Environmental Engineering Reference
In-Depth Information
Table 6.2 VSI and resonant converter comparison
Attribute
VSI
Resonant dc link
Input/output
dc to 3-phase ac
dc to 3-phase ac
Number of active devices
6
6
Switching frequency
2-10 kHz
20-80 kHz
Device ratings
Voltage
1 pu
2.5 pu
Current
1 pu
1 pu
Switching losses
1 pu
0.1 pu
Conduction losses
1 pu
1 pu
Estimated power range of applications
0.5 kW to 1 MW 0.5-300 kW
Size of reactive components necessary (L & C)
0.1-0.5 pu
1 pu/1 pu
Current regulator bandwidth necessary
200 Hz to 2 kHz
1-8 kHz
Cost
Low
Medium
Performance
Acceptable
Acceptable
6.4 Space vector PWM
The most widely used inverter modulation scheme for hybrid propulsion drives is
SVPWM because it yields higher effective voltages at the machine terminals than
conventional sine-triangle PWM or current regulated PWM. In fact, it is true to say
that SVPWM is the most widespread modulation scheme used in all traction drives.
In Section 6.6, the various alternative modulation schemes such as natural sam-
pling, regular sampling, synchronized sampling and others are discussed and
compared for application to hybrid propulsion drives. Most of these techniques are
free running PWM in which the modulating signal and fundamental output signal
frequencies are independent of each other. When the switching frequency is an
integer multiple of the fundamental output frequency, the technique will be called
synchronized sinusoidal modulation. Figure 6.11 is an illustration of synchronized
sampling, also referred to as regular sampling. In this figure, the frequency of the
sampling triangle wave is ten times that of the fundamental wave.
Sinusoidal modulation exhibits good performance for modulation index,
m
i
,
ranging from 0 to 1. If
m
i
>
1, sinusoidal modulation is no longer possible because
of pulse dropping. In some systems, predefined modulating waveforms are used for
overmodulation. Consider a square wave of amplitude
U
dc
. The square wave has an
rms value of
U
dc
and a peak fundamental value of
4
p
U
dc
U
s
1
¼
ð
6
:
13
Þ
U
s
1
U
dc
¼
1
:
27
m
0
¼
