Electrifying Electrologic!!!

6/7/2015 ElectrifyingElectrologic!!!

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ElectrifyingElectrologic!!!

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PROTECTIONOFALTERNATORCHAPTER4INDRODUCTIONAlternator is themost importantandcostlyequipment inpowersystem.Thecost ishighbecause the alternator unit is accompanied by primemover, excitation system, voltageregulator, cooling system etc. The protection system provided should be thereforebecomeselaborateandcomplex.Assoonasthefaultdevelopsonalternator,itshouldbeimmediatelydisconnectedfromheothergeneratingplanttoavoidthefaultymachinefrombeingfedbyothersources.ABNORMALITIESANDFAULTSFollowingTableshowsabnormalconditions(Faults)andprotectionschemesemployedforthealternator.SN AbnormalCondition Effect Protection

1 Thermaloverloading:i) Continuous

overloadingii) Failure of cooling

system

Overheating of statorwinding and insulationfailure.

Thermocouples of resistancethermometer impeded in stator slotsand cooling system. Statoroverloading protection with overcurrentrelays.

2 External fault fed bygenerator

Unbalanced loadingstressesonwindingandshaft,excessiveheatingfor prolonged shortcircuit.

Negativephasesequenceprotectionforlargemachines.Overload protection for smallgenerators.

3 Statorfaults:i)Phasetophaseii)Phasetoearthiii)Interturn

Winding burnout,welding of corelaminationsdown

Biased differential protection,sensitiveearth faultprotection, interturnfaultprotection.

4 Rotorearthfaults Single fault does notharm second faultcauses unbalancedmagneticforcescausingdamage to shaft,bearings

Rotorearthfaultprotection

5 Lossoffield:Tripping of field circuitbreaker.

Generator runs asinduction generatorderiving excitationcurrents from busbar.Speed increasesslightly.

Loss of field or Field failureprotection.

6 Motoring of generator.When input toprimemoverstops, the generator drawspower from busbars andruns a synchronous motorinthesamedirection.

Effect depends upontypeofprimemoverandthe power drawn fromhebusduringmotoring.

Reverse power protection bydirectional power relays direct thereversalofpower.

7 Overvoltages Insulationfailure Lightning arresters connected neargeneratorterminals.

8 Overfluxing of generatortransformer and auxiliarytransformer.

Heatingofcorerelay. Connectedinvoltageregulatorcircuitgenerator.

9 Underfrequency Failure of blades ofsteamturbines.

FrequencyRelays.

