AbsｔｒａｃｔInｃrｅａsinｇaｔtention haｓｂｅｅn showｎin receｎt yｅａｒｓin the a.ｃ. eleｃｔriｆiｃatｉon ofｒａilｗay tｒａｃtｉon sｙstems aｎｄａnｕｍber of esｔａblishｅd sysｔemｓｎｏw eｘｉst throughout the worｌｄ. Wherｅｔhe tracｔiｏn sｙstｅｍis supplied dirｅｃtｌｙfrｏm ａhigｈ-voltagｅnational-grid nｅtwork, a common ｃomｐonenｔｏf suchｓｃheｍｅs ｉs a steｐdｏwn tｒansf ｏrmer conｎecｔioｎｂetweｅｎtｈe twｏsｙsｔems. Ｔhｅeｌecｔrical，ｍeｃhanical and theｒmａl ｄesign oｆｔhｅse transfｏｒｍers is subject to a number ｏf sｐecｉａl consｉdｅｒatiｏnｓｎｏt norｍａlｌy eｎcounｔeｒｅd in the ｄｅsign of dｉｓtribuｔｉon-ｔype ｔransformers oｆｓimilａr rating and voltａｇe clasｓ. T ｈｅpaｐer reviews the operaｔing cｏｎdｉtiｏns ｐeｃuｌiａr tｏｒailwayｔra ｃtｉoｎsucｈaｓoｖeｒvoｌｔａｇes,shｏrt circuｉts, cｙｃliｃand ｐｅak loa ｄings aｎｄｄiｓcｕssｅs hｏw ｔhesｅconｄitionｓｉnfluｅｎce thｅdeｓign and cｏnｓtrｕｃtｉoｎof siｎglｅ－pｈase transforｍers sｕpplyｉng poｗer foｒｔraction purpｏseｓ. Tｈe pａpｅｒｄesｃribesｔheｅｎgｉnｅering pｒactiｃｅand ｏpeｒating expeｒience obtained ｗith the Brｉtｉsｈ25 ｋV a.c. ｒailｗay－tracｔｉｏn sｙstem in partｉcｕlar, ｂut much of the ｓｙstｅm and transforｍeｒ-desi ｇn philoｓｏphy and oｐeｒaｔｉonal expeｒiｅnce ｒeferrｅd tｏaｐpｌｉes ｔo ａ.c. ｔｒａction ｓysteｍｓelsｅｗhere in the wｏｒld.Key Wｏｒｄｓ: Pｏｗer transformers, Raｉlwａｙｓ, Tractioｎ1 IntrodｕctiｏnIn Mａrch１９5６the ＢriｔisｈTｒaｎsｐort Commission ａnnouｎcｅｄｔheir inｔentiｏn tｏaｄoｐt a 2５kＶsingｌe－phase ５０Ｈz a．c.sｙｓteｍaｓthｅstａndaｒd fｏr futuｒe elecｔrification of Briｔish Railways in all Ｒeｇions eｘcept ｔｈe Southern, ｗhere extensｉon oｆtｈe existiｎｇtｈird-rail sｙsｔeｍ, whｉch had beｅnｏperatiｎg saｔisfacｔｏｒily for some yｅars, wasｃonsｉdered ｔo ｂe thｅmoｓt satｉsfａctory courｓｅto follow.The commｉｓsiｏnｈaｄｅaｒｌiｅr ａｕthorized a studｙto be mａdｅof the ｃｏmpａrａtｉve cｏｓts of electrifｉｃatｉon at15０0 V d．ｃ. and at ２5 000 V single－ｐhaｓe50 Hz a．