Method and device for checking latent insulation defect alternating current withstand voltage of optical fiber for transformer

Abstract

The invention discloses an alternating-current withstand voltage assessment method for latent insulation defects of an optical fiber for a transformer, and the method employs an optimized ball-plate electrode structure, guarantees the uniform distribution of an electric field, and avoids the edge effect and bubble interference of an electrode. The arrangement mode and the fixing method of the optical fibers adopt horizontal and vertical crossed arrangement, and the stability and the accuracy of the optical fibers in the voltage loading process are ensured through a non-conductive clamping device. Setting an assessment voltage and a pressurization gradient according to an electric field simulation result to ensure that the test field intensity covers the operation condition. And determining the performance boundary of the optical fiber by taking no breakdown and no abnormal partial discharge under the voltage of 120kV as a qualified criterion.

Description

Technical Field

[0001] This invention relates to condition monitoring and assessment technology for high-voltage power equipment, and in particular to a method and apparatus for AC withstand voltage testing of latent insulation defects in optical fibers used in transformers. Background Technology

[0002] Fiber optic temperature measurement technology, due to its advantages such as distributed measurement, high insulation, and resistance to electromagnetic interference, has been widely used for monitoring the temperature of internal windings, bushings, and leads in converter transformers, becoming an important means of accurately assessing equipment operating status. However, fiber optic systems are typically buried in high electric field regions, and their outer sheaths, cladding layers, or connection points may suffer from latent defects such as mechanical damage, poor cladding, and micro-air gaps during long-term operation. These latent problems are difficult to detect in conventional optical testing. Once exposed in actual operation, they may lead to partial discharge and insulation breakdown, thereby affecting fiber optic transmission performance and even causing equipment misjudgments. Therefore, there is an urgent need to develop an assessment method for the electrical reliability of fiber optics, to identify potential insulation defects in advance, and to improve the reliability of fiber optic temperature measurement systems under high-voltage and complex operating conditions.

[0003] According to the methodologies in GB / T 1408.1-2016 Electrical Insulation Materials - Test Methods for Electrical Strength - Part 1: Power Frequency Withstand Voltage Test and DL / T 2551-2022 Test Methods for Fiber Optic Temperature Measurement Devices for Oil-Immersed Power Transformers, the fiber optic AC withstand voltage test uses a Rogowski electrode structure with a diameter of 200mm (10mm gap). However, some problems were found during the experiment, including:

[0004] 1. Small electrode size and significant edge effect: The Rogowski electrode used in the test has a diameter of 200mm, which produces obvious electric field distortion in the edge area, which can easily cause electrode edge discharge or local breakdown, interfering with the accurate determination of the insulation performance of the optical fiber body.

[0005] 2. The test subjects are mostly new optical fibers, lacking aging assessment: The current test subjects are mostly newly manufactured optical fibers, and there is no established method for evaluating the withstand voltage performance of optical fibers aging in operation, which cannot cover the long-term performance evolution under actual service conditions.

[0006] 3. Lack of correspondence between test field strength and operating conditions: The field strength formed by applied voltage is usually much higher than the actual operating field strength of the equipment, and there is a lack of systematic research on the equivalence and margin design between the two.

[0007] 4. Validity of oil gap breakdown interference test criteria: In the test, the entire electrode-oil-fiber system often breaks down, making it difficult to accurately determine the location and cause of the breakdown. Especially when the fiber cladding is undamaged, it is impossible to confirm whether it is due to fiber body insulation failure or oil gap breakdown, which seriously affects the authenticity and relevance of the test results.

[0008] 5. Lack of a unified standard for selecting the test voltage value: The existing standard recommends testing at 100kV, but the fiber optic distance for testing is relatively short. Once the distance is increased, the corresponding oil gap breakdown dispersion increases, and the test voltage cannot be increased linearly. Therefore, the rationality of the selection of the test voltage value is a major challenge.

[0009] 6. Surface discharge lacks a normal component, making it difficult for surface discharge to occur: In current standards, optical fiber arrangements are mostly attached, and the electric field is mainly distributed along the surface, lacking a normal field strength in the vertical direction. Surface discharge is difficult to be effectively excited, which is not conducive to exposing potential insulation defects inside the optical fiber sheath. Summary of the Invention

[0010] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes an AC withstand voltage test method for latent insulation defects in optical fibers used in transformers, which can solve the problem that the prior art cannot detect inherent defects in optical fibers in a timely manner, especially potential electrical problems such as microcracks and poor coating, and make up for the deficiencies in the prior art.

