Impedances seen by relays during power swings with and without faults

WE are at present in a period of power-system development during which it is essential that the best use be made of manpower and materials. To meet this requirement, equipment must be re-evaluated and refinements made in the design of apparatus and methods of system control. Loads on existing lines must be increased with continuity of service maintained. Of vital importance at this time is a thorough understanding of the operation of distance relays. The characteristics of the relays, and the system impedances “seen” by them, determine their performance. If it is known exactly what impedances are seen by a system of relays, their performance during power swings, with or without a fault on the system, can be predicted; and more important, relays having characteristics best suited for specific applications can be selected.