46 • SPRING 2011 using comtrade records to test Protective relays

diffeRential Relay opeRation duRing tRansfoRmeR eneRgizationThe first example is the case of transformer differentialprotectionoperatingduringenergizationduetolowsecondharmoniccurrentcontentintheinrushcurrent.Thiseventwasrecordedbythenumericalrelayprotectinga400MVA230/115kVautotransformer thatwas energized from thehighsidewhilethelowsidewasopen.Theautotransformerisconnectedtoa230kVstraightbusthroughamotorizeddisconnect switch. The CTs are wye connected on bothsides. The 230 kV CTs are on the transformer bushingsconnectedwiththefullratio(1200:5).NotethatatertiarywindingfeedsthestationserviceloadbutdoesnothaveCTsconnectedtothebankdifferential.

Figure 1: Autotransformer Energization from High Side

This is an excellent case to use the COMTRADE record captured by the relay since you can test transformer differential protection to ensure it does not operate during inrush for many applications; that is most two winding transformers and autotransformer banks with five ampere rated CT secondary values on the high side.

Figure 2: High Side CT Secondary Fundamental versus 2nd Harmonic Current

FEATURE

uSIng Comtrade reCordS to teSt proteCtIve relayS By Steve turner

Beckwith Electric Co., Inc.

ThispurposeofthispaperistodemonstratehowtouseCOMTRADErecordstotestprotectiverelays.COMTRADErecordsthathavebeencapturedbynumericalrelaysanddigital fault recordsduringactual systemeventsareofparticular interest sincetheseprovidetheabilitytotestprotectionforcriticalfaultsordisturbancessuchasanout-of-stepconditionthatarehardtocreateusingofftheshelftestsetsoftware.Utilitiesandothercustomerscanbuildalibraryoftestcases.

NETAWORLD • 47using comtrade records to test Protective relays

Figure 2 shows there was very little restraint current andhighmagnitudedifferentialcurrent inB-phaseduringthetransformerenergization.ThetripoccurredwhentheratioofB-phase2ndharmonictofundamentalcurrentdroppedtoolow.

Therelevantcurrentphasorsmeasuredby the relayat thetimeofthetripalongwiththe2ndharmoniccontentsarelistedtotherightinFigure3.

The numerical transformer differential relay that trippeduses internal zero-sequence current compensation topreventunwantedoperationsduringexternalgroundfaults,since the current transformers are connectedwye and thetransformerisanautotransformerbank.

Calculating the phase-to-phase current automaticallyeliminateszero-sequencecurrentasfollows:

Ia=I1+I2+I0

Ib=a2I1+aI2+I0

Ic=aI1+a2I2+I0

Iab=Ia–Ib=I1(1–a2)+I2(1–a)

Ibc=Ib–Ic=I1(a2–a)+I2(a–a2)

Ica=Ic–Ia=I1(a–1)+I2(a2–1)

If the transformer differential relay uses phase-to-phasecurrent to eliminate zero-sequence current then Ibc isthe most depleted of 2nd harmonic content and alsocorresponds to the phase that actually tripped (B-Phase).Figure5illustratesthefollowing:

• Ibc

• fundamentalcomponent

• 2ndharmoniccontent

• ratioof2ndharmonictofundamental

The ratiodecreases inmagnitudeover thefirst two cyclesfollowingenergization.Therelaytrippedatthepointwhenthe ratio dropped to 14 percent. Note that transformerdifferentialrelaysaretypicallysettorestrainat15percent.

Figure 3: Current Phasors Measured at the Relay with 2nd Harmonic Current

Figure 4: Current 2nd Harmonic Restraint Logic

Figure 5: Current Phasors Measured at the Relay with 2nd and 4th Harmonic Current

test RequiRementsYou will need a three-phase test set that can playbackCOMTRADErecords.Threecurrentchannelsarerequired.Connect the three-phase test set to the relay as shown inFigure6A(Seefollowingpage).

FEATURE

Ratio = diff

nddiff

II 2

100%

48 • SPRING 2011

Figure 6A. Test Connections

Figure 6b: Test software

Figure6Bshowsoff-the-shelfsoftwareavailabletoplaybackthisparticularCOMTRADErecordthroughthetestsettotherelay.

test pRoceduRe 1. Firstplaybacktheinrushcasetotherelaywith

harmonicrestraintdisabled.

2. Therelayshouldtripwhenharmonicrestraintisdisabled.

3. Iftherelaytripsthenplaybacktheinrushcaseagainwithharmonicrestraintenabled.

4. Therelayshouldnottripwhenharmonicrestraintisenabled.(SeeFigure7)

Figure 7: Transformer Inrush Test Procedure Flow Chart

advanced test – adjusting the level of 2nd haRmonic contentItispossibletoreducetheamountof2ndharmoniccontentpresent in the inrush current during the injection test.You can reduce the level of 2nd harmonic current untilthe restraint no longer blocks the differential protection.For example, 10 percent is typically the minimum levelacceptable toset the2ndharmonicrestraint;otherwise, ifit were set lower, tripping might be significantly delayedforheavyinternalfaultsduetoharmonicsgeneratedbyCTsaturation.ThesoftwareshownbelowinFigure8illustratesthisprocess:

1.Firstisolatethefundamentalcomponentand2ndharmoniccomponentinB-Phasecurrent(IB).

