Flexible AC transmission systems or FACTS are devices which allow the flexible and dynamic control of power systems. This paper is aimed toward the benefits of utilizing FACTS devices with the purpose of improving the operation of an electrical power system. Introduction Power quality is an issue that is becoming increasingly important to electricity consumers at all levels of usage. Sensitive equipment and non-linear loads are commonplace in both the industrial and the domestic environment, because of this a hightened awareness ofpower quality is developing.The sources of problems that can disturb the power quality are: power electronic devices, arcing devices, load switching, large motor starting, embedded generation, sensitive equipment, storm and environment related damage, network equipment and design.The solution to improve the energy quality (PQ-Power Quality) at the load side is of great important when the production processes get more complicated and require a bigger liability level, which includes aims like to provide energy without interruption, without harmonic distortion and with tension regulation between very narrow margins. The devices that can fulfil these requirements are the Custom Power; a concept that we could include among the FACTS, but that is different to them because of their final use. In fact the topologies that they employ are identical to the ones in the FACTS devices with little modifications and adaptations to tension levels, therefore they are most oriented to be used in distribution networks of low and medium tension, sometimes replacing the active filters.Recent developments in electrical power systems such as deregulation, open access, and cogeneration are creating scenarios of transmission congestion and forced outages. Addition of new transmission lines is an almost impossible solution due to environmental and other considerations, and developing new approaches to Power System Operation and Control is the need of the hour for overload relief and efficient and reliable operation. Flexible AC Transmission Systems (FACTS), with the underlying concept of independent control of active and reactive power flows, offer an attractive alternative for achieving the objectives.The use of static power converters in electricity networks has the potential of increasing the capacity of transmission of the electric lines and improving the supply quality of the electric energy. The devices used to achieve this, are the FACTS (Flexible Alternating Current Transmission Systems). The FACTS technology has a collection of controllers, that can be used individually or co-ordinated with other controls installed in the network, thus permitting to profit better ofthe network's characteristics of control.
