Method for evaluating oil paper insulation electrical strength of converter transformer through alternating current and direct current voltage superposition

Abstract

The invention relates to the field of insulation electrical strength evaluation, and particularly discloses a method for evaluating the oil paper insulation electrical strength of a converter transformer through alternating-current and direct-current voltage superposition. Comprising the following steps: step S01, collecting breakdown voltage data of an oil paper insulation sample; step S02, analyzing breakdown strength distribution of the oil paper insulation sample; step S03, analyzing breakdown characteristics of an unaged oil paper insulation sample; step S04, analyzing breakdown characteristics of a thermally aged oil paper insulation sample; s06, outputting a breakdown characteristic analysis result of the oil-paper insulation sample, and analyzing the influence of alternating-current and direct-current voltage superposition and thermal aging on the breakdown characteristic; according to the method, the breakdown performance of the oil paper insulating material under the alternating current / direct current superposed voltage and thermal aging conditions is deeply analyzed, the influence on the performance of the oil paper insulating material is obtained, and a theoretical basis is provided for optimization and reliability improvement of insulating materials in a high-voltage direct current system and a converter transformer.

Description

Technical Field

[0001] The present invention relates to the field of insulation electrical strength evaluation, and in particular to a method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages. Background Art

[0002] Converter transformers are a crucial component of power systems and play a key role in AC / DC hybrid transmission systems, enabling power conversion between AC and DC systems. In high-voltage direct current (HVDC) transmission systems, converter transformers, as key components of the converter unit, connect the grid (AC) side and the valve (DC) side. As the primary insulating material in these transformers, oil-paper insulation is subjected to both AC and DC voltages, resulting in superimposed voltage stress. This superimposed voltage stress, combined with long-term high-temperature exposure, significantly impacts the aging process of the oil-paper insulation. Long-term exposure to superimposed AC / DC voltages, coupled with thermal aging, accelerates the breakdown of the oil-paper insulation, leading to degradation of its dielectric strength and increasing the risk of insulation failure. This degradation manifests as partial discharge, insulation breakdown, or even catastrophic failure of the converter transformer, compromising the reliability and operational safety of the HVDC transmission system. To ensure the long-term performance and safety of converter transformers, in-depth research is needed to investigate the synergistic effects of superimposed AC / DC voltage stress and thermal aging on the breakdown characteristics of oil-paper insulation.

[0003] In summary, research on the breakdown characteristics of oil-paper insulation materials has primarily focused on the influence of single factors, such as thermal aging, temperature, humidity, AC harmonic content, and DC voltage content. However, research on the combined effects of multiple factors, particularly the synergistic effects of superimposed AC / DC voltage stress and thermal aging on breakdown behavior, has been lacking in-depth exploration.

[0004] Therefore, this application will focus on exploring the combined effects of superimposed AC / DC voltage stress and thermal aging on the breakdown characteristics of oil-paper insulation. Furthermore, the study will analyze the effects of different AC / DC voltage components and thermal aging time on the insulation performance of oil-paper materials. Summary of the Invention

[0005] In order to overcome the above-mentioned defects of the prior art, an embodiment of the present invention provides a method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages, so as to solve the problems raised in the above-mentioned background technology.

[0006] To achieve the above-mentioned object, the present invention provides the following technical solution: a method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages, comprising the following steps: step S01: collecting breakdown voltage data of oil-paper insulation samples, step S02: analyzing the breakdown strength distribution of the oil-paper insulation samples, step S03: analyzing the breakdown characteristics of unaged oil-paper insulation samples, step S04: analyzing the breakdown characteristics of thermally aged oil-paper insulation samples, step S05: analyzing the effects of AC and DC voltage superposition and thermal aging on the breakdown characteristics, and step S06: outputting the breakdown characteristics analysis results of the oil-paper insulation samples.

