GB1600277A - Measurement of voltages - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE MEASUREMENT OF VOLTAGES (71) We, CENTRAL ELECTRICITY GENERATING BOARD, a British Body Corporate, of Sudbury House, 15 Newgate Street, London, EC1A 7AU, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to electro-optical transducers for the measurement of voltages.

Particularly with high voltage transmission lines and other equipment used in power transmission systems, it is convenient to make use of passive electro-optical transducers.

In the Paper entitled "Method for Simultaneous Measurement of Current and Voltage on High Voltage Lines Using Optical Techniques" by Dr A J Rogers, (Proc.I.E.E. Viol. 123 No.10, Oct.1976). there is described a device making use of electro-gyration effects for the determination of the magnitude of an electric field.

Linear birefringence in a medium is that phenomenon whereby two orthogonal linear directions of polarisation of light propagating in the medium travel at different velocities. In a material, such as quartz, there is sensitivity to both magnetic and electric fields. There are two distinct electric field effects:- the electro-gyration effect and the electro-optic effect.

The present invention is concerned with the use of the electro-optic effect. The electro-optic effect is the name given to the dependence of linear birefringence on the external electric field. If one considers, for example alpha quartz, this is a positive uniaxial crystal with trigonal 32 symmetry. It possesses one three-fold symmetry axis, the optic axis, and three two-fold symmetry axes. If one considers orthogonal axes OX1, OX2 and OX3 forming a right-handed set, then OX3 is the three-fold symmetry optic axis, OX1 is one of two-fold symmetry axes and OX2 is at right angles to both OX1 and OX3. In the system of the above-mentioned Paper using the electro-gyration effect, the electric field is along the axis OX1 and the light propagation is substantially along the axis OX3. It may be shown that one can utilise the electro-optic effect to measure the electric field by propagating light in the direction of one of the two-fold symmetry axes (e.g. OX1) while applying the electric field in the direction of the same two-fold symmetry axis. The electro-optic effect may be described in terms of the change of shape brought about by the field in the index ellipsoid.

The index ellipsoid is the ellipsoid which defines the three dimensional birefringence which is consequent upon the crystal anistotropy. For any given propagation direction in the material, there corresponds a direction in the ellipsoid and, where the plane, which lies at right angles to this direction and which also contains the origin, cuts the ellipsoid surface, there will be defined an ellipse which, via its major and minor axes, defines the directions of the birefringence fast and slow axes and also their velocity difference for that direction.

When an electric field is applied to the crystal, the ellipsoid is distorted and the electro-optic effect is characterised via a quantification of this distortion. Thus the ellipsoid defines the way in which the two velocities due to the birefringence vary with direction of propagation when referred to the crystallographic axes. The electro-optic effect thus is the result of a distortion of the index ellipsoid caused by application of an electric field. The consequence of the electro-optic effect is. in general, that a linearly polarised beam becomes elliptically polarised as a result of its passage through the medium.

The velocity difference between the components may be, for example, proportional to the field (Pöckels effect) or to the square of the field (Kerr effect). The resulting phase difference between the components will also be proportional to the beam path length within the crystal. The ellipticity of the polarization ellipse (for the emerging beam) resulting from the phase difference thus depends on the electric field, and can be used to measure it.

Consider a rectangular block of crystalline uniaxial (one optic axis) material and suppose that a linearly polarized beam of light is incident normally on one of the faces and passes through the block to exit from the opposite face. Take the crystallographic fast axis, in the plane of the front face, as the reference direction, X, i.e. this is the direction in which the light must be polarized in order to travel at maximum velocity. The direction at right angles to this, Y, will then be the slow axis. Suppose that the polarization direction of the incident light makes an angle 0 with X, The electric vector of the propagating beam will be resolved into components along the axes which may be written: Ex = Eo cos H cos (i)t Ey = E, sin 0 cos ot Eo being the incident amplitude, #)/2# the frequency of the light and t the time.

