Adaptive Virtual Impedance Control for LVRT of Grid-Forming Energy Storage PCS in HVDC Receiving-End Grids

With the large-scale integration of high-voltage direct-current (HVDC) transmission systems, the receiving-end AC grid exhibits weak voltage-support capability. Consequently, grid-forming (GFM) battery energy storage converters are required to satisfy both current-limiting and voltage-support requirements during fault conditions. To address the challenge of maintaining GFM characteristics while limiting overcurrent, this paper proposes an improved low-voltage ride-through (LVRT) control strategy based on adaptive virtual impedance. The inherent limitations of switching-based strategies, which may cause overcurrent and instability due to control delays, are analyzed. The feasible region of the adaptive virtual impedance is determined, and a closed-loop regulation scheme based on real-time current-amplitude feedback is developed. To verify the feasibility and effectiveness of the proposed control strategy, hardware-in-the-loop (HIL) experiments are conducted, in which the proposed strategy is compared with existing current-limiting strategies. The results demonstrate that the proposed strategy limits both transient and steady-state fault currents to approximately 1.5 pu while maintaining high converter capacity utilization, with a reactive-power increment of approximately 0.8 pu. Furthermore, under different three-phase voltage sag depths and asymmetric fault conditions, the proposed strategy effectively limits fault currents while maintaining reactive power support, thereby enhancing the transient voltage-support capability of the HVDC receiving-end AC grid.