Page 352 - Power Electronic Control in Electrical Systems

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336 Transient studies of FACTS and Custom Power equipment

When the D-STATCOM is in full operation, the DC voltage increases to nearly
28 kV. As shown in Figure 8.54(b), in this period the D-STATCOM absorbs active
power from the AC system to charge the capacitor and maintain the required DC
link voltage level. The reactive power exchange between the AC system and the
compensator is shown in Figure 8.54(c).
The results achieved through the digital simulations clearly show the capability of
the D-STATCOM to mitigate voltage sags providing a continuously variable level of
shunt compensation. The response of the controller is fast, and even when simple, it
is effective for the operating conditions considered in the example.
The D-STATCOM has plenty of applications in low-voltage distribution
systems aimed to improve the quality and reliability of the power supplied to the
end-user. It can be used to prevent non-linear loads from polluting the rest of the
distribution system. The rapid response of the D-STATCOM makes it possible to
provide continuous and dynamic control of the power supply including voltage and
reactive power compensation, harmonic mitigation and elimination of voltage sags
and swells.
There are several factors that must be considered when designing the STATCOM
and associated control circuits. In relation to the power circuit the following issues
are of major importance:
. DC link capacitor size
. coupling transformer reactance and transformation ratio
. output filter equipment.
These elements must be properly selected bearing in mind the application's require-
ments, voltage regulation and power compensation. The DC capacitor has direct
influence on the harmonic distortion of the output voltage generated by the STAT-
COM and the speed of response of the controller. If the capacitor is undersized the
controller's response will be fast but the DC link voltage will have excessive ripple
and consequently the output voltage will contain high levels of harmonic distortion.
Moreover, high transient overshoots will exist. On the other hand, an oversized
capacitor will improve the output voltage waveform shape and reduce the transient
overshoots but at the expense of a sluggish controller's response. Besides, some
oscillations will appear in the STATCOM response and if care is not taken in the
adjustment of the PI parameters, the system may become unstable.
The selection of the coupling transformer parameters has a large impact on the
performance of the STATCOM. It plays an important role in the value of voltage
regulation and power compensation that the STATCOM can provide. In essence, the
amplitude and the phase angle of the voltage drop across the transformer reactance
define the active and reactive power flows between the STATCOM and the AC
system. This reactance can be seen as a smoothing reactor that will attenuate medium
and high order harmonics in the STATCOM output voltage.

8.7 Dynamic Voltage Restorer (DVR)

Similarly to the D-STATCOM, the DVR consists of a VSC, a switching control
scheme, a DC energy storage device and a coupling transformer that in this case, is