Pwm Techniques 4x3c4h
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Power Electronics
Chapter 6 PWM Techniques
Power Electronics
The most widely used control technique in power electronics DC/DC
AC/AC
Pulse Width Modulation (PWM) (Chopping control)
DC/AC
AC/DC 2
Power Electronics
Outline 6.1 Basic principles 6.2 Some major PWM techniques in DC/AC inverters 6.3 PWM techniques with control 6.4 PWM rectifiers
3
Power Electronics
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. 4
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
5
Power Electronics
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
6
Power Electronics
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
7
Power Electronics
Triangular-wave natural sampling Uni-polar PWM in single-phase VSI V1 Ud
+
V3
VD1 R
L uo
V2
V4
VD2 Control signal
VD3
VD4
ur
Carrier
uc
Mudulation Carrier
Uni-polar sampling is used to realize uni-polar PWM.
8
Power Electronics
Triangular-wave natural sampling Bi-polar PWM in single-phase VSI V1 Ud
+
V3
VD1 R
L uo
V2
V4
VD2 Control signal
VD3
VD4
ur
Carrier
uc
Mudulation Carrier
Bi-polar sampling is used to realize bi-polar PWM.
9
Power Electronics
Triangular-wave natural sampling In 3-phase VSI
Three-phase bridge inverter can only realize bi-bolar PWM therefore should be controlled by bipolar sampling. 10
Power Electronics
Triangular-wave uniform sampling Easier to realize by computercontrol
Modulation factor
11
Power Electronics
Selective harmonics elimination PWM (SHEPWM)
12
Power Electronics
Frequency relationship between triangularwave carrier and control signal Asynchronous Modulation Synchronous Modulation
13
Spectrum of 1-phase bridge PWM inverter output voltage
1.4
a=1.0 a=0.8 a=0.5 a=0
1.2 1.0
No lower order harmonics The lowest frequency harmonics is wc and adjacent harmonics. wc has the highest harmonic content.
Magnitude(%)
Power Electronics
Harmonics in the PWM inverter output voltages
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 (nωc +kωr )
14
Spectrum of 3-phase bridge PWM inverter output voltage No lower order harmonics No harmonics at ωc. The lowest frequency and highest content harmonics are ωc±2ωr and 2ωc±ωr.
1.2
a=1.0 a=0.8 a=0.5 a=0
1.0 Magnitude(%)
Power Electronics
Harmonics in the PWM inverter output voltages
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 (nωc +kωr )
15
Power Electronics
Ways to improve utilization of DC input voltage and reduce switching frequency Use trapezoidal waveform as modulating signal instead of sinusoidal
16
Power Electronics
Ways to improve utilization of DC input voltage and reduce switching frequency Use 3k order harmonics bias in the modulating signal
u 1
uc
t
O -0.5
O
t urU
urV
urW
uc
t
-1 uUN'
ur3
Ud 2
ωt
O −
ur1
urW1
-1 uP
ur1
O
urV1
O
u 1
u
urU1
Ud 2
t
uVN'
ur uc
u
O
t
uWN'
O
ωt
O
t
uUV Ud O
t
-Ud
17
Power Electronics
Connection of multiple PWM inverters
Purposes – Expand output power rating – Reduce harmonics 18
Power Electronics
6.3 PWM techniques with control Current hysteric control Voltage hysteric control Triangular-wave comparison (sampling) with control
19
Power Electronics
Current hysteretic control In Single-phase VSI
20
Power Electronics
Current hysteretic control In 3-phase VSI
21
Power Electronics
Voltage hysteretic control
Ud 2 Ud 2
Filter
+ u* u
u
22
Power Electronics
Triangular-wave comparison (sampling) with control
23
Power Electronics
6.4 PWM rectifiers Operation Principles
a) Rectification mode
c) Reactive power compensation mode
b) Inversion mode
d) Current leading by ϕ
24
Power Electronics
PWM rectifiers Three-phase circuit
25
Power Electronics
PWM rectifiers Indirect current control Triangular-wave u*d
+ ud
PI
id
uR + + R - uA,B,C sin(ωt+2kπ/3) (k=0,1,2) uL XL cos(ωt+2kπ/3) (k=0,1,2)
R
L ua,ub,uc
ud + Load
26
Power Electronics
PWM rectifiers Direct current control
27
Chapter 6 PWM Techniques
Power Electronics
The most widely used control technique in power electronics DC/DC
AC/AC
Pulse Width Modulation (PWM) (Chopping control)
DC/AC
AC/DC 2
Power Electronics
Outline 6.1 Basic principles 6.2 Some major PWM techniques in DC/AC inverters 6.3 PWM techniques with control 6.4 PWM rectifiers
3
Power Electronics
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. 4
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
5
Power Electronics
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
6
Power Electronics
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
7
Power Electronics
Triangular-wave natural sampling Uni-polar PWM in single-phase VSI V1 Ud
+
V3
VD1 R
L uo
V2
V4
VD2 Control signal
VD3
VD4
ur
Carrier
uc
Mudulation Carrier
Uni-polar sampling is used to realize uni-polar PWM.
