High-voltage cable joint main insulation internal defect 3D reconstruction system and method

Abstract

The invention discloses a high-voltage cable joint main insulation internal defect 3D reconstruction system and method, the system comprises a nonlinear ultrasonic detection platform and a signal processing computer, the nonlinear ultrasonic detection platform comprises a nonlinear ultrasonic signal generating device, a nonlinear ultrasonic signal receiving device and a two-dimensional plane moving device; the nonlinear ultrasonic signal generating device and the nonlinear ultrasonic signal receiving device are installed on the two-dimensional plane moving device and connected with the signal processing computer through communication cables. According to the invention, based on a nonlinear ultrasonic detection technology, non-destructive detection of tiny defects at the main insulation part in the high-voltage cable joint can be realized, the detection rate of the tiny defects is high, and three-dimensional reconstruction of the defects at the main insulation part in the joint can be realized, so that inspection personnel can judge the types and degrees of the defects more intuitively.

Description

Technical Field

[0001] The present invention belongs to the technical field of non-destructive detection of internal defects of cable joints, and particularly relates to a 3D reconstruction system and method for internal defects in the main insulation of high-voltage cable joints. Background Art

[0002] Compared with traditional overhead lines, high-voltage cable power transmission has the advantages of easy maintenance and less space occupation. The underground cable project can avoid the problem of urban cobwebs. At present, the construction scale of high-voltage cable projects is gradually expanding, and both the capacity and length are continuously increasing. The quality and operating conditions of cables are closely related to the safety and stability of power transmission. During the laying process of high-voltage cable projects, the laying distance is often long, and the transmission channel needs to be changed when passing through different regions. During laying, a large number of intermediate joints are often used for connection, and intermediate joints are also required when the line is extended later.

[0003] Although the basic materials of high-voltage cable joints are the same as those of the cable body, the internal structure is relatively complex. During operation, the change of the internal physical field is quite different from that of the cable body, and faults are most likely to occur. According to relevant accident statistics, the accidents of high-voltage cable joints exceed more than half of the total number of accidents. At present, the high-voltage cable intermediate joints are mainly prefabricated, that is, they are installed on site after production, which requires extremely high production technology and installation experience. Limited by technology and experience, small cracks, scratches, and pits will be caused to the main insulation when stripping and installing high-voltage cable joints. The existence of small defects has a serious impact on the insulation ability at the cable joint, becoming an important hidden danger during the operation of the cable.

[0004] The main insulation of high-voltage cable joints is located in the internal area. When small defects such as scratches, cracks, and pits occur, it is difficult to conduct stripping inspection. The existence of small defects seriously affects the electric field distribution at the interface near the main insulation, and due to the change of the structure, there is a phenomenon of uneven stress at the relevant composite interface. After long-term operation, it is likely to age and cause insulation breakdown, ultimately causing serious damage to the power grid operation. When installing intermediate joints, it is necessary to rely on manual stripping of the cable body and then install it through manual operation. Therefore, small internal defects of high-voltage cable joints mainly exist near the main insulation, and existing detection methods cannot effectively detect them. Usually, physical quantities such as voltage, current-carrying capacity, sheath circulating current, and temperature that can be directly measured during the operation of cable joints are used for indirect analysis and calculation of the internal conditions. However, the above detection methods cannot identify internal defects and can only judge the operating state through physical quantities, increasing the workload of operation and maintenance.

[0005] Based on the above problems, aiming at the difficulty of detecting tiny defects in the main insulation inside high-voltage cable joints, there is an urgent need for a non-destructive and penetrative detection method to detect internal defects. In recent years, the non-linear ultrasonic detection technology has developed rapidly and is widely used in fields such as chemical materials, engineering construction, and equipment detection. Compared with traditional ultrasonic detection based on linear theory, which detects through parameters such as sound velocity, sound attenuation, and reflection coefficient, the non-linear ultrasonic detection technology has a more sensitive detection advantage for tiny defects. By applying a non-linear ultrasonic signal to the target and then performing high-order harmonic analysis on the reflected wave, the condition of the defect can be manifested through the degree of non-linearity, and the morphology of the defect can be restored using the degree of non-linearity, which has good convenience and intuitiveness. Therefore, based on the non-linear ultrasonic technology, the present invention proposes a 3D reconstruction method for internal defects in the main insulation of high-voltage cable joints, providing support for the research on non-destructive detection technology for internal defects in the main insulation of high-voltage cable joints and the reconstruction of defect morphology. Summary of the Invention

