Unit Unit --ii: Protective Relaying Protective Relaying
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UNIT - I Protective Relaying
What is System Protection? System protection is the art and science of detecting problems with power system components and isolating these components.
• Problems on the power system include: 1. Short circuits 2. Abnormal conditions 3. Equipment failures
Why do we need Protection?
• To detect abnormalities (faults) • To eliminate such abnormality-by isolating smallest portion of the system in a shortest period of time. • To prevent injury to personnel • To prevent damage to equipment
What components (equipment) do we Protect? Power System Components 1. Generators 2. Transformers 3. Transmission Lines 4. Feeders 5. Motors 6. Capacitor Banks 7. Bus Bars
Types of Protection
Apparatus Protection
Transmission Line Protection Transformer Protection Generator Protection Motor Protection Bus bar Protection System Protection Out-of-Step Protection Under Frequency Relay Islanding System Rate of Change of Frequency Relay
Faults in Power System Definition: A fault in an electrical equipment/apparatus is defined as a defect in the electrical circuit due to which current is diverted from the intended path.
Causes of faults
•
Natural calamities
•
Equipment related
•
Malfunction of protective systems
Natural calamities
The damages caused by unpredictable happenings such as •
blowing of heavy winds,
•
tree falling across lines,
•
vehicles colliding with towers or poles,
•
birds shorting lines,
•
aircraft colliding with lines,
•
line breaks,
•
ice and snow loading,
•
earthquakes, direct lightning strokes, switching surges etc.
Equipment Related
In the case of cables, transformers, generators and other equipment , the causes of faults are: •
failure of the solid insulation due to aging, heat, moisture or over voltage,
•
mechanical damage,
•
accidental contacts with earth or earthed screens,
•
flashover due to over voltages etc.
Malfunction of Protective Systems
•
Circuit breakers may trip
due to errors in the switching operation,
testing or maintenance work, •
wrong connections,
•
defects in protective devices,
•
faulty system design etc.
Effect of Faults
Short circuits have following effects on power system: •
Damage to the equipments due to over heating and high mechanical forces set up.
•
Fire hazards.
•
Reduction in supply voltage of healthy feeders resulting loss of industrial loads.
•
Heating of rotating machines due to unbalancing of supply voltages and currents.
•
Loss of system stability
•
Interruption of supply to consumers and loss of revenue.
Types of Faults in Power System Series faults (Open conductor) Voltage, frequency increases and current decreases
Three phase fault
L-G
L-L
L-L-G
Shunt faults (short circuit) Voltage, frequency decreases and current increases
11
Faults Statistics Faults in Various Elements of power System Element
% of Total Faults
Overhead Lines
50
Underground Cables
9
Transformers
10
Generators
7
Switchgears
12
CTs. VTs, Relays Control Equipment, etc
12
Different Types of Faults on Overhead Lines Types of Faults
% of Total Faults
Severity
Line to Ground (L-G)
85
Least severe
Line to Line (L-L)
8
Double Line to Ground (L-L-G)
5
Three Phase (3-Φ)
2
Most severe
Essential Qualities of Protection
The basic requirements of a protective system are as follows: (i). Selectivity or discrimination (ii). Reliability (iii) Sensitivity (iv) Stability (v) Fast operation
Components of a Protective System
F
Protective Relays
A relay is a logical element which process the inputs (mostly voltages and currents) from the system and issues a trip decision if a fault within its jurisdiction is detected. Inputs to a relay are Current from a current transformer. Voltage from a voltage transformer.
Principle of Operation
Efficient , reliable and fast operation
Senses the fault, locates it and sends command
Current, voltage, phase angle (direction) and freqency
Evolution of Protective Relays •
Electromechanical relays -First Generation
•
Static relays –Second generation
•
Numerical relays-Third Generation
Electromechanical relays
First generation of relays.
Uses the principle of electromechanical energy conversion.
Immune to electromagnetic interference and rugged.
Solid State Relays
These relays were developed with the advent of transistors, operational amplifiers etc. Their functionality is through various operations like comparators etc. Their advantages are:
More flexible.
Self checking facility.
Less power consumption and low burden.
Improved dynamic performance characteristics.
High seismic withstand capacity.
Reduced panel space.
Numerical Relays
Operation of a numerical relay involves analog to digital conversion of voltage and currents obtained from VT and CTs. These samples are fed to the microprocessor or DSP where the protection algorithms process these signals and necessary decisions are taken. Its advantages are:
Maximum flexibility.
Provides multiple functionality.
Self checking and communication facility.
It can be made adaptive.
Comparison between the different generations of protective relays
Protective Zones
Primary and Back-Up Protection
There are three types of back-up relays
i.
Remote Back-up
ii.
Relay back-up
iii. Breaker back-up
i.
Remote Back-up
It is the cheapest and simplest form of back-up protection.
Widely used back-up protection for transmission lines.
II.
Relay Back-up
It is the costly back-up protection.
It is recommended where remote back-up is not possible.
It needs separate current and potential transformers.
III. Breaker Back-Up
This is called local break-up
It is used for bus-bar protection
Relays…
Performance of Protective Relays
i.
Correct operation- correct operation either wanted or unwanted.
ii.
Incorrect operation- misapplication, incorrect settings, personnel errors, equipment malfunction.
iii.
No conclusion
Classification of Protective Relays
I.
Based on Technology
a.
Electromechanical relays (i) electromagnetic (ii) thermal
b.
Static relays
c.
Numerical relays
Relays… II.
Based on Speed of Operation
a.
Instantaneous relays
b.
Time-delay relays
c.
High-speed relays (60 m sec.)
d.
Ultra high-speed relays (< 5 m sec.)
III. Based on Generation of development a.
First-generation relays : Electromechanical relays
b.
Second-generation relays: Static relays
c.
Third-generation relays: Numerical relays
Relays… IV. Based on Function a.
Over current relays
b.
Under voltage relays
c.
Impedance relays
d.
Under frequency relays
e.
Directional relays
V.
Based on Relay as Comparators
a.
Single-input comparator (over current relay)
b.
Dual-input comparator ( distance & differential)
c.
Multi-input comparator
Relays… VI. Based on timing characteristics a.
Instantaneous over current relays
b.
Definite time lag relays
c.
Inverse time current relays
d.
Inverse definite minimum time (IDMT) over current relays
e.
Very inverse relay
f.
Extremely inverse relay
Classification of Protective Schemes (i). Over current protection (over current relay) Example: distribution lines, large motors, equipment etc. (ii). Distance protection (Impedance, reactance, mho relay) Example: transmission and sub transmission, 33kV, 66kV, 132kV (iii) Carrier-current protection Example: EHV and UHV lines (>132 kV) (iv) Differential protection (CT) Example: generators, transformers, motors of very large size, bus zones, etc.
Current Transformers
Current transformers are used to perform two tasks:
Firstly, they step down the heavy power system currents to low values that are suitable for the operation of the relays and other measuring instruments (meters) connected to their secondary windings.
Secondly, they isolate the relays and meters circuits from the high voltages of the power system.
The standard current ratings of the secondary windings of the CTs used in practice are 5A or 1A.
The measure of a current transformer performance is its ability to accurately reproduce the primary current in secondary amperes.
•
Ideally, the current transformers should faithfully transform the current without any error. But in practice, there is always some error. The error is both in magnitude and in phase angle. These errors are known as ratio error and phase angle error.
•
The exciting current is main source of these errors of a CT.
•
The error in magnitude is due to error in CT ratio which is called ratio error and the error in phase is called phase angle error.
CT Burden
•
It is defined as the load connected across its secondary, which is usually expressed in volt amperes (VA).
•
When the relay is set to operate at current different from the rated secondary current of the CT, the effective burden of the relay can be calculated as follows:
where
Is Pe = Pr Ir
2
Pe = effective VA burden of the relay on CT Pr = VA burden of relay at given current setting Ir Is = rated secondary current of CT Ir = current setting of the relay
Ratio Error (current error)
No min alratio − Actualrati o % ratioerror = × 100 Actualrati o N − Na % ratioerror = × 100 Na
N− % ratioerror =
Ip Is
Na
× 100
Where N = Nominal (rated) ratio = rated primary current/rated secondary current Na = Actual ratio = Ip/Is Ip = Primary current Is = secondary current
The ratio error is largely dependant upon the iron-loss component of the exciting current.
Phase-Angle Error
Im β= NI s •
Phase angle error largely dependent upon the value of magnetizing component of Im of the exciting current.
Ratio correction factor
Rcf where
1 = 1+ ε
N − Na ε= Na
Voltage Transformers
Voltage transformers are used to reduce the power system voltages to standard lower values and to physically isolate the relays and other instruments (meters) from the high voltages of the power system.
The standard voltage ratings of the secondary windings of the VTs used in practice is 110 v line-to-line 110/√3 volts line-to-neutral.
The measure of a current transformer performance is its ability to accurately reproduce the primary current in secondary amperes.
