A Capacitance Reduction Technology of FBSMMMC based on Second-harmonic Circulating Current Injection

In a modular multilevel converter (MMC) system, the sub-module(SM) capacitors account for a large proportion of the cost, volume and weight. This paper proposes a method to reduce the total capacitance of MMC based on second-harmonic circulating current injection (SCCI). The full bridge sub-module (FBSM) is used as SM topology. FBSM can generate negative voltage, which can increase the AC voltage and handing the DC-link short circuit fault. An optimized SCCI considering modulation index is introduced, which is used when the modulation index is less than 1.414. When the modulation index equals to 1.155, the total capacitance is minimum. Simulation results confirm the feasibility and validity of the proposed SM capacitance reduction method.