Environmental Engineering Reference
In-Depth Information
14.4.2 Control of the Rectifier Leg
As mentioned before, an important function of the rectifier leg is to inject the right amount
of current to maintain a stable DC-link voltage. This can be achieved by introducing a PI
controller, as shown in Figure 14.6(b). Because there are ripples in the DC-link voltage at the
doubled supply frequency, a low-pass filter, e.g. the hold filter
e
−
Ts
/
2
Ts
1
−
H
(
s
)
=
,
/
2
where
T
is the fundamental period of the supply, can be adopted to remove the ripples of
V
DC
for feedback. The output of the PI controller actually plays the role of the reference peak
amplitude of the current drawn from the supply. The reference current drawn from the supply
is then obtained by multiplying it with the phase signal sin
t
of the supply voltage provided
by the synchronisation unit shown in Figure 14.6(a). This makes sure that no reactive power
is drawn from the supply. The reference current of
i
s
for the rectifier leg is then obtained by
subtracting
i
a
from the reference current drawn from the supply. As a result, all the harmonic
current components in
i
a
are automatically diverted into the reference current of
i
s
and no
extra effort is needed to suppress the current harmonics in the supply current. What is left is
to design a current controller so that the current
i
s
tracks the reference current.
Many current controllers, e.g., hysteresis controllers (Tilli and Tonielli 1998) that have a
variable switching frequency and repetitive controllers (Hornik and Zhong 2011a) that have a
fixed switching frequency, can be applied to track the reference current of
i
s
. Here, a simple
hysteresis controller, which is easily implemented, is adopted. For this purpose, the current
i
s
is measured for feedback. Since both the supply voltage and the supply current
i
ω
=
i
a
+
i
s
are
available, a power meter can be easily embedded into the system for metering.
It is worth noting that a suitable band for the hysteresis controller should be selected. A
small band offers good tracking performance but leads to high switching frequencies while a
large band offers low switching frequencies but leads to a large tracking error.
14.4.3 Control of the Neutral Leg
The neutral leg included in Figure 14.4, which consists of switches
Q
3
and
Q
4
, one inductor
L
r
and two split DC-link capacitors, was studied in detail in Chapters 10-13. It is the combination
of a split DC link and a neutral leg with the advantage of being controlled separately (Zhong
et al
. 2006). Assume the voltages across the capacitors
C
N
+
and
C
N
−
with respect to the
neutral point
N
are
V
+
and
V
−
, respectively. Then the DC-link voltage is
V
DC
=
V
+
−
V
−
(14.3)
and the voltage of the neutral point
N
with respect to the mid-point of the DC link, which is
the reference point common to the supply voltage and the phase voltages generated, is
V
DC
2
V
+
+
V
−
V
ave
=
V
+
−
=
.
(14.4)
2
The main task of the neutral-leg controller is to maintain the voltage
V
a
v
e
close to zero via
controlling
Q
3
and
Q
4
. At the same time, it provides a return current path for the rectifier leg
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