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2. Thesis for the Degree of Doctor of Philosophy
A Study of High Resolution
Isolated Multi-Level Inverter for
Grid-Tie Renewable Generation
System
by
Noh Sik Park
Department of Mechatronics Engineering
The Graduate School
Pukyong National University
February 2009

3. A Study of High Resolution
Isolated Multi-Level Inverter for
Grid-Tie Renewable Generation
System
신재생 발전 시스템의 계통 연계를 위한
고정도 절연형 멀티레벨 인버터에 관한 연구
Advisor: Prof. Soon Jae Kwon
by
Noh Sik Park
A thesis submitted in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
in Department of Mechatronics Engineering
The Graduate School
Pukyong National University
February 2009

5. - i -
Tableofcontents
Tableofcontents······································································································ⅰ
ListofFigures··········································································································ⅳ
Listoftables··············································································································ⅶ
ListofSymbols·········································································································ⅷ
ListofAbbreviation·································································································ⅹ
Abstract·······················································································································ⅺ
CHAPTER
Ⅰ.INTRODUCTION······························································································1
1.1Trendinthetechnicaldevelopmentofgrid-tiedinverter···················1
1.2Objectiveandcontentsofresearch··························································10
1.2.1Objectiveofresearch·············································································10
1.2.2Contentsofresearch··············································································11
1.3Compositionofthepaper············································································12
Ⅱ.MULTI-LEVEL INVERTER······································································15
2.1Theprincipleofmultilevelinverter························································15
2.2Thetypesandstructureofexistingmultilevel inverter············18
2.2.1Diode-clampmultilevelinverter························································20
2.2.2Improveddiode-clampmulti-levelinverter·····································23
2.2.3Flyingcapacitormulti-levelinverter·················································25
2.2.4H-bridgemulti-levelinverter······························································28
2.3Modulationmethod························································································31

6. - ii -
2.3.1Staircasemodulationmethod·······························································31
2.3.2Sinusoidalpulsewidthmodulationmethod·····································36
2.3.3.Spacevectormodulationmethod·······················································36
2.4Grid-TiedTechnology··················································································38
Ⅲ.THE PROPOSED GRID-TIDE HIGH RESOLUTION
MULTI-LEVE INVERTER········································································40
3.1Thestructureofproposed3phasemulti-levelinverter····················40
3.2Thecontrolmethodofproposed3phasemulti-levelinverter·········42
3.2.1Regularspacevectormodulation ······················································42
3.2.2Vectorcontroloftheproposed3phasemulti-levelinverter ···46
3.3Analyzinglossoftheproposedmulti-levelinverter···························51
3.3.1Factoroflossinmulti-levelinverter···············································51
3.3.2Analyzinglossforeachfactor····························································53
3.4Anti-islandingtechnologyofpowerconvertoringrid ·····················64
3.4.1Passivemethod························································································66
3.4.2Activemethod··························································································68
3.4.3Gridsplittingconditiontopreventislanding··································71
Ⅳ.COMPUTER SIMULATION·······································································75
Ⅴ.DESIGN andEXPERIMENTS···································································84
5.1DesignofPowerConverter········································································84
5.1.1Inrushcurrentprotectioncircuit·························································85
5.1.2DC linkvoltagecontroller····································································87

7. - iii -
5.1.3Isolatedmulti-levelconverter······························································91
5.2DesignofDigitalControlSystem·····························································95
5.2.1DesignofPowercontroller··································································96
5.3Prototypeof30kW powersystem··························································102
5.4ExperimentalResults··················································································106
Ⅵ.CONCLUSIONS··························································································119
요 약 ·····················································································································122
REFERENCES·········································································································123
AppendixA.DesignSchematics········································································131

8. - iv -
ListofFigures
Fig.2.1Schematicof3Phasemulti-levelinvertersystem·························16
Fig.2.2Outputvoltagewaveform of5-levelinverter··································17
Fig.2.3ClassificationofDC-AC inverter·························································18
Fig.2.4Structureofdiode-clampmulti-levelinverter··································19
Fig.2.5Structuresofimproveddiode-clampmulti-levelinverter·············23
Fig.2.6Structureofflyingcapacitormulti-levelinverter···························25
Fig.2.7StructureofH-bridgemulti-levelinverter·······································28
Fig.2.8Operationalprincipleof9-levelHBMLinverter······························29
Fig.2.9H-bridgeswitchingfunction·································································30
Fig.2.10Switchingfunctionsandoutputwaveform of
staircasemodulation·············································································31
Fig.2.11Thecalculationmethodofswitchingangleusing
equalarea································································································34
Fig.2.12Spacevectorrepresentationofathree-levelinverter··················37
Fig.3.1Thestructureof3phasemultilevelInverter ·······························41
Fig.3.2Normalizedvectorsof3-levelIHMLinverter··································44
Fig.3.3Normalizedvectorsofcascade3-phaseIHML································45
Fig.3.4NormalizedvectorofIHML usingthreetransformer····················45
Fig.3.5Representationofthelocuswithhighestproximityvector·········46
Fig.3.6Calculateratepolarityofoutercoveringfunctionof
highestproximityvector·········································································48
Fig.3.7Flowcharttocalculatethehighestproximityvector······················49
Fig.3.8Thecomponentofmultilevelpowerconvertersystem················52
Fig.3.9EquivalentmodelofcapacitorusingESR·········································58
Fig.3.10Equivalentcircuitandv-ispecificofpowerdiode······················56
Fig.3.11Thelossgraphsofpowerdeviceincaseofresistanceload
andinductionload················································································63
Fig.3.12PowercurrentofPV system andGrid ··········································64

9. - v -
Fig.3.13CurrentcommandusingSandiafrequencyshiftmethod············69
Fig.3.14Simulationofislandingusing Sandiafrequency
shiftmethod·····························································································70
Fig.4.1Simulinkmodelforcomputersimulation···········································75
Fig.4.2Transformerandoutputvoltageaccordingmodulationindex·····77
Fig.4.3Line-lineandoutputvoltageaccordingtomodulationindex······79
Fig.4.4FFT analysisaccordingtomodulationindex···································81
Fig.4.5OutputvoltageTHD andDFaccordingtomodulationindex·····83
Fig.5.1Theproposedmulti-levelconvertersystem·····································85
Fig.5.2Theinrushcurrentprotectioncircuit··················································86
Fig.5.3Thedesignedbuckandboostconverterwithcommon
reactor··········································································································87
Fig.5.4Operatingmodesofbuckandboostconverter································88
Fig.5.5Data-sheetandmagneticcharacteristicsofcorematerial············90
Fig.5.6The3-phaseisolatedmulti-levelinverter·········································92
Fig.5.7Transformerpowercapacityaccordingtotherateof
modulation·································································································93
Fig.5.8Manufactured3-phasepowertransformer········································94
Fig.5.9Blockdiagram oftheproposedpowerinverter·······························96
Fig.510Signalflow from powersystem todigitalcontroller····················97
Fig.5.112ndoderactiveanti-aliaslow powerfilter····································98
Fig.5.12FrequencycharacteristicsofdesignedIIR filter····························99
Fig.5.13Proposedphasedetectingmethod····················································100
Fig.5.14Anti-windupPIcontroller··································································101
Fig.5.15System layoutofdigitalcontrollerandIPMs······························102
Fig.5.16DSPcontrollerandsensorboards···················································103
Fig.5.17GatedriverandIPM connection······················································104
Fig.5.18Prototype30kW grid-tiedtypemultilevelpowerinverter······105
Fig.5.19Outputandtransformervoltages(=0.9)·······································106

10. - vi -
Fig.5.20THD characteristicsofoutputvoltages·········································107
Fig.5.21Experimentalresultsatloadvariation············································109
Fig.5.22Experimentalresultsat25% load····················································111
Fig.5.23Experimentalresultsat50% load····················································112
Fig.5.24Experimentalresultsat75% load····················································113
Fig.5.25Experimentalresultsat100% load··················································114
Fig.5.26Monitoringandmeasurementofpowersystem··························115
Fig.5.27Inverterefficiencyaccordingtooutputpower·····························116
Fig.5.28Experimentalresultofgridconnectionwithsoftstart·············117
Fig.5.29Experimentalresultofgriddisconnection·····································118
Fig.A.1SchematicofDSPandpowerofdigitalcontroller······················131
Fig.A.2Schematicoffilterandamplifier·······················································132
Fig.A.3SchematicofIPM controlboard·······················································133
Fig.A.4SchematicofkeyandLCD displayboard·····································134
Fig.A.5Schematicofsensorboard··································································135
Fig.A.6SpecificofGeneral30kW 3phasePWM invert
forgrid-tied·····································································································136

11. - vii -
Listoftables
Table2.1Switchstateofdiode-clampmulti-levelinverter························21
Table2.2Switchstateofflyingcapacitormulti-levelinverter··················26
Table2.3Protectivetechnologyofpowerconverterforgrid-tied·············39
Table3.1Featureofsystem drivingaccordingtoloads······························67
Table3.2VariationoffrequencyusingSandiafrequency
shiftmethod···························································································71
Table3.3Troublecleartimeaccordingtoabnormalvoltage
atgrid-tiepoint····················································································71
Table3.4Troublecleartimeaccordingtoabnormalfrequency
atgrid-tiepoint····················································································72
Table3.5Protectorequipmentfrom abnormalstates
ingrid(Korea)························································································73
Table3.6Standardofdisconnectiontimefrom gridafterislanding·········74
Table3.7Protectorequipmentforanti-islanding············································74
Table5.1Electricalspecificationofpowerconverter····································84
Table5.2Specificationof30kW Transformer·················································94
Table5.3Loadtestresults··················································································110
Table.6.1Comparedresults················································································120

12. - viii -
List of Symbols
 :Switchingangle
 :Intersectionoflevelvoltage& referencevoltage
 :Seriesconnectedeachinputcapacityeachvoltage
ofmulti-levelinvert
 :Numberofnods
 :Intersectionnumberoflevelvoltage& referencevoltage
 :Y-connectionloadA-phasecurrent
 :NumberofTransformerusingattheIHMLI
 :Maximum levelnumberofmulti-levelinvert
 :Rateofmodulation
 :NeutralofY-connectionload
 , :FactorOf′ ,′ inregularaxisincomplexplane
 :Numberofclampdiodesusingeach phase
 :Numberofcapacitorusingeach phase
 :Numberofusablespacevector
 :NumberofdeviceusingatIHMLI
 :Statesof  thswitch
 :Turn-ratiosofIHMLI'stransformer
 , :Voltageof , axisincomplexplaneofoutputvoltage
′ ,′ :Voltageof, axisinregularcomplexplane
ofoutputvoltage

13. - ix -
 :''thtransformer's-phaseinputvoltageofIHMLI
 :''thtransformer's-phaseoutputvoltageofIHMLI
 :IHMLI's-phaseoutputvoltage
 :IHCML'sphasetophaseoutputvoltage
 :Y-connectionloadA-phasevoltage
 :DC inputsource
 :Referencevoltageof, axisincomplexplane
′ :Referencevoltageof, axisinregularcomplexplane
 :''thfrequencycomponentsofm-leveloutputvoltage
 :Invertoutputvoltage
 
:RMS voltageofinverter'soutput

14. - x -
List of Abbreviation
CT :Currenttransformer
DCC :Diode-clampcircuit
DCMLI :Diode-clampmulti-levelinverter
DF :Distortionfactor
EMC :Electromagneticcompatibility
FACT :FlexibleAC transmissionsystems
FCC :Flyingcapacitorcircuit
FCMLI :Flyingcapacitormulti-levelinverter
FFT :Fastfouriertransform
GTO :Gateturn-offthyristor
HBC :H-bridgecircuit
HBMLI :H-bridgemulti-levelinverter
IHMLI :IsolatedH-bridgemulti-levelinvert
IPM :Intelligentpowermodule
LCD :Liquidcrystaldisplay
MC :Magnetcontactor
PT :Potentialtransformer
PWM :Pulsewidthmodulation
SPWM :Sinusoidalpulsewidthmodulation
SVM :Spacevectormodulation
SMPS :Switchingmodepowersupply
THD :Totalharmonicdistortion

15. - xi -
AStudyofHighResolutionIsolatedMulti-LevelInverter
forGrid-TiedRenewableGenerationSystem
NohSikPark
DepartmentofMechatronicsEngineering,TheGraduateSchool
PukyongNationalUniversity
Abstract
The objective of this paper is to contribute to the commercialization of
high resolution 3 phase multi level inverter that uses new control method
whichispropertothegreatcapacityinverterusedforGrid-tiedRenewable
GenerationSystem.Forthispurpose,thebasicoperationprincipleandtypes
of the conventional multi level inverter is divided to describe the each
merit and demerit, the modulation technique applicable to multi level
inverter, the domestic and overseas standard and protecting technology of
grid-tied technology which is indispensable to grid-tied inverter. The loss
for each factor in 3 phase multi level inverter that uses low-frequency
transformer was analyzed with structure of 3 phase multi level, theoretic
investigationontheregularspacevectormodulationmethodandanewcontrol
technique so as to calculate all loss that may occur in the proposed
inverter. Further, this research explained the control technique appliedto
anti-islanding, an essential function of grid-tied inverter and confirmed
outputwaveformandTHDforeachrateofmodulationbyusingMatrap/Simulink
so as to check the technical feasibility of multi level inverter using 3
phase low-frequency transformer. As 3 phase multi level inverter in 30kW
isolatetypewasdesigned, producedand experimented,itwasconfirmed that
the performance which was not inferior to that of the existing 2 level
inverter method was embodied. Since there is no particular difficulty in
productioncomparedtotheexistingproductswhichareproducedbydepending
on many know-hows and less part was technically restricted in producing
inverterwithcapacity largerthanthatofinverterproducedforresearch,
it presents the possibility of localizing power transformer for Grid-tied

16. - xii -
RenewableGenerationSystemwhichgetslarger.

