Modular multilevel converter based on unipolar-current full-bridge submodules for medium voltage variable speed drive applications

With the continuous development of Modular Multilevel Converters (MMC), the introduction of new Sub-Modules (SMs) and their operational characteristics has become favourable for optimizing the performance of many applications such as Variable Speed Drives (VSD), where the conventional ac/dc/ac MMC converters based on Half Bridge (HB) face challenges in low- frequency operation. This paper focuses on the main features of the Unipolar Current (UC) Full- Bridge (FB) SM (UC FB-SM) and benefits of its integration into the MMCs for VSD applications. Basic operation methods of the HB and UC FB based MMCs are analysed and compared, and the duality between their operational modes is highlighted. Main advantages of utilization of the UC FB MMCs in VSD applications are identified and illustrated in the simulation results presented.