Improved Modulation Strategy for Semi-Full-Bridge Submodule in Modular Multilevel Converter

The modular multilevel converter (MMC) has become the most attractive multilevel converter for medium and high-power applications, specifically for HVDC. In each arm, the most popular and widely used SMs are half-bridge submodule (HBSM) and full-bridge submodule (FBSM). In order to make the tradeoff between power losses and the protection function of DC side short circuit, the novel semi-full-bridge submodule (SFBSM) topology and related modulation strategy which used three phase-shifted carriers were proposed. However, this modulation strategy will make different charging states between two capacitors at certain intervals in one line cycle and lead to the larger current spike. To solve this problem, in this paper, the new improved modulation strategy was proposed to make two capacitors have the same charging states all times, extremely reduce the current spike and reduce the capacitor voltage ripple. Finally, the simulation results show the effectiveness of the proposed modulation strategy in semi-full-bridge submodule.