Power System Protection2.pdf

ELECTRICAL PROTECTION Prof. JJ (Jerry) Walker WALMET TECHNOLOGIES (Pty) Ltd

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JJ (Jerry) Walker • Background –Practical –Academic

• Contact [email protected] WALMET TECHNOLOGIES (Pty) Ltd

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Contents • Basic Principles • System Earthing • Protection Equipment – Faults in a three-phase system – Calculation of short-circuit currents – Fuses – Instrument Transformers – Relays WALMET TECHNOLOGIES (Pty) Ltd

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• • • • • •

Overcurrent Protection Unit Protection Transformer Protection Busbar Protection Motor Protection Generator Protection

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Basic Principles

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Protection of Power Systems • Customers always demand power on a continuous basis without interruptions. • Hence it is necessary to foresee the likely interruptions that may occur in the distribution system to detect failures and to isolate only the faulty sections. • Protective equipment or protective relay is used in a power network to detect, discriminate and isolate the faulty equipment in the network to ensure that the rest of the system is fed with continuous power and at the same time, damage to faulty section is minimized. WALMET TECHNOLOGIES (Pty) Ltd

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Power system protection basic requirements 1. To safeguard the entire system to ensure continuity of supply. 2. To minimize damage and repair costs. 3. To ensure safety of personnel.

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Power system protection basic qualities 1. Selectivity: To detect and isolate the faulty item only. 2. Stability: To leave all healthy circuits intact to ensure continuity or supply. 3. Sensitivity: To detect even the smallest values of fault current or system abnormalities and operate correctly at its setting before the fault causes irrepairable damage. 4. Speed: To operate speedily when it is called upon to do so, thereby minimizing damage to the surroundings and ensuring safety to personnel.

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Protection philosophy • Selectivity - Stability - Speed - Sensitivity • Emphasis on Speed for the following reasons: – To minimise damage and repair costs. – To reduce production downtime. – To prevent undue thermal and magnetic overstressing of healthy equipment on through fault. – To keep voltage depressions as short as possible in the interests of plant stability. (SISHEN mine) – Above all, to ensure the safety of personnel (Flashes).

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Power system protection qualities -- facets of reliability

It MUST trip when required

It must NOT trip when not required

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System Earthing Basic Definitions and Preliminaries • • • • •

Reasons for Power Systems Earthing Reasons for Equipment Earthing (Bonding) Touch and Step Potentials Power System Earth Systems (LV, MV and HV) Earthing and International / National Standards

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Reasons for Power Systems Earthing • Systems and circuit conductors are grounded to: – limit voltages due to lightning, line surges, or unintentional contact with higher voltage lines, and – to stabilize the voltage to ground during normal operation

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Typical Earthing Arrangement in a Power System

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Types of Neutral Earthing in Power Systems • In the early times power systems were mainly Neutral ungrounded due to the fact that the first ground fault did not require the tripping of the system. An unscheduled shutdown on the first ground fault was particularly undesirable for continuous process industries.

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Importance of Neutral Grounding • Neutral Earthing offers many advantages over an ungrounded system, like: – Reduced magnitude of transient over voltages – Simplified ground fault location – Improved system and equipment fault protection – Reduced maintenance time and expense – Greater safety for personnel – Improved lightning protection – Reduction in frequency of faults. WALMET TECHNOLOGIES (Pty) Ltd

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Method of Neutral Earthing There are five methods for Neutral earthing. • Unearthed Neutral System • Solid Neutral Earthed System. • Resistance Neutral Earthing System. – Low Resistance Earthing. – High Resistance Earthing.

• Resonant Neutral Earthing System. • Earthing Transformer Earthing. WALMET TECHNOLOGIES (Pty) Ltd

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Ungrounded Neutral Systems • In ungrounded system there is no internal connection between the conductors and earth. However, as system, a capacitive coupling exists between the system conductors and the adjacent grounded surfaces. Consequently, the “ungrounded system” is, in reality, a “capacitive grounded system” by virtue of the distributed capacitance WALMET TECHNOLOGIES (Pty) Ltd

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Advantage: • After the first ground fault, assuming it remains as a single fault, the circuit may continue in operation, permitting continued production until a convenient shut down for maintenance can be scheduled. Disadvantages: • The interaction between the faulted system and its distributed capacitance may cause transient over-voltages (several times normal) to appear from line to ground during normal switching of a circuit having a line-to ground fault (short). These over voltages may cause insulation failures at points other than the original fault. • A second fault on another phase may occur before the first fault can be cleared. This can result in very high line-to-line fault currents, equipment damage and disruption of both circuits. • The cost of equipment damage. • Complicate for locating fault(s), involving a tedious process of trial and error: first isolating the correct feeder, then the branch, and finally, the equipment at fault. The result is unnecessarily lengthy and expensive down downtime.

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Solidly Neutral Grounded Systems

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Advantage: • The main advantage of solidly earthed systems is low over voltages, which makes the earthing design common at high voltage levels (HV). Disadvantages: • This system involves all the drawbacks and hazards of high earth fault current: maximum damage and disturbances. • There is no service continuity on the faulty feeder. • The danger for personnel is high during the fault since the touch voltages created are high. • Applications: • Distributed neutral conductor. • 3-phase + neutral distribution. • Use of the neutral conductor as a protective conductor with systematic earthing at each transmission pole. • Used when the short-circuit power of the source is low. WALMET TECHNOLOGIES (Pty) Ltd

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Resistance earthed systems Resistance Grounding Systems limits the phase-to-ground fault currents. The reasons for limiting the Phase to ground Fault current by resistance grounding are: • To reduce burning and melting effects in faulted electrical equipment like switchgear, transformers, cables, and rotating machines. • To reduce mechanical stresses in euipment carrying fault currents. • To reduce electrical-shock hazards to personnel caused by stray ground fault. • To reduce the arc blast or flash hazard. • To reduce the momentary line-voltage dip. • To secure control of the transient over-voltages while at the same time, – improve the detection of the earth fault in a power system

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Types of Resistance Earthing There are two categories of resistance grounding: • Low resistance Grounding. • High resistance Grounding. The difference between Low Resistance Grounding and High Resistance Grounding is a matter of perception and, therefore, is not well defined. Generally speaking high-resistance grounding refers to a system in which the NGR let-through current is less than 50 to 100 A. Low resistance grounding indicates that NGR current would be above 100 A WALMET TECHNOLOGIES (Pty) Ltd

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Low Resistance Grounded

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Advantages: • Limits phase-to-ground currents to 200-400A. • Reduces arcing current and, to some extent, limits arc-flash hazards associated with phase-to-ground arcing current conditions only. • May limit the mechanical damage and thermal damage to shorted transformer and rotating machinery windings. Disadvantages: • Does not prevent operation of over current devices. • Does not require a ground fault detection system. • May be utilized on medium or high voltage systems. • Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must be served through an isolation transformer. Used: Up to 400 amps for 10 sec are commonly found on medium voltage systems

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High Resistance Grounded • High resistance grounding is almost identical to low resistance grounding except that the ground fault current magnitude is typically limited to 10 amperes or less. High resistance grounding accomplishes two things – The first is that the ground fault current magnitude is sufficiently low enough such that no appreciable damage is done at the fault point – The second point is it can control the transient overvoltage phenomenon present on ungrounded systems if engineered properly WALMET TECHNOLOGIES (Pty) Ltd

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High Resistance (Dist Tfmr) Earthing

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Advantages:

• • • • • • • •

•

Enables high impedance fault detection in systems with weak capacitive connection to earth Some phase-to-earth faults are self-cleared. The neutral point resistance can be chosen to limit the possible over voltage transients to 2.5 times the fundamental frequency maximum voltage. Limits phase-to-ground currents to 5-10A. Reduces arcing current and essentially eliminates arc-flash hazards associated with phase-to-ground arcing current conditions only. Will eliminate the mechanical damage and may limit thermal damage to shorted transformer and rotating machinery windings. Prevents operation of over current devices until the fault can be located (when only one phase faults to ground). May be utilized on low voltage systems or medium voltage systems up to 5kV. IEEE Standard 141-1993 states that “high resistance grounding should be restricted to 5kV class or lower systems with charging currents of about 5.5A or less and should not be attempted on 15kV systems, unless proper grounding relaying is employed”. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral loads must be served through an isolation transformer.

