depends upon the impedance of the relay and lead wiring that is connected across it. The higher this impedance is the bigger the iron-circuit

22

Purpose of Protective Relays and Relaying

1.13

of the c.t. must be to cope with the IZ voltage across the load. Hence in the interest of economy of C.t. cost and space requirements the lead runs must be short and the relays must be sensitive, i.e. the power they require to close their contacts firmly under borderline conditions must be as little as possible. The power required to operate the relay is called the relay burden. In the U.K. it is expressed as the volt-amperes (VA) at pick-up and this is an index to the sensitivity of the relay and is a constant value irrespective of the rating. In the U.S.A. the relay burden is usually given as VA at C.t. rated current or impedance at rated current. The latter gives no idea of the relays sensitivity but enables the total burden on the c.t. to be calculated more easily, especially if the relay saturates below C.t. rating. Nowadays in all countries there is a tendency to provide a curve of relay current circuit impedance over a range of current and this enables the C.t. performance to be calculated at short-circuit currents that are expected in the protected circuit. In particular it enables the c.t. secondary voltage to be calculated at maximum fault current so that a suitable C.t. can be chosen. Where the relays are remote from the c.t's the burden due to the leads will be high and, in order to have a C.t. of reasonable size, it is often desirable to use a lower secondary current rating. In the U.S.A. 5 ampere c.t's are almost standard and sometimes they are very large; in the U.K. 1 ampere or even t ampere c.t.s are used which is bad for standardisation of c.t's and relays but results in a more economical design. It is important to design test-gear and switchboard components so that there is no risk of open-circuiting the secondary of a c.t. because in this condition it can produce an extremely high secondary voltage which may break down the insulation and destroy the c.t. This is because, although the C.t. iron may saturate at a sinusoidal secondary voltage of a few hundred, the rate of change of flux near the zero points of the cycle can produce enormous voltage peaks. In later chapters and in volume II the subject of C.t. behaviour is considered in more detail. 1.13.2. Potential Transformers

Magnetic p.t's usually have a VA capacity which is more than sufficient to maintain their ratio with ordinary relay burdens. Their accuracy decreases at low voltages but is acceptable down to about 1 % of normal. Capacitor type p.t's of the voltage divider type have less good performance and, with rated burden, have an error of the order of 5 % and 5° at 3 % of normal v:oltage. In short, the secondary potential supply seldom creates any problem but the secondary current supply frequently does.

23

2 Belay IJeSigD alUl (;oD8tructioD Characteristics-Choice of Measuring Units-Construction of Measuring Units-Construction of Timing Units-Details of Design-Cases-Panel Mounting-Operation Indicative-Finishes N the design of a protective relay, the first stage is to select the characI teristics which will give the clearest distinction between faults in the protected section and all other conditions. Fig. 2.1, for example, shows an R/X diagram on which the characteristic circle of a mho relay fits around a

x FllUlt in l~ll~n9 pha.sc

)( Fllult in Ill99in9 pha.s~

------~~----~~~---,

FIG.

2.1. Characteristic of a mho relay enclosing the possible fault impedances (shown shaded)

shaded area which includes the impedances (including arc resistance) for all positions of fault within the protected zone; with such a characteristic the relay will not trip during power swings, or on faults other than those in the protected section and involving the phase with which the relay is associated (47) (58), The second stage is to choose a suitable relay construction; the third is to design the movement for the utmost reliability so that it will operate correctly even under the most adverse conditions. These three stages will be considered first of all in the light of general requirements, and then in terms of practical execution. The industrial trend is towards standardised designs

24

Relay Design and Construction

2.1

KIAI2_K'IBI2+ IAIIBICos (-B) =0

FIG.

2.2. Characteristic of any relay comparing two vector quantities A and B

of movement which can be applied as appropriate to meet the required characteristics of protection gear systems. 2.1. GENERAL REQUIREMENTS

The characteristics should be chosen and plotted so as to provide the greatest amount of information in the fewest possible curves. They should show clearly the conditions for tripping and blocking and, where pertinent, the times of operation under various operating conditions. 2.1.1. Factors Affecting Design and Construction (a) The characteristic of the relay must be such that it always operates for

the type of fault which it is intended to protect against, and not for any other conditions. (b) The relay must have a range of adjustment to permit it to operate selectively with other relays. (c) It should meet the specifications of the country where it is to be used. (d) A relay must be immune from transient effects, e.g. drop in voltage, peak currents, d.c. signals and harmonics. This applies particularly to high-speed relays. (e) The construction should be simple and accessible, so as to facilitate maintenance. (f) The wiring and terminal arrangement should facilitate testing and the tracing of faults. (g) The construction should facilitate the making of minor modifications to meet unusual conditions of temperature, humidity, corrosive atmospheres, vibration, mechanical shock, etc. 2.1.2. Reliability

The most important consideration in the design of a relay is reliability, which should be in the designer'S mind when making every decision. Simplicity of construction and circuitry makes reliability of operation easier to achieve. The most important constructional feature in this respect is contact pressure, since the main purpose of a protective relay is to close its contacts

25

2.2

Protective Relays

effectively and correctly, even under adverse conditions and in the event of inadequate maintenance having been carried out (l05). It must be remembered that a relay spends at least 99·999% of its life stationary, during which time there is a tendency for contacts and bearings to deteriorate so that, when a fault does occur, the relay may not be able to respond properly unless it is designed with these conditions in mind. 80me of the general rules which are followed by most responsible manufacturers are: (a) The use of wire not less than 0·002 in. diameter; the proper support and wrapping of the beginning and end wires and their junctions to the external leads (see Coil Design, section 2.6.9). (b) Design for maximum torque/friction ratio in order to promote accuracy and avoid 'sticking' after long periods of non-operation (see Bearings and Backstops, sections 2.6.1 and 2.6.2) or failure to trip due to contact corrosion. (c) Design for minimum contact maintenance; for this the contacts should be bounce-proof since otherwise arcing, with consequent pitting of the silver, will ensue (see Bounce-proof Contacts, section 2.6.4). 2.2. RELAY CHARACTERISTICS

To recognise a faulty circuit most electrical protective relays measure the current entering the protected circuit and compare it in magnitude and/or phase relation with some other convenient quantity; these include the current emerging from the protected circuit, the local bus-bar voltage and constant quantities such as gravity of the force of a calibrated spring. It follows that such relays must be able to carry out addition, subtraction, multiplication and division of scalar and vector quantities, as required. In some cases these electrical quantities are compared in more than one phase, or more than one circuit. The purpose of this chapter is to consider the various devices which are capable of measuring and comparing electrical quantities. Since the two input quantities can produce torque singly or in co-operation, the equation for the characteristic of the relay at the threshold of operation under steady state conditions, when plotted on a diagram whose axes are

I~I

cos tP and

jl~l sin rp, must be of the form (2.1) KIAI2-K'IBI 2 +IAIIBlcos(4)-8)-K'' = 0 where A and B are the two electrical quantities which are being compared, K and K' are scalar constants, K W is a constant representing a bias which would take the form of a mechanical restraint in an electromagnetic relay, 4> is the phase angle between A and B, 8 is a fixed predetermined angle, e.g. in an electromagnetic relay 8 is the value of tP which provides maximum relay torque.

26

2.3

Relay Design and Construction

The equation (2.1) represents all the circular and straight-line characteristics which can be obtained from any two-input relay. This equation is applicable to most of the common types of relay and simplifies the explanation of their operation and characteristic curves (as shown in fig. 2.4). K" is finite only in single quantity relays where it is used as a level indicator; it is made substantially zero in relays that compare two input quantities and in this case the equation represents a circle or a straight line on a complex plane (polar diagram). This can be demonstrated by rewriting equation (2.1), making K" = 0 and dividing throughout by K'A2: K K'

_1~12 A

+

I~ICOS(-(}) = 0 A

(2.2)

K'

Moving the :, term to the right hand side and adding

(~,) 2 to each side,

equation (2.2) becomes

(2.3)

I~r -I~I cos ~,- (}) + 12~T = :' + 12~T Equation

·IBI . 'I'"" A sm

J

(2.3) represents a circle on a complex plane having I~I cos and

as co-ord·mates, th e ra d·IUS b· emg

and the centre being at

JK + 11 12 J1+4KK' K'

2K' =

2K'

2~' from the origin at an angle () from the reference

axis, as in fig. 2.2. The axes of the steady state characteristic diagram have been designated

I~I p and j I~I q for I~I cos and j I~I sin respectively.

This will be discussed more fully in Chapter 3. In the case of distance relays A will be current and B voltage; the coordinates of the diagram will be

WI cos and j WI sin ,

in other words

R andjX. 2.3. CHOICE OF MEASURING UNIT

In broad terms protective relays are in two categories of construction, (a) those which are wholly electromagnetic or electrothermal, in which the

comparison or measurement is done by the relay itself by balancing two forces or magnetic fluxes and (b) using a static comparator in which the comparison is done in a static circuit by comparing two or more currents or voltages and feeding the resultant output into a slave device which takes the required action. The slave devices are at present electromagnetic relays but, when static devices of adequate capacity and reasonable price become available, such as controlled silicon rectifiers, they will enable wholly static relays to be

27

Protective Relays

2.3

ATTRACTED ARMATURE RELAYS

-a 00

Pol(1t1sc:d

Ba.la.nccd

P lung e r

b~a.m

MOVING COIL RELAYS

A. i",lIy moving coli

Roto..ry moving c.oil

DynQJl'lom e tcr

INDUCTION CUP RELAYS

~ 2

4

pol~

~ 8 pol e

po l ~

Induction

dyno.momctc.f"

m Sp lit cup

INDUCTION DISC RELAYS

E.M

EM.

P M.

THERMAL RELAYS

o

=

0

Unimcta.lli c

Bil1'l c\o.lI i e

FIG.

2.3. Some electromagnetic relay units

28

P.'"

Relay Design and Construction

2.3

produced commercially. For convenience, the second category (b) will be referred to as static relays. Table 2.1 compares eight types of relay construction, four electromagnetic and four static. Fig. 2.3 shows diagrammatically the physical arrangement of some electromagnetic types. Tables 5.9 and 5.10 (in Chapter 5) show some static relay arrangements and a number of illustrations and diagrams of polarised and unpolarised slave relays are provided in the present chapter. The numbers in Table 2.1 represent order of merit rather than degree, since the degree would depend very much upon the actual design. The highest number, 8, represents the best performance. It should be noted that the total 2.1 Evaluation of Comparator Units TABLE

Moving Armature Electromagnetic Quality

Attracted Induction Induction Thermal Armature Cup Disc

Low Cost Accuracy Speed Output Quality Sensitivity Stability Robustness Simplicity Experience Total

5 2 5

1

4

1

5 8 6 37

7 3 2 4 2 4 4 6 8 40

4 5 4 3 3 2 6 5 7 39

8 1 1 2 1 3 3 7 5 31

Static Comparators with Slave Relay Electronic Transistors R~tifier MalP!etic BrIdges Amplifiers

1 8 8 7 8 6 1 2 2 43

2 7 7 6 7

1

7 1

1

45

3 6 6 8 6 8 2 4 4 47

6 4 3 5 5 5 8 3 3 42

for anyone class does not necessarily represent the view of industry or its acceptance by it; anyone class may be precluded by poor performance, or attainment in anyone quality, or for reasons which are not entirely technical. Reliability is not listed because it is covered under stability, simplicity, robustness, etc. Stability includes overtravel and transient overreach. Experience is confined to their use in protective relays. In the column headed 'Quality' the fourth item 'output quality' refers to the steadiness of the torque or force in the case of electromagnetic relays and to the smoothness and range limitation of the output voltage or current in the case of static relays. 2.3.1. Attracted Armature Relays

This group (first column) includes plunger, hinged armature, balanced beam and moving iron relay~. As measuring units they are handicapped by inherently low reset/pick-up ratio and inadvertent operation on sudden changes in circuit conditions. In this family there are also magnetised-shot relays; these consist of steel balls in a container surrounded by a coil which, when energised, causes the

29

~

sepa.ra. c SO no VI term

wlndln~

Va.nd I

8-75°

1(-/(#-0

8-90°

/('·1( 1i _0

K-K'-O

-K"

<

/K'

1

z<jf, K'

co.(~-8)

Z

1'sln ~
FIG.

V

.!: vce

xce

2.4. Common forms of induction disc and cup units

All eha.ra.etcristics a.re loci of Vor I for zero torque

COO

--

Q!.sl 1

--::,us-$~ •

--

Polcv dla.gra.ms Admltto.ncc or current Impeda.nec or potentia.l

~~~~. j ~' ~·9".

I

I

I

~~

-=

/~ /I

=~

-

/

.J

.J

~ .J

.J

'll

.

i

.

.J

:r

"0

'll

~ .. 'o~~~ ".

• [(I i ~ • ~'k\? I •."eo -GH ~

:::o--t--a voe

v<jf{ -K' VI COl (~-6»KN

~ coe

Rcla.y

1>$

Rela.y pic kup

+ VIeos (~-6) ~K" (Where oJI K's a.re torque eonsta.nt)

K/ 2·K'V'

-VI CoS (~-750)_x'V2

-Vlsin

VlcO'(~-6)-K'"

-K~V~

K/2.V/COs~-/l)

Genera.l rela.y «qua.tion -KI" -K'V"

Itnpcda.nc) (Ohm unit

Otrect,ona.J Impcdcu>ec (mho unit)

Rea.cta.nce (ohm unit)

Oireetiona.J

wind ings hcncc no I' or VI terms

No current

KI'-K"

No potcntla.1 windings hence no V' or Vlterm.

Oyereurrent

UndtrYOlta.ge

Result ing equa.tlon

Conditions

Type rda.y

~

It>


~

Qj

:tJ

~.

~.....

'"tJ

W

Relay Design and Construction

2.3

shot to move in the axial magnet field with considerable force after a time interval depending on the current magnitude, during which they align their magnetic axes. 2.3.2. Induction Disc Relays

These relays may have either shaded-pole or wattmetric type magnets driving discs or vanes (17). 2.3.3. Induction Cup Relays

The induction principle is one of the most widely used throughout the world. Fig. 2.4 shows some of its many applications to disc and cup relays. It has been applied to the great majority of relay functions and is backed by 30 years of experience. Its most attractive features are its steady, nonvibrating torque and its simple armature which requires no flexible connection. Figs. 2.5 and 2.6 show typical relays using the induction principle. With a cup-shaped armature, the induction relay can be made for fast operation with reasonable immunity from system transients and, properly designed, it can be given a very large operating range. Its drop-out is within a few percent of its pick-up, so that it can be used where normal and abnormal conditions are very close together. These relays can be 2- or 4-pole, single phase or 8-pole, three-phase. This class of relay includes a split-cup 4-pole unit which is similar to the 4-pole induction dynamometer relay; there are shaded-pole arrangements also. 2.3.4. Thermal Relays

The thermal types include bimetallic strips or spirals, unimetal strips and thermometric devices such as sylphons. In some early German relays they were used as comparators; the thermal movement acted as a current operated tripping unit and an electromagnetic or second thermal unit energised by the restraining quantity was arranged to control the position of a contact and hence the operating time. In motor protection, three thermal spirals energised with current from the three phases control differential contacts in a similar manner; this is described more fully in Chapter 9, section 9.2.3. Their advantages are simplicity and smooth consistent operation. Their principal disadvantage is low torque per VA input. 2.3.5. Electronic Relays

The high ratings of the relays based on electronic valves arises from the form of assessment adopted, and should not overshadow the fact that these relays have failed, over a period of 30 years, to obtain acceptance in the powerindustry (2) (26) (60) (131), This is notwithstanding the absence ofcontacts or bearings, with ensuing ease of maintenance and very fast operation, even when close to pick-up level. The reasons for non-acceptance are not hard to find (11); in spite of a number of excellent operating features these relays

31

2.3

Protective Relays

FlG.

2.Sa. Exploded view of induction disc relay unit with C-type magnet core

FIG.

2.5b. Exploded view of induction cup relay unit (4-pole)

32

Relay Design and Construction

o (a)

(b)

(c) flO.

2·6.

33

2.3

Protective Relays

2.3

failed, other than for special purposes, on grounds of complexity and the short life of vacuum tubes. The basic arrangements of two electronic relays are shown in fig. 2.7.

--u-

Transactor

+

=

Tro'p' cool

Rect,f,er

(a) POlorls lng tube

,...---.....-- +

v~ll\

Peak,ng tranSformer

y

00__----..., 1~ " 11 o~ 11

Thyratron

"

(b)

2.7. Electronic relay units (a) Time-overcurrent relay (b) Mho relay based on a pentode FIG.

2.3.6. Transistor Relays Tran~istor relays are being developed by several large electrical manufacturers, but at present there is insufficient experience with them to justify pronouncements about their future role and status. They appear to have the advantages of electronic relays based on thermionics, without their disadvantages, but it will be several years before it is known how consistent and stable their characteristics are and how effective are the means for protecting them, e.g. against voltage surges. When the necessary life tests have been done, transistor relays should be smaller, cheaper and faster than electromagnetic relays and should require little or no maintenance. In addition their great sensitivity will permit much smaller c.t's to be used and more sophisticated characteristics to be obtained. In existing relays transistors are used only as switches or amplifiers, so that changes in their characteristics due to temperature or ageing will not affect relay calibration. The first use of transistors in protective relaying was in the field of carrier protection. It should be noted, however, that at the time of going to press a

34

Relay Design and Construction Input

Input

B

A

(a)

2.3

Block diagram ofstatic relay

Collector 00

Emitter

(b)

Multiple inputs to transistor base

Trip

R

relay

i

A ~--------L-

ee)

________

~~

________________L---Q+

Inputs to muched trans ilto rs

FIG.

2.8. Transistor comparators used as relays

35

2.3

Protective Relays

Input

~ A Operate level '1\ ~~ Reset level I IV i '" -~-~-~---~ ~ Z.OI~~ I . I . I . I

21

i

Output

i

i

-.IlLL (d)

Op ..... tion 01 Schmitt level detector

Output

c

...rLn..I1.. Input

(I)

FIG.

Int.,,,,inl level d .. ector

2.8. Transistor comparators used as relays

36

Relay Design and Construction

2.3

substantial body of literature already exists on transistor and semi-conductor protection (12) (13) (14) (27) (28) (29) (59). Two elementary arrangements of relays based on transistor comparators are shown in figs. 2.8 (a) and (b). A comparison of the relative sizes of a transistor and an electronic tube of comparable performance is shown in fig. 2.9.

FIG.

2.9. Comparison of a transistor and an electronic tube

2.3.7. Rectifier Bridge Relays

There are very effective competitors to transistor relays in the form of relays based on semi-conductor diodes, some of which have been marketed for a number of years; the most common are relay comparators based on rectifier bridges (15) (62) (73) (109), These bridges can be arranged as either phase or amplitude comparators and are used in conjunction with a sensitive output relay, usually of the polarised moving iron or moving coil type. 2.3.8. Magnetic Amplifier Relays

These relays occupy an intermediate position between electromagnetic relays and electronic relays, because they have the robustness of electromagnetic relays together with some of the properties normally associated with static relays. The magnetic amplifier relay, if used as an amplitude comparator (transductor), is limited in its sensitivity by the sensitivity of the slave relay in its output circuit; if used as a phase comparator to obtain greater sensitivity, it

37

2.3

Protective Relays

is dependent upon an external a.c. supply which is sometimes difficult to arrange. Figs. 2.23 (a), (b) and (c) show some typical magnetic amplifier relay circuits. The use of the transductor has been described in references (16) (23) (24). 2.3.9. Summary

Examination of Table 2.1 shows a relatively small range of merit which indicates that the different constructions each have their own virtues and useful properties and that there is no universal type. For instance, although rectifier bridge relays have been given the highest merit score, it is doubtful if the induction disc relay will be displaced for time-overcurrent protection of feeders for many years to come. The induction disc relay is in almost universal use throughout the world as an overcurrent relay and as the basic element for a number of relays of more complex characteristics. The induction cup unit is in widespread use throughout the world, particularly for directional and distance relays, but its position is being challenged by polarised d.c. relays supplied from rectifier bridge comparators. Furthermore, these static elements are very competitive with transistor relays in their present state of development, other than for carrier relaying. Static relays require lower burdens than electromagnetic relays but tend to have less accurate characteristics because of feedback between the inputs. This limitation can easily be overcome, however, by amplifying the output of the comparator. It is obvious that two types of relay units are necessary, one for high speed and one where time delay is required. Both may be of induction type; for example, figs. 1.1 and 2.5 show assembled and exploded views respectively ofthe induction disc type which is suited for time delay, 'and figs. 1.2 and 2.6a the corresponding views of the induction cup type which, with its higher torque/inertia ratio, is suited for higher speeds of operation. Either type can perform any a.c. relay function and both are simple and robust in design. In the future it is ineVItable that static relays will supersede all electromagnetic types except the attracted armature type. The latter will continue because it can control many circuits for a low price and provide high insulation between them. For d.c. relays, small contactors, slave relays and instantaneous alternating current or voltage applications, a small hinged armature unit is commonly used, as shown in fig. 2.10. This unit can also be used as the movement for auxiliary relays and for indicating flag or target relays.

2.4. TYPES OF CONSTRUCTION FOR MEASURING UNITS

The relay units will not be described here in detail since such descriptions appear in the literature (17) (20) (22), including manufacturers' publications; modern improvements in design will, however, be mentioned. 38

Relay Design and Construction

FIG.

FIG.

2.l0a. Hinged armature relay

2.lOb. Blow-out magnet for contacts of hinged armature relay

39

2.4

2.4

Protective Relays

2.4.1. Induction Disc Relay

Early induction disc relays consisted of a watthour meter unit with a pair of bridging contacts substituted for the dial register. Modern designs employ a C-shaped magnet with shaded poles as, for example, in fig. 2.Sa and have three main advantages: (a) higher torque per VA input to the relay; (b) the single coil permits the use of large wire diameter; (c) two electromagnets and a damping magnet can be mounted on the

same disc for conversion into a differential relay. On the left-hand side of the disc in fig. 1.1 can be seen an Alcomax permanent magnet with high coercive force, which is smaller and more stable than the earlier chrome-steel magnets. It can be mounted on the side of the frame as shown and permits removal of the disc without disturbing the position of the driving magnet and hence the relay calibration. High contact pressure is obtained partly by the use of a magnet system which produces a high torque on the disc and partly by the use of a single contact with a flexible lead-in, rather than the older method of bridging contacts. In addition, by using cylindrical contact surfaces at right angles, the maximum contact pressure per unit area is achieved without using a fixed point of contact; this ensures that the arc which forms on closure is not localised. Fig. 2.Sb shows an electl;omagnet in which the shape of the time-:current curve is adjustable so that calibration. is made easy even with wide variations of magnetic characteristics of the laminations. It is also somewhat more efficient in torque per VA from the c.t. (18). The torque of these relays is proportional to the sine product of the two magnetic fluxes cutting the disc. Where both fluxes are produced by the same quantity, as in .an overcurrent relay, the torque is proportional to the square of the current, i.e. T = [(J2; K decreases with increase of current owing to magnetic saturation and is used to give a definite minimum time feature to the characteristics of overcurrent relays. The vector diagram of the induction relay units, illustrated in figs. l.la and 1.1 b, is shown in fig. 2.11, whereas that for another shape of driving magnet is shown in fig. 2.12a. The former, as would be expected from its simpler shape, is more efficient than the latter in terms of the proportion of total flux ~t which is available at the air-gap for the production of torque. In general, no induction disc relay is very efficient viewed from this particular standpoint, although some are considerably better than others. The other major limitation on torque producing properties is the maximum angular displacement between the working fluxes which can be produced by shading. This, together with the theory of torque production, is dealt with in section 2.4.3. In inverse time-overcurrent relays it is important that the disc or cup should not continue moving under its own momentum after the fault current has been interrupted at another relaying point; otherwise, additional circuit-

40

Relay Design and Construction

2.4

breakers may open. In modern relays the operating torque is high and they must have a correspondingly high damping torque to meet a given time current curve; this heavy damping serves to stop the disc very promptly. At twenty times tap setting the contacts of such relays would not close if the current should be shut off 0·04 second before the moment at which they would have closed if the current had continued. 2.4.2. Induction Cup Relay

For high-speed operation, where polarising and/or differential windings are required, a 4-pole or 8-pole electromagnet is used in the fashion of an induction motor (see figs. 1.2 and 2.6). This is a very flexible arrangement, used in many countries, and will produce almost any steady-state relay characteristic with identical components except for the coil windings. The high operating speed is obtained by forming the induction disc into an inverted cup, so that its inertia is greatly reduced and by designing the pole !;ystem to give maximum torque per VA input. In the 4-pole unit almost all the eddy currents induced in the cup by one pair of poles appear directly under the other pair of poles, so that the torque per VA is about three times that of the induction disc even when the latter is equipped with a C-shaped magnet. The greater efficiency of the induction cup is due to the arrangement of its magnetic circuit to minimise magnetic leakage and to reduce the resistance of the rotor induced current paths. By designing to avoid magnetic saturation, the 'operating characteristics of the relay can be made linear and accurate over a very wide range, with pick-Up and reset values close together; this simplifies application and testing. The relay tends to be inherently self-compensating for d.c. transients originating in the power system and proper design can minimise its response to other system transients as well as those associated with c.t.s and the relay circuit. This relay unit is particularly suited to a role as a directional or phasecomparison unit, not only on account of its sensitivity and speed but because it has a steady non-vibrating torque and its parasitic torques due to current or potential alone are small; furthermore, these particular parasitics can be eliminated by using a vane on the cup and a flat on the rotatable centre core, respectively. Interaction between the pole fluxes can be minimised by the· use of an iron base (lower part of fig. 2.6a) which also forms a machining reference for the accurate location of bearings, etc. Since it has no connecting leads, the rotor or the contacts can be removed quickly without disturbing the contact settings. As in the case of the induction disc relay, minor modifications can be made to provide special characteristics, most of the parts remaining standard. For instance, an 8-pole electromagnet (fig. 2.6b) can be substituted for the standard 4-pole stack of laminations if an electromagnetic polyphase directional relay is required (32). Furthermore, a holding magnet can be added to provide either harmonic restraint or a voltage restraint. 41

2.4

Protective Relays

The torque of the induction relay is proportional to the sine product of the two magnetic fluxes produced by the two pairs of poles, i.e. 1~111~21 sin (X where ~1 and ~2 are the magnetic fluxes and (X the angle between them. If ~1 and ~2 are produced linearly by two electrical quantities A and B respectively, the torque is proportional to IAIIBI cos (q,-O), where q, is the angle between A and B and (J is the value of t/J which gives maximum torque. 2.4.3. Theory of Induction Relay Torque

The two magnetic fluxes ~1 and ~2' which together create the torque, each generate eddy currents in the disc or cup; each flux reacts with the eddy current produced by the other flux to produce two torques, the sum of which is a steady, unidirectional torque. To produce torque, the induction relay or instrument must have its two working fluxes displaced in time-phase. In the bi-polar type of magnet core, illustrated in fig. 2.12, there are two separate exciting windings which give two out-of-phase fluxes in two separate magnetic circuits. In the case of relay cores of the type shown in fig. 2.11, there is only one exciting m.m.f. and the necessary pair of two out-of-phase fluxes are produced by shading action. The principle involved is illustrated in iig. 2.13. The flux ~ is shown divided into two components, ~1 and ~2' in the shaded and unshaded portions of the pole respectively. ~1 acts as the mutual flux of a transformer of which the shading ring or band is the secondary. The e.m.f. elf induced in the shading band produces therein a circulating current (an eddy current in the case of a flux passing through an induction disc) i,,; the m.m.f. of the singleturn shading band is clearly F"t = i" amp-turns. The difference between F. t and F., the effective m.m.f. of the shading band, is the m.m.f. responsible for leakage flux. The total m.m.f. of the pole is F1 = F2 + F". It should be noted that F. has a component F2 cos

(~- A.)

= F" sin A. in phase opposition

to Flo and thus has a demagnetising effect. The disc, itself a shading element, thus also has a demagnetising effect which must be considered in deriving the steady-state torque equation of an induction relay (fig. 2.l3c). The air-gap fluxes ~1 and ~2 and the eddy currents i1 and i2 they produce in the induction disc or cup are shown vectorially in fig. 2.13b. Torque is produced by the interaction of each flux with the disc current induced by the other flux and is of the form 't' = ~li2-~2i1' In the Appendix 2.11 it is shown that the torque works out to (2.4) where K is a design constant, 11 and 12 are the r.m.s. coil currents producing the magnet air-gap fluxes, (J is the angle between 11 and 12 and A. is the phase angle of the disc impedance. In the shaded-pole magnet the fluxes are produced by the same coil current so the torque is 't' = Kro12 sin (J cos A. (2.S)

42

Relay Design and Construction

Nonshad.d

pol. (a)

2.4

Ii /'i,'

Ii

I'

I I

I , ,4>E ,i , '' ,, :, , ,,

.

,, "',1 .'

(b)

" ' - -- - Ref.ronce FIG. 2.11. Inverse time-overcurrent relay with C-type magnet core (a) Magnet shape, (b) Scale diagram of flux distribution throughout the relay core

43

~

(b)

, .. - - - - - - - - - - - - -

------

2.12. Inverse time-overcurrent relay with wattmetric type magnet (a) Magnet shape. (b) Scale diagram of flux distribution throughout the relay core FIG.

---------

ta)

.----. ----~

T

~

iii

~

(1)

:0;'

~....

."

~

N

F;--------------------(b)

(a)

~~~------------,_--------~~~2

(c)

(d)

2.13. Principle of shading in an induction disc relay (b) Flux and m.mJ. vector diagrams of (a) (a) Simple shaded pole (c) Disc currents and gap fluxes (d) A typical shaded-pole electromagnet. FIG.

45

F

2.4

Protective Relays

In either case the steady-state torque is 't

=

K'wIII2 sin 0

(2.6)

In the case of high-speed induction cup mho relays where memory action is used in the polarising circuit, a beat frequency torque may appear if the polarising potential circuit is tuned to natural resonance at frequency n instead of at the system frequency wand the torque is 't

= KI l12{(n- w) sin [(w+ n)t+l/I] +(n + w) sin [(w- n)t- I/I]}

(2.7)

where 1/1 is the phase displacement between the two currents at t = O. Since (w - n) is usually small this simplifies to 't ~

2wK/I I2 sin «w - n) t - 1/1)

(2.8)

(.) Equivalent dl<1gram of induction cup unit

I

1...

I,..

(b)

Vector di<1gram of on. pole of mductlon cup unit

~ I

b' /

/

~

................. .{

I

I

I

~r~

'

,

'f.

I.J

Ill'r

I

I

......

/..'~----- -~~ / S .....

'..... ..........

..... ,

.-/ '1z

/

(c) Flux di<1gram of both pol.circUlh of Induction C,,!, unit

flO.

2.14. Vector relationship of current and fluxes in induction cup relay

In an induction disc relay of the shaded-pole type the torque depends on the proper ratio of the shaded to the unshaded pole face areas and the crosssectional area of the shading rings. At 50 to 60 cycles the shaded area should be approximately equal to the unshaded area. The shading rings should be of as low resistance as possible and wound as closely as possible around the pole tips to minimise magnetic 46

Relay Design and Construction

2.4

leakage. Their cross-sectional area is usually about one-third of the pole face area but has to be compromised with the dimensions required for minimum frequency error. The mathematics governing the design of shading rings are very complicated and a certain amount of trial and error is necessary. 2.4.4. Attracted Armature Relay

Most manufacturing companies make a wide variety of auxiliary relays and contactors; this variety tends to arise because the original design has had to be modified from time to time to meet new requirements. Now that most of the requirements for protection are known it has been found possible to make one standard hinged armature unit to fulfil the great majority of requirements for auxiliary relays, a.c. and d.c. instantaneous voltage and current relays and electrical operation indicators (68). Such a unit is shown in fig. 2.10. It is applicable to a.c. or d.c. and can be self, hand or electriCally reset; it can have an operation indicator and up to four' pairs of contacts, one or more of which can be latched in. It can be the operating unit of all auxiliary relays such as annunciators, semaphores, alarm relays, etc. In the interest of universal application its VA consumption is low, being of the order of 0·08 watt at pick-up with one contact, or 0·2 watt with four contacts. The same unit can be polarised for sensitive d.c. applications by the addition of a permanent magnet. It can also be used for controlling heavy currents by the addition of a small blow-out magnet for d.c. or contacts of special material for a.c. (fig. 2.10b). The pull on the armature of an ordinary non-polarised attracted armature relay is proportional to the square of the flux in the gaps. The gap flux is proportional to the coil current below the level at which core saturation takes place; it decreases inversely, however, almost as the square of the total magnetic gap, i.e. the armature gap plus any other gaps which there may be in the magnetic circuit, such as those introduced by joints, (20) (21). The pull on the armature in C.O.s. units is F

=

2n(NI) 2

A(Ro+~r

(2.9)

where N is the coil turns, I the coil current, A the pole face gap area, Ro is the reluctance of the iron circuit, x the air gap at the pole centre. In the open position Ro is small compared with

i:

so that the pull can be written

F = 21t(N~2A

x

(2.10)

A comprehensive study of the design of these relays has been made by Wagar and Peck (146). The hinged armature relay increases its attractive force as the armature

47

Protective Relays

2.4

approaches the pole-piece (20) (21); referring to fig. 2.15, this increase in attractive force tends to give a snap action and a drop-out value at a low level of energisation unless the pressure built up by the contact springs is approximately matched. Line S shows the build-up of pressure of the restraining spring. Line C is the additional pressure due to the contact brushes, and CR is the pressure between the contacts when closed; this should be at least 15 gm. Curve R is the force on the armature; this force may tend to level off as the relay core saturates in spite of the decreasing magnetic reluctance of the armature gap as it closes. The ratio of pick-up to drop-out current is OC/OR. To obtain a high value of drop-out current, C must be close to R; it is not possible to make C lie R EFFECT

0,.

SATVRATION

OC

t:

OROP OFF

o R = PICK-

c

YP.

S'c' : CONT-'CT PRESET.

CONTACT PRESSURE.

PYL.L.. A.T MIN. VOL..TS.

6

-Wipe

o FIG.

G.... P -TRAVEL. .

2.15. Attractive force or 'pull' characteristic of a hinged armature relay

very close to R, however, because it is a normal requirement that tripping relays must operate down to 75 % of normal voltage although saturation helps this situation, as is apparent from fig. 2.15 (see dotted curve to R'). Fig. 2.16 shows how the attractive force on the armature varies with ampere-turns. The attractive force increases as the square of the flux and hence square of the ampere turns until point A is reached. It thereupon increases less rapidly from B to C as saturation occurs. For the relay with the characteristic shown in fig. 2.16 the drop-out is low due to the effect of remanence which tends to hold the armature in the operated position down to a low value of ampere-turns. Low remanence iron such as radiometal is used for correcting this and for obtaining drop-out at a higher value of ampere48

2.4

Relay Design and Construction

DROP' OUT

100,%

FlO.

2.16. Hysteresis curve of flux in a hinged armature relay

turns; a less efficient arrangement for the same purpose is an air-gap in the iron circuit or a brass pin in the pole face, which prevents the armature from touching the pole and limits the flux value at point B. 2.4.5. Balanced Beam Relay

This construction was once popular for high-speed differential and impedance relays (fig. 2.17 (61)). Its popularity is now waning because of its overreaching on faults with offset current due to high X / R ratio. Unless means are employed to slow down the speed of operation, a fast balancedbeam unit will follow the half cycles of energising current and/or voltage so that the ratio of operating to restraint quantities depends upon their phase relationship. Furthermore, the resetting value of a balanced-beam unit is low compared with its operating value on account of the fact that the magnetic gap is small under one pole in the normal position and large under the other; in the operated position, this situation is reversed. The force at each. end of the beam is proportional to the square of the gap flux, as in the attracted armature relay. The gap flux is proportional to the current and decreases inversely approximately as the square of the total air gap length in the magnetic circuit.

It is capable of very fast operation but is not accurate because of the tendency to phase angle error mentioned above, and because of its susceptibility to incorrect operation with d.c. transients in the energising quantities. The phase angle error can be eliminated by modifying the voltage restraint magnet; a split-pole or three-legged construction should be used wherein the flux is split into at least two components which are equal in magnitude and as near to 900 out of phase as possible for two components, or 1200 apart for three components. An alternative is to rectify the restraining quantity. The discrepancy between the pick-up and drop-out values can be reduced to a certain extent by limiting the motion of the beam and introducing an air gap into the magnetic circuit of the current pole; this latter device minimises the total change in reactance when the beam moves. c

~

Protective Relays

2.4

(a)

I

(b)

Tonlon hca.d

Inntl~

diScs

Fixed conl"cl

Air

-."po

(c)

2.17. Balanced beam relay unit (a) Basic arrangement (b) Beam unit with filter for reducing d.c. transients in the current circuit and a phase-splitting device in the voltage circuit (c) Balanced vane unit FIO.

50

Relay Design and Construction

2.4

The most troublesome of the transients to which the balanced beam is susceptible is assymmetrical d.c. in the current wave. Since the operation is proportional to the square of the current, a 50 % d.c. transient will increase the torque by more than a factor of 2, and may give rise to overreaching, i.e. tripping on faults external to the protected section. To prevent this, a d.c. filter may be used as shown in fig. 2.17b but a reactor of very high

Q( =

a:) is required.

Fig. 2.17c shows another form of balanced attracted armature relay which has the foregoing weaknesses to a slightly less degree because it is slower and has a small amount of inherent damping. To summarise, the balanced beam relay is a simple and economical relay with limited precision; it may be used as a starting device for other units in a protective scheme but it is not suitable for any primary role. 2.4.8. Moving Coli Relays

The polarised d.c. moving-coil relays (fig. 2.1S) are the most sensitive electromagnetic relay units available, but they are generally more expensive than induction cup or moving iron relays. They are adapted to a.c. measurement and comparison by using tHem in conjunction with rectifier bridge comparators. Two kinds of these relays are available, those with a rotary moving coil, and those with an axially moving coil. The rotary moving coil type has jewelled bearings and is cheaper since it uses standard d.c. instrument construction, which is easy to assemble and align (62). Furthermore, it is unaffected by tilting. Its arrangement is after the fashion of a moving coil ammeter with contacts (fig. 2.1Sa). The axially moving coil type is twice as sensitive from an electromagnetic viewpoint since it has only one radial air gap in its magnetic structure; it has, however, to be mounted within a few degrees of vertical and encounters the problem of coil supports. Early types had six connection ligaments (22) and two suspension ligaments (fig. 2.1Sb) to support the two ends of the coil. Fig. 2.1Sc shows a construction in which the spiral diaphragms support the moving coil and also act as electrical connections; in this type the coil has only axial movement, so that the air gap can be made small; this enables the same sensitivity to be obtained with a smaller device. The attractive force of the relay is directly proportional to the coil current. Typical sensitivity figures are 0·2 to 0·5 mW for just closing the contacts. The speed of operation depends upon the damping; two cycles minimum time is possible with a properly damped coil with an aluminium former. Copper can be used for heavier damping and slower operation. Insufficient damping reduces the capacity of the relay to withstand shock and vibration. In cases where there is only one coil, such as in the parallel rectifier bridge circuit of fig. 2.21a, the aluminium former can be eliminated since damping is supplied by the coil itself, and the rectifier bridges provide a path for the 51

2.4

Protective Relays

lC<1d-in.

Cont4cts

Yoke

Plrma.nlnt ma.gnlt Moving call

{al

52

Relay Design and Construction Flud conta.ct MovIng contl1.C:t

L..ca.f ,uspenslon

Loca.ting tong

Soft .ttOl yoke

(b)

3 conta.cU on

Pcrspu cowr

ruihQnt disc

a.nd side ring

Pcrma.ncnt magnet

Coil winding

(c) FIG. 2.18. Moving coil relays (a) Arrangement of rotary moving coil relay (b) Constructional details of axially moving coil relay (c) Constructional details with spiral diaphragm coil supports

53

2.4

Protective Relays

2.4

damping current induced in the coil by its own movement. Where the relay has separate operating and restraining windings, the metal former is necessary for damping (22). Recent improvements in permanent magnet materials have increased the sensitivity of moving coil relays so that the rotary moving type can now be made with 0·1 milliwatt sensitivity and yet be mechanically stable (50 g) and employ wire of 0·002 inch diameter or larger; units with such powerful magnets must be sealed against the ingress of iron filings. 2.4.7. Polarised Moving Iron Relays

This type of relay is the inverse of the moving coil because the iron armature moves. The stationary coil results in a much more robust relay and permits a remarkably high ratio of continuous rating to pick-up. It is shown in figs. 2.19. and 2.20d. The normal pick-up is 1 milliwatt for shock-proof opera-

flO.

2.19. Sensitive polarised d.c. moving iron relay with plug-in transistor amplifier

tion (30 g) but the coil will stand 5 watts continuou~ly. In fig. 2.19 the relay is fitted with a plug-in transistor amplifier which increases its sensitivity to I microwatt for pick-up. Figs. 2.20a, b, c, d and e show five typical polarised moving iron relays employing the flux shifting principle. The choice between them depends on the characteristic considered most important, such as speed, sensitivity, robustness, contact pressure, etc. Most relays of this type use leaf spring supported armatures, but the one shown in fig. 2.20a uses jewel bearings. The type shown in fig. 2.20b is highly resistant to mechanical shock because normally it is held strongly in position by its permanent magnet; when the armature flux is diverted through the small electromagnet E, the armature is

54

2.4

Relay Design and Construction s

N

4 RadiomctaJ o=--i+--tt"ir. pote pteces """,:::.r--w.._ ,

- . ,.........d"\

Radiomda.l bridge pieces

(a)

(b)

A-B

s

N

(d)

(c)

~~

N

A

B Hingcd <1I"mClturc

(f)

2.20. Polarised moving iron relays (a) Polarised rotating iron vane relay (b) Flux shifting type, releasing armature (c) Flux shifting type, attracting armature (d) Balanced version of flux shifting type (c) (e) Bistable pattern of flux shifting type of relay (0 Remanence type relay FIG.

released and it is moved to the operating position by the very strong spring. This relay is capable of tripping in two milliseconds. Fig. 2.2Oc shows an adaptation of the telephone type relay (20) (21); when energised it diverts flux into the armature instead of away from it. It is self-resetting. Fig. 2.20d shows an improved version of fig. 2.2Oc with a balanced armature. It is a very practical combination of speed, sensitivity, 55

Protective Relays

2.4

low cost and resistance to shock; it has contacts of 5 ampere rating and will stand a shock of 30 g when adjusted for a sensitivity of 1 milliwatt. Fig. 2.20e is the well-known Carpenter type in which the polarity of the lower poles is controlled by the direction of the current in the coil. It will pick up on 0·2 milliwatt but it has a very short contact travel and is suitable for only low voltage circuits. Fig. 2.20f is a remanence type relay which is inexpensive to manufacture and extremely sensitive (15). It operates when m.m.f. ofthe operating coil (A) "vercomes that of the restraining coil (B), which is normally energised, and kills the remanent flux. The relay operates quickly enough for the armature to open under the influence of the spring before the surplus of operating m.m.f. can build up the flux in the opposite direction and hold in the armature. TABLE

2.2

Performance of Polarised d.c. Relays Fig. No. Pick-up Coil Continuous Rating Operating Time at 5 x Pick-up Contact Pressure Contact Gap Armature Travel Shock Resistance Insulation Test Contact Rating Reset

2.20f

2.20b

2.20c

2.20d

2.20e

lOmW 1W

10mW SW

1mW 7·SW

0·03 mW 1W

0·03 mW 2W

2mS lOOO gm. 0·04 in. 0·03 in. 10 g 2500 V lOA Hand

ISmS 8gm. 0'025 in. 0·025 in. sg 2500 V 1A Self

ISmS 1 gm. 0·05 in. O·04in. 40g 2S00 V SA Self

2'smS 3gm.

2mS 4Ogm. 0·03 in. 0'02 in. 30g 1500 V SA Electrical

0·002 in. 0·003 in. sg SOO V 0·02 A Electrical

2.4.8. Rectifier Bridge Comparators

These may be arranged either as amplitude comparators (fig. 2.21a) or phase comparators (fig. 2.22a). The former type have been used in Europe for many years but the latter are not yet popular, although they have distinct advantages for directional and distance relays involving phase angle. (a) Amplitude Comparator Bridges. The amplitude comparator may work on either the voltage balance or the circulating current principle (15) (22) (62). The latter is more efficient (fig. 2.21a) because the non-linear resistance characteristic of the rectifiers provides a limiting action so that all the spill (i.e. difference) current goes through the relay at low currents whereas only a small fraction of it does so at high currents (fig. 2.21c). This implies that the sensitivity of the relay is greatest at low currents, which is the desirable condition. This characteristic can be accentuated by using different types of rectifiers in the two bridges, such as germanium in one and selenium iti the other. 56

Relay Design and Construction

2.4

Another way of expressing this is to say that output of the circuit to the tripping relay increases rapidly (fig. 2.21e) near the threshold of operation but the rate of increase diminishes at higher currents, so that the relay sensitivity is greatest near pick-up. The reason for this characteristic can be appreciated by reference to figs. 2.21a to 2.21e. Normally the restraining current preponderates and current flows in the winding of the polarised relay in the blocking direction.

eel c ",grUT OI8"!!!'YT19N "'_N !o"O 8P i~ Is ~u..,

VOLTS

API'>UEO TO ~Ew\Y.

Or---,-.--,-----k~~-------

.(,o,A.t-

-~ FIG.

i o ·i1'

(!)~ 1i!E:r1"'1E'Ii!-BIlIClGE

Lo-lr

--.

C~I2ISTIC,

2.21. Rectifier bridge amplitude comparator

Small values of ir will cause a current to flow in the relay, as in fig. 2.21b, the voltage drop across the slave relay being - V volts; this voltage - V serves as a bias in the forward direction of bridge 1. If ir is increased further the voltage drop across the relay will rise to a value - V, the threshold of bridge 1 and it will conduct, the current distribution being as shown in fig. 2.21c; the current through the relay consists of fairly flat-topped halfwaves corresponding to the case of io < ir as in fig. 2.21d. 57

2.4

Protective Relays

The reverse is true if bridge 1 only is .energised; the voltage drop across the relay will now be V, in the reverse direction from formerly, and this will bias the restraint rectifier in its forward direction. When the voltage drop across the relay attains a value V" corresponding to the threshold voltage of two rectifiers in series, the surplus current from bridge 1 is spilled through bridge 2. This corresponds to the case of io > i, in fig. 2.21d. When both bridges are energised simultaneously, the complete relay arrangement is acutely sensitive to small differences between io and i, without a delicate setting for the slave relay or a high thermal rating for its coil. The composite characteristic for the relay is shown ideally in fig. 2.21e. The current in the relay is a function of the difference between io and i" shown in fig. 2.21a. The current circulating between the bridges is the smaller of the two input currents plus some of their difference which appears as reverse current in the bridge with the larger current. The voltage V across the comparator cannot exceed twice the forward drop (toe voltage) in one of the rectifiers and is usually around 1 volt. The maximum current that can flow in the relay is the saturating voltage of the rectifier Vs divided by the relay coil resistance. With three inputs and three parallel rectifier bridges, elliptical and hyperbolic characteristics can be obtained. This subject is discussed in Vol. II, Chapter 12. (b) Phase Comparator Rectifier Bridge. This bridge is shown in fig. 2.22a and its operation can be followed from fig. 2.22c and 2.22f. It is a circulating

T

(a) Gtncro.l Qtra.nqcmcnt

(b) Wo.yc. forms of input currents

58

2.4

Relay Design and Construction

•

I,

1,-i2

2"

(c) i,> i 2 ; both + Output.

2',

(d)'2>I,jboth +

12(q+q)- '2 R2

tt

Output-

2l,!J = '1R

21,

Block

Block

21,

2~

(~)i2>i1;

'1 is-

Output

~

-2;,

:2.1',

(t)(,>i 2 ;i,io-

-1- -',R

Output

2i,

2/,

(h)'2>i,; bot~­ Output- 2i,

(9) '1>'2; bothOutput

= - i2(~+~) = -i 2R

~ i2(~+~)

2<,

(j)

(k)l, >;2 j i2

i 2 >i,i 12 "Output'"

-2(,.q =-i , R FIG.

59

',R

2i, i.-

Out.put:.

2.22. Rectifier-bridge phase-comparator

~-

-i2(~+~J= -' 2 R

2.4

Protective Relays

current bridge whose output current is equal to the smaller of the two currents inputs. The path of the current through the bridge is established by the larger of the two currents and depends upon their relative instantaneous polarity. If 11 > 12 the current will flow in the top and bottom rectifiers if 11 is positive (fig. 2.22c) and in the diagonal rectifiers (fig. 2.22d) if 11 is negative. If 12 > 11 the current flows in rectifiers I and 3 if 12 is positive (fig. 2.22e) and rectifiers 2 and 4 if negative (fig. 2.22f). If 11 and 12 have the same polarity the current in the pol~rised relay R flows in the tripping direction, if opposite polarity it will be in the blocking direction. The limiting action of this bridge is less pronounced than in the amplitude bridge comparator and a non-linear resistor, such as thyrite, is usually connected across the polarising input so that the sensitivity at minimum fault conditions can be raised to a satisfactory level, by increasing the polarising current i1 without exceeding the rating of the rectifiers at maximum fault conditions. This bridge produces more circular characteristics than the amplitude comparator bridges and hence is preferable for mho and directional relays. It is limited to two inputs. 2.4.•. Magnetic Amplifier Relays

Magnetic amplifiers are special transformers having two sets of windings, a.c. and d.c., which are not magnetically coupled but which may use the same core or cores. They can be voltage or current operated and their load or output can be series or shunt connected. They can be operated as phase or amplitude comparators. The subject of phase and amplitude comparison is considered in Chapter 3. The control circuit can be in series or parallel with the output. The earliest application to protective relays was an impedance relay (23) of Swedish manufacture, illustrated in fig. 2.23a which used a currentoperated shunt-controlled magnetic amplifier known as a transductor. Here the current required to operate the relay increases linearly with the voltage because, as the latter increases, it reduces the shunt impedance of the transductor, thus acting as an amplitude comparator. A similar principle is used in England (16) (24). A current operated series controlled transductor has been used as an amplitude comparator for differential and impedance relays. It is shown in fig. 2.23c comparing two electrical quantities in magnitude, irrespective of their phase relationship. Such a relay produces excellent electrical characteristics but has to be slowed to a minimum time of around 3 cycles by the damping winding shown in fig. 2.23c. Fast operation is prevented by the fact that the restraint dies down slowly due to the inductance of the control winding which is shortcircuited by rectifiers. Faster operation can be obtained from the Ramey amplifier circuit shown in fig. 2.23b, which is a half-wave magnetic amplifier. It is a shunt-controlled phase comparator in which alternate half-waves of the one input quantity

60

Relay Design and Construction

(a)

Trip winding Block winding A

B

B

Opcr
Rt.stra.i n

Pol
(b)

RutroJnt

(c)

r--_t-.::f::=;-_~Trl p rola.y

1_J:~====~

D.C. output 1. N.twork

(d)

(e)

(a) (b) (c) (d) (e)

FlO. 2.23. Magnetic amplifier relays Current-operated, shunt-controlled (transductor)-amplitude comparator Voltage-operated, shunt-controlled (Ramey)-phase comparator Current-operated, series-controlled (bias)-amplitude comparator Voltage-operated, series-controlled (magamp)-phase comparator Transductor relay-amplitude comparator.

61

2.4

2.4

Protective Relays

are polarised by the other input quantity. In fig. 2.23b the relay compares the phase relationship of the two input quantities regardless of their amplitude. The original magnetic amplifier (fig. 2.23d) developed in Sweden (23) and the U.K. and known by various trade names, such as magamp and amplistat, etc., has been used by a French manufacturer in a static distance relay as an amplifier to minimise the burden on supply c.t.s and to miniaturise components. The amplifier is supplied by a 500 cycle transistor oscillator energised from the station battery. In distance relays, where accurate measurement is required over an extremely wide range, it is necessary to have an output device which operates with less than a milliwatt. This means either an extremely sensitive relay or the provision of an amplifier, which can be a magnetic amplifier. Fig.2.23e shows an attracted armature relay which has a transductor built into it, making a compact device which is well suited as a low cost impedance fault detector. 2.4.10. Thermal Relays

Thermal relays will receive only brief treatment here since they are not generally used for protection of transmission systems, although they are widely used for protection of motors against overloads and unbalanced currents (25). The actuating elements are generally bimetallic strips (fig. 2.24a) which are often wound into a spiral to increase their length and thereby increase their sensitivity. An alternative design which, although less sensitive, Microswltch

Bi -motoJhc

strip

(a)

Adjusta.bltt thttrma.1 insula.tor

(b) FlO. 2.24. Thermal relays (a) Three-phase bimetallic strip thermal relay (b) Single-phase unimetallic strip thermal relay

62

Relay Design and Construction

2.4

is extremely shockproof consists of a brass strip bent into a hairpin of which one leg is heated by current passing through it or through a heating coil (fig. 2.24b); this type is also inherently temperature-compensated. Thermal elements are sometimes heated by passing the current through them but more usually by a heater situated immediately below them, the whole unit may be encased in thermal insulation or left open, depending upon the characteristics to be obtained. For instance, a bimetallic spiral indirectly heated in an insulating cover would be used where precision and a high resetting time is required. For the other extreme ofless precision and a fast resetting time, the brass hairpin design would be used without a cover. The bimetal strip has the limitation that its characteristic is affected by the load current which may be flowing through it prior to its operation. The hairpin unimetal design is very little affected by the previous load current because this tends to heat up both legs and cause them to expand equally, resulting in no movement of the contact. This effect can be controlled to suit a given application introducing thermal insulation at the bend of the hairpin. The equivalent torque of it bimetallic strip is proportional to the heat supplied which is proportional to the [2R of the strip. The deflection of an 1 )[2. · all'IC stnp . .IS 7.7(T26 -T h /.IS t he 1ength and I nvar- Brass bImet .. JOc hes, were 10 .W w is the thickness, both in inches. 2.4.11. Electronic Relays

Here the movement is replaced by electronic components and circuitry (2) (11). In amplitude comparators the two a.c. quantities to be compared are rectified and applied in opposition in the control grid circuit of an electronic tube (26) (60) (129) (131), so that operation occurs when one quantity exceeds the other by an amount depending on the bias. In phase comparison circuits, one a.c. quantity can be connected to the control grid of an electronic tube and the other a.c. quantity to the screen grid of the tube, as in fig. 2. 7b; operation occurs when the two quantities are in phase. Another technique is to connect each of the two a.c. quantities to the control electrode of one of a pair of tubes with their output electrodes in series or parallel so that the tubes are jointly conductive, or not, depending on the phase relationship of the a.c. quantities. The reliability of this class of equipment is dependent upon the quality of the components and their connections. The latter subject is discussed in section 2.6.10. The main drawbacks of electronics based on thermionic tubes are: (a) Constant drain on the station battery for the heater supply.

(b) The provision of a minimum d.c. voltage of 120 V, for plate (anode) circuits, which necessitates a battery voltage of at least 180 V to allow for negative grid bias supplies and a suitable margin for voltage drop during the closing of a circuit breaker.

63

Protective Relays

2.4

(c) The relays in use in the greatest quantities, such as time-overcurrent

relays, are more complicated in electronic design than their electromagnetic counterparts; the basic arrangement is shown in fig. 2.7a. (d) Limited output capacity which necessitates a mechanical relay for tripping the circuit breaker (attempts to use a thyratron for this purpose were not overly successful). (e) Greater problems in ensuring correct operation under transient conditions than with electromagnetic relays. The main difficulty here, as with transistor relays and other relays based on semiconductors, is that the inherent times of operation of electronic relays is well within the time-constants of the power system and associated current and voltage transformers. (f) Uncertain life of the electronic tubes. 2.4.12. Transistor.

Transistors have similar limitations. Items (c), (d) and (e) above apply directly. Item (f) does not apply but, in some transistors now available, some change in characteristics can be expected during their life. Transistors do not impose a constant drain on the station battery but they do require a separate low voltage d.c. supply. The present solutions to this problem are either to provide a small nickel-cadmium storage battery with a trickle charger and a relay to disconnect the battery from the transistor circuit while charging or to rectify the output of a saturating auxiliary C.t. and stabilise it with a limiting device. The circuitry of transistors has some similarity to that of electronic tubes; those at present available differ in having low input impedance and are current-fed devices. A phase comparator can be made either by connecting two transistors back-to-back (13) (14), as in fig. 2.8c, or by applying the input signals in parallel through diodes (fig. 2.8b) so that the transistor acts as an 'and' device (12) causing the transistor to cut off if either of the input quantities is positive. In either of the above circuits, current of constant magnitude will flow in the collector circuit only when the input a.c. quantities are simultaneously negative; a relay in the collector circuit will pick up when the overlap angle exceeds a certain value, i.e. when the mean d.c. level in the collector circuit exceed the relay pick-up level as a result of phase coincidence. Inserting inductance in the collector circuit causes the collector current to rise exponentially, instead of being constant, during the period of phase coincidence; this enables a trigger circuit to be added, which operates on the basis of current level. Alternatively, capacitance can be used and the voltage level measured; either method has the following virtues: (a) Permits a single tripping device to be used for all phases and zones. (b) Makes the pick-up level independent of voltage. (c) Permits instantaneous resetting using an integrating circuit.

64

2.4

Relay Design and Construction

A third method of phase comparison is by gating the current pulses with a transistor whose base-emitter circuit is polarised by the other a.c. quantity; this is after the fashion of the electronic relay systems proposed by Loving (27). Transistors can also be used in series for phase comparison and they can also be used as amplitude comparators. Both electronic and transistor relays at present have the disadvantage of insufficient capacity to trip a breaker and transistors are very vulnerable to transient overvoltages but, within the next five years, these limitations will probably be overcome. There is a clear case for the application of transistors to relaying systems using a carrier channel (28) (29), and to automatic synchronising (123). 2.4.13. The Hall Effect (31) (132)

So far, no relays have appeared on the market using the Hall Effect for vectorial and scalar multiplication, probably because of the small output and the high temperature error; however, at the time of writing this book, it has been found that Indium Arsenide has a practicable level of Hall Effect 1'--- - V - - --9

Trip ,ola.y G.rmC1llium cryst<1i

FIG.

2.25. Arrangement of Hall Effect directional relay

output with negligible temperature variation. The principle of the Hall Effect is illustrated in fig. 2.25. If a magnetic flux 1<1>1 sin wt proportional to one a.c. electrical quantity is arranged to pass through the surface of a flat crystal of N type germanium (or other metal with good Hall Effect) and a current III sin (wt + ¢) proportional to another a.c. electrical quantity is passed through the crystal from the middle of one edge to the middle of the opposite edge, then a d.c. e.m.f. Eh will appear between the midpoints of the remaining pair of edges. If H is the field strength, I the current in amperes, w the thickness of the 65

2.5

Protective Relays

germanium and R is the Hall coefficient then the Hall Effect voltage is RIH Eh = = w = =

K<1>1

K'[I\\/\ cos cf> - \<1>\\/\ cos (2wt+cf»]

(2.11 )

(2.12)

It will be noted that the first term is a d.c. voltage proportional to the vectorial product of
The operation of a relay can be delayed by a circuit component which delays the build-up or decay of operating voltage by changing its impedance slowly when current flows in it. This includes thermistors, filament lamps and R/C circuits. Longer time delays can be obtained with R/C circuits when used with electronic relays than with electromagnetic relays because smaller current can be used which takes longer to charge up a capacitor. This is evident from comparison of the times shown in Table 2.3. The operation of an electromagnetic relay can also be delayed by reducing the rate of build-up or decay of magnetic flux in its operating magnet or by impeding the motion of its contact arm. The former can be done by a copper ring around the magnet pole and the latter by friction, damping, gearing or thermal expansion. There are seven broad classes of timing units in which these principles are employed: 1. Magnetically operated devices with mechanical damping. 2. Mercury tubes. 3. Thermal devices employing the expansion of an electrically heated metal strip. 4. Motor-operated devices. 5. Electrical circuits containing reactance and non-linear impedance. 6. Electronic circuits. 7. Semiconductor circuits.

66

2.5

Relay Design and Construction 2.5.1. Mechanical Damping

An a.c. or d.c. solenoid operates contacts either directly or through a spring, against the delaying action of a dashpot, pneumatic, inertia, escapement or magnetic induction device. The spring provides a constant driving force irrespective of variations in the solenoid pull due to voltage, temperature, etc. A summary of their timing characteristics is given in Table 2.3. 2.3 Performance of Timing Devices TABLE

(a) Mechanical Delay Devices Type Oil Dashpot Pneumatic Escapement Magnetic Pot Friction Governor Mercury Tube Mercury Dashpot Synch. Motor

Range of Adjustment

Max. Time Setting

Accuracy

(Fig. 2.26)

20-1 100-1 10-1 15-1 10-1 None None 10-1

5 min. 10 min. 120 sec. 120 sec. 10 sec. 50 sec. 500 sec. No liIDit

5-10% 5-10% 5% 5% 5% 10% 5% 1 cycle

a b c d g e f h (Fig. 2.27) a b

Filament Lamp Resonance

(b) Electrical Delay Devices 10-1 4 sec. 5% 2 sec. Oper. 10-1 5% 30 sec. Reset 10-1 5 sec. Oper. 10% 50 sec. Reset None ! sec. 10% None 0·05 sec. 10%

Thermionic Semiconductor Diode Transistor

(c) Electronic Delay Devices 20-1 60 sec. 5% 10-1 8 sec. 7% 10-1 60 sec. 5%

(Fig. 2.28) a b c

Flux Decay Capacitor (d.c.) Capacitor (a.c.)

c d e

(a) Dashpots consist of a magnetically operated plunger which is retarded by oil flowing through an adjustable orifice in the plunger (fig. 2.26a). Where quick reset is required the orifice is a check valve which opens on the return stroke. (b) Pneumatic damping originally used leather or rubber bellows which were compressed by the solenoid, the time being controlled by adjustment of a needle valve. In modern pneumatic timers the bellows have been replaced by a metal chamber and diaphragm which can give greater consistency of timing, for two main reasons; the chamber and diaphragm can be more accurately made and the air can be in a closed circuit so that there is less likelihood of dirt blocking the needle valve (fig. 2.26b).

67

Protective Relays

2.5

-(c) E SCQ.pt.m.~nl

t (~) Mc.rcuty tube

(9) Fridion govc rnar

FIG.

(h)

~olor

(t) ~"C Ut y

dorllpot

opcrc.lcd

2.26. Mechanical and electromagnetic time delay devices

(c) Escapements similar to those used on clocks are less affected by temperature than dashpots or pneumatic devices but must be designed for easy starting (fig. 2.26c) by suitable design of the tooth angles. (d) Magnetic damping pots consist of a copper induction cup enclosing a permanent magnet and a soft iron pot which provides the return path for the

68

Relay Design and Construction

2.5

magnetic flux, as shown in fig. 2.26d. This device is more reliable in starting than one depending upon the escapement principle because it provides no retardation until the cup is in motion. With modem high coercive force magnets, time delays up to 120 seconds can be obtained. (e) Friction governor; an adaption from the phonograph. It consists oftwo weights located at the ends of two resilient arms which are parallel to the shaft when the mechanism is at rest. As the speed increases the centrifugal force makes the weights fly outwards until, at the rated speed, the weights rub on a conical surface which prevents them from going any faster (fig.2.26e). 2.5.2. Mercury Delay

Mercury is prone to corrosion and hence must be used in closed containers. The most common use is in a tube which, when tilted, causes the mercury to flow through an orifice to a new position, bridging the contacts. Recently it has been used successfully in the form of a dashpot. (a) Tilting Tubes. In this type the mercury not only provides the delay by the time taken to flow to a new position under gravity but also acts as the moving contact, by bridging the terminals which pass through the walls of the glass tube (fig. 2.26e). Great care must be taken in the design of the tubes to avoid leaks which would admit air. Existing tubes are unsatisfactory for short delays because the mercury may splash and break the circuit. High-speed designs have been developed, however, in which the glass tube with its stationary contacts are moved and the mercury stays still. (b) Mercury Dashpot. Fig. 2.26f shows a typical arrangement with hermetically sealed contacts in a metal or glass shell. A hollow magnetic plunger floats in the mercury and is pulled downwards by a solenoid, displacing the mercury which in turn displaces some inert gas from an inverted thimble. After sufficient gas has been expelled from the thimble through the porous ceramic plug, the mercury in the main pool completes the circuit to a small pool which forms the other contact. De-energising the solenoid releases the plunger which then floats up to its original position and allows the inert gas to return between the contacts. This is a fairly accurate device but it is difficult to provide any adjustment of the operating time; hence it is usually provided for a fixed time delay. 2.5.3. Thermal Delay

The most common arrangement is a bimetal strip or spiral which is heated directly or indirectly by a.c. or by d.c. and operates contacts as it bends due to the unequal expansion of the two metals in the strip with increasing temperature. In section 2.4.10 of this chapter a device using a unimetal (brass) strip was described which has the advantage of being shockproof, whereas the bimetallic strip is not. 69

Protective Relays

2.5 2.5.4. Motor Operated

On a.c. circuits a synchronous motor can be used which is interlocked with the system frequency and hence accurate within one cycle. On d.c. circuits a friction governor is generally employed but it is of course much less accurate (fig. 2.26g). A clutch between the motor and the contacts provides instantaneous resetting and, for this reason, the motor usually rotates continuously in order to reduce the effect of inertia on starting. Fig. 2.26h shows a typical arrangement.

2.5.5. Electrical Damping

This method employs either a short-circuiting ring (slug) around a relay solenoid pole, or a circuit containing reactance, or non-linear resistance, or a resonant circuit. The timing of such relays is affected by the supply voltage unless a stabilizing device such as a non-linear resistor is provided between the supply and the relay circuit. (a) Short-circuiting Ring. In attracted armature relays this consists of a heavy copper ring around part of the length of the iron core or a copper tube inside the coil (fig. 2.27a). To obtain maxiinum delay on pick-up the slug is put at the armature end of the core and, with a large armature gap and a stiff restraining spring (or many contact springs), operating times of 0·1 second can be achieved. To obtain maximum delay on drop-out the slug is placed at the other (heel) end of the core and, with a light spring load and a short lever arm, release times up to 0·6 second can be achieved. On relays with ground pole faces the drop-out delay due to the copper ring can be up to 4 seconds with 10% accuracy but is non-adjustable. The drop-out time can be made adjustable by adding a neutralising winding which opposes the main coil and is left energised after the main coil is open-circuited, as shown in fig. 2.27a. The rheostat can adjust the time from 0·3 to 4 seconds. Since both windings are energised from the same voltage source, the effect of voltage variation on the operating time is small. This is further improved and the drop-out time increased by making the relay iron saturate at 20 %of normal voltage with the armature closed. (b) Capacitance. Connecting the relay coil in parallel with a capacitor which has to be charged through a series resistor (fig. 2.27b) gives longer time delays. Only slightly longer drop-out times are achieved but the pick-up time can be increased to half a second with a capacitor of reasonable size. The time of operation is adjustable by the rheostat R and is given by r V(R+r») t =CR - - Iog ( 1 + --

R+r

v,.

C

(2.13)

where r is the resistance of the relay coil, and v is its pick-up voltage. The

70

Relay Design and Construction

(a l

2.5

(h )

(d)

(e)

(e) FIG.

2.27. Electromagnetic time delay circuits (a) Inductive flux decay (b) Capacitor charge delay (c) A.C. capacitor decay (d) Lamp filament heating delay (e) Resonance build-up delay

71

2.5

Protective Relays

operation of the relay can be made more positive and the timing more consistent by connecting a neon lamp in series with the trip coil. On a.c. applications a rectifier is used to produce delay on drop-out, a capacitor being across the relay winding, as shown in fig. 2.27c. (c) Ballistic Resistance. Time delay on pick-up can be obtained by connecting a metal filament lamp across the relay coil and a rheostat in series with it. The lamp short-circuits the winding and reduces the magnetic flux to zero for a short time until its filament becomes incandescent; the lamp filament then has ten times its cold resistance and permits pick-up (fig. 2.27d). Alternatively a carbon filament lamp or a thermistor can be connected in series with the coil. At room temperature the thermistor resistance is high which limits the coil current. As the current heats the thermistor its resistance falls until the coil current is sufficient for pick-up. (d) Resonance. With a.c. instantaneous overcurrent relays a time delay of up to 3 cycles can be obtained on pick-up by providing a secondary winding which is connected to a high-Q series tuned circuit. The delay is caused by the resonant circuit current building up to its steady value (fig. 2.27e). (e) Remanence. About 1930 a remarkable timing device was developed as an inverse time-current relay but never used commercially. It consisted basically of a test-tube 20% filled with steel balls about 0·1 in. diameter and surrounded at the upper end by a solenoid coil. When a.c. potential was applied to the coil, the balls would rotate for a certain time and then suddenly rise en masse to the middle of the coil. The balls arrived in the new position with considerable speed and force, sufficient to operate a breaker trip latch directly. The time of repeated operations was consistent but its VA burden was high. 2.5.8. Electronic Devices

The pick-up of an attracted armature relay can be delayed accurately up to 30 seconds by an electronic circuit, such as the one shown in fig. 2.28a. The capacitor C is charged through the resistance R and, when it reaches the value necessary to make the electronic tube conductive, current flows in the plate circuit and picks up the relay. The electronic tube is of the hard (high vacuum) type where self-resetting is required; it can, however, be a thyraton if the circuit is de-energised elsewhere, such as by an auxiliary switch on the circuit breaker. 2.5.7. Semiconductor Circuits

The principle of the R-C charging circuit can also be used with semiconductor diodes (rectifiers) or triodes (transistors), as explained below. (a) Rectifier-type Timing Unit. When the starting contact closes (fig. 2.28b) the current 11 rises to a value below the pick-up value of the relay. The capacitor voltage VB rises exponentially from zero at a rate determined by CR. When VB exceeds VA. a current 13 flows in the second relay coil and 72

2.5

Relay Design and Construction

raises the relay operating force above pick-Up. Variation in R (or C) changes the time between start and operate. (b) Transistor Timing Unit. A simple circuit for a static timing unit using a transistor is shown in fig. 2.28c. A constant direct voltage is applied to an R-C network. The capacitor C is charged through the resistor R and, when it

I o.c. +

~s\art

con\QC\

I,

(b)

(c)

Two winding

rela.y

r

R

L FIG.

-.,:e I

O.C.volts

"'ux.r~lQ,y

I

c

.J..

2.28. Static time delay circuits (a) Electronic (b) Semi-conductor diode (c) Transistor

reaches the value necessary to make the transistor conductive, an emitter current flows and picks up the relay. A battery B provides the power for tripping the auxiliary relay. This operation is similar to that of the electronic timer but some advantages in performance are obtained; these include smaller size, longer life, lower voltage supply and the elimination of cathode heaters which consume power and can fail. 73

2.6

Protective Relays

By putting the auxiliary relay in the emitter circuit instead of in the collector circuit, negative feedback provides a longer time delay and improved stability with temperature variation. 2.1. DETAILS OF RELAY DESIGN

2.6.1. Bearings

The most common type of bearing for precision relays, such as in the induction type, is the pivot and jewel bearing similar to that used in watthour meters; the modem types have spring-mounted jewels and are designed so that shocks are taken on a shoulder and not on a jewel (fig. 2.29). For special applications requiring high sensitivity and low friction, a single ball bearing running between two cup-shaped sapphire jewels has been used.

Endstone

~-~-:-~I-- bedr i ng

FIG.

2.29. Induction cup jewel bearing

Multi-ball bearings are popular on the Continent and it is claimed that miniature bearings less than 116 in. diameter are now available, which provide as low friction as jewel bearings but have greater resistance to shock; furthermore, they can combine side-thrust and end-thrust in a single bearing. In hinged armature relays, knife-edge bearings, pin bearings or resilient strips are used instead of pivot bearings because they are designed for operating many contacts rather than for precision. 2.6.2. Backstops

In cases where a relay has a strong resetting torque under nornial conditions but has been given a very sensitive pick-up setting, there may be trouble from sticking against the backstop. This may be due to the use of normally non-magnetic materials which have become magnetic due to 74

Relay Design and Construction

2.6

fabrication, or it may be due to molecular 'sticktion', the tendency for the contact arm and backstop to interlock their microscopically rough surfaces under the influence of a.c. vibration. The magnetic adherence can be overcome by making the backstop of nonmetallic material. The molecular adherence can be prevented by the use of hard surfaces rounded to a large radius, the ideal combination being a rounded metal contact arm and a smooth agate or nylon backstop. 2.6.3. Contacts

Contact performance is possibly the most important item affecting the reliability of protective relays. Corrosion or a particle of grit can prevent a relay from tripping. Consequently, the material and shape of contacts are of considerable interest to the designer and user (20) (64) (146). (a) Contact Design. It has been estimated that line contacts have about half the resistance of square flat contacts of the same length and are commonly used for stationary contacts such as plug contacts. Cylindrical contacts at right angles provide the most reliable arrangement for relay contacts because they provide the optimum high pressure of a point contact without concentrating the current at an actual point which would tend to burn and erode away. Silver is the most widely used metal for relay contacts since it has the lowest resistance and its oxides and sulphides, though readily formed in air, are broken down more readily than those of other metals. Copper circuits are not used in relays because the resistance of clean, new copper contacts is eleven times that of silver ones and oxidation raises the resistance of copper contacts several hundred thousand times. When large currents are to be handled, such as in a.c. tripping, special alloys of silver are available, such as silver cadmium oxide, which have a low resistance like silver but do not weld or become sticky. For small currents and very light contact pressures an alloy is used which consists of 67 % gold, 26 % silver and 7 %platinum. In sensitive (low input) relays, where the contact pressure may be very low, non-corroding metals such as gold, palladium or rhodium are used. Since these metals do not corrode there is no need for high pressure to break through tarnish; on the other hand, such relays are not recommended in protective schemes because high contact pressure may still be needed to squeeze out dust and lint from between the contacts in order to make contact. For maximum precision of electrical characteristics as well as maximum contact pressure, the protective relay should have only one contact; if additional contacts are required they should be provided on an auxiliary relay. Bridging contacts make the contact pressure uncertain and introduce inaccuracy if the two contacts do not make at precisely the same current; a better arrangement is to use a flexible connection to a single moving contact. Double contacts in parallel are almost ideal because, if there is one chance

75

2.6

Protective Relays

in 10,000 of failure with a single contact, there is only one chance in 100,000,000 with two contacts in parallel. The maintenance of contacts can be minimised, if not eliminated, by high contact pressure, hard smooth contact surfaces, bounce-proof contact design and the use of dust-tight relay cases. The smooth contact surface created by a burnishing tool will permit about ten times as many operations as a filed surface, because the minute ridges caused by filing are melted by the arc, causing roughness and hastening the need for reconditioning. Emery paper should never be used because emery particles adhering to the silver may prevent electrical contact. The exclusion of dust is very important; most contact failures are caused by lint or grit stuck to them by varnish from overheated coils and tarry combustion products in the air. It is difficult to cure completely the phenolic resin varnish commonly used on coils and a hot coil can cause volatile components of the varnish to leave the coil and condense on electrical contacts, with the risk of open-circuiting them. However, polyester and epoxide varnishes are now available which cure completely and at a lower temperature so that this difficulty is avoided, especially if the coil is encapsulated in resin. (b) Corrosion. The resistance of the contacts is partly that of the contacts themselves (which depends upon their material and dimensions) and partly that of the actual contacting surfaces. For clean, dry silver contacts R = C/P" where R is in ohms and P is in grams. For silver n = 0·8 and C depends upon in. the contact shape and dimensions. For cylindrical silver contacts diameter C = 0·04. The resistance of a clean contact has also (64) been expressed as ;;,

n-

where p is the resistivity of the metal (1'7 microhm. em for silver) and a is the radius of the contact area. a

= 1·11

;j~ where

P

= contact

pressure in grams, r = radius of the two similar cylindrical rods in contact at right angles (in em.) and E is the elastic modulus for the metal (7 x 108 gm/em2 for silver). It has been said that silver oxide is a good conductor. Actually it can be classified as an insulator since it has.a resistance of the order of 40 megohms! em3 but it is very readily broken down by heat (200°C) and electrical or mechanical pressure. Fortunately, silver oxide does not form readily and seldom to a thickness of more than 10 A, so that it is easily moved aside by pressure or sliding wipe and enough heat is generated at the point of contact to reduce the oxide to silver. On the other hand, silver sulphide forms very readily in industrial areas or where coal fires are used or sulphur cured rubber is near the contacts, especially in the presence of heat and humidity. It does not break down so easily as the oxides but it is as soft as lead and can be squeezed aside with a force of 300 gm. with crossed silver rods of 0·03 in. diameter; the resistance

76

Relay Design and Construction

2.6

of silver sulphide is relatively low (0·017 ohm/cm3). The resistance of silver sulphide also decreases with temperature; at 170°C it is only one thousandth of its value at room temperature. At voltages below 100mV the resistivity of the tarnish is constant and not related to the voltage. Hence the tarnish resistance Rr = (1(J)/na 2 where (1 is the resistivity of the tarnish, (J) the thickness and a the radius of the contact area. a = 1·11

3/Pr !tJ E as before (64). The actual contact area is very

small compared with the contact surface; it consists of the total area of the tops of the irregularities in the contact surface. With round contacts the contact area is theoretically a point and the actual area is finite only because of plastic yield of the silver. The voltage at which a corroded silver contact becomes conductive is of K'(J) the form: V =. - P where K' depends on the nature of the corrosion and

(J) is the thickness; this applies to oxide and sulphide coatings. P is the pressure in grams. The breakdown voltage for silver sulphide is 0·8 to 1·5 volts per 100 A, or 106 volts/cm. The thickness of the coating is in,dicated by the colour. Up to 50 A the bright silver appearance is retained; a brown tarnish indicates roughly 250 A; a blue colour the 500 A region and violet

l,oooA.

It should be remembered that K ', although small, has a definite value so that it is hazardous to employ silver contacts with less than O'S gm. steady pressure for voltages down to SO volts or 2 gm. at 24 volts in clean atmosphere; much higher values around 30 gm. apply in polluted atmospheres; the alternative to this high pressure is sliding wipe which can scour off enough tarnish to establish initial contact so that heat from the ensuing current will quickly overcome the tarnish as already described above. In short, the higher the contact pressure the better. In reasonable atmospheres, contact pressures of the order of a gram are satisfactory with cylindrical silver moving contacts of measuring relays in circuits above 50 volts d.c., because the collision of the contacts momentarily increases the pressure per square inch sufficiently to break down thin coatings of silver oxides and sulphides so that current can flow. Below 50 volts 8 gm. is recommended. However, stationary contacts, such as coil taps and drawout case contacts, should have at least a 500 gm. pressure because, in some circuits, the voltage across them may be very small. A thin coating of petrolatum has been found to reduce metal transfer and corrosion of contacts without increasing their resistance. It is beneficial in polluted atmospheres and for relays which have a very large number of operations. A dust-tight relay case fitted with a filter breather is essential for contacts operating below 100 volts d.c. because a minute particle of sand or hard grit can prevent flashover contact at low voltages. A special problem exists in relays with poor ventilation, especially in

77

2.6

Protective Relays

sealed relays. High resistance polymers can appear on the contacts due to organic emanations from coil insulation, especially where traces of iron or copper are rubbed into the surface during manufacture (75). Contacts containing palladium are the most affected and gold plated contacts the least. Phenolic resin varnish impregnation is the worst offender but all insulation gives off organic vapour to a certain degree except PTFE (Teflon). Ventilated relay cases with dust filters minimise the effect but the ideal solution is separately encapsulated contacts, i.e. like reed relays. (c) Making and Breaking Capacity. The short-time carrying capacity of contacts depends upon their weight, thermal conductivity, electrical conductivity and surface resistance. The last two items control the heat produced ([2R) and the first two control the heat absorbable. The continuous carrying capacity of contacts depends upon their surface area, the volume and thermal coefficients of the contacts and their supporting members, their electrical conductivity and their surface resistance. For modern silver contact designs with cylindrical contacting surfaces, the relationship between the short-time (tripping) and continuous capacity is shown in Table 2.4. The contacts are mounted so that their cylinders or ridges meet at right angles. TABLE 2.4 Contact Capacity

Contact Class .05"

~

O~A6"

A. Auxiliary relay

. + 04"

~

o

> 110 :;j>

c:::l 13/16 1

C. Sensitive relay

> 110 :;j>

7

15

4

14 x;os

4

7·5

2

110

> 110

Continuous Carrying Capacity Amps. 7

13'~

110

Ya"

B. Protective relay

.04'~

Making Capacity for 200mS Circuit Circuit Volts Amps. :;j> 110 30

15

X;04

2

Table 2.4 assumes that the contacts will be operated not more than 5,000 times before maintenance; there is an inverse relationship between the contact duty and the number of operations that can be done before the relay goes outside the performance specification pertaining to the relay. The number of operations is drastically reduced if the bouncing period of the contacts exceeds the time constant of the circuit; it can be significantly increased if a seal-in relay is used.

78

Relay Design and Construction

2.6

In Table 2.4 the current values given refer to linear inductive circuits such as the trip coils of circuit breakers and the coils of auxiliary relays. In such circuits the current starts from zero at the moment of contact closure and builds up exponentially. In a resistance circuit the continuous rating would be the same but the 200 mS values should be halved. In a circuit dominated by a capacitance or a metal filament lamp the values would be much less because the initial inrush current could be many times the steady value. Protective relays (Class B in Table 2.4) are not normally expected to interrupt any power because the circuits they set up are usually interrupted by an auxiliary switch on the circuit breaker. In some cases, however, they have to interrupt the coil circuit of an auxiliary relay or a timer. Modern protective relays will interrupt 100 VA a.c. up to 120 volts or 10 watts d.c. inductive. With a spark-quenching circuit they should interrupt 50 watts d.c. up to 250 volts. The corresponding values for sensitive protective relays are less, depending on the design. Auxiliary relays (Class A in Table 2.4) will interrupt about 500 VA a.c. or 50 watts d.c. inductive. Such a relay is illustrated in fig. 2.l0a but, when equipped with blow-out magnets (fig. 2.10b) it will interrupt 3 kW in a highly inductive circuit (LjR = 0·05). Further data is given in Vol. II, section 2.4.1. In a.c. tripping (see Chapter 4, section 4.6) the current is transferred to the trip coil by opening a contact which normally short-circuits it, so that the contact does not actually interrupt any current. This transfer is done by an auxiliary tripping relay (Class A in Table 2.4), and the transfer capacity of these contacts expressed as the product of the current before opening times the voltage across the trip coil after opening, is about 3,750 VA for silver contacts, assuming a maximum of 3 seconds for the current to be flowing through the contacts. This can be raised to 15,000 VA by using elkonite contacts. 2.6.4. Bounce-proof Contacts

The pmblem of contact bouncing arises particularly in the case of a highspeed relay in which the armature and contacts have to be accelerated to a relatively high speed in about 20 milliseconds and then stopped abruptly against the stationary contact without any rebound. This requires some means of absorbing the kinetic energy of the moving parts, such as a miniature friction-type shock absorber, or an inclined tube containing a ball, as shown in fig. 2.30b. The method most commonly used employs flexible contact brushes which slide against each other at their contacting tips; this method can be made much more effective if twin contacts are used which have different spring rates so that their bouncing periods do not coincide. The most ingenious and effective solution is a tiny capsule on the moving contact which is half-filled with tungsten powder and which 'sandbags' the contact closed by the powder flying up to the other end of the capsule when the contact is 79

2.6

Protective Relays

stopped suddenly (fig. 2.30a). A friction clutch between the armature shaft and the moving contact gives as good results when properly designed (fig. 2.30d) and has the additional advantage of reducing the tendency of the relay to operate undesirably under the influence of circuit transients.

I

b)

(0)

~hln

~

flexible

"striP

Ie)

Stationary contact

MOVing

(d )

contact Ie)

FIG. 2.30. Bounce-proof contacts (a) Tungsten powder in a moving contact (b) Ball mounted on an incline (c) Pivoted contact type (d) Axial clutch type (e) Radial clutch type

A simpler approach is to use a loose contact suspended freely in the middle; in this design the rebounds from the two ends tend to cancel each other out (fig. 2.30c). This solution is superior to the tungsten container because it does not increase the total inertia and thus retard the relay. It can be combined with the friction clutch for optimum results. It is sometimes possible to absorb the energy of the moving contact by providing a backstop behind the fixed contact which is made of material with low resilience and high internal resistance, i.e. material of similar consistency to that of the pad of a human finger. Such a material is Vinyl acetate-vinyl chloride copolymer of 60 Durometer A hardness. 2.6.5. Spark-Quenching Circuit.

Comparator relays usually have contacts of rather small rating; when such contacts are used to control auxiliary relays or timing devices it is necessary to protect them with a spark-quenching circuit. The simplest sparkquenching circuit consists of a series resistor and capacitor connected across the contacts, as in fig. 2.31a.

80

Relay Design and Construction

2.6

For the best action, i.e. sparkless interruption of an inductive circuit and no contact welding on closure, the parameters of the spark-quenching circuit are given by Re = 0·2 VCO. 2 , where V is the circuit voltage and Rand C

=-

<>---------1-1

(b) FIG.

L

R~

1

P. R.

o

00>__----0

2.31. Spark-quenching circuits. PR = protective relay (a) Across contacts. (b) Across load

are in ohms and mfd respectively. In the case of a highly inductive load, L ReC = - where Land RL refer to the load. Values of C and R can be RL

calculated by solving these two equations. An alternative is to put a capacitor across the load, as shown in fig. 2.3lb; in this case C =

0·3~. RL

2.6.6. Contact Pressure Augmentation

Geartrains and linkages are avoided in modern protective relays; although they can increase contact pressure they introduce friction and inaccuracy and increase the resetting time. Near the threshold of operation of a relay, especially at low values of current or voltage, the torque may not be sufficient to make the contacts complete their circuit electrically because of the presence of small amounts of dirt and oxide, or sulphide coatings which would be broken down under conditions of normal torque. This difficulty can be overcome by designing the relay to increase its torque as the contacts approach. Attracted armature relays inherently possess the latter feature; in induction cup or disc relays, however, it is necessary to provide auxiliary means, such as a slot in the disc or a small auxiliary armature, to provide the additional closing force at contact closure. With moving coil relays it is necessary to provide a seal-in relay or to connect the contacts so that, in addition to their main function, they also divert some extra current through the operating coil (fig. 2.32a). Fig. 2.34 shows the contact pressure for different types of induction relays D

~

2.6

Protective Relays

with silver contacts. Even with the highest contact pressures, silver contacts would rapidly deteriorate if they had to handle currents above SA, especially if they bounced at all. However, if a fast seal-in relay is used to protect the contact, by paralleling them with its own contacts, these relays can handle quite heavy currents, provided that the seal-in relay operates within 0·010 second and has at least 25 gm. pressure on cylindrical contacts which do not bounce. Fig. 2.35 shows the contacts of an induction disc relay with a shaded pole electromagnet and a fast seal-in relay; the illustration indicates negligible contact deterioration after 500 closures of a 40 amp, 250 volt trip coil. Seal-in relays or augmentation of the operating coil current, as is practised in some foreign relays, is not a complete solution for this problem because they do not work unless electrical contact is made in the first place. On the other hand, such methods are valuable for making the contact action more positive if the contacts are inclined to chatter or if they are of small capacity.

--

C.S. t rip coil

+o----o~ I I I

A

ra) Torque O.lJgmentmg by a.dditlona.1 coil currtnt

,.... S

(b) Ca.pa.dtor cha.rgt a.nd dlScha.'gecircuit FIG.

2.32. Circuit arrangements for augmenting contact pressure Quantity A operates; B restrains

Fig. 2.32b shows a circuit which makes the action of a polarised d.c. relay more positive. When the contacts close the capacitor discharges through the transformer, producing a circuit-closing impulse in the operating coil of the polarised d.c. relay; this causes firm closure and then decays exponentially to cause no contact rebound. On opening, the charging of the capacitor produces an impulse in the reverse direction causing a clean break of the contacts. In the case of ultra-sensitive relays, used with some static relays, the contact pressure can be very small and, in order to ensure intial contact, non-

82

Relay Design and Construction

2.6

tarnishing (royal) metals can be used, such as gold or rhodium. Contacts of 67 % Au, 26 % Ag and 7 % Pt are reputed to work reliably down to 20 micrograms. The foregoing applies to measuring relays, especially those operating on very small power inputs. In the case of auxiliary relays the input can be made as high as their thermal capacity will permit. In cases where very fast action is required, still greater input may be necessary; fig. 2.33a shows a circuit which

.

-0. .t c

u

+0--0

T

t

0

l!J

0

-0 (a) Circuit for a.ccc.lcro.ti ng p ci k.up

"O--O-f -0

',I

R V\IWIIII\

(b) Circu it for Q.C.Cc 'crCLtlnq drop-out

l"'

0

~

0

''"0'

R

.,

.

Go

0

i3

0

~ u

Tim e

c

:::0

. c

t

... ~

Time

'0 U

(c) Circu it for Q.cctlc.ra.t lng both pick - up a.nd drop-out

FIG.

:::o

ij

. c

Go

o

2.33. Circuit arrangements for accelerating pick-up and reset of auxiliary relays (a) Fast pick-up. (b) Fast drop-out. (c) Fast pick-up and drop-out

temporarily increases the relay coil current while the capacitor is charging and thus accelerates the pick-up. Fig. 2.33b shows a means of accelerating drop-out of an auxiliary relay by using the inductive inertia of a parallel inductor to reverse the coil current and hence suppress the core flux rapidly. Fig. 2.33c shows both principles used to obtain fast action on both pick-up and reset. 83

Protective Relays

2.6

_~

30+---+

J

_

Conta.ct pr«ssure of typica.l gea.rless induction rcla.ys with 3 V A -+-_~ burden a.t setting Note:- The induction disc rela.ys ha.vc sa.tura.ting cha.ra.ctcristic to produce inverse-definite time current curve

20+----+---++

o+-~~~==~==t=~c==t===c==±=~ 234567 e 9 _ - L_ _

FIG.

