In this research, a protection of transmission lines using Global Positioning Systems (GPS) is presented. The proposed idea has the feature of unit protection relays to protect large power transmission grids based on phasor measurement units. The principle of the protection scheme depends on comparing positive sequence voltage magnitudes at each bus during fault conditions inside a system protection center to detect the nearest bus to the fault. Then the absolute differences of positive sequence current angles are compared for all lines connecting to this bus to detect the faulted line. The new technique depends on synchronized phasor measuring technology with high speed communication system and time transfer GPS system. The simulation of the interconnecting system was applied using Matlab Simulink. The new technique can successfully distinguish between internal and external faults for interconnected lines. The time of fault detection were estimated by 0.00345 msec for all fault conditions and the relay were also evaluated as a backup relay based on the communication speed for data transferring.
