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1. EEE 360
Energy Conversion and
Transport
George G. Karady & Keith Holbert
Chapter 9
Introduction to Motor Control and Power
Electronics
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2. Lecture 28
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3. 9.5.1 Voltage Source Inverter
with Pulse Width
Modulation
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4. 9.5.1 Voltage Source Inverter
• The amplitude of the harmonics can be reduced
by using the pulse width modulation (PWM)
technique.
• The basic concept of the PWM method is the
division of the on-time into several on and off
periods with varying duration.
• The rms value of the ac voltage is controlled by
the on-time of the switches,.
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5. 9.5.1 Voltage Source Inverter
• The most frequently used PWM technique is
sinusoidal pulse width modulation.
• This approach requires a bridge converter with
IGBT or MOSFET switches shunted by an
anti-parallel connected diode.
• The diode allows current flow in the opposite
direction when the switch is open.
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6. 9.5.1 Voltage Source Inverter
• These freewheeling diodes prevent inductive
current interruption
• This provides protection against transient
overvoltage, which may cause reverse
breakdown of the IGBT and MOSFET
switches.
• The typical circuit diagram is shown in Figure
9.44.
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7. 9.5.1 Voltage Source Inverter
Rload
Idc
Vdc
Lload
Vac
S1
S4
S3
S2
Figure 9.44 Single-phase voltage source converter.
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8. 9.5.1 Voltage Source Inverter
• During the positive cycle, S1 and S2 are
switched by the high frequency pulse train
shown in Figure 9.45.
• During the negative cycle, the pulse train
switches S3 and S4.
• The load inductance integrates the generated
pulse train and produces a sinusoidal voltage
(Vac) and current wave, as shown in Figure
9.45.
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9. 9.5.1 Voltage Source Inverter
• The width of each pulse is varied in
proportion to the amplitude of a sine
wave. A typical PWM waveform is
shown in Figure 9.45.
• The switches in this converter are
controlled by gate pulses.
• The gate signal contains several pulses
distributed along the half-cycle.
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10. 9.5.1 Voltage Source Inverter
PWM Output
Voltage
Load
Current
Load
Voltage
0s 10ms 20ms 30ms 40ms 50ms
I(V5)*10 V(L1:1,VOUT-) V(V5:+,V5:-)
Time
400
0
-400
Figure 9.45 Gate pulse input signal, and ac voltage and current
outputs of a pulse width modulation (PWM) converter.
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11. 9.5.1 Voltage Source Inverter
• The control circuit produces the gate pulse
train by generation of a triangular carrier wave
and a sinusoidal reference signal.
• The two signals are compared, and when the
carrier wave is larger than the reference signal,
the gate signal is positive.
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12. 9.5.1 Voltage Source Inverter
• When the carrier wave is smaller than the
reference signal, the gate signal is zero.
• This results in a gate pulse with variable width.
• Figure 9.46
– (a) shows the carrier wave and reference sine wave;
– (b) depicts the resulting gate signal with variable
width pulses. It has to be noted that several other
methods are used for generation of PWM signals
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13. 9.5.1 Voltage Source Inverter
1.0V
0V
-1.0V
Carrier wave Reference signal
0s 5ms 10ms 15ms 20ms 25ms
V(PWM_TRI1.E1:IN+) V(PWM_TRI1.Vtri:+)
Time
(a) Triangular carrier wave and sinusoidal reference signal
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14. 9.5.1 Voltage Source Inverter
Gate pulse with variable width
0s 5ms 10ms 15ms 20ms 25ms
Time
V(PWM_TRI1:s)
1.0V
0V
-1.0V
(b) Variable-width gate pulse signal
Figure 9.46 Pulse width modulation (PWM) signals.
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15. 9.5.1 Voltage Source Inverter
• The frequency of the reference sine wave
determines the frequency of the generated ac
voltage.
• The amplitude of the ac voltage can be
regulated by the variation of the reference
signal amplitude.
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16. 9.5.1 Voltage Source Inverter
• The amplitude of the fundamental component
of the ac voltage is:
V = V =
ac V mV
dc dc
control
carrier
V
• The modulation index is the ratio of the peak-to-
peak ac voltage (2Vac) to the dc voltage.
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17. 9.5.1 Voltage Source Inverter
Freewheeling diode
• The inverter interrupts the current several
times each cycle.
• The interruption of an inductive current would
generate unacceptably high overvoltage.
• This overvoltage generation is eliminated by
providing freewheeling diodes connected in
parallel with the switches.
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18. 9.5.1 Voltage Source Inverter
Frdeieowdheeeling
• When the
switches
open, the
current, if
inductive, is
diverted to
the diodes, as
shown in
Figure 9.47
Current when switches S1 and S2 closed and S3 and S4 open
Current when switches S1 and S2 open and S3 and S4 open
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Rload
Vdc
Vac
Idc
Lload
S1
S4
S3
S2
Figure 9.47 Freewheeling diode operation.

