Calibration Service for Current Transformers

1 Page 1 NISTMeasurement Services: ACalibrationServicefor CurrentTransformers NlST Special Publication 250-36 JohnD.RambozandOskarsPetersons U.S.DepartmentofCommerce NationalInstituteofStandardsandTechnology Open at page → 3 Page 3 NISTMEASUREMENTSERVICES: ACalibrationServicefor CurrentTransformers JohnD.RambozandOskarsPetersons ElectricityDivision ElectronicsandElectricalEngineeringLaboratory NationalInstituteofStandardsandTechnology Gaithersburg,... Open at page → 4 Page 4 NationalInstituteofStandardsandTechnologySpecialPublication250-36 Natl.Inst.Stand.Technol.Spec.Publ.250-36,60pages(June1991) CODEN:NSPUE2 U.S.GOVERNMENTPRINTINGOFFICE WASHINGTON: 1991 ForsalebytheSuperintendent ofDocumen... Open at page → 5 Page 5 INTRODUCINGTHEELECTRONICSAND ELECTRICALENGINEERINGLABORATORY TheElectronicsandElectricalEngineeringLaboratory(EEEL) isthecomponentoftheNational InstituteofStandardsandTechnologychargedwithprovidingInstituteactivitiesands... Open at page → 6 Page 6 J Open at page → 7 Page 7 —PREFACE— CalibrationsandrelatedmeasurementservicesoftheNationalInstituteofStandardsandTechnology (NIST)providethemeansforindustry,governmentagencies,academia,andotherstorelatethe measurements theymaketonationalstandards... Open at page → 8 Page 8 \ Open at page → 9 Page 9 TableofContents 1INTRODUCTION 1 1.1GeneralRequirements 1 1.2PrinciplesofTransformersandNatureofTheirErrors 3 1.3TermsRelatedtoTransformerCalibration 5 1.4OtherPrecisionTransformer-LikeDevices 7 1.4.1 CurrentComparator 8... Open at page → 10 Page 10 4.4OverallMeasurementUncertainties 44 4.5SpecialHigh-AccuracyTransformers 45 4.6InternationalComparisons 46 5ACKNOWLEDGMENTS 46 6APPENDIX 47 7REFERENCES 51 viii Open at page → 11 Page 11 ListofFigures 1.Two-WindingTransformer 3 2. CurrentTransformerConnectedtoSourceandBurden 4 3. PhasorDiagrams Illustrating TermsUsedWithInstrument TransformerRatios 7 4. CurrentComparatorUsedtoCalibrateaCurrentTransformer... Open at page → 12 Page 12 ListofTables 1. TurnsArrangement intheLow-RangeCompensated Current Comparator 17 2. TurnsArrangement intheMid-Range Compensated Current Comparator 18 3. TurnsArrangement intheHigh-RangeCompensated Current Comparator 19 4... Open at page → 13 Page 13 ACALIBRATIONSERVICEFORCURRENT TRANSFORMERS AcalibrationserviceattheNationalInstituteofStandardsandTechnology (NIST)forlaboratory-quality currenttransformers isdescribed.Theservice providesmeasurements ofthecurrentratioan... Open at page → 14 Page 14 2 INTRODUCTION (NIST)providescalibrationservicesforlaboratorystandardsforeachofthesedevices. ThisSpecialPublicationdescribestheserviceofferedforthecalibrationofcurrent transformers.OtherNISTpublicationsdescribecalibratio... Open at page → 15 Page 15 1.2PrinciplesofTransformersandNatureofTheirErrors 3 adiscussionofthevarioustypesofprecisiontransformersandthenatureoftheir errors,andasummaryoftheterminologypertainingtocurrenttransformers.The Appendixpresentsinformation... Open at page → 16 Page 16 4 INTRODUCTION Theconventional equivalent circuitofanactualtransformerbasedonanideal