High-frequency ultrasonic optical fiber sensing probe for partial discharge detection

Abstract

The invention belongs to the technical field of on-line monitoring of power equipment, and discloses a high-frequency ultrasonic optical fiber sensing probe for partial discharge detection, which comprises a sensing optical fiber, and the end face of one end of the sensing optical fiber is provided with a plano-concave optical resonant cavity structure of the end face of one end of the sensing optical fiber. According to the principle that detection is carried out through deformation of a vibrating diaphragm and a sound-sensitive material under the action of sound waves, the refractive index of the polymer material is changed through the elasto-optical effect of sound pressure on the polymer material of the polymer curved surface layer, and external ultrasonic sensing is achieved. The antenna is not limited by the inherent mechanical resonant frequency of materials, and has the technical characteristic of wide frequency band; the diameter of the ultrasonic sensor is only the diameter of an optical fiber, and the ultrasonic sensor has the structural characteristics of tiny structure and flexible detection, is of a fully fixed structure, has no additional vibrating diaphragm, no space light path alignment part and the like, and is higher in system structural stability.

Description

Technical Field

[0001] The invention belongs to the technical field of online monitoring of electric power equipment, and in particular relates to a high-frequency ultrasonic optical fiber sensing probe for partial discharge detection. Background Art

[0002] At present, more stringent technical requirements are put forward for the state perception of power equipment: First, the strong uncertainty and volatility brought about by the access of a high proportion of new energy to the power system causes power equipment to withstand more extreme and drastic operating conditions, requiring panoramic and in-depth monitoring of the power equipment status to complete timely fault warning; Second, to improve the utilization efficiency of energy and equipment and extend the service life of equipment, it is necessary to accurately and dynamically grasp the equipment operating parameters based on the power equipment state perception technology, so as to realize the effective evaluation of equipment life and status information.

[0003] Partial discharge is the most common insulation abnormality in power equipment. Reliable monitoring of partial discharge signals of equipment is of great significance for the perception and evaluation of insulation status, fault warning and diagnosis of power equipment. The partial discharge process is accompanied by acoustic signals. By monitoring the partial discharge acoustic emission signals, the generation of partial discharge, discharge type and fault location can be effectively judged. However, facing the more stringent technical requirements for the state perception of power equipment in the current digital power grid construction, the existing partial discharge ultrasonic sensing technology shows its technical limitations: ① Existing fiber optic ultrasonic sensors generally use vibrating membranes. Limited by the inherent mechanical frequency limitation of the vibrating membrane, the measurement frequency band is generally lower than 200 kHz. However, broadband and high-frequency ultrasonic detection is conducive to the detection of low-discharge partial discharges and the recognition of voiceprints of partial discharge information of equipment; ② Existing ultrasonic sensors are generally large in size and use rigid packaging structures. It is difficult to realize ultrasonic signal detection on equipment with complex shapes or narrow spaces, and it is also impossible to complete the intrinsic fusion with power equipment to form a key component of intelligent power equipment, and it is impossible to realize the in-depth and in-situ perception of partial discharge information of equipment; ③ The electrical detection method has poor electromagnetic compatibility performance and is easily interfered by electromagnetic signals during operation. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a high-frequency ultrasonic fiber optic sensing probe for partial discharge detection. The present invention can solve the problems of narrow response frequency band, large size and low structural stability in the existing fiber optic partial discharge ultrasonic detection technology.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows: A high-frequency ultrasonic optical fiber sensing probe for partial discharge detection comprises a sensing optical fiber, wherein one end face of the sensing optical fiber is provided with a plano-concave optical resonant cavity structure.

[0006] Preferably, the plano-concave optical resonant cavity structure comprises a first reflective film layer disposed on the end face of the sensing optical fiber, a polymer curved surface layer disposed on the surface of the first reflective film layer, and a second reflective film layer disposed on the curved surface of the polymer curved surface layer.

[0007] Preferably, the core end face of the sensing optical fiber is located at the reflection focus of the second reflection film layer.

[0008] Preferably, the shape of the polymer curved surface layer is a spherical segment.

[0009] Preferably, the shape of the polymer curved surface layer is hemispherical.

[0010] Preferably, the reflectivity of the first reflective film layer is 45% to 55%, and the reflectivity of the second reflective film layer is above 90%.

[0011] Preferably, the material of the polymer curved surface layer is cured optical UV glue or PDMS.

[0012] Preferably, the sensing optical fiber is a single-mode optical fiber or a multi-mode optical fiber.

[0013] The present invention also provides a detection system based on the above-mentioned high-frequency ultrasonic fiber optic sensing probe for local discharge detection of the present invention, comprising a narrow-band laser light source, a photodetector and the above-mentioned high-frequency ultrasonic fiber optic sensing probe for local discharge detection, wherein the other end of the sensing fiber is connected to the narrow-band laser light source and the photodetector via a fiber optic circulator.

