电力系统故障毕业论文中英文资料外文翻译文献

FaultsonPowerSystemsEachyearnewdesignofpowerequipmentbringaboutincreasedreliabilityofoperation.Nevertheless,equipmentfailuresandinterferencebyoutsidesourcesoccasionallyresultinfaultsonelectricpowersystems.Ontheoccurrenceofafault,currentanvoltageconditionsbecomeabnormal,thedeliveryofpowerfromthegeneratingstationtotheloadsmaybeunsatisfactoryoveraconsiderablearea,andifthefaultedequipmentisnotpromptlydisconnectedfromtheremainderofthesystem,damagemayresulttootherpiecesofoperatingequipment.Afaultyistheunintentionalorintentionalconnectingtogetheroftwoormoreconductorswhichordinarilyoperatewithadifferenceofpotentialbetweenthem.Theconnectionbetweentheconductorsmaybebyphysicalmetalliccontactoritmaybethroughanarc.Atthefault,thevoltagebetweenthetwopartsisreducedtozerointhecaseofmetal-to-metalcontacts,ortoaverylowvalueincasetheconnectionisthroughanarc.Currentsofabnormallyhighmagnitudeflowthroughthenetworktothepointoffault.Theseshort-circuitcurrentswillusuallybemuchgreaterthanthedesignedthermalabilityoftheconditioninthelinesormachinesfeedingthefault.Theresultantriseintemperaturemaycausedamagebytheannealingofconductorsandbythecharringofinsulation.Intheperiodduringwhichthefaultispermittedtoexist,thevoltageonthesysteminthenearvicinityofthefaultwillbesolowthatutilizationequipmentwillbeinoperative.Itisapparentthatthelateconditionsthatexistduringafault,andprovideequipmentproperlyadjustedtoopentheswitchesnecessarytodisconnectthefaultedequipmentfromtheremandingofthesystem.Ordinarilyitisdesirablethatnootherswitchesonthesystemareopened,assuchbehaviorwouldresultinunnecessarymodificationthesystemcircuits.Adistinctionmustbemadebetweenandanoverload.Anoverloadimpliesonlythatloadsgreaterthanthedesignedvalueshavebeenimposedonsystem.Undersuchacircumstancethevoltageattheoverloadpointmaybelow,butnotzero.Thisundervoltageconditionmayextendforsomedistancebeyondtheoverloadpointintotheremainderofthesystem.Thecurrentintheoverloadequipmentarehighandmayexceedthethermaldesignlimits.Nevertheless,suchcurrentsaresubstantiallylowerthaninthecaseofafault.Servicefrequentlymaybemaintained,butatbelow-standardvoltage.Overloadsarerathercommonoccurrencesinhomes.Forexample,ahousewifemightplugfivewaffleironsintothekitchencircuitduringaneighborhoodpart.Suchanoverload,ifpermittedtocontinue,wouldcauseheatingofthewiresfromthepowercenterandmighteventuallystartafire.Topreventsuchtrouble,residentialcircuitsareprotectedbyfusesorcircuitbreakerswhichopenquicklywhencurrentsabovespecifiedvaluespersist.Distributiontransformersaresometimesoverloadsascustomersinstallmoreandmoreappliances.Thecontinuousmonitoringofdistributioncircuitsisnecessarytobecertainthattransformerssizesareincreasedasloadgrows.Faultsofmanytypesandcausesmayappearonelectricpowersystems.Manyofusinourhomeshaveseenfrayedlampcordswhichpermittedthetwoconductorsofthecordtocomeincontactwitheachother.Whenthisoccurs,thereisaresultingflash,andifbreakerorfuseequipmentfunctionsproperly,thecircuitisopened.Overheadlines,forthemostpart,areconstructedofbareconductors.Therearesometimesaccidentallybroughttogetherbyactionofwind,sleets,trees,cranes,airplanes,ordamagetosupportingstructures.Overvoltagesduetolightingorswitchingnaycauseflashoverofsupportingorfromconductortoconductor.Contaminationoninsulatorssometimesresultsinflashoverevenduringnormalvoltageconditions.Theconductorsofundergroundcablesareseparatedfromeachandfromgroundbysolidinsulation,whichnaybeoil-impregnatedpaperoraplasticsuchpolyethylene.Thesematerialsundergosomedeteriorationwithage,particularlyifoverloadsonthecableshaveresultedintheiroperationatelevatedtemperature.Anysmallvoidpresentinthebodyoftheinsulatingmaterialwillresultsinionizationofthegascontainedtherein,theproductsofwhichreactunfavorablywiththeinsulation.Deteriorationoftheinsulationmayresultinfailureofthematerialtoretainitsinsulatingproperties,andshortcircuitswilldevelopbetweenthecableconductors.Thepossibilityofcablefailureisincreasediflighteningorswitchingproducestransientvoltageofabnormallyhighvaluesbetweentheconductors.Transformerfailuresmaybetheresultofinsulationdeteriorationcombinedwithovervoltageduetolightningorswitchingtransients.Shortcircuitduetoinsulationfailurebetweenadjacentturnsofthesamewindingmayresultfromsuddenlyappliedovervoltage.Majorinsulationmayfail,permittingarcstobeestablishedbetweenprimaryandsecondarywindingsorbetweenwindingandgroundedmetalpartssuchasthecoreortank.Generatorsmayfailduetobreakdownoftheinsulationbetweenadjacentturnsinthesameslot,resultinginashortcircuitinasingleturnofthegenerator.Insulationbreakdownmayalsooccurbetweenoneofthewindingandthegroundedsteelstructureinwhichthecoilsareembedded.Breakdownbetweendifferentwindingslyinginthesameslotresultsinshort-circuitingextensivesectionofmachine.Balancedthree-phasefaults,likebalancedthree-phaseloads,maybehandledonalineto-neutralbasisoronanequivalentsingle-phasebasis.Problemsmaybesolvedeitherintermsofvolts,amperes,andohms.Thehandingoffaultsonsingle-phaselinesisofcourseidenticaltothemethodofhandingthree-phasefaultsonanequivalentsingle-phasebasis.Faultsmaybeclassifiedaspermanentortemporary.Permanentfaultsarethoseinwhichinsulationfailureorstructurefailureproducesdamagethatmakesoperationoftheequipmentimpossibleandrequiresrepairstobemade.Temporaryfaultsarethosewhichmayberemovedbydeenergizingtheequipmentforashortperiodoftime,shortcircuitsonoverheadlinesfrequentlyareofthisnature.Highwindsmaycausetwoormoreconductionstoswingtogethermomentarily.Duringtheshortperiodofcontact.Anarcisformedwhichmaycontinueaslongaslineremainsenergized.However,ifautomaticequipmentcanbebroughtintooperationtoserviceassoonastheareisextinguished.Arcsacrossinsulatorsduetoovervoltagesfromlightingorswitchingtransientsusuallycanbeclearedbyautomaticcircuit-breakeroperationbeforesignificantstructuredamageoccurs.Becauseofthischaracteristicoffaultsonlines,manycompaniesoperatefollowingaprocedureknownashigh-speedreclosing.Ontheoccurrenceofafault,thelineispromptlydeenergizedbyopeningthecircuitbreakersateachendoftheline.Thebreakersremainopenlongenoughforthearctoclear,andthenrecloseautomatically.Inmanyinstancesserviceisrestoredinafractionofasecond.Ofcourse,ifstructuredamagehasoccurredandthefaultpersists,itisnecessaryforthebreakerstoreopenandlockopen.电力系统故障每年新设计的电力设备都使系统的可靠性不断提高，然而，设备的使用不当以与一些偶然遇到的外在因素均会导致系统故障的发生。发生故障时，电流、电压变化得不正常，从电厂到用户的送点在相当大得内不令人满意。此时若故障设备不立即从系统中切除的话，则会造成其他运行设备的损坏。故障是由于有意或无意地使两个或更多的导体接触造成的。导体间本来是有电位存在的，而这种接触可能是金属性接触，也可能是电弧引起的。如果是前者造成的故障，则两部分导体之间电压下降为零；若为后者，则电压变得很低，超常的大电流经过网络流至故障处。此短路电流通常会大大超出导线以与供电发电机的热承受能力，其结果，温度的升高会导致导体烧毁或绝缘焦化。在允许的期限内，最靠近故障处的电压会变得很低，致使用电设备无法正常运行。显然，系统设计者必须事先考虑到故障可能发生在什么地方，能够推测出故障期间的各种情况，提供调节好的设备，以便驱动为将故障设备切除所必须断开的开关能够跳闸。通常希望此时系统无其他开关打开，否则会导致系统线路不必要的修改。过负荷与故障是两个概念。过负荷仅指施加于系统的负荷超过了设计值。发生这种情况时，过负荷处的电压可能很低，但并不等于零。这种电压不足的情形可能会超过过负荷处蔓延一定距离，进而影响系统其它部分。过负荷设备的电流变大而超过预定的热极限，但是这种情况比发生故障时的电流要小。此时，供电虽然往往能维持，但电压较低。过负荷的情况在家里发生，例如请街坊邻居聚会时，女主人可能将五个化夫饼干烘烤器的插头同时插入厨房的插座，诸如次类的过负荷倘若不能迅速处理的话，就会造成电力线发热甚至酿成火灾。为了避免这种情况的发生，须采用保险丝或短路器来保护住宅区电路免受损坏。断路器会在电流超出预定值时迅速切断电路。当用户安装的用电器增加时，也会超过变压器负荷能力，因此有必要不时地监视配电线路以确保在负荷增加时变压器的容量也相应增加。电力系统会发生各种类型，由各种原因引起的故障。我们在家里看到过破损的照明灯电线，使得其两根导线相触，并会发出弧光。如果此时断路器或保险丝能够正常工作，则电路能被自动切断。大部分架空明线是用裸导体假设的，又是由于风、雨、雷、或大树、起重机，飞机与支撑物的损坏等因素会使导线偶然碰