GB242354A - Improvements in or relating to electric protective systems - Google Patents

Abstract

242,354. Reyrolle & Co., Ltd., A., Clothier, H. W., and Biles, R. W. Aug. 7, 1924. Cut-out arrangements.-A protective system for a feeder having a power transformer connected to one end thereof, affording complete protection with a minimum number of relays and pilot wires, comprises current transformers at each end of the feeder producing a balance of E.M.F.'s in a field wire circuit, and fault protective current transformers disposed on the side of the power transformer remote from the feeder and a fault co-operating, directly or indirectly, with the pilot wire system to disturb the balance. As shown in Fig. 1, current transformers G, G<1> in the feeders and a transformer F in the earthed neutral connection of the lowtension side B, B<1>, B<2> of the transformer are connected in series and balanced against the similar transformers G<2>, G<3> and the earth-fault ring transformer F<1> at the other end of the feeder. There is also connected in the pilotwire circuit J, J<1> the interconnected secondaries N, N<1>, N<2> of three current transformers in the high-tension side of the power transformer which produce a current only in the event of an earth fault on the high-tension side. The pilot wire system acts upon the protective relays H, H<1>, one at each end of the feeder and, in order that high settings may be used normally, a diverter relay L, L<1> which acts, on the passage of heavy straight-through currents in the feeder to introduce a resistance M, M<1> and thereby decrease the sensitiveness of the tripping relay, has its operating winding ocnnected across the pilotwire circuit between the protective relay and the current transformer secondary windings, as described in Specification 200,876. Instead of connecting the fault protection transformers N, N<1>, N<2> into the field wire system these transformers may operate a separate tripping relay, and the main circuit-breaker on the side of the power transformer remote from the feeder in opening serves to close an auxiliary switch shortcircuiting the pilot wires and bringing out the remote circuit-breaker. In order to deal with interphase faults in the windings D, D<1>, D<2> of the power transformer, earth leakage transformers may be provided which embrace not only the outgoing leads from the delta coils D, D<1>, D<2> but also the leads connecting consecutive coils with one another. Instead of the balancing transformer G, G<1> - - G<3> the arrangement shown in Fig. 4 and described in Specification 242,355 may be used. In this arrangement, the three-phase feeder is provided at each end with the equivalent of iron core ring-type transformer S, S<2> through which the three phases of the feeder are passed two in one direction and the third in the opposite direction and in which one phase is arranged to have a greater or less effect on the secondary winding than either of the other. In the arrangement shown in Fig. 4 a greater effect of phase A<1> is produced by introducing auxiliary transformers S<1>, S<3> into phase A<1>. The invention is applicable to the case where a power transformer is provided at each end of the feeder by providing at the right-hand end of the feeder an arrangement similar to that shown at the left-hand end.