Patent GB-472663-A (GB)
Title: Improvements in or relating to protective gear for alternating current systems
IPC: H02H3/16
Filed: 1936-03-28
Abstract: 472,663. Protective cut-out systems. ROSS, T. W., KAUFMANN, M., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. March 28, 1936, No. 9278. [Class 38 (v)] An earth-fault protective system for bus-bars for interconnection of a plurality of lines (to which feeders, generators, or transformers may be connected) comprises a system of relays respectively associated with the lines or groups thereof, and each responsive to residual current in the line or group to control the tripping of the breaker or breakers governing the connection of the line or group to the bus-bars, and means are provided responsive to flow of current in the earthing conductor or conductors of the frame-work of the switchgear and bus-bar system and are arranged to govern the effectiveness of the relays to trip the breakers, whereby the relay for a line or lines is only rendered effective to produce tripping when the flow of residual current in the line or lines is accompanied by flow of current in the earthing conductor or conductors. In Fig. 1, the framework 23 is earthed over a conductor 22 in which are current-transformers 21, 24, the former being connected to a relay 20 with contacts 18, 25, of which the latter control the parallel-connected trip coils TS1, &c. of section-breakers S1 &c., connected between the bus-bar sections B1 &c. When on bus-bar-earth fault, S1 &c. are tripped open, contacts AS1 &c. are closed to make, over closed contacts 18 and transformer 24, the circuit of the operating windings 15 of operating magnets O1 &c. of relays P1 .. R9 connected in the respective lines 1 .. 9. Each relay has a fixed winding 11 energized by the residual current of a transformer group 12 .. 14 in the associated line, and has a moving element, as MR1, adapted when the windings 11, 15 are energized in the proper sense, to close trip contacts 10 for the appropriate breaker CB1 &c. Since the opening of S1 &c. isolates the faulty bus-bars, the proper sense is that due to inflowing earth-fault currents into the bus-bar section, the other lines having been relieved of fault current by the opening of S1 &c. Fig. 2 shows how the above arrangement may be modified to obtain discriminative operation of the section breakers S1, S2. such operation being obtained by comparing the input to, and output from, a section B1 or B2 or B3. Thus section B1 has current transformers T1A .. TIC connected to be energized by residual current in lines 1 .. 3 and balanced against the residual current in current transformer T1D energized from B2, as shown, or from B1 adjacent the breaker S1. A similar set of transformers T4A .. T4D is provided for the section B3, and the spill-over current is arranged to energize operating windings 30 of relays S1A, S2A, the field windings 29 of which are energized in series with field windings 31 of relays S1B, S2B from a current transformer 24 in the earth connection 22 from framework 23. The operating magnet windings 32 of these relays are energized, respectively, by current transformers T2A .. T2E and T3A . . T3E. Transformers T1D, T3E have their primaries connected in series across the associated residual-current transformer group 34 .. 36, a similar arrangement applying to other pairs. Relays S1A, S2A are respectively responsive to faults on sections B1, B3, and relays S1B, S2B are responsive singly or together for faults on section B2, the former closing trip contacts A3, A5 in series with closed contacts 40 of leakage relay 19. Relays S1B, S2B, have two sets of contacts 44, 44a and 46, 46a, respectively, each pair controlling the trip coils of both breakers S1, S2, which will therefore be tripped upon operation of either relay. Where there is more than one intermediate section, the above double-tripping arrangement is applied to each such section, but the Specification refers also to a single circulating-current system and relay for each intermediate section. Relays R1 . . R9 are provided, but not shown, in Fig. 2. Their field coils may be energized by the transformer groups in the appropriate lines, and the operating windings of R1 .. R3 may be energized by opening of S1, those of R4 .. R6 by opening of S1, S2, and those of R7 .. R9 by opening of S2, these operating windings being energized by a transformer similar to 24 and controlled by contacts such as 18. Alternatively, these windings may be independent of the operation of S1, S2, and may be given a time-lag which ensures their operating, if at all, after the breakers S1, S2 have isolated the faulty section. The relays employed are preferably of the induction type and may be of the " translay " type having biassing windings on the field system providing a restraint that increases with the current flowing in the associated circuit, whereby relay-operation by creeping and through-faults is prevented. In a further arrangement, the associated with each line, instead of being energized in the way shown in Fig. 1, may be energized so as to be responsive to the residual current in a group of lines, for example, as shown in Fig. 2, for relay S1A, each relay still however controlling the circuit-breaker of the associated line as in Fig. 1. The Specification states also that the framework-leakage detective means may comprise relay means governing a trip-circuit for the breakers through the contacts of the relays in the lines alternatively, or in addition to, governing the circuit of windings of the relays in the lines.