Transcript Power Semiconductor Devices

Chapter 6
AC to AC Converters
( AC Controllers and
Frequency Converters )
Classification of AC to AC converters
Power
Same frequency
variable magnitude
AC power
AC power
AC controllers
Variable
frequency
AC power
Frequency converters
(Cycloconverters)
AC to AC converters
2
Classification of AC controllers
Phase control: AC voltage controller
(Delay angle control)
Integral cycle control: AC power controller
Power
AC controller
PWM control: AC chopper
(Chopping control)
On/off switch: electronic AC switch
PWM: Pulse Width Modulation
3
Classification of frequency converters
Power
Frequency converter
(Cycloconverter)
Phase control: thyristor cycloconverter
(Delay angle control)
PWM control: matrix converter
(Chopping control)
Cycloconverter is sometimes referred to
– in a broader sense—any ordinary AC to AC converter
– in a narrower sense—thyristor cycloconverter
4
Outline
6.1 AC voltage controllers
Power
6.2 Other AC controllers
6.3 Thyristor cycloconverters
6.4 Matrix converters
5
6.1 AC voltage controllers
6.1.1 Single-phase AC voltage controller
6.1.2 Three-phase AC voltage controller
Power
Applications
Lighting control
Soft-start of asynchronous motors
Adjustable speed drive of asynchronous motors
Reactive power control
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6.1.1 Single-phase AC voltage controller
Resistive load
u1
VT1
io
O
wt
uo
VT2
uo
Power
u1
R
O
io
wt
O
wt
u VT
The phase shift range
(operation range of phase
delay angle):
O
wt
0ap
7
Resistive load, quantitative analysis
RMS value of output voltage
Uo 


p
1
p
a

1
p  a (6-1)
sin 2a 
2p
p
2U 1 sinw t d w t   U 1
2
RMS value of output current
Power
Uo
Io 
R
(6-2)
RMS value of thyristor current
2
U1
1  2U 1 sinw t 


IT 
d
w
t


2p a 
R
R

p
1
a sin 2a
(1  
)
2
p
2p
Power factor of the circuit
U
P U I
1
p a
  o o  o 
sin 2a 
S U1 I o U1
2p
p
(6-3)
(6-4)
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Inductive (Inductor-resistor) load,
operation principle
u1
VT1
Power
u1
VT2
uo
wt
O
io
uG1
R
The phase shift range:
ap
0.6
O
uG2
wt
O
uo
wt
O
io
wt
O
wt
uVT
O
wt
9
Inductive load, quantitative analysis
Differential equation
di
L o  Rio  2U 1 sinw t
dt
(6-5)
io w t a  0
Power
We have
q
sin(a  q   )  sin(a   ) e tg 
140
a wt
tg
q/(°)
2U 1
[sin(wt   )  sin(a   )e
Z
(6-6)
a  wt  a  q
Considering io=0 when wt=a+q
io 
ã
90¡
= ¡ã
75 ¡ã
60 ¡ã
45 ¡ã
30 ¡ã
15 ¡ã
0
Solution
180
]
100
60
20
0
20
60
100
a/(°)
140
180
图4-3
(6-7)
The RMS value of output voltage, output current, and thyristor
current can then be calculated.
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Inductive load, when a < 
The circuit can still work.
Power
u1
The load current will be
continuous just like the
thyristors are short-circuit,
and the thyristors can no
longer control the
magnitude of output
voltage.
The start-up transient will
be the same as the
transient when a RL load is
connected to an AC source
at wt =aa < .
wt
O
iG 1
p
Oa
wt
iG 2
O
io
iT1ap
Oa q