Disadvantages:

• •

Generates extensive earth fault currents when combined with strong or moderate capacitive connection to earth Cost involved. Requires a ground fault detection system to notify the facility engineer that a ground fault condition has occurred

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Resonant (Petersen coil )earthed system

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Advantages: • Small reactive earth fault current independent of the phase to earth capacitance of the system. • Enables high impedance fault detection. Disadvantages: • Risk of extensive active earth fault losses. • High costs associated. WALMET TECHNOLOGIES (Pty) Ltd

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Earthing Transformers

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Reasons for Equipment Earthing (Bonding) • Conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, are grounded to limit the voltage to ground on these materials and to facilitate overcurrent device operation in case of ground faults. • Bonding is used to reduce the risk of electric shock to anyone who may touch two separate metal parts when there is a fault somewhere in the electricity supply or electrical installation. • By connecting together the particular metal parts with bonding conductors, bonding reduces the voltage there might have been. WALMET TECHNOLOGIES (Pty) Ltd

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Touch and Step Potentials • Touch voltages typically appear between a hand and one or both feet of a person touching a temporarily livened conductive part. • Step voltage is the voltage between two points on the earth’s surface that are 1 m distant from each other, which is considered to be the stride length of a person. WALMET TECHNOLOGIES (Pty) Ltd

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Body current limit • 1 mA: Threshold of perception • 1 mA to 6 mA: Let-go currents • 9 mA to 25 mA : Painful, difficult to release energized objects • 25 mA to 60 mA : Muscular contractions, breathing difficult • 60 mA to 100 mA : Ventricular fibrillation • Ventricular fibrillation does not respond to resuscitation and the threshold of this condition is thus of major concern. WALMET TECHNOLOGIES (Pty) Ltd

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• From experimental data the upper limit for body current (Ik) is a function of current duration (t , in sec) according to the equation •
 

. √

, for 0,03 sec < t < 3 sec

• Ik ≈ 0,060 amps r.m.s. at 50 Hz.

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Limit of safe Touch Potential = IK (RK + 0,5RF) . ..  = (1000 + 1,5Ps)  √

√

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Limit of safe Step Potential = IK (RK + 2RF) . ..  = (1000 + 6Ps)  √

√

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National Standards • SANS 10200 - Neutral earthing in medium voltage industrial power systems • SANS 10292 - Earthing of low-voltage (LV) distribution systems

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PROTECTION EQUIPMENT The definitions that follow are generally used in relation to power system protection: • Protection System: a complete arrangement of protection equipment and other devices required to achieve a specified function based on a protection principal (IEC 60255-20) • Protection Equipment: a collection of protection devices (relays, fuses, etc.). Excluded are devices such as CT’s, CB’s, Contactors, etc. • Protection Scheme: a collection of protection equipment providing a defined function and including all equipment required to make the scheme work (i.e. relays, CT’s, CB’s, batteries, etc.) WALMET TECHNOLOGIES (Pty) Ltd

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Basic components of protection 1. Voltage Transformers and Current transformers: To measure the parameters of a system. 2. Relays: To convert the signals from the above devices and give instructions to open a circuit or to give alarms under faulty conditions. 3. Fuses: To protect the downstream equipment being protected by self destruction. 4. DC Batteries: To give uninterrupted power to the relays and breakers independent of the main power source being protected.

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Faults in a Three-phase System Short-circuits can be: • Phase-to-earth (80% of faults) • Phase-to-phase (15% of faults). This type of fault often degenerates into a three phase fault • Three-phase (only 5% of initial faults)

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Types of three-phase faults

(A) Phase-to-ground (B) Phase-to-Phase

(E) Three Phase-To-ground (F) Phase-to-Pilot *

(C) Phase-to-Phase-to-ground (D) Three Phase

(G) Pilot-to-ground * * In mines

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Magnitudes of fault currents • Normally impedance decides the value of fault currents - But impedance can not be reduced below a certain value • ground currents can be limited by grounding the neutral of the source and choosing suitable grounding method • Phase fault currents can not be controlled

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Transient and permanent faults • Transient faults - do not damage insulation permanently (eg. Tree branches on O/H line), re-closing will be successful • Permanent - the insulation has broken down permanently requiring repair to restore insulation levels (re-closing will fail)

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Consequences of short-circuits At the fault location, the presence of electrical arcs, resulting in: • Damage to insulation • Welding of conductors • Fire and danger to life On the faulty circuit: • Electrodynamic forces, resulting in - Deformation of the busbars - Disconnection of cables

• Excessive temperature rise due to an increase in Joule losses, with the risk of damage to insulation WALMET TECHNOLOGIES (Pty) Ltd

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On other circuits in the network or in near-by networks • Voltage dips during the time required to clear the fault, ranging from a few milliseconds to a few hundred milliseconds • Shutdown of a part of the network, the extent of that part depending on the design of the network and the discrimination levels offered by the protection devices • Dynamic instability and/or the loss of machine synchronisation • Disturbances in control / monitoring circuits WALMET TECHNOLOGIES (Pty) Ltd

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ASYMMETRICAL AND SYMMETRICAL SHORT CIRCUIT CURRENTS • Symmetrical and Asymmetrical are terms used to describe the symmetry of the short-circuit current waveform around the zero axis. If a short-circuit occurs in an inductive reactive circuit at the peak of the voltage waveform, the resulting short-circuit current will be totally symmetrical. • If a short-circuit, in the same circuit, occurs at the zero of the voltage waveform, the resulting short-circuit current will be totally asymmetrical. If a short-circuit, in the same circuit, occurs at some time between the zero and peak of the voltage waveform, the resulting shortcircuit current will be partially asymmetrical. WALMET TECHNOLOGIES (Pty) Ltd

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Symmetrical vs. Asymmetrical • Symmetrical faults

• Asymetrical faults - displays a DC offset which decays

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Symmetrical vs. Asymmetrical • Symmetrical represents the steady state fault conditions while asymmetrical is experienced during commencement of faults • The amount of offset in asymmetrical depends on the X/R (power factor) and decays to steady state • The First peak can be as high as 2.55 times the steady state level.

