WO2014081326A2

WO2014081326A2 - Guided optical polarimetric sensor based on lithium niobate for measuring the ac/dc electric fields

Info

Publication number: WO2014081326A2
Application number: PCT/RO2012/000026
Authority: WO
Inventors: Rudi BRATOVICH; Angelo Barberis
Original assignee: A.O.T. Advanced Optical Transducer Company S.R.L.
Priority date: 2012-08-16
Filing date: 2012-09-21
Publication date: 2014-05-30
Also published as: RO128236A0; WO2014081326A3

Abstract

The invention refers to an optical sensor for measuring the AC/DC electric fields. The suggested sensor may be used in air or in different fluids without affecting the electric field. The main industrial application of the sensor is measuring in real time the health of the HV component used in the AC/DC transmission line or in the electricity transmission substance. The optical polarimetric sensor for measuring the electric field is formed of at least a "z- cut" LiNbO₃ electro-optic crystal as sensitive optical material (10). a probe (6), with physical axes parallel with the crystal axes, whose birefringence characteristics are modulated by the electric field in contact, as well as a light source (1), an optical fibre (2) and some dielectric analysis components, including a GRIN lens (7), a polaroid (8), a retarder plate λ/4, (9), a dielectric mirror (12) and a photo diode (5) protected by a dielectric case (13, 13'). The device also includes a Y derivation (3) for transmitting a light source to the probe T (6) and from it to the photo diode (5), coupled to an optical fibre (2) coupling (4), and inside the probe (6) the electro-optical crystal (10) is framed by the polaroid (10) and the retarder plate λ/4, (9), of the one part and the retarder plate λ/8, (11) and the dielectric mirror (12) of the other part; in order to align the light beam wit this analyzer assembly, the GRIN lens (7) with attached optical fibre (2) is introduced in a cased block (14) aligned with the polaroid (8). As part of a real invention example, the GRIN lens (7) with attached optical fibre (2) is fixed in a dielectric sphere (15) perforated axially, that can rotate between two plates (16) and (17) fixed in the cased block (14).

Description

GUIDED OPTICAL POLARIMETRIC SENSOR BASED ON LITHIUM NIOBATE FOR MEASURING THE AC/DC ELECTRIC FIELDS

The invention refers to an optical sensor for detecting the electric field - of alternating or continuous current - and measuring its intensity by using the Pockels effect which consists of generating variable birefringence by an electro-optic crystal through a variable electric field applied to it. The sensor in question may be used in air, as well as in different fluids without affecting the parameters of the respective field. The main industrial application is measuring in real time the health of the HV component used in the AC/DC transmission line or in the electricity transmission substance.

The defects of the electricity transmission line are in general caused by the defects of the HV component, thus these shall be checked periodically. Several techniques have been developed and applied over the years for identifying a HV component defect of the electricity transmission line:

the simplest and the most direct is the visual and acoustic identification, which in the presence of electric discharges or the corona defect it is possible to detect the IR/UV electromagnetic waves or the acoustic waves;

The second technique is that based on measuring the electric field around the HV component;

The visual and acoustic identification of HV line defects may only identify the advanced deterioration states and does not allow to identify initial degradations, which are of extreme importance.

The identification of the electric field around the HV component allows measuring, preventing discharge events and identifying abnormal variations in the electric field due to the visible internal defects. The electric field can be measured with an electric or optic sensor.

The disadvantage of the electric sensor is due to its metallic components which modify the distribution of the measured electric field, its large size and the fact that it cannot differentiate between different components of the electric field. An optical sensor is usually fully dielectric, does not affect the measured field, is small and has a large broadband width response.

Different electric field measuring devices with an electro-optic crystal are available, just like the one described in document: JP2000105256 A.

