WHEN EXISTING RECOMMENDATIONS FORPST PROTECTION CAN LET YOU DOWN Z. Gaji 1), M. Podboj 2), B. Traven 3), A. Kraovec 4) 1) ABB SA Products AB; Sweden; zoran.gajic@se.abb.com 2) ABB d.o.o.; Slovenia 3) ELES d.o.o.; Slovenia4) Istrabenz Gorenje Ineniring d.o.o.; Slovenia Keywords: phase shifting transformer, differential protection. Abstract Thepaperwillpresentimplementedprotectionschemeon two,symmetrical,doublecoredesignPhaseShifting Transformers(PST)[6],eachratedat600MVA,400kV, 866A,50Hz,40withintotal64tappositions(32advance and32retard).Thus,everyindividualtapintroducephase angle shift of 1,25. TheusedPSTprotectionschemefollowstheprotection recommendationspublishedbyIEEE[1].Itshallbenoted thatthisIEEErecommendationismainlybasedon information available in reference [2]. Howeverduringexecutionofthisprojectnewchallenges were faced due to following reasons: Slightly different construction of PSTNumericaldifferentialrelaysareusedinsteadof electro-mechanical ones originally suggested in [1,2] Due to these differences this paper will: Indicate atwhich statements IEEEpublication[1] is wrong due to slightly different PST design Providedesignandsettingguidelinesfor87Pand 87Sfunctionswhenmodernnumericaltransformer differential protection is used 1Introduction References[1]and[2]providequiteclearguidelinesfor protectionschemeofsymmetrical,doublecorePSTdesign. Howevertheyarewrittenmorethantenyearsagofora particular variant of double core PST design [6].In reference [2] PST with the following rated data were used formostcalculations:450MVA,345kV,753A,60Hz,25 withintotal32tappositions(16advanceand16retard). Thus,everyindividualtapintroducephaseangleshiftof 1,56.ItisobviousthatnewPSThasmuchbiggernumberoftap positions.Inprinciplethisshallnotinfluencetheprotection schemebecauseofthefactthatproposedschemein[1,2]do not need any tap changer information for its correct operation. Howeveritwillbeshownthatthisstatementisactually wrong!ThusevenforsymmetricaldoublecorePST,the designoftheprotectionschemecanactuallybeconstruction dependent! Proposed protection schemein [1]and[2] wasbasedon two differentialprotectionrelays.Thefirstdifferentialrelay (called primary protection system or 87P in [1,2]) is based on the first Kirchhoffs low. Thus in principle one can say that it issimilartothestandardbusbarorgeneratordifferential protectionrelay(i.e.87Bor87G).Thisdifferentialrelayhas three restraint inputs and measures three CT sets marked with numbers1,2and3inFigure1.NotethatallthreeCTsets shall be star connected in accordance with [1,2]. Theseconddifferentialrelay(calledsecondaryprotection systemor87Sin[1,2])isbasedontheampere-turn-balance betweenthreewindingsexistinginsideoftheseries transformer. Thus, one can say that it is the same principle as usedby the differential protection relays applied for standard powertransformers(i.e.87T).Ampere-turn-balanceofthe series transformer is used because this is the only transformer whichhasfixednumberofturnsinthistypeofPSTdesign. The87Sdifferentialrelayhasthreerestraintinputsand measuresthreeCTsetsmarkedwithnumbers1,2and4in Figure1.FirsttwoCTsetsshallbedeltaconnectedinorder tofacilitatephaseangleshiftcompensationandzero-sequencecurrentreduction,whilethethirdCTsetisstar connected in accordance with [1,2]. However it shall be noted that these two differential relays do not provide complete redundancy for each other. For example the87Pdonotprovideanyprotectionforfaultswithinthe secondarywindingofeitherseriesorexcitingtransformer. Thereforetwoadditionalgroundfaultovercurrentrelaysare suggestedinthetwostarpointsoftheexcitingtransformer [1,2]. Their pickup and time delay settings depend very much onparticularPSTconstructiondetails(e.g.threeversusfive limb core design, test winding arrangements, etc.). NotethatseparateCTcoresarerequiredforthesetwo differentialrelaysbecauseofthedifferentmainCT connections required on S- and