Abstract:
A high voltage measuring device is disclosed for measuring the voltage between two spaced-apart points using a poled optical fiber that is extended between the points and subjected to an electric field associated with the voltage. A control unit of the device has a light source, which transmits light into the fiber. The device also has a detecting unit for receiving light transmitted through the fiber and for generating a signal (S out ) corresponding to a change in refractive index of the fiber caused by the electric field.

Description:
TECHNICAL FIELD 
     The invention relates to a high voltage measuring device for measuring the voltage between two spaced-apart points. 
     BACKGROUND ART 
     High voltage measuring devices for use in power transmission and power distribution plants usually consist of electromagnetic voltage transformers or capacitor voltage transformers (capacitive voltage dividers). Both types are fairly expensive, heavy and bulky, largely due to the high insulation requirements. 
     Optical voltage transformers are also known. These transformers are all based on point sensors and require complicated high voltage insulation. The measurement is either of the full voltage, requiring advanced high voltage insulation, or at a reduced voltage level, requiring complicated electrode arrangements or capacitive voltage dividers. In all these cases the required high voltage insulation results in bulky and heavy voltage transformers. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to obtain a voltage measuring device of the kind referred to above, which offers a high measuring accuracy together with a simple design and low price, low bulk and weight, and easy installation. 
     The measuring device according to the invention measures a voltage as a line integral of the electric field. It uses a poled optical fibre extending between the two points, the voltage between which is to be measured, such as between a power line and ground. Light is transmitted through the fibre. The electric field at any part of the fibre causes a phase shift of the light, and the phase shift will be proportional to the field. The total voltage is measured as a line integral of the electric field by measuring the accumulated phase shift along the whole length of the fibre. 
     What characterizes a voltage measuring device according to the invention will be clear from the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail in the following with reference to the accompanying drawings  1 - 5 . 
     FIG. 1 shows a section through a measuring device according to the invention, using a transversely poled fibre. 
     FIGS. 2 a  and  2   b  show the coordinate system used in explaining the function of the device of FIG.  1 . 
     FIG. 3 a  shows in greater detail the arrangement of the optical fibres of the device of FIG.  1  and their connection to a light source and an optical detector, FIG. 3 b  shows the output from some of the fibres, and FIG. 3 c  shows the signal processing equipment in greater detail. 
     FIG. 4 shows an embodiment where a poled helical core fibre is used. 
     FIGS. 5 a  and  5   b  show an embodiment where a series of Bragg gratings are written into the poled fibre. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a device according to the invention. It is arranged for measuring the voltage between ground and a high voltage power line or bus bar  1 . The device has metal end plates  2  and  3 , which are provided with lugs  4  and  5  for mounting the device and for connecting it electrically to the line or bus bar and to ground. The device has a central rod or tube  6  of a fibre reinforced polymer material, such as glass fibre reinforced epoxy. A poled fibre  7  is wound around the rod and forms a helix with a constant pitch angle. A second, unpoled, fibre  8  is also helically wound around the rod and forms a helix which is interleaved with the helix formed by the fibre  7 . Fibres  7  and  8  are wound in parallel in order to achieve similar stress and temperature conditions for the two fibres. 
     At the lower end of the device the two fibres  7  and  8  are connected to the first and second leads of a so-called 2×2 fibre-optic coupler  10 . A fibre  9  is connected to a third lead of the coupler and supplies linearly polarized light to the two fibres from a control unit  15 . At the upper end of the device the two fibres are connected to two of the leads of a 3×3 optical coupler  11 . The light from the two fibres interferes and is coupled into the three output arms of the 3×3 coupler, supplying three phase-shifted output signals through fibres  12   a ,  12   b ,  12   c  to the control unit  15 . 
     The device according to the invention is intended for high voltage use, that is, for use at voltages from about 1 kV and upwards. It is primarily intended for use in electrical power transmission and power distribution systems, and for use up to the highest voltages used in such systems. To obtain the necessary mechanical protection of the fibres, to obtain a sufficient leakage path length, and to obtain the necessary protection against precipitation and pollution for outdoor use, outer layers  13  of polymeric materials are applied outside the fibres and provided with sheds  14 . 
     The poled fibre  7  is a silica fibre which is poled in its transverse direction. The poling of the fibre may be produced by means of UV-excited poling in the presence of a strong electric field in the manner described in the paper “Electro-Optic Effect Induced by UV-excited Poling in a Silica Fibre” by T. Fujiwara et al, Electronic Letters  31 , pp 573-575, 1995. 
     A poled fibre exposed to an electric field, which has a component E p  in the poling direction of the fibre, experiences a change Δn in the index of refraction for light transmitted through the fibre. The relation between Δn and E p  is given by 
      Δn=rE p   (1) 
     where r is the electrooptic coefficient of the poled fibre. It has been shown that values of r of about 5 pm/V may be obtained. 
     In the device of FIG. 1 the poled fibre is mounted in a helix with a known and constant pitch angle ξ. FIG. 2 a  shows an element of length ds of the fibre  7 . In the orthogonal coordinate system used in the drawing, 
     the z-axis is coincident with the longitudinal axis of the rod  6 , 
     R is a radius in the x-y-plane from the z-axis to the fibre element ds, 
     Θ is the angle between radius R and the x-axis, 
     z′—z′ is a line through the fibre element and parallel to the z-axis, and 
     ξ is the pitch angle of the helix, that is, the angle between the fibre element and the line z′—z′. 
     Assuming that the electric potentials at the upper and lower ends of the helix are V h  and V o respectively, the voltage to be measured by the device is 
     