Somethingaboutdiagrams: The differential protection scheme is applied for 3phase

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alternator. For simplicity of understanding the star and delta connections are drawn insomedifferentway.DIFFERENTIALPROTECTIONThis is the most common scheme used for alternator protection differential protectionschemeforastarconnectedalternator.SixidenticalCTsareusedforthisscheme.ThesecondarysofCTareaconnectedinstarthroughpilotwires.Therelayoperatingcoils(R1,R2,R3)areconnectedacrossneutralpilotwireend.TherelaysareconnectedbetweenequipotentialpointsrespectivelinepilotwiresofCTsecondaryside.Therelaysusedareelectromagnetic type.Operationundernormalworkingconditions,thecurrententeringalteringthealternatorwindingisequaltocurrentleavingthealternatorwinding.ThereisinducedcurrentinsecondarysofCTthesecurrentsarealsosameinmagnitude.Thesecurrentscirculateinpilotwiresbutnotthroughtherelaycoils.Whena fault develops, sayearth fault orphase tophase fault, thecurrententering thealternatorwindingisnotequaltocurrentleavingthealternatorwinding.SoCTsecondarycurrents are no longer equal and difference current flows through operating coil. Thus,relayisenergized,andtripsignaloperatestherespectiveCBtoisolatethefaultypart.Percentage Differential Protection of Alternator/Biased Differential Protection ofAlternatorFig. (a) and (b) shows percentage differential protection scheme for star and deltaconnectedalternators.For star connected alternator the neutral is earthed through a resistance. TheCTs areconnected instar.Onemorecoilcalled restrainingcoil isadded in thecircuitasshown.Therestrainingcoilisconnectedcentrallyinthepilotwires.Therespectiveoperatingcoilsareconnectedatcentreofrespectiverestrainingcoils.UndernormaloperatingconditionsthesecondarycurrentsofCTsareequal.Thus,thereisbalancedcirculatingcurrentinthepilotwiresandtherestrainingcoilandcurrentdoesnotflowintheoperatingcoil.EARTHFAULTPROTECTIONOFALTERNATORWhenthefaultcurrentflowsthroughearthreturnpath,thefault iscalledearthfault.Theearthfaultsarefrequent.Soearthfaultprotectionisimportant.Residual earth fault relay: When earth fault does not take place, the sum of threesecondarycurrentsofCTiszero.i.e.IR+IY+IB=0.Thesumofthesecurrentsiscalledresidualcurrent.Whenearthfaultdevelops,thesumofthreecurrentsisnotzero,theresidualcurrentflowsthroughearthfaultrelay.This protection is also called unrestricted earth fault protection. In another scheme, therelayisconnectedinneutralofthetransformer.The neutral acts asCT, primary.When fault occur, the fault current flows through thisearthconnectionandsecondarycurrentincreasescausingearthfaultrelayoperation.Theamountofneutralcurrentflowingdoesdependontypeofearthingsystemprovided.Thisisalsocalledunrestrictedearthfaultprotection.Earth Fault Protection with Core balance Current Transformer (CBCT)/zeroSequenceCT.AcorebalanceCT isused in thisscheme.ThecoreofCBCThas largecrosssectionalareasothatitdoesnotsaturatewhenfaultcurrentproduceshighfluxdensityinit.The core embraces the three conductors R, Y, B. Under normal conditions, thecomponentsoffluxesproducedbythreecurrentsarebalancedandCTsecondarycurrentisnegligibleandrelaydoesnotoperate.WhenearthfaultdevelopsthebalanceisdisturbedandsufficientcurrentisinducedinCTsecondary,thusrelayisoperated.RESTRICTEDEARTHFAULTPROTECTIONFORALTERNATORTheCTsareaconnected instarasshowninFig.Theremainingconnectionsareasperpercentage differential protection scheme. In addition, one restricted earth fault relay isplacedinthestarconnectionofCTsecondarysasshowninFig.Whenearthfaultoccursonthewinding,theimbalancecurrentflowsthroughtheoperatingcoilsandrestrictedearthfaultrelay.Thus,therestrictedearthfaultrelayoperatesandtripsignalisproducedwhichoperatestheCB.INTERTURNFAULTPROTECTIONORSTATORINTERTURNFAULTPROTECTIONThe normal differential protection scheme can protect the alternator against phase in

