System and method for detecting damp state of internal interface of high-voltage cable joint

Abstract

The invention discloses a system and a method for detecting the damp state of an internal interface of a high-voltage cable joint. The system mainly comprises a distributed sensing optical fiber, a data modulation processor and an interface damp state calculation strategy based on the strain of the distributed sensing optical fiber. According to the method, the damp state is judged by utilizing strain increase after volume expansion in the damp aging state of the interface of the main insulation and the prefabricated part in the high-voltage cable joint, and the strain magnitude is represented by utilizing light scattering change characteristics caused by strain near the optical fiber. Compared with a traditional stress detection method, the method is low in cost and high in sensitivity, the method can be achieved through a communication optical cable in the production process, a sensor does not need to be additionally arranged, damage to internal insulation is avoided, and the insulation performance of the cable connector is not affected in the live-line operation process.

Description

Technical Field

[0001] The invention belongs to the field of internal detection of high-voltage cable joints, and in particular relates to a system and method for detecting the moisture state of the internal interface of a high-voltage cable joint. Background Art

[0002] In recent years, the problem of internal moisture in high-voltage cable joints has become increasingly serious. When the internal interface is damp during operation, it often causes insulation breakdown accidents. Cable joints are usually damp through the lead seals with poor melting process. In the dark and humid channel environment, moisture is affected by the temperature difference between the inside and outside and penetrates along the lead seals, and adheres to the internal interface of the high-voltage cable joint through the capillary effect at the interface. In order to enhance the insulation performance of the interface, silicone grease is applied to the interface between the prefabricated silicone rubber and the main insulation cross-linked polyethylene during the construction and installation of high-voltage cable joints. However, there are errors during manual on-site operations, and there are tiny gaps at the interface. When the cable is operating normally, there is a certain temperature difference between the main insulation near the cable conductor and the prefabricated silicone rubber. In an environment with high humidity or soaking in water, water vapor adheres to the silicone grease at the interface between the two and condenses to form liquid water, causing it to become damp.

[0003] After being exposed to moisture, silicone grease decomposes and produces small molecules after long-term operation. The small molecules migrate to the silicone rubber of the prefabricated parts and undergo cross-linking reactions, which reduce the cross-linking density of the silicone rubber and increase the free volume. This exerts a strong extrusion force on the interface and changes the strain of the cross-linked polyethylene of the main insulation. In addition, the entire aging process is accompanied by severe high-resistance heating, which not only affects the efficiency of cable power transmission, but also hardens when the aging degree of the material reaches the limit, which can easily cause collapse or explosion, and is a serious latent fault.

[0004] There are few direct detection methods for the internal moisture status of existing cable joints. Usually, indirect detection is performed by using electrical and thermal signals after material aging. During the detection, internal sensors and other operations need to be installed, which have a certain impact on the insulation performance of the cable joints. The direct detection method usually involves power outage and dissection of the cable joints for inspection after an accident, and it is impossible to make a real-time judgment. Therefore, the existing detection of the moisture status of the internal interface of high-voltage cable joints cannot be performed online with power on, and the hidden dangers are high and difficult to detect. With the rapid development of distributed optical fiber technology, based on its passive, low-cost and high-sensitivity characteristics, its application advantages in cable joints are obvious. When the factory produces cable joints, communication optical fibers are pre-buried in the main insulation, so there is no need to add additional sensors to monitor the internal status of the joints using changes in optical signals.

[0005] How to use optical fiber to detect the inside of cable connectors is a current problem. Summary of the invention

[0006] In view of the above problems, the present invention provides a system and method for detecting the moisture state of the internal interface of a high-voltage cable joint with low detection cost and high sensitivity.

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

[0008] A high-voltage cable joint internal interface moisture state detection system includes a data modulation processor, the data modulation processor includes a pulse light generator, an acousto-optic modulation device, a unidirectional optical isolator 1, a photoelectric detector, an optical fiber body, a filter amplifier, a laser generator, a unidirectional optical isolator 1 and a computer.

