RO128236A0 - Controlled polarimetry optical sensor based on lithium columbate for measuring ac and dc electric fields - Google Patents

Abstract

The invention relates to an optical sensor for measuring alternating and direct current electric fields. According to the invention, the sensor consists of a light source (1) which emits a depolarized coherence light which is propagated, through an optical fiber (2) and a coupling (4), up to a probe (6), the intensity of the light modulated by the probe (6) being measured by a photo diode (5) coupled to the optical fiber (2) by means of a Y-shaped derivation (3), where the probe (6) comprises an electro-optical crystal (10) of "z-cut" type LiNbO, with physical axes parallel to the crystallographic axes, whose birefringence characteristics are modulated by the electric field they contact, the crystal (10) being confined, on the one hand by a polaroid (8) and a λ /4 delay plate (9), and on the other hand, by a λ /8 delay plate (11) and a dielectric mirror (12), and for aligning the light beam to the analyzing assembly which forms the probe (6), a GRIN lens (7), which is coupled to the optical fiber (2), is introduced into a unit (14) aligned to the polaroid (8).

Description

OPTICAL, POLARIMETRIC, DIRECTED, LITHIUM-BASED POLYMETRIC SENSOR

FOR MEASUREMENT OF ELECTRIC FIELDS AS AND DC

The invention relates to an optical sensor for detecting the electric field - alternating or direct current - and measuring its intensity by using the Pockels effect which consists in the production of variable birefringence by an electrooptic crystal by means of a variable electric field applied to it. The proposed sensor can be used both in air and in various fluids without affecting the parameters of that field. The main industrial purpose of the sensor is for real-time measurement of the health of the high voltage component used in the AC and DC high voltage power transmission line or in the power transmission substation.

Defects in the power transmission line are generally caused by defects in the high voltage component, so they will need to be checked regularly. Over the years, several techniques have been developed and applied to detect a defect in the high voltage component of the power transmission line:

the simplest and most direct is the visual and acoustic detection, which in the presence of electric discharge or corona effect detects emissions of electromagnetic IR / UV waves or acoustic waves;

the second technique is based on measuring the electric field around the high voltage component.

The visual and acoustic detection of defective cmor of the high voltage line can only detect the advanced states of damage and does not allow the detection of the initial signs of degradation, a fact of extreme importance, which is easy to understand.

The detection of the electric field around the high voltage component allows the measurement, the prevention of the discharge event and the detection of the abnormal variation of the electric field profile due to the invisible internal defects. The electric field can be measured by means of an electric or optical sensor.

The electric sensor has the disadvantage that, due to its metal components, it changes the distribution of the measured electric field, it is large in size and cannot differentiate between the different components of the electric field. An optical sensor is usually completely dielectric, does not affect the measured field, is small in size and has a high bandwidth response.

Various electric field measuring devices are known by means of an electro-optical crystal, such as the one described in document JP2000105256 A. It ^ -2012-00602-1 6 -08- 2012 has a polarimetric sensor with optical fiber and Pockels crystal type element electro-optical, polarizer, pl / 4 delay plate and analyzer.

Also, the document US2009 / 0066952A1, fig.2, also presents a sensor for measuring the electric field with crystal · electro-optical, in particular, of lithium niobate, which also consists of a laser light source, a fiber optics, a polarizer, a quarter-wave delay board, a GRIN (gradient index) lens, a dielectric mirror, a photodetector and an oscilloscope. The laser beam is transmitted from the sensor block of the sensor to its head, which includes a GRIN lens, a quarter-wave delay plate, the electro-optical crystal and a dielectric mirror reflecting the laser beam, modulated inside the electro-optical crystal by a external electric field, back to the analyzer block - through the sensor head, where it is converted into an electrical signal by means of a photodetector after passing through the polarizer board.

This device has the disadvantage that it makes an insufficiently compact assembly and is less reliable in case of accidental mechanical stresses and less convenient to use in unusual conditions, for example for measuring the electric field of a current conductor in a liquid environment such as oil or water.