[0014] The working method of the detection system of the present invention comprises: The wavelength of the narrowband laser light source is set to the middle area of ​​the rising edge of any interference period of the interference spectrum output by the plano-concave optical resonant cavity structure. When the spectrum of the plano-concave optical resonant cavity structure drifts and changes under the action of acoustic waves, the optical power received by the photodetector also changes accordingly. The acoustic wave signal is demodulated by analyzing the optical power.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention is different from the traditional fiber optic ultrasonic sensing method, which uses the deformation of the diaphragm and the acoustically sensitive material under the action of sound waves for detection. The present invention proposes to use the elastic-photoelectric effect of the sound pressure on the polymer material of the polymer surface layer to change the refractive index of the polymer material to achieve the sensing of external ultrasound. The present invention is not limited by the inherent mechanical resonance frequency of the material, has the technical characteristics of a wide bandwidth, and can achieve ultra-high frequency ultrasound sensing; the ultrasonic sensor of the present invention is a fiber optic sensing probe, and the sensor diameter is only the diameter of the optical fiber. It has the structural characteristics of a tiny structure and flexible detection, and has application advantages in the anisotropic curved surfaces and narrow areas of power equipment. The ultrasonic sensor is a fully fixed structure, without additional diaphragms, spatial optical path alignment and other parts, and the system structure has higher stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is an overall scheme diagram of the detection system of the present invention; Figure 2 It is a plano-concave optical resonant cavity structure of the present invention and a graph showing the emission and reflection characteristics of a light beam inside the cavity; Figure 3 is the output spectrum of the optical fiber sensor and the wavelength setting value of the demodulation light source in the embodiment of the present invention; Figure 4 It is the waveform of the optical fiber sensor in the embodiment of the present invention responding to 5 MHz ultrasonic sound waves.

[0017] In the figure, 1-sensing optical fiber, 2-first reflective film layer, 3-polymer curved surface layer, 4-second reflective film layer, 5-narrow-band laser light source, 6-optical fiber circulator, 7-photoelectric detector. DETAILED DESCRIPTION

[0018] The present invention is described clearly and completely below in conjunction with the accompanying drawings and embodiments of the present invention. The described embodiments are only a part of the embodiments of the present invention, not all embodiments.

[0019] The present invention prepares a polymer curved surface layer 3 on the end face of the sensing optical fiber 1, and coats the surface of the polymer curved surface layer 3 with a high reflective material to form a plano-concave optical resonant cavity structure, thereby forming the optical fiber sensor of the present invention (that is, the high-frequency ultrasonic optical fiber sensing probe for local discharge detection of the present invention). The structure of the optical fiber sensor of the present invention is tiny, only as thick as the optical fiber, simple and stable, and retains the flexibility of the optical fiber. Different from the traditional optical fiber ultrasonic sensing method, that is, the principle of detection by using the deformation of the diaphragm and the acoustic sensitive material under the action of sound waves, the present invention proposes to use the photoelastic effect of sound pressure on the polymer material to change the refractive index of the polymer curved surface layer 3, thereby causing a change in the interference spectrum to realize the perception of external ultrasound. With the photoelastic effect, it is not affected by the inherent mechanical frequency of the material\structure, so it has the characteristics of a wide bandwidth and can realize limited detection of ultra-high frequency ultrasound.

[0020] Specifically, refer to Figure 1 and Figure 2 The high-frequency ultrasonic optical fiber sensing probe for partial discharge detection of the present invention comprises a sensing optical fiber 1, a first reflective film layer 2, a polymer curved surface layer 3 and a second reflective film layer 4. The first reflective film layer 2 is located at the head end face of the sensing optical fiber 1, the polymer curved surface layer 3 is located on the upper surface of the first reflective film layer 2, and the second reflective film layer 4 is located on the curved surface of the polymer curved surface layer 3.

[0021] The detection system of the present invention comprises a narrowband laser light source 5, an optical fiber circulator 6, a photodetector 7 and the above-mentioned high-frequency ultrasonic optical fiber sensing probe for partial discharge detection. The output of the narrowband laser light source 5 is connected to port ① of the optical fiber circulator 6, port ② of the optical fiber circulator 6 is connected to the tail end of the sensing optical fiber 1, and port ③ of the optical fiber circulator 6 is connected to the photodetector 7.

[0022] The method for forming the polymer curved surface layer 3 comprises: dripping a certain volume of polymer solution on the first reflective film layer 2, the polymer solution stably forms a smooth spherical segment under the action of liquid surface tension, and the curvature radius of the spherical segment is R, and then solidifying the polymer to form the polymer curved surface layer 3. The curvature R of the polymer curved surface layer 3 can be adjusted by controlling the volume V of the dripped liquid, and the calculation formula of the curvature R is expressed as:

[0023] Wherein, V is the volume of the dripping liquid, and A is the radius of the sensing optical fiber 1.

