Measured impedance at relaying point for faults on line behind

Distance relays are one of the mostly used protective devices in transmission systems. These relays are used as main, first zone and a part of second zone, and backup, the rest of second zone and third zone, protective devices. Fault resistance is a source of error for distance relays in the form of the measured impedance deviation from its actual value. This paper investigates the measured impedance by distance relays for the faults on the line behind by means of presenting the measured impedance at the relaying point and distance relay ideal tripping characteristic. The variation of ideal tripping characteristic duo the changes in the power system parameters is investigated.