Gas-insulated switchgear device with optical current sensor

DESCRIPTION

Technical field

The invention relates to a gas-insulated high-voltage switchgear device with an optical current sensor.

Background

Conventional current and voltage transformers for gas-insulated switchgear (GIS) are commonly based on electro-magnetic induction. These transformers are heavy and voluminous modules and are integral components of a GIS system. The current transformer may be mounted be ^¬ tween a disconnector/earthing switch and the circuit breaker. The voltage transformer may be mounted on the opposite side of the disconnector/earthing switch near the bushing. The significant size of the transformers is in contrast to the main goal of GIS, i.e. overall com ^¬ pactness and space savings. In recent years, more compact electronic and optical sensors have been disclosed. DE 4025911 Al de ^¬ scribes a compact arrangement of optical voltage and cur ^¬ rent sensors for single-phase encapsulated GIS. The volt ^¬ age is measured via the piezoelectric effect in disk- shaped quartz crystals positioned on the inside of a metal ring. The pre-assembled sensor module is mounted between two GIS compartments and forms a part of the GIS encapsulation. The piezoelectric deformations of the quartz caused by the electric field around the bus bar are detected with an optical fiber. The module may also contain a mechanical support for an optical fiber coil for magneto-optic current measurement (Faraday effect) .

The fiber coil may be placed inside or outside the GIS gas compartment (i.e. the gas-filled chamber) . A drawback of the arrangement is the need for gastight fiber feed- through (s) from the GIS chamber to the outside and the need for protection of the fiber against aggressive de ^¬ composition products of SFg produced in electric arcs.

Ref . 1 describes a compact module with com ^¬ bined electronic current and voltage sensors. Here, the current is measured with a Rogowski coil and the voltage is measured with a capacitive divider. The module is again mounted between two GIS compartments.

EP 1710589 Al discloses a further arrangement of optical current and voltage sensors. Here, the me ^¬ chanical support structure consists two ring-shaped parts (one electrically conductive, the other non-conductive) which are also mounted between two GIS compartments and again are part of the GIS encapsulation. Between the ring-shaped parts is a groove to accommodate an optical fiber for current sensing. The non-metallic part has a dead end bore (accessible from the outside) that houses a Pockels cell for optical voltage measurement. Again, tem ^¬ perature dependent changes in the dimensions and in di ^¬ electric constants may affect the voltage measurement.

The approaches above have in common that ret- rofit or module exchange is not possible without taking the switchgear from the line and at least partially dis ^¬ mantling it .

WO 2005/111633 discloses a concept for stress-free packaging and orientation of the sensing fi- ber of a fiber-optic current sensor, e.g. for the precise measurement of high direct currents at aluminum smelters.

US 6,348,786 and US 5,936,395 disclose opti ^¬ cal voltage sensors based on electrically poled fibers.

EP 522 303 describes current and voltage sen- sor comprising a toroidally wound coil for the current measurement as well as a hollow cylindrical sensing elec ^¬ trode for the voltage measurement.

Summary of the invention

The problem to be solved by the current in ^¬ vention is to provide a gas-insulated switchgear device with a current sensor that yields accurate current meas ^¬ urements while being easy to assemble.

This problem is solved by the device of claim 1. Accordingly, a magneto-optic fiber is wound at least once around the bus bar at a distance therefrom. The fi- ber is carried by a flexible carrier strip, and a shield wall separate from the carrier strip is arranged between the carrier strip and the chamber.

This design allows to locate the fiber out ^¬ side the chamber, which obviates the need for gastight fiber feed-throughs . At the same time, the sensing assem ^¬ bly consisting of the fiber and its carrier strip can be manufactured and calibrated separately from the rest of the switchgear and be mounted to the switchgear without disassembly of the switchgear. This simplifies installa- tion, maintenance or retrofitting.

