GB489037A - Improvements in and relating to means for controlling and protecting electric circuits - Google Patents

Abstract

489,037. Protective cut-out systems. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 17, 1936, No. 34726. Convention date, Dec. 17, 1935. [Class 38 (v)] A protective arrangement for a feeder circuit connected to a network through a transformer comprises means connected to be energized in accordance with a predetermined function of the voltage to earth of one of the feeder circuit conductors and the voltage between two of the network circuit conductors and operative in response to an earth on the feeder circuit to effect the opening of a circuit interrupter to interrupt the supply to the network from the feeder. In Fig. 1, the bus-bars 19 are connected by a breaker 22, having an overload trip 21, to a feeder 10 which is inductively connected to a circuit 11 over a delta-star transformer 12 with earthed neutral. The feeder may be one of a number supplying a network 16, and has a switch 23 for earthing one or more of the conductors when breaker 22 has been opened, as described in Specification 396,853. A relay 24 is adapted to trip breaker 17 in response to an earth fault on a feeder conductor and is for this purpose made responsive to the difference between the voltage-to-earth of one of conductors 10 and the voltage between two conductors 11. The former voltage is derived from the secondary 27 of a transformer having its primary 26 connected across part of a high earthing-resistance 25, and is adjusted so that the difference is normally substantially zero. An earth fault trips breaker 22 and disconnects the feeder 10 from the bus-bar-neutral 20. If the fault is on the right-hand conductor 10, its voltage-to-earth after 22 opens becomes zero whereas the network voltage does not alter materially, so relay 24 is energized. The earth may be artificially applied by closing switch 23 for selectively disconnecting the feeder from the network. If either of the other conductors is earthed, the voltage-to-earth of the right-hand conductor changes from star to delta-voltage and relay 24 operates. In Fig. 2, the breaker 17 is adapted to be opened by an energydirectional relay 28 as disclosed in Specification 361,193, the restraining voltage windings 32 of which are energized by a potential transformer 33 under control of contacts 39 of a relay 38, which in turn is controlled by a directional relay 34. Operation of the latter opens contacts 39 to remove the restraint and permit the relay 28 to effect tripping provided breaker 22 has been opened. The relay 34 has a winding 36 energized by the voltage between two network conductors 11, and a winding 35 energized by the difference-voltage between that voltage and the voltage-to-earth of a conductor 10, operation being dependent on the product of, and phase-angle between, the two voltages. Relay 34 may, alternatively, be adapted to trip breaker 17 directly, as in Fig. 1, In Fig. 3, the difference between the voltage across two conductors 11, as derived from the transformer primary 26, and the voltage-to-earth of a conductor 10, as derived from earthing-resistance 25, is applied to a rectifier system 41, and as this difference is normally zero, there is no voltage between the grid 45 and anode 46 of a discharge valve 40. When an earth occurs discharge of the valve is effected and relav 24 energized.