19. 9.5.1 Voltage Source Inverter
Rload
Current when switches S1 and S2 closed and S3 and S4 open
Current when switches S1 and S2 open and S3 and S4 open
Idc
Vdc
Lload
Vac
S1
S4
S3
S2
Figure 9.47
Freewheeling
diode
operation.
• The diagram shows the current path when switches S1 and S2 are
closed, and switches S3 and S4 are open.
• When switches S1 and S2 open (now all switches are open), the
current diverts through the diodes of switches S3 and S4.
• This current diversion prevents the interruption of inductive current.
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20. 9.5.2 Line Commutated Inverter
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21. 9.5.2 Line Commutated Inverter
• The current source inverter operation requires
both ac and dc sources
• In addition to an inductance that maintains the
dc current constant or at least assures
continuous dc current.
• The inverter operation requires a delay angle
between 90° and 180°.
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22. 9.5.2 Line Commutated Inverter
• Figure 9.48 shows bridge
circuit generated waveforms
for 90° and 170° delay angles.
– (a) shows that the delay
angle between the ac
voltage
– The square-shaped ac
current is 90°
– The average dc voltage is
zero
– Because the magnitude
and duration of the
positive and negative
segments of the dc voltage
are identical.
200
160
120
80
40
0
40
80
120
160
200 0 30 60 90 120 150 180 210 240 270 300 330 360
w×t
deg
(a) Delay angle of 90°
Vac(t)
V
Vdc( t,a)
V
Iac( t,a)
A
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23. 9.5.2 Line Commutated Inverter
• Figure 9.48 shows bridge
circuit generated waveforms
for 90° and 170° delay angles.
– (b) shows that the delay
200
160
120
80
40
0
40
Vac(t)
V
Vdc( t ,a)
V
Iac( t,a)
A
angle between the ac voltage
80
and the square-shaped ac
120
160
current is 170°,
– The average dc voltage is
negative
– Because the duration of the
positive part of the dc
voltage is almost non-existent
(i.e., 10° out of 180°).
. This implies that the power flows
200 0 30 60 90 120 150 180 210 240 270 300 330 360
w×t
deg
•(b) Delay angle of 170°
from the dc to the ac side
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24. 9.5.2 Line Commutated Inverter
• The practical use of the line-commutated inverter
requires the regulation of the dc voltage to maintain
constant dc current.
• Figure 9.49(a) shows the bridge converter circuit that
produces the voltages shown in Figure 9.48.
• Using the Thévenin equivalent, the converter can be
replaced by a dc source and impedance, as shown in
Figure 9.49(b).
• The average dc current in this equivalent circuit is:
-
V V
I dc_inv dc_source
dc
R
=
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25. 9.5.2 Line Commutated Inverter
• The practical use of the line-commutated inverter
requires the regulation of the dc voltage to maintain
constant dc current.
• Figure 9.49(a) shows the bridge converter circuit that
produces the voltages shown in Figure 9.48.
Vdc_inv
Idc
Th1
Th3
Vac Vdc_inv Vdc_source
A) Circuit diagram
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Vdc_source
Vac
AC
Th4
Th2
Idc
Voltage R
difference
Voltage
difference
A B
Figure 9.49 Single-phase line-commutated inverter B) Equivalent circuit

