Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Patent
1993-10-29
1995-12-12
Nguyen, Vinh P.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
324117R, 3242441, G01R 1900
Patent
active
054752981
DESCRIPTION:
BRIEF SUMMARY
This invention relates to measurement methods and apparatus dependent on the Faraday effect; that is the rotation of the direction of polarization of light in a magneto-optic medium when a magnetic field is applied in the direction of propagation.
The invention is particulary useful for sensing the magnitude of an electric current flowing in a conductor by sensing the rotation of polarization of light in an optical fibre wound around the conductor.
Optical fibre measurement devices offer several important advantages for electrical power system applications.
Amongst other parameters which may be measured are magnetic field strength, voltage, temperature and strain or pressure.
The advantages of using an optical fibre technique to measure current in electricity supply systems are that electrically passive devices may be used and the sensor is free from electromagnetic interference and saturation effects. The optical fibre acts both as the transducer element and an insulating telemetry medium and may be interfaced easily to long distance communications links. It is also possible to measure a large bandwidth, typically up to 10 MHz. In optical fibre systems the costs are low and are not strongly dependent on the system voltage, as is the case for conventional current transformers.
In known optical fibre current measurement devices, such as are described in patent specifications EP 356670, FR 2475230 and U.S. Pat. No. 4,563,639, a laser beam is linearly polarized and is launched into a single-mode fibre which loops one or more times around a conductor which carries a current. The fibre then returns to a ground based detector system. Cladding modes are stripped off and the linearly polarized light propagating in the fibre core has its polarization direction rotated by the longitudinal magnetic field around the loop, via the action of the Faraday magneto-optic effect. The magnitude of the final rotation p is proportional to the line integral of the magnetic field H around n loops material. But, by Ampere's circuit theorem loop and the conductor may vibrate without affecting the magnitude of the rotation.
The resulting rotation may be sensed with, for example, the aid of a Wollaston prism, which resolves the emerging light signal into two orthogonal, linearly polarized components. Each of these components is separately detected with a photodiode, and the difference between the two intensities is normalized to their sum, to give a parameter P which is proportional to the polarization rotation, .rho., independent of received light power, and hence of laser drift and variations in path attenuation.
The device is capable of a.c. and d.c. measurement.
The primary difficulty which exists with optical fibre current measurement devices for operational use is the sensitivity of the fibre to variations in propagation conditions, such as temperature or pressure variations. Some known current measurement devices make use of the fact that the Faraday rotation is non-reciprocal, that is, the absolute sense of tile rotation of the polarization direction is constant with respect to the direction of the longitudinal magnetic field and is independent of the direction of the propagation of the light. This has the effect of reversing the sign of the of Faraday rotation with a reversal of the light propagation direction. The devices described by patent specifications GB 2104213, U.S. Pat. No. 4,539,519, U.S. Pat. No. 4,560,867 and WO 91/01510 all reflect a linearly polarized laser beam which has been launched into magneto-optical medium back along the same path in order to cancel out any reciprocal effects, either intrinsic or induced. The devices described in patent specifications U.S. 4,563,646, U.S. Pat. No. 4,542,338 and U.S. Pat. No. 4,370,612 use two beams of light which propagate in opposite directions through the magneto-optical medium. However, a persistent and universal problem is that of vibrational noise, which can be prohibitively large in many power system applications.
The effect of vibration is to exert pressure on the
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British Technology Group Ltd.
Nguyen Vinh P.
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