A High Efficiency Hybrid Modular Multilevel Converter with a SiC Device-Based External Shaping Module

In order to improve the waveform quality and efficiency of medium voltage modular multilevel converter (MMC), a hybrid MMC (HMMC) topology with an external shaping module is proposed. Each arm of the HMMC consists of N half-bridge submodules (HSMs) based on Si IGBT, while a full-bridge submodule (FSM) based on SiC MOSFET on the ac side. Furthermore, a 2N+1 level hybrid modulation strategy is proposed for this topology. The HSMs operate in the low-frequency state to output the staircase wave voltage, while the FSM operates in the high-frequency state to reduce the harmonics of the output voltage. It makes full use of the advantages of low on-state loss of Si devices and low switching loss of SiC devices. Additionally, the energy fluctuation of the HMMC submodules is analyzed in detail, and a voltage balancing scheme is proposed. In the end, the feasibility and effectiveness of the proposed topology and modulation method are validated by simulation and experimental studies on a single-phase HMMC. Moreover, the comparison of cost and loss between HMMC and existing MMCs proves that the proposed HMMC can better balance the device cost and operating loss.