[0011] The present invention also proposes an apparatus for testing the AC withstand voltage of latent insulation defects in optical fibers used in transformers, as described above.

[0012] The method for AC withstand voltage testing of latent insulation defects in optical fibers for transformers according to a first aspect of the present invention is characterized by comprising the following steps:

[0013] Determine the recommended test field strength value for optical fiber;

[0014] Based on the recommended test field strength value, the test voltage is set;

[0015] The optical fiber to be tested is arranged in a ball-plate electrode structure and immersed in insulating oil;

[0016] An AC voltage is applied to the optical fiber under test according to the set test voltage to conduct a withstand voltage test; and the partial discharge signal is monitored.

[0017] The insulation performance of the optical fiber under test is evaluated based on preset criteria.

[0018] The AC withstand voltage test method for latent insulation defects in optical fibers used in transformers according to embodiments of the present invention has at least the following beneficial effects:

[0019] 1. Addressing the issues of small electrode size and significant edge effects: This invention adopts a ball-plate electrode structure, which increases the electrode size and makes it geometrically symmetrical, optimizes the electric field distribution, significantly reduces edge electric field distortion, avoids edge discharge interference, and improves the accuracy of evaluating the insulation performance of the optical fiber body.

[0020] 2. To address the lack of aging fiber assessment: The test scheme allows for the use of aging-treated fiber samples for testing, enhancing the method's adaptability and representativeness to the insulation performance of in-service aged fibers, and providing support for full life-cycle condition assessment.

[0021] 3. To address the issue of the disconnect between test field strength and operating conditions: This invention is based on the actual installation environment inside the converter transformer. It determines the boundary value of the test field strength through multiphysics simulation and derives the test voltage accordingly, ensuring that the test field strength and operating conditions have engineering equivalence and reasonable margin.

[0022] 4. Regarding the effectiveness of the oil gap breakdown interference criterion: By optimizing the electrode structure and fiber optic deployment, the breakdown type can be accurately identified, effectively distinguishing between fiber optic insulation defects and oil gap discharge behavior.

[0023] 5. Regarding the lack of a unified standard for the test voltage: Based on the electric field simulation results and the actual field strength under operating conditions, a recommended test field strength of 4.5kV / mm is clearly proposed, and the standard test voltage value of 120kV is calculated, forming a unified and repeatable test benchmark, which improves the consistency and comparability of the test.

[0024] 6. To address the difficulty in exciting surface discharge: In the experiment, the optical fiber perpendicularly passes through the region with the highest electric field intensity, and the electric field direction has a significant normal component, which effectively excites surface discharge, helps to expose potential defects inside the optical fiber cladding layer, and improves the ability to detect hidden dangers.

[0025] According to some embodiments of the present invention, the step of determining the recommended test field strength value includes:

[0026] Based on the electrothermal coupling multiphysics field simulation of the actual optical fiber installation area inside the converter transformer, the field strength distribution of the key insulating medium is obtained.

[0027] Based on the maximum allowable field strength control value obtained from simulation and considering the engineering safety margin, the recommended test field strength value is determined.

[0028] According to some embodiments of the present invention, the recommended test field strength value is 4.5 kV / mm.

[0029] According to some embodiments of the present invention, the step of setting the test voltage includes:

[0030] Based on the recommended test field strength value, and through electric field simulation of the ball-plate electrode structure, the power frequency AC voltage value that enables the optical fiber deployment area to reach the recommended test field strength value is determined as the test voltage.

[0031] According to some embodiments of the present invention, the test voltage is 120kV.

[0032] According to some embodiments of the present invention, in the ball-plate electrode structure, the radius of the ball electrode is 100 mm, a through hole is provided in the center of the plate electrode, and the gap between the ball electrode and the plate electrode is set to 50 mm.

[0033] According to some embodiments of the present invention, the step of arranging the optical fiber to be tested in the ball-plate electrode structure involves installing the optical fiber vertically along the central axis of symmetry of the electrode and passing it through a through hole in the center of the plate electrode.