[0009] The pulse light generator is connected to the acousto-optic modulation device and connected to the computer at the same time; the acousto-optic modulation device is connected to the unidirectional optical isolator 1, the unidirectional optical isolator 1 is connected to the optical fiber body and connected to the photodetector at the same time, the photodetector is connected to the filter amplifier, and the filter amplifier is connected to the computer; the optical fiber body is connected to the unidirectional optical isolator 2, the unidirectional optical isolator 2 is connected to the laser generator, and the laser generator is connected to the computer;

[0010] The optical fiber body is arranged inside the main insulating cross-linked polyethylene in the cable joint.

[0011] The optical fiber body is a distributed sensing optical fiber or a communication optical cable, and the distributed sensing optical fiber includes an optical fiber, a steel pipe and a polyethylene sheath arranged in sequence from the inside to the outside. The distributed sensing optical fiber can directly use the communication optical cable pre-buried in the main insulation of the cable body without additional changes to the cable joint.

[0012] The cable joint comprises a cable conductor, a conductor shield, a crimping tube, a main insulating cross-linked polyethylene, an interface silicone grease layer, a prefabricated silicone rubber and a copper shell which are arranged in sequence from the inside to the outside.

[0013] The data modulation processor is packaged in a polypropylene shell and is hung on the inner wall surface of the cable channel.

[0014] Furthermore, a computer is used to control a pulse light generator to emit pulse light, which is converted into a pulse light with a frequency of v-Δv through an acousto-optic modulation device, and then enters the optical fiber body after passing through a unidirectional optical isolator 1; a detection light with a frequency of v is emitted by a laser generator, and then enters the optical fiber body after passing through a unidirectional optical isolator 2.

[0015] Furthermore, when the frequency difference Δv between the pulse light with frequency v-Δv and the detection light with frequency v is equal to the Brillouin backscattered light frequency shift v at a certain position in the fiber body, B At the same time, stimulated Brillouin scattering is generated, the detection light is amplified and after other scattered light is removed by a filter amplifier, it is detected by a photodetector and transmitted to a computer for processing.

[0016] Furthermore, by controlling the emission times of the pulsed light and the probe light through a computer, the scattering position in the optical fiber body can be determined.

[0017] A method for detecting the moisture state of the internal interface of a high-voltage cable joint, comprising the following steps:

[0018] S1. Calculate and determine the relationship between the frequency shift amount and the stress change around the optical fiber body;

[0019] After obtaining the relationship between the frequency shift amount and the stress, obtain the internal strain change in the main insulation cross-linked polyethylene under different moisture levels of the interface through experiments, and establish the relationship between the frequency shift amount and different moisture levels of the interface.

[0020] S3. Judge the moisture state of the internal interface of the high-voltage cable joint.

[0021] The detection method in the present invention is an interface moisture state calculation strategy based on distributed sensing optical fiber strain, mainly including: the principle of Brillouin scattering frequency shift mapping the strain near the optical fiber; the principle of strain change caused by moisture in the interface of the high-voltage cable joint; establishing a corresponding method between the moisture level of the interface and the Brillouin scattering frequency shift to realize the detection of the moisture state of the interface.

[0022] Furthermore, when the frequency difference Δv value between the pulsed light with a frequency of v - Δv and the probe light with a frequency of v is consistent with the Brillouin backscattering light frequency shift amount v at a certain position in the optical fiber body B Stimulated Brillouin scattering occurs. After the probe light is amplified and other scattered lights are removed through a filter amplifier, it is detected by a photodetector and transmitted to a computer. Based on this, the corresponding relationship between the frequency shift amount v B and the strain ε can be established, so as to analyze the scattering position of the optical fiber body and the strain generated around the optical fiber through the detected frequency shift amount of the probe light.

[0023] Furthermore, for the principle of strain change caused by moisture in the interface of the high-voltage cable joint, both silicone grease and prefabricated silicone rubber are macromolecular composite materials with a silicon-oxygen bond as the main chain. However, the molecular size of silicone grease is smaller, while the average molecular size of silicone rubber molecules is larger. After the silicone grease gets wet, it ages and decomposes to produce small molecule substances during long-term operation and migrates into the prefabricated silicone rubber to undergo a cross-linking reaction, causing the main chain at the original silicone rubber to break, reducing the cross-linking density of the silicone rubber and increasing the free volume, exerting a strong extrusion force on the interface, changing the strain at the main insulation cross-linked polyethylene, and the optical fiber body is located inside the main insulation cross-linked polyethylene. Therefore, the strain magnitude can be reflected by the frequency shift amount, specifically:

[0024] v B (ε) = v B (0)[1 + (Δn + ΔE + Δk + Δρ)ε]

[0025] In the formula, vB (ε) is the function of the frequency shift amount with respect to the strain ε, v B (0) is the frequency shift amount when the strain ε is 0, Δn represents the Taylor expansion component of the fiber refractive index when the strain is 0, ΔE represents the Taylor expansion component of the Young's modulus when the strain is 0, Δk represents the Taylor expansion component of the Poisson's ratio when the strain is 0, and Δρ represents the Taylor expansion component of the density when the strain is 0.