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groundorphasetophasefaults.Itcannotprotectthealternatorfrominterturnfaults.Thisisbecausecurrententeringand leaving, thealternatorwinding remains thesame.Eventhoughsome turnsofanyphaseareshorted.SoCTsecondarycurrentsdonotchangeandoperatingcoildoesnotcarryanycurrent.Theinterturnfaultslaterondevelopearthfault,sointerturnfaultmustbedetectedatanearlystage.Fig.showsarrangementforinterturnfaultprotectionscheme.Wehaveshownitforonephaseonlyforsimplicityofunderstanding.Forthreephasesalternatorstherewillbesuchthreegroups.Thisschemeisusedformultiturnalternator,thewindingofeachphaseisdividedintotwoequal sections (S1 and S2) as shown in figure. Two identical CTs are connected inrespectivewindings.TheCTsecondarysareconnectedinphaseoppositionandarelayRisconnectedinCTsecondaryloop.Thefunctioningofthisschemeissimilartodifferentialprotectionprinciple.Under normal operating conditions, the currents through S1 and S2 are same, so CTsecondariescarrysamecurrent.AndnocurrentflowsthroughrelayR.IftheadjacentturnsofS1areshorted,currentssharedbyS1andS2differshencethereisdifference inCT secondary currents. The difference current flows through the loop andrelaynowcarriescurrentandproducestripsignalforCB.OVERHEATINGPROTECTIONStatorOverheatingProtection:Overheating instatormaybecausedby the failureofthecoolingsystem,overloadingorcorefaultlikeshortedlaminations.Modernalternators9above2MW)employtwomethodstodetectoverheating.Inonemethod,theinletandoutlettemperaturesofcoolingmediumaremeasured.Coolingmediummay be gas (hydrogen) or liquid (water). The inlet and outlet temperatures ofcoolingmediumarecomparedand ifdrasticdifference is there, itmeansoverheating isoccurring.Thus,overheatingcanbedetected.In secondmethod, temperature sensorsareembedded in stator slots.The temperaturesensorsmaybeaRTD(ResistanceTemperatureDetector),thermistor,thermocouple.Whetstone'sbridgeprincipleisusedtodetectoverheating.P,QandSarethefixedvalueresistances.TheRTDisconnectedinonearmasshown.ThisRTDiskeptinstatorslotofalternator.Whentemperatureiswithinlimits,pointaandbarehesamepotentialandrelaycoil does not carry any current i.e. bridge is balanced.When overheating occurs, RTDresistancechangescausing imbalanceandsomepotentialdifference iscreatedacrossaandb.Therelaycoilisthusenergizedandrelayoperatesthealarmcircuit.NegativeSequenceProtectionofAlternatorAgainstUnbalanceLoadWhenstatorcurrentsareunbalanced, theyproducenegativesequencecomponent.Thiscomponent rotates at synchronous speed and in opposite direction to rotor. This causedouble frequencycurrents tobe induced in the rotor.So, rotor iron losses increaseandheatingofrotoroccurs.Thedoublefrequencycurrentalsoproducesvibrationsandheatingofstator.Figureshowsaschemeforprotectionagainstunbalanceloads.ThreeCTsareconnectedinstarandthesecondarysareconnectedtonegativesequencefilter,overcurrentrelayisconnectedtosequencefilter.Thenegativesequencefiltersconsistsofnumberofinductorsandresistors.Thisnegativesequencefilterdetectspresenceofnegativesequencecomponentduetounbalanceandoperates.REVERSEPOWERPROTECTIONThe turbine drives the alternator. Alternator is connected to supply system throughtransformers,busbarsetc.Andata timemanyalternatorsareconnected to thesupplysystem.Butwheninputtoanyoneturbineisstopped,thealternatorcontinuestorotateasamotor i.e. it takes electrical power from supply and supplies mechanical input to theturbinesoitdrivestheturbine.Sopreviouslythepowerflowwasfromturbinealternatorsupplysystemandnoitisfrom supply system motor (alternator acting as a motor) turbine i.e. power flowreverses.Whenelectricalpowerflowsinreversedirection,itstillmaintainsthebalanceinthreephases.Soreversepowerflowcanbedetectedbyusingasinglerelayinanyonephases.Directional relay can be used to detect reverse flow. Figure shows induction typedirectionalrelaytodetectreversepowerflow.The shunt magnet coil and series magnetic coil are excited from any one alternatorwinding.When power flow direction is correct the disc rotates in normal direction. But



PostedbySureshApreat08:17

Labels:PROTECTIONOFALTERNATOR

whenpowerflowreverses,thediscrotationis inoppositedirectionandoppositerotationcausestripcircuitcontactstoclose.Thus,tripsignalisproduced.SOLVEDEXAMPLEA20MVA11kV,3phasestarconnectedalternatorisprotectedbyMertzPriceprotectionscheme.Thestarpointisearthedthrougharesistanceof5.IfCTshavearatioof1000/5andtherelayissettooperatewhenthereisanoutofbalancecurrentof1.5Amp.Calculate:

i)Thepercentageofeachphaseofthestatorwindingwhichisunprotected.ii)Theminimumvalueofearthingresistancetoprotect90%ofthewinding.

Solution:i)LetX%ofthewindingisunprotected,Earthingresistancere=511x103

Vph=Voltageperphase==6350.85volts3Theminimumfaultcurrenttooperatetherelay=1000/5x1.5=300Amp.e.m.f.inducedisX%ofstatorwinding,=Vphx(X/100)=63.508xVolt.Earthfault,currentthatwillflowbecauseofvoltageinducedinX%winding=63508X63.508==12.7Xamps.re5Thiscurrentshouldbeequaltominimumfaultcurrentrequiredtooperatetherelay.12.7X=300soX=23.62%ii)Letre=minimumearthingresistancerequiredtoprotect90%ofthestatorwinding.The10%windingwillbeunprotected.63.508x10i.e.X=10%300=63.508X/re300==12.7XAmp,re=2.12.re

EXERCISE

1.Statetheabnormalitiesandfaultsinalternatorwithnecessaryprotection.

2.Drawadiagramofdifferentialprotectionschemeforastarconnectedalternatorandexplainitsworking.

3.Drawadiagramofpercentagedifferentialprotectionschemefordeltaconnectedalternatorandexplainitsworking.

4.ExplainearthfaultprotectionofalternatorusingCBCT.

5.WithaneatsketchexplainoverheatingprotectionusingWheatstonebridge.

6.Drawaprotectionschemefordoublefrequencycurrentusingnegativesequencefilters.

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