A fault-responsive control strategy for improving superimposed distance protection in mixed GFES–IIDG grids

In high-penetration inverter-based resources (IBR) system, the widespread deployment of grid-forming energy storage (GFES) and inverter-interfaced distributed generation (IIDG) introduces complex and time-varying behaviors in power frequency fault component (PFFC) networks. These behaviors, shaped by diverse low-voltage ride-through (LVRT) strategies, lead to significant uncertainty in the IBR PFFC impedance, undermining the performance of power frequency superimposed distance protection (PFSDP). This paper first constructs fault sequence component equivalent networks under representative GFES and IIDG control strategies. Impedance-plane analysis is used to characterize the transient evolution of characteristic impedance and identify key contributors to PFSDP performance degradation. To address these challenges, a fault-responsive sequence voltage control (FRSVC) strategy is proposed. This control scheme bypasses the outer-loop controls post-fault and output actively shaped sequence current components to emulate synchronous generator impedance behavior. The proposed strategy stabilizes the PFFC impedance, enables PFSDP to maintain correct operation under mixed-source conditions. Simulation results verify the effectiveness of the proposed method in improving protection sensitivity in systems with both GFES and IIDG.