(Power) Measurement AFE for mV Output Sensors Interfaced to SAR ADC

47 Pages·2015·3.86 MB·English

by  Texas Instruments, Incorporated

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
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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
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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.
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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
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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
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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
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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
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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)
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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.
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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
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