On emergence. a phase difference # will have resulted from the birefringence and hence we shall have the new components: Ex' = Eo cos 0 cos (#t + t) = = Eo E,, sin 0 cos o)t (ignoring any attenuation due to absorption or reflection) If now we set a polarizing analyser so that it accepts only that component which lies at an angle ss to the X direction, then the amplitude of the accepted component will be: Ef, = Ex' cos ss + E,7 sin ss.

The output light intensity will be proportional to |Ess|2 IE!2 hencehence we have I = Io (sin2 t sin ss + cos2 0 cos2 ss + sin 20 sin 2t3 cos 4;) where It, is the incident intensity.

If now we set p = 0 = #/4 we obtain I = 2 (1 - cos 2 If we then provide ip with a bias of -z/2 and represent the phase variation caused by the electric field by o, we obtain I I 3 = (1 - cos 2 + w)) = 2" (1 - sin o) and hence, as before, if o4 is small: 61=-1Y a result analogous to that for the Faraday effect (magneto-optic rotation) For the Pöckels (linear) effect o will be proportional to the electric field E and, for a uniform field, to the path length within the crystal, i.e.

O = C E f where C is a constant characterizing the effect. It should also be noted, in passing, that a given electro-optic phase change o will produce only half the output signal resulting from an equal polarization rotation o.

In the aforementioned Paper, a device is described making use of a pair of transducers situated on opposite sides of a current conductor. Light from a laser beam is directed through these two transducers and thence passed separately through polarisation analysers to two separate photo-detectors. From the two outputs, signals can be obtained which separately represent the magneto-optic and electro-gyration effect. From the output representative of the electro-gyration effect, it is possible to obtain an indication of a magnitude of the electric field at the location of the transducers. Such an arrangement suffers from a number of drawbacks. In particular it is necessary to measure separately the amplitude of two different light beams. In practice, this presents difficulties, particularly in measurements out-of-doors for example an overhead transmission lines. A photo-diode has a sensitivity which is critically dependent on the exact area of the diode on which the beam is incident and hence any vibration or relative movement of components can cause variations in output signals.

It is an object of the present invention to provide an improved form of voltage measuring device making use of an electro-optic effect.

According to this invention, a voltage measuring device comprises an electro-optical transducer element in the form of a fibre defining a light path, the fibre extending between points of different potential in an electric field, a light source arranged for producing a beam of polarised light for transmission through said element from one end to a receiver receiving light emergent from the other end of said element, the fibre forming the transducer element being arranged to have electro-optical sensitivity along the direction of its axis and said receiver being arranged to given an output dependent on the magnitude of the linear birefringence (as hereinbefore defined). Conveniently the transducer comprises an optical fibre of material with its electro-optical axis along the direction of the axis of the fibre. By using a fibre as described above, the determination of the magnitude of the electric field is integrated along the whole length of the light path through the electro-toptical medium. If an electro-optical fibre is used, it will integrate the electric field between the points of different potential irrespective of the position of the fibre in the electric field. Thus it is possible to use a length of fibre which, provided its two ends are located at the points in the field between which the voltage is to be measured, will enable this voltage to be measured irrespective of any movement of the fibre or irrespective of its actual path between the two points. It will be immediately apparent that this provides a very convenient technique for determining the voltage on a conductor such as a high voltage transmission line, busbar or the like. In such a case, it would generally be convenient to have the light source and the light receiver close together and at points remote from said conductor at the same potential. To achieve this conveniently two such fibres are employed one extending from the light source to a point of high potential adjacent to the conductor and the other extending to the receiver from another point adjacent to the conductor, with reflecting means comprising two reflectors for reflecting light through 1800 from the end of one fibre into the end of the other, a 90" polarisation rotator being provided between the two reflectors so that any polarisation effect introduced by the first reflection is cancelled by the second and also so that any residual linar birefringence introduced by the first fibre is cancelled by the second fibre. Alternatively a single continuous fibre may extend from the light source around the conductor and thence to the receiver, if the fibre is fabricated to have negligible intrinsic birefringence. It will be seen therefore that, by these arrangements, the electric field is effectively integrated along its whole length to give a measure of the voltage to be determined.