It is formed of a polarimetric sensor with optical fibber and electro-optic crystal Pockels type element, polarizer, retarder plate λ/4 and analyzer. Also, document US2009/0066952A1 , f^'ig.2, also presents an electric field measuring sensor with electro-optic crystal based on lithium niobate, which also includes a laser type light source, an optical fibre, a polarizer, a quarter wave retarder plate, a GRIN (gradient index) lens, a dielectric mirror, a photocell and an oscilloscope. The laser beam is transmitted from the analyzer block of the sensor to its head, which includes a GRIN lens, a quarter wave retarder plate, the electro-optic crystal and a dielectric mirror that reflects the laser ray, modulated inside the electro-optic crystal on an external electric field, back towards the analyzer block - through the head of the sensor, where it is converted into an electric signal through a photocell after it passes the polarizer.

The disadvantage of this device is the fact that it has an insufficiently compact assembly and is less reliable in case of accidental mechanical strains and less comfortable to be used in unusual conditions, for e.g. for measuring the electric field of an electricity conductor in a liquid environment such as oil or water.

The technical issue solved by this invention foresees the execution of an optical polarimetric sensor for measuring the electric field having at least a LiNb0₃ electro- optic crystal as sensitive optical material, with a light source, optical fibre and dielectric analysis components chosen and placed in such a way in order to form a compact and reliable assembly, with correctly aligned elements, that can be easily fine tuned, but which can be also used in unusual situations, for e.g. for measuring the electric field of an AC/DC electricity conductor in a liquid environment such as oil or water.

According to the invention, the optical polarimetric sensor for measuring the electric field based on at least a LiNb0₃ "z-cut" electro-optic crystal as sensitive optical material, eliminates all the above disadvantages and solves this technical problem through the fact that it is formed of a light source, an optical fibre and some dielectric components of an analysis probe and a photo diode, all protected by a dielectric case. Moreover, the sensor also includes an Y derivation for transmitting the light beam towards the analysis probe and from this towards the photo diode, coupled to an optical fibre coupling. Also, inside the analysis probe, the electro-optic crystal is framed by a polaroid and a λ/4 retarder plate, of the one part and a λ/8 retarder plate and a dielectric mirror of the other part; the light beam enters this analyzer assembly through a GRIN lens with an attached optical fibre and is introduced into a block aligned with the polaroid of the analysis probe. In a specific example of performing the invention, the GRIN lens with attached optical fibre is fixed in a dielectric sphere axially perforated that can rotate between two plates fixed in the housing block after adjacency of the alignment, the dielectric sphere being blocked between the two plates with three screws. The housing has some circular brackets where the Polaroid, the retarder plate λ/4, the retarder plate λ/8 and the dielectric mirror are fixed; all these steps are made in order to accurately rotate and orientate these components during assembly.

The optical, polarimetric sensor, equipped with a manoeuvrable probe and manufactured entirely out of dielectric material, according to the invention, has the following main advantages:

- it is small, manageable and has a linear amplitude response;

- it can be used in the air or in different fluids without affecting the electric values of the field measured;

- the dielectric construction of the sensor allows it to be used in a high voltage environment without compromising the safety of its use in the presence of a high voltage conductor;

- the directivity of the adjustment makes it possible for the components of the electric field to be measured in turn, thus making it possible to trace the distribution of the electric field;

-by analyzing the variation between the field profile of the intact component and that of the damaged component it is possible to detect the exact location of the damage, and this allows this technique to be used in checking online the status of the high voltage component;

- the directivity of the sensor allows thorough study of the effect the defects have on the electric field distribution;

- the probes of the proposed sensor are waterproof, allowing measurement of the electric field in fluids or gas, this function extends the application field, for example, to the measurement of the electric field of some components immersed in dielectric oil or gas;

- it offers the possibility to measure the electric field of an AC or DC or the electric field generated by the presence of electric charge on dielectric materials: this measurement makes it possible to characterize the dielectric material and charge degree of static electricity. The optical, polarimetric sensor in accordance with the invention is shown in detail below, in several variants of execution, in connection with Figures 1-12 which represent: Fig. 1 : - Schematic representation of the optical sensor for measuring the electric field.

Fig. 2: - Overview of the probe elements by orienting its components.

Fig. 3: - View by orienting the LiNb0₃ crystal inside the probe.

Fig. 4: - Cross view of the first suggested probe with cell-case.

