a third dimension

Paperacceptedforprentationat2003IEEEBolognaPowerTechConference,June23-26,Bologna,Italy

AnInnovativeModularMultilevelConverter

TopologySuitableforaWidePowerRange

ar,rdt

Abstruct-Thispaperprentsanewmultilevelconverter

topologysuitableforveryhighvoltageapplications,especially

fundamentalconceptandtheappliedcontrolschemeis

tionresultsofa36MW-networkintertie

ble

structureoftheconverter-controlispropod.

IndexTerm-HVDCconverters,highvoltagetransmission,

multilevelconverter,space-vectorPWM

UCTION

HEderegulationofinternationalenergymarketsandtheTtrendtodecentralizedpowergenerationareincreasingthe

s

applicationfieldmultilevelconverterswithahighnumberof

voltagelevelsemtobethemostsuitabletypes,becauof

theneedforriesconnectionofmiconductorsin

combinationwithlowvoltagedistortiononthelineside

[1]-[3].Besidesthepoints,alotofotherimportantaspects

havetobetakenintoaccount

technicalandeconomicalaspectsforthedevelopmentof

multilevelconvertersare:

0Modularrealization:

-scalabletodifferentpower-andvoltagelevels

-independentofthestateoftheartoffast

developingpowerdevices

expandabletoanynumberofvoltagesteps

dynamicdivisionofvoltagetothepowerdevices

Multilevelwaveform:

-

-lowtotalharmonicdistortion

-

-uofapproveddevices

-redundantoperation

-

-avoidanceofmechanicaldestruction(high

Investmentandlifecyclecost:

-standardcomponents

-modularconstruction

0Highavailability:

Failuremanagement:

failsafeoperationondevicefailures

currentmagneticforcesandarcing)

TheauthorsarewiththeInstituteforPowerElectronicsandControl,

UniversiBtderBundeswehrMiinchen,85577Neuhiberg,Germany(e-mail:

ar~unibw,-,rdt@)

0-7803-7967-5/03/$17.0002003EEE

TOFTHENEWMODULARMULTILEVEL

CONVERTERM*LC

pleofQLC

Inordertofulfiltheabovementionedrequirements,a

convertersystemsolelycompodofanarbitrarynumber

of

sakeof

stringentmodularandscalablerealization,additional“central”

-linkcapacitorof

conventionalvoltagesourceinvertersprentsanexampleof

suchacomponent-independentofitsrealizationoutofa

systems

ofthenewconceptaretwoterminaldevicescompodof

switchesandalocalDC-storagecapacitor(CO).Noadditional

externalconnectionorenergytransmissiontothesubmodules

isneeded,forfull4-quadrantoperationoftheconverter

system.

Fig.(1)illustratesaninverterlegconsistingofn

ststep,thesubmodulescanbe

lessofthe

signofthecurrent&,theterminalvoltageV,,ofeach

submodulecanbeswitchedtoeither0VortothevoltageVc.

ForeasierexplanationallvoltagevaluesVCareassumedtobe

chinganumberofsubmodulesinthe

upperandlowerarm,ilar

manner,thevoltageVACcanbeadjustedtoadesiredvalue.

erlegconsistingof2nsubmodules

TABLE1

COMMONLYUSEDCONTROLSTAnlSOFASUBMODULETABLE

I.---

Thefollowingequationshowsthelimitationofthevoltages

vd(t)andVAC@respectivetothenumberofsubmodulesper

arm:

Whenchoosingvd=:

thentheamplitudeoftheoutputvoltageisrestrictedto:

(3)

Asuitableandsimplerealizationofthesubmoduleisgiven

erfaceiscompodsolelyoftwoelectrical

reducesthecostsformanufacturingandmaintenance,

voltageofanysubmodulecanbefi-eelycontrolledby

ividualvoltagesofthesubmodulesmayeven

nbeudtoincreathenumberof

resultingvoltagesteps(erwithPWM-operation).In

contrasttotheconventionalVSIacommoncentralcapacitive

storageisfortheconceptofM'

advantageeastheprotectionoftheconverteragainst

mechanicaldestructionincaofashortcircuit,significantly.

