Research on control strategy of MMC DC power electronic transformer based on DAB mode

High-power DC power electronic transformer (DC PET) is becoming the key equipment in the DC power network. Face-to-Face (F2F) DC transformers based on modular multilevel converter (MMC) have the modular structure and can flexibly design systems based on different voltage and power levels. Previous studies show that the control strategy of the F2F topology regulates the MMCs separately. Moreover, the voltage ratio of the DC sides depends on the intermediate AC transformer. In the current study, a new control strategy is proposed, which can change the DC voltage ratio by turning on/off submodules of each MMC bridge flexibly and set relevant parameters with different application scenarios. The algorithm enables the DC PET with F2F topology to operate in a dual active bridge (DAB) circuit mode. Thereby reducing the overall system size and meeting different demands of medium and high voltage application scenarios. According to the control strategy used, a submodule capacitor voltage equalization method is proposed to eliminate the capacitance voltage deviation and reduce the capacitance voltage stress. At the same time, the current characteristics of the AC conversion part in the DC transformer are analyzed. Finally, the feasibility of the proposed method is verified by simulation and experiments.