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