Closed-loop Control Strategies for Minimizing Submodule Capacitance in MMC

In modular multilevel converter (MMC), sub-module (SM) capacitors account for a significant proportion of the volume, weight and cost. Suppressing capacitor voltage fluctuations is an effective way to reduce the capacitance requirement, thereby reducing the cost and volume of SM capacitor and MMC. This paper discusses the closed-loop control methods for reducing SM capacitor voltage fluctuation by two direct ways. Firstly, the expressions for the voltage of the SM capacitor and the arm energies are analyzed, and the components of the fluctuation are obtained. Then, taking the average SM capacitor voltage fluctuation and arm energy fluctuations as control objectives, respectively. ultimately achieving the suppression of the SM capacitor voltage fluctuation, and thus to reduce SM capacitance requirement. The simulation results show that both methods can reduce the capacitance value of SM capacitors without negative impact on the converter.