A modified submodule of modular multilevel converter using active power decoupling method for reducing capacitor voltage ripple under low‐frequency operation

Abstract

Abstract In this paper, a modified submodule (MSM) method is proposed to reduce capacitor voltage fluctuation by integrating a bidirectional buck‐boost type active power decoupling (APD) circuit into each SM for the low‐frequency operation of a converter system. The capacitor voltage fluctuation of the proposed MSM‐MMC can be significantly reduced compared to conventional SMs with the same capacitance by transferring the low‐frequency ripple power components to the auxiliary energy storage capacitor, resulting in a lower voltage fluctuation of the main capacitor in the SM, which requires a lower total SM capacitance. Additionally, a closed‐loop controller is proposed to control the APD capacitor voltages by inheriting the advantages of the repetitive controller and proportional‐integral (PI) controller to enhance the performance of SM capacitor voltage ripple suppression. The method can improve the start‐up performance of medium voltage drives without injecting high‐frequency components, which can avoid the common‐mode voltage imposed on the AC side of the system. The operating principle, auxiliary capacitor voltage control, and capacitor voltage ripple suppression control of the proposed MSM‐MMC method are analyzed in detail. The feasibility of the proposed method has been confirmed by simulation results based on case studies using MATLAB/SIMULINK software and further verified with hardware‐in‐the‐loop simulation (HILS) using the real‐time simulator OPAL‐RT platform.

Keywords

• active power decoupling (APD)
• low‐frequency operation
• modified submodule (MSM)
• modular multilevel converter (MMC)