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A simulation system for power grid with concentrated based on the electro-magnetic transient model of wind turbine （DFIG）, which is built by real-time digital simulator （RTDS） large-scale wind farm integration is established equipped with doubly-fed induction generator Using the hardware communication interface of RTDS, a closed-loop testing experiment is accomplished to study the impacts of large-scale wind farms on the existing relay protection devices for wind farm outgoing transmission line. This paper points out problems existing in current relay protection devices as follows： fault phase selector can select unwanted phase due to the changes of fault features caused by special network connection of wind farms; blocking condition for distance protections needs to be re-examined due to the weak power-feed characteristics of wind farms; and power frequency parameter based relay protection devices cannot accurately operate due to the special transient voltage and current characteristics of wind farms during fault period. Results lay the foundation for improving the performances of the existing relay protection device and developing new principle relay protection.