High-precision dielectric loss measurement method for polypropylene cable insulation defects

Abstract

The invention discloses a high-precision dielectric loss measurement method for polypropylene cable insulation defects. The method comprises the following steps: S1, preparing a cable sample; s2, setting a resonant cavity test device; and S3, measuring and recording the change conditions of the resonant frequency and the quality factor of the resonant cavity before and after the cable sample is introduced to obtain the dielectric constant and the loss factor of the cable sample. According to the invention, high-precision measurement can be conveniently and efficiently carried out on the dielectric loss of the cable.

Description

Technical Field

[0001] The invention relates to a high-precision dielectric loss measurement method for insulation defects of a polypropylene cable, which is used in the field of cable performance detection. Background Art

[0002] In modern electronics and communication technology, accurate measurement of dielectric properties of materials is critical. Accurate measurement of dielectric constant and loss factor in cable insulation testing technology can help workers quickly understand the insulation of materials and ensure the reliability and efficiency of cables in different environments. In order to meet these needs, a variety of measurement techniques have been developed, including resonant cavity method, waveguide resonance method, coaxial line method, frequency domain reflectometry method, time domain reflectometry method, dielectric spectroscopy method and partial discharge measurement method.

[0003] The waveguide resonance method uses the electromagnetic wave propagation characteristics in the waveguide to obtain the dielectric properties by measuring the changes in the waveguide resonant frequency and quality factor after the introduction of the material. This method is suitable for measurements in the high-frequency range, especially in the field of communications. However, precise control of environmental conditions such as temperature and humidity is required, as these factors may affect the accuracy of the measurement results. In addition, these methods usually require expensive equipment and a high level of operating skills, which may limit their application in some resource-limited environments.

[0004] The coaxial line method measures the dielectric properties of the material through the transmission characteristics in the coaxial cable, and evaluates the dielectric constant and loss factor through the insertion loss and reflection coefficient. This method can be used in a wide frequency range and is applicable to a variety of materials. However, its precision and accuracy may be affected by the loss of the measurement system itself, and it may not be ideal for measuring non-uniform or multi-layer media.

[0005] Frequency domain reflectometry and time domain reflectometry analyze the dielectric properties of materials through signal reflections in cables or waveguides. These techniques can provide dynamic information about the dielectric response of materials and are suitable for on-site testing and rapid diagnosis. However, they have limited ability to handle multipath reflections and complex signals, which may lead to errors when resolving the dielectric properties of complex structures.

[0006] Dielectric spectroscopy involves measuring the dielectric response of a material over a wide frequency range. It can reveal the dielectric behavior of a material at different frequencies and is widely used in the development and performance evaluation of new materials. However, this method usually requires long-term data acquisition and high-performance analytical instruments, which may not be practical in commercial environments that require fast measurement results.

[0007] The partial discharge measurement method is used to test insulating materials under high electric fields. It evaluates the dielectric properties of materials by monitoring and analyzing discharge phenomena, which is particularly important for high-voltage equipment in power systems. However, this method needs to be operated in a high-voltage environment, has extremely high requirements for operational safety, and is very sensitive to noise in the test environment. For example, slight changes in air humidity and temperature may affect the results.

[0008] In addition, all of these methods may require special calibration steps in actual operation to ensure the repeatability and accuracy of the data. The calibration process itself can be complex and time-consuming, especially in scientific research or industrial applications with high precision requirements. Therefore, when choosing an applicable measurement technology, it is necessary not only to consider the theoretical advantages of the technology, but also to evaluate its practical feasibility in a specific application environment, including cost, ease of operation, availability of required equipment, and environmental conditions of expected use. Summary of the invention

[0009] The purpose of the present invention is to overcome the deficiencies of the prior art and to provide a high-precision dielectric loss measurement method for polypropylene cable insulation defects, which can conveniently and efficiently perform high-precision measurement of the dielectric loss of the cable.

[0010] A technical solution to achieve the above purpose is: a high-precision dielectric loss measurement method for polypropylene cable insulation defects, comprising the following steps:

[0011] S1, prepare cable samples;

[0012] S2, setting up the resonant cavity test equipment;

[0013] S3, measure and record the changes in the resonant frequency and quality factor of the resonant cavity before and after the cable sample is introduced, and obtain the dielectric constant and loss factor of the cable sample.

[0014] Furthermore, S1 specifically includes selecting a cable sample that meets the inspection size standard, and then cleaning the cable sample.