The technical problem solved by the present invention consists in the realization of a polarimetric optical sensor for measuring the electric field having as minimum sensitive optical material an electro-optical crystal of LiNbOs, with light source and optical fiber and dielectric analysis components thus chosen and arranged so as to form a compact and reliable assembly, with all the elements correctly aligned, which at the same time allows the necessary fine-tuning adjustments to be made in a simple and easy way but which can also be used in unusual conditions, for example-for measuring the electric field of a alternating or direct current conductor in a liquid medium such as oil or water.

The polarimetric optical sensor for measuring the electric field, having as minimum sensitive optical material an electro-optical crystal of LiNbO ₃ type z-cut, according to the invention, eliminates all the above disadvantages and solves this technical problem by being composed of a source light, an optical fiber and some dielectric components of an analysis probe and a photodiode, all protected by a dielectric housing. In addition, the sensor according to the invention further comprises a Y-branch of light beam transmission to the analysis probe and from there to the photodiode, coupled to an optical fiber coupling. Also, inside the Analysis Probe, the electro-optical crystal is framed by a polaroid and a delay plate Λ / 4 j_ on one side and a delay plate λ / 8 and a dielectric mirror on the other side; the light beam enters this analyzer assembly, is made by a GRIN lens with fiber optic attached and inserted

ί \ - 2 Ο 1 2 - Ο Ο δ G 2 - 1 6 -08- 2012 in a housing block aligned with the polaroid of the Analysis Probe.

In a concrete embodiment of the invention, the GRIN lens with the attached optical fiber is fixed in an axially perforated dielectric sphere, which can rotate between two plates fixed in the housing block, after reaching the alignment, the dielectric sphere being locked between the two plates with three screws. The housing is provided with circular supports in which the polaroid, the delay plate Λ / 4, the delay plate λ / 8 and the dielectric mirror are fixed; all these measures being taken for the correct rotation and orientation of these components during assembly.

The optical, polarimetric sensor, provided with a maneuverable probe and made entirely of dielectric material, according to the invention, has the following main advantages:

- is small in size, airshipable and has a linear amplitude response;

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

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

- the directivity of the setting allows the measurement of the components of the electric field 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 the damaged component, it is possible to detect the exact location of the damage, which allows this technique to be used to check the health status of the high voltage component online;

- the directivity of the sensor allows the in-depth study of the effect of defects on the distribution of the electric field;

- the probes of the proposed sensor are waterproof, allowing the measurement of the electric field in liquids or in a gaseous environment; this function extends the field of application, for example, when measuring the electric field of components immersed in dielectric oil or dielectric gases;

- offers the possibility to measure the electric field of an alternating or direct current and to measure the electric field generated by the presence of electric charge on dielectric materials: this measurement offers the possibility to characterize the dielectric material and its degree of static electricity charge.

The optical sensor, polarimetric according to the invention, is presented in detail below, in several embodiments, in connection with figures 1-12 which represent:

^ - 2 0 1 2 - 0 0 6 0 2 - 3

6 -Ofl- 2012

Fig. 1: - schematic representation of the optical sensor for measuring the electric field.

Fig. 2: - overview of the probe elements with the orientation of its components.

Fig. 3: - view with the orientation of the crystal TJNbO ₃ inside the probe.

Fig. 4: - first probe proposed with cell-housing, cross-sectional view.

Fig. 5: - overview of the probe in Figure 4.

Fig. 6: the proposed second probe, cross-sectional view, and cover view;

Fig. 7; longitudinal section by probe, with lens mounted in dielectric tube; Fig. 8: the probe in Figure 6, without the components of polarization and radiation analysis. Fig. 9: probe alignment system of Figure 6 and Figure 8, different from that of Figure 7. Fig. 10: overview of the probe in Figure 4, with spacers.