Ratio Error (voltage error)
No min alratio − Actualrati o % ratioerror = × 100 Actualrati o K − Ka % ratioerror = × 100 Ka Vp K− Vs % ratioerror = × 100 Vp Vs % ratioerror =
KV s − V p Vp
× 100
Where K = Nominal voltage ratio = No. of primary turns/No. of secondary turns Na = Actual ratio = Vp/Vs Vp = Primary voltage Is = secondary voltage
Phase-Angle Error
•
The phase difference between the primary voltage and reversed secondary phasors is the phase angle error of the VT.
Electromechanical Relays I.
Electromagnetic relays
II. Thermal Relays
ii.
Induction relays
a.
Induction disc relay
Plunger type
shaded pole type
c.
Balanced beam type
Watt hour meter type
d.
Moving coil type
b.
Printed disc relay
c.
Induction cup relay
i.
Attracted armature relays
a.
Hinged armature type
b.
e.
Polarized moving-iron type
f.
Reed type
Attracted armature Relays
Introduction
Attracted armature relays are the simplest type which respond to ac as well as dc.
They operate through an armature which is attracted
to an
electromagnet or through plunger which is drawn into a solenoid.
All these relays use the same electromagnetic attraction principle for their operation.
The motion of the moving element is controlled by an opposing force generally due to gravity or a spring.
The attracted armature relays are fast relays. They have fast operation and fast reset because of small length of travel and light moving parts.
They are described as instantaneous, But their operating time does vary with current. Slow operating and resetting times can be obtained.
On the other hand, very high operating speeds are possible. One modern relay has 0.5 millisecond of operating time. The current/time characteristic is hyperbolic.
Ratio Rest to Pick-up can be as high as 90-95% for a.c. relays and 60-90% for d.c. relays, means of special design features.
These relays do not have directional feature unless they are provided with additional polarized coil.
As they are fast and operate on d.c. and a.c., they are affected by transients. The transients contain d.c. component in addition to a.c. wave. Therefore, though the steady state value may be less than relay's pick-up the relay may pick-up during transient state.
VA burden depends on construction, setting etc. For a typical relay it is of the order of 0.2 to 0.6 VA for current range 0.1 to 0.4 A.
Modern attraction armature relays are compact, robust, reliable.
Operating Principe
The electromagnetic force exerted on the moving element is proportional to square of the flux in air gap. If saturation is neglected it is proportional to square of operating current.
F = K1I 2 − K2 where F = net force K1 = a constant K2 = restraining force including friction When the relay is on the verge of the operation, F is zero hence
K1I
2
= K2 and
I =
K2 K1
Attracted armature… a.
Hinged armature type relay
The actuating quantity of the relay may be either ac or dc. In dc relay, the electromagnetic force of attraction is constant. In the case of ac relays, sinusoidal current flows through the coil and hence the force of attraction is given by
1 2 2 F = KI = K (I max sin wt) = K (I max − I max cos2wt) 2 2
2
The relay is an instantaneous relay. The operating speed is very high (5 ms).
It is faster than the induction disc cup type relays.
The restraining force is provided by a spring.
The reset to pick-up ratio is 0.5 to 0.9.
The voltage burden is low. i.e. 0.08 W.
The relay is an instantaneous. The operating speed is very high (5ms).
Attracted armature relays are compact, robust and reliable.
They are affected by transients as they are fast and operate on both dc and ac.
This type of a relay is used for the protection of small machines, equipment, etc.
It is also used for auxiliary relays, such as indicating flags, slave relays, alarm relays, annunciators, semaphores, etc.
Attracted armature… b.
Plunger type relay
Attracted armature… c.
Balanced beam relay
Attracted armature…
Neglecting spring effect, the net torque is given by
T = K1I12 − K2 I 22 where T = net torque K1, K2 = constants I1 = current in operating coil I2 = current in restraining coil at the verge of the operation, T is zero therefore,
K1I12
= K2 I 22 , hence
I1 = I2
K2 K1
Attracted armature… d.
Moving coil relay
It is also called a polarized dc moving coil relay. It responds to only dc actuating quantities.
It can be used with ac actuating quantities in conjunction with rectifiers.
It is most sensitive type of relay with sensitivity of 0.1 mW.
These relays are costlier than induction cup or moving iron type relays.
The VA burden of moving coil relays is very small. These are used as slave relays with rectifier bridge comparators.
There are two types of moving coil relays: i. rotary moving coil ii. axial moving coil
Attracted armature…
Rotary moving coil relay
The components are : a permanent magnet, a coil wound on a nonmagnetic former, an iron core, a phosphor bronze spiral spring to provide resetting torque, jeweled bearing, spindle, etc.
The moving coil assembly carries an arm which closes the contact. Damping is provided by an aluminum former.
Attracted armature…
The operating time is about 2 cycles.
The operating torque is produced owing to the interaction between the field of the permanent magnet and that of the coil.
The torque exerted by the spring is proportional to deflection.
The relay has an inverse operating time/current characteristic.
Attracted armature…
Axial moving coil relay
As this type has only one air gap, it is more sensitive than the rotary moving coil relay.
It is faster than rotary moving coil relay because of light parts.
An operating time of the order of 30 msec. can be obtained.
Sensitivities as low as 0.1 mW can be obtained.
Attracted armature…
Its coils are wound on a cylindrical former which is suspended horizontally. The coil has only axial movement.
The relay has an inverse operating time/ current characteristic.
The axially moving coil relay is a delicate relay and since the contact gap is small, it has to be handled carefully.
Attracted armature… e.
Polarized moving Iron relay
A relay whose operation depends on the direction of current or voltage.
The fig. shown is a flux shifting attracted armature type construction. Polarization increases the sensitivity of the relay.
A permanent magnet is used for polarization. Polarization increases the sensitivity of the relay.
Attracted armature…
The permanent magnet produces flux in addition to the main flux. It is a dc polarized relay, meant to be used with dc only. However, it can be used with ac with rectifiers.
It is used as a slave relay with rectifier bridge comparators. As its current carrying coil is stationary, it is more robust than the moving coil type dc polarized relay.
Its operating time is 2 msec. to 15 msec. depending upon the type of construction.
Attracted armature… f.
Reed relay
A reed relay consists of a coil and nickel-iron strips (reeds) sealed in a closed glass capsule, as shown in Fig.
The coil surrounds the reed contact. When the coil is energized, a magnetic field is produced which causes the reeds to come together and close the contact.
Reed relays are very reliable and are maintenance free.
Attracted armature…
As far as their construction is concerned, they are electromagnetic relays. But from the service point of view, they serve as static relays.
They are used for control and other purposes. Also used as a protective relays. They are completely bounce free and are more suitable for normally-closed applications. Their speed is 1 or 2 msec.
Electromagnetic Relays
Advantages
Can be used for both ac and dc.
They have fast operation and fast reset.
These are almost instantaneous. Though instantaneous, the operating time varies with current. With extra arrangements like dash pot, copper ring etc. slow operating and resetting times can be obtained.
High operating speed with operating time in few milliseconds also can be achieved.
The pick-up can be as high as 90-95% for dc operation and 60 to 90% for the ac operation.
Modern relays are compact, simple, reliable and robust.
Electromagnetic…
Disadvantages
The directional feature is absent.
Due to fast operation the working can be affected by the transients. As transients contains dc as well as pulsating component, under steady state value less than set value, the relay can operate during transients.
Electromagnetic…
Applications
The protection of various ac and dc equipments.
The over/under current and over/under voltage protection of various ac and dc equipments.
In the definite time lag over current and earth fault protection along with definite time lag over current relay.
For the differential protection.
Used as auxiliary relays in the contact systems of protective relaying schemes.
Induction Relays
Induction…
Introduction
Induction relays use electromagnetic induction principle for their operation. Their principle of operation is same as that of a single-phase induction motor. Hence they can be used for ac currents.
In both Induction disc and Induction cup relays, the moving element (disc or cup) is equivalent to the rotor of the induction motor.
Two sources of alternating magnetic flux in which the moving element may turn are required for the operation of induction-type relays.
In order to produce an operating torque, the two fluxes must have a phase difference between them.
Induction… a.
Induction Disc Relay
i.
Shaded pole type induction disc relay
Induction…
In this type of relay a metal disc is allowed to rotate between two electromagnets.
The electromagnets are energized by alternating currents. The fields produced by the two magnets are displaced in space and phase.
The torque is developed by the interaction of the flux of one of the magnets and the eddy currents induced in the disc by the other.
Referring to Fig. the shading ring is a copper band or a coil. Effect of shading ring is to produce flux in the shaded portion of the magnet (Ф1) which is displaced in phase and space from the flux in the remaining portion (Ф2).
The flux Ф1 induces e.m.f. E1 is the disc at 90° to Ф1. The e.m.f. E1 produces currents I2 lagging behind E1 by small angle.
Induction…
The interaction between I1 and Ф2 produces torque, which is proportional to Φ2 I1 cos α, where I1 cos α is component of I1 in phase with Φ2. Greater the angle θ, greater is the torque.
