Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Patent
1999-01-08
2000-09-05
Strecker, Gerard
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
3242441, G01R 33032, G01R 1524, G01R 1902
Patent
active
061148466
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and an arrangement for measuring an alternating magnetic field. An alternating magnetic field is understood to be a magnetic field which has in its frequency spectrum only frequency components differing from zero.
BACKGROUND OF THE INVENTION
Optical measuring arrangements for measuring an electrical current in an electrical conductor are known which are based on the magneto-optic Faraday effect, and are therefore also designated as magneto-optic current transformers. In a magneto-optic current transformer, linearly polarized measuring light is transmitted through a Faraday sensor device which is arranged in the vicinity of the electrical conductor and includes an optically transparent material exhibiting the Faraday effect. Because of the Faraday effect, the magnetic field generated by the current causes a rotation of the plane of polarization of the measuring light by a rotational angle .rho., which is proportional to the path integral over the magnetic field along the path covered by the measuring light in the sensor device. The constant of proportionality is the Verdet constant V. The Verdet constant V is generally a function of the material and the temperature of the sensor device, as well as of the wavelength of the measuring light employed. In general, the sensor device surrounds the electrical conductor, so that the measuring light runs at least once around the electrical conductor in a virtually closed path. The rotational angle .rho. is, in this case, essentially directly proportional to the amplitude I of the current to be measured, in accordance with the relation electrical conductor. The Faraday rotational angle .rho. is determined polarimetrically by performing a polarization analysis of the measuring light running through the sensor device, in order to obtain a measuring signal for the electrical current.
It is known for the purpose of polarization analysis to use an analyzer to decompose the measuring light, after it has traversed the sensor device, into two linearly polarized light components L1 and L2 having planes of polarization, which are directed perpendicularly with respect to one another. A polarizing beam splitter can be used as the analyzer for this polarization analysis. Specifically, some of the types of polarizing beam splitters that can be used in this analysis include a Wollaston prism or a simple beam splitter having two downstream polarizers whose axes of polarization are rotated by .pi./2 or 90.degree. with respect to one another. Each of the two light components L1 and L2 is converted by one assigned photoelectric transducer into, in each case, an electrical intensity signal T1 or T2, which is proportional to the light intensity of the light component L1 or L2, respectively. A measuring signal intensity signals T1 and T2, as described in PCT Application No. WO 95/10046, is formed from these two electrical signals.
Disregarding interference effects, this measuring signal T is given by between the plane of polarization of the measuring light on being coupled into the Faraday element and a distinctive intrinsic optical axis of the analyzer.
Although, according to equation (1), the Faraday measuring angle .rho. is itself a linear, and thus unique, function of the current I, according to equation (4) the measuring signal T is a unique function of the measuring angle .rho. only over an angular range of at most .pi./2 (or 90.degree.). Consequently, it is possible using these polarimetric magneto-optic current transformers to measure uniquely only those electrical currents which lie in a current measuring range (current measuring interval) MR with an interval length of the current measuring range MR of a magneto-optic current transformer can be set by the selection of materials having different Verdet constants V for the Faraday element and/or by the number N of revolutions of the measuring light around the electrical conductor. A larger current measuring range is obtained by setting the product N.multidot.V in t
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Bosselmann Thomas
Menke Peter
Siemens Aktiengesellschaft
Strecker Gerard
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