(Power) Measurement AFE for mV Output Sensors Interfaced to SAR ADC PDF
Preview (Power) Measurement AFE for mV Output Sensors Interfaced to SAR ADC
TI Designs Voltage and Current (Power) Measurement AFE for mV Output Sensors Interfaced to SAR ADC TIDesigns DesignFeatures ThisTI designisafixedgainamplifierstagefor • Op-AmpBasedFixedGainAmplifierStagefor measuringlowamplitudevoltageandcurrentinputs VoltageandCurrentInputsWithOutputCompatible accuratelyoverawidedynamicrangeusingSARADC toADS8688Input Rangeof±2.56, ±5.12,and forpowermeasurementapplications.Thisdesign ±10.24V providesamplificationtothelow-amplitudeACinputs • DifferentAmplifierConfigurationsProvidedfor fromresistornetwork-basedvoltagedividers,current MeasurementofVoltageandCurrentInputs transformers, orsplitcorecurrentsensorswitha333- • ConfigurationsProvided: mVoutput.Theoutputoftheamplifieriscompatible withADS8688inputrequirements.Thisapplication – VoltageInput WithResistorDivider includessub-metering,machinemonitoring,calibration >1-MΩ Impedance(NoExternalPTRequired) checkofmetersinstalledatconsumerlocation,power – 333-mVACVoltageOutputTypeCurrent measurement,powerdataloggingandpowerquality SensorInterface analysis.ThissubsystemcanbeusedinFTU, DTU, – CTSecondaryInput WithBurdenResistor: andRTUapplications.TheadvantageofthisTIdesign Single-EndedandDifferential canbeseenwheninterfacedwiththeADS8688orany otherSARADC.Theamplificationstageensuresthe • OperatesFromSingle 5-VDCInput useofthefullrangeofADCimprovingmeasurement • Generates±15-VPowerSupplyUsingSplit-Rail accuracy. ConverterWithDualOutputs(Positiveand Negative) DesignResources • CanbeInterfacedWithADS8688-BasedTIDesign TIDA-00493 DesignFolder • OnboardProgrammableReferenceProvidedfor Single-EndedMeasurementApplications TPS65131RGET ProductFolder OPA4180IPW ProductFolder FeaturedApplications OPA180IDBVR ProductFolder LM4041BIDBZ4 ProductFolder • MultifunctionProtectionRelays TIDA-00307 ToolFolder • RTU/DTU/FTU • BayControllers • PowerQualityAnalyzers ASKOurE2EExperts WEBENCH®CalculatorTools • MergingUnits • EnergyMeters • SolarApplications • DataLoggerwith333-mVInput +15 V OPA4180_3 OPA4180_4 +15 V I1_SE I1_Diff Connectors: A A Connectors: seFcoorn CdaTr y I2_SE B -15 V -15 V B I2_Diff seFcoorn CdaTr y single-ended differential input current I3_SE I3_Diffinput current (~400 mV at C C (~400 mV at mcauxrrimenutm) I4_SE D ecxotIennrntneearcflta oscirge fn oar l D I4_Diff mcauxrrimenutm) processing +15 V OPA4180_1 inbteorafardc eo rt o OPA4180_2 +15 V (C~pFmiopn4roodnoopa0rtinutux0v ee>etiti cmpdn m cwt1uetituioV-oiatrtmM nhr l as) :t VVVV1234 BCDA-15 V TATIDDI CAD -eE0s0Vig3Mn0 7/ -15 V ABCD 333333333333 mmmmVVVV____1234 FCopouorvcsrootn oupe3utnlreunt3traepctse3 gcutown-iteotmrtoi t nrhV: Op-amp2 +15 V +15 V LM4041BID OPA VREF_Diff +5 V cDonCv-eDrCte r BZ 180 DGND TPS65131 -15 V TIDUA01–June2015 VoltageandCurrent(Power)MeasurementAFEformVOutputSensors 1 SubmitDocumentationFeedback InterfacedtoSARADC Copyright©2015,TexasInstrumentsIncorporated SystemDescription www.ti.com AnIMPORTANTNOTICEattheendofthisTIreferencedesignaddressesauthorizeduse,intellectualpropertymattersandother importantdisclaimersandinformation. 