LI_o~p~crLa.~tl~n~g~cu~r ~c~nt~a.~s~m~U~lt~iP~IC~O~fs~ct~ti~ng~I ~j __

2.34. Torque curve showing the development of contact pressure with operating current for different types of induction relays

2.6.7. Dust Proofing

The importance of excluding dust from a protective relay cannot be overemphasised. The most common cause of contact failure is lint or grit, especially in humid atmospheres, or where there is a sticky deposit on the contacts from manufacturing processes in the neighbourhood, or an overheated coil in the relay. Modern relays have dustproof cases of pressed steel, cast aluminium or moulded plastic. Some can be made dust-tight by sealing their joints with an adhesive filler. Such cases must then be provided with a dust filter (fig. 2.37) so that breathing can take place without building up a pressure which would force air past the cover gasket. Modern cover gaskets are usually of neoprene rubber, since this is resistant to tropical conditions and insects. If a dust-tight relay leaves the factory in a clean condition and if it has a high torque it can be left in service for many years without maintenance. This may be a method of solving the difficulty of obtaining sufficient staff to cope with the relay maintenance of an expanding system. 2.6.8. Mechanical Stability

Increasing demands for sensitivity and speed have encouraged the use of light movements and very short contact travels. This has made modem relays more susceptible to undesirable operation due to mechanical shock or vibration; where these conditions cannot be avoided there is a danger of incorrect relay operation. 84

Relay Design and Construction

2.6

Many manufacturers have installed apparatus for applying calibrated impact vibrating forces to their relays and have modified their designs to stand relatively rough conditions. These modifications include, where possible, stiffening of contacts, balancing of armatures, bracing of weak supports and designing for a low resonant frequency. Clearly static relays are less susceptible to shock than those with moving parts. The method of analysis is to prepare a diagram similar to an electrical analogue of the problem in which friction, inertia and resilience are represented respectively by resistance, induction and capacitance. From this analogue the vibration and shock can be determined which will cause the contacts to close undesirably and what must be done to prevent this. The remedy may be confirmed by inspection of high-speed cine films of the relay action, which slow it down so that the weak or oscillatory members can be easily detected and modified. Specifications for mechanical stability have not yet been formulated for general acceptance but there is evidence that an impact of 30 g. normal to the panel surface as near as possible to the armature should be an adequate test for stability of the contacts. Such a blow causes complex rotary and translational movements of the relay due to shock waves in the panel surface and has been found to be more severe than any reasonable service conditions. It is probable that a controlled applied vibration of the form F sin 2 w/. will be the basis of a calibration for resistance to shock and vibration. Unfortunately the wprk has not progressed far enough at the time of going to press to include further information. 2.•. 9. Coil Design

Relay windings must stand a high potential or flash-over test of 2,500 volts for I second. Actually, their insulation must be designed to withstand at least 4 kV because: (a) It is customary to measure voltage on the supply side of the test

transformer; thus the voltage applied to the relay may be amplified considerably by resonance betw~en the secondary reactance of the transformer and the capacitance to earth of the relay, especially if it is mounted on a large switchboard. (b) Moisture condensing on the relay when it is brought from a cold store-room into a warm test-room reduces its normal insulation. With modem insulating materials, however, these circumstances present no problem. What is more difficult is to design the coil to stand mechanical abuse and the effects of heat, humidity, corrosive atmosphere and bacteria. Mechanical robustness is achieved by a protective wrapper, varnish impregnation and the proper anchoring of the leads, especially in the case of fine wire coils. A common method is to use a harness taped to the coil, which firmly supports the lead and the end of the winding where they are joined. A common cause of failure is a loop in the wire, due to imperfect tension 85

Protective Relays

2.6

control, which causes wires to cross each other and the insulation to break down between turns. An external wrapper improves the appearance of the coil and protects it against accidental blows, such as from a screwdriver slipping off a screw head; such a wrapper is a trap for moisture and bacteria in tropical climates, however. Corrosion is prevented by the avoidance of acid-forming insulating materials or soldering fluxes and taking precautions against hand perspiration; in the case of d.c. coils it is important to connect one end of the coil directly to the negative pole in order to avoid electrolysis. The larger the wire, the more mechanical abuse it will stand and the longer it will take for corrosion to cause an open-circuit. It is considered unwise to use wire of less than 0·002 in. diameter, even if all the foregoing precautions are taken; this is because of the risk of a kink in the wire which, although not noticed in winding the coil, might break later in service after a few hundred daily temperature cycles. It is considered good practice to use not less than 0 ·004 in. diameter wherever possible and especially on coils directly concerned with tripping the circuit-breaker. Where fine wires are unavoidable, a precision governor for the wire tension should be used and encapsulation is recommended, i.e. 'potting' in a sealing compound such as epoxide or polyester resin. The heat dissipation of a relay coil in watts is about 0·2 A, where A is the superficial area in square inches for 50°C rise. Encapsulation makes the coil practically impervious to subsequent external conditions. Flaws in the enamel and imperfect winding can be detected by an induced voltage tester, described in Chapter 13, section 13.11.1. The use of this device greatly reduces the possibility of subsequent failure of the coil in service. For relays with tapped coils or for air-gap transformer reactors requiring a constant

~ ratio,

the stranded coil technique is recommended (fig. 4.20);

in this technique many wires are wound in parallel in the form of a tape whose strands are connected in series with taps brought out at the junctions. Such X

a winding has constant magnetic leakage and R ratio for all taps because all the turns traverse the same path. This technique is particularly valuable for eliminating tap error on time-overcurrent relays 2.6.10. Electrical Connections

Until recently, all permanent internal connections of relays were soldered. Dry joints were theoretically avoided by cleaning the joints thoroughly before soldering and making sure that the soldering iron was at the proper heat so as to ensure that the connections were properly wetted with solder before joining. Quality control of soldering can reduce failures to one in 50,000; this can be improved to one in a million by first twisting the conductors around each other at least three times. 86

Relay Design and Construction

2.7

Brazing is superior to soldering for wires above 0·01 in. diameter. Crimping and wire wrapping are still better for wires above 0·01 in. diameter because they use a cold-welding technique and the high pressure (over 1,000 Ib/sq. in.) at the contact eliminates failure due to imperfect cleaning. Here again, twisting the conductors around each other before crimping tends to make failure almost impossible. Stranded wires are preferable to single wires for relay connections because a single wire can break due to vibration, especially if it has a nick due to stripping or a crack due to sharp bending. Furthermore, stranded wire makes a more reliable soldered or brazed joint. The type of connection that is considered reliable in a conventional relay circuit may not be acceptable for static comparator circuits which operate with milliampere currents at less than one volt. At the time of writing this book, a great deal of research is being conducted in this field. Of the permanent forms of connections, the wire-wrapped type appears to have ten times the reliability of soldered connections, assuming that skilled operators and correct tools are employed in each case. A bolted connection using a flexible nickel-plated device like a "Speed Nut" has been approved for connections to printed circuit modules in some telephone equipment. Pressure connectors with gold-plated contacts are used in important equipment such as guided missiles but are relatively expensive. A technique liable to replace them in protective relays is to gold plate the printed circuit conductors at the edge of the module; on the assumption that the module wi.11 be removed and re-inserted not more than 500 times in its life, a gold flash on top of 0·0002 in. of nickel is satisfactory provided that the method of depositing the gold leaves a non-porous layer so that the nickel and copper will be sealed off from the atmosphere and thus prevent corrosion. The nickel is used primarily for ensuring good bonding; if it is not used. a thicker layer of gold is necessary to protect the copper. The contact pressure required is about 60 gm. 2.7. CASES

Modem relay cases have the same width and depth. Only the length varies with the number of relay units in a case. This enables the width of the switchboard panels to be standardised, makes it possible to use pressing tools for blanking out the panel, facilitates the layout of relays on the panel and gives an improved appearance. In the U.S.A. the tendency in recent years has been towards the flush mounting of relays to improve the appearance of the panel and minimise the effect of dust. This necessitated a relay unit withdrawable from the front, so that the American manufacturers have standardised on drawout relays. The main advantage of this construction, however, is that it also permits the use of a test plug; this speeds up testing time by a factor of about 5 and tends to eliminate the possibility of a wrong 87

2.7

Protective Relays

FIG.

2.35. Appearance of induction disc contacts after 500 operations at 40 A, 250 V to illustrate the benefit of a seal-in unit

connection or a bad contact due to an improperly tightened nut or some pinched lead insulation. This topic is treated later in Chapter 13. Fig. 2.36 shows an English drawout relay (68) (105) which can be used for either flush or projection mounting; detachable clamps are provided on the sides of the case for flush mounting and bosses are provided on the rear for the holding studs necessary for projection mounting. The terminal arrangement is the same for all relays; this facilitates switchboard wiring and the use of a permanently wired test plug. The relay cases have withdrawable relay units which are interchangeable in their cases so that a unit can be replaced quickly and the insertion of the relay can be postponed until the construction work is finished and the station clean. Furthermore, if the protective scheme has to be modified to meet system requirements, new relay units can often be put in existing cases on the panel, so that no plugging of holes is necessary. By sealing the case so that it is practically air-tight and providing a dust filter for breathing purposes, protective relays can be operated reliably in extremely dusty or dirty atmospheres (fig. 2.37). For extreme tropical conditions of heat, humidity and bacteria, or for factories where a corrosive atmosphere exists, a sealed case is desirable which completely protects the relay from these conditions (fig. 2.38). However, if the case is completely air-tight it is necessary to expel all moisture and volatile materials before 88

FIG.

2.36a. Operation of test plug in a drawout case

FIG.

2.36b. Relay withdrawn from drawout case

89

2.7

Protective Relays

FIG.

2.37. Sealed relay case with dust filter

sealing and to provide means for either detecting or removing them if they occur during service. A limited amount of moisture can be absorbed by a silicagel unit (shown just above the nameplate in fig. 2.38). The moisture changes the silicagel crystals from blue to red; the unit also acts as a leak detector if the relay is intially dry. Most volatile organic materials can be minimised by an activated carbon 'getter' in a porous container. Proper baking of the whole relay before sealing the case avoids the risk of overloading the getter. 90

Relay Design and Construction

FIG.

FIG.

2.38. Airtight relay case

2.39. ContInental European type of case

91

2.8

2.8

Protective Relays

2.•. ECONOMICS IN PANEL MOUNTING

In most countries protective relays are mounted on panels to the rear of the instrument and control boards but some progress has been made in mounting relays in racks instead of panels, i.e. a system of parallel horizontal straps bolted to vertical posts of L section so that the straps can be moved vertically to suit the length of the relay. Wiring is cabled and supported by cleats on the horizontal straps. This system is considered to be cheaper than cubicles and is generally attached to a wall behind the control board; it is also more flexible since the relays can be rearranged or replaced simply by moving the straps instead of re-drilling or blanking the panel. Continental European practice (135) is to put complete protective schemes such as 3-step distance relays in one large case (fig. 2.39), whereas in Britain, Sweden and the U.S.A., individual relays are mounted in separate cases, several of which may form a complete scheme. The single large case facilitates factory testing and reduces the external wiring but separate relays are easier to handle, giving greater flexibility of mounting and permitting changes in connections; this last feature enables modifications of the basic scheme to be made, which are necessary to meet the demands of different applications and which would be more difficult with a large case in which the wiring was already in cable form. Furthermore, separate relays with plug testing facilities permit one unit to be tested while the remainder of the units are still in service.

2.9. OPERATION INDICATORS

The general advantages of electrical as compared to mechanical operation indicators have already been mentioned in Chapter 1, section 1.11, but some constructional details will be considered here. The series electrical operation indicator shown in fig. 2.10 has one tap so that it is suitable for a wide range of trip coils; one coil covers 80 %of the trip coil ratings with less than 5 % voltage drop. Such an indicator is shown mounted on a protective relay in fig. 1.1. The shunt electrical operation indicator is preferable where the trip current is variable or unknown; but it does, however, require an extra contact on the protective relay to prevent the operation indicator from operating when the circuit-breaker is tripped manually or by another relay. Fig. 1.10 shows typical connections for series and shunt flag and seal-in circuits. The electrical operation indicator is generally combined with an auxiliary contactor which provides extra contacts for other functions such as remote alarm and also protects the main relay contacts by sealing in around them; this has already been discussed in section 2.6.6. The following is a comparison between mechanical, shunt and series flags from a practical viewpoint.

92

Relay Design and Construction

FIG.

2.40. Mechanical operation indicator

93

2.9

2.10

Protective Relays

Mechanical flags are the cheapest (fig. 2.40) but they may not be as visible because they must be mounted on the relay armature. Two electrically separate contacts are required on the protective relay if the remote alarm is on a supply of different voltage from the trip circuit. The work done to trip the flag, though small, affects the pick-up of a relay and prevents mechanical flags from being used on very sensitive relays or those with short armature travel. The absence of electrical connections on the flag simplifies stocking of relays where the trip current does not exceed 5 A. The worst shortcoming of mechanical flags is the difficulty of setting them so that they are released exactly when the relay contacts make; if this is not assured a flag may fail to operate, or may operate erroneously when an induction disc relay resets before making contact. Shunt electrical flags are easier to apply than series flags in a complex control or trip scheme; tripping is delayed, however, by their operating time and may be prevented if, for instance, the fine wire coil of the shunt coil is open-circuited by corrosion. The shunt flag cannot be sealed in except by an extra wire to an auxiliary switch on the circuit breaker and requires three contacts on the protective relay. The most common arrangement is 'shunt reinforcing' (fig. 1. lOb ) where the protective relay opens the flag coil circuit when it resets after the fault is cleared. Series flags are used in most countries and offer the most practical solution. It introduces no delay in tripping and there is negligible risk of failure to trip because of an open-circuited coil, because the coil wire is of heavy gauge. Its only limitation is imposed by the very wide range of current over which it has to operate; some trip coils take 30 A while some tripping relays take only 0·1 A. This situation normally requires a choice of three coils, each having a tap, but a sensitive polarised flag has recently been developed in England which will pick up at 0·1 A and has a resistance of only 1 ohm full coil and 0·1 ohm on its tap, so that it is applicable to trip currents up to 30 A.

2.10. FINISHES

Since relays are liable to be kept in damp surroundings in transit or storage, they must be constructed of materials resistant to rust and corrosion or they must be plated or painted with a protective coating. When stainless steel, titanium and magnetic nickel steels become cheaper there will be no problem; at present the following protective finishes are the most commonly used. Steel Parts: Copper-nickel where the appearance is important, with, as an alternative, tin zinc. Either aluminium or black paint for large parts such as cases, with an undercoat of primer or phosphate. Brass: Nickel for good appearance, otherwise bare. Aluminium: Anodising or irriditing finish.

94

Relay Design and Construction

2.10

Laminations: Varnish impregnation after either painting or phosphating to prevent rust. Relays normally energised in service tend to preserve their finish but those stored in damp atmospheres tend to deteriorate in appearance unless the room or container is kept at least SoC above ambient temperature.

2.10.1. Tropicalisation

The protection of relay parts by special finishes is seldom completely reliable; it is preferable to provide a few watts of heat in relays which are not normally energised so that the temperature of the air inside the case may be raised 10°C to prevent condensation, especially during storage. An anti-bacterial and fungicidal varnish is available which should be painted on mouldings and around the inside of the case or cover every two years. Wrapping relays in strong waxed paper or sealed plastic bags for shipment or storage in the tropics is effective provided that the relays were warm and dry at the time of sealing the bags, and preferably equipped with a bag of silica-gel.

2.10.2. Corrosion

Unless proper precautions are taken at the factory, fine wire coils are liable to subsequent failure on open-circuit, usually near one of the leads but sometimes at a kink or crossed turns, due to the fine wire having been eaten through by corrosion. The primary cause of this corrosion is the condensation of moisture on the coil surface when its temperature is lower than that of the surrounding air. The condensed moisture rapidly absorbs CO 2 , fatty acids from handling at the factory and other acid-forming impurities, including bacteria, so that a weak acid is formed which acts as an electrolyte; this results in electrolysis, the acid radical combining with the copper to form an acid salt. In the case of CO 2 , which is absorbed from the air, the salt is CuC0 3 , a green deliquescent powder of low resistance, which attracts more moisture and tends to spread, accelerating the corrosion. Failures from this cause were much more common when coils were connected to the positive end of the d.c. circuit, because the coil became the electrode to which the acid ions were attracted. There is statistical evidence that coils wound with 0·006 in. wire are no less liable to failure than those wound with 0·002 in. wire, although they may take somewhat longer to fail. Experience has shown that corrosion does not. occur on coils that are continuously energised since a rise in temperature, even as low as 10°C above ambient, prevents the deposit of moisture. For this reason trouble is very seldom experienced with a.c. coils; on the other hand, d.c. auxiliary coils and

95

2.11

Protective Relays

trip coils which are not normally energised will corrode if not properly manufactured, unless they are connected to the negative pole of the battery and are separated from the positive pole by the contacts of a relay or switch. It has been found that condensed moisture or dew starts depositing at the bottom of cracks in the coil rather than on the surface, whereas rain water deposits on the surface and tends to bridge over the microscopic cracks. The formation of the dew at the bottom of the cracks is, of course, the worst place as regards corrosion, but it can be prevented by encapsulation or by proper impregnation of the coil or the application of a hard wax over an existing coil. Corrosion of other parts of the relay can arise from the same causes and is accelerated at stress points or where two components made of dissimilar metals, or having dissimilar finishes, are in contact, promoting galvanic action. This is especially common in tropical countries where excretions of bacteria provide a source of acid which, in a damp atmosphere, forms the electrolyte. Astonishingly large and brightly coloured growths of metal salts sometimes form during the periods between maintenance, especially in certain factories in-the tropics such as rubber refineries. For the relays to operate reliably in such localities it is essential to house them in a sealed case. 2.10.3. Metal Whiskers

Metal whiskers are hairlike single crystals of the order of 0·0001 in. diameter and up to 0'25 in. long, occurring mostly on electronic parts plated with cadmium, tin, zinc, tin-zinc or tin-cadmium. They are flexible but of immense strength and can puncture solid insulation so that they are dangerous to miniaturised equipment. A tin whisker t in. long and 0·0001 in. thick has a resistance of 50 ohms and will carry 10 mAo They grow most readily in warm, humid air and on thinly electro-plated brass or copper parts, especially if they are subject to mechanical stress or high frequency vibration. The ideal conditions appear to be 63°C and high humidity. They can be prevented by the use of plating other than tin, cadmium pr zinc or by avoiding plating. Humidity can be excluded in some cases by varnishing or encapsulation. On the other hand, silver contacts are also susceptible to metal whiskers in the presence of a suitable catalyst such as sulphur and certain volatile components of phenolic varnish. APPENDIX 2.11. CALCULATION OF INDUCTION RELAY TORQUE

Consider a moving element made of thin conducting material and prevented from rotating about its axis. Let two alternating magnetic fluxes Cl>1 and Cl>2 be applied to the element in such a way that their normal components
96

Relay Design and Construction

2.11

are given by:

= klIll s~n(wt+1X1)} CP2 = klI21 s1O(wt+ 1X2) CPl

(2.11.1)

where 1111 and 1121 are the amplitudes of the respective input currents and k is a constant, characteristic of the magnetic system producing the fluxes. If the effective impedances presented by the element to currents induced by the two fluxes are equal and given by Z j A, the total currents are given by:

-~l

I

Z

i l = ZjA = - wk l11 cos (Wt+lX l -A)

) (2.11.2)

I i2 = ZjA = - Z 12 coS(Wt+1X2- A) -~2

wk l

and the currents i12 and i21 reacting with the resultant fluxes within the element by:

. 112 . 121

Wkk11 11 I cOS(Wt+1X 1-A) = kIll. = - -Z

) (2.11.3)

= kl12. = - rokkl -Z I12IcoS(rot+1X2- A)

where kl is a constant assumed equal for both currents and defined by: (2.11.4) Neglecting the effect of the induced currents upon the applied fluxes, the torque developed within the element is then given by: (2.11.5) where k2 is a constant. Thus: 'T

wkk ll 12 I . coS(Wt+1X2-,1.). = k2{k I11 I s1O(wt+lXl)-Z -k1I21 sin (rot + 1X2) . ro~klllli Cos(rot+a 1-A)}

2 . ~ 2-A)= rok Zk1k 21 11 I 12 I{sm(wt+al)' cOS(Wt+1X -sin (wt+a2) . cos(rot+al-A)} =

rok2~lk2IIII112IHsin(2wt+al +a2- A)+sin(al- 1X2+ A)-sin (2rot+al +a2-A)-sin(a2-al +J..)}

:. 'T

I I.

= rok2kl Z k21 11 12 s1O(al- a2) COSA

97

(2.11.6)

2.11

Protective Relays

Using r.m.s. values and considering II 't'

22klk2

=

12 in a time-current relay:

= 2wk Z-l sm (!Xl -!X2) cos A. = KW]2

2

•

sin () COSA.

98

(2.11.7) (2.11.8)

3 TIw Main Cha,.trete,.istics of p,.otective Belays Phase and Amplitude Comparators-Relay Characteristics-General Equation for Characteristics-Inversion Chart-Resonance-Appendix 3.1. GENERAL CHARACTERISTICS AND EQUATIONS OF PROTECTIVE RELAYS

In this chapter a general mathematical relationship for relays will be developed which is applicable to all types of relay movement. A graphical method of showing the complete performance of any relay at pick-up will be discussed. 3.1.1. Relays as Comparators

All protective relays of the electrical type operate when an electrical quantity of the protected circuit either changes from its normal value or changes its ratio and/or phase relation with respect to another electrical quantity of the circuit. In other words, the relay may measure one quantity, usually the current entering the protected circuit, and compare it either with a standard or with another quantity with which it has normally a certain magnitude or phase angle relation. In some simple types of relay, used as level detectors, the second quantity is constant, as in the case of an overcurrent relay where a spring or gravity may oppose the force produced by the current in the operating electromagnet. The spring acts as a standard of comparison and prevents the electromagnet from moving the armature and closing the relay contacts until its force has been overcome, which requires a current representing the calibration level of the relay. On account of the fact that the fault current level changes with generating conditions, it is seldom possible to obtain selectivity on the basis of current magnitude alone and most applications require the addition of a time function so that the relays nearest the fault location, having the most current, will trip first and before the others in unfaulted circuits can do so. Because of the difficulty of obtaining selectivity on the basis of single quantities like current, potential, phase angle, etc., without employing time delay, most high-speed relays measure a derived quantity which is a com-

99

Protective Relays

3.1

bination of several simple quantities; for example, admittance, current ratio, etc., in which two simple quantities are compared in magnitude and/or phase relation. An example of magnitude or amplitude comparison is the differential current relay where the current entering the protective circuit is compared with the current emerging (fig. 3.1), which should normally be the same.

FIG.

Restra.in

Rutra.in

(a)

(b)

3.1a, b. Differential current protection (longitudinal) (a) normal (b) faulted

An example of phase comparison is the power directional relay which compares the direction of current flow relative to the bus potential (see fig. 2.4, third row). The comparison is usually made in relays by turning the electrical quantities into forces, torques, m.m.f.s or e.mJ.s proportional to the two quantities compared. This physical aspect of relays was considered in Chapter 2 in connection with different types of relay movements. In the present chapter we will consider the different types of characteristics which those movements can be used to produce. The movements are divided into two groups: (a) Relay Movements or Circuit Arrangements which inherently make Amplitude Comparisons. Balanced beam relay (fig. 2.17a) Induction disc element with shaded pole driving magnets (fig. 2.5a) Opposed rectifier bridges (fig. 2.21a) Transductor relay (fig. 2.23a). (b) Relay Movements or Circuit Arrangements which inherently make Phase Comparisons. Induction cup relay (fig. 2.6) Induction disc element with wattmetric type of driving magnet (fig. 2.12a) Induction dynamometer (fig. 2.3b) Polarised rectifier bridge (fig. 2.22) Hall effect crystal (fig. 2.25) Magnetic amplifier relay using Ramey circuit (fig. 2.23b). In inherent amplitude comparators the two quantities are opposed and the relay operates when the operating quantity exceeds the restraining quantity in magnitude, irrespective of phase relation. 100

3.1

The Main Characteristics of Protective Relays

An example is a balanced beam impedance relay (see Chapter 2, fig. 2.17a) with the current I tending to operate the relay and the voltage V tending to restrain it. The relay operates when Kill exceeds IVI or when IZI < K, irrespective of the phase angle between I and V (fig. 3.6).

FIG.

3.2. Balanced current protection (N is the number of coil turns) Restrdining coli ~_ i(l.. +sl.)

----JM~~

__~~~ _ L__

.

__

. ~~~~

(a.) Opcta.t.ing coil ________--:-'______ Pi lot

!i~

_____ ____'________ TcrminoJ

®

(a) Opna.ting T

Opera.ling

co il

coil

\

~-:}II II ........._ _ _ _.1...- _ _ ..../

I

\

I

\I

1\

I

\

\

' - - _ _ _ _........._ _- - ' ' - '

(b) FIG.

I

®

3.3. (a) Circulating current pilot wire protection showing alternative positions of restraining coil (a and b). (b) Balanced voltage pilot wire protection

In inherent phase comparators the two quantities interact causing the relay to operate when one quantity has a certain phase relation relative to the other, irrespective of their magnitudes. An example is a wattmetric relay movement which is a natural phase comparator; the current and voltage co-operate to produce a torque proportional to IVI III cos cP where cP is the phase angle between V and 1. This means that the device produces a positive torque when cos cP is positive, i.e. 90° > cP > - 90°. 101

Protective Relays

3.1

3.1.2. Relationship between Amplitude and Phase Comparators

The above is only part of the matter, however, since an inherent amplitude comparator becomes a phase comparator and vice versa if the input quantities are changed to the sum and difference of the original two input quantities .. Consider a relay which operates when IA I > IBI, i.e. an amplitude comparator. If the input quantities are changed so that it operates when A-B

~------~~------~

B

(a) A \

, , t,

,

I

':v, ,

'''',,

,

'l.

to/ I I

tfJ B

(b) . FIG.

(c)

3.4. Vector diagrams of amplitude comparator used for phase comparison (a) ; > 90° when > + (b) ; = 90° when A - B = A + B (c) ; < 90° when A - B < A + B

)A - B) )A B)

IA + BI > IA - BI it is now a phase comparator because A and B must have the same sense or polarity for the relay to operate. This is illustrated in fig. 3.4. Similarly, a directional relay whose torque is proportional to a vectorial product of A and B is a phase comparator which operates when A and B have the same direction. If, however, the input quantities are changed to

FIG.

(c) (b) (a) 3.5. Vector diagrams of phase comparator relay used for amplitude comparison (a) A < B when A > 90° (b) A = B when A = 90° (c) A > B when ..t < 90°

(A +B) and (A - B), as in fig. 3.5, the relay becomes an amplitude comparator because (A + B) and (A - B) have the same polarity only if IAI > IB I· This can also be proved algebraically by taking specific cases of relays. For instance, a balanced beam relay operates when the pull of the operating magnet at one end of the beam exceeds that of the restraining magnet at the

102

The Main Characteristics of Protective Relays

3.1

other end, i.e. when IAI2 > IBI2. If we change the input quantities, as discussed above, the relay operates when IA+BI2 > IA-BI 2, Le. when IA2+B2+2AB cos (4)-8)1 > IA2+B2_2AB cos (4)-8)1 -where 4> is the angle between A and Band 8 is a design angle, or when 4AB cos (4)-8) > 0 Le. when (0+90°) > cP > (0-90°) Similarly, in an induction cup relay, the torque is proportional to the vector product IAIIBI cos (4)-0) and the relay operates when (0+90°) > 4> > (0-90°). If we change the input quantities as before, the torque oc IA +BIIA - BI sin ex where ex is the angle between (A + B) and (A - B) which must be 90° for maximum torque. Thus the torque oc IA 12 -IBI2 and the phase comparator has become an amplitude comparator. 3.1.3. Graphical Representation of Threshold Conditions

It would seem obvious that the operating characteristic of a comparator would be most clearly demonstrated by plotting in a polar diagram the ratio of the amplitudes of the two quantities compared for different angles between them. Nevertheless, prior to 1930, it was customary to show the characteristics of an amplitude comparator, such as a differential relay or an impedance relay, in a diagram along whose axes were plotted the two quantities -compared and to use a polar current diagram for phase comparators, such as directional relays, showing the characteristics at certain fixed voltage or current values. In the early thirties, when the mho relay was evolved (5), the amplitude and phase characteristics were shown in the same diagram in a single characteristic (figs. 3.6, 3.9b and 3.lOb) at the suggestion of J. Neher, the coordinates being the real and quadrature components of VII, i.e.

WI cos 4> =

Rand j

WI sin 4> =

X.

Clearly this same idea can be applied to other relays comparing the two quantities vectorially. For instance, fig. 3.14b shows the characteristics of a current differential relay with axes

I~:I cos 4>

and j

I~I sin 4>;

this diagram

tells a great deal more about the performance of the relay than some present manufacturers' bulletins, which use the amplitude comparison diagram of' fig. 3.14a. It has already been shown, in Chapter 2, that the characteristics of all relays comparing two quantities A and B are circles in a diagram whose coordinates are the real and imaginary or quadrature components of (~), viz.

I~l, = l~l cos 4>

and j

103

l~t = j I~I sin 4>

(3.1)

3.1

Protective Relays

FIG.

3.6. Impedance relay characteristic on impedance diagram

\ \. Oi rection of I for mQJC. torque

(a)

.x

(b) FIG.

3.7. Directional relay characteristic on (a) current diagram (b) impedance diagram

104

The Main Characteristics of Protective Relays

3.2

where A and B are the two quantities compared. As A and B can be either currents or voltages, points in the diagram then indicate either current ratios, impedance or admittance. The advantage of such a diagram is that the phase and magnitude relations are clearly indicated, but it is not applicable to relays with non-linear characteristics except as a series of graphs at different levels of the quantities compared. Phase angles are considered positive when the numerator quantity leads the denominator quantity, i.e. when A leads B where

~ is plotted. For example,

where; is plotted, j

1;/ =

X is positive

when V leads I by the positive (counter-clockwise) angle ljJ. Where A is potential and B is current the real ordinate and j

I~t

I~t is resistance

is reactance; hence the diagram in which the characteristic is

plotted has ordinates R andjX and is called an impedance diagram. Similarly, where

I~\ is

plotted, the diagram has ordinates G and jB and is called an

admittance diagram. Other names are the Z-plane and the Y-plane characteristics (see figs. 3.9 and 3.10). Unfortunately, there are no words like impedance and admittance that apply to the ratio of two currents or to the general case of

I~I or I~I. Con-

sequently, in the interests of clarity and brevity, these diagrams will be referred to respectively as the (X-plane and the p-plane diagrams. 3.2. PARTICULAR TYPES OF RELAYS

The characteristics of instantaneous relays measuring a single quantity cannot be plotted because they are in fact defined by their pick-up and reset values. Time-current relay characteristics are usually defined by limits of operating time at certain current values such as 2 x and 10 x pick-up and, although shown as time-current curves, are very seldom represented by an equation. However, they can be expressed in the form t =

-#-+ B where m -1

m is the multiple of pick-up current and A and B are constants. For I.D.M.T. relays, A is of the order of 2 and B is of the order of 0 '1. A is of the order of 6 for very inverse relays. n is 0 for a definite time relay, 1 for a very inverse and 2 for an extremely inverse relay. In comparator relays the relay cha:racteristic is the locus of zero torque conditions; the relay operates for conditions on one side of the locus and resets for conditions on the other side. This locus is the result of plotting the equation of balance (zero torque) in terms of the two quantities compared. We will now consider various well-known types of comparator relays, comparing currents and voltages in magnitude or phase or both. 105

3.2

Protective Relays

3.2.1. Current Differential Relays

The most effective principle of protection is 'Unit Protection', in which the relay compares the magnitude and/or direction of the currents entering and leaving the protected circuit (fig. 3.1). For generators and motors, the magnitudes ofthe phase currents entering and leaving will be the same under normal conditions. This will also apply to transformers after correction for their tum ratio. Any difference will indicate an internal fault and is detected by a relay whose operating winding receives the vectorial difference of the currents at the two ends of the protected section. For transformers and large generators, this is called longitudinal differential protection. In order to avoid undesirable operation on heavy external faults due to C.t. errors, a restraining winding is provided which is energised by the through current and has fewer turns than the operating winding. If the restraining and operating magnets are similar and the ratio of their turns is S, operation occurs when

II ;121> III ;I2[ for a static comparator, +1 [2 for an electromagnetic comparator. or II -1212 > II-T I

/-T

(3.2) (3.2a)

S is usually ·05 or 5 % for generators and 0·1 to 0·4 for transformers, the higher values being used if the transformer ratio is varied by a tap changer. S is also the ratio of restraining turns to operating turns in symmetrically designed relays. Equations (3.2) and (3.2a) represent the same characteristic at balance. By dividing equation (3.2) by II and substituting an expression for marginal operation 11-a-jbl

~ 11

= a + jb we obtain

= ~ 11+a+jbl which is

shown in the Appendix 3.7.1 to be the equation of a circle on a diagram having ordinates

c=

I~I

~p

and j

I~I,. whose radius is ~q

1-

(S~) '

and whose centre is at

1+(~y

( 2 " The negative location of the cen're is due to the convention

1-

~)

of calling the difference current (II - 12 ), If the directions of II and 12 were considered relative to the protected circuit 12 would be normally negative relative to II and their difference would be written II + 12 which would make c positive; The mathematical steps for the calculation of rand c are given in Appendix 3.7.1. The characteristic is illustrated in fig. 3.14b for S = 0·1, which is referred to as a 10% slope (fig. 3.14a).

106

3.2

The Main Characteristics of Protective Relays

In the case of a circuit with more than two ends, such as a three-winding transformer or a multi-circuit bus, the c.t's are polarised with respect to the direction of power flow from the bus or transformer so that the operating quantity is now the vector sum of the currents which, by Kirchhoff's Law, should be zero under normal conditions (this method of polarisation makes the expression for c positive). In the protection for such multi-ended circuits the operating coil receives the vector sum of the currents and the restraining coil or coils the scalar sum of the currents or the squares of the currents. The equation for marginal operation is

111

+Iz; ... 1,,\

=

111Iz+llzI2 ... 11,1

(3.3)

2

Another variation in the design of relays for protecting two-ended circuits, such as generator windings, is the principle of product restraint, where the restraining torque is proportional to the product of the two currents, i.e. the net relay torque is of the form

I

r-ll1111zl

I 1- I z coS'r (3.4) S I where l' is the angle between 11 and 12 , This arrangement permits S to be non-linear and large at high currents so that there is a very high restraining torque (high stability) for heavy external faults. It also gives very fast action on internal faults because 1111 1121 cos T then becomes negative and hence adds to operating torque.

The characteristic is a circle of radius S

c=

J + (~) 1

2

(1 + S2) when plotted on a diagram with axes I~I12 and 2

the same circle when plotted in a diagram with axes

p

I~I 11

and centre at j

I~I. It is also 12

and j p

q

I~I. This is 11 q

analysed mathematically in Appendix 3.7.2. In all these arrangements the stability can be further increased by making the restraint non-linear, increasing the through current, or by introducing saturation into the operating coil circuit so that the pick-up increases sharply with the high through current. The effect on the equation for marginal operation is to increase the value of S at high currents. 3.2.2. Current Balance Relays

In the case of parallel lines or split-winding generators, where the currents in two parallel paths are normally equal (fig. 3.2), balanced current relays are used in the system known as transverse differential protection. Because either one of the two currents can be the larger during a fault it is necessary to have two relays in each of which the restraining winding has 10% more turns than the operating winding, so that an excess of 10% of current in either circuit can be detected. Relay operation occurs when 1111 > 1·1 1121 in one unit or 1121 > 1·1 1111 in the other, where subscripts 1 and 2 refer to the two parallel circuits. 107

3.2

Protective Relays

The equation for balance is

ItI

= K, which gives a circle of radius K

with its centre at the origin. This direct· comparison of the two currents is less effective than the system described in the previous section 3.2.1 because the latter is of the order of ten times as sensitive at low currents. It is used for parallel line protection because line faults seldom involve less than load current and because it is necessary to distinguish the faulty line which could not be done with a single relay measuring the difference current (see figs. 3.1 and 3.2). 3.2.3. Pilot Relays

Longitudinal differential protection can also be applied to lines and cables in the same way as it is to generators and transformers but, if the relay is located at one end of the line, some means of tripping the remote end by a superimposed signal (known as transferred tripping) must be provided. Although this is theoretically preferable it is not adopted in practice; instead it is customary to have relays at both ends, in each of which the local current is compared with the current in the pilot wire. With this arrangement the performance of the relays is limited by the current by-passed through the capacitance of the pilot wires and by the deliberate introduction of nonlinear devices in order to keep the pilot voltage down to a reasonable level; the pilot wire current is the current supplied by the c.t's at one end minus the current by-passed through these shunt paths and the linear current taken by the local relay. Depending on the relative polarity of the currents at the two ends of the line, the pilot wire current under normal conditions may either be zero (balanced voltage scheme) or equal to the through current (circulating current scheme) assuming negligible capacitance and leakage. These schemes will be described in more detail in Chapter 8. (a) Circulating Current Scheme. In this system, under normal conditions and during an external fault, the current circulates around the pilot wire loop because the line current flows in at one end and out at the other. When an internal fault occurs the current tends to flow inwards from both ends so that it no longer circulates and the difference current flows in the operating coils of the relay, as shown in fig. 3.3a; the restraining coil can be connected either on the pilot wire side of the operating coil (position 'a') or on the c.t. side (position 'b'). (i) Restraining Coil on Pilot Side. Considering the first position (a), the. equation for balance in the relay at end A is K tIIA-1IBI = "2IIA+1IBI + P.U. (3.5) K is a relay parameter which depends upon the ratio of the operating and restraining coil turns and the impedance of the operating coil circuit. P.U. is the pick-up current ofthe relay, which will be neglected because the critical 108

The Main Characteristics of Protective Relays

3.2

conditions of operation occur at high currents where P.U. is negligible. ')I is the propagation constant and ~~ is a, a vectorial quantity of the form a+jb. (3.6) where m is the attenuation constant and n is the phase shift constant. Z is the series impedance and y the shunt admittance per mile of pilot. It is more usual to use the symbols ')I = a+jp (151), but they have been avoided here to prevent confusion with the a- and p-planes. For zero attenuation and phase shift ')I = 1/0°. ')I

= ..)Z". y = m+jn

To plot the characteristic in the a-plane

(a

=

~~), we must divide equa-

tion (3.5) by I B which gives

(3.7) In Appendix 3.7.3 this is shown to be the equation of circle of radius

2K 21')11 and whose centre in the a-plane is located at c = ')11 +K:. 1-K 1-K

This is also the characteristic of the relay at terminal B plotted in the p-plane because a is local current/remote current at end A. and p is local/ remote current at end B (fig. 3.8a). It is more usual to plot the characteristic of the relay at terminal A. in the p-plane, i.e. in terms of

1~:I. To do this we divide equation (3.5) by 1,( which

gives (3.8) Following a similar procedure, which is given in Appendix 3.7.4, this gives the equation of a circle whose radius is =

1':'~21~1 and whose centre is at

!(1+K2 )

l-K2 . This is the same circle as for the characteristic of the relay at terminal B plotted on the a-plane. These results can be summarised as follows:

c

')I

3.1 Circulating Current Relay Characteristics TABLE

a plane for end A or

pplane for end A or

Radius

2K 1- K21l'1

2K 111 1 - K2 l'

Centre

1 +K2 ,'I-K2

Plane Quantity

Pplane for end B

109

a plane for end B

! (1 + K2) l' 1- K2

3.2

Protective Relays

To take an example, if l' = °'655/-24° for a 20-mile pilot and K = 0·31. The a-plane characteristic for the relay at A is a circle of.radius 0'448, centre at 0'792/- 24°; this is also the p-plane characteristic for the relay at terminal B. The p-plane characteristic for the relay at end A is a circle of radius 1·045 and centre at 1,85/24°; this is also the characteristic for the a-plane characteristic of the relay at end B. These characteristics are shown in fig. 3.8a. For a zero length pilot l' = 1/0° and the a- and p-plane characteristics are the same for the relays at either end, viz. a circle of radius 0·685 and

II~ (. )

-..L c ·, r-~ l-/I(Z , , - 1 _/1(2

}I!.!I

CA -

C, =~ . 0

I, '{

I

r, = ~

II~ ~---+-4-4------1I+---+-- I~lp

FIG.

3.8. Charactcoristic of circulating current pilot relays on the ex plane (a) Restraint coil on pilot side (b) Restraint coil on C.t. side

centre at 1'21/°°. This is the middle circle of fig. 3.8a. This circle cuts the . I-K I+K real aXIs at op = - - and OP' = - ---. I+K l-K For the relay to be stable on load and external faults it must not trip when IA. = IB' i.e. the point 1,0 must be well within the circle. To do this and allow for modification of the circle due to 1', the relay constant K and the phase angle

110

The Main Characteristics of Protective Relays

3.2

of the operating coil circuit must be related to y. To permit the use of a constant value of K, the impedance of the operating coil circuit can be made equal to the characteristic impedance Zo of the pilot wires. This is explained in Chapter 8, section 8.4.1 (a). (ii) Restraining Coil on C.t. Side. For long lines and G.P.O. pilot schemes, the restraining coil is usually connected in position (b). Here the relay at the end A is at the threshold of operation when

1r,.-yIBI = KIIAI

(3.9)

In Appendix 3.7.5 it will be shown that the operating characteristic in the a-plane is a circle of radius

~IYI and centre at y - 1 1 2' l-K -K

Zo in series with the operating coil in fig. 3.3a is adjusted so that the operating coil currents in the relays at the two terminals, t(IA - sIB) and t(IB- SIA)' are in phase with each other for single-end feed, i.e. with I B = O. This compensates for the phase shift in the pilot so that y can be taken as scalar and equal to s (fig. 3.8b). The equation for balance then becomes

(3.10) This results in a circle in the a-plane whose radius is

sK 2 and whose l-K

centre is at _s_ /0°. The characteristic for the relay at terminal B is a circle l-K-

of radius ~ whose centre is at ! /0°. The circles for terminals A and Bare s sinterchanged when plotted in the p-plane. The effect of pilot wire voltage limiters is to make the circle bulge to the right. (b) Balanced Voltage Scheme. In this system (fig. 3.3b) the currents at the two ends are turned into voltages by passing them through reactors with secondary windings (transactors). The transactors are connected so that their output voltages are opposed during normal conditions and no current flows in the pilot wire, neglecting pilot capacitance. During an internal fault the voltages are nearly in phase and current flows in the pilot wires and in the relay operating coils which are in series with them. The relays compare the pilot wire current with the current in the local restraint circuits and operate when K 1/01 > 1/,1· Now, 10 is effectively proportional to VA - VB and IR to VA; since VA and VB are proportional to the currents producing them, the relay at terminal A operates when K IIA-yIBI > IIAI, which is the same basic equation (3.9) as that of the Circulating current pilot scheme for long lines (restraining coil on the c.t. side). Hence it will be seen that circulating current and balanced voltage pilot schemes have similar equations and hence similar characteristics. Balanced voltage schemes are more difficult to apply to multiterminallines 111

/I:)

=

III ;121

KIIA\

lIB - "IAI = KIIBI

=

KIIB + )'IAI

Ditto

=

KIIA + )'IBI

\IA - )'IBI

lIB - )'IAI

Ditto

=

Circulating current pilot scheme B and balanced voltage scheme

IIA - )'IBI

III -12/2 = Illllhi cos IX

-S-

Ih;hl

Equation

Circulating current pilot scheme A (Restraint on pilot side)

Current differential with :::: product restraint

Current differential

System

3.2

(~r

11~1 KI~I

B

1- KI!I I+K)'

Cl-K2 +K2)!. )'

1 l-K I)'I

1 +KI!\ l-K)'

1 +K II l-K )'

c+r

2

1-~

2

1+~

OP'

3.2.3 (a) (ii)

3.2.3 (a) (i)

3.2.3 (a) (i)

3.2.1

3.2.1

Section

(1 - K) I~I (1 + K) I~I 3.2.3 (a)(ii)

1 1 +K I)'\

l-K 1 +KI)'I

1 +K2 l-K2)'

1 l-K2)'

:XK2) +

c-r

S;)/oo

(1 +

2

1+~

(~rW

1- 2

K l-K2 1)'\

(1

2K

1- K2 1)'1

SJ1 + (~r

1-

2

1-~

(~r

S 1+

OP

c

r

A

B

A

Both

Both

Terminal

a.-Plane Characteristics of Differential Current Relays

TABLE

3.7.4

3.7.4

3.7.3

3.7.3

3.7.2

3.7.1

Appendix

3.Sb

3.Sb

3.Sa

3.Sa

3.14b

3.14b

Fig. No.

w

'"

~

iii

Cb

::0

Cb

~

.....

Cb 0

-.

'b

a ....

N

The Main Characteristics of Protective Relays

3.2

due to the fact that the voltage limiting devices upset the voltage balance that is required to prevent tripping on external faults. (c) Criterion for Stability. Table 3.2 summarises the values for the radii and centre locations for the various applications of differential current relays. As previously explained, the point 1,0 must be well within the characteristic to ensure stability. This will be the case if 1,0 is well between the points P and P' where the circle cuts the real axis. Where the centre of the circle lies in the real axis, as in the case of differential current relays, OP = c-r and OP' = c+r, but, in the case of the pilot wire relays, the data apply only where compensation is provided for 'Y so that the centre of the characteristic circle lies on the real axis. In the Appendix, section 3.7, the cases are considered of relays not having this compensation and which are treated as follows. Referring to fig. 3.21c, if the centre of the circle is located at ao, bo, the points P and P' where the circle cuts the real axis are given by a1 = ao-(r2-b 2)t and a2 = ao+(r2-b 2)t, (3.11) respectively. 3.2.4. Impedance Relays (58)

Where the length of the line makes pilot wire protection expensive and where the importance of the line does not justify carrier protection, distance relays are used. These relays compare the local current with the local potential in a given phase or phase pair. In relays of the electromagnetic type, such as the balanced beam, the current magnet exerts a force K 1112 tending to close the contacts and is opposed by the potential magnet whose force is K'1V12, where K and K' are constants corresponding to those in equation (2.1) in the previous chapter. Operation occurs when K 1112 > K' 1V12, i.e. when

< Kor IV/2 I K'

IZI

<

JKK"

The characteristic on an impedance diagram is obviously a circle of radius

J~,

and centre at the origin (see fig. 3.6).

Such a relay is set to operate when the impedance measured by the relay is less than that of the protected section of line AB, i.e. the relay is adjusted so that

J:,

is made equal to the impedance of the protected zone. The

application of impedance relays is discussed more fully in Chapter 5. 3.2.5. Directional Relays

Relays such as overcurrent relays and impedance relays will respond to . faults in either direction (fig. 3.6). To prevent such relays from tripping unfaulted lines they are monitored by directional relays (fig. 3. 7b) which respond only to currents flowing in the direction from the bus to the line. E 113

3.2

Protective Relays

The torque of the relay is IVIIII cos (4)-9) where 4> is the angle between V and I and 9 is the value of 4> for which the relay has maximum torque. This is the third term in equation 2.1 in the previous chapter. 3.2.6. Ohm Relays

This term has been used by the author to define a relay which measures a particular component of impedance JZII!!. In the U.S.A. it has been called an angle-impedance relay. The threshold characteristic is a straight line on

x

,1

/I

II

:12K-zl

R

(a)

(b) -8

Ref V

-G--~7-r-.---~---G

Trip

Trip

(d)

(c) FIG .

3.9. Ohm relay characteristic on different graphs (K is the impedance setting ZI!. of the relay) (a) Voltage. (b) Impedance. (c) Current. (d) Admittance

an impedance diagram (fig. 3.9b) or a circle on an admittance diagram (fig. 3.9d). This can be produced on a phase comparator relay by comparing the phase relationship of the current KI with the vector difference (V - KI) as shown in fig. 3.9a. The relay operates when a < 90°. For example, in an induction cup relay, the torque ex: the product III [K III - IVI cos (4) - 9)] and

114

3.2

The Main Characteristics of Protective Relays the relay operates when

III [K III - IVI cos (4) -

0)] > 0, i.e. when

Z cos (4)-0) < K (see fig. 3.9b). In an amplitude comparator, magnitude of the voltage V is compared with that of the vectorial difference (2KI - V). It can be seen from the dotted lines in figs. 3.9a and 3.9c that this relationship also defines the same locus. In the voltage diagram, fig. 3.9a, the characteristic represents a voltage locus for a particular value of current. For double the value of I the locus would be twice as far as from the origin. Similarly, fig. 3.9c is the current locus for a particular value of V. The circuitry of the phase and amplitude comparator relays is given in Chapter 5. Figs. 3.9b and 3.9d show the duality of linear and circular characteristics on impedance and admittance diagrams. Each one has a single universal locus for any values of V and I, whereas in figs. 3.9a and 3.9c a family of curves would be necessary to cover a range of current or voltage. A particular case of the ohm relay is the reactance relay which measures IZI /90°. The itnpedance characteristic of such a relay is a straight line parallel to the R axis, i.e. measuring constant reactance. 3.2.7. Mho Relays

This is again a term used to define, in this case, a relay which measures a component of admittance IYI /0. Angle impedance is the alternative name, used in the U.S.A. The characteristic has an inverse relation to the ohm relay as would be expected from the relation terms 'ohm' and 'mho'. It is a circle in an impedance diagram (fig. 3.lOb) and a straight line in an admittance diagram (fig. 3.10d). It will be seen that it is an inherently directional impedance relay. In a mho relay of the phase comparator type the quantities V and

(V - ;,) are compared in phase relation; operation occurs when

r:t.

> 90°,

as in fig. 3.lOa. In an induction cup relay the torque oc the product

IVI [III cos (4) - 0) - K' IVI1 and the relay operates when IYI cos (4)-0) > K'. In an amplitUde comparator the magnitude III of the current is compared

with that of the vectorial difference (2K'V-/), as shown in fig. 3.lOc. Here again it will be seen that this relationship results in the same characteristics. The circuitry of these relays is discussed in detail in Chapter 5. Further dualities can be seen by comparing the four diagrams of fig. 3.9 with the four of fig. 3.10. The admittance and impedance circles have radii 1 1 -1 1 2K and 2K' and centres at 2K /0 and 2K' /0. The straight lines are distant

from the origin K and K' at an angle o. 115

3.2

Protective Relays A

I

""' I

"

I, I I I

v'

V~AD

~

"

I

I

x

12V_1, I X I

Zt' :2Z-x'1 I

I"

,'v_l~

,

I

I

R

(b)

(a)

-8 --~-r------~~ G

(c) FIG.

(d)

3.10. Mho relay characteristic on different graphs (K is the admittance setting YI!!. of the relay) (a) Voltage. (b) Impedance. (c) Current. (d) Admittance

3.2.8. Offset Mho Relay

This is the term given to a relay whose circular characteristic in an impedapce diagram does not pass through the origin (fig. 3.lla). This characteristic can be obtained by adding current bias to a mho relay; the bias provides an extra 1/12 term which results in an equation of the general form KI/12 - K'1V12 + IVII/I cos (¢-e) = 0 and represents a circle of radius .. b r = .JI+4KK' 2K' and centre d·Istant f rom t h e ongm y cI =l 2K'e ~

In a phase comparator mho relay the current bias can be introduced by inserting a biassing impedance Zb in the current circuit and introducing its IZb voltage drop into the potential circuit. The same characteristic can be derived from an amplitude comparator impedance relay using the same method but reversing the current bias, as shown in fig. 3.11c. This is referred to as a modified impedance relay. In the amplitude comparator mho relay the current bias is achieved by changing the current term in one of the input quantities so that IKII is compared with 12V-K"II. This is discussed more fully in Chapter 5, sections 5.2.1 and 5.2.4. 116

3.3

The Main Characteristics of Protective Relays

When the offset mho characteristic is plotted on an admittance diagram it is again a circle but its radius and distance of the centre from the origin are inversely related to those of the circle drawn on an impedance diagram.

x

, '"

....... - ......

... , , ,

x

,

\. \

I

I

I I I I

R

\

\

'v'"

"

(a)

R

'"

'--

-..-;

",/

(b)

8

(c)

3.11. (a) Offset mho characteristic on impedance diagram (b) Offset impedance characteristic on impedance diagram (c) Offset mho or impedance characteristic on admittance diagram FIO.

3.3. GENERAL EQUATION OF COMPARATORS

In order to compare the design constants of phase and amplitude comparators and also to produce a characteristic equation which directly fits any sort of relay, including static relays, a more general approach is necessary. The treatment here is given for two inputs under threshold conditions; a treatment extending to multiple inputs and characteristics other than straight lines and circles is given in Vol. II. The two quantities A and B can be supplied to the relay in any combination (Kl A +K2B) and (K3A +K4B). If A is used as the vector of reference they can be written: (3.12) KlIAI+K2IBI [cos (c/>-O)+j sin (C/>-O)] and

K3IAl+K4IBI [cos (c/>-O)+j sin(c/>-O)] 117

(3.13)

3.3

Protective Relays

This is shown vectorially in fig. 3.12. K 1 , K 2, K3 and K4 are design constants. In most relays at least one of them is zero and two of them are often equal. This makes the practical case relatively simple. It will be shown in sections 3.3.l and 3.3.2 that, for a given characteristic, A B

A B

FIG.

3.12. General case of vector quantities for supplying comparators

the equations for the amplitude comparator and for the phase comparator are of the same form (equation 2.2) but with different values for K 1 , K 2 , K3 and K 4 • On the other hand, if the same values of K are used for both comparators their characteristic circles will be orthogonal. 3.3.1. Amplitude Comparator In this case the two quantities are opposed and their moduli will be equal at the threshold of operation for any phase angle between them, the locus of which is the relay characteristic. Equating the moduli of expressions (3.l2) and (3.l3) we have [KlIAI+K2IBI cos (ljJ-8)Y = [K3IAI+K4IBI cos (ljJ_8)]2

+ [K2IBI sin (ljJ_8)]Z + [K4IBI sin (ljJ-8)Y

Rearranging the terms:

IBlz

(Ki-K~) IAI2+2(K 1K 2-K 3 K 4) IAIIBI cos(ljJ-8)+(K~-K~) = 0 (3.14) Dividing through by (K~ - K~) IA 12 for plotting in the p-plane

1~lz + 2(Kl~~=~iK4)1~ICOS(ljJ_8) + ~~=~1 = 0 The last term can be rearranged in a form indicating the location of the characteristic circle in the p-plane, viz.

I~r + 2cl~ICOS(ljJ-8)+C2 = r2

(3.15

This will be seen to be of the same form as equation (2.3) in Chapter 2, and is illustrated in fig. 3.21 b. Since

c=

-K 1K z -K 3 K 4 K~-K~

,

C 2 must be ( K 1K 2 -K 3 K)2 4 K~-Ki

118

3.3

The Main Characteristics of Protective Relays

Hence the right-hand side of the equation must be .2 = (KIK2- K 3K 4)2 Ki-K~ I K22 - K24 K22 - K24

-2KIK2K3K4 +K~K~+KiK~ (K~-K~)2

= (KIK4-K2 K 3)2 K~-Ki

Consequently the equation now becomes (A._B) (KIK2- K 3K 4)2 (A~)2 + 2(KIK2-K3K4)1!!lc K~-Ki A os + K~-Ki Of'

= (KIK4 -K2 K 3)2

(3.16)

Ki-Kl

This equation represents a circle of radius r = KIK~ -K;K3 and centre at K2-K4 (see fig. 3.13), where c = -

Amplitude compara.tor

j~lq

FIG.

r

K1K -/(.

c

K,K~-K(! K K

K.

J(.

-K

C/B,

Pha.se com a.ra.tor K1K~-K2K'}

2)(, K K1K~+K2:K3 21(.

3.13. Threshold characteristic of any phase or amplitude comparator comparing the vector quantities (KIA + K2B) and (K3A + K4B)

In order to obtain the radius and centre of the circle plotted in the ex-plane we have to return to equation (3.14) and divide it through by (Ki-K~) instead of (K~-Kl) and this gives

IBI2

IAI2

2 (KIK2-K3K4)A co (A._B) Ki-Kl = 0 (~)2 B + Ki-K~ B s Of' + Ki-K~ This is a circle of radius

KIK4-K2 K 3 Ki-K~

whose centre is at

3.3

Protective Relays

3.3.2. Phase Comparator

The two quantities to be compared are of the same form as equations (3.12) and (3.13) except that the constants are different, i.e. (3.17) KIIAI+KzIBI [cos (cp-O)+j sin (cp-O)] (3.18) and K;IAI+K4IBI [cos (cp-O)+j sin (cp-O)] The relay operates when the product of (3.17) and (3.18) is positive. Considering expressions (3.17) and (3.18), if ct is the phase angle of one quantity and p that of the other, the threshold of operation is when ct - p = ±90°, because the product is greatest when the two quantities are in phase. i.e. when tan (ct- P) = ± 00 tanct-tanp - - - - ' - = +00 i.e. when 1+tanct tanp 1+tanct tanp = 0 i.e. when -1 tanct= - (3.19) or I tanp K21BI sin (cp-O) KlIAI+KzIBI cos (cp-O) K41BI sin (cp-O) and tanp = K;IAI +K4IBI cos (cp-O) Substituting for tan ct and tan p in equation (3.19) we get From fig. 3.12

tanct =

K1K41BI2 sin2(cp-O) = - K1K;IAI2-K1K4IAIIBI cos (cp-O)-K1K;IAIIBI cos(cp-0)-K1K4IBI2 cos 2(cp-9) i.e. K1K4IBI2+K;'K;IAI2+(K;'K4+K;K;)IAIIBI cos (cp-O) = 0 Dividing through by K5. K~ IAI2 gives the characteristic equation for the p-plane !!12 K1K4 +K1K;I!!1 (,1,.-0) KIK; = 0 1A + K:zK4, A cos,+, + K2K.~ The last term can be rearranged in the form of the circle equation (3.15) to give th~ following: !!/2 (KIK4+K1K3)/'!!leos(,I,._O) + (K 1K4+K1K 3)2 /A + K;K4 '+' 2K1K4

AI

= (K;'K4-K1K3)2 2K~K4

' . 1 f d' K;'K4-K1K; This IS a CITe eo ra lUS r = 2K;K4 and centre at

c

= _ (K;'K 4+K;K 3)10

2K 1K 4

-

In order to obtain the radius and centre of the circle plotted in the 120

3.4

The Main Characteristics of Protective Relays

(X-plane we have to return to equation (3.19) and divide it through by K; K31BI2 instead of K2 K41AI2 and this gives

K;K~+K2K31:i1 ('/"-0) K2K~ = 0 1 ~12 B + K;K~ B cos + KI K 3 'I'

When this is rearranged in the form of the circle equation (2.3), the equation represents a circle of radius

K1K~-K2K;

2K1X;

_ K1K~+K2K~ 10 2K1K; -

whose centre is at

3.4. COMPARISON OF CONSTANTS

Since the same characteristic (fig. 3.13) is to be produced by both comparators rand c must be the same and, if equated, should give the relations' between the constants for the two comparators. Considering the values for the p-plane: TA.!JLE

3.3

Amplitude Comparator

r=

K,K4 - K2K3 K22 - K42

(3.20)

(3.21)

Phase Comparator

r=

K{K4-K2K3 2K2K4

c = - K{K4+K2K~ /(J 2K2K4 -

(3.22)

(3.23)

The corresponding values for the (X-plane are given by substituting Kl for K2 and K3 for K4 in the above expressions in their denominators only. 3.4.1. Typical Example of Comparison of Constants

Let us consider a percentage differential relay which operates if the difference of the currents entering and leaving the protected circuit exceeds 5 % of the sum of these currents, or 10% of the mean through current

11 ;1

2•

Such a relay operates when one current is 10% greater than the other

and is said to have a 10% bias and, when plotted on a graph (fig. 3.14a), the characteristic has a 10% slope. Fig. 3.14b shows the same characteristic plotted on the preferred diagram with axes

1~lp and j 1~lq'

(a) Amplitude Comparator. The differential current relay must operate when

(11 -1

2)

exceeds S

(It ;12) where Sis the slope expressed as a fraction.

The operating winding is supplied with currents (Il - 12) and has N turns. 121

Protective Relays

3.4

s;

The restraining winding is supplied with currents (II +12 ) and has turns. The common term N can be included in the relay constants K t •

Kl , K3 and K4 • The operating quantity is therefore (11 - /2) = Kd/ll +K2 1/2 I(cos I{> +j sin I{».

I,+h

-2-

(a)

-+-------t---:--t----I*l

p

r=O·1004

(b) FIG.

3.14. Operating characteristic of differential current relay (a) Scalar diagram. (b) Polar diagram

The restraining quantity is (/1 +/2) ~ = K31/11 +K41I21(cos I{>+j sin I{»where c/> is the angle between 11 and 12 • () is the angle of 12 from a reference axis and can be made zero and hence neglected.

From equations (3.20) and (3.21) the characteristic is a circle given by

S S

-+2 2

K 1 K 4 -K 2 K 3 r =

K~-K~

=

(S)2

1- -

2

122

S

=

(S)2

1--

2

(3.24)

The Main Characteristics of Protective Relays

c= -

K1K2- K 3 K 4 K2 K2 2-

=

1+(~r = (1 +!2) 1-

4

3.4

(S)2

2

1-

S2

4

(3.25)

This checks the results given in section 3.2.1 and Appendix 3.7.1. If the slope Sis 10 %we can substitute S = 0·1 in the above equations, which gives r = 0·1004 and c = 1·004. This is the characteristic shown in fig. 3.14b which has the general equation (3.26) It is evident from equations (3.24) and (3.25) that, if S is small, C = 1 and r = S. Therefore the approximate equation for the characteristic of a differential current relay can be taken as

1~:12 -21~:1 cos q, + 1 =

(3.27)

S2

(b) Phase Comparator. In order to find the constants for 11 and]2 in the quantities that must be supplied to a phase comparator to give the same differential current characteristic, i.e. a circle of the same rand c, we must use the same values as in equations (3.24) and (3.25).

and

c

= _

c-r = -

K~K~+K;K; (1 + !2) 2K' K' S2 =

2

2K~K~ 2-K-;-K-~

4

K~

=-

K;

=

=-

K; -K4 =

1-4 S2 S 1-1+ 4 -S 2 S2 = --S-

1- 4 S2

and

c+r = -

2K;K; -2K-;-K-~

1+"4 +S S2 1-4

S

Kl K~ 2- 1 -=-=-K2 K; S 2+1

123

1+2 S

1+2 =--

1-~

2

3.4

Protective Relays

which is fuI1iUed when

Kl =K:"=~-l 2

(3.28)

S

Ki=K3 = 2-+ 1 Therefore the quantities supplied to be compared in a phase comparator to give the same characteristic as the amplitude comparator considered in section 3.4.1 (a) are the vectorial sums

[11(~-1)+/2(~+1)]

and

[11(~+1)+12(~-1)]

(3.30)

For a 10% slope characteristic, S = 0·1 (fig. 3.14), S S "2 - 1 = - 0'95 and "2 + 1 = 1·05 The two quantities must therefore be the vectorial expressions (- 0·95 11 + 1·05 12 ) and (1'05 11 - 0·95 12 ) Since amplitude is not important in a phase comparator, all the quantities can be diVided by 1·05 to give the simpler forms: (-0'90511+12) and (11-0'91512) (3.31) These can be cQmpared with the expressions for the amplitude comparator which were

(11 -/

2)

and

~ (11 +12 ), These expressions apply particularly to

biassed current differential relays used for the protection of generators and power transformers. The approximate equation of the characteristic on again

1~12 _ 2 \~\ cos tP +

1~1.p and j I~I. axes is

1 = S2 and the locus is again as shown in

fig.3.13b. (c) Other examples o/Comparison o/Constants. The same method can be applied to any other type of relay. In fact it is a good exercise for students to work out this method with different types of relays. Table 3.3 shows the summarised results for the common types of relays comparing one current with a voltage or another current. It will be seen that there are never more than two constants, so that the general equation (2.2) given in Chapter 2 fits all the normal types of protective relays. Consideration of the constants calculated in this manner will indicate which type of comparator is preferable for a given relay characteristic. In general an inherent comparator is better than the converted type because, if one quantity is large compared with the other, a small error in the large quantity may cause an incorrect comparison when their sum and difference are supplied as inputs to the relay. 124

ti

-0.

(I

+ ~)h -

(1 - K)IA - )lIB

IIA - )lIBI

Amplitude Phase

~)Il -

Ditto Scheme B and balanced voltase scheme

(1-

III-hi

l)h

(1

1

1-

1

~

K K-I

K K

2K K

K K

K K

Kl

KIIAI

)lIB

1 I-K

0 -1

-1 0

0 -1

0 -1

K2

-)I -y

-y

(I + K»)I

2

I+~

-I

-K - (K+ 1)

(1 - K»)lIB 1 - K -

+ )lIBI

+ K)IA -

KIIA

+ K)IA -

(I + ~)h + (1 - ~)12

i lh + hl

S

+ (K -

Ih +121

(K + 1)11

Amplitude IIA - )lIBI Phase (1 - K)IA - (1 + K)yIB (1

Phase

Amplitude

Amplitude Klh-hl (K - 1)11 - (K + l)h Phase

+ 12

Ihl

KIIll K1l-h K1l

IVI

VI

Circulating current Pilot Scheme A

Percentage differential current (Slope = S)

Differential current

Balanced current Amplitude Phase

12K1KI

Amplitude Phase

Ohm KI- V

1- K1+2VI V

IKiI

Amplitude Phase

Mho K1-V

IKiI IVI

KI+ V

K1-V

Amplitude Phase

Comparator Quantities to be Compared in Amplitude or Phase

Impedance

Relay

3.4

p-Plane Characteristics for General Equation

TABLE

(1

K

+ K)

K 1 +K

- (I +~)

S 2

1 K+I

0 K

0 K

-K 0

0 K

K3

(~r

1-

1+

(~r

(~r

K2+ 1 K2-1

0

00

K 2

0

c

0 -)I

K~I

II~/- )I

K)I 2K \1\1 + K21111 - (1 - K)y 1 - K2 Y 1 - K2 Y -)I

I-~

1-

S

S 2

2

2K K2-1

K

00

K 2

K

r

1 - (K-I)

1 1

1 -1

1

2

14

3.5

Protective Relays

3.5. INVERSION CHART FOR COMPLEX QUANTITIES

The diagrams in figs. 3.9b, 3.10b and 3.lla can be inverted to the diagrams 3.9d, 3.10d and 3.l1c respectively by vector algebra or they can be plotted from the chart (fig. 3.15). The chart has axes the corresponding form

I~\p and j 1~lq'

To convert a value of

1~lp + j 1~lq it is only necessary to refer to the values

of the semi-circles which intersect at the point If A is potential and B is current

eX),

1~lp + j I~t to

1~lp' j I~t on the graph.

1~lp is resistance (R), j 1~lq is reactance

I!!!Ap is conductance (G), j I!!\Aq is susceptance (B), ~B is impedance (Z) and

~ is admittance (Y).

007

008

•,,

0 ,09

,, ,,I

' ,0

o

U~~::lii~0~'~4~0~'=3~0;;:'2~5=0~';: 2::I0='18=l-:lO':-:'16~~-:--tJ::-~::--:0~'1-1% Ip o

2

FIG.

3

I 4 5 Va.lu~ of

lA/al..,

3.15. Conversion chart from

126

6

7

1~lp ± jl~lq to 1~lp ± jl~lq

10

The Main Characteristics of Protective Relays

3.6

If it is desired, for example, to find the admittance corresponding to the impedance value 3'8+j7 ohms, first locate the point 3'8+j7 on the graph; then note the values of the two semi-circles that intersect at that point; viz.

0'061~1" and 0·11 j I~". The equivalent admittance is Y = 0·06+jO·11 mhos. 3.6. RESONANCE IN RELAY CIRCUITS

Tuned circuits are used in protective relays to obtain frequency selectivity, amplification, time qelay, memory, phase-shifting and power factor correction. Owing to the resistance of the circuit, however, there are three ways of tuning a circuit to resonance, depending upon which ofthese results is desired; in a zero resistance circuit the three methods of tuning would become identical. The three types of resonance are as follows: Natural resonance exists when the natural period of oscillation of the system is the same as that of the applied e.m.f. Phase resonance exists when cjJ, the phase difference between the current and the supplied e.m.f., is zero. Amplitude resonance exists when a change in the frequency of the e.m.f. reduces the current amplitude. Resonance is used in an unbiassed relay for differential protection, as described in Chapter 9 for generators, Chapter 10 for restricted earth protection of transformers and Chapter 11 for bus zone protection. The build-up time of the oscillations in the tuned coil of the relay provides a delay in operation to override transient conditions at the inception of a fault and also reduces the c.t. burden and makes the relay insensitive to harmonics. The same relay uses non-linear resonance to act as a limiter since saturation of the relay coil at high currents de-tunes the circuit. In directional and mho relays the torque of the relay must be maintained during a fault even if it is close enough to reduce the voltage to zero. By tuning the series potential circuit of the relay for natural resonance at system frequency it will continue to oscillate at system frequency and the voltage across the potential coil will be maintained for a few cycles after the line voltage has disappeared. If it is tuned for amplitUde or phase resonance it will oscillate at a different frequency which will cause a momentary phase shift in the potential coil current and a momentary shift in the phase angle of the relay characteristic which, in a mho relay,. can cause overreaching. This means that, if the series potential circuit of the relay coil is tuned with a voltmeter or an ammeter, it should peak at a frequency equal to the system frequency divided by

J(

1-

~Z)·

Phase resonance is used in protective relay circuits to produce current in phase with the applied potential at a desired frequency. In frequency relays, amplitude resonance is employed because the relay must be more sensitive to a given frequency so that it can operate when that 127

3.6

Protective Relays

frequency is reached. Either series or parallel tuning can be used, depending upon whether the relay is wound for current or potential. This subject is dealt with mathematically in Appendix 3.8. In the following table the amplitudes of oscillation Q and I for an impressed e.mJ. at the frequency which produces the kind of resonance are given in the left-hand column. The period of oscillation T corresponds to that frequency. In the case of natural resonance it is also the frequency at which the circuit continues to oscillate when the e.m.f. is removed. TABLE

Period of Oscillation

Kind of Resonance

(T)

21CVLC

I. Natural

2. Phase

J 1 LC

R2 4L2

Amplitude of Oscillation (Q)

Amplitude of Oscillation (I)

tan '"

E"v'LC

1R'C !i / 4L 4L -1 J1 - R2C RJ1- 3R2C R" I _ 3R2C - 2)R2C 4L 16L 16L

I

21CVLC

VLC

3 Amplitude ) - 1 - -R2 . LC 2L2

3.6

21CVLC

)1 _ R2C 2L

EVLC R EVLC RJ1- R2C 4L

E R E R

ct:)

- )4L _ 2 R2C

3.6.1. Non-Linear Resonance

By permitting saturation in the inductive reactance of a resonant circuit, other useful effects can be obtained. In the circuits of figs. 3.16 and 3.17 the reactance of the condenser Cis resonant with that of the saturating reactor Ls above the knee of the volt ampere curve (fig. 3.16a). In the series circuit, fig. 3.16b, the current increases linearly up to a certain value of voltage V at which saturation of the reactor has caused resonance because VLs = Ve. This causes the current to increase suddenly to i on the upper solid curve. If the voltage is then decreased, the current follows down the upper curve. Fig. 3.16c shows the behaviour of a parallel non-linear resonant circuit in which the roles of the current and voltage are interchanged, i.e. the parallel circuit is the dual of the current circuit. The dotted portion of the curves connecting the upper and lower sections is the locus of values that are obtained by calculation but which cannot be obtained in practice except by introducing an exact amount of regulation. 3.6.2. Non-Linear Resonance in Relay Circuits

If a relay is connected in the series circuit, the operating characteristic would be as shown in fig. 3.18. The abscissae shown in brackets are for the 128

3.6

The Main Characteristics of Protective Relays

I

(a) I

v

~

~f-o s C

C

I

V

(c)

(b) FIG.

3.16. (a) Volt ampere relation of a capacitor and a saturating reactor (b) Ampere/volt relation of a series resonant circuit (c) Volt/ampere relation of a parallel resonant circuit

.!;.L

.~~L~ R

C

00

V'"

R

I-

...~...

~'"

'h'.U

Cf

Ls

mw

I'"

FIG.

o

~

(a)

0

J~:

(b)

3.17. Non-linear resonant circuits (a) Series. (b) Parallel

I

(or V)

FIG.

3.18. Operating characteristic of a non-linear resonant relay

129

3.7

Protective Relays

parallel circuit of fig. 3.17b and the unbracketted ones for the series circuit (fig. 3.17a). The arrows show the curves followed in increasing and decreasing the voltage or current. This phenomenon is useful in the design of a regulating device to make it more sensitive over a small change in voltage or frequency. In a relay the pick-up and drop-out can be closely controlled in the region x - y of fig. 3.18.

~ Ilnll¢"~ Dorltctoona.l rcla.y

FIG.

3.19. Circuit for maintaining torque of directional relay at low voltages

The distance between x and y decreases with resistance. The associated analysis is very complicated and is dealt with in references (6), (7) and (145). This principle has also been used in the potential restraining coil of a distance relay starting unit to obtain a high pick-up under normal conditions and a low pick-up under fault conditions. It has also been used to

v. I

I

I

I

I I

I

I

I

v ( a.) FIG.

(b)

3.20. Constant phase angle characteristic of relay in fig. 3.19 (a) Coil current versus line voltage (b) Vector diagram of circuit voltages

maintain the torque of a directional relay at low voltages. Fig. 3.19 shows the circuit of the polarising potential coil. The non-linear resistor N limits the coil voltage and the coil current is maintained (fig. 3.20a) down to 10% of normal voltage while the phase relation of the coil current relative to the system potential is kept constant (fig. 3.20b) and hence the maximum torque angle of the relay is constant. 3.7. APPENDIX 1

The threshold equations of relays can be transformed to represent threshold characteristics in suitably chosen planes. This appendix deals with a few typical cases. 130

The Main Characteristics of Protective Relays

3.7

3.7.1. Differential Current Relay

The relay is on the threshold of operation when

III ~I21 = III ;121

(3.7-1)

where II and 12 are the two currents fed into the relay and s is a design parameter.

x

(c)

(b)

FIG. 3.21. Characteristic circle of differential relays (a) In rectangular co-ordinates (x - g)2 + (y - f)2 = r 2 (b) In polar co-ordinates fJ2 + 2/lc cos 8) + c2 = r 2 (c) In polar co-ordinates a b • = ao =F -vi r· - b~

«(I -

The threshold characteristic of the relay can conveniently be presented in the (X-plane, where: (3.7-2) or in the p-plane, where:

P= 12 -=a+J·b

(3.7-3)

II Consider the characteristic in the

(X~p1ane. Equation (3.7-1) can be expressed in term.s of (X and transformed as follows:

II 2(a+jb)-I 21= KII 2(a+jb)+I 21 la+jb-11 = Kla+jb+ll (a_l)2+b 2_K2(a+l)2_K 2b2 = 0 where K =

2

131

3.7

Protective Relays 2 2 1+K2 a +b -2--a+1 =0 1-K2

(3.7-4)

This is of the form X2+y2_2gx-2fy+c=0 which can be written '(x-g)2+(y-f)2=g2+f2-C. Referring to fig. 3.21a this is the equation of a circle of radius Vg2 +f2 - c whose centre is at (g,!). It can easily be shown that the threshold characteristic of the relay in the p-plane is also represented by equation (3.7-4). Thus in both the a-plane and the p-plane the threshold characteristic of the relay is a circle of radius r and centre defined by vector c, where: r = {(1+K2)2 1-K2

_1}t

=

2K 1-K

2'

c = 1+K2 1-K2

(3.7-6) (3.7-7)

and K is defined by equation (3.7-5). 3.7.2. Differential Relay with Product Restraint

The threshold equation of the relay in terms of the two input currents 11 and 12 and the design parameter s is

III ~/212

=

1/1/1/21 cou

(3.7-8)

where 'f is the phase angle of 11 with respect to 12, Consider the threshold characteristic of the relay in the a-plane.

II

a=-==a+J'b

(3.7-2) 12 :. arga == 't' (3.7-9) Equation (3.7-8) can be expressed in terms of a and transformed as follows:

\I2(a+jb)-/ 212 = Kllz(a+jb)\\12\ cou la+jb-W = Kla+jbl cos T (a_l)2+b 2 = Ka :.

a2+b2-2a(1+~)+1==0

(3.7-10)

where: K == S2 (3.7-11) It can be shown that the threshold characteristic of the relay in the p-plane is also represented by equation (3.7-10). Thus in both the a-plane and the p-plane the threshold characteristic of the relay is a circle of radius r and centre defined by vector c, where: r = {( 1 +

IY - r 1

K 2 and K is defined by equation (3.7-11).

c=I+-

132

= { K (1

r

+ ~)

(3.7-12) (3.7-13)

3.7

The Main Characteristics of Protective Relays

3.7.3. Circulating Current Pilot Scheme with the Restraining Coil on the Pilot Side

The two relays situated at the two ends A and B of a pilot are on the threshold of operation when: (3.7-14)

IIA -yIBI = KIIA +yIBI IIB-yIAI = KIIB+yIAI

and

(3.7-15)

respectively, where IA and IB are the two currents fed into each relay and K is a design parameter; y is the propagation constant of the pilot. Consider the threshold characteristic of relay A in the a-plane. IA 'b a=-=a+J

(3.7-16)

y = m+jn

(3.7-17)

18

Equation (3.7-14) can be expressed in terms of a and transformed as follows: IIB(a+Jb)-lim+jn)1 = KIIia+jb)+IB(m+jn)l, (a-m)2+(b-n)2-K2{(a+m)2+(b+n)2} = 0 2 2 I+K2 I+K2 2 2 a +b -2ml_K2a-2nl_K2b+(m +n )=0

(3.7-18)

Consider the threshold characteristic of relay B in the a-plane. Equation (3.7-15) can be expressed in terms of a and transformed as follows: IIB-(m+jn)IB(a+jb)1 = KIIB+(m+jn)IB(a+jb)l, 1(I-am+bn)-j(bm+an)1 = KI(I+am-bn)+j(bm+an)l, (l-am+bn)2+(bm+an)2-K2(I+am-bn)2-K2(bm+an)2 = 0, 2 2 I+K2 a +b - 2m (I_K2)(m2+n2)a+

+2n(1

-

1 2)b + - 2- 2 = 0 m +n m +n

I+K2 K2)( 2

(3.7-19)

It can be shown that equations (3.7-18) and (3.7-19) also represent the threshold characteristics of relays A and B in the p-plane respectively, where:

P=IB- = IA

a+J"b

(3.7-20)

Thus the threshold characteristics of relay A in the a-plane and of relay B in the p-plane are identical circles represented by equation (3.7-18), of radius r and centre defined by vector c, where: r=

{(m2+n2)G~~:r -

2K = lyll_K2

)Y

(m 2+n 2

(3.7-21)

133

3.7 and

Protective Relays

I+K2

C

= m l-K2

. I+K2

+ Jn l _ K2

I+K 2

= y I-X2

(3.7-22)

Similarly, the threshold characteristics of relay B in the DC-plane and of relay A in the p-plane are identical circles represented by equation (3.7-19), of radius r and centre defined by vector c, where: r =

{

l}t

(m 2+n 2)(I+K2)2 (m 2+ n2)2(1- K2)2 - m2 + n2

1

2K

(3.7-23)

= 1Yl (I-K2) and

.

I+K2

C

I+K2

= m (m 2+n 2)(I-K2) - In (m 2+n2)(I_K2)

1 (I+K2) = y(I-K2)

(3.7-24)

3.7.4. Circulating Current Pilot Scheme with the Restraining Coil on the c.t. Side

The threshold equations of the two relays situated at the two ends A and B of a pilot are (3.7-25) I/A-IBI = KIIAI (3.7-26) and IIB-IAI = KIIBI where, as before, IA and IB are the two currents fed into each relay, K is a design parameter and y is the propagation constant of the pilot. Consider the threshold characteristic of relay A in the DC-plane.

IA 'b DC=-=a+J IB y = m+jn

(3.7-16)

(3.7-17) Equation (3.7-25) can be expressed in terms of DC and transformed as follows:

IIaCa+jb)-(m+jn)IBI = KllaCa+jb)1 (a_m)2+(b_n)2_K 2a2_K 2b2 = 0 m n m 2+n 2 a 2+ b2 - 2 (1- K2) a - 2 (1- K2) b + 1- K2 = 0

(3.7-27)

Consider the threshold characteristic of relay B in the DC-plane. Equation (3.7-26) can be expressed in terms of DC and transformed as follows:

IIB-(m+jn)IaCa+jb)1 = KIIBI 1(I-am+bn)-j(bm+an)1 = K (l-am+bn)2+(bm+an)2-K2 = 0 22 m 11 l-K2 a +b - 2 2b+ 2 2=0 2 - 2 a+2 2 m +n m +n m +n 134

(3.7-28)

The Main Characteristics of Protective Relays

3.8

It can be shown that equations (3.7-27) and (3.7-28) also represent the threshold characteristics of relays B and A in the p-plane respectively, where: 'b P= I- B = a+j

(3.7-20)

IA

Thus the threshold characteristics of relay A in the ex-plane and of relay B in the p-plane are identical circles represented by equation (3.7-27) of radius r and centre defined by vector c, where: r = {m 2+n 2 _ (m 2 +n 2)(1_K 2)}t (1- K2)2 (1- K2)2 K

= lyl 1 _ K2 and

c

(3.7-29)

1

.

1

= m 1 _ K2 + jn 1 _ K2 1

(3.7-30)

= Y 1-K2

Similarly, the threshold characteristics of relay B in the ex-plane and of relay A in the p-plane are identical circles represented by equation (3.7-28), of radius r and centre defined by vector c, where: '{ m 2+n 2 1-K2 r= (m2+n~2-(m2+n~

}t

K

IYI and

(3.7-31)

m

.

n

c=----j---

m 2+n 2

m 2+n 2

1

(3.7-32

Y 3.7.5. Some Special Values of a and

P

The two points at, 2 at which any of the threshold characteristics discussed in this appendix intersects the real axis can be determined from the already known values of rand c, as shown in fig. 3.2lc. at,2

= ao±(r2-b~)t

(3.7-33)

where ao and bo are the moduli of the real and imaginary parts of c respectively. 3.B. APPENDIX 2

Resonance is of vital interest in the design of relay circuits. This appendix deals with three typical cases of linear resonance in a series circuit. 3.B.1. Series Resonance in Terms of Charge in the Capacitor

Consider a series circuit containing a resistance R, an inductance L and a capacitance C. Let e, i and q denote the instantaneous values of the electro-

135

3.8

Protective Relays

motive force applied to the circuit, the current in the circuit and the charge in the capacitor, referred to its current facing terminal, respectively. At every instant 1'R

di q + L --+-=e

c

dt

.

where:

dq dt

1=-

(3.8-1) (3.8-2)

Equations (3.8-1) and (3.8-2) can be restated using the Laplace transformation

iR+L{pi-i(o)} +!I= e c

(3.8-3)

i = pq-q(o)

pq{R+LP +

:c}

= e+(R+Lp)q(o)+Li(o)

(3.8-4)

where e, i, and q are the transforms of e, i, and q respectively and i(o), q(o) are the corresponding initial values. Assuming initial quiescence (3.8-5) i(o) = 0

q(o)

=0

(3.8-6)

and an alternating electromotive force e = Esihrot

(3.8-7)

equation (3.8-4) can be transformed to

q= where:

Ero 1 L [(p+oc)z+pZ] [pz+ro Z]

R oc=2L Z 1 RZj fJ = LC- 4Lz

(3.8-8)

(3.8-9)

Using equation (3.8-7) and a table of standard transforms Ero p sin (rot + cP1)+roe-·' sin (fJt + cPz) q = --;-L-'--P-::-ro-.JT-'4oc=;Z;:::ro~z=+=:(=;ocz;=+=fJ7.z;=_=ro~Z)::;;z"":""'='

(3.8-10)

where:

cP1 = arc tan ( Z 2oc~ pZ) (steady state) 00 -oc -

(3.8-11)

and

cPz = arc tan ( Z 2oc~ pZ) (transient) 00 +oc -

(3.8-12)

Thus, under steady-state conditions

q = Q sin (rot+cP1)

136

(3.8-13)

The Main Characteristics of Protective Relays

Q= L-J

where:

41X 2 C0 2

3.8

E

+ (1X2 + p2 _ C( 2)2 E

LJR 2 co 2 + (~_ L2 LC

(3.8-14) C(

2)2

and CP1 is given by equation (3.8-11). The period T of sinusoidal oscillation of angular frequency co is given by T = 21t

co

(3.8-15)

Natural resonance occurs when the circuit is energised at its natural angular frequency p, given by equation (3.8-9). The amplitude Q, period T and phase shift cP, characteristic of the oscillation, are then obtained from equations (3.8-14), (3.8-15) and (3.8-11) respectively:

co=P=J~-fu

J

I_ R2C 4L

= T

RJl- 3R

(3.8-16)

(3.8-17) 2

C 16L

21t

21tJLC

= - = ---;====:;:=

P Jl- R 2 C

(3.8-18)

4L

CP1 =

arc tan

4LJLc 1 R2) ( -"R - 4L2

=a.rctan(

-2J;~c-l)

(3.8-19)

Phase res.onance occurs when the frequency at which the circuit is energised is adjusted so that its power factor becomes unity: 1

coC

coL-- =0

1 co = -JLC 137

(3.8-20)

Protective Relays

3.8

The amplitude, period and phase shift, characteristic of the oscillation under such conditions, are, as before, obtained from equations (3.8-14), (3.8-15) and (3.8-11).

Q= L

E

J

R2 1 L2' LC

E

+

(1

1

LC - LC

)2

1-

(3.8-21)

=-yLC R

T

2n

I-

= -ro = 2nyLC

(3.8-22)

2~ _1_ 2L' JLC

(

)

(3.8-23) Amplitude resonance occurs when the frequency at which the circuit is energised is adjusted so that the steady state amplitude Q of charge q becomes a maximum. Differentiating Q, equation (3.8-14), with respect to ro and equating the derivative to zero / dQ E {R2 - = - -2L -ro 2 + ( - 1 _ro 2 )2}-3 2{R2 -2ro+2 ( - 1 _ro 2 )( -2ro)} = 0 dro L2 LC L2 LC

Leads to:

ro=

J/;C-ffi

J

l - R2C 2L

= ----,-0=-

(3.8-24) JLC The amplitude, period and phase shift, characteristic of the oscillation under such conditions, are obtained using equations (3.8-14), (3.8-15) and (3.8-11).

J

(3.8-25)

=--;===== R2C

R

T = 2n = ro

1-4L

2nJLc

Jl- RC 2

2L 138

(3.8-26)

The Main Characteristics of Protective Relays

3.8

(3.8-27) 3.8.2. Series Re.onance In Term. of Current

The results of section 3.8.1 can be expressed in terms of current as follows: q = Q sin (wt+tPl) (3.8-13) . dq 1=(3.8-2) dt i = 1 cos (wt+tPl) (3.8-28) where i = wQ (3.8-29) Consider natural resonance. Q and ware given by equations (3·8-17) and (3.8-16) respectively. Thus

EJl- R 2 C 1=

4L

(3.8-30)

RJI_ 3R2C 16L

The period T and phase shift tPl are given by equations (3.8-18) and (3.8-19) respectively. Consider phase resonance. Q and ware given by equations (3.8-21) and (3.8-20) respectively. Thus

1 __1_ EJLc_~ -..[Lc R -R

(3.8-31)

The period T and phase shift tPl are given by equations (3.8-22) and (3.8-23) respectively. Consider amplitude resonance. Differentiating 1, equation (3.8-29), with respect to wand equating the derivative to zero dI dw

= Q+ w dQ = 0 dw

E{R2L2 W2+ (1LC - w2)2}-t - w 2LE{R2IJ w2+ (1LC - w2)2}-3/2

L

{~: 2w+ 2 (L1C -

(

2) ( -

2W)} = 0

leads to

1 w = .../LC

139

(3.8-32)

3.8

Protective Relays

which is the same as for phase resonance, equation (3.8-20). Thus E I = Ii

(3.8-31)

and the period T and phase shift lPl are given by equations (3.8-22) and (3.8-23) respectively. 3.8.3. Linear Resonance in More Complex Circuits

The method of treating linear resonance, used in sections 3.8.1 and 3.8.2, can be used with more complex circuits. As the complexity of the circuit increases, however, it may become difficult to solve the resulting differential equations rigorously. In such cases, suitable numerical or analogue methods can be used to obtain the required information. In some cases of existing circuits empirical methods are the simplest.

140

4 Overcurrent Protection Time-Current Characteristics-Application-Limits of ErrorRatings-Directional Overcurrent Protection-A.C. TrippingSchemes for Radial Feeders-Construction-Application-Problem 4.1. TIME·CURRENT CHARACTERISTICS

Fault current can be used as a basis for selectivity only where there is an abrupt difference between its magnitude for a fault within the protected section and a fault outside it, and these magnitudes are almost constant. Where this is so, a current magnitude device can be used, such as a fuse or an instantaneous relay or trip device and selectivity can be obtained by grading current. A typical case where current grading can be used is shown in fig. 4.1, where there ~SUPPIY

'I~

Tra.nsformer

O.C.trip device

y

FIG.

4.1. Current grading through transformer impedance (Fault at X heavier than fault at Y)

is a high impedance unit such as a transformer which makes the fault much less for faults beyond the transformer. In fig. 4.2 the fuses are current magnitude devices but, although they are considered instantaneous, they have an inverse time-current characteristic, i.e. the higher the fault current the faster they blow. In the diagram which may represent the electrical circuit of a hotel or a factory, a fault on one of the circuits at the right-hand end of the diagram may draw 50 amperes which will be ten times the rating of the end fuses, five times that of the next group 141

4.1

Protective Relays

@

InIWta.ncOUI

--C1--1

FU5C5

FIG.

4.2. Current grading with fuses

and so on. Owing to the steep time-current characteristic of fuses the fuse nearest the fault blows well before the others can do so.

(]3 t

= K)

4.1.1. Definite Time-Current Relays

In radial or loop circuits, where there are several line sections in series, there is no difference in current between a fault at the end of one section and a fault at the beginning of the next one; consequently, it is necessary to

FIG.

4.3. Lack of current selectivity where Z./Z. is high (Current similar for faults at X and Y)

add time discrimination, as shown in fig. 4.3, with the time settings increased towards the source. Where there are many sections in series the tripping time for a fault near the power source may be dangerously high (fig. 4.4). This is obviously unTime 2scc.

11scc. lscc.

~SUb.l ~

Sub. 2 flO.

Sub.3

I

I

lsce.

Sub.4

4.4. Definite time grading on radial circuit

desirable because such faults involve large currents and are very destructive if not removed quicldy. In fact, the fundamental weakness of time-graded overcurrent relays is the fact that the heaviest faults are cleared slowest. In the next paragraphs methods of dealing with this problem will be discussed. 4.1.2. Inverse Time-Current Relays

Where Za (the impedance between the relay and the power source e.mJ.) is small compared with that of the protected section Zl' there will be an appreciable difference between the current for a fault at the far end of the section (] =

z.! z)

and the current for a fault at the near end (] =

142

:J

4.1

Overcurrent Protection

In such a case a relay whose time is inversely proportional to the current (It = K) would trip faster for a fault at the end of the section nearer the power source; the ratio for the tripping time at the near end to the time at the far end is

~.

Z.+Z/ The resultant time-distance characteristics, compared with those of definite time relays, are shown in fig. 4.5, and it will be seen that the inverse ,../

----

:

I I t--_D::...:~""fin",-it:..::.~-----1- _ -:;...-_~~___________ ....J __ -

~ \!\~~rsr;

-

-

~_:::

___ /

____ -

II I

_ --________ ...JI

I===_i-r---'-:~:::

QWl~_ _ _ _~_ _ _ _ _ _~_ _~+I=== \.Y1lsub.l

Sub.2

FIG.

Sub.3

Sub.4

4.5. Definite versus inverse time current relaying

time relay can provide faster clearing times than the definite time relay, assuming the same selective intervals, S. The tripping time can be still further reduced by using e. more inverse characteristic, such as Ilt = K. On systems solidly grounded at each station Z. is small so that excellent selectivity on ground faults can be obtained with inverse time current relays. 4.1.3. Definite versus Inverse Time Relays

There are two conditions however which can reduce the advantage of the inverse time characteristic. First Z. can be so high on impedance grounded systems that the ratio

~

is not sufficiently lower than unity to give any Z.+Z/ appreciable reduction in tripping times. This will occur at the end of a long system where Z. is large. Secondly, Zs will vary if the generating capacity is varied, becoming larger during weekends and at night when there is less load and hence less generating capacity connected. This increase in Z. will not interfere with selectivity because the inverse curve increases the time discrimination at low currents, but it does increase the tripping times and hence defeats its purpose of reducing them. Definite time relays have always been popular in Central Europe because their time is not dependent on current magnitude and because the synchronous type of relay in common use is more accurate than the induction disc unit used for inverse time relays. On the other hand it is obvious that, on long radial or loop systems, lower tripping times can be achieved with inverse time relays. In other words, definite time relays are better on isolated systems and for use as back-up to differential relays and distance relays, but inverse time relays are advantageous on interconnected systems and solidly grounded systems, i.e. where Z./Z/ < 2. 143

4.1

Protective Relays

4.1 .4. Inverse-Definite Time Relays (17) (18) (19)

In the U.K. and the U.S.A. a compromise curve is popular, which is known as an inverse curve with definite minimum time (I.D.M.T.). This was introduced about 1920 and could have been a happy solution but it was ruined by the method of execution which was to obtain the definite minimum time by saturation of the electromagnet. The effect of the spring which controls the pick-up at the low current end and the saturation at the high current end is to produce an irregular curve which follows no particular law (see fig. 4.7, curve 'b' and fig. 4.15). Nevertheless, with a certain amount of patience, the I.D.M.T. curve can be applied successfully by cut-and-try methods, preferably using a plastic curve with an outline corresponding to the I.D.M.T. time-current characteristic. Fig. 4.6a shows a typical family of I.D.M.T. curves. Theoretically, their time ordinates should be proportional to the time multiplier setting (contact

10

"-

~

1\ :-...: 1\ ~~ 1\ I-

i'''~

T....S.

~ 62

~ gie-.

05 3

0·2

~ 0·'

.,

0 ,

FIO.

10 MU'ltlpru 01 r1lJ9 lIttlin.9 currc.n.l

100

4.6a. I.D.M.T. time current characteristic to B.S.S.

travel) so that, if the times for a given current were divided by the time multiplier setting (T.M.S.), all the curves should be coincident. Unfortunately, the inertia of the disc makes this impossible at low current values because it takes a little time for the disc to accelerate from standstill to its steady speed. This is taken care of by publishing a family of curves such as is shown in fig. 4.6a. In the U.K. the curves are coincident within B.S. tolerances down to 0·1 T.M.S. at ten times the tap value and the nameplate shows a single 144

4.1

Overcurrent Protection

curve (fig. 4.15) or logarithmic scale (fig. 4.6b) which gives the time at full travel (1·0 T.M.S.). The actual time is given by multiplying the time given on the scale by the T.M.S. At low currents the time is given by the curves of fig. 4.15 which are part of the national specification B.S.S. 142. Plug setting multiplier

2

I

10

2·5 I

FIG.

I

8

I I

3 I

4 I

6

i

I

I

5 I

I

6 I

5 4 Time in seconds

I

7

II

3,5

8

9 10

I, I

I,

3

12

II,

2·8

14 16 18 20 I' , II

2·6

2·4

I I

2,2

4.6b. Logarithmic scale for I.D.M.T. relay

In the printed. disc type of relay, the family of curves is unnecessary because the disc has very low inertia and the proportionality is within ± 5 %down to 0·1 T.M.S. at twice the tap value. The advent of static relays has made it possible to obtain accurate control of n in the characteristic I"t = K. This is an ideal arrangement because n can LogT

(e) Definite time rOt:/(

Log! FIG.