Inaddition,adefectivesubmodulecanbereplacedbya

redundantsubmoduleinthearmbycontrolactionwithout

sultsinanincreadsafetyand

availability.

0vxl0-

ureofasubmodule

Table1showsthecommonlyudcontrolstatesofa

eIGBTSFis

switchedon,thevoltageV,=ythevoltageVcto

theterminals,of

switchingoffbothIGBTs,theimpresdvoltagetothepower

devicesislimitedbythecapacitorvoltageVc.

tofcontrolledvoltagebalancing

Inordertokeepthecapacitorsonthesamevoltagelevel

andtoensureequalstressforthepowerdevicesthefollowing

algorithmisappliedforeacharm:

Thevoltagesofthecapacitorsareperiodicallymeasured

withatypicallysampling-rateinthemillicond-range.

Accordingtotheirvoltagethecapacitorsaresortedby

ofpositivecurrenttherequirednumberof

submodules,determinedbyoutputstateEontroller,withthe

ore,thelected

ecurrentinthecorresponding

armisnegative,thedemandednumberofsubmoduleswith

highestvoltagesarelected.I3ythismethod,continuous

ntly,

thisconceptsupportsanoptimi:zedutilisationofthestored

energyandevenlydistributedpowerlosfortheinstalled

onally,thepowerlosscanbekept

lowbyswitchingthesubmodulessolelywhenachangeofthe

outputstateisrequested.

Withregardtothemodularandscalabletopologyofthe

M'LC,theappliedcontrol:schemezihouldbeeasily

isinmind,the

ralall

concepts,badonthespace-vectorP'WMtheory,are

compatiblewiththefollowingfundamentalalgorithm:

Thefirsttaskisthetransformationofthethreepha

voltagesintothetwo-dimensionalspacebyusingtheequation

(4)

Thecondstepistofindthethreeactiveswitchingvectors

adjacenttothereferencevectorv,:eeactiveswitching

pointsnexttothereferencevecctorhavetobelocatedto

y,duetotheexpressionofthe

referencevectorbythedeterminedsurroundingvectors,the

correspondentdwellingtimeshavetobecalculated[3]-[6].

2,89

2,31

1,73

1,15

0,58

0.00

-1,15

-1,73

-2,31

-2,89

-3.33-2,67-2,OO-1,33-0,670,OO0.671.332.00'2573,33

Fig.3Space-vectordiagramofa5-levelconverter

Thenumberofnssadjustableswitchingstatesforathree-

phanlevel-multilevelconverterissimplygivenby:

Thenumberofdiflerentvoltagevectorsnvforathree-

nlevel-multilevelconvertercanbecalculated

pha

from:

AppliedtothetopologyofM2LC,Fig.3showsa5-level

space-vectordiagram,assumingthatthecapacitor-voltagesof

is

itfollowsthatthenormalizedterminal-voltagesVxofthe

oints

illustratethepossiblevoltagevectorsinthetwo-dimensional

space.

TheM2LCoffersthedegreeoffreedomtocontrolthe

possibilitytovarythecommon-modevoltagewithunchanged

phavoltagescanbeutilizedtoavoidhighrateofrior

highamplitudeofthecommon-modevoltage,whichleadsto

pod

controlschemeadoptsthefundamentalprinciplesofthe

additiontothetwodimensions,thecommon-modevoltageis

hree-dimensionalspacearis,

showninFig.4.

Fig.4Space-vectortrajectory(5-leveltopology)forapossible

three-phaoutputwithsinewavecommon-modevoltage

Forsynthesizingthereferencevectorbyusingthemost

convenientswitchingstatesasuitablealgorithmshallbe

broadlydescribed:

Inthesameway,likeintheplanereprentation,thepha

voltageshavetobetransformed(4).Thethirddimensionis

tsurrounding

switching-statevectorstothereferencevectorhaveto

be

edtotheplanegraph(surroundingtriangle

builtby3points)now4pointshavetobedetermined,which

encathereferencevectorbyatetrahedron(Fig.5).