1 System Description 1.1 Introduction to Power Measurement Electricalpowermeasurementisattheheart ofnumerousapplicationsintheGridInfrastructuresectorfor 1. Electricalpowersupplymanagement, 2. Electricityusagecontrol(sub-metering), 3. Conditionmonitoring,and 4. Portablepowerquality analysis. Electricalpowersupplymanagementistheprimaryapplicationfield, asitisessentialtoanyindustrialand businessactivity.Itmainlyconcernscompaniesrelatedtopower generationanddistribution, butalso industryprofessionalsmonitoringtheirpower qualityandpower factortocontrolrateoftariffsimposedby theirutilities,especiallywhenoperatingunderlow-power-factorloads. Energysub-meteringisgainingimportanceamongfacilityandplant managersasitallowstracking and allocatingenergycosts.Powersupplysizingandbillingisoftendependentonthepeakconsumption,and adynamicmanagementoftheoverallsystem enablesbothcostreductionandfailureprevention.Energy sub-meteringisrequiredtounderstandandmanagethemainsenergyconsumption.It alsohelpsin identifyingtheenergywastesthataregenerallycausedbydefectiveappliancesorinefficientfacilities usage(forexample,inappropriatelighting,heating,orair conditioning). Conditionmonitoringrequiresimmediatefailuredetectionandreactiontoprevent damagetoequipmentor interruptionofcriticalprocesses.Electricalpower measurementprovidesacomprehensivesetof information(current,activepower,power factor,frequency, andsoon)that reflectstheloadbehavior(for example,conveyer,bearing,pump,cuttingtool,andsoon). It oftenprovidesfasterdetectionofabnormal behaviorsthantraditional sensorssuchastemperature,pressure, vibration,andsoon.Ananalysisof theseelectricalparametersevenenablestheanticipationoffailures,whichallowsfor planningeffective predictivemaintenance. Portablepowerqualityanalysisisusedtomeasure, record, anddetect power qualityissueslike harmonics,demand,inrushandpowertransients. Someofthepower qualityapplicationsinclude: • Measurementandrecordingofpowersystem quantity(kW,VA,VAR) • Determineharmonicproblemsoriginatingfromsourceorload • Monitorphasebalances • Troubleshootingofpowerdistributionpanelsandindividualmachinery • Motorstart-upanalysis 2 VoltageandCurrent(Power)MeasurementAFEformVOutputSensors TIDUA01–June2015 InterfacedtoSARADC SubmitDocumentationFeedback Copyright©2015,TexasInstrumentsIncorporated www.ti.com SystemDescription 1.2 Current Transformer / Current Sensor Currenttransformers(CTs)aidinmeasuringalternatingcurrent.CTsprovideameansofscalingalarge primary(input)currentintoasmaller,manageableoutput(secondary)current for measurementand instrumentation.ACTusesthestrengthofthemagneticfieldaroundtheconductortoformaninduced currentonitssecondarywindings.Thisindirect method ofinterfacingallowsfor easyinstallationand providesahighlevelofisolationbetweentheprimarycircuit andsecondarymeasurementcircuits. ACTisan"instrumenttransformer"that isdesignedtoprovidecurrentinitssecondary,whichis accuratelyproportionaltothecurrentflowinginitsprimary. Someofthecurrenttransformersproducea 333-mValternatingvoltagewhentherated currentismeasured(either30Aor50A).Sometransducers producea5-V DCoutputora20-mADCcurrent output attherated value. 1.2.1 CurrentSensorRequirements Someofthekeyspecificationsare • Accuracy:Inmostapplications,measurementaccuracydirectlyimpactstheefficiencyofanoverall system.Theaccuracyofthepowercalculationisdependentontheaccuracyofthecurrent sensors.A class1powermeterrequirescurrentsensorswithaccuracymuchbetter than1%. • Drift:Thedriftofasensorisrelatedtothesustainabilityofareadingovertime.Somevariationsof its characteristicsmaybecausedbychangesintheambienthumidityandtemperature,componentaging, andsoon. • Linearity:Thelinearityofthesensorreferstothestabilityofitscharacteristicswithinthefulloperating mode.Ahighlinearityoftheanalog-sensingpartisessentialtomeasureawiderangeofprimary currentsaccurately,especiallyatlowcurrent