Fig. 5: - Overview of the probe from Figure 4.

Fig. 6: Cross view of the second probe and view of the cap;

Fig. 7; Longitudinal section through the probe, with a lens mounted in a dielectric tube; Fig. 8: Probe from Figure 6, without radiation polarization and analysis components. Fig. 9: Figure 6 and 8 probe alignment system, different from Figure 7.

Fig. 10: Overview of the Figure 4 probe, with spacing.

Fig. 11 : Cross view of the third suggested probe, without the upper cap and the left side of the case.

Fig. 12: Top view of Figure 1 probe.

The scope of the invention is an optical polarimetric sensor for measuring the electric field, where the sensitive optical material is a LiNb0₃ electro-optical crystal: the birefringence characteristics of the crystal are modulated by the electric field in contact and are measured through the configuration illustrated in Figure 1 and Figure 2 which translates the optical polarization variation into optical intensity. The LiNb0₃ crystal is a "z-cut" crystal with physical axes parallel with crystal axes (Figure 3).

Figure 1 illustrates the system schematic: it is formed of a light source 1 , an optical fibre 2, a Y derivation 3 coupled to an optical coupling 4 of the optical fibre 2, a photo diode 5 and a probe 6. The role of the Y derivation 3 is to transmit the light source towards probe 6 and from it towards the photo diode 5. The depolarized light of low coherence generated by source 1 propagates through a standard monomode or through a monomode that curves the insensitive optical fibre 2 passing through the coupling 4 and reaching the probe 6 (Figure 2). The intensity of the light modulated by probe 6 returns through the 2^nd fibre and is measured by the photo diode 5. Probe 6 is a sensor probe where the light is modulated by the electric field and directly demodulated by the configuration that transforms the modulated polarization into the intensity of the signal measured by the photo diode 5.

According to the invention, the sensor has probe 6 from Figure 2 formed of a GRIN lens 7 with optical fibre 2, a polaroid 8, a retarder plate λ/4 9, (inhibitor), an LiNb0₃ electro-optic crystal 10, or two such crystals, a retarder plate λ/8 11, and a dielectric mirror 12. Light leaving fibre 2 is collimated by GRIN lens 7 and linearly polarized by polaroid 8 vertically oriented. The light linearly polarized with vertical vector E, which spreads through plate 9 oriented at 45 ° has become circularly polarized. Subsequently the polarization of the circularly polarized beam is modulated by the birefringence of the electro-optical crystal 10 induced by the electric field. The polarization leaving the electro-optical crystal 10 makes a dual-pass through the retarder plate λ/8 11 vertically oriented and a reflection on the mirror 12. The retarder plate λ/8 11 is used to create a fixed birefringence to force the configuration to function in the linear area. The beam then propagates for a second time through the electro-optical crystal 10 and through the retarder plate λ/4 9. After the second propagation through the retarder plate λ/4 9, the elliptical polarization becomes linear and the orientation of this linear polarization is modulated by the birefringence of the electro-optical crystal 10 induced by the external electric field.

This 45 ° orientation for the null electric field depends linearly on the delay between the specific status of the electro-optical crystal 10, which is modulated by the electric field.

Then the polarization modulated linearly is analyzed by polaroid 8 vertically oriented: through fibre 2 the light then reaches the photodiode 5 where its intensity is measured.

The measurement is not influenced by the intrinsic or induced birefringence of the optical fibre 2, because the light generated by source 1 is depolarized and because in the signal that returns information is contained only in its amplitude.

The formula that expresses the variation of the signal according to the delay of the status specific to electro-optical crystal 0 is:

5(t) = ^cos²(^ + 45°)

where

is the phase induced by E_Y with a LiNb0₃ crystal of length L and with dimensional characteristic D_y: the electric field E_Y is the field impinging the crystal parallel to the Y crystallographic axes indicated in Figure 3. In Figure 3 is indicated the orientation of the electro-optical crystal 10 with z-cut inside the setup in Figure 2: the light propagating parallel to Z axes with null electric field don't experience birefringence because the X e Y crystallographic axes have the same index refractions. The configuration from Figure 2 is sensitive only to the electric field parallel with Y axes, because it measures only the birefringence induced into the crystal by E_Y: the sensitivity of the configuration is maximum for E_Y, 7% for E_x, and zero for E_z.

