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Apptovsllssue Course 23S Module 7 - Generator ProtectionNOTES & REFERENCES

Module 7

GENERATOR PROTECTIONOBJECTIVES:

c) Generator phase unbalance,d) G e n e r a t o r l o s s o f f i e l ~e) Overexcitation.f) Generator underfrequency,g) Generator out of step,h) Excitation rectifier overcurrent.i) Motoring.For each of the schemes listed in 7.2, give an example of a faultrequiring the protection scheme to operate and the consequence tothe generator if the protection scheme failed to operate.

After completing this module you will be able to:Describe Class A, B, C, and D turbine generator protection trips.Explain how each of the following protection schemes could beusedto provide protection of a generatOr:a) Generator differential protection,b) Generator ground fault protection,

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INSTRUCTIONAL TEXT

INTRODUCTIONThis module concentrates on the protection schemes used for the protectionof generators and the consequences of their failure to operate. A briefdiscussion will also be given on the operation of the various types ofprotective relays, but you will not be required to memorize thisinformation.

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Course 23S Module 7 - Generator Protection

As with electrical motor protection, the protection schemes that will bediscussedhave some similarities andoverlap. This is advantageous. sincenotall generators have all of the protection schemes listed in this module. In fact,there are manyprotectionschemes available. onlythemorecommonones arediscussed. here.CLASSES OF TURBINE GENERATOR TRIPSThere are different classes of protecove nips for generators. -each withdifferent actions, depending on the cause and potential for damage. Each ofthe four Classes of nip (A, B, C, &D) are discussed below.Class A trips will completely separate the generator from tbe grid, andsbut down tbe turbine generator (ie. it will nip the turbine and the fieldbreaker). Typical causes could be generator electrical protection, maintransformer electrical protection, groundfaults or,any othercausewhichmaydirectly affect the unit's safe electrical output.Class B trips will disconnect the generator from the grid, but will leavetbe turbine generator supplying the unit load. Typical initiation of thisevent is a grid problem, thus resulting in this loss of load.Class C trips are generator overexcitation trips, and are activated only i fthe generat9I' is not connected to the grid (it may still be supplying the unitloads). Typical causesof this overexcitation are manually applying toomuchexcitation, or applying excitation currentbelow synchronous speed(this willbe discussed later in this module).Class D trips the turbine and then trips the generator after motoring(motoringis discussedin the 234Turbine and-Auxiliaries course). Thecausesof this type ofnip are associatedwith mechanical problems with the turbinegenerator setEach of these trips, along with their causes and exact effects, will bediscussed further in your station specific training.GENERATOR DIFFERENTIAL PROTECTIONDifferential protection, as described in Module 3, can be used to detectinternal faults in the windings of generators, including ground faults,sbort circuits and open circuits. Possible causes of faults are damagedinsulat;9n due to aging, overheating, over-voltage, wet insulation andmecbanical damage.Examplesof theapplicationofdifferential protection are shown inFigure 7.1which considers a generator winding arrangement with multiple windings,two per phase (this type of differential protection is also called split phaseprotection for this reason).

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Generator Tel111lnals

B.- - - - - - - -.

ooooooooooooo

aj HeahhyPhase

ooProtection :Zones ,o,",,,,,,Neutral Connection

To Groundbl F a u ~ e dPhase c) Open Circuit inthe Phase

Figure 7.1: Differential Protection for GeneratorWindings (Split Phase Protection)In Figure 7.1 aJ, the currents in the two windings will be balanced, causingthe ClllTents in the protection circuit to be balanced. Hence in this case, thedifferential relay will not operate.In Figure 7.1 bJ, a ground fault is shown on one of the windings. In this casethe faultcutTent direction is shown, and it will be unbalanced. Thiswill resultin unbalanced secondary currents in the protection circuit, causing thedifferential relay to operate. Similarly, a"short circuit" within awinding willcause the two winding currents to be unmatched, causing the differentialrelay to operate.In Figure 7.1 c), an optn circuit is shown, resulting in no current in the onewinding. Again, the unbalanced currents will cause the differential relay tooperate.In generators with singlewindings perphase, differential protection could beused across each end of the windings.

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This latter type ofdifferential protection scheme could be used to protect thewindings of the generator and the main transfonner, byoptimum placementof thecurrent transformers. Figure7.2 shows thedifferential schemewith thecurrent transformers located at the output side of the main transformer, theconnection for the unit service transfonner and on the generator winding atthe center of the star connection. This puts the generator winding and maintransformer windings within the zone of protection for this differentialscheme. Note that the current transformerswill require a different ratio, sincetheane current nansformeris on theoutput sideof the main tl'ansfonnef"l'.