[0007] Step S01: collecting breakdown voltage data of the oil-paper insulation sample: pre-treating the oil-paper insulation sample and performing a breakdown test on the oil-paper insulation sample under preset experimental setting conditions, and collecting breakdown voltage data at the moment of breakdown;

[0008] Step S02: analyzing the breakdown strength distribution of the oil-paper insulation sample: using a two-parameter Weibull distribution to analyze the breakdown strength distribution of the oil-paper insulation sample;

[0009] Step S03: Analyzing the breakdown characteristics of the unaged oil-paper insulation sample: analyzing the breakdown characteristics of the unaged oil-paper insulation sample under a DC preset voltage condition;

[0010] Step S04: Analyzing the breakdown characteristics of the thermally aged oil-paper insulation sample: analyzing the breakdown characteristics of the thermally aged oil-paper insulation sample under a DC preset voltage condition;

[0011] Step S05: Analysis of the influence of AC / DC voltage superposition and thermal aging on breakdown characteristics: Obtain the AC / DC superposition breakdown strength of unaged and thermally aged oil-paper insulation samples at a preset DC voltage, and analyze the influence of AC / DC voltage superposition and thermal aging on the breakdown characteristics respectively;

[0012] Step S06: Outputting the breakdown characteristic analysis results of the oil-paper insulation samples: Outputting the breakdown characteristic analysis results of the unaged and thermally aged oil-paper insulation samples, and the analysis results of the influence of AC / DC voltage superposition and thermal aging on the breakdown characteristics.

[0013] Preferably, the specific method of pre-treating the oil-paper insulation sample is as follows: the oil-paper insulation sample comes from a 220kV converter transformer, has a thickness of about 0.2mm, and is subjected to heat aging treatment in transformer oil at an aging temperature of 130°C for 10 days.

[0014] Preferably, in step S01, the specific execution method of collecting the breakdown voltage data of the oil-paper insulation sample is as follows:

[0015] An AC / DC superposition breakdown test was conducted, where a DC voltage was applied at a ramp rate of 0.1 kV / s. When the preset DC voltage was reached, a prestressing stage was maintained for 30 seconds. Subsequently, the AC voltage was gradually increased at a rate of 0.5 kV / s until breakdown occurred. At the moment of breakdown, the breakdown voltage data was recorded, which included both the AC breakdown voltage and the DC breakdown voltage.

[0016] Preferably, in step S02, the specific execution content of analyzing the breakdown strength distribution of the oil-paper insulation sample is as follows:

[0017] A two-parameter Weibull distribution model is established as follows:

[0018] Where F(t) represents the Weibull breakdown probability of the oil-paper insulation sample, t represents the breakdown field strength of the oil-paper insulation sample, exp(·) represents the exponential function with base e, α is the scale parameter, which represents the field strength when the breakdown probability reaches 63.2%, and β is the shape parameter.

[0019] Preferably, in step S03, the specific contents of the breakdown characteristic analysis of the unaged oil-paper insulation sample are as follows:

[0020] Step S031: increasing the DC preset voltage from 0 kV to 4 kV, recording the AC breakdown field strength ACE1 of the unaged oil-paper insulation sample at this time; increasing the DC preset voltage from 5 kV to 7 kV, recording the AC breakdown field strength ACE2 of the unaged oil-paper insulation sample at this time;

[0021] Step S032: When the DC preset voltage increases from 0kV to 4kV, analysis shows that the AC breakdown field strength ACE1 of the unaged oil-paper insulation sample gradually decreases; when the DC preset voltage increases from 5kV to 7kV, analysis shows that the AC breakdown field strength ACE2 of the unaged oil-paper insulation sample initially increases and then decreases; when the DC preset voltage is 6kV, the AC breakdown field strength of the unaged oil-paper insulation sample reaches a maximum value.

[0022] Preferably, in step S04, the specific contents of the breakdown characteristic analysis of the thermally aged oil-paper insulation sample are as follows:

[0023] Step S041: increasing the DC preset voltage from 0 kV to 4 kV, and recording the AC breakdown field strength ACE3 of the thermally aged oil-paper insulation sample at this time; increasing the DC preset voltage from 5 kV to 7 kV, and recording the AC breakdown field strength ACE4 of the thermally aged oil-paper insulation sample at this time;

[0024] Step S042: When the DC preset voltage increases from 0kV to 4kV, the analysis shows that the AC breakdown field strength ACE3 of the thermally aged oil-paper insulation sample gradually decreases; when the DC preset voltage increases from 5kV to 7kV, the analysis shows that the AC breakdown field strength ACE4 of the thermally aged oil-paper insulation sample initially decreases and then increases.

