There are two techniques for controlling the STATCOM. The first technique, referred to
as phase control, is to control the phase shift α to control the STATCOM output voltage
magnitude. The other technique referred to as Pulse Width Modulation (PWM) on the
other hand allows for independent control of output voltage magnitude and phase shift; in
this case, the DC voltage is controlled separately from the AC output voltage.
The STATCOM increases voltage stability margin of the system by providing active and
reactive power at the connected bus. Moreover, this device does not significantly alter the
existing system impedance, which is an advantage over SVC.
In summary, STATCOM has better characteristics over SVC; when the system voltage
drops enough to force the STATCOM output to ceiling, its maximum reactive power
output will not affect by the voltage magnitude. Therefore, it exhibits constant current
characteristics when the voltage is low under the limit. The steady state power exchange
between the controller and the AC system is mostly reactive, as active power is only
consumed to supply for the internal losses.