L-side of the PST. 2Influence of slightly different construction First it shall be understood that voltage in the secondary, delta connected winding of the series transformer must be swopped (i.e.reversedfor180)foradvanceandretardmodeofPST operation.Thisswoppingcanbeachievedindifferentways dependingonparticularconstructiondetailsofeach individual double core PST.PSTdescribedin[1,2]hadintotalonly32tappositions(16 advance and 16 retard). Thus, it was technically possible and mosteconomicalthatthisvoltagereversalisintegratedand performed within theon-load tap-changer itself, byreversing directionofcurrentflowthroughthesecondarywindingof the exciting transformer. Therefore if one looks from outside PSTsuchswitchingarrangementhaseffectofreversinga vector group connection of the exciting transformer from Yy0 toYy6foradvanceandretardoperatingmodesrespectively. Suchswitchinghasnoinfluenceonsecondarydifferential relaywhichisbasedonampereturnbalanceoftheseries transformer windings. The PSTused inourprojecthasin total64 tappositions(32 advanceand32retard).Thus,itwasnottechnicallyfeasible to arrange this voltage reversal in the same way as in previous design.Insteadanadvance-retard(A/R)switch,aseparate mechanicaldevicefromtheon-loadtap-changeritself,is locatedwithinconnectionstowardsthedelta-connected winding of the series transformers (see Figure 1). Therefore if onelooksfromoutsidePSTsuchswitchingarrangement has effect of reversing a vector group connection of the series transformerdelta-connectedwindingfromd3tod9(seethe nextsectionofthepaperformoredetailsregardingseries transformerclocknumbers)foradvanceandretardPST operatingmodesrespectively.Suchinternalswitching arrangement has great influence on the secondary differential relaywhichnowneedsextrafacilitiesinordertoremain stableduetothiseffectivechangeoftheprotected transformer vector group during normal relay operation.AdditionallythisinternalPSTswitchingisdoneinaquite specialway.Namelyallcornersofthesecondary,delta-connectedwindingoftheseriestransformersarefirst temporarygrounded(forcoupleofseconds)whilethis switchingisperformed.Thiscorrespondstoanintentional three-phaseshortcircuitwithintheprotectedzoneofthe differentialrelay.Thus,thesecondarydifferentialprotection relaywillbeentirelyde-balancedduringthisswitching time and its restraint stage must be completely blocked while the operation of the advance-retard switch is in progress.Tosummarize:ForthisPSTconstructionthesecondary differentialrelayneedsthefollowingthreeexternalbinary signals for its proper operation: Advance mode of PST operation Retard mode of PST operation Advance-retard switch operation is in progress The firsttwobinarysignalsare requiredinorder to facilitate effectivevectorgroupchangeoftheprotectedseries transformer.Thisforexamplecanbedonebyintentionally inverting(i.e.turningfor180)individualphasecurrents measuredintheexcitingtransformersecondarywindingstar point, during retard operating mode of the PST. Alternatively, someotherwaytofacilitatetheprotectedtransformervector groupchangecanbeengineereddependingonaparticular differentialrelayusedintheproject.Thethirdbinarysignal shallbeusedinordertoblocktherestraintstageofthe secondarydifferentialrelay.Commissioningtestshallbe done in order to verify that this binary signal has good timing incomparisonwithoperationoftheprimarycontactsofthis switch.Ifthisisnotthecasesomeotheralternativesolution shallbearranged.Itisstrictlyrecommendedtoperformthis timing test in bothoperating directions of the tap-changing sequence(i.e.fromadvancetoretardmodeandvice-versa). Note that operation of the advance-retard switch is done when on-loadtap-changer is in middle (zero)position (i.e.for PST phase angle shift of zero degrees). S LSeriesTransformerExcitingTransformer2 1351200/1 1200/11000/11000/1 3000/1A/R SwitchN1N2N1UQuadU463000/1Other phase of the delta winding Figure 1: PST construction