Patent GB-1114995-A (GB)
Title: A device for measuring electrical current
IPC: G01R15/24
Filed: 1966-04-12
Abstract: 1,114,995. Protective arrangements. ETABLISSEMENTS MERLIN & GERIN S.A. 12 April, 1966 [10 April, 1965; 3 March, 1966], No. 15812/66. Heading H2K. [Also in Division G1] The current in a high voltage line is measured by three photo-electric systems symmetrically arranged in a housing each system comprising a respective source 10 at earth potential, directing light to an optical member 18 common to the three systems, which mixes the separate beams for passage through a common guide tube 19 to a further common optical member 20 located near the high voltage line where the beam is again split into a plurality of separate light beams, each of which is passed through a respective polarizing device 26, a respective Faraday effect device 27, whose coil 28 is excited by the high-voltage line current so as to rotate the plane of polarization in proportion to this current, and guided to a respective photo-electric detection device 29-33 at earth potential where the degree of rotation of the polarization plane is measured. In the embodiment described, three independent light sources (10, 11, 12, Fig. 2, not shown) which may emit visible, infra-red or ultra-violet radiation, are equally spaced from one another around the inside of the cylindrical insulator housing at earth potential. Light from each source is directed towards a triangular reflecting prism 18, through respective lenses 13. The prism (or, alternatively, a mirror arrangement) mixes the beams together and directs them through a tubular light guide 19 to a similar prism 20 located in a housing 14 associated with the high voltage line. The beam is split up again into three beams at 120 degrees to one another and each separate beam is reflected by a respective prism 21, polarized by a respective polarizer 26 and passed through a respective transparent body 27 of high Verdet constant. The coil 28 surrounding the body is connected in series with the high voltage conductor and the light beam passing through undergoes a rotation of its plane of polarization corresponding to that current. The light beam passes down a light guide 22 to a similar Faraday effect device 29, 30 which rotates the polarization plane in the opposite sense to that of device 27, 28. The compensating rotation is arranged to align the plane of polarization of the beam emerging from device 29 with that entering device 27 when the current in the compensating coil 30 is proportional to the high voltage current. This is done by directing the light beam from device 29 via optical system 31 and beam splitter 32 arranged at Brewster's angle to the beam, to detectors 33, 34, the difference in the outputs of which is used to control an amplifier 36, thereby avoiding effects due to intensity fluctuations in the light beam. The amplifier output feeds the coil 30 of device 29 and also a load 37. Since three separate measuring systems are available, three different types of load 37 can be introduced into the coil circuit, e.g. a current measuring device, an overload protection device, or a short-circuit protection device, each of which demand a different order of amplification and can thus be provided with a suitably matched amplifier.