Environmental Engineering Reference
In-Depth Information
V +
R
~
C N+
p
S 7
S
~
N
V DC
u N
i L
L N
i C
S 8
T
~
i N
C N-
p
V -
neutral
Figure 13.1
Independently-controlled neutral leg under consideration
Since the control objective is to maintain the point N as a neutral point, the average voltage
V + +
V
V a v e =
(13.1)
2
is expected to be as close to 0 as possible all the time. As a result, i L is expected to be almost
equal to i N . This means the majority of the neutral current should flow through the inductor
L N but not through the capacitors. A small current i C would lead to a small variation of the
neutral point N . The capacitors do not have to be chosen to be very large, like those needed in
the conventional split DC link.
The block diagram of the independently-controlled neutral leg is shown in Figure 13.2,
where p is the average of the control signal over one switching period normalised with respect
to half of the DC-link voltage. This is very similar to Figure 12.3, but in a different form to
highlight that the control objective is to make i C as small as possible while maintaining the
stability of the system. The duty cycle for the upper switch is
+
p
1
=
.
d
2
The resistance of the inductor is taken into account and the capacitors C N + and C N could be
different.
i N
1
V
u N
i L
+
i C
p
+
DC
sL
+
R
-
2
+
N
N
V c
1
s
(
C
+ +
C
)
N
N
Figure 13.2 Block diagram of the independently-controlled neutral leg
 
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