[0015] Furthermore, after the cable sample is cleaned, the cable sample is placed in a constant temperature and humidity environment for static preparation.

[0016] Furthermore, the constant temperature and humidity environment is specifically that the temperature is maintained at 25° C. and the relative humidity is 50%.

[0017] Furthermore, according to a high-precision dielectric loss measurement method for insulation defects of polypropylene cables according to claim 1, it is characterized in that S2 is specifically to calibrate the resonant cavity by using a standard sample according to the size and material properties of the cable sample.

[0018] Furthermore, it is characterized in that, in S3, for the cable sample, the resonant frequency change is described by the following formula:

[0019]

[0020] Where: ΔW e is the change in electric field energy before and after the cable sample is introduced; W e is the original electric field energy of the resonant cavity, f0 is the original resonant frequency of the resonant cavity, and △f is the change in resonant frequency;

[0021] For a volume V s The cable sample causes the electric field energy change to be:

[0022]

[0023] Where, ∈0 is the vacuum dielectric constant, ε' is the sample dielectric constant; E is the electric field strength;

[0024] Combining the above formulas, the resonant frequency shift formula is obtained:

[0025]

[0026] Where: V c is the volume of the resonant cavity, the above formula can be further written as:

[0027]

[0028] Where: E and H are the electric and magnetic field intensities in the cable, μ' is the magnetic permeability of the sample, and μ0 is the magnetic permeability of vacuum.

[0029] Furthermore, in S3, for the cable sample, the quality factor reflects the energy loss of the polypropylene cable in the electromagnetic wave, specifically:

[0030]

[0031] Among them, ∈″ and μ″ are the dielectric loss and magnetic loss factor of the material respectively, and ω is the grid frequency;

[0032] Due to the difference in power loss before and after the introduction of the material, combined with the expression for power loss, we can get:

[0033]

[0034] Among them, Q is the quality factor, W is the energy stored in the equivalent capacitance of the cable, and here It can be regarded as the power loss of the system when there is no lossy material, which is zero and can be ignored.

[0035] Furthermore, in S3, the dielectric constant ∈ rIt reflects the degree of response of the medium to the electric field and is calculated by the following formula:

[0036]

[0037] Here V c is the volume of the resonant cavity, and V s is the volume of the sample.

[0038] Furthermore, in S3, the loss factor tan(δ) represents the absorption capacity of the medium to electromagnetic wave energy and is calculated by the following formula:

[0039]

[0040] ΔQ represents the change in quality factor, which refers to the difference in quality factor caused by changes in material properties during the measurement process, and Q′ refers to the quality factor measured when the material is present.

[0041] Furthermore, by introducing the correction factor C f to adjust the model to accommodate the diversity of sample shapes and sizes, namely:

[0042] ∈ r ′=∈ r ·(1+C f )

[0043] Among them, C f The mathematical expression is:

[0044]

[0045] Among them, E sample and E cavity are the electric field distributions in the sample and the cavity, respectively; H sample and H cavity are the magnetic field distributions inside the sample and the cavity, respectively; f(r) and g(r) are the field distribution correction functions related to the position r, which take into account the inhomogeneity of the field distribution and the local enhancement effect of the sample on the electromagnetic field.

[0046] This patent proposes a high-precision dielectric loss measurement method based on cavity perturbation technology, aiming to overcome many limitations in existing dielectric property measurement technology. Through the innovative cavity perturbation method, this method has shown significant advantages in measuring the dielectric constant and loss factor of cable insulation materials, specifically:

[0047] 1. Easy operation and low equipment cost:

[0048] Compared with traditional methods, the operation process of cavity perturbation technology is simpler and does not require complicated calibration process.

[0049] The required equipment is relatively simple and the cost is significantly reduced, making it more suitable for on-site testing and quality control in large-scale production.

[0050] 2. High-precision measurement:

[0051] By measuring the changes in the resonant frequency and quality factor of the resonant cavity after the introduction of the sample, the dielectric constant and loss factor can be accurately calculated.

[0052] It can quickly detect the dielectric properties of insulating materials to ensure that the produced cables meet the design requirements and have high reliability.

[0053] 3. Wide applicability:

[0054] This technology has low requirements on the shape and size of the sample and can measure cables of different types and specifications.

[0055] No destructive processing of the sample is required, making it suitable for a variety of application scenarios, including quality control and fault diagnosis of high-voltage cables.