17. - 1 -
CHAPTERⅠ
INTRODUCTION
1.1Trendinthetechnicaldevelopmentofgrid-tiedinverter
Owing to the issue of scanty energy and high oilprice,diverse
researches and efforts are recently in progress to utilize renewable
energy.Further,manyresearchesareinprogressthesedaystoconnect
micro-grid system and series.Composed ofenvironment-friendly and
reliablepowersupplierssuchasmicroturbinethatcanbecontrolledby
independent active and reactive power,fuelcell,Photovoltaic Power
Generation and Wind Generation System,micro-grid mustbedesigned
so thatelectric powerofthese systems can be finally connected to
system inordertosupplyelectricpowertobus.Inthemicro-grid,each
generator is connected to system through power convertor.As the
powergeneratedbyPhotovoltaicPowerGenerationandWindGeneration
System isquitevariabledepending on thenaturalconditions,grid-tied
powerconvertorthatcanregularlycontroltheelectricpowerisessential
tostablesystem connection.Thetypesofpowerconvertorarevarious
dependingonthemethodandusetoconvertelectricpower,forinstance,
AC-DC convertor that generally converts AC source to DC source,
DC-DC convertor thatconverts DC source to other DC source and
DC-AC inverterthatconverts DC source to AC source,etc.Among

18. - 2 -
them,grid-tiedpowerconvertormaybethedevicethatcomprehensively
uses power conversion technology which is used in the mostpower
convertorexplained above.The powerconversion technology used for
thispurposehasbeendevelopedinmanystagessofar.Consideringthe
recenttrendwhererenewableenergy ismoreutilized,commercialization
technologyofgrid-tiedpowerconvertorisdeveloping considerably.The
advanced makerofgrid-tied powerconvertorthatcurrently occupies
Renewable Generation System market launched and commercialized
grid-tied powerconvertorin the unitcapacity of250kW and 500kW,
which areimported and used in thecountry.To maintain orincrease
theexistingmarketshare,theseadvancedcompaniescontinueR&D and
investmenttocommercializegrid-tied powerconvertorwith largerunit
capacity.
On thecontrary,when itcomestothecommercialization ofdomestic
grid-tiedpowerconvertor,themakersofgrid-tiedpowerconvertorthat
havetheirown technology arevery few and mostofthem aresmall
and medium companies which are unable to commercialize grid-tied
powerconvertoroflargeunitcapacitythatneedsenormousdevelopment
cost.Thoughtherearecommercialdomesticgrid-tiedpowerconvertors
in thegradeof30kW,thestandardoflargecapacity,they arenotyet
trusted in price,technology and reliability. This tendency is more
conspicuousin largerunitcapacity.Considering domesticand overseas
situation wherenew andrenewableenergy aremoreutilizedrapidly,it
isrequiredtoresearchgrid-tiedpowerconvertorwithlargecapacityin
diverse aspects in the country as well.Itis also necessary to make

19. - 3 -
much effort to commercialize products that can compete with the
advancedproductsthatoccupythepresentmarket.
Thispaperintendsto describethestructurewhich isdifferentfrom
thelargecapacity grid-tied powerconvertorwhich isused in mostof
existingsystem anditsnew controltechnique.Infact,3phasegrid-tied
power convertor in 30kW is to be produced so as to present the
excellent performance and potentialof commercialization of products
suggestedthroughexperiment.
The composition ofpower convertor or the types and methods of
appliedcontroltechniquevary,dependingonthetypes,volume,capacity
andfrequencyofinputandoutputvoltageaswellasqualityofAC and
useofpowersource.
The structure and controltechnique used in mostgrid-tied power
convertorthatis currently used in the Renewable Energy Generation
System in the marketis 2-levelinverter method thatconverts into
desiredAC Powerbyusing 0V or±Vdc
levelofDC input.Forinstance,
it has been intensively used in AC motor control inverter, AVR
(AutomaticVoltageController)thatimprovesthequalityofpowersource,
CVCF(constantvoltageconstantfrequency),UPS(Un-interruptablepower
source)andSMPS(Switching Modepowersource).Further,itsscopeof
applicationwillbeexpandedinthefuture
[1]-[3]
.
PWM(PulseWidth Modulation)controltechniqueusing thechangein
pulsewidthisgenerallyusedin2-levelinverter.Astheperformanceof
powerdevice such as FET,IGBT,etc is improving,grid-tied power
convertor that requires high quality power can reduce THD(Total

20. - 4 -
HarmonicDistortion),themainassessmentfactorofhighqualitypower
byincreasingswitchingfrequency.
As2-levelinverterneedstomaintain DC-Linkvoltagethatishigher
than peak value ofAC voltage to acquire AC voltage,the insulation
voltage ofpower device used in grid-tied power convertor mustbe
higherthan600V toconnecttothesystem ofAC 220V orAC380V.As
switching loss is higher when insulation voltage ofpower device is
higher, loss increases owing to switching frequency heightened to
reduce THD and surge voltage also increases owing to high voltage
switching.Asvoltagealsoincreases,theirrationalcondition iscaused,
thatis,itisrequired to selectpowerdevicethathastheproperty of
higherinsulation voltagesoasto supplementsnubbercircuitorsurge
compensation.When load capacity increasesby switching high voltage
in high speed(dv/dt),the adverse effectmay occur,thatis,switching
loss ofEMC (Electro Magnetic Compatibility)and powerdevice itself
thatmay occurby stressandhighspeedswitching duetoloadcaused
bylargecurrentwhenloadcapacityincreasesbyswitchinghighvoltage
inhighspeed
[4]-[6]
.
Asmoregeneration system thatusesnew andrenewableenergy are
recently produced and installed worldwide,inverterthattakes low DC
poweroccurring in fuelcellorsolarcellismoreneeded.However,the
existing 2-levelinvertermodecannotreachthecommercialAC voltage
necessary forgrid-tied withoutusing convertorthathas high rate of
voltage increase.Thus,grid-tied power convertor thatrequires high
quality powersourceneed toheighten switching frequency forcontrol.

21. - 5 -
In realizing high resolution grid-tiedinverterthatsatisfieshigh quality
and high efficiency,the existing 2-levelinverter mode cannoteasily
make commercialproducts withoutsystematic design as perincreased
capacityinproductionandtechnicalandempiricalknow-how acquiredin
thesite.
Sincethegeneration capacity ofrenewableenergy generation system
is increasing,the existing 2-levelinvertermode cannoteasily acquire
elementthatsatisfieshigh insulation voltageand high speed switching
property in selecting technology and powerdevicelikeFET orIGBT.
Afterall,these situations disable the domestic smalland medium to
commercializeproductsthatcancompetewiththeadvancedproduct.
Many researches on the theme ofmultilevelinverterwith diverse
structuresareperformedtomakeinverterwith largerunitcapacity by
supplementing thedemeritoftheexisting 2levelinverterandtoapply
powerconvertorwith multilevelstructuretolargecapacity renewable
energy generation system.While the existing 2 levelinverter uses
single DC power,multilevelinverter uses severalindependent DC
powerandgetshighvoltagebyaccumulatingthesevoltages
[7-10]
.
Sincetheinputvoltageofpowerdeviceusedforeachunitinverteris
low, power device of high insulation voltage is unnecessary and
switching frequency is low,the stress on elementis low in spite of
increased outputcapacity,because current is divided to each power
device.Thelow switchingfrequencymaylowertheoccurrenceofEMC
considerablycomparedto2-levelmode.Asthereisnolimitinselecting
elementduetoswitchingpropertyandthepartsofpowerdeviceinthe

22. - 6 -
character oflarge currentoflow insulation voltage can be acquired
easily,invertercan bedesigned and produced easily.In control,ifthe
specificswitching patternthatcanacquirehighqualitypowersourceis
found,controlby software is possible and can be easily applied to
inverterofotherarea
[7-10]
.
However,multilevelinverteralsohasdemerit.Manypowerelements
arenecessarytoincreasevoltagelevel.Thoughunitpriceofelementis
reducedaslow insulation voltagepowerdeviceisused,thetotalprice
maynotbequitedifferentfrom thatof2-levelinvertermodeowing to
the increased number.As driving circuitfor independentpower part
increases,powerpartcircuitmay becomplicatedandthecosttomake
circuitmayincreasemoreorless.Further,sourceformanyindependent
power supplies is necessary to accumulate voltage.The multilevel
inverterto be constructed in this papersupplements this demeritby
combining transformer. Multi level can be constructed in diverse
methodsdependingontheinputpowersourceandoutputpowersource,
whose method and control have been researched intensively and
continuously
[11-[15]
.Multilevelinverterand2-levelinverterarecompared
and arranged in the aspect of performance to produce high quality
power.
◈ Levelofoutputvoltage
WhilemultilevelinvertercanraisevoltagewithoutseparateDC-DC
convertor as booster by accumulating voltage level,it needs more
powerdevicethan2-levelinverter.

23. - 7 -
◈ Insulationvoltageofpowerdevice
AsDC linkvoltageofmultilevelinverterislow,insulationvoltageof
powerdevicedoesnothaveto behigh,making iteasy to getpower
device.However,thecompositionofdrivecircuitandpowerpartcircuit
todrivepowerdeviceiscomplicatedthan2-levelinverter.
◈ ReductionofEMC
Assurgevoltageoccurringinswitchingduetolow pressureandlow
speedswitchingislow,theinfluenceofEMC diminishes.
◈ Reductionofswitchingloss
In low frequency switching,theexcessiveswitching loss ofelement
byswitchingisreduced.
◈ Convenientutilization
Aspowerelementscanbecontrolleddifferentlyasperconstructionof
voltagelevel,diversedesignsarepossibleinaccordancewithpurposeof
inverter.However,itisdifficulttoembodythealgorithm tosatisfy the
performancerequiredinthefirstdevelopment.
Forallthesedemeritsofmultilevelinverter,itisrecentlyrecognized
as new area ofinverter,thanks to its meritlarger than demeritin
producing grid-tied powerconvertorin large capacity thatcan output
highAC outputvoltageinrenewableenergygenerationsystem.Further,
itsapplicationareasareexpandinggradually.