[0025] Preferably, in step S05, the specific content of the analysis of the influence of AC / DC voltage superposition and thermal aging on the breakdown characteristics is as follows:

[0026] Step S051: when the DC preset voltage is increased from 0 kV to 1 kV, the AC / DC superimposed breakdown strength of the unaged oil-paper insulation sample and the AC / DC superimposed breakdown strength of the thermally aged oil-paper insulation sample are obtained. Analysis shows that the AC / DC superimposed breakdown strengths of the unaged and thermally aged oil-paper insulation samples both decrease. When the DC preset voltage is further increased, the AC / DC superimposed breakdown strengths of the unaged and thermally aged oil-paper insulation samples both recover. When the DC preset voltage reaches 4 kV or 5 kV, the AC / DC superimposed breakdown field strength is equal to the AC breakdown field strength observed at the DC preset voltage of 0 kV.

[0027] Step S052: Increasing the DC preset voltage from 5 kV to 6 kV to obtain the AC / DC superimposed breakdown strength of the unaged oil-paper insulation sample and the AC / DC superimposed breakdown strength of the thermally aged oil-paper insulation sample. Analysis shows that the AC / DC superimposed breakdown strength of both the unaged and thermally aged oil-paper insulation samples increases, and the increase in the unaged oil-paper insulation sample is significantly greater than that of the thermally aged oil-paper insulation sample.

[0028] Step S053: Increasing the DC preset voltage from 6 kV to 7 kV, the breakdown strength of both the unaged and heat-aged oil-paper insulation samples increased. At the DC preset voltage of 7 kV, the AC / DC superposition breakdown strength exceeded the DC breakdown strength.

[0029] Technical effects and advantages of the present invention:

[0030] 1. This invention provides valuable insights into the breakdown behavior of oil-paper insulation materials under AC / DC superposition voltage and thermal aging conditions. These findings contribute to a deeper understanding of the effects of thermal aging and AC / DC superposition on the performance of oil-paper insulation materials, and provide a theoretical basis for optimizing and improving the reliability of insulation materials in HVDC systems and converter transformers.

[0031] 2. This invention can accurately simulate the complex voltage stresses experienced by converter transformer oil-paper insulation during actual operation, thereby revealing its AC / DC superposition breakdown characteristics under different DC preset voltages and thermal aging conditions. This helps researchers gain a more comprehensive understanding of the breakdown mechanism of oil-paper insulation and provides a theoretical basis for material optimization.

[0032] 3. The present invention uses a two-parameter Weibull distribution to analyze the breakdown strength data, which can more accurately describe the relationship between the breakdown probability of oil-paper insulation materials and the field strength, thereby improving the accuracy of the assessment; this helps to more accurately predict the life and reliability of oil-paper insulation materials in actual operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention is further described with reference to the accompanying drawings. However, the embodiments in the accompanying drawings do not constitute any limitation to the present invention. A person skilled in the art can obtain other drawings based on the following drawings without inventive effort.

[0034] Figure 1 The figure is a flow chart of the method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superposition of AC and DC voltages according to the present invention. DETAILED DESCRIPTION

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0036] See also Figure 1 As shown, the present invention provides a method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages, comprising the following steps: step S01: collecting breakdown voltage data of oil-paper insulation samples, step S02: analyzing the breakdown strength distribution of the oil-paper insulation samples, step S03: analyzing the breakdown characteristics of unaged oil-paper insulation samples, step S04: analyzing the breakdown characteristics of thermally aged oil-paper insulation samples, step S05: analyzing the effects of AC and DC voltage superposition and thermal aging on the breakdown characteristics, and step S06: outputting the breakdown characteristics analysis results of the oil-paper insulation samples.

[0037] Step S01: collecting breakdown voltage data of the oil-paper insulation sample: pre-treating the oil-paper insulation sample and performing a breakdown test on the oil-paper insulation sample under preset experimental setting conditions, and collecting breakdown voltage data at the moment of breakdown;

[0038] In this embodiment, it should be specifically explained that the specific method of pre-treating the oil-paper insulation sample is as follows: the oil-paper insulation sample comes from a 220kV converter transformer, with a thickness of about 0.2mm, and the oil-paper insulation sample is subjected to heat aging treatment in No. 25 transformer oil, the aging temperature is 130°C, and the treatment time is 10 days.

