Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Patent
1993-08-30
1995-09-12
Weider, Kenneth A.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
3242441, 324117R, 25022714, 25022717, 356345, G02F 101
Patent
active
054500060
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and a method for sensing or measuring magnetic fields and hence also to apparatus and a method for sensing or measuring electric current. Temperature may also be measured. A particular application is in measuring high fault currents in electrical power and distribution systems. The invention could, for example, be used in mapping magnetic fields around high power transmitters or for condition monitoring in electrical machinery.
Considerable research has been carried out into the use of the Faraday effect, which is described below, in measuring large electrical currents. Measurement depends on the Verdet constant V(.lambda.,T) which is both dispersive and temperature dependent. Most techniques have concentrated on single mode optical fiber as the sensing element because of its well-known desirable properties; electrical isolation, flexibility, linear response, large bandwidth and zero hysteresis. However, optical fibers still suffer from several serious problems. Firstly, environmental birefringence perturbations, the most serious of which arises from vibration, which causes changes in the state of polarization (SOP) of the optical beam with concomitant sensitivity and scale factor fluctuations. Various techniques have been investigated to reduce this effect including the use of very low intrinsic birefringent fiber, the development of spun high birefringence fiber, the use of jelly filled cabling to mechanically isolate the optical fiber and the deployment of low birefringence fiber in a helical configuration to take advantage of geometrical birefringence effects. These techniques have met with varying degrees of success, but all suffer from varying problems. A major problem is the low Verdet constant of silica fibers (.about.5.times.10.sup.-6 rad A.sup.-1 ) which then requires many turns of fiber to increase the sensitivity. However, this then leads to greater birefringence in the fiber introduced when the fiber is deployed in a loop around the current carrying conductor, and hence larger diameter loops are required; the birefringence is proportional to (1/R.sup.2) where R is the loop radius. In addition this technique cannot be used for small sensing elements, for example localized sensors for mapping magnetic fields around high power transmitters or for condition monitoring in electrical machinery. Optical fiber has recently been employed as the sensing element in constructing an electric current sensor for fault protection, up to 60 KA, on a 145 KV power line. However, the loop diameter is .about.1 m and up to 7 turns of fiber are required to give the required dynamic range. This technique is inappropriate for the smaller sensing configurations mentioned previously, and where lower currents, 1 to 1000A, are required; the reduction in coil diameter combined with the increase in the number of fiber loops required would induce a serious degree of linear birefringence thus degrading the SOP of the light in the sensing element with a concomitant reduction in measurement sensitivity and with increased environmental sensitivity.
Techniques to increase the sensitivity, and thus allow smaller and easier methods to deploy sensors, have been investigated based predominantly on using materials which exhibit higher Verdet constants. Generally, these have been glass blocks made from flint or lead glass (see: T. Yoshino, "Optical fiber sensors for electric industry", SPIE, 586, 30, 1985; and Y. Kuroda, Y. Abe, H. Kuwahara and K. Yoshinaga, "Field test of fiber-optic voltage and current sensors applied to gas insulated substation", SPIE, 586, 30, 1985) with limited investigation of FR-5 glass (See: R. P. Tatam and D. A. Jackson, "Remote probe configuration for Faraday effect megnetometry", Optics commun. 72, 60, 1989). FR-5 has a Verdet constant of .about.10.sup.-4 rad A.sup.-1, typically 20 times greater than silica fiber. It is a glass doped with paramagnetic Nd ions and therefore V(.lambda., T) follows a 1/T dependence; silica based optical fib
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British Technology Group Limited
Wardas Mark
Weider Kenneth A.
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