This paper focuses on the application of the distance relays for the protection of transmission systems using versatile electricity transmission controllers like the Unified Power Flow Controller (UPFC). Firstly, an in-depth model of the UPFC and its control is papered and so it's integrated into the gear for the needs of accurately simulating the fault transients. The simulation results show the impact of UPFC on the performance of a distance protection relay for various fault conditions and for various fault location.