Fig. 3 (a) Single line diagram of DSTATCOM (b) Phasor Diagram
Fig. 3(a) shows the single line diagram of single phase DSTATCOM
connected to a single phase system. Consider fig 3(b) which shows phasor
diagram of compensated single phase system.
Here, id is direct axis compensating current,
iq is the reactive component of compensating current,
Vt is the load voltage which is needed to be kept at 1
p.u. Current ‘i ’ is the necessary compensating current which is
to be injected to the system and it is the vector sum of
id and iq
The compensated current i is given as:
i =
I*sin( \(\omega t+(\phi_{1}\mathbf{+\ }\delta\mathbf{\ }\))) (2)
which is resolved to,
i =
I*cos \(\mathbf{\ }\delta\)*sin( \(\omega t+\phi_{1}\))
+
I*sin \(\mathbf{\ }\delta\)*cos( \(\omega t+\phi_{1}\)) (3)
i = id*sin( \(\omega t+\phi_{1}\)) +iq *cos( \(\omega t+\phi_{1}\)) (4)
where id = I*cos δ and
iq = I*sin δ (5)
Here, id = 0 since constant DC source is connected
across VSC terminals and iq is the q-axis current
component which is required to maintain PCC voltage at 1 p.u.
Here one should note that for extraction of reference signals for the
other two phases the reference signal generated for the respective
phases should be displaced by an angle of 120 degrees each.
Fig. 4 shows the control block diagram for reference signal generation
for DSTATCOM using concept of single phase d-q theory for VCM operation.
From the fig. it is seen that load voltages Vta,
Vtb, Vtc are individually compared with
1 p.u value and the error in these values are sent to three separate PI
controllers. These errors in each of the three phases generate
iqa, iqb, iqc from the
PI controllers. iqa, iqb,iqc are q-axis reactive current component as observed in
eq.(5). Now as seen from equation (4), iq current obtained from PI
controllers are then multiplied with cosωt to obtain
i*a. For obtaining
i*b and
i*c the function cosωt is
shifted by 120 and 240 degrees respectively. Here, a constant voltage
source is used for the VSC and thus the d-axis current component or
active current component id is zero.