Evaluation method of EPR cable terminal stress intensity model based on terahertz detection

Abstract

The present invention discloses an evaluation method for an EPR cable terminal stress strength model based on terahertz detection, which belongs to the field of high voltage electricity and insulation technology, and establishes a stress-strength interference model of an EPR cable terminal to evaluate the health of the EPR cable terminal and determine a safety threshold. The method includes sample and detection equipment preparation, sample simulation test and data acquisition and processing, establishment of a stress-strength interference model, characterization of the health of the EPR cable terminal, setting a reasonable safety threshold, using the model as a reference, and setting parameters at will according to specific requirements in actual engineering, and calculating the required other parameter values, that is, evaluating the safety threshold of the EPR cable terminal according to its operating environment. The present invention has strong practical significance and reference value in the evaluation of EPR cable terminals for high-speed railways.

Description

Technical Field

[0001] The invention belongs to the technical field of high voltage electricity and insulation, and in particular relates to an EPR cable terminal stress strength model evaluation method based on terahertz detection. Background Art

[0002] Ethylene propylene rubber (EPR) cable terminals are important components for powering high-speed trains and connecting different electrical equipment, and are widely used in the field of rail transit. Their working status directly affects the safe operation of vehicles. Because cable terminals may be affected by various factors such as vibration, torsion, high temperature, and high voltage during operation, along with insulation aging, cables may experience delamination and electrical breakdown. In order to avoid the impact of the above factors on cable terminals and eliminate safety hazards during vehicle operation, it is urgent to detect the above health conditions of cable terminals and evaluate safety thresholds.

[0003] At present, the fault diagnosis of cable terminals is mainly based on XLPE cables, while the experimental research related to EPR is very limited, and the related fault diagnosis research is mostly focused on the analysis of electrothermal aging, lacking effective discussion on EPR cable breakdown. Cable breakdown, as the final representation of cable failure, is the result of the combined action of multiple factors, and it is necessary to comprehensively consider multi-source information, the most important of which are temperature stress, mechanical stress, and electrical stress.

[0004] Terahertz waves are electromagnetic waves with a frequency of 0.1-10THz and a wavelength of 30μm-3mm. They are between infrared optics and microwave millimeter waves, and contain a wealth of physical and chemical properties. Therefore, when terahertz waves penetrate objects, they can collect physical and chemical changes in the object being measured to determine the composition of defects. Because of their great scientific value and broad application prospects in object imaging, environmental monitoring, medical diagnosis, radio astronomy, etc., terahertz electromagnetic waves and related detection technologies for their applications have become a "hot" field in the scientific community.

[0005] Our school's patent CN202011368042.2 "An EPR heat-shrinkable cable terminal internal defect detection system and detection method thereof" studies and discloses the defect detection method of EPR cable terminals applied to terahertz technology. This technology is a non-contact, non-destructive testing technology developed in recent years. It can effectively ensure the integrity of the detection object while achieving deep detection. Compared with traditional detection methods such as infrared and ultraviolet detection, ultrasonic detection and X-ray detection, terahertz detection, in addition to its non-destructive characteristics, also has the characteristics of strong penetration ability, good material recognition ability, strong anti-interference ability, and high recognition. The application of this detection technology in the research and development of cable terminals has gradually achieved results.

[0006] Based on the problems in the above background technology, comprehensive EPR cable terminal fault diagnosis is carried out, combined with the temperature stress, mechanical stress and electrical stress represented by it, and terahertz detection technology is applied to construct a stress-strength interference model to realize the monitoring of the operating status of the EPR cable terminal of the EMU and the evaluation of the safety threshold. Summary of the invention

[0007] Another object of the present invention is to provide an EPR cable terminal stress strength model evaluation method based on terahertz detection, so as to establish a stress-strength interference model of the EPR cable terminal to evaluate the health status of the EPR cable terminal and determine the safety threshold.

