* Prof.Ph.D.D.Sc.Eng.AdamSzeląg,FacultyofElectricalEngineering,WarsawUniversityofTechnology.

ADAMSZELĄG*

25KVACRAILWAYLINEWITHIN3KVDCINFRASTRUCTUREINPOLAND–ANALYSIS

OFOPERATINGCONDITIONS

ANALIZAWARUNKÓWWPROWADZANIA DOPOLSKIEJINFRASTRUKTURYKOLEJOWEJLINIIZELEKTRYFIKOWANEJWSYSTEMIE25KVAC50HZ

A b s t r a c t

Electrified transport is strongly assorted, due to long term development and use of diversified solutions,particularlyinthescopeofelectricalengineeringconsideringthefollowingaspects:nominalvoltageofapowersupplysystem,vehicles’maincircuitsandelectricaldrives.Theproblemofcompatibleworkofvariedrailwaysystemsinmutual interactionregions,despiteofvariousstudies,stillappearsasignificant issue.ThemainpurposeofthepaperistopresentthenegativeimpactofmutualinteractionofACDCrailwayregionsontheelectrifiedrailwaylinesandpowergrid.AMatlab–SimulinkmodeloftheDCandACtractionsystemshasbeendeveloped.Inthearticle,apartfromthemainproblemswhichcouldappearintheAC-DCtransitionzones,thevoltageunbalanceanalysiscausedbytheACtractionsubstationiscoveredandtheinfluenceofasymmetricinputvoltageoftherectifiertractionsubstationontheDCvoltagequalityisanalysed.

Keywords: traction power supply system, asymmetry, harmonics, mutual influence of AC and DC power systems

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

Zelektryfikowany transport jest silnie zróżnicowany zewzględu na długi okres rozwoju oraz stosowa-nierozmaitychrozwiązań,wszczególnościwzakresierozwiązańtechnicznych,zuwzględnieniemtakichaspektówjak:napięcieznamionoweukładuzasilania,główneobwodypojazdóworaznapędyelektryczne.Pomimoprzeprowadzeniaszeregubadań,problemkompatybilnościdziałaniaróżnychsystemówkolejo-wychnaobszarachwzajemnegooddziaływanianadalpozostajeistotnymzagadnieniem.Niniejszyartykułmanaceluprzedstawieniezagadnienianegatywnegowpływuwzajemnegooddziaływaniaobszarówkole-jowychAC-DCnazelektryfikowaneliniekolejoweorazsiećenergetyczną.WtymceluopracowanomodelMatlab–SimulinksystemówtrakcyjnychDCiAC.Wartykule,opróczgłównychproblemów,jakiemogąpojawićsięwstrefachstykusystemówAC-DC,przedstawionorównieżanalizęasymetriinapięciapowo-dowanejprzezpodstacjętrakcyjnąACorazprzeanalizowanowpływasymetrycznegonapięciazasilającegopodstacjiprostownikowejnajakośćnapięciawsiecitrakcyjnejDC.

Słowa kluczowe: system zasilania trakcji elektrycznej, asymetria, harmoniczne, oddziaływanie wzajemne systemów zasilania AC i DC

TECHNICAL TRANSACTIONSELECTRICAL ENGINEERING

1-E/2016

CZASOPISMO TECHNICZNEELEKTROTECHNIKA

DOI: 10.4467/2353737XCT.16.036.5298

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1. Introduction

Highspeedrailwaydevelopment requiresACelectrification,due tomuchhigherpowertransfer capacity – vehicles collect lower currents from catenary. As a result, countriespreviouslyelectrifiedonlywithDCsystemhave todealwithmutual interactionofACandDCsystemsoperatinginadjacentareas[1,3,4,6,11–13].Thelattercooperationandrelatedissuesiscommonincountriesthatusemorethanonetractionpowersupplysystem(e.g.25kVAC50Hzrailwayandatramsystemsuppliedfrom600VDCcatenaryor15kVAC16.7Hzrailwayand750DC3rdrailurbansystem).Allthesesystemsmustoperatewitharequiredhighlevelofsafetyandreliability,especiallyinthecities–highlyurbanisedareas,wherethetracksofneighbouringsystemsareoperatingintheirvicinity[4].Despitevariousmeasurestaken,theproblemofcompatibleoperationofvarioustractionpowersupplysystemsinthecontactareasisasignificantareaofconcernandshouldbetakenintoaccountatthefirststagesoftheproject.

