GB423955A - Improvements in or relating to protective systems for sectionalised electric supply networks - Google Patents

Abstract

423,955. Protective cut-out systems. ROSS, T. W., 10, Broadoaks Road, Sale, Cheshire, and ASSOCIATED ELECTRICAL INDUSTRIES, Ltd., Crown House, Aldwych, London. Sept. 5, 1933, No. 24622. [Class 38 (v)] In order to render a sectionalized network immune from undesired cutting out due to accidental excess voltage in, or interruption of, a pilot circuit (particularly where that circuit includes a telephone or other line which is normally not under the jurisdiction of the supply authority), a normally-closed pilot circuit is controlled by protective relay means at each end of the section so as to be interrupted at each section end through which fault current flows, except when power flows into the section at that end, and relay means for the respective ends of the section are included in the pilot circuit so as to be operated by current flowing therein, thereby to prepare the trip circuits of the breakers at the ends of the section to which they appertain, which circuits however, are only completed if and when the fault occurs on the section. Each pilot circuit P has at each end normallyclosed contacts C1 of an overload or earth leakage relay, contacts D1 of a directional relay adapted to close with incoming fault current, and the coil of a relay OT which is substantially instantaneous in operation, is normally, used for instance, for telephone arrangements, and controls contacts T1 in a local circuit containing both a trip coil or relay O and normally-closed contacts O1 which open when the breaker trips. If fault current enters the section only at, say, the left end, contacts D1 at that end close and after a time-delay, contacts C1 open and contacts C2 close to complete the trip circuit prepared by T1 remaining closed. The pilot circuit is not interrupted at contacts C1 of the right-hand end, neither is the trip circuit at that end completed by its contacts C2. If, however, both ends feed an internal fault, both contacts D1 close to maintain the continuity of the pilot circuit, so that contacts T1 at both ends prepare the trip circuits which are subsequently completed by contacts C2. With a through fault, the pilot circuit is broken at the outgoing end, so that both relays OT are de-energized, and open contacts T1 to break both trip circuits. Interruption of the pilot circuit from any other cause produces de-energization of OT and opening of contacts T1. In these circumstances the relays OT may be arranged to give a warning signal. If a fault occurs in the section when the pilot circuit is broken by other-than-protective causes, back-up protection operates to cut out with a time-delay. Moreover, excess voltage in the pilot circuit has no effect on the trip circuits. In a more detailed embodiment, Fig. 2 (not shown), an intermediate relay of the induction type is interposed between the contacts C2 and the trip coil O, the relay having an operating winding in series with those of contacts C1, D1 and controlling windings in series both with contacts C2 and with contacts which remain closed so long as relays OT remain energized.