Fig. 4 Reference signal generation during voltage unbalance
(VCM)
After obtaining the reference current values,
i*a,
i*b,
i*c are sent to hysteresis controller
where these signals are compared with the actual value of DSTATCOM
injected currents and switching pulses to VSC are generated such that
error is minimum.
3. Reference signal generation during Dual mode:
Now, the working of DSTATCOM in dual functional mode i.e. in CCM and
VCM, and its effective switching between these modes is shown in the
fig.5. The scheme proposed in this paper is based on sensing the source
voltage. If the source voltage in each phase is within the limit, the
DSTATCOM operates in CCM and if the source voltage is out of the limit
then the DSTATCOM operates in VCM.
During normal operation the DSTATCOM works in CCM mode i.e. it makes the
source current sinusoidal and in phase with the source voltage. During
this phenomenon it continuously senses the source voltages of each
phase. If the source voltage of each phase is not maintained at 1 p.u,
then the DSTATCOM switches itself from CCM to VCM mode of operation and
maintains the load voltage of each phase at 1 p.u. Now as soon as the
source voltage comes to its original desired value, DSTATCOM again
switches to CCM operation to maintain unity power factor. This can be
understood more
clearly by observing the control block diagram as shown in fig.6.