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ANALYSISOFADIFFERENTIALANDOVERCURRENTOPERATIONONA345KVHIGHVOLTAGELINEREACTOR
Authors:
Eric Schroeder P.E., Cross Texas Transmission, Amarillo, Texas
Jerry Burton, Cross Texas Transmission, Amarillo, Texas
Luke Hankins, SynchroGrid, College Station, Texas 77845
Joe Perez P.E., SynchroGrid, College Station, Texas 77845
Presented before the
69th Annual Texas A&M Protective Relay Conference
College Station, Texas April 4th – April 7th, 2016
ANALYSISOFADIFFERENTIALANDOVERCURRENTOPERATIONONA345KVHIGHVOLTAGELINEREACTOR
Eric Schroeder P.E., Cross Texas Transmission, Amarillo, Texas [email protected]
Jerry Burton, Cross Texas Transmission, Amarillo, Texas [email protected] Joe Perez P.E., SynchroGrid, College Station, Texas, [email protected] Luke Hankins, SynchroGrid, College Station, Texas, [email protected]
I. INTRODUCTION
Highvoltagelinereactorsareusedinlongtransmissionlinestomitigatethehighvoltagelevelscreatedbythelinechargingcapacitance.Theapplicationoflinereactorsshouldbestudiedcarefullysinceanincorrectoperationcanisolatenotonlythereactorbutalsothetransmissionlineitself.Thistypeofapplicationcomeswithchallengesconcerningthesystemandprotectiverelays.Reactorspresentdifficultiestodifferentialalgorithmsduringin-rushsituations.Thisisbecausereactorswillexperiencethesameamountofcurrentsonboththehighandlowsides.Asaresult,thedifferentialcurrentmagnitudecouldbezero,leavingnomagnitudestoextractthesecondharmonicwaveformusedforblockingofthe87function.Theprotectionengineermustunderstandtheprotectionfunctionandoperationalgorithmsoftherelaysbeingappliedinordertoproperlyprotectthereactorandavoidmis-operationsduringin-rushconditions.Thispaperdescribestheanalysisofareactorin-rusheventwherethebackuprelaytrippedondifferentialduringenergizationwhereastheprimaryrelaydidnottripondifferential.Inaddition,thispaperdescribesindetailtheprotectionalgorithmconcepts,waveformbehavior,anddifferentialcharacteristicsoftheprimaryandbackuprelays.Thiswillallowustoseehowthetwoalgorithmsdifferinsecurity,reliability,andsensitivity.II. SYSTEMONELINE
Figure1showsthesystemonelineofthelinereactorsconfiguration.Thenormalreactoroperationisdonethroughtheuseofacircuitswitcher.Thereactorisprotectedbyaredundantsystemusingtwodifferentdifferentialrelaysfromdifferentmanufacturers.ManufacturerAwillbereferredtoastheprimaryandmanufactureBwillbereferredtoasthebackup.ThedifferentialzoneisboundedbyCTswithinthereactor’shighsideandlowside,asshowninfigure2.Ifafaultisdetectedinsidethezoneofprotection,thedifferentialrelayssendatransfertripsignalviaGOOSEtothelinerelaystoopenlocalandremotelinebreakers.Thecircuitswitcheropens30cyclesafterthelinebreakersopen.
BKR_G120
BKR_G130
345 kV Line 1
345 kV 345 kV
Figure1:SystemOneline
BKRG130
Line1toTesla
GY-R3
SWRGR34
BackupPrimary
BKRG120
Primary Backup
345kV
Figure2:ReactorDifferentialProtection
III. SEQUENCEOFEVENTS
Table1showsasummarizedversionofthesequenceofeventsthatoccurredduringthisevent.Asshownbelow,operatorsaretoldtoclosethereactorat09:26:30.887.Fivecyclesafterclosingthecircuitswitcher,thelinerelaysreceiveatransfertripsignalviaGOOSEfromthebackupreactordifferentialrelay.Twocyclesafterthedifferentialtripwasissued,thelocallinebreakersareopened.21cycleslater,theprimaryrelaytripsongroundinstantaneousovercurrent.Finally,34cyclesaftertheinitialdifferentialtrip,thecircuitswitcheropens.