[0034] According to some embodiments of the present invention, the step of applying AC voltage to perform a withstand voltage test includes:

[0035] The voltage is increased to a first preset voltage at a constant rate and held for a first preset time;

[0036] The voltage is then gradually increased to the target test voltage in a step-by-step manner, with each voltage step maintained for a second preset time.

[0037] Maintain the target test voltage for a third preset time.

[0038] According to some embodiments of the present invention, the first preset voltage is 96kV, the first preset time is 10 minutes, the target test voltage is 120kV, and the third preset time is 10 minutes.

[0039] According to some embodiments of the present invention, the monitoring of partial discharge signals is performed in real time using a high-frequency partial discharge detection system in the frequency band of 2.5-30MHz.

[0040] According to some embodiments of the present invention, the preset criteria include one or more of the following:

[0041] No electrical breakdown or flashover occurred during the test;

[0042] The partial discharge level did not exceed the set background noise threshold.

[0043] The optical fiber operates stably under the target test voltage, and its power frequency breakdown voltage is not less than 120kV.

[0044] According to a second aspect of the present invention, a transformer fiber optic latent insulation defect AC withstand voltage testing device is characterized in that it comprises:

[0045] A ball-plate electrode system is used to generate the test electric field;

[0046] An insulating oil tank is used to accommodate the electrode system and the optical fiber under test.

[0047] High-voltage power supply, used to provide the AC voltage required for the test;

[0048] A partial discharge monitoring system is used to monitor partial discharge signals during the testing process.

[0049] According to some embodiments of the present invention, the plate electrode in the ball-plate electrode system has a through hole at its center for the optical fiber to pass through.

[0050] According to some embodiments of the present invention, the ball-plate electrode system is fixed and electrically isolated by a polyethylene support structure.

[0051] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0052] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0053] Figure 1 This is a typical wiring diagram for a power frequency withstand voltage test provided in an embodiment of the present invention;

[0054] Figure 2 for Figure 1 The diagram shown is a typical power frequency withstand voltage test wiring diagram, in which electrode G is a Rogowski electrode.

[0055] Figure 3 This is a schematic diagram of the installation path and minimum bending radius position of the fiber optic sensor provided in an embodiment of the present invention;

[0056] Figure 4 A schematic diagram of the AC electric field distribution provided in an embodiment of the present invention;

[0057] Figure 5 This is a schematic diagram of the standard electrode electric field simulation results provided in an embodiment of the present invention;

[0058] Figure 6 This is a schematic diagram of the simulation results of the ball-plate electrode electric field provided in an embodiment of the present invention;

[0059] Figure 7 This is a diagram of the AC withstand voltage test after closing, provided in an embodiment of the present invention. Detailed Implementation

[0060] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0061] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0062] In the description of this invention, "several" means one or more, "more than" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0063] In the description of this invention, unless otherwise explicitly defined, terms such as "set up," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0064] Based on the methodologies in GB / T 1408.1-2016 Electrical Insulation Materials - Electrical Strength Test Methods - Part 1: Power Frequency Withstand Voltage Test and DL / T 2551-2022 Fiber Optic Temperature Measurement Device for Oil-Immersed Power Transformers, the fiber optic AC withstand voltage test employs a 200mm diameter Rogowski electrode (10mm gap) structure, with partial discharge detection performed by boosting the power frequency voltage to 80kV. The criterion is that the partial discharge should not exceed 10pC. The test circuit and electrode structure are as follows: Figure 1 and 2 As shown.

[0065] The aforementioned traditional method, when implemented using Rogowski electrodes, suffers from a series of problems that make it difficult to meet engineering requirements. These problems include:

[0066] 1. Small electrode size and significant edge effect: The Rogowski electrode used in the test has a diameter of 200mm, which produces obvious electric field distortion in the edge area, which can easily cause electrode edge discharge or local breakdown, interfering with the accurate determination of the insulation performance of the optical fiber body.

[0067] 2. The test subjects are mostly new optical fibers, lacking aging assessment: The current test subjects are mostly newly manufactured optical fibers, and there is no established method for evaluating the withstand voltage performance of optical fibers aging in operation, which cannot cover the long-term performance evolution under actual service conditions.

[0068] 3. Lack of correspondence between test field strength and operating conditions: The field strength formed by applied voltage is usually much higher than the actual operating field strength of the equipment, and there is a lack of systematic research on the equivalence and margin design between the two.

