A method for identification and localization of air gap defects in xlpe power cables

Abstract

The application discloses a kind of XLPE power cable air gap defect identification and positioning method, obtain XLPE cable sample containing air gap defect;XLPE cable sample containing air gap defect is tested by terahertz time-domain spectroscopy reflection mode, and the terahertz time-domain spectroscopy reflection of XLPE cable sample containing air gap defect is obtained;Based on the terahertz time-domain spectroscopy reflection of XLPE cable sample containing air gap defect, determine the phase information of main wave and air gap reflection echo;According to the phase information of main wave and air gap reflection echo, construct reflection model, based on reflection model, the accurate positioning of air gap defect is completed by optical calculation.The application provides a kind of detection mode with high precision, fast operation speed, low energy, safe and reliable for XLPE power cable defect identification method, can realize real-time monitoring and accurate positioning to XLPE power cable air gap defect.

Description

A method for identifying and locating air gap defects in XLPE power cables Technical Field

[0001] This invention relates to the field of high-voltage insulation materials technology, specifically a method for identifying and locating air gap defects in XLPE power cables. Background Technology

[0002] Cross-linked polyethylene (XLPE) is widely used in medium and high voltage power cables due to its excellent electrical and mechanical properties. However, XLPE is affected by various environmental and human factors during production, laying, and operation, leading to micropore defects. Numerous studies have shown that micropore defects cause localized electric field concentration and partial discharge, resulting in electrical treeing and severe degradation of insulation performance. Therefore, studying the location of air gap defects in XLPE power cables and replacing cables in a timely manner are crucial for the stable operation of the power grid.

[0003] Traditional methods for studying air gaps mainly include: identifying defects such as bubbles and cracks in basin insulators using laser-ultrasound technology; locating thermal aging and nuclear radiation aging defects in cables using wideband impedance spectroscopy; and locating local defects in XLPE cables using the amplitude-frequency characteristics and phase difference of PD signals based on partial discharge methods. However, these methods generally suffer from the following problems: 1. They require high-voltage, high-current experimental environments, which are relatively dangerous; 2. They cannot meet the requirements for large-scale non-destructive testing of XLPE cables. Summary of the Invention

[0004] The purpose of this invention is to provide a method for identifying and locating air gap defects in XLPE power cables, so as to overcome the defects of the existing technology. This invention has the advantages of high identification accuracy, fast operation speed, low energy consumption, and safety and reliability, and is suitable for actual power production environments with high voltage and high current.

[0005] This invention is achieved through the following technical solution:

[0006] A method for identifying and locating air gap defects in XLPE power cables includes the following steps:

[0007] Obtain XLPE cable samples containing air gap defects;

[0008] Terahertz time-domain spectral reflectance mode test was performed on XLPE cable samples containing air gap defects to obtain terahertz time-domain spectral reflectance diagrams of XLPE cable samples containing air gap defects.

[0009] Based on the terahertz time-domain reflectance spectrum of an XLPE cable sample containing air gap defects, the phase information of the main wave and the air gap reflected echo was determined.

[0010] A reflection model is constructed based on the phase information of the main wave and the air gap reflected echo. Based on the reflection model, the air gap defect is accurately located through optical calculations.

[0011] Furthermore, obtaining the XLPE cable sample containing air gap defects specifically includes:

[0012] Block-shaped XLPE cable samples were obtained by diaphragmatic cutting of the cable sample.

[0013] Air gap defects were reconstructed on the block-shaped XLPE cable specimens to obtain pinhole-type air gap defect specimens, i.e. XLPE cable specimens containing air gap defects.

[0014] Furthermore, the air gap defect reconstruction of the block XLPE cable sample specifically involves: using the tail end of a steel needle to penetrate the block XLPE cable sample to reconstruct the air gap defect.

[0015] Furthermore, after obtaining the XLPE cable sample containing air gap defects, the XLPE cable sample containing air gap defects was placed in a vacuum environment for heat preservation.

