I. Introduction
Electricity is part and parcel of our life. No one can live without electricity. The demand of electricity is increasing day by day. The dependency on electricity has been increased tremendously. All the consumers expect full and pure supply from utility. Power disruption and power outages are problems due to increasing demand. Due to large requirement of power, transmission systems are running closer to their stability limits. [1] The major problem faced by all industries includes non matching of supply and demand. In large power system, stability problem leads to power disruption and blackout which will easily hamper the economic growth of nation. One of the suitable methods for reducing these problems includes use of FACTS devices. According to IEEE the FACTS can be defined as, “Alternating current transmission systems consisting of power electronics based and other controllers to enhance controllability and available power transfer capability (APTC) of the system; so that one or more AC transmission system parameters like voltage, current and powers can be effectively controlled. The devices under FACTS categories are having very fast response and can be used in multiple applications. The basic classification of FACTS controllers includes series FACTS devices, shunt FACTS devices, series series FACTS devices and series-shunt FACTS devices. Apart from this FACTS technology also based on thyristor controlled FACTS devices and VSI based FACTS devices. [2] Due to resonance phenomenon seen in the thyristor controlled devices, VSI based devices are better than earlier one. The only limitation of VSI based controllers is requirement of self commutating power electronic semiconductor devices like GTO, IGBT and IGCT etc. Basically FACTS controller provides voltage support at critical buses in the system (with shunt connected controller) and regulate power flow in critical lines (with series connected controllers) [3]