Brief description of the drawings

Further embodiments, advantages and applica ^¬ tions of the invention are disclosed in the dependent claims as well as in the following description, which makes reference to the figures. These show:

Fig. 1 a sectional view of a first embodiment of a GIS (upper half only) with current and voltage sen ^¬ sor,

Fig. 2 a sectional view of a second embodi ^¬ ment of a GIS (upper half only) with current and voltage sensor, Fig. 3 a sectional view of a carrier strip with embedded fiber,

Fig. 4 a sectional view of a multiple-winding arrangement of a carrier strip with embedded fiber,

Fig. 5 a sectional view of a carrier strip with several windings of an embedded fiber, Fig. 6 a sectional view of a third embodiment of a GIS (upper half only) with current sensor,

Fig. 7 a sectional view of a fourth embodi ^¬ ment of a GIS (upper half only) with current sensor,

Fig. 8 a sectional view of a fifth embodiment of a GIS (upper half only) with current sensor,

Fig. 9 a sectional view of a sixth embodiment of a GIS (upper half only) with voltage sensor,

Fig. 10 a sectional view of a seventh embodi ^¬ ment of a GIS (upper half only) with voltage sensor, and Fig. 11 a sectional view of a poled fiber.

Embodiments of the invention

Definitions:

The following definitions are used in the present text and claims:

The direction "axial" designates the longitu ^¬ dinal axis of the bus bar in the region of the voltage and current sensor.

The direction "radial" designates any direc ^¬ tion perpendicular to the axial direction.

The term "flexible carrier strip" refers to a carrier strip that can be non-destructively bent from a stretched configuration to a radius corresponding to the typical radius of the enclosure of a GIS, i.e. to a ra ^¬ dius of approximately 20 cm or larger.

Current sensor arrangements: Fig. 1 shows a sectional view of a first em ^¬ bodiment of a gas-insulated switchgear. It comprises an enclosure 6 enclosing a chamber 40 filled with an insu-

lating gas, such as SFg, or vacuum. Typically, enclosure 6 is arranged cylindrically and concentrically around a bus bar 5 carrying the current through the switchgear. Chamber 40 is divided into a plurality of partitions axi- ally separated by partition insulators 1. Each partition insulator 1 comprises a partition wall 41 extending from the enclosure 6 to the bus bar 5. Partition wall 41 is surrounded by a metal embracing 3.

In a first embodiment at least one partition insulator 1 of the GIS is modified such that it accommo ^¬ dates an optical current sensor and/or an optical voltage sensor. The current sensor makes use of the magneto-optic effect (Faraday effect) in an optical fiber 11. A pre ^¬ ferred sensor version is an interferometric sensor as de- scribed in Ref. 2, 3 and WO 2005/11633. The magnetic field of the current produces a differential phase shift of left and right circularly polarized light waves propa ^¬ gating in a sensing fiber 11. The phase shift is detected e.g. by a technique as known from fiber gyroscopes. The invention is not restricted to interferometric fiber ^¬ optic current sensors, however, but may be used as well for others, in particular polarimetric, sensors. In a po- larimetric sensor the magneto-optic effect is detected as a rotation of a linearly polarized light wave. The sensing fiber 11 is packaged in an e.g. substantially rectangular, flexible carrier strip 2, for example of fiber re-enforced epoxy resin, as dis ^¬ closed in WO 2005/111633. A sectional view of the carrier strip comprising and its embedded fiber 11 is shown in Fig. 3.

Metal embracing 3 of the partition insulator 1 forms a shield wall, separate from carrier strip 2, ar ^¬ ranged between carrier strip 2 and chamber 40. In other words, carrier strip 2 is arranged outside chamber 40, e.g. under normal atmosphere. Metal embracing 3 is ar ^¬ ranged radially outside partition wall 41 and is con ^¬ nected thereto.