Analoguetotheconventionaldutycyclecomputationthefour

dwellingtimesoftheircorrespondentswitching-statevector

canbecalculated.

common-mode

"voltaget

Fig.5Referencevectorencadbyatetrahedron

Simulationresultsofthethree-phavoltagesandthe

thepropodspace-vector

common-modevoltageusing

PWMareshowninFig.6an7.

5

-5d

33316672000

tirne[rns]

Fig.6Normalizedline-to-linevoltages(5-leveltopology)

1.33

-1.33I

03.336.6710.0013.3316.6720.00

tinK?[ms]

Fig.7Exampleofcontrolled,normalizedcommon-modevoltage(5-level

topology)

TIONRESULTS

Anetworkintertiemodelhasbeenbuiltandtestedtoverify

purpothesimulatorprogram

odel10inverterlegsare

connectedwithaDC-linktoonethree-phapowerconverter

(powersupply)andoneone-phapowerinverter(load).Each

tional,

centralcapacitiveenergystorageattheDC-sideisinstalled

(Fig.8).

Inpracticedifficultiesoccur,whenacommonnetwork

intertiehastobeputoutofthede-energizedconditioninto

operatingcondition(“blackstart”).Thenewpowerconverter

ollowing

ordertoaccomplishthisprocedure,onlyoneauxiliaryvoltage

sourcewitharelativelylowoutputvoltage(Vbd=V,)is

necessary.

’1d

tionmodel

Startingunderde-energizedcondition,lheoutputterminal

ofthissupplyisconnectedtotheDC-busofthemultilevel

erterleganuniberof(2n-1)IGBTsSFare

TSFofthecapacitor,whichhastobe

chargedandallremainingIGBTsSRintheinverterlegarenot

ecapacitorreachestheoperationvoltage,

aneously

wayallcapacitorsinthearmsaregraduadlychargedtothe

operationvoltage(Fig.9).Finally,thevoltagesourcehasto

bedisconnectedbyriesdiodesormechanicalswitching.

time[ms]

Fig.9Processofchargeperinverterarm

10bareundersteady

glephaandthethreephavoltages

aresynthesizedby21levels(+1redundantsubmodule).The

ripplecontentofthe2”dharmonic:ontheDC-linkvoltagehas

noeffectontheinputandoutputpowerclnaracteristic(Fig.

lob).Thecapacitiveenergystorageofallsubmodulesisthe

sameasudinaconventionalnetworkintertieofthesame

powerrating(Pd=36MW).

-40

........................................................................

.............................

..............................................................

ndoutputpowervariationcurve

sobtainedwiththeparameters:

TransmittedrealpowerPd=36MW;CO=2mF;

one-pha-system:Vl,ms=25.0kV,fi=25Hz;

three-pha-system:V3yms=26.5kV,f3=60Hz

............................:............................:...................................................>...........................

UREOFM’LC-CONTROL

Consistingofthreedifferentfundamentalunits,the

followingcontrolstructureforM’LCispropod(Fig.11).

Thesupervisoryunittakesoverthefeedbackandsupervisory

ingtothetpointvalues

thefeedbackcontrolsuppliesthecentralcontrolunitinreal-

alsignalprocessor@SP)orFPGAiswell

ntedconceptofvoltage

putstate

controllerwhichhastodeterminetheoptimizedoutputstates

andtheoperatingquenceforthenextPWM-periodis

integrated,-generatorcalculatesthedwelling-

timesfortheappendantswitchingstates.

