電力系統(tǒng)故障畢業(yè)論文中英文資料外文翻譯文獻

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.電力系統(tǒng)故障每年新設(shè)計的電力設(shè)備都使系統(tǒng)的可靠性不斷提高，然而，設(shè)備的使用不當以與一些偶然遇到的外在因素均會導致系統(tǒng)故障的發(fā)生。發(fā)生故障時，電流、電壓變化得不正常，從電廠到用戶的送點在相當大得內(nèi)不令人滿意。此時若故障設(shè)備不立即從系統(tǒng)中切除的話，則會造成其他運行設(shè)備的損壞。故障是由于有意或無意地使兩個或更多的導體接觸造成的。導體間本來是有電位存在的，而這種接觸可能是金屬性接觸，也可能是電弧引起的。如果是前者造成的故障，則兩部分導體之間電壓下降為零；若為后者，則電壓變得很低，超常的大電流經(jīng)過網(wǎng)絡(luò)流至故障處。此短路電流通常會大大超出導線以與供電發(fā)電機的熱承受能力，其結(jié)果，溫度的升高會導致導體燒毀或絕緣焦化。在允許的期限內(nèi)，最靠近故障處的電壓會變得很低，致使用電設(shè)備無法正常運行。顯然，系統(tǒng)設(shè)計者必須事先考慮到故障可能發(fā)生在什么地方，能夠推測出故障期間的各種情況，提供調(diào)節(jié)好的設(shè)備，以便驅(qū)動為將故障設(shè)備切除所必須斷開的開關(guān)能夠跳閘。通常希望此時系統(tǒng)無其他開關(guān)打開，否則會導致系統(tǒng)線路不必要的修改。過負荷與故障是兩個概念。過負荷僅指施加于系統(tǒng)的負荷超過了設(shè)計值。發(fā)生這種情況時，過負荷處的電壓可能很低，但并不等于零。這種電壓不足的情形可能會超過過負荷處蔓延一定距離，進而影響系統(tǒng)其它部分。過負荷設(shè)備的電流變大而超過預(yù)定的熱極限，但是這種情況比發(fā)生故障時的電流要小。此時，供電雖然往往能維持，但電壓較低。過負荷的情況在家里發(fā)生，例如請街坊鄰居聚會時，女主人可能將五個化夫餅干烘烤器的插頭同時插入廚房的插座，諸如次類的過負荷倘若不能迅速處理的話，就會造成電力線發(fā)熱甚至釀成火災(zāi)。為了避免這種情況的發(fā)生，須采用保險絲或短路器來保護住宅區(qū)電路免受損壞。斷路器會在電流超出預(yù)定值時迅速切斷電路。當用戶安裝的用電器增加時，也會超過變壓器負荷能力，因此有必要不時地監(jiān)視配電線路以確保在負荷增加時變壓器的容量也相應(yīng)增加。電力系統(tǒng)會發(fā)生各種類型，由各種原因引起的故障。我們在家里看到過破損的照明燈電線，使得其兩根導線相觸，并會發(fā)出弧光。如果此時斷路器或保險絲能夠正常工作，則電路能被自動切斷。大部分架空明線是用裸導體假設(shè)的，又是由于風、雨、雷、或大樹、起重機，飛機與支撐物的損壞等因素會使導線偶然碰