[0024] The first reflective film layer 2, the polymer curved surface layer 3, and the second reflective film layer 4 together form a plano-concave optical resonant cavity structure, the reflectivity of the first reflective film layer 2 is between 45% and 55%, and the reflectivity of the second reflective film layer 4 is above 90%; In order to make the outgoing light beam of the sensing optical fiber 1 reflect and resonate efficiently in the plano-concave resonant cavity, the core end face of the sensing optical fiber 1 should be located at the reflection focus of the second reflective film layer 4, that is, the curvature R of the polymer spherical layer 3 should be equal to its film thickness L. , the volume V of the polymer liquid dripped on the end face of the sensing optical fiber 1 is:

[0025] The sensing principle of the ultrasonic sensor is as follows: the first reflective film layer 2, the polymer curved surface layer 3 and the second reflective film layer 4 form a plano-concave optical resonant cavity structure. After the sensing beam is output by the sensing optical fiber 1, the sensing beam forms optical interference with a high quality factor in the plano-concave optical resonant cavity structure. Finally, the sensing optical fiber 1 outputs a high-precision interference spectrum, and the peak wavelength of the interference spectrum is:

[0026] In the formula, N is the interference order, n poly is the optical refractive index of the polymer surface layer. According to the elastic-optical effect of the polymer, when external sound pressure acts on the polymer, its optical refractive index changes with the sound pressure intensity, that is,

[0027] In the formula, P (t ) is the external acoustic signal. Therefore, by analyzing the changes in the output spectrum, the sensing of the external acoustic signal can be achieved.

[0028] The demodulation method of the optical fiber sensor is as follows: the wavelength of the narrowband laser 5 is set to the middle area of ​​the rising edge in any interference period of the interference spectrum output by the optical fiber sensor. When the spectrum of the optical fiber sensor drifts and changes under the action of the sound wave, the light power received by the photodetector 7 also changes accordingly. The demodulation of the sound wave signal can be achieved by analyzing the output light power.

[0029] In the above solution of the present invention, the sensing optical fiber 1 can be a single-mode optical fiber or a multi-mode optical fiber. In order to ensure the signal-to-noise ratio of the optical fiber sensor, it is preferably a polarization-maintaining single-mode optical fiber; The material of the polymer curved surface layer 3 can be optical UV glue or PDMS (i.e. polydimethylsiloxane); the UV glue is cured by UV lamp to form the polymer curved surface layer 3; PDMS is formed by thermal curing method, and optical UV glue is preferred because of its fast curing speed, which reduces the sensor production time.

[0030] In the above scheme of the present invention, the ultrasonic sensor (i.e., the optical fiber sensor) utilizes the surface tension of the liquid to form a plano-concave optical resonant cavity structure, which has a better interference quality factor than the traditional optical fiber interference structure, and thus has a better ultrasonic sensing sensitivity; The ultrasonic sensor is a fully fixed structure without additional diaphragms, spatial optical path alignment and other parts, the system structure is more stable, and the preparation process is simple.

[0031] The high-frequency ultrasonic sensor based on material refractive index modulation proposed in the present invention is a fiber optic sensor that is no longer limited by the traditional mechanical natural vibration frequency and can realize the perception of ultra-high frequency ultrasound. The sensor is an integrated optical fiber structure, and the sensor diameter is only the optical fiber diameter (such as 250μm), realizing a probe-type ultrasonic sensor, which has the technical characteristics of small size and flexible perception, and is conducive to the real-time and in-situ perception of ultrasound in the narrow space of power equipment.

[0032] Example In this embodiment, the sensing optical fiber 1 adopts a 1550-band polarization-maintaining single-mode optical fiber with a core diameter of 4 um and an optical fiber diameter of 250 um; the polymer curved surface layer 3 adopts an optical ultraviolet adhesive material; The first reflective film 2 is formed by a magnetron sputtering coating process to prepare a multilayer dielectric reflective film on the end face of the optical fiber, and the reflectivity is 50%; like Figure 2 As shown, the first reflective film layer 2, the polymer curved surface layer 3 and the second reflective film layer 4 form a plano-concave optical resonant cavity. The light beam is reflected multiple times inside the plano-concave optical resonant cavity, and finally outputs an interference spectrum structure with a high quality factor.