[0056] 4. Significant application value:

[0057] In the quality control process of high-voltage cables, cavity perturbation technology can quickly and accurately measure the dielectric properties of insulation materials, ensuring the stability and efficiency of cables in different environments.

[0058] In cable fault diagnosis, this technology can detect the degree of degradation of insulation materials, timely discover and prevent potential fault risks, and improve the overall safety and reliability of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 The present invention is a flow chart of a high-precision dielectric loss measurement method for polypropylene cable insulation defects. DETAILED DESCRIPTION

[0060] In order to better understand the technical solution of the present invention, the following is a detailed description through specific embodiments:

[0061] See also Figure 1 A high-precision dielectric loss measurement method for polypropylene cable insulation defects of the present invention comprises the following steps:

[0062] S1, prepare cable samples.

[0063] Select a representative cable sample and clean it carefully to ensure that there is no impurity on the sample surface. This step is very important because any surface contaminants may affect the accuracy of the test results. In addition, the sample should be in a constant temperature and humidity environment to avoid the influence of environmental factors on the measurement and ensure the consistency of the test conditions. The constant temperature and humidity environment is specifically maintained at 25°C and the relative humidity is 50%.

[0064] S2, setting up the resonant cavity test equipment.

[0065] The resonant cavity needs to be adjusted according to the size and material properties of the sample. This adjustment step is to ensure that the parameters of the resonant cavity match the physical characteristics of the sample, so that the measurement is more accurate. Subsequently, the equipment is calibrated by using standard samples to ensure that the equipment operates in the best condition and can provide accurate measurement results. Calibration is a necessary process that can correct the systematic errors of the equipment and ensure the reliability of the measurement data. After the equipment is ready, the cable sample is precisely placed in the resonant cavity. The position of the sample must be accurate to ensure that the sample can correctly couple electromagnetically with the resonant cavity during the measurement process.

[0066] S3, measure and record the changes in the resonant frequency and quality factor of the resonant cavity before and after the cable sample is introduced, and obtain the dielectric constant and loss factor of the cable sample.

[0067] After starting the equipment, the changes in the resonant frequency and quality factor of the resonant cavity before and after the cable sample is introduced are measured. These changes reflect the electromagnetic properties of the sample, especially its dielectric constant and loss factor.

[0068] After recording the data, the mathematical model is used to analyze the data in detail. Through model calculation, the dielectric constant and loss factor of the cable can be accurately obtained. These parameters are key indicators for judging the insulation performance of the cable. In the process of data analysis, error analysis is also required to eliminate possible external interference factors and ensure the accuracy and credibility of the measurement results.

[0069] Finally, the measured dielectric constant and dissipation factor are compared with industry standards to evaluate whether the cable insulation performance is qualified. This step determines whether the cable meets the requirements for use and ultimately generates a detailed test report. This report not only contains measurement data and analysis results, but may also include a record of the test process and a description of the equipment status. Through this systematic process, cavity perturbation technology can efficiently and accurately detect the performance of cable insulation materials, and is widely used in a variety of application scenarios from production line quality control to on-site fault diagnosis.

[0070] The mathematical model of measuring the dielectric properties of cables using cavity perturbation technology considers the basic behavior of electromagnetic waves in the resonant cavity. When a dielectric sample is placed in the resonant cavity, the electromagnetic properties of the medium will disturb the electromagnetic field in the cavity. This disturbance is reflected in the changes in the resonant frequency and quality factor.

[0071] The mathematical model is as follows:

[0072] 1. Change of resonant frequency

[0073] When a polypropylene cable sample is placed in a resonant cavity, its unique dielectric properties affect the distribution of the electromagnetic field in the cavity, resulting in a change in the resonant frequency. For polypropylene cables, this frequency change can be described by the following formula:

[0074]

[0075] Where: ΔW e is the change in electric field energy before and after the sample is introduced; W e is the original electric field energy of the resonant cavity, and f0 is the original resonant frequency of the cavity.

[0076] For a volume V s The small sample, the electric field energy change caused by it is:

[0077]

[0078] Where: ∈0 is the dielectric constant of vacuum; E is the electric field strength.

[0079] Combining the above formulas, the resonant frequency shift formula is obtained:

[0080]

[0081] Where: V c is the volume of the resonant cavity. The above formula can be further written as:

[0082]

[0083] Where: E and H are the electric and magnetic field intensities in the cable respectively, and μ0 is the magnetic permeability of vacuum.

