Power Electronics
Chapter 6 PWM Techniques
The most widely used control technique in power electronics
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DC/DC
AC/AC
Pulse Width Modulation (PWM) (Chopping control)
DC/AC
AC/DC
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Outline
6.1 Basic principles 6.2 Some major PWM techniques in DC/AC inverters 6.3 PWM techniques with feedback control 6.4 PWM rectifiers
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6.1 Basic principles of PWM
Similar response to different shape of impulse input
The equal-area theorem: Responses tend to be identical when input signals have same area and time durations of input impulses become very small.
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Power Electronics
Basic principles of PWM
Application of the equal-area theorem This is sinusoidal PWM (SPWM) The equal-area theorem can be applied to realize any shape of waveforms
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A list of PWM techniques
Triangular-wave sampling
Natural sampling Uniform sampling
Calculation
Calculation based on equal-area criterion Selective harmonics elimination
Hysteric control Space Vector Modulation (SVM, or SVPWM) Random PWM
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6.2 Some major PWM techniques
Natural sampling Uniform sampling Selective harmonics elimination Some practical issues
Synchronous modulation and asynchronous modulation Harmonics in the PWM inverter output voltages Ways to improve DC input voltage utilization and reduce switching frequency Connection of multiple PWM inverters
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Triangular-wave natural sampling
Uni-polar PWM in single-phase VSI
V1 Ud + V2 VD2
Control signal Carrier
VD1 R uo
V3 L V4
VD3
VD4
ur uc
Mudulation Carrier
Uni-polar sampling is used to realize uni-polar PWM.
Triangular-wave natural sampling
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Bi-polar PWM in single-phase VSI
V1 Ud + V2 VD2
Control signal Carrier
VD1 R uo
V3 L V4
VD3
VD4
ur uc
Mudulation Carrier
Bi-polar sampling is used to realize bi-polar PWM.
Triangular-wave natural sampling
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In 3-phase VSI
Three-phase bridge inverter can only realize bi-bolar PWM therefore should be controlled by bipolar sampling.
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Triangular-wave uniform sampling
Easier to realize by computercontrol
Modulation factor
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Selective harmonics elimination PWM (SHEPWM)
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Frequency relationship between triangularwave carrier and control signal
Asynchronous Modulation Synchronous Modulation
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Harmonics in the PWM inverter output voltages
Spectrum of 1-phase bridge PWM inverter output voltage No lower order harmonics The lowest frequency harmonics is wc and adjacent harmonics. wc has the highest harmonic content.
Magnitude(%) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 k 1 n 0 0 + -5 0 + -2 + -2 + -4 0 + -1 + -3 + -4 1 2 3 (nc +kr ) a=1.0 a=0.8 a=0.5 a=0
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Harmonics in the PWM inverter output voltages
Spectrum of 3-phase bridge PWM inverter output voltage No lower order harmonics No harmonics at c. The lowest frequency and highest content harmonics are c2r and 2cr.
1.2 1.0 Magnitude(%) 0.8 0.6 0.4 0.2 k 1 n 0 0 + -2 + -4 0 + -1 + -3 + -5 0 + -2 + -4 1 2 3 (nc +kr ) a=1.0 a=0.8 a=0.5 a=0
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Ways to improve utilization of DC input voltage and reduce switching frequency
Use trapezoidal waveform as modulating signal instead of sinusoidal
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Ways to improve utilization of DC input voltage and reduce switching frequency
Use 3k order harmonics bias in the modulating signal
u ur1 ur3 O ur1 u ur uc
u 1 O -1 uP O -0.5 u 1 O -1 uUN'
Ud 2
urU1
urV1
urW1
uc
t urU urV urW uc
O
Ud 2
uVN' O uWN'
O uUV Ud O -U d
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Connection of multiple PWM inverters
Purposes
Expand output power rating Reduce harmonics
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6.3 PWM techniques with feedback control
Current hysteric control Voltage hysteric control Triangular-wave comparison (sampling) with feedback control
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Current hysteretic control
In Single-phase VSI
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Current hysteretic control
In 3-phase VSI
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Voltage hysteretic control
Ud 2 Ud 2
Filter
+ u* u
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Triangular-wave comparison (sampling) with feedback control
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6.4 PWM rectifiers
Operation Principles
a) Rectification mode
b) Inversion mode
c) Reactive power compensation mode
d) Current leading by
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PWM rectifiers
Three-phase circuit
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PWM rectifiers
Indirect current control
Triangular-wave u*d + ud
PI
id
uR + + R - uA,B,C sin(t+2k/3) (k=0,1,2) uL XL cos(t+2k/3) (k=0,1,2)
R ud + Load
L ua,ub,uc
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Power Electronics
PWM rectifiers
Direct current control
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