Patent GB-351033-A (GB)
Title: Improvements in or relating to electric protective systems
IPC: H02H3/13
Filed: 1930-02-21
Abstract: 351,033. Protective cut-out arrangements. KAPP, R. 0. and CARROTHERS, C. G., 25, Randolph Crescent, Maida Hill, London, and WHITCHER, J., Rosegarth, Manor Road, Ruislip, Middlesex. Feb. 21, 1930, No. 5875. [Class 38 (v).] A polyphase A.C. potential arrangement for sectionalized transmission systems of the type in which the phases are coupled together at junctions between sections by polyphase reactors with earthed neutrals such as star-connected primary windings of distributing transformers as shown in Figs. 2 and 3 of Specification 312,725, has, in addition to impedance relays at the ends of sections, a relay arrangement responsive to current below a predetermined value in one or more phases accompanied by current above a predetermined value in another phase. As shown in Fig. 1 the lines 1, 2, 3 of a section controlled at one end by a circuit-interrupter 4 have current transformers 10, 11, 12 each feeding corresponding relay coils 8. The contacts of the relays 8 are cross-connected in the manner shown in series with the circuit breaker release coil 20 in such a manner that when one of the relays is in the operated position with the other two unoperated, due to an earth fault in the corresponding phase, a tripping circuit is completed between its upper contacts and one of the sets of lower contacts of the other two relays. Thus isolation of both ends of the faulty section is produced. In order to deal also with an open-circuit connection unaccompanied by an earth fault such as may occur in the case of a fractured overhead transmission line, a set of open-circuit relays can also be arranged to be fed from the current transformers 10, 11, 12, Fig. 2 (not shown), such relays in a similar manner producing tripping in the case of a cessation of current in one phase accompanied by normal flow of current in the other phases. A further modification, Fig. 3 (not shown), involves a current balance relay adapted to prevent tripping by the earthfault relays in the case of an earth fault in an adjacent section but to permit the tripping to occur in the manner described when the earth fault is in its corresponding section. In the arrangement shown in Fig. 4 a relay which may be of the induction type is employed, the actuating electromagnet 46 being connected in series with the lead joining the neutral 38 of the current transformers 10, 11, 12 with the neutral point 43 of the restraining electromagnets 39, 40, 41. Each of these electromagnets is adapted to be saturated at low current values and tripping takes place by a preponderating torque of the operating electromagnet 46 only if appreciable current flows in one only of the restraining electromagnets. The circuit-breaker is controlled preferably with a slight time lag and the relay arrangements described operate independently of the impedance relays (not shown) which are connected at the ends of sections. The impedance relays are arranged in known manner to operate with timelag discrimination so that, for example, if a fault occurs near the end of a section even if the currents flowing in the sound phases do not fall to zero or a low value the section is quickly isolated at that end by the substantially instantaneous operation of the adjacent impedance relay so enabling the conditions at the other end of the section to effect rapid isolation in the manner described above.