A Hybrid MMC Control Strategy to Enhance the Resilience of DC Transmission System

Offshore wind power through flexible dc transmission system faces the challenge of comprehensively improving the transient and steady-state performance. However, existing control strategies often fail to address both simultaneously. This article proposes a coordinated control strategy based on the hybrid modular multilevel converter that can enhance the transient and steady-state resilience of the flexible dc transmission system. First, the coupling relationship between the submodule capacitor voltage and active power is analyzed, and submodule virtual capacitance control (SVCC) is proposed to enhances the dc inertia. Second, the rate of change of dc current is introduced into the dc control loop. The virtual inductance control (VIC) is proposed, which can actively limit the rate of rise of dc current at the initial stage of dc fault. The link of the hysteresis comparator is designed to avoid frequent misoperation of VIC control caused by dc voltage disturbance. At the same time, the positive adjustment capability of the SVCC is used to reduce the fluctuation of the capacitor voltage during the dc fault ride-through process, avoid the switching of the working mode of the ac loop controller, and realize the cooperative control of the transient and steady-state performance. Finally, the effectiveness of the proposed control strategy is verified through simulations and experiment.