The torque equation of single quantity induction relay may be expressed as:
T = K1I 2 − K2 where T = Net torque I = Current in relay coil K1, K2 = constants
Induction…
where Φ1 = flux in shaded portion of magnet Φ2 = flux in un shaded portion of magnet E1 = e.m.f induced in the disc due to Φ1 I1 = current in the disc induced by E1 Torque α Φ2 I1 cos α, where α is angle between Φ2 and I1
Induction… ii.
Watt-hour meter type Induction disc relay
Inverse characteristics
Induction…
The construction of this relay is similar to the watt-hour meter commonly used everywhere.
It
consists
of
an
E-shaped
electromagnet
and
a
U-shaped
electromagnet with a disc free to rotate in between.
The E-shaped magnet produces flux Ф1 and the U-shaped magnet produces flux say Φ2.
The phase angle θ between the fluxes is
adjusted by a reactance in parallel with the secondary winding.
Torque is produced by interaction between flux and the eddy currents in the disc (produced by flux Ф1 and Φ2).
The relay coil is tapped at several points. The current setting is selected by inserting a knob to take desired number of turns of the coil in the circuit.
Induction…
Highlights
The operation of induction relay can be controlled by opening secondary coil, as opening of this coil makes relay inoperative.
The time/current characteristics of induction disc relay is inverse characteristic . The time reduces as current increases.
The VA burden depends on rating. It is of the order of 2.5 VA.
Modern induction disc relays are robust and reliable.
These are used for over current protection.
In these relays, there is a facility for selecting the plug setting and time setting such that the same relay can be used for a wide range of current, time and characteristics.
Induction…
Plug Setting bride is provided with induction disc relays and it provides a wide range of current settings.
The plug setting refers to the magnitude of current at which the relay starts to operate. The plug setting bridge comprises connections tapped from relay coil. By inserting the plug, in a particular gap in the bridge, a certain number of turns of the relay coil are brought into circuit.
Time multiplier setting is generally in the form of an adjustable backstop which decides the arc-length through which the disc travels, by reducing the length of travel, the time is reduced.
Induction…
The effect of d.c. offset may be neglected with inverse time single quantity induction relay, because they are generally slow. The d.c. offset may effect fast relays.
Ratio of reset of pick-up is high because operation does not involve any change in air gap. The ratio is above 95%.
Inverse time characteristic is obtained by disc relays, is 10 to 60 sec.
Induction…
Torque Equation
Induction… b.
Induction Cup Relay
Induction…
This relay has two, four or more electromagnets, in stator. These are energized by the relay coils. A stationary iron core is placed as shown in Fig.
The rotor consists of a hollow metallic cylindrical cup. The rotor is free to rotate in the gap between the stationary iron and the electromagnets.
In this type of relay, the eddy currents are produced in the metallic cup. These currents interact with the flux produced by the other electromagnet and torque is produced. The theory is similar to that of the disc type induction relay.
In Fig. structure employing four poles is shown. It has an iron core at the centre and a metal cup between the core and electromagnet.
Induction…
Fig. shows a two pole structure. The two fluxes Ф1 and Ф2 are at right angles and produce eddy current in the cup. Thereby torque is produced.
Induction…
Highlights
Modern induction cup relays have 4 or more poles. A control spring and moving contacts are carried on an arm attached to the spindle of the cup.
The relay can be responsive to voltage or current, Similar structures are used in either cases.
The double actuating quantity relay can be responsive to both voltage and current.
The operating time characteristic depends on the type of structure. The relays have inverse time characteristic.
A modern induction cup relay may have an operating time of the order of 0.010 second.
Over Current Protection
Introduction
A protective relay which operates when the load current exceeds a preset value, is called an over current relay.
The value of the preset current above which the relay operates is known as its pick-up value.
Over current relays offer the cheapest and simplest form of protection.
An over current protection scheme may include one or more over current relays.
At present, electromechanical relays are widely used for over current protection. The induction disc type construction is commonly used .
Over Current…
With the development of numerical relays based on microprocessors or micro controllers, there is a growing trend to use numerical over current relays for over current protection.
These relays are used for the protection of distribution lines, large motors, power equipment, industrial systems, etc.
Over current relays are also used on some sub transmission lines which can not justify more expensive protection such as distance or pilot relays.
Over Current…
Several protective devices are used for over current protection. These include - Fuses - Miniature circuit-breakers, moulded-case circuit-breakers. - Circuit-breakers fitted with overloaded coils or tripped by over-current relays. - Series connected trip coils operating switching devices. - Over-current relays in conjunction with current transformers.
Over Current…
Applications
1.
Motor protection
2.
Transformer protection
3.
Line protection (feeders)
a.
Instantaneous over-current relays.
b.
Inverse time over-current relays.
c.
Directional over-current relays.
Lines can be protected by impedance, or carrier current protection also.
4.
Protection of utility equipments
Over Current…
Relays used in over current protection The choice of relay for over-current protection depends upon the time/current characteristic and other features desired. The following relays are used.
1. For instantaneous over-current protection. Attracted armature type, moving iron type, permanent magnet moving coil type, static. 2. For inverse time characteristic. Electromagnetic induction type, permanent magnet moving coil type, static.
Over Current… 3. Directional over-current protection. Double actuating quantity induction relay with directional feature. 4. Static over-current relays. 5. HRC fuses, drop out fuses, etc. are used in low voltage medium voltage and high voltage distribution systems, generally up to 11 kV. 6. Thermal relays are used widely for over-current protection.
Over Current…
Time Current Characteristics of Over Current Protection Depending upon the time of operation the relays are categorized as:
1.
Definite –time over-current relay
2.
Instantaneous over-current relay.
3.
Inverse -time over-current relay.
4.
Inverse definite minimum time (IDMT) over-current relay.
5.
Very Inverse -time over current relay.
6.
Extremely Inverse -time over current relay
Figure. Characteristics of various over-current relays: (a) definite time , (b) IDMT, (c) very inverse, and (d) extremely inverse
1.
Definite –time over current relay
A definite-time over current relay operates after a predetermined time when the current exceeds its pick-up value.
Pick-up (level): the threshold value of the actuating quantity (current, voltage, etc.) above which the relay operates.
The operating time is constant, irrespective of the magnitude of the current above the pick-up value.
The desired definite operating time can be set with the help of an intentional time-delay mechanism provided in the relaying unit.
2.
Instantaneous over current relay
An instantaneous relay operates in a definite time when the current exceeds its pick-up value.
The operating time is constant, irrespective of the magnitude of the current. There is no intentional time delay.
This characteristic can be achieved with the help of hinged armature relays.
It operates in 0.1 s or less. Some times the term like “high set” or “high speed” is used for very fast relays having operating times less than 0.1s.
3.
Inverse – time over current relay
An inverse-time over current relay operates when the current exceeds its pick-up value.
The operating time depends on the magnitude of the operating current. The operating time decreases when the current increases.
4.
Inverse Definite Minimum Time (IDMT) over current relay
This type of a relay gives an inverse-time current characteristics at lower values of the fault current and definite-time characteristics at higher values of the fault current .
Generally, an inverse-time characteristics is obtained if the value of the plug setting multiplier is below 10.
For values of plug setting multiplier between 10 and 20, the characteristic tends to become a straight line, i.e. towards the definite time characteristic.
IDMT relays are widely used for the protection of distribution lines. Such relays have a provision for current and time settings.
5.
Very Inverse-time over current relay
A
very
inverse-time
over
current
relay
gives
more
inverse
characteristics than that of a plain inverse relay or the IDMT relay.
Its time-current characteristic lies between an IDMT characteristic and extremely inverse characteristic.
The very inverse characteristic gives better selectivity than the IDMT characteristic. Hence, it can be used where an IDMT relay fails to achieve good selectivity.
These relays are recommended for the
cases where there is a
substantial reduction of fault current as the distance from the power source increases. They are practically effective with ground faults because of their steep characteristic.
6.
Extremely Very Inverse-time over current relay
An extremely inverse time over current relay gives a time-current characteristic more inverse than that of the very inverse and IDMT relays.
When IDMT and very inverse relays fail in selectivity, extremely inverse relays are employed. IDMT relays are not suitable to be graded with fuses.
The electromechanical relay which gives the steepest time-current characteristic is an extremely inverse relay.
This type of relay is very suitable for the protection of machines against overheating. (I2 t = K)
Extremely …
This type of relays are used for the protection of alternators, power transformers, earthing transformers, expensive cables, railway trolley wires, etc.
The rotors of large alternators may be overheated if an unbalanced load or fault remains for a longer period on the system. In such case, an extremely inverse relay, in conjunction with a negative sequence network is used.
This relay is quite suitable for differentiate between sustained short circuit current and momentary over loads.
This relay is also used for reclosing distribution circuits after a long outage.
Extremely …
This relay is able to distinguish a fault current and inrush current due its steep time-current characteristic. Therefore an extremely inverse relay is quite suitable for the load restoration purpose.
Method of defining shape of time current characteristics
The general expression for time-current characteristics is given by
t=
K I n −1
The approximate expression is
t=
K In
For definite-time characteristic, the value of n is equal to 0.
Method of defining...