[0016] Furthermore, the insulation temperature is 70°C, and the insulation time is 12 hours.

[0017] Furthermore, the phase information of the main wave and the air gap reflected echo is determined based on the terahertz time-domain spectral reflectance diagram of the XLPE cable sample containing air gap defects, specifically as follows:

[0018] Based on the terahertz time-domain reflectance spectrum of the XLPE cable sample containing air gap defects, the amplitude information of the main wave and the amplitude information of the air gap reflected echo are obtained. Based on the amplitude information of the main wave and the amplitude information of the air gap reflected echo, as well as the peak finding module embedded in the Origin software, the phase information of the main wave and the air gap reflected echo is determined.

[0019] Furthermore, the construction of the reflection model based on the phase information of the main wave and the air gap reflected echo specifically involves:

[0020] Calculate the reflection angle θ2 and refraction angle θ3 of the terahertz wave on the surface of an XLPE cable sample containing air gap defects during terahertz time-domain spectral reflection mode testing:

[0021] Based on the phase information of the main wave and the air gap reflected echo, the optical path difference Δt between the terahertz wave reflected directly on the XLPE surface and reflected on the air gap surface is calculated.

[0022] The reflection model is obtained based on the reflection angle θ2 and refraction angle θ3 of the terahertz wave on the surface of the XLPE cable sample containing air gap defects, and the optical path difference Δt between the terahertz wave reflection path directly on the XLPE surface and the reflection path on the air gap surface.

[0023] Furthermore, the reflection angle θ2 and refraction angle θ3 of the terahertz wave on the surface of the XLPE cable sample containing air gap defects are calculated as follows:

[0024] θ1=θ2=45°,

[0025]

[0026] Where, ε s ε0 and ε1 are the dielectric constants of the XLPE cable sample and the vacuum dielectric constant, respectively; θ1 is the incident angle; n0 and n s These are the vacuum refractive index and the refractive index of the XLPE cable sample, respectively.

[0027] Furthermore, the optical path difference Δt between the terahertz wave's direct reflection path on the XLPE surface and its reflection path on the air gap surface is calculated as follows:

[0028] The terahertz wave reflected directly on the XLPE surface is the terahertz path that generates the main wave signal, denoted as path 1; the terahertz wave reflected on the air gap surface is the terahertz path that reflects the echo signal, denoted as path 2.

[0029]

[0030] Where c is the speed of light in vacuum, L1 is the optical path of the terahertz wave in path 1 from the surface of the XLPE cable sample to the reflector, L2 is the optical path of the terahertz wave in path 1 from the reflector to the off-axis parabolic mirror, L3 is the optical path of the terahertz wave in path 1 from the off-axis parabolic mirror to the terahertz detector, L1' is the optical path of the terahertz wave in path 2 from the air gap surface to the surface of the XLPE cable sample, L2' is the optical path of the terahertz wave in path 2 from the surface of the XLPE sample to the reflector, L3' is the optical path of the terahertz wave in path 2 from the reflector to the off-axis parabolic mirror, and L4' is the optical path of the terahertz wave in path 2 from the off-axis parabolic mirror to the terahertz detector.

[0031] Eliminating spatial distances that consume the same amount of time in the path, we obtain:

[0032]

[0033] Among them, Δl1=L2-L3′, Δl2=L2′-L1, Δl3=L3-L4′.

[0034] Furthermore, the precise location of the air gap defect based on the reflection model is achieved through optical calculations, specifically by calculating the air gap depth, as expressed in the following expression:

[0035]

[0036] Where h is the air gap depth.