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Power factor vs. total asymmetry factor

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Calculation of Short Circuit Currents • Accurate fault calculations adopt the use of symmetrical components to calculate fault currents in three phase networks. This is based on the principle that any set of unbalanced vectors can be represented by a set of 3 balanced vectors namely positive, negative and zero sequence vectors and involve: – Calculation of the Symmetrical Fault Current – Calculation of the Assymmetrical Fault Current – Calculation of the contribution of Rotating equipment (motors) to the total fault current

• However, simple methods are available for field calculations which give adequate accuracy WALMET TECHNOLOGIES (Pty) Ltd

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Assumptions for Simple Calculations • Ignore the arc resistance • Ignore cable impedance values when cable lengths are small. • Ignore resistance component for transformer impedance and consider directly transformer reactance to avoid complex algebra simplify calculations • Ignore source impedance when transformers are involved WALMET TECHNOLOGIES (Pty) Ltd

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Calculation methods • Ohmic Method - Where all impedances are expressed in Ohms • Percentage impedance Method - Where a Base MVA is selected and all impedances referred in percentage related to the base MVA • Per Unit Method - Similar to % impedance method removing the % factor (Preferred Method)

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3ø Short Circuit Calculation: Per-Unit Method The per-unit method is generally used for calculating short-circuit currents when the electrical system is more complex. After establishing a one-line diagram of the system, proceed to the following calculations: – Select a common MVA (kVA) base for the system = Sb. – Calculate the p.u. reactance for the Utility (source) X(p.u.-utility) =

 /

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• Calculate the p.u. reactance of the transformers: X(p.u.-transformer) =

% !"# $   $  !"#

• Calculate the p.u. reactance* of the other components: X(p.u.-component) =

% $  &' (

Note: Sb = MVA

* Consider the Resistance of the component? WALMET TECHNOLOGIES (Pty) Ltd

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Xp.u.(total) = Xp.u.(utility) + Xp.u.(transformers) + Xp.u.(components) IS/C =

 ) $ &' $ *.+.!

kA Note: Sb = MVA

Note: If the resistances of the components cannot be ignored: – Calculate: Xp.u.(total) – Calculate: Rp.u.(total) – Calculate: Zp.u.(total) =

,-... / 01

2

3 4-... / 01

2

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Fuses

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Fuses • The fuse is a reliable overcurrent protective device. A “fusible” link or links encapsulated in a tube and connected to contact terminals comprise the fundamental elements of the basic fuse. – Electrical resistance of the link is so low that it simply acts as a conductor. However, when destructive currents occur, the link very quickly melts and opens the circuit to protect conductors and other circuit components and loads. – Fuse characteristics are stable. – Fuses do not require periodic maintenance or testing. – Fuses have three unique performance characteristics: WALMET TECHNOLOGIES (Pty) Ltd

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• Modern fuses have an extremely “high interrupting rating”—can withstand very high fault currents without rupturing. • Properly applied, fuses prevent “blackouts.” Only the fuse nearest a fault opens without upstream fuses (feeders or mains) being affected—fuses thus provide “selective coordination.” (These terms are precisely defined in subsequent pages.) • Fuses provide optimum component protection by keeping fault currents to a low value…They are said to be “current limiting.” WALMET TECHNOLOGIES (Pty) Ltd

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Fuses - types • Re-wireable Type Fusible wire – Disadvantages - Open to abuse - incorrect rating used to keep circuit in rating drops as time goes by – Advantage - Fail safe

• Cartridge Type – Silver element enclosed in a barrel of insulating material (sometimes filled with quartz sand)

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Fuses - types CARTRIDGE TYPE Advantages : – fault energy contained by insulating tube – Sealed hence does not deteriorate as fast as open type – Better grading possible – Quartz sand absorbs energy and melts across ionized metal path to quench arc – Faster and can handle very high currents up to 100 kA – Normal currents are closer to fusing currents today due to improved materials and design

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General Purpose Fuses IEC • The fuse comply with standard EN 60269-2 section II and standard BS88 part 2. • This fuses are designed for : – “general purpose use” protection, (gG type) – motor protection (gM type)

• This fuse range insures an excellent current limitation for all overloads on a large range of applications. • Their size cannot allow exchange by other fuses of higher rating in the F type range. • They are compact and can be connected with clips. WALMET TECHNOLOGIES (Pty) Ltd

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Applications • Power cable protection • Distribution panel protection • Control panel • Main circuit • Lighting, heating and electrical equipment • Capacitor, batteries • Can be used with circuit breakers

F1,F2 types - gG curve Features /Benefits • Compact design • Curve gG and gM • Very current limiting • Tested in DC • Voltage 415VAC • High breaking capacity : 80kA @ 415VAC • Connection by clips or appropriate fuse holders

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Time/current data 4 hrs

1 000

Fuse-link ampere rating 100

10

1

0.1

0.01 0.005 1

10

100

1 000

10 000

100 000

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2 It

characteristics

107

106

105

104

103 Operating I2t at 415 V. pre-arcing I2t 102

101

0

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Energy “ Let Through” I (rm s)

I (rm s)

Energy (I2t) let through by fault of one cycle duration

2

Energy (I t) let through by H.R.C. Fuse-link Time Peak

H.R.C Fuse-link c ut-off

H.R.C Fuse-link duration Fault current one full cycle (0.02 second)

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Fuse Selection • In order to select the proper protective device, the following parameters and criteria need to be considered: – – – – – –

What is the normal operating current of the circuit? What is the operating voltage? Is the circuit AC or DC? What is the operating ambient temperature? What is the available short-circuit current? What is the maximum allowable I²t? WALMET TECHNOLOGIES (Pty) Ltd

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– Are there in-rush currents available? – Is the protective device being used for shortcircuit protection, over-load protection, or both? – What are the physical size limitations? – Is the PCB surface mount or thru-hole? – Does the fuse need to be "field-replaceable"? – Is resettability an issue? – What safety agency approvals are needed? – How will I mount the device? – What are the cost considerations?

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Instrument Transformers

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Instrument transformers The main tasks of instrument transformers are: • To transform currents or voltages from a usually high value to a value appropriate for relays and instruments (1 or 5 Amps for CT’s and 110 volts for VT’s) • To insulate the relays, metering and instruments from the primary high voltage system. • To provide possibilities of standardising the relays and instruments etc. to a few rated currents and voltages.