c. This showed ｂoth econoｍic and ｔeｃｈnｉcａl aｄｖａntages in faｖor ｏf the a.c. sｙstem, ａnd tｈｅｄecｉsion was ｔａｋen to ado ｐt it. Another dｅciｓiｖeｆactoｒwas the successｆul ｏｐeｒatｉonａl expｅrienｃｅgaiｎed by the French Raiｌway Ａuthoｒitieｓfrom an exｐｅｒｉmentaｌline, iｎstａlｌed ａｆew yｅarｓeaｒｌｉer iｎNoｒth Eaｓtern Fraｎｃｅ, opeｒa ｔing ａt the ｓaｍｅvｏltage andｆrｅquencｙ.Iｔｗａs conseｑuenｔly decideｄtｏadopｔa 5０Ｈｚa.c systeｍto introｄｕcｅit oｎｔhｅＳcottish, Eaｓterｎaｎd London Mｉdland Rｅgions.25kV was chｏsen as the gｅnｅral ｓtanｄａrｄforｍaｉn-lｉne seｒviｃes and ６-２５kＶａｓａｓuｂsidｉaｒy standａrd foｒuse ｉn ｔhｏｓe ａreas, ｍainｌy suburban, where ｉt was not pｒａcｔｉcablｅto modify exｉｓting stｒuｃｔures,ｓuch as tｕnnｅls ａｎd overｈｅad ｂrｉｄｇes,ｗhicｈａｔtｈaｔtiｍe ｉｍpo ｓed reｓtrictiｏns on the eｌectrical ｃlｅarａnｃeｓto ｌiｖeｃonｄuｃtoｒｓ.As a ｒｅsuｌｔoｆｏｐerational exｐerｉence, ６25kV sysｔemｓaｒe now being ｃonverted ｔｏ２5 kＶ, and iｔis vｉsuａｌiｚedｔhaｔeventuallｙthｉs wｉll be tｈe opｅｒatｉnｇvoltage for ａll regiｏnｓ.Ｔｈiｓｐａper revｉews the opeｒaｔiｎｇcｏndｉｔions ｐecuｌiar tｏrailｗay ｔｒactｉｏｎ，resulｔinｇfrｏm ｏver vｏｌtagｅｓ，shorｔcircuits, cyclｉcａnd peａｋloａdiｎｇ,and dｉscussｅs how tｈｅse conｄitions infｌuｅncｅthe elｅcｔriｃal, mechaｎｉｃal ａnd ｔｈermａl desｉgｎａnd ｃonstrｕｃt ｉon oｆthe siｎgle-phasｅtｒansｆormers suppｌying power fｏr tｒacｔion from thｅＮaｔｉonal Ｇriｄsｙｓtem to Briｔish Rａil at2５kV50 Hz a.c. Altｈoug ｈthｅpapeｒdescrｉbes Britisｈpｒactiｃe ｉｎpartiｃｕlar, muｃh of ｔhe ｓｙsｔｅm ａnd tｒansformｅr ｄｅｓｉｇｎphiｌosophy anｄoｐeratｉonal experieｎc ｅｒeferｒｅd to iｓapｐlicable to ｓyｓtｅmsｅlsｅwｈere in the woｒld．2 Ｓyｓtem dａta2.1 Supｐly to ｔｈe rａｉlway－ｔrack ｆeedｅr stａtionsＢulk ｓuｐplｉｅs are taｋen from ｔhe １32 kＶand275 kV grid systｅｍs ｖia ｓingle-phａse ｔrａｃtiｏn－supply tｒansformeｒsａnｄfｅd to ｔhe ｆeedeｒｓtationｓaｔ２５kV．Ｔhe feeder stａtｉｏnsａｒe lｏcated aｄjａce ｎt tｏｔｈe rａil traｃks at40or ５0km inｔervals,and whereveｒｐｏs ｓiｂle in cｌosｅproximity ｔo gｒid sｕbｓtatiｏns ｔo ａvｏid the disadvaｎt ａｇe oｆｌonｇfeeders. On ｔhｅfｉrst Regioｎs to be eｌeｃｔrｉfied, ｓｕpp ｌies ｗｅre ｔａkｅn in ｄuplicate,but on rｅcentｅxtensｉons to the Ｎorth Ｗｅsｔeｒｎaｎd Eastern Regionｓ a ｓingle tｒansfｏｒmer has ｂｅen providｅd at everｙalteｒnaｔe ｓｕpply ｐoint, anｄｔｈe ｐｏssibilｉty of ｉnstａllｉnｇonly onｅｔransformer iｎfutｕre atｅach substａｔion iｓnow bｅｉng ｃonsｉdereｄ.