A design method of hybrid modular multilevel converter with negative output generated by FBSM

Abstract

Hybrid modular multilevel converter (MMC) is considered as a promising topology to deal with DC fault in MMC based high-voltage direct current (HVDC) due to the negative output from full-bridge sub-module (FBSM). In addition, the modulation index can be promoted with the negative output, which can enhance the AC voltage and transmission power in steady state. This paper focuses on this situation that FBSMs generate negative output in steady state. A design method of hybrid MMC is proposed, which contains a way to determine the proportion of FBSMs and a means to determine the capacitance of sub-module capacitor. Proceeding from the requirements of AC voltage boosting and DC fault riding through, principles of FBSM proportion design are delivered. Based on a capacitor voltage balancing scheme, the physical process of capacitor voltage fluctuation in half-bridge sub-module (HBSM) and FBSM is analyzed in detail with power factor no less than 0.9. Formulas and diagrams are demonstrated to portray the capacitor voltage fluctuation. And the quantitative design method of the sub-module capacitor is brought out. Simulation in PSCAD/EMTDC is implemented to verify the feasibility and validity of the proposed method.