A cell capacitor energy balancing control of Modular Multilevel Converter considering the unbalanced AC grid conditions

This paper presents a control scheme for the regulation of cell capacitor energy balancing of a Modular Multilevel Converter (MMC) for HVDC transmission systems, considering the unbalanced AC grid conditions. It is essential that the MMC balancing control should be valid not only for the balanced normal operations but also for the asymmetrical grid fault conditions. This paper proposes the control scheme that has the ability of seamless mode change between balanced and unbalanced grid condition. Applying the proposed method, the capacitor energy balancing operation is successfully realized with improved dynamic responses. Finally, the simulation results verify the validity of the proposed method.