The short-circuit current characteristic of doubly-fed wind turbine is different from traditional synchronous motor, which brings new problems to algorithms and principles of protection. The deviation expression of the full-wave Fourier algorithm caused by speed frequency components is deduced, and the impact of double-fed wind turbines short circuit current on the full-wave Fourier algorithm is analyzed. Then simulation demonstrates the correctness of theoretical analysis. Meanwhile considering that wind farms are generally weak fed, for distance protection, taking into account different types of ground faults and different value of transition resistance, measured impedance of distance protection of wind farm feeders is calculated and verified by simulation using the DIgSILENT software. The impact of speed frequency short-circuit and weak fed character of doubly-fed wind farm on distance relay are analyzed and the pattern of measured impedance of distance relay varying with the speed frequency current is revealed. The conclusion can provide reference for the protection and configuration of wind power connect-in system.