Metal embracing 3 has a groove or recess 4 on its radially outward facing side to accommodate one or several loops of the flexible carrier strip (see Fig. 3 - 5) . It is important that there is no current flow through metal embracing 3, since the current sensor should only measure the current in the bus bar 5, but not be affected by any current in the GIS enclosure 6. The embracing 3, which is arranged axially between two neighboring tube sections of enclosure 6, is therefore electrically isolated from one or both of tube sections by means of one or more non-conductive seals 7. The seals 7 also prevent leakage of gas (commonly pressurized SFg gas) from the gas-filled chamber 40 of the switchgear. The flanges 8 of the adjacent tube sections of enclosure 6 are held together by bolts and screws (the location of one of which is shown in a dashed line under reference number 42 in Fig. 1) . The bolts penetrate the metal embracing 3 preferably on the radial outside of the sensing carrier strip 2. Any current through the bolts will then not disturb the current measurement. If the bolts penetrate the metal embracing 3 on the radial in ^¬ side of the carrier strip 2, they must be electrically isolated from at least one of the flanges 8 in order to prevent current from flowing through the bolts. Current flowing in the enclosure 6 is guided around the carrier strip through a conductive part 9 electrically connecting the flanges 8. Part 9 is arranged radially outside the fiber loop of the carrier strip, again in order to pre- vent its current from being measured. A conducting con ^¬ nection 10 between the metal embracing 3 and the part 9 or the GIS enclosure 6 ascertains that the metal embrac ^¬ ing 3 is on the same electric potential as the enclosure 6 (ground potential) . The sensing fiber 11 is preferably a sin ^¬ gle-mode fused silica fiber with low intrinsic birefrin ^¬ gence. The bare fiber (without coating) is accommodated

in a thin fused silica capillary 12 as shown in Fig. 3 and as described in EP 1 512 981, the disclosure of which is herewith incorporated by reference. The capillary 12 is coated for protection with e.g. a thin polyimide coat- ing and is filled with a lubricant to avoid friction be ^¬ tween the fiber and the capillary walls. The capillary is embedded in silicone or a resin in a groove 13 of the carrier strip 2. The groove 13 may be, for example, of rectangular or triangular shape. Preferably, the longitu- dinal capillary axis is in the neutral plane of the car ^¬ rier strip (at half the thickness of the strip) so that bending the strip 2 does not strain the capillary 12.

This way of fiber packaging avoids any packaging related stress on the fiber over a wide range of temperatures, which is crucial for high stability and measurement accuracy of the sensor. The carrier strip 2 serves as a robust mechanical protection of the capillary 12 and also ascertains a reproducible azimuth angle of the fiber, a further prerequisite for high scale factor repeatability, see WO 2005/111633, the disclosure of which is herewith incorporated by reference.

The fiber 11 forms an integer number of loops around bus bar 5 to ascertain that the sensor measures a closed path integral of the magnetic field. The signal is thus independent of the magnetic field distribution and unaffected by currents flowing radially outside the fiber loops. The sensing fiber length therefore corresponds to an integer multiple of the circumference of the partition insulator 1. In order to properly close the carrier strip 2, the carrier strip 2 carries markers separated by the length of the sensing fiber. Preferably, the markers are at or near the fiber ends. The carrier strip 2 is mounted in the groove or recess 4 of the partition insulator 1 in such as way that the markers coincide, i.e. that they are at the same tangential position. A clamp (not shown) keeps the overlapping strip sections in place. Instead of markers or in addition to markers there may be boreholes

(not shown) through the strip 2 (in Fig. 1 in a radial direction), separated by a loop circumference. The fiber coil is then closed by bringing the bore holes to coin ^¬ cide. A pin through these holes and mechanical fixtures may be used to keep the arrangement in place.

At high rated currents one fiber loop may be already sufficient. If more loops are needed, the carrier strip 2 may be mounted in two or more superposed loops as shown in Fig . 4. A particular advantage of this scheme is that the GIS and the sensor can be fully assembled independent of each other. The sensor can be easily added to and re ^¬ moved from the assembled GIS. Provided the GIS has been assembled with an appropriately modified partition insu- lator, a later retrofit of a sensor is possible without any dismantling of the switchgear. The calibration of the sensor can be done without the partition insulator being available at the time of calibration.