and CT information 3 Influence of numerical differential relays References[1] and [2]are written foroldelectro-mechanical differentialrelays.Allsettingcalculationsforsecondary differentialrelayisbasedonak-factor(i.e.seriesunitturns ratio). This k-factor is defined in [1,2] as: 1 12QuadUN NkN U+= =(1) whereN1andN2areexactturnnumbersoftheseries transformerwindingsandUQuadandUareno-loadvoltages acrossseriestransformerwindings.Forexactdefinitionsof all used symbols in Equation (1) see Figure 1.Modernnumericaldifferentialrelayforstandardthree-phase powertransformers[3]typicallyrequiresonlytherateddata oftheprotectedpowertransformer(i.e.ratedapparentthree-phasepower,ratedph-to-phvoltageforeachwinding,rated phase current for each winding and power transformer vector group[5])inordertoautomaticallybalanceitself.Therefore inordertoproperlyapplyfor examplesecondarydifferential protection for a PST application one now need to derive rated dataofanequivalentthree-phasepowertransformerwhich correspondsexactlytotheseriestransformerusedinsideof the PST. 3.1 Solution for the primary differential protection 87P Primarydifferentialprotectionisbasedonthefirst Kirchhoffslow.Howeverinpracticeveryoftenstandard differential protection for a three winding, three-phase power transformer is used instead. Therefore for relay setting values, one needs to enterdatafor anequivalent three-phasepower transformer. In this application one can easily check that the ratedcurrentofaPSTonS-oronL-sideis866Aatrated voltageof400kV.Thatcorrespondsto600MVA.Therefore rateddatafortheequivalentthree-winding,three-phase powertransformerare:600MVA,400/400/400kV; 686/686/686A,Yy0y0.AllthreeCTsetsshallbestar-connected and typically stared towards the protected object as showninFigure1.Zerosequencecurrentreductionshallbe disabled on all three sides in order to maximize the sensitivity oftheprimarydifferentialprotectionforinternalground faults.Foroneparticularmakeofthenumericaldifferential relay [3], actual setting parameters are given in Figure 2. Figure 2: 87P Compensation settings for 600MVA PST Thispaperwillnotgoinmoredetailsregardingthepickup (i.e. operating characteristic) setting for the used 87P relay. 3.2 Solution for the secondary differential protection 87S Secondary differential protection is based on the ampere-turn-balance between three windings installed around the magnetic coreoftheseriestransformer.Inpracticeveryoften differentialprotectionforastandard,threewinding,three-phasepowertransformerisusedforsuchapplication. Thereforeassettingvalues,oneneedstoenterdataofan equivalent three winding, three-phase power transformer for the PSTseriestransformer.Let us assume that CT locatedin PSTS-bushingsareusedforwinding1(W1)input,CT located in PST L-bushings are used for winding 2 (W2) input andCTlocatedinexcitingtransformersecondarywinding star pointare used for winding 3 (W3) input.Thisequivalenttransformercanbeunderstoodinaneasiest way if one imagine a fictitious star point in the middleof the two 400kV windings of the series transformer (i.e. at the point where exciting transformer primary winding is connected). If this is done than the series transformer can be represented as a standard,three-winding,Yydpowertransformer.Nowonly rated power, rated ph-to-ph voltages, rated phase currents and vectorgroupnumbers(i.e.clocknumbers)shallbederived forsuchequivalentstandard,threewinding,three-phase power transformer. The rated voltage of the twoy-connected, 400kV windings connectedinseriesbetweenSandLPSTbushingscanbe calculatedformPSTratingdataandmaximumphaseangle shift as follows: 400 40*2*sin 157, 972 3oQuadkVU kV = = (2) Thus,theph-to-gndvoltageofonestar-windingwillonlybe onehalfofthisvalue(i.e.78,99kV).Thecorrespondingph-to-ph voltage of the equivalent star connected winding will be 3times bigger (i.e.3*78,99kV=136,8kV ). Theratedvoltageofthesecondarydelta-connectedwinding