Since the development of interconnection of large electric power systems, it has been the 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. The power utility worldwide is currently implementing PSSs as effective excitation controllers to enhance the 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 necessitate 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 investigate. Generally, it is not cost-effective to install FACTS devices for the sole purpose of power system stability enhancement.