24. - 8 -
Forinstance,multilevelinverteris often used forindustrialpower
controller in large capacity
[14]-[15]
, FACT(Flexible AC Transmission
system)
[16]-[23],
the transmission equipmentand transportation equipment
[24]-[25]
.
Multilevelinvertercan be divided into 3 types,depending on the
methodtoaccumulatevoltage.
First,thecontrolofstructureiscomplicated,becausethoughfrequency
applied to allpower device composed of Diode-Clamp Circuit(DCC)
modethatutilizesdiodeclamping voltageasoutputlevelpowersource
issame,many capacitorsanddiodesarenecessarytocreateeachlevel
voltageand thevoltageofcapacitorappliedtoeach DC Link mustbe
controlledandbalanced
[26]
.
Second,thoughmultilevelcanbemadebyusingrelativelylesspower
device in the Flying CapacitorCircuit(FCC)mode thatcreates output
levelvoltagebyusinglevelvoltageofseparatecapacitor,thevolumeof
capacitorincreasestotalvolumeofpowerpartsystem duetoincreased
numberofcapacitortocreatelevel
[27]
.
Third,asinsulatedfull-bridgeinverterisonelevelvoltageandthese
voltages are serially accumulated to make Isolated H-bridge Circuit
(IHC)
[27]
soastoconstruct1moduletypeofpowerpartoutputterminal
which is in the shape ofH-bridge in low voltage,each module has
independentDC Link to getthe required outputvoltage.As a result,
highvoltagecanbeacquiredwithlessnumberoflevel.
Theabove3multilevelinvertercanboostoutputvoltageinfinitelyas
per accumulation number oflevelvoltage theoretically.However,the

25. - 9 -
optimum multilevelshould becomposed,depending on theproduction
cost,complexityofcircuit,simplicityofcontrolandpurpose.Infact,the
following mattersmustbeconsidered to producehigh resolution multi
levelinverterapplicabletogrid-tiedrenewableenergygenerationsystem
withhighcapacity.
◈ Highqualityofoutputvoltageandcurrent:DiminishingTHD(Total
harmonicdistortion)bydiminishingtheharmonicwavecomponent.
◈ Reduction ofswitching frequency:Increasing system efficiency by
diminishingexcessivelosscausedbyhighfrequencyswitching
◈ Reduction ofpowerdevice:Realizing maximum voltageleveland
highqualitypowerbyusingtheleastpowerdevice
◈ Embodying composition and algorithm to realize the lowestcost,
theoptimum volumeandtheoptimum performance
Asstatedabove,outputlevelnumbermustbeincreasedinmultilevel
inverter to reduce THD and the increment ofpower device due to
increasedlevelnumbermatbeweakpointineconomicaspect.Thus,it
is necessary to increase the levelnumber of output voltage while
minimizing thenumberofpowerdevice.Itisrequiredtocommercialize
multilevelinverterto connecthigh quality powerto system without
excluding easy production andexcellentperformance.Further,itisalso
necessary to develop high resolution grid-tied multilevelinverterthat

26. - 10 -
containstheown controltechnology ofmultilevelinvertertechnology
as well as the technology which may cope with the intermittent
accidentthatmayoccurinthesystem.
1.2Objectiveandcontentsofresearch
1.2.1Objectiveofresearch
The objective ofthis paper is to decide and produce structure of
30kW high resolution grid-tied multilevelinverterwhich is the unit
capacity thatmay realize the highestprofitin generation ofgrid-tied
renewable energy generation system to experiment the feasibility of
proposedstructureandperformanceofproducedproductsby comparing
withtheperformanceoftheexisting2-levelinvertersoastoverifythe
economic aspect,easiness in production and excellence in performance
and tocheck potentialofcommercialization ofhigh resolution grid-tied
multilevelinverterwith largecapacity asstructurethatcan beeasily
applied to theincrementofunitcapacity in orderto contributeto the
localization ofhigh resolution grid-tied multilevelinverterwith large
capacity thatmayadvancetothepowerconvertormarketofrenewable
energy generation system which is occupied by advanced foreign
products.

27. - 11 -
1.2.2Contentsofresearch
To commercialize high resolution grid-tied multilevelinverterwith
large capacity,this paperproposes multilevelinverterthatuses low
frequency 3 phasetransformerinstead ofPWM which wasused in 2
level inverter and checks the possibility of commercialization.
Supplementing the generaldemeritofthe existing multilevelinverter
anddeciding thestructurethatcanfullyutilizethemeritofmultilevel
inverter,thispaperembodiestheminimum THD toproducehighquality
power by using highestproximity space vector modulation technique
[20]~[48]
andmakesitasprogram torealizecontrolbysoftware.
Tocreatetheindependentmultilevelvoltage,low frequency 3phase
insulated transformerisused toaccumulatevoltageon outputterminal
soastomakecomposition with themostproperpowerdevicedecimal
andmultilevelnumberfortheoreticexplanation.Then,thefeasibilityof
multi level composition is verified by simulation and production
experiment.
Thispapercompletesphasecalculation methodthatusesPLL (Phase
Locked Loop) mode, the stable synchronization technique that is
indispensabletopowerconvertorusedinthesystem andhighresolution
grid-tied multilevelinverterwhich is properto technicalexamination
and regulation on grid-tied technology,system operation program and
sequence composition of renewable energy generation system in the
grid-tied unit inverter group.The feasibility of multilevelinverter
structure and the highest proximity space vector controltechnology

28. - 12 -
composedinthispaperisverifiedbycomputersimulation.
Thefeasibility ofcommercializing highresolutiongrid-tiedmultilevel
inverterproposed in thispaperisverified by designing and producing
the experimental prototype (for each elements) of high resolution
grid-tied multilevelinverterwith 30kW renewable energy generation
system towhich structureand controltechnology verified by computer
simulation are applied and experimenting through connection to
commercialpowergrid.
1.3Compositionofthepaper
Thispaperiscomposedasfollows.
ChapterⅠ Introduction
Theoverallresearch trend,research objective,research contents and
the composition of research paper on the grid-tied inverter are
described.
ChapterⅡ Multilevelinverter
Thebasicoperationprincipleandtypesofexistingmultilevelinverter
is divided and the meritand demeritofeach type is described.With
respectto themodulation techniqueapplied to multilevelinverterand
grid-tied technology ofgrid-tied inverter,the domestic and overseas
standardandessentialprotectivetechnologyarearranged.

29. - 13 -
ChapterⅢ Theproposedgrid-tiedhighresolutionmultilevelinverter
Theregularspacevectormodulationmethodusedtothestructureand
controloftheproposed3phasemultilevelisdescribedandtheoverall
lossiscalculated by analyzing thelossofeach factoroftheproposed
multilevelinverter.Theanti-islandingmethodthatisessentialfunction
ofgrid-tiedinverterisapplied.
ChapterⅣ Computersimulation
Usingproposedcontroltechnique,themovementofcomposedgrid-tied
multilevelinverterandthechangeinoutputwaveform issimulatedper
alteration in modulation rateby Matrap toverify technicalpropriety of
controltechniqueandmulticomposition.
ChapterⅤ Experimentandinvestigation
Toproducetheproposedmultilevelinverter,thedesign factofeach
factorand30kW grid-tied high resolution multilevelinverterproduced
as experimentalprototype are connected to system so as to check
operationstateperchangeinloadandtorecordthefact.
ChapterⅥ Conclusion
Theperformanceiscomparedthroughresultofexperimentalprototype
experimentand the catalog on which the property of2 levelinverter
launched in the existing marketis described.Itis confirmed thatthe
performanceofproposed modeisnotinferiortothatofexisting mode

30. - 14 -
andthereisnoadditionalcontroltechniqueordifficultyinproductionor
technicaldifficulty in the operation to produce by increasing the unit
capacity. Then, the possibility of commercializing high resolution
grid-tiedpowerconvertorwithlargecapacityispresented.

31. - 15 -
CHAPTERⅡ
MULTI-LEVEL INVERT
2.1Theprincipleofmultilevelinverter
Thefigure2.1showstheconceptofmultilevelinvertersystem which
takesDC voltage Vdc
asinputand3phaseasoutput.Asthecapacitor
connected serially isused asenergy tank thatissupplied to inverter,
severalnodesareprovidedtoconnectmultilevelinverter.Ifcapacityof
capacitorusedhereissame,thevoltage Em
willbesameasdescribed
inthefigure.Then,Em
canbedescribedasfollows.
Em=
Vdc
m-1
(2.1)
where,m referstolevelnumber.
Generally,the levelnumber of multi levelinverter refers to the
numberofnodesconnectedtoinverter.InFigure2.1,m nodes(from 0
to m-1)areconnectedtomake m -levelinverter
[30]
.
AsshowninFigure2.1,m-1seriallyconnectedcapacitorswiththe
samecapacity arenecessary to compose m-levelinverters.In Figure
2.1,the voltage, Vag
, Vbg
, Vcg
ofinverter outputphase indicate the

32. - 16 -
outputnode ofeach phase,a,b,cphaseand the voltageof g,the
negativepoleofDC powersourceofinputterminal.
Fig.2.1Schematicof3Phasemulti-levelinvertersystem
Then,thephasevoltageon theoutputterminalload,VaN
,VbN
,VcN
canbedescribedinthefollowingformula.






VaN
VbN
VcN
=
1
3






2-1-1
-1 2-1
-1-1 2






Vag
Vbg
Vcg
(2.2)
In Figure2.1,V1
,V2
,…,Vm
indicatethevoltageofeach nodeand
I1
,I2
,…,Im
indicatethecurrentofeachnode.Thus,theoutputphase
voltageofinverter,Vag
,Vbg
,Vcg
isdecidedby selecting each nodeand
nodecurrentdecidesphasecurrentIaN
,IbN
,IcN
by loadon theinverter
outputterminal.
TheFigure2.2shows1standard phasevoltageandinverteroutput

33. - 17 -
voltageintheoutputvoltageof5-levelinverter.
Fig.2.2Outputvoltagewaveform of5-levelinverter
The Figure 2.2 shows schematization ofvoltage thatcan be output
from ideal inverter where voltage level is acquired ideally on the
positiveandnegativepartofaphase.Foractualimplement,eachswitch
mustcompose by using elementwhere currentmay flow to positive
direction.
Given thisfact,thestructureofmultilevelinvertermustbeableto
create the mostlevels by using elements in smallnumber and the
controlfrequencyofeachswitchingelementmustbeaslow aspossible
tomaximizeefficiencyofinvertersothatstructuremustbedesignedto
lowerswitching loss ofswitch.In this sense,multilevelinverteris
researchedintensively.

34. - 18 -
2.2 The types and structure ofexisting multilevel
inverter
Powerconvertersystem thattakesDC powersourceasinputpower
andconvertsittoAC powerbyusingitslevelisfrequentlyresearched
thesedays.
Fig.2.3ClassificationofDC-AC inverter
Insimplestructure,itcanbedividedinto2levelinverterthatsimply
takesDC voltage0V and +Vdc
or0V and -Vdc
only asoutputlevel
and multilevelinverter thattakes levelover 2 levelas output.As
showninFigure2.3,multilevelinverterhasbeen developedindiverse
formsdependingonthestructureorform.
Therepresentativestructuresofthismultilevelinverteraredivided

35. - 19 -
intodiode-clampmethod,improveddiode-clampmethod,flying-capacitor
method,H-bridgemethodandCascademethod.
Fig.2.4Structureofdiode-clampmulti-levelinverter

36. - 20 -
2.2.1Diode-clampmultilevelinverter
Generally,Diode-clamp multi-level inverter(DCMLI) is a structure
where m levelscan be made in thephasevoltageofinverteroutput
terminalifDC powersource ofinputterminalis composed of m-1
capacitors.TheFigure2.4shows1legof5-levelDCMLIthatcanmake
5levels
[31]
.Ifm-levelisformedinDCMLIstructureasshowninFigure
2.4,the numberofcapacitors necessary forinverteris m-1,thatof
switchingdeviceis 2(m-1),thatofclampingdiodeis (m-1)×(m-2).
Analyzing DCMLIin Figure2.4,4 capacitorsareconnected to input
soastoembodyinverterin5-level.Ifthecapacityofusedcapacitoris
same,thevoltageon each capacitorisexactly divided into1/4oftotal
voltage,Vdc
.Then,the voltage stress applied to each elementis also
limitedto Vdc/4whichisdividedtoonecapacitorbyclampingdiode.
Table2.1 showsthestateofswitch thatcreatestheoutputvoltage
levelofDCMLI.InTable2.1,thestateofswitchin1meansconduction
and 0 means blocking.The switch state in Table 2.1 shows that4
switches must be on always when outputting voltage in any level.
Pairedin(Sa1,Sb1),(Sa2,Sb2),(Sa3,Sb3),(Sa4,Sb4)ateachphase,eachswitch
hasreciprocating switch combination wheresimultaneousconduction or
simultaneousblockingneveroccurs,leavingswitchingonlyonceineach
cycle.Asexplainedabove,thesecombinationsneedtheconduction of4
switchesallthetime.
In conclusion,thefeatureofDCMLIcan belargely summarizedin 3
categories.

