Abstract:  ABSTRACT： Because of the preferable performance, modular multilevel converters （MMCs） have a wide application prospect in some fields including large-scale interconnection of power grids. Firstly, this paper analyzes the operational principle of MMC system, and combines the method of redundant configuration to design the optimal quantity of sub-modules on MMC arms. And then, based on the nearest level modulation （NLM） algorithm, an improved control strategy for capacitor voltage balancing of MMC is proposed, in which redundant sub-module has the charging priority to ensure its voltage stay at the level higher than the lowest standard, so that failure sub-module may be replaced smoothly. Finally, the effectiveness of the proposed control strategy is verified through simulation in PSCAD/EMTDC. 

Key words:  KEYWORDS：  modular multilevel converter, redundant configuration, capacitor voltage balancing strategy, nearest level modulation, redundancy protection of sub-module