With the steady expansion of modern power system and the interconnected region

the time delay problem in the process of measuring signal transmission has aroused extensive concern. The conventional method of tackling the problem consists in finding the time delay margin bearable to the system based on the Lyapunov stability theory to solve the inevitable inequality amplification and contraction in the process of obtaining the criteria. Thus the conservatism of stability criteria has become one of the significant indices to measure the merit of criteria. A novel Lyapunov function is proposed first

followed by using Wirtinger integral inequality to reduce the conservatism of stability criteria when zooming out the derivative of Lyapunov function. Then

delay-dependent stability criteria are derived via linear matrix inequality(LMI)to find the time delay margin of the power system. Finally

a typical second-order delay system and a two-area four-machine system including wide-area damping control(WADC)are provided to verify the effectiveness of the criteria obtained. Simulation results show that the main result obtained by Wirtinger integral inequality can reduce the conservatism of stability criteria. This work is supported by National Natural Science Foundation of China(No. 61104045)

Program of Introducing Talents of Discipline to Universities(“111” Program)(No. B14022)and Fundamental Research Funds for the Central Universities(No. 2014B08014).