We consider the effects of time delay and disturbances on the stable operation of large-scale interconnected power systems, and the H∞ control problem of wide-area power systems is investigated. First, a new augmented vector and Lyapunov-Krasovskii function with more delay information are constructed. Then, the Wirtinger integral inequality, double integral inequality via auxiliary function and a convex combination approach are used to estimate the derivative of the function and a less conservative stability criterion is obtained. A memoryless H∞ state feedback controller is designed, and by introducing a novel staged linearization method, the existing condition of the H∞ controller is obtained in terms of linear matrix inequality. Finally, numerical simulations are given to illustrate that the proposed methods improve the stable operation region of the power systems, and the designed controller has good control performance. This work is supported by the National Natural Science Foundation of China (No. 61973105).