[0084] 2. Quality factor changes

[0085] The change in quality factor reflects the energy loss of polypropylene cable in electromagnetic waves. Due to the low loss characteristics of polypropylene material, the change in quality factor caused by it is small, but it still needs to be accurately measured in high-frequency applications. The power loss in the cable consists of two parts: dielectric loss and magnetic loss. The specific formula is:

[0086]

[0087] Here, ∈″ and μ″ are the dielectric loss and magnetic loss factor of the material, respectively, and ω is the grid frequency.

[0088] Due to the difference in power loss before and after the introduction of the material, combined with the expression for power loss, we can get:

[0089]

[0090] Where: W is the energy stored in the equivalent capacitance of the cable, where It can be considered that the power loss of the system is zero when there is no lossy material, so it can be ignored.

[0091] Reciprocal form The advantage of this method is that it can clearly separate the change in energy loss before and after the introduction of the material. Especially in the case of low-loss materials, the change in the inverse form is more significant and easier to measure.

[0092] 3. Dielectric constant and loss factor

[0093] Dielectric constant∈ r It reflects the response of the medium to the electric field, while the loss factor tan(δ) indicates the medium's ability to absorb electromagnetic wave energy. The dielectric constant and loss factor of polypropylene material can be estimated by measuring the changes in resonant frequency and quality factor. The dielectric constant can be calculated by the following formula:

[0094]

[0095] Here V c is the volume of the resonant cavity, and V s is the volume of the sample.

[0096] The calculation of the loss factor is more complex and requires considering multiple factors of energy loss:

[0097]

[0098] ΔQ is the change in the quality factor, usually the difference in the quality factor due to changes in material properties during the measurement process. Q' is the quality factor measured in the presence of the material (or under certain conditions).

[0099] In addition, the position and size of the sample in the resonant cavity affect the degree of its perturbation of the electromagnetic field, which can be quantitatively described by the cavity filling factor:

[0100]

[0101] Among them, η e is the electric field filling factor, η mis the magnetic field filling factor, ∈′ is the real part of the complex dielectric constant, and μ′ is the magnetic permeability of the material.

[0102] Electric field distribution:

[0103]

[0104] Where: E x , E y , E z is the component of the electric field in the x, y, and z directions; B is the amplitude constant of the field, which represents the amplitude of the maximum value of the electric field; ω is the angular frequency; μ is the magnetic permeability, which indicates the degree of response of the material to the magnetic field; γ is the complex wave number, which indicates the characteristics of electromagnetic wave propagation in the medium; a and b are the dimensions of the rectangular resonant cavity in the x and y directions respectively; m and n are the orders of the mode, which indicate the mode distribution in the x and y directions respectively; e -jωt is the time harmonic factor, which represents the change of the electric field over time.

[0105] Magnetic field distribution:

[0106]

[0107] Where H0 is the amplitude of the magnetic field.

[0108] In the polypropylene cable insulation testing, the cavity perturbation technology introduces the correction factor C f To adjust the model to accommodate the diversity of sample shapes and sizes. This correction factor helps accurately reflect the local enhancement effect and inhomogeneity of the electromagnetic field, especially for the electric and magnetic field distribution characteristics of polypropylene cable materials. The corresponding mathematical model can be described as follows:

[0109] ∈′ r =∈ r ·(1+C f )

[0110] Among them, C f The mathematical expression is:

[0111]

[0112] Among them, E sample and E cavity are the electric field distributions in the sample and the cavity, respectively; H sample and H cavity are the magnetic field distributions inside the sample and the cavity, respectively; f(r) and g(r) are the field distribution correction functions related to the position r, which take into account the inhomogeneity of the field distribution and the local enhancement effect of the sample on the electromagnetic field.

[0113] The dielectric properties of polypropylene materials are affected by environmental conditions, such as temperature and humidity. Cavity perturbation technology can sensitively detect these changes, providing data support for the long-term stability and durability of the cable. For example, as the temperature increases, the dielectric constant of polypropylene may increase, while humidity changes may affect the material's loss factor. By measuring the changes in resonant frequency and quality factor under different temperature and humidity conditions, the dielectric constant ∈ r And the relationship between the loss factor tanδ and time and environmental factors. The specific change formula is as follows:

[0114] ∈ r (t) = v r0 +k t ·t+k h ·h(t)

[0115] Among them, ∈ r0 is the initial dielectric constant, t is the time, k t and k h is a coefficient related to time and environmental factors (such as humidity) h(t).

