Beyond the MMC: Extended Modular Multilevel Converter Topologies and Applications

The unique advantages of modular multilevel converters (MMCs) have led to wide adoption of the converter topology in high voltage dc (HVDC) transmission systems, with great potential for use in medium voltage motor drive and dc grid applications. Inspired by the structure of the MMC, many extended converter topologies have been developed in the literature. These include, in addition to conventional parallel multiphase connection, a variety of series-connected topologies. Extended MMCs integrate the modular nature of cascaded SMs of the MMC but with different topology structure leading to substantially different operating principles. Given the growing number of such topologies, this paper provides a critical review of MMC derived topologies and summarizes individual advantages and disadvantages. The control schemes and application fields of these different topologies are analysed. This review facilitates the classification and understanding of extended MMCs and provides the necessary scaffold for the development of a general framework for new topologies and applications based on the MMC.