#測量電能質(zhì)量和發(fā)展檢測裝置的方法 英文

MethodofMeasuringPowerQualityandDevelopmentofMonitorDevice

ShuyingYang

NorthChinaElectricPowerUniversity,Baoding,P.R.China,071003

Abstract—Thispaperanalyzesthemainfactorinpowersystemwhichinfluencespowerquality,andthenecessityofdevelopingmonitordeviceofpowerqualityisanalyzed.Measuringmethodofpowerqualityindexesandthefeasibilityofmonitordevice’srealizationareresearchedalso.Furthermore,thesoftwareisprogrammedandPS-3powerqualitymonitordeviceisdeveloped.Thehardwaresystemiscomposedofindustrycontrolcomputer,dataacquisitionandanalysismodulebasedon80C198microcontrollerandexternalanalogsignaltransformingbox.Everystandardofpowerqualitycanbetestedandanalyzedbydevicedevelopedinthispaper.Atlast,thedeviceisusedtotestthe35KVbusofasteelfactory.Theresultsshowthatthemethods

adoptedarecorrectandtheprecisionis

higher.

IndexTerms

—Powerquality,measurement,monitordevice

INTRODUCTION

Intherecentyears,asthefastdevelopmentinmetallurgyindustry,chemistryindustryandelectricalrailwaytechnology,morenonlinearelectricalequipments,suchaslarge-sizedelectric-arcfurnace,electricallocomotives,rectifyinginstallations,converterplantsandsoon,areappliedwidely.Thesenonlineardevicesmaketheelectricnetworkmorenonlinear(harmonics),asymmetric(negativesequence),andfluctuating.Atpresent,muchattentionispaidtopowerqualitymanagementineverycountry.TheStateBureauofQualityandTechnicalSupervisionhaspromulgatedsixpowerqualitystandards,whicharefocusedondeviationofpowersupplyvoltage,frequencydeviationofpowersystem,fluctuationandflickerofvoltage,harmonicsofpublicnetwork,unbalancedegreeofthreephasevoltage,instantaneousover-voltageandmomentaryover-voltagetoensurethatelectricalnetworkandelectricalequipmentsrunsafelyandstably.Thesestandardscanundoubtedlypromotethemanagementlevelofpowerqualityeffectively,butcorrespondingpowerqualitymonitordeviceisrequiredtotestandanalyzethepowerqualityindices.

1.

METHODSTOMEASURINGPOWERQUALITYINDICES

1.1.HARMONICSMEASUREMENT

Consideringtheimplementationofdevice,followingprocessesshouldbeincludedtomeasureandcalculatevoltageandcurrentharmonics:firstly,thedistortedvoltageandcurrentwaveformsfromsystemoverPTandCTareconvertedintolowvoltagesignalthroughlinearattenuationbranchoff

voltage.secondly,discreteanalogsignalintodigitalsequencesignalbysamplingandanalogtodigital(A/D)conversion.thirdly,theamplitudeandphaseoffundamentalwave,harmonicsandmuchmoreotherinformationcanbeobtainedafterharmonicsanalysisandcomputationbasedoncomputer.fourthly,statisticalprocessandothertechnologiesareperformedtoanalysisharmonics.

ThebasictheoremofthediscreteFouriertransform(DFT)transitingtothefastFouriertransform(FFT)isusedtocalculatevoltageandcurrentharmonics.AssumethatseparateonecycleTofcontinuous-timesignalf(t)intoNpoints,samplingeveryT/Nseconds.Thenthediscretesignalcanbegotwithdiscretetimepointt=kT/N(wherek=0,1,2…N-1),andsamplingvalueiskfTheFFTtransformformulacanbeexpressedasfollowing:

Thematrixmodelcanbeexpressedas:

WhereFhisthecomplexvalueofeachharmonic,fundamentalcomponentanddccomponent,F0isthedccomponent,F1isthefundamentalcomponent,F2isthesecond-orderharmoniccomponent,etc.