[0116] The specific change formula of loss factor is:

[0117]

[0118] where ε′ and ε″ are the real and imaginary parts of the complex dielectric constant, respectively; Δε′ and Δε″ represent the changes in the real and imaginary parts due to sample size or other perturbations; f0 is the resonant frequency without a sample; Δf0 is the frequency shift due to the introduction of a sample; Q is the quality factor; f r and f sr are the volumes of the cavity and the sample, respectively; and are parameters related to the simulated resonant frequency and frequency shift; Δf is a correction factor related to the frequency shift.

[0119] The core of cavity perturbation technology is to calculate the dielectric constant and loss factor of the material by measuring the changes in the resonant frequency and quality factor after the dielectric material is introduced into the resonant cavity. The configuration of the measurement system is crucial to the verification results. It is necessary to ensure that the design and manufacturing accuracy of the resonant cavity is high to avoid errors caused by cavity irregularities or surface defects. At the same time, the measurement equipment should have high resolution and high sensitivity to accurately capture subtle changes in the resonant frequency and quality factor.

[0120] Take polypropylene cable as an example. The cable used for testing is a polypropylene insulated cable, which is widely used in power transmission and distribution at medium and high voltage levels. The cable has a standard medium-thickness insulation layer with a diameter of 10.7 mm. The test cable is designed to withstand a maximum voltage level of 35kV. All measurements are carried out in a controlled laboratory environment with a temperature maintained at 25°C and a relative humidity of 50%. The measurement results are shown in Table 2:

[0121] Table 2. Changes in dielectric constant and dielectric loss of polypropylene cables under different aging conditions

[0122] Aging <![CDATA[Dielectric constant ∈ r > Dielectric loss tan(δ) Frequency response changes Normal situation 2 0.06 0 Mild aging 2.7 0.072 -0.1％ Moderate aging 3.0 0.091 -0.5％ Severe aging 3.5 0.17 -1％

[0123] Taking polypropylene cable as an example, Table 3 shows the comparison of the original measurement results and the dielectric constant and dielectric loss after correction by the correction factor.

[0124] Table 3. Dielectric constant and dielectric loss of polypropylene cables at different sample radii after introducing correction factors

[0125] Sample radius Original dielectric constant Corrected dielectric constant Original dielectric loss Corrected dielectric loss 5.1 2.03 2.05 0.025 0.027 10.7 2 2.01 0.06 0.063 15.8 1.97 1.94 0.074 0.078 20.3 1.93 1.97 0.09 0.096 25.7 1.9 1.92 0.16 0.163 32.5 1.86 1.89 0.25 0.21

[0126] The correction factor is introduced to compensate for the non-uniform influence of sample size, shape and position on the electromagnetic field distribution, as well as the error caused by edge effect and other factors. The correction factor effectively improves the measurement accuracy, so that the corrected results more accurately reflect the true electromagnetic properties of the material, thereby ensuring that the measurement of the dielectric constant and loss factor of the material under various conditions meets the high-precision requirements.

[0127] Polypropylene cable samples were used as test objects. The cable diameter was 10.7 mm and the withstand voltage level was 35KV. We used 10 groups of samples for measurement under different conditions. The power-on time was 180 hours. Then we compared the insulation parameters with normal cables and obtained the data shown in Table 4.

[0128] Table 4. Measurement results of different samples

[0129]

[0130] The above test results show that the high-precision dielectric loss measurement method based on cavity perturbation technology proposed in the present invention uses cavity perturbation technology to introduce dielectric materials into the resonant cavity and use the changes in the behavior of electromagnetic waves in the resonant cavity to measure the dielectric properties of the material. By accurately measuring the changes in the resonant frequency and quality factor, the dielectric constant and loss factor of the material can be calculated. This method has the significant advantages of simple operation, low equipment cost and high measurement accuracy, and is particularly suitable for on-site detection and quality control in large-scale cable production.

[0131] Traditional dielectric property measurement methods such as the resonant cavity method and the coaxial line method, although they can provide accurate data, usually require complex equipment and precise environmental control. In contrast, the cavity perturbation technology can achieve high-precision measurements without the need for complex calibration by measuring changes in resonant frequency and quality factor. This greatly simplifies the operating process, reduces measurement costs, and improves measurement efficiency.

[0132] The present invention has low requirements on the shape and size of the sample and can measure cables of different types and specifications without destructive treatment of the sample. This means that whether it is a large-diameter high-voltage cable or a small-diameter low-voltage cable, the technology can be used to accurately measure the dielectric properties. This diverse adaptability makes the technology highly practical in a variety of practical application scenarios.

