Reduction in Submodule Capacitance in FB- and HYB-MMCs With Variable DC Voltages

Full-bridge submodule (FBSM)-based modular multilevel converter (FB-MMC) or hybrid MMC (HYB-MMC) composed of FBSMs and half-bridge submodules is necessary in many applications. However, the arm energy fluctuation during reactive power exchange is very high, causing large sizes and high costs of submodule capacitors. Using the negative states of the FBSMs, the FB- and HYB-MMCs can dynamically adjust the dc voltage, and this technique has been used in many commissioned projects. In this article, we treat the dc voltage as a new dimension and propose a new operation mode based on variable dc voltages for FB- or HYB-MMCs to reduce the arm energy fluctuations and submodule capacitance. The basic idea is to reduce the dc voltage in high reactive power and low active power conditions, thereby suppressing the arm energy fluctuations. The theoretical analysis shows that the energy fluctuation and submodule capacitance can be reduced by 30%–40%, and the valve cost can be reduced by 13%. The economic and technical comparison results show that the proposed approach can have similar reduction effects on the capacitance and valve cost but has fewer technical limits. The simulation and experimental results verify the analysis.