[0026] Furthermore, the strain is related to the moisture level of the interface. The calculation relationship between the frequency shift amount and the strain has been obtained above. By experimental means, the relationship data between the frequency shift amount and the moisture level can be obtained. Through machine learning, the data can be trained to obtain the mapping relationship formula between the moisture level and the detected optical frequency shift amount.

[0027] Furthermore, for the experiment, a test platform should be built using cross-linked polyethylene, silicone rubber, silicone grease, and a pressure film sensor of the same material as the high-voltage cable joint. The silicone grease is coated on the surface of the cross-linked polyethylene, and the silicone rubber is crimped on the outer layer. The pressure film sensor is placed between two inner layers of cross-linked polyethylene, and an initial stress is applied to ensure the same initial stress as that inside the actual cable joint.

[0028] Furthermore, the test platform is placed in a temperature and humidity chamber to ensure that the temperature remains unchanged. The moisture level is labeled with different humidities and different moisture absorption times. A series of gradient moisture absorption hours and a series of gradient environmental humidities are used for the experiment. Each time the experimental conditions are changed, a new sample needs to be made. The stress values after different humidities and different moisture absorption times are collected and converted into strain ε. The neural network operation is performed on the strain ε and the corresponding moisture level to obtain the relationship prediction model between the strain ε and the moisture level.

[0029] Furthermore, during the detection of the moisture level of the actual cable joint interface, first, the detected optical frequency shift amount is obtained, and the strain ε is calculated through the above derivation formula. After inputting the strain ε into the relationship prediction model, the moisture level can be judged, thereby realizing the judgment of the moisture level of the internal interface defect of the high-voltage cable joint.

[0030] Compared with the existing technical means, the beneficial effects of the present invention are:

[0031] 1. Compared with the traditional detection method, the present invention can achieve passive detection, ensure the uniformity of the electric field inside the cable joint, and has no impact on the insulation.

[0032] 2. The distributed sensing optical fiber proposed by the present invention can directly use the communication optical cable inside the main insulation cross-linked polyethylene of the cable body, without modifying the cable joint or the cable body, ensuring the normal operation of the equipment.

[0033] 3. The present invention has low cost. The distributed sensing optical fiber has low cost, and only one data modulation processor is needed to monitor multiple cable joints of multiple lines in real time.

[0034] 4. The present invention has high sensitivity. The order of magnitude of strain detection can reach 10 -6 , and it can effectively detect subtle strain states and timely feedback them to the operation and maintenance personnel for early warning. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a schematic diagram of the layout of the distributed sensing optical fiber of the present invention;

[0036] Figure 2 is a radial schematic diagram of the structure of the distributed sensing optical fiber and its internal position in the cable joint of the present invention;

[0037] Figure 3 is a schematic diagram of the principle of the Brillouin scattering frequency shift of the present invention reflecting the surrounding strain;

[0038] Figure 4 is a schematic diagram of the internal structure of the data modulation processor of the present invention;

[0039] In the figure, 1 is the cable conductor, 2 is the conductor shield, 3 is the compression joint, 4 is the main insulation cross-linked polyethylene, 5 is the interface silicone grease layer, 6 is the prefabricated silicone rubber, 7 is the copper shell, 8 is the distributed sensing optical fiber, 9 is the polyethylene sheath, 10 is the steel pipe, and 11 is the optical fiber. DETAILED DESCRIPTION OF THE INVENTION

[0040] The following further illustrates the application method of the technical solution of the invention in conjunction with the drawings and specific examples.

