Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Patent
1997-03-07
1998-12-08
Nguyen, Vinh P.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
324117R, G01R 3100
Patent
active
058475605
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process and device for measuring an alternating electric current.
BACKGROUND INFORMATION
Optical measurement methods and measurement equipment utilizing the magneto-optic Faraday effect to measure an electric current are known. The Faraday effect refers to 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 line integral over the magnetic field along the path traversed by the light with the Verdet constant as a proportionality constant. The Verdet constant depends on the material through which the light travels and on the wavelength of the light. To measure an electric current in a current conductor, a sensor device that shows the Faraday effect is positioned near the current conductor. Such a conventional device is composed of an optically transparent material, usually glass, and may be designed with one or more solid bodies that form an optical path or with an optical fiber. Linearly polarized light is passed through the sensor device. The magnetic field generated by the electric current causes of the polarization plane of the light in the sensor device to rotate by an angle of rotation that can be interpreted by an analysis unit as a measure of the strength of the magnetic field and thus the strength of the electric current. In general, the sensor device surrounds the current conductor in such a way that the polarized light travels around the conductor in a quasi-closed circuit. In this case, the absolute value of the angle of rotation of the polarization is in good approximation directly proportional to the amplitude of the measuring current.
In an embodiment of an optical measurement device described in International Application No. 91/01501 for measuring an electric current, the sensor device is designed as part of a monomode optical fiber surrounding the conductor in the form of a measuring winding. Therefore, the polarized measuring light passes around the conductor N times at one pass, where N is the number of spires in the measuring winding. In the transmission mode, the measuring light passes through the measuring winding only once. In the reflection mode, however, the other end of the fiber is reflected so the measuring light passes through the measuring winding a second time in the opposite direction after the first pass. Therefore, because of the nonreciprocity of the Faraday effect, the angle of rotation is twice as great in the reflection mode as in the transmission mode using the same measuring winding.
European Patent No. 0 088 419 describes an optical measurement device for measuring a current, where the sensor device is designed as a solid glass ring around the conductor. Light from a light source is linearly polarized with a polarizer and then injected into the sensor device. The linearly polarized light passes through the sensor device once and then split by a Wollaston prism as a polarizing beam splitter into two components, linearly polarized component light signals A and B with polarization planes normal to each other. Each of these two light signals A and B is transmitted over a respective optical transmission fiber to a respective light detector and converted to a corresponding electric signal PA and PB. From these two signals PA and PB, a normalized intensity measuring signal M=(PA-PB)/(PA+PB) is formed in a processor. This measuring signal M is independent of fluctuations in intensity of the light source or attenuation in the optical conductors.
One problem with such optical measurement processes and equipment for performing current measurements is the interference due to additional linear birefringence in the optical materials of the sensor device and the optical transmission paths. Such additional linear birefringence can be caused by mechanical stresses due to bending or vibration, for example, or by temperature changes. This linear birefringence caused by interference results in an undesirable change in operating point and measurement sensi
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patent: 4755665 (1988-07-01), Ulmer, Jr. et al.
patent: 4973899 (1990-11-01), Jones et al.
patent: 5656934 (1997-08-01), Bosselmann
Bosselmann Thomas
Menke Peter
Nguyen Vinh P.
Siemens Aktiengsellschaft
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