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Unit ServiceTransformerConnection

DifferentialRelay~ > - - -Unit ServiceTransformer

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Figure 7.2: Differential Protection Scheme For TheGenerator (and Main Transformer)

I f the faults listedearlier are not cleared, then the risk of insulation damagewill occur due to overheating. as well as damage from arcing if theinsulation has already been damaged.GENERATOR GROUND FAULT PROTECTIONIn thepre0.olls section,we have seen how differentialprotection can be usedto protect against a ground fault in the windings of the generator itself.Anothermethodofdetecting faults is tomonitor the neutral connection forcorront flows. In Figure 7.3, the grounding of the generator neutralconnection is dune through a neutral grounding transfonner. A ground faultin the generatorwindings (similar to the case shown in Figure 7.1), tenninalsorequipmenton-linewill causeunbalanced current flows in the phases.Thisunbalanced corrent Dow will cause a corrent Dow to the neutral This clU7'wwillalsorequiTephaseco"ectioll. since the transformaJiotIwill have calUdphaseshifts. How thisp1uJse shift is corrected is beyond the scope of this course.

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Counc 235 Module 7 - Generator Protcetial1

(ground) connection. The current flowing in the neutral to ground willcause a current to be induced on the secondary side of the transfonner aswell. Once the voltage in the secondary side of this transfonner reaches apreset level, the voltage relaywill operate. The resistor seen in the diagram issized to limit the ground fault current and thus minimize the damage to thegeneratorstatorcore and winding insulationwhen a groundfault develops.

Generator Terminals

Neutral Connection

TransformerGrIndingResistor

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Figure 7.3:Ground Protection for GeneratorWindingsPossible causes of ground faults are insulation damage due to aging,overheating, over-voltage. wet insulation and mechanical damage. I f the

. faults are not cleared, then the risk of insulation damage will occur due tooverheating (as a result of high currents), or damage from arcing if theinsulation has already been damaged.

ROTOR GROUND FAULT PROTECTIONThewindings on therotorofan acgeneratorproduce the magnetic field at thepoles. In four pole generators (typical of 60 Hz, 1800 rpm units). theoccurrence of a single ground fault within the rotor generally has nodetrimental effects. A second ground fault, however, can have disastrousresults. It can cause panof the rotor winding to be bypassedwhich alters theshape of the otherwise balanced flux pattern. Excessive vibration and evenrotor/stator contact may result.

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Approvellssue Course 23S Module 7 - GeneralOr ProtectionNOTES & REFERENCES

Obj. 7.2 bi """ A means of detecting the first ground fault provides protection against theeffects of asecond fault to ground on the rotor. Figure 7.4 shows asimplifiedexcitation system with a ground fault detection (GFD) circuit'. The GFD isconnected to the positive side of the exciter source.r---------;:::l.,-

ReidWinding

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Sensitive GFD Relay

Current Umitlng Resistor

IIC AuxmaryA.C. SupplyFigure 7.4: Ground Fault Detectionon Excitation System

A ground fault occurring anywhere within the excitation system and rotorwinding will cause current to flow' through the limiting resistor (the voltageat the fault point will add to the bias voltage and cause a current flow throughthe GFD circuit), the GFD relay, the bias supply to ground and then back tothe fault location. Current flow through the GFD relay brings in an alarm.

Rotor grolllfdfau1l protectionwas tkaltwith in your 335.05-1 Electrit:D1 Systems COUTS#!.

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Course 235 Module 7 - Generator Protection

GENERATOR PHASE UNBALANCE PROTECTIONI f the generatorcontinues to operate with a phase imbalance, CUIrents in thewindings will increase due to additional Induced circulating currents"(these cum:nis will also cause heating of other internal components of thegenerator). This will result in rapid and uneven heating within thegenerator. Possible damage to Insulation and windings (hence, reducedmachine life), and tbermal distortion could occur.A specialized relay to detect these circulating cum:nts, called a NegativeSequence Current Relay (since the "induced" cum:nts are called negativesequence currents"), is used to detect the phase imbalance within thegeneratorduringunbalancedfault conditions. A differential schemecouldbeused between the three phases to detect excessive variations in currentcaused by uneven loading.The unbalanced magnetic forces within the generator due to these cum:ntswill also cause excessive vibration. This may result in bearingwear/dam-age and reduced macbine life, and may result in a bigb vibration trip"".Causes of phase imbalance include unequal load distribution, grid faultsand windings faults.GENERATOR LOSS OF FIELD PROTECTIONWhen a generator develops insufficient excitation for a given load, theterminal voltage will decrease. and the generator will operate at a morelesding power factor with a larger load angle. I f the load angle becomes toolarge, lossofstability and pole slippingwilloccur.More infurmation aboutpole slipping will be presented later in this module, andmore about stabilitywill be presented in Module 8.A lossoffieldcould be caused by an exciter or rectifier failure, automaticvoltage regulator failure, accidental tripping of tbe field breaker, shortcircuits in the field currents, poor brush contact on the sliprings, or acpower loss to tbe exciters (either from the station power supply or from theshaft generated excitation current).Relays that sense conditions resulting from a loss of field, such as reactivepower flow to the machine, internal impedance changes as a result of fieldchanges or voltage decreases, may be used fur the detection of th e loss offield. A field breaker limit switch indicating that the breaker is open alsogives an indication that there is no field to the generator.