Fig. 11: proposed third probe, cross-sectional view, without top cover and left side of housing.

Fig. 12: Top view of the probe in Figure 11.

The object of the invention is a polarimetric optical sensor for measuring the electric field, in which the sensitive optical material is an electro-optical crystal LiNbO ₃ : the birefringence characteristics of the crystal are modulated by the electric field with which it comes into contact and is measured by the configuration illustrated in Figure 1 and Figure 2 which transduces the variation of optical polarization into optical intensity. The LiNbO ₃ crystal is a z-cut crystal, with physical axes parallel to the crystallographic axes (Figure 3).

Figure 1 illustrates the scheme of the system: it is composed of a light source 1, an optical fiber 2, a Y-branch 3 coupled to an optical coupling 4 of the optical fiber 2, a photodiode 5 and a probe 6. Y, 3 has the role of transmitting the light beam to the probe 6 and from it to the photodiode 5. The low coherence depolarized light produced by the source 1 propagates through a standard single-mode or through a single-mode bending insensitive optical fiber 2 passing through the coupling 4 and reaching the probe 6, illustrated in Figure 2. The light intensity modulated by the probe 6 returns through the fiber 2 and is measured by the photodiode 5. The probe 6 is a sensor probe in which the light is modulated by the electric field and directly demodulated by the configuration that transforms the modulated polarization into the signal strength measurable by the photodiode 5.

The sensor according to the invention has the probe 6 of Figure 2 composed of a GRIN 7 lens with optical fiber 2, attached, a polaroid 8, a delay plate Λ / 4,9, (inhibitor), an electro-optical crystal 10, of LiNbO ₃ , or two such crystals, a delay plate λ / 8 11, and a dielectric mirror.12.

^r \ -2 ϋ '1 -? 0 Ț <î 6 -08- 2 (12

The light leaving the fiber 2 is collimated by the GRIN 7 lens and linearly polarized by the vertically oriented polaroid 8. The light polarized linearly with the vector E vertically, which propagates through the plate 9 oriented at 45 ⁰ became circularly polarized. Subsequently, the polarization of the circular polarized beam is modulated by the birefringence of the electro-optical crystal 10 induced by the electric field. The polarization starting from the electro-optical crystal 10 makes a double-pass through the λ / 8 11 delay plate oriented vertically and a reflection on the mirror

12. The delay plate λ / 8 11 is used to create a fixed birefringence to force the configuration to operate in the linear area. Then the beam propagates for the second time through the electro-optical crystal 10 and through the delay plate Ă / 4 9. After the second propagation through the delay plate λ / 4,9, the elliptical polarization becomes linear and the orientation of this linear polarization is modulated by birefringence of the electro-optical crystal 10 induced by the external electric field.

This orientation, of 45 ⁰ for the zero electric field, depends linearly on the delay between the self-stations of the electro-optical crystal 10, which is modulated by the electric field.

Then, the linearly modulated polarization is analyzed by the vertically oriented polaroid 8: through the fiber 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 fiber 2, because the light generated by the source 1 is depolarized and because in the returning signal the information is contained only in its amplitude.

The formula that expresses the variation of the signal according to the delay of the eigenstands of the electro-optical crystal 10 is:

S (r) = ^ cos ² (<9 + 45 °) where

4.

is the phase induced by E _Y , with a LiNbO ₃ crystal of length L and with the dimensional characteristic D _Y ; the electric field E _Y is the field that reaches the crystal parallel to the Y crystallographic axes indicated in Figure 3. Figure 3 shows the orientation of the electro-optical crystal 10 with z-cut inside the configuration in Figure 2: light propagated parallel to the Z axes with electric field null does not feel birefringence because the crystallographic axes X and Y have the same refractive index. The configuration in Figure 2 is only sensitive to

6V 2 Ο 1 2 - Ο Ο 6 ϋ 2 - 1 6 -08- 2012 the electric field parallel to the Y axes, because it measures only the birefringence induced in the crystal by E _Y : therefore, the sensitivity of the configuration is maximum for E _Y , it is

7% of this for E _x , and zero for E _z .

