Low Voltage Ride-Through Control Strategy for Grid-Forming MMC-HVDC Based on Current Change Rate Virtual Impedance

Grid-forming Modular Multilevel Converter- High Voltage Direct Current (MMC-HVDC) are widely used in large-scale renewable energy generation systems, short-circuit faults on the gird side can lead to extremely high short-circuit currents, severely impacting the system's stability. Therefore, this paper proposes an adaptive virtual impedance control strategy based on the rate of change of current to achieve fault ride-through. A mathematical model for the fault current of grid-forming MMC is established, and the characteristics of fault current and power angle are analyzed in detail. During the fault period, the combined effect of transient and steady-state components results in a higher rate of change of the fault current compared to that during the steady-state fault period. While the traditional adaptive virtual impedance control strategy based on current threshold can effectively limit the steady-state component of the fault current, it remains insufficient in suppressing the transient component. By leveraging the difference in the rate of change of the current, the proposed adaptive virtual impedance control strategy based on current rate of change not only enhances the dynamic characteristics of the current during the fault period but also significantly suppresses the transient current. Consequently, this ensures that during the fault period, the MMC maintains its grid-forming characteristics while generating reactive current to support the grid. Simulation results validate the correctness and effectiveness of the proposed control strategy.