eColi Br i u M • J u n e 2010 32

Harmonics demystifiedTim Makris, Danfoss Australia

ABsTrACTThisarticleprovidesaquickoverviewofharmonicsinelectricalpowernetworks,thecauses,themitigationmethodsandapplicablestandards.Thearticlealsodiscussesbestpracticeapproachtoachievetheoptimumbalancebetweencost/performanceformoderninstallationswithintheHVACindustry.

F O R U M

inTroDuCTionHarmonicsarefairlycommoninmodernelectricalinstallationsandformonepartofameasurementofpowerquality.Equipmentsuchascomputerpowersupplies,lightingwithelectronicballasts,UPSsystemsandvariablespeeddrives(VSD)distortthecurrentdrawnfromthenetwork,causeharmonicsanddisturbotherconsumersorotherequipmentconnectedtothesamesupplynetwork.Harmonicdistortion,ifnotmitigated,canleadtoincreasedconsumptionofpower,reducedpowersystemutilisation,increasedlossesincablesandtransformersandequipmentmalfunction.

Mitigationforharmonicscanbeappliedatvariouspointswithintheelectricalnetworkofabuildingdependingonthesourceofthedistortion,thelocationandpowerratingofthesourceofthedistortionrelativetothesupplycapacity.Filteringcanbeappliedattheterminalsofthedevice,atanMCCorswitchboardfeedinganumberofdistortingloadsoratthetransformer.Agoodanalogyistoconsiderapipingsystem,withthewaterbeingthepowersupplyandharmonicsacontaminantwithinthewater.Themitigatingfilterisafilterthatremovesthecontaminantfromthewatersupply.Thelocationofthefilterthendetermineshowfarwithinthebuilding’snetworkthecontaminant,theharmonics,areallowedtoflow.

Whateverharmonicsareunfilteredwillalsofindtheirwayintotheelectricalnetworkfeedingotherconsumersofelectricity.Tomaintaincompatibilityandareasonablelevelofpowerquality,standardsexistlimitingtheamountofdistortioneachconsumerisallowedtoproduce.Electricaldistributorshavepowertoimposespecificlimitstoeachconsumerorbuilding.

Asthelocationandpowerratingoftheharmonicproducingequipmentdeterminesthemethodoffiltering,sotoothetypeoffilterandthelocationwithinthenetworkwilldetermineoverallcostandimprovementtothebuilding’selectricalnetwork.Whichfilterisusedandwhereitisusedwillalsogovernwhethercompliancewithstandardsorlimitsimposedbyanelectricaldistributororaspecificationbyaconsultantcanbemet.

WHAT Are HArMoniCs?Theidealsupplyvoltagewaveformisasinewave.InAustraliathevoltageis230V(+10%,–6%)withafrequencyof50Hzforsingle-phasesupplies,and400V/50Hz(+10%,–6%)forthree-phasesupplies.AnexampleoftheperfectsinewaveisshowninFigure1.

Figure 1: Perfect single-phase sine wave

Whenaload(equipment)isconnectedtotheelectricalsupply,currentisdrawnbytheload.Inthecontextofharmonics,loadscanbelistedaseitherlinearornon-linear.Theloadiscategorisedbythecurrentitdraws.Forexample,incandescentlightbulbs,resistanceheaters,directon-line(DOL)connectedmotorsarelinearloads.Thecurrentdrawnbylinearloadsisasinewave,althoughmaybeoutofphasewiththevoltagesupply(displacementpowerfactor).

Non-linearloadsconsistofcomponentsdrawingnon-sinusoidalcurrent.Single-phaserectifiers(powersupplies),lightingwithelectronicballasts(fluorescentlights),VSD,alldrawnon-sinusoidalcurrent.Figure2showstypicalcurrentfromasingle-phaserectifier[1].Figure3showsatypicalcurrentwaveformfromathreephaseVSDwithoutanyharmonicfiltering[1].

