Comparison of modulation strategies for a cascaded H-bridge StatCom: Part 1: theoretical background

Phase shifted carrier modulation has become an industry standard in its application to multi-level H-bridge Stat-Coms. The technique uses the cancellation of harmonics within each phase-leg to significantly improve the harmonic performance relative to the switching frequency. The purpose of this paper is to investigate whether the practical implementation of the technique deteriorates the harmonic cancellation. The effects of non-uniform DC bus voltage, transient capacitor voltages and phase shift error are investigated. A comparison with alternative modulation strategies is developed in terms of a defined set of performance indicators. Part 2 of this paper applies the developed analysis to a particular StatCom and investigates the effect of increasing the StatCom level number.