Time-delay influences on the link of measurement and control need to be carefully considered in power system wide-area controller design. Most of the time-delay stability criteria based on the Lyapunov stability theory. After creation of a Lyapunov functional

a sufficient condition for a stable system with time delay is obtained via amplification and minification. Such criteria are inevitably conservative to a certain degree. It will become a hot topic of sustained concern to explore how to validly reduce the conservatism of stability criteria in this field. A new Lyapunov functional is established by introducing integral quadratic forms to an exited one. Furthermore

improved time-delay stability criteria are derived via the linear matrix inequality (LMI). Finally

a typical second-order delay system

a single-machine infinite-bus delay system and a WSCC 3-generator 9-bus power system including remote feedback links are taken to verify the effectiveness of the proposed method. Numerical results show that the conservatism of time-delay stability criteria can be substantially reduced by introducing integral quadratic forms.This work is supported by National Natural Science Foundation of China (No.51277128).