A Reduced-Switching-Frequency Modulation Method for Hybrid MMCs Under Over-Modulation Conditions

Abstract

A hybrid modular multilevel converter (MMC), consisting of full-bridge sub-modules (FBSMs) and half-bridge sub-modules (HBSMs), is an efficient and DC-fault tolerant topology, which can be used in overhead-line HVDC transmission application. Additionally, it can work under over-modulation condition (i.e., arm submodule clusters may generate negative voltage during a fundamental cycle) to improve the utilization of submodules (SMs). Regarding the over-modulation issue, this paper proposes a reduced-switching-frequency modulation method, considering the different dynamics of FBSMs and HBSMs, for hybrid MMCs. Moreover, an improved wave-distributing approach which can reduce losses and make the operating time and losses of the two types of SMs more even than the conventional method is presented. Finally, the effectiveness of the proposed method is verified by an over-modulation hybrid MMC simulation model built based on PSCAD/EMTDC platform.