Figure 2: Typical single-phase rectifier input current waveform

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Figure 3: One phase of a three-phase VSD waveform without harmonic filtering

Thesedistortedwaveformsarerepetitiveandconsistofcomponentwaveformsatdifferentfrequencies(harmonics)ofthefundamental50Hzwaveform.Thesecomponents,orharmonics,combinedproducetheresultantwaveformsshowninFigures2and3.

Figure 4: Three phase VSD harmonic spectrum without harmonic filtering

Acommonwaytodisplayharmonicsisasaspectrum(frequencydomain).Figure4[1]showsthespectrumoftheVSDwaveforminFigure3.Eachfrequencyisshownasabarwithaparticularmagnitudeasapercentageofthefundamental,50Hz.

Theharmonicvaluesarecombinedtoprovideasinglevaluethatcanquantifytheamountofharmonicspresent–totalharmonicdistortion,orTHD.HarmonicsinbothcurrentandvoltagecanbeexpressedasTHD:THDIandTHDVrespectively.TheformulaforTHDisexpressedas:

WhereI(h)isthecurrentatthecomponentharmonicnumbers(forexample5th,7th,etc.)I(1),isthefundamentcurrent.ThewaveforminFigure3hasaTHDofapproximately104%.

THe AffeCT of HArMoniCsIfthenon-linearcurrentdrawnbytheloadisconsumedbytheloadandonlytheload,thenwhatistheproblem?Theproblemofdistortedcurrentisthatitcreatesadistortedvoltageinthesupplythatisseenbyotherequipmentorconsumersconnectedtothesamesupply.

Thesupplynetworkconsistsofanimpedance.Thesource,thetransformer,andthecableallhaveimpedances.Whencurrentwithharmonicdistortionisdrawnthroughthesupplyimpedance,avoltagedropiscreatedateachparticularharmonicfrequency.ThesevoltagedropsdistortthevoltagewaveformfromtheidealsinewaveasseeninFigure5.

Figure 5: Distorted voltage waveform

Bothharmoniccurrentandvoltagedistortionaffectthenetworkandotherequipmentorconsumers.Asthenon-linearloadcreatescurrentharmonics,theseharmonicsincreasetheRootMeanSquared(RMS)currentoftheload–averagecurrentovertime.TheincreaseinRMScurrentduetoharmonicseffectthesupplybycausingincreasedthermallossesinsupplytransformersandgenerators.Thehigherfrequenciesoftheharmonicsalsoincreaselossesintransformersandgeneratorsbyincreasingtheeddycurrentlosses.Theseincreasedlossesaredissipatedasheat,raisingthetemperatureofthetransformer,itswindingsandinsulation,decreasingitslifetime.

Harmonicsalsohaveaneffectoncablesthatconductdistortedcurrent.AswiththeeffectontransformerstheharmoniccurrentsincreaseRMScurrentandthereforeincreasecablelosses.Anadditionaleffecttheharmonicfrequencieshaveoncablesisknownaskineffect.Skineffectdescribesthephenomenonofhowdifferentfrequenciesofcurrentaredrawnwithinaconductor.Thefundamentalfrequencycomponentofcurrent(currentat50Hz)isdistributeduniformlyacrossthecross-sectionoftheconductor.Theharmonicsathigherfrequencies,typicallyabove500Hz,aredistributedclosertotheouterlayersoftheconductor,reducingtheeffectivecross-sectionalareaandincreasingthecableresistanceandincreasingthermallossesincable.

Singlephasenon-linearloadssuchascomputerpowersuppliesandelectroniclightballast,createharmonicsknownastriplens–multiplesofthethirdharmonic.Thesetriplensaddupintheneutralconductorofthesupply,placingextraloadontheneutral.Itiscommonpracticetooversizeneutralconductorstoaccommodatetheadditionalharmonicloadingsingle-phasenon-linearloadsgenerate.

