Patent GB-440902-A (GB)
Title: Improvements in protective arrangements for alternating current electric circuits
IPC: H02H3/30
Filed: 1934-08-17
Abstract: 440,902. Protective cut-out systems. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, WILSON, W., and CLIFF, J. S., c/o General Electric Co., Ltd., Witton, Birmingham. Aug. 17, 1934, No. 23752. [Class 38 (v)] A protective arrangement for A.C. feeders in series, with a branch connected to their junction (or for a single feeder with an intermediately-tapped branch) comprises a circulating current system with current transformers operatively associated with the feeders at their remote ends and interconnected by a pilot circuit, local circuits connected in parallel with the pilot circuit at the ends thereof, and biassed relay means with an operating coil included with a resistor in the local circuit and a restraining coil included in the pilot circuit, adjacent each remote feeder end adapted to compare the currents flowing in the pilot and local circuit at that end and thereby to effect interruption of the current flowing into the adjacent feeder end through divergence from the normal value of the ratio of said currents. An auxiliary current transformer circuit adapted to carry current proportional to that in the branch is connected to an intermediate point in the pilot circuit, the two circuits in parallel across which the auxiliary current transformer circuit is connected being of substantially equal resistance. As shown, the branch consuming or generating circuit 18 is connected to the junction 3 of feeders 1, 2 provided at their outer ends with current-transformer groups 8, 9, connected, as shown, so as normally to provide a residual circulating current for the pilot circuit 10. Two of the transformers in each group may be of unequal ratio to protect against faults between the corresponding phases. A local shunt circuit 11, 12 at each end of the pilot circuit includes a resistance 23, equal in value to half that of the pilot circuit, and the operating coil 21 of a relay 13 or 14, the restraining coil 19 of which is in the pilot circuit, whereby each relay operates on divergence from the normal ratio, conveniently unity, between the currents in the pilot and local circuit. The relays may be of the biassedbeam type, with displaced fulcrum. The auxiliary circuit 15 is connected to electrical mid-points of the pilot and local circuits and contains an impedance 31 equal to that of each relay coil. Its local circuit contains an impedance 32 and resistance 33, the impedances 31, 32 being formed by the coils and armature of a relay similar to those 13, 14, whereas the resistance of 33 is one quarter that of the pilot circuit. Thus the output of the transformers 16, 17, which are connected in the feeders I, 2, respectively, normally divides equally between the local circuit 32, 33 and the group 8 or 9, depending on which end the power is entering. When the branch 18 is without load, the groups 16, 17 only circulate current amongst themselves, but when it is on load, they inject in the pilot circuit a current which compensates for the difference between the outputs of 8, 9, so preventing operation of the relays 13, 14. On fault in either feeder, the operating coil of the relay at the entering end overcomes the restraining coil to trip the breaker at that end. The groups 16, 17 may be replaced by a single group in the branch 18 itself, but with either arrangement the impedances 31 .. 33 may be dispensed with and the transformers associated with the branch may be directly connected to the pilot, their turn ratios being then made half that of the transformers of groups 8, 9. An independent circulating-current system, each with its two relays, may be used on each phase of the feeders. The coils 31, 32 may serve as overload relays to trip local switches at the junction.