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Those transmission lines that are on the same tower present unique problems to distance relays, especially for cross-country faults. In this paper, a novel first-zone distance relaying algorithm of parallel lines using only single-line sampling data is proposed for cross-country nonearthed faults. The relation between six phase currents and six sequence currents of parallel lines is derived. Moreover, the sequence currents of the adjacent parallel line at the relay location are expressed only by the sequence currents of the concerned line. Finally, the distance relaying algorithm using the one-end sampling data from the concerned line is presented. The computer simulations with PSCAD/EMTDC software and practical transmission-line parameters are carried out. The results show that the proposed algorithm can obtain the accurate measuring impedance for cross-country nonearthed faults and the accuracy is not affected by fault resistances, load currents, and source impedances.