Patent GB-542993-A (GB)
Title: Improvements in and relating to protective equipment for electrical power systems
IPC: H02H7/26
Filed: 1940-05-03
Abstract: 542,993. Protective cut-out systems. ROSS, T. W., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. May 3, 1940, No. 7988. [Class 38 (v)] In a protective system of the kind wherein tripping of circuit-breakers at each end of a section is allowed or prevented by directional relays at each end of the feeder in association with a pilot circuit or the like, for example as in Specification 523,360, tripping is prevented when, after clearing a through fault beyond the feeder, the direction of power-flow in the feeder reverses and the directional relay at the end thereof at which power is now out-flowing lags behind the other relay, by providing a timedelay means for maintaining the pilot circuit in the non-tripping condition to which it has been set by the lagging relay and for an interval sufficient to allow that relay to assume its new position. In an arrangement possessing some of the features of Specification 505,656, the pilot is first energized by a rapid-acting fault-detecting relay, whereupon the directional relay is made effective to allow or prevent tripping according to the fault-location, tripping being effected by subsequent de-energization of the pilot. The breaker trip-coil 46 is energized by a faultdetecting relay 4A, the coil 7 of which is normally short-circuited by contacts 39 of a relay coil 41, controlled by contacts 43 of relay 32 in the pilot 25, 26, and by contacts 44 of a hold-in relay 27 having locking-in contacts 33. On fault, winding 6 of fault-detecting relay 4 makes over its contacts 12 and closed contacts 34 of relay 2,7 the circuit of a main pilot relay winding 20, the contact 23 of which energizes relay 32, so making at 30 the circuit of winding 27 which opens contacts 34 and removes a shortcircuit from contacts 16 and auxiliary pilot relay winding 17, and as contacts 36 also open a shortcircuit of current coil 8 of directional relay 5, the latter is rendered effective. If the power is entering, contacts 16 of relay 5 open and, as contacts 34 have opened, no energization of relays 17 occurs and relay 20 is de-energized. With internal faults the far-end relays behave similarly, the pilot is de-energized at both ends and both relays 32 allow contacts, 43 to reclose and, as contacts 44 are closed, to remove the short-circuit from each coil 7, which trips. If the fault is external, and assuming power is outflowing at the end shown, relay contacts 16 which are biased closed, remain closed and energize windings 17, 20 in series to maintain the pilot energized and prevent tripping of either end, notwithstanding the relay 5 at the far end conditions the pilot for tripping. If on clearance of the external fault the power reverses, the relay at the feeder-end shown may open its contacts 16 and produce tripping before the far relay has operated its contacts to maintain energization of the pilot and prevent tripping. The relay 17 is, however, caused to operate in both directions with a time-lag, and remains closed for a time sufficient to enable the far relay to reverse. Since this relay is only energized when power flows initially out of the section at the associated end, it is not affected by internal faults and no delay in tripping is introduced by it.