A Simple and Cost-effective Precharge Method for Modular Multilevel Converters by Using a Low-Voltage DC Source

A simple and cost-effective precharge method for modular multilevel converter (MMC) by using a low-voltage direct current (dc) source is proposed in this paper. The submodule (SM) capacitors in MMC are required to be precharged to their nominal voltage values at start-up to ensure MMC work normally. Conventional methods perform the precharging either through grid side or dc side by using costly high-voltage bypass breakers, charging resistors or a high-voltage dc source. Voltage of the high-voltage dc source must be equal to SM capacitor voltage and could be as high as thousands of volts. By contrast, the proposed method employs a very low voltage dc source on dc bus through a series-connected blocking diode, and requires no charging resistor and bypass breakers. It takes advantages of the existing power devices, arm inductors, and SM capacitors in MMC, and configures them into boost circuit for the precharging. The proposed method is simple and cost-effective. Moreover, the characteristics of the boost circuit render the proposed method flexible. For instance, the voltage selection of the dc source is flexible and not restricted to a fixed value. The SM capacitors can be charged in different groups. In addition, the method is applicable to different MMC SMs, including SM with different capacitor voltages. The effectiveness of the proposed method is verified through simulation and experiment.