transformerandaT-network,figurelb,explainswhytheactualtransformerdeviates fromideal.WhiletheT-networkrepresentation appliestoanynetwork... Open at page → 17 Page 17 1.3TermsRelatedtoTransformerCalibration 5 ratio.Theprimaryleakageimpedance,Zi,hasnoeffectonthecurrentratio.In practice,thedesignerofacurrenttransformerwillminimizetheratiocorrectionsand phaseanglesubjecttootherconstructi... Open at page → 18 Page 18 6 Besidestheabove,thefollowingtermsarealsofoundinthetechnicalliterature: Nominalratio.Thistermisusuallyusedasasimplenumberandissometimes substitutedforthemarkedratio.Forexample,themarkedratioofacurrent transformermightbe... Open at page → 19 Page 19 1.4OtherPrecisionTransformer-LikeDevices 7 Ij=nl 2(l+a)e I 2 (1+a )e (a)PolarCoordinateRepresentation I 1 =nI 2 (1+a J- jb j) I 2 (1+a 1 - jb (b)CartesianCoordinateRepresentation Figure3.PhaserDiagramsIllustratingTermsUs... Open at page → 20 Page 20 8 TRANSFORMER BEING CALIBRATED CURRENT COMPARATOR MAGNETIC SHIELD I1/I2 = /N21 Figure4.CurrentComparator(atRight)UsedtoCalibrateaCurrentTransformer (at Left) byMooreandMiljanic [5]describessomeofthemandalsoprovidesfurthe... Open at page → 21 Page 21 1.4OtherPrecisionTransformer-LikeDevices 9 1.4-2Two-StageTransformer Aregularcurrenttransformercanbecombinedwithathree-windingtransformer operatedinadifferentialmode(figure5a).Thistransformercanthenbeusedasa referencesta... Open at page → 22 Page 22 10 INTRODUCTION TR1 TR2 Nll/Ni2=N21/N22 N 22 =N 23 ll/(I 2 +I3) =N22/^21=^12^^11 (a)AsTwoSeparateTransformers N2=N 3 I 1/ (I 2 +I 3) =N2/N 1 (b)AsOneUnit Figure5.Two-StageCurrentTransformer Open at page → 23 Page 23 1.4OtherPrecisionTransformer-LikeDevices 11 Open at page → 24 Page 24 12 CALIBRATIONSYSTEMATNIST 2CALIBRATIONSYSTEMATNIST 2.1CalibrationCircuits CurrenttransformersatNISTarecalibratedbycomparingthecustomer'stransformer (referredtoasthetesttransformer) toastandardcurrentcomparatorwhose corr... Open at page → 25 Page 25 2.1CalibrationCircuits 13 addition,thecompensationwindingandtheoutputofthetestsetareconnected tothejunctionofA^2andA^22-Thepowersourceandacurrent-sensingshuntfor theinputofthetestsetareinserieswithA^22-Foratesttransforme... Open at page → 26 Page 26 14 CALIBRATIONSYSTEMATNIST Thecompensatedcurrentcomparatorservesbothasaprecisionratiodeviceandas apowertransformer,transferringinthiscasethepowerfromthesecondarytothe primarywinding.Thepowerrequirementsintheprimarycircui... Open at page → 27 Page 27 2.2DetailsofCompensatedCurrentComparatorsandCalibrationHardware 15 POWER SUPPLY COMPENSATED CC 21 TESTSET 0.in TEST CT BURDEN N22/N2i=^2 /Nj =n Ij/I2^ n [1-I f/ ( I 77 +I 22^^23' n[l-I ^ /I2) Figure9.CurrentTransformerCa... Open at page → 28 Page 28 16 CALIBRATIONSYSTEMATNIST POWER SUPPLY AUXILIARY COMPENSATED CT CC TEST CT 21 TESTSET 0.Ifl BURDEN N22/N21=N32/N 31 =N 2/N 1 II/I2^n [1-I c / (I 22 +I 23) 1 ^n[1-I /I c 2 =n Figure10.CalibrationofaCurrentTransformerHavi... Open at page → 29 Page 29 2.2DetailsofCompensatedCurrentComparatorsandCalibrationHardware 17 Table1.TurnsArrangementintheLow-RangeCompensatedCurrentComparator