4.7. Inverse, definite and I.D.M.T. characteristics

be varied (118) to suit the application and the value of Z8/Z" Furthermore, combinations of definite, true inverse and instantaneous units (fig. 4.1) are much easier to apply than the present I.D.M.T. curves. 4.1.5. Voltage Monitoring

It is obvious that overcurrent relays can be used only where the minimum fault current exceeds the maximum load current. Where there is a wide variation in generating conditions and the minimum fault current is below maximum load current it is possible still to use overcurrent relays if they are monitored by undervoltage re1ays, since the voltage does not fall appreciably during load. In most cases of this type, however, it is preferable to use distance relays which are described in the next chapter. F 145

4.1

Protective Relay·s

4.1.1. Instantaneous Overcurrent Relays

Another tool for reducing the tripping time for faults near the source is the high-set instantaneous relay which reduces the overall tripping times to a minimum because each relay, whether definite or inverse, can be given the same time-multiplier setting, since it has only to be selective with the instantaneous relay in the next section (fig. 4.8). Like the inverse relay the instantaneous relay is effective only where Z, is large compared with ZS' Fortunately this is true near the power source so that it provides tripping at a place where the fault currents are heaviest and where the longest tripping times would otherwise have to be accepted. In order that these instantaneous units shall be' selective with each other, each one is set to pick up at a progressively higher value towards the source so that no relay can operate on the lower current value of a fault in the next section away from the source. Instantaneous relays cannot be applied where difference in current between faults at the two ends of the protected section is exceeded by the difference in current for a fault at the far end of the section for maximum and

. . . cond'ltions, . . were h Zs+ Z, < --ZZs+ Z, were hZ" mlD1mUm generating I.e. -s IS ZS

Zs+,

the value of Zs under reduced generating conditions, i.e. where

Z;-Zs>

Z, (1 + ::).

An example of this is the short section 1-2 in fig. 4.8. In such cases fast

. II

.::

----

Power

source

FIG.

_ ...... ...... ...

---- ......... Sui>-stG.tion Sub-sta.tion No.1 No.2

Sub-sta.tion No.3

4.8. Reduction of time-settings by addition of high-set instantaneous overcurrent units

clearing of faults can be obtained by the use of one-step distance relay described in Chapter 5, section 5.4.1, which cuts off at the same distance from the relay regardless of the magnitude of the fault current. Assuming, however, that conditions are suitable for the application of the instantaneous unit, it should be set to pick up at a current value for a fault near the end of the protected section under maximum generating conditions. The percentage of the section that can thus be protected depends upon the 146

4.1

Overcurrent Protection

tendency of the relay to overreach on offset waves. The current is proportional to oc _1_ so that, with 100% offset current Zs+Z, transient, pick-up would occur with half the symmetrical value of current, i.e. double the value of Zs+Z" Since Zs is fixed Z, is correspondingly increased, i.e. the overreach K can be obtained from Z.+KZ, = 2(Z,+Z,),

Zs K = 2+(4.1) Z, hence with 100% offset current wave a truly instantaneous overcurrent relay would overreach to more than twice the length of the protected section. Actually the overreach will be reduced by the operating time of the relay because the d.c. component of the fault current will be decaying exponentially, so that i.e.

Emu. sin (wt+l/I-t/» I. = ../R2+(Lw)2 .

R - -t

+

e L sin(l/I-t/» ../R 2+(Lw)2

Emax

= 1max [sin (wt+l/I-t/»+Ae -

(4.2)

~t]

(4.3) The first expression is the steady symmetrical component and the second is the decaying d.c.; where t/> is the phase angle of the circuit (tan -1 ~),

if; is the time in radians after voltage zero at which the fault occurs and t is the time after the inception of the fault. Lw = X (see fig. 5.12). On a system with high XI R ratio- the operating time of the relay would have to be increased by several cycles to avoid overreach; the delay can be calculated from the time constant. A preferable alternative is to use a d.c. filter. In the U.S.A. induction cup instantaneous units are used because they are less sensitive to the d.c. offset component. A less expensive solution is to

I

1J~k

I71

(a) FIO.

(b)

4.9. Instantaneous overcurrent relays with negligible overreach (a) Tapped secondary (b) Continuous adjustment

147

4.1

Protective Relays

use the arrangement of fig. 4.9 which not only eliminates the overreach but also provides a drop-out to pick-up ratio of over 90 %. In systems such as are described in section 4.7, where the instantaneous unit is given a relatively low setting, it is important that the relay reset upon the return of normal load conditions, i.e. the reset value of current should be as close to the operating value as possible. The normal drop-out/pick-up ratio of attracted armature relays is SO % or less because of the change in reluctance as the armature gap closes but a ratio of 90 % can be achieved by matching the pull to the mechanical load over the range of travel of the armature. This can be done mechanically by auxiliary spring arrangements, as explained in Chapter 2, section 2.4.4, fig. 2.1S, or by non-linear resonance, as illustrated in fig. 4.9 of this chapter, or by an adjustable air-gap inside the solenoid. 4.1.7. Extremely Inverse Time Overcurrent Relay (/2t = K)

For cases where the generation is practically constant and discrimination with low tripping times is difficult to obtain, because of the low impedance per line section, an extremely inverse relay can be very useful (i.e. one in which the time is inversely proportional to the square of the current) since only a small difference in current is necessary to obtain an adequate time difference. This relay is also very desirable for protection of apparatus against overheating, since [2t = K is also the current versus heating characteristic of most apparatus. Typical applications are earthing transformers, power transformers, expensive cables and railway trolley wires. Fig. 4.10 shows typical heating curves for SO cis generators and transformers. An application of particular importance is that of large generators which may receive damage to their rotors by overheating if an unbalanced fault or load is permitted to remain too long on the system. In this case a relay of extremely inverse time-current characteristic is supplied through a negative sequence filter and the constant K is set by the time multiplier scale according to the type of machine. Such a relay must be well designed since it may have to operate in a fraction of a second at heavy currents or several thousand seconds at low currents. It also has to be very accurate in order to exactly match the heating characteristics of the generator so that it will not be taken off too soon or unnecessarily, which is important if it is a big machine. The best relay of this type at present available uses a shrouded ball and jewel bearing (see Chapter 2, section 2.6.1) and is shown in fig. 9.11. Other useful applications of this relay are for accurate discrimination with fuses (fig. 4.11), which is impossible with the LO.M.T. curve, and also for reclosing distribution circuits after a long outage (see last paragraph of section 4.2.6). 4.1.8. Special Characteristics

Owing to the American manufacturers having used a superlative in the term 'Extremely Inverse' relay it is difficult to find descriptive names for the

148

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4.10. Application of 121 relay to generator-transformer unit

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4.1

Protective Relays

relays which are still more inverse, i.e. those with a time-current characteristic I"t = K where n > 2. Fig. 4.11 shows that the characteristics of enclosed fuses approximate to the law [H t = K, so that the [2t = K relay characteristic is not too good a match, but it is the best that can be done with electromagnetic relays (35). With static relays, however, it is easy to obtain an [HI = K curve, using a circuit of the form shown in fig. 4.12.

FIG.

4.12. Circuit of

rt relay for n up to 3·5

L -_ _ _ _ _ _
FIG.

4.13. Time-current curve required for mercury arc rectifiers,

FIG.

4.14. Circuit of

r1= K 8

rt relay for n above 3·5

The shape of the time-current curve can be changed so as to increase the value of n at higher currents by interchanging the positions of the resistor S and the capacitor C. Other variations can be obtained by interchanging S or C with the position of the output V2 •

150

Overcurrent Protection

4.2

The steepest time-current curve required so far is ]8 t = K for mercury arc rectifier protection. Fig. 4.13 shows the curve and fig. 4.14 the circuit employed to produce it. 4.2. APPLICATION OF TIME-CURRENT RELAYS

In their role as back-up relays their pick-up currents must be adjusted so that each will operate for all faults in the immediately adjoining circuits and their time settings must be just long enough to permit the relays In a faulted circuit to work first. This is facilitated by using relays with a similar timecurrent characteristic, for instance, differential and other fast relays should have a definite time relay for back-up. On feeders each relay backs up the one in the next section further from the power source so that the timecurrent characteristic of the back-up relay should be intermediate between the characteristics of the relays on each side of it; for example, with fuses on the tapped-off loads and LD.M.T. relays on the supply, the extremely inverse relay would be used at the sub-station nearest to the fuses and a very inverse relay at the sub-station next to the supply, so that there would be a selective difference between them at all current levels; this assumes that suitable taps and multiplier settings have been chosen to prevent any crossing of their time-current characteristics at likely fault current values. Before considering the setting of these relays, the following definitions will be helpful. Time Lever Setting (T.L.S.-U.S.A.) or Time Multiplier Setting (T.M.S.U.K.). A means of adjusting the moveable backstop which controls the travel of the disc and thereby varies the time in which the relay will close its contacts for given values of fault current. It should be noted that the T.L.S. scale marking is 0-10 in the U.S.A., divided into ten equal divisions, while the T.M.S. scale marking is 0-1·0 in the U.K., the scale markings being calibrated to be proportional to operating time at ten times the tap value. The latter system has the advantage that only the max. T.M.S. time curve is required (instead of the family of curves for different T.L.S.) and the operating time is that indicated by the curve times the T.M.S. Tap Block (U.S.A.) OR Plug Setting Bridge (U.K.). A device providing a range of current settings at which the relay will start to operate. Pick-up Errors. Allowable errors in the current value at which the disc starts to move and at which its contacts close, expressed as a multiple of the plug setting. In the U.S.A. the start and close values must be within ± 2 % of the tap value. In the U.K. the contact closing value must exceed the disc starting value by a value between 0 and 5 %. Overshoot. This is defined as the time to close the contacts at twenty times tap current with maximum disc travel minus the time to reach the point where the current must be shut off in order to prevent the contacts from closing, due to the momentum of the disc. 151

4.2

Protective Relays

4.2.1. Rules for Setting I.D.M.T. Relays on Phase Faults

(a) The relay must reach at least up to the end of the next protected zone. For example, in fig. 4.16, relay Rl must operate for a fault at R3 with minimum fault current (for phase relays this is a phase-to-phase fault at minimum generation). (b) The current setting must not be less than maximum load, usually 1·5 x c.t. rating, unless monitored by an undervoltage relay. (c) In estimating the current setting, allowance must be made for the fact that in England the B.S.S. until now permitted the relay pick-up to vary from 1·05 to 1·3 times tap value (see fig. 4.15). B.S.S. is quoted here because other countries do not have exact specifications for time-current relays. (cl) Where the generation varies widely, a low pick-up is preferable so as to allow most faults to operate on the definite time part of the curve; on the other hand, where the generation is sufficiently constant, higher pick-ups should be used so as to operate on the inverse part of the curve and thereby obtain the lowest overall operating times and the easiest discrimination, see fig. 4.5. (e) The time multiplier must be chosen to give the lowest possible time for the relays at the end of the system most remote from the source, but the contact gap employed should not be so small as to permit accidental tripping due to mechanical shock. In the following sections of line the time multiplier should be chosen to give the desired selective interval from the previous relay at maximum fault conditions (for phase relays this a 3-phase fault just beyond the next bus with maximum generation). The time multiplier setting should allow not only for the time of the next breaker but also for overshoot and errors in the relays, as will be discussed in the next section. (/) Directional control should be used at places where there is not sufficient difference between the currents in the faulted and unfaulted incoming feeders on a given substation bus to permit current grading. This may be at a substation at the receiving end of two parallel lines or it may be at a substation in the middle of a loop remote from the power source. Setting I.D.M.T. Relays on Ground Faults. The setting of ground relays to provide discrimination between breakers is much easier than with phase relays, because they are energised with residual current which is zero under normal conditions. Furthermore, the zero sequence impedance of the system is larger than the positive or negative and is terminated in the nearest grounded transformer. This means that the residual current varies chiefly with distance to the fault and is less affected by generating conditions (34). The same rules apply as for setting phase relays, except item (b) can be ignored. Faster tripping times can be achieved because (a) the pick-up settings can be below load values, (b) there will be a greater difference in current for faults at the near and far ends of the protected section, so that the time for a fault at the far end will tend to be high enough for discrimination, even if it is set low for a fault at the near end. 152

4.2

Overcurrent Protection

(a) The quickest and most effective way of obtaining time-current relay settings is to use log-log paper and a plastic monogram corresponding to the time-current curve. The current tap value moves the monogram along the time axis so that, by moving the monogram to the pick-up and time values required, the current and time settings are immediately given. With U.S. relays the family of T.L.S. curves on a transparent tracing is used instead of a plastic curve. 4.2.2. Typical Application of I.D.M.T. Relays

Fig. 4.15 shows the errors permissible by B.S.S. and 4.16 shows the effect of these tolerances on the discriminative interval required to ensure selectivity between successive relays. It will be seen that a fault drawing 100

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4.15. B.S.S. permissible errors at T.M.S. = 1

1000 amperes will cause the I.D.M.T. relay R 2 , which operates in 2·8 seconds and is set at 1·0 T.M.S., to have an error of 0·07 x 3 = 0·21 second; relay R3 operates in just over 2 seconds and has a permissible error of 0·14 second. These errors can be ± and must therefore be added so that the total selective interval required is 0·35 second for error plus 0·05 second for overshoot and 0·1 second fol' the breaker time, making a total of 0·5 second, assuming the breaker never exceeds its clearing time of 0·1 second even for low current faults. For the Rl relay (3·8 seconds at 1000 amperes) a selective interval of 0·6 153

4.2

Protective Relays

second is required but for the lower time settings the selective interval can be smaller because the errors are a percentage and therefore less. Actually, it is common practice to use a fixed selective interval of 0·4 second but it would be much better to use an interval of 0·2+0·1t where t is the operating time of the next relay away from the source at maximum fault conditions, assuming five cycle breakers. The 0·2 second allows for breaker time plus relay overshoot and the 0·1 t is the sum of the errors on two neighbouring relays. 4.2.3. Improvement of Selectivity

When a distribution system is supplied from a high tension power system and the maximum time allowed for relays on the distribution system must not exceed a figure of say 1·5 seconds, it is often extremely difficult, if not impossible, to set the time-overcurrent relays on the distribution system so as to provide adequate selective intervals. There are several ways of solving this problem. (a) By setting the relays so as to operate on the inverse part of the I.D.M.T. curve. (b) Using 'very inverse' relays, to obtain greater time selectivity for a given difference in current and permit lower current settings. (c) Obtaining more accurate relays. (el) The addition of instantaneous current or impedance units. The first two suggestions have already been discussed above but relays corresponding to item (c) have not been available until recently. Induction disc relays are now available in which the overshoot is less than a third of the B.S.S. value (about 0·03 second); the coil tap error has been eliminated by a new winding technique described below and errors due to magnetic variations in electromagnets can be minimised by an adjustment on the relay. The elimination of overshoot is, achieved by the use of a light disc and an electromagnet which has extremely high torque and hence requires a correspondingly strong damping magnet to meet the B.S. CUllVe, with the result that the high damping force stops the disc almost instantly after the fault current is interrupted. The method of elimination of tap error used by the English Electric Co. and its licensees is to wind a multi-conductor strip on the core instead of a single conductor, connecting the end of each conductor to the beginning of another, and so on, so that all the conductors are in series and each conductor traverses essentially the same path (fig. 4.19). Taps are connected to the junctions of the conductors so that each tap involves a different number of turns but all the turns are equivalent magnetically. This gives time-current curves of identical shape on all taps whereas, with the ordinary method of a single conductor winding, the magnetic leakage varies with the average distance of the turns from the core and from the ends of the winding, and can create a pick-up error up to 5 %, which is eliminated in the multi-strand winding. Fig. 4.20 shows typical coils of this type. 154

4.2

Overcurrent Protection 4.2.4. Application of Instantaneous Unit

The last solution (d) mentioned above, is the use of instantaneous units (fig. 4.8) set to cover as much of the line as possible so that the time setting of the next relay towards the source need only be about 0·3 second with five cycle breakers (fig. 4.17) instead of the time setting of the previous I.D.M.T. relay plus the errors and the breaker time (fig. 4.16). By using these instantaneous units in every section all faults are cleared instantaneously except R,

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4.16. Calculation of I.D.M.T. relay settings

those in the end zone which would be cleared in most cases in less than

! second. This is particularly important for the relays near the source because

(a) this is where the most severe faults can occur, (b) the time settings of these relays must be less than those of the relays at the power source, (c) this is generally the easiest place to apply them, because the difference in fault currents at the two ends of the line section is greatest near the source. 155

4.2

Protective Relays

Unfortunately there are many cases where there is not sufficient impedance in the line sections of the distribution system to provide a reasonable difference in current at the two ends of each protected section. In such cases the instantaneous overcurrent unit can be replaced by a single-step impedance unit, or mho unit (directional impedance). An impedance unit or a mho unit has the advantages that its reach is unaffected by current magnitude and, if of proper design, it is not affected by transient conditions, such as offset current waves, so that it can be set to

10,000

5,000

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1,000

500

200

100

current In C1IIIpa

FIO. 4.17. Calculation of settings of instantaneous O.C. relays Inverse relays: All relays set at 12S%. Rl set at SOOA on 0'1 T.S.M. R2 set at 12SA on O'IS T.S.M. R3 set at 62'5A on 0·1 T.S.M. Instantaneous relays: Rl set at 3000A. R2 set at 1400A. R3 set at SOOA

cover up to 95 % of the protected section and thereby provide instantaneous clearing of 90 %of the faults. This unit is described in more detail in Chapter 5, section 5.1.1 (c) on Distance Relays. The most rewarding place to put either instantaneous overcurrent units or mho units is on the line section nearest the source because, not only is the difference in fault current greatest near the source, but there is the greatest need for fast tripping because of the heavy fault currents there. 4.2.5. Application of Very Inverse Time Current Relays

The very inverse type of overcurrent relay is particularly suitable in cases where there is a substantial reduction of fault current as the distance from the 156

4.2

Overcurrent Protection

power source increases. The characteristics of this relay are such that its operating time is approximately doubled for a reduction in current from seven to four times the relay plug setting multiplier. This permits the use of the same time setting multiplier for several relays in series. In the example shown in fig. 4.18 it has been assumed that the maximum fault current at substations C, B and A is 1225, 700 and 400 amperes respectively, i.e. in the ratio of the 7 to 4 between successive substations. It is seen

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4.18. Calculation of settings of very inverse relays

that with the stated relay settings a difference of 0·3 second in the tripping times of adjacent circuit-breakers is obtained although all relays have the same time multiplier setting of 0·15. A comparison is given in the same figure to show the impossibility of obtaining the same time settings using a standard LD.M.T. relay. This very inverse relay not only reduces the tripping time between the

157

4.2

Protective Relays

breakers but invariably allows a lower time multiplier setting to be employed and, as the errors are only 7 % of the minimum operating time, down to a minimum value of 0·1 second (B.S.S.), the margin between the time grading steps can be reduced. Very inverse time-current relays are particularly effective with ground faults because of their steep characteristic, coupled with the fact that the zero sequence current varies with distance to the fault much more than with phase faults (section 4.2.1). This permits lower time multiplier settings to be used without losing discrimination between stations. 4.2.6. Application of Extremely Inverse Time-Overcurrent Relays

It is sometimes difficult to find an inverse relay having characteristics suitable to grade with fuses and at the same time to remain inoperative on switching current. This problem has been successfully solved by the long time characteristics of the extremely inverse relay at normal peak values. Fig. 4.11

Currcnlla.ppings fIG.

4.19. Relay coil arrangement for zero tap error

gives typical curves showing the application of this relay on an 11 kV system supplying a distribution transformer through a high-voltage fuse, and indicates a difference of 0·45 second between the operation of the relay and the clearing time of fuses at the maximum fault level, which allows for the circuitbreaker time and variations in the fuse operating time. This relay can also be used in conjunction with a negative phase sequence network for the protection of large generators, as its characteristics can be made to follow closely the permissible negative phase sequence current of any generator by varying the time setting multiplier and/or current setting, thereby ~ring a range of adjustment of I 2 t from 7 to 70. The characteristics of this relay permit such close settings as to prevent a generator being taken out of service unnecessarily, but to trip before any damage can result from abnormal conditions occurring on the power system. It is also possible to use an extremely inverse relay for the protection of generators against overload and internal faults. The average generator under fault conditions reaches its sustained value of short-circuit current in 3 or 4

158

Overcurrent Protection

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4.2

Protective Relays

seconds, this value being often below twice the generator's continuous current rating, depending upon the value of excitation, and whether it has manual or automatic control. In the past, difficulties have arisen in the correct selection of suitable relay settings when using the standard I.D.M.T. relay, as it is necessary to select a setting which not only discriminates with other overcurrent relays on the system and is inoperative on momentary overloads, but is operative on the sustained short-circuit current of the generator, or at least operates before it reaches the sustained value. The problem is further complicated by the damping effect of offset currents on the relay itself. An extremely inverse relay applied to a generator-transformer equipment is shown in fig. 4.10. It will be apparent that the characteristics are such that the relay gives adequate protection at the lower values of overload at a time corresponding to the safe thermal rating of both the generator and the

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4.21. Various time-current characteristics

160

4.3

Overcurrent Protection

transformer, while leaving ample time for discrimination with other overcurrent relays on the system. Fig. 4.21 compares the characteristics of the extremely inverse relay with those of the I.D.M.T. and the very inverse type. Another use for extremely inverse time-current relays is load restoration (40) (49). After an outage long enough to permit the motors of refrigerators, factory equipment, etc., to stop, the impedance of the dead load is about onetenth of normal and when the breaker is reclosed the inrush current is comparable to fault current. With ordinary I.D.M.T. relays the faulted feeder would be tripped out again after reclosing but, with the extremely inverse relay, the square law time gradient is steep enough in most cases to distinguish between the persistent high current of a fault and the rapidly decaying inrush current of the load, as light filaments become incandescent and motors increase in speed. 4.3. PERMISSIBLE LIMITS OF ERROR

The British Standards Institution Specification B.S. 142, 1953, allows errors of + 30 % in pick-up, ± 12 % in time from two to four times pick-up and ± 7 %in time from four to twenty times pick-up. The 30 % error in pick-up is far too great but was allowed in the early days of inefficient mechanical flags. The sudden transition from 12 % to 7 % error at four times pick-up is equally unrealistic and a more logical method of defining permissible error is explained in the following paragraphs. 4.3.1. A New Method of Defining the Limits of Error

For inverse time-overcurrent relays the time of operation approaches infinity at pick-up current and, for small increases of current just larger than pick-up, the time of operation diminishes very rapidly. Thus a small change in current causes a considerable increase in operating time. At larger currents and correspondingly shorter operating times the characteristic has become less steep; thus a small percent deviation in current causes an even smaller percent deviation in time. For a true I.t = K characteristic the relay would have no restraining spring to provide a fixed pick-up current value and the slope of its time current characteristic would be constant; hence the percent time error always = K, the corresponds to the percent current error. If we therefore write above case is when n = 1. It is shown on page 164 that, for any other value of n, the percent time error is equal to the percent current error multiplied by n, where the small change in current tends to zero. The proposed method of defining the error is embraced by the following statement:

r.t

The maximum percent error permissible shall be equal to the class index and shall be defined in terms of current or time, whichever produces the larger time error.

161

4.3

Protective Relays

This implies that, for n < 1, the error shall be defined in terms of percent time error for any particular current and that, for n > 1, the error shall be defined in terms of percent current error for any particular time. Normally, for the purpose of any test or measurements, I is the independent variable and t the dependent variable; it may therefore be preferable to have all the errors tabulated in terms of time errors for the chosen values of current. For any particular characteristic the maximum permitted percent current error may easily be converted into the maximum permitted percent time error for chosen values of current. An example of this is shown on each of the two curves (shown at figs. 4.22 and 4.23) when the Time Multiplier Setting is equal to 1·0. For this purpose the curves should preferably be drawn on log-log graph paper as the law In. t = K is then a straight line. (i) Draw a tangent to the curve such that the law for the tangent is I.t = K. (ii) Note the value of current 10 at which the tangent touches the curve. (iii) For all values of current in excess of 10 and less than Imax the permitted percent time error for any given current will equal the class index C. (iv) For all values of current less than 10 , down to and including pick-up, the permitted percent current error for any given time will be equal to the class index C. (v) At any selected current less than 10 and more than pick-up, draw a tangent to the curve at this point. (vi) The law for this tangent will be r. t = K. Determine n. (This is conveniently determined by reading from graph any two points on the tangent and equating thus

(12)n = !!..) II t2

(vii) The maximum permitted time error at the chosen current is now given by n. C where C is the class index. (viii) If v to vii are repeated for values of current over the required range, a table of permissible percent time errors can be constructed. This has been done for B.S.S. 3 sec. characteristic and also the extremely inverse characteristic; the tables are given in Tables 4.1 and 4.2 respectively. Consideration ofthese tables will show that, for values of current diminishing towards pick-up, the maximum permitted percent time error increases according to the steepness of the time-current characteristic and in this way the permissible time error is made to correspond with the requirements of the characteristic, even to points on the characteristic fractionally above pick-up and also for characteristics approaching a definite time. In this way errors may be quoted from and including pick-up, up to the upper limit of the effective range Imax' This is because the permissible pick-up 162

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4.22. Method of defining error of very inverse relays

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4.23. Method of defining error of extremely inverse relays

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4.3

Protective Relays

error is equal to the class index and the permitted time error fractionally above pick-up corresponds with nx the class index. If the time approaches infinity at pick-up so does n, and hence the permissible percent time error also approaches infinity. To summarise, th,is is a very simple and most effective way of defining errors because: (a) (b) (c) (d)

The percent pick-up error is equal to the class index C. For n < 1 the percent current error is equal to the class index C. For n < 1 the percent time error is equal to the class index C. This method is applicable to all types of inverse time overcurrent relays whatever shape the characteristic may be.

4.1 B.S.S. 3 sec. Characteristic TABLE

I Multiples of Tap Value 1·3 1·5 2 2·5 3 3-S 4 5 10 20

t

(Sees.)

32·6 18-4 10 7·46 6·22 5·47 4-97 4·3

3

2·2

Max. permitted percent time error

5-94 C 2-95 C 1·37 C H2C C C C C C. C

4.2 Extremely Inverse Characteristic TABLE

I

Multiples of Tap Value 1·3 1·5 2 3 4 5 10 14 18 20

t (Sees.)

Max. permitted percent time error

166 43·5 14·5 5-13 2·76 1·79 0·6 0·404 0·33 0·308

12·72 C 6·125 C 2-82 C 2-33C 2·03 C 1-818 C 1·307 C C C C

In both tables above nC is the permitted percent time error for n C is the class index.

164

> 1.

4.3

Overcurrent Protection

Tables 4.1 and 4.2 show the percent time errors obtained with this formula for the B.S.S. characteristic and the extremely inverse characteristic. An alternative method considered in the U.S.A. is to measure the error in a direction normal to the time-current curve, i.e. to express the permissible error as 8 = C sin A. (4.4), where A. is the slope angle and is calculated in the same manner as before, i.e. by taking two points on the tangent and calculating

~ 1 - ~2 which is the tangent of the slope angle i.e. A. = tan -1 dd~

'1 -12

1

A formula somewhat more suitable to test conditions is 8 = C cos hA. (4.5) provided that no times are measured below twice the top value of current. 4.3.2. Method of Defining Shape of Characteristic

It is necessary to be able to time grade Extremely Inverse relays with fuses and, possibly, other devices using a heating effect. The short-time heating effect is dependent on ]2.t = K; it is therefore desirable that the characteristic of the relay should match this law at the higher currents. All devices 'affected by heating have a maximum continuous rating and, in our case, this is the maximum current that can be supplied for infinite time. For correct matching, this current should match pick-up current of the relay, and at pick-up the time of operation should approach infinity. In this case the law for the relay would be: K t = ]2_12

"

(4.6)

where t = time in seconds ] = multiple of tap current I" = the multiple of tap current at which pick-up occurs. K = constant which determines time delay. Most electromagnetic relays will match this law up to some maximum current. If this current is exceeded the time of the relay "usually becomes longer than that obtained from the above law. This is due to the shortcomings of the relay; for example, saturation and inertia of the moving systems. Future developments which might improve these characteristics should not be prevented by standardising on a characteristic which is dependent upon given amounts of these undesirable effects. The system suggested should therefore quote accuracies to the above law over some particular current range. The counterpart of this for the Very Inverse relay is to use the expression K t=(4.7) ]-1" Most of the above arguments still apply for this less inverse characteristic. An important point is that grading will be more accurate at high currents if the relay characteristicS match these laws, instead of tending to a definite time at high currents. It will also result in great convenience to be able to calculate the shape of the characteristic in such a simple manner. 165

4.4

Protective Relays

It should be noted in conjunction with these laws that n can never be less than 1, which is an argument in favour of the method of defining errors = K.) given in section 4.3.1. (n is the index of I in the relationship

r.t

4.4. RATINGS OF OVERCURRENT RELAYS

In the U.K. it is customary to express the pick-up of overcurrent relays as a percentage of the c. t. secondary current but in the rest of the world it is expressed directly in amperes. For example, a ground relay supplied by 5 ampere c.t's may have seven tappings for a range of 1 to 4 amperes (20 % to 80%). The percentage marking used in the U.K. has the slight advantage of being the same for different c.t. ratings but the latter is easier for a man testing the relays and eliminates the necessity for specifying the c.t. secondary current. Almost all other countries mark the taps in amperes. 4.4.1. Phase Faults

The standard range of current taps for phase fault relays in the U.K. is: Percent

5 amp c.t. 1 amp c.t.

50

2t

0'5

75 3'75 0·75

100

5 1'0

125 6'25 1'25

150 7'5 1'5

175 8'75 1'75

200

10 2·0

%

amps amps

In the U.S.A. until recently the standard range was 4/5/6/8/10/12/16 amperes for the standard 5 ampere c.t's, which are invariably used for feeder protection. Lately there has been a move towards a 3 to 12 amperes range because the taps above 12 amperes are seldom used except on instantaneous overcurrent relays. 4.4.2. Ground Faults

Residual current relays (fig. 4.28) can be set much more sensitively than phase relays because the residual current is normally zero except for phase unbalance. In most countries a popular range of taps is 1/1t:ll;;/2/lt/3/4 amperes for a 5 ampere c.t. which is 20 %to 80 %of c.t. rating. A more sensitive range sometimes used is 10% to 40%. Where 1 ampere C.t. secondaries are used the ampere ratings will of course be correspondingly reduced where marked in amperes. 4.4.3. Geometrical versus Arithmetical Tap Progression

In most countries the taps increase in whole numbers or in arithmetical progression. A typical phase relay for instance may be marked 3/4/5/6/8/10/12 amperes. Other examples are given in the table above which has 25 % increments. This custom results in inaccuracy in the relay pick-up on each tap or necessitates trimming turns which are expensive and reduce the reliability of the relay; furthermore, the arithmetical taps give varying accuracy of adjustment.

166

4.5

Overcurrent Protection

If geometrical progression of tap values were used the coil construction would be simpler and there would be less tap error and a constant accuracy of adjustment. This is because the tap values are inversely proportional to coil turns so that round numbers for taps require fractional turns on the coil taps. A constant increment of turns on consecutive taps would enable the pick-up of the relay to be set within the same accuracy on any tap. Preferred tap ratings are as follows: Percent 5 amp C.t. 1 amp c.t.

50 2·5 0·5

60

3·0 0'6

75 3·75 0·75

100 5·0 1'0

120

6·0 1·2

150 7·5 1·5

200 10 2·0

%

amps amps

It will be seen that, with preferred taps, there is an almost constant ratio between consecutive tap values so that settings can be made within the same accuracy anywhere in the range. With the standard taps given in the previous table there is an increment of 50 % between the lowest two taps and only 13 % between the highest two. 4.5. DIRECTIONAL OVERCURRENT PROTECTION (33) (38) (39)

It is obvious that the relays on a single radial line need not be directional but, in the case of parallel lines or loop lines, the current magnitude may be the same in two feeders on the same bus except for the relative direction of the fault current; furthermore, the direction may change with the location of the fault. Fig. 4.24 shows a typical network where the direction of the current depends upon the location of the fault and where, apart from direction, the current may be the same in two relays at a station near the middle of the loop. For instance, a fault at X would produce similar fault currents in relays D and E, except that the current in D is incoming and that in E outgoing. Similarly a fault at Y causes similar currents in relays G and H. Directional control of these relays would prevent the relays D and G from disconnecting their sound lines. Fig. 4.24 shows no loads or power infeeds. Where these exist they have the effect of increasing the current in the faulted section and, in some cases, reducing that in the sound sections so that directional control of timeovercurrent relays is seldom necessary, especially where those relays have inverse time characteristics or are on the inverse part of an I.D.M.T. curve. In general, directional control should be used with instantaneous-overcurrent or definite time relays where the maximum fault current flowing through the relay into the bus for a fault on another feeder exceeds 80 %of the minimum current outgoing for a fault at the far end of the protected section. 167

4.5

Protective Relays

In the case of induction disc time-overcurrent relays a lower percentage applies because of overtravel of the disc and because the operating time does not vary much if the currents are on the definite part of the LD.M.T. curve.

FIG.

4.24. Directional overcurrent protection

The decision must obviously depend upon the application in question but a figure of 25 % is safe for general application with inverse time or LD.M.T. relays. Directional control is also required where the load current is flowing towards the bus normally and a low pick-up current setting is required. This, of course, does not apply to ground fault relays. 4.5.1. Phase Faults

Because of the possibility of sudden reversal of the current direction when the fault is cleared and load resumed, the directional unit should be fast and induction cup units are popular for this purpose. The contacts of the directional relay· can be connected either in series with those of the overcurrent relay or in series with a winding which prevents the overcurrent relay from operating unless its circuit is completed. The latter is called 'directional control' and is preferable because it permits the overcurrent relay to start moving only for a fault in the tripping direction, 168

Overcurrent Protection

4.5

thereby eliminating the risk of wrong tripping if the current direction reverses after the fault is cleared. Fig. 4.2Sa shows how an overcurrent relay with a split-pole magnet is directionally controlled. Fig. 4.2Sb shows the method used for the older wattmetric type overcurrent magnet. In the case of attracted armature relays, such as are used for instantaneous overcurrent units, directional control is more difficult because there is seldom a convenient coil across which the contacts of the directional unit can be connected to effect control.

(a)

I

Directiona.l unit

(b) FIG.

4.25. Directional control of time-current relays (a) Shaded pole type. (b) Wattmetric type

If the relay operating winding has sufficient impedance it can be normally short-circuited by the contacts of the directional relay but this method is effective only with relays operating below lA, because the resistance of the contacts is comparable with that of the current coil which is to be shortcircuited. One solution is the use of a secondary winding, as in fig. 4.9, across which the a.c. potential is sufficient to enable effective contact to be made. 169

Protective Relays

4.5

The directional unit uses current from the same phase as the overcurrent relay and is polarised by one of the line-to-line potentials, usually the one in quadrature with the current, i.e. Vbc with current la' because (a) this gives maximum torque with current lagging the unity power factor position by 60°, using an induction cup unit having no phase shifting means; (b) this connection is less affected by phase shifts that occur when there is a wyedelta connected power transformer between the relay and a fault. In the early days, induction disc watt-hour meter units were used for directional purposes. Since this unit had maximum torque at unity power

x

--~--------------~~--~R

FIG.

4.26. Impedance characteristics of directional relays with and without voltage restraint

factor it was necessary to use the adjacent delta voltage to polarise it, i.e. to use Vca with la and this gave maximum torque at 30° lagging the unity p.f. position on three-phase faults or 60° lagging on phase-to-phase faults. The subject of the connections of single-phase directional units has been discussed at length by various authors (33) (38). (a) Voltage Restraint. On many systems the connected generating capacity varies with load conditions to such an extent that the minimum fault current at one timc may be less than maximum load current at another time. This of course makes it dfficuIt to set overcurrent relays unless the directional unit is provided with voltage restraint. Under even the heaviest load or power swing conditions the potential will be near normal, preventing the directional unit from operating but, during a fault in the protected section, it will fall to a value permitting the operation. The directional unit becomes, in effect, a mho type fault detector, such as is described in Chapter 5, with an impedance pick-up between the load and fault values, which fortunately are widely different. The directional unit associated with the overcurrent relay in the a phase also has a phase current fa; it is polarised by the quadrature potential Vbc and restrained by the voltage Vab • 170

Overcurrent Protection

4.5

No voltage restraint is required for ground relays because they work on zero sequence currents which are normally zero under load conditions and hence avoid any requirement for blocking their action. (b) Two Relays versus Three. Overcurrent relays in two of the phases and one in the C.t. residual circuit give complete protection for all faults on lines or cables but, in the case of a A - Y transformer feeder, the current in one phase may be twice that in the other two phases so that three phase relays are required. On an ungrounded system where there is no relay in the residual circuit, there should be either relays in all three phases or, if only two are used, they should be in the same two phases in all feeders; otherwise there is a risk of a 'cross-country' fault, (i.e. two simultaneous single phase ground faults on different feeders) involving the two phases not having overcurrent relays so that time selectivity may be upset. Discrimination between relays can be upset on interphase faults with relays in only two phases (a) if there is a wye-delta power transformer between the relays, (b) if there is a light fault and a heavy load. To summarise, two phase relays and one ground relay are sufficient on grounded· systems and three phase relays are desirable on systems grounded through high impedance or ungrounded. In the case of directional overcurrent relaying the same considerations apply if the directional units are polarised by quadrature potential. If polarised from any other potential, three phase relays and one ground relay should be used. (c) Single-Phase Directional Units. Single-phase directional units can use wye or delta current and can be polarised by any of the wye or delta potentials. Certain combinations, however, are preferable because their maximum torque angle (M.T.A.) is more consistently close to the fault phase angle for all types of faults. The M.T.A. is the angle () by which the potential applied to the relay leads the current for maximum torque (or the angle by which the current lags the potential). The best known connections have names by which they are known in the U.K. and the U.S.A.; the names are given in the left-hand column of the following tables, 4.3 and 4.4. The connection chosen depends upon the type of relay unit. Early directional relays were adaptations of watthour meters and had maximum torque at unity power factor; the 30° connection was the most suitable for such units because its M.T.A. was 30° lagging the unity power factor position under three-phase conditions and up to 90° lagging the faulted voltage for a phase-to-phase or double-ground fault. This gave reliable directional torque for all interphase faults on lines of average (50°) phase angle, but it could operate incorrectly during leading load conditions on a single-phase ground fault on the remote side of a wye-delta connected power transformer. The induction cup unit replaced the wattmetric induction disc unit because of its superior speed and efficiency; it has maximum torque with current 30°

171

4.5

Protective Relays

leading the voltage applied to its potential coils; hence the quadrature connection was the most suitable for it. With this arrangement M.T.A. was 600 lagging for all faults. The constant M.T.A. makes this arrangement the most reliable one because the relay is never near the threshold conditions due to voltage phase-shifts caused by the fault and opposite current phase-shifts due to an intervening power transformer or during leading load conditions (33) (38). The torque of a single-phase directional unit is proportional to 1111 cos (¢-O) where ¢ is the angle by which the potential leads the current for a particular fault condition and 0 is the M.T.A. Fig. 4.29 shows a condition which can cause wrong operation of a single-

Vi

/

c~--~--------~b

FIO.

4.27. Currents ani" potentials used in directional relays for phase faults

phase directional relay using wye current but not one using delta current nor a polyphase unit. At both ends the wye currents in the three phases are in phase and, since the potentials polarising the single phase directional relays are 1200 apart, at least one of them will operate in the wrong direction if wye currents are used. Single-phase directional units are much more used at the present time than polyphase directional units because, until now, there has been a tendency to consider all protection phase to phase. However, polyphase relays offer economy of equipment and panel space and immunity to certain fault conditions associated with Y -!1 power transformers which can cause wrong operation of single-phase directional relays (32) (104). (d) Polyphase Directional Units. Tables 4.3 and 4.4 show the various possible connections for single-phase directional units and the sum of the torques of three such units, one of which is connected this way and the other two with corresponding connections in the other two phases, to form a polyphase unit. The torque of a polyphase directional unit is of the form

P1ICt+O + P2 ICt +O+ Po~

(4.8)

where Ct is the phase angle of the chosen potential V relative to the current I 172

-'" ...... (0)

be

ab

ab

ab

ab

60°

- ea

a

30°

- ea

be

a

Quadrature

ab

/60°

/120°

0°

/30°

/90°

/30°

/60°

-e

a

a

/120°

b

a

/60° /60° 0°

Pt!120° + P2/12O° PI/& + P2/6O°

/30°

/30°

/90°

0°

-/60°

/60°

)I

PI +P2

PI/120°

+ P2/120° + Po PI/60° + P2/6O° - Po PI/30° + P2/30° PI/90° + P2/90° PI/30° + P2/30°

PI +P2+PO

0°

a

a

3 Phase Torque

ex

V

I

Adjacent

Connection

TABLE 4.3 Wattmetric Unit (() = 0°)

/)1

+ Po/60° PI/60° + P2/6O° + Po/60° PI/60° + P2/6O° - Po PI/60° + P2/6O° PI/60° + P2/6O° PI/60° + P2/6O° Pt!6O° + P2/6O° PI/60° + P2/6O° PI/60° + P2/6O°

PI/60° + P2/6O°

Torque with V shifted by

~

...ill

(I) (")

.... g.

"\)

a

::J ....

(I)

~

c:::

0 ~

~

.....

/30°

+ P2/3O°

- ea /60°

/120° PI/30°

/30°

PI/90° + P2/90°

be

ab

ab

/90°

PI/30° + P2/30°

0°

ab

ab

60°

/60°

PI +P2

/30°

- ea

+ P2/60° PI/60° + P2/60°

+ P2/60° PI/60° + P2/60° PI/60°

PI/60°

PI/60° + P2/60° 0°

a

/90°

30°

be

a

PI/60° + P2/60°

-

+ P2/60° -

Po

+ P2/60° - + Po/60° PI/60°

PI/60°

PI/60° + P2/60° + Po/60°

Torque with V shifted by 11..

/120°

/30°

/30°

/90°

"I

Quadrature

/30°

+ Po/30° PI/90° + P2/9O° + Po/30° PI/30° + P2/30° - Po/30° PI/60° + P2/60° PI/60° + P2/60°

PI/30° + P2/30°

3 Phase Torque

a

ab

/60°

/120°

b

a

-c

0°

a

a

a

IX

V

I

A~jacent

Connection

4.4

Induction Cup Unit (9 = 30°)

TABLE

".

.

'"

"0;

Qj-

::0 II)

II)

..;:

0

II)

2.

."

UI

4.5

Overcurrent Protection

with balanced unity power factor conditions, 9 is the angle of the current relative to the potential applied to the directional unit for maximum torque, i.e. the M.T.A. of the unit alone without phase shifting devices. Polyphase units are not recommended for single-phase ground faults, it being the practice to use separate residual directional relays with a torque Po cos (cfJ-9). It will be seen that the Po components in the expression for polyphase torque are either missing or at undesirable phase angles. In evaluating actual torques it must be remembered that P2 and Po flow in the opposite direction from PI because V2 and Vo originate at the fault whereas VI originates at the generator. Using one of the connections of Table 4.4 giving no zero sequence torque a polyphase directional relay would not work wrongly in the above case of fig. 4.29 which deceived the single-phase directional relay. This is because it would work correctly at terminal s and not work at all at terminal L where there is only zero sequence current. A polyphase directional relay can be used to control three overcurrent relays, through a multi-contact auxiliary relay, with a saving of cost and panel space; there is also the advantage that it will not operate incorrectly on certain conditions that would deceive a single-phase directional relay (33) (104). 4.5.2. Ground Faults (34) (37)

A single-phase ground fault on any phase produces the same magnitude of zero sequence current and potential and in the same phase relation. Consequently, only one directional overcurrent relay is required for ground faults and it is energised from the residual circuit of the c.t's and p.t's, i.e. with Ires = 3/0 and Vres = 3 Vo. It is connected as shown in fig. 4.28.

a----T-------------------------------b----+-~----------------------------c----+--4---r------------------------C.Ts

Phc.se relc.ys

FIG.

4.28. Connections of residual current relay

The zero sequence current 10 can flow only from a grounded neutral to the

=

E Z where Z1> Z1+ Z 2+ 0 Z2 and Zo are the total system impedances viewed from the fault. Zero sequence current cannot pass through a wye-delta transformer because, 175

point where a ground fault occurs. Its value is 10

Protective Relays

4.5

even if the neutral of the wye side is grounded, the residual current can only circulate around the delta. Fig. 4.29 illustrates this point. Only positive and negative sequence currents are supplied from the ungrounded power source at the left and only zero sequence current from the grounding transformer at the right. Zero sequence current can pass through a wye-delta-wye transformer from one wye winding to the other if both are grounded. Residual current relays (fig. 4.28) can operate erroneously due to spurious zero sequence current caused by c.t. inequalities or by non-simultaneous

rm~-------------r~ ttII t I'

,

,~~~:X,~ FIG.

[IE, abc

jjj 10

4.29. Residual current can flow only between a grounded neutral and a ground fault

closing of three-phase circuit-breaker contacts (34). Hence it is desirable to limit the speed of instantaneous ground relays to a minimum time of 2 cycles; an alternative is a stabilising resistor, as explained in section 4.5.2 (d). Instantaneous operation is permissible when the relay is supplied from a corebalance c.t., i.e. one which surrounds all three phases. A residual overcurrent relay will also operate on the spurious zero sequence current caused by an open-circuited c.t. secondary; tripping under this condition may not be undesirable. In systems grounded through. a reactor which is tuned to the system capacitance to ground, the fault current is automatically. blocked because it divides between the reactor and the capacitance to ground of the healthy conductors which form a high impedance parallel resonant circuit. This is known as a Petersen coil or a neutral compensator coil. Analytical treatment orthese circuits is given in Volume II. (a) Polarisation of Ground Relays. The directional unit can be polarised by residual voltage Vres = 3 Vo or by neutral current In, or by both. The neutral current obviously can be used only where there is a system neutral grounding point at the station; otherwise, potential grounding is necessary. Where the system ground is not always available (because the grounded transformer may be out of service) both types of polarisation are necessary, either on two relays or both on the same relay. This is because the zero 176

4.5

Overcurrent Protection

sequence potential is the IoZo drop between the relay and the nearest grounding point and will be very low if the grounding point is at the station; on the other hand, current polarisation will not work when the local ground is not available. (b) Residual Current, Potential and Power Relays. Fig. 4.30a shows the zero sequence current distribution in a system grounded solidly at each substation. It is clear that excellent discrimination is provided by zero sequence

1 f A---tl

1

(a)

(b) FIG.

4.30. (a) Distribution of 10 in multiple grounded system (b) Distribution of zero sequence kW and kVar

directional current relays because of the fact that most of the zero sequence current comes from the neutrals at the two ends of the protected section and very little from the other sources. An alternative to a directional overcurrent relay is a zero sequence timepower or time-VA relay. Fig. 4.30b shows the distribution of zero sequence kW and kVar in a multiple grounded system. This relay is more selective on a single grounded system but it is more difficult to calculate its settings.

Currant

(a) FIG.

(b)

4.31. (a) Very inverse time-current characteristic lot = K (b) Current-distance relation for lot = K relay

Directional overcurrent relays have tended to supersede the directional power relay because it is easier to set. Furthermore, with a very inverse timecurrent characteristic, it is as selective as the time-power relay. Fig. 4.31 shows that the very inverse relay has a time-distance characteristic because G 1n

4.5

Protective Relays

the time T is inversely proportional to the current I which is inversely proportional to Zoo Hence TocZo and the relay operating time increases with the distance to the fault. Fig. 4.32 illustrates this on a multiple grounded system and fig. 4.33 on a single grounded system. Time

I

I

./

,I

/

I

I

I

J

-'

/

/

I

/

I

I

Distance

A

FIG.

o

B

4.32. Time-distance characteristics of lot

=

K relay on a multiple grounded system

The addition of an instantaneous zero sequence overcurrent relay provides the same benefits as in phase relaying, but it is equally important to use one with negligible transient overreach on offset fault current waves. Tlm~

FIG.

4.33. Time-distance characteristics of lot = K relay on a system grounded only at the source

On systems with neutral compensators or isolated neutrals a sensitive wattmetric relay can be used for detecting single-phase ground faults and energising an alarm if the fault is not self-extinguishing. Fig. 4.34 shows the

f

! (a)

)A (b) FIG.

4.34. Zero sequence quantities on system with neutral compensator (a) Distribution of kVar. (b) Distribution of kW

178

4.5

Overcurrent Protection

zero sequence kW distribution on a system with a neutral compensator. An alternative to the residual power relay on an ungrounded system is a residual potential relay, but it is less selective. (c) Po/arising Sources. Current polarisation is obtained from the grounded neutral of a power transformer or from a delta tertiary, depending on the ______~------------~S~t~~t~io~n~~~$~·------------------_r---a

----~-+--------------------------------_r_t---b --~~-4--------------------------------,--r-t---c

P.T!,

C ·T.5 '1--+--+----i

FIG.

4.35. Potential polarising with wye-broken delta p.t.

R~Ia.y i>O l~ri $ing

coil

(a)

ConnectlOl1s of otMr

wind ings I mm~t.rial

(b)

FIG

4.36. (a) Current polarising with wye-delta power transformer (b) Current polarising with zig-zag power transformer

179

Protective Relays

4.5

overall connections. Either method is satisfactory in a wye-delta transformer (fig. 4.36a). In a power transformer with one grounded zig-zag connected winding and the other winding wye or delta, the neutral of the zig-zag winding is a reliable polarising source (fig. 4.36b). In the case of an auto-transformer with a delta tertiary, a c.t. in the delta winding is preferable to the neutral (fig. 4.37a), but c.t's in the three delta

(a)

(b) FIG. 4.37. (a) Polarising with auto-transformer wi~h delta terti~ry (b) Polarising with wye-wye power transformer With delta tertiary

windings must be paralleled if load is taken from the tertiary. A c.t. in the neutral can be used if Z

~t

t+ ,+

Z < VII where suffixes t, h, 1 and s refer to &.

l'I

tertiary, high side, low side and source. In a wye-delta-wye transformer with both wye neutrals grounded it is important to parallel the two neutral c.t's. There will always be a resultant current, even for a through fault, because the windings have different currents inversely as the ratio of their voltages. In fact, the directional relay can use current from the neutral in the protected line side instead of the residual current, as shown in fig. 4.37b. Polarising potential is usually provided by a grounded wye-broken delta p.t., as shown in fig. 4.35, or by a p.t. connected across the neutral grounding impedance, if available. 180

Overcurrent Protection

4.5

Potential polarisation is not possible with two open-delta high side p.t's unless the neutral is located by deriving the third phase from the Lt. side and compensating for the transformer drop. High side zero sequence potential can be obtained, however, even if there is only one high side p.t., by subtracting from the high side wye potential the corresponding low side delta potential; allowing for the transformer ratio these should be the same except for Vo. An alternative is to use a negative sequence power relay; this relay is also effective where mutual coupling between power lines prevents the use of a zero sequence power relay. (d) Restricted Earth Protection. This is an English term which may be misunderstood in other countries. It refers to the differential protection of generators or transformers against ground faults. It is called 'restricted' because the relays operate only for ground faults within the protected windings (fig. 4.38). Kirchhoff's law can be applied to grounded neutral circuits in the same way as it is applied to bus protection, i.e. the sum of the neutral currents and residual currents should be zero at a given location. Fig. 4.38 shows restricted earth protection of the grounded wye windings of a generator or transformer. The neutral current In will normally be equal

flO.

4.38. Restricted earth relay with transient blocking

to the residual current of three phases, which is the sum of the current in the three phases, and no current will flow in the relay unless there is a ground fault in the protected equipment which will upset the balance. For the same reason, no current should flow in the relay in an external ground or phase fault but in practice it is possible in a heavy through fault to saturate the c.t's unequally due to fault current asymmetry or to remanence flux in their iron cores and hence to cause the spurious difference current long enough to operate the relay. This subject will be dealt with in more detail in Chapter lIon Bus Protection. For the moment it is sufficient to say that this spurious spill current can be prevented from causing undesirable tripping in the following manner. In fig. 4.38 the choke and capacitor in the relay circuit are tuned to system frequency so that harmonics (the spurious spill current has a very distorted wave) and the d.c. components are rejected and a short time delay (1 to 2 cycles) will be introduced, by which time the effect of the remanence flux will have largely disappeared. Finally, a stabilising resistor can be connected in series with the relay, as shown in fig. 4.38, which makes the c.t's saturate on

181

Protective Relays

4.5

an external fault and not on an internal fault, as will be explained in Chapters 9 and 10 on restricted earth protection of generators and transformers respectively. 4.5.3. Zero Sequence Power Relays

These relays are of the wattmetric type with two co-operating windings, one (the operatinKwinding) in the residual circuit of the c.t's of the protected line and the other (the polarising winding) which can be energised by either Ineut or Vres ' depending upon whether there is a transformer on the local bus which has a solidly grounded neutral (34). Usually, only the operating coil is tapped for adjusting the relay pick-up; consequently in these relays the pick-up is a square-root function of the tap turns. With neutral current In polarisation, the time is inversely proportional to the current product 10 , In. With v;.es polarisation it is proportional to the VA plus product Vo. 10 , The setting of these relays for time discrimination is rather complicated and directional overcurrent relays are usually preferred for protection against ground faults, except where low cost or minimum panel space is considered important. Furtherm ore, it is possible to have incorrect tripping on parallel lines with current product relays polarised by neutral current. Referring to fig. 4.39 a fault near bus A with breaker 4 open will cause a current I in the unfaulted line 12 and a current I+kIin the faulted line 2, so that the relay at I_

FIG.

4.39. Wrong tripping of zero sequence directional relay

breaker 1 receives the current product 12 and the relay at 3 gets kI(J + kI). If the relay 1 in the unfaulted line is not to trip the good line 12 unnecessarily, it must be slower than relay 3, i.e. 12 < 12(k+k 2) k2+k-l > 0

k > 0·64 In other words, to avoid wrong operation, Ib must not exceed 64 % of I" if instantaneous overcurrent relays are used which will open breaker 4 at once leaving a race between relays 1 and 3. This condition will not occur on single lines and is less likely to happen where instantaneous high set overcurrent units are not used. 182

Overcurrent Protection

4.6

4.5.4. Dual Polarisation (39)

In the U.S.A. it is becoming general practice to provide a double polarising coil, one part being a potential coil energised with Vo and the other a current coil energised with neutral current In. This arrangement not only prevents the directional ground relay from failing to operate if the grounding transformer is removed from service, but it also gives more consistent operation. When the grounding transformer is in service In is large for a fault in the protected section, especially if the fault is near the relay bus; Yo, on the other hand, approaches zero as the fault location approaches the bus. When the grounding transformer is out of service In is zero but Vo is as high as at the fault location. For neutrals grounded through resistance, intermediate values of Vo and In pertain and in this case dual polarisation is especially valuable. 4.6. A.C. TRIPPING

Where a d.c. source is not available for tripping the circuit breaker (such as at a small unattended station) the energy must come from the a.c. power system itself. This can be derived either from the c.t's or the p.t's. If c.t's are used they must have enough output to trip the breaker at low currents. If p.t's are used their potential output may not be available during a fault so it is necessary to rectify the energy output to store it in a capacitor. The early schemes for a.c. tripping from c.t's used a tripping reactor connected in series with each relay and c.t. secondary; when the relay contacts closed each reactor was paralleled by. an individual trip coil on the breaker, as shown in fig. 4.40. The reactor should then have been removed so as to have the maximum energy available for the trip coil, but this required a transfer contact which was capable of transferring twenty or more times normal current and which was not available. Fig. 4.40b shows how this has been done in an English relay. In the case of protective relays with at least 3 VA burden at c. t. rating the transactor can be eliminated and the voltage for the auxiliary relay can be taken from across the coil of the protective relay. The burden of the switching relay is zero normally, so that the timing characteristic of the protective relay is not affected; the burden during tripping is only 1 VA at c.t. rating. The contents of the tripping relay must be capable of transferring at least 100 amperes at 1 ohm or 150 volts. Silver contacts would stick and weld even if their operation were sparkless. Most alloys of silver which resist welding have too high a resistance; the one exception is an elkonite, silver cadmium oxide. Potential or 'capacitor' tripping, of course, imposes no burden on the c.t's and has no limit of fault current. It also requires only one trip coil and is applicable to any kind of protective relay. Its main disadvantage is that it gives only one short tripping impulse instead of a sustained pull on the trip latch and requires much more careful adjustment and maintenance of the trip mechanism. 183

4.7

Protective Relays sta.lion bus

~

C.T.'s

H~I~y

'uuu u-

A-~I__~____________cO_'I_s~~~,-__~~=~~~~~~~~

~~

2OVA.

-vvv

~

~p

(a) Stenion bus.

Prol~ct ivc r~IQy

Protected lin&

(b) flO.

4.40. A.C. tripping

(a) With reactors. (b) With relays 4.7. PROTECTIVE SCHEMES USING TIME-OVERCURRENT RELAYS

Rural lines in the U.S.A. are generally protected by fuses but, in order to minimise the patrolling of the lines and the replacement of fuses, faults on the lines are initially cleared by a low-set instantaneous overcurrent relay at the source (fig. 4.41) and reclosed. This prevents unnecessary blowing and replacement of fuses. If a permanent fault occurs the fuse blows because the instantaneous relay is cut out after the first trip long enough for the fuse to blow and thus locate the faulty feeder or tap-off line. An I.D.M.T. overcurrent relay provides back-up protection if the fuse does not blow. 184

Overcurrent Protection

4.8

15 kV radial feeders in France do not use fuses but are protected by definite time overcurrent relays and a recloser, as follows. A fault anywhere on the line operates an instantaneous overcurrent unit which clears the fault in approximately 0·2 second, including the breaker time. This gives time for the fault to burn through if it is caused by a falling object such as a tree branch. If the fault is still there after automatic reclosing, it is cleared the

FIO.

4.41. Protection of rural lines with relays at source only

second time in 0·6 second. The next reclosure is delayed 15 seconds, after which it is finally cleared in 0·6 second and locked out. If the fault is on a tap line an instantaneous overcurrent relay operates a notching relay which trips the breaker if the fault is there after the second reclosure. Their philosophy is that the fuses are expensive in manpower as well as in capital cost and do not clear high resistance permanent faults which require patrolling of the whole line anyway. I.D.M.T. relays have higher burden and less accuracy than definite timers. Transient faults are eliminated after the first trip and reclosure. Semi-permanent faults are eliminated after the second trip and delayed reclosure. Many 15 kV feeders in Belgium are protected by the Ramelot scheme, which uses instantaneous overcurrent relays at the ends of each line section. These relays are interlocked so that each outgoing relay, when it operates, blocks the incoming relay, on the incoming side of the same bus, which is slightly delayed. There are two trips and reclosures and the incoming relays operate as back-up relays if the fault is not cleared by an out-going relay after its short delay. 4.1. CONSTRUCTION OF TIME-OV£RCURRENT RELA VS

The first induction disc time-overcurrent relays (I.D.M.T.) used a modified watt-hour meter electromagnet in which the upper magnet acted as a transactor to supply the lower magnet (fig. 4.42a). About 1920, the shaded pole single-coil electromagnet was introduced in the U.S.A. (fig. 4.42b). Its efficiency (torque/VA burden) is about double that of the watt-hour type magnet because its flux leakage is much less, i.e. less amp turns and volt-ampere burden is expended in magnetic flux that does not drive the disc. 185

4.8

Protective Relays

(a)

Shunt tor obto.'"'"9

So.turo.t ion o.djustmtnt

~xtrf:mdy InVlrSI. t l m~ curve.

I

(b)

I

(c) FIG. 4.42. Electromagnet constructions (a) Wattmetric. (b) Shaded pole. (c) E-type

186

Overcurrent Protection

FIG.

4.8

4.43. Printed disc inverse time relay

Fig. 4.42c shows a hybrid construction used in Germany and the U.S.A. which is reputed to be as efficient as the shaded-pole magnet and easier for adjusting time-current curves (17) (18). The efficiency of an induction disc relay with even the best electromagnet design does not exceed 0·05 %, which is extremely poor. Figs. 4.43 and 4.44 show a printed disc dynamometer type which has 50 to 100 times the efficiency and very much more accurate time-current characteristics. The printed disc relay has inherently a pure inverse time-current characteristic. Other characteristics are obtained by non-linear resistance networks connected between the disc printed circuit and the rectified current input (141).

FIG.

4.44. Printed disc [2/ = K relay

187

Protective Relays

4.9

4.9. APPLICATION PROBLEM

Application of Inverse Time-Overcurrent Relays Problem Determine the current and time settings and plot the grading diagram for the overcurrent relays shown in fig. 4.45. 5MVA 15%

rv F.L.C.- 440A S.C.C.= 2,900A

8 6' 6KV

WOM" (a,700A)

If

I

10 *

f

200/5

t

f

*I *I Rtk1y I 2

r

6'6KV 7S MVA - --'-"""T1- -

(6,550 A)

o

6·6KV/415V I,OOOKVA

>:<

Typo Sttting Thermal Thtrma.ll05 with H.S. H.S.-IOX F. L. I.D.M .T. o/e with H.S.

%

3

I.D.M.T·oIe

4

Volta.gt FontrOlltd I.D.M .T. o/e

41SV 10MVA

(eeOA)

x-WB 150/5

8

300.1. FU'2 FIG.

4.45. Application of time-current relays to a distribution system

The reactance of the 1000 kVA transformer may be taken as 7 %, and the motor starting current as six times full load (assume full load of the motor to be 150 amperes). HRC Fuse. The time-current characteristic of the 300 ampere fuse is shown in fig. 4.46.

Relay (1) Type 'Mn' Thermal Relay for protection of the motor at M; current settings 105 %and 125 %of 5 amperes; there is no provision for adjusting the operating time with this type of relay. Its timecurrent characteristics are shown in fig. 4.47. It has an instantaneous (high-set) overcurrent unit continuously adjustable between 400% and 1600% of 5 amperes. 188

z

.....

t

o

FIG.

0'10

I

;

)

t

;

o

8-,.

~

c

.,.,

:;:;

E

.£

§

.. ..

..,c..

10I

20

•

50•

100

1\

1\

\

1

\

1

.

50 100 200 Prlma.ry current In a.mps. (6 ·6 KV.)

\

500

1,000

4.46. Time-current characteristic of 300 amp HRC fuse

20

\

.\

50

I

, 1

t

;

10•

... 20

.~

.S

.

::

3

i

100•

200•

500•

1,000

\.

\\

FIG.

\

\.

"

\.

2

"-

.......

.....

r---

-....

.....

.......

"

--.... .......

e

:.: r--

B

5 3 4 Multiples of ra.ted currcnt

6

7

-

- -- - -- _, A

- - - - Sta.rting (cold) cha.ra.tcrisllc - - - Running (hot) cha.ra.lcrl5tic

'"

'-,

4.47. Thermal time-overcurrent relay (12/ = K)

\

" '," \ \ ,

\ I"

2

3

4

7 6 5

8

.

~

0

~

II)

E

'E

C

~

Co

g'

(j;

...c:: .0:: .. ....:;, l= ." a .... ~ ....

!l :I

o

2

4

16

4.'

Protective Relays

Relay (2) Type CDGll. I.D.M.T. Overcurrent Relay with current tap settings 50%, 75%,100%,125%,150%, 175%,200%of5amperes. Its time-current characteristics are shown in fig. 4.6a. Its operating time is continuously adjustable between 10 and 0·8 seconds at two times the current setting. Its instantaneous high-set overcurrent unit is continuously adjustable between 400 % and 1600 % of 5 amperes. Relay (3) Same as relay (2) except no instantaneous unit. Relay (4) Type CDV22 I.D.M.T. Overcurrent Relay, voltage controlled. Current tap settings 50%, 75%,100%, 125%, 150%, 175%,200% of5 amps. Its operating time is continuously adjustable between 20 and 2 seconds at two times the current setting. 100 80

,

60 40

n

...•c

820

:

1\

.s

l~crIOC1d characteristic Normal voltf

~

~

-="'" I..... I"'t"

4

FaU~cn.tIC

~

f"""..

2

1

fiG.

1

2

4

6

8 10

Current,CllllpS

20

i' 40

6080100

4.48. Voltage controlled time-overcurrent relay for generator back-up protection

The relay characteristics are shown in fig. 4.48 for a 5 ampere relay having a current setting of 100 % and the maximum time setting of 1·0. Referring to the normal volts curve, it will be seen that at 10 amperes it will take 20 seconds for the relay to operate and it can be assumed therefore that, if the relay is given a time setting of 0·1, it will take 2 seconds to operate at 10 amperes. Similarly, when the relay characteristic changes due to a collapse in volts, with a time setting of 1 and 10 amperes through the relay, it will take 4·3 seconds to operate for the same current.

190

5 Bistance Belalls General Principles-Special Characteristics-Limitations-Application to Lines-Settings-Multi-terminal Lines-ConstructionA.C. Potential Supply-Simultaneous Ground Faults-Autoreclosing Zero Sequence Compensation 5.1. GENERAL PRINCIPLES 5.1.1. Distance Measurement

As has been previously stated the most positive and reliable type of protection compares the current entering the circuit with the current leaving it; on transmission lines and feeders the length, voltage or arrangement of the line very often make this principle uneconomical so, in a distance relay, instead of comparing the local line current with the current at the far end of the line, the relay compares the local current with the local voltage in the corresponding phase, or suitable components of them. For a fault at the far end of the line the local relay voltage will be the IZ drop of the line. It follows that the current to voltage ratio for a fault at the far end will be VII = Z where Z is the impedance of the line, fig. 5.1. For a fault internal to the protected section of line VII < Z. For a fault beyond the next section, VII> Z. Since Z is proportional to the line length between the relay and the fault it is also a measure of the distance to the fault; hence the term distance relay. Thus it can be seen that comparing the local current with the local voltage is an alternative to comparing it with the far end current. It is not as accurate, however, because the voltage changes gradually with the location of the fault whereas the far end current reverses for a fault beyond the C.t. at the far end of the line, thus providing an abrupt discontinuity which makes selectivity easy and automatic. On the other hand, we shall see that the distance relay has other advantages which outweigh this consideration, such as back-up protection and the elimination of the pilot channel (47). Meanwhile-in order to measure the same distance on all faults involving more than one phase (fig. 5.tb) the distance relay compares the potential between the two faulted phases with the vectorial difference of their currents, e.g. for a b-c fault the relay measures VbcI = Z1> the positive sequence Ib-

e

impedance of the line between the relay and the fault (57). 191

5.1

Protective Relays

Intc;'n~ fa.ult

(a) Ja.~

vab{ :

lb-

RCIa.y setting

Externa.l fa.ult

a.-------

Fault

~

c

(b) FIG.

(c)

S.l. Principle of impedance distance measurement (a) Relay trips when V < IZI (b) Interphase fault (c) Phase-to-ground fault

Similarly, for phase to ground faults, the relay measures the impedance of a similar kind of loop, this time along the faulted line conductor to the fault and back via the ground return path to the neutral of the system. Hence for a phase c-to-ground fault the relay measures

Yc-..

Ie-I..

= Zl but,

since the current I" in the ground return path is inaccessible, the relay is given the equivalent current which is a function of the C.t. residual current and the phase c relay measures I

e

-~I

res

which is also Z1'

The mathematical justification for this is given in the Appendix of this chapter, section 5.10.1. Tables 5.12 and 5.13 summarise in the left-hand column, the currents and voltages supplied to each phase and ground relay and the measurements that result during different kinds of faults (58). (a) Impedance. The earliest distance relays were designed to operate in a time proportional to the impedance between the relay and the fault, and hence to the distance to the fault, irrespective of fault current magnitude. This was an improvement on the inverse time overcurrent relay whose operating time was considerably affected by the generating conditions. A typical induction type relay had contacts held open by a potential magnet whose pull was roughly proportional to voltage. The current magnet exerted on the induction disc a torque roughly proportional to current. The disc torque was opposed to the potential magnet pull through a spiral spring 192

5.1

Distance Relays

(fig. 5.2a). The time taken by the current magnet to wind up the spring until it dislodged the potential magnet causing tripping was thus roughly proportional to potential and roughly inversely proportional to current. The operating time was thus roughly proportional to

~ oc Z, I

see fig. 5.2b. These

relays were inaccurate for the simple reason that the torque of an induction

~.~~~_ Pctcnt.ia..t holdil"l9 fIIaqnc.t

(al

01

l s-+U-b-1 ------Sur-b-2---Sut-b-3------0,.ta+-nce

(b) 5.2. (a) Principle of time distance relay (b) Application of time distance relay

FIG.

relay tends to be proportional to [2 and the pull of the holding magnet to V 2 so that the time tended to be proportional to Z2 and no adequate compensating means could be found which would give a linear time/impedance characteristic. The actual characteristic was curved (see fig. 5.2b) and hence somewhat difficult to apply selectively. The first high-speed impedance relays (61) used a balanced beam construction (see fig. 5.3a). A potential magnet normally held the contacts open against the pull of a current magnet at the other end of the beam. The relay tripped when the current pull exceeded the potential pull, i.e. when J(J2 > V2 or when Z < .JK. The high speed of this unit made it subject to undesirable tripping by overreaching on offset waves and also liable to elliptical distortion of the impedance circle. Later models found solutions to these difficulties; the d.c. offset component was extracted from the current 193

5.1

Protective Relays

wave by a filter and the voltage restraint was smoothed to give a circular impedance characteristic by rectification or by phase-splitting (117). Three such units, one tripping directly and two others tripping through time-delay relays, produced the stepped time-distance characteristic of

/

e.ClIonc:c ",c lght

Stop aer e-

Voltag-c. COIl

(a)

FIG.

5.3. (a) Balanced beam impedance relay (b) The stepped time characteristic

fig. 5.3b, which gave faster clearing times than the time-distance relay of fig. 5.2. (b) Reactance. In 1928, the,author of this book designed an induction disc type distance relay for an American company, which had a reactive VA magnet instead of the usual potential restraining magnet. Slots in the disc caused the relay to balance at a position on a graduated scale proportional to VI sin cfJ1I 2 , i.e. to X. A three-step time-distance characteristic was obtained by connecting stationary contacts on the reactance scale to a timing unit, as shown in fig. 5.4a. The time-distance characteristic was stepped as 194

5.1

Distance Relays

in fig. 5.3b. The impedance (R - X) diagram for the three time zones is shown in fig. 5.4b. In 1934 the author designed the first high-speed reactance relay, using a 4-pole induction cup instead of an induction disc. Two opposite poles had

l~~~~ I

}o,.lIlftlt~1-

I

I

I

I

(a)

X

oI1m~s

___

~

__--\-Zone3 Contact

Q{- ---+-----''f------1<-.:....-------4- - - - --

Reactn.nco

unit

Contact P

- -- - f -Zonc 2 -- - --\-----1--1--------+ -'~-:Zoncl

(b) FIG.

5.4a. d.c. connections Qf early reactance relay

(b) Impedance diagram of early reactance relay

current windings and the other two had opposed current and potential windings; the torque was proportional to J(Kl- V sin tP) and the relay operated when V s~n tP < K, i.e. when X < K, the ohmic setting of the relay. Figs. 5.Sa, band c show the advantage of the reactance relay over the impedance relay in its immunity to the effect of fault resistance. Fig. S.Sb shows the ordinary directional unit characteristic DD' used with impedance relays. Fig. 5.4b shows the closed directional characteristic required for reactance relays to prevent them from tripping on load current. This feature 195

5.1

Protective Relays

was obtained by adding a voltage restraining magnet to an ordinary directional relay. At low voltage the characteristic still passes through the origin on an impedance diagram but as the voltage increases the current required to operate the unit 'increases which in turn increases the impedance value of x

0 L-------------~1

x x-----,+----T'----=.:~+---x ,

Rela.y setting

----~~--------------R (c) FIO. 5.5 (a) Additional voltage drop in fault resistance (b) Reduction of impedance relay reach by fault resistance (c) Reactance relay unaffected by fault resistance (d) Fault area and tripping area of reactance relay

pick-up so that the straight line VI characteristic is bent around into a circle as shown for the starting unit in fig. 5.4b. Another way of expressing this is to say that the unit balances when KVI cos (<jJ-e) = V 2 or when Z = K cos (<jJ-e) where <jJ is the angle between V and I and () is the value of <jJ for maximum operating torque. This gives the maximum value of Z when <jJ = e. (c) Admittance. The impedance relay was prevented from operating on faults on other feeders on the same bus by a simple directional relay with a straight line characteristic on an impedance diagram. The admittance relay is a combined directional-impedance relay which was developed by the author in 1932 for the protection of extremely long lines (45). In this relay a 4-pole induction cup unit had potential windings on two opposite poles and an opposed current and potential windings on the other 196

Distance Relays

5.1

two poles. The torque equation at balance was V{KI cos (cf>-9)- V} = 0 so that Z = K cos (cf>-9) which is the impedance characteristic shown in fig. 5.6a. The torque of this relay would theoretically be zero for a fault close enough to the bus to make the voltage zero. This was overcome by using a

• Arcing fa.u lt 10n&

R

I

I

,

I

I

I

(b) FIO.

5.6. (a) Mho characteristic (*faults in other phases)

(b) Effect of power swing on impedance and mho relays

resonant circuit in the polarising potential winding so that the voltage across this winding is maintained by 'memory action' long enough to ensure operation. This matter will be discussed further in section 5.1.3. (b). On extremely long lines the impedance measured by the relay during power swings can be as low as for a fault and may cause an impedance relay to trip undesirably. Fig. 5.6b shows how the mho relay is relatively immune to tripping on power swings, 5.1.2. Time Steps

The very fact that the relay measures the ratio VII means that its cut-off point is accurate only within the accuracy of the measurement by the relay. 197

5.1

Protective Relays

Five per cent accuracy would mean ± I mile on a 20-mile line. For this reason it is necessary to make the relay cut-off at a point 5 % to 25 % short of the end of the section, depending on the accuracy of the relay. Faults in the end zone must be cleared by a second distance measuring unit which will reach beyond the next bus and will have enough time delay to prevent it from tripping on a fault in the next section, which should normally be cleared by the relay in that section. This delay is not serious (0'25 second) with modern relays and breakers. This second zone of protection also provides back-up for the relay in the next section for faults close to the bus (fig. 5.3b). A third relay with still more delay will give complete back-up for all faults at all locations. Consequently, most modern relays are 3-step relays with three time-distance zones, as shown in fig. 5.3b. Such a relay provides fast selective tripping for faults over most of the protected section and close back-up protection for the next section. In continental Europe, one or two additional time steps are provided, making a total of five (63). The fourth step is controlled by the overcurrent or impedance fault detectors through the directional unit, and the fifth step by the fault detectors alone. In distance relays where the fault detectors are directional, only the fourth step is provided. 5.1.3. Distance Measurement Problems

The heart of the distance relay is the measuring unit, which compares the current and voltage in each of the phase-to-phase and phase-to-ground circuits. This unit must not only compare] with V accurately but also must ignore fault resistance and transient line conditions which may cause] and V temporarily to have incorrect values. This appears to be impossible in fast relays but it has been achieved, as will be explained in section 5.1.4. The various methods for measuring distance are discussed in the following sections and their circuitry is shown in Tables 5.9 and 5.10 (47). (a) Fault Resistance. One source of error is shown in fig. 5.5c, where the relay measures the voltage OB instead of OA because of the additional component contributed by the voltage drop in the fault itself, due to arcing at the fault or to a high tower footing resistance; this shortens the reach of the impedance relay from OA to OA' (fig. 5.5b). Fault resistance has two components, the resistance of the arc (54) and the resistance of the ground (55); in a fault between phases only the arc is involved. Fault arc resistance is given by the Warrington formula (41) R arc = 87501 ]1-4

(5.1)

where 1 is the length of the arc in feet in still air and] is the fault current. I will initially be equal to the conductor spacing but it will increase in the presence of a cross wind (which generally accompanies a lightning storm) because the arc has no inertia. A 15 m.p.h. wind, for instance, will increase I up to 11 ft. each side in t second (see figs. 5.7a and 5.7b). For this reason it is 198

5.1

Distance Relays Wind

l+

JN~""""'" Conduc tor

Arc

(a)

(b)

(c) FIG. 5.7. Stretching of arc by wind (a) With cross wind. (b) With wind along line (c) Typical power arc

generally assumed that arc resistance will have little effect on the accuracy of the Zone 1 unit of a high-speed impedance or mho relay (except on very short lines) but a fault at the end of the section (Zone 2) may not clear if the time setting is too long (see section 5.3.1 (b)). Where time is involved the formula becomes

(5.2) R arc = 8750(S+3ut) 1 1 '4 where S is the conductor spacing and u is the wind velocity in miles per hour and t the duration in seconds. 199

5.1

Protective Relays

The formula allows for a certain amount of deionisation of the arc due to the cooling effect of the wind, in addition to the lengthening of the arc (fig. 5.7a). This arc formula has been confirmed by tests in Russia, France and the U.S.A. Lower values have been claimed on various occasions but each one investigated has used improper methods of starting the arc or improper electrodes. The electrodes should be smooth and the arc should be started by a fuse consisting of the finest iron wire that will support its own weight. Lead or copper fuses form a metallic vapour which gives an arc resistance which is much lower than that of an arc started by lightning, a van de Graaff generator or the fine iron wire (41). The most effective way of preventing the fault resistance from making the distance relay underreach is to design the measuring unit to measure the reactance rather than the impedance of the faulted circuit. Reactance relays are therefore used for short lines where the fault resistance may be comparable with that of the protected section and also for ground faults where the ground resistance may be very high. There is considerable confusion concerning the effect of double end fed arcing faults (48). It has been erroneously stated by some manufacturers that there can be considerable error in a reactance relay due to the fictitious reactive component of the arc impedance during double end feeds of different phase angle. The reactance error is Yare sin a where IA is the current fed in from one fA

end and a is the angle between that current and the total fault current (see fig. 5.8b). The larger the current IB fed in from the other end, the more a approaches the angle between the two currents and the greater the fictitious reactance but, on the other hand, (a) a+ f3 is a small angle because it is the angle between the bus voltages at the ends of the protected section, and (b) the larger f B is the smaller the arc voltage is because it decreases as the 1·4 power with current magnitude. For example, if Yare is 4500 volts on a 30 mile 132 kV line, IA = 600 Oh fi .. . 4500 sin 50 h Th 600 0·290 m. e amperes an d a = 5 , t e ctItlOUS reactance IS reactance of the line is about 18 ohms and the error is therefore 1·6 %which is negligible compared with the errors in impedance relays due to arc resistance. (b) Direction. Impedance type distance relays generally contain a directional unit which prevents them from operating on faults behind the bus (characteristic DD' in fig. 5.5b). The reactance relay requires a directional unit with a voltage restraint which gives the circular characteristic shown in fig. 5.4b and prevents the reactance relay from tripping on overloads which represent a vector near the R axis. Admittance (mho) type distance relays are inherently directional because their RjX characteristic passes through the origin (fig. 5.6). 200

5.1

Distance Relays

Since the operating torque of a directional or a mho relay is the vector product VI cos (cP - 8) it must become small and unreliable at low voltages which may occur for faults very close to the relay. The three possible solutions are (a) ultra-sensitivity, (b) memory action, (c) polarisation with potential from an unfaulted phase. On overhead lines a flashover, even at the bus, will always provide at least 3 % of normal voltage because of the arc resistance which, at the minimum value of 400 volts per ft. and a minimum spacing of 0·08 ft. per kV, gives 32 x

Error

(c)

A

I

8

1A=:t

+~

1=IB

I

If

(a)

L-----------------------

R

S.S. Spurious reactance due to arc during double-end feed (a) Fault location. (b) Impedance diagram. (c) Current phase relation FIG.

volts per kV or 3'2%. The spacing may, however, be momentarily less due to a bird flying into the line so a directional sensitivity of I % of normal volts is usually provided. Any lower value would be beyond the accuracy of the p.ts. On cables, due to the smaller spacing, the minimum fault voltage can be as low as 0·1 % but these faults are invariably to the grounded sheath so that the ground relay can be polarized by zero sequence potential or current or alternatively part of the zero sequence potential (which has a negative value) can be subtracted from the faulted phase potential. Memory action has been successfully used but sometimes causes inaccurate measurement in high-speed relays in faults that involve another phase during operation. Polarisation with potential from another phase is popular in Europe but it reduces the accuracy of the distance measurement. Neither of these methods is effective when closing in on a line with a solid three-phase fault caused by leaving ground clamps on after maintenance if the p.ts. are on the line side of the circuit-breaker. In such a case a high-set instaneous overcurrent relay can be used to open the breaker. Alternatively, the mho relay can be given a slight current 201

5.1

Protective Relays

bias to do the same thing. In cases where the minimum fault current is comparable with maximum load current the instantaneous overcurrent relays can be monitored by an 'a' switch on the breaker and can clear the fault through an auxiliary relay which has 4-cycle delay in pick-up. The circuit is shown in fig. 5.9. When closing in on a fault, the trip circuit is closed for 4 cycles and thereafter is open-circuited to prevent tripping on overcurrent during normal conditions. The ideal scheme would appear to have the following features: (a) Employ the fault voltage for directional polarisation as well as

restraint, but add to the polarising voltage 5 %of the voltage from one of the sound phases, shifted in angle to be in phase with the fault voltage during fault conditions.

-

O.C

-

O.C.

-

O.C.

~

a.

b

e

-~ po~

Trip

4'" P;ck.up

+ FIO.

5.9. Circuit for clearing close-in faults

(b) Provide memory action of at least I cycle duration, and an auxiliary

relay which seals-in the contacts of the directional fault detector for 4 cyles after it has operated. (c) Provide an overcurrent relay in each phase with contacts in series with an 'a' auxiliary switch on the breaker for tripping only during the 4 cycles after reclosure (fig. 5.9).

(c) Overloads and Power Swings. The impedance measured or 'seen' by a distance relay during normal load is shown in fig. 5.6b. Normally this would be outside the tripping zone of the distance relay but, on a very long line where the length of the line in miles exceeds the system kV, the circular impedance characteristic may have to be made so large as to involve the point L. Furthermore, as the load increases L moves towards the relay characteristic in the direction of the arrow and, during a power swing, it may oscillate up to a point such as P.S. where it may enter the tripping zone of the relay, even on a medium length line. To overcome this the admittance (mho) relay was developed (45) (51), which is sensitive only to a component of current at about the same phase 202

Distance Relays

5.1

angle as that of the protected line, so that it is insensitive to high powerfactor current conditions such as loads and power swings. The characteristic of the mho relay is shown in fig. 5.6. It can be shown that the mho relay will not trip on any overload or power swing from which the power system can recover without going out-of-step. Another virtue of the mho relay is that its characteristic fits so snugly around the fault area (fig. 5.6a) that it will not operate for faults in other phases (marked x), and of course it is inherently directional. (See Appendix 5.10.3). This subject has been dealt with much more fully in reference (58) and in Volume II. 5.1.4. Transient Conditions

(a) At Fault Inception. At the inception of a fault the sudden change of impedance due to a short-circuit causes the currents and voltages to go through some abnormal values before settling down at the correct fault values. If the X to R ratio of the primary circuit is high the current may have a temporary d.c. component which offsets the normal sine wave long enough to affect the relay. The potential may have superimposed on it (a) transient oscillations due to resonance between the line capacitance and the leakage reactance of potential transformers, (b) a d.c. offset transient due to trapped flux decay in a power transformer or the system reactance, (c) parasitic oscillations in the relay itself where phase shifting circuits are used. The relay current can contain (a) a d.c. offset, (b) a transient due to differences in the time constants of the primary and secondary circuits of the c.t. All these transient conditions can affect the current aqd voltage supplied to the relay so that it may overreach and trip for a fault beyond its first zone setting if the relay operating time is less than the time for them to expire, which is usually the case. An analysis of these transients is given in Volume II. In a homogeneous system it is possible to cancel out the effect of an offset fault current with the help of an impedance which is the replica of the impedance of the protected line section on a secondary basis. If the replica impedance is used in the current circuit, the voltage across the replica is compared with the line voltage, which is the voltage drop between the relay and the fault. For a fault at the far end of the protected section the two voltages should be equal for both transient and steady state conditions. If the replica impedance is connected in the potential circuit, the current through it is compared with the c.t. secondary current; the two currents are equal for a fault at the end of this section. Where the source impedance is more lagging than the line impedance, the transient response of the line potential will not be the same as that of the current and the currents or voltages compared in the relay will not be matched for a fault at the end of the protected section, so that a fast relay can over-

203

Protective Relays

5.1

reach if the fault current is considerably offset. In practice this is prevented by the slower operation of the relay near cut-off and the shorter time constant of the less lagging lines. The current offset in the relay coil circuit can be eliminated by a transient shunt (fig. 5.11) consisting of a very high Q reactor in parallel with the c.t. and small resistance in series with the relay coil. This shunt effectively passes ------~------ ~

----~

'A.pllca. ""pedant' PotcntiaJ coil Currcfllt co jl Rcla.y

FIG.

5.10. Use of replica impedance in potential circuit

the d.c. component. As more sensitive relays become available, WIth lower burdens. the ordinary iron-cored c.t. will be. replaced by linear couplers which minimise the d.c. component and hence eliminate this source of transient error. Meanwhile, such a shunt is necessary with impedance relays supplied from iron-cored c.t's. Primary voltage transients can be blocked by a filter which permits only the system frequency to reach the relay potential coils. Transient voltages

FIG.

5.11. Transient shunt for bypassing d.c. component

within the relay can be suppressed by proper damping of any phase-shifting circuits containing Land C and, where possible, by tuning such circuits to be resonant at system frequency to avoid temporary phase shifts. relative to other circuits, which may cause undesirable tripping. (b) Likelihood of Transients in Faults. The instantaneous fault current can be expressed as

sin(wt+I/J-cf» Emax sin (I/J-cf» -!!, .jR2+(WL)2 + .jR2+(wL)2 . e L

Emax

i=

(5.3)

where cf> is the phase angle of the primary circuit (tan -1~). I/J is the time after voltage zero at which the fault occurs and t is the time after inception of the fault.

204

5.1

Distance Relays

From the second term it can be seen that (a) the duration of the offset increases with L/R, (b) the maximum value of the offset occurs when

if! = ~ + cP (i.e. at current maximum), and

(c) that it is zero when

if! = cP

(i.e. at current zero); see fig. 5.12. Since flashovers occur when the voltage approaches the maximum value, i.e. when rot = ~, the offset is usually small because, on lines where it can be appreciable, cP tends towards ~ and hence rot tends towards cP, the condition for zero offset.

~

cp

y I

/

V

irf- cp HI I

/

~

~

'" " /

~ -.....

V

\

'" \

t=O

FIG. (

\

• •

I

'\

1\

~'i',-:..

V

\

\

\

r\

\.

5.12. Conditions controlling d.c. offset .I.'

lrurumum at I/f = '1': maxunum at I/f =

.I.

'I'

+ 11:2"

The reason why flashovers normally occur near maximum voltage is because, on overhead lines, there are two main causes for flashover, (a) induction from lightning discharges, and (b) conductors swinging together. In (a) the line potential is raised by induction until the sum of the induced potential plus wye voltage exceeds the line insulation level, which obviously will occur first on the phase nearest to voltage maximum; in (b) the rate of change of sinusoidal voltage is much faster -than the rate of reduction of insulation tan ABC as the conductors swing together, so that the phase pair with highest voltage flashes over first. The only time when a flashover can occur cP radians later than voltage 205

5.1

Protective Relays

maximum, i.e. when", = ~ + cp, is on closing a breaker in a solid fault such as when grounding clamps have been left on. Even in this case the rate of reduction of insulation due to the speed of contact approach must exceed

~'

1\

\

",iiV'"

~l\

VmQ••- 107·5

L A

r--

c FIO.

1

~r- t-

~r.--I4>

1/

L

V

~, r---..,

~ r-- r- ~ 1f1z-

r-

I

r-

B

'"

~, ~

5.13. Likelihood of maximum offset also controlled by breaker speed, i.e. by slope of A'B

the rate of change of voltage in the quarter cycle after voltage maximum (tan A'BC), i.e. it must exceed

:t (V max sin rot) = - V maxro cos rot = - V maxro cos(cp + ~)

../2

On a 132 kV system Vmax = 132 ../'3 = 107·5 kV and

,'. '!:: =

cp = 86°

107.211: . 50 sin 86° = 33700 kV /sec. (see fig. 5.13).

Although such speeds are attainable in some high-speed breakers, there are no recorded cases known to the author of overreach from this cause. Automatic oscillograph records from U.S.A. on actual faults show very few faults starting at more than 45° from voltage maximum and very few cases of overreaching by modem distance relays of other than the balanced beam type. Complete treatment of the subject should include the effect of load current flowing prior to the fault. The offset in the fault current will depend upon the ins~taneous value of the load current at the moment the fault occurs; hence load current can add or subtract from the offset component and change the point of wave at which the maximum offset occurs. The greatest effect is when the current prior to the fault is about 90° leading the 206

5.1

Distance Relays

potential and is large compared with the fault. Since neither of these conditions can occur to an appreciable extent in practice the effect of current flowing prior to the fault is generally to reduce the offset component. (c) Transients on Loss of Potential. On a radial line, if a breaker is opened between the relay and the power source, the line voltage will not immediately go to zero but will decay exponentially due to the stored energy in the capacitance of the line and will also have some low frequency oscillations due to resonance between the line capacitance and the self-inductance of the potential transformers. The energy discharged through the relay is very considerable in this case because, viewed from the relay terminals, the line capacitance is of the order of one farad for a 30-mile 132 kV line. Where capacitors are used in phase-shifting circuits in the relay the currents in the restraining and polarising potential circuits will be affected differently, causing erroneous transient response. For this reason the induction cup mho relay is most affected and may trip wrongly upon loss of primary voltage. A reactance relay using an amplitude +

\1"

]'"

t

yL..1o_,c___..... yIT..,.2_ _--'y IT3

I

Trip FIG.

5.14, Zone 1 monitored by instantaneous overcurrent unit

comparator would also be affected. On the other hand a phase comparator reactance relay, such as an induction cup reactance relay, would not be affected because it has only one potential circuit; nor would a mho relay using an amplitude comparator such as a rectifier bridge (see Chapter 2). The remedy for this condition is to monitor the instantaneous (Zone 1) unit by an instantaneous overcurrent unit, fig. 5.14. It is not necessary to monitor the time delay zones because they are not subject to this transient tripping and also because a third zone fault may not draw enough current to operate the instantaneous unit. However, the instantaneous unit must have a drop-out current value not less than 90 %of its pick-up current in order to reset on load current. The instantaneous overcurrent unit also prevents undesirable tripping on loss of secondary potential (blown fuse or bad contact at fuse clip, etc.) in most cases, but it is advisable also to provide a fuse-failure relay which not 207

5.2

Protective Relays

only opens the trip circuit when a fuse blows but also sounds an alarm, calling attention to the fact. An alternative to the overcurrent relay is the rate-ofrise-of-current relay which can be used where fault currents are extremely low. Both these relays are described in Chapter 12, section 12.4.4. Where capacitance p.ts are used no remedy is necessary because (a) the tuned circuit of the device excludes most of the d.c. component, (b) it is customary to provide a I! cycles delay to prevent wrong tripping on transient oscillations between the capacitance of the potential device and the inductive reactance in the primary circuit such as power transformers .. (d) Summary. Overreaching by distance relays for electrical causes (not mechanical causes such as shaft resilience and catapult action) can give very little trouble in service because (a) the fault must be initiated near zero on the voltage wave, (b) it must be located just outside the protected section; this combination of circumstances very seldom occurs. On lines of above 132 kV, supplied by very large generator-transformer units, the XI R ratio of lines can exceed 7 and that of the source can exceed 20 so that there is a risk of transient overreach due to offset fault current waves, especially when closing in with very fast breakers on a solid fault with only a short line between the fault and the source, giving a high ZslZL ratio. For this reason the latest distance relays are designed for the elimination of current and voltage transient components and parasitic oscillations so that they will not overreach appreciably even on a fault initiated at voltage zero in a circuit of

i

> 20. The same precautions will generally be effective

to prevent tripping on loss of primary or secondary potential but a series overcurrent relay and a fuse-failure relay are necessary for complete protection because the blowing of one fuse may result in a distorted set of potentials which may look like a fault to the relay in one of the phases. 5.2. SPECIAL DISTANCE RELAY CHARACTERISTICS (47)

The following are various ways in which the standard impedance, admittance and reactance characteristics can be modified to have more tolerance to fault resistance and less susceptibility to power swings. 5.2.1. MocHfied Impedance

The standard impedance characteristic can be moved outwards along the R axis by current biassing the potential circuit with the Ir drop across a resistor so that it has more tolerance for fault resistance, as shown in fig. 5.15. The maximum permissible offset is when the circle nearly passes through the origin, because the torque then approaches zero for a fault close to the relay. This scheme is used in Europe for lines up to 50 kV (62). In the U.S.A. the arc tolerance was increased by biassing in the - X direction as well as the R direction, so that a bigger impedance circle resulted whlch tended towards a reactance characteristic (134).

208

Distance Relays

5.2

Introduction of the Ir bias changes the equation for balance of the amplitude comparator to IKW-IV+1rI2 = 0 K212_ V2-12r2-2Vlr cos¢ = o. or Dividing through by - [2 we get:

Z2+2Zr cos¢-K2+r2 = 0

(5.4)

which can be shown to be a circle passing through the origin with its centre on the R axis and r from the origin if r = K ohms. In the phase comparator this method would be unecolJ.omical; better results can be obtained from a mho relay, making () = o.

FIG.

5.15. Modified impedance characteristic

Such schemes are not as effective or accurate as the reactance relay mentioned in section 5.1.3 (a) and they also have the disadvantage that the more bias is given the more liable is the relay to trip on power swings (fig. 5.15). 5.2.2. Admittance and Conductance

By swivelling the mho relay characteristic in the leading direction (clockwise on the impedance diagram) its tolerance to fault resistance can be increased with less vulnerability to power swings than in the modified impedance relay (fig. 5.16). In the limiting position along the R axis it becomes a conductance relay. This has also been done in Europe but it is applicable X only to medium voltage lines up to tan -1 R < 60° because the accuracy of the distance measurement goes off rapidly on lines more than 60° lagging, as the mho circle becomes almost tangential to the line impedance vector ZL' In the conductance relay there is no change in the balance equation from the mho relay, H

~ () = K, except that () is now 0 instead of the normal cos( - ) 209

5.2

Protective Relays

value of 600 but the ohmic setting of the relay has to be multiplied by

Z,J.. cos 'f'

so that the impedance cycle will still pass through the ohmic value Z/4>. Chapter 3 explains how the mho characteristic can also be obtained by )(

R

x

R

(c) FIG.

