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The large AC generators in a power plant are core facilities for providing stable production and supply of electric power in many applications. The generator protection system has very important role in maintaining stability in a power system. The generator protection systems in South Korea have been imported and operated through a turn-key from overseas entirely. Recently, with increase in the power demand, and the aging of the existing protection devices, it has become necessary to consider replacing them or construct new power plants. Therefore, researches on the AC generator protection relays for the next-generation ECMS and efficient operation of protection control system are required. These require developing both the detailed generator modeling and internal fault analysis, and prototyping for multifunctional IED device dedicated to next-generation production. In this paper, an over-excitation protection and time over-current with voltage restraint algorithm were proposed. The DFT-based gain compensation algorithm was adopted for the frequency measurement. The 345 kV power system data of South Korea modelled by EMTP-RV and synchronous machine data by ATPDraw5.7p4 were used for simulations of performance evaluation.