[0039] In this embodiment, it should be specifically noted that the preset experimental setup conditions are as follows: a DC isolation capacitor and a DC isolation resistor are used in the experimental apparatus to isolate the effects of the DC voltage on the AC power supply, while an AC isolation resistor is used to reduce the effects of the AC voltage on the DC power supply. The superimposed AC and DC voltages are applied via a spherical electrode above the sample; simultaneously, a cylindrical electrode is placed below the sample, which is grounded. The superimposed AC and DC voltages are measured using a voltage divider circuit.

[0040] In this embodiment, it should be specifically explained that the parameters of the components are as follows: the DC isolation capacitor is 0.1 μF, the DC isolation resistor is 10 MΩ, and the AC isolation resistor is 1 MΩ.

[0041] In this embodiment, it should be specifically explained that in step S01, the specific execution method of collecting the breakdown voltage data of the oil-paper insulation sample is as follows:

[0042] An AC / DC superposition breakdown test was conducted, where a DC voltage was applied at a ramp rate of 0.1 kV / s. When the preset DC voltage was reached, a prestressing stage was maintained for 30 seconds. Subsequently, the AC voltage was gradually increased at a rate of 0.5 kV / s until breakdown occurred. At the moment of breakdown, the breakdown voltage data was recorded, which included both the AC breakdown voltage and the DC breakdown voltage.

[0043] Step S02: analyzing the breakdown strength distribution of the oil-paper insulation sample: using a two-parameter Weibull distribution to analyze the breakdown strength distribution of the oil-paper insulation sample;

[0044] In this embodiment, it should be specifically explained that the specific execution content of analyzing the breakdown strength distribution of the oil-paper insulation sample in step S02 is as follows:

[0045] A two-parameter Weibull distribution model is established as follows:

[0046] Where F(t) represents the Weibull breakdown probability of the oil-paper insulation sample, t represents the breakdown field strength of the oil-paper insulation sample, exp(·) represents the exponential function with base e, α is the scale parameter, which represents the field strength when the breakdown probability reaches 63.2%, and β is the shape parameter, which is used to characterize the discreteness or variability of the data.

[0047] In this embodiment, it should be specifically explained that the reliability of the insulating material is evaluated in combination with the parameter β of the two-parameter Weibull distribution; the larger the β value, the smaller the data discreteness, indicating that the performance of the insulating material is more stable and the reliability is higher.

[0048] Step S03: Analyzing the breakdown characteristics of the unaged oil-paper insulation sample: analyzing the breakdown characteristics of the unaged oil-paper insulation sample under a DC preset voltage condition;

[0049] In this embodiment, it should be specifically explained that in step S03, the specific contents of the breakdown characteristic analysis of the unaged oil-paper insulation sample are as follows:

[0050] Step S031: increasing the DC preset voltage from 0 kV to 4 kV, recording the AC breakdown field strength ACE1 of the unaged oil-paper insulation sample at this time; increasing the DC preset voltage from 5 kV to 7 kV, recording the AC breakdown field strength ACE2 of the unaged oil-paper insulation sample at this time;

[0051] Step S032: When the DC preset voltage increases from 0kV to 4kV, analysis shows that the AC breakdown field strength ACE1 of the unaged oil-paper insulation sample gradually decreases; when the DC preset voltage increases from 5kV to 7kV, analysis shows that the AC breakdown field strength ACE2 of the unaged oil-paper insulation sample initially increases and then decreases; when the DC preset voltage is 6kV, the AC breakdown field strength of the unaged oil-paper insulation sample reaches a maximum value.

[0052] In this embodiment, it should be specifically noted that at all test voltage levels, the DC breakdown strength is consistently higher than the AC breakdown strength. The lack of voltage reversal in DC voltages imposes more stable and predictable stress on the insulating material. Furthermore, the application of a preset DC voltage reduces the AC breakdown field strength. This reduction is due to the combined effects of AC and DC voltages, which lead to synergistic degradation of the insulating material. The presence of DC voltage alters the charge distribution and produces a polarization effect, thereby weakening the insulation performance and making it more susceptible to breakdown under AC voltage stress.