[0008] In order to solve the above problems, the technical solution of the present invention is:

[0009] 1. An EPR cable terminal stress strength model evaluation method based on terahertz detection, characterized in that the method comprises the following steps:

[0010] S1. Preparation of samples and testing equipment;

[0011] S1.1, sample preparation;

[0012] Cut the EPR cable terminal sample into annular sheets and polish them with sandpaper to control the thickness to 1mm±0.05mm with uniform thickness and smooth surface;

[0013] S1.2. Preparation of temperature stress and mechanical stress test equipment;

[0014] The temperature stress and mechanical stress test equipment includes: a torsional vibration table and a temperature monitoring device installed in the electric heat aging box. The temperature monitoring device is connected to a control console, which is located on the outer wall of the electric heat aging box. The control console can collect and record all temperature data in real time and transmit it to the computer through a data cable;

[0015] S1.3, preparation of electrical stress test equipment;

[0016] The electrical stress test equipment includes: high voltage circuit and electrical breakdown device;

[0017] The high-voltage circuit includes a loop consisting of a three-phase power supply, a voltage regulator, a transformer, a current-limiting resistor, and a test sample; a compensation capacitor is provided between the voltage regulator and the transformer in the loop; the test sample is provided in an electrical breakdown device;

[0018] The sample electrical breakdown device comprises: a transformer oil loading container composed of a bottom plate and a visible insulating cover arranged thereon, wherein the loaded transformer oil covers the sample; an insulating support is arranged on the bottom plate, a plate-plate electrode clamped in a group of conductor rods is arranged above the insulating support, the sample of the EPR cable terminal is clamped in the plate-plate electrode, the top of the conductor rod above is connected to the high-voltage line, the high-voltage line is connected to the transformer, the conductor rod below is connected to the bottom of the bottom plate, the bottom plate is made of stainless steel, and the plate-plate electrode is made of copper;

[0019] S1.4, terahertz detection and imaging system preparation;

[0020] Select any existing terahertz detection and imaging system and complete debugging;

[0021] S2, sample simulation test and data collection and processing;

[0022] S2.1, temperature stress and mechanical stress test data collection;

[0023] Fix the sample prepared in S1.1 on the torsional vibration table in the electric heat aging box, and conduct the test after reinforcing it with insulating support. Meanwhile, record the intensity and number of torsional vibrations (i.e., frequency) of defects in each sample, as well as the time of torsional vibration;

[0024] S2.2, data collection of terahertz detection test;

[0025] The samples with defects in S2.1 are tested by terahertz, and the relevant parameters are obtained through terahertz time domain and frequency domain spectrograms to calculate the refractive index, dielectric constant, and dielectric loss;

[0026] S2.3, electrical stress test data collection;

[0027] The sample with defects in S2.1 is clamped between the plate-plate electrodes and pressed Figure 3 and Figure 4 Wiring, the transformer adjusts the voltage level through the voltage regulator, gradually increasing it until electrical breakdown occurs, and the voltage level of each sample at breakdown is recorded;

[0028] S2.4, data collection of terahertz detection experiment 2;

[0029] Refer to the test steps in S2.2, and perform the same terahertz test on the sample broken down in S2.3. Obtain relevant parameters through terahertz time domain and frequency domain spectra, and calculate the refractive index, dielectric constant, and dielectric loss;

[0030] S3, establish a stress-strength interference model to characterize the health status of the EPR cable terminal and set a reasonable safety threshold;

[0031] The stress-strength interference model is essentially based on fatigue theory, involving basic theories such as matrix theory, mathematical statistics, fatigue mechanics, and material mechanics. The purpose is to meet the requirements of reliability and safety, cope with various conditions under normal operation, complete the established functional tasks, and achieve the required expected lifespan; at the same time, during the specified service life, the probability of failure of the EPR cable terminal under temperature stress, mechanical stress, and electrical stress is minimized; therefore, it is feasible to establish the mechanical theory of the stress-strength interference model under the condition of clear fatigue mechanism;

