J. Basic. Appl. Sci. Res., 1(11)2219-2225, 2011 © 2011, TextRoad Publication ISSN 2090-4304 Journal of Basic and Applied Scientific Research www.textroad.com *Corresponding Author: Morteza Nasiraghdam,Department of Computer Engineering, Ahar Branch, Islamic Azad University, Ahar, Iran. Email: m_nasiraghdam@iau-ahar.ac.ir Analysis the Effect of UPFC Location in Low Frequency Oscillation Using Intelligent Search Method Based on Fuzzy Morteza Nasiraghdam 1 and Mehdi Nafar 2 1 Department of Computer Engineering, Ahar Branch, Islamic Azad University, Ahar, Iran 2 Department of Electrical Engineering, Marvdasht Branch, Islamic Azad University,Marvdasht, Iran ABSTRACT Unified Power Flow Controller (UPFC) is use for controlling the real and reactive power on the transmission line and the bus voltage simultaneously and independently. An additional task of UPFC is to increase transmission capacity as result of power oscillation damping. The effectiveness of this controller depends on its optimal location in the power system network. This paper proposed various cases studies to find the optimal location of the UPFC controllers and Eigen value analyses are used to assess the most appropriate input signals (stabilizing signal) for supplementary damping control of UPFC to damp out the inter-area mode of oscillations .The placements of UPFC controllers have been obtained for the base case and for the dynamic critical contingences. The effectiveness of the proposed method of placement is demonstrated on practical network. KEY WORDS: Flexible AC Transmission System, UPFC, Inter-Area Oscillation, Power System Control, Optimization Approach, Fuzzy Logic Program, Low Frequency Oscillation, Optimal placement. INTRODUCTION Power system stability is concerned with the ability of a power system either to reach a new stable operating point or to come back to the original stable equilibrium after a disturbance. Power system stability is a very complex problem .Many different phenomena have been identified as power system stability problems. Classifications of power system stability have been proposed to address such problems .Power system stability problems are classified: • According to the underlying phenomenon into angle and voltage stability, • According to the size of the disturbance into large- and small-disturbance, and • According to the dynamics involved in into short and long-term stability. Angle stability is interested in the capability of the generators to remain in synchronism. Voltage stability is concerned with the ability of the generators to supply the loads at acceptable voltage levels. A disturbance is large when the non-linear differential equations that describe the power system dynamic behavior cannot be linearized for analysis purposes. In contrast, a disturbance is small when the non- linear differential equations can be linearized for analysis purposes. In other words, a linear model characterizes accurately the dynamic behavior of the power system. Powerful techniques can be used for analysis and control of linear systems [Fouad A.A. and V. Vittal, 1992]. Short-term stability is determined by the dynamics of the synchronous generators and their primary controls (voltage and load-frequency). Long-term stability is also affected by the dynamics of the energy sources of the generators (boilers, nuclear reactors, hydro stations with complicate hydraulic circuits, etc) and the Automatic Generation Control and Secondary Voltage Control [(H.F.Wang' 1999),(Erick, T.E., 2000)]. Power system variables such as voltages and power flows may exhibit low frequency poorly damped oscillations. Such oscillations are usually known in the literature as electromechanical oscillations since they are due to the mechanical oscillations of the generator rotors. Electromechanical oscillations lie in the frequency range between 0.1 and 2 Hz. Their frequency depends on the number of generators involved in: local oscillations are of higher frequency since they involve few generators whereas inter-area oscillations are of lower frequency because many generators are involved. Electromechanical oscillations are natural oscillations of power systems. The damping of the electromechanical oscillations is affected by generator voltage controllers. First instances of poorly damped oscillations occurred when hydro stations equipped with static excitation systems were connected through long transmission lines to the load centers. Power System Stabilizers (PSSs) have been very effective devices to 2219