[0037] Compared with the prior art, the present invention has the following beneficial technical effects:

[0038] This invention presents a method for identifying and locating air gap defects in XLPE power cables. Based on the transient, coherent, and non-contact characteristics of a terahertz time-domain spectral reflectance module, the method achieves non-destructive testing and precise location of air gaps through optical computation by measuring the terahertz time-domain spectral reflectance of XLPE samples. This invention utilizes terahertz time-domain spectroscopy to identify air gap defects in XLPE power cables, which benefits cable manufacturers by improving processing techniques and enhancing production quality. It also helps monitoring personnel quickly grasp the actual defect status of the cables and replace them in a timely manner, thereby ensuring the safe and stable operation of the power system. The defect identification and location method for XLPE power cables described in this invention has the advantages of high identification accuracy, fast operation speed, low energy consumption, and high reliability, making it suitable for high-voltage, high-current actual power production environments. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 is a flowchart of a method for identifying and locating air gap defects in XLPE power cables according to the present invention.

[0041] Figure 2 is a structural diagram of an XLPE cable sample containing air gap defects in an example of the present invention.

[0042] Figure 3 is a terahertz time-domain reflectance spectrum of an XLPE sample containing air gap defects in an embodiment of the present invention;

[0043] Figure 4 is a schematic diagram of the terahertz time-domain spectral reflectance mode.

[0044] Figure 5 shows the reflection model of terahertz waves acting on a defective sample. Detailed Implementation

[0045] The present invention will now be described in further detail, but this description is intended to explain rather than limit the invention.

[0046] Example 1

[0047] This invention provides a method for identifying and locating air gap defects in XLPE power cables, as shown in Figure 1, including the following steps:

[0048] Obtain XLPE cable samples containing air gap defects;

[0049] Terahertz time-domain spectral reflectance mode test was performed on XLPE cable samples containing air gap defects to obtain terahertz time-domain spectral reflectance diagrams of XLPE cable samples containing air gap defects.

[0050] Based on the terahertz time-domain reflectance spectrum of XLPE cable samples containing air gap defects, the phase information of the main wave and the air gap reflected echo was determined.

[0051] A reflection model is constructed based on the phase information of the main wave and the air gap reflected echo. Based on the reflection model, the air gap defect is accurately located through optical calculations.

[0052] This invention presents a method for identifying and locating air gap defects in XLPE power cables. Based on the transient, low-energy, and broadband nature of terahertz time-domain spectroscopy, the method measures the terahertz time-domain reflectance spectrum of an XLPE sample containing air gap defects. Combining this with a reflection model of terahertz waves acting on the defective sample, a series of optical calculations are used to obtain the defect depth of the XLPE sample, thus achieving accurate identification of the defect location. This invention utilizes a terahertz time-domain spectroscopy reflection module for defect identification in XLPE power cables, which benefits cable manufacturers by improving processing techniques and enhancing production quality. It also helps power grid monitoring personnel quickly grasp the actual defect status of cables and replace them in a timely manner, thereby ensuring the safe and stable operation of the power system. The defect identification method for XLPE power cables described in this invention has advantages such as being non-destructive, having a high signal-to-noise ratio, high reliability, convenience, and speed, and can be widely applied in actual power grid operation.

[0053] Example 2

[0054] The specific implementation steps of the method for identifying and locating air gap defects in XLPE power cables of the present invention are as follows:

[0055] 1) A block-shaped XLPE cable sample with a thickness of approximately 5.35 mm and a diameter of approximately 88.5 mm was obtained by radial cutting of a 110kV cross-linked polyethylene cable.

[0056] 2) The tail end of a steel needle is inserted vertically along the axial direction into the block XLPE cable sample to reconstruct the air gap defect, forming a pinhole-type air gap defect sample with a diameter of about 0.25 mm and a depth of about 1.502 mm from the surface, i.e., an XLPE cable sample containing air gap defects, as shown in Figure 2.