[0041] As Figures 1-4 shown, a detection system for the moisture ingress state of the internal interface of a high-voltage cable joint includes a data modulation processor. The data modulation processor includes a pulsed light generator, an acousto-optic modulation device, a unidirectional optical isolator 1, a photodetector, an optical fiber body, a filter amplifier, a laser generator, a unidirectional optical isolator 1, and a computer.

[0042] The pulsed light generator is connected to the acousto-optic modulation device and is also connected to the computer; the acousto-optic modulation device is connected to the unidirectional optical isolator 1, the unidirectional optical isolator 1 is connected to the optical fiber body and is also connected to the photodetector, the photodetector is connected to the filter amplifier, and the filter amplifier is connected to the computer; the optical fiber body is connected to the unidirectional optical isolator 2, the unidirectional optical isolator 2 is connected to the laser generator, and the laser generator is connected to the computer;

[0043] The optical fiber body is arranged inside the main insulation cross-linked polyethylene in the cable joint.

[0044] The optical fiber body is a distributed sensing optical fiber or a communication optical cable, and the distributed sensing optical fiber 8 includes an optical fiber 11, a steel tube 10 and a polyethylene sheath 9 arranged sequentially from the inside to the outside. The distributed sensing optical fiber can directly utilize the communication optical cable pre-buried in the main insulation of the cable body without additional changes to the cable joint.

[0045] The cable joint comprises a cable conductor 1, a conductor shield 2, a crimping tube 3, a main insulating cross-linked polyethylene 4, an interface silicone grease layer 5, a prefabricated silicone rubber 6 and a copper shell 7 which are arranged in sequence from the inside to the outside.

[0046] The data modulation processor is packaged in a polypropylene shell and is hung on the inner wall surface of the cable channel.

[0047] A method for detecting the moisture state of the internal interface of a high-voltage cable joint comprises the following steps:

[0048] S1. Calculate and determine the relationship between the frequency shift and the stress change around the optical fiber body;

[0049] S2. After obtaining the relationship between the frequency shift and the stress, the internal strain change of the main insulating cross-linked polyethylene under different degrees of interface moisture is obtained through experiments, and the relationship between the frequency shift and the different degrees of interface moisture is established.

[0050] S3. Determine the moisture status of the internal interface of the high-voltage cable connector.

[0051] In the implementation case, the communication optical cable inside the main insulation cross-linked polyethylene of the cable body is used as the distributed sensing optical fiber 8, without additional modification of the cable connector or the body. The communication optical cable is located inside the main insulation cross-linked polyethylene 4.

[0052] Figure 3 The curve is drawn according to the relationship between the frequency shift and the strain proposed in the present invention. According to the curve drawn according to the relationship, it can be directly observed that there is a positive correlation between the two. Therefore, the strain at the location can be calculated based on the measured frequency shift.

[0053] S1: In the implementation case, the moisture state of the internal interface of the high-voltage cable connector is indirectly diagnosed by detecting the optical frequency shift. First, it is necessary to determine the relationship between the frequency shift and the strain around the optical fiber body.

[0054] When the frequency difference Δv between the pulse light with frequency v-Δv and the detection light with frequency v is equal to the Brillouin backscattered light frequency shift v at a certain position in the fiber body, B When the frequency is consistent, stimulated Brillouin scattering occurs. The detection light is amplified and other scattered light is removed by the filter amplifier. It is then detected by the photodetector and transmitted to the computer. Based on this, the frequency shift v can be established. BThe corresponding relationship with the strain ε, so as to analyze the position of the scattering of the optical fiber body and the strain generated around the optical fiber through the detected optical frequency shift amount of the detected light.

[0055] The calculation process of the frequency shift amount is as follows:

[0056]

[0057] Among them, v B is the frequency shift amount, v0 is the initial frequency of the detected light, c is the speed of light in vacuum, n is the refractive index in the optical fiber, E is the Young's modulus, k is the Poisson's ratio, ρ is the density, and v0 is obtained by performing a Taylor expansion at 0:

[0058]

[0059] Among them, v B (ε) is the function of the frequency shift amount with respect to the strain ε, v B (0) is the frequency shift amount when the strain ε is 0, n’, E’, ρ’, k’ are the first-order derivatives of n, E, ρ, k respectively, and n(0), E(0), ρ(0), k(0) are the values of n, E, ρ, k at the 0 point respectively. Through simple substitution, the following can be obtained:

[0060] Let

[0061]

[0062] In the formula, Δn represents the Taylor expansion component of the refractive index of the optical fiber when the strain is 0, ΔE represents the Taylor expansion component of the Young's modulus when the strain is 0, Δk represents the Taylor expansion component of the Poisson's ratio when the strain is 0, and Δρ represents the Taylor expansion component of the density when the strain is 0.

