Environmental Engineering Reference
In-Depth Information
The line-to-line voltage at the load can be found for phases a to b as follows:
U ab ð a n Þ¼ U dc ½ a ða n Þ b ða n Þ
1
2 ½ 1 þ m i sin ðw t Þ
1
2
2
3 p
U ab ð a n Þ¼ U dc
1 þ m i sin w t þ
ð 6 : 12 Þ
p
2 m i U dc sin a n þ
p
6
U ab ð a n Þ¼
The load line-to-line, and hence, line-to-neutral voltage are completely defined
by (6.12) for the stated dc link voltage, modulation index and modulating function
waveform. Other investigators, Kaura and Blasko [6], for example, have worked to
extend the linearity of sinusoidal PWM into the pulse dropping region depicted in
Figure 6.8 by adding a square wave to the modulating function discussed above.
With this scheme, the modulator maintains linearity from full PWM through the
pulse dropping region and up to six step square wave mode.
The pulse dropping region defines the zones about the vertices of the hexagon
where the velocity of the voltage vector must increase with increasing modulation
depth. In the limit, as the voltage vector reaches the limit of the outer circle that
inscribes the hexagon, it exists only at the vertex points and has infinite velocity
along the sides of the hexagon as the voltage vector steps from one inverter voltage
state to the next. This is the six step square wave mode.
6.3 Resonant pulse modulation
That a vast number of inverter topologies exist that are potentially better suited to
matching the performance levels of classical PWM voltage source inverters (VSIs)
was the contention posed by Professor Divan [7]. Indeed, all power converters can be
classified into hard or soft (i.e. resonant) switching. Soft switching inverters have
significantly reduced device stresses and can be found in topologies ranging from
resonant converters, resonant link converters, to resonant pole inverters. The most
popular appear to be topologies that employ a high frequency resonant LC circuit in
the main power transfer path. There have been numerous attempts over the years to
incorporate series resonant and parallel resonant elements into the power transfer path.
This section discusses the more popular high frequency resonant dc link converter.
Figure 6.10 is the schematic of the power stage and representative waveforms of
the resonant dc link inverter. In this circuit, the resonant dc bus is implemented by the
addition of a small inductor and capacitor along with one additional semiconductor
switch at the input of a conventional six switch VSI. When the dc link switch is gated
ON, current from the supply charges the inductor linearly. When the link converter
switch is gated OFF, the inductor discharges into the capacitor forming a resonant
pulse at the VSI inverter input. The main switches in the VSI inverter are then gated
ON when the dc link voltage resonates to zero, thereby permitting true zero voltage
switching. Rather than implementing this auxiliary switch, it is more common to
utilize the existing VSI bridge switches to charge the link inductor.
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