Modelling of custom power equipment using harmonic domain techniques

A new and comprehensive harmonic domain model of a three-phase, six-pulse PWM STATCOM is presented in this paper. The model takes proper account of the DC capacitor effect and it comes in the form of a three-phase Thevenin equivalent expressed in the harmonic domain, where switching functions are used to represent with ease the PWM control firing sequences. The harmonic impedance of the Thevenin equivalent shows high cross-couplings between phases and between harmonics, an effect which is strongly influenced by the STATCOM capacitor size. Results are presented which show that the PWM STATCOM observes quite different harmonic voltage response when it is made to operate as a reactive power source and when it is made to operate as a sink. This effect can not be observed with steady state models that use a voltage source representation to model the STATCOM