2. AC and DC railway systems interactions

Afterintroductionof25kVACtractionintoPolishrailwayinfrastructure,themanagementof mutual interaction areas will be required. These areas occur at various points of theinfrastructure(Fig.3):• onthePowerUtilitySystem(PUS)side-whentractionsubstations(TSs)ofbothsystems

aresuppliedfromthesamePointofCommonCoupling(PCC).DCTSwillbefedbyunbalancedvoltagewhereasACTSbydistortedvoltage;

• ontherailwayside(ERL).UnbalancedinputvoltageofDCTScausesfluctuationofitsoutputvoltagewithdoubledPUS

frequency(inPoland100Hz).Suchphenomenonleadstotheoccurrenceofadditional,previouslynot present, non-characteristic higher order harmonics (HH) derived from the operation ofarectifierunit-bothonthePUSandERLsides.Asaresult,anACTSconnectedtothesamePCCisfedwithlowervoltagequality.ThelattercausesthepresenceofHH,intheACTSoutputaswell.SignificantlydecreasedpowerqualitymayresultinadditionalissuesrelatedtothefilteringdevicesinDCTS–deteriorationoftheHHfilterperformance.Installationoftworesonancefilterstunedtoparticularrectifierfrequencies(600Hzand1200Hz)resultinparallelresonanceinthemiddleofthosefrequencies(Fig.1).Ingeneral,itshouldnotbeaproblem,howeverincaseofnon-characteristicHHpresence;furtheranalysisofthefiltereffectivenessmayberequired.

Independent bulk supply points for theAC andDCTSs reduces the issues of powerqualitydescribedabove,however,doesnot resolve the issuerelated tomutual interactionareaspresentontheERLside.Railwaylinesinteractwitheachothermainlydueto[9]:• inductive coupling–ACERLhas an impact on theDCERL.50Hzvoltage (and as

aresultcurrents)ispresentintheDCsystemcircuitry.Additionally,HHcurrentsflowingintheDCcatenarymayalsoresultininducedvoltage(andcurrent)intheACcircuitry.However,thiseffectisnegligibleduetorelativelylowmagnitudesofthosecurrentsandenough,evensmall,separationbetweentheACandDClines;

• galvaniccoupling–returncurrentsofonesystemmaybepresent intheneighbouringrailsofadjacentsystem.Thisphenomenonderivesfromtheearthingmethodology–therailsoftheACsystemarewell-earthedwhereastherailsoftheDCsystemareisolatedfromtheground(Fig.3).

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Fig.1ExemplarycharacteristicsofthefiltersusedinaDCtractionsubstationsinPoland (simulationresults)

Fig.2.CurrentflowinACtractionsystems.Fromthetop:25kV,25kVwithareturnconductor, 25kVwithreturnconductorandbooster-transformers,2×25kV)

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Duetoinductivecoupling,thewell-knownandsafevalueof120VpotentialinDCrails[9]isnolongeracceptable.ThisiscausedbytheinfluenceofACcomponents,whichtogether(superimposed)withtheDCvoltage,increasetheriskofelectricshock(mixedvoltages–containingACandDCcomponents[9]).50HzcomponentinducedinDCrailscanalsohaveanegativeimpactontheoperationofoldertypestrackcircuits(TC),whichoperateonthebasisofisolatedblocks.Additionally,duetogalvaniccoupling,the50HzcomponentmayoccurinDCrails(togetherwithaninducedcomponent).DependingonthetypeofanACsystem(25kV,25kVwithareturnconductor,25kVwithabooster-transformersor2×25kV),thenegativeimpactofgalvaniccouplingseenfromtheDCsystemschanges,duetotheamountofreturncurrentflowingintheearth(Fig.2).

As faras theadjacentoperationofPUS linesandERLisconcerned,describedabovedescriptionmaybeapplicableaswell.

GalvaniccouplingresultsinthesignificantimpactoftheDCsystemontheACsystem–DCcomponentpresentintheACrailsnotonlyincreasestheriskofelectricshock(mixedvoltage[9]), but alsonegatively influences anACsystem infrastructure equipment (transformers,booster-transformersandautotransformerscoressaturation).DCstraycurrentsflowpathisclosedbytheelementsoftheadjacentACsysteminfrastructure.