Description TimeStampValue(hrs:min:sec:ms)CircuitSwitcherGR34closes 09:26:30.887
(5cycleslater)backuprelayoperatesonBPhaseDifferential 09:26:30.972
1/4ofacycleafterdifftrip,linerelaysreceivetransfertripfromthereactorbackupdifferentialrelay
09:26:30.974
2cyclesafterdifftrip,breakersG13&G12open 09:26:31.005
20cyclesafterdifftrip,theprimaryreactordifferentialissuesatripongroundinstantaneousovercurrent
09:26:31.308
21cyclesafterdifftrip,thebackupreactordifferentialissuesatripongroundinstantaneousovercurrent
09:26:31.324
34cyclesafterdifftrip,R34opens 09:26:31.537
Table1.SequenceofEvents
Thesummaryofthesequenceofeventstellsusthattherearequiteafeweventshappeningthatneedfurtherinvestigation.Wecanseethatonlythebackuprelaydetectsadifferentialfault.Thisalready
raisesseveralquestionsabouttheseoperations.Isthereaninternalfaultinthereactor?Whydidn’ttheprimaryrelayseeaninternalfault?Whydobothrelaysoperateoninstantaneousgroundovercurrent?
Thefirststepinanalyzinganeventistochecktherelaysettingsthatareinservice.Thisprocedurecanbelaborioussinceitrequiresthatallsettingsberecalculated.Thisallowsustouncovererrorsthatmighthaveslippedduringinitialcommissioning.Thenextsectiondigsdeeperintheanalysisoftherelaysettingsanddescriptionofitsuse.
IV. DIFFERENTIALRELAYSETTINGS
Thisreactorapplication,asshowninthesystemoneline,usestworedundantcurrentdifferentialrelaysforprimaryandbackupprotection.Inaddition,twodifferentrelaymanufacturersareutilizedwheretheprotectionalgorithmsarecalculateddifferently.Asummaryofthesettingsfromtheprimaryrelayusedtoprotectthereactorisshownintable2.
Description Setting ValueDiff.ElementOperatingCurrentPickup(p.u.) O87P 0.5Slope1Setting(%) SLP1 35Slope2Setting(%) SLP2 75UnrestrainedElementCurrentPickup(p.u.) U87P 1.00IncrementalOperateCurrentPickup(p.u.) DIOPR 1.2IncrementalRestraintCurrentPickup(p.u.) DIRTR 1.2EnableHarmonicBlockingDifferentialElement E87HB YEnableHarmonicRestraintDifferentialElement E87HR NSecond-HarmonicPercentage(%) PCT2 10
Table2:PrimaryRelayDifferentialSettings
Thesettingsarecomposedofaminimumpickupdifferential087PalongwithahighinstantaneousU87Psetting.The087Psettingoffersaverysensitivethresholdthatallowstherelaytoisolateinternalfaultsveryquickly.TheU87Pisaninstantaneoussettingwheretherestraintcurrentisnottakenintoconsideration,makingitsuitableforhighmagnitudeinternalfaults.Itoperatesdirectlyasthesummationofthefiltereddifferentialcurrents.Wenoticedthatharmonicblockingandharmonicrestraintsettingsarealsoavailablewithharmonicrestraintnotbeingused.Thesecondharmonicsettingissetto10%ofthefundamental.Thismeansthatiftherelaydetectsasecondharmoniccontentabove10%,therelaywillblockthedifferentialelementfromoperating.Harmonicblockingandrestraintareusedinordertoincreasethesecurityanddependabilityofthealgorithmduringin-rushorexternalfaultevents.
Theslopecharacteristicforthisrelayisshowninfigure3.Thisrelayusesoneoftwoslopesaspartofthedifferentialcharacteristic.Therelay’sinternalalgorithmsdecidewhichslopetousebasedonthebehaviorsofthecurrents.Slope1,setat35%,istypicallyusedtoincreasetherestrainoftherelayinordertoavoidoperationsduetoCTerrors,transformerlosses,highloadconditions,etc.Slope2,setat75%,givestherelaymoresecuritybyincreasingtherestrainregionofthedifferentialplaneandisusedforhigh-throughfaults,highclose-inexternalfaults,CTsaturation,etc.Increasedrestrainisnecessarywhendealingwitherrorsofahighermagnitude.
Figure3:PrimaryRelayDifferentialPlane
Thebackuprelayusessimilarsettingstotheprimaryrelay.Asummaryofthesettingsfromthebackuprelayusedtoprotectthereactorisshownintable2.
