Phase-Shifting Control Strategy for MMC-Based DC Transformer Using DAB Virtual Models

This paper presents a phase-shift control method for an MMC-type DC transformer for carrier phase-shift modulation, aimed at suppressing the backflow power generated in this topology. Traditional DAB typically uses extended phase-shift and equal phase-shift strategies to suppress power backflow. However, the complex primary topology structure of the MMC topology and the use of carrier phase-shift modulation on the MMC side result in a complex voltage waveform on the high-frequency transformer primary side, with different carrier ratios and modulation ratios generating different waveforms. Therefore, it is difficult to directly apply DAB phase-shift strategies to this topology. In this paper, a DAB virtual model is proposed, and a quantification factor K is introduced to measure the number of primary-side voltage levels. The MMC topology is equivalent to the DAB topology to simplify the modal analysis process. By calculating the expression of the AC-link inductance current, the phase-shift angle backflow power suppression value range is given, and a phase-shift strategy based on the DAB virtual model is proposed. The effectiveness of the conclusions is verified through simulation, and the simulation results are further used to narrow the range of phase-shift angles to determine the optimal phase-shift angle value, resulting in a phase-shift operating point with approximately zero backflow power.