Patent GB-676100-A (GB)
Title: Improvements in or relating to electrical signalling systems
IPC: G08C19/12
Filed: 1948-12-07
Abstract: 676,100. Protective cut-out arrangements. GENERAL ELECTRIC CO., Ltd., and HUMPHREYS, C. Dec. 7, 1949. [Dec. 7, 1948] No. 31671/48. Class 38 (v). [Also in Group XXXVIII] In an electric signalling system, a transmitter producing two signals of the same frequency includes control means whereby the relative phases of the signals are variable, and a receiver responsive to the relative phases of the two signals includes a phase discriminator to which oscillations derived from the meaning signals are applied to produce an output operating indicating and/or control means when the oscillations applied to the discriminator input are in phase quadrature. In a system (Figs. 1, 2) for inter-tripping power circuit breakers, carrier frequencies f1 f2 from audio oscillators 1, 2 are modulated with a low frequency (e.g. 50 c.p.s.) from the power source 5, filtered, and connected to pilot cable. A phase shifter 6 in the modulation circuit of f1 is controlled by two local circuit breakers through relays 7, 8 to respectively advance or retard the phase of the modulating signal when each breaker. opens. At a remote point the pilot cable is connected to parallel channels each comprising a filter 15 accepting the appropriate frequency, a demodulator 16, an amplifier 18, and a limiter 19. The f1 channel demodulated signal energizes a transformer 21 and the f2 channel signal feeds a transformer 22 through a 90 degrees phase shifter 20. The secondary of the transformer 21 is connected through dry rectifiers 26, 27 to the grids of the relay operating valves 28, 29, normally biased to cut off, while the secondary of the transformer 22 is connected between the centre tap and the junction of the equal resistances 30, 31 and condensers 32, 33 connected to the valve grids. The circuit operates as a phase discriminator and since the demodulated signals are normally in quadrature the relays 42, 43 are inoperative, while on operation of either breaker at the transmitter the signals are either in phase or in opposition so that one or other valve is biased to operate its relay and open the corresponding breaker at the receiver. Operation of both breakers at the transmitter modulates f1 with two L.F. signals which are respectively advanced and retarded 90 degrees in phase, so that both breakers at the receiver are thereby operated. Supersonic carriers may be used, and a common carrier may be modulated alternatively by the two L.F. signals. The carriers may be modulated on radio waves. Signals are transmissible over the power lines. In a modification, the carriers are normally balance modulated by 40 c.p.s. in phase, to give a suppressed carrier signal which is balance demodulated at the receiver to supply in phase 80 c.p.s. signals to the relay operating phase discriminator, so that opening either breaker at the transmitter modulates the appropriate channel with a 45 degrees phase shifted signal, to operate the appropriate breaker at the receiver. A balanced phase discriminator of the kind described in Specification 676,112, [Group XXXVI], is preferred.