Table3:BackupRelayDifferentialSettings
Thisrelayalsohasminimumandhighoperatingpickupsettingsthataresettothesamepickupastheprimaryrelay.Eventhoughthesamepickupcriteriaisusedforbothrelays,thesettingsthemselvesappeartobedifferentbecauseeachrelaycalculatedtheperunitvaluedifferently.Thisrelayusesharmonicblockingandaninhibitin-rushsettingcalledadaptivesecondharmonic.Similartothepreviousrelay,thesecondharmonicissetto10%ofthefundamental.Inaddition,thesecondharmonicvalueascomparedtothefundamentalmustbehigherthanthesettingsinatleast2outof3phasesinorderforthedifferentialtobeblocked.Thiswillbeveryimportantinformationthatwillbevisuallyexplainedinthewaveformanalysissection.
Description Setting ValuePERCENTDIFFERENTIAL Function EnabledPERCENTDIFFERENTIAL Pickup 0.100pu(0.5A)PERCENTDIFFERENTIAL Slope1 25%PERCENTDIFFERENTIAL Break1 1.570puPERCENTDIFFERENTIAL Break2 7.840puPERCENTDIFFERENTIAL Slope2 98%PERCENTDIFFERENTIAL Inrush
InhibitFunction
Adapt.2nd
PERCENTDIFFERENTIAL InrushInhibitMode
2-out-of-3
PERCENTDIFFERENTIAL InrushInhibitLevel
10.0%fo
PERCENTDIFFERENTIAL Function EnabledPERCENTDIFFERENTIAL Block OFF
Theslopecharacteristicofthebackuprelayisshowninfigure4.Thisrelayalsousesthedualslopecharacteristictoimprovethesecurityoftherelays.However,unlikethepreviousrelay,theslopesarefixedforbothinternalandexternalfaults.
Figure4:BackupRelayDifferentialPlane
V. PROTECTIONALGORITHMS
Inordertounderstandthereasonsbehindthereactordifferentialrelayoperation,itisnecessarytounderstandhoweachindividualrelayalgorithmworks.Eachmanufacturerhasitsownpreferredmethodofprotectionalgorithmthateithermakesthemmoresecureandlesssensitiveorlesssecureandmoresensitive.BelowisareviewonhowbothrelaysestablishtheoperateIOPandrestrainIRTcurrentsalongwiththeharmonicrestraintandblockingthatareneededindifferentialapplications.
OperateandRestraintCurrents:
Theprimarycalculatestheoperateandrestraintcurrentsonaperphasebasis.TheformulasforAPhasedifferentialforthehigh(IAT)andlowside(IAW)ofthereactorphasorcurrentsareshowninequations1and2below.
IOPA = |IAT + IAW|(1)
IRTA = |IAT| + |IAW|(2)
ThebackupcalculatestheoperateandrestraintcurrentsforAPhaseasshownbelow:
IOPA = IAT + IAW(3)
IRTA = MAX |𝐼𝐴𝑇| , (|𝐼𝐴𝑊|) (4)
Noticethatinbothrelays,thedifferentialcurrentsIOPAiscalculatedbyaddingthephasorcurrentsthatareprotectingthereactor.Theabsolutevalueofaphasorcalculationresultsintakingmagnitudesonly
andnottheangles.Themajornoticeabledifferenceishowtherestraintcurrentiscalculated.ThiswillbeshownvisuallyinsectionVII.Ineithercase,bothsetsofcurrentdifferentialmethodshavetoovercometherestraincurrentalongwiththeslopesettinginorderfortherelaytooperateasshowninequation5.
IOP > IRT ∗ SLP(5)
HarmonicRestraintandBlocking:
Bothrelaysofferextrasecurityalgorithmsforin-rushconditions.Theprimaryrelayhastheabilitytoapplyharmonicrestraintandblockingbeforethedifferentialfunctionissuesatrip.
