Wide-area signal has been proposed to be used as feedback signal to enhance the dynamic performance of large interconnected power systems， thereby the impact of time delays introduced by remote feedback signal transmission and processing has to be considered. In this paper， the wide area feedback loop is chosen according to residue matrix. Pade approximation method is used to express time delay and state space model is established with the time delay considered. Additional inter-area damping control（AIADC） is proposed for generators’ excitation controllers， and the effect of delayed feedback signal is demonstrated by using eigenvalue analysis. The control parameters of AIADC are derived by using linear optimal control approach. Simulations in a 4-machine test system are given to demonstrate the advantages of the proposed AIADC.