HowtM#cwrenuanfortMd isbeyoNithescopcoftlUscOIIFH. y ~ nadonJy.blow that thueCIln'DW circld4ting in 1M genuQ/Of' will causft tJddilioMl hbUing due to PR losses.

.. Trips of the whine-generator initiated by high vibration signals an discussed in the 234Twbinu" AwcilUviuCOfll'$ft.

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Course 235Module 7 - Generator Protection.

SUMMARY OF THE KEY CONCEPTS Cl&Ss A trips will completely separate the Unit from the grid, and shutdown the turbine generator. Class B trips will disconnect the generator from the grid, but will leave

the turbine generator supplying the unit loads. Class C trips are generator overexcitation trips. Cl&Ss D trips will trip the turbine and then trip the generator after

motoring. Generator differential protection c an be used for protection against

winding ground faults, shorts and open circuits. The flow of fault currents can arise from insulation damaged due toaging, overheating, moisture ormechanical damage. Ground faults can also be detected by current through the neutral

grounding transformer. Ground faults on the rotor are detected by a ground fault detectionsystem connected. to the positive bus of the exciter circuit. The firstground fault is alarmed, allowing action to be taken to prevent theconsequences of a second ground fault Ph&Se imbalance can be caused by unequal load distribution, grid faultsand windings faults. Phase imbalance will induce circulating currents.which will result in rapid. uneven heating within the generator. This willresult in damage to insulation and windings (hence. reduced machinelifel, and thermal distortion. Unbalanced magnetic forces within thegenerator will also cause excessive vibration, resulting in a possiblehigh vibration trip.

Anegative sequencecurrent relay can be used to detect phase imbalanceconditions and initiate protective action. A differential scheme couldalso be used.

Loss of field protection will prevent the generator from pole slipping.which can result in mechanical shocks to the turbine generator. This canbe caused by an exciter failure, automatic voltage regulator failure,accidental tripping of the field breaker, shorl circuits in the fieldcurrents, poor brush contact on the sliprings, or ac power loss to theexciters.

Loss of field can be detected by special relays that sense reactive powerflow to the machine or internal impedance changes.

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C01D'Be235 Module 7 - Generator Protection

GENERATOR OVEREXCITATION PROTECTIONI f the generator is required to produce greater than rated voltage at ratedspeed (or rated voltage below rated speed), the field current must beincreased above normal (generated voltage is proportional to frequency andflux*). Th e excess current in the rotor and generated voltage will result inoverfluxlng of the generator stator iron, and the iron cores of the main andunit service transformers. Damage due to overheating ma y result in thesecomponents. Ovcrvoltage ma y alsocause breakdownofinsulation, resultingin faults/arcing.This problem may occur on generators that are connected to the grid i f theyexperience generator voltage regulation problems. It ma y also occur forunits during start-up or ,.....ynchroniZing following a trip (the field breakershould open when the turbine is tripped. At low frequencies, the fielddischarge resistor should prevent tenninal voltage from reaching dangerouslevels**). Overexcitation in these instances may be a result of equipmentproblems o r operator e rro r in applying excessive excitation prematurely(excitation should not be applied to the generator until it reaches ncarsynchronous speed).A specializcd voltslhcrtz relay is used to detecrthis condition, and will tripthe generator i f excessive volts/hertz conditions are detected.GENERATOR UNDERFREQUENCY PROTECTIONWhile connected to a stable grid, the grid frequency and voltage are usuallyconstant I f the system frequency drops excessively. it indicates that therehas been a significant increase in load*. This could lead to a seriousproblem in tbegrid, and it is of little us e to supply a grid tbat may bea b ou t t o collapse. In this case, the generator would be s eparat ed f r om tb egrid. Th e grid (or at least portions ofit) ma y well collapse. Th e systemcanslowly rebuild (with system generators ready to restore power) to proper,pre-

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Course 235 Module 7 - Gena:ator Protection-

