JEE_V5_N2_RP2 D. Dib A. Rezaiguia Z. Abada A. Haddouche Journal on Electrical Engineering 2230 – 7176 5 2 8 13 Unified power Flow Controller (UPFC), Flexible AC Transmission Systems (FACTS), Electrical Network, Power flow, Modeling, Identification This paper describes the theory and simulation by Matlab/Simulink of flexible Alternative Current Transmission Systems (FACTS) devices used in the disturbed power systems. One of these devices, Unified Power Flow Controller (UPFC) will be chosen for a specific application, detailed in this paper. Simulation investigate the effect of UPFC on the voltage of the related bus, it's also considers the effect on the amount active and reactive power flowing through the transmission system. The UPFC is designed to control and improve the power flow stability in the transmission lines. The aim of this paper is modeling, the identification of the references (decoupled Watt-Var, instantaneous power) and control of UPFC and studying its influence on the electrical Network. Finally simulation results have been presented to indicate the improvement in the performance of the UPFC to control voltage in power systems. December 2011 Copyright © 2011 i-manager publications. All rights reserved. i-manager Publications http://www.imanagerpublications.com/Article.aspx?ArticleId=1576