Alternatively, the sensor may have only one loop of carrier strip containing several capillary loops with the fiber 11 inside, as shown in Fig. 5. In this case the carrier strip is mounted first in the groove or recess 4 of the partition insulator 1. Subsequently, the capillary loops are wrapped into the groove of the strip. Here, it must be ascertained that the sensing fiber length is an integer multiple of the perimeter length of the carrier strip. For practical reasons, the sensor can be added to the partition insulator 1 before the parti ^¬ tion insulator 1 is installed at GIS. Sensor calibration preferably is done with the fiber mounted on the insula ^¬ tor. Alternatively, the partition insulator may be de ^¬ signed such that the carrier strip including the fiber loops can be prepared and calibrated independent of the insulator. To this purpose the metal embracing of the partition insulator 1 may be divided into two parts so that the carrier strip can be slid on one of the parts from the side and the second part attached subsequently.

Instead of a fiber with low intrinsic bire ^¬ fringence, the fiber may be a highly birefringent spun fiber as known from Ref. 4. This type of fiber is more stress tolerant and therefore may be embedded into the epoxy strip or metal embracing (Fig. 1 - 5) without a capillary, and without a removal of the coating.

Figs. 6 - 8 show further alternatives for the current sensor arrangement. In Fig. 6, the capillary with the sensing fiber is embedded into a recess of the metal embracing of the partition insulator. In this case a carrier strip is not needed. Fig. 7 shows an arrangement where the carrier strip is mounted in a metal embracing without a partition insulator. Fig. 8 shows an embodiment without a carrier strip and without a partition insula- tor. The capillary alone is mounted in a recess or groove 4 of the metal embracing and embedded for example in silicone. A lid 43 closes the groove or recess and pro ^¬ tects the capillary.

It must be noted that the sensor described here can also be mounted at a position separate from a partition insulator, i.e. instead of a partition insulator an independent mount without an insulator can be used. This is illustrated in the embodiments of Figs. 6 - 8. The mount is essentially equivalent to the metal em- bracing of a partition insulator and again installed between two GIS modules, i.e. between two tube-shaped sec ^¬ tions of enclosure 6.

Voltage sensor arrangements The partition insulator of Fig. 1 contains, apart from the current sensor, a voltage sensor that measures the voltage between the bus bar 5 and the enclo ^¬ sure 6. The voltage sensor is arranged within the parti ^¬ tion wall 41. In the embodiment of Fig. 1, the voltage is measured by an electro-optical voltage sensor performing a line integration of the electric field. An electro- optical voltage sensor is based on measuring electric-

field-induced refractive index changes. An electric field applied to the material induces birefringence or changes the birefringence of an intrinsically birefringent mate ^¬ rial. The voltage sensor of Fig. 1 comprises a light guiding element, such as a crystalline fiber, extending radially inwards from metal embracing 3.

Advantageously, the sensor utilizes the lin ^¬ ear electro-optic effect (Pockels effect) in a crystal ^¬ line, electro-optical fiber or rod 14, preferably of Bi4Ge3θ]_2 (BGO) . Suitable technologies for measuring the voltage using electro-optical fibers are e.g. described in US 4 269 483, the disclosure of which is herewith in ^¬ corporated by reference.

The fiber or rod resides in a radial bore 19 of the insulating partition wall 41 between the bus bar 5 and the metal embracing 3 and preferably is operated in reflection. The radially outer end of fiber or rod 14 extends into a recess 16 in a radially inward facing sur ^¬ face of the shield wall or embracing 3, and the radially inner end of fiber 14 extends into a recess 16 in a ra ^¬ dially outward facing surface of bus bar 5. This design ensures that the line integral of the field along the fi ^¬ ber or rod corresponds to the full electric potential difference between the bus bar and the metal embracing. The recesses 16 have preferably a large depth-to- diameter-ratio so that the field strength in the recesses is small. As a result small displacements of the fiber or rod 14 have little effect on the signal, even if the fi ^¬ ber ends are not in direct electric contact with the bus bar 5 and the embracing 3. Alternatively, or in addition thereto, the ends of fiber or rod 14 may carry a metal shielding in the form of a metal cap or a conductive layer electrically connected to the bus bar and the em ^¬ bracing, respectively, for example via spring contacts. The fiber or rod 14 can be held in a socket 17 in embrac ^¬ ing 3 and be connected to the light source/detection mod ^¬ ule of the sensor (not shown) by one or several optical feed fibers 18.

