Reactive Power Compensation and Active Filtering Capabilities of the Step-down AC Chopper

141 Pages·2013·6.83 MB·English

Reactive Power Compensation and Active
Filtering Capabilities of the Step-down AC
Chopper
Ole Christian B Spro
Master of Energy and Environmental Engineering
Submission date: August 2013
Supervisor: Marta Molinas, ELKRAFT
Norwegian University of Science and Technology
Department of Electric Power Engineering

Problem Definition
The aim of the study is to investigate the AC Chopper circuit, design a suitable control
system,establishthesystembehaviorinsimulationandtestthebehavioronalaboratory
prototype. Thecircuitwillbeconnectedtoa50Hzsinglephasegrid. Thefinalgoalisto
clarifyifapossibilityexiststousetheACChopperasanactivefilter. Tofurtherspecify
theprojectgoals,threegoalswereset:
1. During the initial investigation of the AC Chopper, some unexpected correlations
werediscoveredconcerningtheopenloopbehavior. Onetaskofthemasterproject
wastovarykeydeviceparameterstoverifythisnewfoundrelationinsimulation.
2. ThecapabilityoftheACChoppertosupplyreactivepowertoagridhasbeenestab-
lishedinsimulation. Thedevicealreadyhasthetraitsofbeinglessexpensiveand
moreefficientinreactivecompensationthanaSTATCOM.Itisinterestingtoseeif
theareasofusecouldbeexpandedtoincludeactivefiltering. Thestudentwillmap
the active filtering capabilities of the AC Chopper using the predeveloped control
systemindq-referenceframe.Changescanbemadetothecontrolsystemifneeded
toachievethenecessaryfilteringcapabilities.
3. Finally a prototype of the AC Chopper will be constructed to verify the results
obtainedinsimulation. Theprototypewillworkatavoltageof400Vwitharating
ofapproximately1kVAR.
Theprototyperatingswerelaterchangedtosimplifytheprototypecircuitandreducethe
requirementsofthecomponentspecifications.
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Abstract
TheStep-downACChopper(herebyreferencedas"ACChopper")hasbeenpresentedin
recentacademicpapersasapossiblesolutionforreactivecompensation. Thegoalofthis
projectwastoinvestigateandevaluatethetopologytoseeifthepraisecouldbejustified.
Inaddition,thisprojectsetagoaltotaketheresearchonestepfurtherbyinvestigatingif
theusesoftheACChoppercouldbeexpandedtoincludeactivefiltering.
The ability of the AC Chopper converter to produce harmonic current was investigated.
ThiswasthefirststepindevelopingtheACChopper’sactivefilteringfunctionality. The-
orems were developed and simulations were performed with both open and closed loop
control. TheresultsshowthatusingtheACChoppertoproduceharmoniccurrentisvery
difficult if not impossible. This is due to intrinsic characteristics of the topology which
leadstotheconverterproducingmultipleharmonicsforasinglecontrolinputfrequency.
Hence it was concluded that the AC Chopper is not fit for performing active filtering
andthattheresultsdonotsupportcontinuinganyfurtherresearchontheACChopper’s
filteringcapabilities.
Aprototypeoftheconverterwasdevelopedandbuilttocomparethesimulationsagainst
realobservedbehavior. Thedevelopmentprocesshasbeenwelldocumentedinthisthe-
sis. Openloopcontroloftheprototypewassuccessfullyimplemented. Theresultsshow
agoodrelationbetweenthetheoryandthemeasuredvalues. Therearecertainchallenges
connectedtotherealizationoftheACChopper,andthewaysofdealingwiththesechal-
lengeshavebeendescribedherein.
Thetopologywasalsocomparedtoavailabletechnologytoevaluatethecompetitiveness
regarding reactive compensation at fundamental frequency. The AC Chopper was not
foundadvantageouscomparedtotheVSI,amodernavailablesolution. Theefficiencyof
the AC Chopper has been found to be slightly higher in simulation, but this has yet to
bevalidatedwithtestsonrealprototypes. Inadditiontolackingthefilteringcapabilities,
the AC Chopper has other disadvantages including expensive components and greater
volume. Together the findings in this report suggests that the AC Chopper should be
limitedtouseinsinglephasesystemsforreactivecompensationatfundamentalfrequency
inopenloopcontrol.
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Contents
ProblemDefinition i
Abstract iii
TableofContents vii
ListofTables ix
ListofFigures xiv
ListofAbbrivations xvi
1 Introduction 1
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 BriefHistoricOverviewoftheACChopper . . . . . . . . . . . . . . . . 2
1.3 ConclusionsofthePreliminaryInvestigationoftheACChopper . . . . . 3
2 ACChopperTopologyandOperatingPrinciple 5
2.1 CircuitLayout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 SwitchingStrategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 BasicRelation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4 OperatingPrincipleforReactiveCompensation . . . . . . . . . . . . . . 9
2.5 ActiveFiltering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.6 SystemValuesForSimulations . . . . . . . . . . . . . . . . . . . . . . . 11
