电力专业英语4-继电保护2

电力专业英语4

--继电保护2电气工程学院12/27/20231继电保护2DistanceRelay距离保护UndervoltageRelay低电压保护OvervoltageRelay过电压保护BlockingRelay闭锁继电器PhaseAngleorOutofStepRelay失步保护FrequencyRelay频率继电器DirectionalPowerRelay方向功率继电器InstantaneousOvercurrentRelay瞬时过流保护DifferentialRelay差动保护12/27/20232GroundingSystemFormerly,manypowersystemswereungroundedsystemsusingdelta-deltatransformers.Ungroundedsystemssufferformseveralproblems,includingtransientovervoltagesthatnecessitatetheuseoftwosurgearrestersforasingle-phasetransformer(oneoneachsideofthewinding),andtroublewithrapid,automaticclearingoftheverycommonphasetogroundshortcircuit.以前，许多电力系统采用角型接线变压器，为不接地系统。不接地系统会带来很多严重的问题，包括暂态过电压，因此，即使是单相变压器，也需要分别在原边和副边绕组各安装一个避雷器。而且对于非常常见的相接地故障的快速自动切除非常困难。12/27/20233Anungroundedsystemisconnectedtogroundonlybystraycapacitances,soasinglephasetogroundshortcircuitdrawsverylittlefaultcurrent.不接地系统通过隔离电容接地，因此单相接地故障会产生非常小的故障电流Toallowrapiddetectionandautomaticclearingofgroundfaults,andtoreduceinsulationandarrestercosts,groundedsystemshavebecomemorecommonthanungroundedones.为了快速检测和去除接地故障，并且降低投资在隔离设备和避雷器上的费用，接地系统的应用越来越广泛。12/27/20234Inmanycases,thepowertransformersinagroundedsystemareconnecteddeltawye,withthewyeneutralgroundedtothesubstationgroundmat.很多情况下，接地系统中的电力变压器的连接方式为三角星型接线，星型接线的中性点和变电站的接地网相连。Generatorsandstep-downtransformersservingonlythree-phaseloadsareoftengroundedthrougharesistancetolimitthegroundfaultcurrenttolowvalues.发电机和只带三相负荷的降压变压器一般通过电阻接地，以降低接地故障电流的大小。12/27/20235Generatorstep-uptransformersandstep-downtransformersservingfour-wiredistributionsystemareusuallyconnecteddeltaontheprimaryandwyeonthesecondary.Rememberprimarymeansinputandsecondmeansoutput,soastep-uptransformerprimaryisthelow-voltagesidebutastep-downtransformerprimaryisthehigh-voltageside.发电机升压变压器和向四线配电系统供电的降压变压器通常在原边侧为三角型连接，在副边侧位星型链接。需要注意的是原边指输入，副边指输出，因此升压变压器的原边为低压侧，但降压变压器的原边为高压侧。12/27/20236Distributiontransformersonfour-wiresystemsareconnectedphasetoneutral(singlephase)orwye(threephase)ontheprimaryside.Thesecondarysideofthree-phasedistributiontransformersmaybeconnectedineitherwyeordelta.四线系统的配电变压器的原边连接在相和中线上〔单相变压器〕或连接成星型〔三相变压器〕。三相配电变压器的副边可以星型或三角型连接。12/27/20237Thewye-wyedistributiontransformerisusedforfour-wire(threephaseconductorsandaneutralconductor)systems,andthewye-deltadistributiontransformerisusuallyconnectedwiththebankneutralnotconnectedtoground.星星连接配电变压器用于四线系统〔三相导线和中线〕，星角接线配电变压器的中性点通常不接地。12/27/20238Iftheneutralisgrounded,thebanktriestogroundthefeeder,andwhengroundfaultsoccurontheprimaryside,thedistributionbankswillsupplygroundcurrent.Thisusuallycausesalargenumberofblownfusesonwye-deltadistributiontransformerswithgroundedneutrals.如果中性点接地，….，当在原边侧发生接地故障时，配电变压器组中有接地电流流过。这会导致星型中性点接地接线的配电变压器的保险熔断。12/27/20239Floatingthebankneutralalleviatestheproblem,butmaycauseovervoltageproblemssuchasferroresonance.Wye-wyebanksavoidtheferroresonanceproblemsincetheyinvariablyhavegroundedneutralsonbothsides.变压器中性点浮地能减轻这一问题，但可能造成过电压问题，例如铁磁共振。星星连接变压器组不会产生铁磁共振问题，因为在原边和副边中性点均接地。12/27/202310Instrumenttransformers仪用〔或测量用〕互感器Instrumenttransformersaretheinputtransducerforprotectiverelays,butalsometering(bothfrorevenuemeteringandfordataacquisition).Thetwomaintypesofinstrumenttransformersarecurrenttransformers(CTs)andpotentialtransformers(PT’s).Thelatterarealsocalledvoltagetransformers(VT’s),withnodifferenceintended.测量用互感器指的是保护继电器的输入传感器，也指计量仪表〔例如电费计量和数据采集〕的输入传感器。两种主要的测量用互感器为电流互感器〔CT〕和电压互感器〔PT〕,后者也简称VT。12/27/202311TheCTactstostepthecurrentdown,sayfrom1000Aontheprimaryto5Aonthesecondary.ThiswouldgiveaCTratioof1000:5Aor200:1A,andwouldrequire200turnsonthesecondary(assumingoneprimary“turn〞).CT的作用是降低电流，例如将原边1000A的电流降至副边的5A。那么CT的变比为1000:5A或200:1A，副边绕组的匝数为200〔假设原边绕组匝数为1〕12/27/202312Currenttransformersecondariesaretypicallyratedat5A,althoughsomeareratedat1A.Thisisthecontinuousrating,anditmaybeexceededbyafactorof10or20forashorttime,especiallyduringashortcircuitinsupplyingrelaysthatprovideovercurrentprotection.电流互感器二次侧典型额定电流为5A，有时为1A.这只是额定标称值，其二次侧允许短时过电流10-20倍，尤其是作为过流保护的输入传感器，有短路电流流过时。12/27/202313Currenttransformersforrelayingpurposesaregivenaccuracyclassratings,suchasC400orT400(sometimeswrittenas10C400or10T400).TheletterCindicatesthatitsperformancemaybecalculatedfromitsmagnetizationcurve.TheletterTindicatesthatitsperformancemustbedeterminedbytest.保护用电流互感器均给出其〔测量〕精度等级，例如C400或T400〔有些标为10C400或10T400〕。字母C表示其特性通过其磁化曲线计算出来。字母T表示其特性由实验决定。12/27/202314ThenumberfollowingtheletterCorTisthevoltagerating:thesecondaryvoltageproducedby20timesratedsecondarycurrent