[0069] 4. Validity of oil gap breakdown interference test criteria: In the test, the entire electrode-oil-fiber system often breaks down, making it difficult to accurately determine the location and cause of the breakdown. Especially when the fiber cladding is undamaged, it is impossible to confirm whether it is due to fiber body insulation failure or oil gap breakdown, which seriously affects the authenticity and relevance of the test results.

[0070] 5. Lack of a unified standard for selecting the test voltage value: The existing standard recommends testing at 100kV, but the fiber optic distance for testing is relatively short. Once the distance is increased, the corresponding oil gap breakdown dispersion increases, and the test voltage cannot be increased linearly. Therefore, the rationality of the selection of the test voltage value is a major challenge.

[0071] 6. Surface discharge lacks a normal component, making it difficult for surface discharge to occur: In current standards, optical fiber arrangements are mostly attached, and the electric field is mainly distributed along the surface, lacking a normal field strength in the vertical direction. Surface discharge is difficult to be effectively excited, which is not conducive to exposing potential insulation defects inside the optical fiber sheath.

[0072] To address the numerous limitations of existing technologies, this application provides a method for AC withstand voltage testing of latent insulation defects in optical fibers used in transformers. This method includes at least the following steps:

[0073] Step S100: Determine the recommended test field strength value for the optical fiber.

[0074] In converter transformers, fiber optic sensors are typically pre-positioned in winding clamps or pads to effectively monitor temperature rise in localized hot spots. For example... Figure 3 As shown at points A and B, the minimum bending radius of the optical fiber mainly occurs in the oil channel area near the electrostatic ring, with a typical value of not less than 30–50 mm. Especially in the corner of the clamp or the optical fiber lead-out point area, it is necessary to avoid micro-bending loss and mechanical stress concentration to ensure the stability of the temperature measurement signal and the integrity of the optical fiber structure.

[0075] Considering the complex electrothermal coupling effects in the region where optical fibers are located, this invention performs multiphysics field analysis on typical structural units inside the converter transformer based on electrothermal coupling and uncoupling simulation models. Taking the area around the winding clamp as a representative region, the simulation results of the key insulating dielectric field strength under power frequency AC withstand voltage conditions are as follows: Figure 4 As shown in Table 1.

[0076] Table 1 shows the recommended test field strength values ​​based on simulation results.

[0077]

[0078]

[0079] The insulation design standard for integrated converter transformer equipment under operating conditions specifies a maximum allowable internal electric field strength of 3kV / mm. Considering a safety margin factor of 1.5, the recommended electrical test electric field strength for optical fibers is ultimately determined to be 4.5kV / mm. This value can be used as the engineering boundary condition for screening the withstand voltage performance of optical fibers and as the basis for voltage conversion in subsequent test design.

[0080] Step S200: Based on the recommended test field strength value, set the test voltage.

[0081] To evaluate the applicability of the Rogowski electrode structure used in current standards for AC withstand voltage testing of fiber optic temperature measurement devices, this embodiment first performs electric field simulation analysis on a standard electrode structure (D = 200 mm, gap 10 mm). Figure 5 As shown, in this simulation, the optical fiber sample is vertically arranged at the center of the electrode to simulate its typical arrangement in transformer oil, so as to observe the electric field intensity distribution and edge effect at the location of the optical fiber.

[0082] Simulation results show that, under the standard Rogowski electrode structure, the electric field is mainly concentrated in the electrode edge region, exhibiting a significant edge electric field enhancement phenomenon, with the maximum electric field strength reaching 4.5 kV / mm. The electric field distribution in the electrode center region (a typical fiber deployment location) is relatively uniform, but the field strength is much lower than at the edges. These results indicate that the electric field distribution in this structure is significantly affected by the geometric boundaries, and the electric field in the central region is insufficient to reflect the high field strength conditions that the fiber may experience during operation.

[0083] The standard electrode structure has limitations in its applicability to fiber optic withstand voltage testing. The electric field strength is mainly concentrated at the electrode edges, while the fiber is typically positioned centrally symmetrically. This leads to a misalignment between the actual area of ​​electrical stress and the target under test, making it impossible to effectively assess the insulation performance of critical fiber components. The small 10mm gap restricts the fiber's placement space, especially when connectors, protective sleeves, or multi-segment arrangements are present, leaving insufficient operational margin. The electric field direction is approximately parallel to the fiber axis, lacking a significant normal component along the surface, making it difficult to induce surface discharge processes that may occur under real-world conditions, thus reducing the severity and representativeness of the test. Test results are more susceptible to edge effects, hindering the effective identification of weak areas in the fiber's local insulation.

