Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Patent
1996-04-01
1998-12-01
Nguyen, Vinh P.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
324117R, 3242441, G01R 3100
Patent
active
058444092
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and arrangement for measuring an electric current in a current conductor, in particular with the aid of a Faraday element assigned to the current conductor.
BACKGROUND INFORMATION
Optical measuring arrangements are known for measuring an electric current in a current conductor, using the Faraday effect, which are also designated magneto-optic current transformers. By the Faraday effect is understood the rotation of the plane of polarization of linearly polarized light as a function of a magnetic field. The angle of rotation is proportional to the path integral over the magnetic field along the path traced by the light with the so-called Verdet constant as the constant of proportionality. The Verdet constant is generally dependent on material, temperature and wavelength. To measure the current, a Faraday element made of an optically transparent material such as, for example, glass is arranged in the vicinity of the current conductor. The magnetic field generated by the current effects a rotation of the plane of polarization of linearly polarized light transmitted through the Faraday element by an angle of rotation which can be evaluated as a measured signal. Generally, the Faraday element surrounds the current conductor, so that the measuring light circulates around the current conductor in a closed path. The amount of the angle of rotation is in this case to a good approximation directly proportional to the amplitude of the current to be measured.
The Faraday element can be designed as a solid glass ring around the current conductor as described in European Patent Application No. 0 088 419, or can also surround the current conductor in the form of a measuring winding made of a light-conducting monomode fiber (fiber coil).
Advantages of the magneto-optic current transformer with respect to conventional inductive current transformers are its galvanic isolation and insensitivity with respect to electromagnetic disturbances. However, in the use of magneto-optic current transformers, problems are presented by their temperature and vibration sensitivity.
The Internation Patent Application No. 92/13280 discloses one embodiment of a magneto-optic current transformer having an optical fiber designed as a Faraday measuring winding around the current conductor. In this known embodiment, by the provision of an optical fiber with a high intrinsic circular birefringence .rho.0, compared with the Faraday rotation, in conjunction with suitably selected signal processing, vibration influences are largely compensated. The measuring principle in this known embodiment consists in transmitting into the fiber coil two linearly polarized light signals S and T propagating in opposite directions. Both light signals S and T, after the passage through the Faraday element, are split into two partial light signals S1 and S2 or T1 and T2 with planes of polarization orthogonal to each other, and these partial light signals S1 and S2 or T1 and T2 are converted into corresponding electric intensity signals IS1 and IS2 or IT1 and IT2 with the aid of photodetectors. These four intensity signals IS1 and IS2 and IT1 and IT2 are divided by squares of the amplitudes, which correspond to the total intensity IS1+IS2 or IT1+IT2, and are thus intensity-normalized. A measured signal M is derived from the four intensity-normalized signals is1 and is2, or it1 and it2, which is equal to M=((is1-is2)-(it1-it2))/((is1-is2)+(it1-it2)). Now, the Faraday effect is a non-reciprocal effect, so that the two light signals S and T are rotated in the opposite sense of rotation, through the same Faraday angle .rho.. The linear birefringence in the material of the fiber is, on the other hand, a reciprocal effect and, therefore, effects the same modulation of the two light signals S and T. The measured signal M formed with this known method is, to a good approximation, equal to .rho./.rho.O and thus specifically still contains the information about the Faraday rotation .rho. of the two light signal
REFERENCES:
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patent: 4973899 (1990-11-01), Jones et al.
patent: 5063290 (1991-11-01), Kersey
J.P. Dupraz, "Fiber-Optic Interferometers for Current Measurement: Principles and Technology", Alsthom Review, N. 9, Dec. 1987, Paris, pp. 29-44.
Bosselmann Thomas
Menke Peter
Nguyen Vinh P.
Siemens Aktiengesellschaft
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