HarmonicRestraint:Whentheprimaryrelayissettoharmonicrestraint,theoperatingcurrentiscalculatedasfollows:
IOPA > IAT + IAW ∗ 𝑆𝑙𝑜𝑝𝑒 + 𝐾2 ∗ 𝐼𝑂𝑃𝐴𝑝ℎ2 + 𝐾4 ∗ (𝐼𝑂𝑃𝐴𝑝ℎ4)(6)
K2andK4arethe2ndand4thharmonicsettingsandIOPAph2andIOPAph4arethe2ndand4thharmonicvaluesfoundintheoperatecurrentorcurrentdifferentialsummation.Equation6showsthattheoperatingcurrentIOPA“mustovercomethecombinedeffectsoftherestrainingcurrent,IRTA,andtheharmonicsoftheoperatingcurrentfortheelementtoassertatripoutput.Anymeasurableharmoniccontentprovidessomebenefittowardthegoalofpreventingdifferentialrelayoperationduringin-rushconditions”[1].Harmonicrestraintisgenerallyslower,buthasimproveddependabilitywhenenergizingafaultedtransformerorreactor.Also,becausetheharmonicsaresummed,harmonic restraintismoresecureduringin-rushconditions.Therestraintmethodisshowninfigure5below.
•10
•K4th
+
Σ
Σ
•f(SLP1,SLP2)IRTA
I2nd
I4th
IOPA
• • •
TRIP
Figure5:PrimaryRelayHarmonicRestraintSupervision
HarmonicBlocking:Whentheprimaryrelayisintheharmonicblockmode,theIOPAoperatingcurrentis“independentlycomparedwiththerestraintcurrentandtheselectedharmonics.”[3]Theharmoniclogicisshowninfigure6.Weseethattheoperatecurrentstillhastoovercometherestraintandslopesettingasshowninequation7.
IOPA > IAT + IAW ∗ 𝑆𝑙𝑜𝑝𝑒(7)Beforeadifferentialtripisdeclared,theamountofsecondharmoniccontentischeckedwithintheoperatecurrent.Ifthe2ndharmonicmeasuredvalueisgreaterthanthepercentsetting,thedifferentialtripisblocked.Forexample,fora2ndharmonicsettingof10%,whenthefundamentaloperatecurrent
hasavalueof10Aandthe2ndharmonicfoundintheunfilteredoperatecurrenthasavalueof1A,therelaydifferentialelementwillbeblocked.“Whentheharmoniccontentisbelowthespecifiedthreshold,theharmonicblockinghasnoeffect.”[3]
+
+
+
+
IAM4
IOPA•f(SLP1,SLP2)IRTA
IPU
AND AND
OR
2nd-HarmonicBlocking
87BL
87R
xth-HarmonicBlocking
TRIP
ph2
ph4
IAM2
Figure6:PrimaryRelayHarmonicBlocking
Inaddition,theprimaryrelayusescommoncrossblockingwhichblocksthedifferentialofany2ndharmonicphasethatisabovethegivensetting.Thisisshowninfigure7below.
OR87BL1
TRIPAND
OR
87BL287BL3
87R187R287R3
Figure7:PrimaryRelayHarmonicCrossBlocking
Thebackuprelaydoesnotofferarestraintharmonicblockingfeature.However,itdoesoffertwotypesofharmonicblockingtechniques:traditionalandadaptive2ndharmonicblocking.Thetraditional2ndharmonicrestraintrespondstotheratioofmagnitudesofthe2ndharmonicandfundamentalfrequencycomponents.Ifthe2ndharmoniccontentfoundinthedifferentialcurrentishigherthanthegivensettings,therelayblocksthedifferentialsetting.Thisissimilarorthesameastheharmonicblockingtechniqueoftheprimaryrelay.Theadaptive2ndharmonicrestraintrespondstomagnitudesandphaseanglesofthe2ndharmonicandthefundamentalfrequencycomponent.Thebackuprelaymanufacturerclaimsthattheadaptiveharmonicrestraintalgorithmsuccessfullyrestrainstrippingwhenfacedwithlowlevelsofsecondharmoniccurrentduringanin-rushevent[3].Theharmonicblockinglogicofferedbythebackuprelayisshowninfigure8.
ANDSlope
Functionladlar
DisabledAdapt.2nd
Trad.2nd
=0
=1
Disabled5th
=0=1
lad2>=LEVEL
lad5>=LEVEL
1outof32outof3Average
lad2
lad5
2ndHarmonicBlock
5thHarmonicBlock
ORA-PhaseDiffOpB-PhaseDiffOpC-PhaseDiffOp
DiffOp
Figure8:BackupRelayHarmonicandCrossBlocking
Sincethesecondharmoniciscalculatedonaperphasebases,therelayoffers4differentmodesofharmonicblocking.