According to the British standard, the following are the important characteristics of over current relays:
i.
ii.
iii.
0.14
IDMT :
t=
Very inverse :
13.5 t= I −1
Extremely inverse :
t=
I 0.02 − 1
80 I 2 −1
Current Setting
The current above which an over current relay should operate can be set.
The operation of the relay requires a certain flux and ampere turns. The current settings of the relay are chosen by altering the number of turns of the current coil by means of a plug.
The plug-setting (current-setting) can either be given directly in amperes or indirectly as percentages of the rated current.
An over current relay which is used for phase-phase fault protection, can be set at 50% to 200% of the rated current in steps of 25%.
Current Setting...
The usual current rating of this relay is 5 A. So it can be set at 2.5 A, 3.75 A, 5 A,..., 10 A. When a relay is set at 2.5 A, it will operate when current exceeds 2.5 A. When relay is set at 10 A, it will operate when current exceeds 10 A.
The relay which is used for protection against ground faults (earth-fault relay) has settings 20% to 80% of the rated current in steps of 10%.
The current rating of an earth-fault relay is 1 A.
The actual r.m.s. Current flowing in the relay expressed as a multiple of the setting current multiplier (PSM).
(pick-up current) is known as the plug setting
PSM can be expressed as PSM = Secondary current / Relay current setting = Primary current during fault (Fault current) / Relay setting x CT ratio
Time Setting
The
operating time of the relay can be set at a desired value. In
Induction disc type relay, the angular distance by which the moving part of the relay travels for closing the contacts can be adjusted to get different operating time.
There are 10 steps in which time can be set. The term time multiplier setting (TMS) is used for these steps of time settings.
The values of TMS are 0.1, 0.2,...0.9,1.
Directional Power Relay
The directional relay means the relay operates for the specific direction of the actuating quantity in the circuit.
The directional power relay operates when power in the circuit flows in the specific direction.
The construction and principle of operation of this relay is similar to the induction type watt hour meter relay. The difference is that in watt hour meter type relay the torque produced is due to the interaction of the fluxes produced by only the current derived from the secondary of CT, while in directional power relay the torque is produced due to interaction of fluxes produced from both voltage and current of the circuit.
Directional…
Directional…
The relay has two windings, one act as voltage coil and while other as current coil , similar to a watt meter.
The upper magnet carries a voltage coil or potential coil which is energized from PT, while the lower magnet carries a current coil which is energized from CT in the line to be protected.
The number of tapping's are provided to the current coil with which desired current
setting can be achieved. The restraining torque is
provided by the spiral spring.
The spindle of disc carries the moving contacts which make contact with tripping circuit terminals when the disc rotates.
Directional…
The voltage coil provided on the upper magnet produces the flux Ф1. this lags the voltage by 90o.
The current I is sensed by the current coil on lower magnet which produces the flux Ф2. This is in phase with current I. the current I lags voltage V by an angle Ф.
Directional…
The interaction of fluxes Φ1 and Φ2 produces the torque. Hence we can write.
T αφ 1φ 2 sin α
φ 1α V
but while therefore
φ 2α I
and
α = 90 − φ T α VI sin( 90 − φ )
T α VI cos φ
α
power in circuit
Directional…
Under normal working conditions, the driving torque acts in the same direction as that of restraining torque. This moves the moving contacts away from the fixed tripping circuit contacts. Thus, relay remains inoperative as long as power flow is in one particular direction.
But, when there is a current reversal and hence the power reversal then the driving torque acts in opposite direction to the
restraining
torque in such a manner that the moving contacts close the tripping circuit contacts. This opens the circuit breaker to isolate the faulty part.
This relay is used for providing the reverse power protection to synchronous machines. This relay can be single phase or three phase.
Directional Induction Type Over current Relay
The directional power relay is not suitable to use as a protective relay under short circuit conditions. This is because under short circuit conditions the voltage falls drastically and such a reduced voltage may not be sufficient to produce the driving torque required for the relay operation.
Hence in practice, directional induction type over current relay is used. This relay operates almost independent of system voltage and power factor.
The directional
induction type over current relay uses two relay
elements mounted on a common case these elements are: 1. Directional element which is directional power relay 2. Non directional element which is non directional over current relay
Directional…
Directional…
Directional Element
The directional element is nothing but a directional power relay which operates when power in the circuit flows in a particular direction.
The voltage coil of this element is energized by
a system voltage
through a potential transformer. The current coil on the lower magnet is energized by the system current through a current transformer.
The trip contacts of this relay
(1-1’) are connected in series with
secondary winding of non-directional element.
Directional…
Non-Directional Element
The plug setting bridge provided in this element
to adjust current
setting as per the requirement.
The trip contacts of this relay
(1-1’) are connected in series with
secondary winding of non-directional element. So unless and until trip contacts (1-1’)
are closed by the movement of disc of directional
element, the non-directional element can not operate. Thus, the movement of the directional element.
non-directional element is controlled by the
Directional…
Operation
When the fault takes place the current or power in the circuit has a tendency to flow in reverse direction. The current flows through the current coil of the directional element which produces the flux. The voltage coil produces another flux.
The two fluxes interact to produce the torque due to which the disc rotates. As disc rotates
the trip contacts (1-1’) get closed. So
directional element must operate first non-directional element.
to have the operation of the
Directional…
The following conditions must be satisfied to have the operation of the entire relay. i. the direction of current in the circuit
must
reverse
to operate
directional element. ii. the current value in the reverse direction must be greater than the current setting. iii. The high value of current must persist for a time period which is greater than the time setting of the relay.
Directional…
Directional Characteristics V = relay voltage through P.T I = relay coil current through C.T θ = angle between V and I Φv = flux produced by voltage V ΦI = flux produced by current I Tα therefore
Φv ΦI sin (θ+ Φ)
T = K V I sin (θ+ Φ), where K is a constant
Directional… Maximum torque occurs when sin (θ+ Φ) is 1 i.e. θ+ Φ = 90 The condition for the maximum torque is shown dotted in the figure the torque is zero when sin (θ+ Φ) is zero i.e. θ+ Φ = 0 or 180
Directional…
Protection of Parallel Feeders and Ring mains
At the sending end of the feeders (at A and B), non-directional relays are required. At the end of other feeders (at C and D) directional over current relays are required.
Here the directional relays can operate for fault current flowing in a particular direction shown by arrow
In the diagram, the double headed arrow
indicates non-directional
relay.
If a fault occurs at middle of the line BD the directional relay at D trips, as the direction of the current is reversed . The relay at C does not trip, as the current flows in the normal direction. The relay at B trips for a fault. Thus the faulty feeder is isolated and the supply of the healthy feeder is maintained.
Earth Fault Protection
Earth fault relay and Over current relay
A fault which involves ground is called an earth fault (L-G, 2L-G).
Faults which do not involve ground are called phase faults.
Relays which are used for the protection of a section ( or an element) of the power system against earth faults are called earth faults relays.
Similarly, relays used for the protection of a section of the power system against phase faults are called phase fault relays or over current relays.
The operating principle and constructional features of earth fault relays and phase fault relays are the same. They differ only in the current levels of their operation.
Earth Fault…
The plug setting of earth fault relays varies from 20% to 80% of the CT secondary rating in steps of 10%.
Earth fault relays are more sensitive than the relays used for phase faults. The plug setting for phase fault relays varies from 50% to 200% of the CT secondary rating in steps of 25%.
Earth Fault…
Earth Fault Protective Schemes
An earth fault relay may be energized by a residual current.
Figure (a) shows the connections of earth fault relay.
ia, ib, ic are currents in the secondary of CTs of different phases.
The sum (ia+ib+ic) is called residual current. In the absence of earth fault, the residual current is zero.
When an earth fault occurs, the residual current is non-zero. When it exceeds pick-up value, the earth fault relay operates.
The manufacturer provides a range of plug settings of earth fault relay from 20% to 80% of secondary rating in steps of 10%.
Figure (b) and (c) shows earth fault relays used for the protection of transformer and an alternator, respectively.
Earth Fault…
Earth Fault…
Phase Fault Protective Schemes
This scheme is mainly for the protection of the system against phase faults.
Figure shows the connections of phase fault relay. Three over current relays.
Earth Fault…
Combined Earth Fault and Phase Fault Protective Schemes
Figure shows the two over current relays (phase to phase fault relays) and one earth fault relay.
When an earth fault occurs, the burden on the active CT is that of an over current relay (phase fault relay) and the earth fault relay in series. Thus, the CT burden becomes high and may cause saturation.
Earth Fault…
Directional Earth Fault Relay
For the protection against ground faults, only one directional over current relay is required.
Its operating principle and construction is similar to the directional over current relays.
It contains two elements, a directional element and an IDMT element.
The directional element has two coils. One coil is energized by current and the other by voltage.
The current coil of the directional element is energized by residual current and the potential coil by residual voltage, as shown in figure (a).
This connection is suitable for a place where the neutral point is not available. If the neutral of an alternator or transformer is grounded, connections are made as shown in figure (b).
