A Novel Method Based on Self-Power Supply Control for Balancing Capacitor Static Voltage in MMC

The modular multilevel converter is one of the most attractive converter topologies for high-voltage dc transmission systems, but it needs at least 10 min during the system uncontrolled precharge stage to verify the stability and reliability of submodules (SMs), making the capacitor static voltage balancing a key issue. This paper proposes a novel method based on self-power supply control for balancing capacitor static voltage. Because of the influence of self-power supplies on capacitor voltages, the method can keep the capacitor static voltage balanced by controlling the input characteristic of SMs self-power supplies. The control signals of self-power supplies have a fixed frequency and duty ratio, and they can be determined based on capacitor voltage sorting results and self-power supply output support capacitor. Compared with previous works, this method has less computation, and it does not rely on IGBTs and additional complex circuits except for the self-power supplies. This can save on the software and hardware costs. The proposed method also leads to improved equalizing resistances and reduced active power losses of the SMs. Simulations and experimental studies were conducted, and the results confirm the effectiveness of the proposed method.