levels. • Phaseshift:Theaccuracyofthetrueactivepower orenergycalculationisnotonlyrelatedtothe accuracyandlinearityoftheACcurrentandvoltagesensorsintermsofamplitude,butalsotothe phaseshiftthatmayoccurbetweenthemeasurementofthesecorrelatedvalues.Thephaseshift shouldbeaslowaspossible. • Integration:Beingself-powered,theCTsdonotrequireanyotherwiringthana2-wire output connectiontothemainpowermonitorunit.Thetypical1-Aand5-Aor333-mVoutputsarecompatible withmoststandardpowermetersonthemarket.Currentoutputsarealsoalmostinsensitiveto interferencesandarepreferabletovoltageoutputswhenlongcablesarerequiredtoconnect the sensorstothepowermeter. • Price:Thepriceofthesensorsisimportantwhenaccuratecurrentsensorsarerequiredfor 3-phase powermeasurement. TIDUA01–June2015 VoltageandCurrent(Power)MeasurementAFEformVOutputSensors 3 SubmitDocumentationFeedback InterfacedtoSARADC Copyright©2015,TexasInstrumentsIncorporated SystemDescription www.ti.com 1.3 Current Transformer Types • Precisionsolid-coreCTs • Split-coreCTs • Clamp-oncurrentsensors • FlexibleorrigidRogowskicoilcurrentsensor • ACcurrenttransducers 1.3.1 PrecisionSolid-CoreCTs Powermeasurementsystemsgenerally implement contactlesscurrent sensorsratherthanshunts becausethelattercausepowerlossesaswellasinstallationandsafetyissues.Traditionalsolid-core currentsensorsarebasedontheprincipleofatransformer, meaningtheprimaryandsecondarywindings magneticallylinkedbyacore.ThesebasicCTsaredesignedtomeasuresinusoidalalternatingcurrentsin thetypical50-or60-Hzrange.Solid-core currenttransformersprovidealow-amperagecurrentoutput proportionaltothelinecurrentandarefor useinbuildingautomationandmeteringapplications.Solid core CTsareveryaccurate(0.3%maximumerror),smallinsize,andinexpensive.However, power mustbe turnedoffandthecircuitopened,generallyatacircuit breaker, sothat thesolidcoreCTcanbeslipped overthepowerline.Afterinstallation,thepower wiremust bereconnectedtoclosetheelectricalcircuit. CTsarenotsuitable,however,forthenumerousapplicationsinvolvingpower monitoringofexisting machinesandfacilities,whereitwouldbenecessarytoshutdownpower anddisconnectcablesbefore retrofitting thesolid-coresensorsinalltheplaceswheretheymightbeused.Installingpower metering systemsisgenerallynotpossible,prohibitivelyexpensive, orevendangerousifitrequiresaservice interruption,evenforashortwhile(forexample,stoppingaproductionline,atelecomordatacenterpower supply,somenuclearplantequipment,andsoon). Figure1.PrecisionSolid-CoreCT Applications: • 0.2or0.5classmetersinHVCTandMVCTfor power plant,sub-station, andindustrialcomplex • Loadsensorfortheloadcenter • In-homedisplay(homeenergymanagement) • Invertersforsolarandwindturbinesystems 4 VoltageandCurrent(Power)MeasurementAFEformVOutputSensors TIDUA01–June2015 InterfacedtoSARADC SubmitDocumentationFeedback Copyright©2015,TexasInstrumentsIncorporated www.ti.com SystemDescription 1.3.2 Split-CoreCT Split coretransformersareintendedforsemi-permanent installations.Theyconsist ofatransformerwhere oneofthelegscanbeopenedorremovedtoplacearoundtheconductorandthenbesecuredwitha latchorsomeothertypeoffastener. Theycanbeinstalledinelectricalcontrolpanels—thusavoidingcomplexwiring —toremotelymonitor devicesthatsometimesoperateininaccessibleorharshenvironments.Thebeautyofthesplit-core transformersisthattheycanberetrofittedintoaliveinstallationwithoutdisturbing it,whichoftenmake themtheuniquechoiceforengineersdesigningpower meters. Split-core,orclamp-on,CTsprovideanalternativetodirectlywiringtomeasurementorrelayCTsin