1. Per-phase:Inper-phasemode,therelayperformsin-rushrestraintindividuallyineachphase.
2. 2-out-of-3:In2-out-of-3mode,therelaychecksthesecondharmoniclevelinallthreephasesindividually.Ifanytwophasesestablishablockingcondition,theremainingphaseisrestrainedautomatically.
3. Averaging:Inaveragingmode,therelayfirstcalculatestheaveragesecondharmonicratioand
thenappliestheinrushthresholdtothecalculatedaverage.
4. 1-out-of-3:In1-out-of-3mode,allthreephasesarerestrainedwhenablockingconditionexistsonanyonephase.1-out-of-3modetypicallyrevertsbacktoper-phasemodeafterashorttimedelaytoallowtrippingincaseaninternalfaultoccursduringenergization.
VI. WAVEFORMANALYSIS
Thissectionwillusetheinformationthatwasexplainedabovetodeterminethebehaviorofthetwodifferentialrelaysduringthereactorin-rush.
Figure9showsaCOMTRADErecordofthereactorin-rushthatwasobservedduringenergizationcapturedbythebackuprelay.Equalphaseshavebeensuperimposedwitheachotherinordertoshowtheirangleseparation.
Figure9:ReactorIn-RushCurrentsfromBackupRelay
Ascanbeseen,thewaveformsignatureandphasevectorsindicatethateachequalphaseis180degreesfromeachother.Forexample,thehighsideandlowsideofphaseAare179.7degreesapart,indicatedbychannels1and5respectively.TheBphasesare135degreesapartandtheCphasesare179degreesapart.Aninternalfaultisdeclaredwhentheanglebetweenthecommonphasesarelessthan90degrees.Itisclearthatthereisnointernalfaultbasedonthewaveformanalysis.However,thebackuprelaydeclaresadifferentialoperationontheBPhase.Thisresultsintrippingthelinebreakersand345kVlineoutofservice.Theprimaryrelaydoesnotseeadifferentialoperationduringtheenergization.
Let’sevaluatetheperformanceofeachrelaybylookingintotheoperate,restraint,and2ndharmonicblockingfunctionsbyusingtheformulasoutlineintheprotectionalgorithmssection.
Inordertocalculatetheoperateorcurrentdifferentialofthisrelay,onemustfirstfilterthecurrentsusingaFastFourierorCosineFilter.Thisisdoneinordertoextractallharmoniccomponentsfromthewaveformexceptthe60Hzsignal.Relaysoperateonlyonthefundamentalsignalforallprotectionfunctions.UsingWavewin,wecaneasilyfilterthefundamentalsignalforouranalysis.
1. Takethefundamentalofeachphasecurrent.2. Calculatetheoperatecurrentforeachphasedifferential.
IOPA = |IAT + IAW|IOPB = |IBT + IBW|IOPC = |ICT + ICW|
3. TaketheRMSofeachoperatecurrent.
SincetheBphaseisthecurrentthatoperated,theIOPBwascalculatedfirst.Figures10and11showtheIOPBoriginallygivenbytheprimaryandbackuprelayrecordsalongwiththeIOPBcalculatedbyWavewin.NoticethattheoriginalIOPBwaveformsareslightlydifferentfromtheonescalculatedbyWavewin.Thisisbecauseofthesamplingrateofeachrelay.Inaddition,thebackuprelaycutsoffitsmeasurementwhenthecurrentsvaluefallsbelow0.1A.Anyvaluebelow0.1Aisnottakenintoaccountinprotectionfunctions.Nevertheless,theWavewin-calculatedvaluessimulatetheoriginalrelaysignalsverywell.
Figure10:IOPBCurrentDifferentialforPrimaryRelay
Figure11:IOPBCurrentDifferentialforBackupRelay
Theminimumoperatecurrentofbothrelaysaresettotripata0.5Asecondary.BasedontheinformationseeninFigures10and11,bothrelaysIOPBreachedbeyondthesettingpointof0.5Asecondary.Theprimaryrelayshowsavalueof0.51ampsandthebackuprelayshowsavalueof0.45ampsor0.095p.u.Thesesmalldiscrepanciesaremostlikelyduetothesamplingrateoftherelaydonefortheprotectionfunctions.
