Multi-objective optimization of circulating current control considering operating conditions of modular multilevel converter

Abstract

For the modular multilevel converter, the submodule capacitor constitutes a considerable proportion in the footprint size and weight of the whole converter. In order to achieve the light design of the converter, the requirement of submodule capacitance should be minimized. It is widely acknowledged that the sizing of submodule capacitance can be reduced by the appropriate circulating current injection, which however leads to the increase of conducting loss. In order to improve the overall operating efficiency of the system, this paper addresses the issue of multi-objective design of circulating current injection in modular multilevel converter. Firstly, considering that the submodule capacitance is sized according to the maximum capacitor voltage ripple, the operating conditions of modular multilevel converter with maximum capacitor voltage ripple are found out. Then, a novel design of circulating current injection considering the operating conditions of the converter is proposed. Under the operating condition with significant capacitor voltage ripple, the circulating current injection is designed to minimize the capacitor voltage ripple to reduce the sizing of submodule capacitance. As for the rest operating conditions, the proposed design expects to minimize the magnitude of circulating current injection to reduce the converter conducting loss. Finally, case studies of the modular multilevel converter based offshore wind power transmission system are built on PSCAD/EMTDC to validate the effectiveness of the proposed method. It can be concluded that a reasonable setting of boundary of operating conditions is essential to achieve the minimized sizing of submodule capacitance and decreased conducting loss simultaneously.