ABSTRACTWith the continued expansion and growth of the electric utility industry, electricity has become a highly engineered product; it is increasingly considered and managed as a commodity. In the evolving utility environment, financial and market forces require, and will continue to require, more optimal and profitable operation of the energy system with respect to generation, transmission and distribution. Power electronics-based equipment, Flexible AC Transmission Systems (FACTS), provide proven technical solutions to address these new operational challenges presented today. The potential benefits of FACTS equipment are now widely recognized by the power systems engineering and T&D communities. This paper focuses on summarizing the issues and benefits of applying FACTS controllers to AC power systems. The overall process for system studies and analysis associated with FACTS installation projects and the need for FACTS controller models is also discussed. Finally, an introduction to the basic circuitry of several FACTS controllers is provided with a focus on the performance characteristics of their systems. INTRODUCTION FACTS technologies enable better transmission system operation with minimal infrastructure investments, environmental impact and implementation times compared to building new transmission lines. FACTS technologies provide advanced solutions as cost-effective alternatives to building new transmission lines. Regarding FACTS equipment, voltage-fed converter (VSC) technology, which uses self-switching thyristors/transistors such as GTO, GCT, IGCT and IGBT, has been successfully applied in numerous installations around the world for synchronous static compensators ( STATCOM), Unified Power Flow Controllers (UPFC), Convertible Series Compensators (CSC), Back-to-Back DC Links (VSC-BTB), and VSC Transmission. These transmission system installations mentioned above join the previous generation of power electronic systems that use line-commutated thyristor technology for static Var compensators (SVC) and thyristor-controlled series compensators (TCSC). Although power electronics based equipment is prevalent in each of the three areas, Generation, Transmission, Distribution the focus of this document is on transmission. Transmission system limitations can take many forms and may involve the transfer of power between areas ( defined here as transmission bottlenecks) or within a single area or region (referred to here as regional constraint) and may include one or more of the following characteristics: Steady-state power transfer limit