5.16. Phase angle biassing of mho relay (IP - 8) to increase tolerance to fault resistance (a) 20°. (b) 40°. (c) 55°

comparing the phase relations of { I cos (4) - 0) -

IZp + I -

parator or by comparing the moduli Vp

VI and Vin a phase comv,. and IVp - I + Vrl in an Zr Zp Zr

amplitude comparator where the suffix p refers to polarising and the suffix r to restraining. 5.2.3. Reactance and Angle Impedance (Z/j)

There is unfortunately no standard term for a relay which measures a component of impedance somewhere between resistance and reactance, but such relays are used and have been called angle-impedance relays. A-ISo impedance relay has been used in the U.S.A. as a 'blinder' to prevent distance relays from tripping on very severe power swings on long lines, fig. 5.17. In rare cases a heavy power feed from the other end of a protected line 210

Distance Relays

5.2

can make a reactance relay measure a small negative component of reactance in the resistance of a fault; overreaching in this case can be prevented by moving the reactance characteristic slightly leading (clockwise) so that

Line Impcdancc-7

'---_ _-LJ

R

---

----

power swing locus

FIG.

5.17. Use of ohm unit as blinder

x

Modified

rC
characteristic ___

Ra.rc

--------+---------~~~---R

----

FIG.

5.18. Modified reactance characteristic

it measures impedance at 85° lag instead of 90° lag (see section 5.1.3 (a), para. 8. This is illustrated in fig. 5.18, where the effect has been considerably exaggerated. The balance equation of the ZjljJ relay is the same as for the Z/90° or X relay, viz. Z cos (cjJ-O) = -K. It was explained in Chapter 3 that this is obtained in the amplitude comparator by comparing

1:'1:::;: II - ;.1 or,

in

the phase comparator, by the phase relation between I and I-V cos (cjJ - 0). 5.2.4. Offset Mho

Unlike the Zone I characteristic, the Zone 2 and Zone 3 characteristics need not pass through the origin of the R-X diagram. By offsetting the third zone characteristic to overlap the origin it can also be used for power swing blocking, as will be explained in section 5.4.6. 211

5.2

Protective Relays

The offset characteristic can also be reversed for starting carrier in the carrier-blocking scheme (see Chapter 7, section 7.3.1.) In this case memory action (see section 5.1.3.2) is used to make the unit operate quickly for lowvoltage faults near the relay bus and the offset increases the steady torque. The offset is caused by biassing the mho relay to have torque on current alone. The introduction of this current torque puts the [(]2 term back into the equation of the mho unit, thus making it the general equation, K[2-K'V2+ VI cos (t/>-(J)

=0

The offset of the impedance circle is secured by adding to the line potential a biassing potential, IZb , proportional to the current, which has the effect of moving the characteristic impedance circle bodily by an amount Zb. Substituting V + IZb for V in the equation for the mho relay, we get -K'(V +IZb)2+(V +IZb)1 cos (t/>-(J) = 0

Dividing through by 12 , the equation becomes -K'(Z+Zb)2 + (Z+Zb) cos (t/>-(J) = 0

or

Z = cos (t/>-(J) - Zb

(5.5)

K'

This shows the characteristic circle is the same as before except moved through an impedance Zb (see fig. 5.19b). Fig. 5.19c shows how the offset of the mho unit is obtained by introducing, in series with the supply potential, a biassing potential that is obtained from a reactor in the current circuit. Every point on the impedance characteristic of the unit thereby is moved through an impedanCe equal to that of the reactor. In order to reduce the burden imposed on the current transformers by the reactor, it is provided with two windings. The primary winding has few turns and is in the current circuit; the secondary has many turns and is in the potential circuit. It is thus a combination of a transformer and a reactor and is called a transactor. The bodily shifting of the characteristic circle necessitates resetting the ohmic reach. This can be avoided by applying the bias to the polarising circuit only, which has the effect of keeping the ohmic setting fixed and moving only the part normally going through the origin. It is obvious that an offset mho circle enclosing the origin can be obtained equally well by shifting the mho circle towards the origin or shifting the impedance circle in the opposite direction. In the amplitude comparator of the circulating current rectifier bridge type, it is easier to shift the impedance circle than the mho circle, this can be done by introducing current bias into the potential circuit so that the relay operates when

I;, -kII

<

III where Zr is

the replica impedance in the potential circuit. Multiplying both sides by

~' the relay operates when IZ - kZ, I < IZ, Iwhich is a circle on the impedance diagram with its centre offset by Zr from the origin.

212

5.2

Distance Relays x

(a)

Rela.y

(c) FIG. 5.19. Offset mho characteristic (a) Diameter increased. (b) Circle shifted (c) Transactor used for circle (b)

------.--------------r-----------o+ B

Timing unit

RI.I.tor

Trip FIG.

5.20. Out-of-step blocking circuit for mho relay

213

5.3

Protective Relays

5.3. LIMITATIONS OF OHM AND MHO UNITS

In addition to difficulties in measurement caused by fault resistance, power swings and power infeeds there is severe limitation imposed by the fact that the relay may have to operate over a 30 to 1 range of potential which, in an electromagnetic relay, may result in a 900 to 1 variation in torque with consequent design difficulties. 5.3.1. Minimum Length of Line

When a fault occurs the current increases and the potential decreases from normal. Since the current and the potential balance at the cut-off point it follows that the potential torque cannot decrease below a certain limit without impairing the accuracy of the measurement. Since the potential is proportional to the length of the line to the fault the shortest line that can be protected by the distance relay is a function of the minimum voltage down to which the relay can remain accurate. It is also limited by fault resistance which may be comparable with the impedance of a short line, as explained in section 5.3.1 (b) of this chapter. (a) Limitations Due to Relay Sensitivity. This can be expressed as a percentage of normal voltage or conversely as the ratio of Zs/ZL where Z. is the impedance from the relay to the power source and ZL is the impedance of the protected line section. The reactance relay can operate down to a lower voltage than the mho relay because it is polarised by current whereas the mho relay is potential polarised. Induction cup mho relays can usually measure within ± 5 % accuracy down to 8 volts or Zs/ZL = 14, which is sufficient for all normal overhead lines. Modem induction cup reactance relays measure accurately down to 3 volts or Zs/ZL = 37 and are immune to fault resistance. Replacing the induction cup unit by a rectifier bridge circuit feeding a very sensitive polarised relay increases the voltage range, because of its limiting action, and enables a mho relay to be designed with a Zs/ZL ratio of 30 or more which can be used to protect cable feeders where fault resistance is generally very low. However, on most overhead lines fault resistance prevents the use of a ratio higher than 12 (see section 5.3.1 (b) ). It is difficult to make rectifier bridge relays as accurate as the induction cup relay without resorting to rather complicated compensation by nonlinear resistors. This is because the rectifier bridge circuit is very sensitive to burden and even the small burden of a d.c. polarised moving coil relay upsets it. The European solution is to use a contact-making micro ammeter as the relay because its burden is extremely low. A better solution is to use a more robust relay and to reduce the burden on the rectifier bridge by interposing a transistor amplifier. The minimum length of line can also be estimated directly in miles as follows: If V is the minimum secondary voltage for accurate measurement, therefore 2IZ = VRp in primary volts (phase-to-phase) where I is the minimum 214

Distance Relays

5.3

fault current, Z is the minimum ohmic length of line that can be protected and Rp is the potential transformer ratio, 2 / 0.9 0.631 ...

= V. k V. 10 3

115

I =7V kV' 1 T mles

(5.6)

where 1 is the minimum length of line, 0·63 ohm is the impedance per line mile, 0·9 is the fraction of the protected section covered by Zone 1 and kV is the system kilovolts (47). Assuming V = 8 volts for mho and impedance relays and 3 volts for reactance relays, both relays being of the induction cup type, kV from equation (5.6) I = 56 . / miles corresponding to Z./ZL = 14 (mho and impedance)

= 21

k;

(5.7)

miles corresponding to Z./ZL = 37 (5.8) (reactance) On a 60-cycle line the ohms per mile are 1·2 times that of the 50-cycle line so that proportionately shorter lines can be protected. (b) Limitations Due to Arc Resistance and Economics. It so happens that the limiting values of Z,/ZL = 14 for mho relays and 37 for reactance relays due to relay performance tend to agree with limitations imposed by fault resistance on overhead lines. In ground faults the resistance of the fault path through the tower, the tower footing resistance and the earth return are unpredictable (55) (142) so that, in most countries, reactance relays are used automatically for ground faults. Exceptions are cables, very long overhead lines and lines in localities where there is a low ground resistance and excellent ground return arrangements (copper ground wires, etc.). In phase faults, where arc resistance only is involved (sectioI). 5.1.3 (a) ), or ground faults where the resistance of the ground fault path is low, the fault . . pred'lctable. I n still'" h I 'IS the resistance IS arr It IS 8750 /1'4 0 h ms per " loot were current in the arc. (i) EFFECT OF ARC RFSISTANCE ON ZONE 1 FAULTS. Arc resistance shortens

. . JX 2 +(R+R arc)2 2 2 where X and the reach of an lffipedance relay by the ratio X +R R are the reactance and the resistance of the line between the relay and the fault, see fig. 5.2Ia. The effect on a mho relay is slightly greater because of its smaller impedance circle but, by making the mho characteristic angle 0 less lagging than cp, the line impedance angle (tan -1 ~), the effect can be reduced to less than that on the impedance relay (see fig. 5.16) but, for the sake of simplicity, they will be assumed equal. 215

5.3

Protective Relays

Fig. 5.21b shows the error due to arc resistance for various secondary values of X and I, assuming 5 ampere c.t. secondaries. The values of X must be multiplied by 5 and the values of I divided by 5 for 1 ampere c.t.S. The

(al 50

40

10 5

OL---~--~2~--=3----4~--=5--~6~--7~--~8~~9~--1~0 Secondary ohm rca.ctanc:c FIG.

S.21. Effect offault resistance on impedance relay reach (a) Impedance vectors. (b) Relay error

values of X are those of the fault loop, i.e. twice the positive sequence or phase to neutral values. The values of R are also secondary values. The %error is = 100 JX 2+(R+R arc)2 (5.9) Il X2+R2 If X

= 2R, i.e. tan- 1 XIi = 63° lag, the %error is Il = 100 216

J

26'4 1 + (IX)2

4·6

+ IX

5.3

Distance Relays

based on an assumed arc resistance of 500 volts per foot of arc length, i.e. 5 % of system voltage. The error can be much greater at lower currents where the arc voltage exceeds 500 volts per foot or during a cross wind. This applies obviously to overhead lines and not to cables. Fig. 5.21b shows that fault conditions involving not more than 20% of system voltage at the relay (Z./ZL > 5) should be checked for the effect of arc resistance. In general, if the arc resistance exceeds one third of the im-

'-----(a)

x

r--.------'"

Zone 3

outside eire'"

R

(b) FIG. 5.22. Effect of arc resistance on Zone 1 (a) Upon Zone 1 reach. (b) Upon Zone 2 reach

pedance of the protected section (see fig. 5.22a), the reach of a relay set to cover 85 % of the section can shrink to 60 % of the section so that only faults in the middle 20 % can be cleared instantaneously from both ends. It is further supported by the facts that 132 kV overhead lines are seldom less than 10 miles long and the minimum short-circuit power is seldom less than 200 mVA, which gives Z,/ZL = 12. Similar minimum values for a 275 kV system are 25 miles and 500 mVA which gives Z./ZL = 11·5. 217

Protective Relays

5.3

In the rare cases where Zs/ZL > 12, reactance relays are required in any case to avoid the error due to arc resistance. Cables have lower impedance than overhead lines so that ZL values are smaller but the Zs values are also smaller for the same reason and because Z. tends to be smaller on lower voltage systems. Fault resistance adds to the impedance measured by a mho relay and hence shortens its reach. (ii) EFFECT OF ARC RESISTANCE ON ZONE 2. A small reduction of the instantaneous zone due to fault resistance is acceptable, but the intermediate zone always must reach beyond the next bus, that is, faults within the section must be cleared in Zone 2 time or less and not in Zone 3 time, or selectivity between stations will not be possible. In fig. 5.22b RII is the arc resistance, ZL is the impedance of the protected section, t/J is its phase angle (tan -1 X/ R) and K is the fraction of ZL which the second mho unit Y2 reaches beyond the end of the section. It is assumed that the mho unit characteristic is given the same angle t/J as the line. The circle in fig. 5.22b is the characteristic of a mho unit for the second distance step and is assumed to reach a short distance KZL beyond the end of the protected section ZL' It is to be noted that a2 + b2 = diameter 2 and hence (R!+K 2Zi-2KZLR Il cos r!J) + (R!+zi+2Z LR Il cost/J) = Zi(1+K)2 :.

2R!-2KZi+2RaZL(1-K) cost/J = 0

RII = ~L[J{cos2¢(1-K)2+4K}-(1-K) cost/JJ

= K'ZL

(5.10)

which equals maximum arc resistance to permit Zone 2 time, or less, for all faults within the section. A similar formula covers the reduction in reach of Zone 1 from 80 % or 90 % to the point where insufficient faults are cleared instantaneously from both ends. If the adjacent line sections are approximately equal Y2 will be set for 50% beyond the next bus, i.e. K = 0·5. Assuming ¢ = 60°, then, from the formula (1), RII = 0·6Z or ZL = 1·68R. If the adjacent line section is shorter the Zone 2 setting will be reduced to perhaps 20% beyond the next bus, i.e. K = 0·2 which gives RII = 0'29ZL or ZL = 3·45Ra. Since we are considering Zone 2, the arc resistance can easily treble its initial flashover value, given by the above formula, if there is much wind (section 5.1.3 (a» because of its stretching and deionising effect. Because of this and because line sections are seldom equal, it is better to take the value of K = 0·2 to be on the safe side so that Ra > 0'29ZL (maximum permissible value). I. l' • '11' Now (5.1) Ra = 8750 ~ lor an arc lD s11 air 218

5.3

Distance Relays where Is is the conductor spacing,

1=

and for a phase-to-phase fault. :.

kV.10 3 2(Zs+ZL +R,,)

(5.11)

)1.4

z +Z +R Actual R" = 1·481. ( • k~ "

(5.12)

(iii) EFFECT OF R" ON MAXIMUM PERMISSmLE Zs/ZL RATIO. The values of actual and permissible R" were calculated (see Table 5·1) for low values of system mVA and line length and the results were plotted to find at what values of Zs/ZL the actual R" exceeded the permissible R". The limiting values of Z./ZL varied from 2 at 11 kV to 12 at 275 kV (see Table 5.2). By making the angle of the mho characteristic less lagging than the line, the tolerance for arc resistance is increased as shown in the right-hand column of Table 5.1. At 275 kV Zs/ZL can exceed 12 under certain conditions without exceeding the permissible value of R" but this is not recommended because 275 kV lines are usually very important and it must be remembered that (a) the arc formula was based on still air and a cross wind could cause much higher values of R" and (b) the relay measures higher values of R" on a three-phase fault. On the other hand these values of ZS/ZL were based on assumed conductor spacings and high values could be tolerated where smaller spacings are used. Furthermore, a calculated risk could be taken that a fault would not occur close to the end of the protected section at the time of minimum generation. The columns headed 'permissible tR,,' are given with and without swivelling the circle to a less lagging angle. tR" is taken instead of R" because the relay measures ZL +tR" in a phase-to-phase fault. The actual values of Zs/ZL that are permissible depend upon the many factors indicated in the table and should be worked out for any particular application where Zs/ZL > 12. In cases where mho relays are not applicable (because of arc resistance) reactance relays should be used. The foregoing applies to single-phase faults in which only arc resistance is involved but, in the case of single-phase ground faults, a much more conservative approach is necessary because the current path includes the tower footing and the earth return in parallel with any ground wires in addition to the arc. Hence, the resistance to single-phase ground faults is liable to be very high indeed in certain localities and it is advisable to check any application with Zs/ZL > 2 or preferably to use reactance relays for ground faults. The formulae used for calculating the various columns in Table 5.1 are as follows: kV2 Z=-(5.14) MVA 219

5.3

Protective Relays

z +Z )1.4

Actual !Ra = 0·741. ( ·kV

L

(5.15)

(see Table 5.1). In equation (5.15) the Ra term was omitted from inside the brackets of equation (5.12) because, for maximum values of Z., Ra is negligible especially as it is added almost at 90°. Permissible !Ra = K'ZL where K' is calculated from equation (5.10). The application of mho relays for interphase faults is limited to the Z./ZL value at which the actual arc resistance exceeds the resistance derived from equation (5.12). i.e. when

0.741 S

(ZS+ZL)1.4 > K'Z kV

1

(5.16)

This can be found graphically by plotting the two expressions and noting the value of Z./ZL for which the two curves intersect or it can be calculated from Z kV(KZ (5.17) Z::} ZL 1.48t

)0.7

The minimum length of line that can be protected by a mho relay without loss of selectivity due to arc resistance can be deduced roughly from the preceding formulae. Because this is the minimum length of line, the adjacent line sections must be at least as long, consequently K can be taken as 0·5 and ZL = 1·68Ra•

8750

kV

kV

= 1·68 ]1-4 • 12.J3 = 715 ]1-4 for a 60° line assuming 12 kV per foot spacing. Assuming the line impedance to be 0·7 ohm per mile, this gives the minimum length as: 1010 :.~ miles for mho relays

(5.18)

A simpler formula is produced if the arc resistance is assumed to have a 440 kV . fixed value of 440 volts per foot, so that Ra = - . r assummg 12 kV per ] 12y3 . kV. kV foot spacmg so that Ra = 21.21 . From thIS Zl = 1·68Ra = 35·5 I for mho relays. Assuming 0·7 ohm per mile for the line impedance, this gives the minimum line length for mho relays as 50 k V miles. However, this value cannot be I applied to ground relays and should be checked against the other method for borderline cases, the safest method being to use reactance relays for ground faults. Accepting the values based on still air, the distance relay should be designed to be accurate for Z./ZL ratios of 30 for reactance relays and 12 for

220

Distance Relays

5.3

5.1

TABLE

ARC Resistance in Phase Faults

-~I~ ... ~

"I

----r:-

Permissible tRa

~

...:: '
Line

Source System

MVA

Zs

Miles

ZL

0

Actual (} - 75° (} = 60° Z./ZL tRa K'= 0·35 K'= 0·5

5000 15-15 2000 37·78 275 kV 1000 75·75 500 151·5 '" = 75° 15·15 L. = 24' 5000 2000 37-78 1000 75·75 500 151'5

10 10 10 10 20 20 20 20

5·3 5·3 5-3 5·3 10·6 10·6 10·6 10·6

2-86 7-13 14·28 28'6 1·43 3'56 7-14 14'3

0·42 1·22 2-96

2000 1000 132kV 500 250 '" = 75° L. = 12' 2000 1000 500 250

5 5 5 5 10 10 10 10

3-5 3·5 3·5 3·5 7·0 7·0 7·0 7·0

2·49 4·97 9·95 19·90 1'24 2-49 4·97 9·95

0·35 0·67 1·57 3-89 0·46 0·84 1'77 4-19

33kV '" = 60° L.=6'

11 kV '" =45° L, = 2·5'

750 500 250 100 50 750 500 250 100 50

750 500 250 100 50 750 500 250 100 50

8·70 17041 34-81 69'62 8·70 17·41 34'81 69·62

1'45 2-18 4·36 10·9 21·8 1·45 2-18 4·36 10·9 21·8

0·16 0·24 0·48 1·21 2-42 0·16 0·24 0·48 1·21 2'42

1·5 1·5 1·5 1·5 1'5 3 3 3 3 3

0'5 0·5 0'5 0·5 0·5 1 1 1 1 1

1'27 1·27 1·27 1'27 1·27 2·55 2'55 2·55 2·55 2'55

0·42 0·42 0·42 0·42 0·42 0·85 0·85 0'85 0·85 0·85

Without Temp. Rise

1-14 1·71 3-42 8·55 17-1 0·57 0·85 1·72 4·27 8·55

With Temp. Rise 75° (} = 60° K'= 0·35 K'= 0'5

(} =

1'86

2'66

1'6

2-16

3·72

5'31

3·2

4'3

1·23

1·75

1-06

1'42

2-46

3'51

2·12

2-84

7-44

0'59 1'44 3·21 7-89

0·12 0·19 ·0·37 1·1 2·69 0·24 0·29 0'5 1·25 2·91

0'38 0·57 1·14 2-85 5-69 0'19 0·28 0'57 1·42 2-84

221

0·02 0·04 0·05 0·12 0·27 0'06 0·07 0·10 0·16 0·34

(} = 60° (} = 30° K'= 0·29 K'= 0'56

() =

60° (} = 30°

0·37

0·71

0·25

0'5

0·74

1·43

0'50

1'01

() = 45° () = 30° () = 45° K'= 0·23 K'= 0·40

() = 30°

0·10

0'17

0·043

0·10

0·19

0·34

0·07

0·20

Protective Relays

5.3

mho relays, although the permissible value of Z.lZL for a particular application may be much less. Consequently, reactance relays should be used for short lines, i.e. less than 12 miles at 66 kV, 35 miles at 132 kV and 50 miles at 275 kV. Reactance relays should also be used for ground faults on any 5.2 Maximum Z./ZLfor MHO Relay Permitted by Arc Resistance TABLE

SystemkV

.p

Line Length (Miles)

()=.p

275 275 132 132 33 33 11 11

75° 75° 75° 75° 60° 60° 45° 45°

20 10 10 5 3 1·5 1·0 0·5

7-95 10'0 6·45 8·15 2·62 3-45 1·74 2'25

Z./ZL Limit

.p

()'

10·6 13·2 8·65 ]0·85 4·8 6·05 2-85 3·7

60° 60° 60° 60° 30° 30° 30° 30°

()'<

-----_.-

length of line because the fault resistance includes ground and tower footing resistance as"Well as arc resistance. Exceptions can be made where there are excellent ground wires or where the ground resistance is known to be low. Another factor which reduces the permissible source/line _ratio is the increase in conductor resistance due to temperature. In overhead lines, unlike iron-cored electrical equipment, the effective a.c. resistance is only 10 to 15 % more than the d.c. resistance and hence is directly affected by temperature. Most overhead lines are designed for a maximum temperature of 85°C during maximum load in the summer (B.S. 159) with permissible conductor sag and the conductor resistance is given at 20°C. The increase in d.c. resis. 65 x 100 tance from 20°C to 85°C IS 234'5 = 27'7% The corresponding increase in a.c. resistance is 19·4 %. 5.3 Permissible Zs/ZL Ratio Including Temperature Effect TABLE

Z,/ZLLimit 8' <

SystemkV

.p

Line Length (Miles)

()=.p

275 275 132 132 33 33

75° 75° 75° 75° 60° 60° 45° 45°

20 10 10 5 3 1·5 1·0 0'5

7·0 8·8 5·75 7·25 1'75 2·4 0'25 0-8

11 11

222

.p

9·0 11·2 7·25 9·2 3'5 4'45 1'8 2'25

f}'

60° 60° 60° 60° 30° 30° 30° 30°

Distance Relays

5.3

This increase in conductor resistance must be subtracted from the permissible value of -iRa in Table 5.1, which will reduce the permissible value of Z./ZL· In Table 5.1 the resistance values of permissible -iRa w!11 have to be reduced by 19'4% of ZL cos cp. For example, in the 10 mile 275 kV line at the top of the Table, the permissible !Ra values will be reduced by 0·194 x 5·3 cos 75° = 0·266 ohm so that they become 1·59 for K' = 0·35 and 2·39 for K' = 0·5. In Table 5.2 this reduces the permissible Z./ZL values for a 275 kV 10 mile line to 8·8 for 9 = 75° and 11·2 for 9 = 60°. Table 5.3 is similar to Table 5.2 except with the increase in conductor resistance/temperature taken into account. 5.3.2. Choice of Measuring Unit

The factors affecting the choice are Z./ZL ratio, fault resistance and economy. From the previous section it was clear that reactance relays should be used for short lines but, owing to the high resistance component often found in ground faults (41) (55), the reactance type measuring unit has also become the standard preference in most countries for ground distance relays. With cables, it is the sensitivity of the relay rather than the fault resistance that determines the minimum length of cable that can be protected (63) (117). The minimum length of cable that can be protected by mho and reactance relays was given in section 5.3.1 (a) in the formulae (5.7) and (5.8). The conductance relay (fig. 5.16c) is a specialised type promoted by a German company (63) (see section 5.2.2), more tolerant to fault resistance than mho or impedance units but more economical than the reactance unit because it needs no directional unit. It is applicable to distribution lines both overhead and cable. Distance relays are used for both phase and ground faults on resistancegrounded distribution lines and on important high-voltage interconnections. On solidly grounded systems adeq~ate protection can be obtained from directional inverse time-overcurrent relays with instantaneous overcurrent attachments. The cost of one time-overcurrent relay in the residual circuit of the line c.t's is only one tenth of the cost of three ground distance relays and their auxiliaries. The occasional slower clearing time with overcurrent relays is unimportant because single-phase ground faults have negligible effect on system stability. Furthermore, the direct trip of a high-set instantaneous overcurrent unit is faster than the tripping of ground distance relays through blocking auxiliaries. Where there is a solid ground at each substation the selectivity is excellent because the zero sequence current in the faulted section is mostly supplied from the ground neutrals at the two ends of the section; hence the zero sequence current in the faulted section is therefore much higher than in the adjoining line sections and discrimination is sharp so that most ground faults 223

Protective Relays

5.3

can be cleared by the instantaneous overcurrent unit provided it is of the type described in Chapter 4, with negligible transient overreach. 5.3.3. Maximum Length of Line

There is no maximum limit to the length of a transmission line section that can be protected by mho relays because, if the phase angle of the mho characteristic is the same as that of the impedance the latter becomes a diameter of the circular mho characteristic. The terminal voltages during a power swing severe enough to trip the mho relay would have to be 90° apart (fig. 5.23) which means that the generator e.mJ. would have to be more

FlO.

5.23. Mho relay stable on power swings

than 90° apart and hence the power system would be unstable. Conversely it can be argued that, within the limits of stability, no line section could be long enough to require a Zone 1 mho relay setting capable of causing tripping on overload or power swing conditions. Reactance, conductance and impeddnce relays however have impedance characteristics which can be crossed more easily during power swing conditions (42). It can be estimated that the longest line section to which a reactance or conductance relay should be applied is 500

k;

k;

miles where

I = minimum fault current. The corresponding figure for an impedance

relay can be estimated at about 1000

miles, with no upper limit for the

mho relay. 5.3.4. Effect of Faults on Relays in Unfaulted Phases

With high ohmic settings that may be necessary on mho units for back-up on long lines, it is always possible for wrong flagging or even wrong tripping 224

Distance Relays

5.3

to occur due to the effect of load, charging current or zero sequence current on the relays in the unfaulted phases. Similar improper operation may occur in reactance relays where the starting unit has a very high ohmic setting in order to permit the reactance unit to operate on an extremely high resistance fault. Where such conditions occur it is advisable to make vector diagrams similar to those given in reference (58) so as to determine whether a calculated risk of wrong operation can be taken or whether relay operation for extreme conditions must be sacrificed. This subject is analysed in an A.LE.E. paper by the author (58) and is further considered in the Appendix 5.10.3 of this chapter and in more detail in Vol. II of this book. In the case of a short section of a long transmission line of high X/ R ratio, the charging current may cause inaccurate reactance measurement in a relay near the power source. This can occur in the c-a relay during a b - c fault because the phase c will have fault current and the phase a charging current, so that the relay current fe - fa will tend to become in phase with the voltage Vea (fig. 5.25b) and the c-a relay will tend to measure zero distance. This can be prevented by overcurrent units in the lagging of the two phases associated with the reactance unit. In section 5.5.9, it is shown that the same arrangement of overcurrent units can be used to block ground reactance relays from wrong tripping on interphase faults. An alternative for short lines is to adjust the reactance unit to measure impedance at 80° leading the current instead of 90°. Wrong operation can also occur in a reactance relay protecting a short line section on a long transmission line where the potential triangle has collapsed and the relay is located between the power source and the neutral grounding point. For example, during a b - c fault the starting unit of the a-b relay may be operated by lagging zero sequence and/or load current in phase a (fig. 5.25c) and the ohm unit current fa - fb can tend to be in phase with the potential Vab causing overreaching. Fortunately, this condition is difficult to produce and can usually be remedied by either making the 10 compensation K < t in the starting unit or by making the ohm unit measure Z/80°. Reactance relays which have starting units using phase-to-neutral potential can operate on faults in the reverse direction, unless the currents in restraining and polarising windings of the starting unit have a phase relation such that a slight restraint is produced during complete collapse of the potential triangle where the two voltages are in phase. Finally, wrong tripping can occur due to a race between the starting and ohm unit contacts at the inception or clearing of a fault, due to the fact that the reactance unit may measure negligible or even negative reactance during load and may not open its contacts before the starting unit contacts close on a Zone 2 fault. This trouble can be avoided at the inception of the fault by connecting 225

5.4

Protective Relays

the starting unit to control the torque of the ohm unit (directional control) but this adds time delay during tripping; a better method is to use very little wipe on the contacts of the two units and to prevent contact bounce by other means (discussed in Chapter 2), such as a contact back-stop of glyptal succinate or vinyl-acetate chloride co-polymer. If the ohm unit has too little travel, the contact race may be lost in the other direction. When a fault occurs in the neighbouring section of line the starting unit closes and the ohm unit does not at first close. If the fault is promptly cleared by the relays in the faulted section, load will be restored and the starting unit will reset while the ohm unit may close its contacts. Unless the starting unit can open its contacts before the ohm unit contacts close, wrong tripping will occur. Normally, this race is always won by the starting unit and no undesirable tripping occurs, but it is possible for a starting unit to have been wrongly adjusted so that its contacts have too much wipe, while the ohm unit may have too little travel. To avoid this, an auxiliary contact T can be provided on the timing unit which opens the Zone 1 trip circuit four cycles after the starting unit operates, by which time the ohm unit should have tripped if it is a Zone 1 fault, and the circuit is now set up for faults in Zones 2 or 3 only (fig. 5.29). If a second fault should occur in Zone 1 of the protected section another relay, S, will reclose the Zone 1 trip circuit a cycle after the ohm unit closes its contacts, i.e. tripping will occur for a subsequent Zone I fault but not for a load condition that may cause the starting unit and the ohm unit contacts to be closed simultaneously for an instant while taking up their normal load positions. 5.4. APPLICATION OF DISTANCE RELAYS

Distance relays can be used to protect almost any type of equipment or circuit so that they have been growing in popularity steadily since their introduction about 1920. 5.4.1. Single Step

Sometimes instantaneous overcurrent relays cannot be used for reducing the fault clearing times in time-overcurrent protection of feeders (see Chapter 4) because of the shortness of the line sections or because of the wide variations of generating capacity and hence fault current. In such cases a single step impedance or mho unit provides the ideal solution; it is simple, reliable and unaffected by fault current variation; the modern mho unit also is inherently directional and insensitive to power swings and offset current waves. Fig. 5.24 shows that the overall time-distance characteristic, using instantaneous overcurrent units with existing time-overcurrent relays, is comparable with that of the much more expensive three-step distance relay, shown dotted.

226

5.4

Distance Relays I I I

Time

I I I I I

_____ 1

y

t:=:===::::::.---!--------+----Dista.ncc Sub.l

Sub.3

Sub.2

5.24. Mho with time-current relay back-up (3-step distance characteristic shown dotted)

FIG.

5.4.2. Directional Time-Overcurrent Relay

Conventional time-overcurrent relays will not clear faults during periods of minimum generation if the minimum short-circuit current is less than their setting. On the other hand, their setting must be above the maximum load current with maximum generation, unless the settings of the overcurrent relays are changed between maximum and minimum generating conditions. The substitution of a mho unit for the conventional directional unit solves this difficulty by permitting overcurrent relay settings of less than normal load to be used all the time. In other words, if the directional relay has a voltage restraint it becomes a mho unit and will not operate nor, with the usual directional control arrangement, permit the overcurrent unit to operate unless the impedance presented to the relay falls sufficiently to indicate a fault on the line, whatever the setting of the overcurrent relay. A widely used type of directional overcurrent relay has a 4-pole induction cylinder directional unit with alternate potential and current coils around the cylinder. It is necessary only to replace one of the current coils with a potential coil to provide a V2 restraining torque, while the remaining coils provide the VI cos (cP - lJ) directional torque and thus change the directional unit into a mho unit. An overcurrent relay with this voltage restrained directional unit can be provided with current taps from 20 %to 80 %of c. t. rating on the overcurrent unit instead of the usual 50 % to 200 % taps. The more sensitive overcurrent settings permit operation on low-current faults and faster operation can be obtained on heavy faults. 5.4.3. Multi-step Distance Relays (50) (51) (53) (63) (117)

The conventional distance rel~y consists of a directional starting unit, a timing unit and one or more distance measuring units. An instantaneous step, and two time-delay steps are usually provided, the three steps being controlled in distance reach either by three distance measuring units or by one unit whose distance reach is increased in steps by the timing unit, progressively increasing its ohmic reach.

227

5.4

Protective Relays

Crou pola.riaing

tra.nla.ctor ~

(a)

\VQb

r"

Ie

-r,'{

lc

IQ -If!.

Ib

Ib

VeQ

(c)

(b)

5.25. Errors in reach of quadrature polarised mho relay (a) a.c. connections of distance relay for b-c phase pair (b) Effect of charging current on c - a relay (c) Effect of load and/or 10 on a - b relay

FIG.

FIG.

5.26. Basic d.c. circuit of 3-step impedance relay

Tripping times for

brea.ker A

72 7; A 7; T2

---~-------------

FIG.

Tripping times for

brea.kcr B

5.27. 3-step time-distance characteristics

228

5.4

Distance Relays

The first zone unit is used for instantaneous faults up to 10 % from the remote end of the protected section. The second zone unit clears faults in the neighbourhood of the next bus in a delayed time, and the third zone provides back-up protection for the relays and breaker in the next section beyond. The overall characteristic is shown in fig. 5.27 (a) Mho Distance Protection. Fig. 5.28a shows the basic d.c. connection for one phase of a three-step mho relay having separate mho units for each phase and time-step. Series flags indicate the time-step in which the relay has Seal in (S.I.) and phase flag

1

'I

II

5.1.

I

lr------------jo+

1

1

Y, I

OY31

I

T31

I

O.C

To similar circuits ~in the other two phases

T

Trip alarm

L------oTrip

(a)

j~IF-ox---~~---F-o~~I,-F~ob~FO-'---O+ 15.1.

Trip

(b)

5.28. Basic d.c. circuit of 3-step mho relay (a) With series flags. (b) With shunt flags

FIG.

tripped. Instantaneous overcurrent units (O.C.) prevent the relays from tripping on accidental loss of potential. A typical modern three-step mho relay has two mho units per phase (Fig. 5.28b), one of which is used for Zones 1 and 2 and the other, an offset mho unit, is used as a fault detector (F.D.). No directional unit is necessary because a mho unit is inherently directional; this fact simplifies the circuit and reduces the number of contacts in the trip circuit. In Fig. 5.28b a rate-ofrise relay (R.R.) is used instead of overcurrent units and shunt-connected flags are shown. The third step mho unit, OY 3' uses a transactor to offset its impedance characteristic so that it overlaps the origin, as shown in fig. 5.19a. If polarised in the same direction as the first and second steps, Y 1 and Y 2, the OY 3 229

5.4

Protective Relays

unit can be used for out-of-step detection as well as for time delay back-up protection of lines in which the electrical centre of the system is located. Alternatively the OY3 can be reversed in direction so that it can provide improved back-up protection (see section 5.5.3) and can at the same time start the carrier signal (Chapter 7) when used for directional carrier protection. (b) Reactance Distance Protection. As explained previously (5.1.3 (a» the directional unit of a reactance relay must be of the mho type in order to prevent it from tripping on loads which may have negligible or negative reactance. This directional unit is generally known as a starting unit or fault r-----------------~O+

1 1

5eaJ-in

5.1.1 5.1·1

~

Alarm

Timer

Trip FIG.

5.29_ Basic d.c. circuit of 3-step reactance relay

detector. Fig. 5.29 shows the basic d.c. connections for one phase of a threestep reactance relay. In modem 3-step reactance relays one pair of contacts of the starting unit are in series with those of the reactance unit of the same phase and the other controls a timing unit (fig. 5.28). The normal setting of the reactance unit controls the first zone reach. If the fault is beyond the reach of the first zone the timing unit changes the ohmic setting of the reactance unit after a time delay so that it can reach past the next bus. If the fault is too far away for the second step the timer again resets, the reactance unit providing a third timedistance step. In some reactance relays the third step is provided by the starting unit through the timer because, although the distance measurement of the starting unit varies with the type of fault, it is sufficiently accurate for the third step because it varies in any case due to the effect of power infeeds. For instance, in fig. 5.36 the distance relay at A measures too much impedance for a fault at F because the voltage drop in section BF is increased by the current fed in at D which does not appear in the relay A. On the other hand, where fault resistance may be high, a third reactance step is provided instead of relying on the starting unit which has less tolerance to fault resistance. This latter arrangement is the most flexible and effective one.

230

5.4

Distance Relays 5.4.4. Extended First Zone

Where instantaneous automatic reciosing of the breaker is employed the distance relay can be arranged to trip instantaneously all faults within the protected line section. This is done by setting the instantaneous zone for 105 to 110% of the impedance of the section instead of 85% or 90%. The 105 % setting is used for relays having an abrupt cut-off to their Zone I reach. With sections of equal length this gives a 10% chance of tripping for a fault in the next section but the likelihood of this occurring is reduced by any power infeed at the intervening bus. Occasionally therefore (fig. 5.30) a fault

B -=- F

A

flO.

C

5.30. Extension of Zone 1 reach to end of section

in the next section Be, close to the bus B, would cause breakers at both A and B to trip. To preserve service continuity the tripping of breaker A operates a relay which resets the Zone 1 ohm unit from 110% to 90% of section AB so that, when reclosing occurs, if the fault is permanent, only breaker B operates the second time. In other words instantaneous clearing of all faults in the protected section is obtained for the price of a very occasional extra trip and reclose of the breaker and the momentary loss of voltage on one substation bus. Fig. 5.31 shows how the main d.c. connections of a distance relay can be modified to achieve this result merely by adding the auxiliary relay RX which is controlled by an auxiliary switch 'b' on the circuit breaker which opens when the breaker closes. The auxiliary relay RX has instantaneous

+

I

yb

Va \

II T2

IIr

11o,

\IRX

Ib

5

F2 a

T.C.

flO.

T

OX

5.31. Basic d.c. circuit for 100% Zone 1 coverage

231

RX

+

Protective Relays

5.4

pick-up and 1- second reset time to permit only the relay in the faulted section to clear the fault when the breaker recloses. The Zone I jZone 2 transfer relay OX has contacts in the a.c. circuit which increase the ohmic reach. 5.4.5. Bus Protection

Where a bus section is separated from other circuits by reactors, as shown in fig. 5.32 reactance ohm units have been used in the U.S.A. to provide protection for the generators and for the bus. Each generator has a reactance ohm unit set to reach into, but not beyond, any of the bus-tied feeder reactors. Feeders

FIG.

5.32. Bus protection with reactance units

A fault in a generator will be cleared immediately because its ohm unit will measure reactance in the reversed direction. A fault on the bus will disconnect all the generators because it will be within the ohmic settings of their ohm units (47). Tripping is delayed 0·5 second in order to permit the line relays to clear a fault in a feeder reactor. The main difficulty of conventional differential bus protection is the balancing of the current transformers in all the circuits around the bus to ensure that there is no differential current to operate the relay during an external fault. This difficulty does not appear with the ohm units because they are connected only to the one set of current transformers in the generator leads. This form of protection has also been used for bus-tie reactors. Each reactor is protected by two reactance ohm units (fig. 5.33) with their contacts in series. The ohm units are polarised away from the reactor so that a fault in the reactor is in the reverse direction for both of them and causes them to trip and isolate the reactor. Both ohm units are set to operate up to half way through the feeder reactors so that a fault on the bus causes both ohm units to operate, one because it is within its ohmic setting and the other because the fault appears to it in the reverse direction (47).

232

5.4

Distance Relays Bus-tie reactor

Brea.ker C.T.

C.T. Breaker Trip

r=:-+-----+------t------i--'~_coil

o---t----+ Rtla.ctance

rela.ys

FIG.

5.33. Bus-tie reactor protection

An alternative that has been used in the U.K. is two mho relays each set to reach 75 %through the bus-tie reactor and connected so as either can trip both breakers. This arrangement will clear faults in the reactor only, and is applicable to stations without feeder reactors. 5.4.6. Out-of-Step Blocking (42) (58)

When a short-circuit occurs for which the relay should operate, its voltage, current, and the phase angle between them, instantly change from their normal values to the value capable of operating the relay (fig. 5.l9a). But, during a power swing, the voltage, current and phase angle change more slowly from values incapable of operating the relay to the necessary operating values. This fact enables tripping to be blocked on power swings without holding up tripping under fault conditions. The out-of-step condition is detected by making the OY3 impedance characteristic concentric with the Y 2 characteristic, as shown in fig. 19, and somewhat larger, so that the changing impedance seen by the relays during a power swing will always operate the OY 3 unit before the Y 2 unit. The OY3 unit then is arranged (fig. 5.20) to pick up a blocking relay B with a small time delay; blocking is prevented on faults because the Y 2 unit de-energises the blocking relay on drop-out if the OY 3 does not operate first by the margin of the pick-up time of the auxiliary relay. The auxiliary relay is generally connected to prevent automatic reclosing after a trip on out-of-step conditions but can also be used to prevent tripping. It should be remembered, however, that this is necessary only if the relay characteristic circle includes the electrical centre of the system, because a mho unit will not trip on out-ofstep unless its characteristic is out by the power swing locus. Zone 3 is not blocked because a fault may conceivably occur during a swing and must be cleared. A better alternative is to reset the blocking circuit by means of a negative sequence current relay. With high-speed distance relays the out-of-step blocking relay B is required in only one phase because a power swing is a three-phase condition and, even if caused by a single-phase fault, the fault should be cleared and the 233

5.4

Protective Relays

asymmetrical conditions gone in plenty of time to permit blocking of the swing that follows it. If the prevaience of lightning is such as to expect a fault during a power swing as a common occurrence, three blocking circuits like fig. 5.20 are needed and a connection to permit the removal of blocking from Zones 1 and 2 if one of the three fault detectors resets. 5.4.7. Blinders

One application of the ohm unit is-to extremely long lines where distance relays other than mho relays are liable to tripping on power swings because their impedance characteristic has to be large in order to cover the long line. In such a case the straight line characteristic of the ohm unit may be arranged in parallel with the line impedance vector so as to cut off one or both sides of the tripping characteristic (figs. 5.17 and 5.35) and confine the tripping zone to a strip wide enough to permit tripping on arc resistance. This scheme was the first attack on the problem of relaying long or heavily loaded lines (45) and is applicable to any form of relay including overcurrent relays. In most applications only one blinder is necessary because the major flow of power generally is in one direction. It is only on interconnections where the maximum power flows are the same in either direction that two blinders are necessary. 5.4.8. Out-of-Step Tripping (47) (56)

When synchronism is lost the impedance measured by a distance relay will progress through the line impedance vector on a locus from right to left if the relay is located at the leading or fast end of the system and from left to right if at the lagging end. Two blinders Oa and Ob are arranged to operate an auxiliary relay T a or Tb if the impedance crosses the characteristic of one of them before the other, in either direction. This auxiliary relay can be arranged to operate an alarm, trip a breaker, or initiate some form of control (fig. 5.34),

08

I

FIG.

Trip

5.34. Out-of-step tripping circuit

234

5.5

Distance Relays The advantages of this form of out-of-step relay are:

(i) Its operation is not affected by variation in the location of the electrical centre of the system. (ii) It will not trip under any fault conditions. (iii) It trips instantly after the first half cycle of system oscillation. Existing relays require the machine to slip several poles before they will operate. (iv) Distinction can be made between speeding up and slowing down of the local generation.

-R

R

-x FIG.

5.35. Out-of-step tripping characteristics

A single-phase relay is adequate for tripping on out-of-step because the regular protective relays will trip during a fault. If the swing continues after the faults have been cleared or if the swing was caused by switching of load or generation without a fault, only a single-phase out-of-step relay is required for tripping because all three phases act similarly. 5.5. THE SETTING OF DISTANCE RELAYS

The ohmic setting or reach of the relay may be controlled either from the current operating circuit or the potential restraining circuit or both. Since potential decreases from normal during a fault while the current increases, it follows that a high torque level can be obtained by tapping the current circuit and leaving the potential circuit alone. An elegant arrangement is to provide coarse taps in the current circuit and fine taps in the potential circuit (or a rheostat if the potential circuit is at unity p.f.). The following information is needed in order to determine distance relay settings for phase-to-phase and double-ground faults. (i) Voltage and frequency of the line. (ii) d.c. control voltage. (iii) Trip coil current at normal voltage.

(iv) c.t. and p.t. ratios, and connections.

235

5.5

Protective Relays

(v) State whether a.c. potential is supplied from the line or the bus side of the breaker and advise whether magnetic potential transformers, coupling capacitors or bushing potential devices will be used. (vi) Transient reactance and resistance of line (phase-to-neutral). If in percentage or per unit, give the kVA or MVA base. (vii) If potential supplied from low side of power transformer bank, give connections of transformer (Y-L\, Y-Y, etc.). (viii) Maximum three-phase short-circuit current for a fault at the near end of the protected section, and the minimum current for a phaseto-phase fault at the far end of the section, or complete information from which this data can be calculated. The following additional information is required for setting ground distance relay settings. The same information is required for setting directional residual time-overcurrent relays. (ix) Will ground relays be polarised by current transformers connected in neutral of power transformer or by Y-open-delta connected potential transformers 1 (x) Maximum phase and residual current for a single-phase ground fault at near end of the protected section, and the minimum current for a single-phase ground fault at the far end of the section (or positive and zero phase sequence diagram of the system). (xi) Ratio of zero to positive sequence reactance of protected section of line. (xii) Mutual coupling with any parallel lines. 5.5.1. Setting of Zones 1 and 2

The Zone 1 setting of the relay is 0·9Zntfnp where Z is the impedance of the protected section in primary phase-to-neutral (positive sequence) ohms, nl is the c.t. ratio, np is the p.t. ratio and 0·9 is the fraction of the section covered, assuming a modem relay with ± 5 % accuracy over the range of fault current expected. Since the relay slows down towards the cut-off point, Zone I should not be set to cut off at 90 % but to reach Zone 2 time at 90 %; otherwise Zone 2 may begin at 85 %. In the case of reactance relay, X is calculated for the setting instead of Z. Where a mho relay is used with a characteristic angle of the relay, e, not equal to the phase angle of the line (c/J = tan -1 ~), the ohmic setting must be increased by dividing it by cos (c/J - ()), see fig. 5.42. Overhead transmission lines have approximately the following phase angles on interphase faults. Ground fault phase angles depend upon the terrain (55). Cables are more complicated because their phase angle c/J depends upon the division of the fault current returning through the sheath and through the earth, assuming a ground fault because phase faults not involving the sheath

236

5.5

Distance Relays

TABLE 5.4 Phase Angle of Overhead Lines kV

11

50 cycles 60 cycles

45°

33

132

275

400

55°

70° 72°

75° 76°

81° 82°

60°

50°

are rare. For cables between 11 kV and 33 kV,

=

f(0·00466log 2:S + 0.000506) ohms/mile IO

where S is the equivalent. spacing of the conductors in feet and d is their diameter in inches. For a single-phase 2-wire line, S is the actual spacing. For a three-phase line S = ~ Sa"· S"c· Sca where Sa", S"c and Sca are the spacings between the conductors a, band c. Assuming these impedances to have been correctly calculated, the accuracy of the relay setting will depend upon the c.t. and p.t. performance. The real setting will be the calculated setting times the following factor: Rp cos (

237

5.5

Protective Relays

where Rp = Ratio correction factor of the potential transformer

Rc = Ratio correction factor of the current transformer fjJ

=

Power-factor angle of the transmission line

() = Phase-angle of the relay characteristic (}p = Phase-angle error of the potential transformer (positive

if

leading) (}c = Phase-angle error of the current transformer (positive if leading). The subject of c.t. and p.t. errors will be discussed in detail in Vol. II. Sometimes Z is known as a percentage impedance or reactance and the ohmic impedance is then calculated from

Z= 10kV2 • Z% kVA

(5.20)

where kVis the line-to-line system voltage in kV and kVA is the base of Z%. The values of line impedance so obtained must then be converted to secondary .. c.t. ratio o hms b y m u1tiplymg by . . p.t. ratio Zone 2 must not reach as far as the Zone 1 setting of the relay in the following line section (see fig. 5.3b), i.e. it must not be set to reach farther than 75 %of the next section. If there is an infeed Ip at the next bus this will reduce the reach of Zone 2, because the extra current will not be included in the relay current I R , although the extra voltage drop will increase the apparent ohms seen by the relay. In other words, the infeed at B of fig. 5.3 will decrease the Zone 2 reach of the relay at A from AD to AB, where AE = ZL

+ BD .IR IR+lp

This cannot ever cause serious trouble because however much infeed there is at the next section, it can never cause Zone 2 to fail to cover the next bus B, which is its chief duty. 5.5.2. Zone 3 Setting

The third zone is essentially back-up and, whereas Zones 1 and 2 are for preserving continuity of service, Zone 3 is for preventing the destruction of equipment and danger to personnel. Zone 3 is set to cover the whole of the neighbouring section and, whereas Zones 1 and 2 cannot overreach without upsetting selectivity, Zone 3 cannot underreach without giving inadequate back-up protection. Whereas Zones 1 and 2 are set for the actual impedance of the line sections, ignoring infeed, Zone 3 must be set for maximum infeed conditions. Zone 3 must be set for at least Z1 + WZ{ ohms, where Z1 is the impedance of the protected section, Z{ is the impedance of the next section and W is the

. . IR+IF ' the next section . to t h e maxnnum ratio - 0 f th e tot a currentl entenng IR

amount flowing through the protected section (see fig. 5.36).

238

5.5

Distance Relays

Sometimes this setting becomes high enough for Zone 3 to operate on overload conditions. This difficulty can be ameliorated by (a) using an elliptical R-X characteristic (fig. 5.37), by (b) reversing the direction of the

G

(a)

I I I

I I

J

---------_/ I>'

(b) FIO.

5.36. (a) Effect of power infeed on back-up setting

(b) Current distribution with a multi-terminal line

Zone 3 units, as explained in the following section 5.5.3, or by (c) using a rate-of-rise of current monitoring relay. Where it is impossible to provide back-up on Zone 3 without tripping on overload, some European relays have two more time steps with very long

FIO.

5.37. Elliptical characteristic for Zone 3, to prevent tripping on load current

time delays, one of non-directional overcurrent and then a final step controlled by a pure directional unit. 5.5.3. Reversed Third Zone

Referring to fig. 5.38, the relay at A normally provides the third zone back-up protection for the section Be but there is no reason why it should not be provided by the relay at B; in other words, by reversing all the third zone relay units so that they will cover the next section behind them instead of the

239

5.5

Protective Relays

next section in front of them. The same protection will be provided but the ohmic setting of each third zone unit will be reduced by the impedance of the protected section, e.g. the relay at A has to reach a distance Be which is less by AB and may thereby eliminate the risk of operating on overload. Ta.p line

Brea.ker fa.; led Trip on ba.Ck-UP\

/

Fa.ult

o

B

A

c

TQjJ lin~

FIG.

5.38. Reversed Zone 3 reach to maintain supply to tap lines

Furthermore, with back-up provided by the relay nearest the fault, tap lines on the unfaulted section AB will remain in service whereas they would be l~st without the conventional arrangement of back-up. On the other hand, the reversed back-up relay has the same a.c. and d.c. supplies as the relays it is backing up so that it may fail for the same cause. 5.5.4. Transformer Feeders

Where there is a transformer in series with the line between two buses, the ohmic setting of the distance relay includes the impedance of the transformer. At the end remote from the transformer this enables 100% of the line to be covered by Zone 1, i.e. instantaneous tripping for all line faults, provided that ZL < 1.lZe where ZL is the line impedance and Ze the transformer impedance (see fig. 5.40) but, at the transformer end, Zone 1 is reduced

~. WMre Zt > 2ZL it will be necessary to have the ZL+Zt potential transformers on the line side of the power transformer in order to obtain selectivity; this will enable the relay to measure distance along the line directly. In other words, when the c.t's or p.t's are not connected to the protected line but are on the other side of a power transformer, the ratio and phase shift of the power transformer must be taken into account. In order to obtain the overall ratios, the p.t. ratio must be multiplied by the power transformer ratio and the C.t. ratio must then be divided by the power transformer ratio in calculating the secondary reactance of the protected line. In the case of distance relays for interphase faults, the phase angle shift in wye-delta or delta-wye power transformer connections can be compensated for simply by choosing wye instead of delta quantities. This is because the

in reach by the ratio

240

Distance Relays

5.5

phase relay uses line-to-line potential and delta current to measure the ohms in line-to-line faults and, when these are viewed through a Y -Ll or Ll- Y transformer, the delta quantities look like wye quantities. In the case of ground distance relays, special zero sequence current compensation would be necessary because the zero sequence components of current and potential are blocked on the delta side of the power transformer. The current compensation for a Y - Ll or Ll- Y shift in the power transformer is very simple. A distance relay having Ia- Ib from line side c.t's (fa in one winding and - Ib in the other) would have - Ib in both windings in series from c.t's on the low side (Table 5.5). The proper ohmic setting is then obtained by taking a c.t. ratio of

I

times the actual c.t. ratio, where N

is the turn ratio of the power transformer. The turn ratio is the voltage ratio of the line-to-line ratio on the delta side to the line-to-neutral voltage on the wye side. The reason for taking only half the turn ratio is that the two coils are in series, which doubles the ampere-turns. The potential compensation for a Y - Ll or Ll- Y shift in the power transformer in the case of distance relays for interphase faults is similar to current compensation. Either the wye potential on the low-side is used or the equivalent of high-side potential by means of an auxiliary Ll- Y p. t. to compensate for a Y - Ll power transformer and vice versa. The second method is preferred because the wye potential is not affected by ground faults in the low tension system. No correction factor is necessary 'f h 11 .. 'd d high-side line-to-line potential 1 t e overa p. t. ratio IS consl ere as . relay volts Sometimes it is necessary to check the reach of Zones 2 and 3 through a transformer on the bus at the end of the protected section in order to coordinate their time settings with those on the lines (generally a distribution system) beyond the transformer bank. If the transformers are in wye-wye or delta-delta this will present no difficulty for phase relays since it is only necessary to add their reactance to that of the line. If the transformers are in Y - Ll or Ll- Y the phase-shift complicates matters and makes a tedious calculation necessary. The result of this calculation is given in Table 5.5. The reach of the ground relays terminates in the transformer bank if either of the windings through which the current passes is in delta, because the zero sequence currents circulate in the delta and do not emerge from it. In the case of wye-wye transformers their impedance to phase faults is taken as their phase-to-neutral or positive-sequence-reactance. Their impedance to gt;ound faults is the same for phase faults if the neutrals of both are grounded, but it is infinite if either side is not grounded. A wye-connected auto-transformer presents the same impedance to phase and ground faults if its neutral is grounded, but it presents a much higher impedance to ground faults if its neutral is not grounded, since its windings then act like series reactors. 241

~

b

3

STANDARD COfm'CTION

SECONDARY FAULT

RELAY LOCATION (PRIMAR.Y)

F

b

3

Fig. B

I agrou~bcl

a

2

"""-

Fig.C

I

3

~ --~---

,T

ar.;ound

)

h

2

ALTERNATIVE CONNECTION

SECONDAR.Y FAULT

RELAY LOCATION (PRIMARV)

Fig.D

--~- --

~.

-.

Ib 0

.... '"::0

UI

i.II

I

~

iii

Ib

Ib

cl ....a"'tI

:5

I r.grOUnd I

n. b

2

Figs. A and B are the standard power transfonner connections with the l.t. side or secondary 30 deg.lagging the h.t. side or primary. Figs. C and D are the opposite connections which make the secondary 30 deg. leading the primary. LEGEND: R = Fault resistance. p, n, 0 subscripts denote positive, negative and zero sequence components. C = Fraction of the positive and negative sequence fault currents that flows through the relay. Z~ = Impedance of line and transfonner between the relay and the fault. Rn is the total negative sequence resistance between the fault and the ends of the system in parallel. Ro is the corresponding zero sequence resistance. Note 1: The ground relay measures the same whether it has zero sequence current compensation or not. Note 2: The reactance terms are always positive, but the resistance terms, due to the phase shift, can create negative reactance terms in some phases, thus causing the relay to overreach. In such cases, however, the starting unit pick-up is raised so as to prevent operation.

Fig.A

2

5.5 Effect of Grounded Neutral 11- Yor Y-11 Power Transformer Between Distance Relay and A.rcing Fault TABLE

5.5

Distance Relays TABLE

Protection

Sec. Fault

5.5 (conI.)

Relay Excitation Fig. A Fig. C Secondary &B &D

Zmeasured

Phase

be

V12 h-lz

lz-I)

~

Vb 10

Z' +R.. +R, l' Y3C

Phase

be

V23 lz-I)

V31 I)-h

V. I.

Z'1 ' - R .. +.R, Y3C

Phase

be

I)-h

~

VI2 h-12

Va la

00

Phase

a-Grd

VI2 h -12

V23 lz-I)

Vb Ib

Z'p

Phase

a-Grd

Phase

+

21CZo

+ ;[(2R.. + Ro + 3R,)

12- I)

I)-h

~

V.. I..

Z'P+2C 1 Zo - Y3(2 2C R.. +Ro+3R,)

a-Grd

V31 I)-II

VI2 h -/2

Va la

Z'p

Ground

be

VI II

V2

Ground

be

V2 lz

Ground

be

Ground

~

V.. b la-lb

z'p - C V3 (R .. + R,)

I)

Vb. Ib-i.

Z'p

I)

VI II

V... I. -Ia

Z'p

+ ~3 (R .. + R,)

a-Grd

VI II

V2 lz

Va. la -Ib

Z'p

+ Zo + 2R.. + Ro + R,

Ground

a-Grd

V2 12

V3 I)

Vb. h-l.

00

Ground

a-Grd

VI 11

Veil le-Ia

Z'l'

V3

V3 I)

lz V3

2C

+Zo 2C

2Y3C

2R..

+ Ro -

R,

2Y3C

5.5.5. Transformer Voltage Drop Compensators

When h.t. potential is not available, impedance distance relays may be supplied by potential transformers on the l.t. bus. Where the power transformer is part of the line the relay can be set to include the transformer impedance but, where it is connected to the bus and supplies other circuits as well, the current producing the transformer voltage drop is not the line current in the relay. For this case transformer drop compensators should be used

243

Protective Relays

5.5

(see fig. 5.39). They are supplied with power transformer current and produce a drop which is added vectorially to the Lt. potential to obtain the correct h.t. potential for the distance relays. The compensators used are transactors, i.e. reactors energised by current transformers in either the high or the low side, and equipped with secondary windings which add vectorially a potential proportional to the reactance of the power transformer and the current flowing through it. lt is unnecessary to provide a resistance unit because the XI R ratio of a transactor is similar to that of a power transformer. The secondary winding of the transactor not only avoids sneak circuits between the c.t. and p.t. circuits but reduces the burden on the c.t's to a minimum because the

FIG.

5.39. Transformer drop compensator

effective impedance of the transactor is its secondary voltage divided by its primary current and the turns can be arranged to step up the secondary voltage. On a 5 ampere basis the windings are arranged to give an equivalent impedance up to 3 ohms in 1 % steps by providing the secondary with nine 10% taps and ten 1 % taps, as shown in fig. 5.41. lt is customary to slightly undercompensate because overcompensation would make the distance relay have a small reversed voltage for a fault close to the bus; this would prevent the relay in the faulted line from operating and could cause wrong tripping on other lines. Normally one would not use a transformer drop compensator for a transformer feeder because the distance relay could be set as though the transformer impedance were part of the line impedance but, if the line is short and the transformer small, the error in measurement could warrant a compensator. On the other hand, the compensator adds its error to the total. If 8, is the per unit relay error, L is the line impedance and T is the transformer impedance, the error in measurement without the compensator is 8,(L+T) = 81' With the compensator the total error is the compensator error 8e plus the relay error 8eT+8,(L+8 eT) = 82 (see fig. 5.40). 244

5.5

Distance Relays Sta.tHion 1

(a)

(b)

g ~g

--------11

Sta.tion 2

r-T---*+(-----L--+l-·7~·1 Error = (R( T +L)

(c)

IEC~~~-L---'7~'1 Error =ER(L +EcT)

FlO. 5.40. Error introduced by inaccurate transformer drop compensator (a) Transformer feeder. (b) Error without compensator. (c) Error with compensator

Potential Nine 10')10 taps

T.n

I%taps~

0 )0 )0 ' 10 )0 )0, )0' )0 )0 ' )0

FlO.

5.41. Transactor type transformer drop compensator

R

FlG.

5.42. Correction of mho relay setting with mho circle less lagging than line impedance Zr = ZL!COS (r/> -8)

245

5.5

Protective Relays

To justify the compensator 82 must be less than 810 or or

8eT+8r{L+8eT) < 8,(L+T) 8eT+8,BeT < 8rT Be < Br{1-8 e)

or

8r

i.e.

If the relay accuracy

.

IS

Be -Be

> -1- > Be{1 +8e)

± 5%, then Br =

(5.21)

1 1-c 1 20 and Be = 20 or 8e = 21 per

unit, i.e. ± 4'9% accuracy. This means that a transformer drop compensator is a disadvantage unless its accuracy is at least equal to that of the relay. To be of tangible benefit the compensator should be at least 5 times as accurate as the relay, say ± 1 %. Alternatively, not more than 85% of the transformer impedance should be compensated for (in order to avoid overreach) with a compensator of the same accuracy as the relay. 5.5.8. Magnetising Inrush

On transformer feeders it is possible to have a high magnetising inrush current upon energising the feeder so that, for a few cycles, the impedance seen by the distance relay at the transformer end is below the value for a fault at the end of the transformer feeder. Some types of distance relays tend to overreach and trip undesirably for these conditions; others are negligibly affected. Those most affected are amplitude comparators such as the balanced beam type and the rectifier bridge type. Least affected are the induction type which have either the operating coil or the polarising coil tuned to provide the phase shift necessary for maximum torque in mho and reactance units. The effect of the magnetising inrush can be minimised by a tuned filter in the current circuit which rejects d.c. and harmonics. There is no record, known to the author, of trouble with distance relays of the induction cup type from this cause. 5.5.7. Examples of Setting Distance Relays

Example 1: A 60 mile, 60 c.p.s 154 kV line with 300/5 c.t.s and 4/0 copper conductors spaced 15 ft. in a horizontal row. Calculate the Zone 1 ohmic setting of the phase relay.

Solution:

Assuming ± 5 % accuracy of the relay and ± 5 % accuracy of the data, the Zone 1 coverage should be 90 %of the line. The equivalent delta spacing is iY15.15.30 = 1·26x 15 = 18·9 ft.

The diameter of a 4/0 conductor is 0'46 in. so that

SId = 18'9/0·46 = 41.

246

Distance Relays

5.5

0·9

V 0·8

/v

/

0·7

1/

./

~

!

eO'6

i/

.c

o

0'5

bO

(j

M

/

,l'/

V

v

/

V

V.,-:;0")/

0'4

/

--

1/

0'3

V/

V/ V

0·2

/./ v ... V 0·1 V 0'1

0'2

0'5

2'"

V

G'i~-----

1'0

2

5

10

V

20

50

5/D= Ra.tio of (quival(nt II spacing in fnt to diam(ter of conductor in inch(s

FIG.

5.43. Inductive reactance per mile of transmission line

From the graph in fig. 5.43 the reactance is 0·86 ohm per mile and the relay setting is 0·9 x 60 x 0·86 = 46·4 ohms. The secondary relay ohms 300 115 = 46·4 x 5 x 154000 = 2·08 ohms phase to neutral.

Example 2: 154kV line of 12% reactance on 50000 kVA base. With 1000/5 c.t.s on 13·8 kV side of a Y-L\ power transformer bank; 154000/115 p.t.s connected in open delta on the line side. 10kV2.X% X line = kVA base lOx 1542 X 12 (5.22) = = 57 ohms c/>-to-N 50000 Assuming the first zone set to cover 90 % of the protected section, the first zone setting will be: c.t. ratio Xrelay = 57 x 0·9 x . (overall) p.t. ratIo

= 57xO.9 x 1000 x (13.8 XV3) x ~ x 5

=0·6 ohm. 247

154

154000

(!)2

Protective Relays

5.5

This would require a reactance relay because the setting is too low for a mho relay. The (1) term is because the two current coils of the relay are in series. Example 3: 115 kV line of reactance 24 ohms phase-to-neutral, with 2000/5 c.t's on the 13·8 kV side of a 40000 kVA Y-A power transformer of reactance 9·1 %. Three wye-connected p.t's are connected A - Y on the low side of the bank with 66·5 volt secondaries. There is no line breaker so that the transformer must be considered as part of the line. T

fi rans ormer reactance

=

10xkV2xX% k VA

=

10x115 2 x9'1 40000

= 30 ohms phase-to-neutral = 30+24 = 54 ohms.

Total reactance Assuming the first zone to cover 90 % of the section Overall c.t. ratio x 0·9 X I = 2X· x --::---:.,----::-re ay pnmary Overall p.t. ratio

If wye current is used for the phase relays on the Lt. side and the relay current coils are connected in series, the overall

. will be 2000 . IS . ratio - x -13·8 x -J3 = 41 '6/1 . The p. t. ratiO 5 115 2 1000/1. Assuming a 90% setting for Zone 1, 41·6 Zrelay = 54 x 0·9 x 1000 = 2·2 ohms.

Example 4: If we go back to Example 3 but use compensators the procedure is as follows: Secondary reactance of transformer 30 = - x 2·2 = 1·44 ohms 54 The compensator is therefore set to 1'44/3 x 100 = 48 %. The relay then is set for the reactance of the line alone which is 24 ohms primary. The overall c.t. ratio is again 41'6/1 and the p.t. ratio 1000/1. The Zone 1 setting is therefore 0·9 x

:~ x 24 = 0·9 ohm.

5.5.8. Commissioning

Having set the distance relay it is often desirable to test it on site to ensure that it is in good condition and that the manufacturer's calibration is correct. This will be dealt with in detail in Chapter 13 but a brief mention of the method will be made at this juncture. 248

Distance Relays

5.5

Obviously the best way to test a relay is to subject it to conditions which are as nearly as possible similar to those under which it will operate in service. A common method of testing distance relays, i.e. setting the voltage and phase angle and raising the current until the relay operates, is practically a waste of time. The current and potential and phase angle are not static but change suddenly from normal to fault values, and the relay may behave quite differently in service and in the laboratory. The static test takes no account of mechanical rebound and transient electrical conditions. The old method called for a phase-shifter, a phase-angle meter or wattmeter, a timer, one or more load boxes and ammeters for the current circuit, and potentiometers and voltmeters for the potential circuit. Each reading of impedance or reactance was a slide-rule function of V, I and l/J or

W [X =

1

sin (cos -1

;:)].

Errors in reading the voltmeter, ammeter or

phase-angle meter, or in their calibration, could make a large total error in ohms. This equipment was not only expensive to buy, and fragile and heavy to transport, but it was tedious to wire up each time and required two test men. The modem method employs a test box (128) containing the test circuit wiring, reactances and resistances representing the line and fault impedances, an ammeter and an equivalent of the breaker trip circuit. This equipment is robust and compact and does not require shock-proof transportation (see Chapter 13). It can be used to test any type of distance relay from a singlephase source of supply. It employs only one instrument, an ammeter, which does not have to be accurately calibrated or precisely read, as the setting of the relay is based solely on the calibrated fault impedance. The method is to present the relay suddenly with the impedance it is to measure by closing a single-pole switch. The change from normal to fault conditions exactly duplicates service conditions. With this equipment the behaviour of the relay can be checked over all possible line conditions in a few minutes and without calculation except the mUltiplication of the fault impedance by a dial setting. 5.5.9. Ground Distance Relays (44) (52) (57)

Whereas the distance relays for interphase faults are set directly in terms of the positive sequence impedance of the protected line section, the distance relays for ground faults have an extra adjustment to consider, viz. the zero sequence current compensation. As explained in section 5.1.1 and Table 5.13 of this chapter, the ground relays are supplied" with line-to-neutral voltage and the current from the same phase plus a fraction of the residual current, so that the relay measures Vwye f' d Zo-Z~ , , --~-=----. I K IS ma e equal to Z' ,where Zo and Zl are the zero

I wye - KIresidual

1

and positive sequence impedances of the protected section, the relay then measures Zl, as do the interphase relays. The mathematical proof of this is given in the Appendix 5.10.1.

249

Protective Relays

5.5

It has been explained in section 5.5.1 how to calculate Zl, and Zo can be obtained from Zl by using the ratios given in the following Table, 5.6, or it can be calculated from zo = ro+jxo ohms/mile, where

ro =

'1 +0'00477/

(5.23)

De xo = 0'00466/log 10 G.M.R.

(5.24)

In these formulae/is the system frequency, , is the conductor resistance per mile, De is the equivalent depth of the earth return and G.M.R. is the geometric mean radius which can be obtained from data tables for standard conductors. The fault current returning through the earth is very widely diffused but its electrical effect is as if it went down until it reached an imaginary conductor at a depth Da below the earth's surface. De = 2160

J$

feet

(5.25)

where p is the resistivity of the earth in ohms/cm 3 • p varies from 10 ohms/cm 3 for wet ground to 109 ohms/cm 3 for sandstone but 100 is an average value. TABLE

Values

5.6

0/ ZO/Zl 0/ Overhead Lines

Conductor Arrangement

Min.

Max.

Average

Single circuit. No ground wire Single circuit. One ground wire Two or more ground wires Double circuit. No ground wire Double circuit with ground wires

2·8 1'8 1'5 4·2 2·0

4 3·0 2·5 6·5 4·0

3·5 2-3 2·0 5:5 3·0

The residual current transformer which supplies KIrcsidual in the current compensation scheme is sometimes supplied with a tapped tertiary winding for compensating the distance relay against the effect of mutual induction from a parallel line. Fig. 5.44 shows how much the reach of the relay is reduced by the current in another line on the same tower. Unfortunately this method has to be closely checked before using it because it can trip the unfaulted line as well on a close-in fault. This is because the voltage at the relay is low, due to the proximity of the fault, and, if the current in the faulted phase is high, the compensating current in the good line may be enough to look like a Zone 1 fault. Distance relays of the reactance type will give a practically constant coverage of 90% of the section for instantaneous tripping; this superior operation may not only minimise damage to insulators, etc., but also may prevent the arc from involving another phase and thereby endangering stability.

250

5.5

Distance Relays 100 I

90

I

BO

JI

..

!:

a.

a60

I 1 .. 1

'0"

~

...

.;§I .~I

&1 .}: I "1

50

it

:;

.... d

.~I

.." ". ~30 S40 c: d

20

10

I

1 1

I 1

I

I 1

1

"'I 1 1 1 1 1 1

I

1

1 1

I

~ 70

I

1 1

I

1

1 1

1 I 1

•

.2 .-~

---- ... x~

II

0--i

I

0-1--

(a) Fo.ult o.t fo.r tnd

~/

I

~

(b) Fault o.t nto.r end

I I

r !

1"'" '" ~ ~" 'O~."

lint covtrs only 6B'Ya due to. mutua.l inductien frem current In unfaulttd line

1 1 1

o 0L---2..l.0---4LO--6...J.0-~-B.J..0--l..J00 Fo.ult Icco.tien

FIG.

%ef line Itngth

5.44. Effect of induction coupling from a parallel line

The same reactance relay may be used for either phase or ground protection but, as a ground relay, it requires an auxiliary relay to prevent it from working on phase faults, since the current in the leading phase may be almost in phase with the wye potential and hence measure zero reactance. This is illustrated in fig. 5.45. Fa.

-

--

sx~x

Fe

~l Trip

5.45. Circuit for blocking ground reactance relays from overreaching on double-ground faults SX = starting unit auxiliary; X = reactive unit; F = phase flag; G = residual overcurrent relay FIG.

251

5.6

Protective Relays

In a b-c double ground or phase-to-phase fault it is the b phase relay that is in trouble as the reactance unit in this phase must be monitored by the fault detector in the c phase. This can be ensured by designing the fault detectors so that the one in the lagging phase has the lower pick-up. In switched relays wrong tripping is avoided by supplying the reactance unit with the lagging instead of the leading of the two currents in a double ground fault. 5.5.10. Multi·terminal Lines

Where the protected section has more than two terminals (fig. 5.36b), the distance relays at one terminal can measure the correct distance up to the junction J of the lines to the other two terminals, but from there on their reach along one of them is affected by the magnitude and direction of the current in the other. For exalJlple, the relay at A, for a fault at F, measures

IA.+IC) ZA. = ZA.J+ZJF ( ~ This may be considerably more or less than the line impedance ZAP and may interfere with selectivity; if there is much generation at C the relay at A will underreach, causing sequential tripping; if there is no generation at C but there is a low impedance path (dotted) from C to B, fault current will leave the section at C and cause the relay at A to overreach, because its value is negative in the above expression for Z A.; this may cause unnecessary tripping and disruption of the system. This is one of the penalties for economising in circuit-breakers. If there were an extra breaker at C so that direct connections were made with A and B this difficulty would not occur. In fact, this is the only solution in cases where the variation in Z. at one of the terminals is sufficient to prevent discrimination between internal and external faults. 5.6. CONSTRUCTION OF DISTANCE RELAYS

As explained in Chapters 2 and 3, distance measuring relays can either be phase comparator, such as induction cup relays and electronic or transistor comparators; or they can be amplitude comparators, such as balanced beam, rectifier bridge relays and transductor relays. This is illustrated in the Tables 5.9 and 5.10. There are a great number of types of construction available because any combination can be made of the following choices. (a) Amplitude or Phase Comparators.

(b) Current or Voltage Measurement. (c) Electromechanical, Transductor, Electronic, Semiconductor or Hall

Effect Measuring Units. The general characteristics of the various comparators have been 252

Distance Relays

5.6

5.7 Amplitude and Phase Comparisons for Distance Relays TABLB

Amplitude Comparator Operating Restraining Directional

II + :'1

Impedance

III

Reactance

11 - :'1

Mho

II/

Offset Mho

III

1/ -

~I

Zr,

IfI IfI /1 -

~I

Zr,

1(1 + Ko)lr - Zr, ~I

Phase Comparator Operating Polarising

IZr

V

IZr - V

IZr + V

IZr - Vsin c/>

IZr

IZr - Vcos (c/> - ())

V

IZr - V cos (c/> - ())

V

+ kIZr

described in detail in Chapter 3. In the following paragraphs the types now in use will be discussed briefly as distance relay units. Recapitulating from earlier chapters, the amplitude comparator operates when the operating force Fo exceeds the restraining force Fr. Relating this to a design constant this means when IKII > IVI in an impedance relay. The phase comparator works when IvI cos (fjJ-O) is positive; 0 is 900 for an induction cup unit and 00 for a circulating current rectifier bridge comparator. Relating this to a directional relay, it operates when VI cos (fjJ-(J+a.) is positive where a. is the phase angle of the voltage circuit. It was also explained in Chapter 3 that an amplitude comparator could become a phase comparator and vice versa by substituting (I - V) and (I + V) for I and V. Tables 5.9 and 5.10 show the arrangements in most common use. Electronic comparators have not been shown because it is unlikely that they will be used in view of the superiority of transistors. Hall Effect comparators have not been shown because at present they have certain limitations such as temperature error and low output which have not yet been overcome. 5.B.t. Balanced Beam (53) (61) (134)

The advantages of the balanced beam unit are simplicity and speed. The disadvantages are high reset impedance, overreach due to offset current waves or currents of high XjR ratio, elliptical impedance characteristics and a tendency to chatter. Very little can be done about the high reset impedance except to use the minimum beam travel. The overreach on offset waves can be controlled by a d.c. transient trap having a low resistance high-Q choke across the current source and a small non-inductive resistance in series with the relay current coil. 253

~

C7I

I\:)

Serio r.cllfi., bridge (VOlta.ge comparison)

Tran.d uc tor with rectified re.tra.int (Ma.gndic comparison)

Parallel reciltier bridge (Current compa.rison)

E I.ctro-mechanlcal ba.la.nccd beam (Torque comparison)

Type

0

00

o

P

[J

Trip

~M

~'

o o

!I

Impeda.nce

01

0

0

V

lp

o

I

P

00

':u

G

S

P

T

{illl

0

:l"Mt

V

II.

~

o

~TriP

TABLE 5.9 AMPLITUDE COMPARATORS Admittance

~M

XR

V

~ ~ ~- a

~TriP

Reactance

0

R

Uneconomical

I

~c

@J@I'

o

-

\L~

TriP

ry = fi

ro--

D,rectional

UI

~

~

~

iii

~.

o .....

."

en

Distance Relays

5.6

The ellipticity of the operating characteristic on an impedance diagram is at right angles to the ellipticity of the reset characteristic so that the operate and reset values are close at one phase angle and separated at 90 on each side of this angle. The impedance characteristics can be made circular by smoothing the potential flux through rectifiers and a smoothing capacitor at the expense of speed or by phase-splitting using a three-pole magnet having windings on two legs, one of which has a series capacitor to advance the flux, the unwound leg carrying the combined return flux. 0

5.6.2. Induction Cup (45) (46) (51) (136)

The induction cup is slower than the beam but has nearly perfect impedance characteristics, no vibration, almost equal operate and reset values and small effect on offset waves. It was described in more detail in Chapter 2. Its construction is compact and robust and it is the most popular high

......-- v --~ c, Operating


Polarising

v

(c) Note: For simplicity only one phase of the current circuit is shown. The circuit not shown is

similar but has minus the current in the next

l'Uin, ph.se.

I

I' (0)

v= I

=

rPv

=

h

=

;, =

l' =

1m =

ZI = Izi =

ICI = IRI = VRC =

Z2 £2 IZ2 £'2

=

= = =

Line potential Line current Potential flux Current flux Current in dephasing coil of current. pole Component of I which corresponds toh Magnetising component of I Potential coil impedance Current through coil ZI Current through CI Current through Rl Potential across Rand Cl Phase shifting coil e.m.f. in Z2 Volts drop in Z2 Potential across Z2 FIG.

V90 0 :::..

.pI

>9t

'0

:::..

'0 VI

:l

J

~

...J

v (d)

5.46. Operation of induction cup reactance unit

(a) Potential, current and flux vectors (b) Potential circuit (c) Current circuit (d) Diagram showing only reactive component of V is effective

255

~ Polarising potentia.l wdgs.

~ Restraint potcntia.1 wdgs.

~currcnt ~wdg.

(a)

(b)

= Fault current = Faulted (restraint) potential V, = Directional (polarising) potential

I

I Vr

;r .,; Current in restraint coils

Rcstra.int

if. = Current in polarising coils fI

= Power factor angle

fJ

= Angle betw.een i p and ir

ex =

Angl~tween

I and i p

120° + (180° - If) + IX + 8 = 0 :. = IX - 8 - 300°

=P-8+6O°

I

Operating torque = ipI sin ex = ipI sin (fJ - /I + 60°) Restraining torque = i,Ir sin P = Xi!! Vr sin P 'k h Vr sin(p-fl+600) h RI eaYPlcsUpWenj< sinp oms

(c)

5.47. Vector diagrams of starting unit (a) Potential and current circuits (b) Currents and voltages during interphase fault (c) Vector diagram of relay quantities FIG,

~::r--o PolClrising circuit

.~

~--------~c-----~v

RCltra.int circuit

0

D

0

Current circuit

v=

If.

= = Ire = 10 = r

I

'I

=

Line voltage Current in polarising potential coil Current in restraining potential coil Fault current at maxim.um torque angle Current in parallel RC circuit Current in operating coil

5.48. Vector diagram of mho unit (a) Potential and current circuits (b) Vector diagram of relay quantities

FIG.

256

Distance Relays

5.6

speed electromagnetic unit, especially in Switzerland, France, Sweden and the U.S.A. Phase comparators of the induction type differ from all other types of phase comparator because they are sine product devices whereas the others are cosine product (wattmetric) devices. For this reason the phase shift necessary for the induction cup relays in the Table is 90° different from the others. This property makes it suitable for polyphase application as discussed in Chapter 6, section 6.8. Fig. 5.46 shows the vector diagrams for a reactance relay of the induction cup type. For simplicity only one current circuit is shown but in practice there is another, inductively coupled to it and energised from the residual current or another phase (see section 5.1.1). Fig. 5.47 shows the vector diagram for the mho type fault detector with quadrature potential polarisation. The relay in phase R would have current JR , restraining potential the wye voltage VR and polarising potential the delta voltage V yB , The mho measuring unit (fig. 5.48) diagram is similar to fig. 5.47 except that the polarising potential is supplied from the same phases as the restraining potential. 5.6.3. Rectifier Bridge Comparators (50) (63) (117)

As amplitude comparators these devices tend to elliptical distortion of their impedance characteristics unless the slave relay or output device is slow and has a very low burden. The rectifier bridge current amplitude comparator is popular in Germany and the U.K. The rectifier bridge voltage amplitude comparator is used in Norway, France and the u.K. The other systems are not yet in general use. The advantage of a rectifier brirlge amplitude comparator of the current or parallel type is its limiting action. Using a very sensitive output relay the sensitivity near the balance point can be very high but the relay is protected at higher differentials by the decreasing non-linear resistance of the rectifiers. With the other comparators, limiters are necessary to produce this effect. Distance relays using phase comparator rectifier bridges are not yet on the market but an English company will be manufacturing them by the time this book is available. The phase comparator bridge, like its induction cup counterpart, has superior ohmic characteristics and R - X circles and, unlike the amplitude comparator, any distortion at low voltage has an oblate rather than a prolate tendency, i.e. the circle bulges sideways to give more tolerance for fault resistance. This is an excellent feature since it occurs only at low voltage where it is most needed and where power swings within system stability limits cannot cause wrong tripping. Transistor comparators are not described here because they are not yet on the market, but they are discussed in Chapter 2 and in more detail in Vol. II.

257

5.6

Protective Relays

5••• 4. Magnetic Amplifiers

Like all other comparators these can also be used for phase or for amplitude comparison. The amplitude comparator (transductor) was first used in Sweden and later in England (16). It is most effective as an impedance relay; when used as a directional, admittance or reactance relay it has a tendency to transient inaccuracy which can be overcome only by introducing a time delay through the damping winding D. The phase comparator (Ramey half-wave type) is an excellent device but so far has not been used by the major protective relay manufacturers; it is uneconomical as an impedance relay but very effective as a directional, admittance or reactance relay. Table 5.10 shows circuits employing a polarised relay with operating and blocking windings. The two windings have equal currents when the two circuits to be compared are in phase. The output relay need not be as sensitive as is required for the other circuits in Tables 5.9 and 5.10 because the milliwatts in the control circuit are magnified about 1000 times in the relay windings. The differentially connected transformers should be as efficient as possible using mu-metal or HCR clock-spring cores saturating at low voltage (about 10 volts). 5.6.5. Sensitive Tripping Devices (24) (50) (63) (117)

Perfect characteristics can be obtained from an electromagnetic relay, such as the induction cup 'Jnit, with reasonable burdens, because it uses a torque comparison and the work done to close the contacts has no effect on the magnetic fluxes which produce the torque. In static relays, on the other hand, the comparison is an electrical one, done in a network, and good characteristics are obtained only if the burden imposed by the tripping device is negligible compared with the power in the network. The most sensitive electromagnetic tripping relays are of the moving coil or moving iron types, polarised by a permanent magnet and described in Chapter 2. The moving coil type can work onO'25 milliwatt but it is not shockproof; the moving iron type is shockproof but requires 2 milliwatts. For good characteristics the output relay should not take more than 0·1 % of the input to the comparator. Allowing for a 25 to 1 range of adjustment and assuming a maximum input to the- comparator of 3 watts, this requires a sensitivity of 50 microwatts of the output device for a mho relay. This has been achieved by a transistor pre-amplifier supplying a shockproof (30 g) polarised moving iron relay. The ideal solution is a high power output transistor. At the present time controlled silicon rectifiers are available with sufficient capacity to trip a breaker having a trip coil current of 40 amperes at 250 volts for 60 milliseconds, but their cost is too high compared with the cost of an attracted armature relay. In a few years no doubt these or equivalent devices will be available for a reasonable price. The cost of the amplifier is offset by the reduction in size of the comparator and the auxiliary transformers which supply it.

258

co

C1I

....,

Ma.gnt.llC a.mplil,c, (Amplitud~ comparison)

(Curnnt cOincidence)

Compara.tor

TrCln.i.tor pa.ra.llel

Rectifier bfldge CompClrCltor a.nd limilor (L) (Phase comparison)

Elf.ctro -mocha.nlca.1 Induction cup (Torque. compa-rlson)

Type

p

I

V

Unc:c.of"lomica'

Zr

polarised relay.

TranSductor

Relay

~'

~

CT

-

~.,

I-V

(I+V) "

,ill~-0

Zr

Impeda.nce

~

,dt.tcctar V

Levr~

Zr

PC

V

.

PC

'/

T

PC

1';1

V

~

Lucl

~~

R

'=8~

Rcacta.nc.e.

o

= operating coil.

r;'l

V B = bias restraint coil.

I

tij)~ ~'

S

'"''';~

c.lrc. ..ut

V

~~nIlJt

Aux . C .T.

,~@,

TABLE 5.10 PHASE COMPARATORS Adm;tla.nco

V

4

cD? Aux C.T.

•

k

o&[J

T~PC

-¢--hQ-

LJ L J

V L.... I td.. tector....

Zr

':::JI~

I

Directiona.l

c"

UI

~

iii'"

Cb

::0

Cb

(")

~ ::;,

I:)

5.6

Protective Relays

5.'.'. Relay Sensitivity

The sensitivity of a distance relay measuring unit can be defined as the minimum voltage Vm down to which it will measure accurately. It was shown in section 5.3.1. that this could also be expressed as inversely ZsIZL' Since the VA burden of the potential circuit oc V2 it follows that

~ where VA is the burden of the potential circuit at normal voltage. "VA This is also affected by the sensitivity of the relay unit. In a rectifier bridge Vm ex:

circuit it requires mW milliwatts to operate the unit, so that Vm ex:

~.

"VA In the case of the induction cup relay, the polarising circuit co-operates

with the potential circuit to produce torque and Vm ex: J

1 where VA. (VA)p

(VA)p is the burden of the polarising circuit which may be energised by

current (reactance relay) or potential (mho relay). The burden of the operating current winding or circuit is related to the sensitivity in the same way but it is not a limiting factor because it is only on during fault conditions, whereas the potential burden is a maximum during normal conditions. The most important aspect of the current burden is to make sure that the c.t. is capable of handling it for the maximum fault current Imax for a fault at the end of Zone 1. The knee point voltage VK of the c.t. secondary should at least equal the total voltage drop across the burden, i.e.

V= K

Imax

(R

+ Rleads +

e•t .

;,~) Kt

(5.26)

where VA is the relay burden at its rated current Ir and Kt is the d.c. offset ratio which is a function of the time constant (LI R) of the primary circuit.

K= (1 + X) R 2!!... te t

where tr is the relay operating time and t. =

(5 •27)

~; the factor 2 comes from the

fact that the time of energisation of the relay at a given level need not be more than half the time taken to close the contacts. Kt

Now

:r.

=

(1+ :)~ tr

(5.28)

=(~+co)~ is small compared with co and can be neglected. Hence K, =

co

"2

tr = nf.lr

260

(5.29)

5.6

Distance Relays

If f = 50 and tr = 0'04, K t = 6·28. Actually K can be taken as about 2 for most systems because almost all faults on overhead lines occur near the point of Vrnax in the' cycle and the full offset conditions are rarely encountered. The rare case of the worst conditions would only cause a short delay in tripping. (5.30) 5.1.7. Performance Curve.

The circular characteristics of a distance relay on an impedance diagram are important only if they depart seriously from circularity in the quadrant of the R-X diagram in which faults can occur. An exception is the case ofa very long line where power swings may intersect the circular locus if the circle is too wide (oblate). Another case is that of a very short line where arc resistance can prevent operation of a mho relay if the circle is too narrow (prolate). The accuracy of the measurement can be clearly shown on a graph of per unit accuracy Zr/Zn to the base of voltage, where Zr is the relay reach in '20

110 f//,

100 WI'. rK71

'//. 'h '//. '//.

f/I

f/I !.lli. 1'1/1; illIh il///, I/h

W

f/I

f/I

f/I 'I'.'

,/1 '//1 W/, V/h V// h ;t//h Wh '/I'///. 'Ih '//. '/h

90 80

Shc1dcd arc" dcnotCi a.cCUl'"G.Cy tol crC1nCC . ±

5"0)

40 30 20

,0

o

o

10

20 FIG.

30

40

so

60

Few It vo Ita.qc

70

80

90

100

110

5.49. Distance relay performance curves (ohms/volts)

ohms for a fault at the relay setting Zn (fig. 5.49). Operating time can be shown best on a curve of time to the base of distance so that the slowi~g up of the relay near the cut-off point can be clearly seen. The complete performance of the relay can be shown on a contour diagram (50), plottingZ//Zr against a base ofZ./Zr whereZ/ is the impedance between the relay and the fault and Zr is the relay setting; this is equivalent to plotting the accuracy along the Y axis against a base of the reciprocal of the voltage along the Y axis (see fig. 5.50). 261

5.7

Protective Relays

x - Y curves are plotted for several relay operating times so that there is a family of such curves and contours. This method is a good way of evaluating the performance of one relay against another but it takes much longer to make the necessary tests and is somewhat more difficult for the user who is

I I

T -

-

o.'St.ancc rclQ)' contour

100

- r---

.'"

'--

o

~

t

If,

~.

I

~

........ r-

.......... .......

60

'r--. i'.

n :J.)rM

I --,--

--

SlIa.dcd orca on boundary

CUI"IIICI

//

;t.,

~

~ '~fo i - .. ( 8ClJ<1ncc)

~

--I"- ........

3~~.00lScc, r'\

o0,

02

0 '5

I

1\

5

Source /llnr impedance

FIG.

I

I

\

2

~

1\5001 Sc ""'6001 Sr< .

~\

I

-...:...

",,-

Q.

20

. ;,;;

10

\

20

50

100

rQtlO

5.50. Contour diagram of distance relay performance

more interested usually in the operating times for faults at different points along a given section, i.e. for a given value of Z,/Z". To do this he plots a vertical line and notes where it intersects the various contour curves. 5.7. A.C. POTENTIAL

For accurate measurement a distance relay must be supplied with a secondary voltage linearly proportional to and in phase with the primary voltage of the circuit it is protecting. 5.7.1. Sources of a.c. Potential

One of the principal difficulties with the use of distance relays is the availability of a suitable voltage supply. On low voltage systems a suitable supply is generally available and the cost of potential transformers is not great, but on high voltage lines they are a costly item and must be considered in the overall comparison of the cost of different protective schemes. For protection against phase faults only, two single-phase p.t's are used in many countries and are connected in open delta to provide the three lineto-line voltages. For ground distance relays three p.t's are required to provide 262

Distance Relays

5.7

the three wye voltages and small auxiliary p.t's in wye-broken delta for providing the residual voltage, an alternative being double windings on the main p.t's. Potential for all the distance relays on a busbar can be obtained from a singl(1 three-phase p.t. of the magnetic type connected to the bus. Magnetic p.t's are preferable to those of the capacitance type (65) because they have a greater VA capacity and are more reliable for high speed (1 cycle) relays because they do not have parasitic oscillations when the potential is suddenly changed in value, such as by a fault. The use of a common p.t. on the bus bars reduces the total cost of distance protection considerably and is common practice in the U.S.A. and Canada. Potential can also be taken from the 1.t. side of power transformers, a voltage drop compensator being provided where necessary. This subject has been discussed previously in this chapter, section 5.5.5. Compensators are seldom more accurate than the relays so that they should be avoided where the fault current passes directly through the power transformer. Furthermore, complicated bus systems sometimes make low tension potential impractical. Tap-changing transformers present another problem. 5.7.2. Loss of a.c. Potential

Accidental loss of a.c. potential can cause undesirable tripping of distance relays. In section 5.1.4 (c) it was explained how loss of primary voltage due to the opening of a breaker can cause heavy transient torques from the decaying potential. Wrong tripping on load current can be caused by loss of secondary voltage due to the blowing of a secondary fuse or accidental opening of the circuit during testing. The following methods are used to avoid this undesirable tripping. (a) In the U.S.A. it is claimed that the use of heavy (60 ampere) potential

fuses in the distance relay circuits and light (1 ampere) fuses for other switchboard devices prevents blowing of the secondary fuses due to transient short-Circuits, such as a falling tool or a stray strand of wire. (b) The trip circuit can be opened in 7 or 8 milliseconds and an alarm given by a relay connected to measure the potential across each fuse. Fig. 5.51 shows such a relay with three operating magnets, one across each fuse, so that only one contact is inserted in the trip circuit. (c) A rate-of-rise-of-current relay is the most effective arrangement. It completes the trip circuit only when there is a sudden increment of fault current equal to 20 % of the c.t. rating. This prevents tripping on loss of potential due to accidents or power swings, and permits tripping only on a fault. The rate-of-rise relay shown in fig. 5.52 uses a transistor as a switch. (d) A cheaper protection against tripping on loss of a.c. potential, but which does not include power swings, is the use of an instantaneous

263

5.7

Protective Relays

FIG.

