Shateri - Iran University of Science and Technology Tehran
Jamali - Iran University of Science and Technology Tehran

The increasing demands for electricity energy and the limits for rights for transmission lines paths, lead to utilization of double circuit lines in power systems. The lines are loaded as high as possible and this leads to decrease in the security margin of the system. Here, accurate protective system is required for satisfactorily operation of the power system. Distance relays are widely used in the protection of transmission systems. Main and backup protection of transmission lines are provided by distance protection, which is based on the measured impedance at the relaying point. For the faults on the first circuit, in the case of zero fault resistance, the measured impedance at the relaying point is independent of the power system conditions. But in the presence of the fault resistance, the measured impedance deviates from its actual value according to the system structural and operational conditions and specially the magnitude of the fault resistance. In the case of second circuit, the measured impedance even deviates from its actual value in the absence of the fault resistance. However, this paper studies the measured impedance for the faults on the second circuit of a double circuit transmission line. The tripping characteristic is presented for the faults on the second circuit. The variation of this tripping characteristic is studied as a function of the changes in the operational and structural conditions of the power system.

Distance relay, Double circuit line, Fault resistance, Measured impedance, Tripping characteristic