       
         ΔV=V h −V o   (2) 
       
     
     The voltage measurement is achieved by sending light through the poled fibre. Because of the field-dependent index of refraction of the fibre, the light will be phase-shifted by each element of length of the fibre, and the phase shift will be proportional to the electric field component in the poling direction of the element. As will be shown below, the accumulated phase shift Δφ along the whole length of the fibre will be proportional to the voltage ΔV. 
     The light source would normally comprise a source of sufficiently coherent light and, thus, would normally comprise a laser or a near-lasing device such as a superluminescent diode. 
     The electric field E will have varying direction and magnitude at different parts of the fibre. The change in index of refraction in the fibre may be assumed to be a function of z 
     
       
         Δn(z)=rE p (z)  (3) 
       
     
     The electrooptic coefficient r is independent of z, as the poling is constant along the length of the fibre. The change in refractive index in an element of length ds causes a phase shift dφ which is also a function of z 
     
       
         dφ=dφ(z)  (4) 
       
     
     which may be written as 
     
       
         dφ=(2πΔn/λ)ds  (5) 
       
     
     where λ is the wavelength of the light used for the measurement. 
     FIG. 2 b  shows the fibre element of length ds viewed along radius R inwards toward the axis of rod  6 . The line t—t is an element of the curve or arc of the rod in the x-y-plane, and p is the poling direction of the fibre. From FIG. 2 b  it is seen that 
     
       
         ds=(1/cos ξ)dz  (6) 
       
     
     and 
     
       
         ds=(1/sin ξ)RdΘ  (7) 
       
     
     The resultant E p  in the poling direction of the electric field components E x , E y  and E z  is given by 
     
       
         E p =E z  sin ξ+cos ξ(E x  cos Θ+E y  sin Θ)  (8) 
       
     
     By combining (3) and (5) dφ can be expressed as a function of E p (z) 
     
       
         dφ(z)=(2π/λ)rE p ds  (9) 
       
     
     which, when combined with (8), gives 
     
       
         dφ(z)=(2π/λ)r[E z  sin ξ+cos ξ(E x  cos Θ+ E y  sin Θ)]ds  (10) 
       
     
     The totally accumulated phase shift for the light transmitted through the fibre is 
     
