Circulating-current pilot-wire differential protection of multiended circuits

A general analysis is made of symmetrical multiended pilot-wire circuits terminated by impedances in parallel with the current sources. The fundamental-frequency-transmission characteristics of the pilot circuits are accounted for, and compensation for pilot phase shift is applied by a suitable choice of parallel impedance. A range of alternative amplitude-comparator relay characteristics is presented graphically on the basis of the complex ratio of the relaying current at any relay location to the phasor sum of relaying currents at the remaining terminations. A method is described by which the insertion of a simple network at the electrical centre of a system of symmetrical pilot circuits permits the application to multiterminal lines of schemes of protection requiring the correct termination of the pilot circuit in its characteristic impedance.