Capacitor Voltage Ripple Control Strategy of Lightweight Modular Multilevel Converter for Offshore Wind Power Transmission.

In offshore wind power (OWP) projects, converter valves are the largest equipment within offshore converter stations. A low ripple voltage of submodule (SM) capacitors is helpful in reducing the capacitor volume in converter valves, thereby minimizing the volume and weight of offshore converter stations. This paper proposes a comprehensive control strategy for the voltage ripple of SM capacitors in the modular multilevel converter (MMC), capable of eliminating the second-order harmonic in the capacitor voltage. Compared to the conventional circulating current suppression control (CCSC), the proposed control strategy reduces the voltage fluctuation and submodule capacitance by approximately 50%, providing an opportunity for converter volume optimization. Additionally, the effective value of arm currents is also reduced to avoid additional loss. Finally, a two-terminal MMC based high voltage direct current (HVDC) model was built in PSCAD/EMTDC to verify the effectiveness and feasibility of the proposed method.