A pilot protection method for direct-drive wind farm transmission lines based on the variance of high-frequency impedance

Concerning the difficulty in identifying faults on the transmission line of direct-drive wind farm connected to DC transmission system, a pilot protection method for the wind farm transmission line based on the variance of highfrequency impedance is put forward. First, the topologies of the transmission line farm-side converter and DCside converter in different conducting states are analyzed, and combining the high-frequency characteristics of different system components, the equivalent high-frequency 1-mode models of the farm-side system and the DCside system are built. Then, the high-frequency 1-mode fault models of the transmission line under internal faults and external faults are established, and the high-frequency 1-mode impedances on ends of two lines are derived. Finally, a protection criterion is constructed according to the matching relationhip between the high-frequency 1-mode impedances and different fault scenes. Experimental tests are conducted on the RT-LAB platform and verify that the method can accurately identify internal faults from external faults, unaffected by the control strategy of the wind farm.