A Unified DC-Link Switching Ripple Suppression Modulation Method for Modular Multilevel Converters Including Step-Down Mode

The dc-link switching ripple issue of modular multilevel converters (MMC) is traditionally considered as a conflict issue with ac side total harmonic distortion (THD). This paper studied the fundamental reason towards that problem. Based on the analysis, a simple and unified end-to-end modulation method for MMC is proposed. In this method, the pulse edges of six arm voltages are arranged in a uniformly aligned manner. The instantaneous voltage error between total arm voltage and dc bus voltage can be zero. Therefore, zero dc side switching ripple is achieved. Depending on the type of arrangement, the proposed method can be equivalent to 2N+1 pulse-width-modulation where the low ac side THD is also guaranteed. This breaks the barrier of traditional conflict. Besides, the switching ripple problem of stepdown MMC is further analyzed. The end-to-end modulation is proved to be still effective to minimize the ripple being 89% less. Finally, simulation results compared the proposed modulation with classic carrier-based modulation and validated its effectiveness.