In consideration of the influence of sequence impedance of doubly fed induction generator（DFIG），an analytical method is used to find out the factors affecting the correct action of the phase-selection elements in wind farms.The expressions for the sequence impedance considering the Crowbar protection are derived，which shows that the positive and negative sequence impedance of DFIG are functions of the equivalent rotational speed and crowbar resistance.It is then pointed out that the common hypothesis that the positive and negative sequence impedance of the power sources is nearly equivalent cannot be considered valid for the DFIG plants，which then causes great differences in the current distribution coefficients of the line protections.Based on the above analysis，this paper discusses in detail the reasons why the phase-selection principles based on variation and sequence components cannot be applied to DFIG plants.The simulation results using RTDS have further proved the theoretical analysis.