Thisprovesthatthecurrentdifferentialleveldidgoabovetheoperatesetting.However,therelayshavetocheckthe2ndharmoniccontentofthewaveformbeforeitdeclaresaninternaltrip.Thiswasexplainedintheprotectionalgorithmsection.Inasimilarmanner,wearegoingtouseWavewintochecktheharmoniccontentofeachdifferentialcurrentforeachrelay.
1. Takethefundamentalofeachphasecurrent.2. Calculatetheoperatecurrentforeachphasedifferential.
IOPA = |IAT + IAW|IOPB = |IBT + IBW|IOPC = |ICT + ICW|
3. Extractthe2ndharmonicsignalofeachunfilteredoperatephasedifferential.
Figures12and13belowshowtheratioofthesecondharmoniccontentascomparedtothefundamentalIOPBsignal.Figure12showstheratiocalculatedbytherelayandWavewinshownasIHB2
andthe2ndHarmonicratiofortheprimaryrelayshownasIB.Figure13showstheharmonicspectrumshowingthesameresult.Itcanbeseenthattheharmonicratioshowninbothsignalsisaround50%whichisabovetherelaysettingof10%.Sincetheharmonicratioisabove10%,thedifferentialelementwillbeblockedfortheprimaryrelay.Inaddition,sincethisrelayusesharmoniccrossblocking,allthreephasedifferentialfunctionswillbeblocked.
Figure12:PrimaryRelayRatioof2ndHarmonicandFundamentalforIOPB
Figure13:PrimaryRelayHarmonicContentofUnfilteredIOPB
Figures14showsthe2ndharmonicratioascomparedtothefundamentalcalculatedbythebackuprelayandWavewin.Basedonthewaveform,wecanobservethattheratioisabout50%whichissimilartowhattheprimaryrelaycalculated.Figure15showstheharmonicspectrumshowingthesameresult.
Figure14:BackupRelayRatioof2ndHarmonicandFundamentalforIOPB
Figure15:PrimaryRelayHarmonicContentofUnfilteredIOPB
Basedonthisanalysis,theBphasedifferentialproducedenoughharmoniccontenttoblockthedifferentialfunctionfromoperating.Sowhydidthedifferentialfunctionstilloperate?Let’slookattheothertwophases’2ndharmoniccontentandseewhattherelaycalculated.
Figures16and17showtheharmoniccontentoftheAphasedifferential.Noticethatthe2ndharmoniccontentcalculatedbytherelayiszero.Ourcalculationshowsacontentof29%.ThechallengewiththissignalisthatthehighandlowsidesofphaseAarealmostidentical.Sowhenyouaddthetwosignalstogetthedifferentialmagnitude,theresultproducesalmostnocurrent.Asaresult,thereisno2ndharmonicsignaltoextract.Inaddition,theactualcurrentvaluesaresosmallthattheygobelowthethresholdcutoffoftherelay.Atthatpoint,therelayinterpretsthatthereisnocurrenttobemeasured.
Figure16:PrimaryRelayRatioof2ndHarmonicandFundamentalforIOPA
Figure17:BackupRelayRatioof2ndHarmonicandFundamentalforIOPB
Figures18and19Error!Referencesourcenotfound.showthe2ndharmoniccontentoftheCPhase.Themeasuredvaluecontinuestobeachallenge,butthesignaturereflectsamuchcleanershapethantheBphase.The2ndharmonicvaluecalculatedbytherelayiszeropercentsincethemeasuredvalueswerebelowthe0.1Athreshold.Ourcalculatedvaluegivesa2ndharmoniccontentofover100%.
Figure18:PrimaryRelayRatioof2ndHarmonicandFundamentalforIOPC
Figure19:BackupRelayRatioof2ndHarmonicandFundamentalforIOPC
Thesecondharmonicanalysisofallthreephasesrevealedthattherewasenoughsecondharmoniccontentinallthreephases.However,theAandCphases2ndharmoniccontentsdefaultedtozeroduetotheminimummeasurementcutoffoftherelays.ThatlefttheBphaseastheonlycurrentwithenough2ndharmoniccontenttoblockthedifferentialelementonbothrelays.Sincetheprimaryrelayusesharmoniccrossblocking,therelayonlyneedstoseeonephasetoblockthedifferential.However,thebackuprelayneedsatleasttwophasestoblockthedifferential.Asaresult,thebackuprelaytrippedondifferential.
