Implementation of a Single-Phase Electronic Watt …

1 ApplicationReportSLAA494A May2011 RevisedMay2013 Implementationofa single -PhaseElectronicWatt-HourMeterUsin gtheMSP430 AFE2xxFengHou, ,injury, ,injury, : a a May2011 RevisedMay2013 Implementationofa single -PhaseElectronicWatt-HourMeterUsin gtheMSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013, energymeasurementandhavethenecessaryarch itecturetosupportit. TheMSP430 AFE2xxdeviceshavea powerful12-MHzcentralprocessingunit(CPU) (ADCs)basedonasecond-ordersigma-delta( ) ADCs(SD24) common-modevoltageofupto-1V ,it 16-bitx (RMS)currentandvoltage,activeandreactive powerandenergy,powerfactor, single -PhaseElectronicWatt-HourMeterUsin gtheSLAA494A May2011 RevisedMay2013 MSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013.

2 TexasInstrumentsIncorporatedABCTESTREAC MAXLCDDISPLAYkWkWh20-pin20-pinWirelessDa ughterCard EMKI nterfaceVCCGNDISOLATEDac/dcPOWERSUPPLYVC CGNDSPIUSBSPISPIMSP430F6638 APPLICATIONPROCESSORUSB COMMTO PCSCLKI/OF6638 JTAGL2N2ac mainsELECTRICALISOLATIONBOUNDARYAFE JTAGGPIOVCCXT2 INXT2 OUT8 MHzHFcrystalSCLKSIMOSOMIURXD0 UTXD0 RSTVSSAFEMSP430 Analog toDigitalI1-I1+V1+V1-VREFSPIRELAYCIRCUIT RYRS 232 BRIDGEUART TO PCLOADVCCGNDI InV InCapacitivePower supplyac mainsL1N1 Optical a slavemetrologyprocessorandtheMSP430F6638 is (SPI)oruniversalasynchronousreceiver/tra nsmitter(UART). May2011 RevisedMay2013 Implementationofa single -PhaseElectronicWatt-HourMeterUsin gtheMSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013,TexasInstrumentsIncorporatedUTXD0V 1 - /(O )SPI(I)URXD0XT2 INXT2 OUTA nalog toDigitalI InV InFrom utilityCTV1+I1+I1-I2+I2-NLRSTVSSVCCMSP43 0 AFEA pplicationinterfaceUART or SPILOADV1-VREFPULSE1 PULSE2HF crystal(Up to12 MHZ) showsthehigh-levelinterfacefora single -phasetwo-wirestarconnectiontothem ainsis ,anda voltagedivideris (CT)

3 Determinedbythecurrentrange,gainsettings oftheSD24ontheAFE, selectedtoensurethemainsvoltageis ,seetheMSP430x2xxFamilyUser'sGuide(SLAU1 44) single -PhaseElectronicWatt-HourMeterUsin gtheSLAA494A May2011 RevisedMay2013 MSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013,TexasInstrumentsIncorporatedR1US20K 275L2N2N_APPL_APPLLNN26262222C28+ /400VV0+V0+DVCC_APP_PLV0 150 FC36C37+ NCNCL1 LINEL51 HLINE_AV1R1US20K275470n/ 400VC49R2256R / +2200 C251 TPS770335 INOUTIC334 ENFBGND2ZD1 DNP( )C26C2310 DVCC_PLDGND_AFEDGND_AFECONNECT AGND and N1 NEUTRAL1 (RC)PowerSupplyFigure3 showsa simplecapacitorpowersupplyfora directlyfromthemainsvoltageof110/220 VRMS alternatingcurrent(ac).

4 FeddirectlytoanRC-basedcircuitfollowedby rectificationcircuitrytoprovidea directcurrent(dc) Figure3, (SLAA024). If additionaldriveis required,eitheranNPNoutputbufferora transformer- ,a switching-basedpowersupplyis differentialandrequiresthattheinputvolta gesatthepinsdonotexceed 500mV(gain= 1).Tomeetthisspecification, ,theSD24allowsa maximumnegativevoltageof-1V;therefore, May2011 RevisedMay2013 Implementationofa single -PhaseElectronicWatt-HourMeterUsin gtheMSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013, pFV1+V1 15 usually230V or110V andmustbebroughtdowntoa rangeof500mV(seeFigure5). Theanalogfrontendforvoltageconsistsofspi keprotectionvaristors(notshown)

5 Followedbya showstheanalogfrontendforthevoltageinput sfora ,whichis 495mVpeak, , thatcanbeusedexternallyandalsoasa common-modevoltage,if ,becausetheinputimpedancetothepositivete rminalis muchhigherand,therefore,a lower-valueresistoris thisdifferenceis notmaintained,a single -PhaseElectronicWatt-HourMeterUsin gtheSLAA494A May2011 RevisedMay2013 MSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013,TexasInstrumentsIncorporatedAVCC_AF ECUR1+ R171K1KR18R18 GNDC1747pF47pFI1+I1 C3015nFCUR2+CUR2 +C3415nFIN theburdenresistorthatis selectedbasedonthecurrentrangeandtheturn s-ratiospecificationoftheCT(notrequiredf orshunt).