IfthemutualinteractionareasappearatbothPUSandERLside(e.g.ACandDCTSsaresuppliedfromthesamePCCandERLsoperateincloseproximity),thelevelofnegativeinterference,describedabove,maybehigher.

Fig.3.MutualinteractionareasofneighbouringACandDCrailwaysystems

3. Analysis of voltage unbalance at the PCC feeding AC TS

Feedingdiagram(Fig.4)wasproposedfortheexemplaryanalysisofthePUSsidemutualinteractionareas.ACTSsupplies tworailwaysections(25kmeach)– lefthandside( toACTC2)andrighthandside(tosectioningpost)–50kmintotal.Forthepurposeofthisanalysissectioningpostsandsub-sectioning(paralleling)postsbetweenACTSshavenot

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beenpresentedonFig.4.Fig.5showstheTSACoutputpowerduringapeakperiodundernormalfeedingconditions(whenneighbouringTSis inoperation).Theknowledgeof thepowerdemandcurvewill be significant to assess theminimumshort-circuitpower (faultlevel)atPCCbusbarswhich is required tomeet theunbalance requirementsdescribed inregulations[10].

Fig.4.ContactareaofACandDCrailwaysystemsatthePUSside(ACandDCTSssuppliedfromthesamePCC)

Relevant regulations governing maximum levels of voltage unbalance in Poland [8,10].Polishregulationstatesthat[10]:“…Withineachweek,95%ofresultsfromthesetof10-minuteaverageRMSvaluesofanegativephasesequencevoltagecomponentshouldbeintherangeof0%to1%ofapositivephasesequencevoltagecomponent”.

TheratiooftheRMSvalueofanegativephasesequence(nps)voltagecomponenttotheRMSvalueofapositivephasesequence(pps)voltagecomponentisdefinedasavoltageunbalancecoefficientαu[2,6,8,9].RMSvalueaveragedover600swindowisunderstoodas a onevalue (calculated from the set of instantaneousvalues in 10minuteswindow)mentionedintheregulation[10].Oneweekcomprisesof1008ofthelattervalues,whatmeansthat95%ofαuvalues(RMS)mustbesmallerthan1%–itconstitutes958values.Therefore,ifitisassumedthatapeakperiodlastsfor8hperday(2×4h),thenthepeakperiodconstitutes33%ofoveralltime(8×7=56h).Intheanalysedcase,theexceedanceofαumayoccuronlyduringashortpeakhourperiods.Ifduringallofthepeakperiodstheαuvaluescalculatedinlinewiththerequirements[10]arehigherthan1%,thenonly67%ofthosevalueswillbeacceptable(fromtherequired95%).Insuchcase,tobefullycompliantwith the requirements set out in regulation [10], duration of the peak periodshouldbenolongerthan1.2h.TheoverallpowersupplysystemforACrailwaysshouldbedesignedproperly(withpossiblyhighshortcircuitpowerattheTSHVbusbars)toprovidetheenvisagedlevelofvoltageunbalance(upto1%)throughouttherequiredperiod(min.95%overtheweek).

Takingintoaccounttheforecastedtraffic,onedefinedpowerdemandofanACTSandperformedtheanalysisofmutualinteractionoftheACandDCrailwaysystemsatPUSside(thesamePCCsuppliesDCandACTSs).Thefirststepwastoprovidesuchshort-circuitpowerattheATTSHVbusbarsthat,foragivenpowerdemandcurve(Fig.5),TSwillnotintroduceunacceptable[10]voltageunbalance.

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Fig.5.ForecastedloadofanACTS–4-hourpeakperiod[11]

Fig.6.Exceedanceofthe1%unbalanceαulimitforvariousfaultlevels(short-circuitpowers) atPCC(busbars)feedingtheACTS

Figure6showshowmanyofthe10-minutevalues(describedin[10])exceedthepermissiblelimitversustheshort-circuitpower(faultlevel)atthePCC.Figure7confirmsthatthevoltageunbalance, caused by theAC railwayoperation, is acceptablewhenminimum short-circuitpoweratPCCisatleast3GVA.InthiscasethevoltageunbalancelevelatthePCCbusbarswillbeupto1%(Fig.8)[ofcourseifnounbalancecomesfromthePCCupstreamside].

Therefore,as faras thevoltageunbalance isconcerned, theshort-circuitpowerat thePCCshouldbetheconsideredasthemostsignificantareaofanalysis.

