Today’s interconnected transmission networks with long distance between generation and load centers necessitate exploiting permissible capability of the power transmission line. As a result, power system has become increasingly complicated, and therefore the reliability of such a system is reduced. Flexible AC Transmission Systems (FACTS) are considered to be a partial solution for these problems. Static voltage-sourced compensators are the most important part of FACTS controllers. If such a compensator is connected in parallel with a system, voltage of the connection point is regulated by the injected or absorbed reactive power of the compensator. On the other hand, series compensators, which inject capacitive or inductive series voltage into a transmission line, can control the line current and thereby the transmitted power. Static Synchronous Compensator (STATCOM) and Static Synchronous Series Compensator (C) are examples of series and shunt compensators, respectively. The third one is Unified Power Flow Controller (UPFC), which is based on two back-to-back voltage source converters, is a multi-functional FACTS controller. The UPFC compensator is a combination of series and shunt static voltage-sourced compensators which are coupled via a common DC link. This compensator, by means of angularly unconstrained series voltage injection, is able to control concurrently or selectively, the transmission line voltage, impedance, and load angle or, alternatively, the reactive and real power flow in the line. The UPFC may also provide an independently controllable shunt reactive compensation. On the other hand, Protection of transmission lines is a very important concern in power systems. Distance relays are frequently used to protect transmission lines, due to their simple and precise operation. The protection design of a transmission line equipped with FACTS controllers involves selection of the proper settings and characteristic of its protective relays. In such a line during fault, the relay’s voltage and current signals will be affected by the FACTS intervene. Therefore, studying the impact of these controllers on the performance of distance relays is important. This thesis introduces accurate models for positive, negative and zero sequence networks of STATCOM, C, and UPFC during fault. Based on these models, the apparent impedance seen by distance relay can be easily calculated for different fault types. For verification of the presented models, these compensators with control loops details are simulated. Simulation results confirm the accu Key Words: FACTS, Voltage Converter, STATCOM, C, UPFC, Distance Relay, Over-reach, Under-reach
