GB119937A - Protective Devices for Alternating Current Electric Systems. - Google Patents

Abstract

119,937. McColl, A. E. Nov. 14, 1917. Protective arrangements for circuits and apparatus employing biassed relays operating when the inflow exceeds the outflow by a predetermined percentage, the arrangement is such that the percentage bias is unaffected by change in the direction of the flow of power, but the relays are immediately operative, independently of the percentage bias, if current flows simultaneously into both ends or sides of the circuit or apparatus. In the arrangement shown in Fig. 1, in which the invention is applied to a transformer serving as an inter-connecting link between two power stations, current transformers S<1>, S<2> in the two sides of the transformer circulate current through the restraining coil R of a mechanically-balanced relay, the coil being short-circuited by a resistance L whereby the percentage bias may be varied. The operating-coil 0 which has, shy, ten times as many turns as the coil R is connected between equipotential points Q<1>, Q of the circulating current system, the point Q<1> being the mid-point of the winding R. Excess current in either of the transformers S', S<2> passes between the points Q<1>, Q; that is, through the whole of the large operating-winding and through half the small restraining winding. Consequently, as the unbalancing increases, the operating effect grows more rapidly than the restraining effect and, when it reaches the predetermined percentage, the relay operates to isolate the transformer.. If power flows into both sides of the transformer at the same time, the current transformers will circulate all their current through the operating coil and through each half of the restraining coil in opposite directions so that the restraining coil is inoperative and the relay operates at once, without restraint. In a modification of this form, the transformers circulate current through a resistance across which the restraining coil is connected, the operating coil being connected between normally equipotential points, as before, one of the points being the mid-point of the resistance. In another arrangement, shown in Fig. 3, the current transformers circulate their currents through the two series-connected elements E<1>, E<2>, of a reverse-current relay, the elements being shunted by a resistance or reactance L<3>. The normally equi-potential points Q<1>, Q<2> are connected together. With excess current in one of the series transformers, current is diverted from the corresponding element of the relay through the resistance &c. L<3>, and, when the excess current reaches the predetermined percentage, the current in such element is reversed so that the relay operates. If current flows into the faulty transformer from both sides, the series transformers circulate their currents through the elements E<1>, E<2> in opposite directions, and the relay operates without restraint. The application of the arrangements shown in Figs. 1 and 3 to three-phase transformers and generators employing three relays is described and illustrated in the Specification and, by crossconnexions of the three series transformers, it is shown that the same protection can be obtained with two relays only. The application to singlephase feeders or interconnectors is similar to that described for transformers. In the case of threephase feeders, Fig. 18, three current transformers having different capacities are employed at each end connected in star and to the pilot-wire system so that they normally supply single-phase current to the pilot-wire system. In the arrangement shown in Fig. 20, current transformers of equal capacity (and connected as shown) supply current to summation transformers S, the secondaries of which are connected up to the pilot-wire system. Various connexion arrangements of the current transformers to the summation transformer are described and the arrangement shown in Fig. 18 may be used with summation transformers. In the arrangement shown in Fig. 32 for a single feeder, two current transformers are employed at each end, the extra transformers S', S<4> supplying current to resistances R', R<2>, which are in parallel with the pilot-wire. The operating coils are connected across equi-potential points P<1>, P<2>. In this arrangement, as in all the others, the reverse-current relay arrangement of Fig. 3 may be substituted for the mechanically-balanced arrangement of Fig. 1, and this arrangement may be adapted to three-phase circuits in the manner above described. The relay may be of the pivoted beam or of the induction type. Specifications 28868/11, 4738/15, 101,151 and 104,571, are referred to.