6 Theanti-aliasingcircuitryconsistingofR andC a fullydifferentialinputwitha voltageswingof ,thecommon-modevoltageis selectabletoeitheranalogground(AGND_AFE) discussedin (SD24)ADC,clocksystem,timer,andwatchdogt imer(WDT).7 SLAA494A May2011 RevisedMay2013 Implementationofa single -PhaseElectronicWatt-HourMeterUsin gtheMSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013,TexasInstrumentsIncorporatedRESETHW setupClock, SD24, Port pins,Timer,USARTC alculate RMS values forcurrent, voltage; Activeand Reactive PowerMain Power OFF?1 second of Energyaccumulated? Wait foracknowledgement fromBackground processGo to LPM0 YesNoYesNoWake-upSend Data out , single -phasesystem, (fM) is derivedfroman8-MHzexternalcrystal(ACLK).

7 Thesamplingfrequencyis definedasfS= chosenas256,andthemodulationfrequencyfM, is chosenas1 MHz,resultingin a + + CurrentI2(Neutral) + single -PhaseElectronicWatt-HourMeterUsin gtheSLAA494A May2011 RevisedMay2013 MSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013,TexasInstrumentsIncorporated901Re( )( )Samplecountnactpvni nPKSample count= = ( ) ( )1 Samplecountv ni nnPKActpSample count ==2( )1 SamplecountinphnIKRMSiSample count ==*2( )1 SamplecountvnphnVKRMSvSample count ==* ,Clocksystem,general-purposeinput/output (GPIO)(port)pins, checkis madeifthemainpoweris ,thebackgroundprocessnotifiestheforegrou ndprocessthrougha statusflageverytimea frameofdatais ,a ,voltage, ,current, theprevioussections,simultaneousvoltagea ndcurrentsamplesareobtainedfromthreeinde pendent convertersata 1 secondis (n)= Voltagesampleata sampleinstant n'i(n)= Currentsampleata sampleinstant n'Samplecount= Numberofsamplesin 1 secondKv= ScalingfactorforvoltageKi= frame' (samplecount) (n)

8 = Voltagesampleata sampleinstant n' shiftedby90 Kp= Scalingfactorforpower9 SLAA494A May2011 RevisedMay2013 Implementationofa single -PhaseElectronicWatt-HourMeterUsin gtheMSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013, thencalculatedbasedontheactivepowervalue foreachframe,similartothewaytheenergypul sesaregeneratedin thebackgroundprocessexceptthat:EACT= PACT SamplecountForreactiveenergy,usethe90 phaseshiftapproachfortworeasons: It allowsaccuratemeasurementofthereactivepo werwithverysmallcurrents. It ,it is discussedin realizedbyprovidinganN realizedbya fractionaldelayfilter( ). triggertocollectvoltageandcurrentsamples (threevaluesin total).

9 Thesesamplesarefurtherprocessedandaccumu latedin (onesecondworth)havebeenaccumulated,thef oregroundfunctionis triggeredtocalculatethefinalvaluesofVRMS , IRMS, power, alsowhollyresponsibleforenergyproportion alpulses, single -PhaseElectronicWatt-HourMeterUsin gtheSLAA494A May2011 RevisedMay2013 MSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013,TexasInstrumentsIncorporatedSD24 Interrupts @3906/secRead Voltages V1 Read Currents I1,and I2 Store readings and notifyforeground processPulse generation in accordanceto power accumulationCalculate frequencyCalculate power factorReturn From Interrupt1 second of energycalculated?

10 NoYesYesa. Remove residual dcb. Accumulate samples for instantaneousPowerc. Accumulate for Ifor both currents fullydifferentialinputand,therefore,noad deddcoffsetis neededtopreconditiona signal,whichis a removedindependentlyforV andIbysubtractinga long-termdctrackingfilter' synchronizedtothemainscycletoyielda : Accumulatedsquaredvaluesofvoltageandcurr entforVRMSandIRMS calculations. Accumulatedenergysamplestocalculateactiv eenergy. Accumulatedenergysampleswithcurrentand90 May2011 RevisedMay2013 Implementationofa single -PhaseElectronicWatt-HourMeterUsin gtheMSP430 AFE2xxSubmitDocumentationFeedbackCopyrig ht 2011 2013,TexasInstrumentsIncorporated360360 == ooffininDelay resolutionDegOSRffsmSD16 OSRx = 32f cycles:M323240 Load SD16 PREx:SD16 PREx = 8 PreloadAppliedDelayed ConversionResultTimeConversionDelayed ,whenusedasa sensor,andtheinputcircuit' a functionofinputfrequency(fin), OSRandthesamplingfrequency(fs).