37. - 21 -
Table2.1Switchstateofdiode-clampmulti-levelinverter
OutputV0
SwitchState
Sa1 Sa2 Sa3 Sa4 Sb1 Sb2 Sb3 Sb4
V4=Vdc
1 1 1 1 0 0 0 0
V3=3Vdc/4 0 1 1 1 1 0 0 0
V2=2Vdc/4 0 0 1 1 1 1 0 0
V1=Vdc/4 0 0 0 1 1 1 1 0
V0=0 0 0 0 0 1 1 1 1
First,thenumberofclampingdiode
Though each switch requires rated voltage of Vdc/(m-1),clamping
diodes to which differentvoltage is applied,depending on the switch
combination have different reverse voltage.Thus,using diode with
differentreverse voltage is notgood economically and practically.To
reduce such inconvenience,the reverse voltage ofclamping diode is
composedassametothatofswitchandthenumberisexpressedinthe
followingformula.
ND= (m-1)×(m-2) (2.3)
When trying toincreaselevelin thecaseofDCMLI,thenumberof
diodes increase considerably as stated in the formula 2.8.Thus,this
modeisnoteasilyapplicablewhenalotoflevelisrequired.

38. - 22 -
Second,theratedcurrentofswitchelementisnotsame.
AsstatedinTable2.1,whenswitchisinconduction,levelvoltageat
eachpositionistotallydifferent.Forinstance,Sa1
isinconductiononly
whenoutputvoltageis V0=Vdc
andsoisswitch Sa4
onlywhenoutput
voltageis V0=0.Likethis,whenvoltageisdifferentinthesameload,
the quantity of current that flows through switch element varies,
thereby changing theratedcurrentofeach element.Thus,considerable
inconvenience may be caused ifrated currentis used by connecting
switchelementinparallelatthesameratedvalue.Liketheabovecase,
thismaybeimpropertothesystem thatneedsmuchlevel.
Third,imbalanceofcapacityvoltage
Asamatteroffact,thesamecapacitorvoltagepropertyordischarge
propertycannotbefoundandthisminutedifferencemakesthequantity
ofcurrentsupplied from capacitordifferent.As the discharge time of
eachcapacitorchangesowing tothiscause,voltagebetweencapacitors
is in unbalance.If this is used in the state,switching function is
necessarytoremoveimbalanceinfunction,therebycausing difficultyin
control.Thismaybealsoobstacleinfreeincrementoflevel.

39. - 23 -
Fig.2.5Structuresofimproveddiode-clampmulti-levelinverter
2.2.2Improveddiode-clampmulti-levelinverter
As different reverse voltage is applied to clamping diode,DCMLI
solved theproblem by connecting many diodeswith samerated value
serially to increase rated value.Owing to increased number,however,
the property is notsame in 100%,leaving inaccurate distribution of

40. - 24 -
voltage among serialdiodes.The structure ofdiode-clamp multi-level
inverterimprovedtosolvethisdemeritisshowninFigure2.5
[32]
.
ComparedtoexistingDCMLImode,thenumberofdiodeorswitchof
Improved diode-clamp multi-level inverter(IDCMLI) do not diminish.
However,theratedvalueofallswitchesandcapacitorsaresameasall
switches arecomposed so thatthey can be switching in unitmodule
modeand2-levelswitchingisdoneofeachunitmodule.
Viewing only the movementunitthatis separately composed,each
modulecontains m-1elementsin therightdirection orreturn current
routeandperformsthemovementofgeneral2-levelinverter.However,
someconditionsmustbesatisfiedtomeetthemovement.
First,m-1adjacentswitchesthatcan bein conduction allthetime
arerequired.
Second,ifcomposedof2adjacentswitches,externalswitch mustbe
alwaysinconductiononlywheninternalswitchisinconduction.
Third,ifcomposed of2 adjacentswitches,internalswitch mustbe
alwaysblockedonlywhenexternalswitchisblocked.

41. - 25 -
Fig.2.6Structureofflyingcapacitormulti-levelinverter
2.2.3Flyingcapacitormulti-levelinverter
Showing thestructureofflying capacitormulti-levelinverter(FCMLI),
Figure2.6describesonelegofinverterthatcanoutput5-level.
To output 5 levels,the voltage of each capacitor bank must be
chargedby Em
,2Em
,3Em
,4Em
[27]-[29]
.

42. - 26 -
In Figure2.6,allcapacitorbanksexcept 4Em
arein floating andthe
inverterwiththisstructureiscalledflying-capacitorinverter
[33],[34]
.
TheplacementofFCMLIswitchshowsthattheplacementisopposite
tothatofDCMLImodeinFigure2.4.
Table2.2Switchstateofflyingcapacitormulti-levelinverter
OutputV0
SwitchState
Sa1 Sa2 Sa3 Sa4 Sb4 Sb3 Sb2 Sb1
V4=Vdc
1 1 1 1 0 0 0 0
V3=3Vdc/4 1 1 1 0 1 0 0 0
V2=2Vdc/4 1 1 0 0 1 1 0 0
V1=Vdc/4 1 0 0 0 1 1 1 0
V0=0 0 0 0 0 1 1 1 1
The shape ofembodimentofvoltage levelin FCMLIshows thatit
hasswitching function similartothatofDCMLI.m levelsareformed
on thephasevoltageand 2m-1levelsareformedon thelinevoltage.
Ifcapacitorandswitching elementhasthesamevoltagecharacter,the
numberofcapacitorto make m-levelis m-1.Thus,the numberof
capacitornecessary foreach phase can be described in the following
formula.
NC= ∑
m
i=1
(m-i) (2.4)

43. - 27 -
Ifformula2.4isapplied,10capacitorsarenecessaryforeachphaseto
embody5-levelinverter.Table2.2showsthestateofswitchwherethe
voltagelevelin 5-levelFCMLIofFigure2.6 isformed.In Table2.2,
the condition ofswitch in 1 refers to conduction and 0,blocking.In
fact,4conditionscanbecombinedtogetvoltageof3Vdc/4and Vdc/4in
levelby FCMLI and there are 6 combinations to output 2Vdc/4.If
switchiscombinedasTable2.2,however,eachswitch isin switching
onlyonceineachcycle.
Figure 2.6 shows thatthe largestfeature ofFCMLIis no need of
clamping diode and the parallelvoltage ofcapacitor.Further,there is
room in the levelofinnervoltage,because there is room in voltage
levelif2ormoreeffectiveswitchescanbecombinedintoone.
Thisroom involtageregulatesthecapacitorvoltage.AsFCMLIisin
thestructurewheremany combinationsarepossibletoallow capacitors
charge and discharge preferentially,capacitorvoltage can be regulated
flexibly so thatthe value can be maintained properly.Such flexibility
easily regulatescapacitorvoltageandkeepsitinpropervalue.Further,
it is in the structure where switch can be combined to generate
intermediatevoltage(3Vdc/4,2Vdc/4,Vdc/4),theregulating voltagelevel
foronecycleorseveralcyclestomakethechargeanddischargetime
ofcapacitorparallel.Thus,this structure is generally very suitableto
converteffective power.However,the combination ofswitch is very
complicated to convertthiseffectivepower.So frequency ofswitching
mayincreasetomakecontroldifficult.

44. - 28 -
Fig.2.7StructureofH-bridgemulti-levelinverter
2.2.4H-bridgemulti-levelinverter
Amongtheexistingmult-levelinverters,thisisoneofthetypesmost
widely used for single-phase. This mode serially connects several
H-bridgein low voltageto composeunitmodulethathasindependent
DC-Link.Figure2.7showsHBMLIwhichcanoutput9-levelvoltageas
structurethathas4H-bridgeunitmodules.

45. - 29 -
Figure2.8 shows operationalprincipleofFigure 2.7
[9],[34],[35]
.Different
from the previous diode clamp mode orflying capacitormode,serial
multi-levelinverter in Figure 2.7 does not need clamping diode or
additionalcapacitor.Thus,the powercircuitcan be composed simply
comparedtoothermodesandtheexpansibilityoflevelisexcellent.
Inv1
Inv2
Inv3
Inv4
VO
-Vdc
Vdc
-Vdc
Vdc
-Vdc
Vdc
-Vdc
Vdc
Vdc
2Vdc
3Vdc
4Vdc
0
Inv4
Inv3
Inv2
Inv1
t
t
t
t
t
VO
-4Vdc
-3Vdc
-2Vdc
-Vdc
(a)Numberofleveleachcell (b)Switchingfunctionfor9level
Fig.2.8Operationalprincipleof9-levelHBMLinverter
Asthereisnoproblem inthebalancecontrolofDC linkvoltageand
powerpartcan be in fullmodule even when levelis increased,itis
very suitabletoembody inverterin high voltagethatuseselementin
low voltage.
The operationalwaveform ofeach partin Figure 2.8 shows that

46. - 30 -
9-levelvoltageof4Vdc
,3Vdc
,2Vdc
,Vdc
,0,-Vdc
,-2Vdc
,-3Vdc
,-4Vdc
is
outputby4H-bridgemodule.ComparedtoDCMLIorFCMLIexplained
before,such H-bridge multi-levelinverterhas morelevelthatcan be
outputby switching elementin the same number.Ifthere are many
levels,dV/dtdecreasesso thatoutputvoltagewaveform closeto sine
wavewith low distortion ofwaveform can beacquired.However,the
increaseoflevelnumberneedstheprovision ofinsulated directpower
sourceinHBMLI.Thus,levelcannotincreaseinfinitely
[8],[9],[35]
.
Figure2.8(b)describesswitching function to get9 leveland output
voltageisindicatedasthesum ofoutputvoltageofeachmodule.
+
Vdc
-
Vo
+
-
+Vdc
-Vdc
A1
A2
A1 A2
Vo
p
1a 1ap-
1ap+ 12 ap-
p
Fig.2.9H-bridgeswitchingfunction
Figure2.9describesswitchingfunctionformoduleof1H-bridge.The
outputofswitchingissimilarsquarewaveandisnotaffectedbypulse
widthofoutputwaveform.Sinceitisinconductionby180°always,the
stressbycurrentflowinginswitchingelementissame.