Inthecalculationofharmonics,N=128isusedassamplingpoints,fs=128×50Hz=6.4kHzisusedassamplingfrequency,whichisgreaterthan2×2.5kHzandmeetstherequestofsamplingtheorem.Δt=T/N=20/128msisthesamplingtime.Assumethattheinstantaneousvalueofnetworkvoltageu(t)andcurrenti(t)is:

978-1-4244-4813-5/10/$25.00?2010IEEE

AfterSamplingforu(t),i(t),andA/Dconversion,FhcanbecalculatedusingFFT.Thenthefollowingresultscanbeobtained:

Harmonicvoltageratiois:

Harmoniccurrentratiois:

Totalvoltageharmonicdistortionis:

1

f

Totalcurrentharmonicdistortionis:

1.2VOLTAGEFLUCTUATIONANDFLICKERMEASUREMENT

Tomeasurevoltagefluctuationandflicker,equivalent10HzvoltageflickerΔU10,short-termflickerseverityPstandlong-termflickerseverityPltareusuallyconcerned.

(1)Equivalent10HzvoltageflickerΔU10[3]

WhenFFTisusedtoanalyzespectrum,N=8isconsideredforlowfrequency(slowspeed)analysis,andthesamplingtimeΔt=20/8ms.Inthiscase,therewillbe400samplingpointspersecondandthevoltageafterFFTcanbeeducedaccordingtoformula(1).

Inordertodetectthecurveofamplitude’senvelopeof50Hzvoltagefromvoltagewaveform,the“squaredetect”methodrecommendedbyIECisused.Afterbeingfilteredthrough0.05~35Hzband-passfilterandspectrumanalysis,thecorrespondingmodulatedwavecanbeobtained.Furthermore,theequivalent10Hzvoltageflickervaluecanbegotby:

Where

f

isflickersensationcoefficientatfrequency

f,andΔUfistheaverageofRMSofthesine

wavecomponentwithfrequency

modulatedsignal.

f

whichcanbecalculatedfromthespectrumanalysisofthe

(2)Short-termflickerseverity

st

P

(1~15mins)

TaketheF1atthefrequency

f

=50HzfromthesequenceafterFFTandcalculatetheRMS(

fi

U

)

offundamentalwavecorrespondingtoF1.TherewillbeanaveragevalueofvoltageUfiineach

minute.If1000/20=50pointsaretakenpersecond,thentherewillbe3000

F

and3000

corresponding

fi

U

perminute.Gettheaverageofthe3000

U

fi

:

TakeUastheactualRMSoffundamentalwave,whichcanbeexpressedas

f

U

.Accordingto

thetimepercentofeveryfiU,whichexceedsfU,thecumulativeprobabilitycurve(CPF)canbe

plotted,andprobabilitystatisticcanbeperformed.Consequently,thefollowingfunctionisgot:

Ifmake

f

U

(%)asabscissa,CPF(%)asordinate,thecumulativeprobabilityfunctioncurvecanbe

obtained,whichisshownasFig.1.

If

P0.1,P1,3P3,P10,P50

arepickedoutfromtheCPFcurve,thatcumulativeprobability

exceeds0.1%,1%,3%,10%,50%respectively.Shorttimeflickerseverity

st

P

canbeexpressed

as:

(3)Long-timeflickerseverity

lt

P

(1h~7days)

P

lt

canbecalculatedaccordingtoseveral

st

P

valuepointsinthemeasuringterm.Theformula

isasfollowing:

Assumethatmeasurementisdoneonceeveryother10mins,thenintheperiodoftwohours,

twelve

P

st

valueswillbemeasured,anditwillbe144inaday,and1008insevendays.

1.3THREE-PHASEEUNBALANCEFACTORMEASUREMENT

Thedegreeofthree-phaseunbalanceinthethree-phasesyetemisdefinedasthree-phaseunbalancefactor.Thealgorithmofthree-phaseunbalancefactorissymmetricalcomponentsmethod.Thepercentofreversecomponentstoforwardcomponentsofvoltageorcurrentisusedtorepresentunbalancefactor.

Accordingtotheamplitudeandangleofthree-phasefundamental-wavevoltageandcurrentobtainedfromharmonicsanalysis,everysequencecomponentoffundamentalwavecanbecalculated;

furthermorethree-phaseunbalancefactorcanbegot.Three-phasevoltageunbalancefactor

U

and

three-phasecurrentunbalancefactor

I

canbeexpressedasfollowing,respectively:

1.4FREQUENCYDEVIATIONMEASUREMENT

ThemethodrecommendedbyEuropeGenerateandTransmitElectricityAllianceisused,inwhichthedistributionofmeansquaredeviationisusedtoexpressthedispersedegreebetween

frequencyfandnominalvalue

N

f

.Sampleoncepersecondandworkoutameansquaredeviationσ

valueeveryother15minutes,whichcanbeexpressedasfollowing:

Wherenisthesamplingtimesoffrequencyf,distributing

curveofσisshownasFig.2.