To deduce the phase ^ (r) and calculate E _Y using the signal S (t) it is possible to use the following ratio:

5 (() =

-45 ° where A / 2 is the value of the signal with E _Y zero. The sensor shown is not influenced by temperature, as it does not have a temperature-dependent component. The light beam propagated through the Z axes of the electrooptic crystal 10 does not measure the variation of birefringence as a function of temperature, because the crystallographic axes X and Y have the same refractive index: the two refractive indices are equal and change in value with respect to temperature variation. The other components of the configuration in Figure 2 that could be sensitive to temperature are the delay plates Ă / 4, 9 and Ă / 8, 11: the effect of temperature is limited by the use of the zero order delay plate.

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

It is also an inventive element of the invention to propose another type of housing, 13 ', for the probe. Figure 4 and Figure 5 contain the drawing of a first housing 13 proposed for the configuration of Figure 2: a quartz cell is used.

in Figure 4 the probe is closed and the components are inserted sequentially: all components must be dimensioned respecting the internal cell size of the housing 13 and the orientation required for the setting in Figure 2. The sketch of Figure 5 is the decomposed view of Figure 4: lens element 7 with fiber 2 attached, it is inserted into a housing block 14 which can be moved easily to aid alignment: if alignment is obtained, the lens 7 and the housing block 14 are fixed together in the housing cell 13.

An inventive element, of an inventive character, of the invention is also the use of a component with a single housing block 14 in the sketch of Figure 4, obtained by fixing together all the components in a suitable way.

An inventive element of the invention is also a second variant of the housing 13 'proposed, from Figure 6, Figure 8 and Figure 9. This housing 13' simplifies the construction of the probe, c \ - 2 O 1 1 - OO 8 il - / 6 -08- 2012 allowing the introduction inside it of all components, including the housing block 14 in Figure 8. The sketch in Figure 9 shows the proposed system of alignment between the lens 7 and the mirror 12 of the setting Figure 6: lens element 7 with fiber 2 attached, is introduced in -an axially perforated dielectric sphere 15, which can rotate between two plates 16 and 17, fixed in the housing block 14. After reaching the alignment, the dielectric sphere 15 can be locked between the two plates 16 and 17 with three screws a.

In Figure 6, the housing 13 'must be of the same material as the chassis, and the seal must be impermeable.

An inventive element of the invention is also the use of a massive component to prevent a possible error in the orientation between the polaroid plate 8 and the delay plate λ / 4, 9, (inhibitor). This massive component consists of a polaroid plate and an inhibitor plate properly aligned and fixed together.

The object of the invention is to use a solid component which includes all the components aligned and fixed together inserted in the housing of Figure 6.

To measure a DC direct current electric field it is mandatory that the crystal 10 in LiNbO ₃ has a high value of the relaxation time:

G where G is the volume of the electrical conductivity of the crystal, and ε ₀ and ε _Γ is the vacuum dielectric constant and the relative dielectric constant of the crystal, respectively. The nominal relaxation time value for the LiNbO ₃ crystal is 7x10 ⁶ seconds.

Since the value τ of the electro-optical crystal 10, of LiNbO ₃ , is usually lower than the expected nominal value, a subject of the invention is also the thermal treatment of the crystal to regain its nominal value τ. The heat treatment of the annealing LiNbO3 crystal is performed in the oven by heating with approx. 27min to 400 ° C with maintenance for 6 hours and cooling with the oven, in approx. 2 hours.

The housing 13 of Figure 6 and Figure 7 allows the extraction of the crystal 10 without deviation from the alignment of the setting for performing the heat treatment.

The τ value of the LiNbO ₃ crystal is influenced by prolonged exposure to IR radiation or by the handling provided by the assembly of the housing 13. An inventive element of the invention is the use of a film strip around the crystal to maintain the nominal characteristic of the relaxation time. IR radiation.