[0084] To achieve standardized electric field loading and highly repeatable test evaluation, this experiment uses a ball-plate electrode structure to assess the AC withstand voltage performance of optical fibers. Initially, a standard 200mm metal ball electrode was used for modeling. Subsequently, electric field simulation analyses were conducted under different combinations of ball diameters and electrode spacings, such as... Figure 6As shown in the figure. Simulation results show that when the diameter of the sphere is 25mm, the electrode spacing is set to 22–25mm, and the applied AC voltage is 100kV, the electric field strength in the central symmetry axis region of the sphere is concentrated and stably distributed, reaching the target electric field strength of about 4.5kV / mm, which is highly consistent with the recommended performance value.

[0085] Considering engineering factors such as electrode manufacturing errors, wiring disturbances, and the complexity of fiber optic installation structures, and to ensure sufficient safety margin and defect detection capability in the test results, the final designed test voltage is 120kV (50Hz power frequency AC). This value effectively covers the recommended test field strength, possesses good redundancy and defect excitation capability, and helps to achieve sensitive detection of latent insulation problems in the fiber itself.

[0086] Step S300: Arrange the optical fiber to be tested in the ball-plate electrode structure and immerse it in insulating oil.

[0087] This embodiment uses a ball-plate electrode structure, such as Figure 6 As shown in the diagram, the spherical electrode has a radius of 100mm, while the plate electrode features an asymmetrical chamfered design with an upper radius of 150mm, a lower radius of 170mm, a thickness of 20mm, and a side corner radius of 20mm. To facilitate fiber optic cabling and installation, a 2mm diameter through-hole is formed in the center of the plate electrode. The gap between the spherical electrode and the plate electrode is set to 50mm. Simulation results indicate that this structure can achieve a field strength of approximately 5kV / mm at the fiber optic path when 120kV is applied.

[0088] The electrode support structure is made of polyethylene (PE), which has high dielectric strength and stable mechanical strength, effectively supporting the electrode system and achieving electrical isolation. The relative permittivity (ε) reflects a material's ability to store electrical energy and polarize in an electric field, expressed as a multiple of vacuum. A higher value indicates a stronger "buffering" ability of the material against the electric field, resulting in a smaller electric field strength at the same voltage. In this experiment, the relative permittivity of the oil, paper, and support plate are as follows: ε oil =2.2, ε paper =3.2, ε board =4.5. Because oil has the lowest dielectric constant, the electric field will be more concentrated in the oil gap, and it is easier for partial discharge or breakdown to occur; paper and board have higher dielectric constants, and their effect on dispersing the electric field is stronger.

[0089] To ensure testing accuracy, the electrode design fully considers a symmetrical oil drainage structure. Oil drainage channels are designed into the internal structure and connection points of the sphere to avoid oil stagnation zones or air bubble traps within the electrode cavity. Furthermore, the sphere undergoes vacuum pre-oil impregnation before installation, and an inverted oil injection method is used to ensure no air bubbles remain in the electrode system, improving electric field uniformity and result reliability.

[0090] To ensure testing accuracy, the electrode design fully considers a symmetrical oil drainage structure. Oil drainage channels are designed into the internal structure and connection points of the sphere to avoid oil stagnation zones or air bubble traps within the electrode cavity. Furthermore, the sphere undergoes vacuum pre-oil impregnation before installation, and an inverted oil injection method is used to ensure no air bubbles remain in the electrode system, improving electric field uniformity and result reliability.

[0091] Step S400: Apply an AC voltage to the optical fiber under test according to the set test voltage to conduct a withstand voltage test and monitor the partial discharge signal.