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Theory of operation • Follows the basic Transformer principle to convert voltage on primary to an appropriate value on secondary through a common magnetic core. • Voltage Transformers - Connected across the open circuit ends of the point of measurement • Current Transformers - Connected in Series to carry the full rated / short circuit current of the circuit under measurement

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Voltage transformers Two types: Electromagnetic type (VT) and Capacitor type (CVT) 1. Electromagnetic Type Voltage Transformer

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Voltage transformers 2. Capacitor voltage transformer (CVT)

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Application of voltage transformers • Voltage transformers are used to supply inputs to protection relays as well as for Power Measurement and Energy Measurement. • CVT’s are used at higher voltages (275 kV and 400 kV) and also allow injection of communications channels onto the O/H line for protection relaying and inter-tripping

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Measuring voltage transformer error limits Accuracy Class

0.8 - 1.2 x rated voltage 0.25 - 1.0 x rated burden at 0.8pf Voltage Ratio Error (%)

Phase Displacement (min)

0.1

± .1

±5

0.2

± .2

± 10

0.5

± .5

± 20

1

±1

± 40

3

±3

Not Specified

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Additional limits for protection voltage transformers Accuracy Class

0.25 - 1.0 x rated burden at 0.8pf 0.05 – Vf x rated primary voltage Voltage Ratio Error (%)

Phase Displacement (min)

3P

±3

± 120

6P

±6

± 240

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Voltage transformers: Permissible duration of maximum voltage Voltage Factor Time Duration Primary winding connection/system earthing (Vf) conditions 1.2

continuous

Between lines in any network. Between transformer star point and earth in any network

1.2

continuous

1.5

30 sec

Between line and earth in an effectively earthed network

1.2

Continuous

1.9

30 sec

1.2

Continuous

1.9

8 hours

Between line and earth in a non-effectively earthed neutral system with automatic earth fault tripping Between line and earth in an isolated neutral system without automatic earth fault tripping, or in a resonant earthed system without automatic earth fault tripping

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Construction The construction of a voltage transformer takes into account the following factors: • output – seldom more than 200-300VA. Cooling is rarely a problem • insulation – designed for the system impulse voltage level. Insulation volume is often larger than the winding volume • mechanical design – not usually necessary to withstand short-circuit currents. Must be small to fit the space available within switchgear WALMET TECHNOLOGIES (Pty) Ltd

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Three-phase units are common up to 36kV but for higher voltages single-phase units are usual. • Voltage transformers for medium voltage circuits will have dry type insulation, but for high and extra high voltage systems, oil immersed units are general. • Resin encapsulated designs are in use on systems up to 33kV.

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Connection of VTs • VT’s can be connected between phases or between phase and neutral (CVT’s only phase - ground)

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Residual voltage connection • Normally used for ground fault detection • VT’s are connected in opendelta on primary and the vectorial sum of the 3 phase voltages will appear across the secondary output • When there is a ground fault present, the residual voltage will be non-zero WALMET TECHNOLOGIES (Pty) Ltd

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Current transformer types 1. Wound Primary

Primary

Secondary

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Current transformer types 2. Bar Primary

Sec ondary

Primary

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Current transformers • As with all transformers - Ampere - turns balance must be achieved • E.g. 1000Amps x 1 turn (bar primary) = 1 Amp x 1000 turns (secondary side) • Error introduced into measurement by magnetising current (Refer Vector diagram)

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Measuring CT Error Classes Accuracy Class

± Percentage Current (Ratio) Error

± Phase Displacement (minutes)

5

20

100

120

5

20

100

120

0.1

0.4

0.2

0.1

0.1

15

8

5

5

0.2

0.75

0.35

0.2

0.2

30

15

10

10

0.5

1.5

0.75

0.5

0.5

90

45

30

30

1

3.0

1.5

1.0

1.0

180

90

60

60

% current

(a) Limits of error accuracy for error classes 0.1 - 1.0 Accuracy Class

± Percentage Current (Ratio) Error % current

50

120

3

3

3

5

5

5

(b) Limits of error for error classes 3 and 5

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Protection CT Error Limits for Classes 5P and 10P Class

Current Error at Rated Primary Current (%)

Phase Displacement at Rated Current (minutes)

Composite Error at Rated Accuracy Limit Primary Current (%)

5P

±1

± 60

5

10P

±3

10

Standard accuracy limit factors are 5, 10, 15, 20, and 30 Composite Error This is defined in IEC 60044-1 as the r.m.s. value of the difference between the ideal secondary current and the actual secondary current. It includes current and phase errors and the effects of harmonics in the exciting current.

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CT magnetization curve VK = Knee Point V (volts)

10%

Saturated Region IE

50%

Unsaturated Region

V

Initial Region IE (amps)

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CT Mag - curve characteristics • Toe-point is non-linear region at bottom of B-H curve where magnetisation current is still a significant proportion of total current •

Knee-point - is point at which a 10% increase in voltage requires a 50% increase in excitation current

•

Linear region is from toe point to knee-point - we must ensure that the CT operates mainly in this region by selecting an appropriate CT ratio

•

Metering CT’s can saturate quickly

•

Protection CT’s shall not saturate quickly WALMET TECHNOLOGIES (Pty) Ltd

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Safety when working with CTs • A CT should never be open circuited when current is flowing in the primary winding. • Dangerously high voltages may appear in the secondary circuit!

• A CT may be shorted out on the secondary at any time • This is what the test blocks do when pulled on a panel

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Specifying CTs • A CT must develop enough VA to be able to drive the current through the external circuit - this is specified as the VA rating at a rated current • It must be accurate for the purpose intended - Accuracy class • The limit of it’s accuracy must be stated (the ALF) E.g. A 15 VA 10P10 CT is a protection class CT that will produce 15 VA, have a composite error of 10 % at rated primary current and maintain this accuracy to 10 x rated primary current (ALF) (at the chosen ratio)

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Class ‘X’ CTs • A class X CT is a special class of CT used for differential protection such as busbar protection , Differential protection of a transformer, etc. • These CT’s must be matched as closely as possible to ensure very little differences in output either in phase or magnitude to ensure speed and accuracy of the protection relays • The magnetisations curves shall be closely matched

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Relays

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Relay types • Electromechanical Type • Static Type • Analog • Digital • Electromechanical type dominated market earlier. • Analogue type received with mixed response and in today’s installations digital relays find more importance due to technology advancement • Electromechanical relay characteristics are followed in Static Relays WALMET TECHNOLOGIES (Pty) Ltd

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Principle of IDMTL relay

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Operation of IDMTL relay • • • • •

Speed is proportional to braking torque. Speed is proportional to driving torque. Speed is proportional to I2. Speed = Distance / Time. Time = Distance / Speed = 1/I2.

• This gives an inverse characteristic. (the higher the current the shorter the rotating time)

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Characteristic of IDMTL relay

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Current setting IDMT relay • Plug setting: This adjusts the setting current by means of a plug bridge which varies the number of turns on the upper magnet • This setting determines the level of current at which the relay will start or pick-up • BS142 says - relay must definitely operate at 130% setting and definitely reset at 70% setting • Normally the relay picks up at about 105% - 130% of it’s plug setting

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Time multiplier setting • TM setting : This rotates the tripping bar attached to the disc closer to or further away from the tripping contacts • effectively moves the curve DOWN the axis • this curve shows the relay will operate in 3 seconds at 10 times the plug setting (with the time multiplier =1) WALMET TECHNOLOGIES (Pty) Ltd

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Different curves • The most common type used - NORMAL INVERSE CURVE. • Characteristic shows a 3 second operation at 10 times the current plug setting ie if the plug bridge is set at 1 A and when 10 A flows through, the relay will close its contacts after 3 seconds - sometimes called a 3/10 relay • Other characteristic curves are also available: • Very Inverse • Extremely Inverse • Refer Manual for the different curves.

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IEC 60255 inverse curves • t = k x β /( (I/I>)α - 1) where: • t = operate time in secs. • K = time multiplier • I = measured current • I> = set starting currentI • α & β are constants for curve selection see next slide and manual

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Constants for different characteristic curves α

β

• Normal • Very • Extreme

0.02 1.00 2.0

0.14 13.5 80.0

• Long Time

1.0

120.0

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Example • Calculate the plug setting and time multiplier setting for an IDMTL relay on the following network so that it will trip in 2.4 seconds.