ExcessiveharmonicvoltagedistortioncancausetorquepulsationsandincreasedlossesinDOLconnectedmotors.Typically,a10°Criseintemperatureabovetheratedtemperaturecanreducemotorinsulationlifeupto50%.Thehighfrequencyharmonicspresentinadistortedvoltagewaveformcauserotatingfluxwithinthemotoratfrequenciesabove50Hz,whichinteracttocreatetorqueopposedtothenormal,ratedtorqueoutput,leadingtoshaftpulsations.

10

20

40

60

80

100

3 5 7 9 11 13 15 17 19 21 23 25

harmonic

% F

und

(1)

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CircuitbreakersaredesignedtomonitorRMScurrentandpeakcurrent.AdistortedcurrentwaveformhashigherRMSvaluesandpeakvaluesaboveastandardsinewave.Thecircuitbreaker,ifnotcapableoftrueRMSmonitoringmayneedtobeadjustedtoaccommodateharmonicsifnuisancetripsoccur.Similarly,conventionalmeteringequipmentcanalsoindicateincorrectvalueduetoharmonics.MeteringequipmentcapableoftrueRMSreadingsisrecommendedifmeteringofnon-linearloadsisrequired.

Powerfactorcorrectioncapacitorscanbesusceptibletoproblemsifconnectedonasupplywithsignificantlevelsofharmonicspresent.Capacitorsabsorbharmoniccurrentsand,ifnotfilteredordimensionedfortheharmoniccurrents,canbedestroyed.Resonancecanalsooccurwhenthecapacitor,combinedwiththesupplyimpedanceattheharmonicfrequencies,magnifytheamplitudeofharmonicscurrentsandvoltagesleadingtodamagetoplantandequipment.De-tuningreactorsarenecessarytoprotectthepowerfactorcorrectioncapacitors.

Thepowerfactoroftheinstallationisalsoaffectedbyharmonicproducingnon-linearloads.Non-linearloadshaveadisplacementpowerfactorclosetounity.However,thetruepowerfactoralsofactorsharmonics.Theformula(2)hastheapparentpowerS(thepowerdeliveredtotheequipment),realpowerP(thepowerrequiredtodowork),reactivepowerQandharmonicpowerD.

Assumingalmostunitydisplacementpowerfactorforanon-linearloadsuchasaVSD,truepowerfactorcanbeapproximatedas:

ForaVSDwithaTHDIof45%,thenthetruepowerfactorwouldbe0.92.

Theabovearejustsomeoftheaffectsofharmonicsinthesupplynetwork.Anunfortunateside–effectofenergy-savingdevicessuchasfluorescentlightswithelectronicballastsandvariablespeeddrivesisthattheycreateharmonics.Someoftheaffectscanbemeasured,andtheircostlyimpactquantified.AnincreaseinRMScurrentandareductioninpowerfactorhaveadirectimpactonelectricityusage,efficiencyandplantlifetime.

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eColi Br i u M • J u n e 2010 36

F O R U M

MiTiGATion MeTHoDsConsumersincorporateharmonicmitigationwithintheirbuildingsforvariousreasonssuchastonegateaproblem,tocomplywithaspecificationorwithlimitsimposedbyanelectricaldistributor.Thequestiononwhethermitigatingharmonicsimproveenergyefficiencydependsonvariousfactors,basedontheseverityofharmonics,thetypeofloads,thelimitsrequiredtomeet,andthemethodofmitigation.Iftheimprovementstoelectricityconsumptionoutweightheaddedlossesoftheadditionalharmonicmitigationequipment,thenfurtherimprovementscanbemadetoenergyefficiency.Areviewofcommonmitigationmethodsaresummarisedbelow,focussingonbalancedthree-phaseequipment(suchasVSDload).ItisassumedthataVSDhassomeformofintegralharmonicmitigationssuchasACorDCchokes.ThemitigationmethodsreviewedareadditionaltotheintegralfilteringpresentwithinaVSD.