[0092] The optical fiber was fixed in place by the ball-plate electrode structure, and the electrode and optical fiber were completely immersed in transformer insulating oil. The optical fiber was arranged horizontally and perpendicularly to the surface of the plate electrode. A voltage of 2 kV / s was continuously applied to 96 kV and held for 10 minutes. Each subsequent voltage step (2–5 kV) was held for 10–30 seconds until the voltage reached 120 kV (the electric field strength along the fiber surface reached 5 kV / mm), and held for 10 minutes. The test was conducted as follows. Figure 7 As shown. During the experiment, partial discharge and breakdown signs were observed in real time, and breakdown voltage, abnormal current, and acoustic and optical signals were recorded.

[0093] The experiment employed a high-frequency partial discharge (PD) detection system for real-time tracking. This system uses a high-frequency current transformer (HFCT) to capture partial discharge signals in the area where the optical fiber is located, and employs digital bandpass filtering technology to ensure effective identification and amplification of weak partial discharge signals. The system's filtering frequency band was set to 2.5–30 MHz to minimize power frequency interference and other low-frequency noise, ensuring accurate detection of partial discharge phenomena related to the fiber's insulation.

[0094] Before the test began, the partial discharge monitoring system was calibrated by injecting a 200pC standard square wave signal into the high-voltage side to ensure the sensitivity and accuracy of the test system. As the voltage gradually increased to 120kV, the system monitored and recorded any partial discharges in real time, and evaluated the electrical performance of the optical fiber based on the discharge amplitude and duration. If the discharge signal exceeded the set threshold, the system would immediately acquire data and mark it as an abnormal event, further analyzing whether there were insulation defects or breakdown risks.

[0095] Step S500: Evaluate the insulation performance of the optical fiber under test according to the preset criteria.

[0096] In this test, the withstand voltage performance of the optical fiber was determined using several preset key parameters to ensure that its insulation capability under high-voltage conditions met the requirements. The specific criteria are as follows:

[0097] (1) No breakdown or flashover: If the sample does not experience electrical breakdown or flashover during the test, and there is no abnormal current, spark or partial discharge intensity exceeding the set threshold, then the insulation performance of the optical fiber is considered to meet the basic requirements.

[0098] (2) Partial discharge monitoring: The amount of partial discharge throughout the process should be consistent with the background partial discharge, that is, there is no significant partial discharge signal exceeding the noise level, indicating that the optical fiber has no latent defects or electrical performance degradation during the voltage application process.

[0099] (3) Power frequency breakdown voltage requirement: The power frequency breakdown voltage of optical fiber should not be lower than 120kV, and the corresponding electric field strength along the fiber surface should reach 5kV / mm, which meets the insulation strength requirements of optical fiber in high voltage environment. If the optical fiber can operate stably under this voltage and no breakdown occurs, its withstand voltage performance is deemed qualified.

[0100] The above criteria served as the final evaluation basis for this test, ensuring the safety and reliability of optical fibers in high-voltage environments and enabling them to meet the long-term operating requirements of high-voltage equipment such as converter transformers.

[0101] In ±400kV DC transmission projects, the AC withstand voltage test method for latent insulation defects in optical fibers proposed in this invention has been verified through practical application. Initially, the optical fiber temperature sensing devices used in the project were provided by two foreign manufacturers. Both were specialized optical fiber products designed for long-term operation in oil-immersed environments, with sheath materials primarily composed of polyimide and fluoroplastic composites, achieving temperature resistance levels above 200℃. The sensor components employed a cladding reinforcement process. Conventional performance indicators showed that this type of optical fiber possessed high heat resistance and mechanical stress resistance. However, during AC withstand voltage testing, some samples failed when the breakdown voltage was below the design margin, exposing latent insulation weaknesses.

[0102] Subsequently, domestic manufacturers developed a structurally improved fiber optic sensor, employing a high-strength quartz fiber core and a modified fluoroplastic sheath. A stress transfer layer was added at the junction of the fiber and the sensing probe to enhance stability under surface discharge conditions. Tested using the method of this invention, the domestic fiber optic samples showed stability at a uniform test voltage of 120kV, without early breakdown or abnormal discharge, and their withstand voltage margin was significantly better than some foreign products.

[0103] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs.