       
         Δφ=ƒdφ  (11) 
       
     
     which, if E x  and E y  may be regarded as constant during one turn of the helix, will equal 
     
       
         Δφ=(2π/λ)rƒE z  tan ξdz  (12) 
       
     
     If E x  and E y  are not constant, the result (12) is only approximately correct. The accuracy of the approximation will increase as the pitch angle of the helix is increased, and for this reason it is to be preferred to use a long fibre wound many turns with a large pitch angle. The accuracy of the approximation is also increased with reduced radius R. 
     Under the assumption above (E x  and E y  constant during each turn) the voltage drop ΔV along the axis of the helix is the line integral of E z  from V h  to V 0   
     
       
         ΔV=ƒE z dz  (13) 
       
     
     By combining (12) and (13) the totally accumulated phase shift along the fibre is obtained as 
     
       
         Δφ(2π/λ)r tan ξΔV  (14) 
       
     
     The accumulated phase shift is thus proportional to the voltage ΔV between the ends of the helix, that is, proportional to the voltage to be measured. 
     FIG. 3 a  schematically shows how the fibres of the measuring device of FIG. 1 are connected to the control unit  15 . This unit has a light source  16  in the form of a laser diode, which emits monochromatic and linearly polarized light into the fibre  9  which transmits the light to the fibre-optic 2×2-coupler  10   a  and thus to the lower ends of the poled fibre  7  and the unpoled fibre  8 . An isolator  10   b  can be provided to prevent reflections back into the light source. If the degree of polarization of the light source is not sufficiently high, a polarizer  10   c  may suitably be included. At the upper end the light from the fibre  7  will be phase-shifted an amount which corresponds to the voltage between the upper and lower ends of the fibres. The light arriving at the upper end of the unpoled fibre  8  will not be subject to any voltage-dependent phase shift. The interfering light from the two fibres is coupled by the 3×3 coupler  11  into the three unpoled fibres  12   a ,  12   b ,  12   c  which conduct the light down to a signal processing unit  17 . This unit transforms the total voltage induced phase shift Δφ into an output signal S out  which corresponds to the measured voltage ΔV. 
     The use of the 3×3 coupler  11  prevents signal fading and also increases the dynamic range. The output powers from the three arms of the 3×3 coupler are ideally three sinusoids with a 120° phase shift between them as shown in FIG. 3 b , where S a , S b  and S c  are the signal outputs from the three output arms of the coupler. As a consequence of this it will be possible to maintain maximum sensitivity for all output levels by working only on the linear parts of the sinusoids while keeping track of which of the sinusoids that is currently used. 
     FIG. 3 c  shows an example of the signal processing equipment. The equipment shown is the subject of Australian provisional patent application PN 3292, filed on May 31, 1995. The three outputs from the fibres  12   a ,  12   b ,  12   c  connected to the 3×3 coupler  11  are fed into photodetectors  28   a ,  28   b ,  28   c  with their following amplifiers  29   a ,  29   b ,  29   c . The amplifier output signals are converted into digital form in analog/digital converters  30   a ,  30   b ,  30   c . The digital output signals from the converters are supplied to a magnitude comparator  31  and to a switch  36 . The magnitude comparator determines which of the three signals that is exhibiting maximum sensitivity at any one time, and the comparator controls the switch  36  so that this maximum sensitivity signal is routed to an inverse sine lookup table  34 . The output from this table is the fine voltage value corresponding to the position within the current fringe. If the magnitude comparator  31  determines that a switch should be made to a different output signal, a phase lookup table  32  is addressed, and the appropriate phase constant as found in the table is supplied to an accumulator  33 . As indicated by the magnitude comparator, when the magnitude comparator switches between output signals, the accumulator will add to or subtract from its contents the appropriate value as obtained from the phase lookup table. The accumulator thus provides a coarse voltage value, and this value is added to the fine voltage value obtained from the inverse sine lookup table  34  in an adder  35 . The output from the adder is the signal S out  which corresponds the voltage to be measured. 