Theprimarywindinghasswitch-selectabletaps.Thefull-scalesecondarycurrentis5A. FULL-SCALE... Open at page → 30 Page 30 18 CALIBRATIONSYSTEMATNIST Table2.TurnsArrangement intheMid-RangeCompensatedCurrentComparator Thesecondaryandcompensationwindingscanhave160,100,or240turnseach.The numberofturnsinthefixedprimarywindingcanbeselectedbetween... Open at page → 31 Page 31 2.2DetailsofCompensatedCurrentComparatorsandCalibrationHardware 19 Considerthefollowingexample:assumethesensitivityfigureof10voltsperampere- turn,oneppmunbalanceina0.5-Asecondarycurrent,anda100-turnsecondary winding.Thes... Open at page → 32 Page 32 20 CALIBRATIONSYSTEMATNIST RATIOCORRECTION ADJUSTMENT oVW^ WV '-O+15V MULTIPLYING DAC VOLTAGE-TO- SUMMING CURRENT AMPLIFIER AMPLIFIER 3^) o CURRENT HONITOR IV= IA 15VO V/V O+15V PHASEANGLE ADJUSTMENT Figure11.TestSetUsed... Open at page → 33 Page 33 2.4Burdens 21 Selectablerangesareavailable(notshownindiagram)whichgeneratefull-scale balancingcurrentsof100ppm,1000ppm,and10000ppmofthenominalsecondary current/i/n,andphaseanglesof100//rad,1000/zrad,and10000//rad.Thephas... Open at page → 34 Page 34 22 CURRENTTRANSFORMERCALIBRATIONPROCEDURES 2.5SourcesofExcitation Thetestcurrentsarederivedfromoneoftwosources,dependingonthecurrent required.Formostofthecalibrationsperformed,electronicallygeneratedsinusoidal voltagesar... Open at page → 35 Page 35 3.2InitialPreparation 23 •preparationoftheReportofCalibration(orReportofTest),andfinalreview andapproval. Eachoftheseisdiscussedbelow. 3.2 InitialPreparation Uponreceiptofthecurrenttransformerfromacustomer,thetransformer... Open at page → 36 Page 36 24 TRANSFORMER BEINGTESTED Figure12.Demagnetization Circuit sametime,thevoltmeterreadingisalsonoted.Thesourcevoltageisthenslowly decreasedtozero,andtheswitchisopened.Thesourceisthenturnedoff.Boththe currentandvoltagevalu... Open at page → 37 Page 37 3.4BurdenSelection 25 Forsomewindow-typetransformershavingasingle-fixedsecondary,demagnetization ateven40Hzmaynotbeachievablewithinthelimitsofthesourcevoltage.Inthis case,aspecial40-conductorcableisfedseveraltimesthrough... Open at page → 38 Page 38 26 CURRENTTRANSFORMERCALIBRATIONPROCEDURES Table4.StandardANSI/IEEEC57.13-1978Burdens ThefollowingtableisexcerptedfromtheAmericanNationalStandardsInstitutepublication C57.13-1978,AmericanNationalStandardRequirements forI... Open at page → 39 Page 39 3.4BurdenSelection 27 alsoapplywhen"zero"burdenisrequested.Theyrepresenttheaddedleadsand otherimpedancesoftheNISTequipment. TheNISTtestsetitselfisusedforaccuratedeterminationoftheburdenimpedance, includingthecontribution... Open at page → 40 Page 40 28 CURRENTTRANSFORMERCALIBRATIONPROCEDURES biand62arethephaseanglesin//radobtainedinthefirstandsecondtests, respectively,and uj=2xf,where/isthesignalfrequencyinhertz. Tofurtherillustratetheprocedure, considerthefollowing... Open at page → 41 Page 41 3.6SummasizatioD oftheMeasurementResults 29 theoffsetinthedatawillbeapparent.Toverifythisasthecauseofdisagreement,the transformer isagaindemagnetizedandthetestrepeated. Ifthelow-currentdatafrom bothrunsofthesecondsetdono... Open