An improved nearest-level modulation method for the modular multilevel converter

This paper proposes an improved nearest-level modulation (NLM) method for the modular multilevel converter (MMC). By injecting an optimal common-mode voltage, the proposed method enables optimal capacitor-voltage balancing and circulating-current control of the MMC. A general approach to determine the optimal common-mode voltage is introduced in detail in this paper. In the proposed method, the level number of the output phase voltage of the MMC reaches the maximum (i.e., 2n+1, where n is the number of submodules in the upper or lower arm of each phase), which leads to reduced harmonics. Both of these advantages are not provided by earlier NLM methods. Furthermore, the proposed method is simple to implement and generally applicable to multiphase systems. Simulation results verify the effectiveness and performance of the proposed method.