gen.protections

7/28/2019 Gen.protections

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GENERATOR

Generating units are the source of the power

system and their security against anyadverse conditions is most important in the

system.

The GENERATOR protection must ensure afast and selective detection of any fault in

order to minimize their dangerous effects.


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Generator is accompanied by excitation system,

prime mover, voltage regulator, cooling system

etc. Hence it is not a single equipment. Theprotection of generator should be co-ordinated

with associated equipment.

Protection of passive elements like transmissionline and transformers is relatively simple which

involves isolation of faulty element from the

system , whereas protection of generators

involves tripping of generator field breaker,

generator breaker and turbine.


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GENERATOR PROTECTION SCHEME


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GENERATOR AND GENERATOR

TRANSFORMER AS A UNIT

Station Transformer: 11/6.6 kv transformer

with LV grounded through high resistance,

feeds the station auxiliary load.

Unit auxiliary transformer: 11/6.6 kv

transformer with LV grounded through high

resistance feeds the unit auxiliary loads


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Any earth fault on the 6.6 kv system cannot beseen by any E/L relay (since the 6.6 kv system

is high resistance grounded ) However 3-O/Lrelays are provided on the 6.6kv side of thestation transformers and unit auxiliarytransformers. An open delta voltage of the 6.6

kv bus PT is connected to an over voltage relaywith a very low setting. Any earth fault on the6.6 kv system will cause the presence of theopen-delta voltage and make the voltage relay

operate which is connected to give alarm. Thefaulty 6.6kv feeder can be identified by trippingthe 6.6kv outlets one after the other.


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GENERATOR HAS THREE

PARTS

1. STATOR

2. ROTOR

3. EXCITATION SYSTEM


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FAULTS IN GENERATOR

EXTERNAL FAULTS

INTERNAL FAULTS

FAULTS RELATED SYSTEM


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EXTERNAL FAULTS

OVER LOADING

UNBALANCE LOADING

SHORT CIRCUIT EARTH FAULT


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INTERNAL FAULTS

PHASE TO PHASE FAULTS IN WINDINGS

PHASE TO EARTH FAULTS

INTER TURN FAULTS


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FAULTS RELATED SYSTEM

UNDER VOLTAGE

OVER VOLTAGE

UNDER FREQUENCY OVER FREQUENCY

REVERSE POWER

LOSS OF EXCITATION LOW-FORWARD POWER


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GENERATOR PROTECTIONS ARE BROADLY

CLASSIFIED INTO THREE TYPES

CLASS-A: (UNIT SHUT DOWN ) This covers all

electrical protections for faults within the generating

unit in which GENERATOR FIELD BRK.,

GENERATOR BREAKER, UAT LV BRK. and TURBINE

should be tripped


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CLASS-B: ( INTERLOCKED TRIPPING) This

covers all MECHANICAL protections of the

turbine in which TURBINE will be tripped

first and following this generator will trip

on REVERSE/LOW FORWARD POWER

protection(GT/UAT/FIELD brks.)


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CLASS-C: This covers electrical protection

for faults in the system in which generator

will be unloaded by tripping of generator

breaker only. The unit will come to house

load operation and the UAT will be in

service.


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SOME OF CLASS-C PROTECTIONS ARE

1. Gen. T/F HV side breaker pole discrepancy

2. Gen. Neg. Ph. Sequence

3. Gen. T/F over current

4. Reverse power protection without turbine

trip


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GENERATOR PROTECTIONS

1. Stator E/F (64)

95% stator E/F protection (64G1)

100% stator E/F protection (64G2)

2. Rotor E/F (64)

First rotor E/F protection (64R1)

Second rotor E/F protection (64R2)


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3. Inter turn fault

4. Negative phase sequence (46G)

5. Loss of excitation protection(loss of field) (40G)

6. Minimum Impedance (Mho backup

impedance) (21G)

7. Differential ( 87G)

8. Overall differential (87 O )

9. Power protection

Low forward power (37G)

Reverse power (32G)


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10.Frequency protection ( 8 1 )

Under frequency ( 8 1U)

Over frequency ( 8 1O)

11.Thermal overload (51G)

12.Voltage protection

Over voltage (59)

Under voltage (27)

13.Out of step (pole slipping ) (78G)

14.Voltage restrained over current (51/27G)

15.Standby earth fault (51N)

16.Inadvertent Energization


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1. STATO E/F PROTECTION

The most common practice is the neutral point of thegenerator is usually earthed to enable detection of

earth faults associated with the stator winding andassociated systems and also to limit the transient overvoltages during faults or switching. The commonlyadopted earthing methods for industrial generators.