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Answer • • • •

Fault Current = 1000 A CT Ratio = 100/5 Hence current into relay = 1000 x 5 /100 = 50A Choose plug setting of 5 A (100%)

• Therefore, current into relay as a multiple of plug setting = 50/5 = 10 times USING THE GRAPHS: • Referring to curves on the next page, read off Time Multiplier setting where 10 times and 2.4 seconds cross …namely 0.8. • Relay setting = Plug Tap 5 A (100%) = Time Multiplier 0.8 WALMET TECHNOLOGIES (Pty) Ltd

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• This technique is fine if the required setting falls exactly on the TM curve. OR: • Go to the multiple of plug setting current and read off the seconds value corresponding to the 1.0 Time Multiplier curve. Then divide the desired time setting by this figure. This will give the exact Time Multiplier setting: • Seconds figure at 10 times =3 • Desired Settting = 2.4 • Therefore Time Multiplier = 2.4/3 = 0.8

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USING STANDARD FORMULA: • t = k x β /( (I/I>)α - 1) • 2.4 = k x 0.14 / (100.02 – 1) • K = 0.8

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Factors influencing choice of plug setting • Load Conditions -- must not trip for healthy conditions e.g., starting currents • Load current redistribution after Tripping • Fault currents: can be high enough to saturate CT’s • Ratio choice: CT performance, mag curve • Internal Resistance: relay burden • Accuracy: Better at top of curve

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Relay burden • Burden is the load imposed on the CT’s by the relay expressed in VA • Electromechanical relays -- this is normally 3 VA • Selection of the plug setting increases or decreases this burden as more or fewer windings are inserted into the circuit • As there is a minimum Ampere-turns required to produce the flux to start the disc turning - The lower the current the more turns are needed • The lower the setting therefore - the higher the burden (more turns) WALMET TECHNOLOGIES (Pty) Ltd

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Relay burden • eg. VA = I2R , hence R = VA/I2

For 5 Amp relay on 200% PS; R = 3/(10x10) = 0.03 Ω For 5 Amp relay on 10 % PS;

R = 3/(0,5x0,5) = 12 Ω

• Misconception !!!! (Electro-Mechanical Relays) The lower tap does not necessarily increase sensitivity as low ratio CT’s cannot always drive enough current through the burden imposed on a CT with a low PS WALMET TECHNOLOGIES (Pty) Ltd

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Relay burdens • Refer Graph for the burden values of Electromechanical relays against Static Relay

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Technical features of digital relay 1 • Low burden on CTs • Accuracy of settings of 1% - operating accuracy’s of 3% possible (EM relays - 7,5% best) • Repeatability (not affected by harmonics) • Negligible overshoot (allowing faster reset and no integration effect for networks with fast ARC) • Closer grading intervals possible - use 0,2 sec intervals for grading • Starting characteristics for digital relays is more accurate - can be used to initiate busbar protection WALMET TECHNOLOGIES (Pty) Ltd

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Technical features of digital relay 2 • More accurate pickup - can be used to initiate busbar protection • Adaptive - Multiple settings in a single relay - more than one set of settings can be stored in memory, when the network configuration changes the relay can be automatically configured to change settings • Integrated high-set overcurrent is more accurate than EM relays

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Overcurrent Protection • Protection against excess current was naturally the earliest protection system to evolve. From this basic principle, the graded overcurrent system, a discriminative fault protection, has been developed. This should not be confused with ‘overload’ protection, which normally makes use of relays that operate in a time related in some degree to the thermal capability of the plant to be protected. Overcurrent protection, on the other hand, is directed entirely to the clearance of faults, although with the settings usually adopted some measure of overload protection may be obtained. WALMET TECHNOLOGIES (Pty) Ltd

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CO-ORDINATION PROCEDURE • Correct overcurrent relay application requires knowledge of the fault current that can flow in each part of the network. • The data required for a relay setting study are: – a one-line diagram of the power system involved, showing the type and rating of the protection devices and their associated current transformers – the impedances in ohms, per cent or per unit, of all power transformers, rotating machine and feeder circuits – the maximum and minimum values of short circuit currents that are expected to flow through each protection device WALMET TECHNOLOGIES (Pty) Ltd

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– the maximum load current through protection devices – the starting current requirements of motors and the starting and locked rotor/stalling times of induction motors – the transformer inrush, thermal withstand and damage characteristics – decrement curves showing the rate of decay of the fault current supplied by the generators – performance curves of the current transformers

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The basic rules for correct relay co-ordination can generally be stated as follows: – whenever possible, use relays with the same operating characteristic in series with each other – make sure that the relay farthest from the source has current settings equal to or less than the relays behind it, that is, that the primary current required to operate the relay in front is always equal to or less than the primary current required to operate the relay behind it. WALMET TECHNOLOGIES (Pty) Ltd

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Definite Time Philosophy (Discrimination by Time) – Coordinate with a definite time of operation between successive relays

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Discrimination by Current • Discrimination by current relies on the fact that the fault current varies with the position of the fault because of the difference in impedance values between the source and the fault. • Hence, typically, the relays controlling the various circuit breakers are set to operate at suitably tapered values of current such that only the relay nearest to the fault trips its breaker.

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Discrimination by both Time and Current • Each of the two methods described so far has a fundamental disadvantage. In the case of discrimination by time alone, the disadvantage is due to the fact that the more severe faults are cleared in the longest operating time. On the other hand, discrimination by current can be applied only where there is appreciable impedance between the two circuit breakers concerned. WALMET TECHNOLOGIES (Pty) Ltd

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Inverse definite minimum time • Inverse Definite Minimum Time – Use Relays with following Characteristics

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Network application - overcurrent grading

Electro mechanical Relays Grading: 0.4 secs Interval

1.6

1.2

0.8

0.4

0

?

0.3 secs Interval

1.6

1.3

1.0

0.7

0.4

0.1

0.3 secs Interval

0.75

0.45

0.15

?

??

???

We can run out of grading intervals with Electromechanical relays.

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RELAY TIME GRADING MARGIN • The time interval that must be allowed between the operation of two adjacent relays in order to achieve correct discrimination between them is called the grading margin. If a grading margin is not provided, or is insufficient, more than one relay will operate for a fault, leading to difficulties in determining the location of the fault and unnecessary loss of supply to some consumers. • The grading margin depends on a number of factors: – – – – –

the fault current interrupting time of the circuit breaker relay timing errors the overshoot time of the relay CT errors final margin on completion of operation WALMET TECHNOLOGIES (Pty) Ltd

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Typical relay timing errors - standard IDMT relays Relay Technology Electro-Mechanical

Static

Digital

Numeric

Typical basic timing error (%)

7.5

5

5

5

Overshoot time (s)

0.05

0.03

0.02

0.02

Safety margin (s)

0.1

0.05

0.03

0.03

Typical overall grading margin - relay to relay(s)

0.4

0.35

0.3

0.3

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IDMT normal inverse curve not for overload

A = Light Overload: Relay trips before transformer cooks. B = Heavy overload. Transformer cooks before relay trips ! WALMET TECHNOLOGIES (Pty) Ltd