47. - 31 -
2.3Modulationmethod
2.3.1Staircasemodulationmethod
The method commercialized first as modulation method to control
multi-levelinverterisstaircasemodulation method describedin 2.10.It
isthemethodthatrealizessinebymakingstaircaseclosesttostandard
sinewave
[9],[33][34]
.
AsstaircasediscontinuousDC voltageofcontinuousanalog standard
voltage,staircasemodulationmethodiscalledquantization-themethod
ofapproximation.
t
t
t
0
Vdc
2Vdc
3Vdc
4Vdc
-2Vdc
-3Vdc
-4Vdc
-Vdc
t
0
0
0
0
V1
V2
V3
V4
Vo
t
a1
a2
a3
a4
Fig.2.10Switchingfunctionsandoutputwaveform ofstaircasemodulation

48. - 32 -
As staircase modulation method does notuse high speed switching,
low speedGTO withlargecapacityisavailable.Thoughthisissuitable
toproduceinverterofhighvoltagewithlargecapacity,manylevelsare
necessary to getpowersourcewith good quality.However,as power
lossduetotransientlosscausedby high speedswitching isless,itis
mostsuitable to controlpower with large capacity.Itis modulation
methodthatcanbeappliedtopowertransmissionanddistribution.
In Figure2.10,outputvoltageisdecided by each α ofswitching.In
thenthfrequencycomponentofm-levelthatusesstaircasemodulation,
levelin odd numberrefersto formula (2.5)and levelin even number
referstoformula(2.6).
Vn,m=
4Vdc
nπ ∑
m-1
2
i=1
cos(nα i);n=odd (2.5)
Vn,m=
4Vdc
nπ


 1
2
+ ∑
m
2
-1
i=1
cos(nα i)



;n=odd (2.6)
If m-levelis outputby using the above formula,itis possible to
eliminate fundamentalwave and m-1 harmonics components.By the
way,itisrequired to interpretnon-linearequation and to calculatein
real-timesoastoeliminatespecificharmonicscomponents.Sincethisis
impossible,however,modulation method to which equalarea method
thatsupplementsthispointisappliedisrecentlyproposed
[9],[33],[34]
.
In the past,specific harmonics components could be eliminated by
properlycontrollingswitchinganglethroughthecalculationoffrequency

49. - 33 -
componentofeachharmonicsinformula(2.5)and(2.6)
[39],[40]
. InFigure
2.9,formula (2.5)can beused aslevelisin odd number.Theoutput
voltage(Vo
)canbedescribedbyformula(2.7).
Vo=
4Vdc
nπ (cos(nα 1)+cos(nα 2)+cos(nα 3)+cos(nα 4)) (2.7)
however,0≤α 1≤α 2≤α 3≤α 4≤π/2
Outputvoltage can be controlled by controlling each switching, α1
,
α2
, α3
, α4
and the factthat4 switching can be controlled shows
harmonics components besides fundamentalwave can becontrolled.In
thecaseof3phaseinverterwheremultipleof3,harmonicscomponents
do not appear,the 5,7 and 11th harmonics components must be
eliminated and can be described in the following formula to eliminate
fundamental wave components and the 5, 7 and 11th harmonics
components.
cos(α 1)+cos(α 2)+cos(α 3)+cos(α 4)=
ma
π
4
cos(5α 1)+cos(5α 2)+cos(5α 3)+cos(5α 4)=0
cos(7α 1)+cos(7α 2)+cos(7α 3)+cos(7α 4)=0
cos(11α 1)+cos(11α 2)+cos(11α 3)+cos(11α 4)=0 (2.8)
where,ma
referstomodulationrate.
Theaboveformulasconvertsswitchingangleintodata,theswitching

50. - 34 -
anglethatisusuallycalculatedinadvancebecausereal-timecalculation
isimpossibleandstoresandusesthem intheform oflook-uptable.In
Figure 2.9,the effective values ofoutputvoltage are given as below
dependingontheswitchingangles,α1
,α2
,α3
,α4
.
Vorms
= Vdc
2
π (8π-α 1-3α 2-5α 3-7α 4) (2.9)
Sincethevaluesofswitchingangle,α1
,α2
,α3
,α4
togettheeffective
valuestobecontrolledbyformula(2.9)arecountless,theformulacanbe
usedtoeliminatespecificharmonicscomponents.However,itcannotbe
easily applied,because real-time calculation is impossible.Equalarea
method supplements the above problem, namely, specific harmonics
components are not eliminated, but THD of output waveform is
improved.
α4
V
t
Vdc
2Vdc
3Vdc
4Vdc
θ4α1 θ1α2 θ2 α3 θ3
0
Fig2.11Thecalculationmethodofswitchingangleusingequalarea
Figure 2.11 shows each switching angle controlmethod thatuses

51. - 35 -
equalarea method.The formula to make shaded area same in the
intersection of each levelvoltage and sine reference voltage is as
follows
[41]
.
θj= sin
-1
(
Vdc
Vp
) (2.10)
In thisformula,Vp
isthemaximum pointofreferencevoltageand if
there is no jth intersection thatsatisfies formula(2.8),intersection is
calculatedbythefollowingformula.
θj=
π
2
(2.11)
The intersection calculated by formula(2.11)and switching anglethat
makes 2 area indicated in Figure 2.16 same is decided by formula
(2.12).
α j= θj-1-
1
Vdc
⌠
⌡
θ j
θ j-1
(VPsin(θ)-jVdc)dθ (2.12)
however,θ0
=0
Sinceequalarea method can becalculated by trigonometricfunction
only asexplainedabove,itcan becontinuously controlled by real-time
calculation.

52. - 36 -
2.3.2Sinusoidalpulsewidthmodulationmethod
Sinusoidalpulse width modulation (SPWM),the modulation method
mostly used for commercial inverter is the method that performs
modulation by comparing standard signalofsinusoidalform andreturn
wavein trigonometricform.Mainly used for2,3-levelinverter
[42]
,the
application to multi-levelis divided by the use ofreturn wave and
standard signal. Then, diverse return waves are synchronized for
sinusoidaleffect
[43]
onthestandardsignaltorealizeSPWM withmulti
standard signalmethod with excellentharmonicspropertiesin thelow
frequency areaby using multireturn wavemethodandmany standard
signals.
2.3.3.Spacevectormodulationmethod
Differently from PWM method thatconsiderseach phaseofinverter
independent, modulation method that uses space vector considers
inverterasonesinglebody forswitching.Compared toPWM method,
space vectormodulation method is controlled easily thatitis widely
usedfor3phasepowerconvertor
[9],[13],[45],[46]
.
Figure 2.12 shows space vector thatcan be expressed by 3-level
multi-levelinverter.IntheFigure,000~222show inverteroutputofeach
phase
[48]
where ‘0’means 0 levelvoltage,‘1’means Vdc
levelvoltage
and‘2’meansoutputof-Vdc
levelvoltage.Forinstance,in210,aphase

53. - 37 -
means -Vdc
level,bphasemeans Vdc
leveland cphasemeans0level.
Fig.2.12Spacevectorrepresentationofathree-levelinverter
InFigure2.12,Vref
referstothestandardvoltagetobeacquiredand
the method to output by using average technique by using vector
closetostandardvectorisfundamentaltogetVref
inthespacevector
modulation method.In Figure2.12,thevectorclosetostandardvector
is 200,210,211 vectorin the selected triangulararea.Ifoutputting
standardvectorby properly using thesevectors,high frequency noise
ofoutputvoltagecanbeminimized,whilekeepingthebalanceofinput
capacitor voltage[13]
.In spite ofthis method,however,the capacitor

54. - 38 -
voltage is not balanced completely due to non-linear property of
switchingelement.Inordertosolvethisproblem,manyresearchesare
recently in progress,whose representative example is voltage control
by wasteloop.Theotherconcernsofspacevectormodulation method
isto decidetooutputwhich vectorclosetodesignated timeforhow
long time when standard vectoris outputto maintain quality ofthe
bestoutputvoltage.Thisisintensively researched
[46]-[50]
.Recently,the
research that can minimize the switching frequency of switch by
calculatingthevectorclosesttooutputvectorisinprogress.
2.4Grid-TiedTechnology
As technology to produce grid-tied power convertor must be
operatedinconnectionwithpowersystem,theeffectiveandineffective
power controlas wellas the function of cooperative technology to
protectsystem is required,differently from generalindustrialinverter
technology.Especially,itis required to properly cope with the boost/
downfallofpower system,shortcircuit,ground fault,etc.The high
resolution interface thatcan overcome them and supply high quality
poweris necessary.As the proposed grid-tied high resolution power
convertor is meaningfulsmallpower plantofphotovoltaic generation
system with capacity of 30kW,it is required to consider technical
stabilitypergrid-tiedandrelevantspecificationandinstallation/operation

55. - 39 -
Table2.3Protectivetechnologyofpowerconverterforgrid-tied
Cause Measure
◈ Accidentingrid-tiedinverter
ConverterorInverterFault
WrongVoltageandfrequency,
OccurrenceofHarmonic
Over voltage or over current in
the case of ground fault or short
circuit
◈ Immediatelyseparatinginverterfrom
grid
Separatingbyusingowninverter
Detectingovercurrentorovervoltage
Separationpoint
Power source, Inverter, grid-tied
point
Consideringseparationorder
overvoltage,overcurrent,frequency,
over current of harmonics,auto load
blocking
◈ Accidentingrid-tied
Overvoltageorovercurrentinthe
caseofgroundfaultorshortcircuit
◈ Anti-islandingofgrid-tiedinverter
Separating grid-tied point in the
accidentofpowergrid
Using detection ofovercurrent,over
voltage
◈ When auto-reclosing method is
applied
Using Feeder CB (Circuit
Breaker)orauto-recloser
Auto-synchronousreclosingincase
ofrestorationfrom powerfailure
◈ Grid-tied inverter is completely
separated from bus by CB or auto-
recloser.
When grid-tied is restored,grid-tied
startsbyauto-reclosermethod.
◈Anti-islandingforgrid-tiedinverter
Powerfailuredueto work in the
gridside
Powerfailureofgrid caused by
actofGodoraccident
◈ Preventing possibility ofindependent
driving caused by failure in grid tie
work,impossibility ofdetecting current
ordelayingridaccident,powerfailure
Using power failure factor like
over/shortvoltage,over/shortfrequency
method.In Photovoltaicgeneration system,itisrequired to consider
technicalstabilitypergrid-tiedandentiresystem.

56. - 40 -
CHAPTER Ⅲ
THE PROPOSED GRID-TIDE HIGH
RESOLUTION MULTI-LEVEL
INVERTER
3.1 The structure of proposed 3 phase multi-level
inverter
Thetypesandstructureofmulti-levelinverterandthemerit/demerit
ofeachmethodaredescribedinChapter2.2.Thispaperproposed IHMI
that uses 3 Phase 60Hz transformer by applying the structure of
insulated H-Bridge multi-levelinverter(IHML,Isolated H-Bridge Multi
-Level).Sinceoutputterminalisinsulatedbyinsulatedtransformer,this
structuredoes notneed multiple independentpowersources,additional
diode and capacitor.Figure 3.1 schematizes the structure of30kW 3
phase multi-inverterusedinthisresearch,showingthatitiscomposed
of3three-phaselow frequencytransformerand9H-Bridgesforvoltage
accumulation.Thisstructurecanindependentlyoperateintheform same
tothe3three-phaseIHML inverterwhichisconnectedserially.Asthe
switching frequency ofeach IHML inverteris properly combined,this
structuremayproducehighqualitypowerinhighefficiencyevenwhen
THD islow.

57. - 41 -
Fig.3.1Thestructureof3phasemulti-levelInverter

58. - 42 -
3.2 The controlmethod ofproposed 3 phase multi-level
inverter
3.2.1Regularspacevectormodulation
Spacevectormodulation(SVM)isthemethodthatexpresses3phase
in2-dimensionalspacevectorbyusing conversionformula(3.1),where
theconversionprocessisasfollows.
v(t)=
2
3(vaS(t)+a∙vbS(t)+a
2
∙vcS(t)) (3.1)
where,
vaS(t)=
VaS(t)
Vdc
(3.2)
a=e
j(2/3)π
= -
1
2
+j∙
3
2
(3.3)
VaS(t) is the standard signalfor vaS(t) which is the sum ofinput
voltage ofIHCML inverter A phase transformer.Using formula(3.1),
(3.2)and(3.3),3phaseisexpressedincomplexplaneasformula(3.4).
v(t)=vα +j∙vβ
(3.4)
Informula(3.4),vα
,vβ
asthefactorofv(t)on α,β axisincomplex
planeisdefinedbyformula(3.5)and(3.6).