Such fibres maybe made of quartz crystalline material having electro-optical sensitivity along the fibre axis and preferably with the axis of electro-optical sensitivity oriented along the fibre axis. Other crystalline materials may be used for example ammonium dihydrogen phosphate and potassium dihydrogen phosphate. Many crystalline materials exhibit electro-optical activity but large numbers of them are readily soluble in water. It is possible however to make use of such material as a crystalline core in a fibre having an outer sleeve of glass or other suitable material. More generally, the fibre may comprise a crystalline core in an outer sleeve to form a light transmission path, the crystalline material being electro-optically active with the axis of electro-optical activity oriented along the fibre axis.

Optical fibre can be classified into monomode fibres and multimode fibres. A fibre with a step index structure may be used with its parameters chosen so that it will allow propagation of one transverse mode at the chosen optical wavelength. If a multimode fibre is used, each individual propagation mode may assume an independent polarisation state and the propagating beam profile then exhibits a complex distribution of polarisation states. For these reasons, it is preferred to use a monomode fibre into which is fed light from a laser, for example a semiconductor laser diode, the light from the laser being linearly polarised.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawing which is a diagram explaining the operation of the device.

Referring to this drawing, there is shown diagrammatically a transmission line 10 which is at a high voltage with respect to a ground plane 11 and it is required to measure the magnitude of this voltage. For this purpose there are provided two electro-optical fibres 12, 13 which extend from points adjacent the conductor 10 to the ground plane. A gear laser 14, for example a helium neon laser produces a beam of light which is circularly polarised by a polariser 15 and directed into the fibre 12 at its lower end. This fibre, like the fibre 13, is formed of crystalline material arranged so that the electro-optical sensitivity is along the axis of the fibre. In a known way, the fibre has a step index structure, that is to say it has an inner core and an outer sheath of light transmissive material with differing coefficients of refraction, the parameters being chosen so that the fibre transmits one transverse mode at the optical wavelength of the laser. At the upper end of the assembly, the light is reflected from the fibre 12 into the fibre 13 by a double 45" mirror arrangement comprising two mirrors 20. 21 with a 90" polarisation rotator 22 between the two mirrors. Any polarisation effect introduced by the reflection to the mirror 20 is cancelled by that at the mirror 21 because of the 90" polarisation rotation introduced by the plate 22. The light is transmitted down the fibre 13 to a receiver comprising a quarter wave plate 24 and analyser 25 and a photodetector 26 which provides an output voltage on a line 27 dependent on the electro-optical effect and hence giving an output which is a measure of the integrated field, that is to say the voltage between the reference plane at the lower end of the fibres and the high voltage adjacent to the conductor 10.

The mode of operation of this device is as follows: The fibres are arranged so that their OX1 axes ('electric' axes) lie longitudinally, i.e.

along the propagation direction. Linearly polarised light passes up through the first fibre and there will then be introduced a phase difference between the OX2 and OX3 axes given by: Iii = it)r + We where Wr is the contribution due to the natural birefringence of quartz in that direction and 4)e is that introduced by the electric field via the electro-optic effect in the material. We also have:

where C is a constant. E is the field strength at a distance 1 along the fibre which is of length L.

The light propagation direction is then turned through 1800 by means of a double 45" reflector arrangement and any polarization effect introduced by the first reflection is cancelled by the second with the aid of the 90" rotation plate interposed between the two.

The light then re-enters the second fibre which has its crystal axes so arranged that the electro-optic effect now bears the opposite relationship to the natural birefringence when compared with the first fibre and also the natural birefringence delay is reversed in sign, i.e.: 92 = Wr + We The total phase delay is thus dependent only on the electro-optic effect and not at all on the natural birefringence. a feature which renders the delay very largely independent of temperature. Formally:

An output proportional to WT (using a conventional electro-optical detector arrangement) will thus provide a true voltage indication (VL) independently of field configuration.

Alternatively the effect of natural birefringence may be reduced by a reduction of the intrinsic birefringence of the fibre, using suitable fibre fabrication techniques to form a fibre with negligible intrinsic birefringence. In this case no polarisation rotatator would be necessary and the fibre could be continuous extending from the light source over the conductor and down to the receiver.

