An improved circulating current control startegy for modular multilevel converters through redundant voltage levels

This paper first introduces an existing circulating current controller for modular multilevel converter (MMC) through redundant voltage levels with detailed analysis for the controlled circulating current. To further reduce the peak-to-peak value of the circulating current at switching frequency and decrease the total control error, a fixed-band redundant state transition principle is put forward in this paper, so that the circulating current can be approximately limited within the fixed-band. Compared to traditional control methods, the main advantages of the proposed strategy are unaffected arm voltage reference, simple implementation, reduced switching frequency and very fast dynamic performance. Simulation results verified the effectiveness of the proposed control strategy.