Differential Protection for Collection Lines in Flexible Low-Frequency Transmission Systems Based on Transient High-Frequency Impedance

This paper proposes a transient high-frequency impedance-based protection scheme for offshore wind power collection lines with low-frequency AC transmission. By selecting a frequency band that satisfies converter linearization, avoids transient resonance, and reduces noise, the scheme establishes a high-frequency impedance network where both line ends exhibit inductive impedance. This enables simplified calculations of operating and braking impedances, with internal faults showing operating impedance larger than braking impedance, and external faults the opposite. Simulations on PSCAD/EMTDC confirm the scheme's ability to accurately identify internal and external faults with high sensitivity, strong resistance to synchronization errors, and robustness against transition resistance.