* Eng.OlegPetruk,Prof.Ph.D.D.Sc.Eng.RomanSzewczyk,M.Sc.Eng.WojciechWiniarski,IndustrialResearchInstituteforAutomationandMeasurements(PIAP),Warsaw.

**M.Sc.Eng.MichałNowicki,InstituteofMetrologyandBiomedicalEngineering,WarsawUniversityofTechnology.

OLEGPETRUK*,ROMANSZEWCZYK*,WOJCIECHWINIARSKI*, MICHAŁNOWICKI**

APPLICATIONOFGRAPHENEANDNEWLYDEVELOPEDAMORPHOUSALLOYSINCURRENTTRANSFORMERS

FORRAILWAYAPPLICATIONS

ZASTOSOWANIEGRAFENUORAZNAJNOWSZEJKLASYMATERIAŁÓWMAGNETYCZNIEMIĘKKICH–

MAGNETYKÓWAMORFICZNYCH WBUDOWIEPRZEKŁADNIKÓWPRĄDOWYCH

NAPOTRZEBYKOLEJNICTWAAb s t r a c t

Paper present practical solutions based on the utilization of graphene and amorphous alloys for construction of DC currenttransformer.Proposedsolutioniscompetitiveasforthemetrologicalpropertiesandoveralldimensions,andiswellsuitedforthemeasurementsoflargecurrentsintherailwaytraction.Useofamorphousalloysasmagneticmaterialsforlow-hysteresisandhigh-permeabilitycores,anddevelopmentofhighlysensitivegraphene-basedHalleffectsensors,allowforsubstantialimprovementsin theopenfeedback loopDCcurrent transformersconstruction. Inorder toverify theusefulnessof thedevelopedDCcurrenttransformer,itscharacteristicwasinvestigated.HighlinearityofthesensorisconfirmedbytheR-squareparameterexceeding0.99.Therepeatabilityofthemeasurementswasintherangeof1%.ThepropertiesofthesematerialsraisetheprospectofchangesintheconstructionoftheDCcurrenttransformersandopenuptheperspectiveofanumberofinnovativeprojectsintherailwayindustry,inthecurrentmeasurementsarea.

Keywords: graphene, amorphous alloys, current transformer

S t r e s z c z e n i e

Artykułprzedstawiapraktycznezastosowaniegrafenu i stopówamorficznychwaplikacjiprzekładnikaprądustałego.Zapropo-nowanerozwiązaniejestkonkurencyjnecodoparametrówmetrologicznychigeometrycznychorazjestdostosowanedopomiaruprądówowysokimnatężeniuwystępującychwkolejnictwie.Wykorzystaniemagnetykówamorficznychjakomateriałówdowyko-naniardzeni,którecharakteryzująsięznacznąprzenikalnościąmagnetycznąiznikomąhisterezą,orazwysokoczułychhallotronówwykonanychzgrafenupozwalanapoprawęwłaściwościprzekładnikówprądustałegozotwartąpętląsprzężeniazwrotnego.Wcelusprawdzeniaużytecznościskonstruowanegoprzekładnikazostałazbadanajegocharakterystyka.Liniowośćcharakterystykiwba-danymzakresiepotwierdzonowspółczynnikiemdeterminacjiliniowejwynoszącym0,99.Uzyskanopowtarzalnośćpomiarównapoziomie1%.Właściwościużytychmateriałówbudząperspektywęzmianwobszarzebudowyprzekładnikówprądowychiotwie-rająperspektywęwieluinnowacyjnychprojektówwkolejnictwiewobszarzepomiaruprądu.

Słowa kluczowe: grafen, stopy amorficzne, przekładnik prądowy

TECHNICAL TRANSACTIONSELECTRICAL ENGINEERING

1-E/2016

CZASOPISMO TECHNICZNEELEKTROTECHNIKA

DOI: 10.4467/2353737XCT.16.035.5297

132

1. Introduction

Thetendencytochangetheelectricenergypaymentmethodinrailways,fromlump-sumtochargeforenergyactuallyused,raisestheneedtodevelopnewdevicestomeasuretheamountofusedenergy,oradaptoldones.Thecurrentdrawby the locomotive isashighasseveralkiloamperes.Themeasurementofsuchcurrents,incombinationwiththePolishpowersupplylinesstandard,isnotaneasytask.

Presently, DC current transformers are most commonly used for this purpose [1].Thereareseveraldifferentkindsofsolutionsforthesedevices.Inthispaper,proposalsforimprovementsintheDCcurrenttransformerwithopenfeedbacklooparepresented,basedonthemodernmaterials:grapheneandamorphousalloys.