2.7 ChapterConclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3 CompetingTechnologiestotheACChopper 13
3.1 VoltageSourceInverter(VSI)andSTATCOM . . . . . . . . . . . . . . . 14
3.2 MatrixConverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3 StaticVARCompensator(SVC). . . . . . . . . . . . . . . . . . . . . . . 15
3.4 ChapterConclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
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4 VerificationoftheBasicRelation 17
4.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2 ComponentVariation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.3 ChapterConclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5 OpenLoopBehaviorwithTimeVaryingDutyCycle 21
5.1 FirstMethodforVaryingtheDutyCycle: InversinganExistingMethod . 21
5.1.1 EliminatingIncomingHarmonics . . . . . . . . . . . . . . . . . 22
5.1.2 MakingHarmonics . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.2 SecondMethodforVaryingtheDutyCycle: SinusoidalDutyCycle . . . 29
5.2.1 DevelopingtheMathematicalExpression . . . . . . . . . . . . . 29
5.2.2 SimulationofSinusoidalDutyCycleandFrequencyResponse . . 31
5.2.3 FrequencyResponseofImpedanceandResultingCurrent . . . . 38
5.2.4 SuperpositionofMultipleSinestoProduceSingleHarmonics . . 39
5.3 ChapterConclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
6 ClosedLoopControlofFundamentalCurrent 45
6.1 ChoiceandImplementationofaSOGIUnit . . . . . . . . . . . . . . . . 45
6.2 DesigningaController . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
6.2.1 NonParametricModeloftheACChopper. . . . . . . . . . . . . 48
6.2.2 DesignofControllerbyLoopShaping . . . . . . . . . . . . . . . 49
6.3 ControllerPerformance . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
6.4 SecondAttemptatDesigningaController . . . . . . . . . . . . . . . . . 52
6.4.1 PerformanceofSecondController . . . . . . . . . . . . . . . . . 54
6.5 ChapterConclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
7 MakinganACChopperPrototype 57
7.1 PrototypeDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
7.1.1 PrototypeRating . . . . . . . . . . . . . . . . . . . . . . . . . . 57
7.1.2 BoomBox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
7.1.3 Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
7.1.4 ComponentSelection . . . . . . . . . . . . . . . . . . . . . . . . 60
7.1.5 PCBdesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
7.2 FinishedPCBandTestRoutines . . . . . . . . . . . . . . . . . . . . . . 65
7.2.1 ComponentTesting . . . . . . . . . . . . . . . . . . . . . . . . . 68
7.3 SoftwareDevelopment . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
7.3.1 SoftwareDesignandUML2.0 . . . . . . . . . . . . . . . . . . . 72
7.3.2 SoftwareUsedinDevelopmentandMonitoring . . . . . . . . . . 72
7.3.3 BoomBoxCharacteristicsInfluenceontheDesign . . . . . . . . 73
7.3.4 SafeSwitching . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
7.3.5 SupportSystems-SOGIandPLL . . . . . . . . . . . . . . . . . 75
7.4 ChapterConclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
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8 TuningthePerformanceoftheACChopperPrototype 79
8.1 TestingwithResistiveLoad . . . . . . . . . . . . . . . . . . . . . . . . . 79
8.2 TestingwiththeDesignedImpedance . . . . . . . . . . . . . . . . . . . 83
8.3 CommutationProblematHighDutyCycle. . . . . . . . . . . . . . . . . 86
8.4 FilterCurrent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
8.5 ChapterConclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
9 OpenLoopControloftheACChopperPrototype 89
9.1 TestwithStaticDutyCycle . . . . . . . . . . . . . . . . . . . . . . . . . 89
9.2 TestwithVariableDutyCycle . . . . . . . . . . . . . . . . . . . . . . . 92
9.3 ChapterConclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
10 FinalRemarksontheACChopperandConclusion 95
10.1 PracticalSidesoftheACChopper . . . . . . . . . . . . . . . . . . . . . 95
10.1.1 HigherEfficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 95
10.1.2 ConsequencesofComponentTechnology . . . . . . . . . . . . . 96
10.1.3 MultiplePhases . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
10.1.4 TwoResonanceFrequencies . . . . . . . . . . . . . . . . . . . . 98
10.2 FinalConclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Appendices 103
A PWMSignalGeneration 105
A.0.1 TransistorSwitchingSignals . . . . . . . . . . . . . . . . . . . . 105
B DimensioningofthePassiveComponentsoftheACChopper 109
B.1 DeviceImpedance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
B.2 InputFilter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
C AnalyticCalculationofthePWM-modulatedVoltageRMSValue 113
D UMLDiagrams 117
D.1 UseCases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
D.2 StateMachineDiagrams . . . . . . . . . . . . . . . . . . . . . . . . . . 117
E ComponentList 119
F Schematics 121
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