At present, protection research and optimization of a power transformer are mostly aimed at the traditional power supply, and there is little work on transformer protection considering an RSC control circuit under the power supply of doubly fed wind farm. When the doubly-fed wind farm is connected to a weak current network, the action performance of the traditional transformer differential protection becomes worse. Based on this, this paper analyzes the characteristics of short-circuit current with a crowbar protection circuit and RSC control circuit when the system fails, and deduces the error expression of short-circuit current extracted by a Fourier algorithm under two operating conditions. The reasons for the degradation of the performance of the traditional differential protection are analyzed, and a new protection scheme is proposed, one which uses a synchronous squeezed wavelet transform to extract the fault features and combines the normalized two-dimensional trajectory diagram to identify and act on the fault. The scheme is not affected by the short-circuit current characteristics of the wind farm. It has better applicability and higher accuracy in the output transformer of a doubly-fed wind farm. Finally, a simulation model of doubly fed wind farm is built on Matlab/Simulink platform to verify the effectiveness of the scheme under the influence of different fault types, inrush current, different noise and asynchronous data transmission. This work is supported by the National Natural Science Foundation of China (No. 61773137).