A Capacitor Voltage Balancing Method for a Modular Multilevel DC Transformer for DC Distribution System

A capacitor voltage balancing method for a modular multilevel dc (MMDC) converter is proposed. The MMDC inherits the desirable features of both a modular multilevel converter (MMC) and a dual active bridge, namely high voltage capability, fault tolerance, zero-voltage switching-on of all the power devices, and a wide operating range. These features are ideal for a dc transformer in any dc distribution system. One key MMDC control issue is balancing of submodule capacitor voltages. Conventional MMC balancing techniques cannot be used as arm currents of an MMDC change direction within each switching cycle. This paper establishes that the average capacitor voltage of each arm is self-balanced and proposes a method to balance voltages within an arm by assigning the gate signals with higher charge difference to the submodules with lower voltages and vice versa. Charge difference is obtained by the voltage difference, hence only the SM capacitor voltages are required for balancing control. Different from the existed methods, the proposed technique is not limited by voltage gain or load conditions. Simulation and experimental results show that capacitor voltages are balanced under different voltage gains, different output power, and also when the MMDC starts up, hence ensuring the stable MMDC operation.