SUMMARYWith the continued expansion and growth of the electric utility industry, electricity has become a highly technical product; it is increasingly considered and treated as a commodity. In an evolving utility environment, financial and market forces demand and will continue to demand more optimal and cost-effective operation of the electricity system with respect to generation, transmission and distribution. Power electronics-based equipment, Flexible AC Transmission Systems (FACTS), provides proven technical solutions to meet 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 article focuses on summarizing the issues and benefits of applying FACTS controllers to AC power systems. The overall process of system studies and analysis associated with FACTS installation projects as well as the need for FACTS controller designs are also discussed. Finally, an introduction to the basic circuitry of several FACTS controllers is provided, with emphasis on the performance characteristics of their systems. INTRODUCTION FACTS technologies make it possible to improve the operation of the transport system with minimal infrastructure investment, environmental impact and implementation time compared to the construction of new transport lines. FACTS technologies offer advanced solutions as cost-effective alternatives to the construction of new transmission lines. Regarding FACTS equipment, voltage fed converter (VSC) technology, which uses self-switching thyristors/transistors such as GTOs, GCTs, IGCTs and IGBTs, has been successfully applied in some number of installations worldwide for static synchronous compensators (STATCOM), unified power flow controllers (UPFC), convertible series compensators (CSC), back-to-back DC links (VSC-BTB) and VSC transmission. These aforementioned transmission system installations are in addition to the previous generation of power electronic systems that utilize line-switched thyristor technology for static var compensators (SVCs) and thyristor-controlled series compensators (TCSCs). Although power electronics based equipment is prevalent in all three areas, Production, Transportation, Distribution, this paper focuses on transportation. Transportation system boundaries can take many forms and involve transfer of energy between areas (here called transportation bottlenecks) or within a single area or region (here called as regional constraint) and can include one or more of the following characteristics: Steady-state power transfer limit