The fiber or rod 14 may reside in a capillary tube made from a dielectric material such as fused sil ^¬ ica. The capillary is filled with an insulating fluid, e.g. silicone oil, for electric insulation. As a result, the fiber or rod 14 is not subject to any mechanical stress. The capillary is embedded in a resin or oil 19. In another realization of the invention the fiber or rod

14 may be installed without capillary. The bore 15 is again filled with a resin or oil 19. In still another re- alization the bore 15 is in gas-exchange with a neighbor ^¬ ing GIS module, as shown in Fig. 2. Gas exchange takes place through a channel 29. Channel 29 may be filled with a cartridge 30 containing a filter, blocking at least part of the SFg decomposition products, namely aggressive SFg decomposition products, such as SF4 or HF. The filter can e.g. comprise a material that acts as a molecular sieve or absorber. These decomposition products may arise in electric arcs during switching. The filter may be made of alkali-aluminium-silicate combined with CaO, e.g. 0.7CaO ^■ 0.3Na ₂ O ^■ Al ₂ O ₃ ^■ 2SiO ₂ ^■ nH ₂ 0 (with n = natural number) . A seal 31 prevents gas leakage to the outside.

Instead of being operated in reflection, the crystalline fiber or rod may be operated in transmission. This requires an extra fiber connection to the bus bar end of the crystalline fiber or rod, however.

Instead of an electro-optic sensor, the bore

15 may contain the sensitive part of a piezo-optic sensor as known from EP 0 316 635, the disclosure of which is herewith incorporated by reference. Here the piezoelec- trie deformation of piezoelectric transducer elements is transmitted to an optical fiber. The induced fiber strain produces a phase shift of light waves propagating in the fiber proportional to the voltage.

Fig. 9 shows a voltage sensor combining a ca- pacitive divider with an optical sensing element 20 car ^¬ rying two electrodes 44, 45 arranged radially outside the metal embracing 3. The divider is formed by the metal em-

bracing 3 as in the previous embodiments and a concentric ring-shaped inner electrode ring 21 separated by at least one dielectric spacer layer 22 or several individual spacer elements. Electrode ring 21 is arranged radially inside metal embracing 3 and extends at least partially around bus bar 5 at a distance from bus bar 5. One elec ^¬ trode 44 of the sensing element 20 is connected to the encasing 3 (ground) , while the other electrode 45 of the sensing element 20 is connected to electrode ring 21. The voltage between the metal embracing and the electrode ring 21 is given by V _Q [1 - ln(R _e /R]_) / ln(R2/R]_)] where R]_, R2, R _e ^are the radii of the GIS bus bar, the GIS en ^¬ closure and the ring electrode, respectively. V _Q is the voltage to be measured between the bus bar and the enclo- sure. The electrode ring 21 is in electric contact with the sensitive part of an optical voltage sensor 20 via a connecting wire 23. The wire 23 is insulated from the metal embracing 3 by an insulation layer 24. Ground potential is supplied to the voltage sensor via a connect- ing wire 25. The voltage sensor may be for example an electro-optic sensor as known from EP 0 682 261 or Ref . 5 or a piezo-optic sensor as known from EP 0 316 619, the disclosures of which are herewith incorporated by refer ^¬ ence. The source, detection, and signal processing compo- nents of the voltage sensor are preferably placed sepa ^¬ rately from the GIS and connected to the optical sensing element 20 by one or several fiber cables 26.

The voltage sensor of Figs. 9 and 10 may again be combined with a current sensor as described above.

