A robust nonlinear state feedback control achieving transient stabilization is designed on the basis of the standard third order model of a synchronous generator connected to an infinite bus. Sudden mechanical power failures, short circuits, infinite bus perturbations may drive the generator out of step. The proposed robust nonlinear excitation control prevents the machine from going out of step in the presence of any mechanical or electrical parameter perturbation. L ∞ and L 2 disturbance attenuation are guaranteed from the power angle and relative speed regulation errors with respect to time-varying parameter perturbations from nominal values. The operating condition is the only equilibrium point of the closed loop system with an explicitly computable stability region, when all parameters are equal to their nominal values.