GB183912A - Improvements relating to protective devices for use with polyphase electric currents - Google Patents

Abstract

183,912. Langrish, N. G., and Metropolitan-Vickers Electrical Co., Ltd. April 28, 1921. Protective arrangentents.-In polyphase circuits, the circuit-breaker trip coil or coils, with or without the interposition of relays, are arranged to be energized from two or more of the phase conductors in such a manner that the resultant ampere-turns in the trip or relay coils are modified, due to alteration in phase relationship on the interruption of one line conductor, to enable the circuit-breaker to open with lower currents flowing in the remaining lines than when the system is operating normally. As applied to a three-phase circuit, Fig. 1, the overload trip coils for the circuit-breaker 1 consist of pairs of coils 5, 6, 7, 8, and 9, 10, each pair being energized from two of the phases such as AC, BC, and BC. These coils are arranged so that the resultant flux in each coil corresponds to the resultant of two-line currents separated by a phase angle of 60‹. On interruption of one phase C, one of the trip coils 7, 8 produces a resultant flux corresponding to the currents in the remaining lines A, B in phase and the circuit-breaker 1 is tripped when single-phase current in the lines A, B reaches 86 per cent of the current required to operate it under normal conditions. In a modification, Fig. 2, the trip coils 11, 12, 13 are supplied from the secondary windings 14, 15, 16 of transformers 17, 18, 19, the primary windings of which each consist of two coils 20, 21, and 22, 23 and 24, 25 connected in the phases A, B, C in a manner similar to the coils 5 - - 10, Fig. 1. As applied to a two-phase three-wire system, Fig. 3, special protection against single-phase operation is only provided when the neutral W is interrupted. Ordinary overload trip coils 28, 29 are connected in series with the lines A, B, each operating its own overload device, but there is also provided a third trip coil operated by the resultant of the phases A, B by means of coils 26, 27 in these lines. The flux in this latter coil normally corresponds to the resultant of the currents in the lines A, B in quadrature, and if the neutral N is interrupted, currents in the coils 26, 27 come into phase so that the overload device operates at 70.7 per cent of the line-tripping current under normal conditions. The invention is not limited to two or three-phase arrangements.