High impedance Earthing:

In this method the neutral of the generator is earthedthrough the primary winding of a groundingtransformer having a loading resistor across itssecondary. This method of earthing of is called highimpedance earthing. In this arrangement , the primaryearth fault current will be limited to between 3 to 25amps depending upon the size of the machine andphase to ground capacitance of the Stator.


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High impedance grounding is generally usedin case of unit connected Generators where

the generators is connected to the systemthrough a delta/star step up transformerwith control breaker on the HV (star) side.The neutral of the HV star winding is solidly

grounded. A neutral displacement relay ( voltage

operated) connected across secondary of thedistribution transformer. The relay is IDMTtype with typical setting range of 5-20 voltsand is tuned to supply frequency.


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Short circuits between the stator winding in

the slots and the stator core are the most

common electrical fault in Generators. Interturn faults, which normally are difficult to

detect, will quickly develop into a ground

fault and will be tripped by the statorground fault protection.


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95% STATOR E/F PROTECTION

The relay is normally set to operate at 5% of maximumneutral voltage with a time delay of 0.3 0.5 seconds. Withthis voltage setting , it protects approximately 95% of thestator winding.

RESULTS IN VOLTAGE SHIFT OF GEN NEUTRAL W.R.T

GROUND DETECTED BY OVER VOLTAGE RELAY CONNECTED

ACROSS GROUNDING RESISTOR

PROTECT APPROX 95% OF STATOR WDG

It also covers the generator bus, the low voltage winding ofthe unit transformer and the high voltage winding of theunit auxiliary transformer.


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100% STATOR E/F PROTECTION

THIRD HARMONIC PRINCIPLE

For Single line to Ground fault near the neutralend of winding , there will be proportionately lessvoltage available to drive the current through theground, resulting in a lower fault current and lower

neutral bus voltage.If an earth fault occurs and remains undetected

because of its location (near the neutral end).A 100 % Stator earth fault protection is designed

to detect earth faults occurring in the regions ofMachine winding close to the neutral end.


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Principle: This works on the principle involving monitoring of the

neutral side and line side components of the third harmonic

voltages produced by the AC generators.

AC Generators in service produce a certain magnitude of third

harmonic voltages in their windings. Under the healthy conditions of

working the third harmonic voltage developed by the machine is

shared between the phase to ground capacitive impedance at the

machine terminal and the neutral to ground impedance at the

machine neutral.


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The 100% stator E/F relay includes 95%

unit which covers the stator winding from

5% of the neutral and a 3rd harmonicvoltage measuring unit -2 which protects

the rest of the stator winding.

The voltage check unit is included to

prevent faulty operation of the relay at

generator standstill or during the machinerunning up or running down period.


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ROTOR E/F PROTECTION

The field circuit of a generator has nointentional earthing and hence a single earthfault occurring in the field system poses noimmediate threat. However, the presence of

the first ground increases the risk of a secondearth fault developing due to increasedinsulation stresses between the field systemand earth. The resulting double earth fault will

cause part of the filed winding short circuitedthere by producing an unbalance in themagnetic field and consequent vibrations andmechanical damage.


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METHODS OF DETECTION

Potentiometer method

A.C injection method

D.C injection method


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MODERN ROTOR EARTH FAULT PROTECTIONRELAY OPERATES ON THE PRINCIPLE OF LOWFREQUENCY INJECTION INTO THE FIELD

WINDING VIA CAPACITORS.

IF AN EARTH FAULT OCCURS, THECORRESPONDING CURRENT OR RESISTANCE ISDETECTED

In general

ALARM 25 K OHM TIME = 1.0 SEC

TRIP 5 K OHM TIME = 0.5 SEC


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POTENTION METHER METHOD

In this scheme, a centre tapped resistor is

connected in parallel with the main field

winding. The centre point of the resistor is

connected to earth through a voltage relay.An earth fault on the field winding will

produce a voltage across the relay. The

maximum voltage occurring for faults at theends of the winding.


