Control Strategy and Main Parameter Design for Three-Phase Modular Multilevel DC-DC Transformer

The modular multilevel DC-DC transformer serves as a critical interface device for interconnecting DC lines with different voltage levels, and research on its topology and control strategy is essential for constructing secure and reliable DC grids. Currently, studies on the topology of modular multilevel DC-DC transformer are primarily focused on optimization design, while a systematic design methodology for its control system has not yet been established. To address this issue, this paper takes the average-value model of the modular multilevel DC-DC transformer as the research object and conducts the following work: First, the topological characteristics and operating principles are thoroughly analyzed. Second, aiming to achieve energy balance in each arm, a decoupling control method is proposed based on the differential-mode/common-mode constraints of the submodule capacitor charging and discharging in the transformer arms. Furthermore, the number of submodules per arm, arm inductance, and submodule capacitance are analyzed and designed. Finally, the effectiveness of the proposed control method is verified in MATLAB/Simulink.