Transient Impedance Fitting-Based Distance Protection for Transmission Lines with Hybrid Renewable Integration

The hybrid operation of grid-following (GFL) and grid-forming (GFM) renewable energy units can lead to phase-reference inconsistencies and distorted transient impedance trajectories, which in turn cause maloperation or failure-to-operate of conventional distance protection. To address this issue, this paper proposes a novel distance protection method based on transient impedance fitting. First, a dynamic phase transformation is applied to map the currents of GFL units into a unified reference frame, enabling consistent representation of heterogeneous currents from the hybrid renewable energy station. Second, a transient equivalent impedance model is established based on the transient voltage-current relationship of the transmission line, revealing the influence mechanisms of the rates of change in current amplitude and phase angle on the transient additional impedance. Finally, the magnitude of the transient impedance within a short post-fault data window is selected as the fitting object. The least-squares method is employed to extract the linear fitting slope and intercept, which characterize the evolution trend and initial position of the transient impedance trajectory, respectively, thereby forming the criteria for distinguishing internal and external faults. Simulation results demonstrate that the proposed method correctly identifies fault sections under various fault locations, transition resistances, and renewable power output conditions, with the protection decision completed within 15 ms. The proposed method effectively overcomes the susceptibility of conventional distance protection to maloperation and failure-to-operate in scenarios with high renewable energy penetration.