substationupgradeorretrofitapplicationswhenitisdesiredtoaddmonitoringandSCADAdata.Thisnon- invasiveapproachprovidesforquickerinstallationwithnodisruptionofservice. Figure2.Split-CoreCT Applications: • Sub-metering • Dataloggerstoanalyzebuildingandmachineryperformance • Digital faultrecorders • In-factorydisplayorin-homedisplay • Invertersforsolarandwindturbinesystems • PowermeasurementdeviceforPLC TIDUA01–June2015 VoltageandCurrent(Power)MeasurementAFEformVOutputSensors 5 SubmitDocumentationFeedback InterfacedtoSARADC Copyright©2015,TexasInstrumentsIncorporated SystemDescription www.ti.com 1.3.3 Flexible or RigidRogowskiCoilCurrent Sensor ARogowskicoilisaspecially-woundtoroidalcoilthat canbeopenedupandplacedaroundaconductor carryinganAC.ThealternatingmagneticfieldgeneratedbytheACinducesavoltageinthecoil.This voltageisproportionaltotherateofchangeofcurrentintheconductor. Thisvoltageisthenelectronically integratedtoprovideanoutputvoltagethat mimicsthecurrentwaveformintheconductor.Rogowskicoils aresuitableformeasurementofcurrentsuptothousandsofamps, arenotsensitivetopositioningaround theconductor,andcanprovideaccuratephaseresponse. ARogowskicoilhasalowerinductancethanCTsandconsequentlyabetter frequencyresponsebecause itusesanon-magneticcorematerial.Itisalsohighlylinear, evenwithhighprimarycurrents,becauseit hasnoironcorethatmaysaturate.Thiskindofsensoristhusparticularlywelladaptedtopower measurementsystemsthatcanbesubjectedtohighorfast-changingcurrents.Formeasuringhigh currents,ithastheadditionaladvantagesofsmallsizeandeasyinstallation,whiletraditionalCTsare big andheavy. Figure3.FlexibleRogowskiCoil CurrentSensor Applications: • Electronicwatt-hourmeters(anti-tampering) • Smart powermetersformobilityapplication • ACcomponentfaultdetectorofinverter inDC • Electricmobility(automotive)andsolarapplication 6 VoltageandCurrent(Power)MeasurementAFEformVOutputSensors TIDUA01–June2015 InterfacedtoSARADC SubmitDocumentationFeedback Copyright©2015,TexasInstrumentsIncorporated www.ti.com SystemDescription 1.3.4 Clamp-OnCurrentSensors Clamp-oncurrentsensorsareavailableinavarietyofmodelsandcurrent rangeswitheitherDCorAC voltageoutputs.Clamp-onsensorsareeasytouse:simplyopentheclampandplaceitaroundone of the current-carryingconductors.Thesesensorsareidealfor temporaryinstallationsandcaneasilybe moved fromsitetosite,althoughtheyaresomewhatmore expensivethanfixedCTs. Figure4. Clamp-OnCurrentSensors Applications: • Invertermonitoringandmeasurement • Energy,powermeasurement,andmonitoring • DCmotorcontrol • Uninterruptiblepowersupplies • Motordrives TIDUA01–June2015 VoltageandCurrent(Power)MeasurementAFEformVOutputSensors 7 SubmitDocumentationFeedback InterfacedtoSARADC Copyright©2015,TexasInstrumentsIncorporated SystemDescription www.ti.com 1.3.5 ACCurrentTransducers Figure5.ACCurrent Transducer AstandardmethodofmeasuringACfor apower line-connecteddeviceistouseanACcurrent transducer,whichconvertsanACtoaDCvoltageora4-to20-mAsignal.Variousoutputsoptionsare available: Unipolar: Bipolar: • 4to20mA(500Ω max) • –20to20mA(500 Ω max) • 0to20mA(500Ω max) • –10to10mA(1kΩ max) • 0to10mA(1kΩmax) • –1to1mA(10kΩ max) • 0to1mA(10kΩmax) • –10to10V(500 Ω min) • 0to10V(500 Ωmin) • –5to5V(250 Ωmin) • 0to5V(250Ω min) • 1to5V(250Ω min) • 0to10-mVDC(250 Ωmin) • 0to100-mVDC(250Ω min) • 0to1-VDC(250 Ωmin) 8 VoltageandCurrent(Power)MeasurementAFEformVOutputSensors TIDUA01–June2015 InterfacedtoSARADC SubmitDocumentationFeedback Copyright©2015,TexasInstrumentsIncorporated www.ti.com SystemDescription 1.4 Potential Transformer and Voltage Transducer With 333-mV Output Thedesigncanalsointerfacewithavoltagetransformer withrated voltagesof110Vor230Vextending upto600Vwithoutputof333mVataratedvoltage. 