Ａtｙpiｃaｌtwo-tｒansfoｒmeｒinｓtaｌlatiｏn is ilｌusｔrated ｉn Fig．1. The 25ｋＶｔrａctioｎsuppｌy iｓtaｋｅｎto ｔhe feeder－sｔatioｎbｕｓbaｒs ｅitheｒｂy coｎcentrｉc2-coｒe oｉl-filled underground cabｌeｓor by sin ｇle－ciｒcuｉt wood-poｌｅoｖerｈｅａd lｉnes.Onｅfeedeｒ－ｓtation bus baｒsupplｉｅs cｕrrent to the oveｒhｅａd－contaｃt sｙstem and aｓｅpａraｔely mounteｄｂuｓbaｒiｓｃonnected by sｈｅaｔhed cable tｏthe tｒａｃｋr ｕｎniｎｇｒail，ｗhicｈ，ｂy vｉrtuｅof tｈe overheａd liｎe supporting ｓtruct ｕre bondeｄtｏｉｔ, iｓwｅll eａｒｔｈed.Thｅduplｉcaｔeｓuｐpｌiｅs ｔo the bｕｓbaｒｓare ｓｅparatｅd bｙa secｔioｎswiｔch which is nｏrmallｙｏｐｅｎ,and ｅach ｃｉrcuit sｕｐpliｅs ａn up ａnd dｏwn sectｉoｎof the tｒack.When ｏutａgｅs are necessary, fｏr mainｔｅnaｎcｅor eｍeｒｇencｙopｅration，the sｅctｉoｎsｗitch aｔthe sｕpply point isｃloseｄ. The remａining transfｏrｍer mｕsｔbe caｐableｏf ｆeedinｇthe wholｅlength of the ｓection oｆtｈｅtrack ｎormally fed bｙtｗo transformｅrs．The ｄesｉｇned rating of the transformerｍuｓt therefoｒe ｂｅadequate to catｅr for the maxｉｍuｍｄｅｍand uｎder this eｍergenｃy condｉtio ｎ. Tｈｉs demand ｍaｙoccuｒat ａｎy timeｏf the ｙear，in either ｓｕmmer or winter cｏnｄitiｏns，ｔherｅfｏre ｔｈｅｔｈｅrmａl ratｉng of the transfｏｒmｅrｓｍust bｅadeｑuate to ｅnsure wｉnｄing and oil ｔｅmpeｒatuｒｅs do ｎoｔexceeｄｔhe ｇuaranｔeeｄmaxｉmｕm ｕｎｄer tｈe pｒevailｉng ａmbient temperature.３．Ｓysｔｅm ｏperａｔｉｏｎal ｆaｃtorｓ3.1 LｏaｄcurrentTｈe lｏad-ｃｕｒｒｅntｄｅmand on a traction-ｓuｐply poinｔiｎｓｔalｌａt ｉon iｓof ａｈｉghlyｆｌuｃtuatｉng patｔern ａnd is no sinusoｉｄal in wａveｆｏｒm. Thｅcurｒeｎt fluctuaｔioｎs are ｒanｄom ｉn timｅanｄmagnｉｔuｄe ａnd depend uｐon the ｄensity of traffic withiｎthe secｔiｏn oｆtractiｏｎcircuit suppliｅd by thｅtraｎsfｏrmeｒｓａnd ｕpon tｈeｍｏde of oｐeratｉng tｈeｌocｏmotｉｖes. Tｈｅｎo ｓinｕsoｉｄal ｃurｒeｎt, Fｉg.2, ｉs ｏｆapｐrｏxｉmateｌy ｓquaｒｅwavefoｒm and ocｃurｓｂｙrｅason oｆtｈe harmonｉcs generated by ｔｈe rｅctifiｅｒequiｐｍｅnｔon thｅｌocomotｉveｓ.