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nJanuary1992engineersinstalledanewcomputersysteminabuildingbelongingtothelargeFrenchinsurancecom-panyGroupamainParis.Soonafteritsenergization,themaincircuit-breakertrippedanddisconnectedthewholesystem.Atthe'expenseofmuchtimeandmoney,theengineersfoundthattheinterruptionhadbeencausedbytheneutralconductorofthebalancedthree-phasesystem.Thecurrentthroughtheconductorwasasurprising65percentofthephasecurrents,buttheratingofitsprotectiverelayhadonly50per-centoftheratingoftherelaysprotectingthephaseconduc-tors.(Abalancedthree-phasesystemshouldhavezeroneutralcurrent,sothe65percentwasquiteunexpected.)IntheUnitedStates,aMidwestutilityreportedthefailureofatransformerwhoseloaddidnotexceeditskilovoltam-pereratingbutconsistedprimarilyofvariable-speedmotordriveswithhighharmoniccontent.Inanotherreportedcase,a300-kVAtransformerfailed,althoughaclamp-onaverag-ing-typeammetershowednooverload.Whenareplacementtransformeralsofailed,soonafterinstallation,amoreappro-priatemeasurementwithatruermsammeterindicatedsignif-icantoverloadwithharmoniccurrents.IntheItalianAlps,nexttothemassifoftheMarmolada,themotivepowerplantforadouble-ropecablewayforskierswasinstalledin1991.Thelift,designedtocarry3600per-sonsperhour,hadtwodcmotordrives,eachratedfor2MW,fedbythree-phase,six-pulsethyristorrectifiers.Atthisspot,themotordriveswerepoweredfromarelativelyweak20-kVgrid.Duringacceptancetesting,theplantcausedavoltagedistortionofabout18percentinthegrid,forcingtheutilitytodisconnecttheplantuntilalesspolluting(12-pulse)recti-fierconfigurationcouldbeinstalled.(A12-pulserectifier,inwhich12successivecommutationstakeplaceineachcycle,producesacurrentwaveformthatisamuchbetterapproxi-mationofthesinewavethantheoneproducedbyasix-pulserectifier.)Elsewhereinthisregion,atanexistingskilift,theinstallationofapassivecompensationsystem-tunedfiltersplusacapacitorbankforreactivevoltampere(VAR)compen-sation-trippedthecapacitorbankofanotherplantafewkilometersaway.ThetwocompensationsystemsresonatedRICHARDREDLELF/SAPAOLOTENTIUniversityofPadova,&J.DAANVANWYKRandAfrikaansUniversityInusefarandwideforimproveddeliveryandcontroloftheelectricitysupply,powerelectronicssystemsarebothcauseandremedyoftheharmonicpolIutionproblemmutuallythroughthelineinductance,sothatlargeharmoniccurrentsoverwhelmedtheca.pacitorbank,exceedingitsrmscurrentcapability.OriginofpollutionAlltheseproblemscanbetracedtoacommonorigin,namely,excessivedistortionofthecurrentorvoltageofthepowersuppliedtotheconsumerbythepowerdistributionsystem.Thisdistortionhasseveraldetrimentaleffects,includ-ingoverheatingofthecomponentsofthedistributionsys-tem,mechanicaloscillationsingeneratorsandmotors,andinsulationorcapacitorfailureduetoelectricresonances,nottomentionincreasedaudionoise,impairedperformanceofpower-linecommunicationsystems,andunpredictablebehaviorofinstalledprotectionsystems.Otherundesirablephenomena,alsobecomingmoreandmoreprevalentonthepowerlines,includerapidvoltagefluctuationsthatmakelightsflicker,andradio-frequencyinterference.Themaincauseofallthesephenomena,whichcanbeviewedasasortofpollutionoftheelectromagneticenviron-ment,isthegrowingpopularityofelectronicequipmentpoweredbytheacline.Computersandcolortelevisionsetsheadthelist,whichalsoincludeselectronicballastsforgas-dischargelamps,solid-statemotorcontrollersovertheentirerangefromwattstomegawatts,batterychargers,andelec-troniccontrolofalargevarietyofindustrialloads.Invirtuallyeverycase,theelectromagneticpollutionflowsfromacombinationofthenonlinearnatureofcurrentrectifica-tionandtheswitching.ThelatterresultsinwaveformsmadeofREDL.TENTI&VANWYK-POWERELECTRONICS'POLLUTINGEFFECTS33HarmonicdistortionlnterharmonicsFlicker[2]Electromagneticpollutionofthepowerlineintroducedbypowerelectronicssystemsincludesharmonicdistortionduetononlinearloads-rectifiers,forexample.lnterharmonics(currentcomponentsunrelatedtothefundamental)aregeneratedbysuchasynchronousactiveloadsascycloconverters,orloadsshowingcycle-by-cyclevaria-tion,suchasarcfurnaces.Electromagneticinterference(EMI)isthehigh-frequencynoisebroughtaboutbythefastedgesofthewave-formsoftheswitchingpowerconverters.Notchesareduetothemomentaryshort-circuitofthelineduringacommutationinterval.Theproductofperiodicallyorcontinuouslychangingloads,flicker(alsocalledvoltagefluctuation)isaseriesofdiscreteorcontinuousvoltagechanges.Thenamestemsfromthefactthatitcreatesanimpressionofunsteadinessofthelightofabulb.discretesteps,ratherthancontinuoussinusoidsordc.Whethersingle-phaseorthree-phase,theaclinesideoftheequipmenttyp-icallyincorporatesarectifiercircuitorcircuitsfollowedbysomeformofaswitchingpowersystem-adddcorddacpowercon-verter.Unlessspeciallydesignednottodoso,almostallrectifiersdrawdistortedlinecurrent.Theworstdistortionisattributedtothesingle-phasecapacitivelyfilteredrectifier,themostpopularchoiceinlow-powerapplications[Fig.11.Electromagneticpollutioncoversawiderangeoffrequencies.Inadditiontotheharmonics,whichpollutethelineinthelowkilohertzrange,high-frequencypollution(uptoseveraltensofmegahertz)iscreatedbythemicrosecondandsubmicrosecon