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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