International Journal of Engineering Research and Development e-ISSN: 2278-067X, p-ISSN: 2278-800X, www.ijerd.com Volume 10, Issue 7 (July 2014), PP.25-35 25 Transient Stability Enhancement during Damping of Low Frequency Oscillations of a Multi-Machine Power System Using Adaptive Neuro-Fuzzy Controller for FACTS devices C.Ganesh 1 , P.Bhaskara Prasad 2 and G.Venu Gopal 3 1 Assistant Professor, Dept of EEE, Annamacharya Institute of Technology & Sciences, Rajampet, Andhra Pradesh, India. 2 Assistant Professor, Dept of EEE, Annamacharya Institute of Technology & Sciences, Rajampet, Andhra Pradesh, India 3 PG Student, Dept of EEE, Annamacharya Institute of Technology & Sciences, Rajampet, Andhra Pradesh, India. Abstract:- Long distance AC transmission is often subject to stability problems, which limits the transmission Capability. Large interconnected power systems often suffer from weakly damped swings between synchronous generators and subsystems. This paper studies the comparative performance of SSSC and UPFC for the improvement of transient stability and damping of power swings of a multi-machine power system using neuro- fuzzy controller. Simulation results are carried out in MATLAB/SIMULINK environment for multi-machine power system to analyse the effects of SSSC and UPFC on transient stability performance and damping of power swings of the system. Index Terms: - Transient stability, Power oscillation damping, Adaptive neuro-fuzzy inference system (ANFIS), SSSC, UPFC, Fuzzy logic Controller (FLC). I. INTRODUCTION This paper presents improvement of transient stability and power oscillation damping in a multi machine power system. Transient stability is the ability of the power system to maintain the synchronism after the sudden large disturbance. These disturbances may be because of the application of faults, clearing of faults, switching ON and OFF surges in EHV system. Methods to improve transient stability are use of breaking resistor, reduction in system transfer reactance, use of bundled conductors, short circuit current limiters, and the placement of FACTS devices [2]. Power systems exhibit various modes of oscillation due to interactions among system components. Most of these oscillations are generally associated with transmission system disturbances and can occur due to step changes in load, sudden change of generator output, transmission line switching and short circuits. Depending on the characteristics of power systems, the oscillations may last for 3-20 seconds after a severe fault. Drawn out oscillations that last for a few seconds or more are usually the result of very light damping in the system and are pronounced at power transfers that approach the line‟s stability limit. During such angular oscillation period, significant cycle variations in voltages, currents, and transmission line flows will takes place. Therefore, it is important to damp out these oscillations as quickly as possible because they cause mechanical wear in power plants and many power quality problems. In the past, power system stabilizers (PSSs) have been extensively used to increase the system damping for low frequency oscillations. The power utilities worldwide are currently implementing PSSs as effective excitation controllers to enhance the system stability. However, there have been problems experienced with PSSs over the years of operation. Some of these were due to the limited capability of PSS in damping only local and not inter area modes of oscillations. In addition, PSSs can cause great variations in the voltage profile under severe disturbances and they may even result in leading power factor operation and losing system stability. Recently Flexible AC transmission systems (FACTS) have gained a great interest due to recent advances in power electronics. By using power electronics controllers a Flexible AC Transmission System offers greater control of power flow, secure operation and damping of power system oscillations. FACTS devices are used in power systems to improve both the steady state and dynamic performances of the systems. The voltage stability, steady state and transient stabilities of a complex power system can be improved by using FACTS devices. FACTS devices can control the various parameters of the power system such as voltage, phase angle and line impedance in a rapid and effective manner [3]. FACTS controllers can be divided into four categories: Series Controllers such as Thyristor Controlled Series Capacitor (TCSC), Thyristor Controlled Phase Angle Regulators (TCPAR), and Static Synchronous Series Compensator (SSSC); Shunt controllers such as Static Var Compensator (SVC), and Static Synchronous Compensator (STATCOM); Combined series-series controllers