at page → 42 Page 42 30 CURRENTTRANSFORMERCALIBRATIONPROCEDURES CURRENT TRP TESTNUnBER:7S2/JL^}'_^5_8_ DPTE: CUSTOMER: MfiKER : MODEL: SERIALNLWBER: CURRENTRfiNGE :To-120(5i5j5/*fr\p€rei RPTEDFREQUENCY:JL^i'-f£9Ji'T'^'t RftTEDBURDEN : J(eMj/... Open at page → 43 Page 43 3.6Summarization oftheMeasurementResults 31 CURRENTTRONSFQRMER CQLIBROTION ISECONDftRY INOi^ 1NOn 1CC rHU 1fK 1Hrnh \UUK 1KH 1 1KW 1 RATIO ERROR DOTft i~ir"iiw» 1 P'livi 1 ^1fiM« PHfibE 1ONGLE DfiTrt 1 uRPD 1uRWU 1uKftD... Open at page → 44 Page 44 32 CURRENTTRANSFORMERCALIBRATIONPROCEDURES 1F 1B 1R 1U 1Q 1R 1R 1 1A 1 1T 1 1 SEC 1 CUR 1 RATIO 1 1 « 1 1 1 AVER 1 1 RCF 1 1 1 II PHASE 1 AVER 1 1 1PHASE 1 1 1 1 1 1UNCER 1DATA iDATA 1DATA II- IIDATA 1 1 DATA 1 1 1 DATA... Open at page → 45 Page 45 3.6SummarizationoftheMeasurementResults 33 SecondaryCurrent,Amperes SecondaryCurrent,Amperes Figure15.RatioandPhase-AngleUncertaintyasaFunctionofSecondaryCurrent currentsoflessthan10%ofratedcurrent. Fortheratiocorrection... Open at page → 46 Page 46 34 CURRENTTRANSFORMERCALIBRATIONPROCEDURES 1 1 1 1 1corn tMT 1 1 1ScC 1 1nlO 1 1LUn 1 ___________ BURDEN nlOni/ 1L\Un/ 1 1 1 1 ao 1 c 1ac 1 1 1i.e 1 e.2 1m 1 1 1C*U 1 13.e e.2 1 140 1 1 1 1 1 1 1s.a 1 1 1 1 1 (a 4 I 1s.e... Open at page → 47 Page 47 3.8TheReportofCalibration 35 figure17.TheReportofCalibration specifiestheinstrumentthatwascalibrated, andincludesthemanufacturer,modelortype,andserialnumber. Ifthetransformer lacksasuitableuniqueserialnumber,NISTwillmark... Open at page → 48 Page 48 36 CURRENTTRANSFORMERCALIBRATIONPROCEDURES U.S.DEPARTMENTOFCOMMERCE NATIONALINSTITUTEOFSTANDARDSANDTECHNOLOGY (formerlyNATIONALBUREAUOFSTANDARDS) GAITHERSBURG,MD20899 REPORTOFCALIBRATION CURRENTTRANSFORMER 10-1200to5Ampe... Open at page → 49 Page 49 3.8TheReportofCalibration 37 CURRENTTRANSFORMER Knun-BetareModel3121/C,SerialNo.1234 PeyewPublicGas&ElectricCo. -2 - SecondaryburdenAconsistedofaresistanceof0.20ohminserieswithan inductanceof140microhenries. Atafrequency... Open at page → 50 Page 50 38 MEASUREMENTUNCERTAINTYANALYSIS 4MEASUREMENTUNCERTAINTY ANALYSIS 4.1GeneralConsiderations Severalconsiderations areimportantinpreparingtheuncertaintyestimatesforthe caHbrationofcurrenttransformers.Thetotalmeasurementun... Open at page → 51 Page 51 4.2AccuracyEvaluationofNISTStandardsandEquipment 39 Table5.SystematicUncertainties inNISTEquipment a=ratioerror,inppm,and/3=phaseanglein/irad,bothofwhicharetest-setreadings. SOURCEOFUNCERTAINTIES Ratio (ppm) PhaseAngle (... Open at page → 52 Page 52 40 ahighlydesirablemeansforcheckingotherapproaches, includingtheindirect techniquesdiscussedinthefollowingparagraphs. Fortunately, theproblem is mitigatedbythefactthatcurrentcomparatorsandsimilardeviceshavepermanently st... Open at page → 53 Page 53 4.3ErrorsOriginatinginTransformersBeingTested 41 currentfrompassingthroughalloftheturnsinawinding.Theyeffectivelyproduce acapacitiveshuntacrosstheratiowindings. Intheprimarywinding,thenumber ofturnsinmanycasesisbelow100w... Open at page → 54 Page 54 42 MEASUREMENTUNCERTAINTYANALYSIS