[0133] In practical applications, due to the diversity of sample shapes and sizes, correction factors are introduced to adjust the theoretical model to improve measurement accuracy. Through regression analysis of experimental data, correction factors can be obtained to optimize the model to ensure its accuracy and reliability under different conditions. This correction method further improves the accuracy of the measurement results and makes it suitable for a wider range of application scenarios.

[0134] Those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A high-precision dielectric loss measurement method for polypropylene cable insulation defects, characterized in that: The steps include: S1, prepare cable samples; S2, setting up the resonant cavity test equipment; S3, measure and record the changes in the resonant frequency and quality factor of the resonant cavity before and after the cable sample is introduced, and obtain the dielectric constant and loss factor of the cable sample.

2. A high-precision dielectric loss measurement method for polypropylene cable insulation defects according to claim 1, characterized in that: S1 specifically includes selecting a cable sample that meets the inspection size standard, and then cleaning the cable sample.

3. A high-precision dielectric loss measurement method for polypropylene cable insulation defects according to claim 2, characterized in that: After cleaning the cable samples, place them in a constant temperature and humidity environment for preparation.

4. A high-precision dielectric loss measurement method for polypropylene cable insulation defects according to claim 3, characterized in that: The constant temperature and humidity environment is specifically that the temperature is maintained at 25° C. and the relative humidity is 50%.

5. A high-precision dielectric loss measurement method for polypropylene cable insulation defects according to claim 1, characterized in that: S2 is specifically to calibrate the resonant cavity by using a standard sample according to the size and material properties of the cable sample.

6. A high-precision dielectric loss measurement method for polypropylene cable insulation defects according to claim 1, characterized in that: In S3, for the cable sample, the resonant frequency change is described by the following formula: Where: ΔW e is the change in electric field energy before and after the cable sample is introduced; W e is the original electric field energy of the resonant cavity, f0 is the original resonant frequency of the resonant cavity, and △f is the change in resonant frequency; For a volume V s The cable sample causes the electric field energy change to be: Where, ∈0 is the vacuum dielectric constant, ε, is the sample dielectric constant; E is the electric field strength; Combining the above formulas, the resonant frequency shift formula is obtained: Where: V c is the volume of the resonant cavity, the above formula can be further written as: Where: E and H are the electric and magnetic field intensities in the cable, μ, is the magnetic permeability of the sample, and μ0 is the magnetic permeability of vacuum.

7. A high-precision dielectric loss measurement method for polypropylene cable insulation defects according to claim 1, characterized in that: In S3, for the cable sample, the quality factor reflects the energy loss of the polypropylene cable in the electromagnetic wave, specifically: Among them, ∈″ and μ″ are the dielectric loss and magnetic loss factor of the material respectively, and ω is the grid frequency; Due to the difference in power loss before and after the introduction of the material, combined with the expression for power loss, we can get: Among them, Q is the quality factor, W is the energy stored in the equivalent capacitance of the cable, and here It can be regarded as the power loss of the system when there is no lossy material, which is zero and can be ignored.

8. A high-precision dielectric loss measurement method for polypropylene cable insulation defects according to claim 1, characterized in that: In S3, the dielectric constant ∈ r It reflects the degree of response of the medium to the electric field and is calculated by the following formula: Here V c is the volume of the resonant cavity, and V s is the volume of the sample.

9. A high-precision dielectric loss measurement method for polypropylene cable insulation defects according to claim 1, characterized in that: In S3, the loss factor tan(δ) represents the medium's ability to absorb electromagnetic wave energy and is calculated by the following formula: ΔQ represents the change in quality factor, which refers to the difference in quality factor caused by changes in material properties during the measurement process, and Q′ refers to the quality factor measured when the material is present.

10. The high-precision dielectric loss measurement method for polypropylene cable insulation defects according to claim 1, by introducing the cavity filling factor and the correction factor C f To adjust the model to accommodate the diversity of sample shapes and sizes, C f The mathematical expression is: Among them, E sample and E cavity are the electric field distributions in the sample and the cavity, respectively; H sample and H cavity are the magnetic field distributions inside the sample and the cavity, respectively; f(r) and g(r) are the field distribution correction functions related to the position r, which take into account the inhomogeneity of the field distribution and the local enhancement effect of the sample on the electromagnetic field.

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