Page 336 - Power Electronic Control in Electrical Systems

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH008.3D ± 324 ± [290±372/83] 17.11.2001 10:29AM

324 Transient studies of FACTS and Custom Power equipment

The switching control of the VSC valves can be executed using two different
techniques:
1. Fundamental frequency switching (FFS), where the switching of each semicon-
ductor device is limited to one turn-on and one turn-off per cycle. With this tech-
nique the conventional six-pulse VSC produces a quasi-square-wave output with
the inherent high harmonic content, as previously discussed in Chapter 6. As will
be described later, when FFS is chosen, several six-pulse units are combined to
form a multi-pulse structure in order to achieve better waveform quality and
higher power ratings.
2. Pulse-width-modulation (PWM), where the semiconductor switches are turned on
and off at a rate considerably higher than the power frequency. The output
waveform is chopped and the width of the resulting pulses is modulated. This
shifts the undesirable harmonics in the output to higher frequencies and filtering is
possible with smaller components as was also illustrated in Chapter 6.
Both, the fundamental frequency and PWM switching approaches suffer from cer-
tain drawbacks for utility applications:

. The fundamental frequency switching approach requires complex transformer
configurations to achieve low waveform distortion. In addition, the low switching
frequency constrains the response rate and precludes incorporation of active
harmonic filtering. Nevertheless, this is currently the preferred approach because
of lower losses and higher semiconductor switch utilization. Present implementa-
tions of the STATCOM in high-voltage applications employ fundamental fre-
quency switching.
. The PWM approach results in high switching losses and the fundamental fre-
quency output obtainable is somewhat reduced. Consequently, at present PWM is
less attractive for utility applications that require high efficiency and large power
ratings. However, it has certain advantages such as faster response and capability
for harmonic elimination, which could be exploited in the future with semicon-
ductor switch improvements. However, PWM is preferred for CP low-voltage
applications.
In low-to-medium voltage CPapplications of VSC, PWM is widely used for suppres-
sion of harmonics and control of DC to AC voltage ratio. However, for FACTS
applications, the high ratings of the converter will require valves of high power ratings,
dictating slow switching speed and increased switching losses. With regards to the VSC
electronic valves, VSCs using GTOs as the power devices are preferred for FACTS
applications whereas VSCs using IGBTs are preferred for low-voltage CPapplications
(Zaho and Iravani, 1994; Edwards and Nannery, 1998; Raju et al., 1997).

8.6.1 STATCOM used as a FACTS controller

The output voltage waveform of the conventional six-pulse VSC contains harmonic
components with frequencies of [6k 1] f (and its input current has related harmonic
components with frequencies of 6kf ), where f is the fundamental output frequency
and k ˆ 1, 2, 3, ... The high harmonic content of the output voltage makes this
simple inverter impractical for high-power applications.