الفهرس 
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Abstract
In recent years, power demand increased substantially and the electricity supply
industry is undergoing a profound transformation worldwide due to market forces, scarcer natural resources. While, the expansion of generation and transmission is opposed by environmental restrictions, land-use, and regulatory pressures that prevent the licensing and building of new transmission lines and generating plants. Therefore, the ability of the transmission system to transmit power becomes impaired and the system is characterized by unregulated active and reactive power flows. This leads to great problems such as, loss of system stability, high transmission losses, voltage limit violations, inability to utilize transmission line thermal capacity, and cascade tripping. Such problems have traditionally been solved by building new power plants and transmission lines. However, this solution is costly to implement, involves long construction times, and opposed from pressure groups. Since expansion process of transmission systems is limited, power system planners and operators should find out other solutions. The analysis of the available options indicates that, the solution of such problems rely on upgrading the existing transmission system by using the latest power electronic equipment and methods. This leads to a new technological thinking comes under the generic title of Flexible AC Transmission Systems (FACTS). The Damping of power system oscillations is very important for the system secure operation. Besides the power system stabilizers (PSSs), Flexible AC transmission system (FACTS) devices are also applied to enhance system stability. However, uncoordinated control of the FACTS devices and PSSs may cause destabilizing interactions. To improve overall system performance, coordination between PSSs and FACTS controllers should be made. The linearized models for power systems including FACTS devices are usually used as a base for the optimization process.