Table6.EstimatedPossibleInstabilitiesinaTypicalTransformerUnderTest Ratio Phase SOURCE Correction Ancle (ppm) (^rad) 1.Coremagnetization ±5to±15±16to±48 2.Uncertaintyinburden ±6 ±19 3.Tr... Open at page → 55 Page 55 4.3ErrorsOriginatinginTransformersBeingTested 43 intheburdendirectlyaffecttheindicatedratiocorrectionsandphaseanglesby changingtheratioRg/Zm(seefigure2).Intheabsenceoftheexternalburden, temperaturechangesaffecttheratioZ2... Open at page → 56 Page 56 44 MEASUREMENTUNCERTAINTYANALYSIS Table7.CalibrationHistoryofaTypicalLaboratoryGradeCurrentTransformer,Marked Ratio10A:5A. Meanvaluesandstandarddeviationsarethecumulativevaluestoandincludingthedate ofcalibration. RATIOCO... Open at page → 57 Page 57 4.5SpecialHigh-AccuracyTransformers 45 isestimatedfromthetypeofinformation intable6computed forthespecific transformer. Consideranexample.Themid-rangecompensatedcurrentcomparator isusedwith thetestsethavingthetotalsystem... Open at page → 58 Page 58 46 ACKNOWLEDGMENTS 4.6InternationalComparisons Whenalaboratory,suchasNIST,attemptstoverifytheaccuracyofitscurrent ratiostandards,avastamountoftimeandeffortisrequired.Onehastobe alwayscautiousinguardingagainstundetected e... Open at page → 59 Page 59 47 6APPENDIX INFORMATIONREGARDING THENISTCALIBRATIONSERVICEFOR INSTRUMENTCURRENTTRANSFORMERS AlTechnicalInformation Onlymetering-typeorlaboratory-standard currenttransformers(includingcurrent comparators) willbeacceptedf... Open at page → 60 Page 60 48 APPENDIX Requests forcalibrations ortestsfromFederalandstateagencies,mustbe accompaniedeitherbyapurchaseorder,orbyaletterorotherdocumentauthorizing thecoststobebilledtotheagency.TheNISTagreementtoperformthecalibration... Open at page → 61 Page 61 49 cansparehisinstrumentforonlyashorttimeusuallycanarrangeshipmentofhis instrumenttoNISTintimetomeetthescheduledstartofcalibration.Generally,the acknowledgment ofthepurchaseordergivestheexpectedcompletiondate. NISTstaffd... Open at page → 62 Page 62 50 APPENDIX shippingorinsuranceinstructionsarefurnished,returnshipmentwillbemadeby commoncarriercollectanduninsured. ShipmentsoftransformerstoNISTforcalibrationshouldbedirectedto: NationalInstituteofStandardsandTechnolog... Open at page → 63 Page 63 REFERENCES 51 7.REFERENCES [1]J.D.RambozandR.C.McAulifF,"ACalibrationServiceforWattmetersand WatthourMeters,"Natl.Bur.Stands.(U.S.),Tech.Note1179,July1983. [2]W.E.Anderson,"ACalibrationServiceforVoltageTransformersandHig... Open at page → 65 Page 65 NIST.114A U.S.DEPARTMENTOFCOMMERCE (REV.3-90) NATIONALINSTITUTEOFSTANDARDSANDTECHNOLOGY BIBLIOGRAPHICDATASHEET 1.PUBUCATIONORREPORTNUMBER NIST/SP-250/36 2.PERFORMINGORGANIZATIONREPORTNUMBER 3.PUBUCATIONDATE June1991 4.TI... Open at page → 67 Page 67 THESP250SERIESONNISTMEASUREMENTSERVICES* SP250-1 SpectralRadianceCalibrations PB87179883 SP250-2 FarUltravioletDetectorStandards PB87227609 SP250-3 RadiometricStandards intheVacuum Ultraviolet PB87227625 SP250-4 FrickeDo... Open at page → 68 Page 68 U.S.DepartmentofCommerce NationalInstituteofStandardsandTechnology Gaitfiersburg,MD20899 OfficialBusiness PenaltyforPrivateUse$300 Open at page →