[0006] Aiming at the deficiencies of the existing technology, the present invention provides a 3D reconstruction system and method for internal defects in the main insulation of high-voltage cable joints. Based on the non-linear physical characteristics of the main insulation of high-voltage cable joints, the non-linear ultrasonic detection technology is used to detect internal defects in the main insulation, and the returned non-linear ultrasonic signals are processed and converted to obtain a three-dimensional stereoscopic image of the internal defects in the main insulation of the cable joint, providing an intuitive, efficient, and non-destructive detection means for operation and maintenance personnel.

[0007] The technical solution adopted by the present invention is as follows:

[0008] A 3D reconstruction system for internal defects in the main insulation of high-voltage cable joints includes a non-linear ultrasonic detection platform and a signal processing computer.

[0009] The non-linear ultrasonic detection platform includes a non-linear ultrasonic signal generating device, a non-linear ultrasonic signal receiving device, a filter, and a two-dimensional plane moving device.

[0010] The non-linear ultrasonic signal generating device and the non-linear ultrasonic signal receiving device are installed on the two-dimensional plane moving device. The non-linear ultrasonic signal generating device and the non-linear ultrasonic signal receiving device are connected to the signal processing computer through communication cables. The non-linear ultrasonic signal generating device, the non-linear ultrasonic signal receiving device, and the two-dimensional plane moving device are powered by the signal processing computer.

[0011] A filter is installed at the port of the signal processing computer to process the received non-linear ultrasonic signals. The signal processing computer is a computer with a power supply system.

[0012] The two-dimensional planar moving device is a robotic arm that moves in the X or Y direction on the two-dimensional plane XY. A non-linear ultrasonic signal generating device and a non-linear ultrasonic signal receiving device are installed at the end of the robotic arm. The non-linear ultrasonic signal generating device can only emit a single pulse signal each time, and the non-linear ultrasonic signal receiving device can only receive a single pulse signal each time. By performing translational scanning in the XY directions, the signal state of the entire defect can be obtained.

[0013] A method for 3D reconstruction of internal defects in the main insulation of a high-voltage cable joint, comprising:

[0014] Step 1: Build a non-linear ultrasonic detection platform;

[0015] The non-linear ultrasonic signal detection method for internal defects in the main insulation of a high-voltage cable joint is to vertically attach the non-linear ultrasonic signal generating device to the surface of the main insulation of the high-voltage cable joint, add a coupling agent at the attachment interface between the two, and use a signal processing computer to trigger the non-linear ultrasonic signal generating device to emit a 5 MHz non-linear ultrasonic signal to pass through the main insulation. When there are internal defects, reflected signals will be generated. After being received by the non-linear ultrasonic signal receiving device, they are transmitted to the filter.

[0016] Step 2: Perform non-linear ultrasonic scanning on the inside of the main insulation of the high-voltage cable joint;

[0017] The order of ultrasonic scanning is to first vertically map the surface of the main insulation of the high-voltage cable joint into a two-dimensional plane. The robotic arm on the two-dimensional planar moving device where the non-linear ultrasonic signal generating device and the non-linear ultrasonic signal receiving device are located starts from the first point in the lower right corner as the starting position, and performs S-shaped scanning in the order of first the X direction and then the Y direction. The scanning positions in the X-axis direction are X = [X1, X2,... X m , and the scanning positions in the Y-axis direction are Y = [Y1, Y2,... Y n .

