电力变压器的选择和冷却

T外文文献：SelectionandCoolingofPowerTransformerTheselectionofthetransformercanhaveamajorimpactonthecostasubstation,sincethetransformerrepresentsthemajorcostitem.Nameplateratingisonlyaguidetotransformerapplication,andshouldonlybeusedasafirststepintheselectionprocess.Thetransformerisavailableasaself-cooledunit,oritcanbepurchasedwithadditionalstepsofforcedcoolingthatusefanorfansandoilpumps.Transformerratingscanbeincreasedfrom25%to66%bytheadditionoffansandpumps.Thenameplateratingisbasedonacontinuousloadproducinga55℃to65℃conductortemperatureriseoverambient.Sincemanytransformersdonotcarrycontinuousloads,advantagecanbegainedfromthethermaltimelagtocarryhigherpeakloadswithoutexceedingthetemperaturelimits.Transformerratingsarebasedontheassumptionthatonlyanextremelyslowdeteriorationofinsulationwilltakeplacewithnormaloperation.Asubstantialincreaseinratingcanbeachievedbyacceleratingthelossofinsulationlife.Thisincreaseinratingmightapproach200%foranhourortwo,andapproximately120%for24hours.Forsubstationsthataredesignedtocarryfullloadundertheoutageofanyonetransformer,ahighemergencyratingfora24-hourperiod(e.g.untilthefailedunitcanbereplaced)couldmeantheselectionofsmallertransformersandasubstantialsavinginsubstationcost.Theselectionofthetransformershouldinvolveacarefulevaluationofanumberofotherfactors：(1)Impedancesshouldbeselectedconsideringtheireffectonshort-circuitdutiesandlow-sidebreakerratingsbothforinitialandfuturestationdevelopments.Inaddition,impedanceisimportanttoachieveaproperloaddivisionintheparalleloperationoftransformers..(2)Noloadtaprangesshouldbeselectedtoprovideanadequatelow-sidebusvoltage.(3)Ifthehigh-sideorlow-sidevoltagesvaryoverawiderangeduringtheloadcycle,itmaybenecessarytoprovidebusregulation.Theactualregulationcanbecalculatedusingthesystemandloadcharacteristics.Ifregulatingequipmentisneeded,itmaybedesirabletoprovideitinthetransformerbyusingloadtapchanger(LTC)equipment.Iftheneedforbusregulationisnotpresentlyevident,butmayberequiredinthefuture,itmaybeeconomicaltoleavespaceinthestationforfutureregulators,andbuytransformerswithoutLTCequipment.Agreatmanytransformers,includingallthelargeonesandallthehighvoltageonesareimmersedinmineraloilwhichservesthedoublepurposeofcoolingandinsulatingthewindings.Smalltransformershaveenoughtanksurfacetoradiateal1theheatcausedbytheirlosseswithoutexceedingthepermissibletemperaturerise.Assizeincreases,thelossesincreasefasterthanthetanksurfacewhichsoonbecomesinadequate.Variousmethodshavebeendevelopedtogettheheatoutofthetankmoreeffectively.Largepowertransformerbeingbuilttodayarecommonlyoftheforced-oil-cooledtype.Inthisdesigntheoilispumpedboththroughtheexternalcoolingdevices(airorwaterheat-exchangers)andthroughinternalchannelsthatarelocatednearestthepointswheretheheatisgenerated.Thusthetransferofheattoandfromtheoilisfarmoreeffectivethanintheplainself-cooledorfan-cooledunitwheretheoilisallowedtocirculatebyconvection.Whenoil-to-airheatexchanger(agroupfansoneachradiator)isused,thiscoolingtypeisdesignatedastypeofFOA.Whileoil-to-waterheatexchangerisused,itiscalledFOW.Forapplicationswhereanyliquid,evenanonflammableone,isobjectionable,thedry-typetransformerisused.Theventilateddry-typeunitiscooledbyacontinuousnaturaldraftofairandconsequentlyisnotsuitableforlocationswheretheairiswetordirty.Fortheselocationsacompletelyenclosedunit,thesealeddry-type,isavailable,havingacore-and-coilinatankthatissealedandfilledwithaninsulatinggas.Dry-typetransformersarecompletelynonflammable,usingorganicmaterialsuchasepoxyresin.中文译文：电力变压器的选择和冷却变压器的选择对变电所的建造成本具有重大影响，因为变压器是主要的成本项目。铭牌定额只是变压器应用的指南,它只应该用作变压器选择过程的第一步。变压器可以是自冷式单位，也可以购买额外的冷却装置,如强制冷却风机或机组和柴油泵。根据额外的冷却机组和柴油泵，变压器评级可以从25%增加到66%O铭牌评级是基于连续负荷产生超过环境55c到65c导体温升而评定的。？？？因为很多变压器不带连续负载,优势可以获得从热滞后时间进行更高的高峰负荷而不超过温度限制。变压器的评级是基于假设：正常操作下的绝缘老化只是一个极其缓慢的过程。通过加速绝缘老化的进程可以大幅度的提高评定等级。在一两个小时左右评级可能增长接近200%,而在在二十四小时左右评级则增长大约120虬因为变电站的设计要保证在任何一个变压器断电的情况下都要满足全负荷运行，为24变电站设计将满载在停机的任何一个变压器,因此在二十四小时内的较高的紧急评级(例如直到失败的单位可以更换)可能意味着选择较小的变压器和节约大量的变电所成本。变压器的选择应该涉及对其他影响因素的审慎评估：(1)阻抗应该被选择的考虑到他们对短路职责和下部断路器评级的影响对初始和未来空间站发展。此外，阻抗在并行变压器的操作中获得一个合适的负载也是十分重要的.⑵无载分接范围应该选择提供一个足够的下部总线电压。(3)如果高压侧或低压侧电压在负载周期内变化很大的话，可能有必要提供总线监管。通过系统和负载特性实际监管和控制是可以被计算和描绘的。如果调节设备是必要的，在变压器内使用负载抽头转换开关（LTC）设备来提供监管是可取的。如果需要总线监管目前是不需要的,但可能在未来需要的，那么在变电站预留未来安装设备的空间并且购买没有LTC设备的变压器是十分经济的。许多变压器,包括所有的大体积的和所有的高电压的变压器都是沉浸在矿物油,矿物油有双重目的：冷却和绝缘绕组。小变压器有足够的容器表面辐射all热引起的损失而不超过容许温度上升范围。随着体积的增加,损失增加的速度会加快，这时容器表面会变得不适宜。很多的方法已经被改进以使的容器的散热更有效。今天一般建造的大型电力变压器都是强制循环油冷却类型的。在这个设计中矿物油通过油泵不仅流进外部冷却设备（空气或水热交换器），而且流过内部最靠近热源的凹槽。因此通过矿物油对热量进行转移的方式