8
Power Electronics
Triangular-wave natural sampling Bi-polar PWM in single-phase VSI V1 Ud
+
V3
VD1 R
L uo
V2
V4
VD2 Control signal
VD3
VD4
ur
Carrier
uc
Mudulation Carrier
Bi-polar sampling is used to realize bi-polar PWM.
9
Power Electronics
Triangular-wave natural sampling In 3-phase VSI
Three-phase bridge inverter can only realize bi-bolar PWM therefore should be controlled by bipolar sampling. 10
Power Electronics
Triangular-wave uniform sampling Easier to realize by computercontrol
Modulation factor
11
Power Electronics
Selective harmonics elimination PWM (SHEPWM)
12
Power Electronics
Frequency relationship between triangularwave carrier and control signal Asynchronous Modulation Synchronous Modulation
13
Spectrum of 1-phase bridge PWM inverter output voltage
1.4
a=1.0 a=0.8 a=0.5 a=0
1.2 1.0
No lower order harmonics The lowest frequency harmonics is wc and adjacent harmonics. wc has the highest harmonic content.
Magnitude(%)
Power Electronics
Harmonics in the PWM inverter output voltages
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 (nωc +kωr )
14
Spectrum of 3-phase bridge PWM inverter output voltage No lower order harmonics No harmonics at ωc. The lowest frequency and highest content harmonics are ωc±2ωr and 2ωc±ωr.
1.2
a=1.0 a=0.8 a=0.5 a=0
1.0 Magnitude(%)
Power Electronics
Harmonics in the PWM inverter output voltages
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 (nωc +kωr )
15
Power Electronics
Ways to improve utilization of DC input voltage and reduce switching frequency Use trapezoidal waveform as modulating signal instead of sinusoidal
16
Power Electronics
Ways to improve utilization of DC input voltage and reduce switching frequency Use 3k order harmonics bias in the modulating signal
u 1
uc
t
O -0.5
O
t urU
urV
urW
uc
t
-1 uUN'
ur3
Ud 2
ωt
O −
ur1
urW1
-1 uP
ur1
O
urV1
O
u 1
u
urU1
Ud 2
t
uVN'
ur uc
u
O
t
uWN'
O
ωt
O
t
uUV Ud O
t
-Ud
17
Power Electronics
Connection of multiple PWM inverters
Purposes – Expand output power rating – Reduce harmonics 18
Power Electronics
6.3 PWM techniques with control Current hysteric control Voltage hysteric control Triangular-wave comparison (sampling) with control
19
Power Electronics
Current hysteretic control In Single-phase VSI
20
Power Electronics
Current hysteretic control In 3-phase VSI
21
Power Electronics
Voltage hysteretic control
Ud 2 Ud 2
Filter
+ u* u
u
22
Power Electronics
Triangular-wave comparison (sampling) with control
23
Power Electronics
6.4 PWM rectifiers Operation Principles
a) Rectification mode
c) Reactive power compensation mode
b) Inversion mode
d) Current leading by ϕ
24
Power Electronics
PWM rectifiers Three-phase circuit
25
Power Electronics
PWM rectifiers Indirect current control Triangular-wave u*d
+ ud
PI
id
uR + + R - uA,B,C sin(ωt+2kπ/3) (k=0,1,2) uL XL cos(ωt+2kπ/3) (k=0,1,2)
R
L ua,ub,uc
ud + Load
26
Power Electronics
PWM rectifiers Direct current control
27
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