Note: Descriptions are shown in the official language in which they were submitted.
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CURRENT MEASURING METHOD AND DEVICE
This application claims the benefit of priority under 35 U.S.C. ~119 from the
European Patent Application Number 99400016.4, filed January 5, 1999 and
U.S. provisional application S.N. 60/116,706, filed January 22, 1999.
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
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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
(8), given in degrees, is defined as
(1 ) A = VHL
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 x T/m)m.
The magnitude of the magnetic induction (B) around an infinite straight
conductor is given by the expression:
(2) B =(~0/4~)(21/a)
where I is the current, ~ 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:
(3) B =ft~H.
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Combining equations 1 through 3 gives a proportional relation between
the rotation and the current such that:
(4) A = VI
where 8 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 (Bp) may be defined as the coefficient relating the
difference in the refractive indices in the stress direction (n(par)) and in
the
perpendicular direction (n(per)), to the magnitude of the applied stress:
(5) n(par) - n(per) = Bp6
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/cm2). The values then are in units of
(nm/cm divided by kg/cm2).
An ideal glass fiber would have a photoelastic coefficient of zero,
thereby nullifying any effect of stress-induced birefringence. However, this
has
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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
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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,
5 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.
DESCRIPTION OF THE DRAWINGS
The single FIGURE in the accompanying drawing is a device for
carrying out the method according to the present invention.
DESCRIPTION OF THE INVENTION
The present invention relates to a method and device for determining
the magnitude of an exceptionally large current. The magnitude is determined
by measuring the angle of rotation that the current creates in polarized light
as
the light is transmitted through a fiber in a magnetic field. The angle of
rotation
is less when the glass from which the fiber is drawn has a low Verdet
constant.
In particular, a fiber produced from a fluoride glass of the present invention
will
have a Verdet constant that is less than 0.45 degrees/kA. Therefore, when the
fiber is exposed to a current up to at least 200 kA in magnitude, it will
register
an angle of rotation less than 90 degrees, and will accurately measure the
current.
Reference to a fiber signifies a clad fiber comprising essentially a fiber
core and an outer cladding layer. The fiber core is the functional member for
current measurement. However, it is well known that a fiber core requires a
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cladding of lower refractive index to prevent loss of light from the core
during
transmission.
Except for refractive index, it is desirable that the properties of a
cladding closely match those of a core glass in a clad fiber. Accordingly, it
is
common practice to use glasses of the same composition family for the core
and cladding. The cladding glass will have the same composition as that of the
core glass, except modified to impart a lower refractive index.
Lead fluoride (PbF2) is added to a selected fluoride glass composition, in
exchange for sodium fluoride (NaF), to produce a fiber core. A glass having
the same composition, but with the PbF2 omitted and the NaF restored, may be
used as a cladding glass. Other exchanges, such as for barium fluoride and/or
zinc fluoride, may be made to lower the index for a cladding glass. For
example, NaF, or another alkaline earth metal fluoride, such as calcium or
magnesium fluoride, may be exchanged for barium or zinc fluoride to provide a
lower refractive index. The two glasses may be melted, and a clad fiber drawn
employing the well-known double crucible technique.
Figure 1 illustrates an embodiment of the device of the present
invention. Preferably, a clad fiber 3, as described above, is utilized.
However,
any glass article, such as a piece of bulk glass (not shown), can be used.
Fiber 3 acts as a path for the polarized light. Conductor 4 carries the
current to
produce a magnetic field. Preferably, fiber 3 is wrapped around conductor 4,
as shown, to increase the length of the light path. Also, it is preferable
that
fiber 3 be insulated from the conductor.
The device also includes a source of light rays 1, the source being
located such that light rays are directed to an input end of fiber 3.
Typically,
the source of light rays 1 is a laser. A polarizer 2 is located adjacent to
source
of light rays 1 such that the light rays are linearly polarized. An analyzer 5
is
located at an output end of fiber 3.
