Comprehensive Analysis of Capacitor Voltage Fluctuation and Capacitance Design for Submodules in Hybrid Modular Multilevel Converter With Boosted Modulation Index

Abstract

Hybrid modular multilevel converter (MMC) with half-bridge sub-modules (HBSMs) and full-bridge SMs (FBSMs) is considered as a promising topology for high-voltage direct current (HVDC) system to deal with dc fault due to its negative output from FBSMs. In addition, the negative output from FBSMs can increase modulation index at steady state. With boosted modulation index, ac voltage can be enhanced and remains unchanged under decreased dc voltage. However, the characteristics of SM capacitor voltage fluctuation for such a hybrid MMC are different from that in traditional MMC since the impacts of FBSMs and HBSMs should be compromised. Then, the SM capacitance design should adapt to the working condition. In this paper, the charging and discharging processes of capacitor in HBSMs and FBSMs are physically analyzed in detail with the piecewise analysis method. Then, the characteristics of capacitor voltage fluctuation in HBSMs and FBSMs are portrayed. Based on the characteristics, the runnable region for hybrid MMC is demonstrated, and the principles to obtain the maximum energy variation in both kinds of SMs are deduced. Furthermore, the quantitative design method of the SM capacitance is brought out. Experiments are implemented to verify the feasibility and validity of the proposed analysis and design method.