International Journal of the Physical Sciences Vol. 6(22), pp. 5273-5280, 2 October, 2011 Available online at http://www.academicjournals.org/IJPS DOI: 10.5897/IJPS11.833 ISSN 1992 - 1950 ©2011 Academic Journals Full Length Research Paper Dynamic stability enhancement and voltage support using UPFC tuned genetic algorithms in a multi- machine environment Shoorangiz Shams Shamsabad Farahani, Mehdi Nikzad*, Mohammad Bigdeli Tabar, Mehdi Ghasemi Naraghi and Ali Javadian Department of Electrical Engineering, Islamshahr Branch, Islamic Azad University, Tehran, Iran. Accepted 15 July, 2011 Unified power flow controller (UPFC) is one of the most important flexible AC transmission systems (FACTS) devices in power systems. It has many applications such as stability enhancement, power flow control, voltage support, etc. In this paper UPFC is used in order to control voltage. In this scope, UPFC is installed on a specified bus and the voltage of the proposed bus is controlled by using UPFC. In order to optimize performance, the parameters of UPFC controllers are tuned using genetic algorithms (GA) optimization method. Simulation results, which are carried out on a multi-machine electric power system, clearly show effectiveness and viability of UPFC in voltage control. Key words: Unified power flow controller, voltage Support, dynamic stability enhancement, multi-machine electric power system genetic algorithms INTRODUCTION The rapid development of the high-power electronics industry has made flexible AC transmission system (FACTS) devices viable and attractive for utility applications. FACTS devices have been shown to be effective in controlling power flow and damping power system oscillations. In recent years, new types of FACTS devices have been investigated that may be used to increase power system operation flexibility and controllability, to enhance system stability and to achieve better utilization of existing power systems (Hingorani and Gyugyi, 2000). Unified power flow controller (UPFC) is one of the most complex FACTS devices in a power system today. It is primarily used for independent control of real and reactive power in transmission lines for flexible, reliable and economic operation and loading of power systems. Until recently all three parameters that affect real and reactive power flows on the line, that is, line impedance, voltage magnitudes at the terminals of *Corresponding author. E-mail: mehdi.nikzad@yahoo.com. Tel: +982188043167 or +989122261946. Fax: +982188043167. the line and power angle, were controlled separately using either mechanical or other FACTS devices. But UPFC allows simultaneous or independent control of all these three parameters, with possible switching from one control scheme to another in real time. Also, the UPFC can be used for voltage support and transient stability improvement by damping of low frequency oscillations (LFO) (Eghtedari et al., 2011; Ozturk and Dosoglu, 2010; Hassan et al., 2010; Farahani et al., 2011; Alasooly and Redha, 2010; Mehraeen et al., 2010; Jiang et al., 2010 a, 2010 b, 2010 c; Faried, 2009). As referred earlier, one of the applications of UPFC is LFO damping. LFO in electric power system occur frequently due to disturbances such as changes in loading conditions or a loss of a transmission line or a generating unit. These oscillations need to be controlled to maintain system stability. Many in the past have presented lead-lag type UPFC damping controllers (Zarghami et al., 2010; Guo and Crow, 2009; Tambey and Kothari, 2003; Wang, 1999). They are designed for a specific operating condition using linear models. More advanced control schemes such as particle-swarm method, fuzzy logic and genetic algorithms (Taher and Hematti, 2009; Taher et al., 2008; Al-Awami, 2007; Eldamaty et al., 2005) offer better