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Power electronic control in electrical systems 261

harmonics at 5th and 7th frequencies. Moreover, 11th and 13th harmonic frequencies
must be dealt with via the design of tuned passive filters. Figure 6.96 shows a typical
passive filter configuration for a 12-pulse rectifier with shunt filters tuned at 5th, 7th,
11th, and 13th harmonics.
The application of active filtering as shunt configuration based on a VSC technol-
ogy has the advantage of low converter ratings and simplicity. Such systems of course
for high power applications are not feasible, as most active filtering schemes pre-
sented in the previous section require high bandwidth PWM converters to success-
fully eliminate harmonics below the 13th. Simply said such a solution is also not cost
effective for high power applications (>10 MW).
Hybrid topologies can be considered. The main motivation to use hybrid solution
is to reduce the ratings of the active filter. Figure 6.97 shows a hybrid active/passive
shunt filter system suitable for 12-pulse rectifiers. Specifically, two VSC based active
filters for the 5th and 7th harmonics are used in combination with series connected
passive filters tuned at 11th and 13th harmonic frequencies. Because the application
involves high power, six-step three-phase inverters can be employed. The rating for
the inverters is no more than approximately 2% of the load kVA rating.

6.10 Advanced concepts in reactive power control
equipment

Many new reactive power control equipment based on the VSC topologies presented
earlier have been under research and development over the last decade. A number of
them have been developed and successfully implemented. These include the UPFC,
and the interline power flow controller (IPFC). The UPFC is not discussed in this
chapter, but it is treated in other parts of the book.

6.11 Conclusion

In this chapter, the conventional thyristor-controlled equipment for reactive power
control in power systems has been presented first. The voltage source converter
topologies were then presented in detail including multilevel converter topologies.
Such topologies will have a major impact on the applications of reactive control and
power quality equipment in the industry. New systems based on energy storage,
namely the flywheel technology and superconductor energy storage systems were
then briefly discussed to highlight the potential of this technology. HVDC systems
with conventional technology and using VSC technology and PWM systems were
also presented as a significant application area of high power electronics in the
electric power systems. Finally, active and hybrid filters based on active and passive
technologies were also discussed where the VSC technology will have a major impact
and significant improvements in performance. Many new applications will become
possible in the near future once the limitations of fully controlled devices such as the
IGBT are overcome. This will potentially allow the full replacement of the thyristor