Start-Up Operation of a Modular Multilevel Converter With Flying Capacitor Submodules

The three-level flying capacitor (3L-FC) submodule significantly reduces the magnitude of circulating currents, voltage ripple, and footprint size and improves the efficiency of the modular multilevel converter (MMC). Due to the above advantages, the 3L-FC submodule becomes an alternative for the conventional half-bridge submodule in the MMC. Each 3L-FC submodule consists of two floating capacitors with different nominal voltage rating. The precharging of floating capacitors without inrush current is one of the major challenges in the 3L-FC-based MMC. This paper proposes a sequence of design steps to precharge the floating capacitors in the 3L-FC-based MMC. The proposed approach is highly effective to charge the outer and inner capacitors of each 3L-FC submodule to their nominal value. The superiority of the proposed approach is verified through the MATLAB simulations and dSPACE/DS1103 experiments on a laboratory prototype of the 3L-FC-based MMC.