ThebasicelementsoftheDCcurrenttransformerare:gappedmagneticcoreandmagneticfield sensor [2].Traditionally, for their construction, coresmade of transformer plates orceramicmagnetics(ferrites)areused.Asthemagneticfieldsensors,Halleffectsensorsorfluxgateswereutilised.Duetotheeasyutilisationandsimplerelectroniccircuits,Halleffectsensorsaremorepopular.TheyaremostlybasedonGaAsorInSbstructures.

Experienceintheuseofamorphousalloysasmagneticmaterialsforlow-hysteresisandhigh-permeabilitycores[3],andthedevelopmentofhighlysensitivegraphene-basedHalleffectsensors[4],allowforsubstantialimprovementsintheopenfeedbackloopDCcurrenttransformer’sconstruction.

TheaimofthepaperistopresentpracticalsolutionsbasedontheutilisationofgrapheneandamorphousalloysfortheconstructionofanewtypeofDCcurrenttransformer.

One of themain advantages of electric vehicles is the possibility of energy recoveryduring braking,which is called recuperation. Recuperation allows for a reduction of theenergy demand by re-using the breaking energy. In recent years, research in the field ofstoringandre-usingregenerativebrakingenergyhasbecomeveryintensive.Thisiscausedbyanincreasingnumberofvehiclesequippedwithrecuperationsystemsandglobaldemandsforreducingtheconsumptionofelectricalenergy.

IntypicalDCsupplysystemsofthepublictransport(tramways,trolleybuses),recoveredenergycanbere-usedbyauxiliaryreceiversinabreakingvehicleorbytheothervehicles.Ifthereisnovehicle,whichisaccelerating,thisenergyisturnedintoheatinbreakingresistors.Inordertoavoidsuchasituation,energystoragesystemscanbeused,e.g.supercapacitorsorflywheels.Nowadays,manyenergystoragesareinoperationintram,trolleybusandmetrosystems.Optimisation of the parameters and location of storage devices for regenerativebreakingenergyhasbecomeverysignificant[1–6].

Storage devices can be divided into two groups: on-board energy storage devices,whichareplacedinthevehiclesandoff-boardenergystoragedevices,situatedintractionsubstationsorbetweenthem.Currentresearchisfocusedmainlyontwoobjectives.Thefirstis to reduce the energydemandby introducingon-board energy storages in light electricvehiclesliketrams,trolleybusesorelectrobuses[7–9].Thelatteristoprovideasupplyingsystemforheavyelectricvehicles(trains,metro)withanoff-boardenergystorage[10–13].Incontrasttothis,alackofresearchinthefieldofoff-boardstorageenergysystemsforlightelectricvehiclescanbenoticed.TramandtrolleybustransportationishighlydevelopedinplentyofEuropeancities.Manytramandtrolleybusesoperatorsconsiderputtingintoserviceoff-boardenergy storage systems.Thismotivated theauthors toanalyse theefficiencyofregenerativebreakinginthetrolleybussupplyingsystemusingoff-boardenergystorage.

133

2. Principle of operation

According to the Ampere’s Law, electric current flow in the conductor generatesamagneticfieldaroundit.Thegeneratedmagneticfieldisdirectlyproportionaltothecurrentintheconductorandinverselyproportionaltothedistancefromtheconductor,asgivenintheequation:

B IR

=⋅µπ2

(1)

where:B –Magneticfieldinductionvalue,I –currentintheconductor,R –distancefromtheconductor,µ –magneticpermeability.

The magnetic field induction vector direction is following the right hand rule. ThegeneratedmagneticfieldisconcentratedbytheDCcurrenttransformercore.Theairgapinthecoreiscuttoaccommodatethemagneticfieldsensor.Thesensormeasurestheairgapmagneticfieldandconvertsitintoavoltagesignalproportionaltoitsvalue.

ThelayoutofthedeviceisschematicallypresentedinFigure1.

Fig.1.Schematicdiagramofcurrentsensor:1–magneticfieldconcentrator, 2–Hallsensor,3–wireundertest

Therelationshipbetweenthemagneticfieldinthecoreandthefieldinthegapisdescribedby theformula(2)[1,6].Thisformula indicates that,as thegapwidth increases, thefieldvalueinthegapdecreases.

B Il

l ll

Bexp

p

pin= −

−µ

µµ0 0

(2)

134

where:Bex– magneticfieldinductioninthegap,Bin – magneticfieldinductioninthecore,I – electriccurrent,l – magneticpathinthecore,lp – airgapwidth,µ – relativepermeabilityofthematerial,µ0 – magneticpermeabilityofthevacuum.