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Fig.8.Estimatedcontribution(%)ofinstantaneous(5-seconds)valuesofαufactorsduringa4-hourpeakperiod[ifthevalueisequalto1itmeansthatαuiswithintherangeof0–1%]forvariousfault

levels(short-circuitpowers)atthePCC

Fig.7.Exceedanceofthe5%limitby10-minutesetsofRMSvaluesαuduringaweek,forvariousfaultlevels(short-circuitpowers)atPCC(busbars)feedinganACTS

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4. Voltage quality analysis at the AC and DC ERL side in case of the same PCC for AC and DC TSs

Even ifAC TS is compliant with the regulation [10] (acceptable voltage unbalancelevel),inputvoltageofDCTSwillnotbefullybalanced.Thisphenomenonresultsintheoccurrence of non-characteristic HH in bothAC and DC side (input current and outputvoltage respectively) of thisDCTS.ThemagnitudesofHHdependon the commutationangleoftherectifierdiodesandthereforeontheDCTSload.

Forthepurposeofthelattervoltagequalityanalysis(ACandDCTSaresuppliedfromthesamePCC),onehasdevelopedaMATLAB–SimulinkmodelwiththeSimPowerlibraryapplication.Inlinewiththepreviousanalysis,the3GVAPCCwasmodelledtosupplyACandDCTS.DCTSoutputpowerisaround6MW,whereasthepowerconsumedbytheACTSisequalto40MVA(whatcausesasymmetryatthePCCaround1.3%).Thefollowingfigures(Fig9,10)showHHpresentintheinputcurrentandoutputvoltageoftheDCTS.Fig.11demonstratestheHHpresentintheoutputvoltageoftheACTS(note:thisanalysisdonottakeintoaccounttheharmonicfootprintcausedbytherollingstock).

As previouslymentioned, the presence of non-characteristicHH in aDC system cancause seriouscomplications [1] relatedmainly to theoperationof signalling systemsandfiltersinaTS[5].LCfiltermaybeconsideredasasolution[7].IntheACTSinputvoltage,ifTS is supplied from the samePCCasneighbouringDCTS, the contributionofHH isincreasedasaresultofthevoltageunbalance(Fig.11).FromthepointofviewofthePUS,electric traction,bothACandDC, is seenasso-called ‘disturbingandunstable’ load [2].Therefore,HHintheACERLarepresentwhenaDCTSoperatesunderloadingandfinally,non-characteristicHHoccurinDCTSrectifiedvoltageonlywhenACTSisinoperation.IfbothTSs(ACandDC)areloadedatthesametimetheresultsofasharedPCCwillbepresentatboth,ACandDCsideoftheERL.

Fig.9.CurrentmagnitudesofHHinACcurrentsupplyingaDCTS,relatedto50Hzfundamental[PCCshort-circuitpower3GVA,αu=1.3%]

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Fig.10.OutputvoltagemagnitudesofHHinaDCTS,relatedtotheDCcomponent[PCCshort-circuitpowerat3GVA,αu=1.3%]

Fig.11.OutputvoltagemagnitudesofHHinanACTS[PCCshort-circuitpower3GVA,αu=1.3%]

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5. Conclusions

The introduction of the railway line electrified with 25 kVAC 50 Hz voltage intoPolish railway infrastructuremight lead to appearance ofAC andDCmutual interactionareas.Phenomenaoccurringintheseareasmustbetakenintoaccountattheconceptdesignstageof theproject.Provisionof independent and reliableoperationofboth linesprovestobeinsufficientinthemutualinteractionareas.It is importanttotakeintoconsiderationcertainissues,suchasmalfunctionofinfrastructureequipmentduetoneighbouringsysteminterference,increasedriskofelectricshock(mixedvoltages–ACandDC),deteriorationofvoltagequalityinthePUSatthePCCandinACandDCtractionnetworks(additionalharmonics).Whenintroducinganew25kVACrailwaylineintothe3kVDCinfrastructurearea in Poland, it is necessary to conduct an additional and comprehensive research andanalysis,whichwillemployacomplexapproachtothecontactareas’relatedphenomena.

The author wishes to thank Ph.D. Marek Patoka for permission to use in this paper results of his rese-arch presented in his Ph.D. dissertation “Analiza oddziaływań zakłócających w strefie styku systemów trakcji elektrycznej 3 kV DC i 25 kV 50 Hz” completed at Warsaw University of Technology in 2014.

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