GB347795A - Improvements in and relating to protective devices for electric circuits - Google Patents

Abstract

347,795. Protective cutout arrangements. BRITISH THOMSON - HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Fitzgerald, A. S. ; 1212, Union Street, Schenectady, New York, U.S.A.) Feb. 18, 1930, No. 5452. Convention date, Feb. 19, 1929. [Class 38 (v).] In a protective system in which high-frequency control current is superimposed on the line and has at each protected point a directional relay the two windings of which are energized respectively by the current corresponding to a characteristic of the protected circuit at that part and by a current in accordance with a demodulated carrier wave transmitted from another point, e.g. the remote end of the protected section ; the directional relay contacts are normally biased to a closed position, and an additional relay is arranged in series in the directional relay control circuit, which additional relay is normally open but closes with an abnormal value of the characteristic of the main circuit in the adjacent portion of the section. Different frequencies may be transmitted from the opposite ends of the potential section. As shown in the Figure each end of the section 1 is provided with a direction relay 11, 110 having coils 25 energized directly from local current transformers and coils 26 energized from demodulated carrier waves which are transmitted from opposite ends of the section. The carrier waves are produced by oscillation generators 14 operating in conjunction with modulating valves 16. the grids of which are controlled according to voltage variations at the ends of resistances 24 included in the circuits of the adjacent current transformers 9, 90. Each end of the section is also provided with a receiving valve 21 the plate current of which feeds the coil 26 of the local directional relay through a transformer 32, the receiving circuit at one end being tuned in accordance with the carrier wave transmitted from the other end of the section. Thus a comparison of the direction of flow of power is made in the directional relays which are arranged so that in the case of a through fault the directional relays prevent the completion of the tripping circuit by opening their contacts 33, but permit tripping by remaining in their normally-closed position in the case of an internal fault. The tripping circuits in each case, in accordance with the invention, are finally completed at contacts 37 of time-lag overload relays 36, may also operate to produce tripping independently of the directional relays, as, for example, in the case of a break in the line 1. The transmitting end receiving valves at each end are cut in by overload relays 27 at each end so that if power is fed from one end only the directional relay at that end cannot be opened.