GB254770A - Improvements in electrical protective arrangements for alternating current systems - Google Patents

Abstract

254,770. King, A. J., Brown, W. J., and Metropolitan-Vickers Electrical Co., Ltd. March 10, 1925. Cut-out arrangements for three-phase alternating current circuits in which pilot wires are dispensed with employ at each end of the section of line to be protected, two oscillators which each supply superimposed currents to the line at periods of the main alternating current cycle which are phase displaced from one another. At each end of the section a protective relay or relays is located tuned to the one or more beat frequencies which are set up between the superimposed currents upon the occurrence of a fault on or between any or all of the phases of the line. The arrangement at each end of the section is the same and, as shown in Fig. 1, consists of two oscillation, generators 3, 4 and a receiver 5. The generators are three-electrode valves connected to produce continuous oscillations, on positive and negative half cycles respectively of the line current which co-operate with the oscillations produced at the other end by the negative and positive half cycles. Normally no beat frequency results, but when, on fault, a phase displacement occurs between the currents at the two ends a beat frequency is produced which operates the tuned receivers. The anodes of the valves are supplied through transformers 12, 13, 14 in the phases, Z-connected, the reversed transformer winding being in the two cases in different phases. In order that one receiver only may be used at each end the four oscillators are chosen so that the beats formed between pairs will be the same. If. for example, the summation beat note is employed, the sum of the frequencies of the first (counting from the left) and third is made equal to the sum of the second and fourth. The difference beat note may, however, be used. A single set of current transformers may be used for both generators at each end. The anodes may be energized independently of the line current and to generate continuous oscillations damped out in accordance with the alternations of the line current as described in Specification 254,771 and the beat frequency may be supersonic as described in Specification 252,810. Specification 252,036 also is referred to.