Environmental Engineering Reference
In-Depth Information
12
H
∞
Voltage-Current Control
of a Neutral Leg
In Chapter 11, a voltage-current controller is designed with classical control techniques to
control an independently-controlled neutral leg. In this chapter, the advanced
H
∞
control
technique is applied to design a controller so that it leads to very small deviations of the neutral
point from the mid-point of the DC link, in spite of possibly very large neutral currents.
12.1 Mathematical Modelling
The independently-controlled neutral leg shown in Figure 10.3 is re-drawn in Figure 12.1
for convenience. The objective of the neutral controller is to generate firing pulses to turn
the switches
S
+
and
S
−
ON or OFF, in a complementary way, so that the neutral point is
very close to the mid-point of the DC link. This can be achieved by maintaining the inductor
current
i
L
equal to the neutral current
i
N
so that no current flows through the capacitors. Thus,
the capacitors maintain constant and equal voltages and, hence, the neutral point remains
balanced. The mathematical model of the neutral leg is established in a slightly different way
from that in Chapter 11, taking into account the resistance of the inductor and the mismatch
of the capacitors.
Assume that all voltages are measured with respect to the neutral point
N
. The DC-link
voltage is
V
DC
=
V
+
−
V
−
,
(12.1)
and the mid-point (or average) of the DC link, which is desired to be 0, is
V
+
+
V
−
V
a
v
e
=
.
(12.2)
2
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