The FACTS technology is not represented by a single high-power controlling device, but it is a collection of all the controllers, these individually or in coordination with the others give the possibility to fast control one or more of the interdependent parameters that influence the operation of transmission networks. These parameters include e.g. the line series impedance, the nodal voltage amplitude, the nodal voltage angular difference, then the shunt impedance and the line current. The design of the different schemes and configurations of FACTS devices is based on the combination of traditional power system components (such as transformers, reactors, switches, and capacitors) with power electronics elements (such as various types of transistors and Thyristors). The development of FACTS controllers is strictly related to the progress made by the power electronics. Over the last years, the current rating of thyristors has evolved into higher nominal values making power electronics capable of high power applications for the limit of tens, hundreds and thousands MW.
In general, FACTS devices can be traditionally classified according to their connection, as,
1) SHUNT CONTROLLERS:
The main devices of shunt controllers are the Static VAR Compensator (SVC) and the Static Synchronous Compensator (STATCOM).
2) SERIES CONTROLLERS:
It includes the devices like the Thyristor Controlled Series Capacitor (TCSC) and the Static Synchronous Series Compensator (SSSC).
3) COMBINED CONTROLLERS:
Elements such as the Thyristor Controlled Phase Shifting Transformer (TCPST), the Interline Power Flow Controller (IPFC), the Unified Power Flow Controller (UPFC) and the Dynamic Flow Controller (DFC) belong to this third category of FACTS.
FACTS devices are also classified according to the power electronics technology used for the converters as,
1) THYRISTOR-BASED CONTROLLERS:
This includes the FACTS devices based on thyristors, namely the SVC, the TCSC, the TCPST and the DFC.
2) VOLTAGE SOURCE-BASED CONTROLLERS:
These devices are based on more advanced technology like Gate Turn-Off (GTO) Thyristors, Insulated Gate Commutated Thyristors (IGCT) and Insulated Gate Bipolar Transistors (IGBT). This group includes the STATCOM, the SSSC, the IPFC and the UPFC.
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