This paper describes a simple and clear approach to analyze harmonic instability of phase controlled thyristor converters with input passive filters. It is shown by analysis and experiment that harmonic instability may occur at the parallel resonant frequency between the passive filter and the ac source even if the ac source impedance is low.
A combined system of series active and shunt passive filters has already been proposed as a new approach to harmonic compensation in power systems by the authors. It is confirmed analytically and experimentally that the series active filter used has the capability to suppress the harmonic instability at the parallel resonant frequency. However, it is also shown that the series active filter may cause harmonic instability adversely at lower frequencies. However, harmonic instability at a frequency except for the parallel resonant frequency does not cause any practical problem. The obtained analytical results are confirmed by experiment in detail.