[0018] Step 3: Receive the returned reflected wave signals and summarize them into a matrix;

[0019] The position matrix T is as follows:

[0020]

[0021] Among them, the returned non-linear ultrasonic reflected wave signals are:

[0022]

[0023] In the formula, U is the reflected wave signal matrix, that is, at each position XY in the position matrix T, there corresponds a reflected wave signal Z. The reflected wave signal Z contains the fundamental wave and the second harmonic of the reflected wave. m and n are the number of scans along the X and Y axis directions respectively.

[0024] Step 4: Extract the fundamental relative amplitude and the second harmonic relative amplitude of the returned reflected wave signal, calculate the second harmonic coefficient of the reflected wave, and then calculate the degree of nonlinearity, and use the degree of nonlinearity to characterize the size of the defect;

[0025] The incident wave signal is assumed to be:

[0026] u i = U i exp(jkx)

[0027] where u i is the incident wave signal, j is the imaginary unit, k is the linear stiffness coefficient of the nonlinear spring model, x is the independent variable, and the reflected wave signal and the transmitted wave signal can be expressed as:

[0028]

[0029] where u r (1) is the fundamental wave signal of the reflected wave, u r (2) is the second harmonic signal of the reflected wave, u t (1) is the fundamental wave signal of the transmitted wave, u t (2) is the second harmonic signal of the transmitted wave, U i , U r , U t are the incident wave coefficient, the reflected wave coefficient, and the transmitted wave coefficient respectively, and U r (1) is the fundamental wave coefficient of the reflected wave, U r (2) is the second harmonic coefficient of the reflected wave, U t (1) is the fundamental wave coefficient of the transmitted wave, and U t (2) is the second harmonic coefficient of the transmitted wave.

[0030] The expressions for the fundamental wave coefficient of the reflected wave and the fundamental wave coefficient of the transmitted wave are:

[0031]

[0032] where φ is the acoustic impedance matching state coefficient, ω is the angular frequency, and z1 and z2 are the acoustic impedances of the media in the incident source direction and the incident target direction respectively.

[0033] Furthermore, the expression for calculating the second harmonic coefficient of the reflected wave is:

[0034]

[0035] where H in the expression for the second harmonic coefficient of the reflected wave is calculated by the following formula:

[0036]

[0037] In the above formula, H is the intensity of the nonlinear interaction of the fundamental wave at the interface. Therefore, there is a proportional relationship between the second harmonic coefficient of the reflected wave and the degree of nonlinearity α.

[0038] The second harmonic coefficient of the reflected wave can be calculated by measuring the relative amplitude A of the fundamental wave signal and the relative amplitude B of the second harmonic signal of the reflected wave:

[0039]

[0040] The relative amplitude A of the fundamental wave signal and the relative amplitude B of the second harmonic signal of the reflected wave are measured and are included in the position matrix T and the reflected wave matrix U.

[0041] Step 5: Convert the magnitude characterized by the degree of nonlinearity into a three-dimensional stereoscopic image of the defect.

[0042] In the operation of the present invention, based on the nonlinear ultrasonic detection technology, it can realize non-destructive detection of micro-defects in the main insulation inside the high-voltage cable joint. It has a high detection rate for micro-defects and can perform three-dimensional reconstruction of the defects in the main insulation inside the joint, enabling the operation and maintenance personnel to more intuitively judge the type and degree of the defects. Description of the Drawings

[0043] Figure 1 It is a schematic diagram of the nonlinear ultrasonic detection platform and experimental circuit of the present invention;

[0044] Figure 2 It is a schematic diagram of the principle of detecting internal defects by nonlinear ultrasonic of the present invention;

[0045] Figure 3 It is a flowchart of the implementation of the present invention;

[0046] Figure 4 It is the waveform of the nonlinear ultrasonic mechanism signal in the example of the present invention;

[0047] In the figure, 1 - signal processing computer, 2 - filter, 3 - two-dimensional plane moving device, 4 - robotic arm, 5 - nonlinear ultrasonic signal generating device, 6 - nonlinear ultrasonic signal receiving device, 7 - target sample, 8 - nonlinear ultrasonic incident signal, 9 - transmitted wave, 10 - reflected wave, 11 - internal defect, 12 - glass plate. Detailed Embodiments

[0048] The following further illustrates the application mode of the technical solution of the invention in combination with the drawings and specific examples.