5.51. Potential fuse failure relay

'V1:bllEf O

° Sens!ti ve polarosed d .c relay

(.)

FIG.

5.52. Rate-of-rise-of-current relays

264

Distance Relays

5.8

overcurrent relay in the instantaneous (Zone 1) trip circuit of the distance relay. 5.B. SIMULTANEOUS GROUND FAULTS

When two phases of an overhead line flash-over simultaneously at the same location or within the same section of line, it is known as a double ground fault. When two phases flash-over simultaneously in different line sections, it is known as a simultaneous ground or 'cross-country' fault (137). Cross-country faults are rare on solidly grounded systems but they are fairly common on systems grounded through high impedance or Petersen Coils. Such a fault is illustrated in fig. 5.53. It appears to the phase distance relays at A and D as a double ground fault at an intermediate point X. It is

FIG.

5.53. Simultaneous ground faults

unlikely to operate the phase relays at Band C but it appears to the ground distance relay at B as a phase b-to-ground fault and to the ground relay at C as a c-to-ground fault. The ground relays tend to overreach because the interphase current flowing from phase b to phase c through the earth is much larger than the ordinary ground current. Another way of looking at it is that their residual compensating current Klres is the sum of the c-phase current from end R and the b-phase current from end P. Under these circumstances the least that can happen is that the breakers at A, B, C and D all trip and service is lost to station Q in fig. 5.53. In Europe, where Petersen Coil grounding is common for systems below 100 kV, the isolation of station Q is avoided by the fact that switched impedance or mho relays are use'd on these systems and they select wye potential and current for all faults involving ground. In the fault shown in fig. 5.53, all the relays would receive phase c current and voltage and only breakers C and D would open, with D probably not tripping until C had tripped. The reason is that the fault appears to the relays at A and D as a phase-tophase fanlt so that they underreach when given c-phase current and potential;

265

5.9

Protective Relays

the c-phase current is flowing in the reverse direction for the relay at B, so it stays open also. After C and D have tripped, the ground fault in section AB still remains but is extinguished by the Petersen Coil. On solidly grounded systems it is necessary to trip all four terminals because there is no Petersen Coil to extinguish the remaining fault. With separate distance relays for phase -and ground faults, breakers A and D would be tripped by their phase relays for the fault in fig. 5.53, breakers Band C by their ground relays. In switched distance relays providing delta current and voltage for double ground faults only, breakers A and D would open. 5.9. AUTO·RECLOSING

The large majority of faults on overhead lines are transient, i.e. they disappear when the line is de-energised by opening the circuit-breakers at both ends of the line. This fact permits immediate resumption of service by reclosing the breakers. Obviously, this does not apply to a cable because the breakdown of insulation is permanent whereas, in an overhead line, the insulating value of the air is restored as soon as the fault current stops and the arc disappears. Radial circuits are the most benefited by automatic reclosing because there is only one source of power and the quicker it is restored the better. Furthermore, there is no problem of synchronising on radial lines. On tie lines or interconnections in a network there is more than one source per bus, so that the loss of a line is not so serious and auto-reclosing is less necessary. Furthermore, on tie lines there is sometimes a problem of connecting the two sections of the system after the source and load ends have drifted apart in phase relationship. On the other hand, where there is only one tieline which must be kept in at all costs, single-pole switching is necessary, i.e. separate circuit-breakers and reclosing relays for each phase; with this system transient single-phase-ground faults (which are the most common) cause no real interruption because, while the reclosing is taking place, the load current formerly in the interrupted phase makes its way back to the neutral through the ground and grounding wires (64). 5.9.1. Multi·Shot Reclosing

On low voltage lines (33 kV and below) the fault may have been caused by something across the conductors, such as a vine or a tree branch, and it may not be burned clear at the first reclosure. On such radial lines an instantaneous reclosure is provided, followed by two or three more delayed reclosures if necessary (70). Statistics in the U.S.A. show that over 80% of the faults are cleared after the first trip, so that the instantaneous reclosure stays in and the reclosing relay resets. About 10 % stay in after the second reclosure which is made after a time delay, usually between 15 and 45 seconds. Less than 2 % require the third reclosure, which is made after 60 to 120 266

5.9

Distance Relays

seconds. About 5 %are permanent faults which are not cleared and result in lock-out of the reclosing relay. Most auto-reclosing relays are of the synchronous motor type with a circular cam or drum having adjustable pegs or equivalent means for operating the reclosing contacts after the desired intervals. Blocking relays are provided to prevent the breaker from reclosing until the reclosing circuit has been re-established. 5.9.2. Single-Shot Reclosing

On transmission lines most faults are caused by lightning flashovers; tree branches are unlikely to cause faults because of the height of the line and the wide right-of-way; vines or wire-dropped by birds would be vaporised instantly by the large amount of power in the arc. Consequently there is no need for more than one reclosure which must be instantaneous to restore the connection before an appreciable phase difference occurs between the two ends of the open line. The drifting apart in phase of the two separated sections of the system is caused by the fact that the source end speeds up and the load end slows down when separated. The inertias of the synchronous machinery involved, however, is very great and the drifting apart in phase is slow. However, the reclosure is made as quickly as is compatible with the de-ionising of the arc which permits successful reclosing after the following dead times in cycles (Table 5.8). TABLE

5.8

Dead Times Required for Automatic Reclosing Line Voltage Min. Dead Time

22 4

33 5

66 6

110 8'5

132 10

220 17

300 24

kV

cycles

5.9.3. Instantaneous Reclosing

A line can be successfully reclosed immediately after a fault only if the two ends have been tripped simultaneously, otherwise the fault arc will not have been extinguished before the breaker at the fast end recloses and the breakers will trip again. Where the relay times are not the same at the two ends of a protected section, reclosing must be delayed long enough for both ends to have tripped. There are two methods of ensuring simultaneous reclosing. (a) A connecting channel for intertripping, such as carrier, space radio or pilot wire. (b) Temporary extension of the instantaneous zone.

267

5.9

Protective Relays (a) Intertripping Channel: Of these methods, the carrier or pilot wire

channel is the most expensive and is suitable only for single lines of great importance or when the channel is necessary for communication. Microwave or radio link is cheaper where repeaters or reflectors are not necessary to circumvent geographical obstructions. Space radio is the cheapest channel and can be used where Government allocations of radio wave-length band permit. The channel can be used for the fast end to signal the slow end and thus to clear both ends instantaneously, even if the fault is close to one end and hence otherwise in the second time zone of the relay at the other end. (b) Extension of Instantaneous Zone: The instantaneous zone, normally set 10 % short of the next bus, is in this case 5 %beyond the bus into the next section, so that all faults in the protected section are tripped instantaneously. A fault just beyond the next bus will cause the local breaker to trip as well as the one in the faulted line. When they reclose the relays are reset so that the instantaneous zone covers only 90 % of the section and, if the fault is still there, only the nearest breaker trips. The other time distance zones operate normally. It should be pointed out that, although the overlap of the section by the instantaneous zone in ohms impedance is 5 %, the overlap in actual distance is generally only 1 % or 2 % because the relay only measures a fraction of the total current fed into the fault through the intermediate bus. The likelihood of a fault occurring in this small length of line is correspondingly remote. On the other hand, it is obvious that this scheme is practical only with very accurate relays, i.e. those with an error of less than ± ~ % over the possible range of fault currents. The effect again is to clear all faults instantaneously at both ends of the protected section. The subject of reclosing has not been discussed in detail because it is outside the scope of a book on protective relays.

5.9.4. Single Pole versus Three Pole Reclosing

Single pole reclosing was first introduced in the U.S.A. in the early 1930's. Instead of a common closing and tripping mechanism for all three phases, each pole of the circuit-breakers was provided with its own mechanism. The relays were connected to control the three mechanisms so that, when a singlephase to ground fault occurred, only the phase involved was interrupted and reclosed. For any multi-phase fault all three phases would be simultaneously tripped and reclosed. With single-phase distance relays of the mho type the selection of the faulted phase is not difficult but, with polyphase relays, such as phase comparison carrier or pilot wire protection, phase selectors are necessary. Where potential transformers are available the phase selectors compare each wye

268

Distance Relays

5.10

potential in amplitude with the delta potential that is in quadrature with it; for instance, a ground fault on phase a will be detected by the fact that Va < 0·5 Vbc ' Where a.c. potential is not available the negative sequence component of current is compared in each phase with the zero sequence component; in the faulted phase the two components are substantially in phase; in the unfaulted phases they are substantially 120° out of phase and hence have a negative product. The advantage claimed for single pole reclosing is that, on a system with transformer neutrals grounded solidly at each substation or line terminal, the interruption of one phase to clear a ground fault causes negligible interference with the load because the interrupted phase current now flows in the ground between neutral points until the fault is cleared and the open phase reclosed. This technique has become popular in France and Sweden especially where there are no parallel low impedance paths in the network through which the load can flow while a faulted line is interrupted. Meanwhile, it has lost favour in the U.S.A. and been replaced by three-phase reclosing for the following reasons: (a) phase selectors cannot always select only the faulted phase;

(b) the protection and control circuits are more expensive and complicated with single-pole switching; (c) most systems will maintain the load on two phases long enough to permit instantaneous three-pole reclosing; (d) the real problem is not to eliminate a brief dissymmetry of the load current but to avoid system instability and this is associated with multi-phase faults which require three-phase tripping and reclosing; (e) owing to the more complex relay circuit and to the longer dead time required,' single-pole reclosure cannot be as fast as three-pole reclosure; this is not only undesirable in itself but can cause trouble with telephone interference and with wrong tripping of the residual relays on a sound line in parallel with the faulted line due to mutual induction.

5.10. APPENDIX 5.10.1. Zero Sequence Current Compensation

During a phase fault to ground the wye potential Va at the relay consists of the following drops in the sequence networks and fault resistance

v" = I 1Z 1+1 2Z 2+/oZo+ IFRF where la is the current in the arc. Now Z1 = Z2 for lines; hence

v.. = (I1 +12+/o)Z1 +/O(ZO-Z1)+IFRF = Z1 {Ia+/ o (ZO~Zl)} + IFRF 269

5.10

Protective Relays

If the relay is supplied with current Ia+1o

Va (Zo -

+1

Zrelay = I a

(ZO~Zl) it will measure

i ;-)-

Z1

0

=Zl+ R F 1+1 (Z-Z) _0 _ _ 1 a

Zl

0

Hence the relay measures correctly except for the error due to fault resistance which is negligible in the case of a reactance relay, unless IF is appreciably out of phase with la and 10 , 5.10.2. Zero Sequence Potential Compensation An alternative to measuring the Z~ of the protected section is to measure Z~. This method has an advantage in accuracy for lines carrying appreciable

reactive current but this is offset by the higher c.t. burdens and a somewhat more complicated circuit. It is advantageous for a switched ground distance relay because no switching is necessary in the current circuit. Since only residual current (Ires) appears in the current circuit, the I1Z~ and 12Z; drops in the line voltage must be removed from the relay potential. In phase a the potential supplied to the relay is Vre1ay = Va-(la-/o)Z~ where Z~ is the impedance of the protected section. Vrelay = I 1Z 1 +12Z 2+/oZ o+IFRF -(I1 +12)Z;, For a fault at the balance point Zl = Z2 Vrelay

= 10Zo +IFRF •

;es = 10

If the current circuit is supplied with 1

Zrelay = ZO+RF

e:)·

the relay measures

Again, the relay measures correctly except for the fault resistance which will be ignored by a reactance relay if the angle between IF and 10 is small. 5.10.3. Impedances seen by Distance Relays

If one substitutes

-! +j .J~ for a and - i - j J~ for a 2 in Tables 5.12 and

5.13, the expressions for the impedance measured by the relays in the different phases appear as shown in Table 5.14. These expressions are vectors that are easy to draw on an R - X diagram. For example, in Fig. 5.54 the impedances 'seen' by the measuring relays in the three phase-pairs for a 2-3 fault are AF310 AF23 and AF12 whereas Fig. 5.55 shows how the same fault appears to the three ground relays.

270

Distance Relays

5.10

5.14 Impedance measured by relays in addition to true Impedance TABLE

-----

Relay

Three Phase

Z'12

RF C

Z'23

RF C

Z'31

RF C

Z'l Z'2 Z'3

Phase 2-Phase 3

M

0

C~j(Znl900 + Zo~ + 3RF130

)

RF 2C

MV3Znl90° +

RF

0

)

00

C~3(Znl90° + Zol30° + 3RFI300)

RFI600 )

3 2CnRF

oo

CRF

+ RFI60

v3Z"W

Phase I-Ground

C

c~j(Znl90° + RF[3O")

~~(ZnI90° + Zo~ + V3RF~0)

RF C

C 3

~-(Znl90° + RFI300) -

~~(Znl90° + Zo130° + V3RFI600)

In Table 5.14190° means lagged in phase by 90°. Similarly 1900 means advanced in phase by 90°. x

S

A..

-x FIG.

5.54. Impedances seen by phase relays during 2-3 fault

The method of construction of these diagrams is literally to draw the vectors given in Table 5.14. For instance, the vector AF31 in Fig. 5.54 is obtained by plotting

J!C z2190° (which is the line FM) and adding the arc -

resistance component

~ 160°.

C is the fraction of the total fault current

which flows through the relay. 271

5.10

Protective Relays

Reference (58) shows how to draw all fault conditions, with and without power swings, and how to determine what any relay in any phase measures during these conditions. The impedance seen by a relay during a power swing can be derived even more simply (Fig. 5.56). First simplify the system to the equivalent single

L

R,

ife ...

. R ______~~~~~---------R

..

FIG.

5.55. Impedances seen by ground relays during 2-3 fault

impedance SL connecting the lumped power sources which are swinging. The power swing locus, as far as its effect on the relay goes, can be taken as the straight line bisecting the line SL at right angles. For any point a on this

-x FIG.

5.56. Impedances seen by phase or ground relays during a power swing

locus ISOL is the angle of separation of the generator e.m.fs and the impedance seen by a relay at any location A is AO. If the relay characteristic be drawn on this diagram and it intersects the power swing locus at 0' then ISO'L is the angle up to which the system can swing apart without operating the relay. A fuller treatment of this subject is given in Vol. II.

272

Co - C Co- C Z2 +Zo +CZ'I +3RF CZ'2 - Z2 Col'o - Zo

(a2- a)C

(a - a2)C

Z2 + CZ'I + RF

Z2 - CZ'2

0

a2C

aC

cZ' +RF

0

Klb

KI,

KVI

KV2

IXCZ'I +RF)

KV..

K

(a2- a)(CZ'! +RF)

KVc,

1 £(ZI HF)

(a -

(1 - a2XCZ'1

KV.c

1

1 £(ZI +Z2 +Zo +RF)

- 3CZ'1 +(a - a2)Z2 +(a - 1)(Zo + 3RF)

(a - a2)CZ'1 - 3Z2 +(a - I)RF

£(ZI +Z2 HF)

(a2- a)(Zo +2Z2 +3RF)

IXZo +3RF)

2(a2- a) (CZ'I + ¥)

(a2-

CZ'I +(a - I)Zo +Col'o +3aRp

CZ'I +(a 2-1)Zo +Col'o +3a2RF

3CZ'1 - (a2- a)Z2 -

a(CZ'J +RF)

KV,

(a - a2)CZ'J +3Z2+RF(I - a2)

(a - a2)Z2 -

(a - a2)CZ,! +aRF - Z2

a2(CZ'1 + RF)

KVb

+RF)

(a2- a)Z2 -

+a2RF - Z2

(al - a)CZ'1

CZ'I +RF

KV.

KVo 2CZ'1 + Col'o + 3RF

c[a (Z2 +¥) +(a - a2) (Zo +¥ +3RG)] - Co (Z2 +f)

2C+ Co

0

C

KI.

2(Zd¥)

c[a2(Z2 + ¥)

Co

0

0

Klo

Kl2

a) (Zo

+¥ +3RG)]- Co (Z2 +¥)

+ 3RG)]-Col'0(Z2 +¥) +3RG(a2RF-Zz)t(a2-a)¥ zo+(a2-1)¥Z2+3a2 Rt

+f) [CZ'I - CoZ'o +3(Zo +~ +2RG)]

Uacl/W p. 272

) ] - 3Zol2 - (I - a)RF(Zo +3RG) - (4 - a)Z2"Z Rp - 9Z2RG - 4 3(l-a)aRF2 +(a - a2)(ZoR +TF+3RG

1[ R F 3RF2] E ZIZ2+ Z2Z0+ZOZI + "Z (ZI +Z2 +Zo) + 3RG(ZI +Z2 +RF) +4

. ,[(a - I) (RF) CZ Zzt"Z

+2(Zo +¥ + 3RG)} + ~F (Z2 +2Z0 +~Rp +6RG)]

Rp 3 +(a - a2)(ZoR +"ZF +3RG) 1+3ZOZ2 +(1-a2)RF(ZO +3RG)t(4-a')Z2"Zt9Z2RG+4(I-a2)Rpl (a2- a) [CZ'I{(Z2 + ¥)

CZ ,I[(1- a2) (RF) Zzt 2"

RF) +3RG(aRF-Zz)t(a-a2)"ZZo+(a-I)TZ2+3aT RF RF RF2 CZI,[a(RF) ZdT +(a-a2)(Rp ZO+T+3RG )] -CoZo'(Zzt"Z

CZ+2 (Z2 +¥) +(a2-a)(zo +¥

(Z2

+ 3RG) (Z2 + ¥) (Zo - CoZ'o)

(Z2 - CZ'2) (zo + ¥

RF)(Zo +2" RF + 3RG ) +"Z RF (Zd Zo +Rd 3RG) cz 'I(Zzt Rd Zo + 3RG)t (Zd T

+ (a2-

(C - C~ (Zzt ¥)

. - COjZ2 +R)

f

' +RF - C(Zo "Z + 3RG )

C(Z2 +R/ +Zo +3RG)

C C

C -C

C

Kh

Phase b-Phase c-Ground

Phase a-Ground

Phase b-Phase c

Three-phase

5.11

Fault

TABLE

I)(CZ'I + R,)

Z'I +~ 2C

Z'I+~C

Z'I+~C

z ••

Z..

+(a-al)z al I -c- Z-c R'

Z'I +(al-a)z -c- Z-ca RF

Z'I+~C

z••

z'

1 E(ZItZ2+RF)

1 E(ZI HF)

(a - al)CZ'1 - 3Zz +(a - I)RF

K

(a -

t')

KV..

2(aZ- a) (CZ'I +

(aZ- a)(CZ'1 + R,)

KV••

CZ'I[(1- al)(Zz +

(a - a2)CZ'I +3Zz +(I - a2)RF

(1- a2)(CZ'I +RF)

KV••

I)(Zo +3RF)

C(a - al)

C(a-I)

K(I. -1.)

I

I)(Zo +3RF)

It)

3C

I)(Zo + 3RF)

+(a - al)Zz +(I - aZ)(Zo +3RF)

(a - al)Z2 +(a 13C

Z'

Z'

1

a2)Zz +(a -

£(ZI +Zz +Zo +3RF)

- JCZ'I +(a -

(aZ - a)(Zo +2Z2l +3(al - a)RF

3CZ'I - (aZ - a)Zz - (aZ-

CZ,I[(a - 1) (RF) Zz +"'2

al) (ZoR + "2F+) lRG] -

R, - [3Zz +(I - a)RFl (Zo + R, (I - a) (RF) Zl + "2 2" 2" +lRG )

Z' I-

ZIt

,

Z' I +~ 2C

f +3RG) +(1- al) (Z2+ f)]

, a) ( Zo + RF) C[(a·2" +3RG +(1

- a) (RF)] Z2+ 2"

RF [3Zz +(1 - a)RFl (Zo + RF) 2" +lRG +(I - a) (RF) Z2+ 2" T

C[(a- al) (zo +

[3Zz +(I - al)RFl (Zo +RF) 2 + 3RG t(1 - al) (RF) Z2+ 2" RF "2

1[ p 1ZI ] E ZIZ2R +Z2Z0 +Zoll + "2 (ZI +Zz +Zo) +lRG(ZI +Zz + RF) +4

+(a -

t') +(a- al) (zo +1' +3RG)] +(1- al) (Zl +~) i +[3Zz +(I -al)RFl (zo +1' +lRG) (al - a) (CZ'I +~) {(ZZ + t') + 2(Zo +t + lRG)}

C[(a -I) (Zz +~) +(a - al) (Zo +~ +3RG)]

C[(al - a) {Zz + 3~F +2(Zo + 3RG)}]

o

2C(al - a)

C(a2 - a)

K(I. - Ie)

-3C

c[(1-a2)(Zztt') +(a-a2)(zo+1 +3RG)]

~.-+c-Ground

3C

-;,

C(a - a2)

~.

C(I- al)

3~

Values of Relay Quantities for Different Types of Faults ~•.(Jround

K(I. -1.)

Quantity

5.12 Currents and Potentials Supplied to Phase Distance Relays TABLE

+ CoZ/o

2CZ'I

+ CoI/o

- CZ' I

+ CoZ'o + 3RF

Z' l

Zi l

+ CoI/o

Z\

- CZ' I

2CZ' I

+ Zz + Zo + 3RF)

+

Z'

+ (aZ I

I

a)Zo - 3aRF

Zo °Z'I

+ (l c- C

a)Zz

Z' I --I- -,-(a_-_a~2)_Z=-2-'.+_(,-'_-_az..:..)Z-,o::....-_3_aZ_R..:..F Z'o C-CoZ'I

2C

CoZ'O Z'I

Z'I+~

1 E(ZI

i)

O

Z' I ( Zz Co Z'

+ 3 (Zo + ~F + 2RG)]

+ RF "2 + 3RG)] -

[CZII - CoZ'o

aZ.~( ) Zo

+ RF) "2

+ RF) "2

I

Z'I

Z

I

-

2

,RF 1) -

Zz

4

+ -3 a2RFz

+ (a -

RF RF 3 aZ) - Zo + (a - I) - Zz + - aRFz 2 2 4 ) Z' R ) + (a -"aZ) (Zo -I- R ,; + 3RG ] - Co Z/~ (ZZ + 2F

R)] Z' ( R. a) (Zo + ,; + 3RG - Co Z/~ Zz + ;

2

RF a) - Zo -I- (a2

Z' I

+ ~F + 2RG) Z' C-Co~

3 (Zo

+ (a2 +R -{) +(aZ -

3RG(aRF - Z2)

c[az (ZZ

3RG(a2RF - Z2)

+ C[ R Cl (Z2 + -f-)

+

Z'I+

R F E1 [ ZIZz + ZzZo -+- ZoIl + 2" (ZI + Zz + Zo) -I- 3RG(ZI + Z2 + RF) + 43 RF2]

[facing p. 272

R F ) -+-(a-a2) ( R F ) ] -CoIo ' (ZZ+"2 RF) -+-3RG(aRF-ZZ>+(a-a2)"2Zo+(a-l)"2Zz+"4RF RF RF 3a 2 CZI,[a (Z2+"2 ZO+-i-+-3RG

RF) +(a2 -a) ( R F )--CoIo ] ' ( ZZ+"2 RF) +3RG(aZRF-Zz)+(a2-a)"2Zo+(aZ-I) RF· RF 3a2 CZII [ a2(ZZ+2 ZO+"2+3RG IZz+TRF2

+

RF) + (a +"2

Z' oI ( Zz a) ( ZoR + "2F +) 3RG] - Co Z'

~:~)(Zz + i)

oPb-4>c-Ground (C - Co RF) + (a2 +"2

(Zz

C [ a ( Zz

C [ a2 ( Zz

Values of Relay Quantities for Different Types of Faults oP..-Ground

+ (a Z-a)Zz + (a Z -1)Zo + CoI/o + + 3azRF CZ ' I + (a - aZ)Zz + (a - I)Zo + CoI/o + + 3aRF

- CZ'I

-

5.13 Currents and Potentials Supplied to Ground Distance Relays

TABLE

6 S'fJitehed "lUI Polyphase lJistaDf!e Bel"ys Reduction of Measuring Units-Automatic Switching SchemesPolyphase Distance Relay-Phase and Amplitude ComparatorsAnalysis of Polyphase Comparators 6.1. REDUCTION OF MEASURING UNITS

The early time-distance schemes had the merit of simplicity. Since the operating time was proportional to the distance of the fault from the relay only one relay was necessary per phase, but the clearing time was high for faults near the end of the protected section (fig. 6.1). The stepped timedistance scheme was introduced in Canada in 1925 by Paul Ackerman. It Time

FIG.

6.1. Faster overall time with stepped time distance characteristic

had the advantage of reducing the overall clearing time by the shaded areas in fig. 6.1 and is now universally used. The normal stepped time-distance scheme consists of fault detectors, distance measuring units and logic units; the latter include timing units, auxiliary relay units and flag indicators. Theoretically, four fault detectors and 18 measuring units are required for providing three time-distance steps for the ten varieties of phase-to-phase and phase-to-ground faults. Because of the cost and panel space so many units would require, their number is reduced in practice by using each measuring unit for more than one purpose. In almost all modern distance relays the K

273

6.1

Protective Relays

number of ohmic measuring units is reduced to a third by using the same units for the three time zones. Their ohmic reach is progressively increased through contacts on a timing unit, in a manner already described in Chapter 5, section 5.4.3. The number of measuring units is sometimes reduced to three by using the same set for phase and ground faults. The distance measuring units in modem schemes of this type are normally connected for phase faults (i.e. with delta potential and delta current) and are switched to wye connections only when a single phase ground fault occurs. This provides immediate clearing of interphase faults and a small delay in clearing single-phase-toground faults. This is considered expedient because of the greater effect of interphase faults upon the ability of the system to transmit load. This arrangement is shown schematically in fig. 6.2c. In continental Europe, since 1930, a single measuring unit has been used for all faults, the proper voltage and current being selected for each kind of fault by a rather complicated connection of the contacts of the fault detectors; this is shown schematically in a simplified form in fig. 6.3. Similar switching circuits are used for the current and potential circuits of the relay; a, b, c and n are fault detectors in the three phases and the residual circuit. Fig. 6.3 gives wye potential for all double ground faults and is used on systems grounded through ground fault neutralising reactors. The economy of using a single measuring unit has to be balanced against the following disadvantages: (a) time delay required for the fault detectors to assess the type of fault,

which results in a minimum tripping time of at least 0·1 second compared with 0·02 second with non-switched relays; (b) complete loss of protection if the single ohmic unit or any of the switching contacts fail; (c) possible wrong tripping if the type of fault changes during operation of the relay (effect of wind on arcing faults); (d) inaccuracy due to differing phase impedance (effect of unsymmetrical transposition of conductors); (e) possible reduction in reliability due to dependence upon a number of contacts in series in the a.c. switching circuits. The effect of these considerations is that schemes with six measuring units are generally used for important lines of 100 kY and above, with solidly grounded neutrals, because they require fast tripping and maximum reliability. For distribution lines, switched distance relays with a single measuring unit can be economically applied as a substitute for the normal time-overcurrent relays where high speed is required. For medium voltage lines, below 100 kY, a number of different switched relay schemes are in use, including the A - Y switched scheme and the interphase scheme in which separate single unit switched schemes are used for phase faults and ground faults. 274

Switched and Polyphase Distance Relays
b c

(a)

.. !I,o.\'OI\ but bOon

' ,",'

p .TI

"'-

"''''

~,

]

d

-"""'"

.-.,

I

V•

-"""""

I

V,

i~

cl:

c .TI,

e ~

Hr

L.!&.

...!I..

I

...l&.

Ie

I

.!S..

' W '"

t ~ -l-.L +

Hf 0.- ~

'I G ~ \MN '

"-

+-~

n1

$" ,.,62e

""iF"

...

g... I I

HtJ

(b)

x---f.-

b

(1

--0

0

c

-X

(1

-

~

(c)

6.2. Time distance relays for all faults (a) Early scheme (b) Later scheme (c) Blocking contacts for double-ground faults FIG.

275

6.1

6.3

Protective Relays

(b) 6.3. One distance relay for all faults (a) Basic switching circuit. (b) Typical European circuit FIG.

6.2. EARLY SWITCHING SCHEMES

The first switched schemes appeared in the late 'twenties and early 'thirties; they were simple but less accurate than modern schemes. Because of their not always employing the particular currents and potentials necessary for precise measurement in each type of fault, they were used only with impedance relays (where phase relation is unimportant) and accepted a certain variation in distance measurement. Most early schemes used wye current for all types of faults, wye potential for three-phase and phase-to-ground faults and half the line-to-line potential for phase-to-phase faults. For double ground faults some schemes used wye potential and some used half the line-to-line potential, as shown in fig. 6.2a. Overcurrent starting units, Sa' Sb' Sc and Sg determined the type of fault and applied the appropriate potential to the measuring relays. Zg is a compensating impedance whose value depended upon the Zo/Z 1 ratio of the protected line. 6.3. DEL TA-WYE SWITCHING

The simple scheme shown in fig. 6.2b can be used for distance relays of the impedance or the admittance type and the timing unit can be started by a polyphase overcurrent or mho type fault detector on phase faults, and by a residual current or power relay on ground faults. With reactance relays, the switching to wye voltage must only be permitted on single phase ground faults and the delta connections must be retained for double ground faults. This is because, on a double ground fault, the current in the leading of the two phases involved in the fault may be almost in phase with the corresponding wye voltage and the relay may measure zero or negative reactance and hence trip inadvertently. This is prevented by connecting the contacts of the fault detector auxiliary relays so that the switch to wye voltage is prevented if more than one of them operates (fig. 6.2c). This scheme has been used in Europe but the rather large number of contacts required tends to offset the saving of three measuring units; in fact,

276

Switched and Polyphase Distance Relays

6.4

for this reason, most manufacturers use six reactance units where high speed (0·03 second) is necessary and a single reactance unit switched for all faults where 0·15 second operating time is acceptable. 6.4. INTERPHASE SWITCHING

This consists of one measuring unit for phase faults and one for ground faults, these units being switched to the appropriate phase or phase pair by the fault detectors. The phase and ground protection are two separate schemes, fig. 6.4 for phase faults and fig. 6.5 for ground faults. A cheaper alternative is to use the switched distance relay only for phase faults and to use residual current relays for ground faults. This in fact is preferable for solidly grounded systems where the fault current magnitude varies sufficiently with the fault location to provide selectivity (see Chapter 4, section 4.1.2), especially on multiple grounded systems where most of the zero sequence current comes from the nearest grounding point, thus ensuring that the current for an internal fault is always much greater than for an external fault. A residual time-current relay with a very inverse time characteristic and an instantaneous unit is commonly used for this purpose and it not only provides economical protection against ground faults but also considerably simplifies the circuitry of the switched distance relay, as is shown in fig. 6.4. Another argument in favour of residual inverse time current relays is the fact that at stations where there is an ungrounded power source or a solidly grounded neutral and no power source enough fault current can return via the healthy phases to cause additional fault detectors to operate and hence cause wrong switching. In fig. 6.4 lop is the current (operating) coil of the mho type distance measuring unit, Vrest is its restraining potential coil and Vpo1 js its polarising potential coil. The fault detectors a and c are instantaneous overcurrent relays in phases a and c with a drop-out current about 60 % of pick-up. This is satisfactory on lines of 33 kV and below where the minimum phase fault current is at least tMce the maximum load current. Where lower phase fault currents are possible the fault detectors require a high drop-out/pick-up ratio and hence can only carry one contact; this necessitates auxiliary relays to provide the extra contacts necessary for current and potential switching. Similarly, in fig. 6.5, the fault detectors a, band care undervoltage relays. On resistance grounded systems the phase-to-neutral voltage drops to 50 % or less during a single-phase to ground fault, permitting the relay to carry the necessary switching contacts. On solidly grounded systems the voltage on the faulted phase may not drop to less than 80 %of normal and auxiliary relays must be interposed to do the switching. In fig. 6.5, lop is the current (operating) coil, 10 is the zero sequence compensating winding, To is the compensating transformer, Vrest is the restraining potential coil of the reactance unit and the directional starting unit and VpOI is the polarising potential coil of the starting unit. Vpol is shown 277

Protective Relays

6.4

Pot~ntiClI

Au • .C.T. lor ohmic a.diu.tmtnt

(a)

(b) FIG.

6.4. One mho relay for interphase faults (a) Diagram of a.c. circuit (b) Photo of relay

278

6.4

Switched and Polyphase Distance Relays

energised by residual potential but on solidly grounded systems it is generally necessary to use the line-to-line potential in quadrature with the current; this requires extra switching contacts. In the U.S.A. a reduction in the number of switching contacts has been achieved by using voltage compensation instead of current compensation. This is illustrated in fig. 6.6. The use of residual current (Ires = 3/0 ) eliminates any current switching. The potential switching between phases requires only To P.T's. ~ Q

ben

b

To

C.Ts.

c

n

FIG .

6.5. One reactance relay for single-phase ground faults

one NO and one NC contact per fault detector so that auxiliary switching relays can be eliminated and the scheme is a very simple one from the point of view of contacts. For this reason it is also applicable to static relays. In fig. 6.5, compensation for the zero sequence voltage drop between the relay and the fault is accomplished by adding a portion of the residual current to the phase current. It was proved in Appendix 5.10.1 of the previous chapter that the relay would measure Zl if KIo were added to the phase current supplied to the Z/O-Z/ I relay, where K = where Z/ 1 and Z/ o were the positive and zero Zl sequence impedances of the protected section of line. The alternative to current compensation is potential compensation. The relay can be supplied only with 10 in the current circuit and the phase-toneutral potential can have the positive and negative sequence voltage components removed. Since IIZ I = 12 Z 2 on transmission lines the compensa. Va- 2/ 1 Z I 10Zo tlon can be 2/1Z I • The relay then measures = - - = Zoo (6.1) 10 10 In order to measure Xo instead of Zo either a phase or an amplitude I

279

6.5

Protective Relays

comparator can be employed. In a phase comparator, such as an induction cup relay, the operating and polarising windings have 10 and the restraining winding the compensated phase-to-neutral voltage which must be switched to the faulted phase by a suitable fault detector (phase sc\ector), as shown in fig. 6.5. Fig. 6.6. shows an amplitude comparator used in the U.S.A. for this purpose. It compares the compensated phase-to-neutral potential V - 2/1Z' 1

Op~ra.t&

FIG.

6.6. Ground reactance relay with Vo compensation

with the vectorial difference of the compensated voltage and 2l0Xo where X' 0 is the replica reactance for the protected section. Balance then occurs when (6.2) -21 1 Z 1 -21 0 Xo = 1V-2ltZ~ from(6.1) i.e. when 11oZo- 2/ 0Xol = I/oZ.o1 R02+(Xo-2XO)2 = R~+X~ or i.e. when Xo=Xo

IV

l

6.5. SWITCHING FOR ALL FAULTS

Impedance and conductance relays with a single measuring unit are popular in continental Europe. Fig. 6.7a shows the circuit of a typical German impedance relay using an amplitude comparator consisting of a circulating current rectifier bridge and a rotary moving coil relay. An interesting feature of these relays is that the d.c. for energising the auxiliary relays is derived from the current circuit through a saturating c.t. and a rectifier, at the cost of 20 VA extra burden at c.t. rating. Ohmic measurement is carried out by comparing the phase voltages with the voltage drop across resistors in the current circuits. This requires an extremely sensitive measuring unit (50 p,W); the contacts are necessarily light but their carrying capacity is increased by a torque augmenting circuit (shown in fig. 2.32a of Chapter 2). These relays are designed primarily for operation on power systems grounded through Petersen coils. For this reason the measuring unit receives wye potential instead of delta potential during a double ground fault so that, if it is of the 'cross-country' type (i.e. two single-phase ground faults on different phases and in different line sections), only one phase is cleared and only one line section isolated, leaving the other phase to be cleared by the Petersen coil. This will be understood by reference to the following table of

280

6.5

Switched and Polyphase Distance Relays Imp,danc. unit

To

en.

DireetlonC1l unit

ReSistors

(a) Q.

Induetlon cup MHO rCIa.y r-----Q.

b To P.Ts.

b

n

c

n

(b) flO.

6.7. One impedance relay for all faults (European) (a) With current shunts. (b) With auxiliary C.ts.

operation. It will be seen that a phase fault to ground causes no relay operation; this is permissible on a Petersen coil grounded system because singlephase ground faults are self-extinguishing but, on other systems, an additional fault detector is necessary. One advantage of the scheme in fig. 6.7a is the fact that the current and potential switching can be similar but the use of the voltage drop across TABLE 6.1 Operation of European Switching Scheme

Fault a-b b-c c-a a-g b-g Current la-lb Ib-Ic la-Ie la-Klo Potential fab Vbc Vac Va

a-b-g b-c-g c-a-g c-g 3 -Ie+Klo la-Klo -Ie+Klo -le+Klo la-I. -Ve -Ve -Vc Va Va.

281

6.5

Protective Relays

the resistors in the c.t. secondaries for supplying the current circuit necessitates an extremely sensitive comparator circuit and relay unless the output of the comparator bridge is amplified before it is fed to the output relay. Where less sensitive relays are used, such as the induction cup, the C.t. secondary current must be supplied directly or through an auxiliary C.t. This means that the current switching circuit must be different from the potential switching circuit because the main c.t. secondaries must be shortcircuited when not connected to the relay; this is illustrated in fig. 6.7b which shows the connections of another German relay giving the same switching of current and potential, except that on a c-a fault the ground relay is connected to phase a instead of phase c. A compromise between these two methods is to replace the resistors in the current circuit by transactors and compare the transactor output voltages with the line secondary potentials in a voltage comparator. This scheme permits the use of an induction cup relay and yet allows the current and potential circuits to be the same. Fig. 6.8 shows the simplified circuitry of an English switched reactance relay. In England, France and the U.S.A., reactance relays are preferred for Phnst o-----r-----<>n

<>----+--r----o b

b

To

e...+":':::::"h~ITtll-oc

P.T~,

To

C,TS. L--...L...+-H-r-o n

FIG.

6.S. One reactance relay for all faults

ground faults because of the risk of high fault resistance which could prevent a mho or impedance relay from operating; for this reason the use of a single measuri.ng unit for phase and ground faults requires that it should be of the reactance type. The scheme shown in fig. 6.8 gives accurate measurement for all types of faults since it provides the correct currents and potentials, as shown in Table 5.7 of Chapter 5. The current circuit is very simple, having only one

282

6.6

Switched and Polyphase Distance Relays

transfer contact per phase. In spite of the extra switching needed for the polarising potential circuit of the starting unit, the total number of contacts is no more than for the simplest switched impedance scheme. The directional polarising coil uses the same voltage as the restraining coils, i.e. the faulted voltage. In order to ensure correct directional action for an interphase fault very close to the bus, the directional polarising voltage is augmented by 5 % of the voltage from the phase or phase pair leading the faulted phase or phase pair. Memory action is ineffective in switched schemes because it cannot be sustained for more than 2 cycles after the inception of the fault. 6.2

TABLE

Comparison of Switched Distance Schemes Measuring

Scheme

Minimum Trip Time Ground Phase Fault Fault

Units Normal (unswitched) Delta-wye Interphase Complete switching Polyphase relays

1 1 5 5 1

6 3 2

1 1

cycle cycle cycles cycles cycle

1 5 5 5 1

cycle cycles cycles cycles cycle

6.6. PHASE SELECTORS

Overcurrent fault detectors are used for phase selection in low and medium voltage systems for interphase faults because the minimum fault current exceeds the maximum load current magnitude. They are also used for singlephase ground faults on such systems when they are solidly grounded. The

a.

d.

/

~/

va

/

/

/

/

/

/ Vc

c~-----+------~b

I

/1

Vpol.= 3Vo

I

I

/

IVc

(a.)

(c) FIG.

6.9. Operation of undervoltage phase selectors

283

6.7

Protective Relays

resetting current of such fault detectors should be at least 90 %of their operating current value in order to ensure that they reset during an overload condition that may exist after the fault is cleared. Medium voltage networks, however, are often grounded through resistance so that the minimum fault current does not necessarily exceed the maximum load current. In such cases undervoltage relays are used for detecting single-phase ground faults and their contacts are sometimes connected in parallel with those of the overcurrent fault detectors. Fig. 6.9 shows how the wye voltage of the faulted phase is reduced by a single-phase ground fault while the other two wye voltages are actually increased. A more reliable phase selector than the overcurrent unit for single-phase ground faults, which can be used on both grounded and ungrounded systems, measures the phase angle between the residual potential and each of the lineto-line potentials. In the faulted phase the line-to-line potential lags the zero sequence potential by about 90°; in the other two phases the angles are about 210° and 330°. Other alternatives are (a) to compare the magnitudes of each pair of wye potentials, (b) to measure the phase angle between the negative and zero sequence components of current in each phase, (c) to compare the wye potential to ground with the corresponding wye potentials to a floating neutral. 8.7. POLYPHASE DISTANCE RELAYS

It would seem that polyphase relays would be more appropriate than single-phase relays for protecting a polyphase system. Unfortunately, it has not yet been found possible to devise the circuitry for obtaining uniform performance on all kinds of faults. One set of connections will give the same performance on multi-phase faults, i.e. three phase, phase-to-phase and double ground, and another set will give the same performance on ground faults, i.e. single and double phase-to-ground. Even if a universal connection were found it would not be entirely practical because the effect of load current could swamp the contribution of a light single phase-to-ground fault. Table 6.3 shows the current and potential vectors at the fault location and also their sequence components. Inspection of these indicates three conditions that exist at the fault location. (a) The line-to-neutral and line-to-line potentials are all in phase for inter-

phase faults. (b) VI = V2 for all interphase faults. (c) VI = V2 + Vo for single-phase ground faults. This indicates that polyphase distance relays (both phase and amplitude comparator types) can be made, using potentials at the relay location and compensating for the IZ drops in the line between the relay and the fault. The nearest approach to a practical solution is a phase comparator compensated for line drops and zero sequence components, to give the phase-toneutral potentials at the fault which give a zero sine product for a fault at the

284

6.8

Switched and Polyphase Distance Relays TABLE F<1ult

6.3 Phase Sequence Quantities in Faults Currt nh

Volt<1g<> Pas.

uro

Ntg.

Ph<1n

lora

Ntg.

Po •.

3

0

PhQ ••

0

<1



A·

0

-

#-'

J..

...",

e

0

A -, .. ~ A

<1

t

J.. '

c

b

0

b

~

o.b c

a.

c

<1-0

<1

~b

0.=0

<1

c-

b

a. c

' y'"

Q_O

b

.,r-.....

~

+ a.

Qbc

<1

<1be

I

..I.

c

0

+

b

b=c=O

c.,r-.....b

e~b

0

~ C" y ..t'

b

- "'y"'" +

e

t

e b

a.


A

e

<1

<1

ebb

0

Ph<1se

a.

<1

btl'

'

..

e

t

c

b

<1

t

b~c=O

balance point. Even this scheme underreaches for a balanced three-phase fault and requires a second relay to clear such faults. Amplitude comparators appear to offer a solution; however, they are not true polyphase devices but rather three single-phase comparators which are paralleled to a single output (tripping) device. They will all be discussed in the following section because they offer some measure of economy over single-phase relays at the expense of individual phase flagging. 8.8. POLYPHASE PHASE COMPARATORS

As early as 1928 European single-phase distance relays have utilised the fact that the faulted phase potential, compensated for the /Z drop to the fault, is zero, i.e. the relay operates when (IZ' - V) is positive (Z' is a replica impedance equivalent to that of the protected line section; V and / are measured at the relay). In 1946, the author suggested a polyphase distance relay for measuring the sine product of the two compensated wye potentials, i.e. {(/,,+K'/o)Z'-v,,} {(/b+K'Io)Z'-Vb} sin IX (6.3) where IX is the angle between the two compensated potentials. In 1948 an American company produced a similar relay for phase faults only (46), omitting the zero sequence compensation K 'loZ'. Fig. 6.10a shows the basic connections for one phase, the other being simila(. The resistor R adjusts the phase angle of the secondary voltage /Z of the transactor to suit the line phase angle, 4J = tan - 1 ~. The voltage supplied to each winding of the relay is of the form (/Z - V). Fig. 6.10b shows a preferred method using current comparison. Each relay winding is supplied with a current of the form

285

(I -;, ), where Z' is

6.8

Protective Relays

c:r.'f-__. , J Aux.c:rr a

Sta.tion bus

StQtion bus

R

P.T.

P.T.

R~pljca.

I

ImpedQnc.,Z.

IZ

ce

]

n

n.

Sin. product r_IQY

o

." e= v"

0

W

n

...,

~ v .. Ole

e-=

a..

Son. product r.la.y

(b)

(a)

FIG. 6.10. Polyphase distance relay (a) Potential phase comparison. (b) Current phase comparison

~

__________________

~ St _Q._lio _ n _b _ u _s

____________

~

_ _ _ _ _ _ a.

~--~----------------------------------~~-------b

R~plica.

im p_dane.:

P.TS<:._-=_...".....

.,

d

i"

...,~

"

b======:---------~-~--~] (a)

(c)

(b) FIG.

6.11. (a) Polyphase mho relay (phase comparator) for phase faults (b) Induction cup phase comparator (c) Static (Hall effect) phase comparator

286

Switched and Polyphase Distance Relays

6.8

the replica impedance. Here again the connections are shown only for one phase; the other is similar. Fig. 6.11 shows schematically the three-phase connections of a potential phase comparator for phase faults only. The torque of the relay is the sine product (6.4) which is zero when the two quantities are in phase or when either quantity is zero. Fig. 6.12 shows the effect of a b - c fault on the wye and delta potentials. a. /1 ~ /1 1\\

// f I

\\ Fa.ulted a.nd compensa.ted

II'I~ I \

/ I / I II

f

\Y

\ \

Fa.ulted

/ ,I \ \ /

Ie ,"

FIG.

I

,I

, ,,, /

/

I

I

'/

/

, I

I

,

f

,

'

I

I

I

\ /Norma.1 \v

\

\

1 \\ I \ \

HI

\\

\

\\

\

\

\

\

\

\

6.12. Effect of b - c fault on potentials at relay

abc is the normal triangle of line-to-line potentials. aFE is the triangle of potentials at the relay location. ab' c' is the potential triangle after -compensation, i.e. the potentials of the relay windings. aHG shows the completely collapsed triangle at the fault. The relay torque is proportional to the sine product of two of the compensated potentials, i.e. to the area of the triangle ab' c'. Fig. 6.13 shows the effect of the compensation on the relay voltages. Fl is the fault position corresponding to fig. 6.12, where the sequence of the relay voltages is the same as normal so that the relay contacts stay open .. F2 is a fault at the balance point where the compensated voltage triangle is completely collapsed and the relay torque is zero. For the internal fault F3 , the potentials are compensated and the base of the triangle is inverted so that the phase sequence of the compensated voltages is reversed, their sine product is negative and the relay contacts close. 287

6.8

Protective Relays Sine

A

F4

----1 ~ P~~~~f~~d rea.ch~

rJ..,-;

--1)>f-f----1~ 1~Ia.y : ia.ult behini relay

product

rz

Rela.y

I

loca.tion : Fa.ult within bala.ncf point

'7

)1

rela.y

B

FI

ofof---:)(~I- -

: Fa.ult a.t bala.ncf point

: Fa.ult beyond bala.ncr point

A ,A. iG. it.

- - System voltagu at bus A ~abc) - - - - Compensator voltages IZ - - Net vOltagos applied to induction cup unit For simplicity fault currents and
FIG.

6.13. Effect of compensation on potentials at relay location

The relay of fig. 6.11 does not operate on three-phase faults because the three-phase fault affects both compensators equally and for an internal fault two of the potentials are reversed so that the phase sequence is reversed by one and corrected by the other, hence the phase sequence remains normal. Consequently, balanced three-phase faults must either be taken as a calculated improbability or must be covered by an additional single-phase distance relay. Fig. 6.14 shows the connections .of a current comparator for protection against both phase and ground faults. It uses wye instead of delta potentials and employs zero sequence current compensation to ensure correct operation on phase-to-ground faults. Fig. 6.12 shows that, in a phase-to-phase fault, compensated wye voltages are also in phase for a fault at the balance point and hence the operation is equally reliable with wye or delta potentials. On ground faults, however (fig. 6.15) the wye potentials are not in phase at the fault location and the relay has zero torque for a single phase-to-ground fault on two of the phases but not on the third. For example, in fig. 6.15, if the relay is energised from phases a and b it will not trip for a fault between phase c and ground. An extra single-phase distance relay is shown in fig. 6.14 which takes care of phase c-to-ground faults and balanced three-phase faults. The foregoing phase comparators are based on 4-pole induction cup relays because they are sine product devices. It would be impractical to use dynamometer relays or any form of static relay now available because they are cosine product devices, and the use of a phase-shifting circuit for one phase would give a tendency to transient overreach. It is impractical to use phase sequence components, either singly or in groups, for phase comparison because the positive and negative sequence components have opposite phase rotation and such a relay would operate differently, depending upon which phases were involved. 288

