A Pilot Protection Method for Transmission Lines of Direct-drive Wind Farms Based on High-frequency Impedance Variance

To address the issue of difficult identification of transmission line faults at the delivery end of direct-drive wind power via DC delivery system, a pilot protection method for direct-drive wind farm transmission line is proposed based on high-frequency impedance variance. Under different conduction conditions, the topologies of the wind farm side converter and DC side converter of the transmission line are analyzed in the first step. Considering the high-frequency characteristics of the system components, the single-mode high-frequency equivalent models for both the wind farm and DC sides of the system are constructed. Base on the basis of model, the single-mode high-frequency fault add-on network of the wind power transmission system is constructed under in-zone and out-of-zone faults. The single-mode high-frequency impedance measured by the protection on both sides of the line is derived. The protection criterion is established based on different fault scenarios and single-mode high-frequency impedance matching characteristics. Finally, the simulation results based on RT-LAB platform confirm that the proposed method remains unaffected by the wind field control strategy and can discriminate faults both inside and outside the zone accurately