In view of the conservativeness of the stability analysis based on the direct method

an improved robust stability criterion is developed. A novel Lyapunov functional is constructed. With the restrictions on the differentiability and the derivative of time-varying delay removed

the conservativeness is reduced. Furthermore

the results obtained that are formulated in the form of linear matrix inequalities (LMI) can be effectively solved using the LMI toolbox. Finally

the single machine system and WSCC 3-generator-9-bus system are employed to calculate the delay margin. The effectiveness and practicality of the method proposed is verified by simulation results.