6.8

Switched and Polyphase Distance Relays ____~--------~S~tn~t~lo~n~b~u.~bd~r~ . ----------------~ ~ ---r1---------------------------~_r b

~~~--------------------------------~~~ c P.Ts.

c:r.

ncb

~,

S.cond~y

-------

pountlar

abc

bu.

Prouct." line

6.14. Phase comparator polyphase distance relay for all faults

FIG.

Rclo.y torque

R.lo.y torque

a. I

/

/

I

I

I

I

T-

\

lZ~\ IZr

Va

I

\\ \

!

-

\

\

_/'

I/'/'

/

\

ct.::::..- - - - - - - - -

/

/

/

I

\ \ IZ~\

,,

\

/'

,,

/ /'/' 1IC:.. ________ _ \

\

(a.) FIG.

6.15. Operation of phase comparator distance relay on a single-phase ground fault (a) External fault. (b) Internal fault

289

6.9

Protective Relays

6.9. POLYPHASE AMPLITUDE COMPARATORS

Amplitude comparators are best adapted to static circuits. They can use either wye or phase sequence quantities; but neither gives much economy over single-phase relays and the use of phase sequence quantities introduces some loss in accuracy due to the use of sequence filters. Where wye quantities are used the operating quantities can be paralleled on one side and opposed to the paralleled restraining quantities so that the strongest operating quantity is matched against the weakest restraining quantity. A preferable arrangement, however, is to parallel the outputs of three single-phase comparators as listed below. The sign is used to denote the summation of the outputs of the three phases in the following equations. Tripping occurs when:

L

Impedance LI{l-K'/o)Z'1 > LIVI Reactance L12(1 - K' I o)X' - Vi > LIVI LI(I -K'/o)Z'1 > LIV -2(l-K'/o)Z'IAdmittance

(6.5) (6.6) (6.7)

Fig. 6.16 shows the basic circuit for the polyphase admittance relay; the others are self-evident. These circuits are advantageous with rectifier bridge circuits but uneconomical to apply to electromechanical relays. With the former, only three comparators are needed for phase and ground faults instead of the normal six. In the case of the polyphase reactance relay a polyphase admittance starting relay is needed to prevent it from tripping on load. This was explained with single phase reactance relays in Chapter 5. Fig. 6.17 shows a circuit using phase sequence components. In these circuits all the rectified potentials are added except the positive sequence voltage V 1 , which is reversed for providing restraint. They are less accurate than the circuits using wye quantities because (a) sequence filters introduce an error which increases with the load on them and (b) when a number of large quantities are combined, producing a small resultant near the balance point, small errors in the large quantities produce large errors in the output. Fig. 6.16 is based on the fact that 1V11-1V21 = 0 at the fault location for all faults except single-phase-to-ground faults where IV11-1 V21-1 Vo 1= O. The fault values of potentials are obtained by compensating the phase potentials at the relay before they are applied to the sequence filters. The underreach of the relay on single-phase ground faults is remedied by a monitoring relay which cuts in the component IVo - 10Zo I on the operating side when Vo > KV2 where K > 1 and depends upon the ratio of Zo to Z1' An alternative is to start with 1V1 - I1Z' 1-1V2 + 12Z' 1-lVo +10Zo I and either cut out the 1V0 +/oZo I component or double the 1V2+12Z'1 component when a double ground fault occurs. This alternative is less beneficial because it involves a race to prevent overreaching, whereas the first method merely involves a slight delay in tripping single-phase ground faults.

290

Q.

e

~ u ~

..,

~

.

C.T.

FIG.

faZa

(a)

V"

~

Vc N

Va

( Iv)

4>-G Fa..u lt

E

( III) ~-4K1 Fa.ult E

( ii) 4>- 4> Fa..ul t E

( I) 34> Fa.ult £

V,

(b)

I't-V~=O

--------------------1

V;'=V2+V~

!,~

!'Zi

J.'I=~

I,Z,

ft~

6.16. (a) Amplitude comparator polyphase mho relay for all faults. (b) Phase sequence voltage distribution during faults

Tra.nsa.clors R lZ

Sta.llon bus

£

E

[J

6.10

Protective Relays , -________________~S~~~ij~~~b~u~s~b~~r~s_______________r----~

+-r-----------------------------------~~---b SenSl!,ve differen!".1 POlo.rl:~ rela.y

. - -- t i c-/ - - ---,

Rtttificr bridgcs

Sequence potentia.l t--+-t--l filhr. t--+--f-+--! Scconda.ry potentia.!

bus

Protected lin' Singlc-phcuc-ground 1a.ult detector FIG.

6.17. Basic circuit of polyphase amplitude comparator distance relay using sequence components

The economical advantage of using three comparators instead of the six required for single-phase distance relays is more or less cancelled by the cost of the filters and the monitoring relay. 6.10. MISCELLANEOUS POLYPHASE RELAYS

The 8-pole induction cup relay makes an excellent polyphase directional relay (see Chapter 4, section 4.5.1 (d)). It can also be used as a starting unit (fig. 6.18) for a polyphase reactance relay for phase faults but it is not

Y.lo---+-==::::::...J

Vao--+-./

FIG.

6.18. a.c. Connections of induction cup fault detector (mho type)

accurate enough for mho or reactance distance measurement. Another useful application of this relay (fig. 6.18) is for directional comparison pilot-wire and carrier schemes, described in Chapter 7. The induction cup unit can also be connected to produce a torque proportional to the sum of the torques of three single-phase reactance units, i.e. 292

Switched and Polyphase Distance Relays

6'11

L{/(I -

V sin cP)} but such a unit would cost about the same as three separate units and could be used only for interphase faults because the operating torque ex: I: + I~ + K/~, ex: I~ + I~ + K' I~ and the II component due to load could cause serious inaccuracy during light single-phase ground faults •

•.tt. ANALYSIS OF POLYPHASE RELAYS The following analysis refers to the phase comparator and the amplitude comparator for all faults. 6.11.1. Polyphase Phase Comparator

The two windings of the sine product relay are energised with currents of the form (I + K'I 0

-

:)

where K' =

Z~~Zi; Z;

is the replica im-

pedance of the protected section and is made equal to the positive sequence impedance Z 1of the protected section; Zo is the zero sequence impedance of the protected section. Alternatively, potentials of the form {(/+K'/o)Z; - V} can be impressed on the windings. Both are equivalent mathematically. Referring to the latter, in phase a we have (Ia + K'/o) Z'l - Va which, during a phase-a-to-ground fault at the relay balance point, becomes

i{(2C+Co)Zi +Co(Z~-ZD-2CZ1-CoZ~-3R,}

(6.8)

This expression was obtained by substituting the values for la, 10 and Va given in the Table 5.7 at the end of Chapter 5. K =

!(Z~ +Zz +Zo+R,)

where R, is the fault resistance and Zh Zz and Zo are total system impedances. It will be seen that all the terms in (6.8) cancel out except -3R,/K, i.e. the expression is zero for a solid fault at the reach setting of the relay. For the same fault the expression for phase b is (Ib + K'Io)Zi - Vi,

= ~ {(Co - C)Zi + Co(Z~-ZD+/jjZz + CZi _(a Z -l)Zo - CoZ~ - 3a ZR,}

r.

1 Z-1)Zo-3a Z} = K{",3JZz-(a R,

(6.9)

Similarly, in phase c, for this phase-a-fault-to-ground we have

(Ic-K'/o)Zi -

=

k

=-

v;,

{(Co - c)Zi + Co(Zo -Zi)-~3jZ2 + CZi-(a -l)Zo -

1 ~-3jZz+(a-1)Zo+3aR,} K{

CoZ~ -

3aR,} (6.10)

It is clear that the product of the phase a quantity with either of the others

293

6.11

Protective Relays

is zero at the balance point if R, = O. If R, is not zero there will be a small error, as in all mho relays, depending on the magnitude of R,. The product of the two phases not involved in the fault times the sine of the angle between them is a negative quantity, which shows that the relay will not trip for a fault in the third phase. Consequently, a single-phase mho unit is required for clearing single-phase ground faults in the third phase as well as three-phase faults. This still makes the scheme practical because two induction cup units are a great deal more economical than the six required for normal phase and ground protection using single-phase relays. On phase-to-phase faults between phases band c we have, for phase a

(III+K'Io)Zl- VII which is - i(2Z2+R,)

(6.11

For phase b we have (Ib+K'Io)Zl- v" which is

~( -j.j3cz1+jJ3CZ1-a 2R,+Z2) = i(Z2-a2R,)

(6.12)

For phase c we have (Ic + K1o)Zl - Vb which is

~(j~3CZ1-jJ3czl-aR,+z2) = ~(Z2-aR,)

(6.13)

Obviously, the sine product of any two of these quantities is zero at the balance point, except for the error due to arc resistance which is present in all mho and impedance relays; hence the relay works correctly for a phase fault between any phase pair. It can be demonstrated that it works correctly also on double-ground faults but it will not trip at all on balanced threephase faults because the compensation effects all phases equally and their product is positive (restraining) whether the compensated voltages are positive or negative. 6.11.2. Polyphase Amplitude Comparator

The relay measures the sum of the compensated voltages of the there phases, viz. (6.14) II2(I+K'Io)ZlFor a phase a-to-ground fault the expressions for the phases are: For Phase a

VI-IVI

~12(2C+Co)Zl +2Co(Zo-Zl.)-2CZ 1-CoZ o-3RA-

~ 12CZl +CoZo+3R,I

= ~12CZl +CoZ o-3RA - ~12CZl +CoZ o+3R,1 (6.15) The difference between these moduli is zero except for the arc resistance terms. 294

Switched and Polyphase Distance Relays

6.11

For Phase c

1 /K 12(C o-C)Zi +2Co(Z~-ZJ.)-v 3jZ 2 + CZi-(a -1)Zo- CoZ~-3aRfl-

-kl-jZn-CZi-(a-l)Zo-CoZ~-3aRfl

=

1 r K I-CZ 1+CoZ~-v 3jZ 2 -(a-l)Zo-3aR f l-

~1.J3]z2-czl +(a-l)Zo+CoZ o+3aRf l

(6.17)

The sum of the moduli in expressions (6.15), (6.16) and (6.17) is zero for a fault at the balance point only if all the impedances are homogeneous in phase angle and Rf = O. This can be demonstrated by drawing the vectors (first changing the a operators to j operators with the help of Table 1.1, column 4). Similarly, for a fault between phases band c, the relay outputs are as follows: Phase a

1

1

KI2(2Z2+Rf )l- KI2(2Z2+Rf)1 = 0

(6.18)

Phase b

/-

r

1

1- K1 /-.J3jCZ 1+a2~f-Z2/

K I-v 3j2CZ1+v 3jCZ 1-a 2Rf +Z2

(6.19)

Phase c

1

K

-

-

1

-

1.J3j2CZ'l-.j3jCZ1-aRf +Z21- K 1.j3jCZ1+aRf -Z21

(6.20)

Here again the moduli of (6.18) (6.19) and (6.20) sum to zero for a fault at the balance point with homogenous impedances. For a three-phase fault the expressions are

k

times the following:

Phase a (6.21)

295

co 0)

I':)

2EZ2 Z1 +Z2 (a 2 - a)CZ'1 - Z2 E Zl +Z2 (a - a2)CZ't - Z2 E Zl +Z2 3Z2 + (a - a2)CZ'1 E Zl +Z2 2(a2 - a)CZ'1 E Zt +Z2 (a - a2)CZ'1 - 3Z2 E Zt +Z2

C Z '1 E Z1 a2C Z '1 E Z1 aCZ'l E Z1 Z'l (1 - a2)C-E Zt Z't (a2 - a)C-E Zl Z' (a - I)C --.! E Zt

Vea

VbC

Vab

Ve

Vb

Va

(Co - C)E Zl +Z2 +Zo Z2 +Zo + CZ't E Z1 +Z2 +Zo (CZ'2 -Z2)E Zl +Z2 +ZO ~CoZ'O - Zo)E Z1 +Z2 +ZO 2CZ'1 + CoZ'O E Z1 +Z2 +ZO (a2 - a)Z2 - CZ'1 + (a2 - l)Zo + CoZ'O E Zl +Z2 +ZO (a - a2 )Z2 - CZ't + (a - l)Zo + CoZ'o E Zl +Z2 +Zo 3CZ'1 - (a2 - a)Z2 - (a2 - l)Zo E Zl +Z2 +Zo (a 2 - a)(Zo + 2Z2) E Zl +Z2 +Zo - 3CZ'1 + (a - a2)Z2 + (a - l)Zo E Zl + Z2 +Zo

Phase a-Ground CE Zl +Z2 +Zo CE Zl +Z2 +Zo CoE Zl +Z2 +Zo (2C + Co)E Zl +Z2 +Zo (Co - C)E Zl +Z2 +Zo

Phases b-c-Ground C(Z2 +Zo)E ZlZ2 + Z2Z0 + ZoZl CZoE ZlZ2 + Z2Z0 + ZoZi CoZ2E ZlZ2 + Z2Z0 + ZoZl Z2(C - Co)E ZlZ2 + Z2Z0 + ZoZl C[a2Z2 + (a 2 - a)Z01 - CoZ2 E Z lZ2 + Z2Z0 + ZOZI C[aZ2 + (a - a2)Zo1 - CoZ2 E Z lZ2 + Z2Z0 + ZoZl CZ't(Z2 + Zo) + Z2Z 0 E ZlZ2 + Z2Z0 + ZoZl ZO(Z2 - CZ' 2) E Zt Z 2 + Z2Z 0 + ZOZt Z2(ZO - CoZ'O) E Zt Z 2 + Z2Z 0 + ZoZ1 Z2(3Z0 + CZ'1 - CoZ'O) E Z1Z 2 + Z2Z 0 + ZoZt CZ't[ a2Z2 + (a 2 - a)Zo1 - CoZ'oZ2 E ZtZ2 + Z2Z0 + ZOZI CZ't[aZ 2 + (a - a2)Zo1 - CoZ'oZ2 E Z,Z2 + Z2Z0 + ZOZI CZ't[(l - a2)Z2 + (a - a2)Zo1 + 3Z2Z 0 E ZlZ2 + Z2Z0 + ZoZl CZ'I(Z2 + 2Zo)(a2 - a) E Z lZ2 + Z2Z0 + ZoZl CZ'l[(a - I)Z2 + (a - a2)Zo1 - 3Z2Z0 E ZlZ2 + Z2Z0 + ZoZt

Table 6.4 is similar to Table 5.11 except that the fault resistance terms have been omitted, which makes the symmetry of the expressions more apparent and facilitates the checking of polyphase measuring units.

0

Vt

Ie

0

a2C E Zl

h

Vo

C E Zl

Ia

0

0

10

V2

0

0

lz

aCE Zl CZ'lE Zl

0

C E Zl

h

(a2 - a)CE Zl +Z2 (a - a2)CE Zl +Z2 Z2 + CZ'l E Zl +Z2 Z2 - CZ'2 E Zt +Z2

Phase b-Phase c CE Zl +Z2 CE Zl +Z2

Three-phase

Fault

TABLE 6.4 Currents and Potentials During Fault Conditions

(I)

iii -.;:

::0 (1)

(1)

0

........-.

"b

0 ...... (1)

...

:..

eft

Switched and Polyphase Distance Relays

6.11

Phase b 12a2CZ~ -a2CZ~ -a2Rfl-la2CZ~ +a 2Rfl

(6.22)

Phase c 12aCZ~ -aCZ~ -aR f l-laCZ 1+aRfl

(6.23)

The moduli of these expressions also all cancel out showing the scheme measures correctly on three-phase faults under the same conditions.

297

7 Directional Pilot Belaying Basic Principle-Pilot Wire Schemes-Carrier Channel SchemesCarrier Signal Checking-Future Trends 7.1. BASIC PRINCIPLE

A 'unit' form of protection may be used when it is important to clear all faults simultaneously at both ends of the protected section of line, such as when high-speed automatic reclosing is used. For unit protection it is necessary to exchange information about the fault conditions at each end of the prott:cted section and either a pilot-wire or a carrier channel is used for this purpose. Two basic principles are employed (a) to compare the direction of power flow at the two ends and (b) continuously to compare the instantaneous phase relation of the currents at the two ends. The first method is the subject of this chapter and the second will be dealt with in Chapter 8. 7.1.1. Directional Comparison

In directional comparison pilot schemes; the direction of power flow is compared by means of the relative position of the contacts of directional relays at the two ends of the protected section. This type of protection utilises the fact that, during an external fault, the power must flow into the protected section at one end and out at the other whereas, during an internal fault, the power can flow inwards at both ends. Directional relays at each end are connected so as to block tripping when fault power flows from the protected line to the bus-bar (fig. 7.1). By suitably interconnecting these directional relays through a pilot wire or a carrier channel, the position of their contacts can be compared and thus the location of the fault determined. An external fault (fig. 7.1 b) will cause the directional relay at the end nearest the fault to block tripping at both ends of the protected section. On the other hand, tripping will not be blocked on an internal fault (fig. 7.1a) because power flow will be from the bus into the line at both ends, or at one end if there is a single end feed. Load current (fig. 7.1a) will have the same effect as an external fault; the relay at the load end will prevent tripping. In Chapter 5, section 5.5.10, it was explained that, for a fault near one 298

Directional Pilot Relaying

7.2

terminal of a multi-terminal line, the direction of the current at another terminal could be outgoing under certain conditions even though the fault was internal. This circumstance could of course prevent tripping by directional comparison relays at all three terminals, except by back-up relays. In

r-o --

__of-o B

A

(a)

o

-
(b)

,

(Open

jf

volts restraint used)

~

(c)

-

Internal -; fau It

~

---..

0--

_0-1-0 o

~

Loo.d

0--

--

i

Extirno.l fo.ult

~

7.1. Basic principle of directional pilot relaying (a) Normal conditions. (b) External fault. (c) Internal fault FIG.

some cases the condition can be remedied only by providing the extra circuitbreaker and length of line necessary to eliminate the line junction, 7.1.2. Information Transfer Between Ends

The communicating circuit between the two ends can be either a pair of pilot wires, or a carrier channel using the power lines themselves, or a v.h.f. radio signal transmitted directly between the line terminals. The choice between these channels is generally made on an economic basis. For short lines, pilot wires are generally used; for long lines, a carrier channel is generally more economical; where 'line of sight' exists between stations, microwaves can be used. The choice is also influenced by the fact that power line carrier can also provide some additional facilities for communication, and a microwave channel can provide 30 or more sub-channels for telephone communication, telemetering, telecontrol, etc. (76). 7.2. SCHEMES USING A PILOT WIRE CHANNEL (73)

Systems of d.c. pilot relaying although excellent, fell into disrepute 25 years ago and are seldom used today (95). Although the difficulties could have been surmounted, a.c. pilot protection (Chapter 8) was becoming very popular, because the number of pilot wires had been reduced to two and great stress was laid on the fact that a.c. pilot protection did not require voltage transformers; facts which were generally neglected were: (a) the cost of p.ts was not serious at lower voltages where pilot wire relays were used; (b) often they were already available in the substation; (c) they were often necessary, in any case, for directional-overcurrent back-up relays.

299

7.2

Protective Relays

Simplified diagrams of the two general arrangements, called series and shunt, respectively, are shown in fig. 7.2. They are clearly duals. (i) In the series scheme, fig. 7.2a, the contacts of the directional relays, D, close for power flowing from the bus-bar to the line and, if they all close, indicating an internal fault, the trip coils T of the circuitbreakers CB will be energised and the fault cleared.

1-----I5l-----o~ Pilot

line section

'--_ _ _ _-0'0 Pilot

(a)

... I

I

: ......L....

F

I

(b) FIG. 7.2. Basic circuits of d.c. pilot schemes (a) Series pilot scheme. (b) Shunt pilot scheme

(ii) In the shunt scheme, fig. 7.2b, the contacts of the directional relays

close for power flowing from the line to the bus-bar (opposite from fig. 7.2a) and, if anyone of them closes, indicating an external fault, all the blocking relays B are energised, thereby preventing tripping at all terminals. None of the D relays will close for an internal fault and the fault detector relays, F, will individually trip their breakers. In the actual circuits shown in fig. 7.4 and 7.5 separate directional relays are used for phase and ground faults because the requisite sensitivity on all

300

Directional Pilot Relaying

7.2

faults cannot be obtained with a single relay. The relays for protecting against phase-faults are preferably polyphase (32) since single-phase relays cost more, are less sensitive and involve more contacts and circuit complexity. Both schemes use the basic principle of directional distinction between external and internal faults but differ in the method of exchanging the information between the directional relays. The series pilot wire scheme is basically an interdependent tripping scheme where open-circuit failure of the pilot wire prevents tripping, although an alarm relay gives a warning if this should occur. The shunt pilot scheme is basically a blocking scheme which permits incorrect tripping if the pilot wire is open-circuited; in the shunt scheme the pilot wire can also be used for transmitting other information. Both schemes are applicable to tapped lines or feeders without any special engineering or any change in setting or equipment. 'Ground preference', i.e. arranging for the ground fault tripping unit to override the phase blocking unit in the event of a heavy incoming load preventing tripping on a light internal ground fault, (82) is not necessary with the schemes above. This may be seen from fig. 7.4, where the tripping units have preference and from fig. 7.5 where voltage restraint can be used to prevent the phase directional units from operating on any load conditions, thereby preventing them from blocking tripping on a light, internal ground fault. . 7.2.1. Early Difficulties with d.c. Pilot Schemes

The two main complaints which have been held against d.c. pilot relaying are: (i) wrong tripping due to a contact race between the directional relays at the two terminals, either at inception or clearing of an external fault; (ii) incorrect tripping due either to cable leakage current or to a.c. in the relay tripping circuit caused by the IZ drop in the ground during the external fault; in this latter case the current path is through the battery grounding point at one station and returns through the capacitance of the pilot cable (fig 7.3b) The first difficulty can be avoided by time delay in the tripping relays or by designing them so that they will keep their contacts open during normal conditions regardless of the direction of the load current. In the series pilot scheme this requires voltage restraint on the directional relays so that both relays will start with contacts in the open position and one of them will stay open during an external fault. In the shunt scheme the relays initiating tripping are fault detectors of the overcurrent or impedance type and their setting can be made high enough to prevent operation on load current only. The second difficulty can be solved by using an ungrounded d.c. source. Referring to fig. 7.3a the difference in earth potentials at the two ends, due to the fault current (I[Z,), causes current to flow through the path shown 301

7.2

Protective Relays

in fig. 7.3b. Some station batteries are ungrounded but most are grounded through fault detecting equipment which can pass enough current to operate the tripping relay of the pilot scheme; this is limited to 60 rnA by the fact that the pilot wire resistance may be as high as 2000 ohms but the voltage across them must not exceed 120 volts. When a ground fault occurs, the voltage drop, 10Zo, in the ground is impressed across the insulation of the line potentia.l I I

Ea.rth potentia.l

_____________ ~_Z~ _____ _

--......0

I

I

I I I I

Externa.l

01--.. . . -t-l-'

Protected line section

,_f.:..a.u:;,.lt_

(a) T

Tripping end

-

:UJJJ

T

I I

Blocking end

I I I

t

~_r--------~I------------------~~

i+- Lea.ko.ge a.nd I

-----------_/

J

co.po.cita.nce pa.th

Fa.ult beyond this termina.l

(b) 7.3. Wrong tripping through pilot capacitance (a) Relay current path through pilot and ground (b) Potential drop·in ground due to fault current

FIG.

pilot cable wire and may well cause 60 rnA a.c. to flow through its distributed capacitance, as shown in fig. 7.3b. The remedy is obviously to use an ungrounded battery for the pilot wire circuit. The maximum pilot wire resistance that can be tolerated on d.c. pilot schemes is 2000 ohms, which limits the length of line protected to 23 miles if it is No. 19 AWG (20 lb/mile) pilot cable. 7.2.2. Series d.c. Pilot-wire Scheme

The basic d.c. connections are shown in fig. 7.4. The meanings of the various relay symbols are as follows: q,D = polyphase mho relay GD = ground directional relay q,F = polyphase overcurrent fault detector GF = ground overcurrent fault detector T = tripping relay A = alarm relay for pilot wire open-circuit indication.

302

7.2

Directional Pilot Relaying

The supervisory alarm relays A are normally energised by current flowing in the pilot wires; a break in the pilot wire circuit causes them to drop out and give an alarm. When a fault occurs the fault detectors rpF or GF pick up, opening contacts rpF and GF, de-energising the A relays and closing contacts rpF2 and GF 2 in the trip relay circuit. If the fault is internal the rpD or GD directional relays operate because power is flowing into the protected section, so that the continuity of the pilot wire circuit is restored via the tripping relays T, which then trip their breakers. If there is no power supply at one station, none of its relays will operate but the alarm A will be energised; tripping occurs only at the other stations with a.c. power sources. If the fault is external, the directional relays close at the terminals where power is flowing into the section but no tripping will occur; this is because the pilot circuit is interrupted at the terminal nearest the fault since its directional relays stay open. Voltage restraint on the phase directional units keeps their contacts open during normal load conditions so that there is no risk of a rpD contact not opening in time to prevent tripping on an external fault. The trip and alarm relay coils should have the same resistance in order to keep the pilot wire current constant. Where different types of relays are

TC Ar---~~----------~--+---------------~C

<po

+

1T A

A


(e)

(c)

~----------,----------------~

FIG.

7.4. Simplified diagram of series directional pilot scheme

used for blocking and tripping, the blocking relays must be at least as sensitive as the tripping relays for all types offaults. This is achieved in fig. 7.4 by putting the fault detector contacts rpF and GF in series with the trip relay. rpF 2 and GF2 can be separate overcurrent relays with higher settings than those of rpF 1 and GF 1 but this is usually an unnecessary precaution on low and medium voltage systems, since there is ample margin for effective relay settings, generally between the maximum load current and the minimum fault current. The advantage of high and low set fault detectors on a tapped line must be weighed against the possibility of preventing tripping on an internal fault on a two-ended line if the current at one terminal lies between the pickup values of rpFl and rpF2' or GF 1 and GF 2 • 303

7.3

Protective Relays

7.2.3. Shunt d.c. Pilot Scheme

Referring to fig. 7.5, in this case the fault detectors ¢F and GF tend to initiate tripping but the directional relays ¢D and GD prevent tripping when they close their contacts for power in an incoming direction, because the pilot wire signal operates the blocking relay, B, at all stations; hence no tripping occurs under load conditions or on an external fault. The auxiliary relay F should be given a delay of 0·01 sec in order to ensure that the B (A)

FIG.

(S)

(C)

7.5. Simplified diagram of shunt directional pilot scheme

relays will operate before the T relays on an external three-phase fault close to one terminal where the D relays may tend to operate slowly due to low voltage. When an internal fault ()ccurs, the directional relays remain open and the B relay is not energised; however, the fault detectors ¢F and GF close and pick up the auxiliary relay FX which then energises the tripping relay T, since the contacts B of the unenergised blocking relay remain closed. In this scheme there is no necessity for two sets of fault detectors because the directional relays do not initiate tripping and hence need no fault detector to limit their sensitivity. On unbalanced faults, the directional units will always pick up at a lower current value than the fault detectors because the latter must be set above full load, whereas directional relays (with quadrature connections) should pick up at less than 2 % of c.t. rating on unbalanced faults. Voltage restraint is recommended for the phase directional units in order to prevent one of them from energising the blocking relays during a light internal fault where the power flow in the unfaulted phase, due to load, is in the opposite direction from the fault current. Voltage restraint is not necessary on the fault detectors unless minimum faults require settings below maximum load current. 7.3. PILOT SCHEMES USING A CARRIER CHANNEL (8) (74)

The same basic principles of blocking and interdependent tripping are employed in directional pilot schemes using a carrier channel. The blocking scheme corresponds to the shunt pilot wire scheme and the interdependent tripping scheme is analagous to the series wire pilot scheme. 304

Directional Pilot Relaying

7.3

In the blocking scheme, illustrated in figs. 7.7 and 7.8, the carrier is started by fault detector relays which are countermanded by directional relays only if the fault current is flowing away from the bus; under external fault conditions this will be the case only at one terminal and carrier will still be transmitted from the other end, so that the carrier signal thus appears on the line and blocks tripping at both ends. The interdependent tripping scheme is known as intertripping or transferred tripping; a carrier signal received from the other end of the line causes local tripping, whereas, in the blocking scheme, it would prevent it. The Zone 1 units of the directional distance relays at the end nearest to an internal fault not only trip locally but send a carrier signal to the other terminal. This signal causes immediate tripping at that end also, although the fault may be beyond the Zone 1 reach of the relays at that end; this is shown in figs. 7.11 and 7.12. Failure of carrier prevents simultaneous instantaneous tripping at the end furthest from the fault, and this results merely in delayed tripping at that end. An alternative to intertripping is 'carrier acceleration', wherein the receipt of a tripping carrier signal from the other terminal increases the Zone 1 reach to Zone 2 reach, resulting in instantaneous tripping at both ends. This is shown in figs. 7.9 and 7.10. The carrier equipment is frequently used for communication and telemetering as well as protection so that only a portion of its high cost can be justifiably charged against protection. Voltage restraint directional relays are preferable in this context for the same reason as given for the pilot wire schemes. Since back-up protection is required for the periods when the carrier equipment is out of service for maintenance, it is convenient to use three-step directional distance relays of the mho or the reactance type, of which the second step is associated with the carrier tripping and the third step with carrier blocking. 7.3.1. The Carrier Channel (74)

The signal is injected into the power line circuit, as shown in fig. 7.6a, through coupling capacitors and is prevented from going outside the protected section by line traps, i.e. parallel resonant circuits tuned to the carrier frequency. The carrier signal is generated by a transmitter consisting of an electronic oscillator and amplifier with an output usually of about 15 to 20 watts at a frequency between 50 and 500 kc/s. Below 50 kc/s the size and cost of the coupling components would be too high; above 500 kc/s the line losses, and hence the signal attenuation, would be too great on long lines. 15 watts output has been found sufficient to cope with the losses of lines up to 100 miles, including the effect of icing, i.e. for a maximum loss of 30 dB. Carrier current can be used only on overhead lines because the capacitance of a cable would attenuate the carrier signal to ineffectual levels. The coupling capacitor consists of a stack of capacitors, series connected, inside a porcelain insulator for injection into and receipt of carrier signal from L

305

7.3

Protective Relays

the line. The drain coil presents a high impedance to the carrier frequency so that the transmitter can inject the carrier signal without permitting any appreciable voltage across it at system frequency. The line trap limits the carrier signal to its own section of line thereby preventing (a) interference with other lines and (b) shorting of the carrIer signal by an external fault. The carrier signal may be of fixed frequency and operate in an on-off fashion or it may be on continuously (at a lower power) and a frequency shift employed to operate the relays at the other end of the protected section. The use of modulation techniques permits a number of signals at slightly different frequencies to be used on the same circuit. The carrier signal can be introduced between one phase and ground or between two phases. The latter is technically better but much more expensive, since it requires two sets of coupling capacitors (this is no problem where the coupling capacitors are also used for a secondary potential supply) and two sets of line traps. This is preferred in the U.K. but the single conductor scheme is more generally used elsewhere. The single conductor scheme requires an earth wire for consistent results and demands more careful engineering since it has higher attenuation and has a higher interference level and stronger coupling with other phases. The presence of a fault on a line protected by blocking carrier causes no trouble, because the carrier is in any case cut off for an internal fault. On the other hand, the carrier channel must be operative for the inter-tripping and carrier acceleration schemes and some allowance must be made for the effect of faults, although the attenuatidn of the carrier signal is generally less than expected. The amount of attenuation varies from about 20 to 50 dB with single-phase ground faults and phase to ground coupling, depending on the location of the fault; it is of course worse with multiphase faults. Frequency shift carrier is considerably better because it permits a more sensitive setting of the receiver since the blocking frequency prevents tripping on spurious H.F. signals from disconnecting switches and arcing external faults. Back-up protection is necessary in either case if three-phase faults to ground are considered a practical risk. Fig. 7.6a is a schematic diagram of the carrier coupling circuit for coupling to two conductors; with single conductor coupling one transformer, one series coil and one coupling capacitor are removed. Fig. 7.6b is the simplified equivalent circuit corresponding to either one or two conductor coupling and shows it to be of the form of a band pass filter (see companion volume). A third type of pilot channel uses microwave (900 to 6000 megacycles) and the signal being beamed by parabolic antennae from one station to the next; up to 90 miles range is possible in flat country but obviously the range is limited by hills and buildings. This system is applicable only where there is a clear line of sight between stations. Because of the high frequency as many as 30 signals can be transmitted spaced about 10 kc/s apart. Each frequency channel can be modulated by a multiplexing transmitter with sub-carrier frequencies of about 500 kc/s. Experiments have been conducted and limited

306

Directional Pilot Relaying

7.3

Prohc'lti hi'll!

1~-----------------+-----

Coa.a to

'l rG.,.." • •l\c.r

(oj

Series

tun, ng

Cou pi i ng

I

.-----'UlICOiUll"~__-i ca.pa.tl t or , I I I

OCi""~'

Shunt

tunt-r

I I

,

FIG.

7.6. (a) Capacitor coupling to transmission line (b) Equivalent circuit of capacitor coupling

success claimed, for v.h.f. transmission, other than line-of-sight, using flat reflecting surfaces mounted at convenient geographical points and without auxiliary electric amplification. 7.3.2. Blocking Carrier Scheme

The principle of operation is shown in a very simplified form in figs. 7.7 and 7.8. When a fault occurs the phase fault detector cf>F or the ground fault detector GF at each terminal de-energises the holding coil H of the receiver relay R; the contacts of R would close if it were not for the fact that the fault detectors also start the carrier transmission with another of their contacts, and the carrier receivers at both ends energise the carrier coils C of the receiver relays thereby keeping their contacts open. On an external fault, the directional relay cf>T or GT at one end will stop the carrier transmission at its end, but carrier will still be produced at the other terminal holding the R contacts open at both terminals and thus preventing tripping. During an internal fault, however, the directional telays at both ends of the line stop 307

7.3

Protective Relays

the local carrier transmission and the receiver relays close their contacts, thereby causing tripping at both ends. The advantage of the holding coils on the receiver relays is similar to that of voltage restraint on a directional relay; the receiver relay contacts are :-----r.,

I

I.

I

_BA ~

-+---;--.----~

T. - - - -.....

o

R

(b)

FIG.

7.7. (a) Distance reach settings for carrier blocking scheme (b) Mho characteristics for carrier blocking scheme

normally kept open so that there is no contact race when an external fault occurs, and blocking must be applied before tripping can occur. Furthermore, discrimination is assisted by the pick-up time of cpTX or GTX and the flux decay time of the receiver relay.

308

7.3

Directional Pilot Relaying

At each station, e.g. station A in fig. 7.7, directional distance or directional overcurrent relays (cfJT and GT) are set to trip for faults within the protected system and a short distance beyond the end of it. Tripping for faults beyond the end of the section is prevented by the receipt of a carrier signal initiated by the cfJB or unit GB at terminal B, the one nearest the fault, where fault current is incoming (fig. 7.8). The blocking signal is usually initiated by disconnecting the control grid of the oscillator tube from the d.c. negative supply terminal (fig. 7.8). This Protected line Line trap r------+---------------r--~------r__o+

(a)

O--~l----~l------~----~l~o+ GT

I

Yl I

I'OY3 (t/IB)

IT

3

R

Y1'Ya and OY3 arc MI:IO units for Zones 1,2 and 3· OY3 Is offset a.nd reversed GTX

t/lTX

FIG.

7.8. (a) Basic d ..c. connections of carrier blocking scheme (b) Use of 3-zone distance relay for carrier blocking

is done in modern equipment by the directional unit (cfJB or GB) at the end of the protected section nearest the fault and this unit is polarised so as to measure outwards from the protected section (fig. 7.7a). If the fault is within the protected section, carrier transmission is shut off at both ends of the protected section by the directional relays which shortcircuit the cfJB and GB contacts in the carrier starting circuit and restore the grid to a negative potential, thereby stopping carrier transmission (fig. 7.8). 309

7.3

Protective Relays

Referring to fig. 7.7a, the reach of the carrier starting relay, cpB and GB, must exceed that of the directional tripping relays cpT and GT, otherwise an external fault beyond the carrier reach would not be blocked. (a) Fault Detectors. If the carrier should fail a fault at X beyond terminal B in fig. 7.7a would cause wrong tripping at terminal A. To prevent this the phase relays cpT usually have voltage restraint which gives them a mho characteristic and limits their reach to just beyond the end of the protected section, as shown in fig. 7.7b. Similarly, cpB is usually an offset mho relay so as to ensure positive operation for a fault close to the bus-bar; this ensures that there will be no risk of failure to send a blocking signal for the case of a fault just outside the protected section. In fig. 7.8 cpB and cpT are drawn as two polyphase units but, in most present day equipment, each consists of three single phase mho units with their contacts connected in series for blocking and in parallel for tripping. For ground faults an impedance characteristic is not necessary for limitation of the tripping zone; this is so because, on most high voltage systems where carrier relaying is liable to be used, there is a grounded transformer neutral at every substation thereby ensuring that faults beyond the next station would draw very little zero sequence current through the protected line section. Consequently GB and GT are usually zero sequence directional units polarised in opposite directions. At grounded stations they are polarised by the transformer neutral current; at ungrounded stations they are polarised by residual voltage. (b) Receiver Relay. The holding coil H is normally energised. Fault current flowing into the protected section causes cpT or GT to pick-up their auxiliary relays cpTX or GTX which then open to de-energise the holding coil H. If the fault is internal, this sequence will occur at both ends of the protected section and other contacts of cpTX and GTX will shut off the carrier transmission as previously explained. With both carrier and holding coils de-energised, the receiver relay will close its contacts, thereby permitting tripping. (c) Distance Relay Back-up. Three-step distance relays are generally used in a double role, to provide independent back-up protection and to provide the tripping and blocking functions of the carrier scheme. In a mho relay the starting or Zone 3 unit is used as cpB for starting carrier. For this purpose the direction of measurement is reversed as shown in figs. 7.7a and 7.7b. It still provides Zone 3 time back-up protection as explained in Chapter 5, section 5.4.3, but it is now located at the other end of the protected section. Reactance carrier is not common because carrier relaying is generally used on medium or long high-voltage lines where mho relays are preferable. Where it is used, however, the starting unit cannot be reversed and a separate impedance or offset mho relay is provided for starting carrier. In most distance relaysZone2is obtained by the timer extending the reach of the Zone 1 measuring unit after the elapse of the time for Zone 2. In the

310

Directional Pilot Relaying

7.3

carrier blocking scheme just described, however, it must be a separate unit because it has to operate immediately for an internal fault. In sections 7.3.3 and 7.3.4 other schemes will be described which do not require a separate unit. (d) Ground Preference (82). Before reversed mho relays were used for fault detectors this particular function was performed by non-directional impedance relays whose reach exceeded that of the directional (mho) tripping units. Hence it was possible, on a very long line, to start carrier on overload and prevent tripping on an internal ground fault. For this reason ground fault relays were connected so as to countermand any blocking or tripping which may have been initiated by the phase relays. The latter practice was discontinued because the ground preference could block tripping on a heavy internal phase fault if spurious zero sequence current was produced at one end due to unequal c.t. characteristics. With the modem arrangements of tripping and carrier starting being controlled by directional relays polarised in opposite directions, no preference is necessary. The only precaution which is now, on occasions, taken is to permit the carrier starting ground relay to prevent the tripping ground relay from shutting off carrier transmission during the transformer magnetising inrush current which may follow the clearing of a single-phase to ground fault. 7.3.3. Carrier Acceleration

In this scheme.a separate Zone 2 measuring unit is not necessary because the carrier signal is used for extending the reach of the Zone I measuring unit to Zone 2. In other words, a fault in the protected section, near one terminal, causes instantaneous tripping in Zone 1 at that end; simultaneously there is transmission of a carrier signal to the other terminal, which puts the distance relays at that end on to their Zone 2 setting so that they also can reach the fault and trip immediately (figs. 7.9a and 7.9b). This scheme is much simpler than the blocking scheme described in section 7.3.1. Furthermore, it is safer because the failure of carrier merely means that a phase fault in the end zone will be cleared in Zone 2 time instead of practically instantaneously. Fig. 7.10 shows the d.c. circuit in a very simplified form, assuming again that polyphase units are used for phase faults. The relay T 2X changes the settings of the phase and ground distance relays <jJT and GT from Zone 1 to Zone 2 reach. In normal distance protection this is done by the timing unit contact T 2 after the expiry of the second zone time; fig. 7.10 shows also that the receipt of a carrier signal from the other end operates the receiver relay which, in tum, operates the zone transfer relay T 2X, thereby putting the fault within the range of the distance relays at that terminal also. Incorrect tripping on an external fault is prevented by setting the initiating Zone I unit to cover less than the line length; a further precaution is usually taken, however, by connecting the receiver relay contacts through the directional fault detector contacts of the local relays CPT3 and GT3 • Although distance relays are generally used for ground as well as phase

311

7.3

Protective Relays

Norma.l rca.ch of

rcla.y. a.t A

Extcnd.d rra.ch of nldYs a.t A

R

(a)

fIG.

7.9. (a) Mho characteristic for carrier acceleration scheme (b) Distance settings for carrier acceleration scheme

Ib o

TrIp fIG.

~

Receiver

7.10. Basic d.c. connections of carrier acceleration scheme

faults in this scheme, it is also applicable where directional instantaneous residual overcurrent relays are used for ground faults, in which case they are arranged so that their pick-up setting can be reduced by the receiver relay so that they can reach to the end of the protected section. 312

Directional Pilot Relaying

7.3

7.3.4. Carrier Intertripping

An alternative to carrier acceleration is inter-tripping. It is similar to the carrier acceleration scheme except that, instead of extending the reach of the Zone 1 distance relays, the carrier signal trips directly. Wrong tripping due to a spurious signal from interference can be avoided by tripping through local directional fault detectors or by coding the carrier signal. This scheme is also applicable to distance relay schemes using directional residual overcurrent protection for ground faults. Fig. 7.12 shows the basic d.c. circuit of the scheme. It will be seen that the receiver relay can cause tripping if it receives a signal from the other terminal, provided that the local fault detectors indicate that this signal is in the tripping direction. Figs. 7.l1a and 7.11b show that with each end able to send a tripping signal to the other end, the overlap of the two characteristics will cause all faults within the protected section to be tripped instantaneously. For phase faults cPT 1 is the Zone 1 measuring unit and cPT 3 is the fault detector. Strictly speaking, cPT 3 does not have to be directional because the tripping of the Zone 1 unit at the other end proves that the fault is within the protected section. Hence, the offset mho type of fault detector is satisfactory . For ground faults, distance relays are connected in the same way as the phase relays but, if directional residual overcurrent relays are used for ground faults, GT 1 is an instantaneous overcurrent unit directionally controlled and GT 3 is the ground directional unit at the other end. Where the variation of ground fault current is too great to employ an instantaneous overcurrent relay for GT 1 , it can be a directional unit. For multi-terminal lines there are several advantages to be gained by intertripping directly without using the local directional fault detectors for monitoring purposes. The equipment is simpler and cheaper because it requires only a single-step distance unit and two receiver relays at each terminal; it avoids failure to trip on an internal fault if the current is reversed at one terminal due to a back-feed (fig. 5.36b). Another alternative is to set the distance relay at each terminal to overreach the other terminals under all conditions and to trip only if the received signals indicate that all terminals have operated. This can be regarded as the dual of the blocking scheme; it has the same advantages and disadvantages except that it is faster since it trips directly the signals are received, whereas a small delay is provided in the blocking scheme in case the blocking signal is tardy. This is sometimes called permissive overreach. With directional units at each end the scheme becomes similar to the series pilot scheme described in section 7.2.1 and the circuit must provide immunity from the effect of contact rebound, i.e. when an external ground fault occurs, the directional relay at the terminal farthest from the fault will send a carrier signal, but no tripping will occur because the directional relay at the end near the fault will be open; when the fault is cleared, there is the 313

Protective Relays

7.3

,1t;"I<-___ Tripp'n g Ion. for

,,10.1 Clt A by ca.rr ..~r Sl9n0..1 from B

,.,

U-I'++*--Dlrc:ct loco.l trippi ng lo ne from both ends

Trtpping lono for rola.ya.t 8 by co.r n c.r IIgnQ.1 from A

------------~~~~-----------R~

(a)

k------ Z~ with in hrtrfp -----------+1 A

I,

(b) 7.11. (a) Mho characteristics for carrier intertripping scheme (b) Distance settings for carrier intertripping scheme

FIG.


1

II

1

'PT,I GT, I c>--------------J

I

p.r

a. Tr ip Tro.nsmlttcr FIG.

7.12. Basic d.c. connections of intertripping carrier scheme

possibility that the sudden reduction of torque on this directional relay may cause its contacts to rebound to the closed position, thereby causing incorrect tripping if the carrier signal is still on. One solution is to cut off the carrier signal after about 4 cycles, so that no tripping will occur if the directional 314

7.3

Directional Pilot Relaying

relay does not close in 4 cycles plus the operating time of the relays and the carrier equipment at the other end. 7.3.5. Summary of Directional Comparison Carrier Schemes

7.1

TABLE

Carrier Scheme

Separate Zone-2 Unit Required

Effect of Carrier Failure

Ground Faults

Blocking Acceleration Inter-tripping

Yes No No

Incorrect trip Zone-2 trip Zone-2 trip

Distance Overcurrent Overcurrent

In the right-hand column of the above table, either distance or instantaneous directional overcurrent relays can be used for ground fault for all three schemes. However, ground distance relays are preferable for the carrier blocking scheme; this practice ensures that, if carrier is turned on for telemetering or telephone communication, tripping will be blocked if there is a ground fault near one end of the line section which draws insufficient current at the other end to operate the relays to shut off carrier transmission. On the other hand, in the absence of ground distance relays, tripping due to this cause, which would be virtually a misapplication, cannot happen if a separate channel is used for communication. Similarly, incorrect tripping with either the inter-tripping or the carrier acceleration schemes can be avoided by separate channels, or by monitoring the tripping signal through the local directional fault detector. Mho relays are also safer than directional overcurrent relays for phase faults in the carrier blocking scheme; this is so for the case of a threeterminal line, where an external fault may divide the outgoing current between two of the terminals so that neither has sufficient current to block carrier transmission at the tripping terminal which, of course, has the total fault current. Correct application is absolutely essential with such system configurations. The carrier acceleration and inter-tripping schemes are preferable for three ended lines since a fault near one of the terminals may be affected by the existence of low impedance path around to another terminal, thereby causing outgoing current at one terminal; this condition would cause blocking at all three terminals with the blocking scheme of directional carrier but would only cause sequential tripping with the other schemes. In the U.S.A. there is a growing tendency to use phase comparison carrier for ground faults, in conjunction with mho relay directional comparison for phase faults. This combination eliminated many such problems, including that of the effect of mutual inductance on ground faults. The best general solution would appear to be carrier acceleration or inter-tripping, with phase comparison as an alternative for ground faults where mutual inductance effects are involved. 315

7.4

Protective Relays

7.4. CARRIER SIGNAL CHECKING

A manual multi-contact switch is usually provided to check the carrier equipment. Operation of this switch starts the transmitter and switches in an arrangement whereby either, (a) the signal received at the other end initiates a return signal which can be read by the operator after he has released the carrier start button, or (b) signals 1800 out-of-phase are introduced at the two ends, which balance each other. Another system provides continuous supervision and uses a clock which periodically sends a signal for a pre-set period of time; the received signal operates a relay which sends a return signal for a similar period. Tripping schemes are checked with the trip-circuit open. The carrier receiver is normally designed to have a flat response characteristic, so that small changes in the receiver signal will not change the input to the receiver relay. In order to check attenuation, such as may be caused by icing of the line conductors, the set is temporarily biased so that it operates at the knee of its characteristic. If icing is proceeding, the received current will gradually be reduced, thereby indicating the necessity for de-icing the conductors. An alternative to bias testing is automatic, hourly monitoring wherein an alarm is given if the signal strength received falls below a predetermined safe value. 7.5. FUTURE TRENDS

In most modern sets, transistors are used instead of electronic tubes (valves). It seems probable that future schemes will employ polyphase mho relays and that in directional distance carrier schemes the inter-tripping and carrier acceleration method will replace the blocking method. For lines of less than 50 miles with 'line of sight' between stations, radio link (microwave) techniques may increase in use as power systems become increasingly automatic. Where the cost of carrier is not justified but pilot wires are available there is an increasing use of an audio tone signal over the pilot wires (99) which can be used for blocking and tripping in the same way as a carrier signal. Where the audio tone signal is used for blocking, or in the carrier acceleration scheme where the received tripping signal is monitored by local fault detectors, a continuous signal is used which also provides supervision for the pilot wires. In the inter-tripping scheme, where the received audio signal is used directly for tripping, it is important to avoid interference from outside sources such as a ripple on the a.c. supply and to design the tripping relay circuits so that they cannot be operated by d.c. surges. This can be done by combinations of tones or by using the frequency shift method. Since the time constants of the audio circuit are longer than that of a carrier circuit a delay of 10 to 15 m.s. is necessary to prevent using tripping on power reversal after an external fault. 316

8 A.C. Pilot Bel"ging Pilot Wire Schemes-Phase and Amplitude Comparators-Effect of Load Current-Multi-terminal Lines-Pilot Wire LimitationsPilot Supervision-Phase Comparison Carrier HEORETICALLY, unit protection provides almost perfect selectivity Tbut, when applied to lines and cables, it is less effective because the pilot channel or link between the terminals, together with the coupling equipment in the case of h.f. carrier, introduces amplitude and phase angle errors in the quantities compared (28) (140). This link may be pilot wires or high frequency carrier channel superimposed on the overhead power lines. The comparison is made between the c.t. secondary currents, making use of the fact that they should be equal under normal conditions and for a fault outside the protected section, in an ideal system. Pilot channels are very expensive; hence it is customary to combine the currents at each terminal into a single current either by means of a summation c.t. or by a phase sequence network (83), so that only one pair of wires or carrier channel is necessary. In the case of pilot wires, errors can be caused by currents induced in the wires by magnetic induction or by the potential gradient in the ground during an external fault (73). Furthermore, if the pilot wires are rented from a telephone company (76), there is always the possibility of their being interfered with in error, during maintenance of other telephone equipment at the exchanges. Carrier channels are affected by icing on the conductors and by high frequency interference due to lightning and to arcing in faults and circuit breakers. Wired pilots are used for short lines but are uneconomical above 10 to 15 miles. The cost of carrier channels is not directly related to their length but they are more expensive and their justification depends upon their joint use for other purposes, such as telemetering and telecommunication (59) (76).

Where the cost of carrier is not justifiable and available pilot wires are not suitable for current comparison audio tone relaying is recommended (99). The audio (voice frequency) signal is transmitted between line terminals and used in exactly the same way as high frequency carrier is used on a power line. 317

Protective Relays

8.1

The frequencies used range between 500 and 2500 c.p.s· and a different frequency is used in each direction. The signal not only blocks tripping on external faults but provides monitoring of the pilot wire circuit. Single tone frequencies are used without risk of wrong operation due to interference signals because tripping is controlled by fault detector relays (see section 8.11.1). 8.t. PILOT WIRE SCHEMES Modem schemes employ biassed relays whose restraining windings (or signals in the case of static relays) increase the relay pick-up for high values of through current ; this reduces the effect of c. t. inequalities and the errors due to pilot wire series-resistance and shunt-susceptance (78) (79). As with all comparator systems, either phase or amplitude comparison can be employed on a current or on a voltage basis. Most present day systems use amplitude comparison in a circulating current system since they are easier to apply to multi-ended lines and are less affected by pilot capacitance; especially when pilot compensation is provided (see section 8.4.1). The quantities available for comparison at each terminal are the·local current and the pilot wire current. Since. the latter is the difference of the currents at the two ends, the quantities correspond to 1A and (IA-IB) in fig. 8.1. C.T.

Prot~ct~d

Ilno soctlon

cr.

Pilot wirrs

o

~Rp

L-------~~r-----~----~~Ar----~

0

RolClY R triPsl A Clnd B

,

I I

I I

v,,=o Clt

midpoint

FIG.

8.1. Ideal current balance system

Another method is to compare the direction of each half-wave of current with the direction of the half-wave received from the other terminal via the pilot channel. This method is used in phase comparison carrier relaying (figs. 8.19 and 8.20) but is not now used with pilot wire channels; it was tried in Germany and the U.K. for some years (77) but was not considered practical because of the difficulty of designing and adjusting the relays so that they would respond to instantaneous reversals of direction at the inception and clearing of faults.

318

8.2

A.C. Pilot Relaying

As discussed in Chapter 3, schemes comparing the local current with the pilot wire current produce a circle offset from the origin along the real axis on a graph with co-ordinates

I~~I

Ap

and

jl~BI ' as in fig. 8.7b, i.e. in the fJ-plane. A,q

A polyphase comparison is made by combining the phase and residual currents from the C.t. secondaries into a single current by means of a summation c.t. at each terminal or alternatively by a sequence network (83). 8.2. DISCRIMINATING FACTOR

The selectivity of a unit scheme can be defined as the ratio of the relay operating current at one terminal for an internal fault to that for an external fault with the same primary current supplied from that terminal. This ratio is called the discriminating factor (D) which would be infinite for a diffetential system with the relay situated midway between perfectly matched c.ts, as in fig. 8.1. In pilot wire schemes, for practical reasons, the relay cannot be in the middle of the. protected section, but must be replaced by a relay at each terminal. This difficulty was overcome in an early system by a third pilot wire, as shown in fig. 8.2, which enables the relays at both ends to be at zero voltage

~~

______________ ________________

FIG.

t~

~

8.2. Three-wire pilot system equivalent to fig. 8.1

position for an external fault. Another solution, which is perhaps not economically practical, would be to have a relay at one terminal only and to produce the effect of its being in the middle of the system by connecting between the relay and the c.ts a resistor-capacitor system equivalent to the pilot wires. Transferred tripping would be necessary to clear the other terminal. In modern systems only two pilot wires are used and the discriminating factor may be relatively low on a long pilot because the relay current for an internal fault is reduced by the pilot resistance while the relay current for an external fault is increased by the capacitance between the wires. This effect is shown in fig. 8.3. 8.2.1. Discrimination Between Internal and External Faults

For proper discrimination, internal faults should be outside the characteristic circle for the particular terminal and all external fault conditions inside the circle. 319

8.2

Protective Relays

With double-end feed, all internal fault conditions are well outside the circle in the opposite quadrants, as shown in fig. 8.7b. With perfect C.t. performance, loads and all external faults should be represented by the point 1,0 which should be well inside the circle. Unfortunately, load current and c.t. errors can upset this theory of discrimination unless certain precautions are taken. The combination of a light

~~i-

_ _ _-::::;;:;;;....oo::::c.-_ _ _ _ _ _ _ RclQ.y seUing

+

FCluit seU Ing

FIG.

c

Fa.ult current Discrimina.tlng fo.ctor = ~g

8.3. Discrimination factor in two-wire pilot system

fault and a heavy through load current may produce a value of ex or p which is inside the circle and hence prevent tripping. Pilot wire attenuation or transient C.t. saturation at one terminal (due to c.t. remanent flux or due to the use of different types of c.ts at the two terminals) may move the external fault value outside the circle, causing erroneous tripping. The former is discussed in section 8.7.1 of this chapter and the latter in section 8.4.1, and in Vol. II. '.2.2. Multltermlnal Line.

For reasons given in Chapter 3 and elaborated in Volume II, discrimination between internal and external faults is more difficult with a voltage balance scheme than with a circulating current scheme. However, the condition described in section 5.5.10 of Chapter 5 can also cause incorrect blocking in a circulating current scheme. This is the case where there is generation only at one terminal and there is a low impedance path between the other two so that current flows outwards at one terminal for an internal fault near the other te~al having no generation. This condition can sometimes be overcome by the use of fault detectors because the current at the blocking terminal is usually small. Sometimes it is convenient to provide no relaying at the blocking terminal (which is usually the shortest leg) and treat the line as a two-terminal line with a tap. 320

A.C. Pilot Relaying

8.3

8.3. PHASE COMPARISON

The phase comparison voltage balance scheme shown in fig. 8.4 has been used in England for at least 25 years (80). It compares the phase angle of the local current with that of the pilot wire current. The actual comparison in the relay is between voltages derived from the currents, using the upper pole of a wattmetric type induction disc relay as a transactor, as shown in fig. 8.4. It will be seen that this pole is also used as a three-phase summation a. b

c

........ ~

Prouct

C.T'

V

S ~ (l dong

f-L'.

~ 0L4 p: ~

~~i4 v FIO.

011

C

D 1S C

8.4. Phase comparison pilot relay (induction disc)

transformer. The same pole carries a secondary winding across which appears a voltage corresponding to a combination of the local currents. The voltage is opposed to the voltage produced by a similar arrangement at the other end of the protected line section, and the difference voltage is impressed on the lower coil of the relay electromagnet. Normally, because the two voltages are equal and opposed, no current flows in the pilot wire or the lower coil and the relay has no torque. On the other hand, when an internal fault occurs, the voltages assist each other and current flows in the pilot wire and relay coils, thereby causing tripping. The torque of the electrical signal is of the form J(KJ - Vp cos cf», where cf> is the phase angle between the local current I and the voltage Vp across the pilot wires. In effect the relay measure~ the apparent pilot wire resistance V coscf> • Rp because the relay balances for K = p I = Rp' For this reason the characteristic is generally plotted on an R- X diagram rather than one with

. co-ordmates

IIBI 'L p and J'IIBI (. q'

This characteristic is shown in fig. 8.5. It is theoretically a straight line but becomes curved through the action of a restraining torque provided by a shading ring on the upper pole in order to prevent it from operating on external faults. Maximum tripping torque occurs on an internal fault; on 321

Protective Relays

8.3

external faults the current in the pilot wire is small because the voltages from the upper poles of the relays are opposed and the resultant voltage is shifted almost 90° in phase because of the predominating effect of the pilot wire capacitance. The advantages of this relay are low cost, simplicity and relative immunity to high pilot wire capacitance. Its limitations are slow speed and its inability G

-t~====~~~------B Interna.l fa.ult

Externa.l fa.ult

FIG.

8.5. Operating characteristic of phase comparator relay

to trip on single-end feed at the far end because the relay produces no torque with the same current in the upper and lower poles. It will operate with pilot loops up to 800 ohms. Fig. 8.6 shows an American scheme (79) using a similar principle with an induction cup relay which gives faster operation and greater sensitivity, Rcstra.int

Pol.

II

To

sum~""~i-~io_n

Pilot

II =------'---'----'00000'---/ wrs ~ Tra.nsa.ctClr

_ _ _ _ _----",

FIG.

8.6. High-speed phase comparison pilot relay

permitting operation up to 2000 ohms pilot loop resistance. The maximum pilot current is 0·1 amperes and the maximum voltage across the pilots 120. Both schemes operate for ground faults down to about 20 %of c.t. rating. The setting for three-phase faults is 52 % and, on interphase faults, 45 % or 90 % depending upon which phase pair is involved. There is no phase comparison pilot scheme available at the present time which uses the circulating current principle instead of voltage balance.

322

8.3

A.C. Pilot Relaying 8.4. AMPLITUDE COMPARISON (73)

Two alternative methods are used, the circulating current method and the balanced voltage method of comparison, both of which produce similar operating characteristics, shown in fig. 8;7. In the former, the c.t's at the two ends are connected so that the currents normally circulate around the pilot I

4

A B

Trip

/

, Trip

/ ' Block

/

(a)

'j'AI lie" ExtHno.l fault (1,0) Block zone

If Ip

Inter nul fa.ults on

A

~d~ou~b~I.-~.-nd~~~----~------~----~------- ~ Iud

Trip zone

FIG.

(b) 8.7. Operating characteristic of amplitude comparator (a) Scalar diagram. (b) Polar diagram

loop (fig. 8.8) but, during an internal fault, the c.t. outputs are opposed so that current now flows in the operating coils. In the balanced voltage system the c.t's are opposed so that no current should flow in the pilots normally but, when an internal fault occurs, current

323

8.4

Protective Relays Protected Iine section

a-t~====~----------~~~~~---b~-+~--~------------------------------c~-+-+&r~-------------------------------

Pilots

(a)

1IRES"RO .[3 m 3 14-k.x (b) FIG.

8.8. Circulating current pilot relay scheme (a) Basic diagram of a.c. circuit (b) Vector diagram of filter output

circulates around the loop through the operating coils of the relay. The one system is the dual of the other. •• 4.t. Circulating Current Scheme

A typical circulating current scheme is shown in fig. 8.8 (78). With a sensitive d.c. polarised relay this circuit will operate reliably on pilot loops up to 2000 ohms. Some manufacturers connect the restraining coil on the c.t. side, as in fig. 8.8a; others connect it on the pilot side (fig. 8.lOc). In general the restraining coil is connected on the C.t. side for long line schemes and on the pilot side for short line schemes with homogeneous pilots, especially where automatic reclosing is involved. This is because, with the restraining coil on the c.t. side during single-end feed, the relay at the end with no power source has little or no restraining current and hence has a lower pick-up than the relay at the source end. This discrepancy is greatest with short lines, hence this connection is used for long line pilot schemes. Conversely, with the restraining on the pilot side, the relay at the power source end has a lower ratio of restraint to operating current than the remote relay and this discrepancy increases with the length of the line; hence this connection is used for short lines. It is, however, possible to compensate for these discrepancies and to control them according to the application. For example, with the restraining coil on the pilot side, simultaneous tripping is obtained even on single-end feed with Zop set for pilot compensation, fig. 8.l0c, permitting the use of 324

A.C. Pilot Relaying

8.4

automatic reclosing; by omitting the compensation the relay at the remote end can be prevented from tripping on single-end feed, which is desirable on a long radial line in difficult country because it saves a journey or extra equipment to reclose the other end. Conversely, with the restraining coil on the c.t. side, the scheme can be used for short as well as long lines if Zop is correctly adjusted to compensate for the pilot wire impedance, fig. 8.lOb. The method of compensation depends upon the position of the restraining coil and the results desired, and is discussed in the following section:;, 8.4.1 (a) and 8.4.1 (b). The purpose of the compensation is to cause a similar ratio of restraining to operating coil current at the two terminals for an external fault. As in the case of generator protection, it is possible to use product restraint. This is described in reference (81) but is not yet used commercially. An analysis of the operating characteristics of different schemes has been given in Chapter 3, section 3.2.3 (a). Fig. 8.7a shows a typical characteristic on a scalar diagram. The dotted line represents external faults with no errors due to c.t. inequalities or pilot wire resistance and capacitance. The shaded area shows the relay currents at A and B where only A trips for an internal fault with a power feed from A only. Fig. 8.7b shows the characteristic in a complex plane with co-ordinates / IA / and j/ IA / which was called the-~ IB p IB q plane in Chapter 3. In Chapter 3, fig. 3.8 showed the effect of the attenuation constant y in moving the characteristic circle from the position shown in fig. 8.7b to positions above and below the real axis. It was explained, however, that by adjusting the operating coil circuit impedance Zop to compensate for the pilot impedance the circle stayed on the real axis. By inspection of fig. 8.7b it will be seen how important this compensation is because, if the circle does not move too much, there is considerable latitude for C.t. error, etc., between the two ends, but incorrect tripping could result if the C.t. discrepancy moved the complex quantity

~ in the

opposite sense from the movement of the

circle due to "I. In fig. 8.7b the circle is really the blocking characteristic for external faults and it can be seen that the point 1,0 should be well within the circle to ensure stability on external faults. Internal faults with double-end feed are normally well outside the circle and in the negative zone because the c.urrents flow in opposite directions. Internal faults with single-end feed are near the real axis, either close to the origin or towards infinity, because one current must tend to zero. Where Zop is adjusted to compensate for the phase angle of "I and turn it into the scalar value S, the circular characteristic of fig. 8.7b results. The circle cuts the real axis at two points P and P' whose distances from the origin OP and OP' depend upon the position of the restraining coil and the terminal considered (see Table 8.1). This indicates that the value chosen for K must be related to the value of y or So for a high discriminating factor. 325

8.4

Protective Relays TABLE

8.1

Intersection of Pilot Relay Characteristic on Real Axis Restraining Coil Relative to Operating Coil

'A' Relay on IX-Plane or 'B' Relay on p-Plane

'A' Relay on p-Plane or 'B' Relay on IX-Plane

op

op

or

Pilot side

l-K 1 +K" S

1 +K S l-K"

l-K 1 1 +K" S

C.T. side

1 1 +K' S

1 l_K' S

1 +K'

1

OP'

1

S

1 +K 1 l-K" S-

1

1

l-K'S

(a) Restraining Coil on c.t. Side. Since this connection is usually employed on long lines and Post Office pilots which have appreciable resistance and susceptance, it is necessary to compensate for "I to ensure discrimination. As previously stated, the object of the compensation is to maintain a similar ratio of restraining to operating coil currents in the relays at both ends' of the pilot. It can be approached by inserting inductance in series with the pilot wires (130) but it can be achieved by providing the correct amount of inductance in series with each operating coil. If the impedances of the operating coil circuit at each terminal (Zo, in fig. 8.10b) are adjusted so as to make them in phase with current fed from one end only, this will have the effect of replacing '1 by a scalar quantity s in the expressions for the radius and centre location of their respective characteristic circles; hence their circles will have their centres on the real axis of the IX- or p-plane. Furthermore, by adjusting the bias of the relay so that K = 1- S2, the expressions for rand c will be identical. This can be seen by substituting for K in the expressions for rand c in Table 3.1. of Chapter 3. This method increases the length of the pilot for which discrimination is possible and is achieved by adjusting an inductive impedance series with each operating coil to provide an impedance.

IZo,1 = -IZol [cot n sin (t/>-{I)+coth m cos (t/>-{I)]

(8.1) where m is the attenuation constant in nepers per mile, n is the phase shift constant in radians per mile, ZoN is the impedance of the pilot loops and Zo,IO is the impedance of the operating coil circuit. Inspection of the equation will show that if Zo, is made as inductive as possible (e.g. (I = 83°) its ohmic value is reduced and the sensitivity of the relay will be increased at the end remote from the power source during singleend feed; t/> is usually about 45° for long pilots. Zo, m and n can be found in handbook tables (151) (152); sometimes n is given in terms ofsin n and cos n. The derivation of equation (8.1) is given in the Appendix, section 8.12. (b) Restraining Coil on Pilot Side. Since this arrangement is usually associated with short lines close compensation for "I is not necessary and,

326

A.C. Pilot Relaying

8.4

owing to the relative positions of the operating and restraining coils, it is possible to obtain the economic advantage of using fixed compensation for a given type of pilot, irrespective of its length. By connecting in series with the operating coil of each relay a capacitance impedance Zo equal -to the impedance of an infinite length of the pilot, the currents in the operating and restraining coils can be made equal in magnitude during an external fault for any length of pilot and hence the operating characteristics will be the same for the relays at the two ends of the pilot. Zo can be obtained by measurement, because for any length of homogeneous cable (152) Zo = Zoc.Zsc where oc means with the pilot opencircuited and sc means short-circuited at its remote end. Zo can be calculated from the parameters of the pilot wires as follows:

- JR+jWL G+jwC

Zo -

=

(R:+W:L:)* jarg.;;[tan-1 ~ _ tan-l~] G + C wC wL W

(8.2)

where R, L, G and C are in ohms, henries, mhos and farads respectively. . With this method of compensation the operating characteristic of the relay at the source end is independent of the pilot length but the setting of the remote relay increases with pilot length because the characteristic circle for the remote (B) terminal on the (X-plane increases in size. Inspection of fig. 8.IOc will show that the operating coil and restraining coil currents at terminal A will be approximately equal and in phase for single-end feed ftom A,· irrespective of the pilot length, and hence the operating circle will have its centre on the real axis. For an external fault beyond B, the operating coil current at B will be reduced and de-phased by the argument of Zop from the current in the restraining coil and the circle will be larger, i.e. some tendency to trip on single-end feed from A. With short pilots, however, this effect will be small and need not be considered. 8.4.2. Causes of Loss of Discrimination

One cause, attenuation due to pilot wire resistance and susceptance, has already been dealt with in section 8.4.1. Another cause is unequal C.t. magnetic saturation. As previously stated, the value of (X or f1 should be within the characteristic circle for an external fault and should be represented by the point 1,0. If, however, one or more of the c.t's at one end saturate, the currents will not be equal during an external fault and the value of (X or f1 may be outside the circle. This saturation may be at high currents due to abnormal lead resistance at one end or due to the use of different kinds of c.t's, one end having c.t's with stalloy cores and the other end C.R.O.S. or H.R.C. steel cores. Generally it is a transient condition brought on by the d.c. component of an offset current wave and aggravated by remanent magnetic flux in one C.t. or by unequal lead resistance or dissimilar c.t. characteristics.

327

8.5

Protective Relays

This aspect of pilot relay design was neglected by manufacturers until recently but caused only occasional trouble because of the lower XI R ratios of power systems and the lower speed of circuit breakers (see Chapter 5, section 5.1.4 (b». Today, however, reputable manufacturers test pilot wire relay schemes on high current test sets (Chapter 13, section 13.11.1 (a» and specify the size of c.t's necessary to provide proper discrimination with their equipment. See formula at end of section 5.6.6 in Chapter 5. 8.4.3. Balanced Voltage Scheme

Here a transactor at each end provides a voltage proportional to the local current and it is opposed to the corresponding voltage from the relay at the other end of the pilot wire. Hence, theoretically, current flows in the pilot wire only during an internal fault (79) (80) (140). The relay coils are connected as shown in fig. 8.9 so that the pilot wire current tends to operate the relay and the potential across the pilot wires tends to restrain it. In this way the relay measures the impedance seen from one end of the pilot wires. This impedance will be high normally and during an external fault. Although no polarising winding is used, the relay will have a phase angle characteristic because the current that flows will depend on the phase relation of VA and VB as well as their magnitudes. In fact, the quantities energising the relay are analogous to those of a conductance relay, viz. III-IKI - Vpl where I is the local current, KI is the voltage it produces across the transactor secondary and Vp is the opposing voltage from the transactor at the other terminal, i.e. the pilot-wire voltage. The impedance characteristic is similar to the curve of fig. 8.5, but this type of relay will trip both ends with reduced sensitivity for an internal fault with single-end feed because it is not polarised and the operating coil is in series with the pilot wire. The ex-plane characteristic is the same as for the circulating current scheme with the restraining coil on the c.t. side of the operating coil and the equation for marginal operation is IIA - yIBI = KIIA I. The discriminating factor is low for long pilots and the non-linear impedances that are used for limiting the pilot wire voltages upset the voltage balance required to prevent tripping during an external fault. In voltage balance schemes, compensation can be effected by the use of a replica impedance equivalent to the series resistance and distributed shunt susceptance of the pilot wires during an external fault. Fig. 8.10a shows one method of applying this compensation (140). 8.5. EFFECT OF LOAD CURRENT ON RELAY SETTING

Referring to fig. 8.12a, IA = IF+IL and IB = Iv where IF is the fault IA IF+IL IF current and IL the load current. Hence a = - = - - = 1 + - (8.3) or IB IL IL

328

A.C. Pilot Relaying

Summntion C ,T,

8.5

-----/:~>(:.'----

~] Lc;,L-----o-: ~

n

'0----

Pilots

iwu;t·l

i i ._.~I.~ Oper.

Rest.

(a)

..

__---------------------8

c

t

::> u

'0 u

o scrlminnting

""

c

fador :

~..

*

a.

o

A

Pr imnry fnu lt current Fau lt sell ing

(b)

~1·~~I::==:==__n _+2~-n~-_-_-_-_-_-_~~ ·~I~

I,

k------n--t-::2:----C> 14

(c) 8.9. Voltage balance scheme (a) Basic a.c. connections (b) Discriminating factor (c) Vector diagram of summation C.t. output FIG.

329

8.6

Protective Relays

c,,{ ~]

\

I \

/

/

\

I

V

I

/\

\

I

I

\

\

,'----Pilots

R

(a)

a. b ~

n

Zp

(c) 8.10. Compensation for pilot-wire capacitance (a) A voltage balance scheme (b) A circulating current scheme (c) Automatic compensation for pilot length

FIG.

IF = 1.( _ 1 = (X _ 1. This means that the fault current to operate the relay 1B

h

during load can be found by measuring PQ, which is

~;.

This value of (X-I is in terms of the summation transformer output current. To obtain the corresponding value of c.t. secondary current requires consideration of the relative phase angles of the summation c. t. for load and fault currents. See section 8.7.1. 8.6. HALFWAVE COMPARISON SCHEME (98)

The basic connections of the circuit are similar to that of the circulating current scheme (fig. 8.11a) but the principle of operation is different. The relays have no restraining coils. Half-wave rectifiers are arranged to 330

A.C. Pilot Relaying

8.6

pass current through the operating coil in the tripping direction only during an internal fault. The resistances RA and RB are made slightly greater than that of the pilot loop Rp- During an external fault one of the resistors RA or RB is shortcircuited by its rectifier, depending upon which half of the cycle is considered a __~A~~__________~pr~o~t.~ct~.d~I~ ln~.~'.~Cl~'O~"__________-=~~B~_

b--~R=~--------------------------------~~~~

(0 )

(0)

G\, , \

./

(a)

8.11. Half-wave comparison scheme (a) Basic a.c. circuit diagram (b) External fault-first half cycle (c) External fault-second half cycle (d) Internal fault-first half cycle (e) Internal fault-second half cycle

FIG.

(figs. 8.11b and 8.llc). The alternate short-circuiting of RA and RB causes the relays alternately to have zero or negative voltage across them so that neither relay operates. The fact that RA and RB are slightly greater than Rp , the pilot wire resistance, makes the relays always have a slightly negative voltage. During an internal fault, the c.t. currents are relatively reversed so that positive voltage appears across both relays during one half cycle and zero 331

8.7

Protective Relays

voltage during the next half cycle. An additional half-wave rectifier is connected across each relay coil to perpetuate the coil current during the dead half cycle. Non-linear resistors protect the c.t's from overvoltage during the dead half cycle when the two c.t's would oth€;rwise be open-circuited (fig. 8.11e). This scheme is used in England for both private and telephone pilots. The relay is simple and inexpensive. It can be used with pilot wires up to 1000 ohms with an ordinary telephone type relay. It is somewhat affected by offset waves since its stability relies upon movement of the equi-potential point from one relay to the other during alternate half cycles of an external fault. 8.7. POLYPHASE SUMMATION OF C.T. SECONDARY CURRENTS

Figs. 8.10 and 8.11a show widely used arrangements of windings for summation c.t's. The output current for a given fault current magnitude in each type of fault is given in Table 8.2 in terms of c.t. rated current. Because the characteristics of pilot wire relays are plotted in terms of ex =

fA fB

or

p = ~ it is unnecessary to bring into the equations the number of fA

turns in the summation transformer primary except for the ratio of the tap turns. It is customary to make the turns between the a and b taps equal to those between band c and use a higher number ofturns (n) between c and n, the c.t. neutral return wire, so as to provide more sensitive action on ground faults. Consequently, the pick-up setting can be expressed in terms of combinations of n and 1 for the various faults and, in this way, add only one more constant to the equations. It will be obvious from fig. 8.9c that the sensitivity of the relay will vary with the type of fault; for example, the pick-up current for a c-a fault will be half the value for an a-b fault and the pick-up current for a phase-a-toground fault will be ~2 times the value for a c - a fault.

. n+

Fig. 8.9c shows that, owing to the 120° between phases, the output for a

r

1

balanced three-phase fault is '" 3, so that the pick-up current is ../"3 times that for an a-b fault. 8.2

TABLE

Effect of Summation on Pick-up Settings Fault

Summation c.t. turns Pick-up current

a-G

b-G

c-G

n+2

n+l

n

1

14%

16·5%

20%

90 0/0/

332

a-b b-c c-a a-b-c

90%

2

3

45%

52%

8.7

A.C. Pilot Relaying

It is possible to have a blind spot where load current is flowing in the opposite direction from a single-phase ground fault. Also, no output will occur when Ib = - 21a = - 21e which represents the current value on the delta side of a Ll- Y power transformer with a phaso-to-phase fault on the wye side. An a- c - G fault also cannot produce an output if the ground current is phase-to-phase current divided by (n + 2). The II +Klo filter network shown in fig. 8.8 originated in the U.S.A. (78). . . h-Ic The output of the filter IS Vp = J Xm + laRl + loRo = K}/l + Ko/o

v"3 '

where Kl and Ko are adjustments in the filter. It is free from blind spots if K is chosen to suit the parameters of the protected circuit. Unlike the summation transformer, the pick-up current of the relay supplied by this filter network does not depend upon which phases are involved but only upon the amount of II and 10 in the fault, which can be calculated from the Table 5.11 in Chapter 5. The general problem, however, is to select a form of network which can, with the minimum of adjustment, suit most system application conditions. This has been studied recently (83) and it has been shown that a combination of the form II - NIz where N > 3 is generally applicable. I

8.7.1. The Effect of Summation Transformer on 2: Ratio

IL

In section 8.5 and fig. 8.12c it was shown that the value of {3 =

!!! to permit tripping of an internal fault, IA

was given by the vectorial addition I

+ IF = IL

IX

= ~ or IB

i.e. to be outside the circle IX.

The value of IIF however L

depends upon which phases are involved because of their different treatment in the summation c.t. Fig. 8.9c shows that the output for a balanced three-phase load lags the unity p.f. position for phase-a by 30°. Consequently, for a ground fault on

IF = KaIIFjnj3'
L

The values of

,I,v3

Kdepend upon the angle of II: and can be calculated or

measured on fig. 8.12 by drawing lines at the appropriate angles of IF and

IL

measuring their length from the point 1,0 to their intersection with the characteristic circle. Fig. 8.12 shows the circle on the reference axis but, as 333

8.7

Protective Relays

explained in section 8.4.1, the circle may be displaced from this axis by pilot capacitance unless it is compensated for. Having measured these values of K, a table can be made of their values and the actual effect of the load upon the pick-up current calculated for

L",=Arg·t /

.0.

c-G

a-c

(c) FIG.

8.12. Effect of load on IX-plane characteristic with single-end feed (a, b) Single phase (c) With summation transformer

various faults during single-end feed. As previously stated, the effect of load current is negligible during double-end feed. The value of IF for tripping with an ~

a-G fault is a/3 from fig.

Consequently, with x% load, the pick-up is : : ; . 334

n+2

8.12.

1~ as a percentage of c.t.

8.7

A.C. Pi/ot Relaying

rating. The values for other faults are given in Table 8.3. To these values must be added the threshold value of pick-up given in Table 8.2. TABLE 8.3 Effect of Load on Pick-up Settings

Fault Summation c.t. turns Pick-up setting

e- G a-b b-e e-a a-b-e

a-G

b- G

n+2

n+l

n

bxV3

exV] lOOn

axV3 l00(n

+ 2)

l00(n

+ 1)

2

1

1

3

dxV3 exV] Ixvi

x

100

100

100

200

x is the load expressed as a percentage of the c.t. rating. The actual values of the setting factors a, b, c, d, e, J, g depend upon the location of the centre of the circle relative to the point 1,0.

.

..~:r

!

0:

Ma.ximum fa.ult current

Fcwlt setting (a)

I

Component for opera.tlon

----i------------l-l---I

Common(through) component

I I I

t

Current (b)

FIG.

8.13. Effect of a voltage limiter (a) Effect on pick-up (b) Effect on discrimination

335

8.8

Protective Relays

8.8. PILOT WIRE LIMITATIONS

Private pilots are usually put in at the same time as the power conductors. In cable systems, the pilot cable is laid in the same trench as the power cable and protected from interference by twisting and sometimes screening. Overhead pilots may be on separate poles on the same right-of-way or may even be combined with the earth wire. Fig. 8.14 shows the apparent impedance of commonly used open and screened pilot cables. An analysis of the effect of pilot-wire impedance and shunt admittance upon pilot relay characteristics is given in reference (140). Pilot wire cables are often rented from telephone companies (76) but they involve certain difficulties which have to be recognised, such as re-routing

10,000 6,000 4,000

.... ..

] 2,000 ..£u

c:: d

i

1,000

E

....c::

..

d Q. Q.

«

600 400

200

~

~ ~~~

100lL- -__ ____ __ 1 2 4 6

8

10

____

~ 20

____

~ ~~.~.~. 40

__ 60 80 100

Pilot length (mil •• )

FIG.

8.14. Apparent impedance of pilot-wire pairs

by the telephone staff causing a change in parameters, the insertion of inductive loading coils and sometimes open or short-circuiting during switchboard maintenance. Also, the voltage across the pilots must not exceed 130 V and the current 60 mA, compared with 200 V and 200 mA for private pilots. This means that limiting devices have to be used in order to provide sufficient relay sensitivity during minimum generating conditions without exceeding the permissible voltage and current during maximum conditions. Limiting devices employ either neon lamps (U.S.A.) which cut off sharply at about 90 V or thyrite which has a characteristic of the order of V4 = kI, where k is a

336

8.9

A.C. Pilot Relaying

constant and is controlled by series resistance. Zener diodes are now coming into use for this purpose. Telephone pilots are usually either 2000 or 3500 ohms with padding resistance to make them up to the nearest value. All equipment connected to these pilots must stand a 15 kV test to ground. B.9. PILOT SUPERVISION

Although private pilots are very reliable, telephone pilots require supervision and in some countries overhead pilots are often stolen for their copper sales value. The effects of open or short-circuiting the pilot wires are as follows: TABLE

8.4

Effect of Open or Short-Circuiting the Pilot Wires Voltage Balance Scheme

Pilot Fault

Circulating Current Scheme

Short-circuit

Fails to trip on internal faults

Trips on full load

Open-circuit

Trips on full load

Fails to trip on internal faults

Automatic supervision is usually applied in conjunction with overcurrent fault detectors which prevent wrong tripping on load. The supervision method usually consists of circulating a few milliamps d.c. in the pilot and providing relays which give an alarm if this d.c. is drastically increased or decreased. The alarm is delayed a few seconds so as not to operate during faults. The d.c. has to be extremely well smoothed so as not to interfere with telephone conversations.

Pilot rclo.ys

10,uF

10.u F

120 V A C.

FIG. M

8.1S. Pilot supervision scheme

337

P,lot rclo.ys

8.11

Protective Relays

Fig. 8.15 shows an effective supervision circuit. Relay Rl and R4 drop out and sound the alarm if the supervisory d.c. is interrupted by an open pilot or failure of the d.c. supply; if the pilot is short-circuited R4 again drops out and R2 picks up on the increased current. R3 drops out if the d.c. supply fails. R4 also drops out if the pilot wires are crossed, reversing the polarity of the received current. Relays Rl and R2 can be combined into one relay with a contact that assumes a central position when de-energised. The settings of these relays are normally of the order of 1·5 rnA pick-up and 0·7 rnA drop-out. 8.10. PROTECTIVE DEVICES

Besides affecting the operation of the relay, the high voltages that can appear between the ends of the pilot due to a.c. potential gradients in the ground can also damage the equipment and injure personnel. Fig. 8.16 shows the modern methods of coping with this problem. A 15 kV insulating transformer insulates the relay from the pilot wires. A Ga.. filled protector tube

15 KV in.ula.ting r - - - - - , tra.nsformer

r------,

Pilot rela.y

Pilot wires

Dra.ina.ge rea.ctor

FIG.

8.16. Pilot-wire protection against over-voltage

neutralising transformer cancels out induced and differential ground potentials but it is not necessary where supervision (S) is not employed. Both transformers are wound so as to have low impedance to circulating currents. A gas-filled protector tube and drainage reactor at each end of the pilot wire protects the relay against longitudinally induced voltages. The drainage reactor is also a transformer wound so as to provide a high impedance across the pilots and a low impedance for currents flowing simultaneously from the pilots to earth. 8.11. PHASE COMPARISON CARRIER

A high-frequency carrier via the conductors of the protected power line may be used for unit protection in the case of overhead lines, but it is impractical with cables because of the very high attenuation due to shunt susceptance. This is a clear-cut case for the application of transistor techniques (28) (29). 338

8.11

A.C. Pilot Relaying

In pilot-wire differential relaying, amplitude comparison is used more frequently because the errors introduced by attenuation are fixed and can be compensated for. In a carrier channel, however, the attenuation varies with atmospheric conditions and hence phase comparison is used. The length of pilot wire channel that can be used is limited by the phase shift due to the capacitance between the wires; similarly, the length of transmission line that can be protected by phase comparison carrier is limited by the phase shifts caused by (a) the propagation time, i.e. the time taken by the carrier signal to reach the other end of the line (up to 0·1° per mile), (b) the response time of the band pass filter (about 5°) and (c) the capacitance phase-shift of the transmission line (up to 10°). Amplitude comparison is limited by error due to phase shift and phase comparison is limited by error due to attenuation (fig. 8.17a). The latter is Typica.l input Squa.r~d

minimum input

, :

I I ,

I

I

I I t

:

I

I

,

Thr~.hold

I

l ' ----:--..J : : i-inttlrva.1

,

Fixed interva.l

I

I

~l interva.l ~

(b)

(a)

FIG. 8.17. Effect of current amplitude on phase comparison (a) Variable interval with sinusoidal wave. (b) Fixed interval with squared wave

minimised in phase comparison carrier by squaring the waveform for any amplitude greater than some value below the minimum signal level (fig. 8.17b). Both of these errors have been recently considered in the literature (28) (29) and receive further treatment in Vol. II. 8.11.1. Principle of Operation

At each terminal a high frequency carrier signal is injected into the line, as explained for directional carrier in Chapter 7, and the signal is modulated at each end by the squared wave of local current, so that blocks of carrier frequency are transmitted corresponding to half-waves of system frequency in one direction, i.e. there are alternate half cycles of carrier signal and no signal (fig. 8.18). During an external fault the half-waves of current are equal in magnitude but 180° out of phase so that a continuous signal, consisting of alternate halfcycle blocks of carrier supplied from each end in turn, is provided and tripping is prevented (fig. 8.19a). During an internal fault (fig. 8.19b) the currents at the two ends are in phase so that both ends produce a similar block of carrier

339

Protective Relays

8.11

signals at the same time for one half cycle with nothing during the next half cycle and this permits tripping. Fault detectors or sequence networks ensure that carrier transmission is started only when there is a fault because continuous transmission would be uneconomical and might cause radio interference. The carrier signal can be

n[\ [\ /\ /\ n FIG.

n[\ !\ [\ /\ n

8.18. Carrier modulation in half-cycle blocks

(a) FIG.

8.19. Transmitted and received signals in phase comparison carrier (a) Continuous signal during an external fault (b) Intermittent signal during an internal fault

introduced between phases or between phase and ground and the basic transmitter and receiver equipment is similar to that used for directional comparison. The phase-to-ground method is cheaper but it requires an earth wire and has somewhat higher attenuation, interference and coupling with other phases. The transmitted power level is 10 to 15 watts and the receivers are designed to permit about 30 dB attenuation. 340

A.C. Pilot Relaying

8.11

8.11.2. Equipment

Fig. 8.20 shows the basic functions of the equipment. The currents from the three c.t. secondaries are combined into one current by a summation network based on a preferred combination of phase-sequence quantities. The resultant current is supplied to a modulator which is associated with a carrier frequency oscillator. The half cycle blocks of carrier frequency are injected into the transmission line via an amplifier and a capacitor coupler

I

I

1

IBO O FIG.

8.20. Effect of stability angle upon operation of phase comparison carrier

(fig. 8.21). The carrier, however, is controlled by fault detectors so that it is started only during faults. In order to cope with the wide range of fault current amplitudes, a limiting device is used at the input of the carrier equipment. The signal received from the other terminal is passed to a demodulator for extraction of the power-frequency component in the form of a cyclic quantity of rectangular wave-form which is supplied to the comparator or phase-discriminator unit, along with the local signal, the latter being suitably limited to bring it down to a level comparable with the remote signal. The output from the phase-discriminator goes to the tripping unit. (a) Summation Network. Summation c.t's similar to those described in section 8.7 of this chapter have been used; modem practice is to use a combination of phase sequence components, the most common being (/1 - NI2) where N is adjustable or in some cases fixed at 4 or 5 (fig. 8.22). This subject is discussed in Vol. II and reference (83). The output of the summation circuit must reach the threshold value of the modulator for all fault conditions for which the starting relays or network function, including the effects of load and capacity currents. To ensure this the output should correspond closely to that of the starting network and both

341

8.11

Protective Relays

should be linear over the fault range in order to avoid error due to unequal phase shifts at the two ends. Finally, the transient response should not produce parasitic frequencies at non-system frequency. (b) Oscillator. The oscillator circuit produces the carrier signal. It is usually arranged to operate continuously and, in modern equipment, its frequency is kept constant by a piezo-crystal unit. (c) Modulator. A continuous high-frequency carrier signal is supplied to the modulator together with the local current at system frequency. The

Oscill<1tor

ModuI <1tor

Tr<1nsmitter

Squ<1ring circuit

't' Fa.ult input

F<1ult detector

~-~-

I

Low set Sta.rt circuit

I

I I

High set

!

I

I I

l _____

I I

+I

+

Time del<1y I IForced Ist<1bility I I

Comp<1r<1tor

DemoduIa.tor

R«cf!iver

I I

+

Reset

FIG.

Trip Circuit

8.21. Block diagram of phase comparison carrier protection

square-wave system frequency signal acts as a cyclic gate, and high-frequency signal is passed through the modulator in half-cycle periods; maintenance of unity mark-space ratio in the modulating signal is assured by design of the squaring circuit. (d) Transmitter-Amplifier. The signal from the modulator is increased to at least 10 watts by a power amplifier which is suitable for the range of frequency over which the oscillator can be adjusted. In some equipment the amplifier is tuned. The output from the amplifier is fed into the power line through a capacitance device, as explained in Chapter 7. (e) Carrier Receiver. The receiver has to accept the carrier frequency which has been chosen for the channel and reject all others that may be used on the conductors of the same line for communication, telemetering, etc. 342

A.C. Pilot Relaying

8.11

Carrier frequencies are used for communication purposes in 4 kc/s bands but for protective purposes, where simplicity is demanded, the frequencies used would be much wider apart than 4 kc/s. Closer spacing and therefore better selectivity could, if necessary, be obtained at the expense of operating range. The receiver input impedance is matched to that of the line and coupling circuit in order to avoid trouble due to signal reflection. Included in the receiver are a demodulator, a voltage amplifier and a limiter which equalises input signals so that the phase comparator or discriminator can operate effectively, without overloading. (J) Phase Comparator. The phase comparator produces a tripping signal when the interval between carrier blocks exceeds about 30° and prevents tripping when the interval is less than this (fig. 8.21). Theoretically, the interval between carrier blocks is 180° on an internal fault and 0° on an external fault, but in practice the 180° interval can be reduced due to (a) phase displacement between the generated e.m.f.s at the ends of the

system; (b) through load current being added to the fault current at one end

and subtracted at the other. The 0° interval can be increased by the previously mentioned phase shifts due to the carrier signal propagation time, the response time of the receiver band-pass filter and the capacitance of the protected line. The effect of line charging current can be compensated by supplying the modulator with some additional current lagging the local potential (or reversing the current through a capacitor) but the first two items cannot be compensated for. The narrowing of the tripping interval due to differences in summation at the two ends of the line can be minimised by using separate protection for each phase, or by using phase-comparison carrier with zero sequence current only, and taking care of phase-to-phase faults (which are rare) by another form of protection, such as directional-comparison carrier. (g) Tripping Circuit. The tripping relay has to operate in the interval between carrier blocks, which should be 180° but may be much less for the above reasons. It is usual to provide a trigger-circuit which measures the interval of time between blocks of carrier and trips if it exceeds 30°, i.e.

l~f

second, where f is the system frequency. Such a circuit is not affected by spurious intervals caused by interference. (h) Starting Devices. As in other forms of pilot relaying, two groups of starting devices are used, high-set and low-set. The low-set group initiate the transmitting circuit and the high-set monitor the tripping circuit. The latter are set 20 %to 50 %higher than the low-set group. In the early American phase comparison carrier equipments the starting devices were usually overcurrent relays on lines up to 69 kV and impedance relays for the higher voltage lines. 343

8.11

Protective Relays

In modem equipment a phase sequence network is used, giving an output which is some combination of sequence components, such as (11 - 512 ), which has been found to give the most uniform output for the various kinds of faults; insensitivity to load is provided by making the positive sequence

FIO.

8.22. (h

+ KJiJ starting network

component dependent upon a high rate-of-rise. Such a circuit is shown in fig. 8.22. Fig. 8.23 shows an alternative arrangement in which a transductor is provided in each c.t. secondary circuit and their rectified outputs are paralleled Tra.nsdu c~t..:.:or_._ _ _ _ _ _~

C.T.

FIO.

8.23. Transductor starting network

and fed to the starting relays or circuits. Each transductor is biassed by the current from another phase so that there is no output during balanced threephase load but at least one transductor gives an output during unbalanced con,ditions. Operation on three-phase faults is arranged by delaying the bias 344

A.C. Pilot Relaying

8.12

by means of a large capacitor so that there will be a brief output during which a sealing-in device will operate. In order to prevent incorrect tripping on an external fault, during the period that the fault current has ceased but the starting devices have not yet reset, a carrier-continuing device is provided. This device is similar in action to the relay used in directional comparison carrier for preventing incorrect tripping due to interference signals which are generated by the arc in the nearer breaker which is interrupting the fault in the next line section. 8.12. APPENDIX

The derivation of equation (8.1) in section 8.4.1 (a) is as follows. Referring to fig. 8.24, Is is the current entering the pilot at the sending end and IR is

Pilots FIG.

8.24. Compensation for pilot y

the current leaving the pilot at the receiving end. Applying Thevenin's theorem and considering one end at a time, the voltage across the C.t. at A, from reference (152) is (8.4)

Z Now}' and ~ are complex ratios such that}' = m + jn and Zop

zZo = IZo z I{cos(4)-9)+j sin (4)-O)} Op

Op

where 4> is the argument of Zo and 9 is the argument of Zop' Equation (8.1) can be written lop = IR {COSh (m +jn) + = IR {coshm

I::J

sinh (m+jn) [cos (4)-9)+j sin (4)-9)]}

cos n + j sinhm

sinn+I~:p \(sinh m cosn + j cosh m sinn) [cos (4)-9)+j sin (4)-lm}

(8.5)

In Chapter 3, Appendix 3.7.3, and in section 8.4.1 (a) of this chapter, it was shown that, for the characteristic of the relays at the two terminals to be on the real axis, the phase shift due to )' must be compensated for so that the currents in the operating coils at the two ends (lop and I R ) must be in 345

8.12

Protective Relays

phase. For this to be so the sum of the imaginary components of equation (8.5) must be zero- Hence sinhm sinn + /Zo ,; {sinh m cos n sin (tfo-8)+cosh m sin n-cos (tfo-8)}=O Zop

COS n . cosh m } IZopl = - IZol { -.-sm(¢-8) + ----;+--h cos (tfo-8) smn sm ,m

= -IZol{cotn sin(¢-O)+cothmcos(¢--O)}

346

(8.6)

9 Proteetio" 0' A.C. MfWmnes Generator Protection-Stator Faults-Rotor Faults-Miscellaneous Faults-Motor Protection-Faults-Unbalanced Conditions-Power Station Auxiliaries-Current Differential Relaying 9.1. GENERATORS

The generators are the most expensive pieces of equipment in the a.c. power system (see fig. 1.7) and are subject to more possible types of trouble than any other equipment. The desire to protect against all these abnormal conditions and yet to keep the protection simple and reliable has resulted in considerable divergence of opinion on the choice of protection. The choice must be carefully made since inadvertent operation of the relays is almost as serious as failure to operate. This is because the unnecessary disconnection of a large generator may overload the rest of the system and cause power oscillations which may disrupt the system. On the other hand, failure to clear a fault promptly may cause expensive damage to the generator. Another difficulty is the fact that, unlike other equipment, opening a breaker to isolate the defective generator is not sufficient to prevent further damage, since the generator will continue to supply power to a stator winding fault until its field excitation has been suppressed; very few generators have an additional three-phase circuit-breaker to disconnect the windings from neutral to break up the fault path. It is therefore necessary to remove the field, shut oft' the steam, water or fuel supply to the prime mover and, in some cases, supply braking. Furthermore, carbon dioxide is pumped into some large machines to extinguish any burning of insulation which has been started by a fault arc and fanned by the rotor movement. Finally, the relays must give reasonable protection and certainly not trip undesirably during the running up of a generator. The pick-up of currentoperated relays is very little changed, whilst the pick-up of voltage-operated relays is reduced at the low frequencies during the running up period because their coil reactance is reduced; this latter is not important, however, since overvoltage is not likely. On the other hand, all relays with phase-shifting circuits or sequence networks will be affected and should be disconnected if they cause undesirable tripping. 347

9.1

Protective Relays

Some of the abnormal conditions that must be dealt with are considered in the following sections. The treatment is brief because this subject has been dealt with in many previous books and articles and only the most modern methods will be mentioned. 9.1.1. Stator Protection

The breakdown of conductor insulation may result in a fault between conductors or between a conductor and the iron core. The breakdown may be caused by overvoltage or by overheating which in turn can be caused by unbalanced currents, ventilation troubles, etc.; it may also be caused by damage to the insulation by conductor movement due to forces exerted by short-circuits or out-of-step conditions. Because of the destructive effects of a ground fault (conductor to core), due to the high temperature of the arc, the fault current is usually limited by impedance in the neutral of the generator which may be a resistance, a distribution transformer with resistance loading, a reactance, or a potential transformer. With the neutral current limited to 250 amperes high-speed relays and breakers will prevent serious core damage. With 5 amperes or less, slow-acting relays are sufficient. The higher the neutral impedance, however, the more the risk of creating another winding fault due to voltage resonance with the capacitance to ground of the stator and the equipment connected to it. 106 The neutral resistance should not exceed Rn = 6njC ohms where C is the capacitance of the stator circuit to earth per phase in microfarads and f is the system frequency. Modern practice is to use a resistance-loaded distribution transformer and its resistance should not exceed Rn =

61t~~N2 where N is the turn ratio of

the transformer. If C is variable or not known a safe value of impedance can be used which will limit the current to between 20 and 200 amperes, usually about 30 amperes. (a) Phase and Ground Faults. Faults between conductors can sometimes be repaired by re-taping or replacing the conductor, but faults between the conductor and the iron laminations are a serious matter because the arc may melt the laminations together, thereby forming a hot-spot which may necessitate rebuilding the core. It is essential therefore to clear winding faults as quickly as possible and this is done in machines over 1000 kW by a high-speed differential relay which compares current in c.t's at the two ends of each phase winding (longitudinal differential protection). If there are parallel windings in each phase and they are brought out to separate terminals, another relay compares their two currents (transverse differential protection); it thus provides back-up protection and detects inter-tum faults. The relays use the same principle in both cases. They measure the difference between the two currents, which should theoretically be zero under

348

9.1

Protection of A.C. Machines

normal conditians. The connections of the differential relays are shown schematically in figs. 9.1 and 9.2 for wye-connected generators. Other connections are discussed in references (85) and (86). The relays shown have a biassing (restraining) winding which provides the characteristic shown in

(a)

1· ~

t

..

:>

u u

.,.

c

c

.~ -6

.."' .

~

~

0

E

1='4

'3

·2 ·1

o

4

Tlmn ra.tln9

fIG.

10

9.1. (a) Schematic connections for longitudinal differential protection of a generator. (b) Typical characteristic of biassed differential relay

fig. 9.lb; a mathematical analysis of the biassed differential type relay was given in Chapter 3, section 3.2.1. In machines below 10 MW time-overcurrent relays may replace the differential relays, but they should be monitored by an instantaneous undervoltage relay. The latter is connected to control the overcurrent relay making it faster and more sensitive if the voltage drops below 50 % indicating a fault in the machine. 349

9.1

Protective Relays

If the c.t's were ideal and the leads from the relay to the c.t's were equal, very sensitive settings could be obtained and the bias or restraint windings, shown in figs. 9.1 and 9.2, would be unnecessary. Unfortunately, neither is the case and the spurious differential current increases with current through the windings so that, on a heavy external fault, the sensitivity of the relay Blo.S

c

b

a.

C.Ts

~(--_--G.ntr(1tor---_~

windings

FIG.

9.2. Schematic connections for transverse differential protection of a generator

must be reduced by the bias which is made proportional to the through current and hence makes the relay stable with negligible loss of sensitivity on light faults. In fig. 9.4a no bias is necessary because a common C.t. is used so that error due to C.t. difference does not occur. There are two varieties of this bias. In the U.S.A. the restraint winding is in two halves (fig. 9.3) which are arranged to produce a product torque so that, on an internal fault, the bias torque reverses and increases the sensitivity of the relay. In the U.K., the bias is usualIy obtained by omitting the restraining windings altogether and putting a resistance in series with the operating coil; the relay is set to pick up at a voltage equal to Vr = Is(Rs + RL) where Is is maximum C.t. secondary current for an external fault, Rs is the resistance of one C.t. secondary and RL is the resistance of the leads from that C.t. to the relay (fig. 9.5). This resistance would appear to reduce the sensitivity of the relay and create dangerously high potentials across the c. t. wiring during heavy external faults but, in practice, this is avoided by putting much of the resistance in extra turns on the relay and hence providing greater sensitivity; high voltages are avoided either by magnetic saturation (fig. 9.4b) or by a non-linear resistor connected across the operating coil circuit. The capacitor is used, partly to make the relay insensitive to the d.c. component of offset current waves 350

Protection of A.C. Machines

9.1

Shc.dlng tub.

0 ..

SIc.nk pol.

t

1-

R..lrc.in

Opuc.t.

(a)

(b)

~ Block==

Trip

Trtp

(e) 9.3. Differential relay with product restraint (a) Basic circuit (b) Arrangement on an 8-pole indu<;:tion cup relay (c) Operating characteristic FIG.

and partly to reject the harmonics, which form a large part of the initial spurious differential current caused by remanence in the c.t's. A combination of the restraining winding and the bias resistor will provide stability with somewhat smaller c.t's. The effects upon differential relays of C.t. transients are dealt with in more detail in Chapter 11 and in references (30) and (94). One solution is to employ very sensitive relays which can be used with ironless c.t's (linear couplers) (113). Generator differential relays are'usually set to pick up at 5 %of the C.t. rating with full load flowing and the slope of the linear characteristic is usually about 10%. In the case of the product restraint relay, the slope is negligible at pick-up and increases in an exponential manner to infinity at 351

0.

b

c

In sl .-clJrrcnt

rClo.y '

(a)

Gl: ncrc.tor windings

-

'.

'.

So.turo.t ,n9 a.u lO

CT

(b)

Sto.b,I,.lng

-

nSt5tor

IA

I.

FIG. 9.4. Differential protection with un biassed relays (a) Application to transverse differential protection. (b) Relay circuit. (c) Photo of relay

352

9.1

Protection of A.C. Machines Glnera.tor wind In9

.T

.T.

a ia.s coils (If UUd) Lco.ch

Lnd. Opera.llng coli

Sta.bill s lng resistor

I Z drop in bien coil (If used)

C.T. l .m.1.

f C~iTl.~.­ C.T.

cC,{,. I

c.m.f.

!~--I (c)

FIG.

9.S. Voltage distribution in C.t. secondary circuit during faults (a) Basic circuit. (b) External fault. (c) Internal fault

six or eight times c.t. rating on through faults, but is of course negative on internal faults. Because of the limited ground fault current where a machine is grounded through impedance, the ground fault relays need to be more sensitive than the phase relays. The modem practice of generator-transformer units solves this problem because the generator stator winding terminates in the low tension winding of the power transformer so that a sensitive zero sequence relay can be used to protect against all ground faults (fig. 9.15). It is usual to design the relay to be insensitive to third harmonic currents and to have a setting of 15 %of the neutral impedor rating. When an inverse time relay is used it is set to pick up at 5 %of the neutral impedor rating and to trip in 0·5 second at 10 times the impedor rating. Ungrounded generators are rare but, where they exist, a stator fault to ground must be detected by an electrostatic ground detector, since the fault currents in this case are the low value capacitance currents fed through the healthy phases.

353

9.1

Protective Relays

Ground faults near the neutral of the generator produce less current than c: I . 10.p.kV where kV is the kV . Is. The lau t h ose near the termma t current IS 3Z.. rating of the machine, p is the location of the fault expressed as a percentage of the winding from the neutral, and Z .. is the impedance in the neutral. Let Q be the relay pick-up expressed as a percentage of the c.t. rating and Prc be the neutral c.t. primary rating; the relay pick-up current is QPrc 100 (primary) amperes. 10. p. kV I

y3Z..

QPrc .

= 100 gIves the relationship between

p=

p and Q

Q. Prc v3Z n 1000 kV

(9.1)

As an example, if the relay pick-up is 1 ampere, the c.t. ratio is 250/5 amperes and the machine is rated at 11 kV with Z,. = 200, then 100 200 ' d Q= 5 = % an

p

=

J3.

20·50. 200 3 % 1000.11 - = 1 0

This means that 31 % of the winding from the neutral is not protected or that l00-p = 69% of the winding from the terminals is protected. (b) Stator Inter-Turn Faults. Longitudinal protection cannot be relied upon to detect interturn faults except those between conductors of different phases which are in the same slot. In lap-wound machines an interturn fault affects only a single-pole pitch but in a wave-wound machine it would affect the whole stator. Such faults involve very high local currents which can cause severe damage to the core. With generators having parallel windings separately brought out to terminals, transverse differential protection will detect faults between turns of the same winding. The most sensitive arrangement is shown in fig. 9.4b. For generators without access to parallel windings it is necessary to rely on the zero sequence component of voltage caused by the reduction of e.m.f. in the faulted phase. See fig. 9.6. Another method of detecting turn-to-turn faults (84) is based on the fact that any dissymmetry of the stator currents creates a negative sequence component; this rotates at the same speed as the armature reaction field but in the reverse direction and thus induces a double frequency current in the field circuit. This can be detected by a suitably tuned a.c. relay in the field which can be monitored by a negative sequence directional relay so that it will detect all stator winding faults but will not operate on unbalanced faults external to the generator. A special c.t. is connected in the field to supply the relay; it has a tertiary winding energised from the exciter voltage to produce ampere-turns which cancel out those produced by the field current and thus avoid magnetic saturation.

354

9.1

Protection of A.C. Machines Volta.ge tra.nsfotmers

Sine product rela.ys

RN Genera.tor sta.tor windings

II'

I

Trip :

I

I

I

~ (a)

::...J..::.

II!I IIiI

II

Ii] ~

(b) FIG. 9.6. (a) Interturn fault detection (b) Effect of delayed fault clearance in a generator

In large generators intertum fault protection is often unnecessary because there is only one turn per phase per slot. The greatest value of this fault protection is for a generator with its neutral ungrounded, or grounded through a high impedance, because here an intertum fault would otherwise have to burn through to another phase before it caused any relay operation.

355

9.1

Protective Relays

(c) Stator Overheating. The main causes of stator overheating are ventilation failure, overloading, short-circuited laminations and failure of corebolt insulation. Two methods are commonly used for detecting overheating; both are used in large machines (above 2000 kVA). One method is to compare the inlet and outlet temperatures of the ventilating medium, which may be air, hydrogen or water. The other method uses temperature indicating devices A.C. volta.gt: sourct:

Rclo.y polo.rising coli St:rit:s bridgt: resistor

:s...f----""d-RCIa.y opcroiing coil Fixcd bridgc resistors

Volta.gt: dropping +o---_~/ rcsista.net:

. Ca.libra.tion pott:ntiomt:tcr _.oN.J\N....."AIVI.JVv.

Very nnsitivc rcla.y

Rcplica. rcsista.ncc_

(b) fiG.

9.7. Resistance temperature detector (a) With induction relay (b) With polarised d.c. relay

embedded in the slots at different points in the winding; a selector switch checks each one in tum long enough to operate an alarm relay. The embedded temperature devices may be either thermocouples, thermistors, or resistance temperature indicators (R.T.D.s). Fig. 9.7 shows typical bridge circuits employed with R.T.D.s. In small machines a replica type temperature relay is used which has a bimetallic strip heated by secondary current from the stator; the housing of the bimetallic strip should be designed to have a heating and cooling characteristic similar to that of the machine. Short-circuited laminations can be detected by a temperature indicator before damage only if one happens to be located near enough to the hot spot.

356

Protection of A.C. Machines

9.1

(d) Overvoltage. Apart from transient overvoltages caused by lightning, etc., the over-voltage can be associated with overspeed or it can be caused by a defective voltage regulator (see section 9.1.3 (g)). On modem steam-driven generators the voltage regulators act sufficiently rapidly to prevent serious overvoltage from occurring when either the generator loses its load and terminal voltage increases due to acceleration, or as a consequence of line charging current. In hydro or gas-turbine driven sets, however, the acceleration is greater because it takes longer to shut off the prime mover than in the case of steam. When a steam turbine set loses its load, the steam can be throttled before any great increase in speed has taken place; any overvoltage associated with overspeed will be controlled by the automatic voltage regulator. In hydro sets, however, the water flow cannot be stopped or deflected so quickly and overspeed can occur. Where the exciter is directly coupled to the machine the voltage tends to go up nearly as the square of the speed. It is customary, therefore, to supply overvoltage protection on hydro and gas-turbine sets but it is not so common on steam turbo-alternators. The most suitable overvoltage relay has two units, an instantaneous unit tripping on 25% (steam) or 40% (hydro) overvoltage, and an inverse time unit starting on 10% overvoltage; both overvoltage relays must be compensated for frequency and must be energised from an unregulated voltage supply. Modem practice is for the high-set relay to insert resistance in the exciter field (assuming no voltage regulator) and, if the overvoltage persists, the lowset inverse time relay will shut down the generator. 9.1.2. Rotor Protection

Rotor windings may be damaged by ground faults or open-circuits; structural parts of the rotor itself may be damaged by overheating due to unbalanced stator currents. The methods of protection are as follows. (a) Ground Faults. If the rotor winding is ungrounded a fault to earth has no effect, but a second fault to earth will increase the current in part of the winding and may also unbalance the air-gap fluxes so that there will be serious vibration which may do serious damage. Furthermore, a single rotor fault to earth raises the potential of the whole field and exciter system and the extra voltages induced by opening the field breaker or the main breaker under fault conditions may cause a second rotor winding fault to earth. Finally, a second fault may cause local heating which may slowly distort the rotor causing dangerous eccentricity; this also can cause vibration and serious damage. Fig. 9.8 shows a modem method of detecting rotor earth faults. The field circuit is biassed by a d.c. voltage which causes current to flow through the relay if a ground fault occurs. The relay is a polarised moving iron relay (fig. 2.19) which will pick up at It %of the field voltage and yet stand the full exciter voltage continuously if a fault should occur near one end of the winding, as indicated by the point X. This method is superior to those which

357

Protective Relays

9.1

apply the biasing voltage at the mid-point of a centre tapped resistor because it has no null point. It is also superior to methods using an a.c. biasing voltage because this will cause current to flow through the capacitance of the rotor

:I 01 0

~--

-,

I

L ____ J:

FIG.

Sensitive rclo.y

9.8. Earth fault detection

winding to its core and thence through the bearings to ground; this current, though small, will pit the bearings unless a special collector brush is fitted to the rotor shaft. (b) Open"circuit. Rotor open-circuits are very rare but, if one occurs, it Bus bo.rs

® (4)

To gen. tro.nsf. protection

®

FCiult B

(b)

9.9. Interlocked overcurrent protection (a) Fault on breaker side. (b) Fault on line side

FIG.

must be dealt with promptly because the ensuing arc may cause damage by burning. The relay to detect a rotor open-circuit is the same one as is used for detecting loss-of-field and is described in section 9.1.3 (c). (c) Unbalanced Stator Currents. The negative sequence component of unbalanced stator currents induces double frequency currents in the rotor during normal synchronous running. lfthe degree of unbalance is sufficiently large, severe overheating can be caused in the structural parts of the rotor which tends to soften and weaken slot wedges and retaining rings; these 358

9.1

Protection of A.C. Machines

components are normally already under great stress in large turbo-alternators (120) (121). The system conditions that would cause these harmful unbalanced conditions are: (a) the open-circuiting of cne phase of a line or the failure of one contact of a circuit breaker; (b) an unbalanced fault near the station which is not cleared promptly by the normal relays; (c) a fault in the stator winding.

The time for which the rotor can withstand this condition is inversely as the square of the negative sequence current, i.e. I~t = K, where K is a constant which varies from a value of 7 in a highly rated steam set (101) with direct cooling to 60 for a salient pole-hydro set with air-cooled stator 1,000

500

200

\' \~

\

100

inverse relo.y Type COG 15

~tlrcmCIY

./

~

\

...

.. 50

\

\ \\ \

\\ \

... c

\\\ l\

o

u

.

~~

.5

!.20

~ ~ K~

10

\\

,

~

5

1\

~

2

1

Direct cooled o.lterno.tor ftiydrogen cooled aJterno.tor Wo.tcrwheel o.lterno.tor

,

~

~" 1

10 20 50 2 5 Negative sequence current in multiples of full load FIG.

9.10. lz2t = K characteristic of generators

359

100

9.1

Protective Relays

(see fig. 9.10). The ability of large generators to stand negative sequence current (and hence the value of K) is becoming progressively less because their specific rating is still increasing although their size has almost reached the limit of present material strengths. It is important for the protective relay to have a time-current characteristic I~t = K which matches that of the machine as closely as possible because, while it is important to disconnect the generator if K is exceeded, it is also very important not to take it off the system unnecessarily. Fig. 9.11 shows a

FIG.

9.11. Negative sequence relay for generators

relay which maintains this characteristic very accurately over a time range of 0·2 to 2000 seconds. It has an induction disc movement with a special electromagnet equipped with magnetic shunts, shown in fig. 9.12a. Another way of doing this is to use non-linear resonance so that the operating coil becomes tuned as it saturates; the latter method, however, is affected by frequency (120). The relay contains a negative sequence network which supplies an instantaneous alarm unit as well as the time-current unit. The alarm unit also starts the time unit and is adjustable from 8 to 40 %negative sequence current 360

9.1

Protection of A.C. Machines

because the ability of generators to stand 12 continuously over this range depends on the type of cooling. The alarm is delayed by a timer to avoid unnecessary alarms on unbalanced loads of short duration. <">c.p bridged by Mu-m.tc.1 shunt

/

Mu-m.tc.1 shunt

;;

W~~:~-;:~_Sha.din9

'"

Opcrc.tlnq

rongs

COil

(a)

R

V,

s (b)

(c) flO.

9.12. (a) Electromagnet of [2/ = K induction disc relay (b) Static circuit for 13•5 t = K relay (c) Time-voltage relation of static circuit

361

9.1

Protective Relays

9.1.3. Miscellaneous

Abnormal conditions that do not directly affect the stator or rotor alone are overspeed, motoring, loss offield, loss of synchronism and bearing failure. (a) Overspeed. When a circuit-breaker opens and a steam turbine therefore suddenly loses its load, the steam may be shut off immediately without causing damage. On the other hand, when a water wheel suddenly loses its load the water flow cannot be stopped quickly for reasons of energy and mechanical and hydraulic inertia. This slow reduction of the water supply following sudden loss of load is responsible for the occasional overspeeding of water wheel generators. Under governor control, depending on the governor adjustment, overspeeds of over 150 % of normal are possible. Steam and hydro sets are provided with mechanical overspeed devices but, because of the slower throttling down of the hydro and gas-turbine sets, overspeed relays are usually provided on the latter. The setting of an overspeed relay may be 115% for steam or 140% for hydro machines. On very large steam sets, relays are sometimes provided to anticipate speeding up due to loss of load. In the U.S.A. an out-of-step tripping relay (see Chapter 5, section 5.4.8) has been used which cuts off the steam when the generator has slipped one pole and is 1800 out of synchronism. Obviously the relay will not cope with overspeed caused by loss of load due to the opening of a circuit-breaker. In England a quick acting relay has been used which operates when the wattful power falls relative to the steam pressure. (b) Motoring. In modem steam turbines the steam may be at a temperature equivalent to red heat and is difficult to envisage as a cooling medium; nevertheless, if the steam supply is reduced sufficiently the heat caused by turbulence of the trapped air while the generator is idling or running as a motor can de-temper and damage turbine blades. Motoring is prevented by a sensitive wattmetric relay (88), fig. 2.8, which operates on about 0·5 % reverse power, its setting depending on the type of steam turbine; the reverse power relay usually has a time delay which varies from seconds to minutes, also depending on the type of turbine. Topping turbines require faster settings than condensing turbines. Sometimes reverse power relays are used to prevent other types of generators from motoring. A diesel set requires a 25 % setting but a hydro machine may require a setting as low as 0·2 %. (c) Loss of Field. When a generator loses its field it speeds up slightly and acts as an induction generator, not having amortisseur windings. Turboalternators tend to overheat the rotor and the slot wedges under these conditions because of heavy currents induced in these parts; and sometimes arcing occurs at metal wedges in the slots. At the same time a large machine running out-of-step with the system may upset the system stability. Furthermore, the wattless current that the machine draws as magnetising current from the system may overheat the stator. On the other hand a machine equipped with a quick-acting voltage regulator and connected to a stiff system, i.e. a system with a large amount 362

9.1

Protection of A.C. Machines

of stored rotational energy, may run for several minutes as an induction generator without harm. Field failure may be caused by a faulty field breaker or failure of the exciter. It can be detected by an undercurrent d.c. relay in the field circuit but some of the larger generators operate over a very wide range of field current and such a relay may be an embarrassment. Furthermore, the loss of field due to exciter failure may not be detected because the undercurrent relay may be held in by a.c. induced from the stator. An undercurrent relay fast enough to drop out on a.c. cannot be used because it would be affected by a.c. induced during synchronising and during external faults. The most reliable field failure relay is either a mho relay or a directional impedance relay with its characteristic in the negative reactance area, fig. 9.13 (114). This is because such a characteristic is affected only by loss of

- - ... _ _

·R

Z locus for loss of 'Synchronl5m

~~~ ...... ~~~~-

Z ehClrClct .... t,e 01 rtlny

R

Nor;~I-~~ condit ion

-x FIG.

9.13. Impedance characteristic of field failure mho relay

field and not by any other condition, such as loss of synchronism which may result from the loss of field. (d) Loss of Synchronism. An out-of-step relay can be provided for detect~ ing loss of synchronism (see Chapter 5, sections 5.2.3 and 5.4.8), but is seldom used on an individual generator because it is unlikely to run out of synchronism with the system or the other generators unless it loses field (which was dealt with in the previous section 9.1.3 (c» or unless the governor becomes defective (56). Automatic synchronising by an electronic relay is common for large machines (123). (e) Bearing Failure. The temperature of the white metal or the oil can be monitored by an instrument with alarm contacts or a syphon device can be located in the bearing oil chamber. Such a device would shut down the generator only in an unattended station. Failure of the oil cooling equip363

9.1

Protective Relays

ment is detected on large machines by comparison of the inlet and outlet temperatures of the oil. (f) Auxiliary Failures. Other tripping functions associated with very large units are loss of vacuum and loss of boiler pressure. It is usual on loss of vacuum to reduce the load until the condition is checked; if, however, the vacuum continues to fall until a dangerous value is reached, a vacuum relay closes its contacts and the set is automatically shut down. A fall in vacuum may be the outcome of station auxiliary failures, so to some extent the loss-of-vacuum relay gives protection against loss of auxiliaries. As a further safeguard against a fall in boiler pressure, a steam pressure device is arranged to remove the load from the turbine. It is also the occasional practice to shut down automatically on the loss of the induced draught fans. (g) Voltage Regulator Failure. As the faulty operation of the voltage regulator may cause inadvertent tripping of the unit, or damage to the rotor, it is necessary to provide some form of protection to guard against its failure. This is particularly important on large generators using direct cooling of the stator and rotor, as explained under section 9.1.4, 'Back-up Protection'. Due to the complexity of the modem quick response voltage regulator, it is more vulnerable to the failure of components which may cause the regulator to apply full field under normal load conditions, resulting in overheating of the rotor. To guard against such failures a definite time d.c. overcurrent relay is provided, which is energised from a shunt or a d.c. current transformer in the rotor circuit. As the rotor will be subject to overcurrent during system faults, the time delay should be set to give the system protection time to clear. If, however, the overcurrent condition persists beyond this setting, the relay will operate and switch the excitation to a predetermined value. While it is usual to supply the regulator reference voltage from a separate voltage transformer in order to minimise the risk of a short-circuit on the secondary wiring causing a fuse failure, it is still felt desirable to provide some form of protection to prevent maloperation of the regulator for voltage failure, whatever the cause. The relay provided for this purpose must respond to the failure of any one fuse on either the h.v. or l.v. side of the voltage transformer; furthermore, its setting must be so chosen that it would remain inoperative for a normal voltage reduction during system faults. An arrangement of undervoltage relays can be provided to detect such failures but a more positive method to achieve a desirable setting is to use either a current bias voltage relay whose setting increases with an increase in stator current, or, alternatively, a voltage balance relay which compares the voltage derived from the instrument transformer with the voltage derived from the voltage regulator transformer. The operation of anyone of these relays returns the field excitation to the follow-up value of the manual field rheostat. 364

9.1

Protection of A.C. Machines

(h) Interlocked Overcurrent Protection. Where, for economic reasons, it is necessary to locate the protective current transformers on one side of the circuit-breaker only, a fault occurring between the breaker contacts and the current transformer secondaries is detected by a special overcurrent relay interlocked with the appropriate unit protection. In fig. 9.9a a fault occurring at point A is immediately detected by the bus-bar protection and, although the breaker opens, it will be seen that the fault remains. Similarly, in fig. 9.9b, a fault at B is detected by the generator differential protection but, as in the case of a fault at A, is not cleared by resultant opening of the circuit-breaker. In order to detect this type of fault without incurring any risk of indiscriminate tripping, the shading winding of an induction pattern relay, having a time setting sufficient to ensure the position of the fault, is connected in series with a normally open contact on the appropriate tripping relay.

.... g ~

Ma.m br.a.ku

Hl

. ".;;.

><

~

.0

c: a "w :l .;:

v

~

"0 " ~

d

v

.

-;:,~

",,,

,,0 -"'0 ,,~

"~" ,

...

.c

J

=

..

~

,..

OQ. Q.

p.. Ec;

~

-II>

" i g g .." UI

.0

~ -

:<

E

Z

H-- -- - - ---+----,.-+_-.

PT

FlO. 9.14. Tripping functions of generator protection D = differential relay; E = restricted earth fault relay; 0 = overcurrent relay (with voltage control) or mho; L = loss of load relay; F = Loss of field relay (undercurrent); F.Y. = loss of field relay (admittance); F.D. = fire detector; G = neutral overvoltage relay; R = rotor earth fault relay; T = stator winding temperature relay; T.D. = embedded temperature detector; V = overvoltage relay (hydro only); N = negative sequence relay ]2t = K; N' = negative sequence alarm

365

9.1

Protective Relays

With a fault at A, the tripping relay associated with the bus-bar protection would complete the shading winding circuit to the interlocked overcurrent relay allowing it to operate and shut down the generator. In the case of a fault at B, the tripping relay associated with the differential protection would complete the circuit to the shading winding of the overcurrent relay, which in turn operates the bus-bar protection tripping relay and clears the faulty section. 9.1.4. Back-up Protection (External Faults) Negative sequence (I~t = K) relays protect the generator against un-

balanced external faults which are near enough to the generator to cause overheating and have not been cleared by the appropriate relays in the faulted circuit (120) (121). The likelihood of an uncleared balanced three-phase fault is very small but it can be detected by a single reactance or impedance type relay set to reach just beyond the bus and given sufficient time delay to give the proper relays a chance to clear the fault. This arrangement also gives some back-up protection on generator faults. For smaller machines, where negative sequence relays would not be justified (122), three such distance relays can be used, one in each phase, with a common timing device. A cheaper alternative is a time-overcurrent relay whose characteristic matches that of the relays beyond the station bus but which is equipped with an instantaneous undervoltage unit; this latter reduces the pick-up of the overcurrent relay and increases its speed if the bus voltage drops below its setting thereby indicating a nearby fault. This scheme works best when the relays beyond the generator have instantaneous settings for nearby faults. Standard time-overcurrent relays can seldom be used for this purpose because the synchronous impedance of a turbo-generator is over 100%, i.e. the fault current may fall to less than normal load before the time-current relay can complete its travel and it is of course not possible to set the relay below normal load, except with the bus voltage monitoring unit just mentioned. However, with automatic voltage regulators the voltage, and hence the current, may be sustained so that standard overcurrent relays can be used. Machines without automatic voltage regulators should employ the voltage monitored time-overcurrent relays mentioned above; this also applies to hydro and diesel machines and to small turbo alternators which do not have differential protection. 9.1.5. Stator Protection

Fig. 9.15 shows the main protection for a large steam turbo-alternatortransformer unit. No circuit breaker is provided between the generator and the transformer. All the fault detecting relays trip the main and field breakers, apply braking, inject CO 2 , shut off the steam and shut down certain auxiliary equipment. The relays protecting against overload, overheating, overvoltage,

366

9.1

Protection of A.C. Machines TABLE

9.1

Generator Protection Rating

Below 1 MW 1 MW up

Differential Restricted earth Turn-to-turn fault Time-overcurrent (voltage monitored) Thermal overload Temperature (thermo-detector) Negative sequence current Loss of load Anti-motoring (loss of steam) . Loss of field Out-of-step Overspeed Overvoitage

x x

x x x

x

X

10 MW up 100 MW up

x x

x x x

x x x

x x x X

X X

X

x

Hydro machines only Hydro machines only

TABLE

9.2

Rotor and Bearings Rating

Below 1 MW 1 MW up

10 MW up 100 MW up

Earth fault Loss of field Vibration indicator Bearing temperature Bearing insulation

x x x x x

X X X

X

9.3 Conditions Operating Alarms Only TABLE

Steam Air-cooled Hydrogencooled

Abnormal Condition Condenser low vacuum Hydrogen pressure, temperature or density abnormal Bearing oil pressure low Seal oil pressure low Unit transformer winding temperature high Bearing temperature high Governor oil pressure Cooling water failure Stator air temperature high Guide vane fails to open Main or unit transformer Buchholz gas Main or unit transformer oil temperature H.T. V.T. Buchholz gas Auto-voltage regulator failure Rotor earth fault Field failure Battery voltage low

367

x x x x

x x x x x x x

x x x x x

x x X

x x x x

Hydro

x x x x x x x x x x x x x x

9.1

Protective Relays

7.

- - t - . . , r - - - 132 KY.

Y

y

switchg~ar

Brco.ker ~ 60011 50011 Synchronising 500/1

~/

o§

In.trument

-"""-t-~-tll.

