Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Reexamination Certificate
1999-07-08
2001-11-20
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
Reexamination Certificate
active
06320366
ABSTRACT:
FIELD OF THE INVENTION
The invention relates broadly to measurement of large currents and production of devices for that purpose.
BACKGROUND OF THE INVENTION
Fiber optic, current sensors, based on the Faraday effect, have a number of advantages for remotely measuring large electrical currents. These include wide dynamic range, fast response, immunity to electromagnetic interference, small size, and low cost. Consequently, a variety of fiber optic, current sensors have been investigated in recent years. Mainly, they have employed a single mode optical fiber (SMF) of clad silica.
These sensors have not yet reached the stage of practical field use due to lack of accuracy and stability. This is mainly due to intrinsic and induced, linear birefringences that distort the Faraday rotation being measured. A particular problem arises from the inability of silica fibers to measure accurately large currents, such as surge or fault currents. Such currents are exceptionally large, as much as 180 kA under some circumstances. They generally occur due to some failure, such as a short circuit.
The Faraday effect is a phenomenon by which a linear, polarized light will rotate when propagating through a transparent material that is placed in a magnetic field in parallel to the magnetic field. The size of the rotation angle (&thgr;), given in degrees, is defined as
&thgr;=
VHL
(1)
where H is the strength of the magnetic field (A/m), V is the Verdet constant of the material, and L is the path length over which the magnetic field acts (m).
The magnetic field strength is measured in terms of Amperes (A) times turns (T) per unit length (AT/m) where m is meters). Since values are expressed in terms of one turn, this factor is usually implicit, rather than explicit. Hence, the strength is customarily given in amperes (A) or kiloamperes (kA) per unit path length in meters (m).
The Verdet constant, V, is the angle of rotation divided by the magnetic field strength per unit length. The angle may be expressed in any of the customary units for angle measurement, but degrees are used here. Verdet constant values, unless otherwise indicated, are given in terms of degrees divided by field strength expressed as (kA×T/m)m.
The magnitude of the magnetic induction (B) around an infinite straight conductor is given by the expression:
B
=(&mgr;
o
/4&pgr;)(2
I/a
) (2)
where I is the current, &mgr;
o
is permittivity of free space, and a is the radial distance of the magnetic field from the conductor. The magnetic field is related to the magnetic induction by the simple relation:
B=&mgr;
o
H.
(3)
Combining equations 1 through 3 gives a proportional relation between the rotation and the current such that:
&thgr;=
VI
(4)
where &thgr; is in degrees, V is the Verdet constant, and I is in kiloamperes (kA). Thus, the sensitivity of a method for measuring the current depends on how accurately the angular rotation can be measured.
The degree of sensitivity in measuring the angular rotation is influenced by another factor; birefringence. Birefringence arises primarily from stresses that result from bending, or otherwise distorting, a fiber in its disposition. The sources of linear birefringence in single mode fibers include residual stress from fabrication, bending, contact, and thermal stresses (Yamashita et al., “Extremely Small Stress-optic Coefficient Glass Single Mode Fibers For Current Sensor”, Optical Fiber Sensors, Sapporo Japan, paper We2-4, page 168 (1996) (“Yamashita”).
The stress-induced birefringence is quantified in terms of a coefficient, called the photoelastic constant (or the photoelastic coefficient). The photoelastic coefficient (B
p
) may be defined as the coefficient relating the difference in the refractive indices in the stress direction (n(par)) and in the pependicular direction (n(per)), to the magnitude of the applied stress:
n
(par)−
n
(per)=
B
p
&sgr; (5)
It may also be regarded as the phase shift measured in units of wavelength in nanometers (nm) per path length in centimeters (cm) divided by the stress in kilograms per square centimeter (kg/cma
2
). The values then are in units of (nm/cm divided by kg/cm
2
).
An ideal glass fiber would have a photoelastic coefficient of zero, thereby nullifying any effect of stress-induced birefringence. However, this has proven difficult to obtain in conjunction with other desired properties.
Therefore, a near-zero value, e.g., a value within a range of −0.2 to 0.2, has been considered adequate for some purposes.
In measuring a surge current, it is important to keep the angle of rotation below 90 degrees. With glass fibers having large Verdet constants, a fault current measurement is apt to create an angle of rotation greater than 90 degrees. The angle of rotation greater than 90 degrees will register the same as an angle of less than 90 degrees. In contrast, a device having a glass fiber with a low Verdet constant will not have as great an angle of rotation when measuring a large fault current. Therefore, it will accurately measure such currents.
It is a purpose of the present invention to provide an improved method and device for measuring large currents, such as surge and fault currents.
Another purpose is to provide a glass that is adapted to use in such improved method and device.
A further purpose is to provide a method of producing a glass having a near-zero photoelastic coefficient in conjunction with a low Verdet constant.
A still further purpose is to provide a method of reducing the photoelastic coefficient of a glass having a low Verdet constant.
SUMMARY OF THE INVENTION
The present invention resides in part in a method of reducing the photoelastic coefficient of a fluoride glass that has a low Verdet constant at a wavelength suitable for measurement, and that contains zirconium fluoride as a primary component of its composition, the method comprising the step of incorporating a small amount of lead fluoride in the glass composition.
The invention further resides in a method of determining the magnitude of a surge or fault current of up to about 200 kA which comprises:
providing a glass fiber, current sensor, the glass having a composition composed predominantly of zirconium fluoride and containing up to about 3% lead fluoride, having a low Verdet constant at the wavelength used for measurement, and capable of causing an angular rotation of polarized light less than 0.45° per kA, per pass at that wavelength,
passing a current through a conductor to create a magnetic field surrounding the conductor,
positioning the current sensor within the magnetic field thus created,
propagating polarized light into the glass fiber, current sensor,
measuring the angle of rotation of the polarized light in the glass fiber sensor, and
determining the magnitude of the current from the angle of rotation of the polarized light.
REFERENCES:
patent: 4253061 (1981-02-01), Ono et al.
patent: 4560867 (1985-12-01), Papuchon et al.
patent: 4894608 (1990-01-01), Ulmer, Jr.
patent: 5136235 (1992-08-01), Brandle et al.
patent: 5272434 (1993-12-01), Meyrueix et al.
patent: 5365175 (1994-11-01), Patterson et al.
patent: 6133721 (2000-10-01), Borrelli et al.
Yamashita, T., et al.;Extremely Small Stress-Optic Coefficient Glass Single Mode Fibers for Current Sensor; pp. 168-171.
Optical Properties of Glass, Donald R. Uhlmann and Norbert J. Kreidl, “Optical Properties of Halide Glasses”, Jacques Lucas and Jean-Luc Adam, pp. 37-85.
Borrelli Nicholas F.
Brocheton Yves A H
Cornelius Lauren K.
Netter Paul L.
Ricoult Daniel L G
Allen Philip G.
Corning Incorporated
Hollington Jermele M.
Metjahic Safet
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