[0057] 3) Terahertz time-domain spectroscopy reflectance mode testing was performed on XLPE cable samples containing air gap defects using the CIP-TDS terahertz time-domain spectroscopy system manufactured by Daheng Optoelectronics, as shown in Figure 4. The system consists of a sapphire femtosecond laser, a terahertz detection device 2, a time delay platform 3 (ESP301 type electric translation stage for time delay control), a beam splitter prism 4, a sample stage 5, and a software control system. After beam splitting by the beam splitter prism 4, the femtosecond laser 1 generated by the femtosecond laser is divided into a pump laser and a probe laser. The pump laser is focused onto a photoconductive antenna through a lens, thereby generating a terahertz wave. The generated terahertz wave passes through an optical system composed of off-axis polarizers, merges with the probe laser, and is recorded by the terahertz detection device 2. By controlling the time delay system composed of the high-precision time delay platform 3, the time delay between the pump pulse and the probe pulse can be adjusted, ultimately allowing the detection of the entire time-domain waveform of the terahertz pulse.

[0058] 4) Select defect-free XLPE cable samples and XLPE cable samples with air gap defects respectively, cut the selected parts, control the size of the parts to be 2cm×2cm, and the thickness to be about 5mm.

[0059] 5) Keep the humidity below 3%, place the reflector in the sample stage, test three times and take the average value as the reference signal to eliminate moisture interference and measurement error;

[0060] 6) Place the XLPE cable sample with air gap defects and the XLPE cable sample without defects in the test stage respectively, and take the average value as the sample signal after three tests.

[0061] 7) The obtained terahertz time-domain reflectance spectrum is shown in Figure 3. The time-domain spectral information includes the phase and amplitude of the signal from the defect-free XLPE sample and the phase and amplitude of the signal from the XLPE sample containing air gap defects. Notably, unlike the defect-free sample, the time-domain spectrum of the XLPE cable sample containing air gap defects exhibits a significant echo signal.

[0062] 8) Based on the terahertz time-domain reflectance spectrum of the XLPE sample containing air gap defects, the amplitude of the spectral data is detected, and the phase information of the main wave and the air gap reflected echo is determined by the peak finding module embedded in the Origin software.

[0063] 9) The reflection model of terahertz waves in XLPE specimens with air gap defects is constructed as shown in Figure 5. The reflection paths of terahertz waves directly on the XLPE surface and on the air gap surface are labeled as path 1 and path 2, respectively. Based on the dielectric constant ε of the XLPE specimen... s The vacuum dielectric constant ε0 and the incident angle θ1 can be used to calculate the reflection angle θ2 and refraction angle θ3 of the terahertz wave on the XLPE sample surface:

[0064] θ1=θ2=45°,

[0065]

[0066] Where, n0, n s These are the refractive index in vacuum and the refractive index of the XLPE sample, respectively.

[0067] 9) Based on the reflection model in Figure 5, the two paths are divided into multiple segments for calculation. The theoretical time difference can be calculated using the following formula:

[0068]

[0069] In the formula, c is the speed of light in a vacuum.

[0070] 10) Eliminating spatial distances with the same time consumption in the path includes:

[0071]

[0072] 11) The air gap depth h can be obtained from the above calculation:

[0073]

[0074] 12) Substitute the optical path difference Δt and the dielectric constant ε of the XLPE sample. s The vacuum dielectric constant ε0, incident angle θ1, and reflection angle θ2 can be calculated in this example to be h≈1.539mm. Compared with the actual air gap depth h0=1.502mm, the error is only 3.7%.

[0075] This demonstrates that hidden defects can be accurately located by using terahertz time-domain spectral reflectance testing and based on the phase information of the reflected echo.