A special five limbs VT which can energize both the earth fault relay as well as the phase fault relays as shown in figure (c).
What is System Protection? System protection is the art and science of detecting problems with power system components and isolating these components.
• Problems on the power system include: 1. Short circuits 2. Abnormal conditions 3. Equipment failures
Why do we need Protection?
• To detect abnormalities (faults) • To eliminate such abnormality-by isolating smallest portion of the system in a shortest period of time. • To prevent injury to personnel • To prevent damage to equipment
What components (equipment) do we Protect? Power System Components 1. Generators 2. Transformers 3. Transmission Lines 4. Feeders 5. Motors 6. Capacitor Banks 7. Bus Bars
Types of Protection
Apparatus Protection
Transmission Line Protection Transformer Protection Generator Protection Motor Protection Bus bar Protection System Protection Out-of-Step Protection Under Frequency Relay Islanding System Rate of Change of Frequency Relay
Faults in Power System Definition: A fault in an electrical equipment/apparatus is defined as a defect in the electrical circuit due to which current is diverted from the intended path.
Causes of faults
•
Natural calamities
•
Equipment related
•
Malfunction of protective systems
Natural calamities
The damages caused by unpredictable happenings such as •
blowing of heavy winds,
•
tree falling across lines,
•
vehicles colliding with towers or poles,
•
birds shorting lines,
•
aircraft colliding with lines,
•
line breaks,
•
ice and snow loading,
•
earthquakes, direct lightning strokes, switching surges etc.
Equipment Related
In the case of cables, transformers, generators and other equipment , the causes of faults are: •
failure of the solid insulation due to aging, heat, moisture or over voltage,
•
mechanical damage,
•
accidental contacts with earth or earthed screens,
•
flashover due to over voltages etc.
Malfunction of Protective Systems
•
Circuit breakers may trip
due to errors in the switching operation,
testing or maintenance work, •
wrong connections,
•
defects in protective devices,
•
faulty system design etc.
Effect of Faults
Short circuits have following effects on power system: •
Damage to the equipments due to over heating and high mechanical forces set up.
•
Fire hazards.
•
Reduction in supply voltage of healthy feeders resulting loss of industrial loads.
•
Heating of rotating machines due to unbalancing of supply voltages and currents.
•
Loss of system stability
•
Interruption of supply to consumers and loss of revenue.
Types of Faults in Power System Series faults (Open conductor) Voltage, frequency increases and current decreases
Three phase fault
L-G
L-L
L-L-G
Shunt faults (short circuit) Voltage, frequency decreases and current increases
11
Faults Statistics Faults in Various Elements of power System Element
% of Total Faults
Overhead Lines
50
Underground Cables
9
Transformers
10
Generators
7
Switchgears
12
CTs. VTs, Relays Control Equipment, etc
12
Different Types of Faults on Overhead Lines Types of Faults
% of Total Faults
Severity
Line to Ground (L-G)
85
Least severe
Line to Line (L-L)
8
Double Line to Ground (L-L-G)
5
Three Phase (3-Φ)
2
Most severe
Essential Qualities of Protection
The basic requirements of a protective system are as follows: (i). Selectivity or discrimination (ii). Reliability (iii) Sensitivity (iv) Stability (v) Fast operation
Components of a Protective System
F
Protective Relays
A relay is a logical element which process the inputs (mostly voltages and currents) from the system and issues a trip decision if a fault within its jurisdiction is detected. Inputs to a relay are Current from a current transformer. Voltage from a voltage transformer.
Principle of Operation
Efficient , reliable and fast operation
Senses the fault, locates it and sends command
Current, voltage, phase angle (direction) and freqency
Evolution of Protective Relays •
Electromechanical relays -First Generation
•
Static relays –Second generation
•
Numerical relays-Third Generation
Electromechanical relays
First generation of relays.
Uses the principle of electromechanical energy conversion.
Immune to electromagnetic interference and rugged.
Solid State Relays
These relays were developed with the advent of transistors, operational amplifiers etc. Their functionality is through various operations like comparators etc. Their advantages are:
More flexible.
Self checking facility.
Less power consumption and low burden.
Improved dynamic performance characteristics.
High seismic withstand capacity.
Reduced panel space.
Numerical Relays
Operation of a numerical relay involves analog to digital conversion of voltage and currents obtained from VT and CTs. These samples are fed to the microprocessor or DSP where the protection algorithms process these signals and necessary decisions are taken. Its advantages are:
Maximum flexibility.
Provides multiple functionality.
Self checking and communication facility.
It can be made adaptive.
Comparison between the different generations of protective relays
Protective Zones
Primary and Back-Up Protection
There are three types of back-up relays
i.
Remote Back-up
ii.
Relay back-up
iii. Breaker back-up
i.
Remote Back-up
It is the cheapest and simplest form of back-up protection.
Widely used back-up protection for transmission lines.
II.
Relay Back-up
It is the costly back-up protection.
It is recommended where remote back-up is not possible.
It needs separate current and potential transformers.
III. Breaker Back-Up
This is called local break-up
It is used for bus-bar protection
Relays…
Performance of Protective Relays
i.
Correct operation- correct operation either wanted or unwanted.
ii.
Incorrect operation- misapplication, incorrect settings, personnel errors, equipment malfunction.
iii.
No conclusion
Classification of Protective Relays
I.
Based on Technology
a.
Electromechanical relays (i) electromagnetic (ii) thermal
b.
Static relays
c.
Numerical relays
Relays… II.
Based on Speed of Operation
a.
Instantaneous relays
b.
Time-delay relays
c.
High-speed relays (60 m sec.)
d.
Ultra high-speed relays (< 5 m sec.)
III. Based on Generation of development a.
First-generation relays : Electromechanical relays
b.
Second-generation relays: Static relays
c.
Third-generation relays: Numerical relays
Relays… IV. Based on Function a.
Over current relays
b.
Under voltage relays
c.
Impedance relays
d.
Under frequency relays
e.
Directional relays
V.
Based on Relay as Comparators
a.
Single-input comparator (over current relay)
b.
Dual-input comparator ( distance & differential)
c.
Multi-input comparator
Relays… VI. Based on timing characteristics a.
Instantaneous over current relays
b.
Definite time lag relays
c.
Inverse time current relays
d.
Inverse definite minimum time (IDMT) over current relays
e.
Very inverse relay
f.
Extremely inverse relay
Classification of Protective Schemes (i). Over current protection (over current relay) Example: distribution lines, large motors, equipment etc. (ii). Distance protection (Impedance, reactance, mho relay) Example: transmission and sub transmission, 33kV, 66kV, 132kV (iii) Carrier-current protection Example: EHV and UHV lines (>132 kV) (iv) Differential protection (CT) Example: generators, transformers, motors of very large size, bus zones, etc.
Current Transformers
Current transformers are used to perform two tasks:
Firstly, they step down the heavy power system currents to low values that are suitable for the operation of the relays and other measuring instruments (meters) connected to their secondary windings.
Secondly, they isolate the relays and meters circuits from the high voltages of the power system.
The standard current ratings of the secondary windings of the CTs used in practice are 5A or 1A.
The measure of a current transformer performance is its ability to accurately reproduce the primary current in secondary amperes.
•
Ideally, the current transformers should faithfully transform the current without any error. But in practice, there is always some error. The error is both in magnitude and in phase angle. These errors are known as ratio error and phase angle error.
•
The exciting current is main source of these errors of a CT.
•
The error in magnitude is due to error in CT ratio which is called ratio error and the error in phase is called phase angle error.
CT Burden
•
It is defined as the load connected across its secondary, which is usually expressed in volt amperes (VA).
•
When the relay is set to operate at current different from the rated secondary current of the CT, the effective burden of the relay can be calculated as follows:
where
Is Pe = Pr Ir
2
Pe = effective VA burden of the relay on CT Pr = VA burden of relay at given current setting Ir Is = rated secondary current of CT Ir = current setting of the relay
Ratio Error (current error)
No min alratio − Actualrati o % ratioerror = × 100 Actualrati o N − Na % ratioerror = × 100 Na
N− % ratioerror =
Ip Is
Na
× 100
Where N = Nominal (rated) ratio = rated primary current/rated secondary current Na = Actual ratio = Ip/Is Ip = Primary current Is = secondary current
The ratio error is largely dependant upon the iron-loss component of the exciting current.
Phase-Angle Error
Im β= NI s •
Phase angle error largely dependent upon the value of magnetizing component of Im of the exciting current.
Ratio correction factor
Rcf where
1 = 1+ ε
N − Na ε= Na
Voltage Transformers
Voltage transformers are used to reduce the power system voltages to standard lower values and to physically isolate the relays and other instruments (meters) from the high voltages of the power system.
The standard voltage ratings of the secondary windings of the VTs used in practice is 110 v line-to-line 110/√3 volts line-to-neutral.
The measure of a current transformer performance is its ability to accurately reproduce the primary current in secondary amperes.
