Hybrid Modular Multilevel Converter With Redistributed Power to Reduce Submodule Capacitor Voltage Fluctuation

This paper introduces a hybrid modular multilevel converter (MMC) with the full-bridge submodules (FBSMs) inserted in the ac side. It is intended for redistributing arm powers of the MMC system to reduce the submodule (SM) capacitor voltage fluctuation by injecting high-frequency powers in upper and lower arms without disturbing the output voltage. New power distribution of the hybrid MMC system can be realized by controlling the SMs and FBSMs working under the high-frequency powers. The proposed FBSMs in the hybrid MMC system also perform as the harmonic power generator to eliminate the common-mode voltages in the ac side. Since the FBSMs locate in the ac side of the hybrid MMC system, the traditional MMC control method can be directly applied in arm control of the hybrid MMC system. Meanwhile, relationship between the arm powers and SM capacitor voltage has been analyzed in detail to reduce the SM capacitor voltage ripples by redistributing the frequency spectrum of arm powers. Simulation and experimental results have also verified the effectiveness of the proposed method.