[0053] Step S04: Analyzing the breakdown characteristics of the thermally aged oil-paper insulation sample: analyzing the breakdown characteristics of the thermally aged oil-paper insulation sample under a DC preset voltage condition;

[0054] In this embodiment, it should be specifically explained that in step S04, the specific contents of the breakdown characteristic analysis of the thermally aged oil-paper insulation sample are as follows:

[0055] Step S041: increasing the DC preset voltage from 0 kV to 4 kV, and recording the AC breakdown field strength ACE3 of the thermally aged oil-paper insulation sample at this time; increasing the DC preset voltage from 5 kV to 7 kV, and recording the AC breakdown field strength ACE4 of the thermally aged oil-paper insulation sample at this time;

[0056] Step S042: When the DC preset voltage increases from 0kV to 4kV, the analysis shows that the AC breakdown field strength ACE3 of the thermally aged oil-paper insulation sample gradually decreases; when the DC preset voltage increases from 5kV to 7kV, the analysis shows that the AC breakdown field strength ACE4 of the thermally aged oil-paper insulation sample initially decreases and then increases.

[0057] In this example, it should be specifically noted that increasing the DC preset voltage from 0 kV to 4 kV decreased the AC breakdown field strength of the thermally aged oil-paper insulation sample, which is consistent with the breakdown behavior of the unaged oil-paper insulation sample. This indicates that applying a DC preload negatively affects the AC breakdown strength due to the combined stress of the AC and DC electric fields on the insulation material.

[0058] When the DC preset voltage increases from 5kV to 7kV, the AC breakdown field strength of the thermally aged oil-paper insulation samples first decreases and then increases. This behavior is opposite to the observations of the unaged oil-paper insulation samples, in which the AC breakdown field strength continuously decreases with increasing DC voltage. The increase in the AC breakdown strength of the thermally aged oil-paper insulation samples at higher DC voltages is the result of the complex interaction of AC and DC stresses, as well as changes in material properties caused by thermal aging. Thermal aging changes the molecular structure and breakdown mechanism of the insulating material, altering its response to superimposed voltage stress, resulting in a breakdown mode different from that of the unaged oil-paper insulation samples.

[0059] Specifically, regardless of whether the oil-paper insulation samples had undergone thermal aging, the DC breakdown field strength was consistently higher than the AC breakdown field strength. This is consistent with the common observation that DC voltage stress tends to exert a more stable and stronger dielectric force on the insulating material, resulting in a higher breakdown field strength. Furthermore, when both AC and DC voltages were applied, the AC breakdown strength was lower than when only AC voltage was applied. This suggests that the presence of DC voltage reduces the insulation material's ability to withstand AC stress due to polarization, ionization, and the overall weakening of the material's insulation properties under the superimposed voltage.

[0060] Step S05: Analysis of the influence of AC / DC voltage superposition and thermal aging on breakdown characteristics: Obtain the AC / DC superposition breakdown strength of unaged and thermally aged oil-paper insulation samples at a preset DC voltage, and analyze the influence of AC / DC voltage superposition and thermal aging on the breakdown characteristics respectively;

[0061] In this embodiment, it should be specifically explained that in step S05, the specific content of the analysis of the influence of AC / DC voltage superposition and thermal aging on the breakdown characteristics is as follows:

[0062] Step S051: when the DC preset voltage is increased from 0 kV to 1 kV, the AC / DC superimposed breakdown strength of the unaged oil-paper insulation sample and the AC / DC superimposed breakdown strength of the thermally aged oil-paper insulation sample are obtained. Analysis shows that the AC / DC superimposed breakdown strengths of the unaged and thermally aged oil-paper insulation samples both decrease. When the DC preset voltage is further increased, the AC / DC superimposed breakdown strengths of the unaged and thermally aged oil-paper insulation samples both recover. When the DC preset voltage reaches 4 kV or 5 kV, the AC / DC superimposed breakdown field strength is equal to the AC breakdown field strength observed at the DC preset voltage of 0 kV.

[0063] Step S052: Increasing the DC preset voltage from 5 kV to 6 kV to obtain the AC / DC superimposed breakdown strength of the unaged oil-paper insulation sample and the AC / DC superimposed breakdown strength of the thermally aged oil-paper insulation sample. Analysis shows that the AC / DC superimposed breakdown strength of both the unaged and thermally aged oil-paper insulation samples increases, and the increase in the unaged oil-paper insulation sample is significantly greater than that of the thermally aged oil-paper insulation sample.