oftheseriestransformercanbeobtainedinoneofthe following ways: By reading it from PST rating plate (if available) Calculated form UQuad by using k-factor (if available) For this particular PST k-factor was available and it has value of1,6156. Thus the ph-to-ph voltage for the delta winding can be calculated by using equation (1) as follows: 157, 9797,81, 6156kVU kV= = (3) Theratedpoweroftheseriestransformercanbecalculated fromthePSTratingdataandmaximumphaseangleshiftas follows: 40600 *2*sin 410, 422oSTS MVA MVA = = (4) Fromthispowervalueandavailableph-to-phvoltagesrated phase current for every winding can be calculated as follows: 1 2410, 4217323*136,8W WMVAI I AkV= = =(5) 3410, 4224233*97, 8WMVAI AkV= = (6) Note that rated current of the first two windings is exactly two times bigger than the PST rated current (i.e. 1732A=2*866A). Thereasonisthatthecalculatedpoweroftheseries transformer(i.e.410,42MVA)isactuallyonlyvalidforthe deltaconnectedwinding.Powerfromthisdelta-connected winding, splits into two equal parts which are then transferred intotwoprimaryy-connectedwindings(i.e.W1andW2). Thusthetwo400kV,series-connectedwindingsareactually only thermally rated for one half of the total calculated power oftheseriestransformer(4).Howeverfortheseries transformerdifferentialprotectionfunctionthebaseisthe maximum power among all three windings [4]! Regarding the clock numbers one need the detail internal PST connections inorderto derive them. However due to the fact thatquadraturevoltage(i.e.90shift)isintroducedbythe seriestransformer,thevectorgroupofthistransformershall reflect this fact and in practice typically is Yy6d9 for advance operating mode and Yy6d3 for retard mode of PST operation. Thereforethedatafortheequivalentthree-winding,three-phasepowertransformeroftheseriestransformeris: 410,42MVA,136,8/136,8/97,8kV;1732A/1732A/2423A, Yy6d9 for this PST.Allthree CT sets shallbestar-connectedand typicallystared towards the protected transformer as shown in Figure 1. Zero sequencecurrentreductionshallbeenabledfortwoy-connectedwindings(i.e.W1andW2)inordertoprevent unwantedoperationofthe87Sdifferentialprotectionfor externalgroundfaultsinsurrounding400kVnetwork.For oneparticularmakeofthenumericaldifferentialrelay[3], actual setting parameters are given in Figure 3. Figure 3: 87S Compensation settings for 600MVA PST Thispaperwillnotgoinmoredetailsregardingthepickup (i.e. operating characteristic) setting for the used 87S relay. Figure 4: SLD and associated protection scheme for the two PSTs and associated bypass bay Series transformer Exciting transformer 4Implemented protection scheme ProtectionblockdiagramforthePSTandassociatedbypass bay is shown in Figure 4. Due to processing capability of used numerical IED [3] complete protection scheme recommended by IEEE [1,2] (and even more) can be implemented with one IEDonly.Thefollowingfunctionsareavailableinasingle protection IED used in this PST installation: 87P differential protection 87S differential protection I> (1)&(2) S-side overcurrent protection I> (1)&(2) L-side overcurrent protection Z< (13) S-side underimpedance protection Z< (14) L-side underimpedance protection I> (6) HV-NP side overcurrent protection I> (9) LV-NP side overcurrent protection I> (7)&(8) HV-NP side earth-fault protection I> (10)&(11) LV-NP side earth-fault protection U>(12) S- and L-side overvoltage protection Note that function symbolsusedin theabove list correspond tosymbolsusedinFigure4.Actualfunctionpickupvalues andsetoperatingtimedelaysarealsogiveninFigure4. ProtectionschemeredundancyisobtainedbyIED duplication.Thus,twoIEDswithidenticalfunctionality,as listed above, are used on each PST (see Figure 4 for details). 