A Closed-Loop Soft Startup Strategy to Suppress Inrush Current for Full-Bridge Modular Multilevel Converters

Aim to charge all submodule (SM) capacitors of the full-bridge modular multilevel converter (FB-MMC) to their nominal voltage values smoothly and effectively, this article proposes a closed-loop soft startup strategy by utilizing the negative output voltage of FB-SM, which completely suppresses the potential inrush currents at the startup process. Based on the analysis of the precharging process of FB-MMC with load, when bypassing the current limiting resistors, the inrush currents are induced owing to the presence of load. To solve this issue, an improved startup strategy is further proposed by intentionally controlling the dc voltage to zero. With this strategy, the main ac contactor is closed with zero current, which is beneficial for extending the service life of mechanical switches. Besides, the overall control system is integrated and optimized because separately designed startup control is not required, thereby reducing the computational burden of digital controllers. Moreover, the SM capacitors can be charged rapidly by a ramp shaped charging current, and equal charging of all SMs during startup process can be guaranteed with the voltage balancing control algorithm. Simulation and experimental results of three-phase FB-MMC rectifier interfaced with grid and 5 kV dc are provided to validate the feasibility and effectiveness of the proposed startup strategy.