文献翻译——电力传输

第1页外文文献资料ElectricpowertransmissionElectricpowertransmissionorhighvoltageelectrictransmissionisthebulktransferofelectricalenergy,fromgeneratingpowerplantstosubstationslocatedneartopopulationcenters.Thisisdistinctfromthelocalwiringbetweenhighvoltagesubstationsandcustomers,whichistypicallyreferredtoaselectricitydistribution.Transmissionlines,wheninterconnectedwitheachother,becomehighvoltagetransmissionnetworks.IntheUS,thesearetypicallyreferredtoaspowergridsorjustthegrid,whileintheUKthenetworkisknownasthenationalgrid.NorthAmericahasthreemajorgrids:TheWesternInterconnection;TheEasternInterconnectionandtheElectricReliabilityCouncilofTexas(orERCOT)grid.Historically,transmissionanddistributionlineswereownedbythesamecompany,butoverthelastdecadeorsomanycountrieshaveliberalizedtheelectricitymarketinwaysthathaveledtotheseparationoftheelectricitytransmissionbusinessfromthedistributionbusiness.Transmissionlinesmostlyusethree-phasealternatingcurrent(AC),althoughsinglephaseACissometimesusedinrailwayelectrificationsystems.High-voltagedirectcurrent(HVDC)technologyisusedonlyforverylongdistances(typicallygreaterthan400miles,or600km);submarinepowercables(typicallylongerthan30miles,or50km);orforconnectingtwoACnetworksthatarenotsynchronized.Electricityistransmittedathighvoltages(110kVorabove)toreducetheenergylostinlongdistancetransmission.Powerisusuallytransmittedthroughoverheadpowerlines.Undergroundpowertransmissionhasasignificantlyhighercostandgreateroperationallimitationsbutissometimesusedinurbanareasorsensitivelocations.Akeylimitationinthedistributionofelectricityisthat,withminorexceptions,electricalenergycannotbestored,andthereforemustbe第2页generatedasneeded.Asophisticatedsystemofcontrolisthereforerequiredtoensureelectricgenerationverycloselymatchesthedemand.Ifsupplyanddemandarenotinbalance,generationplantsandtransmissionequipmentcanshutdownwhich,intheworstcases,canleadtoamajorregionalblackout,suchasoccurredinCaliforniaandtheUSNorthwestin1996andintheUSNortheastin1965,1977and2003.Toreducetheriskofsuchfailures,electrictransmissionnetworksareinterconnectedintoregional,nationalorcontinentalwidenetworkstherebyprovidingmultipleredundantalternateroutesforpowertoflowshould(weatherorequipment)failuresoccur.Muchanalysisisdonebytransmissioncompaniestodeterminethemaximumreliablecapacityofeachlinewhichismostlylessthanitsphysicalorthermallimit,toensuresparecapacityisavailableshouldtherebeanysuchfailureinanotherpartofthenetwork.OverheadtransmissionHigh-voltageoverheadconductorsarenotcoveredbyinsulation.Theconductormaterialisnearlyalwaysanaluminiumalloy,madeintoseveralstrandsandpossiblyreinforcedwithsteelstrands.Copperwassometimesusedforoverheadtransmissionbutaluminiumislowerinweightforonlymarginallyreducedperformance,andmuchlowerincost.Overheadconductorsareacommoditysuppliedbyseveralcompaniesworldwide.Improvedconductormaterialandshapesareregularlyusedtoallowincreasedcapacityandmodernizetransmissioncircuits.Conductorsizesrangefrom12mm2(#6Americanwiregauge)to750mm2(1,590,000circularmilsarea),withvarying第3页resistanceandcurrent-carryingcapacity.Thickerwireswouldleadtoarelativelysmallincreaseincapacityduetotheskineffect,thatcausesmostofthecurrenttoflowclosetothesurfaceofthewire.Today,transmission-levelvoltagesareusuallyconsideredtobe110kVandabove.Lowervoltagessuchas66kVand33kVareusuallyconsideredsub-transmissionvoltagesbutareoccasionallyusedonlonglineswithlightloads.Voltageslessthan33kVareusuallyusedfordistribution.Voltagesabove230kVareconsideredextrahighvoltageandrequiredifferentdesignscomparedtoequipmentusedatlowervoltages.Sinceoverheadtransmissionlinesareuninsulated,designoftheselinesrequiresminimumclearancestobeobservedtomaintainsafety.Adverseweatherconditionsofhighwindandlowtemperaturescanleadtopoweroutages:windspeedsaslowas23knots(43km/h)canpermitconductorstoencroachoperatingclearances,resultinginaflashoverandlossofsupply.Oscillatorymotionofthephysicallinecanbetermedgalloporflutterdependingonthefrequencyandamplitudeofoscillation.UndergroundtransmissionElectricpowercanalsobetransmittedbyundergroundpowercablesinsteadofoverheadpowerlines.HistoryIntheearlydaysofcommercialelectricpower,transmissionofelectricpoweratthesamevoltageasusedbylightingandmechanicalloadsrestrictedthedistancebetweengeneratingplantandconsumers.In1882,generationwaswithdirectcurrent(DC),whichcouldnoteasilybeincreasedinvoltageforlong-distancetransmission.Differentclassesofloads(forexample,lighting,fixedmotors,andtraction/railwaysystems)requireddifferentvoltages,andsouseddifferentgeneratorsandcircuits.Duetothisspecializationoflinesandbecausetransmissionwassoinefficientthatgeneratorsneededtobeneartheirloads,itseemedatthe第4页timethattheindustrywoulddevelopintowhatisnowknownasadistributedgenerationsystemwithlargenumbersofsmallgeneratorslocatednearbytheirloads.In1886inGreatBarrington,Massachusetts,a1kValternatingcurrent(AC)distributionsystemwasinstalled.Thatsameyear,ACpowerat2kV,transmitted30km,wasinstalledatCerchi,Italy.AtanAIEEmeetingonMay16,1888,NikolaTesladeliveredalectureentitledANewSystemofAlternatingCurrentMotorsandTransformers,describingtheequipmentwhichallowedefficientgenerationanduseofpolyphasealternatingcurrents.Thetransformer,andTeslaspolyphaseandsingle-phaseinductionmotors,wereessentialforacombinedACdistributionsystemforbothlightingandmachinery.OwnershipoftherightstotheTeslapatentswasakeyadvantagetotheWestinghouseCompanyinofferingacompletealternatingcurrentpowersystemforbothlightingandpower.Regardedasoneofthemostinfluentialelectricalinnovations,theuniversalsystemusedtransformerstostep-upvoltagefromgeneratorstohigh-voltagetransmissionlines,andthentostep-downvoltagetolocaldistributioncircuitsorindustrialcustomers.Byasuitablechoiceofutilityfrequency,bothlightingandmotorloadscouldbeserved.Rotaryconvertersandlatermercury-arcvalvesandotherrectifierequipmentallowedDCtobeprovidedwhereneeded.Generatingstationsandloadsusingdifferentfrequenciescouldbeinterconnectedusingrotaryconverters.Byusingcommongeneratingplantsforeverytypeofload,importanteconomiesofscalewereachieved,loweroverallcapitalinvestmentwasrequired,loadfactoroneachplantwasincreasedallowingforhigherefficiency,alowercostfortheconsumerandincreasedoveralluseofelectricpower.Byallowingmultiplegeneratingplantstobeinterconnectedoverawidearea,electricityproductioncostwasreduced.Themostefficientavailableplantscouldbeusedtosupplythevaryingloadsduringtheday.Reliabilitywasimprovedandcapitalinvestmentcostwasreduced,sincestand-bygeneratingcapacitycouldbesharedovermanymorecustomersandawidergeographicarea.Remoteandlow-costsourcesofenergy,suchashydroelectricpowerormine-mouthcoal,couldbeexploitedtolowerenergyproductioncost.第5页Thefirsttransmissionofthree-phasealternatingcurrentusinghighvoltagetookplacein1891duringtheinternationalelectricityexhibitioninFrankfurt.A25kVtransmissionline,approximately175kmlong,connectedLauffenontheNeckarandFrankfurt.Voltagesusedforelectricpowertransmissionincreasedthroughoutthe20thcentury.By1914,fifty-fivetransmissionsystemseachoperatingatmorethan70kVwereinservice.Thehighestvoltagethenusedwas150kV.Therapidindustrializationinthe20thcenturymadeelectricaltransmissionlinesandgridsacriticalpartoftheinfrastructureinmostindustrializednations.InterconnectionoflocalgenerationplantsandsmalldistributionnetworkswasgreatlyspurredbytherequirementsofWorldWarI,wherelargeelectricalgeneratingplantswerebuiltbygovernmentstoprovidepowertomunitionsfactories.Latertheseplantswereconnectedtosupplycivilloadsthroughlong-distancetransmission.BulkpowertransmissionEngineersdesigntransmissionnetworkstotransporttheenergyasefficientlyasfeasible,whileatthesametimetakingintoaccounteconomicfactors,networksafetyandredundancy.Thesenetworksusecomponentssuchaspowerlines,cables,circuitbreakers,switchesandtransformers.Thetransmissionnetworkisusuallyadministeredonaregionalbasisbyanentitysuchasaregionaltransmissionorganizationortransmissionsystemoperator.Transmissionefficiencyishugelyimprovedbydevicesthatincreasethevoltage,andproportionatelyreducethecurrentintheconductors,thuskeepingthepowertransmittednearlyequaltothepowerinput.Thereducedcurrentflowingthroughthelinereducesthelossesintheconductors.AccordingtoJoulesLaw,energylossesaredirectlyproportionaltothesquareofthecurrent.Thus,reducingthecurrentbyafactorof2willlowertheenergylosttoconductorresistancebyafactorof4.ThischangeinvoltageisusuallyachievedinACcircuitsusingastep-uptransformer.DCsystemsrequirerelativelycostlyconversionequipmentwhichmaybeeconomicallyjustifiedforparticularprojects,butarelesscommon第6页currently.Atransmissiongridisanetworkofpowerstations,transmissioncircuits,andsubstations.Energyisusuallytransmittedwithinagridwiththree-phaseAC.SinglephaseACisusedonlyfordistributiontoenduserssinceitisnotusableforlargepolyphaseinductionmotors.Inthe19thcentury,two-phasetransmissionwasusedbutrequiredeitherthreewireswithunequalcurrentsorfourwires.Higherorderphasesystemsrequiremorethanthreewires,butdelivermarginalbenefits.Thecapitalcostofelectricpowerstationsissohigh,andelectricdemandissovariable,thatitisoftencheapertoimportsomeportionoftheneededpowerthantogenerateitlocally.Becausenearbyloadsareoftencorrelated(hotweatherintheSouthwestportionoftheUSmightcausemanypeopletouseairconditioners),electricityoftencomesfromdistantsources.Becauseoftheeconomicsofloadbalancing,wideareatransmissiongridsnowspanacrosscountriesandevenlargeportionsofcontinents.Thewebofinterconnectionsbetweenpowerproducersandconsumersensuresthatpowercanflow,evenifafewlinksareinoperative.Theunvarying(orslowlyvaryingovermanyhours)portionoftheelectricdemandisknownasthebaseloadandisgenerallyservedbestbylargefacilities(whicharethereforeefficientduetoeconomiesofscale)withlowvariablecostsforfuelandoperations.Suchfacilitiesmightbenuclearorcoal-firedpowerstations,orhydroelectric,whileotherrenewableenergysourcessuchasconcentratedsolarthermalandgeothermalpowerhavethepotentialtoprovidebaseloadpower.Renewableenergysourcessuchassolarphotovoltaics,wind,wave,andtidalare,duetotheirintermittency,notconsideredbaseloadbutcanstilladdpowertothegrid.Theremainingpowerdemand,ifany,issuppliedbypeakingpowerplants,whicharetypicallysmaller,faster-responding,andhighercostsources,suchascombinedcycleorcombustionturbineplantsfueledbynaturalgas.Long-distancetransmissionofelectricity(thousandsofkilometers)ischeapandefficient,withcostsofUS$0.0050.02/kWh(comparedtoannualaveragedlargeproducercostsofUS$0.010.025/kWh,retailratesupwardsofUS$0.10/kWh,andmultiplesofretailforinstantaneoussuppliersat第7页unpredictedhighestdemandmoments).Thusdistantsupplierscanbecheaperthanlocalsources(e.g.,NewYorkCitybuysalotofelectricityfromCanada).Multiplelocalsources(evenifmoreexpensiveandinfrequentlyused)canmakethetransmissiongridmorefaulttoleranttoweatherandotherdisastersthatcandisconnectdistantsuppliers.Longdistancetransmissionallowsremoterenewableenergyresourcestobeusedtodisplacefossilfuelconsumption.Hydroandwindsourcescantbemovedclosertopopulouscities,andsolarcostsarelowestinremoteareaswherelocalpowerneedsareminimal.Connectioncostsalonecandeterminewhetheranyparticularrenewablealternativeiseconomicallysensible.Costscanbeprohibitivefortransmissionlines,butvariousproposalsformassiveinfrastructureinvestmentinhighcapacity,verylongdistancesupergridtransmissionnetworkscouldberecoveredwithmodestusagefees.High-voltagedirectcurrentHighvoltagedirectcurrent(HVDC)isusedtotransmitlargeamountsofpoweroverlongdistancesorforinterconnectionsbetweenasynchronousgrids.Whenelectricalenergyisrequiredtobetransmittedoververylongdistances,itismoreeconomicaltotransmitusingdirectcurrentinsteadofalternatingcurrent.Foralongtransmissionline,thelowerlossesandreducedconstructioncostofaDClinecanoffsettheadditionalcostofconverterstationsateachend.Also,athighACvoltages,significant(althougheconomicallyacceptable)amountsofenergyarelostduetocoronadischarge,thecapacitancebetweenphasesor,inthecaseofburiedcables,betweenphasesandthesoilorwaterinwhichthecableisburied.HVDCisalsousedforlongsubmarinecablesbecauseoverabout30kmlengthACcannolongerbeapplied.InthatcasespecialhighvoltagecablesforDCarebuilt.Manysubmarinecableconnections-upto600kmlength-areinusenowadays.HVDClinksaresometimesusedtostabilizeagainstcontrolproblemswiththeACelectricityflow.Inotherwords,totransmitACpowerasACwhenneededineitherdirectionbetweenSeattleandBostonwouldrequirethe第8页(highlychallenging)continuousreal-timeadjustmentoftherelativephaseofthetwoelectricalgrids.WithHVDCinsteadtheinterconnectionwould:(1)ConvertACinSeattleintoHVDC.(2)UseHVDCforthethreethousandmilesofcrosscountrytransmission.Then(3)converttheHVDCtolocallysynchronizedACinBoston,andoptionallyinothercooperatingcitiesalongthetransmissionroute.OneprominentexampleofsuchatransmissionlineisthePacificDCIntertielocatedintheWesternUnitedStates.LimitationsTheamountofpowerthatcanbesentoveratransmissionlineislimited.Theoriginsofthelimitsvarydependingonthelengthoftheline.Forashortline,theheatingofconductorsduetolinelossessetsathermallimit.Iftoomuchcurrentisdrawn,conductorsmaysagtooclosetotheground,orconductorsandequipmentmaybedamagedbyoverheating.Forintermediate-lengthlinesontheorderof100km(62mi),thelimitissetbythevoltagedropintheline.ForlongerAClines,systemstabilitysetsthelimittothepowerthatcanbetransferred.Approximately,thepowerflowingoveranAClineisproportionaltothesineofthephaseangleofthevoltageatthereceivingandtransmittingends.Sincethisanglevariesdependingonsystemloadingandgeneration,itisundesirablefortheangletoapproach90degrees.Veryapproximately,theallowableproductoflinelengthandmaximumloadisproportionaltothesquareofthesystemvoltage.Seriescapacitorsorphase-shiftingtransformersareusedonlonglinestoimprovestability.High-voltagedirectcurrentlinesarerestrictedonlybythermalandvoltagedroplimits,sincethephaseangleisnotmaterialtotheiroperation.ControlToensuresafeandpredictableoperationthecomponentsofthetransmissionsystemarecontrolledwithgenerators,switches,circuitbreakersandloads.Thevoltage,power,frequency,loadfactor,andreliabilitycapabilitiesofthetransmissionsystemaredesignedtoprovide第9页costeffectiveperformanceforthecustomers.LoadbalancingThetransmissionsystemprovidesforbaseloadandpeakloadcapability,withsafetyandfaulttolerancemargins.Thepeakloadtimesvarybyregionlargelyduetotheindustrymix.Inveryhotandverycoldclimateshomeairconditioningandheatingloadshaveaneffectontheoverallload.Theyaretypicallyhighestinthelateafternooninthehottestpartoftheyearandinmid-morningsandmid-eveningsinthecoldestpartoftheyear.Thismakesthepowerrequirementsvarybytheseasonandthetimeofday.Distributionsystemdesignsalwaystakethebaseloadandthepeakloadintoconsideration.Thetransmissionsystemusuallydoesnothavealargebufferingcapabilitytomatchtheloadswiththegeneration.Thusgenerationhastobekeptmatchedtotheload,topreventoverloadingfailuresofthegenerationequipment.Multiplesourcesandloadscanbeconnectedtothetransmissionsystemandtheymustbecontrolledtoprovideorderlytransferofpower.Incentralizedpowergeneration,onlylocalcontrolofgenerationisnecessary,anditinvolvessynchronizationofthegenerationunits,topreventlargetransientsandoverloadconditions.Indistributedpowergenerationthegeneratorsaregeographicallydistributedandtheprocesstobringthemonlineandofflinemustbecarefullycontrolled.Theloadcontrolsignalscaneitherbesentonseparatelinesoronthepowerlinesthemselves.Toloadbalancethevoltageandfrequencycanbeusedasasignalingmechanism.Involtagesignaling,thevariationofvoltageisusedtoincreasegeneration.Thepoweraddedbyanysystemincreasesasthelinevoltagedecreases.Thisarrangementisstableinprinciple.Voltagebasedregulationiscomplextouseinmeshnetworks,sincetheindividualcomponentsandsetpointswouldneedtobereconfiguredeverytimeanewgeneratorisaddedtothemesh.Infrequencysignaling,thegeneratingunitsmatchthefrequencyofthe第10页powertransmissionsystem.Indroopspeedcontrol,ifthefrequencydecreases,thepowerisincreased.(Thedropinlinefrequencyisanindicationthattheincreasedloadiscausingthegeneratorstoslowdown.)Windturbines,v2gandotherdistributedstorageandgenerationsystemscanbeconnectedtothepowergrid,andinteractwithittoimprovesystemoperation.FailureprotectionUnderexcessloadconditions,thesystemcanbedesignedtofailgracefullyratherthanallatonce.Brownoutsoccurwhenthesupplypowerdropsbelowthedemand.Blackoutsoccurwhenthesupplyfailscompletely.Rollingblackouts,orloadshedding,areintentionallyengineeredelectricalpoweroutages,usedtodistributeinsufficientpowerwhenthedemandforelectricityexceedsthesupply.CommunicationsOperatorsoflongtransmissionlinesrequirereliablecommunicationsforcontrolofthepowergridand,often,associatedgenerationanddistributionfacilities.Fault-sensingprotectiverelaysateachendofthelinemustcommunicatetomonitortheflowofpowerintoandoutoftheprotectedlinesectionsothatfaultedconductorsorequipmentcanbequicklyde-energizedandthebalanceofthesystemrestored.Protectionofthetransmissionlinefromshortcircuitsandotherfaultsisusuallysocriticalthatcommoncarriertelecommunicationsareinsufficientlyreliable,andinremoteareasacommoncarriermaynotbeavailable.Communicationsystemsassociatedwithatransmissionprojectmayuse:MicrowavesPowerlinecommunicationOpticalfibers第11页第12页中文翻译稿电力传输电力传输或“高压电输出“是指从发电厂到位于人口中心附近的变电站，电能的成批量传输。当地线路的距离是从高压变电站到客户之间的距离，这通常称为线路分布。当输电线路相互联系时，就成为了高压输电网络。在美国，这些通常被称为“电网“或“网格”，而在英国则被称为“国家电网。“北美有三个主要电网：西部互联电网，东部互联电网和德州电力稳定委员会（或ERCOT）网格。在历史上，传输线路和配电线路由同一家公司经营，但在过去十年左右的许多国家都已经开放电力市场，率先将分布业务和电力传输业务相分离。传输线主要使用三相交流电（AC），有时在铁路上电气化系统也使用单相交流电。高压直流（HVDC）技术用于超长距离传输（一般大于400英里或600公里）、海底电力电缆（通常超过30英里或50公里）和连接两个不同步的交流网络。电力是指在高电压（110千伏及以上）下电能的传输，超高压是为了在长距离传输中减少能量损失。电力通常使用架空的输电线路，地下输电具有明显更高的成本和更大的操作局限，但仍然有时在城市地区或敏感位置使用。除了个别情况外,电力分配的一个关键限制因素是电能不能被存储，因此必须根据需要来生成。一个复杂的控制系统必须确保发电量与需求密切匹配。如果供给和需求不平衡，那么发电厂和传输设备将不能关闭，在最槽糕的情况下，将会导致大面积停电，如在1996年的美国加利福尼亚州和西北部，1965年、1977年、2003年的美国东北部就发生过类似的事故。为了减少这类故障的风险，电力传输网络要相互连接在地区间，国家间和大陆间，从而将多余的电力提供给多个备用路线，使电能在故障频发（天气或设备）的区域流动。电力公司做了很多分析来确定每条传输线路最大的物理和热比限制的能力，以确保备用电量在任意网络中可以被使用。第13页架空传输高压架空的导线不被绝缘体覆盖，导体材料中几乎总是铝合金，制成几股，并用钢绞线来加固。铜有时用于架空传输，但铝重量较轻且相比铜仅性能略有降低，而且铝成本较低。架空导线是由全球数家公司提供的一种商品，被改进的导体材料和形状常常用来提高现代化传输电力的能力。导线截面为12平方毫米（美国规定为6）到750平方毫米（1,590,000圆密耳面积），具有不同的电阻和电流承载能力。较厚的电线会导致电容