Nonlinear control of static phase shifters used for power systemstabilityenhancement MUTHANA AL-RIFAI AND MOHAMED ZRIBI Electrical Engineering Department, College of Engineering and Petroleum, Kuwait University, P. O. Box 5969, Safat 13060, Kuwait. Email: alrifm@eng.kuniv.edu.kw ABSTRACT This paper investigates the application of nonlinear control schemes to a static phase shifter used to enhance power system stability. Two sliding mode control laws are proposed for the power system. The ®rst controller is a conventional sliding mode controller; the second controller is a fast terminal sliding mode controller. The two control schemes are developed for a single-machine-in®nite-bus system with a static phase shifter used for stability enhancement. Both control schemes guarantee the stability of the system even in the presence of disturbances. Simulations studies are carried out to illustrate the developed controllers. Moreover, robustness studies of the proposed controllers with respect to changes in the parameters of the system due to the faults are presented and discussed. The simulation results indicate that the proposed control schemes work well. Keywords: Sliding mode controllers, Static phase shifters. INTRODUCTION Static phase shifters (SPS) are an outcome of the concept of Flexible AC Transmission lines (FACTS), where existing AC power transmission lines are utilized to their maximum thermal capabilities. Recent investigations show that SPS can be used for mitigation of small-signal oscillations and enhancement of transient stability, as well as regulation of the ¯ow of power at steady-state (Iravani et al. 1994). This is done by modifying the phase angle dierence between the sending and receiving end voltages of the line by injecting a variable voltage to shift the phase angle of the sending end phase voltage, resulting in control of the power ¯ow through the line and additional damping to the power system. Considerable research attention has been devoted to the realization of the various types of SPS schemes, and the functions that can be assigned to an SPS to enhance the performance of the power system during steady state conditions, small signal dynamics, and large signal dynamics (Wang & Lee 1993, Iravani & Maratukulam 1994, Iravani et al. 1994, Tan & Wang 1995, Wang & Swift 1997). Kuwait J. Sci. Eng. 33(1) pp. 187-214, 2006