Practical medium voltage converter topologies for high power applications

Multilevel power conversion has been receiving increasing attention in the past few years for high power applications. Numerous topologies and modulation strategies have been introduced and studied extensively for utility and drive applications in the recent literature. These converters are suitable in high voltage and high power applications due to their ability to synthesize waveforms with better harmonic spectrum and attain higher voltages with a limited maximum device rating. Trends in power semiconductor technology indicate a trade-off in the selection of power devices in terms of switching frequency and voltage sustaining capability. New multi-level, high power converter topologies have been proposed using a hybrid approach involving integrated gate commutated thyristors (IGCT) and insulated gate bipolar transistors (IGBT) operating in synergism. This paper is further developing and optimizing this approach presenting a hybrid nine-level inverter operating at a 4160 V system voltage. Excellent current and voltage waveforms can be achieved even in weak network conditions. Additionally it is shown, that the multi-level conversion system can further be simplified by utilizing series connected H-bridges without the need of supply transformers and rectifiers.