Patent GB-190811634-A (GB)
Title: Improvements in and relating to Electric Distributing Systems.
Filed: 1908-05-28
Granted: 1909-05-06
Abstract: 11,634. British Thomson-Houston Co., and Wedmore, E. B. May 28. Distributing.-In an alternating-current distribution system, the contacts in the tripping-circuits controlled by the protective relays are so connected that, when one relay begins to operate, the others cannot close the tripping - circuits for the other feeders. The relays and tripping-coils are arranged so that the switches are only tripped when there is a substantial lack of balance of the currents in the several circuits. In the arrangement shown in Fig. 1, the distributing-mains 3, 3<1>, 4, 4<1>, 5, 5<1> are connected to the sub-station bus-bars 1, 2 through oil-switches 6, 7, 8 provided with tripping-coils 6<1>, 7<1>, 8<1>. Reverse-current relays 14, 15, 16 are controlled by series transformers 11, 12, 13 and potential coils 17, 18, 19, and are provided with two sets of contacts. The lower sets 20', 21<1>, 22<1> are normally closed, and are connected in series with the upper sets 21, 22, 20 respectively of the relays next in order. When a momentary reversal of energy occurs, the relays close all the upper contacts so that no tripping-circuit is complete. If a fault occurs on one feeder, for instance the feeder 41, the relay 15 on that feeder closes the circuit of the tripping-coil 7' through contacts 21, 20'. The relays may be constructed with definite positions for different current values, and with a considerable time or space interval between their extreme positions. With the modified arrangement shown in Fig. 2, each relay has only one pair of contacts. The bridging -contacts 23, 24 of the relays normally short-circuit the tripping-coils 61, 7'. If a surge causes both relays 14, 15 to operate, there will be no current in the tripping-circuit, or, if there is a current, it will be too small to trip the switches. If, however, a fault occurs on one of the feeders 3, 4, the corresponding relay will operate to remove the short circuit on the tripping-coil, which is then energized by the out-of-balance current in the secondary circuit of transformers 11, 12. The discriminating action against surges in parallel may be assisted by helping the transformers to saturate on heavy loads by employing a small core &c. The relays 14, 15, Fig. 2, may be provided with upper and lower sets of contacts, so that the operation of one relay closes the circuit of its feeder switch tripping-coil through the lower contacts of the. other relay as in the arrangement shown in Fig. 1. Pivoted contact-arms may be used instead of the bridging-contacts 23, 24, and may be arranged to move over the fixed contacts in proper sequence to prevent both tripping-coils from being energized together. The fixed contacts may be mercury contacts as shown in Fig. 5, and the switches are so arranged that one pair of contacts 20 or 21<1> make contact before the other pair 20<1> or 21 break contact. An auxiliary source 31, Fig. 6, may be employed to supply the tripping-circuit, and the resultant current in the balanced transformer circuit may excite a solenoid switch 32 or other type of instantaneous overload relay. This switch operates with either of the relays 14, 15, and so completes the circuit of the tripping-coil corresponding to the relay that has operated. The relays 14, 15, Fig. 6, may each be provided with two sets of contacts as shown in Fig. 1. Overload relays 33, 34, 35, 36, Fig. 8, are provided at the outgoing ends of the feeders 3, 4, 3<11>, 4" connected to the central-station bus-bars CS' and to the busbarsSS of a sub-station. These relays are electrically interlocked in the same way as the reverse current relays at the other ends of the feeders. This discriminating system of relays may be applied to a bank of transformers or to machines. Each oil switch may be provided with an auxiliary switch operated from the tripping-device before the oil switch opens. This auxiliary switch cuts out the automatic device on the healthy feeder or introduces a time limit on its action. The relays are interlocked in pairs if there is an even number, but, if there is an odd number, three relays are connected so that the initial movement of one disconnects the tripping-circuits of the other two.