An Optimization Method for Reducing the Sub-module Capacitor Voltage Ripple Amplitude of Hybrid MMC

The hybrid Modular Multilevel Converter (MMC) composed of Full-Bridge Sub-Modules (FBSMs) and Half-Bridge Sub-Modules (HBSMs) can cause excessive voltage fluctuation of sub-modules during the over-modulation operation. This paper proposes an optimization method to reduce the amplitude of capacitor voltage ripple by adjusting the modulation index. Firstly, the mathematical relationship between Sub-Modules and the modulation index is deduced. Then, according to the change of the sub-module capacitor energy in the fundamental wave cycle, the functional relationship between the capacitor voltage ripple amplitude and the modulation index and power factor is constructed. At the same time, the FBSM accounts for 2/3 of the system, and the power factor is 0.95 as an example to optimize the modulation index and reduce the sub-module ripple amplitude. Finally, a simulation is built on PSCAD/EMTDC to verify the effectiveness of the sub-module modulation analysis and optimization method. The results show that the voltage fluctuation is reduced by 5.837% after the optimization, and the voltage unbalance between the HBSM and FBSM is reduced to 2/5 of that before the optimization. Among the three control strategies, the proposed method is more effective in reducing voltage fluctuation.