In this paper is tried to coordinate and regulate the flexible ac transmission systems (FACTS) and power system stabilizer (PSS) to increase damping in multi-machine power systems. In multi-machine power system there are local and inter area modes. The inter area mode in multi-machine systems are undamped. The local modes can be damp by using PSS but inter area mode should be damp by using FACTS devices. Tuning and coordination together for several parameters is a complex work and we can not consider all conditions and limitations thus we have to use a global search algorithm to find the best solution. We have used modal analysis to improve damping. Damping ratio has been selected as the objective function. The loads have been modeled as constant impedance. GA has been selected as the optimization method to find the optimum point. IEEE 14 buses standard system has been chosen as the case study. Special conditions such as outage a transmission line considered and the stability of the system has been investigated in these conditions.

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