As the penetration of wind power into power grids increases，higher demands are made on the stability andprotection of power systems.Unlike traditional synchronous generators，wind turbines are different on the short-circuit and impedance characteristics，thus some problems exist in pilot protections of transmission linesconnecting wind farms.When a single-phase fault occurs on a transmission line，the phase selector based on current-difference sudden-change may mistake the single-phase fault for a phase-to-phase fault.This paper studiesthe impedance characteristics of an asynchronous wind generator，and analyzes the behavior of a fault phase selectorunder different conditions.Then，the paper explains that inequality between positive and negative branch coefficientsis the cause for the mal-operation of the fault phase selector.Finally，some simulations are done in MATLAB tovalidate analysis results.