[0076] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the scope of the invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0077] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art. The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for identifying and locating air gap defects in XLPE power cables, characterized in that, The process includes the following steps: obtaining an XLPE cable sample containing an air gap defect; performing a terahertz time-domain spectral reflectance mode test on the XLPE cable sample containing the air gap defect to obtain a terahertz time-domain spectral reflectance map of the XLPE cable sample containing the air gap defect; determining the phase information of the main wave and the air gap reflected echo based on the terahertz time-domain spectral reflectance map of the XLPE cable sample containing the air gap defect; constructing a reflection model based on the phase information of the main wave and the air gap reflected echo; and accurately locating the air gap defect through optical calculations based on the reflection model. Specifically, constructing the reflection model based on the phase information of the main wave and the air gap reflected echo involves calculating the reflection angle of the terahertz wave on the surface of the XLPE cable sample containing the air gap defect during the terahertz time-domain spectral reflectance mode test. With the angle of refraction Based on the phase information of the main wave and the air gap reflected echo, the optical path difference between the terahertz wave reflected directly on the XLPE surface and reflected on the air gap surface is calculated. Based on the reflection angle of terahertz waves on the surface of an XLPE cable sample containing air gap defects. With the angle of refraction And the optical path difference between the terahertz wave's direct reflection path on the XLPE surface and its reflection path on the air gap surface. The reflection model was obtained; the reflection angle of the terahertz wave on the surface of the XLPE cable sample containing air gap defects was obtained. With the angle of refraction The calculation is as follows: ， ;in, 、 The dielectric constant and vacuum dielectric constant of the XLPE cable sample are respectively. Angle of incidence 、 The refractive indices are the vacuum refractive index and the refractive index of the XLPE cable sample, respectively; the optical path difference between the terahertz wave reflection path directly on the XLPE surface and the reflection path on the air gap surface. The calculation is as follows: The terahertz wave is reflected directly on the XLPE surface, which is the terahertz path that generates the main wave signal, and is denoted as path 1; the terahertz wave is reflected on the air gap surface, which is the terahertz path that generates the reflected echo signal, and is denoted as path 2. in, The speed of light in a vacuum. For the terahertz wave in path 1, the optical path length from the surface of the XLPE cable sample to the reflector is... For path 1, the optical path length of the terahertz wave from the reflecting mirror to the off-axis parabolic mirror is... For the optical path of the terahertz wave in path 1 from the off-axis parabolic mirror to the terahertz detector, For path 2, the optical path length of the terahertz wave from the air gap surface to the XLPE cable sample surface. For path 2, the optical path length of the terahertz wave from the surface of the XLPE sample to the mirror. For path 2, the optical path length of the terahertz wave from the reflecting mirror to the off-axis parabolic mirror. Let be the optical path length of the terahertz wave in path 2 from the off-axis parabolic mirror to the terahertz detector; after eliminating the spatial path lengths that consume the same amount of time, we get: in, ， ， The method based on the reflection model uses optical calculations to accurately locate air gap defects, specifically calculating the air gap depth, as expressed in the following expression: in, This represents the air gap depth.

2. The method for identifying and locating air gap defects in XLPE power cables according to claim 1, characterized in that, The process of obtaining an XLPE cable sample containing air gap defects specifically includes: radii-cutting the cable sample to obtain a block-shaped XLPE cable sample; and reconstructing the air gap defects on the block-shaped XLPE cable sample to obtain a pinhole-type air gap defect sample, i.e., an XLPE cable sample containing air gap defects.

3. The method for identifying and locating air gap defects in XLPE power cables according to claim 2, characterized in that, The air gap defect reconstruction of the block XLPE cable sample is specifically performed by inserting the tail end of a steel needle into the block XLPE cable sample to reconstruct the air gap defect.

4. The method for identifying and locating air gap defects in XLPE power cables according to claim 2, characterized in that, After obtaining XLPE cable samples containing air gap defects, the samples were placed in a vacuum environment for heat preservation.

5. A method for identifying and locating air gap defects in XLPE power cables according to claim 4, characterized in that, The insulation temperature is 70 ℃, and the insulation time is 12 h.

6. The method for identifying and locating air gap defects in XLPE power cables according to claim 1, characterized in that, The phase information of the main wave and the air gap reflected echo is determined based on the terahertz time-domain spectral reflectance map of the XLPE cable sample containing air gap defects. Specifically, the amplitude information of the main wave and the amplitude information of the air gap reflected echo are obtained based on the terahertz time-domain spectral reflectance map of the XLPE cable sample containing air gap defects. Based on the amplitude information of the main wave and the amplitude information of the air gap reflected echo, as well as the peak finding module embedded in the Origin software, the phase information of the main wave and the air gap reflected echo is determined.

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