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AC INJECTION METHOD

It comprises of an auxiliary supply transformer, thesecondary of which is connected between earth andone side of the field circuit through an interposedcapacitor and a relay coil.

The field circuit is subjected to an alternatingpotential at the same level through out, so that anearth fault anywhere in the field system will give riseto a current which is detected by the relay. Thecapacitor limits the magnitude of the current and

blocks the normal field voltage, preventing thedischarge of a large direct current through thetransformer.


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D.C Injection Method

The capacitance current objection to the a.c

injection scheme is overcome by rectifying

the injection voltage. The d.c out put of a

transformer rectifier power unit is arrangedto bias the positive side of the field circuit to

a negative voltage relative to earth. The

current is limited by including a highresistance in the circuit and a sensitive relay

is used to detect the current.


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INTER TURN FAULT

PROTECTION

Inter turn faults have commonly been

disregarded on the basis that if they occur

they will quickly develop into earth faults.

This is probably true if the fault is in the slotportion is never attractive and may be

entirely unjustified. There is a possibility of

the machine being very seriously damagedbefore the fault evolves to a condition that

can be detected by the longitudinal system.


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It is generally considered difficult to obtain

a reliable protection against short-circuiting

of one turn if the stator winding has a largenumber of turn per phase.

For generators with split neutrals, the

conventional inter turn fault protectivescheme comprises a time delayed low set

over current relay which senses the current

flowing in the connection between the

neutrals of the stator winding.


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zero sequence voltage measurement

Short circuit of one or more turns will

cause the Generated E M F to containzero Sequence component

Earth faults will also produce a zero

sequence voltage. Most of the voltagewill be expended on Earthing Resistor

Hence, drop across the winding should be

measured


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NEG.PH.SEQUENCE PORTECTION

When the generator is connected to a balanced load,

the phase currents are equal in magnitude anddisplaced electrically by 120 deg. The ampere turnwave produced by the stator currents rotatesynchronously with the rotor and no eddy currentsare induced in the rotor parts.

Unbalance loading gives rise to a negative sequencecomponent in the stator current. The negativesequence current produces an additional ampereturn wave which rotates backwards, hence it moves

relatively to the rotor at twice the synchronousspeed. The double frequency eddy currents inducedin the rotor may cause excessive heating, primarilyin the surface of cylindrical rotors and in the damperwinding of rotors with salient poles.


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LOSS OF EXCITATION

Loss of excitation may occur under normalrunning conditions due to followingreasons:

1. Failure of Brush gear of the pilot or shuntexciter

2. Accidental opening of the field breaker

3. Failure of the regulation system4. An open circuit or a short circuit of the

main field


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The condition is particularly detrimental when

the generator is operating in paralleled with

other generators or with the utility system.When the generator looses its field on load , it

runs as an induction generator, operating at

super synchronous speed. The defaulting

machine absorbs VARs from the system tosupport excitation and continues generating

action. Slip frequency currents are induced in

the field system which, it allowed to persist forlong, cause over heating of the field winding

/rotor iron.


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The maximum active power that can be

generated without loss of synchronism

when the generator losses its excitationdepends on the difference between the

direct axis and quadrature axis synchronous

reactance. For generators with salientpoles, the difference is normally sufficiently

large to keep the machine running

synchronously, even with an active load of

15-25% or rated load.


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For cylindrical turbo generators, the direct

and quardrature axis reactance are

practically equal, and the machine fails outof synchronism even with a very small

active lode, The slip speed increases with

the active load. The stator end regions and

parts of the rotor will be overheated, it the

machine is permitted to run for a long time

at higher slip speeds.


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BACKUP IMPEDANCE

The generator minimum impedance

protection(impedance backup protection) is

primarily provided to protect the generator

against uncleared external short circuits onthe lines emanating from the station bus

bars. The relay has an impedance or offset

MHO characteristic and is et to cover theimpedance of the longest line.

I f t th h th i iti l h t


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In case of generator, though the initial shortcircuit current may be high during the subtransient/transient periods, the steady

state short circuit current may be less thanthe full load current, due to the effect ofarmature reaction. Since the initial short

circuit current is high and almost whollyreactive the armature reaction flux is indirect opposition to the field flux. Thiswould reduce the field flux, which in turn

reduces the induced EMF and hence theshort circuit current. The AVR tends tocompensate the reduction by forcing thefield in case of system faults.

f l f l h


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However, for close up faults, the AVRcompensation may not be enough to boostthe voltage and hence the fault current.Conventional over current relays may not,therefore, be suitable for generatorapplication. Voltage controlled or voltage

restrained over current relays arecommonly adopted in such situations.These relays are designed to become moresensitive with generator voltage reduction

and hence operate positively, even if thesustained short circuit current fails belowthe full load current.


