A nonlinear state-feedback controller design scheme based on immersion and invariance methodology is proposed for single machine infinite bus systems with thyristor-controlled series compensation. A nonlinear damping controller is obtained by choosing a stable target system and some designed mappings, which efficiently avoids choosing Lyapunov function and guarantees that all trajectories of the closed-loop system are asymptotically stable and bounded. Simulation
result shows not only the validity of the proposed methodology but obviously enhances the transient stability of the closed-loop system.