A Novel Directional Pilot Protection of Submarine Cables for Low Frequency Transmission System with Offshore Wind Power

Low frequency transmission system (LFTS) has become a competitive method for long-distance offshore wind power transmission. However, traditional protections based on the fundamental frequency phasor may have poor performance since both ends of submarine cables connect with power electronic devices. First, the adaptability of traditional protections is analyzed. It is found that fault characteristics are weakened by typical control strategies. To enhance the characteristic difference between internal and external faults, an impedance construction method for grid-side converter (GSC) and modular multilevel matrix converter (M3C) is proposed. Then this paper proposes a directional pilot protection. For asymmetrical faults, the angle of negative-sequence impedance of GSC and M3C is determined by references of current loop controller. For symmetrical faults, adjusting coefficients of current loop reference in GSC helps the protection perform well under high-resistance faults. Building the sequence networks under different fault types, a phase selection method based on voltage magnitude is proposed. Finally, simulation tests are conducted in PSCAD/EMTDC, and the results demonstrate that the proposed impedance construction method can provide effective support for fault identification and the proposed protection performs well under various fault scenarios.