[0104] It will be understood by those skilled in the art that all or some of the steps and systems in the methods disclosed above can be implemented as software, firmware, hardware, and suitable combinations thereof. Some or all of the physical components can be implemented as software executed by a processor, such as a central processing unit, digital signal processor, or microprocessor, or as hardware, or as an integrated circuit, such as an application-specific integrated circuit. Such software can be distributed on a computer-readable medium, which can include computer storage media (or non-transitory media) and communication media (or transient media). As is known to those skilled in the art, the term computer storage media includes volatile and non-volatile, removable and non-removable media implemented in any method or technology for storing information (such as computer-readable instructions, data structures, program modules, or other data). Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technologies, CD-ROM, digital versatile disc (DVD) or other optical disc storage, magnetic cartridges, magnetic tape, disk storage or other magnetic storage devices, or any other medium that can be used to store desired information and is accessible to a computer. Furthermore, as is known to those skilled in the art, communication media typically contain computer-readable instructions, data structures, program modules, or other data in modulated data signals such as carrier waves or other transmission mechanisms, and may include any information delivery medium.

[0105] The above is a detailed description of the preferred embodiments of this application. However, this application is not limited to the above embodiments. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of this application. All such equivalent modifications or substitutions are included within the scope defined by the claims of this application.

Claims

1. A method for testing the AC voltage resistance of a transformer with optical fiber latent insulation defects, characterized in that, The method comprises the following steps: determining a recommended field strength value for the optical fiber; setting a test voltage based on the recommended field strength value; arranging the optical fiber to be tested in a sphere-plate electrode structure and immersing it in insulating oil; applying an AC voltage to the optical fiber to be tested for withstand voltage test according to the set test voltage; and monitoring the partial discharge signal; evaluating the insulation performance of the optical fiber to be tested according to a preset criterion.

2. The method of claim 1, wherein, The step of determining the recommended field strength value comprises: obtaining the field strength distribution of the key insulating medium based on the electro-thermal coupled multi-physical field simulation of the actual installation area of the optical fiber inside the converter transformer; determining the recommended field strength value according to the internal maximum allowable field strength control value obtained by simulation and considering the engineering safety margin.

3. The method of claim 2, wherein, The recommended field strength value is 4.5 kV / mm.

4. The method of claim 1, wherein, The step of setting the test voltage comprises: determining the power frequency AC voltage value that can make the optical fiber laying area reach the recommended field strength value as the test voltage based on the recommended field strength value and through the electric field simulation of the sphere-plate electrode structure.

5. The method of claim 4, wherein, The test voltage is 120 kV.

6. The method of claim 1, wherein, In the sphere-plate electrode structure, the radius of the sphere electrode is 100 mm, a through hole is formed in the center of the plate electrode, and the gap between the sphere electrode and the plate electrode is set to 50 mm.

7. The method of claim 1, wherein, The step of arranging the optical fiber to be tested in the sphere-plate electrode structure is to install the optical fiber vertically along the central symmetry axis of the electrode and pass through the through hole in the center of the plate electrode.

8. The method of claim 1, wherein, The step of applying an AC voltage for withstand voltage test comprises: raising the voltage to a first preset voltage at a constant rate and keeping it for a first preset time; then gradually increasing the voltage to a target test voltage in a step-by-step manner, keeping each voltage step for a second preset time; keeping the target test voltage for a third preset time.

9. The method of claim 8, wherein, The first preset voltage is 96 kV, the first preset time is 10 minutes, the target test voltage is 120 kV, and the third preset time is 10 minutes.

10. The method of claim 1, wherein, The monitoring of the partial discharge signal is performed in real time in the frequency band of 2.5-30 MHz by using a high-frequency partial discharge detection system.

11. The method of claim 1, wherein, The preset criterion comprises one or more of the following: no electrical breakdown or flashover occurs during the test; the partial discharge amount does not exceed the set background noise threshold; the optical fiber operates stably at the target test voltage, and its power frequency breakdown voltage is not less than 120 kV.

12. A device for testing the AC voltage resistance of a transformer for latent insulation defects by means of optical fibers for implementing the method according to any one of claims 1 to 11, characterized in that It comprises: a sphere-plate electrode system for generating an evaluation electric field; an insulating oil tank for accommodating the electrode system and the optical fiber to be tested; a high-voltage power supply for providing the AC voltage required for the test; a partial discharge monitoring system for monitoring the partial discharge signal during the test.

13. The apparatus of claim 12, wherein, A through hole is formed in the center of the plate electrode in the sphere-plate electrode system for the optical fiber to pass through.

14. The apparatus of claim 12, wherein, The sphere-plate electrode system is fixed and electrically isolated by a polyethylene support structure.

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