26. 9.5.2 Line Commutated Inverter
Vdc_inv
Idc
Figure 9.50 Voltage difference during
inversion.
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Vdc_source
Voltage
difference
R
• The converter delay angle
and the dc source voltage
must be controlled
simultaneously to maintain a
constant voltage difference
and dc current.
• In a practical circuit, the
required voltage difference is
generally small
• Consequently, both the
inverter-produced voltage
and the source voltage must
be negative, as shown in
Figure 9.50.

27. 9.5.2 Line Commutated Inverter
• The dc current can be
maintained constant by
keeping the voltage difference
constant.
• Consequently, if the delay
angle increases, Vdc_inv is
reduced;
• The maintenance of constant
dc current and voltage
difference requires the
appropriate reduction of the
Vdc_source.
Vdc_inv
Idc
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Vdc_source
Voltage
difference
R
Figure 9.50 Voltage difference during
inversion.

28. 9.5.2 Line Commutated Inverter
Vdc_inv
Idc
Figure 9.50 Voltage difference during
inversion.
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Vdc_source
Voltage
difference
R
• If the firing angle
increases beyond 90°,
– Vdc_inv becomes
negative;
– The maintenance of
constant dc current
and voltage difference
requires the change
of Vdc_source polarity.
• In this case, the
power flows from the
dc to the ac circuit,

29. 9.5.2 Line Commutated Inverter
Vdc_inv
Idc
Figure 9.50 Voltage difference during
inversion.
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Vdc_source
Voltage
difference
R
,
• The current and
voltage have
– the same direction in
the dc source
(generator)
– opposite directions in
the inverter (load) as
demonstrated

30. 9.5.3 High Voltage DC transmission
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31. 9.5.3 High Voltage DC transmission
• High-voltage dc lines are used to transport large
amounts of energy over a long distance.
• A representative application is the Pacific DC Intertie,
which interconnects the Los Angeles area with Oregon.
• The voltage of the DC Intertie is ±500 kV and the
maximum energy transport is 3100 MW.
• More than one hundred dc transmission systems
operate around the world, one of the oldest and most
famous is the cable interconnection between England
and France.
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32. 9.5.3 High Voltage DC transmission
Transformer Inductance Inductance
AC
filter
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AC
filter
DC
filter
DC
filter
DC Line (+)
Converter DC Line (-)
Converter
Transformer
Figure 9.51 Concept of high-voltage dc
transmission

33. 9.5.3 High Voltage DC transmission
• Figure 9.51 show a simplified connection
diagram for a high-voltage dc (HVDC) system.
• The major elements are two converter stations
interconnected by a dc transmission line.
• The converter station can operate in both
inverter and rectifier modes, which permits
energy transfer in both directions.
• Each converter station contains two converters.
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34. 9.5.3 High Voltage DC transmission
• The two converters are connected in series at
the dc side. The series connection node (middle
point) is grounded.
• One of the converters generates the positive,
and the other produces the negative dc voltage.
• The harmonics are filtered at both the ac and
dc sides.
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35. 9.5.3 High Voltage DC transmission
• Each converter is typically supplied by a
transformer. The HVDC system uses two
different transformer types at each converter
station.
– One of the converters is supplied by a wye-wye
transformer
– The other is connected to a wye-delta transformer.
• This produces a 30° phase shift between the dc
voltage outputs of the two converters.
• The phase shift produces a smoother dc output
voltage.
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36. 9.5.3 High Voltage DC transmission
• Figure 9.52 shows the converters in a HVDC
station.
• Each converter contains six high voltage valves,
with several hundred thyristors connected in
series.
• The valves are shielded by rounded aluminum
electrodes as shown in the Fig 9.52
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37. 9.5.3 High Voltage DC transmission
Figure 9.52
Valve hall of a
DC converter
station.
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