Protection of Double-circuit Transmission Line Using Superimposed Current

This article presents a differential protection scheme for a double-circuit transmission line using a superimposed component of current. The proposed scheme is based on the ratio of directional current difference setting value to directional current bias setting value of the respective phase at both ends. These two quantities are derived based on the difference and the summation of the superimposed currents of the respective phase. The proposed scheme provides proper discrimination between in-zone faults and out-of-zone faults during complete loss of generation at one of the buses. Further, it is highly sensitive toward high-resistance faults and provides instantaneous operation to the entire line. Moreover, the relay provides reliability and selectivity for discriminating cross-country faults and internal faults and remains stable against external faults. The proposed scheme has been tested under different fault conditions with varying fault resistance between 0-200 . A series of tests using test signals generated by PSCAD/EMTDC (Manitoba HVDC Research Centre Inc., Winnipeg, Manitoba, Canada) on realistic data of a part of the Indian 400-kV power transmission system network have proved the accuracy and effectiveness of the proposed scheme.