Environmental Engineering Reference
In-Depth Information
dT s
1
0
q
-1
V +
u N
0
V -
(k+1) T s
kT s
Time
Figure 11.1 Firing pulse q and inductor voltage u N
Assume that the average of the firing pulse during a switching period is p (with themagnitude
of
±
1), as shown in Figure 11.1, then the duty cycle of the firing pulse is
1
+
p
d
=
2
and the average of the voltage u N (i.e. the inductor voltage), as shown in Figure 11.1, is
u N =
dV + +
(1
d ) V
1
+
p
1
p
=
V + +
V
2
2
p
2 V DC +
1
2 ε.
=
(11.2)
Combining the Laplace transformation of (11.1) and (11.2), the block diagram of the neutral
leg can be obtained as shown in Figure 11.2.
The amplifying effect of the IGBT bridge (the block
V DC
2
in Figure 11.2) can be can-
2
celled/scaled by
V DC in the controller to be designed and, hence, ignoring this block does
not affect the control design. Moreover, the block 2 in Figure 11.2 can be cancelled by the
proportional gain of the controller to be designed because the setpoint for the shift
ε
is 0.
i N
i L
1
ε
p
u N
1
V DC
sL N
-
sC N
2
1
2
Figure 11.2 Block diagram of the independently-controlled neutral leg
 
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