Flexible AC Transmission Systems, called FACTS, got in the recent years a wellknown term for higher controllability in power systems by means of power electronic devices. Several FACTS-devices have been introduced for various applications worldwide. A number of new types of devices are in the stage of being introduced in practice.
In most of the applications the controllability is used to avoid cost intensive or landscape requiring extensions of power systems, for instance like upgrades or additions of substations and power lines. FACTS-devices provide a better adaptation to varying operational conditions and improve the usage of existing installations.
The basic applications of FACTS-devices are:
• power flow control,
• increase of transmission capability,
• voltage control,
• reactive power compensation,
• stability improvement,
• power quality improvement,
• power conditioning,
• flicker mitigation,
• interconnection of renewable and distributed generation and storages.
In all applications the practical requirements, needs and benefits have to be considered carefully to justify the investment into a complex new device. The basic idea of FACTS for transmission systems. The usage of lines for active power transmission should be ideally up to the thermal limits. Voltage and stability limits shall be shifted with the means of the several different FACTSdevices.
It can be seen that with growing line length, the opportunity for FACTSdevices gets more and more important. The influence of FACTS-devices is achieved through switched or controlled shunt compensation, series compensation or phase shift control. The devices work electrically as fast current, voltage or impedance controllers. The power electronic allows very short reaction times down to far below one second.
In the following a structured overview on FACTS-devices is given. These devices are mapped to their different fields of applications.