A novel design of circulating current control target to minimize SM capacitance in MMC

Abstract

In an MMC station, the submodule (SM) capacitor constitutes a high proportion in both the footprint size and construction cost, which greatly limits the application of MMC in some scenarios, such as offshore wind power transmission system. Since the SM capacitor voltage ripple has a close interaction with the phase circulating current in MMC, an appropriate management of circulating current is widely acknowledged to reduce the SM capacitance. This paper proposes a novel design of circulating current control target to minimize the requirement of SM capacitance in MMC. Firstly, the phase angle of circulating current is designed to significantly reduce the fundamental and second-harmonic capacitor voltage ripple at the same time. Secondly, the magnitude of circulating current is determined to achieve the optimal phase angle or magnitude coordination between the components of SM capacitor voltage ripple. Thus, under the proposed design method, the peak-peak capacitor voltage ripple is minimized and the requirement of SM capacitance can be effectively reduced. Finally, an MMC-HVDC case system connecting to the offshore wind farm is built on PSCAD/EMTDC to validate the effectiveness of the proposed design method.