GB349199A - Improvements in or relating to electric protective systems - Google Patents

Abstract

349,199. Protective cut-out arrangements. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.- (Assignees of Blake, D. K. ; 314, 5th Street, Scotia, New York, U.S.A.) March 19, 1930, No. 8853. Convention date, March 19, 1929. [Class 38 (v).] A circuit-breaker for disconnecting an A.C. supply from a load circuit is provided with a power directional relay energized from a transformer which has a tertiary winding energized by the voltage of the load circuit and exercising a restraint on the relay, which restraint diminishes with reduced load voltage so that a less amount of reverse power is required to operate the relay under these conditions. Each feeder 3 is connected to the load circuit 1 through a transformer 4 and circuit-breaker 5, the trip coil 12 of which is in series with contacts 22 of a power directional relay 15 having a coil 16 energized by the load circuit voltage and a current coil 17 energized by the secondary 18 of a transformer 19 in the feeder 3. A tertiary winding 23 on the transformer 19 is energized in accordance with the feeder voltage, and is arranged so that a restraint against reversepower operation is provided by it in co-operation with the coil 16. Such restraint diminishes with the drop in feeder voltage so that a smaller amount of reverse power is necessary to operate the relay in such conditions. Thus, when a fault develops in the feeder, the station breaker 6 is tripped by its overload relay, and the reverse current from the load circuit 1 causes the relay 15 to open contacts 21 and close the contacts 22, thereby tripping the breaker 5. The amount of reverse current required to trip depends on the feeder voltage. The de-energized relay 15 returns to its original position in which the closing coil 11 is connected in series with contacts 21, 26, so that when the feeder is restored to normal voltage, the coil 11 recloses the breaker 5. In a modification, Fig. 2 (not shown), a limiting resistance may be connected in series with the coil 17 across the breaker 5 so as to maintain the contacts 21 open until voltage conditions on both sides of the breaker 5 are suitable as regards both magnitude and phase. To prevent the winding 23 affecting the reclosing operation, it may be arranged to be connected in circuit only when the breaker 5 is closed. In an application to a threephase feeder, a polyphase relay energizes the closing coil 11 only when the voltage on all three phases of the feeder is high enough, or a balance relay responsive to voltage conditions on both sides of the breaker may be used for this purpose, operation of the relay in both cases being preferably after a predetermined time-delay.