Comparison of Modular Multilevel Topologies for AC to AC Applications

This paper presents a comparison of different AC to AC Modular Multilevel Converter (MMC) concepts for selected distributed energy generation applications. The MMC topology is a highly suited and investigated candidate for high voltage transmission systems ([1]-[3]). The applications selected for this paper are an medium voltage AC to AC grid coupling system and an medium voltage converter based wind generator connection to the AC grid. The comparison is done for different circuit configurations using the most attractive cell topologies. The power capability of the different circuit and cell combinations is shown based on a common power semiconductor and energy storage hardware. Real device limitations including not only semiconductor losses but also real cell capacitor AC voltage and current limits are taken into account. Furthermore realistic application data with respect to AC voltage range, power factor and current capabilities are included. This allows a realistic and application related comparison of the required power electronic hardware based on real device data over the entire P/Q range. Finally, the hardware limitations become evident which allows the evaluation of appropriate hardware changes to adapt the required performance.