M. S. Mahdavi - Department of Electrical Engineering, Amirkabir University of Technology, Tehran, Iran
G. B. Gharehpetian - Department of Electrical Engineering, Amirkabir University of Technology, Tehran, Iran
N. Moaddabi - Department of Electrical Engineering, Amirkabir University of Technology, Tehran, Iran

The ability of network generators to maintain their synchronism after a fault depends on several factors, including the initial conditions of the system before the fault, so it can be expected that by changing the system variables, such as the angle of the generator rotors in the system, the system stability can be against the fault. This change is possible by the Permanent presence of FACTS devices. The critical clearing time of fault as the most accurate criterion for transient stability evaluation can be the objective function of our optimization problem, but the determination of the critical clearing time, which involves the time domain simulation of the network along with the repeated examination of the fault occurrence It is time consuming to adjust the FACTS devices to a power system that is changing at any one time. In this paper, we present a new objective function based on the reduction of the voltage angle difference between generators terminals, which greatly increases the speed of optimization, in order to improve the transient stability of smart grid by setting up the online FACTS devices. Minimizing this objective function by optimizing the FACTS is equivalent to maximizing transient stability. In addition  to optimal tuning, optimal type, number and allocation of them are considered for long term. in determining the optimal number of these devices, the cost of investment is also considered.

Transient Stability, Critical Clearing Time, Voltage Angle Difference Between Generators Terminals, FACTS, PSO