600/-58

144 M.'l.A.

@

Volto.g~ tro.~or~ __

~/

Mo.in trw"formtr 132/13·8 K.V. 450/1

A.V.R.QQ)--/ 6000120 Volto.9~ tro.n.lormcr

:"-_-ir---t'1100/1 Eo.rthlng rulsto.nce

,

Ie +-+--......7

l

~

6.6KV

~~~--.-----r-~----~4'

1----------------.. . '4

~

FIG. 9.15. Typical protection for transformer-generator in the U.K. B. = Buchholz; W.T. = winding temperature; O.T. = Oil temperature; V.T. = vacuum trip; O.S. = overspeed trip 40 = field failure; 46 = negative phase sequence; 51 = I.D.M.T. overcurrent; 511 = interlocked I.D.M.T. overcurrent; 59 = instantaneous overvoltage; 64 = I.D.M.T. earth fault relay; 64A = negative biased earth fault; 641 = instantaneous earth fault; 64R = restricted earth fault; 87 = generator differential protection; 87UT = unit transformer differential protection; 87BB = busbar protection; 87T = overall differential protection

negative sequence current and overheating due to an external fault open the main and field breakers only. (a) Abnormal Conditions Operating Alarms and Causing Shutdown. Tables 9.3 and 9.4 indicate the practice in the U.K., which is basically the same as in other countries. 368

9.2

Protection of A.C. Machines

Hydrogen cooled generators may also have a number of additional indicators connected with auxiliary equipment such as defoaming tanks, water detectors, vapour extractors, oil conditioners, d.c. emergency pumps, etc. TABLE

9.4

Abnormal Conditions Causing Shutdown Abnormal Condition All stator faults All transformer short-circuits External faults operating negative sequence or back-up relays Overspeed Overvoltage

Steam Air-cooled Hydrogencooled

x x

x

x

Hydro

x

X

x

x

9.2. MOTOR PROTECTION

A.c. motors include synchronous motors, synchronous condensers and induction motors. In general they are subject to the same electrical faults as

FIG.

9.16. Effect of delayed fault clearance in a motor

generators (fig. 9.16) and, in the case of motors of comparable size to generators, similar protection is used. Smaller motors, such as are used for power station auxiliaries and in industry, are protected against stator faults, overload, unbalanced phase currents, reversed or open-phase starting and undervoltage. 9.2.1. Stator Faults

Direct acting overcurrent tripping devices on the breaker take care of faults to ground or between phases; these are of the thermal or dashpot type N 369

9.2

Protective Relays

giving an inverse time-current characteristic and usually provide an instantaneous trip at high current. On large motors; above 50 h.p., instantaneous overcurrent relays supplied from c.t's are more common, two in the phases and one in the residual circuit. The phase relays have to be set well above the starting current and the latest type (see Chapter 4, section 4.1.6) can give a more reasonable setting because it is not affected by the d.c. component of the inrush current. Fuses are used for protecting smaller motors but they involve the risk of leaving the motor connected to single phase supply. For stator faults, thermal overload relays with instantaneous overcurrent relays usually comprise the main protection. The thermal relays are slow because their time-current characteristic is matched to the I 2 t capacity of the motor; the instantaneous overcurrent relays are usually set very high because they have to surmount the high starting current of the motor, but they are valuable for clearing winding and terminal faults. For motors of 1500 h.p. and above the saving in cost of repairs by the quick clearing of faults justifies the cost of differential protection. 9.2.2. Overload and Locked Rotor (Stalling)

The ordinary I.D.M.T. time-current curve is not suitable for motor protection. The motor heats according to an [2t function and good protection is provided by thermal overcurrent relays using bimetallic spiral movements (fig.9.18). The slow reset of these relays prevents restarting the motor until it has cooled. Furthermore, the heat storage property of the relay gives it different hot and cold time current characteristics which correspond to those of the motor (fig. 9.17). Superior characteristics can be obtained with a thermistor bridge and a thermal replica device. In single-phase fractional h.p. motors the thermal element usually takes the form of a bimetallic disc which snaps into the operated position above a certain temperature and opens the supply. The [2t relays are set to operate on 15 % overload with continuously rated motors and up to 40% overload with motors having overload capacity, depending upon the service factor. When a motor stalls, either due to trouble with the connected load or low voltage, both the stator and rotor windings will be overheated. Some form of protection should be provided to shut the motor down before the locked rotor current persists long enough to cause damage, but it must not shut the motor down during a normal start. It is not always possible to provide adequate locked rotor protection with the overload device without upsetting the overload protection. The best protection is provided by a thermal device which is only operable during a stalled condition (25). An English relay provides this feature by means of an attracted armature relay which switches a separate thermal unit into the circuit if the current is three times the motor full load current. Tripping will occur if the motor current fails to fall to normal value within the time-current characteristic of the thermal unit. The characteristic curve of 370

9.2

Protection of A.C. Machines

the thermal unit is shown in fig. 9.17 for starting (cold) and running (hot) conditions. The relay can also be arranged to give restart once after a stall, and to lock out if there is a second stall. The thermal element also incorporates an indicating device which integrates the' current during the starting period. The trip setting can be set 10.000

8

6

5

" 3

2 1,000 8 6

5

..9.

"3 2

~

.. 100

go

8

I!

\

\

\\ \

3

\

2

-" \

"'"

10 8 6 5

"'" "-..

...........

~

3

~ r-J""-....

Hot

~

2 1.01 FIG.

2

9.17.

3 " 5 6 ..1.....-J Opcrutill9 current =II/r,+ 3[: ]21

t-9

10X5Cttill9

characteristic for motor protection

at a slightly higher value than the indicated value during starting, thus providing the maximum possible protection against a stalled condition. Overload relays also take care of faults not heavy enough to operate the instantaneous overcurrent relays. Larger motors use temperature detectors (see section 9.1.1 (c) ). 9.2.3. Unbalanced Phases

In the U.S.A. an induction disc, polyphase voltage relay is used to protect motors from starting with one phase open or with reversed phase sequence. The relay is connected as shown in fig. 9.20 and its torque is proportional to the sine product of the two line-to-line voltages and hence to the area of the voltage triangle. The relay will not close its contacts and hence the motor 371

9.2

Protective Relays

will not start unless all three phases are present and in the correct sequence. This method however does not prevent the motor from becoming overheated if an open-phase condition occurs while it is running, such as one fuse blowing or a bad contact on a breaker. In the U.K. unbalanced current relays are used for this purpose (25). One type, fig. 9.18, uses three bimetallic

FIG.

9.18. Thermal overload and unbalance movement

spirals, energised by currents from the three phases, whose contacts are arranged so that, if either spiral moves differently from the others, due to more than 12 % unbalance, their contacts meet and trip the supply breaker. The same spirals also provide overload protection. This is an important feature of motor protection. Due to the difference between the positive and negative sequence reactance of a motor, a small voltage unbalance causes a much higher current unbalance, which results in overheating in one winding. A typical example would be a motor operating at rated load with a 3 % voltage unbalance. This could result in approximately a 25 % increase of current in one line, giving 56 % overheating in one winding. The worst case would be complete loss of one phase of the supply due to a blown fuse or a bad contact. 9.2.4. Undervoltage and Underfrequency

Running on undervoltage will generally cause overcurrent which will cause overload or temperature relays to trip; an exception to this is a fan motor whose load drops sharply with speed preventing the current from increasing. It is usual to provide undervoltage protection having an inverse

372

Protection of A.C. Machines

9.2

time characteristic which will override temporary voltage drops. Underfrequency relays are sometimes used to indicate failure of the power supply to the motor because its load will cause it to decelerate quickly. 9.2.5. Miscellaneous

Under this heading are rotor overheating, loss of excitation, loss of synchronism. Very large motors have individual forms of protection for these failures but the normal procedure is to rely on the protection described in the previous sections. In the U.K. all the protection for a motor, except undervoItage, is assembled in one relay case (fig. 9.19). A static relay has also been developed which gives similar protection but substitutes an If + KIf unit for the three bimetallic spirals. This relay uses a thermistor bridge with thermal storage effect.

FIG.

9.19. Type M-4 motor protection relay

373

Protective Relays

9.2

Incomplete starting of a motor, especially in unattended stations, is prevented from damaging the motor by a definite time relay which disconnects it if it remains on the starting connection too long. Large motors which have a current limiting starting sequence are usually provided with an underpower relay which opens the source breaker circuit if the supply should fail. On large motors, pull-out (loss of synchronism) or stalling on overload is detected by either a separate overcurrent relay or by readjustment of the overload thermal element by a high set instantaneous overcurrent unit. a.

Induction disc

aG.mPing

~===*F===

ma.gnct

(a) '------ob

c R

Bc...----~y

(c) R

y'+-------' B FIG.

(d) 9.20. Open and reversed phase relay

In large synchronous motors a power-factor relay is used to take the full field off when the motor pulls out of synchronism, thus reducing the stator out-of-step currents. Bearing protection is the same as for generators except that thermal indication is not applicable to ball-bearings; with these, failure may cause overload but usually results in vibration which must be detected by mechanical means, such as a seismic pick-up.

374

9.4

Protection of A.C. Machines 9.3. POWER STATION AUXILIARIES (87)

These are divided into two categories, 'essential' and 'non-essential'. The first group includes: Boiler feed pumps Circulating pumps Condenser pumps Exciter sets Forced draft fans Induced draft fans Primary air fans Pulverised coal feeders Stokers Unit-type coal pulverisers The non-essential group include: Coal handling equipment Coal crushers Central coal pulverisers Conveyors Clinker grinders Ventilating fans Air compressors Service pumps The protection of the two groups is usually similar except that the essential group may have more comprehensive protection and usually have full voltage starting, so that they can be restarted as quickly as possible after a power supply interruption. They are usually arranged to be switched to an emergency supply so as to keep them running. Standby pumps are sometimes provided in case of low pressure or lack of flow; these are usually d.c. and have thermal overcurrent protection. 9.4. APPENDIX: CIRCULATING CURRENT DIFFERENTIAL RELAYING

In order to make the explanation as simple as possible without affecting the conclusions, the c.t's in fig. 9.21 have been shown as perfect transformers with a shunt impedance carrying the magnetising component of the primary

-

C.T.

I

A

iR"

Z"''' FIG.

Protected unit

-

C.T.

/

IRa z"'.

~,R. ~R" 9.21. Basic circuit of differential current relay

current and with impedances in series with the secondaries representing the lead resistance and the C.t. leakage reactance and resistance. For further simplification, giving a pessimistic error, all impedances are treated as resistances and added arithmetically. It will be seen from fig. 9.21 that, during an external fault, the through current should circulate between the C.t. secondaries and the only current that can flow in the relay is that due to any difference in the C.t. outputs for the same primary current. Magnetic saturation will reduce the output of a C.t. and the most extreme case of error will be if one C.t. is completely saturated and the other unaffected. This condition can be approached in bus differential 375

9.4

Protective Relays

protection but it is unlikely in generator or transformer differential protection because the fault current would be limited by the impedance of the protected unit. However, it will be considered because the principles now discussed

FIG.

9,22. Magnetising characteristics of c.ts. at A and B

apply to all applications of differential current protection including restricted earth relays. 9.4.1. Stabilising Resistance RR

In this extreme case, the c.t. at one end can be considered short-circuited as in fig. 9.23 and the one at the other end as delivering its full current which will then divide between the relay and the saturated c.t. This division will be

--

.I-

Sa.tura.tIIi

I

A

iRA

C.T.

iRII

ti I

z",

I

FIG.

Fa.ult

B

~

iRA iRa 9.23. Effect of complete saturation of one c.t.

in the inverse ratio of the resistances RR and RB and it is obvious that, if RR is high compared with R B , the relay will be prevented from undesirable operation because most of the current will go through the saturated c.t. The voltage across the relay is hence the relay current is (9.2) and RR can be chosen in relation to RB to prevent the relay from operating. Since the extreme case of one c.t. being unaffected and the other shortcircuited is impossible, RR need not be as high as indicated by RR = IBRB IR

376

Protection of A.C. Machines

9.4

where IR is the relay pick-up and RB is the highest c.t.lead resistance; in fact, for generators and transformers it can be about one third of this value. N. IA = Ip-ImA and N . IB = Ip-ImB where Nis the c.t. turn ratio and ImA and 1mB the magnetising components of the primary current Ip. Hence 1 lR = l A-I B = N(ImB-ImA) (9.3) i.e. the relay current is the difference between the magnetising currents corrected for tum ratio. As RR is increased, IR is reduced so that 1m.( and 1mB are forced towards equality and the c.t. with the lower magnetising current will push equalising current through the other c.t. secondary. On the other hand, the high value of RR will not prevent the relay from operating on an internal fault because, in that case, the c.t. secondary e.m.f.s are additive and combine to force current through the relay. The voltage across the relay will be its IR drop due to the current flowing through it. This current will be shared by the c.t's after deducting the magnetising current necessary to produce the voltage across the C.t. secondaries. The distribution of resistance between the relay winding and its series stabilising resistor depends upon the relay sensitivity and the pick-up setting required on internal faults; in a very sensitive relay the pick-up can be controlled by series resistance; in a less sensitive type the pick-up would be controlled by taps on the relay coil. In determining the actual relay pick-up in terms of primary fault current, the magnetising currents of all the c.t's must be added to the relay current setting, i.e. the actual pick-up in (primary) amperes is (9.4) where N is the c.t. ratio and L . 1m is the sum of the c.t. magnetising currents at the relay voltage setting. The c.t's must be chosen to produce a secondary voltage V equal to at least twice the IR drop in the longest leads to the relay at maximum through fault current (usually taken as the switchgear interrupting rating), . V = 2IR (9.5) I.e. N Also, at ~ volts, the magnetising current must not exceed ~ e~

-

IR) where

n is the number of c.t's or circuits. This follows from equation (9.3). 9.4.2. Through Current Bias

Similar considerations are employed in determining the stabilising resistance for a biased differential relay except that the resistance is divided by the bias ratio. The criterion for operation of a biased differential relay is when the differential current exceeds the characteristic percentage of the through current. During an external fault the stability limit will be when IR > sIB where s is the percent slope of the characteristic, i.e. since

377

9.4

Protective Relays

RB > SRR for a relay with 20% slope or bias, the resistance would be onefifth of the value for a non-biased relay. This is an advantage in favour of the biased relay because the operating voltages are reduced in the same ratio and the c.t's can be correspondingly smaller and cheaper.

9.4.3. C.T. Lead Resistance Matching

In many countries stabilising resistors or high impedance relays are used only for bus differential, and biased relays with low impedance operating windings are used for generator protection. In the early days low impedance I.D.M.T. overcurrent relays were used, even for bus protection, and it was necessary to use very large c.t's that would not saturate under the worst conditions. This was found to be impractical except with air-cored c.t's (linear couplers). In such relays it is necessary either to have very short c.t. leads or to balance them so that the potential across the relay is substantially zero during the maximum external fault. Since the c.t. ratios are the same, their magnetising currents must be the

FIG.

9.24. Wave forms of saturated c.t.

same, but their secondary voltages EA and E B will not be equal unless they have identical magnetising characteristics. In fig. 9.21, below saturation of the c.t's, EA > EB and the voltage across the relay

(9.6) Below saturation, where the magnetising impedances of the two c.t's have a constant ratio, this can be made so by inserting a suitable amount of resistance in series with one of the c.t. secondaries. At higher currents, however, this is not possible because, with the different magnetising characteris378

Protection of A.C. Machines

9.4

tics shown in fig. 9.22, EA. > EB below their crossover point and EA. < EB above it. Furthermore, the currents above saturation will have non-sinusoidal wave-form so that this compensating resistance would have to vary during the cycle. In order to avoid instability due to this cause the c.t's. in relays without stabilising resistances require cores large enough in cross-section not to saturate with the maximum magnetic flux that can occur during an external fault. The total flux <1>, allowed for should include the transient d.c. component and is theoretically (9.7) where X and R pertain to the primary circuit and QC is the steady state a.c. flux required to produce the secondary voltage necessary to drive the current through the various impedances in the c.t. and relay circuit. Since X/R can be as high as 30 in a modern generator, it is clear from Equation (9.7) that very large c.t's would be necessary to prevent tripping on external faults and obviously it is preferable to use the stabilising resistor or the high impedance operating coil in the differential circuit or else to employ linear coupler c.t's.

379

10 Power Transformer Proteetion Types of Faults-Gas Relays-Differential Relays-Magnetising Inrush-Methods of Reduction-Relay Solutions-Grounding Transformers-Generator Transformer Units-Transformer Feeders 10.1. GENERAL

The power transformer is one of the most important links in a power system yet, because of its relatively simple construction, it is a highly reliable piece of equipment. This reliability, however, depends upon adequate design, care in erection, proper maintenance and the provision of certain protective equipment. Adequate design includes insulation of windings, laminations, corebolts, etc., bracing the conductors against short-circuit stresses and good electrical connections. Care in erection includes care to avoid physical damage, leaving or dropping anything foreign inside the tank (tools, nuts, etc.), making good connections and making sure the oil is clean and dry. Proper maintenance includes checking oil and winding temperatures, the cleanliness, dryness and insulation level of the oil and analysing any gas that may have accumulated above the oil. Protective equipment includes surge divertors, gas relays and electrical relays. The gas relay is particularly important since it gives early warning of a slowly developing fault, permitting shutdown and repair before serious damage can occur. Of these various items of protective equipment, only the relays are within the scope of this book. Detailed information on the others will be found in the Bibliography (96) (103). 10.2. TYPES OF FAULT AFFECTING POWER TRANSFORMERS

The varied characteristics of the power transformer with different types of fault greatly affect system conditions, which have tended to become more complicated in recent times. It is thus worthwhile to review the different types of fault generally encountered by a transformer, before considering the application of protective gear. 10.2.1. Through Faults

These can be sub-divided into overload conditions and external shortcircuit conditions; the transformers must be disconnected when such faults 380

Power Transformer Protection

10.2

occur only after allowing a predetermined time during which other protective gear should have operated. A sustained overload condition can be detected by thermal relays (97) which give an alarm so that the situation can be attended to or the supply disconnected, if necessary. For the external short-circuit condition (bus-bar short-circuit, or short-circuit on the main supply network), time-graded overcurrent relays or fuses are usually employed. Proper coordination of this back-up transformer protection should be made with the primary protection of the associated power supply network. The primary protective scheme associated with the transformer itself, however, should be made so that the protective gear does not operate under such conditions. 10.2.2. Internal Faults

The primary protection of a power transformer is intended for conditions which arise as a result of faults inside the protected zone. Internal faults are very serious and there is always the risk of fire; these internal faults can be classified into two groups. Group (a)

Electrical faults which cause immediate serious damage but are generally detectable by unbalance of current or voltage such as: (i) Phase-to-earth fault, or phase-to-phase fault on the h.v. and 1.v. external terminals. (ii) Phase-to-earth fault or phase-to-phase fault on h.v. and 1.v. windings. (iii) Short-circuit between turns of h.v. and 1.v. windings. (iv) Earth fault on a tertiary winding, or short-circuit between turns of a tertiary winding. Group (b)

So-called 'incipient' faults which are initially minor faults, causing slowly developing damage. These are not detectable at the winding terminals by unbalance; they include: (i) A poor electrical connection of conductors or a core fault (due to breakdown of the insulation of lamination, bolts or clamping rings) which causes limited arcing under the oil. (ii) Coolant failure, which will cause a rise of temperature even below full load operation. (iii) Related to (ii) is the possibility of low-oil content or clogged oil flow, which can readily cause local hot-spots on windings. (iv) Regulator faults and bad load-sharing between transformers in parallel, which can cause over-heating due to circulating currents. Generally, for group (a), it is important that the faulted equipment should be isolated as quickly as possible after the fault has occurred, not only to limit the damage to the equipment but also to minimise the length of time that the system voltage is depressed. A prolonged period of low

381

10.3

Protective Relays

voltage may result in loss of synchronism between rotating machines and, if this occurs, the excessive current drawn by an out-of-step machine may well cause other relays to operate and initiate sequential and false tripping. The faults of group (b), though not serious in their incipient stage, may cause major faults in the course of time, and should thus be cleared as soon as possible. It should be emphasised that the means adopted for protection against faults of group (a) are not capable of detecting the faults of group (b), whereas the means applicable to detect the faults of group (b) cannot necessarily detect terminal faults and are not quick enough to clear other faults in group (a). These ideas are basic to transformer protection, and the means for protection against groups (a) and (b) should not be treated as alternatives but as supplements to each other. In the discussion to follow, we will make a brief mention of the means of protection applied to conditions arising due to faults under group (b). Then we will pass on to describe the developments of protective schemes to detect the electrical unbalance due to faults under group (a).

10.3. GAS-ACTUATED RELAYS (89) (93)

Core insulation failures and poor electrical connections create local heat which, at 350°C, caused the oil to decompose into gases which rise through the oil and accumulate at the top of the transformer. 10.3.1. Buchholz Relays

Whenever a fault in a transformer develops slowly, heat is produced locally, which begins to decompose solid or liquid insulating materials and thus to produce inflammable gas. The Buchholz gas-actuated relay operates an alarm when a specified amount of gas has accumulated. Analysis of gases collected in the relay indicates the kind of trouble which causes them. The presence of (a) H2 and C2H 2 indicates arcing in oil between constructional parts; (b) H 2, C2H 2 and CH4 indicates arcing with some deterioration of phenolic insulation, e.g. fault in tap changer, (c) H 2, CH4 and C2H4 indicates a hot spot in core joints; (d) H 2, C2H4 , CO 2 and CaH6 indicates a hot spot in a winding. The importance of gas-actuated relays in detecting various types of 'incipient' faults in a transformer has been described (93) and it is generally accepted that these reiays should be used to supplement differential protection of power transformers. They are also being increasingly used for protection of high voltage v.t.s. where other protection is not available. Fig. lO.1a shows such a relay connected into the pipe leading to the conservator tank, and arranged to detect gas produced in the transformer tank. The conservator pipe must be inclined slightly for reliable operation. As the gas accumulates the oil level falls and, with it, a float (or bucket) F which operates a mercury switch, sounding an alarm. The open-topped bucket shown in fig. IO.1b used by an English manufacturer, has the advan-

382

10.3

Power Transformer Protection

tage of more positive action than the float and eliminates the risk of pinhole leaks which would cause the float to sink and give a false alarm. When the oil level falls, due to gas accumulation, the bucket is left full of oil and the force available to operate the contacts is greater than in the case of hollow

Insul C1.to r

bushing

Tr=sformor tC1.nk

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(a)

v.... (S ;7 'l

--

~--

(b)

FIG.

10.1. (a) Principle of Buchholz relay (b) Modern Buchholz relay

383

10.3

Protective Relays

floats. Windows are provided for indicating the amount of gas generated through a scale marked on the windows. Referring to fig. 10.lb, the gas can be drawn off through the petcock, via a pipe to the ground level, and the analysis of this gas will indicate the kind of breakdown which will occur if suitable action is not taken. The gases caused by arcing include hydrogen, hydro-carbons and carbon monoxide. The 'incipient' faults indicated by the Buchholz relay will include arcing due to circulating currents, where a path has been provided by inadvertent contact between two parts of the core supports which are normally separated. Such arcing can cause oil 'sludging' and damage tn the iron. When the transformer is first put into service, air trapped in the windings may give unnecessary alarm signals but, with H.V. transformers, it is customary to remove the air by vacuum treatment during the filling of the transformer tank with oil. The gas accumulated without this treatment will, of course, be air, which can be confirmed by seeing that it is not inflammable. The relay is adjusted to give an alarm when the gas accumulated has reached a volume which depends upon transformer size, as in Table 10.1. TABLE 10.1 Gas Volume to Operate Alarm Transformer Size

Pipe Diameter

Up to 1 MVA 1 to 10 MVA Over 10 MVA

2·5 cm. (1 in.) 5·0 cm. (2 in.) 7·5 cm. (3 in.)

Setting Range Normal Setting l00-120cc. 185-215 cc. 220-280cc.

110 cc. 220cc. 250cc.

When a winding fault occurs in the oil, the arc generates gas so rapidly (over 50 cm 3 jkW sec.) that it creates a surge in the oil which rapidly moves the vane, V, and causes tripping through contacts attached to the vane. The vane is set to operate for an oil velocity which is above that caused by the starting and stopping of oil pumps, as shown in Table 10.2. TABLE 10.2 Oil Velocity to Cause Operation Transformer Size Up to 1 MVA Itol0MVA Over 10MVA

Pipe Diameter

Setting Range

Normal Setting

2·5 cm. 5·0cm. 7·5 em.

75-125 em/sec. 80-135 em/sec. 95-155 cm/sec.

90 cms/sec. at 5° 100 cms/sec. at 5° 110 cms/sec. at 5°

The angle of displacement of the mercury switch for making contact is about 15° plus the angle of the pipe, which must be as short as possible and with at least 2° inclination to permit gas to reach the conservator. 384

10.3

Power Transformer Protection

In fig. 10.1 b, the surge vane has a bucket similar to that in the gas detector unit. This bucket is used for tripping in the case of complete loss of oil and also provides damping which makes operation on oil pump surges less likely. 10.3.2. Sudden Pressure Relays

In transformers having a gas cushion instead of a conservator tank, the tripping unit of the Buchholz relay is not applicable and is replaced by a 'sudden pressure' relay which is built into the tank and operates on the

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10.2. (a) J>rinciple of sudden pressure relay (b) Modem sudden pressure relay

basis of rate-of-increase of pressure. Such a relay is shown in fig. 1O.2a. It has a diaphragm which is deflected by a differential oil pressure; the diaphragm is by-passed by a hole which equalises the pressure on the two sides of the diaphragm normally and also makes it responsive not to pressure but to rate of rise of pressure. The gas accumulating unit in such transformers is located at the top of the dome. 385

Protective Relays

10.3

In the American relay (90) shown in fig. 10.2b, the diaphragm is not directly immersed in the transformer oil but inside a metal bellows full of silicone oil, the bellows being in the transformer oil. In other words, the diaphragm and switch are separated from the transformer oil by a bellows containing silicone oil which has a flat viscosity/temperature characteristic and provides an inverse time/pressure rise characteristic which prevents

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10.3. Operating time of sudden pressure relay

incorrect operation under conditions of mechanical shock, etc. This unit is located at the bottom of the tank where it is convenient for maintenance. Fig. 10.3 shows the operating time characteristic of this relay. The relay is set to operate on a rate of pressure rise of 5 g/cm 2/sec. and a minimum differential gas pressure of 20 g/cm 2 • 10.3.3. Limitations of Gas-actuated Relays

Too sensitive settings of mercury contacts makes them subject to false operation on shock and vibration caused by such conditions as earthquakes. mechanical shock to the pipe, tap changer operation and heavy external faults; these conditions are in addition to normal mechanical vibration caused by alternating magnetic fluxes. This has been counteracted to a certain extent by improved designs of mercurcy contact tubes. No specific shock tests have been agreed but the manufacturer of the Buchholz relay shown in fig. 10.lb checks the relays to see that they do not trip with a seismic shock up to 0·16 g. acceleration and 60 mm. amplitude horizontally or vibrations vertically up to the following values: TABLE 10.3 Maximum Vibration for Stability

Frequency 25 c.p.s 100 c.p.s 150 c.p.s

Amplitude 0·09 in. (2-3 nun.) 0·023 in. (0'6 nun.) 0'015 in. (0·4 nun.)

386

Equivalent Acceleration 3g. 12g. 16g.

10.4

Power Transformer Protection

The minimum operating time of Buchholz relays is about 0·1 second and an average time is 0·2 second, which is somewhat slow; sudden pressure relays are faster only for very heavy faults. On the other hand, electrical relays can be used for heavy faults where high speed is necessary; they can also be used for bushing flashovers which are outside the oil and hence do not create an oil surge, the Buchholz relay being retained for faults involving only a few turns of the winding and for incipient faults. 10.4. ELECTRICAL RELAYS FOR TRANSFORMER PROTECTION (91) (92)

Flashovers inside the transformers due to lightning or switching voltage surges occasionally occur between the winding and the core or the tank, and protection against them is generally by simple earth fault relays. More usually, a break-down between windings or between end turns may be caused by a steep voltage wave-front or sometimes by movement of the windings due to electromagnetic forces during a heavy short-circuit; this is especially so in an old transformer or one in which the insulation has deteriorated due to overheating. Such faults are rare but differential current or Buchholz relays can detect them. Faults can also occur due to faulty contact operation of on-load tap changers; this in its tum can cause incorrect switching or short-circuiting of the taps between turns. Such faults can be detected by overcurrent and under-voltage relays. Overheating may be detected by thermal image temperature indicators. to.4.1. Earth Fault Relays

Delta windings and ungrounded wye windings are best protected by zero sequence overcurrent relays supplied by c.t's situated at the terminals of the power transformers, as shown in Fig. 10.4. Such a relay can only operate for a ground fault in the transformer since it does not have an earth connection through which to supply an external fault. The relay is usually instantaneous .--_....;Pow~c~rt.;.,ro.nsformcr

c:rll

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ccut.h rclo..y

Residuo.l overeurrent rclo..y FIG.

10.4. Earth fault protection of a power transformer

387

10;4

Protective Relays

but must be of the high impedance type if supplied with the residual current of the paralleled c.t's in the three phases; this is to prevent wrong operation on false residual current from the c.t's during a heavy external fault between phases, due to transient differences in the c.t. outputs (94) (see Chapter 9, section 4). An ordinary overcurrent relay is acceptable if it is supplied from a core balance type of c.t. which encircles the three phase conductors, since the magnetic conditions of the c.t. are the same for all three phases. For a wye-connected winding with the neutral grounded, the restricted earth-fault connection offig. 10.4 is used. This differential connection provides relay current only for a winding fault to ground, but with instantaneous relays it is stable only if the relay circuit is of high impedance. During a heavy external phase fault there should be no current supplied to the residual overcurrent and restricted earth relays, referred to in Chapter 4, section 4.5.2 (d), if the c.t's maintain their ratio; there is, however, in addition to the effects of magnetic unbalance and transient components, which were discussed in the Appendix 9.4, the problem of third harmonics in the exciting current. Since the third harmonic components from each of the three phases are in phase they behave in the same fashion as zero sequence components and add together directly instead of cancelling out as with the fundamental components. Fortunately, balanced three-phase faults are very rare, and the problem does not occur with phase-to-phase faults because both of the affected c.t's have similar conditions. The problem can be solved by tuning the relay to fundamental frequency, or by using a third harmonic filter. During an external ground fault (fig. 10.5) the sensitivity of a low impedance relay is limited by the fact that the magnetising current of the neutral

ZR FIG.

10.5. Equivalent circuit of restricted earth relay

c.t. is three times that of each of the three line c.t's, so that the voltage produced by the neutral c.t. is three times that of the line c.t's if they are of similar design and turns. If they are of the same design the relay will have zero voltage across it only if the leads between the relay and the neutral C.t. have three times the resistance of the leads between the relay and the line 388

10.4

Power Transformer Protection

c.t's. If this resistance balance does not exist, it can theoretically be remedied by adding resistance on the neutral c.t. side, but this is not the practice because the balance would not hold during transient conditions or if the neutral C.t. was saturated. The proper solution is to use a stabilising resistance in series with the relay, or to use a high impedance relay. The value of the total resistance necessary in the relay circuit is calculated in Appendix 9.4. The earth-fault current in a faulted winding in a resistance-grounded transformer depends directly on the voltage between the neutral and the fault point on the winding, and inversely on the neutral resistance, i.e. / y;'"

10kV.p hp'IS the percentage 0 f WIn . d'Ing InVO . Ive, d kV'IS V"3 R n amperes, were

line-to-line voltage in kV; the source and transformer impedances are assumed to be small compared with the neutral resistance, R If the power II •

source is on the delta side, the current on that side is / where

Nis the voltage ratio of the transformer; hence /

=

=

V3· N

lbo /y

NrOR~V'

In a solidly grounded transformer the relation between the fault current and the position of the fault along the winding is more complicated because

B

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FIG.

10.6. Effect of location on fault current

the current is limited by the impedance of the winding which increases as the square of the number of turns involved; furthermore, the voltage is not proportional to the turns involved by faults near the neutral because of increased magnetic leakage. Fig. 10.6 shows the current variation with fault position for one type of transformer. 389

10.4

Protective Relays

For a fault in the delta winding the relation between the fault current and fault position is still more complicated. The current magnitude varies less because the voltage to ground can never be less than 50 %. The impedance of the winding is maximum for a fault at the middle of the winding and may be as high as six times the positive sequence impedance. 10.4.2. Biased Differential Transformer Protection (100) (102) (124)

(a) Basic Conception of Differential Protection Applied to Transformers. Buchholz relays will detect all faults that occur under the oil, but it is possible to have a flashover above the oil at the bushings; although practically all such faults would involve ground, it is usual with large transformers to provide high-speed biased differential protection which detects such flashovers and will also clear other heavy faults faster than the Buchholz (92). Economically, this differential protection may not be justified where instantaneous earth fault relays are used, but it provides effective back-up protection. A transformer differential relay compares the currents in the windings of the transformer through the medium of c.t's whose ratios are such as to make their secondary currents notmally equal except for the core magnetising currents of the transformer which are relatively small. Fig. 1O.7a shows the relay in its simplest form; the polarity of the c.t's is such as to make the current circulate normally without going through the relay, during load conditions and external faults, i.e. the relay coil receives the vector sum of the derived currents which is normally zero. Any fault within the transformer disturbs the balance and the relay operates. (b) Inadequacy of Simple Differential Arrangement. In practice, differential protection in the simple form of fig. 1O.7a is handicapped by three main difficulties. (i) CURRENT TRANSFORMER CHARACTERISTICS. Unless saturation is avoided, the difference in c.t. characteristics due to different ratios being required in circuits of different voltage may cause appreciable difference in the respective secondary currents whenever through faults occur, even though these currents are of equal value at normal load. This is particularly troublesome when different types of current transformers are used, as is frequently the case in transformer protection. Unequal length of current transformer secondary leads may well cause a difference in VA burden between the two sets of c. t's; this generally tends to give a ratio-error between the sets of current transformers (see Chapter 9, Appendix, section 9.4). (ii) RATIO-CHANGE AS A RESULT OF CHANGE OF TAPPINGS. Nearly all large modem power transformers are equipped with on-load ratio-change gear. As the transformer ratio is changed, the ratio between the primary-side and secondary-side current transformers can be made to match at one point only of the tap-changing range. At other points an unbalance current will flow in the differential relay, the effect of which requires biased relays for its control. 390

Power Transformer Protection

10.4

(iii) MAGNETISING INRUSH-CURRENT. When a transformer is energised, the inrush-current may attain peak values corresponding to several times the transformer full-load current and decays relatively slowly. This current Power tra.nsformer

C.T.

C.T.

Rela.y

(a)

Power tra.nsformet

(b)

(c) 10.7. Differential protection of transformers (a) Overall differential relay (b) Percentage differential relay (c) Three-winding transformer differential

FlG.

generally flows in one side of the differentially connected relay only, which will tend to operate if some form of restraint is not provided. To make a differential relay stable because of difficulties (i) and (ii) above, percentage differential relays have been developed and are now adopted as the general practice in the protection of large power transformers. In the case of difficulty (iii), early practice was to desensitise the relays for a short 391

10.4

Protective Relays

time until the magnetising inrush currents on each phase had decayed sufficiently; modern practice, however, is to provide some form of restraint to the relays which depends on the harmonic content of the magnetising inrush current(s) (124) (125). These questions are quite fundamental to successful transformer unittype protection. Item (i) is a special study in itself and is not unique to transformer protection; it has already been mentioned in this volume and will be treated separately in Vol. II. Item (ii) in so far as it concerns the general theory of the biased differential relay has been included in Chapter 3 on the theory of relays. Item (iii) as a feature special to transformer protection has not been mentioned before and will be treated in section 10.5. (c) Percentage or Biased Differential Relays and the Effect of Through Currents. The unbalance, or difference, of the derived currents from the c.t. secondaries due to causes (i) and (ii) above (section 10.4.2 (b)), increase with increase of through current. Thus a relay whose operating current is an appropriate percentage of through current will allow a sensitive setting at low current without danger of tripping on through current. Such a relay is shown in fig. 1O.7b. The operating coil is provided with the vector sum of the currents in the transformer windings and the restraining coil with the through current. The spill current required to operate the relay is usually expressed as a percentage of the through current in the restraining coils and the ratio is generally termed the percent slope. In the case of a transformer with more than two windings, the restraint is based on the scalar sum of the currents in the various windings. In early induction disc relays this was done by providing a restraining electromagnet for each winding and adding their torques; on more recent relays it is done by rectifying the currents and adding the outputs of the rectifiers to supply the restraining winding with the scalar sum of the currents (fig. 10.7c). In the case of a three-phase power transformer, the c.t's associated with the wye-connected windings are usually connected in delta and those for the delta windings in wye (91). This is to correct for the phase-shift of the line currents due to the wye-delta transformation. It also eliminates the zero sequence component of the currents on the wye side, which might otherwise upset the stability due to the lack of a corresponding zero sequence component on the delta side and prevent tripping on an external fault on the wye side. In the case of a transformer with its neutral grounded through resistance, differential protection should be supplemented by restricted earth fault protection, as shown in fig. 10.8, because only 41 %of the winding is protected with a differential relay pick-up setting as low as 20 %of c.t. rating (24). This is due primarily to the elimination of the zero sequence component from the phase currents. A method suggested by Matthews (147) to overcome this difficulty is shown in fig. 10.10, in which the C.t. secondaries are connected in star on the resistance-grounded side and in delta on the delta side of the power trans392

Power Transformer Protection

10.4

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C.t.

error curve and differential relay characteristics

former. Another current transformer of ratio 3 : 1, placed in the groundneutral connection, measures the zero sequence current and injects it to the open delta of the multi-winding line-current transformers on the star side.

393

Protective Relays

10.4

It can be shown that, with this arrangement, the zero sequence current is .added to the relaying circuit in the case of internal ground fault and subtracted from the relaying circuit in the case of external ground fault. It has also been shown, so far as percentage of winding protected is concerned, that M<1in multi-winding C.T. r----------,

Tr<1nsformer

M<1in C.T.

,----------, I

R<str<1inJng coil

Perc
I

St<1bdisJn9 resista.nces

10.10. Percentage differential relay protection of a resistance-grounded star-delta transformer

this scheme approaches restricted ground fault protection, the maximum departure being 16'7 %. It requires only one additional current transformer in the ground-neutral lead. One further system, which has been used in England, was proposed by Wellings and Matthews (139) and uses the principle of magnetic ampereturn balance. When applied to transformer protection, this principle is designed to counter the difference in performance of l.v. and h.v. current transformers by a special arrangement of balancing amp-turns on the high voltage current transformer core, as shown in fig. 10.11. The core of the h.v. current transformer is divided into equal parts P and Q and a number' e' of equal and opposite turns are added in series with the secondary winding of 'a' turns. The turns 'a' are so arranged that, with zero differential current (I = al') in the primary, there will be zero core excitation. If I > al' due to out-of-balance, the differential ampere-turns operate in the same direction as the 'cross' ampere-turns 'el" in core Q, and in opposition in core P. It can be shown by taking a specific excitation

394

Power Transformer Protection

10.5

curve of the core that, provided the flux change takes place in the linear portion of the curve, the average flux-swing to give an output to the relay remains almost constant with a given per-unit differential current in the c.t. primary windings. If the through current increases, there is a consequent increase in the cross excitation 'cl" which brings the core-flux above the

I

FIO.

10.11. Magnetic balance protection for a transformer

knee of the excitation curve; the voltage output to the relay thus decreases for the same per-unit differential current (127) as before in the primary windings. Thus, if the relay operating voltage is properly chosen, it can be made stable at high values of through currents. The discussion above relates only to through fault stability, and it is important to consider relay stability on magnetising inrush currents. In the early days of protection, when time-lag induction disc relays were in almost universal use, magnetising inrush currents had little affect on relays. But, with the use of improved steels in the manufacture of power transformers, and with the growing "application of faster relays to protective gear practice, magnetising inrush phenomena came into prominence. to.5. MAGNETISING INRUSH CURRENT IN A POWER TRANSFORMER 10.5.1. Factors Aftecting Magnetising Inrush Phenomena (136) (137) (138)

When a power transformer is connected to the supply with its secondary circuit open, the steady state flux wave Cl> in the core is normally in quadrature with the supply voltage wave v as shown in fig. 10.12, neglecting the resistance drop of the exciting current in the primary winding. If the transformer is switched~in at allY point of the voltage wave, the asymmetry in the core flux will correspond to the voltage asymmetry, but its starting point will depend upon the residual flux Cl>R in the core. If there is no residual flux and the switch is closed at the zero value of the voltage wave, it is evident from fig. 10.12 that the peak value attained by the asymmetrical flux will be 2Cl>max, where Cl>max is the maximum value of the steady-state flux. If, at the instant of closing the switch, the core has a residual flux value of Cl>R' the resultant peak value of the flux becomes 2Cl>max±Cl>R. If the primary circuit is closed at the peak value of the voltage wave, then the commencement of the flux wave 395

10.5

Protective Relays

will be in accordance with its normal value and the peak will have a value of ~max±cJ)R·

The instantaneous value of the asymmetrical flux linked with the transformer winding will be limited by core saturation and the air-core inductance of the winding under consideration. Since the air-core inductance is small,

FIG.

10.12. Wave-shape of voltage and flux in a transformer under normal conditions

the magnitude of current to produce the required flux is large. If it is assumed that the amp-turns necessary to produce the flux iIi the core up to saturation is negligible, then the ampere-turn (or current) wave shape during the saturation period will be as shown in fig. 10.13; this is the origin of inrush current

FIG.

10.13. Wave-shape of voltage, transient flux and magnetising inrush current in a transformer

phenomena in a transformer. Fig. 10.14 shows how the inrush current wave can be derived from the excitation characteristic. The inrush current gradually decays in successive cycles of the voltage wave due to the resistance R in the primary, i.e. the energising circuit, and the impressed voltage on the primary winding of the transform.er is modified by a small amount equal to the voltage drop in the resistance R. The rate of decay of the transient inrush phenomena will be greater during the first few cycles because of the shorter time constant of the circuit for decreased aircore inductance of the winding with higher saturation of the core. In the determination of this time constant, eddy current loss has some effect during 396

Power Transformer Protection

FIG.

10.5

10.14. Derivation of inrush current wave from excitation characteristics

the first few cycles when the rate of decay is highest, but the influence of hysteresis loss is absent (148) for practical purposes. 10.5.2. Three-phase Transformer Banks

In the case of three-phase transformer banks, it is clearly impossible to switch all the three phases on to the supply simultaneously without producing inrush currents in two phases at least. The instantaneous values of inrush currents in the three phases will be affected by the electrical connections of the energised windings and by magnetic coupling with any other closed windings which are present. Some simple cases will be considered: (i) Core-type transformer with primary winding connected in groundedstar and with no closed winding present.

Assuming that all three primary connections are made at an instant of time when the a-phase voltage is a maximum, and there is no residual flux in the core, then the transient asymmetry of the fluxes in each of the limbs will be as for separate single phases, shown in fig. 10.15; fig. 10.15a shows the first cycle of the flux in accordance with normal conditions in the a-phase limb. If the peak of the transient flux is below saturation level, the magnetising current demanded by the coil can be assumed to be negligible. Fig. 10.15b shows the normal voltage swing of the b-phase, the corresponding normal flux swing of the b-phase and the corresponding normal flux swing in core b (both in dotted lines). The transient flux in this core will start from zero and follow the normal flux swing as shown by a full line in fig. 1O.15b. If the saturation flux level is as shown in the diagram, and if the transient flux swing is above it, then the magnetising current demanded by the coil in the b-phase will be a pulse as shown in fig. 1O.15b, full line. The inrush current will appear during the period that the flux remains above the saturation value

397

Protective Relays

10.5

(a)

1

_ Time in

cycle.

" .....

1

4/

/

1

"2

_-, ,'" /~ Norma.l flux

1

_ Time

In

cycl ..

1_

(c)

Time In

cycle.

PIG.

10.lS. Transient fluxes and inrush currents in a 3-phase transformer, energised at VII (max) (a) a-phase conditions. (b) b-phase conditions. (c) c-phase conditions

in anyone cycle. For phase c, the relevant diagram is given in fig. 1O.15c. It should be observed that the inrush currents in phases band c are of opposite polarities and that they occur at different times in the cycle. These currents have a return path to the supply through the ground-neutral connection. (ii) Core-type transformer with primary winding star connected, neutra not earthed and with a closed delta winding present. This case is shown in fig. 10.16. Assuming the same conditions for switching as in Case (i), the exciting ampere-turns required by the core will remain the same, since it is of no importance whether the core of the transformer obtains its exciting ampere-turns from the primary winding or from any other linked winding capable of supplying ampere-turns. In this case, any core-limb demanding inrush ampere-turns during a certain part of a cycle is provided by the other phases with an auxiliary source by virtue of the magnetic coupling of the closed delta winding. Thus, one third of the required inrush current I, will be supplied by the closed delta winding, as shown by the arrows in fig. 1O.16a, when only phase c is experiencing inrush. Similarly, for the period during which only phase b is experiencing inrush, the situation will be

398

10.5

Power Transformer Protection CL

...,Ii

b

III

--.------,I,

III

g

0.

J

3 Ii

III

tlj

III

b

(b)

,~

,t

~Ii

c

(. )

--

"5/1

!I,

FIG.

10.16. Magnetising inrush current in a wye-connected ungrounded 3-phase transformer, energised at Va(max) (a) c-phase experiencing inrush. (b) b-phase experiencing inrush

as shown in fig. 1O.16b. It should be observed that the forced inrush current through phase a, due to the other two phases experiencing inrushes in opposite directions during different periods in the cycle, will be double-sided. The magnitude of this current will be zero at the instant when the inrush magnitudes in phases band c are equal; at this particular instant the delta circulating current will also be zero. This does, however, assume that the inrush currents in phases band c are equal. (iii) As for Case (ii) but with grounded neutral.

In this case, as before, the inrush current experienced by anyone phase will have an alternative path to the supply through the grounded neutral connection. A division of current will take place through the windings of the other phases and through the ground-neutral return, depending on the respective circuit impedances. This aspect has been discussed in a recent paper (137). (iv) Core-type transformer with primary windings connected in delta. In this case not only the voltage but also the current in each phase is independent of the other phases, and the inrush phenomena of each phase take place individually on a single-phase basis. As far as the line currents are concerned, they will be the instantaneous differences of the respective currents. As before, the inrush peaks are determined by the air-core inductance of the exciting winding. It is likely that energising a transformer through its

399

10.5

Protective Relays

higher voltage winding leads to smaller inrush currents; this is particularly true in the case of concentric transformers in which the higher voltage winding is the outer solenoid and the lower voltage winding the inner one. It may be noted that this arrangement of windings gives higher percentage air-core inductance in the outer winding. 10.5.3. Large Inrush Currents in Power Transformers with Low-Loss Steel Cores

There is a tendency in transformer design towards the use of cold-rolled steels for cores, in which the hysteresis loss per cycle is much smaller than with hot-rolled steels. Allowing the same core losses, the normal working flux density in cold-rolled steels can be made higher than in the earlier types. This is shown in fig. 10.17 where the hysteresis loops of a cold-rolled and a 26,

---------;~-------

.... -

I I

I I

I

,

c

t

I

I

I"

>-

+'

.0;;

., """ ;:;:"

_--

I

26,

I

~--

I

I

I'"

/"

...... -

... - -

.... (

I /' I ____;_---l /I I I I I I

I

I I I I

Working o----+--flux deoslty

-

FIG.

Ma.gnetising force H

10.17. Hysteresis loops of high-loss and low-loss transformer cores; loop (1) for high-loss core, loop (2) for low-loss core

hot-rolled core material show the same loss per cycle; the normal working flux densities are at different values. The attainment of these advantages have been accompanied by an increase of the initial transient magnetising current. It can be seen in fig. 10.17 that, if the maximum flux swings are double the maximum value at normal working conditions in both cases, the magnetising ampere-turns required in the case of cold-rolled steel are very much greater than that in the other type. 400

10.5

Power Transformer Protection

Representative data (136) giving the average values of normal magnetising currents and magnetising inrush currents for different steels used for transformer manufacture indicate the following figures: TABLE 10.4 Average Values of Normal Magnetising Current in Transformers

MVARating of Transformers

Normal Exciting Current as % of Full-load Current Cold-rolled Steel Hot-rolled Steel 2·5 2·2 1·5 1-3 1·0

0·5 1·0 5·0 10'0

50'0

TABLE

2·7 2'7 2'5 2-3 1·8

10.5

Average Values of Crest Magnetising Inrush Current in Transformers MVARating of Transformers 0'5 1·0 5·0 10·0 50·0

Crest Inrush Current as p.u. of Crest Full-load Current Cold-rolled Steel Hot-rolled Steel h.v. l.v. h.v. l.v. 11·0 8-4 6·0 5·0

4'5

16'0 14·0 10·0 10·0 9'0

6'0 4-8 3-9 3-2 2·5

9-4 7·0 5·7 3-2 2'5

10.5.4. Methods of Minimising Inrush Currents

Although no commercial means have become available for the suppression of magnetising currents in a transformer, the following methods have been proposed and attempted (138) for their reduction and, in the case of capacitance compensation, clearly affect the choice of protective relaying system. (a) Resistance Energisation. A series resistance is inserted in the energising circuit by the first step of a two-step switch and it is proportioned so that, when the first contact closes, only about one half of the circuit voltage will be impressed on the transformer. Since the voltage is raised in steps, the transient flux swing can be arranged so as not to exceed the steady-state normal value. This method requires a very large value of series resistance and, although it is capable of reducing the switching surges, it fails to cater for the situation of inrush currents accompanying the recovery voltage, e.g. after an external fault close to the transformer has been cleared. (b) Capacitance for Reducing Residual Magnetism. A capacitor is connected in parallel with the transformer so that, when the transformer is diso 401

Protective Relays

10.5

connected from the supply, the damped oscillation in the tuned circuit so formed eventually eliminates any residual magnetism. This method, however, has not proved very effective (148) in reducing the peak value of magnetising inrushes. This may be due to the fact that modem transformers using improved silicon steel and normally working nearly at saturation flux density may, in some cases, retain considerable residual magnetism after deenergisation. When re-energised, these transformers will tend to draw inrush currents which, in early cycles, may cover the period of nearly a complete voltage cycle. This type of inrush current will contain a high percentage of d.c. component but less second harmonic component; thus any relay which depended for its restraint solely on the second harmonic component of transformer magnetising inrush current would encounter difficulties.

to.5.5. Relay Solutions to the Inrush Current Problem Since the inrush current exists only on the source side of the transformer, the ~nrush current will appear in the differential circuit and operate the relay. There are several solutions to this problem, all of which are somewhat complex and expensive: (a) Even Harmonic Cancellation (b) Harmonic Restraint (c) Harmonic Blocking (d) Resonance Blocking (e) d.c. Bias. The theory and effect of magnetising inrush current on c.t's is considered in detail in Vol. II. (a) Harmonic Cancellation. Owing to the saturated condition of the transformer iron, the wave-form of the inrush current is highly distorted; fig. 10.18 shows a typical wave-form for maximum inrush. The amplitudes of the harmonics, compared with the fundamental (100%) are as follows: TABLE

10.6

Amplitudes of Harmonics in a Typical Magnetising Inrush Current Wave-shape Component Typical

value

d.c. 55%

2nd Harm. 3rd Harm. 4th Harm. 5th Harm. 6th Harm. 7th Harm. 63%

26·8%

H%

2'4%

The d.c. component varies between 40 %. and 60 %, the second harmonic 30 % to 70 %, the third harmonic 10 % to 30 %. The other harmonics are progressively less, the range depending upon the equipment in the circuit, e.g. tooth ripple from a generator. The third harmonic and its multiples do not appear in the C.t. leads since the components circulate in the delta winding of 402

10.5

Power Transformer Protection

(a)

(e) 1O.1S. Typical wave-form of inrush current (a) Theoretical (b) Actual currents in wye-connected windings (c) Actual currents in delta-connected windings

FIG.

the transformer and the delta connected c.t's on the wye side. The d.c. components and even harmonics can be cancelled out in the operating circuit of a rectifier bridge relay and added in the restraint. This leaves only the 5th, 7th, etc., which can either be ignored because of their small amplitude or blocked by a suitable filter. This has been done in a Russian relay (143). (b) Harmonic Restraint (124) (125) (136). A popular method of making differential relays insensitive to magnetic inrush current is to ·filter out the harmonics from the differential current, rectify them and add them to the percentage restraint, as shown in fig. 10.19. Harmonic restraint is obtained Power

transformer

FIG.

10.19. Basic circuit of harmonic restraint relay

403

Protective Relays

10.5

from the tuned circuit XCXL which permits only current of fundamental frequency to enter the operating circuit, d.c. and harmonics being diverted into the harmonic restraining coil. The relay is adjusted so that it will not operate when the second harmonic (restraining) exceeds 15 % of the fundamental current (operating). The minimum pick-up is 15% of C.t. rating and the minimum operating time is about 2, cycles. Owing to the fact that a d.c. offset and harmonic components may also be present in fault current, especially if the c.t's saturate, it is customary to provide an instantaneous overcurrent unit in the differential circuit, which is set above the maximum inrush current but will operate in less than 1 cycle on heavy internal faults. In this way fast tripping is assured for all heavy faults. (c) Harmonic Blocking. An alternative to harmonic restraint is to provide a separate blocking relay whose contacts are in series with those of a biased differential relay and which operates when the second harmonic is less than 15 % of the fundamental. Fig. 10.20 is a simplified diagram showing the basic principle (137).

HQ.fmon lC

blooklng y

,/Ia.

,/ Tra.n!'QCLors

.~+--+:zd--+7'
polarts.d r~la.y

FIO.

10.20. Basic circuit of harmonic blocking relay

(d) Resonance Blocking. This method is similar to the harmonic blocking except that the blocking relay is tuned to twice system frequency and is supplied by rectified current from the differential circuit. The magnetic inrush current of the power transformer, when rectified, gives the number of d.c. pulses per second which correspond to system frequency and the relay blocks. 404

Power Transformer Protection

10.5

During a fault the current will have a large fundamental component which, when rectified, gives twice as many pulses per second and the relay operates, thereby permitting the differential relay to trip (100). (e) D.C. Bias Scheme. The characteristic feature of a shunt-loaded currentoperated transductor, in which the operating current increases linearly with increasing d.c. in the control circuit for a constant voltage output, has been

FIG.

10.21. Fault on transformer primary caused by lightning

utilised in this relay (16); this feature gives a convenient way of obtaining percentage bias on through faults by rectifying the through current and using it to control linearly the output from the a.c. primary winding carrying the differential current from the same phase. The output from this transductor goes to the second conductor which controls a tripping relay. The d.c. component of the magnetising inrush current has been used as 'auto-bias' to the relay in the same transductor element. When the magnetising inrush current is symmetrical and does not contain a d.c. component, the relay is made stable by a 'cross-feed' bias from the d.c. component of the inrush current in another phase. For this purpose another transductor element has been incorporated, as shown in fig. 10.22. This type of protection is simpler and cheaper than harmonic restraint but has the possibility of undesirable tripping on inrush current which may occur in a three-phase transformer if the breaker is closed at the moment of voltage maximum on one phase. The resulting inrush current can have no d.c. component to block the relay. This condition can be overcome at some 405

Protective Relays

10.5

PoWf::r

tra.nsformc.r

SenSItive difle.ent'QJ

r.la.y

FIG.

10.22. Percentage biassed and d.c. component biassed transductor relay for transformer protection

sacrifice of speed and sensitivity when its operation on offset internal fault current is demanded. In another similar scheme the d.c. bias has been replaced by the second hannonic current for restraining the relay on magnetising surges. 10.5.1. Overcurrent Relays

In the case of small transformers, overcurrent relays are used for both overload and fault protection. An extremely inverse (]2t = K) time-overcurrent characteristic is preferable for overload and light faults, with an instantaneous overcurrent unit for heavy faults. A very inverse time residualcurrent relay with an instantaneous unit gives adequate protection for ground faults. Time-overcurrent protection with a very long time setting is also used for stand-by protection of a grounding resistor or reactor to protect it against overheating due to a sustained fault to ground. Such a relay is shown in fig. 10.23 which shows an extra damping magnet for giving the long delay. In regulating transformers, time-overcurrent protection is used for the shunt exciting winding in case a short-circuit should occur in the series winding. Inverse time-current relays are also used for protecting transformers for mercury arc rectifiers and arc furnaces. They are connected on the supply side of the transformer and are set just above maximum load. They are usually of the very inverse or extremely inverse type, because there is a narrow margin of selectivity between peak load and minimum fault, and are usually

406

Power Transformer Protection

FIG.

10.5

10.23. Long time overcurrent relay showing extra damping magnet

provided with an instantaneous overcurrent unit, set just above maximum inrush current and preferably tuned to reject d.c. and harmonics (see Chapter 4, section 4.1.6). 10.5.7. Protection of Grounding Transformers

Grounding transformers are connected either in wye-delta or zig-zag and their sole purpose is to provide a grounding point for the power system. Consequently, when a fault occurs they can contribute only zero sequence R.la.y' prot.ctlng grounding tra.nsfor"'.t

C.T' • . Grounding trcs.nsfor",.r

1-_+-&*-_+-__

Exl.rna.1 fa.ult ba.c~-up r.la.y FIG.

10.24. Protection of grounding transformer

current, hence any positive or negative sequence currents can flow only towards the grounding transformer and not from it. For the above reasons, faults in the grounding transformer bank can be detected very selectively by overcurrent relays fed by delta connected c.t's, as shown in fig. 10.24.

407

10.7

Protective Relays

10.6. THERMAL IMAGE OVERHEATING PROTECTION

In large power transformers warning is given of overheating and overload by temperature indicators in the oil and in each winding. The indicator is either a thermostat or a bulb containing volatile liquid which operates a remote pressure indicator through very thin tUbing. The temperature indicator is put in an oil-filled pocket in the hot oil at the top of the transformer tank. The pocket also contains a heating coil energised from the c. t. secondary current in the associated winding. The thermal time-constant of the heater matches that of the winding so that the indicator measures the difference in temperature of the winding above that of the oil and sounds an alarm when its temperature setting has been reached. On smaller transformers, thermal image overcurrent relays with an I 2 t = K time-current characteristic are used. The relay is usually of the bimetallic strip type; it detects overload but does not detect failure of the cooling system. 10.7. GENERATOR·TRANSFORMER UNIT PROTECTION

In most modem systems there is no 1.v. bus-bar or circuit-breaker but each generator is directly connected to the delta winding of a power transformer whose h.v. winding is in wye and connected through a breaker to the h.v. bus-bar. The normal protection is provided for the generator and the transformer but, in addition, an overall biased differential current relay is connected to ..-_ _A""tux.C:r.

Differentia.l rela.y

Restra.in

Restra.in Differentia.l rela.y C.T.

't-----'

T

Sta.tion scrVice

tra.nsformer

(a) FIO.

lO.2Sa. Overall differential protection

protect the two as one unit (fig. 10.2Sa). The relay is not normally provided with harmonic restraint because the transformer is only connected to the bus-bar at full voltage; however, it is possible for a small inrush to occur when a fault near the bus-bar is cleared, suddenly restoring the voltage. The relay is usually given a 20% pick-up setting and a 20% bias. 408

Power Transformer Protection

10.8

An English relay of this type is shown in fig. 10.25b. It has two shadedpole electromagnets acting upon an induction disc. The operating electromagnet is supplied with the difference current and its shading winding is tuned so that the magnet produces maximum torque at system frequency C.T.

Circuit brca.ker

Pow., ira.nsform.r

Restrain

Opera.t.

(b) PlO.

10.2Sb. Overall differential protection using relay with harmonic rejection

and negligible torque on harmonics. The restraining electromagnet is supplied with the through current and its torque increases with frequency. This arrangement provides the equivalent of harmonic restraint with an extremely simple relay. Since the generator windings and the delta winding of the transformer form an isolated circuit, the earth fault relay can be very sensitively set without risk of operation on an external fault; it can be a simple instantaneous overcurrent relay in the generator neutral. 10.8. TRANSFORMER FEEDER PROTECTION

When no breaker is provided between the transformer and a feeder the two must be protected as a unit. The normal transformer protection is provided except that, if biased differential protection is used, the relays are of the pilot-wire type. Fig. 10.26 shows a typical circulating current pilot wire circuit. The residual winding of the mixing C.t. is not connected since the zero sequence components of current circulate in the delta connections and do not appear in the lines. In order to ----~~~r-------------~

----~~gr---------~

____~~m~--~----~/

Power tra.nsfom.r

FIG.

10.26. Transformer feeder protection

409

Protective Relays

10.8

avoid a blind spot for a phase fault on the wye side ofthe power transformer, which would give currents in the ratio 2/1/1 on the delta side, the middle tap of the summation transformer is off-centre. Such an arrangement is obviously less sensitive for ground faults than the normal residual connection. The most common solution is to provide residual relays at each terminal breaker, each of which will detect a ground fault only on its own side of the power transformer; these residual relays are then arranged either to send a tripping signal to the other end or to unbalance the pilot-wire circuit or inject a tripping impulse so that tripping occurs at the other end too. Another alternative is to close a 'fault-throwing' switch which will create a fault on the feeder of the transformer that can be detected at the other end. Fig. lO.27a shows a typical case of a ground fault cleared at one end but not detected at the transformer end; in this case the displacetnent of the voltage neutral can be used to cause local tripping, the neutral displacement

(b) FIG.

10.27. Ground faults not seen through transformers (a) On line side. (b) On bus side

relay being located on the line side of the transformer. This however is no solution for a ground fault on the other side of the transformer which, owing to the neutral grounding resistance, creates only a very small current in the feeder, as shown in fig. 10.27b; such a fault can be detected by a sufficiently sensitive negative sequence relay. 10.'.1. d.c. Intertripping

In order to avoid undesirable tripping due to a.c. int~rference for the case of a potential gradient existing in the earth, the d.c. intertripping relay must be very insensitive to a.c. but able to work rapidly on a small d.c. signal. Such a relay is called a surge-proof intertripping relay and is usually designed to pick up on 20 rnA d.c. and not to pick up on 5 A a.c. This assumes a 1000 ohm pilot and a maximum induced voltage of 5 kV.

410

10.8

Power Transformer Protection

Fig. 10.28 shows the pilot-wire intertripping circuit and fig. 10.29 shows two typical intertripping relay circuits. 10.1.2. a.c. Intertripplng

This may be through an audio signal modulated by a phase-shifting sequence or a coded signal produced by a vibrator energised from a battery. It is sometimes done by unbalancing the pilot wire circuit. The disadvantage ofthese schemes is that time delay has to be allowed for establishing the code sequence and hence the tripping is slow.

~~--------------T-'~

Pilot

wIres

IR

~--~~~--------------~.--~~~--~

Trip

10.28. Pilot-wire intertripping circuit P.R. = protective relay; T.R. = tripping relay; I.R. = intertrip relay flO.

(a) Relo.y

R

FIG.

L

L

L

TTT

10.29. Typical surge-proof intertripping relay circuits

10.1.3. Fault·throwing Switch

Where the cost of the pilot-wires makes intertripping too expensive, the remote relay is operated by putting a fault on the line side of the power transformer by means of a fault-throwing switch controlled by the local relays. These switches are designed for applying but not for interrupting the fault; 411

10.8

Protective Relays

they are usually connected between one phase and ground on grounded neutral systems and between two phases on insulated neutral systems. Until recently the slow operation of these switches (0'5 second) delayed the final clearing of the fault. An American switch with its contacts in sulphur hexafluoride is now able to apply a fault in 0·05 second; this is due to the very high insulation strength of the S2F6 which permits a contact separation of only 0·01 in. per kV.

412

11 Bus.Zone Protection General Principles-Current Differential Protection-Voltage Differential-Frame Leakage Protection-Directional Comparison-Back-up-Supervision 11.1. GENERAL PRINCIPLES

There is some difference of opinion as to whether this should be called bus-zone protection or bus bar protection. The former term is used by the author because the protection includes all the apparatus connected to the busbars. Bus-zone protection should be fast in order to limit damage, especially in indoor stations. It should be very stable, i.e. it should not have any tendency to operate for faults outside the bus zone for values of current up to the interrupting rating of the switchgear because of the disruption of the system that would result from unnecessarily interrupting all the circuits to a large station. Reliability of operation is equally necessary because failure to clear a bus fault can result in extensive damage to equipment, danger to personnel and disruption of service. Since bus faults are extremely rare (about one in 15 years per installation), periodic testing, either manual or automatic, is necessary to check the pick-up of the relay on internal faults. Statistics collected in the U.K. indicate that over half the bus faults that have occurred were due to equipment insulation failure and flashovers due to lightning. About a third of the faults were caused by human errors and the remaining 10% by miscellaneous causes, such as falling objects and circuit breaker faijures. More than half the faults were to ground. The fact that the isolation of a bus causes the disruption of all the circuits connected to it means the bus protection must be very carefully monitored to prevent inadvertent operation of the relays protecting it. In France it is felt that this is such an important point, and bus failure so rare, that local bus protection should be avoided and bus faults should be cleared by back-up relays at the neighbouring stations. In the U.S.A. and the U. K. the risk of inadvertent tripping is frequently met by providing independent protective circuits, both of which must be satisfied before tripping can occur. In some cases continuous supervision of the c.t. and tripping circuits is provided. In addition, the relays are designed 413

Protective Relays

11.2

for maximum electrical and mechanical stability. Electrical stability is defined as the maximum value of through current that will not operate the relay and is generally above 50 times the c.t. rating. Mechanical stability is defined in Chapter 13, section 13.11.2. Relays for the detection of all types of bus faults make use of Kirchhoff's Law; all the currents entering and leaving the protected electrical circuit

R.la.y

&us

Brea.ku$

C.T> .±-----d:;-------r:I;---'

11.1. Differential current protection of a bus (the relay is connected to trip all breakers)

FIG.

(busbar zone) must sum vectorially to zero unless there is a fault therein. Fig. 11.1 shows the arrangement of the c.t's so that the switchgear, as well as the bus itself, is protected. It will be seen that the sum of the currents will not be zero if there is a bus fault, and the relay will be energised. 11.2. CURRENT DIFFERENTIAL PROTECTION

Current differential protection depends upon the sum of the c.t. secondary currents' being zero when the sum of the primary currents entering and leaving the bus is zero, thus producing no differential current and making the relay inoperative during load or an external fault. During an external fault the c.t. in the faulted feeder has a current which is the sum of the currents in all the other c.t's around the bus but the difference in magnetic conditions of the c.t's may affect their outputs so that, with ironcored c.t's, their secondary currents may not sum to zero as they should. Even with identical c.t's with iron cores large enough to avoid saturation with maximum fault currents, d.c. transient conditions may upset the balance, with a total current containing a decaying d.c. component. i = I (sinwt+B-~t) (11.1) If v is the voltage across the C.t. secondary due to the current flowing

in the relay operating coil, the flux in the core is cP = Iv.dt.

1~8 . dt

maxwells, where n is the number of turns in the C.t. secondary winding. For the sinusoidal component of current this will be of the form

cPa..c = KIZr w where Zr is the impedance of the relay circuit. For the d.c. com414

Bus-Zone Protection ponent of current it will be of the form

l/Jd.c.

= KIRr

11.2

~

where Rr is the total

resistance of the relay circuit. It will be seen that

l/Jd.c. Rr ro.L Rr X l/J•.c. = Zr . R = Zr . R

(11.2)

where L, R and X pertain to the primary circuit. X/R will be of the order of 20 on a power station busbar. This means that the c.t. carrying the fault current will saturate if the fault occurs at voltage zero and that balance with the other c.t's will not be possible. This subject is discussed in more detail in Vol. IT, Chapter 9. In the past, attempts were made to overcome this difficulty with time delay but nowadays power systems have become so large, and fault currents so heavy, that high-speed relays are required. The d.c. time constant for a fault circuit is L/R seconds. Typical time constants for primary circuit components are as follows: Turbo-generators 0·1 second, Transformers 0·05 second, Lines 0·01 second. In sections 11.2.2 and 11.2.3, protective systems are described which now solve this problem directly, but the first step taken (in the 'thirties) was to bias the differential relay which improved its stability considerably but was not a complete solution. 11.2.1. Biased Percentage Differential Protection

This was an early English solution to the problem of instability on external faults due to C.t. saturation. The differential current relay was provided with a restraint derived from the arithmetical sum of all the currents, which were rectified and added. In other words, the operating quantity was the vectorial sum of all the currents and the restraining quantity was their scalar sum. The circuit is shown in fig. 11.2; it will be seen that the operating windings of this relay are a.c. and the restraining or bias winding is energised by d.c. This system is theoretically sound but has been superseded by a simpler unbiased system using a voltage relay. Meanwhile, an attempt to simplify the circuit (112) and hence increase the reliability has been made by the use of summation c.t's so that only one relay is necessary for all phase and ground faults. In this scheme it is of course essential to connect all the mixing c.t's similarly with respect to the phases (fig. 11.3). Early American biased differential relays used induction disc relays with two discs on the same shaft, each having two electromagnets and a damping magnet. The operating electromagnet was connected in the differential circuit so that it received the vector sum of the currents; the other three were restraining magnets connected in the individual circuits or groups of circuits and producing a net torque proportional to the scalar sum of the squares of the currents in these circuits. Where the number of bus circuits exceeded three 415

Protective Relays

11.2

C.T's

Bus

FIG.

11.2. Biassed differential current protection

"_summation QUlC.C.TS.....

~

.....,

L

I I

L...-,

~

I

1

I

~

I

I

.rr--

~

~?

S

..

~

..", r--'

~ .<>

:>

CD

~

.~

A-

U

l~

,.......,

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~

Ii:

fl tt

,,~ill'ing

Op .. a.u ..oi,tor

1

.:...'-

•f1

Rutrain Rcla.y

FIG.

i

1

11.3. Use of summation c.t's to simplify bus protection

416

Bus-Zone Protection

11.2

they were paralleled in three groups, the basic power sources in one group, load feeders that contribute less than 5 % of the total fault current in the second group and other power sources in the third. These early biassed schemes required very large c.t's because they did not use stabilising resistors or high impedance operating coils and hence the c.t's had to balance their secondary currents with maximum throughfault currents including the d.c. component (see previous section 11.2 and Chapter 9, section 9.4.3), so that they theoretically had to produce a total flux of the form =

(I + ~)IIC where IIC is the steady state a.c. flux during

maximum through-fault conditions. 11.2.2. C.t. Voltage Differential Protection On a heavy external fault the C.t. in the faulted phase may saturate and, if it does, its output will be deficient and hence the sum of all the c. t. secondary currents will not be zero. The resultant unbalance current will flow in the relay causing it to operate and clear the bus (fig. ll.4c). If, however, the relay works on a voltage basis instead of current, the saturation of the c.t's in the faulted feeder will cause no trouble since the voltage across the c.t's will be limited to the IR drop in the leads from the saturated c.t. and its secondary winding resistance (see Chapter 9, Appendix 9.4), which is a relatively low voltage. If the c.t. does not saturate the relay voltage will approach zero because it is connected across voltages of opposite polarity. On the other hand, during a bus fault, all the c.t's will be pushing their currents through the relay so that the voltage across the relay will be the impedance of the relay circuit times the total fault current (secondary) minus the c.t. magnetising currents. This will be a much higher voltage, approaching the open-circuit voltage of the c.t. secondaries, and will operate the relay (fig. ll.4d). The minimum voltage that can be presented to the relay during an internal fault is usually many times the maximum value for an external fault, so that it is easy to find a selective setting for the relay which is usually set to pick up at twice the maximum external fault value, or half the saturation voltage of the smallest c.t. For ground faults, a lower setting may be necessary if the system is earthed through impedance. For this principle to be effective, the resistance of the c.t. secondary circuits must be low, i.e. the leads between the c.t's and the paralleling point must be as short as possible and toroidal (bushing type) c.t's should be used. AIl the c.t's should have the same ratio and auxiliary matching c.t's should be avoided because they introduce dissimilarity. (a) High and Medium Impedance Relay Schemes. Fig. ll.4a shows the circuitry of an American high impedance differential relay. Series tuning makes the relay responsive only to the fundamental component of the differential (spill) current of the c.t's and improves the sensitivity of the relay at the expense of a slight time delay. Making the relay insensitive to d.c. and

417

11.2

Protective Relays

Sensitive D,C.

polQris~d t~lay

(a)

(b)

Rtlo.y

C.Ts

(c) Es

I relo.y

--- ..............

--------

I P;In~------------~~------------~~Is -I

I

I

--N

11.4. Differential protection using high impedance relay (a) With linear pick-up control (b) With non-linear pick-up control (c) Basic circuit of high impedance bus protection RCL = resistance of c.t's and leads; Rs = stabilising resistance (d) Vector diagram for an internal fault Ip = fault current (primary); Is = fault current (secondary); n = C.t. turns ratio; Elm = total magnetising currents of the other c.t's FIG.

harmonics make it more stable on heavy through faults which may produce spurious residual spill current due to unequal C.t. performance. The provision of harmonic restraint is impractical because of the risk of preventing tripping on a heavy internal fault. Excessive voltages on internal faults are prevented by the use of nonlinear resistance (thyrite) and a relay connected in series with thyrite provides fast operation on heavy faults, its pick-up being high enough to prevent operation on external faults. 418

Bus-Zone Protection

11.2

Fig. 11.4b shows a slightly different connection of this type of relay in use in England. An a.c. relay is used which is prevented by a series capacitor from operating on d.c. voltages in the presence of d.c. offset current components on external faults above the relay setting. The pick-up setting is controlled by series-connected thyrite units rather than an adjustable linear resistance; this gives the relay a sharply defined voltage setting, enabling the relay current to increase at a high rate when the voltage setting is exceeded, thereby providing fast tripping for values above the setting. The series thyrite units are chosen to provide settings in steps of 25 volts, from 25 to 175 volts. The setting chosen is about half the saturating voltage of the c.t's. Because of the high impedance of the relay circuit in fig. II.4b, another thyrite unit is connected in parallel with it to limit the maximum voltage during internal faults to about 900 for I amp c.t's, and thus prevent damage to the insulation of the panel wiring. The number of circuits on one bus which can effectively be protected by high impedance differential protection depends upon the saturation voltage of the c.t's together with the impedance and sensitivity of the relay. The higher the impedance of the relay, the larger the proportion of the current required for c.t. magnetising current and the smaller the margin between the voltage on an internal fault and the voltage ceiling permitted for panel wiring. Two objections to the high impedance differential relay are (a) the thyrite units are bulky and expensive; (b) their resistance varies with temperature and between units, making it difficult to obtain a precise pick-up setting. Another English manufacturer uses the circuit shown in fig. 1l.5a. The vector sum of the secondary currents from the bus c.t's is supplied to an a.c. overcurrent relay through a small saturating c.t. which limits the maximum relay current to a safe value. The relay circuit is tuned to exclude d.c. offset components and transient harmonic currents; due to the high time constants

(~ ratio) of modem power systems, the d.c. offset component of current may cause prolonged spurious differential residual current on heavy external phase faults but, fortunately, the total transient has a relatively small fundamental frequency content. A linear stabilising resistor in series with the auxiliary c.t. in the relay enables it to be calibrated in voltage which can be more accurately determIned because the resistor is of the linear type. The circuit is simpler than the previous ones and the equipment is small enough to put complete three-phase protection in a single unit relay case, fig. 1l.5b. The tuned circuit adds about a cycle to the operating time of the relay but the net efficiency of the relay is higher than that of the preceding types. The net efficiency involves the sensitivity, speed and stability of the relay. Stability is defined as the ratio of maximum through fault current below which the relay will not operate, to the minimum internal fault current for which it will operate. With both high and medium impedance relays the best results are obtained with c.t's having lower resistance and very low leakage reactance, 419

Protective Relays

11.2

e.g. those of toroidal construction. Auxiliary c.t's for ratio correction reduce the sensitivity and electrical stability and should not be used. All the c.t's should have the same ratio. (b) Setting of Voltage Differential Relays. The pick-up voltage setting is usually made just above the maximum relay voltage for an external fault; this voltage would approach zero if the c.t. performance were linear but, if the c.t. in the faulted circuit is completely saturated and the others are not L

~/O-:___~ ( a)

FIG.

c

11.5. (a) Circuit of one phase of medium impedance relay (b) Complete three-phase medium impedance relay

saturated, it can reach a value of the maximum fault current times the resistance of the c.t. secondary plus that of the leads to the junction point with the other c.t's. Hence the relay should be set to pick up at a voltage Vr = 1·1 Imax(Rsec+ Rleads)

(11.3)

V, and Imax are r.m.s. values; no account is taken of the d.c. offset component of Imax because the relay is assumed to be designed not to respond to it; furthermore, saturation of the c.t's due to the d.c. offset component would reduce their output voltage. In the relay shown in fig. 11.5a, the voltage setting is the product of the current setting of the relay times the resistance of the stabilising resistor. The relay has taps between 0·1 and 0·4 ampere so that, for a 50 volt setting, the stabilising resistor would be 125 ohms on the 0·4 ampere tap less the relay impedance. The corresponding resistance in the high impedance scheme 420

Bus-Zone Protection

11.2

would be 50/0·008 = 6250 ohms, because the current pick-up of the relay is 8 rnA.

During an internal fault the c.t's are virtually open-circuited except for the small current taken by the relay. The voltage assumes a value such that the secondary spill current which would have flowed in the relay if their ratios had been maintained is equal to the sum of their magnetising currents required for this voltage. In other words, the primary currents are all used up as magnetising current instead of producing proportional currents in their secondaries and the c.t's not having power sources receive magnetising current from those that do via their secondaries. Hence the real operating current of the relay is 10 = I, + LIm, where I, is the pick-up current of the relay and 1m is the magnetising current of each c.t.; I, is very small compared with 1m. For values above pick-up an increasing proportion of current goes through the shunt saturating circuits of the relay, hence limiting the c.t. secondary circuit voltage and the relay current. 11.2.3. Ironless C.T's (113)

In c.t's containing iron, the number of circuits permissible on a bus is limited by the fact that the relay receives the differential current minus the sum of the exciting currents for all the c.t's on the bus. Furthermore, the high time-constants possible in modern power systems prolong transients, so that stability on heavy through faults may be difficult to obtain, especially in the case of the ground differential relay. With ironless toroidal c.t's (linear couplers), transient conditions are eliminated and there is no magnetising current, "no magnetic saturation limit and no lead resistance problem. These facts eliminate the difficulties requiring special precautions in differential schemes using ordinary c.t's. It should also be noted that ironless c.t's have great potentialities in dealing with the transient problem in general. Two of the greatest difficulties with relay transient problems are differential saturation and the transference of d.c. through the iron-cored C.t. The problem of differential saturation clearly disappears when the iron is removed altogether. This also results in the c.t. becoming a purely differential device, the instantaneous output quantity becoming a voltage. dip (11.4) Vo=±M' dt where ip is the instantaneous primary current. If ip = Im[sin(rot-cf»+K . a-At] (11.5) where A is the time constant of the resultant power system viewed through the transforming device (linear coupler) then, from equation (11.4),

Vo = ± M . Im[ro. cos (rot-cf»-AK . a-At] (11.6) A may well be of the order of 100 m.seconds, i.e. l/lOth second; thus the d.c. transient applied to the relay, in this simple case, has undergone 90% attenuation without the need of any other relay filtering device. 421

Protective Relays

11.2

The linear coupler has a limited VA output, of the order of 3 VA at 1000 A primary current. It is clear, however, that the future may well bring a greater utilisation of relaying systems based on linear couplers since the order of VA is adequate for most static relays based on semiconductors and the inherently superior transient performance expressed in equation (9.4) potentially provides for high accuracy with fast operation. Linear couplers can be wound to within 1 %accuracy. By distributing the winding uniformly around the core and using several layers, interference from other couplers and from neighbouring iron can be made negligible.

Bu& Sensitive relay

LineCLr couplers

(a) Linca.r coupler secondaries.

Rela.ys

(b) FIG.

11.6. Differential voltage protection using linear coupler c.t's (a) Single line diagram (b) Three-phase schematic diagram

The ratio of maximum external fault current for blocking to minimum internal fault current for tripping is about 25 for a uniformly distributed multilayer winding. The number of circuits on one bus that can be protected effectively is of the order of 15 and depends only upon the sensitivity of the relay (113) which should operate preferably on 5 mW or less. On systems grounded through impedance, the ground relay should be more sensitive than the phase relays (fig. 11.6b) but, in cases where the through current on an external

422

Bus-Zone Protection

11.3

fault may be very heavy, it may be necessary to block the sensitive ground relay on multi-phase faults if this through current exceeds 25 times the relay setting. For maximum sensitivity the impedance Zr of the relay should be made about the same as that of the sum of the impedance Ze of the linear couplers, i.e. Zr = LZ(". It is not necessary to include the lead resistance because it is usually small compared with that of the relay and the linear couplers. Ze is of the order of 10 ohms for 132 kV linear couplers. The current setting I, of the ground relay and its impedance Z, (from fig. 11.5b) are related by the equation

LEe = I,(LZ e+ Z r+ 3Z,) (11.7) where" Ee is the e.m.f. from each linear coupler and Zr is the impedance of each phase relay. If the linear couplers give 5 volts output per 1000 amperes primary current, their e.m.f.s can be calculated and added to give LEe. 11.3. FRAME LEAKAGE PROTECTION

In this form of protection the switchgear framework is insulated from ground (building steelwork) except through the primary of a c.t. whose secondary supplies an instaataneous overcurrent relay with current whenever a ground fault occurs anywhere in the bus or its associated equipment. Frame leakage protection is most effective in the case of isolated phase switchgear and bus construction, which should eliminate interphase faults, although it is by no means limited to this type of equipment. It is easiest to apply in new applications where insulation of switchgear from ground can be included in the layout. The insulation of the switchgear framework from ground is light; anything over 10 ohms is acceptable but care should be taken to ensure that all main and multi-core cable glands are insulated and that it is not possible for any earthed metal to make accidental contact with the switchgear frame. In order to prevent the risk of insulation breakdown due to high voltages induced in the cable sheaths during faults, the main cable gland should have a minimum flash test of 8000 volts. It is not possible to protect separately each set of busbars of a double busbar switchboard, nor is it always practical to apply such protection on outdoor switching stations, but separate protection can be applied with very satisfactory results to a phase-segregated metal-clad board at a comparatively low cost. It is essential to have some check system with a frame leakage scheme in order to prevent a spurious current causing unwanted operation. This usually takes the form of neutral check relays operated from current transformers connected in the neutrals of the system, see fig. 11.7. As an alternative, a core-balance transformer can be fitted in the cable box or three residually connected current transformers on "the incoming equipments to supply an instantaneous overcurrent relay. Should it be found impracticable in a frame leakage scheme to provide a 423

11.4

Protective Relays Frameworks

r---------~I 1 I---\---'~------------' .1 • I I I II 1 1 1 1 3 1-+1+-1- - , . . -_ _ _.,-_+-__ I

flO.

11.7. Frame leakage protection (checked by neutral current relay)

neutral check feature, then an inverse time delay relay should be used for the main scheme. This prevents inadvertent operation of the bus bar protection due to current flowing from the auxiliary wiring to the switchgear frame, the auxiliary circuit fuses clearing the fault before the inverse relay operates. 11.4. DIRECTIONAL COMPARISON

During an internal fault the power will flow towards the bus in all circuits connected to it; during an external fault the power will flow towards the bus in all circuits except the faulted one and there the power will flow outwards. An early scheme utilised this fact and had directional relays in all the bus circuits with their contacts in series with a multicontact relay (fig. 11.8a) which tripped all the breakers; the directional relays CD) closed their contacts for incoming power so that tripping could occur only for a bus fault. This scheme was little used because of its dependence upon a large number of series contacts. It was superseded by a scheme in which the directional relays had double-throw contacts; all the make contacts were paralleled and connected to the trip relay (fig. 11.8b) and all the break contacts were paralleled and connected to a blocking relay B which could block tripping. Discrimination was assured by the fact that during normal load conditions at least one circuit had outgoing power, so that the blocking relay was normally energised and there was no contact race to prevent tripping on an external fault. As a further precaution a 2 cycle delay in the tripping relay T can be provided. The scheme can be simplified by the use of polyphase relays for phase faults. The ground fault relays can be polarised by current from a

424

Bus-Zone Protection

11.4

neutral grounding c.t. or from residual potential if there is no grounding point. Phase relays, if used, would be polarised from the appropriate line-toline potential. The directional comparison scheme is difficult to apply in a large network, especially a cable network with resistance earthing; in this case, the capacitance charging current may be comparable with the minimum ground fault current because the magnitude and phase angle of the outgoing capacitance

P.T.

(a)

P.T.

FIG. 11.8. Directional comparison scheme (a) Series trip scheme. (b) Blocking scheme

current may be close to that of a bus fault to ground. However, this problem can in most cases be solved by the use of voltage restraint because a mho characteristic can discriminate between these conditions, as can be seen by reference to Chapter 5. Negative sequence directional relays will be less affected because charging current has very little negative sequence content. Distance units have been used in the U.S.A. for bus protection where their reach is limited by transformers or feeder reactors, as described in Chapter 5, section 5.4.5. The relays were of the reactance type and were set to reach a short way into the feeder reactors; reactance units operate for currents in the reverse direction so that they detect faults either on the bus or in the generator. Reactance relays have also been used for protecting a bus with two sections separated by a reactor (also described in section 5.4.5).

425

11.6

Protective Relays

11.5. BUS BACK·UP PROTECTION

This has two interpretations. It can mean no local protection at all and dependence upon the second zone tripping of stepped distance relays at neighbouring stations to clear local bus faults. It can also mean the clearing of a fault on a feeder which: because the feeder breaker has failed to operate, must be regarded as a bus fault. The latter fault can be cleared by a timer which is controlled by the relays on the faulted feeder. This is described in more detail in Chapter 12, section 12.4.2. 11.1. SPLIT BUS PROTECTION

Each section of the bus is protected in the same way as a single bus, using one of the schemes described in section 2 of this chapter. A split bus permits the use of a check feature (shown in fig. 11.9) which is not possible with single buses, except by duplication of the c.t's and relay. It will be seen that neither section of bus can be isolated unless the overall bus protection relay 0 operates. In the case of a fault in the middle zone near the bus-tie breaker, all the breakers will be tripped.

FIG.

11.9. Split bus protection

Fig. 11.10a shows a 4·1ine ring bus, where bus differential protection would be complicated and expensive because of the many bus sections and the secondary switching necessary when one section is out. However, from the point of view of interruption to service, a bus fault is no more serious than a line fault because the adjoining halves of any pair of bus sections, such as between G and F, can be relayed as part ofthe line C by arranging the c.t's for line protection as shown in fig. 11.lOb. The advantage of this system is that any breaker can be taken out for maintenance without interrupting any load and without providing a spare breaker; but it is essential to provide automatic reclosing of the breakers and a motor·operated disconnecting switch in each line in order to obtain its full benefit. The disconnecting switch can also ground the line. For a transient fault on a line or a bus section the appropriate two 426

Bus-Zone Protection

11.7

breakers open and reclose. For a permanent fault on a line the breakers open and reclose a predetermined number of times and, after the last trip, the motor-operated line switch opens and the breakers reclose, restoring the ring

[email protected]

(b) FIG.

11.10 (a) Four section ring bus. (b) Location of c.t's and p.t's on ring-bus

bus. If the permanent fault is on a bus section the breakers trip again and lockout leaving the other lines in' but the ring bus open. 11.7. SUPERVISION

In large stations, open-circuits in the c.t. circuits are detected by a very sensitive overcurrent relay with an effective setting of 10% of the rating of the smallest feeder on the bus and connected as shown in fig. 11.11. Owing to

c:r~

FIG.

11.11. c.l. supervision scheme

427

11.8

Protective Relays

the magnetising current taken by the other c.t's the actual setting of the relay must be much lower than the 10 %value. This sensitive relay operates. a time delay relay which sounds an alarm and blocks the bus differential relay from tripping by short-circuiting it through a hand-reset control. Wrong tripping during the time delay is prevented by a checking relay. This system cannot be used where earth fault protection only is provided because the c.t's are paralleled at their terminals and only the residual circuit is brought back to the relay panel. 11.8. TRIPPING CHECK

On account of the many circuits that may be connected to a bus, wrong tripping of the bus-zone protection is a serious matter and all possible precautions are taken to avoid it. Inadvertent tripping in handling the relay, mechanical shock to the panel, etc., is sometimes avoided by the use of two tripping relays with their contacts in series, so that both have to operate to cause tripping. For maximum safety the two tripping relays should operate in different planes and \:>e mounted on different panels. Wrong tripping due to electrical defects in the circuit can be prevented by duplicating the c.t's and the bus differential relay and connecting the contacts of the two relays in series, so that both have to operate to cause tripping (fig. 11.9).

428

12 Btrek-up Protection Basic Principles-Precautions for Reliability Remote Back-upLocal Back-up-Relay Back-up-Breaker Back-up-a.c. Supplies -d.c. Supply 12.1. BASIC PRINCIPLES

The function of a protective relay is to operate in response to a fault on a power system so as to minimise the damage to equipment and the interruption to service by opening only those breakers which will isolate the faulty circuit from the power source. Relays can be prevented from doing this by failure of any of the components in the circuit, viz. the breaker trip mechanism, the switchgear wiring, and the a.c. or d.c. supplies to the relay itself, fig. 12.1. Consequently it is ,8

.."

Onlt pha.se shown

al

Lin. FIG.

12.1. Normal connections of a relay protecting line section A-B

necessary either to provide the relays with characteristics so that relays at one location will back-up those in another location which fail to trip, or to duplicate some or all of the equipment locally. The first solution (remote back-up) has been used for over 30 years. In modern power systems it may sometimes be ineffective because of the effect of infeeds, between the back-up relay and the fault, which may reduce the current and increase the voltage at the relay so as to prevent it from operating. The second solution (duplication of relays, current transformers, etc.) involves extra expense and complication. The best solution is first to take those precautions which will reduce the risk of failure to a very small calculated risk and then to employ remote or local back-up protection to an extent justified by the importance of the circuit.

429

12.3

Protective Relays

12.2. PRECAUTIONS FOR MAXIMUM RELIABILITY

Troubles with breaker mechanisms can be minimised by adequate maintenance (106). Troubles with trip coils, their wiring and breaker auxiliary switches become negligible if the trip coil is connected directly to the negative pole of the d.c. supply and a trip supervision circuit is installed. The relays should be designed for high contact pressure under all operating conditions. Ifnecessary, it should be augmented as the contacts are approaching and almost closed. This is done in certain modern relays (68), for instance, by a notch in the induction disc. The relay case should be made dust-tight and provided with a filterbreather to equalise the pressure inside and outside the case without admitting dust. Testing should be done with the cover on or, in the case of plug testing, a temporary perspex cover to permit dust-proof entry of the test plug (see Chapter 13, Section 13.4). Fine wire relay coils and trip coils should have well-braced junctions between the coil wire and the outside lead so that stress on the latter will not cause an open-circuit. The coils should either be encapsulated in araldite or an equivalent substance, or at least be thoroughly impregnated to exclude moisture. Acid fluxes or acid-producing insulation should be avoided; workers with perspiring hands should not be permitted to handle fine wire without gloves; mechanical removal of enamel from the wire should be avoided. In general, a.c. coils should not use wire less than 0·05 mm. diameter and d.c. coils not less than 0·1 mm.; d.c. coils should not be connected directly to the positive side of the d.c. supply unless all these precautions have been taken. Maintenance testing should be done without disturbing switchboard wiring, see Chapter 13. With relays incorporating the foregoing precautions, maintenance should be done infrequently (about once every five years) except in conditions of severe humidity, new untried components, etc. Infrequent maintenance eliminates the risk of relay failure due to improper adjustment by inexpert personnel, which is one of the commonest causes. Adequate maintenance can often anticipate failures due to a.c. wiring faults, including multi-core cables and current transformers. Failure to trip due to loss of the a.c. potential can be prevented by an over-voltage alarm relay connected across secondary potential fuses (see fig. 12.13). Where devices are used which are too recent for comprehensive reliability statistics to be available, they should be connected so that their failure or deterioration does not cause undesirable tripping or failure to trip. For instance, transistors should be protected not only against voltage surges but also preferably should be connected so that the selectivity of the relay does not depend upon the transistor characteristics. 12.3. REMOTE BACK-UP

This is the cheapest and simplest form of back-up. It is entirely independent of local supplies, wiring, etc., and is essential where there is no bus 430

12.3

Back-up Protection

protection. On the other hand it is unreliable where a local power infeed raises the impedance seen by the back-up relay to a value comparable with that of maximum load conditions. In this solution overcurrent (fig. 4.5) or distance relays (fig. 12.2) at one station can provide back-up protection for the breaker, relays and all their associated equipment in the neighbouring station; it is effective because no

.e

t=

--

Dista.ncr FlO.

12.2. Remote back-up with distance relays

common equipment is used and hence the back-up cannot fail from the same cause as the first line of defence. Until recent years system connections were fairly simple so that back-up protection was effectively provided by the relays at the next station towards the source with enough time delay to permit the relay in the faulted circuit to clear the first fault, if operable. The increase in the number of interconnections and power-infeed points in recent years has reduced the fault current in the circuits, other than the faulted circuit, so that remote back-up relaying is becoming increasingly difficult; at the same time loads are becoming more important and hence demanding better service continuity. Where the limitation of the back-up reach of distance relays is mostly due to line length, an improvement in their operation can be obtained by

.

(a)

e i=

zon.l(A)

G

D

A Ta.p

B

c

lin.

Ta.p lin.

12.3. Reversed third zone of distance relays (a) The time zones. (b) Preservation of tap line

FlO.

locating the Zone 3 unit at the other end of the line section and reversing its direction. Referring to fig. 12.3a, the third step of the distance relay at A normally provides remote back-up for faults in section BC if the breaker at B fails to trip. This back-up can equally well be provided by reversing the third zone unit at B so that it covers section BC. 431

12.4

Protective Relays

This arrangement not only reduces the impedance seen by the back-up relay by the impedance of section AB but also maintains power supply to tapped loads in section AB (fig. 12.3b) which would lose their power source with the normal arrangement where section BC is backed-up by the relay at A. It also prevents the back-up relay from reaching through a large transformer into a distribution circuit. The fact that the back-up impedance relay is now located at the same station as the transformer means that a directional relay in the transformer circuit can be used to block the impedance back-up relay for faults fed through the transformer. Where the limitation of back-up reach is due to power infeed at the intervening bus the excessively high impedance setting of the back-up relay may

x

flO.

12.4. Elliptical fault detector characteristic

cover undesirable tripping on overload. Remedies for this condition are: (a) Provide the back-up zone of the distance relay A with a non-linear

impedance characteristic. (b) Provide it with an elliptical characteristic which will enable it to have a shorter -impedance reach along the R axis than along the X axis

(fig. 12.4). (c) Where the bus is divided a fault detector relay can be connected to

split the bus during a fault and reduce the local infeed. Where remote back-up cannot be used effectively the relay circuit and the breaker circuit must have local back-up. 12.4. LOCAL BACK-UP

This solution (fig. 12.5) involves extra expense and complication and the value of duplicating each component depends on its liability to failure. An analysis of the replies to a questionnaire sent in 1958 to the members of the C.I.G.R.E. Relay Study Committee indicates that the order of likelihood of failure of equipment is as follows: (a) Relays (43 %). (b) Circuit breaker interrupters (13'5 %). (c) a.c. wiring (chiefly multi-core cables) (12 %).

432

12.4

Back-up Protection (d) Breaker trip mechanisms (7 %). (e) Current transformers (7%). (/) d.c. wiring (including trip circuits) (5 %). (g) Potential transformers (3 %). (h) Breaker auxiliary switches (3 %). (j) Breaker trip coils (2·5 %). (k) d.c. supply (1 %). B Line

.

Norma.l rcla.y

::J

IQ

Line

FlO.

12.S. Local back-up: duplication of relays and c.t's

The following were the most common reasons given for the failures under the different items of the questionnaire:

(a) Relays (i) Dirty contacts (low contact pressure). (n) Open-circuit in fine wire coils (d.c.). (iii) Wrong setting. (iv) Incorrect adjustment.

(b) Breaker lfailure to interrupt) (i) Insulation failure of bushings or operating rod. (ii) Damage of main contacts.

(c) a.c. Wiring (i) Breakdown of mineral insulation. (ii) Loose connections. (iii) Wiring errors.

(d) Breaker Trip Mechanism (i) Plunger sticking due to corrosion. (ii) Incorrect adjustment.

(e) Current Transformer (i) Insulation failure of current transformer during lightning. (ii) Ditto due to secondary, opened inadvertently. p

4~

Protective Relays

12.4

(/) d.c. Wiring and Trip Circuits (i) Breakdown of mineral insulation or seals. (il) Loose connections. (iii) Moisture ingress and corrosion. (iv) Accidental physical damage. (g) Potential Transformer (i) Fuse deterioration. (il) Blown fuse. (iii) Insulation failure due to lightning.

(h) A.uxiliary Switches on Breaker (i) Lost motion, in older types, causing early or later closure, and contact bouncing.

(j) Breaker Trip Coil (i) Open-circuit due to corrosion.

(k) d.c. Supply (i) Low voltage. (il) Blown fuse. Electrically, the equipment can be grouped in three zones requiring supervision or back-up: (a) The current circuit comprising the current transformers, the relay current coils, and the wiring connecting them. (b) The potential circuit comprising the potential transformers, the relay potential coils, and the wiring connecting them. (c) The d.c. circuit comprising the trip coil, the breaker auxiliary switches, the relay d.c. coils, and the wiring connecting them.

The least trouble is experienced with item (a), the a.c. current circuits, so that some form of overcurrent relay is the best basis for back-up protection. The grouping of the components in (b) and (c) suggests that local back-up must be divided into two sections, relay back-up and breaker back-up, because failure of either unit or its auxiliaries can prevent tripping. 12.4.1. Relay Back-up

Duplication of the normal relays (fig. 12.5) would provide relay back-up without time delay because they would work in parallel, but it would involve considerable cost and complexity; hence it would be justified only on very important interconnections. The best arrangement is for the back-up relays to use a different operating principle from that of the main relays and to be supplied from separate c.t's. For instance, on transmission lines, distance relays are used to back up pilot (wired or carrier) protection and vice versa. Similarly transverse and longi-

434

12.4

Back-up Protection

tudinal differential protection can give mutual back-up on generators or lines. Buchholz and electromagnetic relays are used together in transformer protection. A solution for overhead lines is to provide one-step (impedance or offset mho) relays which are delayed 0·5 second and which overreach both ends of the protected circuit (fig. 12.6). This gives high grade back-up protection

jA

.

I

E

;::

I

I L ___

-J

FIG.

I I

______ J

I C~

B-"

I I I

B

-+

J

c

I

0

10 . .

12.6. Local relay back-up by an offset mho relay (dotted characteristic)

without excessive time delay or expense. At systems solidly grounded at each station it is simpler to use the one-step distance relay for phase faults only and an inverse time-overcurrent relay with a parallel instantaneous unit for ground faults. This is not only simpler but often faster and more reliable because most of the zero sequence current comes from the local grounding transformer (see Chapter 4, fig. 4.30.) The most economical back-up protection for the relays is a non-directional definite time-current or I.D.M.T. relay (fig. 12.7). Because the reach of the I

I L - -

!I________~IA

--,t1 - c------------

i

I

___ JI

B o

B~

O~

~e

fIG.

12.7. Local relay back-up by a definite time relay (dotted characteristic)

time-overcurrent relay varies with generating conditions and may cover more than one line section it must be given a time setting of at least 1·0 second, i.e. corresponding to the third zone time of a distance relay. Faster back-up times can be achieved with a non-directional inverse time-overcurrent relay with an instantaneous unit (fig. 12.8) supplied from separate current transformers, if available. The overcurrent method is inexpensive and reliable; it relies only upon the current transformer and there is no risk of failure due to fine wire, such as is used in a.c. and d.c. potential circuits. The time-overcurrent relay back-up (fig. 12.7) may be slower than the previous alternative of a single-step mho back-up relay with Zone 2 time but this does not interfere with the selectivity. Referring to fig. 12.7 a fault at X is normally cleared by the distance relay at B if the relay at C fails to operate. If the back-up relay at B cannot operate because of a heavy infeed p* 435

12.4

Protective Relays

at C the time-overcurrent relay at C will do so, thus clearing a fault which could not normally be cleared. The fact that the relay at B will operate before the overcurrent relay at C for a fault near the C bus, causing the isolation of station C, is no different from normal operation without the time-overcurrent relay. In cases where this is unacceptable there is the earlier alternative of duplicating the normal relays. With inteed

a.tC

,'"

,,/

C / "..

I

,-- -- - -'F-------"t--,./

I

I

~~t~n!9-~~~~ ~cJ

FIG.

C

Without inteed

12.8. Local relay back-up by an inverse time current relay with an instantaneous unit

More sensitive operation and often faster tripping can be provided by a negative sequence current relay; it should have a fixed time delay if it is backing up distance or pilot relays and an inverse time characteristic if backing up inverse time relays. This relay ignores balanced three-phase faults, which is practical on most systems. For back-up on ground faults only, a very sensitive zero sequence relay can be used in the same way; an English relay of this type will clear ground faults down to 2 % of the c.t. rating and has a c.t. burden of only 0·007 va at pick-up or 2·5 va at C.t. rating; it uses the polarised d.c. relay shown in figs. 2.19. 12.4.2. Breaker Back-up

When a relay operates because of a fault but the breaker fails to trip, the fault can be regarded as a bus fault and necessitates opening all the other breakers on that bus. Opening the breakers nearest the fault has the advantages of (i) saving teed feeders, (ii) facilitating rec1osure, because all the breakers are at the same station. This method is sometimes complicated to apply where the bus is sectionalised and subject to switching. Breaker back-up can be obtained fairly simply by paralleling all the relay tripping contacts associated with all the circuits connected to a bus (fig. 12.9) and connecting them to operate on a common timing unit when there is a fault on any of the circuits. If the appropriate breaker does not clear the fault when its relay operates then the fault should be regarded as a bus fault and the timing unit, after a suitable delay, should trip all the breakers, clearing the bus. Since the timing relay can clear the bus its inadvertent operation must be avoided; there should be an instantaneous overcurrent supervising relay between the timing unit contacts and each breaker trip c.oil (fig. 12.10), so that the breakers cannot be tripped unless both the timer and the individual

436

12.4

Back-up Protection

overcurrent relays close their contacts. In places where some of the unfaulted circuits may feed in less than full load the overcurrent supervising relays cannot be used but the paralleled contacts of the fault detectors of the relays

FIG.

12.9. Local breaker back-up by delayed trip of other breakers

+

II II II I! 11 11 II I t t t t ~Trip

Protective relays on four feeders

Fault

~ detector

relays

To brea.l<er trip coils

FIG.

relay

12.10. Breaker back-up with interlocking scheme

R i , R 2 , R 3 , R4 can be connected in series with the tripping relay contacts, so that, even if the timer or the tripping relay is operated manually, tripping cannot occur unless one of the fault detectors is operated at the same time. 12.4.3. Current Transformers

Current transformers are unlikely to fail in themselves but they may be inadvertently short-circuited by a test link having been left in the wrong position during maintenance testing; also a current transformer insulation may break down if the secondary circuit is accidentally opened. Where possible, separate current transformers are desirable for back-up relays (or at least separate secondaries on one current transformer) because they increase the reliability of the back-up; on E.H.V. lines this may be necessary because of the limited VA capacity of the current transformers. Where they are not available (such as on H.V. air blast breakers) the effect of separate current transformers can be obtained in the case of distance relays by locating the third zone unit at the other end of the associated line; this method also has the advantages of preventing the loss of teed feeders and of facilitating reclosure. 437

12.4

Protective Relays

Where separate current transformers are available they should be on opposite sides of the breaker so as to provide overlapping zones of protection (fig. 12.11), otherwise a fault between the breaker and the nearest current transformer will not be isolated except by remote back-up. If both sets of c.t's are on-the bus side of the breaker (fig. 12.11a) a fault between the breaker and the nearest current transformer will open the

(b) FIG.

12.11. (a) c.t's on bus side of circuit breaker (b) c.t's on line side of circuit breaker

breaker but the fault will remain as a bus fault, and will not be detected by the bus zone protection which is now limited to busbar protection. With distance relays, the fault may be cleared by remote back-up. If both sets of current transformers are on the line side of the breaker a fault between the breaker and the nearest current transformer (fig. 12.l1b) will operate the bus zone protection and clear the bus, which will cause unnecessary interruption to service. The inference is that it js better to have a set of current transformers on each side of the circuit breaker. 12.4.4. Potential Transformers

Duplicate potential transformers are seldom used because they are extremely reliable devices. Where each line has a set of p.t's and there is another set on the bus, the latter could be used to supply the back-up relays. The most common cause of loss of secondary potential is the blowing of a potential fuse due to a temporary short-circuit during maintenance, such as by a metal tool bridging two terminals in falling from a higher position or due to a whisker of a stranded wire touching a grounded metal part. It has been found that a relatively large fuse (60 amperes) for the relay circuits will 438

Back-up Protection

12.5

survive most of the transient short-circuits that are liable to occur in maintenance. Loss of potential due to a blown fuse, an open-circuit in the wiring or to trouble in the potential transformer can either prevent tripping or cause undesirable tripping in distance relays. Loss of potential on the restraining coil can cause wrong tripping but, in a mho relay which is polarised by a potential winding, loss of potential supplying this winding will cause zero torque, i.e. failure to trip. In order to prevent undesirable tripping due to loss of potential each distance relay can have a supervisory relay which may be a simple instantaneous overcurrent relay in series with the Zone 1 tripping circuit. In the Zone 1 fault current pick-up current f h emaximum rare cases w re. > 0 the maXImum load current drop-out current instantaneous overcurrent relay, a rate-of-rise of current relay can be used as the supervisory relay. In either case, the loss of relay potential must be signalled by an alarm relay working on undervoltage and provided with a time-delay so that the alarm will not sound during faults. Where miniature circuit-breakers are used instead of fuses and have an auxiliary contact to open the trip circuit, it must open before the protective relay trips, i.e. with modem high-speed distance relays it should open in less than one cycle. Where fuses are used, an undervoltage relay can be connected with its coil across the fuses and its contacts in series with the trip circuit; this relay will open very fast when a fuse blows because the relay coil receives the full voltage (see fig. 5.51 and refer to Chapter 5, section 5.7.2). 12.4.5. Battery

The records indicate that this is the component least likely to fail. Nevertheless, it can be included in local back-up by providing an undervoltage delayed alarm on the load side ofthe fuses. No supervision other than the alarm is necessary because failure of the d.c. supply can only cause failure to trip. 12.5. SUMMARY

(a) The goal of protective relays is absolute assurance of tripping the

breaker when it is required. (b) This assurance can be made extremely good by proper precautions in

design, such as high contact pressure. (c) Remote back-up is desirable because it cannot fail for the same reason as the first line of defence. (d) Where remote back-up is not possible, local relay and breaker back-up can ensure selective clearing of faults. (e) The degree of duplication of components in local relay back-up depends on the importance of the protected circuit. (f) The a.c. potential supply should have a failure alarm and distance relays should have supervisory relays working on overcurrent or rateof-rise of current, depending upon system conditions. 439

13 HainienalWe .IUI Testing of Belays Commissioning-Periodic Maintenance-Transfer to Test Circuit -Tools-Safety Measures-Mechanical Tests-Electrical TestsManufacture Tests ROTECTIVE relays are intended to protect expensive electrical equipPment. With proper care they will perform this duty, but when neglected they may become inoperative and could become a hazard in themselves. Since the reliability is the most important quality of protective relays it follows that their maintenance must be first class. In the ordinary course of events modern relays, which have been properly adjusted and correctly set initially, should not require subsequent adjustment and, if periodic inspection and check tests show them to be in good condition, they should not be physically touched. It is generally accepted that protective relays and their trip circuits should be periodically checked in order to ensure that they will always be ready to operate with certainty. The recommended practice is to carry out three types oftest: (a) Acceptance tests at the installation or commissioning of the relays. (b) Periodic tests to check the calibration and condition of the relay. (c) More frequent tests of a simple nature to cause movement of the parts, and to check the continuity of the trip circuit. Before attempting any adjustment or tests, the test engineer should carefully read the proper instructions. He should be entirely familiar with the relay, its application, principle of operation, design features and characteristics. 13.1. INSTALLATION OR COMMISSIONING TESTS

Relays should first be examined for damage in transit. Care should be taken not to bend any light parts when removing packing pieces, such as disc wedges. Other important precautions are (a) to avoid handling contact surfaces or small bare wires because of the risk of corrosion; (b) to dust the cover before removing it; (c) to see that packing pieces are removed and the armatures move freely, (d) to avoid touching permanent magnets with ferrous objects such as screwdrivers.

440

13.1

Maintenance and Testing of Relays

Each relay unit should be given a mechanical inspection to see that the armature moves freely and that the contacts have the necessary travel and wipe to ensure reliable operation, checking the manufacturer's settings, if given, in the instruction book. An inspection light and dental mirror should be used to see that the magnetic gaps are clean before the relay is left in service. Suitable electrical tests to check the performance characteristics of the relay are usually described in some detail in the manufacturer's instruction book accompanying the relay. Typical tests are described in section 13.10, under the heading 'Electrical Tests'. The contacts of each relay should be closed electrically or manually to see that the trip circuit is complete and that the proper alarms are actuated. In order to check the current-transformers, voltage-transformers and wiring associated with the relays it is usual also to make overall tests from the primary circuit. The primary current is usually supplied by a test transformer of about 5 kVA supplied from a low voltage lighting or power source, such Prima.ry

circuit

230.A.C.

"------;---...-

Relay

5 Kva.

FIG.l3.!. Primary injection test circuit

as a 240 volts, 30 amperes source, and tapped for various voltages (say 1 to 10 volts) necessary to give line currents up to 1000 A depending on the impedance of the circuit; this current is sufficient to check the polarity of the connections but not to simulate fault currents, the latter being done in the secondary injection tests to check the relay characteristics (see fig. 13.1). The secondary wiring can be further checked if necessary by a low reading ohmmeter or by the ringing method using a bell and battery. A more detailed account of the tests on each type of relay is given in section 10, 'Electrical Tests'. 13.1.1. Primary Fault Tests

Primary tests with actual fault currents can be done, however, by applying a fault through a portable circuit-breaker, in the case of cable circuits. In the case of an overhead line the simplest method is to shoot an arrow over or between the conductors, the arrow being attached to a length of very fine iron wire, the other end of which is free, for phase faults, or earthed, for ground faults. An alternative method is shown in fig. 13.3 wherein the iron wire is pulled into position with insulating cords. The side view shows how conductor burning is prevented. 441

13.1

Protective Relays

Iron wire is preferred for starting the fault arc because it breaks up into small pieces which are expelled from the arc electromagnetically and has no effect on the arc resistance; copper or fuse wire on the other hand forms a cloud of metallic vapour which creates a very low resistance arc, which is misleading for the application of impedance relays. Flashover arcs, or test . h very t hi' . have a reSIstance . 8750 h "'. arcs started WIt n Iron WIre, /1.4 0 ms per loot In still air, which has a value of about 0·75 ohm at 800 amperes. Alternatively, the arc has a drop of ~~~ volts per foot which is 450 at 800 amperes. It is most important not to underestimate the arc resistance on short lines because selectivity can be lost if, for instance, a distance relay gives third zone time for a fault just inside the far end of the protected section (Chapter 5, section 5.1.1). Few companies do such field tests, however, although the chance of trouble is much smaller with a supervised test fault than with an actual fault which is uncontrolled and going to oCCur anyway. In the U.S.A. these field tests are carried out by power companies whenever they install some new protective scheme which is of fairly recent design, or if they wish to find out something about the behaviour of relays under particular system conditions. These tests are usually carried out in conjunction with the manufacturer and complete records are taken by high-speed portable oscillographs. The power company engineers then have a much better idea of ~hat both the system and the protection will do under fault conditions. The superiority of thin iron wire for this purpose was originally discovered by the author in 1928 during a power arc investigation, in collaboration with Mr. E. E. George, on the 154 kV system of the Tennessee Electric Power Company. During the tests it was found that similar results were obtained with a wet rope with the added advantage that the arc was delayed in striking and the effect on the relay was like that of an actual fault although it was in fact initiated by closing a breaker near the source. 13.1.2. Primary Injection Tests

For checking the polarity and correctness of the primary and secondary wiring it is necessary to inject current into the primary circuit. This is done usually with a distribution type transformer of 5 to 10 kVA rating (as described in section 13.1 and shown in fig. 13.1) with its low voltage winding connected to the primary circuit and its high voltage winding connected to the local supply (120 or 240 volts) through a controlling impedance such as that of a secondary test set. The primary circuit is grounded for safety through the conductors in the desired manner to ascertain that: (a) the current-transformers in corresponding phases are correctly connected to differential relays; (b) their polarity is correct relative to each other (checked by zero spurious residual current);

442

13.2

Maintenance and Testing of Relays

(c) the phase and polarity of each current-transformer is correct relative to the polarising potential in the case of a directional relay; (d) there are no. poor electrical connections (checked by a low-reading ohmmeter); (e) there is the proper ratio between primary and secondary currents at the relay setting, i.e. to see that the burdens of the relay and current wiring are not too great for the c. t. and that it is in fact the correct C.t. 13.2. PERIODIC TESTS

These are usually secondary injection tests because it is not necessary to re-check the polarity of the current-transformers, if the wiring has not been changed since the installation tests, and the condition of the current-transformers can be checked from their secondaries or tertiaries. Whereas the secondary tests at installation were fairly comprehensive. it is only necessary in the periodic tests to check the relay at its actual setting. With modem test plugs this can be done very quickly, as will be explained in section 13.4, under 'Transfer to the Test Circuit'. The frequency of these periodic tests depends upon the application. In clean, dry surroundings once a year is sufficient, or even every three years in the case of a modem relay with a high torque/friction ratio, especially if the Clra.r a.rmour rods

F===X=====lf===:X:===tll

\\

SIDE VIEW

dependent on system KV

FI cxlbl. ground ca.bl. d'sconnectl:d for " , pha. .. to pha..e 'a.ulls

Pull on", or mor.e: difp~ndlng

c Ia.mp~-,_ ,

--·No. 30 ,tul or Iron wire, Len9th

Gla.ss cord-- -

cords

Hot hnrr

on

type 01 1a." It de .. "d ~..p.,._~"""';:"" •.::::'",j-L-JL...-

Flulbl. ca.bl.

S«pa.ra.trr ground'S

FIG.

13.2. Fault initiation on overhead line

tripping contacts are relieved by a seal-in relay. For example there should be negligible effect on the contacts of an induction disc relay with a series seal-in unit after 100 operations tripping a 40 ampere trip coil at 250 volts (fig. 2.35). Relays in dirty surroundings, or having contacts with lower pressure, or a tendency to bounce, need more frequent checking. On the other hand, auxiliary relays with high-pressure bounce-proof contacts can be hermetically sealed and replaced every 6 to 10 years, depending on their contact duty.

443

13.2

Protective Relays

In all tests the relay case should be dusted clean before removing the cover. Fig. 13.2 shows a plastic cover which can be used when testing has to be done in dirty or dusty surroundings with the cover off. It is better practice, however, to test with the relay cover on.

FIG.

13.3. Plastic cover for use while testing

13.2.1. Insulation Tests

Occasional insulation tests should be made with a 1000 volt a.c. supply, (a) between the relay case and each terminal, (b) between a.c. and d.c. circuits, (c) between terminals normally separated by open contacts in the relay.

For this test the earthing points of the secondary wiring should be removed and either a 1000 volt megger or a step-up potential transformer with a current limiting resistance should be used, the former being safer and more convenient. If a potential transformer is used as the high voltage source it should be remembered that a large switchboard may have sufficient capacitance between the wiring and ground to cause series resonance, which will increase the voltage applied to the relays to two or three times the test voltage.

444

Maintenance and Testing of Relays

13.4

Another point to remember is that, in testing an individual relay which has been stored in a colder room, time should be allowed for the relay to assume the temperature of the test room and to evaporate any moisture which may at first condense on the cold surfaces of the relay. 13.3. MORE FREQUENT TESTS

Where it is not possible to do the periodic tests at regular intervals, or where the equipment is complex or on a very important circuit, a pushbutton switch is sometimes provided which energises the relay from a station supply so as to make it go through its operation. This switch may also open or insert resistance in the trip circuit, if desired, by means of an extra contact, so that operation of the relay does not trip the breaker each time. On the other hand, if the system conditions permit, it is valuable to check the continuity of the trip circuit. This sort of test is usually done daily or weekly by a station attendant, and is purely a means of seeing that the protection device is operative. Operating the relay mechanism by hand, although recommended by some manufacturers, is not advisable because (I) a heavy hand may upset the relay adjustments, (2) the contacts might successfully be closed by hand in a case where the normal torque of the relay could fail to do so if the contacts were corroded or out of adjustment. 13.4. TRANSFER TO THE TEST CIRCUIT

Most of the relays now in existence are wired through some kind of terminal board which is provided with links for transferring the relay to a test circuit. After locating the proper terminals with the aid of a panel wiring diagram, current-transformers must be short-circuited, the d.c. trip and a.c. voltage circuits disconnected and a number of connections made between the test equipment and the relay terminal board; after completing the test

FIG.

l3Aa. Test plug

445

13.4

Protective Relays

the above procedure must be reversed. All this takes appreciable time and involves the possibility of a mistake or a poor connection, which may leave a relay inoperative. A better method of transfer to the test circuit has been available for some years which is instantaneous and relatively foolproof; it speeds up testing so that the same staff can maintain several times as many relays. In this method the test terminal block is replaced by a plug-in test block on the front of the panel so that the same man can adjust the test controls and also watch the relay. This test block may be separate from the relays (fig. I3.4b)

FlO.

BO.

13Ab. Test block

13.5. Non-drawout relay with test block

446

Maintenance and Testing of Relays

13.5

but, in some of the drawout types of relay, it is integral with the relay (fig. 13.5). The test plug (fig. 13.4a) is already connected to a portable test set so that no extra connections have to be made; when the plug is inserted, the relay is separated from the switchboard circuits and connected to the test circuit. Withdrawing the plug instantly restores the relay to service. The transfer of connections is automatic and there is no possibility of leaving the relay wrongly connected. Finally, the testing can be done without disconnecting the primary circuit and only one relay need be out of service at a time. To avoid loss of time changing the test plug connections for different relays, the terminal wiring should be standardised as far as possible so that the tester can go from relay to relay without fear of opening current-transformer circuits, etc. This, of course, applies only to a.c. protective relays. The test plug should fit both the drawout relay contacts and the separate test block. In these days of rapidly expanding power systems and shortage of maintenance staff, the test plug method is particularly valuable because many times more relays can be tested per year with the same staff and less skilled personnel are required because no circuits have to be traced behind the board and there is no risk of the circuits being left improperly connected. 13.5. TOOLS

An important item in the maintenance of relays is the provision of proper tools; without these tools it is difficult to avoid maladjustment and even damage. For instance, the tension of the contact brushes of the telephone type relays controls their pick-up and it is important to see that the tension is correct. A tool kit (fig. 13.6) is provided by many manufacturers. An English manufacturer provides one which is compact enough to go into a waistcoat

FIG.

13.6. Relay tool kit

447

13.5

Protective Relays

pocket, but contains the most important relay tools-a contact bender, a dental mirror and a combination burnisher and contact pressure gauge. If cleaning is considered desirable, a burnishing tool only should be used. On no account should any abrasive material be used as this may result in the scratching of contacts and the depositing of insulating particles on their surfaces both of which will increase arcing during operation. Fig. 13.6 shows a burnishing tool attached to the contact gauge; it consists of a strip of metal, the surface of which is roughened by etching, so resembling, in effect, a very fine file. It can be compressed between the contacts and thus ensures that the actual contacting points are cleaned. Being flexible, it is useful for cleaning contacts which are located in normally inaccessible positions. By means of the contact bender, contact gaps can be accurately adjusted to the required setting. The contact springs are bent at their clamping point so that kinking and deformation of the springs is avoided. In addition to the tools mentioned above, a few do-it-yourself tools are recommended, such as a needle, a feather and a feeler gauge, which are used as follows. 13.5.1. Bearings

An ordinary needle is the best tool for exploring the surface of a jewel bearing to detect a crack. It can also be used as a gauge to prevent overlubrication, the amount of oil applied to a meter or timing unit bearing being the size of a drop which will stay on the point of a needle. Most protective relay bearings are run dry and can be cleaned out with petroleum spirits. In the field, a jewel bearing can be cleaned by inserting and twisting a clean piece of pith or watchmaker's peg wood. The pivot can be cleaned by pushing it into a piece of pith, or a hole drilled in watchmaker's peg wood, and twisting it. It is exceptional, however, to find bearings dirty enough to need cleaning unless the atmosphere is very polluted. 13.5.2. Gap Cleaners

Gaps are either blown out with a low pressure air jet or cleaned out with a feather. A thin brass spatula has been used with a magnetic insert for attracting and removing iron filings, but this has been discontinued in most companies because of the risk of demagnetisation of high coercive force steel magnets, i.e. the method is applicable to electromagnets but not to permanent magnets. 13.5.3. Gap Gauges

The normal gap between the relay contacts is often important. Too large a gap would cause slow operation or, in the case of an attracted armature relay, it might cause insufficient contact pressure or failure to make contact; too small a gap might cause inadvertent tripping due to shock and vibration. 448

Maintenance and Testing of Relays

13.6

To check contact gaps and 'follow through' an ordinary feeler gauge is useful. For checking electromagnet gaps a steel drill is the handiest gauge, especially in the case of an annular gap, as in an induction cup relay. 13.5.4. Contacts

Since many relays operate many more times in testing than they ever do in service it is a good idea to use a neon lamp for checking contact closing values. Metal filament lamps should be avoided because their initial current can be 10 times their normal current, which may weld pure silver contacts. Contact resistance should be determined with an ammeter and voltmeter at about normal current and not with a resistance bridge at a few milliamps, because contact resistance is inversely proportional to the current magnitude. 13.6. IMPROVISATIONS

Sometimes equipment needed for a specific test is not available, but suitable substitutes can be found. The following suggestions come from the U.S.A. 13.6.1. Voltage Indication

Where a suitable voltmeter is not available and the voltage is above 90 volts, a neon lamp and a potentiometer can be used. A neon lamp fires at approximately 60 volts a.c., or 85 volts d.c. and the voltage can therefore be checked by connecting the potentiometer across the circuit and moving the slider until the lamp lights. Since the neon lamp takes practically no current, the voltage can be deduced from the proportion of the potentiometer connected across the lamp. 13.6.2. Continuity Test

A neon lamp can also be used in series with a suitable resistor (or using a lamp having an internal resistor) to check the continuity of a circuit or the closing of a contact. It can also be used for checking the condition of a capacitor. For instance, a 1 mF capacitor with a leakage resistance of approximately 300,000 ohms will cause the neon lamp to glow at about once per second when connected to a 125 volt d.c. source supply. If the capacitor is short-circuited the lamp will glow continuously and, if open, it will not glow at all. Of course, care has to be used not to use a test voltage higher than the rating of the capacitor. 13.6.3. Inductive Current Control

It has been stated previously that, in order to have a good waveform, reactance rather than resistance should be used for controlling the current because a lower ohmic value can be permitted which will allow higher test currents. If a suitable adjustable reactor is not available one can be made by

449

13.9

Protective Relays

taking two flat rolls of stranded wire of suitable size (such as 100 yds of 7/029) and varying their position relative to each other to control their mutual coupling and hence their impedance and so provide control for current magnitude. 13.7. SAFETY MEASURES

With portable equipment and temporary test connections there is a constant danger of electric shock. Most companies avoid the use of exposed connections and test clips by using insulated test plugs or switches (section 13.4).

Before testing, all capacitors should be de-energised by short-circuiting them, after opening the supply switch; otherwise a charged capacitor may give an unpleasant shock and cause a convulsive movement which may lead to damage of delicate equipment. Current transformers can develop dangerously high potentials across their secondaries, if not short-circuited. Potential transformers should be checked to see that they are de-energised from the high-voltage side and that they are not re-energised through the test circuit so that a high voltage appears on the primary side which may find its way to equipment with which the operator may come in contact. Pilot wires should also be handled carefully because high induced potentials may occur between their extremities even though the voltage between the pilot wires themselves may be negligible. 13.8. RENEWAL OF PARTS

Protective relays normally operate for years with little maintenance, which may lead people to believe that spare parts for them are not really necessary. However, on account of the vital importance of protective relays and since these parts are relatively inexpensive, it has been found desirable that a suitable stock be kept on hand; for important circuits spare relays are often stocked, especially where they are the drawout type which can be instantly replaced without disturbing other relay circuits. The parts most commonly stocked are contacts, coils, jewel bearings, bearing pivots, resistors and cover glass. If damage occurs to larger parts, such as bases or frames, the relay is generally returned to the factory for inspection and repair. 13.9. MECHANICAL TESTS

The force on the armature of a protective relay is relatively small because the power available from current transformers is small and because its efficiency is low (about 0·05 % for the most common type, i.e. the induction disc relay). Furthermore, the torc;ue is proportional to the square of the current so that it decreases very rapidly near pick-up. For accurate operation it is therefore essential to see that the relay is in good mechanical condition. The armature must move freely, which can be checked by moving it delicately. The shaft should have adequate end-play 450

Maintenance and Testing of Relays

13.10

and the gaps should be examined with a mirror and light to see that there are no foreign bodies in the gaps; for instance, a hair can double the pick-up of an induction disc relay. The moving contact should have adequate travel and wipe. The operation indicator should not drop when the relay panel is bumped. 13.9.1. Shock and Vibration

Although protective relays are normally treated as fairly delicate instruments, they are occasionally subjected to substantial shocks (such as during an earthquake or if a fairly heavy piece of equipment is accidentally bumped into the panel) and the relays should not inadvertently trip under these conditions. A few manufacturers have, in recent years, designed relays to stand considerable shock and vibration and these relays can be mounted on circuit breakers or electric locomotives. No national specifications are yet available but an English manufacturer subjects its relays to impact of 20 g. to 40 g. on the panel near the relay contacts (50 g. for tripping relays) and also applies a sin 2 wt vibration test. Such tests are considerably more severe than any service conditions. 13.9.2. Dust and Corrosion

The same manufacturer also provides relay cases which are dustproof and dust-tight (Chapter 2, section 2.6.8), ordinary relay cases are dustproof but not dust-tight. Fig. 2.37 shows a case which can be sealed to make it dusttight, the normal 'breathing' due to changes in ambient temperature being effected through a dust filter which prevents the entry of even the finest dust but offers negligible resistance to breathing. The dust filter is screwed into the back of the case and contains a replacement unit of shredded nylon to stop dust particles, while iron particles are trapped by a magnetised grating at the outside of the filter. Such relays are suitable for use in flour mills, cement mills, steel mills, etc., where an ordinary relay would require frequent maintenance. t3.10. ELECTRICAL TESTS

The instruction book on the relay should be referred to for suitable tests on each type of relay. The B.S. or A.S.A. recommended tests have already been carried out by the manufacturer, since he has to meet their specifications, but when the relay is tested on site it is only necessary to check it with the setting at which is is to operate and at values of current, voltage, etc., which represent maximum and minimum conditions. If this is done much time will be saved and there is then a definite check on the behaviour of the relay for the conditions under which it is expected to operate. 13.10.1. Time·Overcurrent Relays

The B.S. recommendations are to test inverse definite (I.O.M.T.) relays on site with secondary injection test equipment to measure the operating 451

13.10

Protective Relays

time on the middle tap setting, i.e. 100 % tap for phase relays and 50 % for earth fault relays, with currents equal to twice, five and ten times the setting current, repeating these tests with the time-multiplier set on unity and at 0·5. This can be most conveniently done using the circuit of fig. 13.7 in which a

t;j

Cu"ut

control

Rc. l a.y

i:L~1 ~~:: , .... o l

~ooootao. 23B:~_O_'1_0_(1m_P_ _...y,oo;l FUlc.s

5top

Mc.tct'"ng tra.nsforMcr

Vt.ry fast

conto.ctor

-0

-.::::r Push button

FIG.

510.' t

Synct'lronou s timer

(cycle COUr'ltI:r)

13.7. Testing circuit for overcurrent relays

sinusoidal current waveform is assured by reactance control. This circuit presents a constant low burden on the power source irrespective of the rating of the relay, because it employs an impedance matching transformer. This transformer also maintains good waveform by keeping VL below 20% of the source voltage. Fig. 13.8 shows a portable test kit embodying this circuit.

FIG.

13.S. Portable test set for overcurrent relays

The following method is used in England for determining the overshoot of a time-overcurrent relay. i. Use two identical relays connected in series to a current supply of sine wave form. ii. Arrange for contact closure on relay A to interrupt this common current by a suitable contactor. iii. Set the current to 20 times tap value and relay B to Time Multiplier Setting 1,0. Both relays set on the same current tap.

452

Maintenance and Testing of Relays

13.10

iv. Determine the highest Time Multiplier Setting permissible on relay A consistent with relay B not closing its contacts. v. Measure the time from the application of current to its removal by the contactor of Item (ii). vi. Measure the operating time of relay B with the current now maintained until B has closed its contacts. vii. The overshoot time is the difference between (v) and (vi). Where the actual operating conditions are known, the relay should be set on the actual T.M.S. and tap (plug bridge) setting to be used and the time checked with minimum and maximum fault current, preferably in that order because some relays have a heating effect due to high current which is appreciable at low currents unless the relay has been given time to cool off. The mechanical condition of the relay can be checked by closing the contacts by setting the time multiplier to zero and then quickly returning it to the maximum travel position to see if the relay will reset in the manufacturer's prescribed time, thus indicating that the bearings are free. Friction will also be indicated by the disc speed not being consistent; the bearings and pivot can be cleaned as explained under 'Bearings'. High-set instantaneous units should be checked at their setting with maximum fault currents applied suddenly, preferably in a reactive test circuit, closing the switch at least five times to check any tendency to overreach on offset current waves. 13.10.2. Directional Relays

The phase angle curve should be checked at minimum fault current and normal voltage to make sure that there is no parasitic V2 closing torque which may tend to give incorrect directional action at low current. The test should be repeated at 2 %of normal voltage with maximum short-circuit current to make sure that there is no parasitic [2 torque which will give improper directional action at low voltage. Further testing is unnecessary because, even if the directional characteristic departs from the usual straight line passing through the origin, it will do no harm provided that the two conditions above are met. Actually, considerable tolerance can be allowed, even for these two tests, because parasitic V 2 torque aids selectivity if it is in the resetting direction since it gives the relay a tendency to a mho characteristic. Furthermore, a reasonable [2 torque in the tripping direction is useful in the case of cables, where it is possible to have almost zero voltage for a fault close to the bus, and can be usefully employed where the current for a fault in the protected section close to the bus is sufficiently higher than a fault on one of the other feeders close to the bus. The recommended test circuit is shown in fig. 13.9. An auto-transformer is used for adjusting the voltage because, unlike a potentiometer type voltage divider, it does not cause a phase shift at low voltages. Q 453

13.10

Protective Relays

In order to establish the correct polarity of a ground directional relay at site the following procedure can be followed. Short-circuit the c.t's in phases band c, open the secondary potential lead in phase a and short-circuit the a-phase of the auxiliary wye-broken-delta p.t's supplying residual potential to Rheosta.t

o

Fuses

CD

liZ

o

flO

'II

o

t

CD

230 v. A.C.

Pha.sc rt-:t::l--;:;;l--r.JUi~h

shIfter

FIG.

13.9. Testing circuit for directional relays

the relay. The relay should then close its contacts if the load is outgoing, or open them if it is incoming, if it is correctly connected. The test can, if desired, be repeated for the other two phases. 13.10.3. Distance Relays

Here again it is quite unnecessary to plot the circular characteristics of the ohmic units on an impedance diagram because only four points on the curve are of interest in a mho unit (fig. 13.10a), viz. the conditions for faults at the two ends of the protected zone, the cut-off point ZL and the relay bus, with and without fault resistance Ra. In a reactance relay only two points are of interest (fig. 13.10b), the reactance pick-up, with and without fault resistance. In other words, the fault area is bounded by the line impedance and the arc resistance, forming the shaded zone in fig. 13.10a. It is only necessary to establish that the relay trips on faults within the shaded zone and does not trip for faults beyond the protected section at either end. For this purpose a test circuit should be used which contains highly reactive impedance representing Z., the system impedance behind the bus towards the source, and a faulted line impedance ZL of adjustable X and R which is connected to the relay by a switch, as shown in fig. 13.11. Fig. 13.12 shows a portable test set kit which embodies the circuit of fig. 13.11. By adjusting the line impedance ZL to the impedance of the protected section and closing the switch, the accuracy of the relay reach can be determined by varying ZL a few percent each way. Referring to fig. 13.lOb, the test impedance representing the fault is Zt = X;+(R x +R)2 This is scaled down to the value required for operating the relay by a potential auto-transformer which has nine 10 % and ten 1 % taps. For example, the

454

Maintenance and Testing of Relays

13.10

(a)

X R" 2'

13.10. (a) The four test points for mho relays (b) The two test points for reactance relays

FIG.

,, I

''''p oda.ncc of

I 'ph,. b ...ck to I gc.ncra.t ing IOU ree.

'mp
r-~~~~~~~~

I

Stop

Dist ... "c. rclClY

FIG.

No,,,,a.' IOQ.d

V.,y fa..t

conta.cto r

rcpre.scnt ing to..ult

13.11. Testing circuit for distance relays

455

Millisecond count er

Protective Relays

13.10 impedance 02 is Z, x ratio

~~"

1~

where P is the percent tap setting to give the

The test should be repeated with arc resistance and in all cases the

switch should be closed at least five times to make sure that the relay does not overreach due to transient conditions in the line or the relay circuit. In the test set illustrated in fig. 13.12, ZL is calibrated in ohms for different phase angles. In test sets with separate XL and RL it is important that Ra be non-inductive and that Rx , the a.c. resistance of X, be known accurately.

FIG.

13.12. Portable test set for distance relays

It is then possible to set ZL for a desired angle ¢ because XdRL = tan ¢ and Ra is set for the value RL -Rx (see fig. 13.10a). The test is repeated with ZL very small and the current first in the out-

going direction and then in the incoming direction, to make sure that the relay is directional. These relays with memory action will, of course, have strong directional torque even down to ZL = O. Distance relays lend themselves to preliminary tests which are an effective check on the wiring of the switchboard. For instance, the phase sequence of the potential connections can be verified by the fact that the contacts are held open on a mho type relay with a strong torque when no current is flowing through the circuit. The relative phasing of the current and voltage circuits of each relay can be checked by observing the behaviour of the contacts for different directions of power and reactive kVA. If both are outgoing, a reactance unit should open its contacts and a mho unit should close its contacts when the voltage restraint is removed. The reactance unit should also open its contacts when the reactive kVA is outgoing and the power is incoming, and close them when the reactive kVA is incoming. 456

Maintenance and Testing of Relays

13.10

13.10.4. Differential Relays

The simplest way to test differential current or balanced current relays is to connect one current circuit of the relay to an ordinary portable overcurrent test set and to supply the other current circuit with the same current but reversed and controlled in amplitude by a tapped auxiliary current transformer. A more convenient method is to pass the main current through both halves of the restraining coil and to superimpose the differential current upon one half of the restraining and the differential coil. The differential current can be provided by a tapped step-down c.t. or a c.t. and a large Variac (fig. 13.13). ,:.+----'\J

Current supply from telt,-bCI,ch ----~_

Adjusta.blc ra.tlo '-a.uxllia.ry C.T.

Diffcrcntla.1 rela.y windings

...-FIG.

13.13. Testing circuit for differential relays

The only tests important to ensuring correct operation are to see that the pick-up of the relay agrees with the manufacturer's data and that the percentage slope of the characteristic at maximum fault current is sufficient to be in excess of the current transformer errors at that current. The connections of the current transformers supplying the differential relays should first be checked by inserting an ammeter in series with the operating coils of the differential relays and seeing that the spill current is negligible when simulated load current is passing through the circuit. In the case of a balanced current relay the polarity is unimportant because the windings supplied from the two circuits compared are on separate magnets. 13.10.5. Restricted Earth Fault Relays

The relay and stabilising resistance should be replaced by an ammeter of suitable range. The polarities of the line current transformers should be tested as detailed in section 13.14. In the case of restricted earth fault protection with a current transformer in the neutral connection, the polarity of the neutral current transformer can be checked by injecting current through the primary of one line current transformer and the neutral current transformer with the main transformer primary winding shorted out as shown in fig. 13.14. (The ammeter reading 457

13.10

Protective Relays

should be zero or of the order of a few milliamps for correct polarity.) If it is not possible to short the primary winding due to cable arrangements, short circuits should, if possible, be placed on the secondary side as shown dotted in fig. 13.14. In some cases, however, even with the transformer shorted on the secondary side, the transformer reactance may be too high to enable the C.T~

power transformer