Ratio Error (voltage error)
No min alratio − Actualrati o % ratioerror = × 100 Actualrati o K − Ka % ratioerror = × 100 Ka Vp K− Vs % ratioerror = × 100 Vp Vs % ratioerror =
KV s − V p Vp
× 100
Where K = Nominal voltage ratio = No. of primary turns/No. of secondary turns Na = Actual ratio = Vp/Vs Vp = Primary voltage Is = secondary voltage
Phase-Angle Error
•
The phase difference between the primary voltage and reversed secondary phasors is the phase angle error of the VT.
Electromechanical Relays I.
Electromagnetic relays
II. Thermal Relays
ii.
Induction relays
a.
Induction disc relay
Plunger type
shaded pole type
c.
Balanced beam type
Watt hour meter type
d.
Moving coil type
b.
Printed disc relay
c.
Induction cup relay
i.
Attracted armature relays
a.
Hinged armature type
b.
e.
Polarized moving-iron type
f.
Reed type
Attracted armature Relays
Introduction
Attracted armature relays are the simplest type which respond to ac as well as dc.
They operate through an armature which is attracted
to an
electromagnet or through plunger which is drawn into a solenoid.
All these relays use the same electromagnetic attraction principle for their operation.
The motion of the moving element is controlled by an opposing force generally due to gravity or a spring.
The attracted armature relays are fast relays. They have fast operation and fast reset because of small length of travel and light moving parts.
They are described as instantaneous, But their operating time does vary with current. Slow operating and resetting times can be obtained.
On the other hand, very high operating speeds are possible. One modern relay has 0.5 millisecond of operating time. The current/time characteristic is hyperbolic.
Ratio Rest to Pick-up can be as high as 90-95% for a.c. relays and 60-90% for d.c. relays, means of special design features.
These relays do not have directional feature unless they are provided with additional polarized coil.
As they are fast and operate on d.c. and a.c., they are affected by transients. The transients contain d.c. component in addition to a.c. wave. Therefore, though the steady state value may be less than relay's pick-up the relay may pick-up during transient state.
VA burden depends on construction, setting etc. For a typical relay it is of the order of 0.2 to 0.6 VA for current range 0.1 to 0.4 A.
Modern attraction armature relays are compact, robust, reliable.
Operating Principe
The electromagnetic force exerted on the moving element is proportional to square of the flux in air gap. If saturation is neglected it is proportional to square of operating current.
F = K1I 2 − K2 where F = net force K1 = a constant K2 = restraining force including friction When the relay is on the verge of the operation, F is zero hence
K1I
2
= K2 and
I =
K2 K1
Attracted armature… a.
Hinged armature type relay
The actuating quantity of the relay may be either ac or dc. In dc relay, the electromagnetic force of attraction is constant. In the case of ac relays, sinusoidal current flows through the coil and hence the force of attraction is given by
1 2 2 F = KI = K (I max sin wt) = K (I max − I max cos2wt) 2 2
2
The relay is an instantaneous relay. The operating speed is very high (5 ms).
It is faster than the induction disc cup type relays.
The restraining force is provided by a spring.
The reset to pick-up ratio is 0.5 to 0.9.
The voltage burden is low. i.e. 0.08 W.
The relay is an instantaneous. The operating speed is very high (5ms).
Attracted armature relays are compact, robust and reliable.
They are affected by transients as they are fast and operate on both dc and ac.
This type of a relay is used for the protection of small machines, equipment, etc.
It is also used for auxiliary relays, such as indicating flags, slave relays, alarm relays, annunciators, semaphores, etc.
Attracted armature… b.
Plunger type relay
Attracted armature… c.
Balanced beam relay
Attracted armature…
Neglecting spring effect, the net torque is given by
T = K1I12 − K2 I 22 where T = net torque K1, K2 = constants I1 = current in operating coil I2 = current in restraining coil at the verge of the operation, T is zero therefore,
K1I12
= K2 I 22 , hence
I1 = I2
K2 K1
Attracted armature… d.
Moving coil relay
It is also called a polarized dc moving coil relay. It responds to only dc actuating quantities.
It can be used with ac actuating quantities in conjunction with rectifiers.
It is most sensitive type of relay with sensitivity of 0.1 mW.
These relays are costlier than induction cup or moving iron type relays.
The VA burden of moving coil relays is very small. These are used as slave relays with rectifier bridge comparators.
There are two types of moving coil relays: i. rotary moving coil ii. axial moving coil
Attracted armature…
Rotary moving coil relay
The components are : a permanent magnet, a coil wound on a nonmagnetic former, an iron core, a phosphor bronze spiral spring to provide resetting torque, jeweled bearing, spindle, etc.
The moving coil assembly carries an arm which closes the contact. Damping is provided by an aluminum former.
Attracted armature…
The operating time is about 2 cycles.
The operating torque is produced owing to the interaction between the field of the permanent magnet and that of the coil.
The torque exerted by the spring is proportional to deflection.
The relay has an inverse operating time/current characteristic.
Attracted armature…
Axial moving coil relay
As this type has only one air gap, it is more sensitive than the rotary moving coil relay.
It is faster than rotary moving coil relay because of light parts.
An operating time of the order of 30 msec. can be obtained.
Sensitivities as low as 0.1 mW can be obtained.
Attracted armature…
Its coils are wound on a cylindrical former which is suspended horizontally. The coil has only axial movement.
The relay has an inverse operating time/ current characteristic.
The axially moving coil relay is a delicate relay and since the contact gap is small, it has to be handled carefully.
Attracted armature… e.
Polarized moving Iron relay
A relay whose operation depends on the direction of current or voltage.
The fig. shown is a flux shifting attracted armature type construction. Polarization increases the sensitivity of the relay.
A permanent magnet is used for polarization. Polarization increases the sensitivity of the relay.
Attracted armature…
The permanent magnet produces flux in addition to the main flux. It is a dc polarized relay, meant to be used with dc only. However, it can be used with ac with rectifiers.
It is used as a slave relay with rectifier bridge comparators. As its current carrying coil is stationary, it is more robust than the moving coil type dc polarized relay.
Its operating time is 2 msec. to 15 msec. depending upon the type of construction.
Attracted armature… f.
Reed relay
A reed relay consists of a coil and nickel-iron strips (reeds) sealed in a closed glass capsule, as shown in Fig.
The coil surrounds the reed contact. When the coil is energized, a magnetic field is produced which causes the reeds to come together and close the contact.
Reed relays are very reliable and are maintenance free.
Attracted armature…
As far as their construction is concerned, they are electromagnetic relays. But from the service point of view, they serve as static relays.
They are used for control and other purposes. Also used as a protective relays. They are completely bounce free and are more suitable for normally-closed applications. Their speed is 1 or 2 msec.
Electromagnetic Relays
Advantages
Can be used for both ac and dc.
They have fast operation and fast reset.
These are almost instantaneous. Though instantaneous, the operating time varies with current. With extra arrangements like dash pot, copper ring etc. slow operating and resetting times can be obtained.
High operating speed with operating time in few milliseconds also can be achieved.
The pick-up can be as high as 90-95% for dc operation and 60 to 90% for the ac operation.
Modern relays are compact, simple, reliable and robust.
Electromagnetic…
Disadvantages
The directional feature is absent.
Due to fast operation the working can be affected by the transients. As transients contains dc as well as pulsating component, under steady state value less than set value, the relay can operate during transients.
Electromagnetic…
Applications
The protection of various ac and dc equipments.
The over/under current and over/under voltage protection of various ac and dc equipments.
In the definite time lag over current and earth fault protection along with definite time lag over current relay.
For the differential protection.
Used as auxiliary relays in the contact systems of protective relaying schemes.
Induction Relays
Induction…
Introduction
Induction relays use electromagnetic induction principle for their operation. Their principle of operation is same as that of a single-phase induction motor. Hence they can be used for ac currents.
In both Induction disc and Induction cup relays, the moving element (disc or cup) is equivalent to the rotor of the induction motor.
Two sources of alternating magnetic flux in which the moving element may turn are required for the operation of induction-type relays.
In order to produce an operating torque, the two fluxes must have a phase difference between them.
Induction… a.
Induction Disc Relay
i.
Shaded pole type induction disc relay
Induction…
In this type of relay a metal disc is allowed to rotate between two electromagnets.
The electromagnets are energized by alternating currents. The fields produced by the two magnets are displaced in space and phase.
The torque is developed by the interaction of the flux of one of the magnets and the eddy currents induced in the disc by the other.
Referring to Fig. the shading ring is a copper band or a coil. Effect of shading ring is to produce flux in the shaded portion of the magnet (Ф1) which is displaced in phase and space from the flux in the remaining portion (Ф2).
The flux Ф1 induces e.m.f. E1 is the disc at 90° to Ф1. The e.m.f. E1 produces currents I2 lagging behind E1 by small angle.
Induction…
The interaction between I1 and Ф2 produces torque, which is proportional to Φ2 I1 cos α, where I1 cos α is component of I1 in phase with Φ2. Greater the angle θ, greater is the torque.
