A Single Carrier Rotating Modulation for Modular Multilevel Converter based Isolated DC-DC Converters in EV Charging Station

A modular multilevel converter (MMC) based isolated DC-DC converter is an interesting topology enabling medium-voltage DC (MVDC) grid-connected EV charging stations, an efficient solution for reducing power conversion stages. In modulations applied to MMC based DC-DC converters, a quasi-two-level (Q2L) modulation is suitable for medium-frequency (MF) DC-DC conversion and can reduce stresses on a transformer due to a low dv/dt characteristic. However, attention must be paid to balancing DC capacitor voltages between the submodules (SMs), and it is hard to apply the existing methods in practice because of the complex balancing algorithm. Therefore, this paper proposes a new modulation with inherent SM DC capacitor voltage balancing for the MMC, which is used as a primary converter of MVDC isolated DC-DC converters. The proposed modulation can achieve the natural balancing of the DC capacitor voltages by rotating the SM insertion duration. In addition, improved balancing of SM capacitor voltages can be achieved by applying a controller to compensate for the effects of uncertain parameters such as capacitance tolerances. The effectiveness of the proposed modulation is verified with simulation and experimental results.