The objective of this work is to understand the basic idea of FACTS controller and their application in electrical grid, micro-grid and smart-grid. The aim of this Book is to simulate the small, medium and large power system model which includes generation, transmission & distribution system. The value of this work lies to the understanding of basic laws & principles. Here I have discussed how to work FACTS in modern grid system. Thereby is significantly improving the stability and performance of existing and future grid systems. That is, with Flexible AC Transmission Systems (FACTS), power companies will be able to utilize their existing transmission networks better, substantially increase the availability and reliability of their line networks, and improve both dynamic and transient network stability while ensuring a better quality of supply. The rapid growth in electrical energy used, combined with the demand for the low cost energy, has gradually led to the development of generation site remotely located from the load centers.