Abstract: The short-circuit current provided by a double-fed wind farm under the LVRT strategy has a bias frequency characteristic, which leads to a long delay in the action of the conventional differential protection of the sending transformer. To address these problems, firstly, the short-circuit current characteristics of double-fed wind turbines under LVRT strategy and its bias frequency characteristics are analyzed, and the influence of the Fourier frequency algorithm on the extraction of the second harmonic component of the wind farm send-out transformer is investigated; secondly, the adaptability of the traditional differential protection in the send-out transformer of double-fed wind farms is analyzed; and then, a new differential protection method for the send-out transformer of double-fed wind farms based on the full-phase fast Fourier transform is proposed. Finally, the proposed method is validated by PSCAD/EMTDC simulation, and the results show that the new method can be utilized to not only correctly recognize the excitation inrush current, but also improve the rapidity of the differential protection in case of internal faults.