Different operating conditions of wind farm impose new challenges on the setting of protection relay. Short-circuit characteristic is the basis of relay protections. The mathematical model and electro-magnetic transients of a doubly fed induction generator（DFIG） are discussed. The setting of protection relay and the sequence current model of wind farm are established when fault is present in the grid connection line. The main factors that influence the short-circuit characteristic of wind farm and the relay setting are analyzed. Taking a practical wind farm for example， the grid-connected simulation model is established on the PSCAD/EMTDC platform. Above influence factors of faulty characteristic and protection are studied， including control strategy， capacity， wind speed and type of generator. Simulation results are identical with theoretical analysis. These influence factors should be considered in the protection design of a wind farm. It will help to enhance the effectiveness and reliability of the protection.