Simultaneous Faults on Three-Phase Systems

Abstract

The method of symmetrical components now so extensively used to determine short-circuit currents and stability limits during transient conditions for three-phase transmission systems when a fault involving one or more of the three conductors occurs at any one point of the system, has been extended to apply to three-phase systems during simultaneous faults at two or more points of the system. A general equivalent circuit is developed to replace, in the positive phase diagram, two simultaneous faults involving any combination of the six conductors. An approximate equivalent circuit to be used with the d-c. calculating table when resistance is neglected is also given. Special equivalent circuits are employed to rep, lace two simultaneous faults and the lines upon which they occur, when the lines are unloaded feeders radiating from a common point or lines of equal impedances bussed at both ends. The methods and formulas given in this paper were developed in answer to such questions as the following: 1. Which is a more severe shock to a system, a double line-to-ground fault on one circuit or two single line-to-ground faults occurring simultaneously on two separate circuits? 2. Do simultaneous double line-to-ground faults which involve the same phases, a and b, on two circuits, result in more or less ground current than faults which involve phases a and b on one cir-cuit and phases b and c on the other?