Fundamentals of transformer design

Transcription

1 Fundmentls of trnsformer design y H du Preez, onsultnt trnsformer is stti piee of equipment with omplited eletromgneti iruit. The eletril energy is trnsferred from one eletril iruit to nother through mgneti field. In its simplest form, trnsformer onsists of two onduting oils hving mutul indutne. The history of trnsformers goes k to the erly 1880s nd with the demnd for eletril power inresing, lrge high voltge trnsformers hve rpidly developed. Trnsformers re mongst the most effiient mhines. eing stti devies, they hve no moving omponents, therefore mintenne nd life expetny is long. They re neessry omponents in eletril systems s diverse s distriution of multi-megwtt power from power sttions to hnd held rdio trnseivers operting t frtion of wtt. Trnsformers re the lrgest, heviest nd often the ostliest of iruit omponents. The geometry of the mgneti iruit is three dimensionl; this property ples fundmentl restrint on reduing trnsformer size. The properties of ville mteril limit size nd weight redution. High voltge trnsformers require speifi lernes, nd insultion type nd thikness ditte the size of the unit. Trnsformers re indispensle for voltge trnsformtion in power pplitions. Their ility to isolte iruits nd to lter erthing on ventions n often not e mthed in ny other wy. Speil designs re ville to otin isolted multi-phse supplies for six, 12, 24 nd higher phse (pulse) retifition iruits. Trnsformers re essentilly single-pplition devies designed for speifi requirements. well designed trnsformer is rugged piee of equipment nd, if used in the environment nd pplition for whih it ws speifilly designed, it will give mny yers of troule-free servie with miniml mintenne nd ttention. However, euse trnsformers re stti pssive units they often lk ttention nd mintenne. The si priniples for ll trnsformers re the sme; only the detil design will hnge nd in this short rtile it is impossile to over ll possile winding onfigurtions. The si theory overs ll types from smll high frequeny trnsformers using ferrite ore, urrent trnsformers typilly round wound ore nd toroidl winding to 800 kv power trnsformers. There re no rules whih ditte tht either spirl winding or disk winding hs to e use on prtiulr design; the designer would hve to mke these deisions, s in the se of most eletril mhine designs. There is no unique design for prtiulr trnsformer nd there re mny designs whih ould meet ll the speifitions. Some of these designs would e etter thn others ut they would ll funtion. si theory Eletril energy is trnsferred from one eletril iruit to nother through mgneti field. In its simplest form, trnsformer onsists of two onduting oils hving mutul indutne. In n idel senrio, it is ssumed tht ll the flux linked with the primry winding lso links the seondry winding. This is impossile s mgneti flux nnot e onfined; ut it n e direted so tht most of the flux meets this riterion. The smll portion of flux tht nnot e direted is known s lekge flux nd will link one or other winding nd/or omponent in the trnsformer. Voltge is proportionl to the numer of turns, urrent is inversely proportionl to turns. Generl types The two fundmentl types of trnsformers re the ore nd shell types: the winding irulting the iron ore is the ore-type while in the shell-type, the winding is lrgely enirled y the iron ore. oth single nd three phse trnsformers n e onstruted in either type. ore Figure 1: ore type trnsformer (3 phse). ore Figure 2: Shell-type trnsformer (3 phse). ore onstrution ore steel lmintions re mnuftured speifilly for trnsformers nd motors ut with differene. Motor lmintions re mnuftured (stmped) from non-oriented lmintion steel wheres trnsformer lmintions re mnuftured from grin oriented steel Flux flows with lower losses in the diretion of rolling (grin oriented)