Inelectricalnetworkswithmorethanonesupplytransformer,apossiblemitigationmethodinvolvesover-sizingthesupplytransformertoreducetherelativesupplyimpedanceandvoltagedistortion.Distributingthelinearandnon-linearloadstoreduceaconcentrationofharmonicproducingloadsononetransformercan“dilute”apotentialproblem.Thisformofmitigationcanoccuratthedesignstageofaprojectandwouldincuradditionalcostforlargersupplytransformers.

AddingimpedancetothefrontofVSD,suchasACreactors,reducesthecurrentdistortionandthisreducesthevoltagedistortionattheterminalsoftheACreactorcomparedtothevoltagedistortionattheterminalsoftheVSD.TheACreactoraddsavoltagedropbetweenthesupplyandVSD,whichassistsintheharmonicreductionbuthasanunwantedsideeffectofreducingtheinputvoltageandhence,theoutputvoltage,oftheVSD.TypicalvaluesofACreactorsrangefrom1to5%impedance,withtheequivalentlevelofvoltagedrop.Typically,THDIwouldreducefrom40%to28%asthereactorimpedanceincreasedforagivenVSDonagivensupply.CostislowrelativetothecostofaVSD.

PassiveharmonicfiltersconsistingofinductorandcapacitorcircuitscanbeinstalledupstreamfromasingleVSD,oranumberofparallelconnectedVSDs.Thesepassivefiltersconsistofseriesandshuntcomponents,tunedtoblocktheharmonicsproducedbytheloadfromgettingintothesupplynetwork.Theintegralcapacitorsabsorbtheharmoniccurrents.ThesefilterscanbespecifiedtoprovideacertainlevelofTHDIatfullload,withtypicalvaluesintherangefrom5%to10%.

Passiveharmonicfiltersarerobust,simpletoinstallandrelativelylowcostcomparedtomoresophisticatedmitigationtechniques;however,theydohavesomeshortcomings.TheharmonicperformanceofTHDIbetween5%or10%isattheratedloadofthefilter.Operationatreducedloadswillslightlyincreasetheharmoniccontent.Butthecapacitorswithinthefilterwillalsoprovideleadingpowerfactor,whichcouldbeaproblemongenerators.AnotherimpactofpassivefiltersistheycanincreasethevoltageattheterminalsoftheVSD.Ensuringthefiltershavebeentestedwith,orhavebeentunedtothespecificVSD,willprovidebetterperformanceandoperation.Thefilteringcapabilitycanalsobereducedwithincreasedphaseimbalanceandhighvaluesofharmonicvoltagepre-distortion.

Shuntpassivefilters,consistingofinductorandcapacitorscircuits,inessencepowerfactorcorrectioncapacitorscircuits,

canbetunedtofilterspecificfrequencies.Capacitorsactasharmonicsinks,andbycombiningthemwithdetuningreactors,afiltercanbedesignedtoabsorbspecificharmonicfrequencies.Shuntbranchestoremovethefifthharmonic,theseventhharmonicandsoon,canbeaddedasneeded.Precautionshouldbetakenwhenapplyingshuntpassivefiltersasthefilterswillabsorbharmoniccurrentsfromnotonlytheload,butthesupplynetworkaswell.Incorrectlydesignedfilterscouldleadtomalfunctionandresonance.

Anactivefilterconsistsofacontrolcircuit,capacitorbank,inverterandlinefilter.Currenttransformersmonitortheharmoniccurrentsinthesupplynetwork.Thecontrolcircuitdeterminestheharmoniccomponentandconfigurestheoperationoftheinvertertoinjectharmoniccurrentsincounter-phasetotheexistingharmonicsinthesupply.Thetheoryofoperationcanbecomparedtoaudiblenoisecancelling.

Flexibilityinoperationandinstallationmaketheactivefilterversatile.Itcanbeinstalledattheoffendingload,ataswitchboard,atthetransformer,inneworretrofitinstallations.Themodesofoperationofafiltercanconsistofoverallharmoniccompensation,individualorselectiveharmoniccompensationandreactivepowercompensation(displacementpowerfactorimprovement).Moderndesignshaveintuitivecontrolpanelsandoffersupplymeasurementsandauto-tuningofthemeteringcurrenttransformers,havevariousIPratingstosuitdifferentinstallationlocationsandcanwithstandhighambienttemperatures.Activefilterscanalsooperateathigherlevelsofpre-distortioncomparedtopassivesolutions.

