Due to the increasing of complexity in power systems, re‌structuring of power system and innovative movements toward smart grids, it is developing to use of communication networks as the base of data transmitting. The variety of advances of NCS systems causes their usage in power systems as a necessity. One of the applicative fields of NCS in power systems is to support of ancillary services such as load-frequency control (LFC) in deregulated environment. Frequency control, as a major function of automatic generation control (AGC), has been one of the important control problems in design and performance of electric power systems. In a multi-area interconnected power system, AGC systems are designed and implemented to a group of power plant. Maintain frequency and power interchange with neighboring control areas at scheduled value are the two main objectives of an AGC system. Receiving signal of control areas and transmitting AGC control signals is done by using of communication network. One of the challenges in communication networks is the communication delay in sending or receiving control signals. It is well known that time delays can degrade on performance ofLFC system and even cause system instability.In this thesis, at first we sought to investigate the degree of interaction and dependency between control area in multi-area power system. This investigation is done based on gershgorin bonds. This results show that interaction between area is considerable. So, it is expected that by using of distributed control structure, we can improve the control performance of AGC system. In following, two new methods are presented for designing a robust controller based on distributed control structure for AGC systems. In the first method, the interaction effect between control area in multi-area power system is considered as input in state equation of each control area. In this way, control effort is proposed for each area so that they have knowledge about their states of neighbor control area, besides their ownself. Therefore, controller can do its duty with more information and get to more improvement and system performance. In the second, the effect of interaction is considered as disturbance. The proposed control structure of this method is so that by entering the upperbands of delay in each step of design, and considering the appropriate weight matrices, besides of lower conservativeness, it shows more robust performance versus other controllers. To investigating the performance of proposed control strategies, several simulations are done on three-area LFC schemesunder traditional and deregulated environments. These simulations have been done on different scenarios and in comparing with different papers discussed on this field. The results show that proposed strategy can follow the load variations as well and restores the frequency deviations and power interchange with neighboring control areasto nominal and scheduled by considering communication delay. Although, the presented control gains of other references can not sustain the stability of system in presence of time-varying delay. Keywords: Multi-area interconnected power system,Networked Control System (NCS), Load-Frequency Control (LFC), Automatic Generation Control(AGC), Time delay, Distributed control.