GB252036A - Improvements in electrical protective arrangements for alternating-current systems - Google Patents

Abstract

252,036. Hinton, N. P., and Metropolitan-Vickers Electrical Co., Ltd. Dec. 9, 1924. Cut-out arrangements for alternating-current circuits in which pilot wires are dispensed with employ two sources of oscillating current of predetermined different frequency superimposed on the line and each controlled directly in accordance with the alternations of the line currents flowing in the ends of the section of line to be protected so as to set up beats for time intervals which depend on the phase relationship of the line currents at the ends. The beats are utilized for controlling the operation of relays tuned to the beat frequency. In the arrangement shown in Fig. 1, two oscillation generators 2, in the form of three-electrode valves with grid coil 4 coupled to a tuned anode coil 5 and with anodes supplied from the line section by the secondary of a current transformer, are arranged one at each end of the section. Slightly different high frequency oscillations are thus supplied through a capacity coupling 12 to the line, the frequency supplied at the right hand end being for example 14,000 cycles and that at the left hand end 15,000 cycles. The oscillations generated during the positive half-cycles of the line current beat with one another, the beat frequency, in the example given, being 1,000 cycles per second. At one or both ends of the section a detecting receiver 15 is arranged as shown, with a capacity coupling to the line. Each receiver is tuned to the frequency of the oscillator remote from it so that each receives substantially equal energy from both oscillators. A relay 17 is connected in the anode circuit of each receiver and is tuned to the bevel frequency. This relay maintains its inoperative position as long as the beat frequency persists or persists at a certain strength. When, however, the beat frequency ceases or changes the relay operates and controls the protective gear. The beat frequency ceases or changes when, on fault, the current in the respective ends differs in phase owing to the fact that while on fault one of the oscillation generators will continue to oscillate during the same positive half-cycles as before the other will oscillate during half-cycles which may now be nearly 180‹ out of phase therewith. In a modification, a pair of oscillators may be provided at each end of the line the anode supply current of which are 180‹ out of phase with one another. By these means both halfcycles of the line current are used and continuous oscillations are superimposed thereon. All four superimposed frequencies may be different, but the beat notes from the two pairs should be the same so that a single receiver at one or at each end may be employed. As applied to a threephase line, the anodes of the oscillation generators 3 are supplied with current obtained from series connected current transformers in the three phases, one of the secondaries being crossconnected as shown in Fig. 9.