1、Power ElectronicsChapter 7 PWM TechniquesPower ElectronicsChapter 7 The most widely used control technique in power electronicsPulse Width Modulation (PWM) (Chopping control)DC/DCAC/ACDC/ACAC/DCThe most widely used control tOutline7.1 Basic principles7.2 Some major PWM techniques in DC/AC inverters7

2、.3 PWM techniques with feedback control7.4 PWM rectifiersOutline7.1 Basic principleschapter07-PWM-Techniques-電力電子技術(shù)(第5版)英文版本課件chapter07-PWM-Techniques-電力電子技術(shù)(第5版)英文版本課件A list of PWM techniquesTriangular-wave samplingNatural samplingUniform samplingCalculationCalculation based on equal-area criterion

3、Selective harmonics eliminationHysteretic controlSpace Vector Modulation (SVM, or SVPWM)Random PWMA list of PWM techniquesTriang7.2 Some major PWM techniquesNatural samplingUniform samplingSelective harmonics eliminationSome practical issuesSynchronous modulation and asynchronous modulationHarmonics

4、 in the PWM inverter output voltagesWays to improve DC input voltage utilization and reduce switching frequencyConnection of multiple PWM inverters7.2 Some major PWM techniquesNTriangular-wave natural samplingUni-polar PWM in single-phase VSI Uni-polar sampling is used to realize uni-polar PWM.Trian

5、gular-wave natural sampliTriangular-wave natural samplingBi-polar PWM in single-phase VSI Bi-polar sampling is used to realize bi-polar PWM.Triangular-wave natural sampliTriangular-wave natural samplingIn 3-phase VSIThree-phase bridge inverter can only realize bi-bolar PWM therefore should be contro

6、lled by bipolar sampling.Triangular-wave natural sampliTriangular-wave uniform samplingEasier to realize by computer-controlModulation factorTriangular-wave uniform sampliSelective harmonics elimination PWM (SHEPWM) Selective harmonics eliminatioFrequency relationship between triangular-wave carrier

7、 and control signal Asynchronous Modulation Synchronous Modulation Frequency relationship betweenHarmonics in the PWM inverter output voltagesNo lower order harmonicsThe lowest frequency harmonics is wc and adjacent harmonics. wc has the highest harmonic content. Spectrum of 1-phasebridge PWM invert

8、eroutput voltageHarmonics in the PWM inverter Harmonics in the PWM inverter output voltagesNo lower order harmonicsNo harmonics at wc. The lowest frequency and highest content harmonics are wc2wr and 2wcwr. Spectrum of 3-phase bridge PWM inverter output voltageHarmonics in the PWM inverter Ways to i

9、mprove utilization of DC input voltage and reduce switching frequencyUse trapezoidal waveform as modulating signal instead of sinusoidalWays to improve utilization ofWays to improve utilization of DC input voltage and reduce switching frequencyUse 3k order harmonics bias in the modulating signalWays

10、 to improve utilization ofConnection of multiple PWM invertersPurposesExpand output power ratingReduce harmonicsConnection of multiple PWM invSpace Vector PWM (SVPWM or SVM)acbabbccaabbcca1 1 1T Phase variables (a, b and c) produceline-to-line variables (ab, bc and ca) in plane- Line-to-line variabl

11、es (ab, bc and ca) do not have-component in -coordinate systemVector Space of 3-phase Line-to-Line VariablesSpace Vector PWM (SVPWM or SVMLine-to-Line Voltage Space Vectorwhereabbccavv Space vectorIf Vm is the amplitude of balanced, symmetrical, three-phase line-to-linevoltages, then Line-to-Line Vo

12、ltage Space VecSwitching States for 3-phase Voltage Source Inverter idcvabVdcsascsb010000000000011111111111icibib+ic0iaia+icia+ibia+ib+icSwitching statepnnppnnpnnppnnppnppppnnnvcavbcVdcVdcVdcVdcVdc-Vdc-Vdc-Vdc-Vdc-Vdc-Vdc000000000000iaibicvavcvbsasbscpnVdcidcSwitching States for 3-phase VSwitching S

13、tate Vector pnnab, bccavvSwitching State Vector pnnabSwitching State Vector ppnab, bccavSwitching State Vector ppnabSwitching State Vector pppab, bccaSwitching State Vector pppabSwitching State Vectors3015090-90-1500-3000 ()Sector IIVIIIIIVVIat center pointab, bccaSwitching State Vectors3015090Refer

14、ence Voltage Vector, Vrefab, bccavvwhereIn general,Assumeat center pointrefVrReference Voltage Vector, VrefVref ()V1()V2()T1T2T0TStTotal area of= Area ofFor examplevDefinition of High Frequency SynthesisVref ()V1()V2()T1T2T0TStTotSynthesis of Vref using Switching State Vectorsab, bccavvrefVriaibicva

15、vcvbsasbscpnVdcidcabc101010IIVIIIIIVVIrefVrSynthesis of Vref using SwitchFrom HF synthesis definition, Assume is constant in TS , whererefVrDuty Ratio of Switching State Vectors in SVPWMFrom HF synthesis definition,7.3 PWM techniques with feedback controlCurrent hysteretic controlVoltage hysteretic

16、controlTriangular-wave comparison (sampling) with feedback control 7.3 PWM techniques with Current hysteretic controlIn Single-phase VSICurrent hysteretic controlIn SCurrent hysteretic controlIn 3-phase VSICurrent hysteretic controlIn 3Voltage hysteretic controlVoltage hysteretic controlTriangular-wave comparison (sampling) with feedback controlTriangular-wave comparison (sa7.4 PWM rectifiersOperation Principlesa) Rectification modeb) Inversion modec) Reactive power compe