Experimental Determination of Short-Circuit Currents in Electric Power Networks

Abstract

No means has yet been discovered whereby the abnormal rise of current occurring during short circuits is avoided. Protection against its destructive effects remains, therefore, a subject of major importance. Among the problems requiring short-circuit current determinations, the following are the chief ones: (1) Selection of oil circuit breakers of the required interrupting capacity. (2). Determination of the size of current-limiting reactors. (3) Determination of relay settings in relay systems depending on selective action from over-current and directional relays. (4) Calculation of mechanical stresses in the structural elements of apparatus subject to short-circuit electromagnetic forces. Each of these problems requires the knowledge of the magnitude of short-circuit currents; relay problems frequently require, in addition, the relative phases of currents and voltages at different points of the system during short circuits; in item (4) above the wave-form of the short-circuit current sometimes has to be considered. The available information on the latter subjects, i. e. on phase relations and on wave form during short circuits is relatively meager, probably because it has been required in special cases only. Nevertheless, the demands for these data are increasing — on account of both the tendency towards increased sensitiveness of protective devices and the rapid increase in the magnitude of the short-circuit currents to be handled — and it will be worth while, therefore, if this added information is obtained. This paper is confined to the problems of the determination of the magnitude of short-circuit currents. The magnitude of short-circuit currents depends on a multiplicity of factors which have been enumerated and dealt with in other publications.1 When the impedances of all the circuit elements affected by the short-circuit are known together with the current-time decrement characteristics of all machinery capable of supplying current to the short circuit,2 the problem of short-circuit-current determination resolves itself into one of current division in a given network of electrical conductors under given electromotive forces. In the following paper, the comparative merits of three methods — calculation, a-c. test, and d-c. test — of determining short-circuit currents in networks are briefly discussed. Two d-c. experimental methods applicable to the “short-circuit calculating table” are analyzed in detail. The accuracy of its results, by both methods, is obtained for a variety of circuit conditions. The proper field of use of the short-circuit calculating table, and the best method of its application are determined.