[0049] Figure 1Schematic diagram of the non-linear ultrasonic detection platform and experimental circuit of the present invention. The port of the signal processing computer 1 is connected to the filter 2, and the remaining ports are connected to the two-dimensional plane moving device 3 through control lines. The filter 2 is connected to the robotic arm 4 through a signal line and is connected to the non-linear signal receiving device 6. The two-dimensional plane moving device 3 is connected to the non-linear ultrasonic signal generating device 5 through a control line. The target sample 7 is placed under the robotic arm 4.

[0050] Figure 2 Schematic diagram of the principle of non-linear ultrasonic detection of internal defects of the present invention. The sample 7 is placed on the glass plate 12. The non-linear ultrasonic signal generating device 5 emits an incident wave 8 with a frequency of 5 MHz close to the upper surface of the sample 7. After the incident wave 8 passes through the possible internal defect 11, a transmitted wave 9 and a reflected wave 10 are generated. The reflected wave 10 is received by the non-linear ultrasonic signal receiving device 6 for analysis.

[0051] As Figure 3 shown, a 3D reconstruction method for internal defects in the main insulation of a high-voltage cable joint includes:

[0052] Step 1: Build a non-linear ultrasonic detection platform;

[0053] The non-linear ultrasonic signal detection method for internal defects in the main insulation of a high-voltage cable joint is to vertically attach the non-linear ultrasonic signal generating device to the surface of the main insulation of the high-voltage cable joint, add a coupling agent at the attachment interface between the two, and use the signal processing computer to trigger the non-linear ultrasonic signal generating device to emit a 5 MHz non-linear ultrasonic signal to pass through the main insulation. When there are internal defects, a reflected signal will be generated. After being received by the non-linear ultrasonic signal receiving device, it is transmitted to the filter.

[0054] Step 2: Conduct non-linear ultrasonic scanning on the inside of the main insulation of the high-voltage cable joint;

[0055] The order of ultrasonic scanning is to first vertically map the surface of the main insulation of the high-voltage cable joint into a two-dimensional plane. The robotic arm on the two-dimensional plane moving device where the non-linear ultrasonic signal generating device and the non-linear ultrasonic signal receiving device are located takes the first point in the lower right corner as the starting position, and performs an S-shaped scan in the order of the X direction first and then the Y direction. The scanning positions in the X-axis direction are X = [X1, X2,... X m , and the scanning positions in the Y-axis direction are Y = [Y1, Y2,... Y n .

[0056] Step 3: Receive the returned reflected wave signals and summarize them into a matrix;

[0057] The principle that the nonlinear ultrasonic second harmonic signal can detect the internal defects of the main insulation of cable joints is due to the nonlinear characteristics of the solid dielectric material itself. If there are foreign objects such as cracks and bubbles in a single substance, the overall medium is not uniform, so the ultrasonic wave of a single frequency will change or generate higher harmonics. Nonlinear ultrasound analyzes the harmonic signal, has higher sensitivity to detect tiny defects, and is suitable for the scenario of nonlinear materials. The reflected wave signal is received by the nonlinear ultrasonic signal receiving device and forms a matrix.

[0058] Step 4: Extract the fundamental relative amplitude and second harmonic relative amplitude of the returned reflected wave signal, calculate the second harmonic coefficient of the reflected wave, and then calculate the degree of nonlinearity, and use the degree of nonlinearity to characterize the size of the defect;

[0059] Step 5: Convert the size characterized by the degree of nonlinearity into a three-dimensional stereoscopic image of the defect.

[0060] The defect reconstruction process based on the nonlinear ultrasonic second harmonic signal is to obtain the degree of nonlinearity of the internal defects of the main insulation of the cable joint characterized by the fundamental relative amplitude and second harmonic relative amplitude of the reflected wave signal, use the degree of nonlinearity to represent the size of the defect, the area size of the defect is represented by the position matrix T, the depth size of the defect is represented by the degree of nonlinearity, and the three-dimensional reconstruction of the defect can be realized through the two-dimensional plane XY axis and amplitude.

