Direct Digital Controlled Modular Multilevel Converters with Cell-Distributed Controllers and Hot-Swap Features

This paper presents direct digital controlled modular multilevel converters (MMCs) with cell-distributed controllers and hot-swap features to facilitate cell-module number extension and achieve fault-tolerance mechanism. Design of the cell-distributed control system is to facilitate an extension of cell-module number, increase MMC reliability, and simplify the complexity of fault-tolerance mechanism. The direct digital control can take into account wide filter-inductance variation and source voltage harmonics, and achieve a seamless hot-swap feature with low distortion on output currents and low interaction with grid voltages. In this paper, the cell-distributed control system is introduced first, and then the fault-tolerance mechanism for either cell-module unplugging or plugging-in is presented in detail. Experimental and simulated results obtained from an MMC operated in power-injection and with a hot-swap feature have verified the analysis and discussion.