Environmental Engineering Reference
In-Depth Information
Hysteresis band
Upper band
Actual current
Lower
band
Reference
current
0
π/4
π/2
3π/4
π
ωt/rad
(a) Hysteresis band, actual and reference currents
π/4
π/2
3π/4
π
0
ωt/rad
(b) Gate signal
Figure 1.12 Principle of hysteresis control
input voltage. The error between the reference output voltage
v o
is fed into a PI controller to generate the right amount of the current to be drawn from the
source, which is multiplied with the per-unit input voltage sin(
v or e f and the actual voltage
ω
t ) as a synchronisation signal
to generate the reference input current. The input current of the rectifier is controlled via a
hysteresis controller to track the reference input current.
The principle of the hysteresis control is shown in Figure 1.12. When the actual current is
below the lower boundary of a hysteresis band (HB) around the reference current, the upper
switch is turned on and the lower switch is turned off, which causes the actual current to
increase. When the actual current exceeds the upper boundary of the HB, the upper switch Q 1
is turned off and the lower switch Q 4 is turned on, which causes the current to decrease. As a
result, the PWM signal for the upper switch Q 1 is generated as follows
ON if i
1
2 HB
<
i ref
,
Q 1 =
1
OFF if i
>
i ref
+
2 HB
.
The PWM signal for Q 4 is complementary and the PWM signals for Q 2 and Q 3 can be
determined accordingly.
The relevant curves from the circuit are shown in Figures 1.11(c) and 1.11(d). It can be seen
that the input current is in phase with the input voltage. The output voltage is maintained well
although there are some ripples, which can be addressed with other mechanisms. It is worth
noting that it is possible to control the power factor at other values by changing the phase of
the synchronisation signal.
The same principle can be applied to a three-phase PWM-controlled rectifier shown in Figure
1.13(a), with a slightly changed control strategy shown in Figure 1.13(b) to accommodate the
other two phases. The relevant curves are shown in Figures 1.13(c) and 1.13(d). All the
three-phase currents are controlled to be sinusoidal and in phase with the corresponding
phase voltages. Because the instantaneous power flowing into the converter is constant for a
 
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