Theconvertercubiclesareequippedwithanumberof

identicalsubmoduleswhichareconnectedsolelybyduplex

bmodule

receivesthecorrespondentswitchingcommands,opto-

electronicallyandndsitscapacitor-voltagebacktothe

centralcontrolunit,ersupplyvoltage

fordatatransferanddrivecircuitoftheIGBTsissuppliedby

thecapacitiveenergystorageofthesubmodule

[7].The

measuringsystemforthebranchcurrentmeasurementis

connectedfibre-optically,ltdetection

differentiatesbetweenfaults,whichhavetoleadtofault

instance,incasofovercurrentorpowerlossthefailure

managementhastostartthefailsafeoperation,automatically.

Whereas,corrigiblyfailuresdon’tinduceanautomatic

ailuresmayappearindatatransmission.

Thesourceoferrorcanbelocatedandsavedbythefault

ertime,inaccordancewiththe

scheduledmaintenance,sults

inadistinctimprovementoftheavailabilityand

maintainability.

Fig.11StructureofM2LC

SION

Thispaperintroducedthetopologyofthenewmodular

multilevelconverterM’LCanditsrelevantcharacteristics.

Themodularconceptallowstheapplicationforawidepower

podcontrolschemeiswellsuitedfora

ownthatthecontrol

schemeallowstocontrolpha-voltagesandthecommon

modevoltageatthesametime,tions

havedemonstratedagoodperformanceoftheM’LC-concept.

Thestartunderde-energizedconditioncanbesafelyrealized.

ThestructureofM2LC-controlenablesgoodparationofthe

lowvoltageunitsandthehighvoltageunitsoftheconverter

tly,aprototypeofa2MW-convertersystem

dularsystemwillenablefirther

experimentalinvestigations.

NCES

Jih-ShengLai,FangZhengPeng,“Multilevel

Converters-Anewbreedofpowerconverters”,

at.,vol.32,pp509-517,

May./June.1996

RainerMarquardt,AntonLesnicar,JiirgenHildinger,

NetzkupplungsanwendungbeihohenSpannungen”,

ETG-Fachtagung,BadNauheim,Germany,2002

“ModularesStromrichterkonzeptfiir

JosBRodriguez,Jih-ShengLai,FangZhengPeng,

“MultilevelInverters:Asurveyoftopologies,

controls,andapplications”,.,

vol.49,pp724-738,Aug.2002

ai,ehRad,,“Space

vectormodulationbadonclassificationmethodin

three-phamulti-levelvoltagesourceinverters”,

at.,vol.1,pp597-602,

Sep./Oct.2001

NikolaCelanovic,DushanBoroyevich,“Afast

space-vectormodulationalgorithmformultilevel

three-phaconverters”,at.

vol.37,pp637-641,March/April2001

FeiWang,“Sine-triangleversusspace-vector-

modulationforthreelevelPWMvoltage-source

inverters“,.38,

pp500-506,MarcWApril2002

JiirgenHildinger,RainerMarquardt,“Erzeugung

stabilisierterHilfsspannungenausdemLeistungsteil

vonU-Umrichtern”,ETG-Fachtagung,BadNauheim,

Germany,2002

AntonLesnicarwasborninMunich,Germany,in

ivedtheDip1.-Ing.(.)degreein

2000fromtheUniversityofMunich.

inPowerElectronicsatthe“Universitiitder

Bundeswehr/Miinchen”,Munich,Germanyinthe

engagedinrearchanddevelopmentofnew

powerelectronicsystemsforpowergenerationand

transmission.

RainerMarquardtwasborninHannover,

ivedtheDip1.-Ing.

(.)andDr.-(Ph.D.)inelectronic

communicationandpowerelectronicsrespectively

fiomtheUniversityofHannover,beforejoining

ormednumerous

industrialrearchanddevelopmentprojects

for

highpowerapplicationsinpowertransmissionand

advancedAC-Drivesystemsfortraction

iled48patentsintheareas.

Currently,heleadstheInstituteof“Power

ElectronicsandControl”asanordinaryProfessor

atthe‘‘UniversitatderBundeswehrIMiinchen”in

Germany.