A Hybrid Modulation Strategy for Fractional Level MMC

The modular multilevel converter (MMC) applied in medium-voltage field features a smaller quantity of submodules (SMs) and relatively lower voltage quality. The fractional level MMC (FLMMC) adopting fractional level modulation (FLM) can effectively enhance the voltage quality of the MMC with fewer SMs. By introducing the square wave output mode into the working modes of the fractional level SMs, a hybrid modulation strategy for FLMMC based on FLM and square wave pulse width modulation, namely fractional level square wave pulse width modulation (FL-SWPWM) strategy, is proposed in this paper, which can further improve the voltage quality. The proposed FL-SWPWM is characterized by simple control and good scalability, and equivalently can increase the number of ac output voltage levels. Simulation results based on Matlab/Simulink reveal that under the proposed FL-SWPWM, the low-order harmonic content of the ac output voltage is significantly reduced, the voltage quality of the MMC with fewer SMs is greatly enhanced and the degree of current distortion is considerably decreased.