Patent GB-599187-A (GB)
Title: Improvements in and relating to protective systems for electric circuits
IPC: H02H7/26
Filed: 1943-08-12
Abstract: 599,187. Protective cut out systems. BRITISH THOMSON-HOUSTON CO., Ltd Aug. 12, 1943, No. 13054. Convention date, Aug. 19, 1942. [Class 38(v)] Relates to a protective system of the kind set out in Specification 274,070, [Class 38(v)], wherein comparison of fault-current directions is effected at each section-end by superimposed H.F. currents sent out by transmitters operating alternately for alternate half-cycles from opposite ends and adapted to be received by receivers, in dependence on the polarity of the powerfrequency, whereby, on internal fault, the reversal of current at one end causes both transmitters to operate together in the halfcycles when the receivers are simultaneously rendered inoperative. In the present invention, earth faults are given precedence of phase faults by causing both transmitters to transmit continuously and prevent isolation on external faults. The apparatus is the same at each end of section 10 and comprises a transmitter T and receiver R coupled to the line by transformer 17 and coupling 21, 22, a directional earth relay 67, and a tripping valve 58. The transmitter T comprises a Colpitts oscillator the valve anode of which is connected through winding 20 of transformer 17 to + terminal 35 of a three-wire D.C. supply. The receiver valve 40 has its anode 41 connected to the middle wire 36 of the supply through a load resistance 44, and its cathode connected to a point on a resistance 45 connected across the middle and - wires 36, 37 of the supply. The grid 43 is connected to wire 37 through a tuned circuit comprising winding 19 of transformer 17 and winding 47 of a transformer 48 which is shunted by a rectifier 57. In order to render R, T alternately operative for a half-cycle, winding 47 is in opposition to a winding 54 connected between cathode and control grid 29 of the transmitter valve. The transformer 48 has primary windings connected to current transformers in the line so that a single-phase voltage is produced for all kinds of phase faults. A further winding 55 of the same polarity as 47, supplies through a peaking transformer 64 the cathode-grid 62 circuit of the trip valve 58, the anode circuit of which includes the breaker trip coil. A screen grid 61 is connected to one end of load resistance 44 so as to prevent the tripping valve 58 being rendered operative provided the receiver R is operative to pass a current through 44. The latter is shunted by a condenser 63 for reversing the flow of blocking current. When R ceases to operate, the controlling effect of 61 disappears and tripping occurs, that is, with an internal fault, when the current at one end reverses with respect to that at the other and renders both receivers operative together when neither transmitter is functioning. The earth relay has a residual current coil 68 and polarising coil 69 and operates contact 76 to engage contact 77 or 79 according to the direction. With outgoing current, contact 77 applies a D.C. bias to grid 29 to cause T to operate continuously and exert a blocking action. Contact 79 is closed with incoming fault current and imposes a continuous D.C. bias on tripping grid 62 of valve 58 so that tripping will occur unless receiver R, due to outgoing fault current at the other end, is continuously energised by the continuous carrier transmitted from the other end. R continuously receives only if the output of the transformer 48 falls so low that the voltage across winding 47 is insufficient to cause intermittent blocking of valve 40.