A sept-branch modular multilevel converter for three-phase to single-phase direct AC/AC equal frequency conversion

For medium- to high-voltage AC/AC conversion applications, direct AC/AC conversion scheme by adopting the modular multilevel converter (MMC) shows remarkable advantages on costs and efficiency compared with the AC/DC/AC solution which employs a back-to-back MMC configuration. However, direct AC/AC MMC suffers from branch energy instability when conversion frequencies of input and output are equal. This paper presents a sept-branch MMC (SB-MMC) topology for direct AC/AC conversion applications. The feature of this SB-MMC is its capability of direct AC/AC equal frequency conversion from three-phase to single-phase and vice versa. Balanced branch energy and unity power factor at grid side can be achieved simultaneously under various load conditions. Operating principle as well as control strategy of the SB-MMC are described. Simulation on a 27.5kV/50MVA SB-MMC model for railway power supply is carried out. Simulation results validate the effectiveness of the SB-MMC and the proposed control strategy as well.