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Curves must not cross A

B

Problem - on high fault currents B will trip first

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Ideal co-ordination of setting curves Two Basic Rules! • Pick up for lowest fault level • Must coordinate for highest fault level

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Grading: Fuse to Relay • For grading inverse time relays with fuses, the basic approach is to ensure whenever possible that the relay backs up the fuse and not vice versa. If the fuse is upstream of the relay, it is very difficult to maintain correct discrimination at high values of fault current because of the fast operation of the fuse. • The relay characteristic best suited for this coordination with fuses is normally the extremely inverse (EI) characteristic as it follows a similar I2t characteristic. To ensure satisfactory co-ordination between relay and fuse, the primary current setting of the relay should be approximately three times the current rating of the fuse. WALMET TECHNOLOGIES (Pty) Ltd

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Grading: Fuse to Fuse • The operating time of a fuse is a function of both the pre-arcing and arcing time of the fusing element, which follows an I2t law. So, to achieve proper co-ordination between two fuses in series, it is necessary to ensure that the total I2t taken by the smaller fuse is not greater than the pre-arcing I2t value of the larger fuse. It has been established by tests that satisfactory grading between the two fuses will generally be achieved if the current rating ratio between them is greater than two WALMET TECHNOLOGIES (Pty) Ltd

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Effective Setting of Earth-Fault Relays • The primary setting of an overcurrent relay can usually be taken as the relay setting multiplied by the CT ratio. • The CT can be assumed to maintain a sufficiently accurate ratio so that, expressed as a percentage of rated current, the primary setting will be directly proportional to the relay setting. However, this may not be true for an earth-fault relay. The performance varies according to the relay technology used. WALMET TECHNOLOGIES (Pty) Ltd

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• When static, digital or numerical relays are used the relatively low value and limited variation of the relay burden over the relay setting range results in the above statement holding true. • When using an electromechanical relay, the earth-fault element generally will be similar to the phase elements. It will have a similar VA consumption at setting, but will impose a far higher burden at nominal or rated current, because of its lower setting. For example, a relay with a setting of 20% will have an impedance of 25 times that of a similar element with a setting of 100%. Very frequently, this burden will exceed the rated burden of the current transformers. WALMET TECHNOLOGIES (Pty) Ltd

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Electromechanical relay setting

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Motor ground fault protection

Screen to ground connector!!

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Unit Protection

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Protective relay systems • Basic function of protection is to detect faults and to clear them as soon as possible. • Minimum number of items of equipment should be disconnected. – Called SELECTIVITY.

• Speed and Selectivity are the most desirable features of Protection • But cost also decides the selection

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Various forms of unit protection • Need for dividing into zones and providing unit protection

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Features of protection • Backup protection – Refer to the earlier picture and the importance of backup protection can be understood. • Selectivity – As already indicated cost decides the extent of selectivity – Differential protection provides better selectivity based on direction flow of normal and fault currents

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Differential protection • Compares currents flowing into and leaving a protected zone • Operates when a set value of differential (difference) currents/voltages is reached • Analog is a balancing beam • Two types viz., balanced current and balanced voltage

Iin

Iout

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Balanced circulating current • Compares currents flowing into and leaving a protected zone • Use Two sets of CTs at two ends with relay in between • Require Matching CT’s at both the ends

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Balanced circulating current • External Faults - Stable • Internal Faults Operates

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Balanced voltage system In this arrangement the CT’s are connected in opposition to each other

Normal/External Fault Conditions -- Voltages produced by secondary are equal and opposite --no current flows -stable on through faults WALMET TECHNOLOGIES (Pty) Ltd

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Balanced voltage system

On internal faults currents circulate - relays operate

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Applications • Circulating current systems generally used for Generator, Transformer and switchgear protection - CT’s are situated in same sub-station with common relay

• Balanced Voltage Systems are used on feeder systems where CT’s are away from one another with independent relays at both ends

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Introduction of bias • Lower spill current magnitudes affect sensitivity • Used in circulating current systems where restraining winding is introduced to carry the fault current while the operating coil carries the differential current • Ensures stability under both external and internal fault conditions

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Machine differential • Balanced circulating current systems generally used • A bias is introduced as all CT’s are not absolutely identical - this is to ensure stability • Normal sensitivity is 10% ie. When the differential currents are 10% different - the relay will operate • WITHOUT Bias the relay would be susceptible to through faults: e.g. For a through fault current of 10 X full load, the relay would operate if the “spill’ current exceeds 10% of full load or 1% of the through fault current WALMET TECHNOLOGIES (Pty) Ltd

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Transformer differential • Balanced circulating current systems generally used • Dependant on the vector group of the transformer (e.g. Ynd1) - the CT’s can be connected in Star or Delta: • this is to correct for the phase shift through the transformer and to ensure secondary currents through the relays are in phase.

• prevents the relay from operating incorrectly for an external ground fault on the Star side of the transformer

• Bias is necessary as generally the transformer has a tap-changer e.g. A 132/40kV transformer having tap range of +15% - 5% on MV side WALMET TECHNOLOGIES (Pty) Ltd

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Switchgear differential • Sometimes referred to as Bus Zone protection

• Vectorially adds all currents incoming and subtracts all currents outgoing (from the busbar)

• Different schemes are high impedance or low impedance busbar protection

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Feeder pilot wire protection • Copper pilot wire provides communication of measured current to each end as circulating current will not work - each relay decides if the local breaker must trip • New schemes use fiber-optic communications between two ends of feeder • True unit protection for feeders • Digital relays measure currents and send encrypted information to each end through fiber-optic link • Information is compared and trip initiated when fault detected WALMET TECHNOLOGIES (Pty) Ltd

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Recommended unit protection systems • Cable Feeders – Pilot wire differential (and the newer Fibre Optic current differential)

• Transformers – – – – –

Differential HV Restricted ground fault HV High set Instantaneous Overcurrent (Low Transient overreach) LV Restricted ground fault Gas detection (Buchholz relay/Pressure sensing switch)

• Busbars – Medium/Low impedance schemes for strategic busbars. – Busbar blocking schemes for Radial Networks

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Recommended unit protection schemes • Unit Protection – Should be used where possible throughout the network to remove the Inverse Time Relays (IDMT) from the front line. • IDMT – Must be retained as back-up only to cover for a failure of the main protection.

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Advantages of unit protection • Fast and Selective – Unit Protection is fast and selective and will only trip the faulty item of plant, thereby ensuring the elimination of any network disruptions. • Easy to Set – Unit protection is easy to set (?) and once installed very rarely requires changing as it is independent of whatever happens elsewhere on the system. • No time constraints – Time constraints imposed by the supply authorities do not become a major problem anymore. They only need consideration when setting up the back-up inverse time relays (IDMT) WALMET TECHNOLOGIES (Pty) Ltd

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Advantages of unit protection • Maximum Operating Flexibility – The system can be operated in any switching configuration without fear of a loss of Discrimination. • Better Continuity of Supply – In many applications rings can be run closed so that switching would not be necessary to restore loads resulting in better continuity of supply. • Future expansion relatively easy – Any future expansion that may require another in-feed point can be handled with relative ease without any change to the existing protection.