Carehastobetakenwiththeactivefilters,astheinverterswitchingfrequencycanbeimposedonthesupply.Thiscouldleadtoharmonicfrequenciesbeyondthe50thandpossibleresonance.Well-designedfiltersincorporateautomaticallyvariableswitchingfrequenciesthatarenotconsistentlyatonefrequencyandcanlessenthepossibilityofresonance.Increasingthelinefiltercomponentoftheactivefiltertolessentheimpactoftheswitchingfrequencycanbedone,buttheharmonicinjectioncapabilityisreduced.Withmostdesigns,abalancemustbeachieved.

HArMoniC sTAnDArDs AnD BesT PrACTiCeRegulationofharmonicsexistsviastandards.Standardsforpowerqualityareutilisedandappliedbysupplyauthorities,whichthenapportionlimitstoeachconsumerconnectedtothepointofcommoncoupling,orPCC.ThePCCiswheretwoormoreconsumersareconnectedtothesamesupply.Standardsexistthatgovernlimitsofharmonicsathigh-voltage(HV)andmedium-voltage(MV)networksandalsoforlow-voltage(LV)systemsandLVproducts.

Forexample,AS/NZS61000.3.12:2006,Electromagneticcompatibility(EMC)–Limits–Limitsforharmoniccurrentsproducedbyequipmentconnectedtopubliclow-voltagesystemswithinputcurrent>16Aand<=75Aperphase[3],isapplicableforbalancedthree-phase-harmonic-producingequipmentsuchasVSDs.ThisstandardisapplicabletoeachindividualVSD,andmostreputablevendorshaveacompliantproduct.However,thisstandarddoesnotconsidertheimpactofnumerousVSDsconnectedinanHVACinstallation,forexample.

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Maximum Harmonic Current Distortion in Percent of IL

Individual Harmonic Order (Odd Harmonics)

Isc / IL <11 11≤h<17 17≤h<23 23≤h<35 35≤h TDD

<20 4.0 2.0 1.5 0.6 0.3 5.0

20<50 7.0 3.5 2.5 1.0 0.5 8.0

50<100 10.0 4.5 4.0 1.5 0.7 12.0

100<1000 12.0 5.5 5.0 2.0 1.0 15.0

>1000 15.0 7.0 6.0 2.5 1.4 20.0

Even harmonics are limited to 25% of the odd harmonic limits above

Table 2: IEEE519:1992, table 10.3 harmonic current limits for system voltages to 69kV.

StandardslookingatasystemlevelorPCClevelaremoreapplicable,encompassingthebuildingaswhole.InAustraliathereisAS/NZS61000.3.6:2001,Electromagneticcompatibility(EMC)–Limits–AssessmentofemissionlimitsfordistortingloadsinMVandHVpowersystems(IEC61000-3-6:1996)[2],andrecommendationIEEE519:1992,RecommendedPracticesandRequirementsforHarmonicControlinElectricalPowerSystems[4],arefrequentlyreferencedandapplied.

AS/NZS61000.3.6provideslimitsofharmonicvoltagedistortionataPCCforbothMVandHVconnections(andalsoLV)andisusedbysupplyauthoritiestoensurepowerquality.ThesupplyauthoritiestypicallyprovidespecificlimitsbasedonconsumedpowerforeachconnectiontoensureharmoniclevelsateachPCCarewithinthestandard.CompatibilitylevelsofvoltagedistortionforLVandMVconnectionsareshowninTable1.

IEEE519:1992isarecommendationthatprovideslimitsofcurrentdistortionandvoltagedistortion.Currentdistortionlimitsareprovidedbasedonthefactthatlowercurrentdistortioncreateslowervoltagedistortion.Atpresent,onlyonestateinAustralia,Victoria,utilisesIEEE519:1992forharmonicdistortioncontrol.