59. - 43 -
vα =
1
3
(2∙vaS(t)-vbS(t)-vcS(t)) (3.5)
vβ =
1
3
(vbS(t)+vcS(t)) (3.6)
In inverter,the outputofH-bridge, vaS(t), vbS(t),vcS(t) are in the
form ofintegerlike0,1,-1,etc,butvα
,vβ
calculatedbyformula(3.5),
(3.6) do notbecome integer.However,if vα
, vβ
are normalized by
multiplying with3and 3,vα',vβ'whicharenormalizedlikeformula
(3.7)alwayshavetheintegervalue.
v'(t)=vα'+j∙vβ' (3.7)
where,vα',vβ'areasfollows.
vα'=3vα =2∙vaS(t)-vbS(t)-vcS(t) (3.8)
vβ'= 3vβ = vbS(t)+vcS(t) (3.9)
Vector vα',vβ'calculated by theaboveformula haveintegervalue
and the vector(thatcan be output)ofIHCML inverterthatuses 1
transformerisexpressed in 19normalized vectorsasshown in Figure
3.2with3-level.
If1threephaselow frequencytransformerand3H-Bridgesareused
ininverterstructureinFigure3.1,vectordrawingof3-levellikeFigure
3.2can becreated.Ifmorethan 2transformersareused,theconcept

60. - 44 -
thatexpandsthevectordrawinginFigure3.2canbeused.
Fig.3.2Normalizedvectorsof3-levelIHCMLinverter
When 2 transformers are used and the transforming rate of 2
transformersissupposed tobe Tturn
,Figure3.3showsvectordrawing
thatcan be outputwhen Tturn
increases from 1 to 4 and the output
levelchangedfrom 5to11.Asshowing in theFigure,iftransforming
rate Tturn
exceeds5,outputvectorisdistributeddiscontinuouslyandthe
maximum valueofTturn
becomes4.

61. - 45 -
(a)Tturn
=1(5-level) (b)Tturn
=2(7-level)
(c)Tturn
=3(9-level) (d)Tturn
=4(11-level)
Fig.3.3Normalizedvectorsofcascade3-phaseIHML
Fig.3.4NormalizedvectorofIHMLusingthreetransformer
To lowerharmonics rate ofoutputvoltage as low as possible,this

62. - 46 -
paperuses3transformerssoastoensurethemaximum levelandthe
transformingofeachtransformerissenttoturn-ratiosof1:4:16.
Asshown in Figure3.4,however,discontinuousvectorexistsin the
outermostpartofspace.Thus,if3IHML invertersareused in serial
connection,maximum of36-levelcanbeoutputexcludingbreakpoint.
3.2.2Vectorcontroloftheproposed3phasemulti-levelinverter
Sincestandard signalforinverteroutputiscontinuoussinefunction,
differently from H-Bridgeoutput,standardsignalvector v'ref
cannotbe
integerallthetime.
Fig.3.5Representationofthelocuswithhighestproximityvector
In IHML inverter,vectorwith the lowesterrorforstandard vector
mustbeoutput.AsexpressedinFigure3.5,thevectoroftheproximity
IHML inverterforarbitrarystandardvector v'ref
is vsel
.When v'ref
isin

63. - 47 -
thehexagonoffigure,allvsel
vectorisoutputfrom IHMLinverter.
In this paper,new method of calculating vector is proposed to
calculate proximate vectorwith low errorwithin designated time and
thecalculationprocessingisasfollows.
Processing 1)Normalization v'ref
ofstandardvector(vref
)ofreference
value
Asalloutputineachphaseofmulti-levelinverterisinteger,vectors
ofchangedaxiscanhaveintegervalueifformula(3.8)and(3.9)isused.
Thus,standardvector vref
isnormalizedtothestandardvector v'ref
on
the α',β'axisbyformula(3.8)and(3.9).
Processing2)Convertingstandardvectorintointeger
Asmentionedabove,thenormalizedstandardvectorv'ref
isnotinteger.
Thus,itisrequiredtoconvertitintointeger,Fv'ref
,thevectorcloseto
theoriginofα',β'axis.
Fvα =fix(v'α)
Fvβ =fix(v'β) (3.10)
As shown in Figure 3.6,normalized vectorbecomes vectorofinteger
valueclosetoorigin.
Processing3)Decidingexternalfunction
Toselectproximityvector,thispaperdefinesthe1stexternalfunction
and ha
, hb
are defined as shown in Figure 3.6 to determine the

64. - 48 -
inclination and offsetofouter covering function.The defined ha
, hb
mustsatisfy the following formula,because thelength ofone side of
squareis1andtheareaofallhexagonsmustbesame.
ha+hb=1 (3.11)
Further,absolute value ofinclination ofouter covering function is
expressedby ha
,hb
whicharedefinedasabovebyformula(3.11).
|a|=rate=(ha-hb) (3.12)
(a)Case1 (b)Case2
Fig.3.6Calculateratepolarityofoutercoveringfunctionof
thehighestproximityvector
In the outer covering function,the inclination may be positive as
shown in Figure 3.6(a) or negative as shown in Figure 3.6(b).As
expressedinFigure3.6(b),itcanbeselectedbysum ofstandardvector

65. - 49 -
Fv'ref
factor which is defined and converted into integer in formula
(3.10).Ifsum ofcalculated Fvα
andFvβ
isevennumber,theinclination
offunction is setto positive as shown in Figure 3.6(a)and ifodd
number,itissettonegativeasshowninFigure3.6(b).
Processing 4)Calculatingoutercoveringfunctionforeacharea
Thea,bvalueofthe1stfunctioniscalculatedwithfollowingformula
by using intersection(x1
,y1
)and ha
,hb
thatdecideinclinationin each
areachoseninthepreviousstage.
if[{(Fvα +Fvβ)%2}≡0]
then x1=Fvα
,y1=Fvβ +ha
//even
a=-rate,b=y1-ax1
eles x1=Fvα
,y1=Fvβ +hb
//odd
a=rate,b=y1-ax1
(3.13)
Processing5)Applyingtheboundarycondition
Comparing outercovering function and v'ref
calculated by following
formula,highestproximityvector vsel
decides vl
,or vh
inFigure3.6as
vector.
if{v'β >(av'α +b)}
then vsel=vh
else vsel=vl
(3.14)

66. - 50 -
Processing6)Decidingoutputvector
Setting α', β'axis factor of thehighestproximityvector vsel
selected
in the aboveprocessing as Nα
, Nβ
,itis decided by vh
,vl
which is
decidedasfollowsbytheconditionsofProcessing3,5.
if(Fvα+Fvβ)
then vl=(Fvα,Fvβ), vh=(Fvα+1,Fvβ+1)
eles vl=(Fvα+1,Fvβ),vh=(Fvα,Fvβ+1) (3.15)
Fig.3.7Flowcharttocalculatethehighestproximityvector

67. - 51 -
As which vectorin vl
or vh
is to be outputby highestproximity
vectorvsel
throughtheprocessing5,highestproximityvectorvsel
ofthe
normalized standard vector v'ref
iscalculated by Fvα
,Fvβ
asstated in
formula(3.15).
Sincetheabovehighestproximity vectorcan bemostly selected by
integercalculation andsimplecomparison,thecalculation speedisvery
high.Theaboveprocessingisexpressedintheflowchart-Figure3.7.
3.3 Analyzing loss of the proposed multi-level
inverter
3.3.1Factoroflossinmulti-levelinverter
As shown in Figure 3.8, multi-level power converter system is
composed of factors such as Buck-Boost convertor, multi-level
H-Bridge convertor,low frequency transformer,outputfilter,etc.The
lossofpowerconvertersystem mustbeanalyzed in advanceto draw
thehighefficiencytechniquewhichisthefinalgoalofthisresearch.As
loss is evenly distributed in each component of multi-level power
convertersystem,itiscalculatedforeachfactor.
To begin with,system loss can be largely divided into loss of
reactive elements occurring in reactor,capacitor and transformer and

68. - 52 -
loss of active elements (semi-conductor element)existing in each
component.In reactiveelement,resistanceloss,ironloss,etcoccurand
theabove-mentionedcomponent,namely,Buck-Boostconvertorthathas
2 power semi-conductor switches and 2 diodes and multi-level
H-Bridgeconvertorthathas36powersemi-conductorswitchesand36
internal diodes are used in active element, namely, 38 power
semi-conductorswitchesand38diodeelementsareinuse.
Fig.3.8Thecomponentofmulti-levelpowerconvertersystem
The operation conditions ofthese active elements vary considerably,
namely,duty cycle,switching frequency,permittedvoltageand current.
Further,they may also vary considerably,depending on type ofused
elementorconditionsin the circuit.Sinceswitching pattern may also
resultin many differences,there are countless factors thatshould be
actuallyconsideredforpreciseanalysisoflossinactiveelement.
Eventhough itmay bemuchmorefeasibletoapproachthisproblem
experimentally ratherthan theoretically,theproblem istobeexplained
herein principleforprincipleorinterpretation oflossoccurrence.The
examples ofloss occurring in semi-conductor forpower are turn-on

69. - 53 -
transition loss,turn-offtransition loss,on-state conduction state and
off-state loss.Among the losses,turn-on transition loss and turn-off
transition loss mustbe actually interpreted in consideration ofdiverse
circuitconditions.Sincetheyarenotsodifferent,however,theresultis
not so different even when considering whether element current is
delayedornot.
3.3.2Analyzinglossforeachfactor
Thelossoccurringinthesystem componentmentionedabove,namely,
Buck-Boost convertor,multi-levelH-Bridge convertor,low frequency
transformer,outputfilter,etcaredividedintolossofde-activeelement
andlossofactiveelement.
3.3.2.1Lossof(de-active)element
Inde-activeelement,transformer,inductororcapacitorareconceivable
andresistancelossorironlossmainlyoccur.
3.3.2.1.1Transformerloss
Fortransformerused in thisresearch,directionalcorewasused and
square copper wire suitable for high capacity current was used as
winding toconstructtransformer.Theexamplesoflossoftransformer
are1stand2ndcopperloss,hysteresisandironlossexpressedaseddy
currentloss.Getting thetotalcopperlossfirst,the1stand2ndcopper
lossistotally relatedtothecross-section orlength ofwire.Thus,the

70. - 54 -
expression oftotalcopper loss can be acquired in the 1stand 2nd
windingresistance.
Pcu=I2
rms∙RT
(3.16)
where,RT refers to winding resistance.Further,hysteresis loss is
expressedinthefollowingformula(3.17).
Ph=f⌠
⌡○ HdB = η h∙f∙Bn
m
(3.17)
where ηh
and n is integer per design and the value is given by
designerpergenerallyusedcore.Thevalueofhysteresislossincreases
iffrequencyandmaximum magneticfluxdensityincrease.Calculatedin
consideration oftheresistancevalueforcurrentroutein theiron core,
theresultofeddycurrentlossisdescribedinformula(3.18).
Pe=η e(f∙kf∙Bm)2 (3.18)
where    are integers as per design and the iron loss of
transformercanbeacquiredifcombinedwithhysteresisloss.Theresult
isdescribedintheformula(3.19).
Ph+e=Ph+Pe=η h∙f∙Bn
m+η e(f∙kf∙Bm)2 (3.19)

71. - 55 -
Thetotallossincludingironlossandcopperlossoftransformerfrom
thecontentdescribedaboveisexpressedinthefollowingformula(3.20).
PTR=Ph+e+Pcu=η h∙f∙Bn
m+η e(f∙kf∙Bm)2
+I2
rms∙RT
(3.20)
3.3.2.1.2Inductorloss
Theinductorusedin thisresearch can befoundin outputfilterand
Buck-Boostconvertor.Outputfilteris 3.5[mH]and L ofBuck-Boost
convertoris1[mH].Corelossthatgenerallyoccursininductordepends
onthequantityofcurrentpulsation(ΔI)andcopperlossdependsonthe
effectivevaluewhichisnearlysametoDC component(Imean
)ofinductor
current.As the currentpulsation(ΔI)offilterinductorused in output
filterand Buck-Boostconvertorislow,the change (ΔB)ofmagnetic
flux density is also low.Thus,core loss is low and is sometimes
disregarded in design.In AC inductor, ΔB value increases as high
frequencycurrentalterationislarge.Ascorelossincreasesaccordingly,
bothcopperlossandironlossneedtobeconsidered.Thelossoccurring
in inductoris divided into iron loss expressed as hysteresis loss and
eddy currentlossand resistancelossexisting on theconductor.Then,
ironlossisexpressedinformula(3.21).
Ph+e=Ph+Pe=η h∙f∙Bn
m+η e(f∙kf∙Bm)2 (3.21)
Theaboveformula(3.21)showsthatironlossincreaseswithincrement
offrequency and maximum magnetic flux density.Resistance loss is