2.Nextmultiplythe2ndharmoniccontentbyafactortoreduceitsmagnitudetothepickuplevelselectedforthe2ndharmonicrestraint.Forthisparticularcasetheminimumpickupis10percent.Therefore,themultiplicationfactoris0.7(i.e.,).

3.ReassembletheB-Phasecurrentbyaddingthefundamentaland2ndharmonicbacktogether(DepletedIBinthecaseofthefigurebelow).

4.Finallyinjecttheadjustedcurrentintotherelay.

Figure 8: Adjusted Inrush Current

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using comtrade records to test Protective relays

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loss-of-f ield geneRatoR pRotection opeRationThesecondexampleisanactualloss-of-fieldeventcapturedbyanumericalgeneratorprotectionrelay.TheeventisshowninFigure9below.

Figure 9: Generator Loss of Field

Thenumericalrelaythattrippedandcapturedtheeventwasconfiguredfor115voltsline-to-lineand5amperessecondary.Themaximumcurrentmeasuredbytherelayduringtheeventwas1.9amperes secondary. Figure 10 shows the impedance trajectory (ZAB) corresponding to the lossoffield.It issimplertoviewonephase-to-phaseimpedancesincethelossoffieldisabalancedthree-phaseevent.Notethatthescaleintheimpedancediagramiskiloohms.Figure10showsthecorrespondingpointsintimeinthecurrentchannels.Table1showsthecorrespondingtimeline.Notethattheimpedancetrajectorydoesnothaveaconstantvelocity.

FEATURE

using comtrade records to test Protective relays

50 • SPRING 2011 using comtrade records to test Protective relays

FEATURE

Figure 10: Loss-of-Field

Impedance Trajectory

Figure 11: Loss-of-Field Current

Channels

table 1: Loss-of-Field

Timeline

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Figure 12: Loss-of-Field Impedance Function Characteristic and Impedance Trajectory

Figure12aboveshowsaloss-of-fieldimpedancecharacteristicsuperimposedonthetrajectory.Thecharacteristichasanoffsetequaltothreeohmssecondaryandadiameterof32.5ohmssecondary.

Figure 13: Loss-of-Field Zone 2 Impedance Settings

using comtrade records to test Protective relays

FEATURE

52 • SPRING 2011

Providing a safer working environment for people working

on energized electrical equipment

using comtrade records to test Protective relays

FEATURE

test RequiRementsYou will need a three-phase test set that canplay back COMTRADE records. Threevoltagechannelsandthreecurrentchannelsarerequired. Connect the three-phase test set totherelayasshowninFigure14above:

test pRoceduRe 1.Configuretherelaysothatoneoutput

closeswhenthe78functionfirstpicksup(O1)andanothercloseswhenthe78functiontrips(O2).

2.ConnectthetwooutputstothetestsetsuchthatO1startsthetimerandO2stopsthetimer.

3.Playbacktheout-of-stepcasetotherelay. 4.Youcanmeasurethetimerequiredfor

thefunctiontooperate,whichshouldbe18cyclesplusthetimedelay.

conclusionsThis purpose of this paper is to demonstratehow to use COMTRADE records to testprotective relays. COMTRADE records thathave been captured by numerical relays anddigital fault records recorded during actualsystem events are of particular interest sincetheseprovidetheabilitytotestprotectionforcriticalfaultsordisturbancessuchasanout-of-stepconditionthatarehardtocreateusingoffthe shelf test set software. Utilities and othercustomerscanbuildalibraryoftestcases.

The first case uses a COMTRADE recordcaptured by a numerical transformer relayto ensure your protection does not operateduring inrush. This case is suitable for manyapplications; that is most two windingtransformers and autotransformer banks withfiveampereratedCTsecondaryvaluesonthehighside.

The second example is an actual loss-of-fieldevent captured by a numerical generatorprotectionrelay.

Steve Turner is a Senior Applications Engineer at Beckwith Electric Company. His previous experience includes working as an application engineer with GEC Alstom, an application engineer in the international market for SEL, focusing on transmission line protection applications. Steve worked for Progress Energy, where he developed a patent for double-ended fault location on transmission lines.

Steve has both a BSEE and MSEE from Virginia Tech University. He has presented at numerous conferences including: Georgia Tech Protective Relay Conference, Western Protective Relay Conference, ECNE and Doble User Groups, as well as various international conferences. Steve is a senior member of IEEE.

Figure 14: Test Connections