[0064] Step S053: Increasing the DC preset voltage from 6 kV to 7 kV, the breakdown strength of both the unaged and heat-aged oil-paper insulation samples increased. At the DC preset voltage of 7 kV, the AC / DC superposition breakdown strength exceeded the DC breakdown strength. This phenomenon can be explained by the fact that when the DC preset voltage exceeds 6 kV, the influence of the DC voltage on the insulation performance becomes more dominant, causing the breakdown behavior to be closer to the DC breakdown characteristics.

[0065] Step S06: Outputting the breakdown characteristic analysis results of the oil-paper insulation samples: Outputting the breakdown characteristic analysis results of the unaged and thermally aged oil-paper insulation samples, and the analysis results of the influence of AC / DC voltage superposition and thermal aging on the breakdown characteristics.

[0066] In this embodiment, it should be specifically explained that the specific content of outputting the breakdown characteristic analysis result of the oil-paper insulation sample is:

[0067] Effect of DC Voltage: As the DC preset voltage increases, the AC / DC superposition breakdown strength gradually increases. However, when the DC voltage exceeds 6 kV, the breakdown characteristics tend to be dominated by the DC voltage, exhibiting behavior similar to pure DC breakdown. This indicates that at higher DC voltages, the insulation material is more susceptible to the DC voltage, and the effect of the AC voltage gradually weakens.

[0068] Thermal aging: Thermal aging reduces AC / DC combined breakdown strength, especially at higher DC voltages. Aging degrades the molecular structure of the oil-paper insulation, enhancing the polarization effect under DC voltage and damaging the material's microstructure. These changes lead to increased charge accumulation and electric field distortion, which in turn weakens the material's ability to withstand voltage stress.

[0069] Comparison of AC and DC breakdown strength: For both unaged and thermally aged samples, the DC breakdown strength is consistently higher than the AC breakdown strength. Under superimposed AC / DC voltage, the AC breakdown strength is consistently lower than the pure AC breakdown strength, indicating that the superimposed AC / DC voltage has a more significant effect on the material. This suggests that the presence of DC voltage reduces the material's ability to withstand AC voltage stress.

[0070] Finally: The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0071] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. A method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages, characterized in that: The following steps are involved: Step S01: collecting breakdown voltage data of the oil-paper insulation sample: pre-treating the oil-paper insulation sample and performing a breakdown test on the oil-paper insulation sample under preset experimental setting conditions, and collecting breakdown voltage data at the moment of breakdown; Step S02: analyzing the breakdown strength distribution of the oil-paper insulation sample: using a two-parameter Weibull distribution to analyze the breakdown strength distribution of the oil-paper insulation sample; Step S03: Analyzing the breakdown characteristics of the unaged oil-paper insulation sample: analyzing the breakdown characteristics of the unaged oil-paper insulation sample under a DC preset voltage condition; Step S04: Analyzing the breakdown characteristics of the thermally aged oil-paper insulation sample: analyzing the breakdown characteristics of the thermally aged oil-paper insulation sample under a DC preset voltage condition; Step S05: Analysis of the influence of AC / DC voltage superposition and thermal aging on breakdown characteristics: Obtain the AC / DC superposition breakdown strength of unaged and thermally aged oil-paper insulation samples at a preset DC voltage, and analyze the influence of AC / DC voltage superposition and thermal aging on the breakdown characteristics respectively; Step S06: Outputting the breakdown characteristic analysis results of the oil-paper insulation samples: Outputting the breakdown characteristic analysis results of the unaged and thermally aged oil-paper insulation samples, and the analysis results of the influence of AC / DC voltage superposition and thermal aging on the breakdown characteristics.

2. The method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages according to claim 1, characterized in that: The specific method of pre-treating the oil-paper insulation sample is as follows: the oil-paper insulation sample comes from a 220kV converter transformer, has a thickness of about 0.2mm, and is subjected to heat aging treatment in transformer oil at an aging temperature of 130°C for 10 days.

3. The method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages according to claim 1, characterized in that: In step S01, the specific execution method of collecting the breakdown voltage data of the oil-paper insulation sample is as follows: An AC / DC superposition breakdown test was conducted, where a DC voltage was applied at a ramp rate of 0.1 kV / s. When the preset DC voltage was reached, a prestressing stage was maintained for 30 seconds. Subsequently, the AC voltage was gradually increased at a rate of 0.5 kV / s until breakdown occurred. At the moment of breakdown, the breakdown voltage data was recorded, which included both the AC breakdown voltage and the DC breakdown voltage.