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Course 23S Module 7 - Genetaror Protection

and these rectifiers will also be lost Some stations will allow continuedoperation with minced number of rectifiers in service, but generatorexci tat ion (hence load) will be limited by remaining field currentcapacity. By not having the field. CUITent available to "stiffen" thegenerator's connection to the grid. the system stability is at risk.The rectifiers have an overload capacity, but the duration that they cansustain this overloadis limited. This overloadcapacity is requiml when gridfaults result in mluced voltage, power or frequency changes. A powerstabilizing system, upon"seeing a grid problem", willcall foran increase inexcitation to maintain grid stability. This is known as field forcing. I f thenumber of rectifiers is limited, and field forcing is required, i t can/willoverload the remainingrectifiers. resultingin a total loss ofexcitation (henceproduction).To prevent this, the ability to rreld force is reduced to a valuedependentupon the numberof rectifier sections in service. Thiswill result ina less secure electrical supply.MOTORINGMotoring refers to the process of an ac generator becoming a synchronousmotor, that is, the device changing from'a producer of electrical power to aconsumer of it. Following a reactor trip or setbacklstepback to a very lowpower level. it is beneficial to enter the motoring mode of turbine generatoroperatinn.However, this is not a desirable mode of operation for standbyor emergency generators. They are not designed to operate in this manner.and can be seriously damaged i f power is allowed to flow in the wrongdirection.Ameansofindicaling when the ttansition fromexporterto importerofpoweroccurs is pro\1id.e4 by a device known as a reverse power relay. As its namesuggests, it is triggered by power flowing in a direction opposite to thatwhich is normally desired. This can be used forgeneratorprotection, as is thecase with standby generators, or as a permissive alarm{mterlock for rominegenerator motoring (see Class D trips on page 2). Figure 7.5 shows a typicalarrangementof a reverse powerprotectioncircuit employingboth acr and avoltage transformer (VT) to power the relay, and hence, protect thegenerator. The relay will operate when any negative power flow is detected.

TM stabilizing$JIfDn willdetect vollage. spud tJNipowt!T c1IlJ1ages llwt CtDI btt indicalweo fQ,gridftuJI. TIw. ~ i : i l I g S'J#I!III willlM d i s c ~ in JOfU stationspifU: training... FIITtIwr infonNJlion on turbiMgenuator motoring canlMfound in 1M TlU'bine & AuxiliariescOline 234.0-13.

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r - -AA /' "A G.J:. .,. I'- . / - -.b-- -V.T.

ReversePowerRelay

Figure 7.5: Generator Reverse Power Protection

Generator Protection Scheme CI. . . . ofTripDifferential Protection AGround Fault Protection APhase Unbalance ALoss of Field AOverexcitation A,B,CUnderfrequency BOut nfStep BExcitation Rectifier Overcurrent A*Motoring D

This does not lead to a trip directly, though once excitation collapses, a lossof excitation trip will result.

Table 7.1: Summary of Generator ProtectionSchemes and Trip Classes

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Comse 235 Module 7 - Generator ProteetiOh

ASSIGNMENT1. Discuss each of the four classes of turbine generator protection trips.

Class A:

Class B:

Class C:

Class D:

2. Explain how differential protection is used for the protection of agenerator (in your explanation include consequences to station equip-ment i f this protection fails to operate):

3. Faults that can be detected by generator differential protection are:a)b)c)

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Course 235 Module 7 - GeneraIor Protection

4. Explain how ground fault protection is used for the windings of agenerator (in your explanation include consequences to station equip-ment i f this protection fails to operate):

5. Possible causes of ground faults are:a)b)c)

6. Explain how ground fault protection isprovided for a generatorrotor (inyour explanation include consequences to station equipment if thisprotection fails to operate):

7. Explain how phase unbalance protection is used. for a generator (in yourexplanation include conseque,nces to station equipment if this protectionfails to operate):

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Course 235 Module 7 - Genenuor ProleCtion

8. Two po,sible cause' of phase unbalance are:a)b)

9. Explain how loss of field protection is used for a generator (in yourexplanation include consequences to stationequipment if this protectionfail' to operate):

10. Four possible causes of generator loss of field are:a)b)c)d)

11. Explain how overexcitation protection is used for a genemtor (in yourexplanation include consequences to station equipment if this protectionfail' to operate):

12. A possible cause of overexcitation is:

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Coumc 235 Module 7 - Generaror Protection

13. Explain how underfrequencyprotection is used for a generator (in yourexplanation include consequences to station equipment if this protectionfails to operate):

14. Two possible causes of excitation beingapplied during underfrequencyconditions are:a)b)

15. Explain how out of step protection is used for a generator (in yourexplanation include consequences to station equipment if this protectionfails to operate):

16. Three possible reasons that of out of step operation could occur are:a)b)c)

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Course Module 7 - Genc:rator Protection

17. Explain how rectifier overcurrent protection is used (in yourexplanation include consequences to station equipment i f this protectionf&il' to operate):

18. Two possible causes of rectifier overcurrent are:a)b)

19. The flow of power into a generator can De detected by a

Before you move on to the next module. review the objectives and makesure that you can meet their requirements.

Prepared by: Nick Ritler, WNIDRevised by: Paul Bird.WNID

Revision date: July, 1992

20042907

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