CTTconductedin-rushtestingoftheoriginalrecordbychangingthefollowingsettings:• AdaptiveBlockingModefor2outof3• AdaptiveBlockingModeperphase• TraditionalblockingMode2outof3• TraditionalblockingModeAverage
Table4showstheresultsforthein-rushperformedbyCTT.Eachtimethetestwassettotwooutofthreephasesforharmonicblocking,therelaytrippedduringthein-rush.
TestDescription TripAdaptiveBlockingModefor2outof3 YAdaptiveBlockingModeperphase NTraditionalblockingMode2outof3 YTraditionalblockingModeAverage N
Table4:In-rushTestingResults
Basedonthetestsaboveandharmonicanalysisperformedforthisevent,CTTdecidedtoimplementtheperphasemethodusingthetraditionalsecondharmonicsetting.
VII. PRIMARYANDBACKUPRELAYDIFFERENTIALPLANES
Figures20and21belowshowthedifferentialbehaviorduringthein-rusheventfortheprimaryandbackuprelaysrespectively.Eachmanufacturerhasitsownwayofdeterminingitsoperateandrestraintcurrents.Referringbacktoequations2and4,wecanseethatthereisamajordifferenceinhowtherelayscalculateeachphase’srestraintcurrentvalues.Theprimaryrelayaddsthehighandlowsidereactorphasorcurrentstogetherwhereasthebackuprelayuseswhichevervalueishigherinmagnitude.
Thisresultsintheprimaryrelayrestrainingitsdifferentialcurrentsonamuchlargerscaleascomparedtothebackuprelay.Theincreasedrestraintpresentintheprimaryrelayallowsittooperatecorrectlyduringanin-rusheventsuchasreactorenergization.Thisisthefundamentaldifferencebetweenthetwoalgorithmsthatmakestheprimaryrelaymoresecureandreliable.Ontheotherhand,thebackuprelayisfasterandmoresensitiveduringinternalfaults,butisalsopronetomisoperationsduetothelowerrestraintquantities.Itisveryclearthatthebackuprelayoperatedduringtheenergization.
Figure20:2ndHarmonicDifferentialPlaneforPrimaryRelay
Figure21:2ndHarmonicDifferentialPlaneforBackupRelay
VIII. RESONANCEEFFECT
Analyzingfigure22belowshowsthatthetransmissionlinecurrentsextinguishedapproximately5cyclesafterthereactordifferentialrelaytripped.However,thereactorcontinuedmeasuringcurrentsonthehighandlowsides.BasedontheSER,thecircuitswitcherremainedclosedforapproximately30cycles
afterthereactordifferentialtrip.Thesourceofthecurrentsbeingmeasuredbythereactorrelaysisfromthedischargeenergyfromthe345kVline.Thisenergyisbuiltintothelinecapacitancepropertiesforlonglines.Basedonthewaveformanalysisinfigure23,thereseemstobearesonanceeffectbetweenthelinecapacitanceandthereactorreactanceoscillatingaround47Hz.Thevoltagesandcurrentsstarttooscillateat47Hz,producingnon-sinusoidalsignalswhichmakethisshort-termsystemhighlyunbalanced.ThevoltagelevelsonCPhaseforalinetogroundvaluereachashighas326KV.Thisisalmostashighasthephasetophasevalueof345kV.
Figure22:RecordShowingT-LineOpened5cyclesafterReactorTrip
Figure23:ResonanceEffectduetoLineDischarge
Theresonanceeffectalsoaffectedtheinstantaneousandtimeovercurrentsettingsassertingatripsignal.Thegroundovercurrentwaveformisshowninfigure24.Theinstantaneousground50Gelement
inboththeprimaryandthebackuppickedup20cyclesaftertheinitialdifferentialtrip.Asaresult,bothrelayssentsignalstoopenbothlinebreakers,lockingthelineout.However,thelineswerealreadyopenduetothereactordifferentiallockout.Thispresentsachallengeonreclosingforregularlinetogroundfaulteventssincethecircuitswitcherwillnotopenthecurrentsafter30cycles.Thelinewillbelockedoutbeforethereclosingattemptisperformed.Itisimportantthatthegroundovercurrentelementsdonotpickupforthelinedischargecurrentandresonanceeffectwhenthelineistrippedduringnormallinetogroundfaults.
