Patent GB-499748-A (GB)
Title: Improvements in electric protective arrangements
IPC: H02H3/30
Filed: 1937-06-25
Abstract: 499,748. Protective cut-out systems. BRITISH THOMSON-HOUSTON CO., Ltd., KINMAN, T. H., and HIBBERD, R. G. June 25, 1937, Nos. 17780, 20915, and 32928. [Class 38 (v)] Relates to an arrangement for protecting a circuit-section by impressing carrier oscillations of different frequency on both ends, there being provided at each end a source of oscillations, a demodulating receiver for oscillations, and a means of comparison for two currents corresponding to the line current at each end, oscillations from the source being modulated by a voltage derived from the line-current at the near end and superimposed on the line, while the corresponding oscillations received from the far end are demodulated and applied together with a current corresponding to the near-end line current to the comparison-means. Consists in that the demodulated oscillations received from the near and far ends are applied to the individual primary windings of a transformer which forms the comparison means, any out-of-balance current in phase and/or amplitude flowing in the secondary and being used directly or indirectly to trip a breaker and/or give an indication. In the arrangement shown in Fig. 1, for one of two similar ends of a line or section 2, a voltage proportional to the line current and derived from a current-transformer group 3, is applied through a limiting circuit B to a choke 4 in the anode circuit of oscillator 5, which is preferably crystal-controlled and has its anode circuit tuned to a frequency somewhat above the operating frequency. The oscillations modulated by the line current are impressed on the line through a winding 7 inductively coupled to circuit 6, and coupling unit D. The modulated oscillations are fed also to a tuned circuit 8 coupled to circuit 6, and after being demodulated by a detector 9, arrive at the comparison transformer G as a current proportional to the line current. This current is impressed on one, 10, of two opposing primaries, to the other 12 of which, modulated oscillations received from the far end are fed after amplification at F and demodulation by detector 13. Alternatively, transformer G may have a single primary to which the two sets of oscillations are fed, in opposition, through coupling transformers, Fig. 2 (not shown). Since a percentage modulation of between 20 and 30 per cent is necessary to provide sufficient voltage for comparison at normal full load, and over-modulation would result from through faults of about five times normal full load, it is necessary to reduce the rate of increase of percentage modulation as the line current increases, for example to 85 per cent at 20 times normal full load, by means of the limiting circuit B. Fig. 3 shows such a circuit comprising a limiting resistance 14, and power-frequency responsive acceptor and rejector circuits 15, 16 and 17, 18, respectively. Diode rectifiers D<1>, D<2> reversely connected in parallel, are biassed to break down at 2 to 3 tunes normal full load, so producing a relatively large volt-drop across 14 and limiting the voltage across circuit 17, 18, and therefore across the choke 4. In Fig. 5, the diodes are replaced by a material having a non-linear resistance-voltage characteristic such as that described in Specification 292,110, [Class 38 (ii)]. When the current rises above the limiting value determined by this resistance 19, the percentage modulation may be kept at substantially constant value by means of transformer 20 injecting a compensating voltage c in opposition to voltage E. Alternatively the resistance 19 in Fig. 6, is placed across the primary of a transformer 21, and the compensating voltage is obtained from part of resistance 14. The modulated oscillations may be applied to two phases of the line by taking the lower end of the resistance R of Fig. 1, through a coupler D to another phase. Specification 304,359, [Class 38 (v)], also is referred to. The first Provisional Specification describes arrangements for comparing the demodulated oscillations in gas-discharge valves.