4. The method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages according to claim 1, characterized in that: In step S02, the specific execution content of analyzing the breakdown strength distribution of the oil-paper insulation sample is as follows: A two-parameter Weibull distribution model is established as follows: Where F(t) represents the Weibull breakdown probability of the oil-paper insulation sample, t represents the breakdown field strength of the oil-paper insulation sample, exp(·) represents the exponential function with base e, α is the scale parameter, which represents the field strength when the breakdown probability reaches 63.2%, and β is the shape parameter.

5. The method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages according to claim 1, characterized in that: In step S03, the specific contents of the breakdown characteristic analysis of the unaged oil-paper insulation sample are as follows: Step S031: increasing the DC preset voltage from 0 kV to 4 kV, recording the AC breakdown field strength ACE1 of the unaged oil-paper insulation sample at this time; increasing the DC preset voltage from 5 kV to 7 kV, recording the AC breakdown field strength ACE2 of the unaged oil-paper insulation sample at this time; Step S032: When the DC preset voltage increases from 0kV to 4kV, analysis shows that the AC breakdown field strength ACE1 of the unaged oil-paper insulation sample gradually decreases; when the DC preset voltage increases from 5kV to 7kV, analysis shows that the AC breakdown field strength ACE2 of the unaged oil-paper insulation sample initially increases and then decreases; when the DC preset voltage is 6kV, the AC breakdown field strength of the unaged oil-paper insulation sample reaches a maximum value.

6. The method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages according to claim 1, characterized in that: In step S04, the specific contents of the breakdown characteristic analysis of the thermally aged oil-paper insulation sample are as follows: Step S041: increasing the DC preset voltage from 0 kV to 4 kV, and recording the AC breakdown field strength ACE3 of the thermally aged oil-paper insulation sample at this time; increasing the DC preset voltage from 5 kV to 7 kV, and recording the AC breakdown field strength ACE4 of the thermally aged oil-paper insulation sample at this time; Step S042: When the DC preset voltage increases from 0kV to 4kV, the analysis shows that the AC breakdown field strength ACE3 of the thermally aged oil-paper insulation sample gradually decreases; when the DC preset voltage increases from 5kV to 7kV, the analysis shows that the AC breakdown field strength ACE4 of the thermally aged oil-paper insulation sample initially decreases and then increases.

7. The method for evaluating the electrical strength of oil-paper insulation of a converter transformer by superimposing AC and DC voltages according to claim 1, characterized in that: In step S05, the specific content of the analysis of the influence of AC / DC voltage superposition and thermal aging on the breakdown characteristics is as follows: Step S051: when the DC preset voltage is increased from 0 kV to 1 kV, the AC / DC superimposed breakdown strength of the unaged oil-paper insulation sample and the AC / DC superimposed breakdown strength of the thermally aged oil-paper insulation sample are obtained. Analysis shows that the AC / DC superimposed breakdown strengths of the unaged and thermally aged oil-paper insulation samples both decrease. When the DC preset voltage is further increased, the AC / DC superimposed breakdown strengths of the unaged and thermally aged oil-paper insulation samples both recover. When the DC preset voltage reaches 4 kV or 5 kV, the AC / DC superimposed breakdown field strength is equal to the AC breakdown field strength observed at the DC preset voltage of 0 kV. Step S052: Increasing the DC preset voltage from 5 kV to 6 kV to obtain the AC / DC superimposed breakdown strength of the unaged oil-paper insulation sample and the AC / DC superimposed breakdown strength of the thermally aged oil-paper insulation sample. Analysis shows that the AC / DC superimposed breakdown strength of both the unaged and thermally aged oil-paper insulation samples increases, and the increase in the unaged oil-paper insulation sample is significantly greater than that of the thermally aged oil-paper insulation sample. Step S053: Increasing the DC preset voltage from 6 kV to 7 kV, the breakdown strength of both the unaged and heat-aged oil-paper insulation samples increased. At the DC preset voltage of 7 kV, the AC / DC superposition breakdown strength exceeded the DC breakdown strength.

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