72. - 56 -
related to cross-section orlength ofwire and the skin effectas per
frequency mustbe also considered.Though specialwire such as litz
wireisusedsometimestolessenskineffect,squarecopperwireisused
in thisresearch tomeettheproperty ofcurrentwithlargecapacity.It
isnecessary toconsiderusing litzwireinstead ofsquarecopperwire
forwire ofeach reactorforbetterefficiency later.Resistance loss is
generallyexpressedinformula(3.22).
Pcu=I2
rms∙RL
(3.22)
where,RLreferstowindingresistance.
3.3.2.1.3Capacitorloss
Connectedwithsolarcellinparallel,capacitorisusedin many parts.
Itis also used in Buck-Boostconvertor outputterminal.Figure 3.9
showsequivalentcircuitofcapacitor,wherethelossdescribed in ESR
(equivalentseries resistance)existing inside can be acquired,namely,
capacitorloss by capacitorcurrent.Thatis to say,capacitorloss is
givenbyformula(3.23).
Pcap=
1
T
⌠
⌡
T
0
i2
c(t)Rcdt= RESR∙I
2
c-rms
(3.23)
where,lossrelatedtoeffectivevalueofcapacitorcurrentisshownin
capacitor.Especially,the internalR value ofFigure 3.9,namely,ESR

73. - 57 -
(equivalentseriesresistance)varies,depending on theshapeofetching
pit,oxidizeAluminium(Al2O3)orresistancevalueofelectrolyteandthe
shape of paper fiber,generally taking value within 0.015-0.17[Ω].If
capacitoris used in parallelconnection,parallelconnection ofinternal
ESR canreducethevalueofESR.
R E S R
C [F ]
Fig.3.9 EquivalentmodelofcapacitorusingESR
3.3.2.2 Lossofactiveelement
3.3.2.2.1Lossofsemi-conductorelementforpower
Thesemi-conductorelementforpowerin thesystem iscomposedof
powerswitchsuchasIGBT,MOSFET,etcandelementsuchasdiode.
Since diode is notrelated to currentdelay,itis divided into loss in
conductionstateandlossinoffstate.Incaseofsemi-conductorelement
forpower,however,thelossproperty isclosely relatedtothedelay of
passingcurrent.Thus,itisdescribedasfollowsinthisviewpoint.
3.3.2.2.1.1Lossofdiode
Diode applied to power electron must have high rated current,
withstandhighreversevoltageandhighspeedswitchingproperty.

74. - 58 -
(a)Equivalentcircuitof
diode
Di
0 0DV Dv
Dr
1
Slope =
(b)v-ispecificofdiode
Fig.3.10Equivalentcircuitandv-ispecificofpowerdiode
Generally,powerdiodecan be divided into detection diode and high
speeddiode.Especially,fastrecoverydiodepertainstothecaseoflatter
and ultra-fastrecovery diode is used in this research.As shown in
Figure 3.10(a), the loss occurring in power diode is described in
resistance loss ofstatic resistance existing in equivalentcircuitand
lossby drop ofthreshold voltageagainstpotentialbarrierofdepletion
layer.Getting theaveragepowerin onecycleofinstantaneouspower,
the following formula(3.24)shows the occurrence ofloss by average
valueofcurrentoccursin voltagedrop and lossby effectivevalueof
currentininternalresistance.
  



 ×   



× 


  ×    ×
(3.24)

75. - 59 -
3.3.2.2.1.2Lossofsemi-conductorswitch
Thelossofsemi-conductorswitchvaries,dependingondiversecircuit
conditionsanditsexpression variesespecially becauseofcurrentdelay.
The case with currentdelay is fixed as inductiveload and the other
caseisfixedasloadofresistance.
3.3.2.2.1.2.1 Loadofresistance
Since currentand voltage has same phase,currentand voltage of
elementchangesintheswitching transitionsectiontogether.Thus,load
of resistance refers to the case where mutually overlapping section
exist.AsshowninFigure3.11(a),totallossfor1cyclecanbedivided
into 3 parts,namely,switch turn-on transition section loss(section1),
conduction section loss (section2)and turn-offtransition section loss
(section3).Theexpressionforlossineachsectionisinformula(3.25)~
(3.27).First,getswitch turn-on transition section loss(section 1).In
Figure3.11(a),section whereswitch voltagedecreasesoverlapssection
where switch currentincreases in section 1 where switch turns on.
Thus,turn-ontransitionlossoccursasshowninformula(3.25).
  



  

  
 

  
 
‧ ‧
(3.25)
Ifturn-ontransitionofswitchends,conductionsectionofsection2is
madein Figure3.11(a).In this section,conduction loss occurs dueto

76. - 60 -
switch internalresistance.Depending on switch internalresistance,the
lossoccursasshowninformula(3.26).
  

∙ ∙ 
  

∙  ∙ ∙
(3.26)
Finally,ifswitch conduction ends,turn-offtransition loss occurs in
section 3whereswitch ofFigure3.11(a)isoff.Liketurn-on transition
process,lossstated in formula(3.27)occursin thissection asvoltage
increaseoverlapscurrentdrop.
  
 

   
 
‧  ‧ 
(3.27)
Generalizingtheaboveloss,totalswitchlossofloadofresistancecan
beexpressedinthefollowingformula(3.28).
Psw=Psw-on+Psw-off+Psw-cond
= 1
6
∙ID∙VS∙(ton+toff)∙fsw+ I
2
D∙RDS∙tcond∙fsw
(3.28)
3.3.2.2.1.2.2Inductiveload
Sincecurrenthasdelayfactoragainstvoltage,inductiveloadrefersto

77. - 61 -
thecasewherethealteration section variesin theswitching transition
section ofcurrentand voltageofelement,namely,transition section of
voltagecurrentisdelayed.Liketheload ofresistance,alllossofone
cyclewillbeinterpretedforeachsectionininductiveloadasdescribed
inFigure3.11(b).Itcanbedividedinto3parts,namely,switchturn-on
transitionsection(section1),conductionsection(section2)andturn-off
transitionsection(section3)inonecyclerespectively.Thelossofeach
section isexpressed asfollows.First,switch turn-on transition section
loss(section1)isacquiredasfollows.Checkingsection1whereswitch
isturn-on in Figure3.11(b),asswitch currentincreasesfor tri
hours
and switch voltage decreases for tfv
hours,turn-on transition time is
longerthan load ofresistance,becauseitissum oftwo time(tri+tfv
).
Thus,turn-ontransitionlossoccursasstatedinformula(3.29).
   ∙ ∙  ∙   
   

∙ ∙ ∙ ∙ 
(3.29)
Ifswitch turn-on transition is completed,itbecomes the conduction
state ofsection 2 in Figure 3.11(b).Thus,conduction loss occurs by
switch internalresistance forconduction time (tcond
)as stated in the
followingformula(3.30).
 

∙  ∙
  

∙  ∙ ∙
(3.30)

78. - 62 -
Finally,with respect to switch turn-off transition section loss,as
switchcurrentdecreasesfor tfi
hoursafterswitchvoltageincreasesfor
trv
hours in section 3 where switch is turn-off in Figure 3.11(b),
turn-offtransition timeislongerthan load ofresistance,becauseitis
sum of two time (tfi+trv
).Thus,turn-off transition loss occurs as
statedinformula(3.31).
  

∙ ∙  ∙  ∙  
  

∙ ∙  ∙  ∙ 
(3.31)
Generalizingtheaboveloss,totalswitchlossofinductiveloadcanbe
describedasstatedinformula(3.32).
Psw=Psw-on+Psw-off+Psw-cond
=
1
2
∙ID∙VS∙fsw(ton+toff)+I
2
D∙RDS∙tcond∙fsw
(3.32)

79. - 63 -
0
0
DS
v
di
SD II =
t (ns)
t (ns)
SDS
VV =S
V
DDS ivp =
구간 1 구간 2 구간 3psw
tri tcond tfi
ton tcond toff
Switch
(a)Resistanceload
0
d
i
SD
II =
t (ns)
DS
v
di
SDS VV =SV
tfv trv tfi
Switch
psw
0
t (ns)
DDS
ivp =
tri
구간 1 구간 2 구간 3
t
ton tofftcond
tcond
(b)Inductionload
Fig.3.11Thelossgraphsofpowerdeviceincaseofresistanceload
andinductionload

80. - 64 -
3.4 Anti-islanding technology ofpowerconvertorin
grid
When supplying photovoltaic power to load,grid-tied photovoltaic
system supplies deficient power from commercialpower system and
suppliessurpluspowertocommercialpowersystem.Figure3.12shows
thecurrentflow between photovoltaic(PV)system and grid.Theentire
system is composed of PV system, household and switch (fuse,
reclosingbreaker,etc).
Fig.3.12PowercurrentofPVsystem andGrid
Then,formula(3.33)and (3.34)indicates active powerand de-active
powertobeconsumedbyRLC loadinsystem withgridvoltagesource
andnodeaindicatespointofcommon coupling(PCC)between gridand
PV inverter.

81. - 65 -
Pload=
V
2
a
R
(3.33)
Qload=V2
a[
1
wL
-wC] (3.34)
The detection ofislanding can be checked by measuring change in
abnormalvoltageand frequency atPCC and theflow ofactivepower
andde-activepowerisexpressedbyformula(3.35).
    
    
(3.35)
When theoutputcurrentofPV inverterhasphasesame tothatof
PCC voltage,de-active power, becomes 0.In case of  ,
   indicatesthattheactivepowersupplied by PV inverterwhen
gridisdisconnectedcoincideswiththeactivepowernecessaryforload.
Further,     indicates thattotalpower required by load is
sametototalpowersuppliedbyPV whengridisdisconnected.
Moreover, ≠  means the change in the quantity ofPCC and
 ≠  indicatesthechangeinfrequencyperchangeinphaseofPCC.
 ≠ , ≠  indicatesthattotalpowerrequired by load and total
powersupplied by PV are differentfrom totalpowersupplied by PV
whengridisdisconnected.Thus,thechangein  ismeasuredin
inverterto measure the change in voltage orfrequency in islanding.
Thevoltageandfrequencyareblockedbyprotectivefunctionofinverter
whentherangeisbeyondthespecificationorspecificrangeofinverter.

82. - 66 -
Then,permitted scope ofgrid voltage is specified in 88%～110% of
ratedeffectivevalueandscopeoferrorofgridfrequencyisspecifiedas
59.3～60.5[Hz]inIEEE Std.929-2000.However,thezonethatcannotbe
detected,namely,NDZ(Non Detection Zone) exists in islanding.The
situationofislandingcanbedetectedinthisareaaswell.
3.4.1Passivemethod
Passivemethoddetectsthegenerationquantityandloadquantity,line
voltage,frequency,the change ofharmonics components in phase or
voltage in the state ofislanding.There are methods such as using
excessive voltage/low voltage of inverter,using high frequency/low
frequency anddetecting by monitoring phasedifferenceofPCC voltage
andinverteroutputcurrentandTHD fordetection.
WhenPSC isdesignatedassystem generationoutputandPL,asload
consumption power,operation feature ofsystem per relation between
loadpowerandsolarcellarrayoutputinpowerfailureisstatedinthe
followingTable3.1.
Passivemethodistosendsignaltoinverterallthetimeandtodetect
abnormality by signalchange of inverter when islanding.Since the
activepowerchanging method isto detectchange in load voltageby
changing the quantity of active power through periodicalchange of
currentquantity,the detection ofCASE 6 in Table 3.1 is possible.