At the receiver the quarter wave plate inserts the appropriate polarisation bias and the analyser then converts the polarisation variation, due to the phase delay, into an intensity variation which is determined by the photo-detector.

The output from the photo-detector may be applied to an indicator, e.g. a visual indicator or to a recorder or it may be used for control purposes.

In the foregoing embodiment. a voltage measuring device has been described in which the transducers and light source in the receiver have been arranged particularly to obtain an output representative of the integrated voltage between the points at different potential. As is described in co-pending Application No. 19073/77 (Serial No. 1570802), it is possible, using extra optical transducers of this kind to apply light signals and obtain an output which is dependent on both the electro-optical effect and the magneto-optical effect and then, as described in that application, to process the signal output by determining the magnitudes in two different planes of polarisation, to obtain signal outputs representative separately of the integrated electro-optical effect and of the magneto-optical effect and thereby to measure both voltage and current. The electric field is integrated in exactly the same way as has been described above to provide the required determination of the voltage between points at different potentials.

A single continuous fibre may be used in place of the two fibres 12, 13, mirrors 20, 21 and polarisation rotator 22 of the drawing provided the fibre has negligible intrinsic birefringence. The intrinsic birefringence in fibres is mainly due to two factors. One is non-circularity of the core and the other is stress in the core. Recent improvements in manufacturing techniques have resulted in fibres now becoming available which have sufficiently low intrinsic birefringence to be usable in this way.

Claims (10)

WHAT WE CLAIM IS:

1. A voltage measuring device comprising an electro-optical transducer element in the form of fibre defining a light path, the fibre extending between points of different potential in an electric field, a light source arranged for producing a beam of polarised light for transmission through said element from one end to a receiver receiving light emergent from the other end of said element, the fibre forming the transducer element being arranged to have electro-optical sensitivity along the direction of its axis and said receiver being arranged to give an output dependent on the magnitude of the linear birefringence (as hereinbefore defined).

2. A device as claimed in claim 1 wherein the transducer comprises an optical fibre of material with its electro-optical axis along the direction of the axis of the fibre.

3. A device as claimed in claim 2 and for measuring the voltage on a conductor, by measuring the electric field produced by that voltage, wherein the light source and the light receiver are arranged close together and at points remote from said conductor which are at the same potential and wherein two fibres are employed one extending from the light source to a point adjacent to the conductor and the other extending to the receiver from another point adjacent to the conductor, with reflecting means comprising two reflectors for reflecting light through 1800 from the end adjacent to the conductor of one fibre into the end adjacent to the conductor of the other, a 90" polarisation rotator being provided between the two reflectors so that any polarisation effect introduced by the first reflection is cancelled by the second and also so that any residual linear birefringence introduced by the first fibre is cancelled by the second fibre.

4. A device as claimed in claim 2 and for measuring the voltage on a conductor wherein the light source and light receiver are arranged close together and at points at the same potential and wherein a single continuous fibre extends from the light source around the conductor and thence to the receiver, said fibre being fabricated to have negligible intrinsic birefringence.

5. A device as claimed in any of claims 2 to 4 wherein the or each fibre is made of quartz crystalline material with the axis of electro-optical sensitivity oriented along the fibre axis.

6. A device as claimed in any of claims 2 to 4 wherein the or each fibre is formed of ammonium dihydrogen phosphate. with the axis of electro-optical sensitivity oriented along the fibre axis.

7. A device as claimed in any of claims 2 to 4 wherein the or each fibre is formed of potassium dihydrogen phosphate with the axis of electro-optical sensitivity oriented along the fibre axis.

8. A device as claimed in any of claims 2 to 4 wherein the fibre comprises a crystalline core in an outer sleeve to form a light transmission path, the crystalline material being electro-optically active with the axis of electro-optical sensitivity oriented along the fibre axls.

9. A device as claimed in any of claims 2 to 8 wherein the fibre or each fibre is a monomode fibre allowing propagation of one transverse mode at the wavelength of said light source.

10. A voltage measuring device substantially as hereinbefore described with reference to the accompanying drawing.