With the maturity of the technology of wind generation, more and more large wind farms have been put into operation. The variation of operational mode for wind farms and its weak-source feature will exert some impacts on transmission line relays. The paper studies the basic principle of distance protection using power fault component, and further researches the impact of the change of operational mode and weak-source feature of wind farms on protection. An improvement of traditional distance protection is put forward by analyzing the equation of phase comparison under voltage phase graph. The analysis shows that the operational mode of wind farm can rarely impose impact on distance relay using power fault component when there is occurring metallic fault; because the equivalent impedance of positive and negative sequence of wind farms is bigger than system impedance, it makes phase-to-phase relays can resist little transition resistance, while because the zero sequence impedance of wind farms is smaller than that of the system, ground protection can resist transition resistance to some degree. Under the platform MATLAB/Simulink, a wind farm connecting grid simulation model is given, and the paper studies the impact of wind farms on relays and conducts the operational behavior comparison between improved and unimproved relays when faults occur. The result from simulation is in line with the theoretical analysis.