Abstract

Medium voltage ac drives use multi-pulse, multi-cell or multi-level power converter topologies in order to meet the power quality requirements. In fact, they can easily cancel out low order unwanted harmonics while keeping an even distribution of the load power through out the power semiconductors. Multi-pulse configurations based on active switches are used at the ac/dc stage in order to control the fundamental current component. This allows the control of the power sent to the load while operating at reduced current distortion and unity displacement power factor. For instance, in a 12-pulse configuration, the fundamental component control could be achieved by introducing a chopping angle at the gating pattern as indicated by the Selective Harmonic Elimination technique. Thus, the configuration can handle the fundamental component at expense of the injection of the llth and 13th harmonics into the ac system and the 12th harmonic to the load, as the lowest unwanted harmonics. This paper proposes that by adding just one extra chopping angle at the gating pattern level, the first unwanted low frequency harmonic is the 24th. This improvement is achieved without any penalties in the ac waveforms. In general, for a n-pulse converter, the lowest unwanted harmonic is the (2·n)th harmonic. Simulated and experimental results confirm the theoretical considerations.