In the paper a systemic method of choosing the frequency bias parameter and the integrator gain of the supplementary control is presented by using the Lyapunov technique. The reheat effect of the steam turbines is considered in the state space model. The effect of governor deadband nonlinearity is considered by using the describing function approach and including the linearised equations in the state space model. It is shown that the governor deadband (backlash) nonlinearity tends to produce a continuous oscillation in the area frequency and tie-line power transient response. When proportional feedback of area frequency error is used to stabilise the response it it found that only overshoot is reduced, but settling time remains the same.