H SHAYEGHI - Technical Eng. Department, University of Mohaghegh Ardabili, Ardabil
H.A Shayanfar - Center of Excellence for Power System Automation and Operation, Electrical Eng.Department, Iran University of Science and Technology, Tehran, Iran
a jalili - Islamic Azad University, Science and Research Branch, Tehran
a shayeghi - Technical Eng. Department, University of Mohaghegh Ardabili, Ardabil

The Unified Power Flow Controller (UPFC) is the most versatile device in the FACTS devices which has emerged to enhance power system stability spectrum and dynamic performance. In this paper, a new Multi-Stage Fuzzy (MSF) DC-voltage regulator is proposed for the UPFC to damp power system low frequency oscillations. For a fuzzy rule based control system to perform well, the fuzzy sets must be carefully designed. A major problem plaguing the effective use of this method is the difficulty of accurately constructing the membershipfunctions. For this reason, in the proposed MSF type PID controller the membership functions are tuned automatically using Particle Swarm Optimization (PSO) method. The aim is to reduce fuzzy system effort and take large parametric uncertainties into account. This newly developed control strategy combines the advantages of PSO and fuzzy system control techniques and leads to a flexible controller with simple structure that is easy to implement. The effectiveness of the new proposed control strategy is evaluated under different operating conditions in comparison with the classical controllers to demonstrate its robust performance through time simulation studies and some performance indices.

UPFC, Multi-Stage Fuzzy Controller, PSO, FACTS Devices, Power System Stability and Control