[0061] When there are defects in the main insulation, the size of the defects in the depth direction is positively correlated with the degree of nonlinearity of the reflected wave, and the area of the defects perpendicular to the depth direction is determined by the position matrix T. There is a proportional relationship between the degree of nonlinearity and the second harmonic coefficient of the reflected wave, so the degree of nonlinearity can be indirectly calculated by calculating the second harmonic coefficient of the reflected wave.

[0062] Figure 4 This is the excitation waveform of the nonlinear ultrasonic signal used in the implementation of the present invention. By applying Figure 4 the excitation waveform shown, the reflected wave signal of the nonlinear ultrasonic signal is obtained and extracted.

[0063] The incident wave signal is assumed to be:

[0064] u i =U i exp(jkx)

[0065] where u i is the incident wave signal, j is the imaginary number, k is the linear stiffness coefficient of the nonlinear spring model, x is the independent variable, and the reflected wave signal and the transmitted wave signal can be expressed as:

[0066]

[0067] Among them, u r (1) is the fundamental wave signal of the reflected wave, and u r (2) is the second harmonic wave signal of the reflected wave, and u t (1) is the fundamental wave signal of the transmitted wave, and u t (2) is the second harmonic wave signal of the transmitted wave. U i , U r , U t are the incident wave coefficient, the reflected wave coefficient, and the transmitted wave coefficient respectively. U r (1) is the fundamental wave coefficient of the reflected wave, and U r (2) is the second harmonic wave coefficient of the reflected wave, and U t (1) is the fundamental wave coefficient of the transmitted wave, and U t (2) is the second harmonic wave coefficient of the transmitted wave.

[0068] The expressions for the fundamental wave coefficient of the reflected wave and the fundamental wave coefficient of the transmitted wave are as follows:

[0069]

[0070]

[0071] Among them, φ is the acoustic impedance matching state coefficient, ω is the angular frequency, and z1 and z2 are the acoustic impedances of the medium in the incident source direction and the acoustic impedance of the medium in the incident target direction respectively.

[0072] Furthermore, the expression for calculating the second harmonic wave coefficient of the reflected wave is:

[0073]

[0074] Among them, H in the expression for the second harmonic wave coefficient of the reflected wave is calculated by the following formula:

[0075]

[0076] In the above formula, H is the intensity of the nonlinear interaction of the fundamental wave at the interface. Therefore, the second harmonic wave coefficient of the reflected wave is proportional to the nonlinear degree α.

[0077] The second harmonic wave coefficient of the reflected wave can be calculated by measuring the relative amplitude A of the fundamental wave signal and the relative amplitude B of the second harmonic wave signal of the reflected wave:

[0078]

[0079] The relative amplitude A of the fundamental wave signal of the reflected wave and the relative amplitude B of the second harmonic signal are measured and included in the position matrix T and the reflected wave matrix U.

[0080] During scanning, the positions scanned in the X-axis direction are X = [X1, X2, … X m , and the positions scanned in the Y-axis direction are Y = [Y1, Y2, … Y n , and the position matrix T is as follows:

[0081]

[0082] The returned non-linear ultrasonic reflected wave signal is:

[0083]

[0084] In the formula, U is the reflected wave signal matrix, that is, at each position XY in the position matrix T, there corresponds a reflected wave signal Z. The reflected wave signal Z includes the fundamental wave of the reflected wave and the second harmonic of the reflected wave. m and n are the number of scans along the X and Y axis directions respectively.

[0085] The size of the defect inside the main insulation includes depth and area. The area is represented by the position matrix T stored during scanning. The size of the depth is proportional to the non-linearity degree. Therefore, by calculating the non-linearity degree, the depth size of a certain position point of the defect can be represented. The non-linearity degree is proportional to the second harmonic coefficient of the reflected wave. By calculating the second harmonic coefficient of the reflected wave, the non-linearity degree can be calculated. The second harmonic coefficient of the reflected wave is calculated after measuring the relative amplitude of the fundamental wave signal and the relative amplitude of the second harmonic signal of the reflected wave.

