上海意兰可电力电子设备有限公司Shanghai

上海意兰可电力电子设备有限公司ShanghaiInducTekElectric&ElectronicEquipmentLtd.ShanghaiInducTekElectric&ElectronicEquipmentLtd.Building5#,ZhenZhongXinRd.No.599,XiaoKunShanTown,SongJiang,Shanghai,ChinaZip:201614Tel：[phone]www.inductek.comSelectionandRatingofTransformersA).IntroductionThenumberofnon-linearloads–whichdrawnon-sinusoidalcurrentseveniffedwithsinusoidalvoltage-connectedtothepowersupplysystemislargeandiscontinuingtogrowrapidly.Thesecurrentscanbedefinedintermsofafundamentalcomponentandharmoniccomponentsofhigherorder.Inpowertransformersthemainconsequenceofharmoniccurrentsisanincreaseinlosses,mainlyinwindings,becauseofthedeformationoftheleakagefields.Higherlossesmeanthatmoreheatisgeneratedinthetransformersothattheoperatingtemperatureincreases,leadingtodeteriorationoftheinsulationandapotentialreductioninlifetime.Asaresult,itisnecessarytoreducethemaximumpowerloadonthetransformer,apracticereferredtoasde-rating,ortotakeextracareinthedesignofthetransformertoreducetheselosses.Toestimatethede-ratingofthetransformer,theload’sK-factormaybeused.Thisfactoriscalculatedaccordingtotheharmonicspectrumoftheloadcurrentandisanindicationoftheadditionaleddycurrentloadlosses.Itreflectstheexcesslossesexperiencedinatraditionalwirewoundtransformer.Moderntransformersusealternativewindingdesignssuchasfoilwindingsormixedwire/foilwindings.B).TransformerlossesTransformerlossesconsistsofno-load(orcoreloss)andloadlosses.Thiscanbeexpressedbytheequationbelow.where:PC=coreorno-loadloss,PLL=loadlossPT=totalloss.Coreorno-loadlossisduetothevoltageexcitationofthecore.Althoughthemagnetizingcurrentdoesincludeharmonics,theseareextremelysmallcomparedwiththeloadcurrentandtheireffectonthelossesisminimal.LoadlossesaremadeupofI²Rloss,eddycurrentlossandstrayloss,orinequationform:Where:I²R=lossduetoloadcurrentandDCresistanceofthewindingsPEC=windingeddycurrentlossPSL=straylossesinclamps,tanks,etc.TheI²Rlossisduetothecurrentflowingintheresistanceofthewindings.Itisalsocalledohmiclossordcohmicloss.Theohmiclossisproportionaltothesquareofthemagnitudeoftheloadcurrent,includingtheharmoniccomponents,butisindependentofthefrequency.Itisdeterminedbymeasuringthedcresistanceandcalculatingtheresultinglossusingthewindingcurrentsatfullload.Therearenotestmethodsavailabletodetermineindividualwindingeddycurrentlossortoseparatetransformerstraylossfromeddycurrentloss.Instead,thetotalstrayandeddycurrentlossisdeterminedbydeterminingthetotalloadlossandsubtractingthecalculatedohmiclosses,i.e.,Theeddycurrentlossisassumedtovarywiththesquareofthermscurrentandthesquareofthefrequency(harmonicorderh),i.e.,上海意兰可电力电子设备有限公司ShanghaiInducTekElectric&ElectronicEquipmentLtd.ShanghaiInducTekElectric&ElectronicEquipmentLtd.Building5#,ZhenZhongXinRd.No.599,XiaoKunShanTown,SongJiang,Shanghai,ChinaZip:201614Tel：[phone]www.inductek.comWhere:h=harmonicorder,1,2,3,etc.hmax=thegreatestharmonicordertobeconsideredIh=currentatharmonicorderh,amperesh，IR=ratedcurrent,amperes，PEC,R=eddycurrentlossatratedcurrentandfrequency.Theeddycurrentlossdependsonthesquareoftheconductordimensionperpendiculartotheleakagefluxfield.Attheendsofthewindingthefluxfieldbendsandthelargerdimensionoftherectangularconductorisperpendiculartoavectorcomponentoftheleakagefluxfield.Equalizingtheheightofdieprimaryandsecondarywindings,whichcanbeachievedwithanywindingdesign,reducestheconcentratededdylossatthewindingends.UsingFoilwindingsormultiplestrandsperwindingreducesbotheddycurrentloss.Straylossoccursduetothestrayfluxwhichintroduceslossesinthecore,clamps,tankandotherironparts.Straylossmayraisethetemperaturesofthestructuralpartsofthetransformer.Fordry-typetransformersincreasedtemperaturesintheseregionsdonotcontributetoanincreaseinthewindinghotspottemperature.Straylossesaredifficulttocalculateanditiscommontoassumethatthelosseswillvaryasthesquareofthecurrenttimesthefrequency(harmonicorder),asshownby:C).TransformersandK-factorTherearedifferentapproachestoaccountingforadditionallossesinselectingatransformer.HerewewouldsuggestthewayappliedbytransformermanufacturersinconjunctionwithUnderwritersLaboratoriesintheUnitedStates,itistocalculatethefactorincreaseineddycurrentlossandspecifyatransformerdesignedtocope;thisisknownasthe‘K-factor’.Where:h=harmonicnumberIh=thefractionoftotalrmsloadcurrentatharmonicnumberhManypowerqualitymetersreadtheK-factoroftheloadcurrentdirectly.OncetheK-factoroftheloadisknown,itisasimplemattertospecifyatransformerwithahigherK-ratingfromthestandardrangeof4,9,13,20,30,40,50.Notethatapurelinearload–onethatdrawsasinusoidalcurrent–wouldhaveaK-factorofunity.AhigherK-factorindicatesthattheeddycurrentlossinthetransformerwillbeKtimesthevalueatthefundamentalfrequency.‘K-rated’transformersarethereforedesignedtohaveveryloweddycurrentlossatfundamentalfrequency.D).TransformerdesignconsiderationManytransformermanufacturershavedevelopeddesignsratedfornon-sinusoidalloadcurrentswhileoptimizingtheirproductioncosts.Thedesignprocessinvolvesananalysisoftheeddycurrentlossdistributioninthewindingsandcalculationofthehotspottemperaturerise.Anal