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Transformer Protection

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Transformer Protection • Transformer faults are generally classified into six categories: – winding and terminal faults – core faults – tank and transformer accessory faults – on–load tap changer faults – abnormal operating conditions – sustained or uncleared external faults WALMET TECHNOLOGIES (Pty) Ltd

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Transformer faults/protection Fault Type

Protection Used

Primary winding Phase-phase fault

Differential; Overcurrent

Primary winding Phase-earth fault

Differential; Overcurrent

Secondary winding Phase-phase fault

Differential

Secondary winding Phase-earth fault

Differential; Restricted Earth Fault

Interturn Fault

Differential, Buchholz

Core Fault

Differential, Buchholz

Tank Fault

Differential, Buchholz; Tank-Earth

Overfluxing

Overfluxing

Overheating

Thermal

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Transformer differential protection • Current balance or circulating current scheme

• Internal and External Faults

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Basic Considerations for Transformer Differential Protection In applying the principles of differential protection to transformers, a variety of considerations have to be taken into account. These include: – correction for possible phase shift across the transformer windings (phase correction) – the effects of the variety of earthing and winding arrangements (filtering of zero sequence currents) – correction for possible unbalance of signals from current transformers on either side of the windings (ratio correction) – the effect of magnetising inrush during initial energisation – the possible occurrence of overfluxing

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Two winding transformer diff. Solutions • Connect HV and LV CTs in Star delta opposite to the vector group connections of the primary windings • Relay bias setting will overcome CT mismatch and OLTC effects • 2nd harmonic filters stabilize for inrush magnetizing currents • Delta connection of secondary CT’s provides a zero sequence current path or use a zero sequence shunt

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Delta-Star transformer diff. connections

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Restricted ground fault • A simple overcurrent and ground fault relay will not provide adequate protection for winding ground faults. • Need some ground fault protection. • Degree of ground fault protection is very much improved by the application of unit differential or restricted ground fault systems.

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A restricted ground fault system

CT currents balance - no operating voltage to relay

Any residual current will cause relay to operate for E/F in zone

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Restricted ground fault protection • On the HV side, the residual current of the 3 line CT’s is balanced against the output current of the CT in the neutral conductor. • For the LV side, ground faults occurring on the delta winding may also result in a level of fault current of less than full load. HV overcurrent relays will not provide adequate protection. • A relay connected to monitor residual current will provide restricted ground fault protection since the delta winding cannot supply zero-sequence current to the system.

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Restricted ground fault protection • Both windings of the transformer can thus be protected separately with restricted ground fault protection. • Provides high speed protection against ground faults over the whole of the transformer windings. • Relay used is an instantaneous high impedance type.

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Buchholz protection • Failure of the winding insulation will result in some form of arcing which can decompose the oil into Hydrogen, acetylene and methane. • Localized heating can precipitate a breakdown in the oil into gas. • Severe arcing will cause a rapid release of a large volume of gas as well as oil vapor. The action can be so violent that the build-up of pressure can cause an oil surge from the tank to the conservator. • Buchholz relay can detect both gas and oil surges as it is mounted in the pipe to the conservator. WALMET TECHNOLOGIES (Pty) Ltd

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Buchholz protection

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Buchholz relay

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• The device will therefore give an alarm for the following fault conditions, all of which are of a low order of urgency. – hot spots on the core due to short circuit of lamination insulation – core bolt insulation failure – faulty joints – interturn faults or other winding faults involving only lower power infeeds – loss of oil due to leakage

• When a major winding fault occurs, this causes a surge of oil, which displaces the lower float and thus causes isolation of the transformer. This action will take place for: – all severe winding faults, either to earth or interphase – loss of oil if allowed to continue to a dangerous degree WALMET TECHNOLOGIES (Pty) Ltd

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Transformer overloading • Sustained overloading reduces transformer life • Operating Temperatures also decide the transformer oil life • Operating Temperature Oil Life • • • • • •

60 deg C 70 deg C 80 deg C 90 deg C 100 deg C 110 deg C

20 years 10 years 5 years 2.5 years 13 months 7 months

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Transformer overload protection • Alarm and Trip settings for oil and winding temperature MUST for transformer protection • Recommended temperature settings:(Unless otherwise recommended by the manufacturer). – Winding temperature alarm : 1100C – Winding temperature trip : 1200C – Oil Temperature Alarm : 950C – Oil Temperature Trip : 1050C

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Need for oil testing and maintenance • Main purpose of oil - Cooling, insulation and preservation of cores and assembly • Oil Sample Testing to be done on annual basis • Parameters to be tested • Typical oil test Report • Gas Analysis from Buchholz relay • Interpretation of Test Results

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Protection of transformers • Transformers are expensive and important. • IDMTL relays are not for overload. • Recommended protection – – – – – – –

HV and LV restricted ground fault. HV high set instantaneous overcurrent I>> Buchholz gas and surge relay. Oil and winding temperature. LV overcurrent and ground fault I> and Io Differential protection (optional) Oil Analysis and Dissolved Gas Analysis

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Switchgear Protection

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Busbar Protection • Busbars have often been left without specific protection, for one or more of the following reasons: – the busbars and switchgear have a high degree of reliability, to the point of being regarded as intrinsically safe – it was feared that accidental operation of busbar protection might cause widespread dislocation of the power system, which, if not quickly cleared, would cause more loss than would the very infrequent actual bus faults – it was hoped that system protection or back-up protection would provide sufficient bus protection if needed

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Requirements Speed • Busbar protection is primarily concerned with: – limitation of consequential damage – removal of busbar faults in less time than could be achieved by back-up line protection, with the object of maintaining system stability

The basis of most modern schemes is a differential system using either low impedance biased or high impedance unbiased relays capable of operating in a time of the order of one cycle at a very moderate multiple of fault setting. To this must be added the operating time of the tripping relays, but an overall tripping time of less than two cycles can be achieved. With high-speed circuit breakers, complete fault clearance may be obtained in approximately 0.1 seconds.

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Stability • Notwithstanding the complete stability of a correctly applied protection system, dangers exist in practice for a number of reasons. These are: – interruption of the secondary circuit of a current transformer will produce an unbalance, which might cause tripping on load depending on the relative values of circuit load and effective setting. It would certainly do so during a through fault, producing substantial fault current in the circuit in question – a mechanical shock of sufficient severity may cause operation, although the likelihood of this occurring with modern numerical schemes is reduced – accidental interference with the relay, arising from a mistake during maintenance testing, may lead to operation

In order to maintain the high order of integrity needed for busbar protection, it is an almost invariable practice to make tripping depend on two independent measurements of fault quantities. WALMET TECHNOLOGIES (Pty) Ltd

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Busbar differential protection • Sectionalizing busbars into different zones

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Busbar protection high impedance bus zone advantages • • • •

Relays relatively cheap - offset by expensive CT’s. (class X) Simple and well proven Fast - 15 to 45 milliseconds Stability and sensitivity calculations -easy providing data is available. • Stability can be guaranteed.

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Single line diagram high impedance busbar protection

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Busbar protection high impedance bus zone disadvantages

• Very dependent on CT performance. • CT Saturation – False Tripping on through faults – Sensitivity must be decreased.

• DC Offset on CTs unequal - use filters • Expensive class X CT’s. – Same Ratio – Vknp = 2 times relay setting.

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Busbar protection high impedance bus zone disadvantages • Primary Effective Setting (30..50%) – Limited by number of circuits – Z grounded system difficult for ground faults

• Duplicate systems - decreased reliability • Require exact CT Data – Vknp, Rsec, Imag, Vsetting

• High Voltages in CT Circuits (+-2.8kV) – Limited by volt dependent resistors (Metrosils) WALMET TECHNOLOGIES (Pty) Ltd

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Motor Protection

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Motor Protection The design of a modern motor protection relay must be adequate to cater for the protection needs of any one of the vast range of motor designs in service, many of the designs having no permissible allowance for overloads. A relay offering comprehensive protection will have the following set of features: – – – – – – – – – – – – – –

thermal protection extended start protection stalling protection number of starts limitation short circuit protection earth fault protection winding RTD measurement/trip negative sequence current detection undervoltage protection loss-of-load protection out-of-step protection loss of supply protection auxiliary supply supervision (items k and l apply to synchronous motors only)

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Main causes for motor damage

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Equivalent circuit of squirrel cage

• Heating = I2 (Rs+Rr) • These are Copper Losses • Small mass - heat up rapidly - self destruct

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Motor temperature rise - multiple starts Restart 1

Restart 2

Restart 3

Class B max

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Steady state temperature (with time constants)

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Motor current during start

When considering overload protection: Common incorrect assumption: Staring current decreases linearly with time Truth: starting current lasts up to 90% of the total start period

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Relay providing protection

Relay will trip and protect motor

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Relay not giving stall protection Tr>Ts Relay will not trip

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Voltage unbalance - positive, negative and zero phase sequence

+ve

-ve

zero

• Negative sequence currents tend to apply a braking torque to the rotor - apply negative phase sequence protection

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• The effect of negative sequence voltage unbalance results in higher copper losses causing over-heating • eg. 10% unbalance in a machine that has starting current of 8x full load current will be derated to 53%

Max. continuous output %

Maximum continuous output

Voltage unbalance %

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Ph - ground and Ph to Ph faults

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Phase to phase faults • Normally there is a great amount of insulation between phase windings • Phase to phase faults nearly always develop quickly into ground faults • Generally diff protection is only employed on very large motors > 2 MW as Inst. E/F is usually quick enough

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Summary • Bi-metal Thermal Overload is an effective, economical solution for small to medium sized motors to 22kW • For larger motors OR motors that have severe utilisation requirements under varying operational conditions a digital realy should be used having the following features: • Thermal protection on all 3 phases • Short Circuit Protection • Start up and Stall protection

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Summary • • • • •

Phase unbalance (negative sequence) protection Single phasing protection E/F protection Undercurrent Protection Digital read out of all set values, measured values and peak recoreded values • Self diagnostics • Accuracy • Optimum Philosophy WALMET TECHNOLOGIES (Pty) Ltd

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Generator Protection

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Generator protection • Heart of an electrical power system. • Requires a prime mover to develop this mechanical power • Steam turbines are used by main power utilities. • In industry: – Steam turbines where waste steam available and for base load or standby. – Gas turbines • Peak lopping or mobile applications

– Diesel Engines • Most popular used for standby plant.

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Small and medium-sized units in industry • Small and medium sized generators are normally connected directly to the distribution system.

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Larger generating units • Larger units are connected to EHV grid via a transformer.

22kV

275kV

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Modern large generating unit • Stator winding with associated main and unit transformers (and cooling system). • Rotor with its field winding and exciters (and cooling system). • Turbine with its boiler, condenser, auxiliary fans and pumps. • Typical Power station may have 6 x 600MW units

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Generators - different faults Electrical faults – – – – – – – –

Stator Insulation Failure Overload Overvoltage Unbalanced Loading Rotors faults Loss of excitation Loss of sychronism Reverse Power

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Generators - different faults Mechanical – – – – – – – –

Failure of Prime Mover Low condenser vacuum Lubrication Oil Failure Loss of Boiler firing Overspeeding Rotor distortion Excessive vibration Boiler choking (no ash removal)

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Stator grounding and ground faults • Generator neutral is grounded to enable protection (normally impedance grounded) • Severe arc on the core could damage windings - therefore E/F currents limited to low values • Small generators grounded through a resistor • Power station generators normally grounded through a grounding transformer with a secondary resistor • Relay can be applied to measure transformer secondary current or measuring voltage across resistor WALMET TECHNOLOGIES (Pty) Ltd

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Generator ground fault protection • Current measurement

Voltage measurement

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Overload protection •

Generators very rarely get overloaded

• Normally there is either a thermocouple measurement of the internal stator winding temperature(normally hydrogen cooled at large power stations) or embedded thermistor • The rotor winding is constantly checked by measuring the resistance of the field winding

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Overcurrent protection • IDMT relays are used for overcurrent (short circuit) protection (not for thermal protection) • In power stations with multiple generators line side CT’s are used for the relays

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Overvoltage protection • Can be transient O/V or can be sustained at system frequency • High speed transients are normally protected by surge arrestors on the generators terminals (if suitable) • Power frequency overvoltages are usually as a result of a failure of: – AVR failure – Manual error (operator) – Loss of load eg during load shedding)

• O/V protection is normally employed at unattended machines e.g. At a Hydro station

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Generator - unbalance protection • Unbalanced loads create negative torque on the generator as the generator has to generate negative sequence currents for the load demanding them • This in turn leads to overheating of the Rotor as double frequency (100Hz) currents flow on the field system and body • The eddy currents caused can cause heating of the slot wedges - softening them, they can then protrude into the airgap and cause damage • Install Negative Phase Sequence Protection

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Rotor faults protection • Rotor is fed with DC to create a magnetic N-S pole to generate power in the stator • This DC is normally not grounded - (floating) - if a fault existed one would not be aware of this as there is no path for the current to flow. • The danger exists if a second fault occurred - shorting out part of the rotor winding - damaging it • This may cause vibration in the rotor due to unbalanced magnetic forces damaging the machine • Use Rotor ground fault protection…..as follows….

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Rotor faults protection 1

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Rotor faults protection 2

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Rotor faults protection 3

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Reverse power • Reverse Power Protection is applicable where generators run in parallel with other sets or with the grid to protect against turbine failure

• If the prime mover fails the generator will become a motor and draw power from the busbars - this can cause islanding of the power station

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Field failure (loss of excitation) • If the rotor field system had to fail the machine would continue to rotate as an induction generator • This would mean it would continue operating at a slip frequency • Heating will occur but no major damage will happen immediately • On power stations this is detected with a type of impedance relay

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Out of synchronism (loss of sync.) • A severe system disturbance could cause a generator to lose synchronism (especially in lightly loaded systems or loosely tied systems e.g. Matimba) • This can cause oscillation or a pole slip • This can sometimes cause a field trip if severe • The machine would then be re-synchronized automatically

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Field suppression • If a generator develops a severe fault - say a phase to phase fault on the terminals, the field should be tripped immediately to limit the amount of damage as the generator feeds it’s own fault. • Because the field is DC fed a large arc would be drawn as the field tries to sustain itself and generates a large back emf. - an automatic air circuit breaker with blow-out contacts will extinguish this arc. • On large gen-sets > 5MVA a field discharge resistor is used.

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Typical protection scheme for industrial generator

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THANK YOU

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