Ｔhe significａnce of thｉs type ofｌoad－cｕrrent dｕｔy ｕpon tｈeｄesign ａspｅcts of the trａnｓformer iｓofｐarｔicular ｒｅｌevanceｗｈｅn considerｉng tｈe ratinｇof ｔhe transformｅｒ, the effectｕpoｎｒegulａtiｏｎａnd the mechanical forceｓacting oｎtheｗindings．Fiｇ．3 ｒｅpｒesenｔs a tｙpicalｌoad－currｅｎt－deｍaｎd curve obtaｉned on a supply-poｉnt trａnsｆorｍer．The maｘimum variatioｎｏf cｕrrent can bｅｂeｔween zero ａnd two and a ｈaｌf times tｈe full-load ratｅｄｖalue. Ｔh ｅcombiｎｅd heatｉｎg eｆfect onｔhe tｒansforｍer wiｎdｉngs of a curｒent vａｒｙinｇｉn this maｎｎer,ａnｄhaving the tｙpｉcal nｏｓiｎｕsoidaｌｗavefo ｒm referred tｏ，ｉs greaｔｅｒＦig.1Typicaｌ2５ｋＶ2-ｔranｓforｍｅr pｏwer－ｓupｐly installaｔｉon Supｐly aｕthｏrity supｐｌy raiｌwａy auｔhority suｐply:（a) 1３2/3３kV ３-phase arｅa-board traｎsfｏrmeｒs (ｂ) 132/２5kＶsingle-phａse railｗａｙ-supplｙｔranｓfｏrｍeｒｓ(ｃ) Supply-autｈoｒity ｃircｕiｔbreakers (d) Ｒａilwaｙ-aｕthｏrity ciｒcuiｔbreakｅrs（e)Ｃoncentｒｉc cａbles oｒｏvｅrhｅａdｌines （f) Pｉloｔｃaｂｌes foｒprｏｔectｉon,teｌeｃommunication ａnd suｐervisoｒy dutieｓ.tｈan tｈａｔｗhich wｏuｌd ｂe pｒodｕced by a steaｄy current hａvｉnｇa vaｌｕｅeqｕal to mｅan vａlue ｏf the flucｔuａtｉnｇcurrenｔ, alｔhouｇh bｏtｈｗould indicａte eqｕal MW readings on ｔhe half-hｏｕr ｉntegｒａted mａｘｉmｕm-ｄemand meters.Corｒelatｉon ｏf ｔypicaｌｓupｐlｙ－cｕｒrent-deｍａnｄcｕｒveｓ, obｔained duｒing measurementｓunder both nｏｒｍal ａnd ｅmerｇｅncｙsupplｙ-ｐoｉnt opeｒaｔioｎ，indicａtes tｈat ｉｆa trａｎｓｆｏrmerｉs ｔo ｃater ｆor tｈe wｏrst expｅｃtｅd loaｄcondｉtion wｉthout excｅeｄiｎg temｐｅrａｔｕre-rise liｍitｓ, ｔhe equivａｌenｔｒaｔing haｓto ｂe some 1 －2tｏ1 －3 tｉmes the ｅxpeｃtedｃirｃumｓｔaｎces. Tｈe vａrｙing mａgnｉtude oｆtｈｅloａd curｒｅｎt also hａs an ｅfｆect upoｎｔｈe inteｇrated half hourly maxｉmuｍdeｍand forｔhe ｐarticular operating mechanｉcａl sｔｒengtｈs of the transformer wｉn ｄｉｎｇs. In servｉce,ｔhe windingｓareｓuｂｊecｔｅd to pulsatiｎg forceｓｓimilar, ｉｎmany rｅspeｃｔs，to those ｅxperieｎcedｂyFig．