2 ore loss W/kg Flux diretion 1.7 T 1.5 T 1.3 T 1 T Lmintion 0 Diretion of rolling Figure 3: Losses in grin orientted lmintion steel for vrious diretions of mgnetistion [1]. The purpose of the ore steel is to provide low relutne pth for the mgneti flux tht links the primry nd seondry windings. Lmintion steel is speifilly designed to redue losses in the steel. There re two min omponents to iron losses they re: Hysteresis losses Eddy urrent losses Hysteresis is dependent on frequeny, mteril nd flux density. Eddy urrent is dependent on the squre of the frequeny nd the squre of the mteril thikness. numer of different grdes nd types of lmintion steel re ville. Hot rolled steel High-permeility steel (0,025% l old rolled) (30 to 40%) Domin-refined steel (5 to 8%) morphous steel (80% Iron 20% oron nd Silion) (33,33% improvement t knee point) (1,5 to 1,6 Tesl) ore profile n e squre, round (stepped), ovl or retngle. The joint n lso tke on mny onfigurtions (utt, overlp, mitred, et). ore-mgneti iruit The mgneti flux density is mesured in Tesl (Weers/m 2 ), nd norml vlues for trnsformer rnge etween 1,6 nd 1,8 Tesl. How eddy urrents re voided in the ore (eddy urrents inrese no-lod losses nd rete hot-spots): The ore steel lmintions should e thin The ore steels should e insulted from eh other Smllest urrs possile in oth slitting nd utting s these urrs rete shorts ross the lmintions The ore steel should hve high resistivity Joint etween ore lmintions: In joints the mgneti flux jumps to the djent lmintions, with lol sturtion s result Step-lp joints hve higher sturtion limit ompred with onventionl joints. The mgnetising urrent is lower for the step-lp in this re of the joint Mehnilly, the step-lp joint is weker thn the onventionl joint euse of the smller overlp It is importnt to keep the gp etween the lmintions s smll s possile t the joints The lmping t the joint must e s strong s possile to redue noise, inrese strength nd redue gp losses 45 Step-lp joint onventionl joint Figure 4: Lmintion joints. re of higher flux onentrtion To summrise: Step-lp + Lower losses + Lower noise level - Mehnil strength onventionl + Mehnil strength - Higher losses - Noise level The ore onstrution n tke on mny forms ut must e rigid nd tightly lmped ll lmping must e insulted to eliminte the possiility of irulting urrents s result of the min flux nd or the lekge fluxes lmping must not short-out the lmintion; through olts must e insulte In its simplest form, trnsformer onsists of two onduting oils hving mutul indutne Winding n e done in numer of onfigurtions, nmely onentri or sndwih types. In the onentri type the LV oil is generlly wound ginst the ore nd the HV winding over the LV winding. In ertin pplitions the HV is ginst the ore nd the LV is in on the outside. The sndwih type of winding is ssemled with lternting low nd high voltge winding. HV winding ore LV winding onentri type winding Figure 5: Winding types. ore Sndwih type winding LV windings HV windings There re four types of oils used in trnsformer winding ssemlies - ylindril (Figure 6), oin (Figure 7), dis (Figure 8) nd foil windings (Figure 9). Foil-type winding: Foil wound trnsformers generlly hve the LV wound using luminium or opper foil over the full width of the winding; therefore with one turn per lyer nd the numer of turns equl to the numer of lyers, the foil eing wound with suitle insultion is interleved with the foil.

3 Winding ondutors my e opper or luminium, nd they my e in foil or sheet form, or of round or retngulr setion For high powered trnsformers the low voltge winding my require lrge ross-setionl re to e le to rry the required urrent. In this se, the use of strnded insulted ondutors in prllel my e required to redue the eddy urrent losses in the ondutor. It my lso e neessry to trnspose the ondutors, to redue the irulting urrent within the winding. In lrge trnsformers ontinuously trnsposed ondutors (T) my e used It is importnt when ondutors re used in prllel tht the lengths nd onfigurtion with respet to the ore nd eh other re ll the sme otherwise irulting urrents ould result nd Figure 6: ylindril type winding. Figure 7: oin type winding. Figure 8: Dis-type winding. Figure 9: Twin prllel disk winding with ontinued trnsposition. there would e n uneven distriution of urrent in the prllel ondutors Lrge ross setionl ondutors lso result in eddy urrents nd skin effet oming into ply in the ondutor, whih inreses losses nd therefore lolised heting oil insultion Pper insultion on the ondutor is the insultion generlly used for oil-immersed trnsformers. Nomex, n rmid pper developed y Du Pont, is used extensively in the eletril industry nd lso in oil-immersed nd dry type trnsformers. The oil in trnsformer serves two purposes; one to t s n insultor nd the other, to t s oolnt medium. The pper used redily sors the oil to form uniform insultion medium in the trnsformer. Min insultion In oil type trnsformers, pressord nd wood produts re widely used s the operting temperture is limited y the oil nd pper produts used s insultion. In the se of ore trnsformers, pressord ylinders re used etween the LV nd ore nd etween the HV nd LV windings. Dry type trnsformers would use Nomex or Kpton for ondutor insultion nd Nomex or glss-sed ords for pking nd ylinders s the operting temperture would e muh higher thn oil types. ondutor mteril Generlly opper is used for its mehnil properties nd ondutivity. luminium n, nd hs een used ut its ondutivity is muh lower thn opper nd mehnilly not s good. luminium hs suessfully een used in st resin dry type trnsformers euse the therml expnsion oeffiient of the resin nd luminium re extremely lose. The trnsformer designer should weigh up the pros nd ons of the prtiulr pplition when deiding whether opper or luminium is used s the ondutor mteril - there is no fundmentl rule. Generlly, opper is preferred nd used exept where foil winding re employed. ooling Dry type trnsformers rely on ir irultion through nd round the winding for ooling nd n e nturlly- or fore-ooled with fns. The designer would hve to design ordingly, ering in mind tht the operting temperture would e muh higher nd mterils would hve to e seleted to suit the high operting temperture. Oil-ooled trnsformers rely on the oil to ool the trnsformer nd this is irulted through suitle rditors y nturl onvetion or lterntively, pumped. ommon terminology used: ONN Oil Nturl ir Nturl ONF Oil Nturl ir Fored (fns used to fore ir over rditors) OFN Oil Fored (oil pumped through the trnsformer) ir Nturl Oil should hve the following properties. Low visosity High flsh point hemilly stle nd low impurity ontent High dieletri strength Minerl oil hs trditionlly een used in trnsformers though vegetle oils re now ville with properties tht re limed to e superior; notly high flsh point with flme retrdnt properties owing to the high flsh point. One of the mjor prolems with minerl oils is one they re ignited nd urning, it is extremely diffiult to get the fire under ontrol, prtiulrly in enlosed environments suh s uildings or underground in the mines. Fundmentl trnsformer theory E = (2 x π x f x N x x β)/ 2 = 4.44 x f x N x x β where: f = frequeny N = numer of turns