wt
wt
iT2
图4-5
Start-up transient
11
Harmonic analysis
There is no DC component
and even order harmonics in
the current.
100
80
The higher the number of
harmonic ordinate, the lower
the harmonic content.
In/I*/%
– The current waveform is halfwave symmetric.
Fundamental
60
40
3
Power
20
5
7
a90 is when harmonics is
the most severe.
The situation for the inductive
load is similar to that for the
resistive load except that the
corresponding harmonic
content is lower and is even
lower as  is increasing.
0
60
120
180
a/( °)
Current harmonics
for the resistive load
12
6.1.2 Three-phase AC voltage controller
Power
Classification of three-phase circuits
Y connection
Branch-controlled ∆ connection
Line-controlled ∆ connection
Neutral-point-controlled ∆ connection
13
3-phase 3-wire Y connection
AC voltage controller
ia
U a0'
VT 1
a
ua
VT 3
b
n
u
b
VT 5
Power
VT 4
n'
VT 6
c
u
c
VT 2
For a time instant, there are 2 possible conduction states:
– Each phase has a thyristor conducting. Load voltages are the
same as the source voltages.
– There are only 2 thyristors conducting, each from a phase. The
load voltages of the two conducting phases are half of the
corresponding line to line voltage, while the load voltage of the
other phase is 0.
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3-phase 3-wire Y connection
AC voltage controller
Resistive load, 0  a < 60
VT
VT
Power
VT
VT 4
1
VT 1
VT 3
6
VT
5
u ab
2
ua
VT 6
VT
2
5
u ac
2
u ao'
0
p
3
a
t
1
t
2
2p
4p
5p
3
t
3
3
2 p
3
15
3-phase 3-wire Y connection
AC voltage controller
Resistive load, 60  a < 90
VT
VT
5
VT
Power
u
u
2
u
VT
6
ab
VT
1
u
a
2
a
p
3
t
1
2p
3
t
2
VT
2
4
5
VT
6
ac
4p
3
ao'
0
VT
3
p
t
5p
3
2p
3
16
3-phase 3-wire Y connection
AC voltage controller
Resistive load, 90  a < 150
VT
VT
5
VT VT
u 4
Power
VT
5
ab
6
1
VT VT
u 6u
a
VT
3
VT
2
VT
3
VT
2
VT
5
5
VT
4
VT
4
6
ac
2
u
VT VT
1
5p
2
ao'
3
0
p
2p
3
3
a
p
4p
2p
3
17
6.2 Other AC controllers
6.2.1 Integral cycle control—AC power controller
Power
6.2.2 Electronic AC switch
6.2.3 Chopping control—AC chopper
18
6.2.1 Integral cycle control
—AC power controller
uo
VT1
2 U1
io
O
Power
u1
VT2
uo
Conduction 2pN
= M
angle
R
p
M
2p
M
u1
uo,io
3p
M
4p
M
=M *Line
period =2p
wt
Line period
Control period
Circuit topologies are the same as AC voltage
controllers. Only the control method is different.
Load voltage and current are both sinusoidal when
thyristors are conducting.
19
Spectrum of the current in
AC power controller
Power
There is NO
harmonics in the
ordinary sense.
In/I0m
There is harmonics
as to the control
frequency. As to the
line frequency, these
components become
fractional harmonics.
0.6
0.5
0.4
0.3
0.2
0.1
0
0
2 4 6 8 10 12 14
Harmonic order as to
control frequency
1
2
3
4
5
Harmonic order as to
line frequency
20
6.2.2 Electronic AC switch
Power
Circuit topologies are the same as AC
voltage controllers. But the back-to-back
thyristors are just used like a switch to turn
the equipment on or off.
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6.2.3 Chopping control—AC chopper
Principle of chopping control
Power
The mean output voltage over
one switching cycle is
proportional to the duty cycle in
that period. This is also called
Pulse Width Modulation
(PWM).
Advantages
Much better output waveforms,
much lower harmonics
For resistive load, the
displacement factor is always
1.
Waveforms when the load
is pure resistor
22
AC chopper
Power
Modes of operation
u o>0, io>0:
u o>0, io<0:
u o<0, io>0:
u o<0, io<0:
V1 charging, V3 freewheeling
V4 charging, V2 freewheeling
V3 charging, V1 freewheeling
V2 charging, V4 freewheeling
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6.3 Thyristor cycloconverters
(Thyristor AC to AC frequency converter)
Another name—direct frequency converter (as
compared to AC-DC-AC frequency converter which
is discussed in Chapter 8)
Power
Can be classified into single-phase and threephase according to the number of phases at output
6.3.1 Single-phase thyristor-cycloconverter
6.3.2 Three-phase thyristor-cycloconverter
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6.3.1 Single-phase thyristor-cycloconverter
Circuit configuration and operation principle
N
P
Power
uo
uo
O
aP= p
2
Output
voltage
Z
a P=0
Average
output voltage
aP= p
2
wt
25
Single-phase thyristor-cycloconverter
uo,io
Modes of operation
uo
O t1
uP
io
iP
Power
uP
uo
iN
io
t3
t2
t4
t
uo
t
O
uN
t5
uN
O
uo
t
iP
O
iN
t
O
t
P
Rectifi
cation
Inver
sion
N Blocking
Blocking
Rectifi Inver
cation sion
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Single-phase thyristor-cycloconverter
Typical waveforms
uo
Power
O
wt
io
O
wt
1
3
4
6
5
2
图4-20
27
Modulation methods for firing delay angle
Calculation method
– For the rectifier circuit
uo  U d0 cosa
(6-15)
Power
– For the cycloconverter
output
uo  U om sinw o t
(6-16)
– Equating (6-15) and (6-16)
U
cosa  om sinw o t   sinw o t
U d0
(6-17)
– Therefore
u2
u3
u4
u5
u6
u1
wt
aP3
us2
us3
aP4
us4
us5
us6
us1
uo
wt
a  cos1 ( sinw o t ) (6-18)
图4-21
Cosine wave-crossing
method
Principle of cosine
wave-crossing method
28
Calculated results for firing delay angle
Power
U om
 