2 Onｅcyｃle of tyｐｉcａl ２5 kV anｄ50 Hｚtrａctiｏｎvoltage ａndｃurrenｔrectifier aｎd ａｒc-ｆurnace-supｐｌy tranｓforｍeｒｓ. These forcｅs ｃaｎhaｖe ｄeleterioｕs effectｓon thｅpｅrｆｏrmance ofｔhe windｉnｇs and ａssocｉateｄinsｕｌation ｉn serｖｉｃｅ，ｕnｌess ｍｅasｕｒｅｓare taken ｉn the desiｇn aｎd ｃonsｔructionｏf ｔhｅtraｎsfoｒmerｔｏｅnsｕｒｅthat ｔｈe ｙｃan be wｉｔhｓｔooｄ.Ｆig.3Typｉcalｔｒａctiｏn lｏａd-ｃurｒent-ｄemand curve3.2 Ｈａrmoniｃs and uｎbalanced cuｒrent ｅｆfeｃｔsＤｕring the ｄevｅloｐmｅｎt of ｔhｅraｉlｗａyｓｉnｇlｅ-phase ｔraｃｔion sｙstem, onｅof the maｉn characｔeriｓticｓ，to whiｃh partiｃｕlarａｔｔｅｎtioｎwaｓgiｖeｎ，wａｓthe queｓtｉｏn of sｉngle-phase unbａlanｃed loads and harmonｉcｓimposｅd ｏn the 3－phasｅsupply systeｍ. Ｔhｅhaｒmoｎic ｅffects ａrisｅas a resultｏf thｅｒecｔｉfication eｑuipmeｎtｉｎstaｌｌedｏn tｈe ｌocｏmoｔｉｖｅｓ．1'2'3 Frｏm thｅmany ｉnvｅstｉgatioｎs whｉｃｈｈａｖe ｂeｅn ｍade on thｅsystem, ｉt hａs bｅeｎshｏｗn tｈe ｇrid－supply systeｍiｓlarge enｏuｇｈｔo ａbｓorｂthｅse ｅffecｔs witｈoｕt signｉficａnt inｔｅrｆerence to ｏtｈｅr coｎｓuｍｅrs ｏｒｇeneratinｇｐlant．It will bｅｒemｅｍbereｄ，howeveｒ,that tｈe early deｖelopmｅnt of the system wasｉｎurban aｎｄiｎdusｔｒiaｌareas wheｒｅthe fault leｖeｌs wｅre vｅrｙhigh．In ru ｒal areaｓ, suｃhａs the ｗest coast ｅｘtｅnｓion ｂｅｔwｅenＭaｎchｅstｅr ａnd Glasｇow, ａnd where lｏｗｅr ｆauｌｔlｅvelｓｐrevａil, cｏndｉtioｎｓare margiｎal ａnd provｉsｉon for thｅlater additiｏｎｏf hａrｍonｉc ｆilteｒs ｈａs been madｅｓhouｌd thｅse be reｑuiｒed.4.1 Tranｓforｍｅr deｓigｎ Gｅneｒaｌ specificａtiｏnThe trａction－ｓuｐplｙｔrａnsformｅｒｓａre ｏf coｒe-ｔype ｃonstｒuction ａnd are desiｇnｅd，manufacｔuｒed and ｔested in accorｄancｅwitｈＢＥBＳｐｅcificatioｎT2 and CEGB Ｓｐeciｆｉcaｔion RＴ（19７1), BS1７1and otheｒnatiｏｎal sｐeciｆications whiｃh may bｅreｌevａnt. Aｎｙspecial requireｍents are sｐeｃｉｆｉeｄｉnｐertinｅnt cｏｎtract documeｎts.４.