電力電子技術(shù)chapter07課件

1、The most widely used control technique in power electronicsPulse Width Modulation (PWM) (Chopping control)DC/DCAC/ACDC/ACAC/DCOutline7.1 Basic principles7.2 Some major PWM techniques in DC/AC inverters7.3 PWM techniques with feedback control7.4 PWM rectifiers7.1 Basic principles of PWMSimilar respon

2、se to different shape of impulse inputThe equal-area theorem:Responses tend to be identical when input signals have same area and time durations of input impulses become very small.Basic principles of PWMApplication of the equal-area theorem This is sinusoidal PWM (SPWM) The equal-area theorem can b

3、e applied to realize any shape of waveformsA list of PWM techniquesTriangular-wave samplingNatural samplingUniform samplingCalculationCalculation based on equal-area criterionSelective harmonics eliminationHysteretic controlSpace Vector Modulation (SVM, or SVPWM)Random PWM7.2 Some major PWM techniqu

4、esNatural samplingUniform samplingSelective harmonics eliminationSome practical issuesSynchronous modulation and asynchronous modulationHarmonics in the PWM inverter output voltagesWays to improve DC input voltage utilization and reduce switching frequencyConnection of multiple PWM invertersTriangul

5、ar-wave natural samplingUni-polar PWM in single-phase VSI Uni-polar sampling is used to realize uni-polar PWM.Triangular-wave natural samplingBi-polar PWM in single-phase VSI Bi-polar sampling is used to realize bi-polar PWM.Triangular-wave natural samplingIn 3-phase VSIThree-phase bridge inverter c

6、an only realize bi-bolar PWM therefore should be controlled by bipolar sampling.Triangular-wave uniform samplingEasier to realize by computer-controlModulation factorSelective harmonics elimination PWM (SHEPWM) Frequency relationship between triangular-wave carrier and control signal Asynchronous Mo

7、dulation Synchronous Modulation Harmonics 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 inverteroutput voltageHarmonics in the PWM inverter output voltagesNo l

8、ower order harmonicsNo harmonics at wc. The lowest frequency and highest content harmonics are wc2wr and 2wcwr. Spectrum of 3-phase bridge PWM inverter output voltageWays to improve utilization of DC input voltage and reduce switching frequencyUse trapezoidal waveform as modulating signal instead of

9、 sinusoidalWays to improve utilization of DC input voltage and reduce switching frequencyUse 3k order harmonics bias in the modulating signalConnection of multiple PWM invertersPurposesExpand output power ratingReduce harmonicsSpace Vector PWM (SVPWM or SVM)acbabbccaabbcca1 1 1T Phase variables (a,

10、b and c) produceline-to-line variables (ab, bc and ca) in plane- Line-to-line variables (ab, bc and ca) do not have-component in -coordinate systemVector Space of 3-phase Line-to-Line VariablesLine-to-Line Voltage Space Vectorwhereabbccavv Space vectorIf Vm is the amplitude of balanced, symmetrical,

11、 three-phase line-to-linevoltages, then Switching States for 3-phase Voltage Source Inverter idcvabVdcsascsb010000000000011111111111icibib+ic0iaia+icia+ibia+ib+icSwitching statepnnppnnpnnppnnppnppppnnnvcavbcVdcVdcVdcVdcVdc-Vdc-Vdc-Vdc-Vdc-Vdc-Vdc000000000000iaibicvavcvbsasbscpnVdcidcSwitching State

12、Vector pnnab, bccavvSwitching State Vector ppnab, bccavSwitching State Vector pppab, bccaSwitching State Vectors3015090-90-1500-3000 ()Sector IIVIIIIIVVIat center pointab, bccaReference Voltage Vector, Vrefab, bccavvwhereIn general,Assumeat center pointrefVrVref ()V1()V2()T1T2T0TStTotal area of= Are

13、a ofFor examplevDefinition of High Frequency SynthesisSynthesis of Vref using Switching State Vectorsab, bccavvrefVriaibicvavcvbsasbscpnVdcidcabc101010IIVIIIIIVVIrefVrFrom HF synthesis definition, Assume is constant in TS , whererefVrDuty Ratio of Switching State Vectors in SVPWM7.3 PWM techniques with feedback controlCurrent hysteretic controlVoltage hysteretic controlTriangular-wave comparison (sampling) with feedback control Current hysteretic controlIn Single-phase VSICurrent hysteretic controlIn 3-phase VSIVoltage hysteretic controlTriangular-wave comparison (sampling) with