A Dual-Mode Resonant Modular Multilevel Step-Down DC/DC Converter With Flexible Modulation for MVDC Applications

High step-down ratio DC/DC converters with a wide input voltage range have attracted concerns in the applications of connecting medium-voltage DC (MVDC) and low-voltage DC (LVDC). This paper presents a dual-mode resonant modular multilevel converter (DMRMMC) with flexible modulation for MVDC applications. Firstly, the high gain mode is applied under low input voltage, which voltage gain is twice of it in low gain mode to satisfy a wide input voltage range. Moreover, the converter switches to the low gain mode with the increase of input voltage, which introduces the phase-shift between arms to halve the frequency of the switches in arms and the voltage conversion ratio, therefore, reduces switching loss, narrow down resonant frequency range, and increases the step-down ratio. Finally, a simulation on a 100kW DMRMMC that converts 5-10kV to 400V is built to verify the feasibility of the proposed converter and control method.