５ Coｎsｉｄｅratｉｏｎs afｆecting the choiｃe of traｎｓformer conｓtｒucｔionTｈe paｐer ｈas ｏutlined the different ｍeｔhods of ｃoｎstruction wｈiｃｈhaｖe been ｇenｅrａｌly adopｔed to meeｔtｈe ｒeqｕirements detｅｒmined ｂy facｔors ariｓing ｆrｏm cｏｎsiｄeration of ｔhe ｓystｅm or test. Iｔis bｅliev ｅｄthese deｓignｓare of eqｕal ｍeｒit inｐrovidｉng tｈe best overａllｅco ｎomｉｃand technｉｃaｌadｖantagesａnd this ｈas ｌarｇely ｂeeｎconfiｒmedｂy expｅrｉeｎｃe.An alｔernａtｉｖe fｏrｍoｆｃｏｎｓtｒuction, ｏftｅn consｉdｅred in the ｃontext oｆrailway ｔranｓforｍerｓor ｏther appｌicａｔions whｅre a higｈdｅgrｅe of inhｅrenｔmecｈａnｉcａl streｎgtｈｉｓrequｉrｅd toｗiｔhstand ｓhort-ｃircuit forces, is thaｔofｔｈe shell-type ｃonstrｕｃｔion. In this meｔhod ｏf ｃonstｒuctiｏn, the h.v．andｌ.v.wiｎdiｎgs ａre intｅrleaveｄaxiaｌly，therebｙａvｏidｉngｒadｉal ｆorceｓ, ｔhｅonlｙｆorｃｅcomponent being iｎｔｈe ａxial diｒeｃｔｉon.Aｓthｅwindinｇs arｅｌargely coｎtainｅｄwiｔhin tｈe core wiｎdｏw,ｔｈe core yokｅs pｒｏvｉdｅsolid pｏinｔs fｒom which to ｂｒace the ｗｉｎdingｓaｇａiｎst moｖement iｎtｈe axial direction．Fｒom tｈis aｓｐect alonｅ, theｄeｓｉgｎcoulｄｂe cｏnｓidered supeｒｉoｒto those already ｄescｒiｂed．In otheｒaspeｃts，howeｖｅｒ,the shell－typｅtraｎsformeｒｐresentｓｄecidｅdly unｅcoｎoｍｉｃdiｓadvantaｇes. By reasｏn oｆintｅrleavinｇtｈe ｈ.v. ａnd l.v. wｉｎdinｇｓ, sｈell－typｅtranｓformers have ａｎinherｅntly low reacｔan ｃe and the gｅometｒｙｏf ｔhe wｉndings would haｖe tｏbe adjusteｄat the ｅxｐense of spａce factor to achievｅａn impeｄance of1０％ｏn 15 ＭVA. Also, ｂｅcausｅｏf tｈe 2-poｌe simuｌtaneｏus-impuｌse－tesｔｒequiremenｔ, ｔhe ｅlectrical clearaｎｃes at the centrｅof ｔhe cｏｉl stacｋｈｅiｇht tｏthｅｓurrｏundiｎg coｒe wouldｈａve to be aｄeｑｕate to witｈstanｄａpproximａtely twice ｔhｅａppｌｉed impulsｅ－ｔest vｏltaｇe．Ｏther considerations are the ｎｅｅｄtｏｍinimiｚe eｄdy－cｕrreｎt lossｅsｉｎthｅwiｎｄings，by meaｎsｏf coｎduｃtｏrsｈａｖing a loｗaｘiaｌprofile，ａndｔｈe tｒaｎsposit ｉoｎｏｆｔｈe coｎducｔoｒs ｉf ｗｏuｎd iｎparallel ｉn tｈe axｉaｌｄirection.Deｓｉgn sｔuｄieｓhaｖe ｓhown this form ｏf conｓtruction ｉs noｔｅｃoｎomicaｌｌy viabｌｅｗhen compａred ｗitｈthｅcｏre-type cｏｎstrucｔioｎ,ｐaｒticulａｒly ａt supplｙvoltaｇes oｆ１3２ｋV and ａbovｅ, ａnｄthe ｇreａtｅｒexpeｎｓe iｎvolveｄｉs noｔbelieｖｅd to be jｕstifiｅd ｉn ｔhe ｌｉｇｈt oｆwｈat has bｅeｎachievedｗitｈtｈe existing designｓ.摘要近年来，对交流电气化铁路牵引系统和现在它在全世界存在数量的关注已越来越多。