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DIFFERENTIAL PROTECTION

Differential protection is commonly applied

to generators above 1 MVA rating. This is a

unit protection, which covers both phase,

and earth faults within the machine. Thezone of protection is defined by current

transformers at neutral side and line side of

the stator winding.


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Current transformer at each end of the

protected zone are interconnected by an

auxiliary pilot circuit. Current transmittedthrough the zone causes secondary current to

circulate round the pilot circuit without

producing any current in the relay. A fault

within the protected zone will cause secondarycurrents of opposite relative phase as

compared with the through fault condition.

The summated value of these currents will flowin the relay, thus energizes the relay. The

voltage setting is decided from the secondary

load drop by the following formula.


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The differential relay is usually high

impedance relay. The current transformers

on the generator neutral and the line sideshall have identical turns ration and similar

magnetizing characteristics. Hence under

normal service conditions and external

faults, with unsaturated current

transformers, the voltage across the relay is

negligible.


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GEN OVERALL DIFFERENTIAL


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GEN.OVERALL DIFFERENTIAL

PROTECTION

This protection is used to protect the completebus of generator, generator transformer andhigh voltage bus side of unit auxiliarytransformer.

The special features of the relay are:

1. Through current restraint for external faults

2. Magnetizing inrush restraint

3. Over excitation restraint to counteractoperation at abnormal magnetizing currentscaused by high voltage/low frequency

Th ti i t i t i i d t


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The magnetizing restraint is required to

keep the relay stable when a nearby fault on

an adjacent feeder is cleared. During the time of fault, the terminal voltage

of the main transformer is practically zero

and after fault clearance i.e when the circuit

breaker of the faulty feeder opens, the

transformer terminal voltage quickly rises.

This may cause severe recovery inrush

currents. The inrush restraint is alsorequired when the unit transformer is

energized for the H.V bus.

The o er e citation restraint is important


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The over excitation restraint is important

since there is a possibility of over voltage

when load is suddenly disconnected inwhich the differential relay may trip the

generator and the voltage remains high until

the automatic voltage regulator (AVR)

brought it back to the normal valve.


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POWER PROTECTION

1. LOW FORWARD POWER:In thermal machines, when the steam flow

through turbine is interrupted by closing theESVs or the governor valves, the remaining

steam in the turbine generates (low) powerand the machine enters to motoringconditions drawing power from the system.This protection detects low forward power

conditions of the generator and tripsgenerator breaker after a time delay, avoidingmotoring of generator.

The low forward power relay will be


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The low forward power relay will beprovided with TURBINE TRIP interlock inthermal machines. A setting of 0.5% of rated

active power of generator with a time delay of2.0 sec. Shall be adopted.

2. REVERSE POWER:

Reverse power protection shall be used for

all types of generators. When the input to theturbine is interrupted the machine enters intomotoring condition drawing power from thesystem. Reverse power relay protects thegenerators from motoring condition. In

thermal machines, reverse power conditionappears subsequent to low forward powercondition.

F l tti f


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For reverse power relay, a setting of

0.5% of rated active power of generator

with 2 stage timer as1. Stage-1: with turbine trip interlock, a time

delay of 2 sec. shall be adopted.

2. Stage-2: Without turbine trip interlock atime delay of about 20 sec. can be adopted

to avoid unnecessary tripping of unit

during system disturbance causing sudden

rise in frequency or power swing

conditions.


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FREQUENCY PROTECTION

1. UNDER FREQUENCY

2. OVER FREQUENCY

The generators are designed to give

rated output at rated terminal voltage andrated frequency. Hence an operation abovecertain limit i.e +5% and -5% of ratedfrequency is avoided to protect variousapparatus in a network and also thegenerator and turbine.

