Since the development of the interconnection of large electric power systems, it has been spontaneous system oscillations at very low frequencies in the range of 0.2–3.0 Hz. After starts, it would continue for a long period of time. In certain cases, it continues to develop causing system separation due to the lack of damping of the mechanical modes.

In the past three decades, Power System Stabilizers (PSSs) have been extensively used to increase the system damping for low-frequency oscillations. Power utility worldwide is currently implementing PSSs as effective excitation controllers to enhance system stability. Yet, some problems are experienced with PSSs over the years of operation. Some of these were limited to the capability of PSS, due to damping in local modes and not in the inter-area modes of oscillations.

In accumulation, it can cause huge variations in the voltage profile under severe disturbances and they may even result in leading power factor operation and losing system stability. It has necessitated a review of the traditional power system concepts and practices to achieve a larger stability margin, better operational flexibility, and better utilization of existing power systems.

Flexible AC transmission systems (FACTS) have gained a great interest during the last few years, due to the recent techniques in power electronics. FACTS devices are mainly used for solving various power system steady-state control problems such as voltage regulation, transfer capability enhancement, and power flow control. As supplementary functions, damping the inter-area modes and enhancing power system stability using FACTS controllers have been extensively studied and investigated. Generally, it is not cost-effective to install FACTS devices for the sole purpose of power system stability enhancement.


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 of MW.

In general, FACTS devices can be traditionally classified according to their connection, as,


The main devices of shunt controllers are the Static VAR Compensator (SVC) and the Static Synchronous Compensator (STATCOM).


It includes the devices like the Thyristor Controlled Series Capacitor (TCSC) and the Static Synchronous Series Compensator (SSSC).


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,


This includes the FACTS devices based on thyristors, namely the SVC, the TCSC, the TCPST, and the DFC.


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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