In order to deduce the phase 3(t) and calculate E_Y using the S(t) signal it is possible to use the following report:

where A/2 is the value of the null E_Y signal. The presented sensor is not influenced by the temperature because it does not have components that depend on the temperature. The light beam propagated by the Z axes of the electro-optic crystal 10 does not measure the birefringence variation depending on the temperature, because the X and Y crystal axes have the same refraction index: the two refraction indexes are equal and modify their value with regard to the temperature variation. The other components of the Figure 2 configuration that may be sensitive to temperature are the λ/4 , 9 and λ/8, 11 retarder plates: the temperature effect is limited by using "zero order" retarder plates.

The case suggested for the probe must also be insensitive to temperature because the alignment between the lens 7 and the mirror 12 must be maintained for different temperatures: for this reason, case 13 must be made from a dielectric material with a very low thermal dielectric coefficient such as MACOR (machinable glass-ceramic), alumina or quartz.

Another inventive element of the invention is the suggestion of a different case type 13 for the probe. Figure 4 and Figure 5 include the drawing of the first case 13 suggested for the Figure 2 configuration: is used by a quartz cell.

In Figure 4 the probe is closed and the components are introduced sequentially: all the components must be dimensioned by observing the size of the cell's internal case 13 and the direction required for setting from Figure 2. The scheme of Figure 5 is the collapsed view of Figure 4: the 7 lens element with attached fibre 2 is introduced in a cased block 14 that can be easily moved in order to aid the alignment: if the alignment is correct, lens 7 and the cased block 14 are fixed together in the cell's case 13.

The inventive element, with inventive character of the invention is the use of a component with a single cased block 14 in Figure 4 scheme, obtained by fixing together all the components in an adequate way. An inventive element of the invention is also the second variant of the proposed housing 13' from Figure 6, Figure 8 and Figure 9. This housing 13' simplifies the probe design, allowing the inclusion of all components inside, including the housing block 14 in Figure 8. The sketch from Figure 9 presents the proposed system of alignment between lens 7 and mirror 2 of setting Figure 6: the element lens 7 with the attached fibre 2 is introduced in a dielectric sphere 15 axially perforated, which can spin around two small plates 16 and 17, pinned in the encased block 14. After the alignment is made, the dielectric sphere 15 can be blocked between the two small plates 16 and 17 with three screws a. In Figure 6, case 13' has to be made of the same material as the chassis and the sealing must be waterproof.

Another ingenious element of the invention is the use of a massive component in order to prevent a possible error occurring when the Polaroid plate 8 and the retarder plate λ/4, 9, (inhibitor) are oriented. This massive component consists of a Polaroid plate and an inhibitor plate correctly aligned and conjoined.

The purpose of the invention is to use a massive component that includes all the aligned and combined components which are introduced in the housing from Figure 6. In order to measure a DC current electric field it is mandatory for crystal 10 of LiNb0₃ to have a greater value of the equilibrium time:

G

where G is the volume of the crystal's electrical conductivity, and εο and ε_Γ are the vacuum dielectric constant and the crystal relative dielectric constant, respectively. The nominal value of the equilibrium time for crystal LiNb0₃ is 7x10⁶ seconds.

Because the τ value of the electro-optical crystal 10 from LiNbO-3 is usually smaller than the expected nominal value, another subject of the invention is the thermal treatment of the crystal to regain its nominal τ value. The LiNb03 crystal's thermal treatment of annealing is made in an oven by heating it approx. 2 min until it reaches 400°C, keeping the crystal inside for 6 hours and cooling it off inside the oven, in about 2 hours. Housing 13 from Figure 6 and Figure 7 allows the extraction of crystal 10 without deflecting from the alignment of the setting in order to perform the thermal treatment. The τ value of the LiNb0₃ crystal is influenced by the prolonged exposure to IR radiations or by the manipulation provided by the assemblage of housing 3. An ingenious element of the invention is the use of a film strip around the crystal in order to maintain the nominal characteristic of the equilibrium time, protecting it from IR radiations. Another inventive element of the invention is using a perforated spacer 18.d, d' between the components of probe 6, such as in the drawing from Figure 7 and 10, in order to eliminate the eventual interferences caused by the reflections of different components. This characteristic can also be used to insert a space between the components when these are jointed.