Analyzer 5 derives a rotatory, polarization component produced in
proportion to the current flowing through the conductor 4. Also included is a
means 8 for indicating the measured current corresponding to the output of
analyzer 5. Typically, the means 8 is a light detector 6, and a display device
7.
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Light detector 6 receives and detects the output of analyzer 5. Device 7
receives the output of, and provides a display of, the output of the light
detector 6.
Optionally, the analyzer may be a Wollaston prism, as described in
Yamashita. Then, the light ray output from the fiber is broken into two
orthogonal polarizations. Means 8 detects the output of each signal, and
indicates the measured current corresponding to the output.
Electrical current, in a normal power station operation, can be
determined by employing a glass current sensor, preferably in the form of a
clad fiber. In order to avoid the effect of birefringence, it is desirable to
employ
a glass having a low photoelastic coefficient, preferably zero or near-zero.
The
Verdet constant of the glass may then ordinarily be as large as possible to
enhance the sensitivity of the determination. In measuring exceptionally
large currents, as explained earlier, a low Verdet constant is now required.
This avoids pushing the angle of rotation of the polarized light beyond
90°.
Heretofore, fused silica has provided the smallest Verdet constant
available in an inorganic glass, the value being 0.1 °/kA at 1150 nm.
However,
fused silica also has a large photoelastic coefficient, 3.5 (nm/cm)/(kg/cm2)
at
560 nm. This has led to a search for a glass of comparable Verdet constant
and a low, near-zero photoelastic coefficient.
It has been observed that certain fluoride glasses have relatively small
photoelastic coefficients. Further, these glasses may also have small Verdet
constants. Particular reference is made to a fluoride glass known by the
acronym ZBLAN. This glass is reported to have a composition consisting of, in
mole percent, 53 ZrF4, 20 BaF2, 4 LaF3, 3 AIF3 and 20 NaF. Measurements on
this glass show a desirably low Verdet constant of 0.22°/kA at 633 nm.,
and a
photoelastic coefficient of 0.34 (nm/cm)/(kg/cm2) at 546 nm.
In order to measure exceptionally large currents, an even lower
photoelastic coefficient is desirable. We have found that lead fluoride (PbF2)
can be added to a ZBLAN-type glass composition in an amount up to about 3
mol %. Preferably, the addition is in substitution for sodium fluoride (NaF).
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We have further found that such additions result in glasses having
decreased photoelastic coefficients. Based on these findings, core fibers for
present purposes preferably consist essentially of, as calculated in mol %:
52-56% ZrF4
14-24% BaF2
3-6% LaF3
3-6% AIF3
14-22% NaF
up to about PbF2
3%
SPECIFIC EMBODIMENTS
Three glasses were prepared based on the ZBLAN composition. In two
of the glass compositions, a small amount of lead fluoride (PbF2) was
substituted for sodium fluoride (NaF). Otherwise, the ZBLAN composition was
unchanged.
Measurements were made of Verdet coefficient at 633 nm, and at 1150
nm for one glass, and photoelastic coefficient (B) at 546 nm. TABLE I shows
the PbF2 content in mole and weight % and the measured values for Verdet
and photoelastic coefficients.
TABLE I
PbF2
. ....... ................
_.. '....
_... -
_.
m ~e' .. V (633 V (1150 nm) B (546 nm)
o'e % nm)
% ........
ght
0.0 0.0 0.22 - 0.34
0.7 1.43 0.22 0.12 0.25
2.0 4.28 0.20 - 0.18
These data indicate that increasing PbF2 substitutions will provide a
photoelastic coefficient approaching zero. However, the compositions become
increasingly difficult to melt.
The glasses shown in TABLE I were prepared by mixing an appropriate
batch of fluoride components, placing the batch in a covered, platinum
crucible
to retain fluorine, and melting at 800° C. for about 40 minutes. The
crucible
was then uncovered, and the melt was heat treated for 2-3 hours while being
covered with gaseous sulfur hexafluoride. The melts were then poured into
molds heated at 260° C. and the glasses annealed at that temperature.
The
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annealed glasses were clear, and test pieces were prepared for measurements
as recorded in TABLE I.
