Investigation of cell voltages and arm currents in double-star modular multilevel converters

This study investigates the cell voltages and arm currents in double-star modular multilevel converters (DSMMCs) based on power balance and current tracking control law. DSMMCs have been widely applied to static synchronous compensators, high-voltage direct-current transmission, back-to-back power transfer systems, wind/solar power generation and battery storage systems. The size of capacitors in each cell plays a critical role in designing an MMC, and it requires to find explicit expressions for cell voltages which highly relate to their arm currents. This paper presents unique approaches to deriving cell voltages in terms of arm currents and current tracking control laws, from which cell-voltage ripple reduction can be achieved. Moreover, optimization of cell voltages and arm currents can also be conducted. Simulation results obtained from a 10 kW single-phase MMC have verified the analysis and discussion.