2.HARDWAREDESIGNOFTHEMONITORDEVICEFOR

POWERQUALITY

ThehardwarestructureofaPS-3powerqualitymonitordeviceconsistsofthefollowingthreeparts:

Anexternalanalogsignaltransformbox;

Adataacquisitionandanalysismodulebasedon80C198MCUinterface;

Industrycontrolcomputer.

TheschematicdiagramofhardwarestructureisshowninFig.3.

Thefunctionofeachcomponentisasfollowing:

Transformbox:ThisisalsonamedasanauxiliaryPTandCTunit.ThevoltageratiooftheauxiliaryPTis100V/5V,andtheratioofcurrentandvoltageoftheauxiliaryCTis20A/5V.AlternatingvoltageandcurrentcomefromPTandCTcanbetransformedtothevoltagebetween+5Vand–5Vbytransformerbox,whichisneededinA/Dtransformation.Andtheboxcankeepthesecondarycircuitofcomputerfromtheeffectoftransientover-voltageandsurge.

Dataacquisitionandanalysismodule:low-passfilter(LPF),samplerandholder(S/H),

multiplexswitch(MPX),A/Dconverterandmicrocontrolunit(MCU)areincludedinthispart.

c)Simulatelow-passfilter(LPF):inordertoensureA/Dconversionandavoidaliasingof

frequency,maketheoriginalsignalpassthelow-passfilterwhosebandwidthislowerthan

f

c

(natural

frequency),andthenthecomponentshigherthan

f

c

isfiltered.

Double-portRAMisthedatastoragesharedbybothofindustrycontrolcomputerandMCU.WhentheRAMissharedbyMCUandindustrycontrolcomputer,andabuffermustbeusedtoisolatethem.Forexample,ifMCUusestheRAM,itshouldbeconnected,whileIndustrycontrolcomputershouldbeinsulated.Thefunctionofisolationcanberealizedbybi-directionalbusdispatcher/receiver.

Industrycontrolcomputersystem:thePCsystemcansampletheanalysissignalcontrolledbyA/D,anditcanalsosampleoriginaldata,analyzespectrum,calculate,anddisplaytheresultsofmeasurement.Itsadvantageslieinitsgoodperformanceinreliabilityandstability,aswellasitsgoodprotectionperformancefortheusingoftechnicalgradeaccessory.

3.SOFTWAREDESIGN

Modularizationdesignisusedinthesoftwareandtheprogramisprogrammedbyassemblylanguage.Theprogrammainlyincludesmainmodule,double-portRAM(Figure4)andacquisitionmodule(Fig.5)

10

.

PS-3powerqualitymonitordevicecancollectinstantaneousvalueofthree-phasevoltageandthree-phasecurrentsimultaneously.Afterthecalculationandanalysisofindustrycontrolcomputer,thefollowingitemscanbeshownandprinted:(1)analysisresultsofvoltageorcurrent,(2)

statisticresultsofvoltageorcurrent,(3)waveformchartandspectrumchartofthree-phasevoltageandthree-phasecurrent,(4)totalharmonicdistortionchangecurveofvoltage,(5)changingcurveof

voltageandcurrentharmonics,(6)changingcurveofequivalentvoltageflickervalueΔU,(7)result

ofthree-phaseunbalancefactor,(8)three-phasepowerchangingcurve,(9)systemfrequencychangingcurve.

4.TESTINGRESULTS

PS-3powerqualitymonitordeviceisusedto35KVbusinasteelwork,partoftheresultsareshowninfollowingfiguresandtables.

Table1showstheVoltagefluctuationandequivalentflicker.Fig.7isthedaychangingcurveofthree-phaseharmoniccurrentdistortionratio.Fig.8describesthetendencyofequivalent10Hzflicker.

5.

PRECISIONSOFSOMEIMPORTANTTECHNIQUEINDEXESPrecisionsofsomeimportanttechniqueindexesisexpressedasfollowing:

Frequencyerror≤0.02Hz

Harmonicvoltage(current)ratioerror≤0.2%

Voltagefluctuationerror≤0.2%

Supplyingvoltagedeviationerror≤0.2%

Equivalentflickervalueerror≤0.1%

Three-phasevoltageunbalanceerror≤0.2%

6.Conclusion:

ThefunctionsofPS-3powerqualitymonitordevicearefull.Itcantestfrequency,voltagedeviation,three-phaseunbalance,voltagefluctuation