When the AC fault occurs in the power grid, the protection of unified power flow controller (UPFC) would trip to make the UPFC out of operation. It would cause the electrical quantities in the power grid change suddenly once again. Subsequently, the competition between the disturbance caused by AC fault and that caused by the trip of UPFC protection would have adverse influence on the transmission line pilot protection. The demonstrative project of UPFC in the western Nanjing grid is introduced. Moreover, based on the discussion about the UPFC protection system, the impacts of UPFC on the performances of pilot zero-sequence protection, pilot distance protection and superimposed component direction protection are studied with theoretical analysis and simulation study. It points out that the mal-operation or fail-to-trip of the superimposed component direction protection would occur due to the impact of UPFC, and it is necessary to improve the relevant protection schemes.