Independent Voltage Control Strategy for Cascaded Modular Multilevel Converter -Bidirectional DC/DC Converter under Unbalanced Operation

Modular multilevel converter -bidirectional DC-DC converter (MMC-BDC) has been proposed to integrate the medium voltage (MV) DC bus with the distributed energy storage elements (ESE) in the second generation of vessel integrated power system (IPS). However, the level of imbalance in the sub-module (SM) power distribution of this topology is limited, which is rarely investigated in the existing documents. In this paper, the control method of a cascaded MMC-BDC is studied, and a new method adopting independent voltage control strategy (IVCS) over the SM capacitors is proposed to flexibly adjusting the imbalance boundary according to the load conditions. Comparisons are made between the proposed control method with the traditional control method, which features common voltage control strategy (CVCS) over the SM capacitors. The results indicate that IVCS is characterized by more linear control structure and lower voltage stress of the components than CVCS, but the imbalance boundary of IVCS is more limited. Thus the proposed IVCS is more proper for conditions where more attention is paid to efficiency and the imbalance in the SM power distribution is smaller. The validity of the theoretical analysis is verified with simulation results.