A pre-charging strategy based on composite frequency control for a modular multilevel DC transformer

Modular multilevel DC transformers(MMDCTs) have become a research focus in DC grids with different voltage levels based on their advantages of strong expansibility and high voltage levels. It is necessary to apply pre-charging control to avoid inrush currents generated by the charging of the capacitor and the inrush current of the transformer during the starting process. A fast pre-charging strategy based on compound frequency control is proposed in this study. The independent control of the direct current component and intermediate frequency component is used to realize simultaneous charging of the primary and secondary sides. The primary side adopts double closed-loop control, whereas the secondary side adopts variable-step phase-shift peak current control, which resolves the contradiction between the charging speed and inrush current. Additionally, a quantitative comparative analysis of various pre-charging strategies was conducted. The effectiveness and feasibility of the proposed pre-charging strategy are verified through simulations and experiments. The results demonstrate that the magnitude of the inrush current and charging time were effectively controlled by the proposed method.