One of the effective and low cost means of improving the dynamic performance of the synchronous generators and power system stability is the excitation system control through Automatic Voltage Regulator. Recent development in the control and digital technology introduced different digital controllers as an efficient mean in improving the transient and dynamic characteristics of synchronous generators. Automatic voltage regulators were first designed to keep the voltage at the desired value with varying loads. However, they sometimes contribute to negative damping and the machine has a tendency to oscillate continuously under certain loading conditions. Thus increasing attention has been focused on the effect of the excitation control on damping out machine oscillations. Therefore it has been found useful and practical to incorporate stabilizing signals derived from rotor speed, frequency, power angle or superimposed on the voltage error signal to provide damping torque, this is the main function of the PSS.