4 = ore re (m 2 ) β = flux density in Tesl Voltge trnsformtion rtio = N seondry /N primry Therefore V seondry = V primry x (N seondry /N primry ) urrent trnsformtion rtio = N primry /N seondry nd I seondry = I seondry x (N primry /N seondry ) where N is the numer of turns in the primry nd seondry winding Mgnetising flux Φ M Trnsformer ore urrent is I o only. The I 2 R losses owing to this re negligile. (t full lod the I 2 R losses would e pproximtely 1% or less nd sine the no-lod urrent is of the order of one twentieth of the full lod urrent the I 2 R losses would e 1/400 x 1% = one four hundredths of perent.) onsequently, the power input on no-lod is onerned with the ore nd dieletri loss, the ltter eing negligile exept in very high voltge trnsformers. The no-lod losses mesured on open iruit seondry represent the ore nd dieletri losses; the dieletri losses re generlly negligile ompred to the iron losses. Seondry winding lekge flux Primry winding lekge flux 1 2 opper losses (I 2 R losses) s the voltge hs to e redued to very low vlue if the seondry terminls re short-iruited, the urrent in the seondry ould e full lod urrent while the seondry voltge would e zero euse of the short-iruit. The primry voltge would e smll nd the flux F would likewise e smll. t full lod the input voltge would e 0,05 to 0,1 of the rted voltge. The ore loss is pproximtely proportionl to the squre of the flux nd would e very smll. Therefore, the ore losses would e negligile. Primry winding Seondry winding Figure 10: Mgneti flux distriution. Figure 10 shows the min flux in trnsformer inluding some lekge flux. The lekge though the tnk is not shown. There will lwys e lekge flux in the trnsformer nd into the tnk. The lekge into the tnk would generlly e smll in mgnitude ut would depend on the lerne nd tnk onfigurtion nd ny sreening. Effiieny The trnsformer is not lled upon to onvert eletril energy into mehnil energy or vie vers nd onsequently hs no moving prts. The effiieny is generlly high. Effiieny % = {P output / (P output + P losses )} x 100 The losses re onfined to: ore losses: Eddy-urrent losses nd hysteresis losses I² R losses: Owing to the heting of the ondutors due to the pssge of urrent Stry losses: Owing to stry mgneti fields using eddy urrent in the ondutors or in the surrounding metl, eg tnk Dieletri losses: In the insulting mterils, prtiulrly in the oil nd solid insultion of high voltge trnsformers Regultion The voltge regultion is defined for ny given lod urrent s the rithmeti differene etween the seondry no-lod voltge E 2 nd the lod voltge V 2 expressed s frtion of the no-lod voltge. Regultion % = {(E 2 -V 2 )/E 2 } x 100 No-lod losses On no-lod the seondry iruit is open nd, onsequently, the pri mry Trnsformer onnetions In three phse trnsformers there re five types of winding onnetions. The hoie of onnetion depends on the funtion of the trnsformer in n integrted power system. Str-str onnetion This onnetion is used where the phse reltionship is required to remin the sme nd erths re Yy0 onnetion required in oth sides. It is minly used in smll trnsformers nd lrge trnsmission trnsformers. The trnsformers re Figure 11: Str-str vetor digrm. frequently equipped with n dditionl set of winding onneted in delt to suppress Dyn11 onnetion ny triplen hrmonis. 30 Delt-str onnetion n Dy11, Dy1 nd Dy5 re ommonly used onfigurtions enling the seondry to e Figure 12: Delt-str vetor digrm. erthed diretly or through suitly sized resistor. The delt winding inherently suppresses Yd11 onnetion ny triplen hrmon- 30 is, tht my our in the mgnetising urrent nd distort the voltge. The numeril numer ssoited with the onfigurtion indites the phse ngle reltionship. Figure 13: Str-delt vetor digrm.

5 Str-delt onnetion Essentilly used in situtions where the seondry is not to e erthed nd nnot e used where single phse voltge is required, suh s domesti or smll light industry onneted to the seondry supply. gin, the onnetion n ommodte vrious vetor phse ngle reltionships. uto-wound trnsformers uto wound trnsformers shre ommon str point nd thus ommon erth nd the systems re not isolted from eh other. uto-trnsformers omprise two windings; series nd ommon. uto-trnsformers re typilly used s high voltge system interonneting trnsformers nd in redued voltge strting systems for lrge motors. Zig-zg onnetion This onfigurtion is typilly used where speifi phse ngle Zig-Zg onnetion Yzn shift is required, for exmple, in multiphse n retifier trnsformers nd where it is neessry to hve positive sequene impedne higher thn the zero Figure 14: Str zig zg vetor digrm. sequene impedne. onlusion The sujet mtter on trnsformer design is extensive nd this rtile riefly outlines some theory nd ftors to e onsidered in the design. Referene [1] Przyysz P, Trnsformer Fundmentls. Eskom pulition. iliogrphy Wterhouse T, Design of Trnsformers. Trnsformers. hrt Hevy Eletril Limited. MGrw Hill. Flngn WM, Hndook of Trnsformer design nd pplition. MGrw Hill.