Voltage sensor using electrically poled optical fiber:

A common glass fiber does not exhibit a lin ^¬ ear electro-optic effect. It has been shown, however, that the anisotropy produced by electric poling of the fiber does result in a linear electro-optic effect (Pockels effect) [e.g. Ref. 6] . Commonly, the poling di-

rection is transverse to the longitudinal fiber axis. The fiber is then sensitive to transverse electric fields. Voltage sensors based on poled fiber have been disclosed in US patents 6,348,786 and 5,936,395. In the embodiment of Fig. 10, a transversally poled fiber 27 (or some other poled light guiding ele ^¬ ment) is looped at least once, advantageously more than once, around bus bar 5.

In GIS the electric field distribution is well defined and stable so that field integration is not necessarily needed for accurate voltage measurement. Fig. 10 shows an arrangement of a transversally poled fi ^¬ ber for voltage measurement at GIS. The fiber 27 is wound on a ring-shaped mount 28 on the radially inner side of metal embracing 3 and concentric to the embracing. The mount 28 is made of a dielectric material, such as fiber re-enforced epoxy, so that it does not screen the elec ^¬ tric field from the fiber windings. The fiber windings may be embedded in silicone or an epoxy resin. The elec- trie field strength at the location of fiber windings is given by E (R _f ) = V ₀ / [R _f ln(R ₂ /Ri)] where R _f is the ra ^¬ dius of the fiber windings (neglecting the small influ ^¬ ences of the dielectric mount on the field) . The field at the fiber windings is thus proportional to the GIS volt- age V ₀ .

The fiber is preferably a polarization maintaining (pm) fiber with an elliptical core 33 as shown in Fig. 11. It has a D-shaped cladding 32. The D-shape makes it possible to wrap the fiber onto the mount 28 with a defined orientation of the poling direction 34 and the core 33. The D-shape also allows efficient fiber poling, see Ref. 6 and US patent 6,097,867. The flat cladding surface is in contact with the surface of mount 28. The poling direction is perpendicular to the flat cladding surface and thus parallel to the electric field. The ma ^¬ jor and minor axes, x and y, of the elliptical fiber core 33 (slow and fast axes of birefringence) are parallel and

perpendicular to the flat cladding surface. In order to measure the electric field, two light waves E _x and Ey with orthogonal polarization directions are launched into the fiber (Fig. 11) . The electric field introduces a dif- ferential phase shift between the two waves given by δφ = (2π/λ) • (δn _x - δny) -L. Here, λ is the optical wavelength and δn _x , δn _y are the electro-optic changes in the indices of refraction n _x , n _y for the two polarizations, given by δn _x = (1/2) •r ₁₃ -n _x ³ -E _e ff and δn _y = (1/2) • r ₃₃ • n _y ³ -E _{ef f} . The electro-optic coefficients r]_ ₃ , r ₃₃ are related to each other by the relationship r ₃₃ = 3r _] _ ₃ . E _e ff is the effective field strength at the fiber core. L is the fi ^¬ ber length exposed to the electric field. For a stable geometry and field distribution the optical phase shift δφ is a measure for the voltage V ₀ . Small changes in the field strength E _e ff as a result of thermal expansion and thermal variations in the dielectric constants of the mount and fiber may be compensated by a temperature meas ^¬ urement. The influence of thermal expansion may be re- duced by a mount 28 comprising carbon fiber, e.g. by consisting of a carbon fiber reinforced plastic or having an extra embedded support made of carbon fiber reinforced plastic .

The phase shift δφ can be measured by means known to a person skilled in the art. It can also be measured by using a technique as known from fiber gyro ^¬ scopes with adaptations. This technique requires an extra fiber for compensation of the differential group delay of two orthogonal waves and of thermal changes in the phase difference. The extra fiber may be a section of un-poled pm fiber (preferably of the same type as the poled fiber) with an appropriately chosen length. The poled and un- poled fiber sections are spliced together with a 90°- offset in the orientation of the core axes so that the thermal phase shifts in the two fibers have opposite sign and cancel each other. The un-poled fiber may be wrapped on the same mount 28 as the poled fiber or on an extra

support outside the electric field. The extra support should have sufficient thermal contact to the GIS enclo ^¬ sure in order to keep the temperature difference between the two fiber sections small. If the compensating fiber is placed out of the electric field, it may also be a section of poled fiber.