Figure24:ResonanceEffectonGroundOvercurrentElements
IX. CONCLUSION
Sincedifferentialrelaysoffersolutionsformultipleapplications,onecanconcludethattherelayengineermustdeeplyunderstandnotonlytheelementbehavior,butalsohoweachrelaycalculatesitsprotectionfunctionsforthegivenapplication.Thispaperhasdescribedthebehaviorofareactorenergizationandtheresponseoftwodifferentdifferentialrelays.Inaddition,thispaperhasprovidedinformationthatequipsthecustomerandsettingsengineerwiththenecessaryinformationtoproperlyavoidoperationsduringin-rushconditions.
X. REFERENCES
[1] Behrendt,K.;Fischer,N.;Labuschagne,C.,“ConsiderationsforUsingHarmonicBlockingandHarmonicRestraintTechniquesonTransformerRelays,”SchweitzerEngineeringLaboratoriesInc.,2006.
[2] GEURSeriesInstructionManualGEK-113628A,“T60TransformerProtectionSystem,”Markham,
Ontario,Canada,2015.[3] Hunt,R.;Schaefer,J.;Bentert,B.,"PracticalExperienceinSettingTransformerDifferentialInrush
Restraint,"61stAnnualConferenceforProtectiveRelayEngineers,pp.118-141,1-3April2008.
[4] Nashawati,E;Fischer,N.;Le,B.;Taylor,D.,“ImpactsofShuntTractorsonTransmissionLine
Protection,”38thAnnualWesternProtectiveRelayConference,October2011.[5] SEL-487E-3,-4RelayInstructionManual20150626,“CurrentDifferentialandVoltageProtection,”
SchweitzerEngineeringLaboratoriesInc.,2011-2015.
XI. BIOGRAPHY
EricSchroederjoinedCrossTexasTransmissioninJanuary2013andhasmorethan20yearsofexperiencemanagingelectrictransmissionutilitiesandconsultinginthepowerdeliveryindustry.AtCrossTexasTransmission,Ericisresponsibleformanagingbothfieldoperationsandcontrolcenteroperations.PriortojoiningCrossTexasTransmission,EricwasanexecutivetransmissionmanageratTexasMunicipalPowerAgency,overseeingtheelectrictransmissionbusiness.Priortothat,hewasaprojectengineeratPOWEREngineers,aglobalconsultingengineeringfirm.Ericalsohasownedhisownbusinessintheenergyindustryandisaregularspeakeratenergyandutilityconferences.EricholdsaBachelorofScienceinelectricalengineeringfromtheUniversityofTulsa.
JerryBurtonjoinedCrossTexasTransmissioninNovember2013andhasover18yearsofexperienceintheelectricalfieldonprojectsintheresidential,commercial,oilandgas,processandgeneration/transmissionindustries.Mr.Burtonhasfilledseveralpositionsfromapprenticetogeneralforeman,seniorrelaytechnicianandmostrecentlySubstationSuperintendent.Mr.Burtonhasawidevarietyofknowledgeasitpertainstorelaytesting,commissioning,preventivemaintenanceandsubstationconstruction.Mr.BurtoncurrentlyholdsaTexasDepartmentofLicensingandRegulationJourneymanElectricianlicenseandaSubstationJourneymanElectriciancertificatethroughtheUSDepartmentofLabor.
JoePerezreceivedhisB.S.degreeinElectricalEngineeringfromTexasA&MUniversityin2003.JoeistheauthorofmanyrelayapplicationnotesandhaspresentedtechnicalpapersatWPRC,TexasA&MandGeorgiaTechRelayConferences.JoeistheownerofSynchroGrid,aregisteredprofessionalengineerinthestateofTexasandamemberofPSRC,IEEE,andPES.JoeresidesintheBryan/[email protected]
LukeHankinsisfromCleveland,Texas.HegraduatedfromTexasA&MUniversitywithaBachelor’sofSciencedegreeinElectricalEngineering.HeiscurrentlyanE.I.T.andworkingforSynchroGridasaDesignEngineer.Inadditiontosubstationdesign,Lukeisinchargeofrelaysettingsverificationandmis-operationanalysis.HealsowritescodeinC++andVBAthataidincompanyoperation,automatingtasksandimprovingefficiency.
XII. ACKNOWLEDGEMENT
TheauthorswouldliketothankHaleyTriburforhercontributionsinmakingthispaperpossible.