ThevaluesofcurrentdistortionbasedonvaluesshowninTable2,Limitsareplacedontotaldemanddistortion,TDD,anexpressionofharmoniccurrentdistortionasapercentageofthemaximumdemandcurrentatthePCC,andrangefrom5%upwardsbasedonsupplycapacity(shortcircuitcapacity).Thebiggerthesupply,themoreharmonicsitcanabsorb.

Projectspecificationsmustalsoprovidespecificlimits,butcaremustbetakentonotprovideunnecessarilystrictlimits,inflatingcostsbeyondwhatisnecessary.Forexample,placinglimitsof5%THDIoneachthree-phaseharmonicproducingloadcanensurelowharmonics.Yetthismaybewellbelowtherequirementsfromthesupplyauthority.Somepreliminaryworkatdesignstagecanprovideaworkingsystemthatisnotover-engineered.Knowingthelimitsfromthesupplyauthority,thecapacityofthesupply,thetotallinearandnon-linearloadandmaximumdemand,onecanestimatedistortionvaluesusingavailablesoftwaretools.

ConClusionHarmonicswithinapowersystemaregeneratedbynon-linearequipmentsuchasrectifiers,electronicballastlightingandVSDs.Mitigationisrequiredtomaintainpowerquality,improveenergyefficiencyandsystemutilisationandtoreducethepotentialfordevicefailure.

Odd harmonics not multiple of 3

Odd harmonics multiple of 3 Even harmonics

Order h

Harmonic voltage

%

Order h

Harmonic voltage

%

Order h

Harmonic voltage

%

5 6 3 5 2 2

7 5 9 1.5 4 1

11 3.5 15 0.3 6 0.5

13 3 21 0.2 8 0.5

17 2 >21 0.2 10 0.5

19 1.5 12 0.2

23 1.5 >12 0.2

25 1.5

>250.2 +

1.3*(25/h)

NOTE – Total harmonic distortion (THD): 8%

Table 1: AS/NZS 61000.3.6:2006, compatibility levels for harmonic voltages (in per cent of the nominal voltage) in LV and MV power systems.

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Mitigationmethodsvarydependingontheequipmentgeneratingtheharmonicsandthelocation,andthelevelofharmoniclimitsimposedonabuilding.Passivefilteringcanbeinstalledattheterminalsoftheharmonicgeneratingload(seriesfilters),oratthemainboard(shuntfiltering).Shuntactivefilteringprovidesvariousmethodsofoperationtomitigateharmonicsandimprovepowerfactorandflexibilityininstallationandretrofitapplications.

Harmonicstandardsexistforequipmentandsupplynetworks.Careshouldbetakeninimplementingappropriatestandards.Preliminarydesignworkforanewprojectcanincludesoftwareestimatesofharmoniclevelstoprovideaworkingsystemwithoutunnecessarylevelsoffiltering.

TheauthoracknowledgesthecontinualadviceandknowledgeprovidedbyGregersGeilager. ❚

referenCes[1]ABS,GuidanceNotesonControl

ofHarmonicsinElectricalPowerSystems,AmericanBureauofShipping,2006

[2]AustralianStandard:AS/NZS61000.3.6:2001,Electromagneticcompatibility(EMC)–Limits–AssessmentofemissionlimitsfordistortingloadsinMVandHVpowersystems(IEC61000-3-6:1996)

[3]AustralianStandard:AS/NZS61000.3.12:2006,Electromagneticcompatibility(EMC)–Limits–Limitsforharmoniccurrentsproducedbyequipmentconnectedtopubliclow-voltagesystemswithinputcurrent>16Aand<=75Aperphase

[4]IEEE519:1992,RecommendedPracticesandRequirementsforHarmonicControlinElectricalPowerSystems,InstituteofElectricalandElectronicEngineers.

About the authorTim Makris is an electrical engineer with extensive experience in the application of variable speed drives and harmonic mitigation equipment.