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编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第PAGE8页共NUMPAGES8页第PAGE\*MERGEFORMAT8页共NUMPAGES\*MERGEFORMAT8页Unit9DistanceProtectionADistanceRelayApplicationsforTappedandMultiterminalLinesAtypicallineofthistypeisshowninFig.2.1.ThetapatTmaybeatransformeratorneartheline,sothatwouldbetheimpedancefromthetapplusthetransformerbankimpedance.Sometimesthetaptiesthroughtoabus,asshowninthefigure.Thetapmayserveaload,sothatnegligiblefaultcurrentissuppliedthroughittolinefaults,oritmaytieintoafaultsourceatR,asshowndotted.AnothervariationisillustratedinFig.2.2.Thefundamentalsforsettingdistancerelaysonthesetypesoflinesforprimaryphasefaultprotectionareasfollows:Setzone1forktimesthelowestactualimpedancetoanyremoteterminalforFig.2.1-typecircuits,orforktimesthelowestapparentimpedancetoanyremoteterminalforthespecialcaseofFig.2.2.kislessthan1,normal0.9.Setzone2foravaluegreaterthanthelargestimpedance,actualorapparent,totheremoteterminals.Thezone2timer(T2)mustbesetsoasnottocausemisoperationwhenanyterminalisoutofservicetocausethedistanceunittooverreach.Fig.2.1TypicaltappedormultiterminallineForsomearrangementsofcircuits,theserequirementscanmakeprimaryprotectionquitedifficultand/orlimited.InFig.2.1,considerthattapTisverynearbusG,soissmallandlargewithverysmall.Thenzone1atbreaker1,busGmustbesetat90%of,whichisaverysmallvaluecomparedwith.Thushigh-speedcoverageofthelineisalmostnegligible.Ontheotherhand,Ifthetapisaloadtransformerwhereishighrelativeto,zone1atbreakers1and2canbesetfor90%ofthelinetoprovidegoodhigh-speedlineprotection.IfRisaloadtapinFig.2.1,withnegligiblecurrenttolinefaults,distancerelays(andovercurrent)arenotapplicableatbreaker3,andbasicallynotnecessary,asopeningbreakers1and2terminatesthelinefault.TheworstcaseisasmallgeneratororsourceconnectedtoR,largeenoughtomaintainafaultontheline,butnotlargeenoughtosupportfault-detectingrelays.Inotherwords,theimpedancetoalinefaultfrombusRisverylargeandapproachesinfinity.Fig.2.2Multiterminallinewherefaultcurrentcanflowoutatonetermina