[0086] The non-linear ultrasonic defect reconstruction scheme in the present invention includes a non-linear ultrasonic signal detection method for defects inside the main insulation of a high-voltage cable joint, processing and analysis of the second harmonic signal of the non-linear ultrasonic signal, and a defect reconstruction process based on the second harmonic signal of the non-linear ultrasonic signal;

[0087] Among them, the non-linear ultrasonic signal detection method for defects inside the main insulation of a high-voltage cable joint is to vertically attach the non-linear ultrasonic signal generating device to the surface of the main insulation of the high-voltage cable joint, add a coupling agent at the attachment interface between the two, and use the signal processing computer to trigger the non-linear ultrasonic signal generating device to emit a 5 MHz non-linear ultrasonic signal to pass through the main insulation. When there is a defect inside, a reflected signal will be generated. After being received by the non-linear ultrasonic signal receiving device, it is transmitted to the filter for automatic filtering processing, and then the processed signal is returned to the signal processing computer.

[0088] Processing and analysis of the second harmonic signal of the non-linear ultrasonic signal is to perform second harmonic extraction processing on the received non-linear ultrasonic signal to obtain the second harmonic amplitude for characterizing the non-linearity degree of the internal defect.

[0089] The defect reconstruction process based on the nonlinear ultrasonic second harmonic signal characterizes the nonlinear degree of the defects inside the main insulation of the cable joint through the amplitude of the second harmonic of the obtained nonlinear ultrasonic signal, uses the nonlinear degree to represent the severity of the defects, combines the area where the nonlinear change occurs to determine the scope of the defects, and combines the two to visually show the size and degree of the defects using a data statistical table. By mapping the values into a three-dimensional space, the three-dimensional reconstruction of the defects can be achieved.

[0090] The present invention has the following advantages:

[0091] 1. The present invention proposes a new detection method for the internal micro-defects of the main insulation of high-voltage cable joints based on the nonlinear ultrasonic detection technology, which can ensure the detection of the internal defects of the main insulation of the cable joint on the premise of not shutting down, not powering off, and not damaging the internal structure of the high-voltage cable joint.

[0092] 2. The detection efficiency is high, especially for the micro-defects that are difficult to be found manually inside and outside the main insulation, such as burrs, cracks, pits, and scratches, and has a high detection sensitivity.

[0093] 3. The proposed nonlinear ultrasonic second harmonic analysis method has a smaller error compared with the traditional ultrasonic detection method and has a better penetration for the multi-layer composite interface target.

[0094] 4. The proposed 3D reconstruction method for the internal defects of the main insulation includes the defect position information and degree information. Through the three-dimensional reconstruction, the internal defects can be effectively restored visually, improving the efficiency of the operation and maintenance personnel and reducing the workload.

[0095] 5. The proposed detection platform is easy to transplant, can be applied under different cable lines, and does not require additional equipment, reducing the operation and maintenance cost and increasing the convenience of detection.

[0096] The above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A 3D reconstruction system for internal defects in the main insulation of a high-voltage cable joint, characterized in that, It includes a non-linear ultrasonic detection platform and a signal processing computer. The non-linear ultrasonic detection platform includes: a non-linear ultrasonic signal generating device, a non-linear ultrasonic signal receiving device, a filter, and a two-dimensional planar moving device. The non-linear ultrasonic signal generating device and the non-linear ultrasonic signal receiving device are installed on the two-dimensional planar moving device. The non-linear ultrasonic signal generating device and the non-linear ultrasonic signal receiving device are connected to the signal processing computer through communication cables, and the non-linear ultrasonic signal generating device, the non-linear ultrasonic signal receiving device, and the two-dimensional planar moving device are powered by the signal processing computer. A filter is installed at the port of the signal processing computer for processing the received non-linear ultrasonic signals.

2. The 3D reconstruction system for internal defects of a high-voltage cable joint according to claim 1, characterized in that, The signal processing computer is a computer with a power supply system.

