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227AnalysisOfLowReactanceGroundingProtectionIn110KVMainTransformerNeutralSystemPingshaoxun,Zhouyufang,ZhangxuefengRugaoElectricpowercompanyJiangsAbstract-Thepapersummarizesproduceovervoltageprincipleof110KVmaintransformercentralpointandprobleminprotectmode，suggesthasgroundingprotectionoflowreactanceandthismethodcouldprotectmaintransformerin110KVtransformersubstationeffectively。Keywords-transformer；over-voltage；lowreactance；protection；grounding.I.INTRODUCTIONNeutralpointdirectgroundingmodecanreduceover-voltageandinsulationleveloftransformer’sneutralpoint.However,atthesametime,thiswouldincreasesingle-phaseshort-circuitcurrent.Accordingtorelevantinstructionandthedemandoftheoperationsystem,about2/3transformersruninisolatedneutralmodein110kvmaintransformerneutraleffectivegroundingsystem.Ungroundedtransformerswouldbeinfluencedbylightning,workingfrequencyover-voltageandoperationover-voltage.Whentheprotectionofneutralpointisimproperprotectd,equipmentsmightmalfunction,orthemaintransformersorpowerlinesmightbeoutofrunning,evenworse,theequipmentswouldbedamaged.IntermsofDL/T620-1997Regulation,theneutralpointshouldbeinstalledgapandarresterastheprotectionoftheungroundedtransformers,whichhascausedalotofaccidents,suchasmaintransformermalfunction,linemalfunctionandlightningarresterexplosion,orevencauseinsulationbreakdownamongthecoilsintransformers.Toprotectthemaintransformersandlightningarrester,andavoidmaintransformerandlinemaloperationandpowersupplyinterruptionasaresultofabnormalneutralpointprotection.II.KINDSOFOVER-VOLTAGEATTRANSFORMER’SNEUTRALPOINTA.Lightningover-voltageattransformerneutralpointForungroundedneutralpoint,whenthelightningreachestheneutralpointthroughwindingsofatransformer,impulsevoltagewillincreasetoabovetheneutralpointinsulationlevel,threateningtheinsulationofneutralpoint,over-voltageoftransformerneutralpointbylightningwave:Um=nKub/3（1）WhereUmistheamplitudeofover-voltageatneutralpointoftransformerwithlightningwave;nisthenumberofphaseswhichlightningwaveinvades;Kisthefactorofoscillation,forconnectingwindingKis1.6andforcontinuouswindingKis1.8;andubisarrester’sresidualvoltage(ordischargevoltage)atthetransformeroutput.Itsmaximumhappenswhenlightningwavesinvadeintothree-phasesimultaneously.Atthistime,thevalueofover-voltageinnuetralpointcanbe1.6-1.8timesofarrester’sresidualvoltageinthetransformeroutput.Regardlessofthree-phase,two-phaseorsingle-phasebeinginvaded,theatmosphereover-voltageisharmfultotheneutralpointofclassifiedinsulationtransformer.Althoughthesteepandamplitudeoftheinvasionlightingwavearegreatlyattenuatedalongtheline,alongwiththerestrictionsoftheoverheadarresters,multi-channelcurrentandtransformercoilinductance,arrestersarestillneededtoprotectthecentrepoint.B.On-and-offover-voltageofpartial-phasecircuitbreakerWhentheno-loadtransmissionlinessupplypowertothetransformer,themorecapacitivereactanceofline1/ωCisclosertothetransformerinductanceωL,themoretheintersectionIkofV-Icharacteristicsaresmaller,andthenecessaryexcitationconditionsismoreweaker,theprobabilityofresonancewillbemoregreater,conversely,thecaseisopposite,butwithahighervoltageamplitude.Sowithdifferentexcitationconditions,theresonanceover-voltageissubjectedtothelawsofstatisticsdistribution.Testsandoperationexperiencehasshownthat,aslongascertainoutsideconditionsandparametersaremet,theresultofbrokencircuitmaygeneratethehighharmonics,thefundamentalandsub-harmonicresonanceover-voltage.Resonanceinthefundamentalconditions,duetothefrequencyofneutralpointpotentialandpowerhavethesamefrequency,thesuperpositionofthetwomakesgroundvoltageimbalanceofeachphase:Fortheloadtransformer,whenloopcircuitworksinnon-resonantpoint,thecurrentisinductive,thesecondaryvoltageULoftransformersandpowerEhavethesamephase.whentheloopcircuitworksintheresonantpoint,thecurrentiscapacitive,ULandEhavetheoppositephase.Whentheferromagneticresonanceisgeneratedbyaphaseofthetransformer,givingthattheover-voltageoftheendbandCisUao=KUph,andtemporarilyignoringtheimpactofACpowertransformers.ForNeutralUnGroundtransformerworkingwithlightorno-load,insomecases,whenallphasesofthecircuitbreakerorswitchareNon-simultaneouslyonoroff(includingtheoperationlineswithonephaseortwo978-1-4244-2810-6/08/$25.00©2008IEEE2008InternationalConferenceonHighVoltageEngineeringandApplication,Chongqing,China,November9-13,2008Authorizedlicenseduselimitedto:NORTHCHINAELECTRICPOWERUNIVERSITY.DownloadedonMay28,2010at14:50:17UTCfromIEEEXplore.Restrictionsapply.228disconnection),thesystemcapacitorsandtransformersICPwillhaveFerroresonancewhentwo-apowertransformer,theamplitudeofsteadystateover-voltageintheneutralpointofUngroundedtransformerwillbeashighas2Uph(Uphistophasevoltage).Nowonthediscussionoftheover-voltageintheintheneutralpointofungroundedtransformerwithopenphaseswitchingclosing.(1)Over-voltageintheneutralpointoftransformerwithOne-phaseconnectionFigure1(a)istheconnectiondiagramforthe110KVsystemwithtwophasedisconnection(1).Figure1(b)isdrawnastheequivalentcircuitdiagramof(a),whereC0andC12areshuntcapacitacetotherightsideofbre