Instead of a fiber gyroscope based detection technique, a polarimetric concept may be used as known e.g. from Ref. 7. The technique requires a phase modula- tor, preferably at the compensation fiber, as part of a control loop that keeps the bias phase shift at quadra ^¬ ture for maximum sensitivity, Ref. 8.

The poled fiber may be spliced together from several individual sections of poled fiber in order to realize a sufficient overall length if needed.

Instead of a D-shaped fiber other fiber shapes, as described for example in US patents 6,097,867 and 6,134,356, may be used which make simple fiber orien ^¬ tation possible. Instead of an elliptical-core fiber, other types of polarization-maintaining fiber as known from Ref. 9, such as Panda fiber, may be employed. An ad ^¬ vantage of elliptical-core fiber is its relatively small variation of the differential phase with temperature. The fiber may also be a poled side-hole (twin-hole) fiber, Ref. 10, or a micro-structured fiber (holey or photonic crystal fiber, Ref. 11) .

References

1. Andrzej Kaczkowski. "Combined sensors for current and voltage are ready for application in GIS", CIGRE- CE/SC:12. Session paper. Ref. No: 12-106, 1998.

2. "Temperature and vibration insensitive fiber-optic current sensor", K. Bohnert, G. Gabus, J. Nehring, and H. Brandle, J. of Lightwave Technology 20(2), 267-276 (2002) .

3. "Highly accurate fiber-optic dc current sensor for the electro-winning industry", K. Bohnert, H. Brandle, M. Brunzel, P. Gabus, and P. Guggenbach, IEEE/IAS Transactions on Industry Applications 43(1), 180-187, 2007.

4. R. I. Laming and D. N. Payne, "Electric current sen ^¬ sors employing spun highly birefringent optical fi ^¬ bers", J. Lightw. Technol., 7, no. 12, 2084-2094, 1989. 5. G. A. Massey, D. C. Erickson, and R. A. Kadlec,

"Electromagnetic Field Components: Their Measurement Using Linear Electrooptic and Magnetooptic Effects, " Appl. Opt. 14, 2712 (1975) .

6. P. G. Kazansky, P. St. J. Russell, and H. Takebe, J. Lightw. Technol. 15(8), 1484-1493, 1997.

7. W. J. Bock and W. Urbanczyk, "Temperature-hydrostatic pressure cross-sensitivity effect in elliptical-core, highly birefringent fibers", Applied Optics 35(31), 62676270, 1996. 8. D. A. Jackson, R. Priest, A. Dandridge, and A. B.

Tveten, "Elimination of drift in a single-mode opti ^¬ cal fiber interferometer using a piezoelectrically stretched coiled fiber", Appl. Opt., vol. 19, pp 2926-2929, 1980. 9. J. Noda, K. Okamoto, and Y. Sasaki, "Polarization- maintaining fibers and their applications", J. Lightw. Technol. 4, 1071-1089, 1986.

10. P. Blazkiewicz, W. Xu, D. Wong, S. Fleming, and T. Ryan, J. Lightw. Technol. 19(8), 1149-1154, 2001. 11. T. A. Birks, J. C. Knight, and P. St. J. Russell, "Endlessly single-mode photonic crystal fiber", Optics Letters 22(13), 961-963, 1997.

List of reference numbers

1 partition insulator 2 carrier strip 3 metal embracing and shield wall 4 recess 5 bus bar 6 enclosure 7 seals flanges

9 conductive part

10 conductive connection 11 magneto-optical fiber 12 capillary 13 groove 14 electro-optical fiber or rod 15 radial bore 16 recesses 17 socket 18 optical feed fibers 19 resin or oil 20 optical voltage sensing element 21 inner electrode ring 22 dielectric spacer layer 23 connecting wire 24 insulation layer 25 connecting wire 26 fiber cables 27 transversely poled fiber 28 mount 29 channel 30 filter 31 seal 32 cladding 33 core 34 poling direction 40 chamber 41 partition wall 42 screw or bolt location 43 lid 44, 45: electrodes