Patent GB-418666-A (GB)
Title: Improvements in or relating to protective arrangements for electrical power transmission systems
IPC: H02H7/26
Filed: 1933-02-23
Abstract: 418,666. Protective cut-out systems. KAPP, R. 0., and CARROTHERS, C. G., 25, Randolph Crescent, Maida Hill, and GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, both in London. Feb. 23, 1933, No. 5600. [Class 38 (v).] A discriminative protective system of the kind in which relay means adapted to be influenced by circuit conditions of a section of a transmission system are arranged to exert control over a channel of communication between the ends of the section to which protection is to be applied and in which the channel comprises a pilot wire or wires, have the pilot wire circuit normally shortcircuited so that extraneous voltages cannot affect the protective relays associated therewith. The equipments at the ends of the section are identical with one another and the parts are similarly referenced, the suffix L or R being added where necessary to distinguish between the parts at the left-hand and right-hand ends. The trip coil 1 of the breaker at each end is controlled by overload relays 6, 7 in two phases of a polyphase system, and by current relays 9, 10 in two phases operating at a lower value of current than 6, 7, also by a pair of directional elements 11, 12 in the same phases as 9, 10. Contacts of 9, 11 and of 10, 12 are in parallel with one another and in series with those of an earth leakage relay 13, which is arranged to open its contacts at a lower current than that operating current relays 9, 10 and in doing so renders 9 .. 12 ineffective. Further, 13 and a directional earth-leakage relay 14 operate at a lower current than earth leakage relay 8, and when relay 14 operates it shorts the contacts of relays 9 .. 13. Relays 11, 12, 14 close their contacts when power is flowing out of the section at the adjacent end, and relays 6, 7, 8 may have additionally, or alternatively, a time-delay to give the necessary priority of operation. At each end, a circuit may be traced from source 29 over lead 30 and either over contacts of 9 .. 13 or over contacts of 14, and coil 39 of a relay 5, and a further circuit from source 29, over lead 30 and coil 34 of relay 4, contacts of 6, 7, 8, and contacts 27 of relay 5. A circuit may also be traced from source 29 over contacts of a trip relay 3 and either over back contacts 37 of the breaker (which contacts open when the breaker opens), trip coil 1 and contacts 27 or over contacts 26 and coil 39 of relay 5. The pilot circuit 40, 41 is connected in series with a balancing resistance 43 at each end and with the winding 42 of a duplex relay 2, and is normally shorted by contacts 28 of relays 5. With a through fault, say from left to right, relay 4L is operated by 34L and makes a circuit for battery 44L over contacts 23L, half of coil 42L, and resistance 43L. An equal and opposite current flows in the other half of 42L (which therefore does not close the circuit of trip relay 3) over pilot 40, half of 42R, contacts 24R, and pilot 41. Hence 2R, 3R operate, but the circuit of trip coil 1R is broken by operation of 5R so the section is not cut out. If power flows to a fault in the section at only, say, the left-hand end, relays 4L, 2L, 3L operate and 1L is energized to open the breaker. In this case also, opposing currents flow in the two halves of 42L, but they are unequal so 2L operates. If a fault in the section is fed from both ends, relays 4, 2, 3 at each end operate to energize both trip coils 1. In this case, the voltages of batteries 44 oppose one another over the pilot circuit and upper halves of coils 42, which are therefore not energized, so that the circuit made over the lower halves is effective to close the circuits of the trip relays. Where a section is fed in only one direction, the feed end need not have a relay 5 and set of relays controlling it.