[0063] Then:

[0064] v B (ε) = v B (0)[1 + (Δn + ΔE + Δk + Δρ)ε]

[0065] Among them:

[0066]

[0067] In the formula, P 11 and P 12 are photoelastic coefficients.

[0068] Among them, the relationship between Δρ and k is as follows:

[0069]

[0070] Since ΔE and Δk are difficult to obtain through experiments, they can be estimated to be 2.88 and 1.49 respectively by estimating the material of the quartz optical fiber.

[0071] Calculating with quartz as the optical fiber material, the relationship between the frequency shift and the strain can be obtained as follows:

[0072] Δv B =v B (ε)-v B (0)=4.88v B (0)ε.

[0073] S2: After obtaining the relationship between the frequency shift and the stress, obtain the internal strain change of the main insulation cross-linked polyethylene under different moisture levels at the interface through experiments, and establish the relationship between the frequency shift and different moisture levels at the interface.

[0074] S2.1: The experiment should use cross-linked polyethylene, silicone rubber, silicone grease, and a pressure film sensor of the same material as the high-voltage cable joint to build a test platform.

[0075] First, cut the cross-linked polyethylene and silicone rubber into rectangular sheet samples of 150*100*10 mm respectively.

[0076] Secondly, coat the surface of the cross-linked polyethylene sample with silicone grease, attach a pressure film sensor with dimensions of 20*10*5 to the central position of the outer surface of the cross-linked polyethylene sample, and disconnect the data transmission of the pressure film sensor into the computer.

[0077] Finally, attach the silicone rubber sample to the outer surface of the cross-linked polyethylene sample, and apply an initial pressure of 0.1 Mpa to the other surface of the silicone rubber sample.

[0078] S2.2: Experimental process.

[0079] First, place the test platform in a temperature and humidity aging chamber, ensure that the temperature remains at 25°C, and use relative humidity and moisture time as parameters for the moisture level. The moisture level is characterized as shown in the following table:

[0080]

[0081]

[0082] Secondly, for each sample under the above moisture level, it needs to be remade and experimented to ensure that there is no influence between samples. Five groups of sample experiments are carried out simultaneously under different moisture levels to reduce the errors that occur during the experiment.

[0083] Finally, export and store the continuous data in the pressure film sensor after the experiment. The data is the change curve of the strain ε with time under different moisture levels.

[0084] S2.3: Relationship prediction model construction

[0085] The obtained strain ε variation curves under different moisture levels are put into the input segment of the BPNN inverse neural network according to the moisture level and strain ε change, and the nonlinear excitation function is used for iterative calculation to form a group of control equations that can realize the corresponding relationship between moisture level and strain ε.

[0086] S3: Determine the moisture status of the internal interface of the high-voltage cable connector

[0087] S3.1: Install a data modulation processor at the end of a cable project containing a communication optical cable in operation, capture the detection light returned during the actual operation, and perform analysis and calculation to obtain the frequency shift of the detection light.

[0088] S3.2: Calculate the frequency shift and strain to obtain the internal strain of the main insulating cross-linked polyethylene. After inputting the strain into the control equation group, the input variable is the strain and the output variable is the moisture state, and the moisture state is judged.

[0089] The present invention uses the volume expansion of the interface between the main insulation and the prefabricated part of the high-voltage cable joint under the condition of moisture aging to increase the strain to judge the moisture state, and uses the light scattering change characteristics caused by the strain near the optical fiber to characterize the strain size. Compared with the traditional stress detection method, this method has low cost and high sensitivity. It can be realized by using the communication optical cable brought during production, without the need for additional sensors and avoiding damage to the internal insulation. It has no effect on the insulation performance of the cable joint when it is energized.