Theairgapinthecoreservesasthemountingplaceforthemagneticfieldsensor–Halleffectsensor.Itmeasuresthemagneticfieldvalueandoutputsaproportionalvoltagesignal.

Bymeasuring themagnetic field induced by the primary circuit, and not directly thecurrent,weobtaingalvanicisolationbetweentheprimarywindingandthemeasuringsystem.Itisespeciallyimportantforthemeasurementofhighvoltagecurrents.

3. The technical solution of the developed DC current transformer

Thedesignedmeasuringsystemconsistsofafewbasicelements:theprimarywinding– conductor of themeasured current, the gappedmagnetic core and aHall effect sensor.AschematicdiagramofthedevelopedDCcurrenttransformerisshownonFigure2.

Fig.2.Schematicdiagramofadigitallycontrolledcurrenttransformer

As the magnetic sensor inside the gap, an innovative graphene Hall effect sensor isused.Thesensorsbasedongraphenehaveaveryhighsensitivity(3000V/AT)[4]andgoodresolutionaswellasstabilityoftheparametersasafunctionoftemperature[6].ExemplarygraphenebasedHalleffectsensorcharacteristicisshowninFigure3.

Fig.3.Halleffectsensorcharacteristic

135

M-333magneticcoremadeofamorphousribbon,producedbyMagnetec,wasusedinthedevice.Thecoremadeof suchmaterialhasveryhigh relativepermeability (~30000)[8], linearcharacteristic,external influenceresistanceandlowtemperaturecoefficient[9].Geometricdimensionsofthecore:50×45×5mm.Airgapwidth:2mm.

TheDCcurrenttransformermadeoftheabovementionedelementsispresentedinFig.4.

Fig.4.ConstructedDCcurrenttransformer

4. Measuring stand

InordertoverifytheusefulnessofthedevelopedDCcurrenttransformer,itscharacteristicwasinvestigated.Forthemeasurements,thelaboratoryteststandpresentedschematicallyinfigure5wasbuilt.ItconsistoftheInmelindustrialcalibrator,primarywindingwithn =1000numberofturns,thedevelopedDCcurrenttransformer,Fluke8808AprecisionvoltmeterandMotechlaboratorypowersupply.DataacquisitionwasdoneonaPC.

Fig.5.Schematicoftheteststand

136

Thecalibratorcurrentwassetintherangeof0–200mA,whichtakingintoaccounttheprimarywinding, inducedamagneticflux in the core corresponding to the current rangeof0–200A.

5. Measurement results

TheresultsofthemeasurementsforthedevelopedDCcurrenttransformerarepresentedinFigure5.Itshouldbeindicatedthatalinearcharacteristicofthedevice’soutputvoltagewasachievedforthemeasuredcurrentsintherangeofupto200A.

Fig.6.ExperimentallymeasuredcharacteristicofthedevelopedDCcurrenttransformerwithitslinearfit

Linearfitof the function f(x) = k∙x +b,wherek =–0.0015V/Aandb =7.72Vwasmadebytheleastsquaremethod.HighlinearityofthesensorisconfirmedbytheR-squareparameterexceeding0.99.Therepeatabilityofthemeasurementswasintherangeof1%.

6. Conclusions

The proposed solution is competitive because of the metrological properties and overalldimensions,andiswell-suitedforthemeasurementsoflargecurrentsintherailwaytraction.ThegoodlinearityandhighsensitivityoftheDCcurrenttransformerutilisinganamorphousalloycoreandgrapheneHalleffectsensorwasconfirmed.ThepropertiesofthesematerialsraisetheprospectofchangesintheconstructionoftheDCcurrenttransformersandopenuptheperspectiveonanumberofinnovativeprojectsintherailwayindustry,inthecurrentareaofmeasurements.

This work was partially supported by The National Center for Research and Development within the GRAFTECH Program.

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R e f e r e n c e s

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[4] PetrukO.,SzewczykR.,CiukT.,StrupińskiW.,SalachJ.,NowickiM.,PasternakI.,WiniarskiW.,TrzcinkaK.,Sensitivity and offset voltage testing in the hall effect sensors made of graphene, “Automation2014”,Warszawa2014.

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[6] NałęczM.,JaworskiJ.M.,Miernictwo magnetyczne,WNT,Warszawa1968.[7] XuH.,ZhangZ.,ShiR.,LiuH.,WangZ.,WangS.,PengL.M.,Batch-fabricated high-

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temperature applications,“JournalofAlloysandCompounds”,434,2007,pp.623–627.