Another inventive element is the case 13' from Figure 11 and from the top view from Figure 12 where the Polaroid 8, the retarder plate 9, the retarder plate 11 and the mirror 12 are mounted in a circular support 19.b, c, e and respectively f, that can be rotated ensuring the correct orientation of all the components during assembly.

All the issues related to the optical sensor according to the invention are the following:

Sensitivity: the suggested optical sensor is designed to measure the increased electric field, generated by the HV component. In order to increase the sensitivity, it is indispensable to modify the configuration, the optical reception system and the construction of the probe.

The temperature of the crystal: if the LiNb0₃ crystal is exposed to high temperatures, it shall lose its characteristics to measure the DC electric field; a thermal treatment is required in order to recover its characteristics. However, this phenomenon does not occur within the temperature interval.

Installation: for the monitoring to occur, the sensor must operate close to the HV component. In order to allow an automated measuring, the sensor must be moved by an automated system.

Claims

1. The optical, polarimetric sensor for measuring the electric field, having as sensitive optical material at least an electro-optical crystal (10) of LiNb03 "z-cut" type of a probe (6) with physical axes parallel to the crystallographic axes, whose birefringence characteristics are modulated by the electric field that it encounters, and a source (1) of light, an optical fibre (2) and some dielectric components of analysis, including a GRIN lens (7), a polaroid (8), a retarder plate λ/4, (9), a dielectric mirror (12) and a photodiode (5) protected by a dielectric housing (13, 13'), characterized by the fact that it also includes a Y-junction (3) for transmitting the light beam to probe T (6) and from this to the photodiode (5), coupled to a coupling (4) of the optical fibre (2), and inside the probe (6) the electro-optic crystal (10) is placed between the Polaroid (8) and the retarder plate λ/4, (9), on one side and by the retarder plate λ/8, (11) and the dielectric mirror (12) on the other, in order for the light beam to be aligned with this whole analyzer assembly, the GRIN lens (7) with the attached optical fibre (2), being introduced in a housing block (14) aligned with the polaroid (8).

2. The optical, polarimetric sensor, according to claim 1, is characterized by the fact that, the GRIN lens (7) with attached optical fibre (2) is fixed in a dielectric sphere (15) axially perforated that can rotate between two plates (16) and (17) fixed in the housing block (14), after adjacency of the alignment, the dielectric sphere (15) being blocked between the two plates (16 and 17) with three screws (a).

3. The optical, polarimetric sensor, according to claim 1 or 2, is characterized by the fact that, housing (13, 13') has some circular brackets (19.b, c, e and f) in which the polaroid (8), the retarder plate λ/4 (9), the retarder plate λ/8 (11) and the mirror (12) are fixed in order to accurately rotate and orientate these components during assembly.

4. The optical, polarimetric sensor, according to claim 1 , 2 or 3, is characterized by the fact that, between the probe's components (6), a drilled spacer is placed (18.d, d').

5. The optical, polarimetric sensor, according to claim 1 , 2 or 3, is characterized by the fact that, the housing (13, 13') is made out of dielectric material with a very low thermal expansion coefficient.

6. The optical, polarimetric sensor, according to claim 1 , 2, 3 or 4, is characterized by the fact that, the electro-optic crystal (10) of LiNb03 is thermally treated by heating it approx. 2 min until it reaches 400°C, keeping the crystal inside for 6 hours and cooling it off in 2 hours.

7. The optical, polarimetric sensor, according to claim 1 , 2, 3, 4 or 5, is characterized by the fact that, the electro-optic crystal (10) is surrounded by a film strip protecting it against IR radiation.