83. - 67 -
However,thereispossibilityofmalfunction,becausevoltagechangesto
someextentevenwhengridisnormal.
Table3.1Featureofsystem drivingaccordingtoloads
Division OpeCon OperationFeature LoadCon
CASE 1 PL>PSC
loadvoltagedrop,detectionofvoltageor
frequency
lead,lag
CASE 2 PL<SC
loadvoltageincrease,detectionofvoltage
frequency
lead,lag
CASE 3 PL=PSC
nochangeinloadvoltage,detectionof
frequency
lead,lag
CASE 4 PL>PSC loadvoltagedrop,detectionofvoltage
net
resistance
CASE 5 PL<PSC loadvoltageincrease,detectionofvoltage
net
resistance
CASE 6 PL=PSC nochangeinloadvoltage,islanding
net
resistance
Theactivepowerchangingmethodistoslightlychangefrequencyof
currentso as to detectchange in frequency atload voltage.When
power source is normal, frequency of power voltage is constant
irrespectiveofchangeinfrequencyofoutputcurrentofinverter.Inthe
case ofabnormality,however,load voltage changes as perfrequency
sametocurrent.Then,theislanding statecanbedetectedbydetecting
changing frequency ofload voltage.In thismethod,CASE 6in Table
3.1canbedetectedandthereislessriskofmalfunctiondifferentlyfrom
activepowerchangingmethod.

84. - 68 -
3.4.2Activemethod
Active method is the mode to promote active response by breaking
theequivalencebetween generation quantity in islanding stateandload
quantitybychanging outputvoltage,frequencyorphaseofphotovoltaic
system.Withoutaffecting grid considerably,this method changes PV
invertercurrenttostopPV inverterifthereisabnormalityinvoltageor
frequency in PCC.The examplesofactive method arefrequency bias
method,Sandiafrequencyshiftmethodandfrequencyjumpmethod.
In the proposed power convertor,frequency thatcan be controlled
relativelyeasilyisusedtodetectislanding.Islandingstateisjudgedby
detecting phasedifferenceoccurring when abnormalfrequency isinput
to normalfrequency.Compared to existing frequency,10% isinput.If
gridisinoperation,abnormalfrequencyispromptlyrecoveredtonormal
frequency.Ifgridfails,however,frequencyappearsabnormalinPCC.
If1cycleincreasesin thecycleof30minutes,11% movesafter30
cycles,deviating from the trip occurrence standard setin equipment
standardofPV system.Iftheabovenormalityoccursin2times,tripis
realizedwithin1second.
SandiaFrequencyShift(SFS)methodusedinthispaperchangesdead
timebyformula(3.36).
        (3.36)

85. - 69 -
where, refersto cfwhenthereisnoerrorinfrequency,fa
refers
tofrequency detectedinPCC and referstofrequencyofgrid.And
K is called acceleration gain. If islanding occurs in photovoltaic
generationsystem,currentwith changing deadtimeissuppliedtoload
in PV inverter.Then,frequency changes in PCC and this changed
frequencyisdetectedbyPV inverter.
(a)
(b)
(c)
(a)Originalcurrentcommand(  )
(b)Currentcommandincludedadetime( )
(c)Currentcommandcompare (a)with(b)
Fig3.13CurrentcommandusingSandiafrequencyshiftmethod
In Figure3.13,islanding stateoccurs at0.5 second and K value is
selectedsothatdeadtimecan be10%.Then,currentreferenceofPV
invertershownin loadthathas ≠  andhigh resonancefrequency
isindicated.Next,Figure3.14 describesislanding simulation thatuses

86. - 70 -
SFSmethod.
Fig3.14Simulationofislandingusing Sandiafrequencyshiftmethod
IfPV inverterperformsislanding in thismethod,frequency mustbe
detectedasspecifiedinTable3.2.However,astheresultofsimulation
in Figure 3.14,(b)and (d)shows,ifload condition is  ≠ ,the

87. - 71 -
frequency varies more than the result in Table 3.2 by emitting
frequency.
Table3.2 VariationoffrequencyusingSandiafrequencyshiftmethod
cf
Resonance
Frequency
Measuredfrequency[Hz]
FrequencyBias SFS
10%
59.7 60.0 58.9
60.0 60.3 60.0
60.3 60.6 61.4
3.4.3Gridsplittingconditiontopreventislanding
ReferringtoIEEE Standard1547,thedistributiongrid-tiedguidelineof
USA,dispersedpowersourcegrid-tiedguidelineofJapananddomestic
dispersedpowersourcegrid-tied technology(draft),researchercompared
and analyzed tied conditions such as voltage condition, frequency
condition,currentcondition,etctoreflectthem todesign.
Table3.3Troublecleartimeaccordingtoabnormalvoltageatgrid-tiepoint
VoltageRange(percentageof
standardvoltage)
Troublecleartime(seconds)
V < 50 0.16
50≤ V < 88 2.00
110≤ V < 120 1.00
V ≥ 120 0.16

88. - 72 -
3.4.3.1Voltagecondition(Table3.3)
The troublecleartimeaspergrid-tiepointvoltageisstipulated in
IEEE Standard 1547 and domestic dispersed power source grid-tied
technology(draft).This regulation is used as the standard to correct
UVR (UnderVoltageRelay)andOVR (OverVoltageRelay).
Table3.4Troublecleartimeaccordingtoabnormalfrequencyatgrid-tiepoint
Renewableenergy
capacity
Frequencyrange
(Hz)
Troublecleartime
≤ 30kW
> 60.5 0.16
< 59.3 0.16
> 30kW
> 60.5 0.16
< {59.8~57.0}
0.16~3.00
Adjustable
< 57 0.16
3.4.3.2Frequencycondition(Table3.4)
The trouble cleartime as pertied pointfrequency is stipulated in
IEEE Standard 1547 and domestic dispersed power source grid-tied
technology(draft).This regulation is used as the standard to correct
UFR (UnderFrequencyRelay)andOFR(OverFrequencyRelay).
3.4.3.3Standardofovercurrent
Generally,itisseparated from grid within 10Cycleafteraccidentin
IEEE Standard 1547 and domestic dispersed power source grid-tied
technology(draft)(including cooperationforreclosing).Thissetting isthe
standard value to set OCR(Over Current Relay) and OCGR (Over
CurrentGroundRelay)andthesetvaluevaries,dependingontheresult
of interpreting accident.Thus,it is required to select T.C.C (Time

89. - 73 -
CurrentCharacteristics)curve which satisfies the requirementthatis
separated from grid within 10 Cycle aftergrid accidentby using the
resultofinterpretingaccidentintheresultofinterpretingpowergridof
relevantgrid-tiepoint.Very-inverseandExtremely-InverseT.C.C curve
mustbeavailableforsettingfrom outside.
Table3.5Protectorequipmentfrom abnormalstatesingrid(Korea)
Forpowersource
Protector
equipment(1st)
Protectorequipment(2nd)
Synchronous
Generator
Overcurrentrelay
Shortdirectionrelay,short
directiondistancerelayor
currentdifferentialrelay
Inductivegenerator,
reverseconvertor
Low voltagerelay -
Failuredueto
groundfault
Groundfaultand
currentrelay
Directiongroundfaultand
currentrelayorcurrent
differentialrelay
3.4.3.4Obligationtoinstallprotectorequipment(Table3.5)
IEEE Standard 1547 and domestic dispersed powersource grid-tied
technology(draft)stipulatethatprotectorequipmentfactorofrenewable
energy source over 30 kW must be the 2nd protector.3.4.3.5
Anti-islanding
According to IEEE Standard 1547 and domestic dispersed power
sourcegrid-tied technology(draft),islanding,ifany,mustbe separated
from powergrid-tie pointand the limittime mustsatisfy Table 3.6.
Further,protectorequipmentfactorofanti-islandingisdefinedasTable
3.7.

90. - 74 -
Table3.6Standardofdisconnectiontimefrom gridafterislanding
Division Disconnectiontime
Koreanstandard 1second
Americanstandard 2seconds
Table3.7Protectorequipmentforanti-islanding
Pointof
accident
Typeofaccident Protectorequipment
PowerGrid
Islandingbygrid
accidentor
powerfailure
Noreverse
current
Reverescurrent
Functiontodetect
RPR,UFR,
reversecharging
Functiontodetect
OVR,UVR,OFR,
UFR,islanding

91. - 75 -
CHAPTER IV
COMPUTER SIMULATION
In orderto verify the isolated 3-phase multi-levelinverterforgrid
connection,computersimulationusingMatlabsimulinkisimplemented.
Figure 4.1 shows the simulation modelfor 3-phase isolated power
inverterusingtransformer.
Fig.4.1Simulinkmodelforcomputersimulation

92. - 76 -
ThecalculatedpulsesignalsforeachIPM aresuppliedtoproducethe
outputvoltage according to modulation index.In the simulation,the
inputvoltageisfixedas350[V].Theturnratiosofeachtransformerare
setto1:4:16toproduceoutputvoltage.
Figure4.2showsthesimulationresultsaccordingtomodulationindex
.Eachwaveformsexplaintheoutputvoltageofeachtransformerand
output terminal voltage. The output voltage is increased by the
modulation index andhassinusoidalwaveform.Theswitching numbers
are increased in low power transformer,buthas same frequency to
referencefrequency in high powertransformer.So,wecan reducethe
switching loss in high power transformer to increase the operating
efficiency.Inordertogetahighquality power,theswitching numbers
oflow powertransformerandIPMsareincreased.
Figure 4.3 shows the line-to-line and outputphase voltages ofthe
combinationsoftransformeroutputvoltage.AsshowninFigure4.3,the
line-to-line voltages which are connected to grid bus are very clear
with low harmonic distortion.From Figure 4.2 and Figure 4.3,high
qualitypowercanbetransferredtogridbus.
Figure4.4 shows theFFT analysisresults ofinverteroutputphase
voltage, line-to-line voltage and load phase voltage according to
modulation index.According to the modulation index,the harmonic
factors are decreased.And the totalharmonic distortion ofthe load
outputvoltage is very low.The 3th harmonics are included in phase
output voltage ,but are rejected in line-to-line voltage in 3-phase
powersystem.

93. - 77 -
(a) =0.25
(b) =0.50
Fig.4.2Transformerandoutputvoltageaccordingmodulationindex
(continue)

94. - 78 -
(c) =0.75
(d) =1.0
Fig.4.2Transformerandoutputvoltageaccordingmodulationindex

95. - 79 -
(a) =0.25
(b) =0.5
Fig.4.3Line-lineandoutputvoltageaccordingtomodulationindex
(continue)

96. - 80 -
(c) =0.75
(d) =1.0
Fig.4.3Line-lineandoutputvoltageaccordingtomodulationindex

97. - 81 -
(a) =0.25
(b) =0.5
Fig.4.4FFTanalysisaccordingtomodulationindex
(continue)

98. - 82 -
(c) =0.75
(d) =1.0
Fig.4.4FFTanalysisaccordingtomodulationindex

99. - 83 -
Figure4.5showstheanalysisofTHD(TotalHarmonicDistortion)and
DF according to modulation index.In theanalysis,theTHD is under
3% in wideoperation range.Theproposed 3-phasepowerinvertercan
produceahighqualitypower.
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
THD,DF(%)
Modulation(m)
THD
DF
Fig.4.5OutputvoltageTHD andDFaccordingtomodulationindex

100. - 84 -
CHATER V
DESIGN AND EXPERIMENTS
In orderto verify the proposed multi-levelconverterformicro-grid
system, 30kW isolated multi-level converter and its controller are
designed andmanufactured.In thischapter,thedetailed design flow is
explained.
5.1DesignofPowerConverter
Table 5.1 shows the electricalspecifications ofthe designed power
system.
Table5.1Electricalspecificationofpowerconverter
Powerrating 30[kW]
Inputvoltage 275~600[V]
Frequency 59.3~60.5[Hz]
Outputvoltage 3Φ,220[Vac]
Outputvoltageof
DC-DC converter
350[Vdc]
Powerfactor morethan 99[%]

101. - 85 -
Figure5.1showstheproposed multi-levelconvertersystem forgrid
connection in micro-grid system.The system is consisted by inrush
current protection circuit,DC-link voltage controller using buck and
boostconverter,IMP powermodule and 3-phase transformermodule.
Andtheoutputpoweristransferredtopowerfilterandconnectedbus
grid.
Fig.5.1Theproposedmulti-levelconvertersystem
Inputandoutputterminalsareconnectedtomagneticcontactswitches
forconnectionanddisconnectiontosourceandbus-grid.
5.1.1Inrushcurrentprotectioncircuit
Inrush currentprotection circuitisforsuppresstheinrush currentin
large capacitor connected circuit.The proposed power system has a
large capacitorbank forfiltering ofinputDC source.The DC source