晋东南侧保护测量零序电流补偿方案研究

晋东南侧保护测量零序电流补偿方案研究

0表面处理。Thefirst1000kVultrahighvoltage(UHV)demonstrationprojecthassignificantimportancetotheintending1000kVbackbonepowergridinChinaMuchresearchhasbeendonerecentlyfromdifferentaspects.Itiswellrecognizedthatzero-sequencecomponentsbasedrelaysarewidelyappliedforthemainorbackupprotectionschemes.Forinstance,thefaultphaseselectorcurrentlyusedinChinautilizesthephaserelationshipbetweennegative-andzerosequencefaultcurrent.Meanwhile,inEHV/UHVpowersystems,theneutralpointisalwaysdirectlygroundedtolimittheover-voltageofun-faultyphasesaftersinglelinetoground(SLG)faultwhoseoccurrencepossibilityismorethan85%,causingzero-sequencecomponentsbasedrelaysmoreappropriateandsensitive.Inthebackgroundofthefirst1000kVUHVprojectinChina,aspecialproblemisregardedasnoticeableaccordingtorealtimedigitalsimulator(RTDS)testsontheJindongnan-Nanyangtransmissionline.Thezero-sequencecurrentmeasuredatremoteterminalrelayisseverelydeviatedfromsinusoidaftertheoccurrenceoftheSLGfaultneartheJindongnanendbecauseofthehighamplitudeofinterferencecomponentcomparedtothatofthefundamentalfrequencycomponent.Ofcoursethisisdefinitelyintoleranttothepresentfilteringalgorithmofprotectiverelays.Thereforetheincorrectoperationsofzero-sequencebasedrelayschemesareinevitable.Inthispaper,thespecialphenomenonistheoreticallyanalyzedbasedonLaplacetransformandnodalvoltagealgorithm.Theverycontributortosuchdistortionisconfirmedandhenceaneffectivesolutionisproposed,whichistosparkoutthegapaccordingtothesignalsentbyinitialprocedureimmediatelyafterthefaultinception.Thedistortionisthensignificantlyweakened.Atthesametime,theinfluenceofsignaltransmissiondelayanddampingresistancesettingarealsoinvolved.Finally,correspondingsimulationsverifythecorrectnessandeffectivenessoftheproposedmethod.Itisexpectedthattheinvestigationscouldbehelpful.1目标函数和程序Thefirst1000kVUHVdemonstrationprojectisshowninFig.1.Thecomprehensiveevaluationsandtestsfortheprotectiveschemesofthe1000kVUHVprojectareindispensablebeforeputtingintooperationinfield.Aspecialphenomenonisobservedandrecognizedasnoticeable.WhenaSLGfaulttakesplaceneartheJindongnanendoftheJindongnan-Nanyang1000kVUHVtransmissionline,thezero-sequencecurrentmeasuredatrelay2isseverelydeviatedfromsinusoid,directlythreateningcorrespondingfaultphasediscriminationandrelayoperationsbasedonzero-sequencecomponents.Furtherinvestigationsshallbecarriedoutforseekingthemaincontributorandtheeffectivesolution.SupposingphaseAasthefaultphase,andthefaultinceptiontimeis100ms.Thethreephasecurrentsandthezero-sequencecurrentmeasuredatrelay2areshownintheFig.2(a)andFig.2(b),respectively.ThefastFouriertransform(FFT)isappliedtogettheamplitude-frequencycharacteristicwithinthefirst100msafterthefaultinceptioninordertoevaluatetheinfluenceontheprotectiverelaysasshownFig.3Becausethefrequencyresolutionisinverselyproportionaltothesamplingtimewindow,andthefrequencyresolutionis10Hz.Asexpected,thereexistsaseriesofinterferencefrequencycomponents,especiallythelowfrequencycomponentswiththeamplitudeapproximatetothatofthefundamentalfrequencycomponent.Meanwhiletheamplitudeofinterferencecomponentshasbeendecreasingsincethefaultinception.Itisthereforewellunderstoodthattheamplitudeofinterferencecomponentsmustbemuchlargerthanthatoffundamentalfrequencycomponentintheinitialtimequantumafterthefaultinception.Uptonow,thefull-periodFourierarithmetichasbeenwidelyappliedasthefilteringalgorithm,whosefilteringcharacteristicisshownasFig.4.ObviouslytheDCcomponentandintegerharmonicscanbetotallyfilteredout.However,thefilteringeffectversusinterharmonicsisdecreasedespeciallyforlowfrequencycomponents.Asaresult,thefilteringeffectisdefinitelydissatisfactoryversuslowfrequencycomponentsofsuchhighamplitudeasshowninFig.4.Thefaultphasediscriminationandcorrespondingoperationsoftherelaysbasedonzero-sequencecomponentscouldhardlybeacceptableiftheSLGfaultmentionedabovehappens.Correspondinganalysishastobecarriedouttofindtheinfluentialfactorsaswellasthesolution,whicharetobediscussedasfollows.Thedistributedcapacitoronthe1000kVUHVtransmissionlinecouldhardlybeignored.Meanwhile,theseriescapacitorsshouldalsobetakenintoconsideration.Theamplitude-frequencycharacteristicoftransientcurrentafterfaultsisdecidedbythecircuitparametersbecausetheinvolvedinterferencelowfrequencycomponentsarederivedbythetransientprocessofthecorrespondingcapacitorsandinductors.Inordertofindaneffectivesolution,themaincontributorstotheseveredistortion,whichcouldalsobeprocessedafterthefaultinception,havetobeconfirmed.TheequivalentcircuitoftheaimingsystemisshowninFig.5inwhichthetheoreticalanalysisiscarriedout.ThedetailedparametersinvolvedarelistedinAppendixA.representthezero-sequenceinductancesofthesendingend,receivingendandNanyangsystemrespectively.C1,C2andC3representtheseriescapacitors.Jindongnan-NanyangandNanyangJingmentransmissionlinesaremodeledasTtypewheretheinductancesareexpressedasL1,L2,L3,L4separatelyandthedistributedcapacitorsasC4andC5Thefaultvoltageofzero-sequenceaddedatfaultpoinAisdenotedas‘Uf0’.Meanwhile,thezero-sequencevoltageatpointB,CandDintheequivalentcircuiaredenotedas‘UB’,‘UC’and‘UD’respectively.TheanalysisistobecarriedoutwithLaplacetransform.Thecharacteristicequationversustherighsideofthefaultpointinequivalentcircuitisexpressedbelowandcorrespondingresistancesareomitted.However,theanalyticalsolutionisimpossibletosolveduetotheextremelycomplicatedrelationshipsofallvariablesinvolvedin(1).Onlythenumericasolutioncanbegotas(2).Therearetwolowfrequencycomponents(8.813.2Hz)andhighfrequencycomponents(137.4196.1Hz),causedbytheseriescapacitorandthedistributedcapacitorrespectively.Consequently,theanalysiscanbeachievedwithnodalvoltagealgorithminsteadinordertoconfirmthekeypointofsuchseveredistortion.ThegroundisrecognizedasthereferencepointForpointA,wehaveForpointB,thefollowingrelationshipisobtainedThezero-sequencecurrentmeasuredatrelay2(denotedas‘I20’)canbeexpressedasThusI20canalsobeobtainedaccordingto(4).TheLaplaceformatofthefaultvoltageUAisrepresentedasU(s)forconvenience.Meanwhile,UBcanberepresentedbyUAaccordingto(3)asfollowsCombining(6)~(8),I20canbecalculatedas(9)Itcanbeconcludedthatfivevariablesin(9)areresponsibletothespecialphenomenonatrelay2.However,U(s)isdecidedbythepre-faultoperatingconditionwhileC4,L1andLsareinherentparametersofthetransmissionlineandthesendingsystem.ThereforecorrespondingschemeisexpectedtobeprocessedversusseriescapacitorC1torestrainthedistortionofzero-sequencecurrent.Infact,thistransientprocessiscausedbythedischargingofseriescapacitorC1.3通过结语:非织造程序救济出了自身的真实表达Thefilteringeffectversusinterharmonicsdecreasessignificantlyforlowfrequencycomponents.Thesamplingwindowhastobeextendedforacorrectjudgmentifotheroptimalalgorithmsareconsidered,especiallyforlowfrequencycomponentwithsuchhighamplitude.Theadditionaltimeisnotpermittedfor1000kVUHVtransmissionlineprotection.InordertoacceleratetheattenuationofthedischargingprocessofseriescapacitorC1,anewparallelbranchshouldbecreated.Asaresult,thebestwaytolimitthelowfrequencycomponentistoshuntitwitharesistorafterthefaultinceptionassoonaspossiblebecausethefaultphasediscriminationandoperationsofrelatedrelaysarevulnerabletosuchhighamplitudeofthelowfrequencycomponent.AssumingthacapacitorC1isshuntedwith1uf057resistanceatthefaulinception,thezero-sequencecurrentmeasuredatrelay2andtheamplitude-frequencycharacteristicsarecompared,asthecurvesinFig.7(a)andFig.7(b).ComparingthetwocurvesinFig.6(a),thedistortionofthezero-sequencecurrentissignificantlyweakenedwiththeshuntoperationof1uf057resistanceatthefaultinception.InordertoillustratetheinterferencelevelofthelowfrequencycomponentparameterKisdefinedas(10).TheIInt.Maxrepresentstheamplitudeofthemosseverelowfrequencycomponent,andtheIFunrepresentsthatofthefundamentalfrequencycomponent.TheparameterKcanillustratetheinfluentialdegree.ThefilteringeffectofFourierarithmeticcouldhardlybeacceptableiftheKvalueishigh.AccordingtoFig.6(b),theKvaluegreatlydecreasesfrom102.82%to14.95%duetothisshunoperation.Thezero-sequencecurrentmeasuredarelay2ispredominantlythefundamentalfrequencycomponent,whichcouldbeutilizedbythesubsequenoperationsofzero-sequencebasedrelays.Atthesametime,theamplitudeoffundamentafrequencycomponentaftertheshuntoperationislargerthanthatwithoutanylimitationscheme,asshowninFig.6.Thereasonisthattheshuntoperationincreasestheimpedanceoftherightsidefromfaultpointandhenceincreasesthedistributedcurrentoftheleftside.However,ithasnothreattocorrespondingrelayschemes.Asamatteroffact,theprocessontheseriescapacitorcouldbemorecomplicatedinapracticalpointofview.Furtherconsiderationshavetobedoneformoredetails.1)Initialprocedure.Thefaultphasediscriminationforthecontrolofseriescapacitorsisalwaysstartedbyinitialprocedureonlyifthreecontinuoussamplingpointshaveasuddenchangeinmagnitude.Uptonow,thesamplingnumberisalways24pointspercycleforseriescapacitorcontrolinChina,whichmeansthesamplingtimestepis0.83ms.Theinitialprocedureperceivesthefaultin2.5msafterthefaultinception.Itshouldbeemphasizedthatthecapacitorhastobeshuntedbyresistorassoonaspossibletoearnthetimeforsubsequentfaultphaseselection.Forinstance,thefirstzoneofdistancerelayisusedasoneofthemainprotectionofthe1000kVUHVtransmissionline,whichdependsonthecorrectconfirmationoffaultloop.Meanwhile,thejudgmentbyamicrocomputerinthefirstseveraltimewindowsafterfaultinceptionisalwaysinfluencedbyun-faulteddata.Accordingly,thewidelyusedalgorithmhasaself-checkprocedure.Theresultisnotrecognizedascorrectuntilthreecontinuousjudgmentsareidentical.Similarly,theshuntoperationhasnothreattothemicrocomputeralgorithmofmainprotectionschemes.2)MOVprotectionforseriescapacitor.Nowadays,metaloxidevaristor(MOV)ismainlyutilizedonseriescapacitorsforlimitingover-voltageasshowninFig.7.Afterafault,thedischargingcurrentofseriescapacitorcanberecognizedasshuntedbetweentheMOVloopandtheequivalentloopofanexternalsystem.Itiswellknownthatthedetailedvoltage-currentcharacteristicofMOVisdesignedbydifferentoperatingconditionsoftheconcernedproject.However,thegeneralvaryingtrendofMOVactionasshowninFig.8issame.TheshuntedcurrentbyMOVincreasesexponentiallywhenthevoltageacrosstheseriescapacitorreaches1.2pu.Otherwise,theshuntedcurrentislessthan700A,whichmaybemuchlowerthanthefaultcurrentacrosstheseriescapacitor.ThustheequivalentresistanceofMOVcanberecognizedasdependenttotheover-voltagelevel.Meanwhile,thedampingresistanceDofsparkgaplimitsthemaximumresistanceoftheequivalentparallelbranchoftheseriescapacitor.Thevoltageacrosstheseriescapacitorchangesfrombeinglongitudinaltobeingtransverseduringthefaultprocess,whichalwaysbringsasuddenincreaseinmagnitude.Atthesametime,MOVisinitiatedautomaticallybecauseofthesuddenincreaseofthevoltageacrossitstwoends.Duetoitsstrongnon-linearcharacteristic,maximalcurrentisshuntedbyMOVlooptolimittheinstantaneousover-voltageacrosstheseriescapacitorwithinapermissiblevaluewhichmeansthattheequivalentresistanceofMOVislowenoughtosharemostofthedischargingcurrentdirectlyacceleratingthedischargingprocess.However,theover-voltageisnotthatseriousversusSLGfaultsifthecomparativelyhighfaultpathresistancecausedbyvegetationsortowersistakenintoconsideration.Asaresult,theequivalentresistanceofMOViscomparativelylargeandhenceitsshuntedcurrentislow.Thereforesparkingoutthegapisneededundersuchcondition.TheappropriateselectionofdampingresistanceD,inparallelwithMOV,makestheequivalentresistanceofMOVandsparklooplowenoughtosharemostofthedischargingcurrent.Atthesametime,itmakestheenergyabsorbedbyMOVacceptable.Consideringtherandomfaultconditions,inapracticalpointofview,thetimingschedulecanbearrangedasfollowstoeliminatetheseveredistortion.Theinitialprocedureperceivesthefaultat2.5msafterthefaultinception.Thenitgivessignalsforstartingthefaultphaseselectorandoperationsofcorrespondingrelays.Meanwhile,thissignalshouldalsobesenttothecontrolsystemoftheseriescapacitortosparkoutthegap.3)Sensitivityevaluations.Twoaspectsarefurtherinvestigatedinthissection.Foronething,approximate2.5msafterthefaultinception,theinitialprocedureistosendthesignaltothecontrolsystemofseriescapacitorwithatimedelay.Consideringthefactthatthetimedelaycannotbezeroastheidealsituationinnumericalsimulation,KvaluesareevaluatedversusdifferenttimedelaysinFig.9,showingthatthefasteroperationcouldbemorereliable.NotethattheequivalentresistanceofMOVandsparkgaploopis4uf057forinstanceinthissimulations.Foranother,thisresistancevalueisalsoeffective.Itiswellunderstoodthatsmallerresistancecouldbebetterbecauselagerdischargingcurrentflowsacrossthisresistivebranch,directlyacceleratingthedischargingprocess.Supposingthetimedelayis3ms,thecurveinFig.10verifiesthecorrectnessoftheanalysisabove.Theresistanceissetfrom2uf057to20uf057consideringthattheequivalentcapacitiveimpedanceis19.37uf057inthis1000kVUHVproje