(0  r  1)
U d0
a/(°)
Output voltage ratio
(Modulation factor)
180
1.0
0.9
0.8
0.3
0.2
0.1
150
120
=0
90
 = 0.1
0.2
0.3
0.8
0.9
1.0
60
30
0
p
2
p
3p
2
2p
w0t
Output voltage phase angle
29
Input and output characteristics
Maximum output
frequency: 1/3 or 1/2 of the
input frequency if using 6pulse rectifiers
Power
Input power factor
Harmonics in the output
voltage and input current
are very complicated, and
both related to input
frequency and output
frequency.
Input displacement factor
0.8
0.6
0.4
0.2
0
0 0.2 0.4 0.6 0.8 1.0 0.8 0.6 0.4 0.2 0
Load power factor Load power factor
(lagging)
(leading)
30
6.3.2 Three-phase thyristor-cycloconverter
Power
The configuration with common input line
31
Three-phase thyristor-cycloconverter
Power
The configuration with star-connected output
32
Three-phase thyristor-cycloconverter
Typical waveforms
Power
Output voltage
Input current with
Single-phase output
0
200 t/ms
0
200 t/ms
0
200 t/ms
Input current with
3-phase output
33
Input and output characteristics
The maximum output frequency and the harmonics
in the output voltage are the same as in singlephase circuit.
Power
Input power factor is a little higher than singlephase circuit.
Harmonics in the input current is a little lower than
the single-phase circuit due to the cancellation of
some harmonics among the 3 phases.
To improve the input power factor:
– Use DC bias or 3k order component bias on each of the 3
output phase voltages
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Features and applications
Power
Features
– Direct frequency conversion—high efficiency
– Bidirectional energy flow, easy to realize 4-quadrant
operation
– Very complicated—too many power semiconductor
devices
– Low output frequency
– Low input power factor and bad input current waveform
Applications
– High power low speed AC motor drive
35
6.4 Matrix converter
Circuit configuration
Input
a
b
c
Power
u
S11
S12
S13
v
S21
S22
Sij
Output
S23
w
S31
S32
a)
S33
b)
36
Matrix converter
Usable input voltage
U1m
Power
Um
a)
a) Single-phase input
voltage
3
2
1
2 Um
b)
b) Use 3 phase voltages
to construct output
voltage
U1m
c)
c) Use 3 line-line voltages
to construct output
voltage
37
Power
Features
Direct frequency conversion—high efficiency
Can realize good input and output waveforms, low
harmonics, and nearly unity displacement factor
Bidirectional energy flow, easy to realize 4-quadrant
operation
Output frequency is not limited by input frequency
No need for bulk capacitor (as compared to indirect
frequency converter)
Very complicated—too many power semiconductor
devices
Output voltage magnitude is a little lower as
compared to indirect frequency converter.
38