5Field recordings ThisPSTprotectionschemeisinfullcommercialoperation onbothPSTssinceDecember2010.Itsoperationis completelysatisfactory.Duetolimitedspaceonlyone recordingcapturedduringthePSTcommissioningis presentedinFigure5.Itshowsprocessofchangingthe advance-retardswitchpositionfromretardtoadvancestate. ThefollowingtracescapturedbyIEDareshowninFigure5 (note that all shown currents are given in primary amperes):a)CT1 (see Figure 1) three phase currents waveformsb)CT2 (see Figure 1) three phase currents waveforms c)CT3 (see Figure 1) three phase currents waveforms d)CT4 (see Figure 1) three phase currents waveforms e)87Pdifferentialprotectionbiascurrentandthree-phase differential currents (RMS values) f)87Sdifferentialprotectionbiascurrentandthree-phase differential currents (RMS values) g)Relevant binary signals (see section 2 of the paper) The numbers from 1 to 8 added to Figure 5 indicates the most important events in this record. Their short description is: 1)Inprogresssignaloftheadvance-retardswitchappears and 87S protection is immediately blocked. 2)Indication that PST is in retard operating mode disappears 3)CT4currentsdisappear(i.e.allcornersoftheseries transformer delta winding are shorted to ground). 4)WithoutCT4currents87Sprotectionisunbalancedand calculatesfalsedifferentialcurrentsbutitisalready blocked and do not cause unwanted tripping of the PST. 5)After few seconds CT4 currents reappear again. 6)87Sprotectionstillcalculatesfalsedifferentialcurrents duetoafactthatvectorgroupofseriestransformerhave been changed by the advance-retard switch operation! 7)Indication that PST is in advance operating mode appears. 8)CT4currentsarenowintentionallyinvertedintheIED softwarewhichcausesproperdifferentialcurrent calculationwithin87Sfunction.Afterthatpoint87Strip signal can be enabled again. This record clearly indicates all points discussed in Section 2. At the same time it confirms that 87S differential protection is properly set for this PST. Note the following facts: Record is 11s long Advance-retard switch operation takes about 9,5sCT1andCT2currentsdonotchangeatallduring this record CT3 currents are practically zero because PST has 0 phase angle shift (OLTC is in middle position) 87Pdifferentialprotectionisnotaffectedatall during these events. 6Conclusion ExistingrecommendationforPSTprotectionshallbeused wisely. If on the contrary they are followed blindly one can be verymuchsurprisedwhentroublesareencounteredduring PST commissioning or even PST operation. Thepaperalsoprovidesguidancehowtosetmodern numerical87TdifferentialrelaysfordualcorePST87Pand 87S differential applications. References [1]IEEESpecialPublication,ProtectionofPhaseAngle RegulatingTransformers(PAR),AreporttotheSubstation SubcommitteeoftheIEEEPowerSystemRelaying Committee prepared by Working Group K1, (1999). [2]M.A.Ibrahim,F.P.Stacom.PhaseAngleRegulating TransformerProtection.IEEETransactiononPower Delivery, Vol 9, No 1, January (1994). [3]ABBDocument1MRK504086-UEN,"Technical referencemanual,TransformerProtectionIEDRET670", Product version: 1.1, ABB Power Technologies AB, Vsters, Sweden, (2007). [4]Z.Gaji,Differentialprotectionmethodologyfor arbitrarythree-phasepowertransformers,DPSP2008 Conference, Glasgow-UK, (2008) [5]InternationalStandardIEC60076,Powertransformers, First edition, (1997). [6]InternationalStandardIEC62032/IEEEC57.135.Guide for the application, specification, and testing of phase shifting transformers, First edition (2005).

a) SOURCE IL1 SOURCE IL2 SOURCE IL3t/s0 1 2 3 4 5 6 7 8 9I/A-50050 b) LOAD IL1 LOAD IL2 LOAD IL3t/s0 1 2 3 4 5 6 7 8 9I/A-50050 c) ZV HV IL1 ZV HV IL2 ZV HV IL3t/s0 1 2 3 4 5 6 7 8 9I/A-50-2502550 d) ZV LV IL1 ZV LV IL2 ZV LV IL3t/s0 1 2 3 4 5 6 7 8 9I/A-200-1000100 e) 87B IDL1 87B IDL2 87B IDL3 87B IBIASt/s0 1 2 3 4 5 6 7 8 9I/A010203040 f) 87T IDL1 87T IDL2 87T IDL3 87T IBIASt/s0 1 2 3 4 5 6 7 8 9I/A050100150 g) t/s0 1 2 3 4 5 6 7 8 9BLOKADA 87TARS-INPROGARS-RETARDARS-ADVANCE Figure 5: DR captured by the IED under operation of advance/retard switch during PST commissioning4 6 8 27 3 5 CT1 CT2 CT3 CT4 87P 87S 1