The torque equation of single quantity induction relay may be expressed as:
T = K1I 2 − K2 where T = Net torque I = Current in relay coil K1, K2 = constants
Induction…
where Φ1 = flux in shaded portion of magnet Φ2 = flux in un shaded portion of magnet E1 = e.m.f induced in the disc due to Φ1 I1 = current in the disc induced by E1 Torque α Φ2 I1 cos α, where α is angle between Φ2 and I1
Induction… ii.
Watt-hour meter type Induction disc relay
Inverse characteristics
Induction…
The construction of this relay is similar to the watt-hour meter commonly used everywhere.
It
consists
of
an
E-shaped
electromagnet
and
a
U-shaped
electromagnet with a disc free to rotate in between.
The E-shaped magnet produces flux Ф1 and the U-shaped magnet produces flux say Φ2.
The phase angle θ between the fluxes is
adjusted by a reactance in parallel with the secondary winding.
Torque is produced by interaction between flux and the eddy currents in the disc (produced by flux Ф1 and Φ2).
The relay coil is tapped at several points. The current setting is selected by inserting a knob to take desired number of turns of the coil in the circuit.
Induction…
Highlights
The operation of induction relay can be controlled by opening secondary coil, as opening of this coil makes relay inoperative.
The time/current characteristics of induction disc relay is inverse characteristic . The time reduces as current increases.
The VA burden depends on rating. It is of the order of 2.5 VA.
Modern induction disc relays are robust and reliable.
These are used for over current protection.
In these relays, there is a facility for selecting the plug setting and time setting such that the same relay can be used for a wide range of current, time and characteristics.
Induction…
Plug Setting bride is provided with induction disc relays and it provides a wide range of current settings.
The plug setting refers to the magnitude of current at which the relay starts to operate. The plug setting bridge comprises connections tapped from relay coil. By inserting the plug, in a particular gap in the bridge, a certain number of turns of the relay coil are brought into circuit.
Time multiplier setting is generally in the form of an adjustable backstop which decides the arc-length through which the disc travels, by reducing the length of travel, the time is reduced.
Induction…
The effect of d.c. offset may be neglected with inverse time single quantity induction relay, because they are generally slow. The d.c. offset may effect fast relays.
Ratio of reset of pick-up is high because operation does not involve any change in air gap. The ratio is above 95%.
Inverse time characteristic is obtained by disc relays, is 10 to 60 sec.
Induction…
Torque Equation
Induction… b.
Induction Cup Relay
Induction…
This relay has two, four or more electromagnets, in stator. These are energized by the relay coils. A stationary iron core is placed as shown in Fig.
The rotor consists of a hollow metallic cylindrical cup. The rotor is free to rotate in the gap between the stationary iron and the electromagnets.
In this type of relay, the eddy currents are produced in the metallic cup. These currents interact with the flux produced by the other electromagnet and torque is produced. The theory is similar to that of the disc type induction relay.
In Fig. structure employing four poles is shown. It has an iron core at the centre and a metal cup between the core and electromagnet.
Induction…
Fig. shows a two pole structure. The two fluxes Ф1 and Ф2 are at right angles and produce eddy current in the cup. Thereby torque is produced.
Induction…
Highlights
Modern induction cup relays have 4 or more poles. A control spring and moving contacts are carried on an arm attached to the spindle of the cup.
The relay can be responsive to voltage or current, Similar structures are used in either cases.
The double actuating quantity relay can be responsive to both voltage and current.
The operating time characteristic depends on the type of structure. The relays have inverse time characteristic.
A modern induction cup relay may have an operating time of the order of 0.010 second.
Over Current Protection
Introduction
A protective relay which operates when the load current exceeds a preset value, is called an over current relay.
The value of the preset current above which the relay operates is known as its pick-up value.
Over current relays offer the cheapest and simplest form of protection.
An over current protection scheme may include one or more over current relays.
At present, electromechanical relays are widely used for over current protection. The induction disc type construction is commonly used .
Over Current…
With the development of numerical relays based on microprocessors or micro controllers, there is a growing trend to use numerical over current relays for over current protection.
These relays are used for the protection of distribution lines, large motors, power equipment, industrial systems, etc.
Over current relays are also used on some sub transmission lines which can not justify more expensive protection such as distance or pilot relays.
Over Current…
Several protective devices are used for over current protection. These include - Fuses - Miniature circuit-breakers, moulded-case circuit-breakers. - Circuit-breakers fitted with overloaded coils or tripped by over-current relays. - Series connected trip coils operating switching devices. - Over-current relays in conjunction with current transformers.
Over Current…
Applications
1.
Motor protection
2.
Transformer protection
3.
Line protection (feeders)
a.
Instantaneous over-current relays.
b.
Inverse time over-current relays.
c.
Directional over-current relays.
Lines can be protected by impedance, or carrier current protection also.
4.
Protection of utility equipments
Over Current…
Relays used in over current protection The choice of relay for over-current protection depends upon the time/current characteristic and other features desired. The following relays are used.
1. For instantaneous over-current protection. Attracted armature type, moving iron type, permanent magnet moving coil type, static. 2. For inverse time characteristic. Electromagnetic induction type, permanent magnet moving coil type, static.
Over Current… 3. Directional over-current protection. Double actuating quantity induction relay with directional feature. 4. Static over-current relays. 5. HRC fuses, drop out fuses, etc. are used in low voltage medium voltage and high voltage distribution systems, generally up to 11 kV. 6. Thermal relays are used widely for over-current protection.
Over Current…
Time Current Characteristics of Over Current Protection Depending upon the time of operation the relays are categorized as:
1.
Definite –time over-current relay
2.
Instantaneous over-current relay.
3.
Inverse -time over-current relay.
4.
Inverse definite minimum time (IDMT) over-current relay.
5.
Very Inverse -time over current relay.
6.
Extremely Inverse -time over current relay
Figure. Characteristics of various over-current relays: (a) definite time , (b) IDMT, (c) very inverse, and (d) extremely inverse
1.
Definite –time over current relay
A definite-time over current relay operates after a predetermined time when the current exceeds its pick-up value.
Pick-up (level): the threshold value of the actuating quantity (current, voltage, etc.) above which the relay operates.
The operating time is constant, irrespective of the magnitude of the current above the pick-up value.
The desired definite operating time can be set with the help of an intentional time-delay mechanism provided in the relaying unit.
2.
Instantaneous over current relay
An instantaneous relay operates in a definite time when the current exceeds its pick-up value.
The operating time is constant, irrespective of the magnitude of the current. There is no intentional time delay.
This characteristic can be achieved with the help of hinged armature relays.
It operates in 0.1 s or less. Some times the term like “high set” or “high speed” is used for very fast relays having operating times less than 0.1s.
3.
Inverse – time over current relay
An inverse-time over current relay operates when the current exceeds its pick-up value.
The operating time depends on the magnitude of the operating current. The operating time decreases when the current increases.
4.
Inverse Definite Minimum Time (IDMT) over current relay
This type of a relay gives an inverse-time current characteristics at lower values of the fault current and definite-time characteristics at higher values of the fault current .
Generally, an inverse-time characteristics is obtained if the value of the plug setting multiplier is below 10.
For values of plug setting multiplier between 10 and 20, the characteristic tends to become a straight line, i.e. towards the definite time characteristic.
IDMT relays are widely used for the protection of distribution lines. Such relays have a provision for current and time settings.
5.
Very Inverse-time over current relay
A
very
inverse-time
over
current
relay
gives
more
inverse
characteristics than that of a plain inverse relay or the IDMT relay.
Its time-current characteristic lies between an IDMT characteristic and extremely inverse characteristic.
The very inverse characteristic gives better selectivity than the IDMT characteristic. Hence, it can be used where an IDMT relay fails to achieve good selectivity.
These relays are recommended for the
cases where there is a
substantial reduction of fault current as the distance from the power source increases. They are practically effective with ground faults because of their steep characteristic.
6.
Extremely Very Inverse-time over current relay
An extremely inverse time over current relay gives a time-current characteristic more inverse than that of the very inverse and IDMT relays.
When IDMT and very inverse relays fail in selectivity, extremely inverse relays are employed. IDMT relays are not suitable to be graded with fuses.
The electromechanical relay which gives the steepest time-current characteristic is an extremely inverse relay.
This type of relay is very suitable for the protection of machines against overheating. (I2 t = K)
Extremely …
This type of relays are used for the protection of alternators, power transformers, earthing transformers, expensive cables, railway trolley wires, etc.
The rotors of large alternators may be overheated if an unbalanced load or fault remains for a longer period on the system. In such case, an extremely inverse relay, in conjunction with a negative sequence network is used.
This relay is quite suitable for differentiate between sustained short circuit current and momentary over loads.
This relay is also used for reclosing distribution circuits after a long outage.
Extremely …
This relay is able to distinguish a fault current and inrush current due its steep time-current characteristic. Therefore an extremely inverse relay is quite suitable for the load restoration purpose.
Method of defining shape of time current characteristics
The general expression for time-current characteristics is given by
t=
K I n −1
The approximate expression is
t=
K In
For definite-time characteristic, the value of n is equal to 0.
Method of defining...
According to the British standard, the following are the important characteristics of over current relays:
i.
ii.
iii.
