第1页共4页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第1页共4页SelectionandRatingofTransformersA).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:第2页共4页第1页共4页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第2页共4页Where:I²R=lossduetoloadcurrentandDCresistanceofthewindingsPEC=windingeddycurrentlossPSL=straylossesinclamps,tanks,etc.TheI²Rlossisduetothecurrentflowingintheresistanceofthewindings.Itisalsocalledohmiclossordcohmicloss.Theohmiclossisproportionaltothesquareofthemagnitudeoftheloadcurrent,includingtheharmoniccomponents,butisindependentofthefrequency.Itisdeterminedbymeasuringthedcresistanceandcalculatingtheresultinglossusingthewindingcurrentsatfullload.Therearenotestmethodsavailabletodetermineindividualwindingeddycurrentlossortoseparatetransformerstraylossfromeddycurrentloss.Instead,thetotalstrayandeddycurrentlossisdeterminedbydeterminingthetotalloadlossandsubtractingthecalculatedohmiclosses,i.e.,Theeddycurrentlossisassumedtovarywiththesquareofthermscurrentandthesquareofthefrequency(harmonicorderh),i.e.,Where: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-typetransformersin...
