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Power electronic control in electrical systems 289

7.8 Conclusion

This chapter has discussed the main adverse effects caused by harmonics in electrical
equipment, in particular in industrial installations where capacitors used for power-
factor correction become severely affected by the presence of harmonics. The all-
important problem of parallel resonances caused when banks of capacitors are
combined with the inductance of the AC system, is discussed in some detail. Simple
equations are used to examine this problem from the quantitative point of view and a
numerical example is presented.
Comprehensive harmonic domain models are presented for the TCR, the SVC and
the TCSC. The SVC achieves fast and accurate voltage magnitude control at its point
of connection with the AC network due to the TCR, which can be set to absorb a
variable amount of reactive power with very little delay. However, an operational
drawback of this scheme is that the TCR achieves its main operating point at the
expense of generating harmonic currents. The order and magnitude of these har-
monics being a function of the thyristors' firing angles. The harmonic domain models
presented in this chapter enable realistic studies of both TCR and SVC equipment
connected to AC networks of any size and complexity. The models are developed in
the phase domain to incorporate frequency dependent, multiphase transmission
systems, which may be very unbalanced at harmonic frequencies. Furthermore, the
three-phase SVC representation also caters for TCR imbalances in either its firing
angle control or in the linear inductors or capacitors. Numerical examples are
provided to illustrate the usefulness of these models in power systems harmonic
studies.
The discussion also applies to the TCSC, where the fast acting, regulating char-
acteristic of the TCR enables the TCSC to shorten or to lengthen the electrical
distance of the compensated transmission line with almost no delay. This character-
istic of the TCSC is being exploited in high-voltage transmission installations to
provide instantaneous active power flow regulation. However, TCSC harmonic
generation and the existence of resonant conditions inside the TCSC call for accurate
and comprehensive analysis tools. The harmonic domain models presented in this
chapter are suitable to study such complex phenomena.