0.14
IDMT :
t=
Very inverse :
13.5 t= I −1
Extremely inverse :
t=
I 0.02 − 1
80 I 2 −1
Current Setting
The current above which an over current relay should operate can be set.
The operation of the relay requires a certain flux and ampere turns. The current settings of the relay are chosen by altering the number of turns of the current coil by means of a plug.
The plug-setting (current-setting) can either be given directly in amperes or indirectly as percentages of the rated current.
An over current relay which is used for phase-phase fault protection, can be set at 50% to 200% of the rated current in steps of 25%.
Current Setting...
The usual current rating of this relay is 5 A. So it can be set at 2.5 A, 3.75 A, 5 A,..., 10 A. When a relay is set at 2.5 A, it will operate when current exceeds 2.5 A. When relay is set at 10 A, it will operate when current exceeds 10 A.
The relay which is used for protection against ground faults (earth-fault relay) has settings 20% to 80% of the rated current in steps of 10%.
The current rating of an earth-fault relay is 1 A.
The actual r.m.s. Current flowing in the relay expressed as a multiple of the setting current multiplier (PSM).
(pick-up current) is known as the plug setting
PSM can be expressed as PSM = Secondary current / Relay current setting = Primary current during fault (Fault current) / Relay setting x CT ratio
Time Setting
The
operating time of the relay can be set at a desired value. In
Induction disc type relay, the angular distance by which the moving part of the relay travels for closing the contacts can be adjusted to get different operating time.
There are 10 steps in which time can be set. The term time multiplier setting (TMS) is used for these steps of time settings.
The values of TMS are 0.1, 0.2,...0.9,1.
Directional Power Relay
The directional relay means the relay operates for the specific direction of the actuating quantity in the circuit.
The directional power relay operates when power in the circuit flows in the specific direction.
The construction and principle of operation of this relay is similar to the induction type watt hour meter relay. The difference is that in watt hour meter type relay the torque produced is due to the interaction of the fluxes produced by only the current derived from the secondary of CT, while in directional power relay the torque is produced due to interaction of fluxes produced from both voltage and current of the circuit.
Directional…
Directional…
The relay has two windings, one act as voltage coil and while other as current coil , similar to a watt meter.
The upper magnet carries a voltage coil or potential coil which is energized from PT, while the lower magnet carries a current coil which is energized from CT in the line to be protected.
The number of tapping's are provided to the current coil with which desired current
setting can be achieved. The restraining torque is
provided by the spiral spring.
The spindle of disc carries the moving contacts which make contact with tripping circuit terminals when the disc rotates.
Directional…
The voltage coil provided on the upper magnet produces the flux Ф1. this lags the voltage by 90o.
The current I is sensed by the current coil on lower magnet which produces the flux Ф2. This is in phase with current I. the current I lags voltage V by an angle Ф.
Directional…
The interaction of fluxes Φ1 and Φ2 produces the torque. Hence we can write.
T αφ 1φ 2 sin α
φ 1α V
but while therefore
φ 2α I
and
α = 90 − φ T α VI sin( 90 − φ )
T α VI cos φ
α
power in circuit
Directional…
Under normal working conditions, the driving torque acts in the same direction as that of restraining torque. This moves the moving contacts away from the fixed tripping circuit contacts. Thus, relay remains inoperative as long as power flow is in one particular direction.
But, when there is a current reversal and hence the power reversal then the driving torque acts in opposite direction to the
restraining
torque in such a manner that the moving contacts close the tripping circuit contacts. This opens the circuit breaker to isolate the faulty part.
This relay is used for providing the reverse power protection to synchronous machines. This relay can be single phase or three phase.
Directional Induction Type Over current Relay
The directional power relay is not suitable to use as a protective relay under short circuit conditions. This is because under short circuit conditions the voltage falls drastically and such a reduced voltage may not be sufficient to produce the driving torque required for the relay operation.
Hence in practice, directional induction type over current relay is used. This relay operates almost independent of system voltage and power factor.
The directional
induction type over current relay uses two relay
elements mounted on a common case these elements are: 1. Directional element which is directional power relay 2. Non directional element which is non directional over current relay
Directional…
Directional…
Directional Element
The directional element is nothing but a directional power relay which operates when power in the circuit flows in a particular direction.
The voltage coil of this element is energized by
a system voltage
through a potential transformer. The current coil on the lower magnet is energized by the system current through a current transformer.
The trip contacts of this relay
(1-1’) are connected in series with
secondary winding of non-directional element.
Directional…
Non-Directional Element
The plug setting bridge provided in this element
to adjust current
setting as per the requirement.
The trip contacts of this relay
(1-1’) are connected in series with
secondary winding of non-directional element. So unless and until trip contacts (1-1’)
are closed by the movement of disc of directional
element, the non-directional element can not operate. Thus, the movement of the directional element.
non-directional element is controlled by the
Directional…
Operation
When the fault takes place the current or power in the circuit has a tendency to flow in reverse direction. The current flows through the current coil of the directional element which produces the flux. The voltage coil produces another flux.
The two fluxes interact to produce the torque due to which the disc rotates. As disc rotates
the trip contacts (1-1’) get closed. So
directional element must operate first non-directional element.
to have the operation of the
Directional…
The following conditions must be satisfied to have the operation of the entire relay. i. the direction of current in the circuit
must
reverse
to operate
directional element. ii. the current value in the reverse direction must be greater than the current setting. iii. The high value of current must persist for a time period which is greater than the time setting of the relay.
Directional…
Directional Characteristics V = relay voltage through P.T I = relay coil current through C.T θ = angle between V and I Φv = flux produced by voltage V ΦI = flux produced by current I Tα therefore
Φv ΦI sin (θ+ Φ)
T = K V I sin (θ+ Φ), where K is a constant
Directional… Maximum torque occurs when sin (θ+ Φ) is 1 i.e. θ+ Φ = 90 The condition for the maximum torque is shown dotted in the figure the torque is zero when sin (θ+ Φ) is zero i.e. θ+ Φ = 0 or 180
Directional…
Protection of Parallel Feeders and Ring mains
At the sending end of the feeders (at A and B), non-directional relays are required. At the end of other feeders (at C and D) directional over current relays are required.
Here the directional relays can operate for fault current flowing in a particular direction shown by arrow
In the diagram, the double headed arrow
indicates non-directional
relay.
If a fault occurs at middle of the line BD the directional relay at D trips, as the direction of the current is reversed . The relay at C does not trip, as the current flows in the normal direction. The relay at B trips for a fault. Thus the faulty feeder is isolated and the supply of the healthy feeder is maintained.
Earth Fault Protection
Earth fault relay and Over current relay
A fault which involves ground is called an earth fault (L-G, 2L-G).
Faults which do not involve ground are called phase faults.
Relays which are used for the protection of a section ( or an element) of the power system against earth faults are called earth faults relays.
Similarly, relays used for the protection of a section of the power system against phase faults are called phase fault relays or over current relays.
The operating principle and constructional features of earth fault relays and phase fault relays are the same. They differ only in the current levels of their operation.
Earth Fault…
The plug setting of earth fault relays varies from 20% to 80% of the CT secondary rating in steps of 10%.
Earth fault relays are more sensitive than the relays used for phase faults. The plug setting for phase fault relays varies from 50% to 200% of the CT secondary rating in steps of 25%.
Earth Fault…
Earth Fault Protective Schemes
An earth fault relay may be energized by a residual current.
Figure (a) shows the connections of earth fault relay.
ia, ib, ic are currents in the secondary of CTs of different phases.
The sum (ia+ib+ic) is called residual current. In the absence of earth fault, the residual current is zero.
When an earth fault occurs, the residual current is non-zero. When it exceeds pick-up value, the earth fault relay operates.
The manufacturer provides a range of plug settings of earth fault relay from 20% to 80% of secondary rating in steps of 10%.
Figure (b) and (c) shows earth fault relays used for the protection of transformer and an alternator, respectively.
Earth Fault…
Earth Fault…
Phase Fault Protective Schemes
This scheme is mainly for the protection of the system against phase faults.
Figure shows the connections of phase fault relay. Three over current relays.
Earth Fault…
Combined Earth Fault and Phase Fault Protective Schemes
Figure shows the two over current relays (phase to phase fault relays) and one earth fault relay.
When an earth fault occurs, the burden on the active CT is that of an over current relay (phase fault relay) and the earth fault relay in series. Thus, the CT burden becomes high and may cause saturation.
Earth Fault…
Directional Earth Fault Relay
For the protection against ground faults, only one directional over current relay is required.
Its operating principle and construction is similar to the directional over current relays.
It contains two elements, a directional element and an IDMT element.
The directional element has two coils. One coil is energized by current and the other by voltage.
The current coil of the directional element is energized by residual current and the potential coil by residual voltage, as shown in figure (a).
This connection is suitable for a place where the neutral point is not available. If the neutral of an alternator or transformer is grounded, connections are made as shown in figure (b).
A special five limbs VT which can energize both the earth fault relay as well as the phase fault relays as shown in figure (c).
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