On the Emerging Class of Non-Isolated Modular Multilevel DC–DC Converters for DC and Hybrid AC–DC Systems

Explosive levels of research into modular multilevel dc–dc converters (dc–dc MMCs) for dc and hybrid ac–dc systems are currently being witnessed. A new class of dc–dc MMCs has recently emerged that obviates the traditional intermediate ac link with galvanic isolation by internally circulating ac power among the converter switching cells. This novel power transfer mechanism opens the door to an array of new non-isolated converter structures with broad application in MVDC and HVDC systems. This paper gives a comprehensive overview of dc–dc MMCs that exploit the circulating ac power concept. Emergent topologies suitable for use in dc and hybrid ac–dc grids are identified and collated into the following categories: 1) two-terminal and multi-terminal dc–dc MMCs; 2) dc–dc MMCs with ac grid interface; 3) dc line power tapping devices; and 4) dc line power flow controllers. The survey provides researchers with an overall snapshot of technology development for this promising new class of converters in key areas relevant to smart grids.