3. A 3D reconstruction system for internal defects of a high-voltage cable joint according to claim 1, characterized in that, The two-dimensional planar moving device is a robotic arm that moves in the X or Y direction on the two-dimensional plane XY. The non-linear ultrasonic signal generating device and the non-linear ultrasonic signal receiving device are installed at the end of the robotic arm.

4. A 3D reconstruction system for internal defects of a high-voltage cable joint according to claim 3, characterized in that The non-linear ultrasonic signal generating device can only emit a pulse signal once each time, and the non-linear ultrasonic signal receiving device can only receive a pulse signal once each time. The signal state of the entire defect is obtained through translation scanning in the XY direction.

5. A method for a 3D reconstruction system of internal defects in the main insulation of a high-voltage cable joint according to any one of claims 1-4, characterized in that, It includes: Step 1: Build a non-linear ultrasonic detection platform. Step 2: Conduct non-linear ultrasonic scanning inside the main insulation of the high-voltage cable joint. Step 3: Receive the returned reflected wave signals and summarize them into a matrix. Step 4: Extract the fundamental wave relative amplitude and the second harmonic relative amplitude of the returned reflected wave signals, calculate the second harmonic coefficient of the reflected wave, and then calculate the non-linearity degree, and use the non-linearity degree to characterize the size of the defect. Step 5: Convert the size characterized by the non-linearity degree into a three-dimensional stereoscopic image of the defect.

6. A 3D reconstruction method for internal defects in the main insulation of a high-voltage cable joint according to claim 5, characterized in that In Step 1, by vertically attaching the non-linear ultrasonic signal generating device to the surface of the main insulation of the high-voltage cable joint, adding a coupling agent at the attachment interface between the two, using the signal processing computer to trigger the non-linear ultrasonic signal generating device to emit a 5 MHz non-linear ultrasonic signal to pass through the inside of the main insulation. When there is a defect inside, a reflected signal will be generated. After being received by the non-linear ultrasonic signal receiving device, it is transmitted to the filter for automatic filtering processing, and then the processed signal is returned to the signal processing computer.

7. A 3D reconstruction method for internal defects in the main insulation of a high-voltage cable joint according to claim 5, characterized in that, In step 2, the order of ultrasonic scanning is as follows: first, the main insulation surface of the high-voltage cable joint is vertically mapped into a two-dimensional plane. The robotic arm on the two-dimensional plane moving device where the non-linear ultrasonic signal generating device and the non-linear ultrasonic signal receiving device are located starts from the first point in the lower right corner and performs an S-shaped scan in the order of the X direction first and then the Y direction. The scanning positions in the X-axis direction are X = [X1, X2, … X m , and the scanning positions in the Y-axis direction are Y = [Y1, Y2, … Y n .

8. A 3D reconstruction method for internal defects in the main insulation of a high-voltage cable joint according to claim 7, characterized in that In Step 3, the summarized matrix is as follows: The position matrix T is: The returned non-linear ultrasonic reflected wave signal is: In the formula, U is the reflected wave signal matrix. The reflected wave signal Z contains the fundamental wave of the reflected wave and the second harmonic of the reflected wave. m and n are the number of scans along the X and Y axis directions respectively.

9. A 3D reconstruction method for internal defects in the main insulation of a high-voltage cable joint according to claim 8, characterized in that, In Step 4, the relationship between the second harmonic coefficient of the reflected wave and the non-linearity degree is: where U r (2) is the second harmonic coefficient of the reflected wave, α is the degree of nonlinearity, H is the intensity of the nonlinear interaction of the fundamental wave at the interface, z1 and z2 are the acoustic impedances of the media in the incident source direction and the incident target direction respectively, k is the linear stiffness coefficient of the nonlinear spring model, j is the imaginary unit, and ω is the angular frequency.

10. According to the method for 3D reconstruction of internal defects of the main insulation of a high-voltage cable joint described in claim 9, wherein In the formula, A is the relative amplitude of the fundamental wave signal of the reflected wave, and B is the relative amplitude of the second harmonic signal.

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