~~~--~~

rW~~----

".,"

.'

.:"

Altcrna.tivc position for short-circuit

AC. sourc.

'£III FIG

Rela.y

13.14. a.c. polarity test for restricted earth fault relay

primary injection transformer to circulate sufficient current. If this is the case, the following method can be adopted to check the polarity of the neutral current transformer. Alternatively, a low voltage battery in series with a single pole on/off switch can be connected between one primary conductor and earth, and centre zero d.c. instruments connected across the secondaries of the line and neutral current transformers as shown in fig. 13.15. By means of the switch, Power

tra.nsformcr

,-ANWI/',------

centre zero instruments

FIG.

13.15. d.c. polarity test for restricted earth fault relay

d.c. pulses are passed through the line C.t. and the neutral c.t.; the direction of the two instrument deflections should be noted. The meters should be so connected that the deflections are in opposite directions, in which case the terminals of the c.t's, to which the positive terminals of the instruments are connected, should be joined together to give the correct relative polarities. When these tests have been completed, be sure that all C.t. connections are replaced. 458

Maintenance and Testing of Relays

13.11

t3.11. MANUFACTURERS' TESTS

These are of three kinds: 1. Electrical 2. Mechanical 3. Atmospheric. Most manufacturers develop relays to predetermined specifications, based on a knowledge of the requirements of the countries in which the market is located. Prototypes are subjected to vigorous tests to be sure that specifications have been amply met. t3.t1.1 Electrical Tests

Components, sub-assemblies, relay units, complete relays and relay schemes are tested before leaving the factory. The tests on components are merely to see that their electrical values are correct. Sub-assemblies may have a simple test to avoid replacement after they are mounted in a relay. Coils are tested for correct turns by putting them in an iron core with a standard coil and comparing their induced voltage with that of the standard when the core is energised with magnetic flux. Shorted turns can be detected by tuning the coil with a series condenser and measuring the voltage across the coil with a known voltage across the coil and condenser. Another method of testing for shorted turns is induced voltage testing which also indicates defective turn and layer insulation. About 1 volt per turn at about 60 times normal frequency is induced across the coil by magnetic

[rJ j~._CO- - :i\

::-Li_----Jpv,,,.,,.,

High frequencf source

Sphere ga.p

FIG.

13.16. Induced voltage test

coupling as shown in fig. 13.16. In the case oftapped coils the voltage can be induced across the whole coil by applying a lower voltage between taps. For coils with less than 2000 turns a lower frequency can be used or a minimum of 2000 volts at 3000 cycles be maintained. Relays have more extensive testing to check their calibrations over their range of adjustment. Slow-speed relays have static tests. High-speed relays have dynamic tests. Relay schemes have dynamic tests to check both their calibration and their overall performance. Static tests confirm the accuracy of the relay calibration. Dynamic tests check this calibration during the transient changes in current and voltage that occur when a fault is suddenly applied, as happens on an actual power system. These transients may occur on the power system or in the relay or in the c.t's and p. t's supplying it. 459

13.11

Protective Relays

Experience has shown that the relays most affected are differential current relays, distance relays and instantaneous overcurrent relays. (a) Heavy Current Tests. All forms of differential relays, including pilot wire, are tested on very heavy currents to make sure (a) that they do not operate on faults external to the protected circuit, (b) that they do not fail to operate on heavy internal faults due to C.t. saturation. The equipment is large and expensive (fig. 13.2Ia, b) beca,use, in order to simulate actual fault conditions, the heavy current primary circuit must have an X/R ratio similar to that of the actual system, which may be as high as 30, i.e. a d.c. time constant of about 0·1. The primary current is limited by air-cored reactors and, although they can be built with X/ R > 30, it is difficult to maintain the ratio in the whole circuit because of the resistance of the cables and bolted connections. (b) Artificial Transmission Line. Fig. 13.17a shows the arrangement of a typical three-phase artificial transmission line and fig. 13.l7b shows the Sta.tion A

Paralltlline

r-------1 Line r---,---i r----i impe- r---+r-I r-------1 da.nce r-----+l~ r-------1 ZLT \--Circuit brea.ker-/ Protected line M--flt:=~---i Line

PoTS

Sta.tion B

Source impeda.nce Zs Line impeda.nce ZL 2 r-TlrTT,----,

} To loa.d a.nd A.Co generator

100A

fuses

m

'---y----' '-y---'

To rela.ys on test

(a)

flO. 13.17a. Artificial transmission line circuit

appearance of the control panel. The purpose of this equipment is to duphcate as closely as possible the secondary values of the electrical conditions that would occur during a fault on a power system. The transients in current and potential due to the X/ R ratio of the power system are synthesised by using secondary values in the artificial transmission line equal to the primary values multiplied by the c.t. ratio/p.t. ratio. With large, modem generators and transformers and the high voltage transmission lines, the X/R ratio of Zs can be over 30 (c/J = 80°) and, although an 80° air core reactor can be readily designed, it is difficult to provide an ohmic adjustment because either taps or a variometer control reduce the X/R ratio. The method generally used is to make the reactor ill sections for seriesparallel connection; this also matches the current (thermal) capacity to the Z. value. 460

Maintenance and Testing of Relays

13.11

Transients due to c.t's can be simulated by using c.t's of similar design or by introducing into the current circuit their equivalent impedance, as shown in fig. 13.18. Rc is the ratio of the C.t.

FIG.

13.17b. Artificial transmission line panel

The transients introduced by magnetic p.t's are closely similar, regardless of their ratio, so that the 440/110 volt p.t's of the artificial transmission line are acceptable. Capacitor p.t's, however, must be represented by their equivalent circuit, as shown in fig. 13.19.

(a)

(b) FIG.

13.18. Equivalent circuit of line

C.t.

referred to primary

This type of transmission line is used for type-testing of new distance relay schemes and for individual (production) testing of distance schemes, where special conditions prevail or the scheme is complicated. 461

13.11

Protective Relays

A= Auxllia.ry t ra.ns former

~11:11L

R':(Ci::+CS) 1.r-R'2---1 ~

RA = Ra.tlo of a.uxilla.ry tra.nsformer, A

.....

---~

FIG.

13.19. Equivalent circuit of capacitor p.t. referred to secondary

(c) Test Benches. Fig. 13.23 shows a modern production test bench for miscellaneous a.c. relays. Current is controlled by reactance in a circuit similar to fig. 13.7. Potential is controlled in angle by a phase-shifter and in magnitude by a tapped auto-transformer. Operating time is measured by a synchronous timer for slow-speed relays and a Chronotron or a decatron device for high-speed relays. The equipment for each control circuit is mounted on a sub-panel so that a test bench can be changed, for instance, from single-phase current and potential to three-phase current by exchanging the sub-panel. 13.11.2. Mechanical Tests

The increasing use of protective relays, together with the decreasing availability of maintenance engineers and the stringent specifications for sensitivity and speed, has accentuated some of their physical weaknesses, such as the risk of their wrong operation due to mechanical shock and vibration, and failure to operate due to dirt in bearings and on contacts. Fig. 13.20 shows a shock tester with the relay mounted on a panel which is sUbjected to a very heavy impact from a falling hinged weight, capable of administering blows up to 400 g. In the tests an ultra high-speed cine camera is used to slow down the motion of the relay to demonstrate exactly what happens to it during the shock. These cine films clearly show where design changes are necessary to improve the shock resistance of the relay. From such tests, data can be obtained for setting up equations of inertia, resilience and friction, similar to the R, L, C transient equations of electrical circuits, so that a proper analysis can be made and the shock resistance of the relays can be increased. From such tests it has been found that ordinary jewel bearings can be protected by spring-mounting the lower jewel and providing a shoulder on the bearing to take violent shocks. It has also been found that attracted 462

FIG.

FIG.

FIG.

13.20. Mechanical shock tester

13.2Ia. Controls of G.E.C. 44,000 amp test set

13.2Ib. Switchgear for G.E.C. 44,000 amp test set

463

13.11

Protective Relays

d~i~;;~m~~~:$O ' : sct N$'...: 10MVA a.ltuna.tor

.

'2 ~ -:::r=,!.,,!., - - __ .,

11 KV ~50 MVA .w,tchgoa., -

.

._

,

'21

L-_ _ _..J.

r

~OCB

Intulock.d Exc,t.r ~- isola.tor Accommoda.tlon for C.T'. - 'I whon ttstitl!l Mo.ln 900 A ~ tra.nsform"~-r conta.ctor g, ovoro.lI prot.

t

~l

:;;

~I

J

11'Ma.ko' switch 900A

1 ~

10.J", )

I

6,360 volts

~~

3-14> 39MVA

~:; f T J~'::::::::::::" ~

I

y

~ N

C.T's for oscillogra.phs .• tc

4 4 KA termino.ls R

B

FIG.

13.22. High-current test set connections

FIG.

13.23. Productlun relay test bench

464

Maintenance and Testing of Relays

13.11

armature relays of great sensitivity can be made shock-proof by balancing the armature. In cases where the construction makes it impossible to prevent undesirable closing of the contacts due to shock, such as in the case of an induction cup unit, the problem can be solved by connecting its contacts in series with those of another relay whose armature moves in a different plane, or another induction cup relay with the opposite rotation of the cup to close contacts. There is no problem where there is a strong restraining torque under normal .conditions. Extremely delicate relays can be mounted on shock absorbers between the relay frame and case or between the relay panel and the floor or supports. Vibration tests are carried out to determine if natural resonance of any part of the relay occurs at any multiple of system frequency (or the frequency of nearby equipment) which would result in damage, undesirable contact closing or loosening of parts. The test is usually continued for a long period to see whether any part fails from fatigue. Parts likely to fail can be spotted by stroboscopic light. No definition of mechanical stability has yet appeared but the English company which employs the apparatus shown in fig. 13.20 specifies that protective relays should not trip when an impact of 20 g. is administered near one corner of the relay (this is the most effective spot) or 30 g. for a tripping relay. The g. value is set by the angle () through which the hammer falls. The relation between () and g. was originally determined by g. testers, which are small devices that can be attached to the back of the panel, opposite the point of impact; they contain two steel balls in clips at right angles which are dislodged by a blow of the calibrated value. Such a blow causes a complex combination of translational and rotational movements in the panel and provides a severe test for the relay. However, the difficulty of exactly duplicating the panel in other factories will probably result in the ultimate adoption of a controlled sin 2 wt type of vibration for shock testing. This would have the advantages of combining the shock and vibration tests and being easily reproducible on a magnetic vibrator but it would not duplicltte the complex shock waves which occur in practice when a ladder or a wheeled device bumps the panel. 13.11.3. Atmospheric Tests

Manufacturers subject new relays and new finishes to temperature and humidity cycles in excess of expected conditions. Relays intended for the tropics are generally given better finishes and magnet impregnation than the standard, but many manufacturers 'tropicalise' all relays because the temperature/humidity cycles in storage or on a dockside are just as severe in temperate climates; for tropical use it is only necessary then to apply antibacterial varnish. B.S. 2011 describes approval tests which relays must meet to be approved in the U.K.; it specifies in detail tests for heat, cold and humidity cycles, the salt spray test and fungus tests. 465

13.11

Protective Relays

Owing to the small forces in relays near the borderline of operation, gritty dust in bearings or iron particles in magnetic gaps can adversely affect the operation of the relay. Fig. 2.37 shows a drawout relay sealed against



MtC1SUfI"g

tub.

1~i:3"

Orili« tube. (b)

FIG.

13.24. Test rig for Buchholz relays (a) Photo. (b) Diagram

466

Maintenance and Testing of Relays

13.11

dust and provided with a dust filter through which cyclical expansion and contraction of air takes place through a dust filter. Such an arrangement permits use of protective relays in the most dusty locations, such as cement mills, flour mills, steel mills and desert locations. Fig. 2.38 shows an airtight draw-out relay case which is completely sealed against all ambient conditions. These two cases are described in Chapter 2, sections 2.6.7, and 2.7. 13.11.4. Buchholz Relays

Fig. 13.24 shows the special test rig used by an English manufacturer for testing Buchholz relays. Valves are provided for controlling the oil flow so that velocities corresponding to Table 10.2 in Chapter 10 are produced and the whole equipment can be tilted to simulate the slope of the pipe to the conservator tank. The test rig (fig. 13.24) consists of two oil storage cylinders with interconnecting pipes and valves, so arranged that when put under pressure by the admission of compressed air above oil level in the left-hand cylinder, the oil flows up and round through the orifice tube (selected to suit the size of relay), giving the necessary differential to the flow meter, and thence through the relay being tested to the empty right-hand cylinder. The compressed air inlet is regulated to build up pressure gradually, and this stimulates the rate of flow through the relay. Removable sections of either 1 in., 2 in. or 3 in. diameter pipe used for mounting are interchangeable to suit the size of the relay undergoing test, and the whole rig can be tilted to any angle between 1° and 9°, which gives relay. positions corresponding to various angles in the pipe rising to the conservator. Adjustments are made until the correct trip values are obtained under steady flow conditions, as indicated on the flow meter, and these are followed up by sudden surge tests, surges being created by the quick opening, under pressure, of the left-hand air operated main valve. The action of the top alarm element in response to gas accumulation is simply tested by admitting air to the relay via the top pet cock, whilst running off oil from the relay to waste. The gas scale is calibrated by comparison with the oil forced out of the relay under pressure and rising in the calibrated measuring tube. Used compressed air is occasionally released into the exhaust chamber, and as needed the left-hand tank is recharged with the oil which has accumulated in the right-hand tank, by manipulation of the appropriate valves. The air injection test is carried out by the application of air under measured pressure to the back pet cock provided for this purpose. The relay is also given a mechanical stability test (see Table 10.3). Fig. 13.24 shows the pendulum hammer and graduated scale for administering a calibrated impact. In addition the casing is given a porosity test by being subjected to a pressure of 30 Ibs/in2 (over 2 Kgm/cm2) with cold oil for 12 hours and a short time test with hot oil (l00°C) at 1 Kgm/cm2. The strength of the casing is tested at 150 Ibs/in2 (over 10 Kgm/cm2) for one minute. 467

14 MiseellaReolUl Static Relays-Future of Electromagnetic Relays-d.c. Protection Relays-Protection Engineering as a Career The growing complexity of power systems, coupled with the increasing difficulty of obtaining suitable personnel for maintaining the equipment, poses a problem for which an ideal solution would theoretically be a completely static power system. The components of a power system which are at present non~static are generators, circuit-breakers, meters, instruments and relays. Research is already under way on the direct transformation of heat, nuclear and chemical energy into electrical energy by static means. It is possible that circuit-breakers may one day be replaced by a device whose impedance can be controlled over a wide range, similar to a magnetic amplifier, which would extinguish arcing faults and reduce the current in permanent faults to a value which would not affect the operation of the rest of the power system; isolation of the defective circuit for repair could then be done by isolating switches or static equivalents which would have no difficulty in interrupting the small inductive current that was still flowing. Static meters may soon be available which will integrate the amplitudes of impulses derived at fixed intervals from a static measuring circuit, the indication where necessary being given in printed form or on decatrons. Indicating instruments may work on a similar principle except that indication would be maintained between impulses. 14.1. STATIC RELAYS

Owing to the fact that static measurement and control devices use comparators which are similar to those of protective devices it is probable that protection, instrumentation, metering and control will tend to become combined at each power station and substation and linked to a central control as shown schematically in fig. 14.1. As mentioned in Chapters 2 and 5, considerable work has been done already on static relays. For many years semi-static relays have been available, using transductor or rectifier bridge comparators supplying an electromagnetic slave output relay for tripping the breaker. Thyratrons and controlled silicon rectifiers are available which could take the place of the output relay 468

Miscellaneous

14.1

and thus make the relay wholly static, but they have not yet done so because thyratrons are not considered sufficiently reliable and controlled rectifiers are at present too expensive. It is probable, however, that a suitable device will be available in a few years, owing to the rapid advancement of semi-conductor techniques. The trend towards static relays will be accelerated by the use of higher transmission voltages for the following reasons. At voltages of 275 kV and above, the cost of c.t's of conventional design becomes prohibitive and even

Controlled sta.tion

FIG.

Ana.logdigita.l tra.nsmitter a.nd digita.la.na.log rece.iver

14.1. Static relaying combined with instrumentation and control

linear couplers become somewhat impractical at 400 kV and above; consequently, research is being conducted into the possibility of transmitting optical, acoustic or radio signals to an amplifier at earth potential so as to eliminate the problems of insulation. Present day amplifiers are limited in linear output range to about 20 watts; if this corresponds to a maximum fault current of, say, 20 times normal rating it means that the relay current burdens should not exceed 0·05 VA at C.t. rating. This sensitivity, together with the higher source-to-line impedance ratios of lines at these high voltages, can only be achieved by static comparators with amplifying properties. Although it was hoped that the absence of moving parts in static relays would solve the problem of eliminating maintenance it is unlikely to do so for a number of years because so little is known about the statistical reliability of static components, such as thermistors, transistors and miniaturised capacitors. This is aggravated by the fact that new types constantly appear and the technical improvements which they offer may make it necessary to use them although their recent appearance precludes the possibility of long term life tests. For this reason maintenance is unlikely to disappear although it may be reduced by automatic monitoring. In due course the reliability of static components will have been proved and only then can there be any real reduction of maintenance of protective relays. An American company has developed a static flag indicator in which the relay controls a digital computer type memory core; the state of the core determines whether a neon indicator lamp lights when the circuit is energised by a push-button 'reading' switch. Printed circuits naturally follow the use of static components; they conserve space, eliminate wiring errors and assist standardisation. They are widely used in the radio industry but are new in the protection field. The relay of the future will undoubtedly consist of a number of printed circuit 469

14.2

Protective Relays

cards or modules accommodated in a rack with plug-in or permanent connections. An English firm has adapted the drawout case for this technique using modules about 4 in. x 5 in. each of which represents a complete relay function such as direction, time delay, ohmic measurement, etc. All the modules for a given relay scheme would be mounted in one (or perhaps two) drawout chassis. Initially, lack of confidence in the new technique of static relays will necessitate easily detachable modules and the bringing out of testing points for periodic manual or automatic testing, but, as the reliability of static relays becomes established, the need for maintenance will tend to disappear and it may be possible eventually to have protection built into primary equipment and checked only from a central station. 14.2. THE FUTURE OF ELECTROMAGNETIC VERSUS STATIC RELAYS

Although static relays have obvious advantages of high speed, low burden, etc., it is possible that certain simple relays, such as multi-contact attracted armature relays and inverse time-current relays, will remain electromagnetic for a very long time indeed because their static counterparts are more complicated and more expensive. Furthermore, new forms of instantaneous and time-delay sensitive polarised d.c. relays are now appearing which, with rectifiers, can be made 100 times as efficient as the present a.c. measuring units; this efficiency will enable the size of relays to be considerably reduced and their burdens to be small enough for operation from linear couplers, which may replace iron-cored C.ts on systems below 150 kV because of their adaptability to extra high voltage lines, their linear characteristics and their freedom from transient inaccuracy. In general it can be said that static circuits are cheaper and better for complex protective relays, but not as yet for simple relays and that, when the requisite new static tools appear, even the simpler electromagnetic relays may be replaced. It is possible that the Hall Effect (31) (132) phase comparator may replace existing phase comparators. The advent of a static phase comparator with a sine product output would permit polyphase measurement, which would considerably reduce the price of static relays and hasten their general acceptance. The probable initial resistance of power company engineers to the use of static relays, especially those using transistors, will undoubtedly stimulate the application of great ingenuity to the design of supersensitive electromagnetic relays with high mechanical stability and encapsulated components which will theoretically eliminate the need for transistor amplification and require no maintenance. At present, the lack of knowledge of transistor failure statistics justifies this attitude but the data now available gives cause for optimism; there appears to be a greater tendency to parametric rather than catastrophic failure. Most manufacturers use them in gating and amplifying circuits rather than for measurement. Furthermore, they are improving all the time.

470

Miscellaneous

14.4

14.3. D.C. PROTECTIVE RELAYS

Relays for protecting d.c. equipment are generally less sophisticated than a.c. relays, partly because no phase relation is involved and partly because most electrical equipment is a.c. on account of the ease with which a.c. voltage can be changed. The main uses of d.c. are traction, electrolytic reduction of metals, production of gases and chemical fertilisers and variable speed drives in steel and paper mills. It is probable that the use of d.c. will increase when electrical energy is produced by magneto-hydro-dynamics or by static means (nuclear, chemical, thermal, solar, etc.) instead of by synchronous machinery. Furthermore, d.c. transmission links are becoming more common as transmission voltages increase and the importance of these synchronous power links will stimulate investigation into d.c. protective relaying. D.C. relaying is simpler in execution but more difficult in application than a.c. relaying. Modern, powerful permanent magnets enable very sensitive d.c. relays to be made; the absence of a.c. losses enables solid cores to be used which can be fabricated in much more convenient shapes than are possible with lamination stackings. On the other hand, discrimination is more difficult with the lack of a quality corresponding to phase relationship; furthermore, d.c. transformers are much more complicated and expensive than a.c. instrument transformers. 14.4. PROTECTION ENGINEERING AS A CAREER

In conclusion, the author has no hesitation in recommending protective relaying as a career for a young' engineer. It offers all the essentials of a satisfying life, viz. security, interest and variety. Security stems from the fact that the profession is not overcrowded; in fact, manufacturers of relays have the greatest difficulty in finding new engineers. Few colleges have protective relay courses and many relay engineers are promoted to managerial posts on account of their broad knowledge of the electrical field which they inevitably gain in the course of studying the behaviour of all sorts of equipment so as to learn to protect it. Interest comes from the wide range of knowledge that the relay engineer acquires in the course of his work; to design a relay which, in 0·02 second, accelerates from standstill and travels to a contact which must absorb its momentum with zero bounce, obviously requires a knowledge of transient mechanical as well as transient electrical theory. Then, to make the relay resistant to indu;;trial corrosion and tropical bacteriae requires considerable knowledge of chemistry and metallurgy as well as industrial tooling and processes. Variety comes from the wide range of characteristics required of protective relays and the activities connected with making and selling them, such as aesthetic design, automation study, computer application to stocking, publicity, customer contacts, psychology, travel and foreign languages. 471

14.4

Protective Relays

The design and application of relays is always a challenge because it starts off with detective work, looking for clues to enable the relay to detect a fault and then designing a device which will do the job better than competitive devices, and for less cost. It is one of the few professions where the mathematical and electrical knowledge gained in college is not neglected during the first few years of employment; in fact, not only is such knowledge put to work from the start but, during his career, the relay engineer constantly adds to his mathematical, electrical and general knowledge.

472

Belereaces CHAP.

1

2 3 3 7

2,6 2 2,5

5 2,5 2, 5 2 2,4 4 9 2

REP.

1 A.I.E.E. Relay Committee. 'Bibliography of Relay Literature', re-published 1927-39, 1940-43, 1944-46, 1947-49, 1950-52, 1953-54, 1955-56, 1957-58, Transactions A.I.~.E., 60, 1941, pp. 1435-47; 63, 1944, pp. 705-709; 67, Part I, 1948, pp. 24-27; 70, Part I, 1951, pp. 247-250; 74, Part II, 1955, pp. 45-48; and 76, Part III, 1957, pp. 126-128; 78, Part III, 1959, pp.78-81. 2 ADAMSON, C. 'Electronic Protection of Power Systems', Electrical Times, London, Part I, June 20th, 1957; Part II, July 25th, 1957; Part III, October 3rd, 1957; Part IV, November 7th, 1957; Part V, February 27th, 1958; Part VI, March 6th, 1958. 3 HOLM, J. G. 'Costs Study of 69-346 kV Overhead Power Transmission Systems', Transactions A.I.E.E., 63,1944, pp. 406-422. 4 DAVIDSON, L. A and HINES, V. M. 'A Relay Designed to Meet Automatic Reclosing Requirements of Ring Bus Station', Transactions A.I.E.E. Paper, 60-1267. 5 NEHER, J. H. 'A Comprehensive Method for Determining the Performance of Distance Relays', Electrical Engineering, 56, No.7, July, 1937, pp. 833844. Also Disc., ibid., No. 12, December, 1937, p. 1515. 6 SUITS, C. G. 'Non-linear Circuits for Relay Applicatiom', Electrical Engineering, SO, December, 1931, pp. 763-765. 7 SUITS, C. G. 'Non-linear Circuits Applied to Relays', Electrical Engineering, 52, April, 1933, pp. 244-246. 8 CoRDRAY, R. E. and WARRINGTON, A. R. VAN C. 'The Mho Carrier Relaying Scheme', Transactions A.I.E.E., 63,1944, pp. 228-235. Disc., p. 434. 9 WARRINGTON, A. R. VAN C. 'A Condensation of the Theory of Relays', G. E. Review, 4, No.9, September, 1940. 10 HOARD, B. V. 'An Improved Polyphase Directional Relay', Transactions A.I.E.E., 60, May, 1941, pp. 24-28. Disc., p. 633. 11 WARRINGTON, A. R. VAN C. 'Electronic Protective Relays', C.I.G.R.E. (Paris), 69, Paper No. 325, 1954. 12 BERGSETII, F. R. 'An Electronic Distance Relay Using a Phase-Comparison Principle', Transactions A.I.E.E., 73, 1954, p. 1276. Also 'A Transistorised Distance Relays', Convention Paper, AI.E.E. Summer Convention, San Francisco, June, 1956 (limited circulation). 13 ADAMSON, C. and WEDEPOHL, L. M. 'Power System Protection with Particular Reference to the Application of Junction Transistors to Distance Relays', Proceedings I.E.E., 103, Part A, 1956, p. 379. 14 ADAMSON, C. and WEDEPOHL, L. M. 'A Dual-comparator Mho-type Distance Relay Utilising Transistors', ibid., p. 509. 15 BRATEN, J. L. and HoilL, H. 'A New High-speed Distance Relay', C.I.G.R.E. (Paris), 67, Paper No. 307, 1950. 16 EDGELEY, R. K. and HAMILTON, F. L. 'The Application of Transductors as Relays in Protective Gear', Proceedings I.E.E., 99, Part 11,1952, p. 297. 17 ScINNEMANN, W. K. and GLASSBURN, W. E. 'Principles of Induction-type Relay Design', Transactions A.I.E.E., 72, Part III, 1953, pp. 23-27. 18 SoNNEMANN, W. K. 'A New Inverse Time Overcurrent Relay with Adjustable Characteristics', Transactions A.I.E.E., 72, Part III, 1953, p. 360. 19 LEWIS, U. S. 'Design Features of a Modern Induction Pattern Overcurrent Relay', G.E.C. Journal, 18, No.2, April, 1951. 20 KELLER, A C., WAGER, H. N., PEEK, R. L. and LooAN, M. A 'Design of Relays', Bell Telephone Systems Technical Publications Monograph No. 2180, 1954.

473

Protective Relays CHAP.

REF.

2

22

2

23

2, 10

24

9

25

2A

26

2A

27

8

28

8

29

8,9, 10,11

30

2

31

2,4,6

32

4

33

4

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GROSS, E. T. B. 'Sensitive Ground Relaying of a.c. Generators', Transactions A.I.E.E., 71, 1952, paper No. 51-371. RYDER, C., RUSHTON, J. and PEARCE, F. M. 'A Moving-coil Relay Applied to Modern High-speed Protective Systems', Proceedings I.E.E., 100, Part II, 1953, p. 261. LAMN, U. 'The Transductor and its Applications', A.S.E.A. Journal, 16, No.5,1939. LEYBURN, H. and LACKEY, C. H. W. 'Protection of Electrical Power Systems; a Critical Review of Present Practice and Recent Progress', Proceedings I.E.E., 98, Part 2, Feb. 1951, p. 47-66. BIRD, D. E. and GOLD, L. B. S. 'Induction Motor Protection; a New Type of Relay', Electrical Review (London), 137, July, 1945, pp. 60-67. MACPHERSON, R. H., WARRINGTON, A. R. VAN C. and MCCoNNELL, A. J. 'Electronic Protective Relays', Transactions A.I.E.E., 67, Part III, 1948, p.1702. LoVING (JR.), J. D. 'Electronic Relay Developments', ibid., 68, Part I, 1949, p.233. ADAMSON, C. and TALKHAN, E. A. 'The Application of Transistors to Phasecomparison Carrier Protection', Proceedings I.E.E., 106, Part A, No. 25, February, 1959. FEASTER, W. C. and SCHENEMAN, E. E. 'Applications of Transistors in Power-line Carrier Relaying', Transactions A.I.E.E., 73, 1954, pp. 976-979. HODGKISS, J. W. 'The Behaviour of Current Transformers Subjected to Transient Asymmetric Currents and their Effects on Protective Relays', C.I.G.R.E. (Paris, 1960),72, Paper No. 329. SOROTKA, V. T. 'Protective Relays Based on the Hall Effect', Elektrichestro, 1958, No. 11, pp. 68-71. MCCoNNELL, A. J. 'A Single Element Polyphase Directional Relay', Transactions A.I.E.E., 56, No.1, 1937, pp. 77-80 and 113; No.8, pp. 1025-1028. SONNEMAN, W. K. 'A Study of Directional Element Connections for Phase Relays', Transactions A.I.E.E., 69, Part II, 1950. Also Disc., ibid., pp. 1450-1451. BARNES, H. C. and MCCoNNELL, A. J. 'Some Utility Ground Relay Problems', Transactions A.I.E.E., June, 1955. Wn.soN, R. M. and CANNON, C. E. 'Fundamentals of Co-ordinating Fuses and Relays', Electrical West., 87, July, 1941, pp. 30-31. STEEB, G. 'Relay Inverse Time Characteristic Doubled', Electrical World, 118, October, 1942, p. 1484. HUNT, L. F. 'Sensitive Ground Relay Protection for Complex Distribution Circuits', Transactions A.I.E.E., 65, November, 1956, pp. 765-768; disc., Supplement, 1946, pp. 1180-1181. GRAYBEAL, T. D. 'Factors which Influence the Behaviour of Directional Relays', Transactions A.I.E.E., 61, 1942, pp. 942-952. MORRIS, W. C. 'Dual-polarised Directional Ground Relays', Distribution, April, 1952, pp. 8-9. PRATT, M. G., AUDLIN, L. J. and MCCoNNELL, A. J. 'New Relay Assures Feeder Resumption After Outage', Electrical World, Part I, September 10th, 1949, pp. 99-103; Part II, September 24th, 1949, pp. 95-98. WARRINGTON, A. R. VAN C. 'Reactance Relays Negligibly Affected by Arc Impedance', Electrical World, 98, No. 12, September 19th, 1931, pp. 502-505. ZYDANOWICZ, J. 'Applications of the Idea of Steady State Impedance and Admittance to the Construction of Diagrams intended for the Analysis of the Operation of Distance and Directional Relays', C.J.G.R.E. Conference, 1960, Paper No. 323. WARRINGTON, A. R. VAN C. 'A New High Speed Reactance Relay', A.I.E.E. Paper presented Summer Convention, June 20th, 1932. Abstract, Electrical Engineering, 51, No.6, June, 1932, p. 410. Disc., Electrical Engineering, 52, No.4, April, 1933, pp. 248-252.

474

References CHAP.

5 5 5 5

5

4 5 5 6

5 5 5 5,9 5 5 8 2 5 2 5 5 5 6 6

REF.

44 GOLDBOROUGH, A. L. and SMITH, R. M. 'A New Ground Distance Relay', Electrical Engineering (Trallsactiolls A.I.E.E.), 55, No.6, June, 1936, pp. 697-703. Disc., ibid., 55, No. 11, November, 1936, pp. 1255-1256. 45 WARRINGTON, A. R. VAN C. 'Protective Relaying for Long Transmission Lines', Transac/iolls A.I.E.E., 62, June, 1943, pp. 261-268. Disc., June Supplement, 1943, p. 427. 46 SONNEMANN, W. K., et al. 'Compensator Distancc Relaying', Trallsactions A.I.E.E., June, 1958, 77, Part 3, pp. 372-388. 47 WARRINGTON, A. R. VAN C. 'Application of the Ohm and Mho Principles to Protective Relays" Transactions A.I.E.E., 65, June, 1946, pp. 378-386. Disc., June Supplement, 1946, pp. 490-491. 48 GUTMANN, H. 'Behaviour of Reactance Relays with Short-Circuits Fed from Both Ends', E.T.Z., 1940, p. 514. 49 RAMSAUR, O. 'A New Approach to Cold Load Restoration', Eler-trical World, October, 1952, pp. 101-103. 50 HAMILTON and ELLIS. 'The Performance of Distance Relays', Reyrolle Review, 1956, No. 166. 51 HUTCHINSON, R. M. 'The Mho Distance Relay', Trallsactions A.I.E.E., 65, 1945, pp. 353-360. . 52 DODDS, G. B. and MARTER, W. E. 'Rl:actance Relays Discriminate between Load-Transfer Current and Fault Currents on 2,300-volt Station Service Generator Bus', Transactiolls A.I.E.E., 71, Part III, 1952, pp. 1124--1128. Disc., p. 1128. 53 SONNEMANN, W. K. 'A New Single-Phase-to-Ground Fault Detecting Relay', Transactions A.I.E.E., 61, 1942, pp. 677-680. Disc., pp. 995996. 54 STROM, A. P. 'Long 60 Cycle Arcs in Air', Transactions A.I.E.E., 65, 1946, pp. 113-117. Disc., pp. 504--507. 55 GILKESON, C. L., JEANNE, P. A. and VAAGE, E. F. 'Power System Faults to Ground'. Part II, 'Fault Resistance', Transactions A.I.E.E., 56, 1937, pp. 428-433, 474. 56 MORRIS, W. C. 'One Slip Cycle Out-of-Step Relay Equipment', Transactions A.I.E.E., 68, Part II, 1949, pp. 1246-1248. 57 LEWIS, W. A. and TIPPETT, L. S. 'Fundamental Basis for Distance Relaying on Three-phase Systems', Transactions A.I.E.E., 66, 1947, pp. 694--709. 58 WARRINGTON, A. R. VAN C. 'Graphical Method for Estimating the Performance of Distance Relaying during Faults and Power Swings', A.l.E.E. Paper No. 49-154. 59 NOWICKI, J. R. 'Phase-operated Relay Using Transistors for Power System Protection', Mullard Tech. Commun., 4, July, 1958, pp. 7-13. 60 WIDEROE, R. 'Thyratron Tubes in Relay Practice', Transactions A.I.E.E., 53, June 16th, 1934, p. 872. 61 GOLDSBOROUGH, S. L. and LEWIS, W. A. 'A New High-speed Distance Relay', A.I.E.E. Electrical Engineering, 51, March, 1932, pp. 157-160. 62 NEUGEBAUER, H. 'The Use of Rotating Coil Relays and Rectifiers in Protection', Elektrotechnische Zeitschri/t, August; 1950. 63 BIERMANNS, O. 'Schnelldistanzrelais flir Mittelspannungnetze', A.E.G. Millelungen. 64 GOLDSBOROUGH, S. L. and HILL, A. V. 'Relays and Breakers for High Speed Single Pole Tripping and Reclosing', Transactions A.I.E.E., 61, 1942, pp. 77-81. Disc., p. 429. 65 , The Effect of Coupling Capacitor Potential Devices on Protective Relay Operation', Trallsactions A.I.E.E., 70, pp. 2089-2096. 66 WARRINGTON, A. R. VAN C. 'Control of Distance Relay Potential Connections', A.I.E.E., Electrical Engineering, 53, 1934, pp. 206-213. Disc., pp. 465-466 and 617. 67 AUDLIN, L. J. and WARRINGTON, A. R. VAN C. 'Distance Relay Protection for Subtransmission Lines made Economical', A.I.E.E. Paper No. 43-92, May, 194.1.

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Protective Relays CHAP.

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13

68

WARRINGTON, A. R. VAN C. 'Survey of Methods of Mounting Protective Relays and Arrangements for their Testing and Maintenance', C.I.G.R.E., 1955, Paper No. 334. FEIST, P. K. 'An Analysis of Compensated Polyphase Relays Using the Circle Diagram', Electrichestvo, 1957, No.9, pp. 31-35. SALZMANN, A. 'Relaying for Rural Distribution Feeders', Electrical Times, April 2nd, 1959, p. 529, and April 9th, 1959, p. 572. SALZMANN, A. 'Co-ordination of Phase Fault Protection " Electrical Energy, December, 1958, pp. 480-487. JENKINS, B. D. 'Current Transformers for Protection Circuits', Electrical Times, October 24th, 1957, and November 14th, 1957. A.I.E.E. Relay Committee. 'Pilot-wire Circuits for Protective RelayingExperience and Practice, 1942-50', Transactions A.I.E.E., 72, Part Ill, 1953, pp. 331-336. Also Disc., p. 336. TRAVER, O. C., AUCHINCLOSS, J. and BANCKER, E. H. 'Pilot Protection by Power Directional Relays Using Carrier Current', G.E. Review, 35, No. 11, November, 1932, pp. 566-570. CHAIKIN, SAUL W. 'Mechanics of Electrical Contact Failure caused by Surface Contamination', Electro-Technology, August, 1961. NEHER, J. H. 'The Use of Communication Facilities in Transmission Line Relaying', Transactions A.I.E.E., 52,1933, pp. 595-602. CASSON, W. and LAST, F. H. 'Ultra High-speed Relays in the Fields of Protection and Measurement', Proceedings I.E.E., 96, Part II, 1949, pp. 50-56. . HARDER, E. L. and BoSTWICK, M. A. 'A Single Element Differential Pilotwire Relay System', Electrical Journal, November, 1938, pp. 443-448. NEHER, J. H. and MCCoNNELL, A. J. 'An Improved A-C Pilot Wire Relay', Transactions A.I.E.E., 60, January, 1941, pp. 12-17. FENWICK, W. and MARAIS, C. P. 'The Application of Pilot-wire Protection to long 88 kV Transmission Lines, including intertripping', Transactions South African I.E.E., 36, 1945, p. 60. WARD, R. I. and GILMAN, D. W. 'Pilot-wire Relaying Utilizing the product Differential Relay', Transactions A.I.E.E., 72, Part III, October, 1953, p. 911, and Electrical Engineering, 73, February, 1954, pp. 137-142. LEYLAND, S. C. and GOLDSBOROUGH, S. L. 'The Functions of Ground Preference in Carrier Current Relay Schemes', A.I.E.E. Journal, March, 1944, p. 97. ADAMSON, C. and TALKHAN, E. A. 'Selection of Relaying Quantities for Differential Feeder Protection', Proceedings I.E.E., 107, Part A, No. 31, February, 1960, pp. 37-47. KINrrsKY, V. A. 'Relay Scheme Protects Generators', Electrical World, October 3rd, 1960, pp. 44-45. SONNEMANN, W. K. 'A High Speed Differential Relay for Generator Protection', Transactions A.I.E.E., 59, November, 1940, pp. 608-612. Disc., pp. 1250-1252. MCCONNELL, A. J. 'A Generator Differential Relay', Transactions A.I.E.E., 62, January, 1943, pp. 11-13. Disc., pp. 381-383. A.I.E.E. Relay Sub-Committee. 'Protection of Power House AuxiIiaries', Transactions A.I.E.E., 65, November, 1946, pp. 746-751. Disc., pp. 1115-1116. FOUNTAIN, L. L. 'Motoring Protection for A.C. Generators', Westinghouse Engineer, 6, November, 1946, pp. 190-191. STERNER, V. 'The Protection of Large Transformers', C.I.G.R.E., 1958, Paper No. 348, and C.I.G.R.E., 1960, Paper No. 334. BEAN, R. L. and CoLE, H. L. 'A Sudden Gas Pressure Relay for Transformer Protection', Transactions A.I.E.E., 72, Part III, 1953, pp. 480483. A.I.E.E. Relay Sub-Committee. 'Relay Protection of Power Transformers', Transactions A.I.E.E., 66, 1947, pp. 911-916.

476

References CHAP.

10 10

8,9, 10,11 7 10 10 8 10 10 9 10 10 6 13 12 12 6 2 11 11

11 11 9 8 13 5 4

REF.

92 KUNGSHIRN, E. A., MOORE, H. R. and WENTZ, E. C. 'Detection of Faults in Power Transformers', Transactions A.I.E.E., April 1957, 76, Part 3. 93 MADILL, J. T. 'Typical Transformer Faults and Gas Detector Relay Protection', Transactions A.I.E.E., 66, 1947, pp. 1052-1060. 94 SEELEY, H. T. 'Effect of Residual Magnetism on Differential Current Relays', Transactions A.I.E.E., 2, 1943, pp. 164-169. 95 NEHER, J. H. 'D.C. Pilot Loop Protects 66 kV Cable Circuits', Electrical World, 101, March 25th, 1933, pp. 384-387. 96 DIETSCH, c., HENRIET P. and LARRUE, C. 'Simplified Devices for the Protection of Extra High-voltage Transformers and Results of their Application', C.I.G.R.E., 1950, Paper No. 342. 97 JEAN-RICHARD, CHARLES. 'Thermal Protection of Transformers', C.I.G.R.E., 1950, Paper No. 309. 98 PATRICKSON, J. B. 'Solkor-R Pilot Wire Protection', Reyrolles Pamphlet No. 1296. 99 BROWN, J. E. and EVISON, J. N. 'A Transistorised V.F. lntertripping System for Power Line Protection', Electrical Journal, October, 1960. 100 BERTULA, G. 'Enhanced Transformer Protection through Inrush-proof Ratio Differential Relays', Brown Boveri Review, 32, 1945, p. 129. 101 NEWCOMBE, R. W. 'Electrical Protection of Large Generator Units', The Electrical Journal, 22 May, 1959. 102 SPIESS, H. 'Ein Neues, Einschaltsicheres Differentialstrornrelais filr Transformatoren', Bulletin Oerlikon, No. 306, 1954, p. 84. 103 GRUND MARK, B. 'The Lightning Arresters Protecting the Harspranget Plant, Sweden', A.S.E.A. Journal, 1953, Nos. 7-8. 104 BALDWIN, C. J. and GOFFARD, B. N. 'An Analysis of Polyphase Directional Relay Torques', Transactions A.I.E.E., 72, Part III, 1953, pp. 752759. 105 WARRINGTON, A. R. VAN C. 'Reduction of Maintenance Time for Protective Relays by Simplification of Design and Test Methods', C.I.G.R.E., Paris, May 12th-22nd, 1954, Report No. 330. 106 WARRINGTON, A. R. VAN C. 'Back-up Protection', C.I.G.R.E., Paris, 1960, Report No. 334, Section III. 107 KENNEDY, L. F. and MCCONNELL, A. J. 'An Appraisal of Remote and Local Back-up Relaying', Conferences Paper A.I.E.E., Paper No. 57-560. 108 LoSEV, S. B. and CHERMAN, A. B. 'Study of 3-phase Directional Impedance Relays under Short Circuit Conditions'. 109 FABRIKANT, V. L. 'Relays Based on Semiconductors', Elektrichestvo, No.7, 1958, pp. 41-45. 110 NEWCOMBE, R. W. 'The Development of Bus Bar Protection', The English Electric Journal, 14, No.6, June, 1956. III REID, I. A. 'Busbar Protection', Electrical Review, June 7th, 1957, pp. 1041-1047. li2 RUSHTON, J. 'Bus Bar Protection', Electrical Review, December 27th, 1957, pp. 1156--1159. 113 HARDER, E. L., WENTZ, E. C., SONNEMANN, W. K. and KLEMMER, E. H. 'Linear Couplers for Bus Protection', Transactions A.I.E.E., 61, May, 1942, pp. 241-248. Disc., June Supplement, 1942, p. 463. 114 MASON, C. R. 'A New Loss of Excitation Relay for Synchronous Genera. tors', Tra1lsactio1ls A.I.E.E., 68, 1949, pp. 1240-1245. 115 MCCONNELL, A. J., CRAMER, T. A. and SEELEY, H. T. 'Phase Comparison Carrier Current Protection', Transactions A.I.E.E., 64, 1945, pp. 225-233. Disc., Supplement, 1945. 116 ADAMS, A. W. 'Some Aspects of Testing Related to Meters, Relays and Instruments', The English Electric Journal, Vol. 15, No.6, June 1958. 117 GUTMANN, H. 'The SD-14 High-speed Impedance Relay with Associated Current Transformers', E.T.Z. 118 KREEKON, N. and POWCHROSKI, D. W. 'A New Static Overcurrent Relay' Allis-Chalmers Review, April, 1960.

477

Protective Relays CHAP.

REF.

119 HALMAN et al. 'A New Carrier Relaying System', Transactions A.I.E.E., 63, 1944, pp. 568-572. 120 MORRIS, W. C. and GOFF, L. E. 'A Negative Sequence Overcurrent Relay 9 for Generator Protective', Transactions A.I.E.E., 72, Part III, 1953, pp. 615-621. 121 BARKLE, J. E. and GLASSBURN, W. E. 'Protection of Generators Against 9 Unbalanced Currents', ibid., pp. 282-286. 9 122 GROSS, E. T. B. and LE VISCONTE, L. B. 'Back-up Protection for Generators', ibid., pp. 585-592. 9 123 SEELEY, H. T. 'A Compensated Automatic Synchroniser', Transactions A.I.E.E., 53, 1934, pp. 960-968. 10 124 KENNEDY, L. F. and HAYWARD, C. D. 'Harmonic Restrained Relays for Differential Protection', Transactions A.I.E.E., 57, 1938, pp. 262-271. 10 125 HAYWARD, C. D. 'Prolonged Inrush Currents with Parallel Transformers affect Differential Relaying', Transactions A.I.E.E., 60, 1941, pp. 10961101. Disc., pp. 1305-1312. 11 126 ONYEMELUKERE, C. 'Differential Protection: Notes on Theory and Practice' Electrical Times, December 24th, 1959. 11 127 SEELEY, H. T. and VON ROESCHLAUB, F. 'Instantaneous Bus Protection Using Bushing Current Transformers', Transactions A.I.E.E., 67, 1948, pp.1709-1719. 13 128 WARRINGTON, A. R. VAN C. 'Portable Equipment Speeds Relay Test', Elec. World, 107, February, 1937, p. 764. 2,5,14 129 ADAMS, A. W. and BERGSETH, F. R. 'A Simplified Unit for Distance Relaying', Transactions A.I.E.E., 72, Part III, 1953, fJp. 996-998. 8 130 NEHER, J. H. 'A New Approach to the Pilot Wire Protection of Transmission Lines, using Leased Pilot Wires having relatively Long Electrical Characteristics', A.I.E.E. Paper No. 60-155, Power Apparatus and Systems, June, 1960, pp. 245-252. 2,5,7 131 SEELEY, H. T. and KIss, M. A. 'All Electronic One Cycle Carrier Relaying System', Power Apparatus and Systems, A.I.E.E., April, 1954, pp. 161-195, together with three other papers on the same subject by Messrs. Barnes, H. C. and Kennedy, L. F., Hodges, M. E. and Macpherson, R. H., Price, W. S. and Cordray, R. E. 2,5 132 BARLOW, H. E. M. 'An Experimental Impedance Relay using the Hall Effect in a Semi-conductor', I.E.E. Paper No. 3136M, February, 1960. 6 133 SALZMANN, A. 'Cross Country Faults seen by Protective Relays in Resonant Neutral Earthed Transmission Systems', Electrical Energy, 1, No. 16, December, 1957, pp. 494-500. 5 134 GOLDSBOROUGH, S. L. 'A Distance Relay with Adjustable Phase-angle Discrimination', Transactions A.I.E.E., 63, 1944, pp. 835-838. 2, 5 135 MULLER, M., et al. 'Protection of E.H.V. Systems, Tl\king into Account Single-phase Automatic Reclosure on Very Long Lines', Brown-Boveri Review, 45, No.6, June, 1958, p. 243. 10 136 A.I.E.E. Committee. 'Report on Transformer Magnetising Inrush Currents and its Effect on Relaying and Air Switch Operation', Transactions A.I.E.E., 70, Part II, 1951, p. 1730. 10 137 ROCKEFELLER, G. P., et af. 'Magnetising Inrush Phenomena in Transformer Banks', Transactions A.I.E.E., 77, 'Power Apparatus and Systems', October, 1958, p. 884. 10 138 BLUME, L. F. and CAMILLI et al. 'Transformer Magnetising Inrush Currents and its Influence on System Operation', Transactions A.I.E.E., 63, 1944, p.366. 10 139 WELLINGS, J. G. and MATHEWS, P. 'Instantaneous Magnetic Balance Protection for Power Transformers', B.T.H. Activities, 191, 1946, p. 30. g 140 RUSHTON, J. 'The Fundamental Characteristics of Pilot-Wire Differential Protection Systems', Proceedings 1.E.E., 108, Part A, No. 41, October, 1961. 8

478

References CHAP.

8

REF.

141 142 143 144 145

2

146 147

10 9

148 149 150 151 152 153 154 155

MORETON, P. L. and NELLIST, B. D., 'Printed Disc Inverse Time Overcurrent Relay', I.E.E. Proc. May, 1965 p. 1000. WAGNER, C. F. and EVANS, R. D. Symmetrical Components, McGraw-Hill Book Co., 1933. MASON, C. R. The Art and Science of Protective Relaying, John Wiley Inc., 1956. ATABEKOV, G. I. The Relay Protection of H. V. Networks, Pergamon Press, 1960. PIPES, L. A. Applied Mathematics for Engineers and Physicists, McGraw-Hill Book Co., 1946. PECK, R. L. and WAGAR, H. N. Switching Relay Design, van Nostrand, New York. MATHEWS, P. Protective Current Transformers and Circuits, Chapman & Hall Ltd., 1955. BLUME, L. F., et al. Transformer Engineering, John Wiley Inc., 1938. CoNCORDIA, C. Synchronous Machines, John Wiley Inc. HENRlET, P. Foncionnement et Protection des Reseaux de Transport d'Electrieite, Gauthier-Villars et Cie, Paris, 1958. Federal.Telephone Radio Corp., New York, N.Y. Reference Data for Radio Engineers. The Royal Signals Manual of Military Publications. Handbook of Line Communication. Johnson and Phillips Ltd. The J. & P. Switchgear Book, 5th edition. KAUFMANN, M. The Protective Gear Handbook, Pitman & Co. Ltd. KIMBARK, E. W. 'Power System Stability' Vols. 1 and 2, John Wiley & Sons Inc. New York.

479

Pages

Intlex

Sections

318 183 114, 196, 209 102, 118, 121 210 226 151 441 198 109 29, 47 17, 266

A.C. pilot relaying A.C. tripping ... Admittance relays Amplitude comparators Angle impedance relays Application of distance Application of time current Arc initiation .. . Arc resistance .. . Attenuation constant; compensation Attracted armature relays Auto reclosing ...

8.1 4.6 3.2.6, 5.1.1(c), 5.2.2 3.1.2, 3.3.1, 3.4.1 5.2.3 5.4 4.2 13.1.1 5.1.3 3.2.3(a) 2.3.1, 2.4.4 1.9,5.9

74 15,426,431 49 107 111, 328 363 74 106, 377, 390 234 307 79 436 382 23 232,413

Back-stops Back-up relaying Balanced beam unit Balanced current relays Balanced voltage pilot scheme Bearing failure ... Bearings Biassed differential relay Blinders ... Blocking pilot ... Bounce-proof contacts Breaker back-up Buchholz relay ... Burdens on c.t's p.t's ... Bus protection ...

2.6.2 1.8, 11.5, 12.3 2.4.5 3.2.2 3.2.3(c),8.4.3 9.1.3(e) 2.6.1 3.2,9.4.2,10.4.2 5.4.7 7.3.2 2.6.4 12.4.2 10.3.1 1.13.1, 1.13.2 5.4.5,11.1

109, 327 311 304, 338 316 87 26, 103 18 108, 133, 324 21 85 248, 440 99, 117 243,285 209 86 25 27, 38 66 75 448 225, 226 81 95 265 107 106 375 437 421

Capacitance of pilot wires Carrier acceleration ... Carrier relaying Carrier signal checking Cases ... Characteristics of relays Circuit breaker control Circulating current pilot Oassification of relay schemes Coil design Commissioning Comparators ... Compensators (voltage) Conductance relays ... Connections (electrical) Construction factors ... Construction of measuring units Construction of timing units ... Contacts Contact cleaners Contact co-ordination Contact pressure augmentation Corrosion Cross-country faults ... Current balance relays Current differential relays Current transformers ... C.T. back-up ... C.Ts. without iron

3.2,8.4.1 7.3.3 7.3,8.11 7.4 2.7 2.2,3.1.3 1.10.1 1:2.3,3.7.3,8.4.1 1.12 2.6.9 5.5.8,13.1 3.1,3.3 5.5.5,6.8 5.2.2 2.6.10 2.1.2 2.3,2.4 2.5 2.6.3 13.5 5.3.4 2.6.6 2.10.2 5.8 3.2.1 3.2.1 9.4 12.4.3 11.2.3

405 D.C. biased differential 147, 204 D.C. offset current 299 D.C. pilot relaying

10.5.S(e) 4.1.6, 5.1.4 7.1

481

Index Pages 66, 142 5 74 304 168 113, 167 191 20,102 183 84 175, 387 15,91 451 31,63 72 161, 244 148

2 441 198 94 18,93 423 263,371 148, 158 448 448 382 348 249 175 311 65 403 356,408 192, 210, 227 68 31,41, 255 31,40,321 42, 96 146, 158 444 365 311,410

354 142, 156 126 421 74

414 285 378 204 214 108, 323, 336 421 243, 285 304 432 106 362 207, 263, 438

Definite time relay Definitions Design details ... Directional comparison carrier Direction control Directional relays Distance measurement Duality ... Dual polarisation Dust-proofing ...

Sections 2.5,4.1.1 1.3 2.6 7.3 4.5.1 3.2.5,4.5 5.1.1 1.11,3.1.2 4.5.4 2.6.7

Earth fault relay Economics of relaying Electrical tests ... Electronic relays Electronic time delay ... Error limits Extremely inverse current relays

4.5.2, 10.4.1 1.7,2.8 13.10 2.3.5, 2.4.11 2.5.6 4.3,5.5.5 4.1.6

Faults: Causes of Initiation of test Resistance of ... Finishes ... Flag indicators ... Frame leakage protection Fuse blowing ... Fuse co-ordination Gap cleaners ... Gap gauges Gas actuated relays Generator faults Ground distance relays Ground faults ... Ground preference Hall Effect Harmonic restraint Heating (over) ...

1.1 13.1.1 5.1.3 2.10 1.10.2,2.9 11.3 5.7.2, 9.2.3 4.1.7, 4.2.6 13.5.2 13.5.3 10.3 9.1.1 5.5.9 4.5.2 7.3.2.4 2.4.13 10.5.5(b) 9.1.1(c), 10.6

Impedance relays Inertia method of delay Induction cup .. . Induction dij;c .. . Induction torque theory Instantaneous O.C. relay Insulation tests Interlocked A.C. protection ... Intertripping ... Interturn faults Inverse time relays Inversion chart for complex quantities Ironless c.t's. . .. Jewel bearing ...

5.1.1(a), 5.2.3, 5.4.3 2.5.1(c) 2.3.3, 2.4.2, 5.6.2 2.3.2,2.4.1,8.3 2.4.3,2.11 4.1.6,4.2.4 13.2.1 9.1.3(b) 7.3.3, 10.8.1 9.1.1(b) 4.1.2,4.2.5 3.5 11.2.3 2.6.1 11.1 6.8 9.4.3 5.1.4(b) 5.3 3.2.3, 8.4, 8.8 11.2.3 5.5.5,6.8 7.3 12.4 3.2.1 9.1.3(c) 5.1.4(c), 572, 12.4.4

Kirchoff's Law K-Dar ... Lead resistance to c.t's Likelihood of transients Limitations of distance relays Limitations of pilot wire relays Linear couplers Line drop compensators Line traps Local back-up ... Longitudinal differential Loss of field Loss of potential

482

Index Pages 363 243, 262

Loss of synchronism ... Low tension current and potential

Sections 9.1.3(d) 5.5.5,5.7.1

37,60 68 246, 395 459 27,252 67 84 450 69 96 208 115, 196 369 362 70 51 54 250

Magnetic amplifier relays Magnetic damping Magnetising inrush Manufacturers' tests ... Measuring units Mechanical damping ... Mechanical stability Mechanical tests Mercury timers Metal whiskers Modified impedance Mho relay Motor faults ... Motoring Motor operated timer ... Moving coil unit Moving iron unit Mutual induction (overhead lines)

2.3.8, 2.4.9 2.5.1.4 5.5.6,10.5 13.11 2.3,5.6 2.5.1 2.6.8 13.9 2.5.2 2.10.3 5.2.1 3.2.7, 5.1.1(c) 9.2 9.1.3(b) 2.5.4 2.4.6 2.4.7 5.5.9

358, 371 Negative sequence relays 338 Neutralising transformers 128 Non-linear resonance ...

9.1.2(c), 9.2.3 8.10 3.6.1

116, 211 371 18,93 233 234 141 147,204 362 451 357

Offset mho relays Open-phase Operation indicators Out-of-step blocking Out-of-step tripping Overcurrent relays Overreach Overspeed Overtravel Overvoltage

3.2.8, 5.2.4 9.2.3 1.10.2,2.9 5.4.6 5.4.8 4.1 4.1.6, 5.1.4(b) 9.1.3(a) 13.10.1 9. 1.1(d)

Performance curves (distance relay) Petersen coil ... Phase comparison carrier Phase selector ... Pilot wire relays Pilot supervision Pneumatic damping Polarisation Polyphase directional relay Polyphase distance relay Potential drop compensators ... Potential, loss of supply Potential supply Power rectifier protection Power station auxiliaries Power swings Primary injection test ... Product restraint

5.6.7 5.8 8.11 6.6 3.2.3,7.2,8.2 8.9 2.5.1(b) 4.5.1, 4.5.2 4.5.1(d) 6.7 5.5.5 5.1.4(c), 5.7.2, 12.4.4 5.7.1 4.1.7 9.3 5.1.3 13.1.2 3.2.1,3.7.2

Radial line protection ... Ratings: distance relays overcurrent relays Reactance relays Receiver relay ... Reclosing Rectifier bridge comparator ... Rectifier protection Relay back-up ... Reliability Remote back-up Remote tripping

4.7 5.3 4.4 3.2.6, 5.1.1(b) 7.3.1(b) 1.9,5.9 2.3.7,2.4.8 4.1.7 12.4.1 2.1.3,12.2 12.3 7.3.3,10.8

261 265 338 283 108, 299, 319 337 67 168, 176 172 284 243 207, 363,438 262 148 375 198 442 106,132 184 214 166 114, 194 310 17,266 37,56 148 434 25,430 430 311,409

483

Index Pages 177 127 72 181 239,431 357 104

Residual tripping relays Resonance Resonance time delay ... Restricted earth protection Reversed third zone Rotor faults R-X diagram ...

Sections 4.5.2 3.6 2.5.5(d) 4.5.2 5.5.3,12.3 9.1.2 3.1.3

450 18 13, 141 51,53 258 42 84 226 268 265 80 426 376 343 224,283 348 154 385 332 341 337, 427 276 202

Safety measures Seal-in relays ... Selectivity Sensitive relays Sensitive tripping devices Shaded-pole principle ... Shock-proof relays Single-step distance relay Single pole reC\osing ... Simultaneous ground faults Spark-quenching circuits Split bus protection Stabilising resistance Starting network Starting units ... Stator faults Stranded Coil ... Sudden pressure relays Summation C.t. (pilot) Summation network ... Supervision circuits Switched distance relays System stability

13.7 1.10.2 1.6,4.1 2.4.6,2.4.7 5.6.5 2.4.3 2.6.8 5.4.1 5.9.4 5.8 2.6.5 11.6 9.4:1 8.11.2(h) 5.3.4,6.6 9.1.1 4.2.3 10.3.2 8.7 8. 11.2(a) 8.9,11.7 6.2 5.1.3(c)

Tap error Targets ... Thermal delay ... Thermal relays ... Three-step distance relays Time delay methods Time steps Torque equations Tools Transferred tripping Transformer differential protection ... Transients Transistor relays Transistor timer Transverse differential Tripping check ... Tropicalisation ...

4.3 1.10.2 2.5.3 2.3.4, 2.4.10, 10.6 5.4.3 2.5 5.1.2 2.2,3.3 13.5 7.3.3,10.8 10.4.2 5.1.4 2.3.6, 2.4.12 2.5.7.2 3.2.1 11.8 2.10.1

Unbalanced currents ... Under-frequency Undervoltage ... Universal torque equations

9.2.3 9.2.4 9.2.4 3.3

161 18 69 31, 62,408 227 66 197 26,117 447 311,409 390 203 34,64 73 106 428

95

371 372 372 117

7 Vector conventions 417 Voltage differential relays 364 Voltage regulator

1.3.2 11.2.2 9.1.3(g)

198 Warrington's law for power arcs 96 Whiskers (metal)

5.1.3(a) 2.10.3

104 X-R diagram

...

3.1.3

242 Y-f':,. transformation

5.5.4

269,270 Zero sequence compensation ... 182 Zero sequence power relays '"

484

5.10.1, 5.10.2 4.5.3