Patent GB-284773-A (GB)
Title: Improvements in electrical protective arrangements for alternating current systems
IPC: H02H7/26
Filed: 1926-11-03
Abstract: 284,773. Brown, W. J., Field. A. B., and Metropalitan-Vickers Electrical Co., Ltd. Nov. 3. 1926. Cut-out arrangements.-Relates to means for selectively protecting line sections by the use of oscillatory circuits supplying high frequencies at each end of the section controlled by fault conditions in the line and interacting with one another, for example by means of the beat note, to operate protective relays as described in Specifications 252,936, 252,810, 254,770, 254,771. According to the invention, sources of oscillatory current supply carrier waves under normal conditions, continuously to the two ends of the section and, when a fault occurs, fault relays cause the oscillations at one or other or both ends to be broken up into carrier-wave groups having line frequency. These groups correspond to predetermined half-cycles of the line current and the carrier waves thus modulated co-operate with one another (or with the unbroken carrier wave at the other end) to control the operation of the protective relays of the section. The continuous oscillations are supplied by reacted thermionic valves 33 the anodes of which are energized, from an independent source 38. The fault responsive relays 31, 32 (two at each end are shown) close on the occurrence of a fault to cause the generator to supply oscillations intermittently instead of continuously. In one arrangement the relays change the anode excitations from the independent source to excitation from a source representative of the line current thereby producing the circuit arrangement described in Specification 252,036. Alternatively, and as shown in Fig. 7, the anode is supplied continuously from the independent source and the oscillations are (on the operation of relay 32 and through its contacts 39) periodically quenched by means of line currents or currents derived from them and supplied to the quenching device 40 as described in Specification 254.771. In the arrangement shown, the intermittent quenching is effected by connection to the leads 25, 26 which carry a current which is a measure of the negative phasesequence component of the line currents and is extracted from the section by the system of transformers 19, 20, 21 and the reactance network 30 is described in Specification 284,771. The two generators at the respective ends may supply carrier waves of different frequencies which when a fault occurs beat with one another intermittently instead of continuously, the normal beat note being modulated on fault at line frequency and serving to operate the protective relays. With such an arrangement, when a fault occurs outside the section both fault relays operate but in these conditions the intermittent carrier-waves at the two ends are supplied during different half-cycles of the controlling current by reversely connecting the quenching circuits at the two ends, so that, in the absence of a phasedifference of the currents at the two ends, the protective relay will not operate. With such a phase difference the protective relays are operated by the resulting line frequency fluctuations. In order to avoid incorrect operation due to differences in the times of closing of the two fault relays one at each end as usually employed, two relays 31, 32, at each end are used to give a " back-lash " effect. When, in the example shown, relay 31 is operated at a certain value of fault, the independent source of anode energy is withdrawn from the valve 33 and the tuning condenser 37 is short circuited. At a somewhat higher value of fault, relay 32 is operated to reconnect the independent source and at the same time to introduce the quenching device 40. The time thus. taken before, the actual comparison between ends is effected ensures that stable conditions are reached before comparison. In the example shown, the two carrier-waves are of the same frequency and the fault responsive relays serve to change the frequency and to modulate the changed frequency at the line frequency. The receiving apparatus 42 at each end comprises a carrier wave amplifier 43 supplying carrier waves, which under fault conditions differ by a predetermined amount, to a detector 45. The summation or difference beat so detected is amplified at 46 or 47 responding respectively to a difference beat due to a fault at the different ends and the active amplifier 46 or 47 supplies its rectifier 48 or 49 both connected to the protective relay 50.