[0033] The polymer curved surface layer 3 is formed by dripping a certain volume V of polymer liquid on the first reflective film layer 2, and using the surface tension of the liquid to form a smooth spherical cap with a curvature R on the first reflective film layer 2 at the end face of the optical fiber. The surface curvature of the spherical body formed is different depending on the volume of the dripped polymer solution. In order to achieve efficient reflection of the incident light and form a high-quality resonant structure, the core end face of the sensing optical fiber 1 should be located at the focus of the reflective surface of the polymer curved surface. Therefore, through calculation and analysis, the volume of the polymer solution dripped in this embodiment is 1.3 μL. Then, the end face of the optical fiber is irradiated with ultraviolet light for about 2 minutes, and the polymer is completely cured; After the polymer curved surface layer 3 is cured, a multilayer dielectric reflective film (ie, the second reflective film layer 4 ) is prepared on its surface by a magnetron sputtering method. The reflectivity of the second reflective film layer 4 is better than 95%.

[0034] The optical fiber sensor of this embodiment outputs an interference spectrum as shown in FIG. Figure 3 When sound pressure acts, the optical refractive index of the plano-concave optical resonant cavity changes, causing the output interference spectrum to drift. By monitoring the wavelength drift of the output spectrum, the external ultrasound can be sensed.

[0035] Since the optical fiber sensor of this embodiment is used for high-frequency ultrasound detection, the traditional spectrum analysis method is too slow to be applied. Therefore, the spectrum change is converted into light intensity change for rapid detection. The method is as follows: a spectrometer is used to observe the sensor output interference spectrum, and an interference period is randomly selected in the 1545 nm ~ 1555 nm band, and the central wavelength of the rising edge in the period is observed. λ 0, set the wavelength of narrowband laser source 5 to λ 0. When the sensor interference spectrum drifts under the action of sound pressure, the light power received by the photodetector 6 will change with the ultrasonic signal. By collecting the output power of the photodetector through an oscilloscope, the external ultrasonic signal can be obtained.

[0036] Figure 4 The waveform of the sensor's response to a 5 MHz high-frequency ultrasonic signal shows that the proposed sensor has a frequency band of 5 MHz.

[0037] Finally, it should be noted that the above preferred embodiments are only for illustrating the technical solutions of the present invention and are not limited thereto. For those skilled in the art, various changes such as modification, replacement or optimization can be made in terms of form and details without departing from the scope defined in the claims.

Claims

1. A high-frequency ultrasonic optical fiber sensing probe for partial discharge detection, characterized in that: It comprises a sensing optical fiber (1), wherein one end surface of the sensing optical fiber (1) is provided with a plano-concave optical resonant cavity structure of the end surface of the sensing optical fiber (1).

2. A high-frequency ultrasonic optical fiber sensing probe for partial discharge detection according to claim 1, characterized in that: The plano-concave optical resonant cavity structure comprises a first reflective film layer (2) arranged on the end face of a sensing optical fiber (1), a polymer curved surface layer (3) arranged on the surface of the first reflective film layer (2), and a second reflective film layer (4) arranged on the curved surface of the polymer curved surface layer (3).

3. A high-frequency ultrasonic optical fiber sensing probe for partial discharge detection according to claim 2, characterized in that: The core end face of the sensing optical fiber (1) is located at the reflection focus of the second reflection film layer (4).

4. A high-frequency ultrasonic optical fiber sensing probe for partial discharge detection according to claim 2, characterized in that: The shape of the polymer curved surface layer (3) is a spherical segment.

5. A high-frequency ultrasonic optical fiber sensing probe for partial discharge detection according to claim 4, characterized in that: The shape of the polymer curved surface layer (3) is hemispherical.

6. A high-frequency ultrasonic optical fiber sensing probe for partial discharge detection according to claim 2, characterized in that: The reflectivity of the first reflective film layer (2) is 45% to 55%, and the reflectivity of the second reflective film layer is above 90%.

7. A high-frequency ultrasonic optical fiber sensing probe for partial discharge detection according to claim 2, characterized in that: The material of the polymer curved surface layer (3) is cured optical UV glue or PDMS.

8. A high-frequency ultrasonic optical fiber sensing probe for partial discharge detection according to claim 1, characterized in that: The sensing optical fiber (1) is a single-mode optical fiber or a multi-mode optical fiber.

9. A detection system based on the high-frequency ultrasonic optical fiber sensing probe for partial discharge detection according to any one of claims 1 to 8, characterized in that: It comprises a narrowband laser light source (5), a photoelectric detector (7) and a high-frequency ultrasonic optical fiber sensing probe for local discharge detection, wherein the other end of the sensing optical fiber (1) is connected to the narrowband laser light source (5) and the photoelectric detector (7) via an optical fiber circulator (6).

10. The working method of the detection system according to claim 9, characterized in that: include: The wavelength of the narrowband laser light source (5) is set to the middle region of the rising edge in any interference period of the interference spectrum output by the plano-concave optical resonant cavity structure. When the spectrum of the plano-concave optical resonant cavity structure drifts and changes under the action of the acoustic wave, the optical power received by the photodetector (7) also changes accordingly. The acoustic wave signal is demodulated by analyzing the optical power.

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