- / In this thesis new advanced nonlinear control methods that solve the power system stabilization problem in a more efficient and integrated manner are considered. The proposed methods mainly concern with the primary level control of a power system that plays a central role in maintaining transient stability and obtaining a desired system performance. To this end, several new nonlinear control schemes mainly applicable on power system stabilizers (PSS) have been designed and extensively analyzed. For a theoretical assessment of the system operation the concepts of passivity and passivity margin are analyzed while the concept of Ω--passivity is introduced. Using partial feedback linearization and backstepping design techniques on suitable models of the system under consideration (power system) the Ω--passivity property of the system is proved. This property is further improved through the control in the closed-loop design. To this end, several control schemes are developed and a series of different theoretical problems have been solved on using simple output feedback and advanced nonlinear control methods like sliding mode control, adaptive control or a combination of them. A significant breakthrough has been achieved with the use of fuzzy techniques in these schemes yielding designs with combined advantages of both fuzzy and adaptive control methods. A detailed stability analysis, based on Lyapunov functions, has been used to prove that the proposed controllers guarantee either uniform ultimate boundedness or asymptotic stability for the closed-loop system. The proposed schemes are examined assuming that the system operates under parameter uncertainties as well as external disturbances. The theoretical analysis indicates that regulating some design parameters of the proposed controllers one can significantly improve the robustness and the disturbance attenuation capability of the system. Extensive simulations on a two machines infinite bus test system have been carried out to evaluate the effectiveness of the proposed schemes as these are applied on the PSS of each machine. Hard cases of three phase faults or significant power demand changes have been simulated. The simulation results show that the proposed nonlinear controllers enhance the damping of the electromechanical oscillations with respect to classical AVR/PSS and improve their robustness to parameter uncertainties and disturbance attenuation capability. Using similar techniques, speed governor controls (SGC) are also designed. An adaptive control scheme is proposed that ensures asymptotic stabilization of the closed-loop system as proved by standard Lyapunov techniques. Simulations are carried out for the one generator system connected to infinite bus. The simulation results confirm a significant improvement in the electromechanical oscillations damping compared to conventional speed governor controls. An important contribution of the thesis involves the coordination and management of the controls at the primary level. The design is carried out so that each control is not competing with the action of the other and it is cooperating to complement the action of the other. In this frame, first, the coordinated operation of the designed nonlinear power system stabilizers with the classical AVR/PSS is proposed. Particularly, the sequential operation of these controllers is considered in the following way: immediately after a fault, only the nonlinear controller operates but when the fault attenuates the classical AVR/PSS takes over. In this way, significant transient enhancement and voltage regulation after the large transients can be achieved. To implement this kind of operation a soft-switching logic from the one controller to the other is proposed by using a fuzzy logic mechanism to determine which controller will act each time period. In this way, it is avoided a discontinuous switching that can create a number of problems and may even lead to instabilities. The analysis and the simulation results confirm the validity of this approach since it is shown that the coordinated control scheme has an almost identical transient performance with the nonlinear controllers ensuring simultaneously the voltage regulation at the desired set-point. The same approach is used for the coordinated operation of FACTS with the excitation controllers. To this end, a static var compensator (SVC) controller is developed which, during a fault, acts to improve the transient stability. During the transient period, the SVC uses as inputs not the voltage at the connecting point but suitable signals from neighbor stations in order to contribute to the electromechanical oscillations damping; in the sequel, it returns to its normal operation maintaining the voltage level at the connecting point at a specified value. A soft switching scheme is also applied while signal transmission delays are taken into the account. The simulation results of a three phase short-circuit in a system with a single machine connected to infinite bus through a bus that has a SVC attached, indicate that this coordinated control scheme improves the transient stability even more (in comparison to the previous coordinated scheme). Finally, coordinated control logic is used for the design of both the speed governor control and the PSS. This is needed when the operation of the PSS cannot be considered completely independent and decoupled from the speed governor dynamics (in the case of fast valve operation). In this combined system a parallel design of both the SGC and the PSS ensures the uniform ultimate boundedness of the complete closed-loop system. Moreover, the use of continuous switching through fuzzy logic, as mentioned before, between these controllers and the classical AVR/PSS (for the excitation system) and a PID control (for the SGC) is proposed. The simulation results on the two machines infinite bus system clearly confirm the superiority of the coordinated control scheme with respect to the classical AVR/PSS excitation controllers and PID speed governor controllers. As a conclusion, new combined advanced nonlinear control schemes are analyzed and proposed for power systems. From the stability analysis and the simulation results it is clearly confirmed that the application of these nonlinear controls can be effectively improve the transient behavior as well as the robustness and disturbance attenuation of a power system. It is also proved that, without extreme cost, coordinated control schemes implemented through the proposed soft-switching techniques further enhance the transient and dynamic performance of the system.