Patent GB-353104-A (GB)
IPC: H02H1/00
Filed: 1930-04-30
Abstract: 353,104. Protective cut-out arrangements. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London. -(Assignees of Fitzgerald, A. S. ; 1212, Union Street, Schenectady, New York, U.S.A.) April 30, 1930, No. 13347. Convention date, April 30, 1929. [Class 38 (v).] Cut-out systems in which a source of high-frequency current is superimposed on the line, have two circuits supplied from this source, one of the circuits including a portion of the line and the other circuit being a standard compression circuit having electrical characteristics similar to those of a position of the protected circuit under normal conditions, out-of-balance high-frequency current during fault conditions causing the tripping of the associated circuit-breaker. As shown in the Figure in which a line 10 to be protected is provided with a circuit-breaker 11 with closing coil 13 and tripping coil 12, the high-frequency source 9 is obtained from an oscillator valve 18 and amplifying valve 19. Current from this source is fed through a transformer 20 to two circuits one of which includes a resistance 21 from which it passes through a wave trap 24, 26 and coupling condenser 28 to the protected line and the other includes a resistance 22 and variable inductance capacity and resistance units 28, 29, 30. These units are so adjusted that with normal conditions in the line 10 no current flows in the winding of a transformer 8 which is connected across the ends of the resistances 21, 22 and consequently no appreciable plate current flows in the valve 31. If however an earth fault occurs in the line 10 the balance between the high-frequency currents in the two circuits is destroyed and the outof-balance current in the transformer 8 overcomes the blocking effect of the battery 32 on the grid of valve 31 so causing the energization of relay 33. This relay completes the tripping circuit of the circuit breaker 11 and also operates the circuit of an alarm bell 39 through a relay 36. High-frequency current may be fed to the protected circuit continuously and as shown may be switched on only during overload conditions by an overload relay 48 energizing a relay 49 having contacts in the anode supply to the various valves. To reclose the circuit-breaker a manually-operated switch 50 is provided in the energizing circuit of relay 49. The reclosing is however controlled by a valve 40 to the grid circuit of which if fed from the highfrequency source through a transformer 41 and which contains a rectifier 42 and is also provided with a condenser 44 in parallel with a resistance 45 across the grid circuit so that a time delay occurs before the bias of the battery 42 in the grid circuit can be overcome and the relay 46 in the circuitbreaker reclosing circuit operated. If however the fault in the line 10 is still existent relays 33, 36 will have been operated prior to the operation of relay 46 so that no reclosure will occur as the reclosing circuit will be still open at contacts 37 of relay 36 while the alarm circuit will be closed at contacts 38. To prevent the trespass of the H.F. current beyond the protected portion suitable traps 27 may be arranged at the limits of such portions. A modified arrangement, Fig. 3 (not shown), is described in connection with a polyphase circuit in which suitable sources of polyphase H.F. current, which may be of the type described in Specification 278,466, [Class 38 (ii), Electric currents, Converting &c.], and which may differ in frequency are provided at the ends of a potential polyphase reaction. In such an arrangement protection against interphase faults, in addition to earth-fault protection, is provided for. A modified polyphase earth fault protection arrangement is also described, Fig. 2 (not shown), in which the H.F. current is superimposed on the neutral lead of a three-phase winding such as a transformer winding, the H.F. current being confined by suitable traps between the neutral and the outer ends of the star-connected windings. The system in this form is brought into operation by a sensitive relay situated in the earthed neutral.