[0090] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A detection system for the moisture state of the internal interface of a high-voltage cable joint, characterized in that, Including a data modulation processor, the data modulation processor includes a pulsed light generator, an acousto-optic modulation device, an optical isolator 1, a photodetector, an optical fiber body, a filter amplifier, a laser generator, an optical isolator 1, and a computer. The pulsed light generator is connected to the acousto-optic modulation device and is also connected to the computer; the acousto-optic modulation device is connected to the optical isolator 1, the optical isolator 1 is connected to the optical fiber body and is also connected to the photodetector, the photodetector is connected to the filter amplifier, and the filter amplifier is connected to the computer; the optical fiber body is connected to the optical isolator 2, the optical isolator 2 is connected to the laser generator, and the laser generator is connected to the computer. The optical fiber body is arranged inside the main insulation cross-linked polyethylene in the cable joint.

2. The internal interface moisture absorption state detection system for a high-voltage cable joint according to claim 1, characterized in that The optical fiber body is a distributed sensing optical fiber or a communication optical cable. The distributed sensing optical fiber includes an optical fiber, a steel pipe, and a polyethylene sheath arranged in sequence from the inside to the outside.

3. A detection system for the moisture state of the internal interface of a high-voltage cable joint according to claim 1, characterized in that, The cable joint includes a cable conductor, a conductor shield, a compression joint pipe, a main insulation cross-linked polyethylene, an interface silicone grease layer, a prefabricated silicone rubber, and a copper shell arranged in sequence from the inside to the outside.

4. A detection system for the moisture state of the internal interface of a high-voltage cable joint according to claim 1, characterized in that, The data modulation processor is packaged with a polypropylene shell and is suspended on the inner wall surface of the cable trench.

5. A method for detecting the moisture state of the internal interface of a high-voltage cable joint, characterized in that, Based on the implementation of a high-voltage cable joint internal interface moisture ingress state detection system according to any one of claims 1-4, it includes the following steps: S1. Calculate and determine the relationship between the frequency shift amount and the strain around the optical fiber body. S2. Establish the relationship between the strain and different moisture ingress degrees of the interface. S3. Judge the moisture ingress state of the internal interface of the high-voltage cable joint.

6. A method for detecting the moisture ingress state of the internal interface of a high-voltage cable joint according to claim 5, characterized in that In step S1, the computer is used to control the pulsed light generator to emit pulsed light, which is transformed into pulsed light with a frequency of v-Δv through the acousto-optic modulation device, and then enters the optical fiber body after passing through the optical isolator 1; the laser generator emits detection light with a frequency of v, which enters the optical fiber body after passing through the optical isolator 2. When the frequency difference Δv between the pulsed light with frequency v - Δv and the probe light with frequency v is consistent with the Brillouin backward scattering light frequency shift v at a certain position in the optical fiber body B stimulated Brillouin scattering occurs. After the probe light is amplified and other scattered lights are removed by a filter amplifier, it is detected by a photodetector and transmitted to a computer, and then the corresponding relationship between the frequency shift and the strain is established, specifically as follows: v B v(ε) = B (0)[1 + (Δn + ΔE + Δk + Δρ)ε] where \(v\) B \((\varepsilon)\) is the function of the frequency shift with respect to the strain \(\varepsilon\), and \(v\) B (0) is the frequency shift when the strain \(\varepsilon = 0\), \(\Delta n\) represents the Taylor expansion component of the fiber refractive index when the strain is 0, \(\Delta E\) represents the Taylor expansion component of the Young's modulus when the strain is 0, \(\Delta k\) represents the Taylor expansion component of the Poisson's ratio when the strain is 0, and \(\Delta\rho\) represents the Taylor expansion component of the density when the strain is 0.

7. A method for detecting the moisture ingress state of the internal interface of a high-voltage cable joint according to claim 6, characterized in that Step S2 includes: First, obtain the strain changes inside the main insulation cross-linked polyethylene under different moisture ingress degrees of the interface. Then, the strain-time change curves obtained under different moisture ingress degree levels are respectively placed into the input section of the BPNN backpropagation neural network according to the moisture ingress degree level and the strain change, and iterative calculations are performed using a non-linear activation function to form a control equation set that can realize the corresponding relationship between the moisture ingress degree level and the strain ε.

8. A method for detecting the moisture ingress state of the internal interface of a high-voltage cable joint according to claim 7, characterized in that Step S3 includes: After inputting the strain into the control equation set, the input variable is the strain, the output variable is the moisture ingress state, and the moisture ingress state is judged.

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