Simulative　analysis　shows　that　the　short　circuit　current　frequency　of　doubly-fed　induction　generator　during　low-voltage　ride-through　period　deviates　from　the　fundamental　frequency　according　to　the　working　conditions　before　the　fault.　When　the　transformer　internal　fault　occurs,　the　differential　action　and　braking　currents　of　differential　protection　based　on　phasor　values　fluctuate　widely　and　the　action　conditions　of　differential　protection　based　on　sample　values　are　not　satisfied　at　certain　sampling　points,　resulting　in　the　incorrect　and　instable　operations.　With　an　actual　regional　wind　farm　as　an　example,　the　off-line　action　performance　of　the　main　transformer　protection　of　wind　farm　and　the　step-up　transformer　protection　of　wind　power　group　is　tested　and　analyzed　on　EMTDC　platform,　which　shows　that,　the　fast　action　of　differential　protection　based　on　either　phasor　or　sample　values　can　not　be　ensured　and　the　factors　influencing　the　accurate　operation　of　protections　for　wind　farm　outgoing　transformers　are　generator　type,　operating　condition,　fault　location,　fault　type　and　short　circuit　capacity　ratio.