Direct Power and Torque Control of AC/DC/AC Converter-Fed ...

3. Vector Control Methods of AC/DC/AC Converter-Fed IM Drives – A Review
Further, for DPC needs it is assumed that R is negligible small and the switching
devices in the VSR are ideal (i.e. they do not need dead-time and there is no forward
voltage drops).
From Fig. 3. 8 it can be seen that there are two power loops: for active P , and
reactive Q ones. Command active power P c
is controlled by DC-link voltage loop,
while the command reactive power
Q c
is given from the outside of the control
scheme. Usually reactive power is set to be zero, to obtain a unity power factor
operation. The DC-link voltage is maintaining to be constant by appropriate active
power adjustment. Estimated values of the active power P and reactive power Q are
compared with commanded values. The power errors e P
and e Q
are input signal to
hysteresis comparators. At the output of the comparators are digitized signals S
P
and
S
Q
.
In classical (voltage based) DPC from predefined switching table, based on
signals
S
P
, and S Q
, and position of the line voltage
γ
UL
, the appropriate voltage
vector is selected. In virtual flux based VF-DPC instead of γ
U
the position of the
L
virtual flux
γ
Ψ
are utilized in control algorithm. The outputs from the predefined
L
switching table (Tab. 3. 2) are the switching states S 1
for the VSR. The powers
hysteresis is defined in the same way for active and reactive power, therefore only
for active power is given:
Where,
S
P
= 1 for e
P
> H
P
(3. 39)
S
P
= 0 for eP
< −H
P
(3. 40)
H
P
is a hysteresis band of the active power (Fig. 3. 19).
S P
H P
e P
Fig. 3. 19. Power hysteresis controller - two level
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