OVER LAOD PROTECTION


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OVER LAOD PROTECTION A generator operating on a large system under

continuous supervision is not in much danger of

accidental overloading. The power r that can begenerated is limited by the steam production andhence can not rise un noticed or maintained for anyappreciable period above the programmed level.Overloads in terms of current or MVA as distinctfrom megawatts are possible. Depending on thevoltage regulator setting and type of control relativeto the rest of the system, a given generator may takea disproportionate share of the MVAR load on thesystem. Overloads up to 1.4 times the rated currentare not normally detected by the impedance or overcurrent protection. Sustained loads within this rangeare usually supervised by temperature monitors(RTD/or thermocouples)

OVER VOLTAGE PROTECTION


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OVER VOLTAGE PROTECTION

During the starting up of a generator, prior

to synchronization, the generator terminalvoltage is obtained by the proper operationof the automatic voltage regulator (AVR).

After synchronization, the terminal voltageof the machine will be dictated by its ownAVR and also by the voltage level of thesystem and the AVRs of nearby machines. It

is not possible for one machine to cause anyappreciable rise in the terminal voltage aslong as it is connected to the system.

Increasing the field excitation, owing to a fault


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g , gin the AVR, merely increases the reactive MVARoutput, which may ultimately lead to tripping

of the impedance relay or the V/Hz relay.Maximum excitation limit prevents the rotorfield current and the reactive output powerfrom exceeding the design limits.

This protection is used for the insulation levelof the generator stator windings. Severeovervoltage will occur, if the generator circuitbreaker is tripped while the machine is

running at full load and rated power factor, thesubsequent increase in terminal voltage willnormally be limited by a quick acting AVR

However, if the AVR faulty or at this particular


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, y ptime switched over to manual control, over voltagewill occur. This voltage rise will be further increasedif simultaneous over speeding should occur, owing

to a slow acting turbine governor.Modern unit transformers with high magnetic

qualities have a relatively sharp and well definedsaturation level, with a knee point voltage between1.2 and 1.25 times the rated voltage (Un). A suitablesetting of the over voltage relay is, therefore,between 1.15 and 1.2 times Un and with a definitedelay of 1 to 3 sec.

An instantaneous high set voltage rely can be

included to trip the generator quickly in case ofexcessive over voltage following a sudden loss ofload and generator over speeding.

OUT OF STEP (POLE SLIPPING)


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OUT OF STEP (POLE SLIPPING) An generator may lose synchronism with the

power system, without failure of the excitationsystem, because of severe system faultdisturbance operation at a high load with aleading power factor and hence a relatively

weak field, In this condition, which is quitedifferent from the failure of field system, themachine is subject to violent oscillations oftorque, with wide various in current, powerand power factor. Synchronism can be

regained if the load is sufficiently reduced butif this does not occur within a few seconds it isnecessary to isolate the generator and thenresynchronize.

The impedance of the generator measure at the


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The impedance of the generator measure at thestator terminals changes mostly whensynchronism is lost by the machine. The

terminal voltage will begin to decrease and thecurrent to increase, resulting in a decrease ofimpedance and also a change in power factor.A pole slipping protection comprising of two

ohm relays is used to detect out of stepoperation. The relay monitors the loadimpedance at the machine terminals andoperates when the terminal impedance locussequentially crosses both ohm relay

characteristics which corresponds to one poleslip between the defaulting machine and thesystem

VOLTAGE RESTRAINED


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VOLTAGE RESTRAINED

OVER CURRENT

This will operate when the fault currentfrom the generator terminal becomes low

due to excitation system characteristic with

under voltage criteria. It operates as abackup protection for system faults with

suitable time delay.


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STANDBY EARTH FAULT

This relay monitors the current in the

generator transformer neutral. It can detect

earth faults in the transformer HV side or in

the adjacent network.

INADVERTENT ENERGIZATION


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INADVERTENT ENERGIZATION

Many instances of inadvertent energization1. Closing the generator breaker with the

machine at standstill

2. Closing a unit service breaker with themachine at standstill

3. High voltage breaker flashover near

synchronism4. Closing of generator disconnect with unit

breaker closed

F ll lt i ti f hi t


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Full voltage energization of a machine at

standstill does not produce an enormous

magnitude of current, but it does supply anextreme impact of torque and mechanical

damage to the shaft or bearings may occur.

The resulting current is of sufficient

magnitude that fast removal is necessary if

thermal damage to the generator is to be

avoided.

gen.protections

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