It is an inventive element of the invention to use a perforated spacer 18.d, between the components of the probe 6, as in the drawing of Figures 7 and 10 to eliminate any interference caused by reflections of the various components. This c \ ~ 2 Ο 1 2 - Ο Ο 6 Ο 2 - Î 6 -08- 2012 feature can also be used to insert a space between components when joined.

It is an inventive element of the invention and a housing 13 'from Figure 11 and from the top view of Figure 12 where the polaroid 8, the delay plate 9, the delay plate 11 and the mirror 12 are mounted in a circular support 19.b, c, d and e, respectively, which can be rotated to ensure the correct orientation of all components during assembly.

Other problems related to the optical sensor according to the invention are the following: Sensitivity-, the proposed optical sensor is designed for measuring the high electric field generated by the high voltage component. In order to increase the sensitivity, it is essential to change the configuration, the optical reception system and the construction of the probe.

Crystal temperature: if the LiNbO ₃ crystal is exposed to high temperatures, it loses its characteristics to measure the electric field of direct current; in order to regain its characteristics, a heat treatment is needed. However, this phenomenon does not occur within the predicted temperature range.

Installation: The sensor must work close to the high voltage component in order to be able to monitor. However, for an automated measurement to be possible, an automated system must move the sensor.

Claims (6)

claims 1. Polarimetric optical sensor for measuring the electric field, having as a minimum sensitive optical material an electro-optical crystal (10) of LiNbOs type z-cut of a probe (6), with physical axes parallel to the crystallographic axes, whose characteristics of birefringents are modulated by the electric field they come in contact with, as well as a light source (1), an optical fiber (2) and some dielectric analysis components, including a GRIN lens (7), a polaroid (8), a delay plate Λ / 4, (9), a dielectric mirror (12) and a photodiode (5) protected by a dielectric housing (13, 13 '), characterized in that it also comprises a Y-branch (3) for transmitting the light beam to the T-probe (6) and from there to the photodiode (5), coupled to a coupling (4) of the optical fiber (2), and inside the probe (6) the electro-optical crystal (10) is framed by the polaroid (8) and the delay plate Λ / 4, (9), on one side and the delay plate A / 8, (11) and the dielectric mirror (12) on the other p arts, for alignment of the light beam with this analyzer assembly, the GRIN lens (7) with the optical fiber (2) attached, being inserted into a housing block (14) aligned with the polaroid (8). Polarimetric optical sensor according to Claim 1, characterized in that the GRIN lens (7) with the attached optical fiber (2) is fixed in an axially perforated dielectric sphere (15), which can rotate between two plates (16). ) and (17) fixed in the housing block (14), after reaching the alignment, the dielectric sphere (15) being locked between the two plates (16 and 17) with three screws (a). Polarimetric optical sensor according to Claim 1 or 2, characterized in that the housing (13, 13 ') has circular supports (19.b, c, d and e, respectively) in which the polaroid (8) is fixed, delay plate Λ / 4 (9), delay plate Λ / 8 (11) and mirror (12), for the correct rotation and orientation of these components during assembly. Polarimetric optical sensor according to Claim 1, 2 or 3, characterized in that a perforated spacer (18.d, d ') is arranged between the probe components (6). Polarimetric optical sensor according to Claim 1, 2 or 3, characterized in that the housing (13, 13 ') is made of dielectric material with a very low coefficient of thermal expansion. A polarimetric optical sensor according to claim 1; 2, 3 or 4, characterized in that it has the electro-optical crystal (10) of LiNbOs heat-treated by annealing at 2 ° / min at 400 ° C with maintenance for 6 hours and slow cooling in 2 hours. Polarimetric optical sensor according to Claim 1, 2, 3, 4 or 5, characterized in that it has an electro-optical crystal (10) surrounded by a strip of protective film against IR radiation.