电力英语9

Unit9DistanceProtectionADistanceRelayApplicationsforTappedandMultiterminalLinesAtypicallineofthistypeisshowninFig.2.1.ThetapatTmaybeatransformeratorneartheline,sothatTRZwouldbetheimpedancefromthetapplusthetransformerbankimpedance.SometimesthetaptiesthroughTRZtoabus,asshowninthefigure.Thetapmayserveaload,sothatnegligiblefaultcurrentissuppliedthroughittolinefaults,oritmaytieintoafaultsourceatR,asshowndotted.AnothervariationisillustratedinFig.2.2.Thefundamentalsforsettingdistancerelaysonthesetypesoflinesforprimaryphasefaultprotectionareasfollows:1.Setzone1forktimesthelowestactualimpedancetoanyremoteterminalforFig.2.1-typecircuits,orforktimesthelowestapparentimpedancetoanyremoteterminalforthespecialcaseofFig.2.2.kislessthan1,normal0.9.2.Setzone2foravaluegreaterthanthelargestimpedance,actualorapparent,totheremoteterminals.3.Thezone2timer(T2)mustbesetsoasnottocausemisoperationwhenanyterminalisoutofservicetocausethedistanceunittooverreach.Fig.2.1TypicaltappedormultiterminallineForsomearrangementsofcircuits,theserequirementscanmakeprimaryprotectionquitedifficultand/orlimited.InFig.2.1,considerthattapTisverynearbusG,soGHnZissmalland(1)GHnZ-largewithTRZverysmall.Thenzone1atbreaker1,busGmustbesetat90%of()GHTRnZZ+,whichisaverysmallvaluecomparedwith(1)GHnZ-.Thushigh-speedcoverageofthelineisalmostnegligible.Ontheotherhand,IfthetapisaloadtransformerwhereTRZishighrelativetoGHZ,zone1atbreakers1and2canbesetfor90%ofthelinetoprovidegoodhigh-speedlineprotection.IfRisaloadtapinFig.2.1,withnegligiblecurrenttolinefaults,distancerelays(andovercurrent)arenotapplicableatbreaker3,andbasicallynotnecessary,asopeningbreakers1and2terminatesthelinefault.TheworstcaseisasmallgeneratororsourceconnectedtoR,largeenoughtomaintainafaultontheline,butnotlargeenoughtosupportfault-detectingrelays.Inotherwords,theimpedancetoalinefaultfrombusRisverylargeandapproachesinfinity.Fig.2.2Multiterminallinewherefaultcurrentcanflowoutatoneterminalforinternalfaults.Forthecaseoffig.2.2,currentcanflowoutRterminalforaninternallinefaultneartheHbus.Thusdistanceordirectionalrelaysatbreaker3seetheinternalfaultasexternalfornooperationuntilafterbreaker2hasopened.Thusprotectionoftappedandmultiterminallinesismorecomplexandrequiresspecificdataonthelineimpedance,locationandtypeoftaporterminal,andfaultdatawithcurrentdistributionsforthevarioussystemandoperatingconditions.Mostoften,exceptforsmalltransformerloadtaps,thesetypesoflinesareprotectedbestbypilotrelaying.Three-phasevoltageisrequiredandprovidesreferencequantitiestowhichthecurrentsarecompared.Forphasedistancerelays,eitheropen-deltaorwye-wyevoltagetransformers(VTs)orcouplingcapacitorvoltagedevices(CCVTs)canbeutilizedandconnectedeithertothebusortothelinebeingprotected.Botharewidelyusedandthedecisioniseconomicaswellasinvolvinguseofline-sideCCVTsforradio-frequencycouplingforpilotand/ortransfertriprelaying.Thesevoltagesourcesinvolvefuses/primaryandsecondaryforVTsandsecondaryforCCVTs.Thesefusesshouldbegenerouslysized,carefullyinstalled,andwellmaintained,asalossofoneormorephasevoltagesmayresultinanundesired,unwantedrelayoperation.Wherethisisofgreatconvern,overcurrentfaultdetectorscanbeaddedtosupervisethetripcircuitofthedistancerelays.Forlossofvoltageintheabsenceofanactualfault,theovercurrentunitswouldnotoperate.Thedisadvantagesaretheneedforadditionalequipmentandthelossofthefeatureofdistancerelaysoperatingforfaultlevelslessthanmaximumload.Recently,detectorsmeasuring0Vbutno0Ihavebeenusedtosupervisetherelays.Thisrequireswyegrounded-wyegroundedvoltagesources.UsefulExpressionsinPassageA1)Multiterminallines多端线路2)apparentimpedance视在阻抗3)remoteterminals远程终端4)misoperation误动5)abreaker断路器6)currentdistributions电流分布7)line-side铁路沿线8)voltagesources电压源BDifferentialProtectionTransformersareeverywhere-inallpartsofthepowersystem,betweenallvoltagelevels,andexistinginmanydifferentsizes,types,andconnections.Smalltransformersofabout3to200kVAcanbeobservedmountedonpowerdistributionpolesinmanyareas.Usually,circuitbreakersorotherdisconnectionmeansareavailableatornearthewindingterminalsofthetransformerbanks.However,economicssometimesdictatesomissionofabreaker.Thustransformerbankscanbeconnecteddirectlytoabus,line,orgenerator.Differentialprotection,whereapplicable,providesthebestoverallprotectionforbothphaseandgroundfaults,exceptinungroundedsystemsorwherethefaultcurrentislimitedbyhigh-impedancegrounding.Intheselatterlow-ground-faultcurrentsystems,differentialprovidesonlyphase-faultprotection.Generally,differentialprotectionisappliedtotransformerbanksof10MVAandabove.Thekeyistheimportanceofthetransformerinthesystem,sodifferentialmaybedesirableforsmallerunitsinsomecases.Inapplyingdifferentialprotection,severalfactorsmustbeconsidered:1.Magnetizinginrushcurrent.Thisisanormalphenomenonwhichhastheappearanceofaninternalfault(currentintobutnotoutofthetransformers).2.Differentvoltagelevelsandhencethecurrenttransformersareofdifferenttypes,ratios,andperformancecharacteristics.3.Phaseshiftsinwye-delta-connectedbanks.4.Transformertapsforvoltagecontrol.5.Phaseshiftand/orvo