     FIG. 4 shows an alternative embodiment of the invention. It uses a helical core poled fibre  7 ′ instead of the transversely poled fibre. A helical core fibre has a core which follows a helical curve in the manner described by Birch RD, Electronics Letters 23, pp 50-52, 1987. If the core is poled with the poling field having a significant component in parallel with the fibre axis, the helical form of the core will result in a similar effect as with the transversely poled fibre wound in a helix and discussed above. The result will be that the electric field component in the z direction will have a component in the poling direction of the core. This gives the desired field-dependent index of refraction, and the accumulated phase shift along the fibre will correspond to the voltage to be measured in the same manner as that described above. The fibres  7 ′,  8 ,  12   a ,  12   b ,  12   c  may follow straight lines. The device shown in FIG. 4 is suitably provided with a protective outer layer with sheds (such as layer  13  of FIG.  1 ). 
     An alternative embodiment of the invention uses a fibre poled so that its poling direction has a component in the longitudinal direction of the fibre. If the light in the fibre is presupposed to have a significant longitudinal field component, the fibre will be able to sense an electrical field which is parallel to the longitudinal direction of the fibre, and such a poled fibre may be disposed in a straight line in the same manner as that shown in FIG.  4 . 
     Another alternative embodiment of the invention is to employ a poling direction which at a point a distance z along the fibre axis is at a fixed angle less than 90° to the fibre axis, but rotates about that axis as z increases. The response of the fibre to transverse components of the external electric field averages (because of the rotation) to near zero, leaving substantially only response to the external field component parallel to the fibre axis. A fibre poled in this way may be disposed in a straight line in the same manner as that shown in FIG.  4 . 
     FIG. 5 shows still another embodiment of the invention. FIG. 5 a  shows an element of an optical fibre  20  with a core  21 . The fibre is poled in the transverse direction. As shown in the drawing, a plurality of alternate closely spaced regions  23  are poled while the intermediate regions  22  are left unpoled. The poling may be made by irradiating the fibre by UV-light through a suitable mask while subjecting the fibre to a strong transversal electrical field. The regions  23  will have an index of refraction n 2  which is dependent on the external electric field, and which is in general different from that—n 1 —of the unpoled regions  22 . The regions  22  and  23  form a Bragg grating. A plurality of such gratings are distributed along the length of the fibre. The centre wavelength (the wavelength of maximum reflection of light arriving at each grating through the fibre) will shift in response to an external electric field, and the wavelength shift of the reflected light will then be a measure of the change of the local external electric field. 
     FIG. 5 b  schematically shows how the fibre  20  is disposed in a helix between two points at potentials V h  and V o  for measuring the voltage ΔV between the points. A light source  25  emits light into the fibre through a fibreoptic 2×2 coupler  24 . The light reflected from the series of Bragg gratings is coupled through a fibre  26  into unit  27 , which by means of suitable signal processing decodes the wavelength shift into an output signal S out  corresponding to ΔV. 
     The gratings may be dimensioned so that their zero-field centre wavelengths are different and their reflection bands non-overlapping. The light source  25  may then be a single broadband source for interrogating the gratings (and thereby the corresponding local electric fields). 
     In devices according to the invention it is preferable to use highly birefringent fibre, such as bow tie fibre, in order to reduce polarization changes caused by mechanical stress and temperature effects on the fibre. 
     Additional information may be obtained by employing both polarizations of guided light, and by employing several different wavelengths. Such additional information may be used (by means of suitable signal processing) to reduce or eliminate the contribution of unwanted components of the electric field. 
     The device according to the invention may be used for measuring both ac and dc voltages. 
     In the embodiments described above, the fibres, their support and the surrounding protective layers with sheds etc. form a self-contained entity. Alternatively the fibres and their support may be arranged in the interior of a support insulator or suspension insulator, which is used for supporting some kind of electric apparatus. The voltage measuring device according to the invention may also be used as a support insulator for other apparatus.