1.5 Voltage and Current Measurement AFE — TI Design Advantages TheADS8684andADS8688are16-bitdataacquisitionsystemswith4-and8-channelanaloginputs, respectively.Eachanaloginputchannelconsistsofanovervoltageprotectioncircuit,aprogrammable gain amplifier (PGA),andasecond-order,anti-aliasing filterthat conditionstheinput signalbeforebeingfed intoa4-or8-channelanalogmultiplexer(MUX).TheoutputoftheMUXisdigitizedusinga16-bitanalog- to-digital converter(ADC),basedonthesuccessiveapproximationregister(SAR)architecture.Thisoverall systemcanachieveamaximumthroughput of500kSPS,combinedacrossallchannels. Thedevices featurea4.096-V internalreferencewithafast-settlingbufferandasimpleSPI-compatibleserialinterface withdaisy-chain(DAISY)feature.Thedevicesoperatefromasingle5-Vanalogsupplyandcan accommodatetruebipolarinputsignalsupto ±2.5 × V .Thedevicesofferaconstant 1-MΩ resistive REF inputimpedanceirrespectiveofthesamplingfrequencyortheselectedinput range.Theintegrationof multichannelprecisionanalogfront-endcircuitswithhighinput impedanceandaprecisionADCoperating fromasingle5-Vsupplyoffersasimplified endsolutionwithoutrequiringexternalhigh-voltagebipolar suppliesandcomplicateddrivercircuits. Table1liststheavailablerangesintheADS8688: Table1.ADS8688InputRanges INPUTRANGE POSITIVEFULLSCALE NEGATIVEFULLSCALE FULL-SCALERANGE LSB(µV) ±2.5×V 10.24V –10.24V 20.48V 312.50 REF ±1.25×V 5.12V –5.12V 10.24V 156.25 REF ±0.625×V 2.56V –2.56V 5.12V 78.125 REF 0to2.5×V 10.24V 0V 10.24V 156.25 REF 0to1.25×V 5.12V 0V 5.12V 78.125 REF Table1indicatesthattheADS8688canmeasurebipolarandunipolarinputs.TheTIDA-00310 demonstratessensingunipolarandbipolartransducer outputsasdescribedinSection1.3.5. TI designTIDA-00307demonstratesmeasurementcapabilitiesoftheADS8688includingdaisychaining. ThepowermeasurementAFE TIDA-00493isafixedgainamplifierstagethat couldbeusedalong with theTIDA-00307orTIDA-00310toincreasethemeasurementaccuracyandusemultiplemeasurement rangecapabilitiesoftheADS8688.AnytypeofACcurrent sensorortransducerinput canbemeasured usingcombinationoftheseTIdesigns. TheamplifieroutputiscompatibletotheADS8688input range.TheAFEboardcanbewiredtothe ADS8688ADC-basedTIDA-00307,TIDA-00310,ortheADS8688evaluationboardeasily. TIDUA01–June2015 VoltageandCurrent(Power)MeasurementAFEformVOutputSensors 9 SubmitDocumentationFeedback InterfacedtoSARADC Copyright©2015,TexasInstrumentsIncorporated KeySystemSpecifications www.ti.com 2 Key System Specifications Table2.KeySystemSpecifications SERIALNUMBER PARAMETERS SPECIFICATION 1 Numberofamplifierconfigurations 4 3 Channelsperamplifierconfiguration 4 3 DirectACvoltageinput ≤350-VAC (NoexternalPTrequired) RMS 4 Voltageoutputcurrentsensorrange 10-mVto333-mV (400mVmax) RMS 5 Currenttransformersecondaryrange 10mV,400mVwith22Rsecondaryburden 6 Amplifieroutputvoltage ≤7V RMS 7 Inputfrequencyrange DCor50/60Hz(Applicationandcurrentsensordependent) 8 DCpowersupplyinput 3-Vto5.5-VDC 9 DCpowersupplyforamplifiers Programmablefrom±10-Vto±15-VDC 10 Voltagereference Programmable:1.5-Vto10-VDC 11 Inputprotection ESDandSurgeasperIEC61000-4-2,IEC61000-4-5 Screw-typeconnectorsforInterfacetoADS8688ADCor 12 Outputinterface otherEVMs 10 VoltageandCurrent(Power)MeasurementAFEformVOutputSensors TIDUA01–June2015 InterfacedtoSARADC SubmitDocumentationFeedback Copyright©2015,TexasInstrumentsIncorporated
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