Abstract-This paper presents simulation results of the application of distance relays for the protection of transmission systems employing flexible alternating current transmission controllers such as the unified power flow controller (UPFC). Firstly a detailed model of the UPFC and its control is proposed and then it is integrated into the transmission system for the purposes of accurately simulating the fault transients. The simulation results show the impact of UPFC on the performance of a distance protection relay for different fault conditions and for different fault location.