[0032] The data of S2.1-S2.4 are summarized, and their distribution satisfies the normal distribution, that is, they are concentrated in a certain interval, or they satisfy the exponential distribution, or they satisfy the logarithmic distribution characteristics. Based on this law, the relevant parameters of the data, such as the mean, variance and mode, are summarized, and used as references in turn to establish the stress-strength interference model;

[0033] S4. Application of results;

[0034] Taking the stress-strength interference model established in S3 as a reference, according to the specific requirements of the actual project, the parameters can be set at will, and the required other parameter values ​​can be calculated according to the model;

[0035] That is, the safety threshold of the EPR cable terminal is evaluated according to its operating environment.

[0036] Furthermore, the process of establishing the stress-strength interference model in step S3 is as follows:

[0037] S3.1 General formula of one-dimensional stress-strength interference model:

[0038] Known fatigue stress amplitude s a The fatigue strength amplitude S a The one-dimensional probability density functions are f(s a ) and g(Sa), then the general formula of the one-dimensional stress-intensity interference model is:

[0039]

[0040] Fatigue stress and fatigue limit are two independent normal variables. When they satisfy the normal distribution, the calculation formula of the stress-strength interference model can be obtained:

[0041]

[0042] In formula (9), μ and σ are the mean and variance of the matrix, μ1 and σ1 are the mean and variance of stress, and μ2 and σ2 are the mean and variance of strength;

[0043] Formula (9) comprehensively considers the fatigue limit of stress and strength, and uses reliability as the basis for judgment, so this formula can also be defined as the fatigue reliability formula;

[0044] S3.2 General formula of two-dimensional stress-strength interference model:

[0045]

[0046] In formula (10), g is the probability density function of fatigue limit amplitude, Sa and Sm are two-dimensional fatigue extreme data (Sa, Sm);

[0047] According to the two stress-strength interference models established above, the fatigue mechanism and working threshold of the samples are characterized.

[0048] Furthermore, the specific test process in step S2.1 is:

[0049] The samples prepared in S1.1 are fixed on the torsional vibration table in the electric heat aging box and reinforced with insulating supports. The number of samples is 100, and the test is carried out in 10 groups. The number of samples that can be fixed at one time is 10;

[0050] Set the temperature of the electric heat aging box, set the intensity and number of torsional vibrations (i.e., frequency) of the torsional vibration table, and the time of torsional vibration;

[0051] The first setting of the torsional vibration time is 800 hours. After the torsional vibration table stops working for the first time, the continuous operation time is set to decrease by 100 hours each time starting from the second time. Each time the torsional vibration table stops working, the samples are observed, and the defective samples are taken out and the normal samples are retained until all samples have defects.

[0052] The temperature acquisition frequency in the electric heat aging box is 5 minutes / time;

[0053] At the same time, the intensity and number of torsional vibrations (ie, frequency) of defects in each sample, as well as the duration of torsional vibrations were recorded.

[0054] Furthermore, the specific calculation process in step S2.2 is as follows:

[0055] The sample with defects in S2.1 is tested by terahertz. The terahertz transmission and reflection coefficients can be expressed by the Fresnel formula as follows:

[0056]

[0057] In formula (1), ω is the angular frequency of the THz wave; is the complex refractive index of THz wave propagating in air; is the complex refractive index of the THz wave propagating in the sample; θ and β are the incident angle and refraction angle respectively; a and s are the air medium and the sample medium respectively;

[0058] The refractive index of the sample is:

[0059]

[0060] in

[0061]

[0062] κ s (ω) is the absorption coefficient;

[0063] The complex dielectric constant of the sample is:

[0064]

[0065] The real part of the complex dielectric function is:

[0066] ε′(ω)=n 2 (ω)-κ 2 (ω) (5)

[0067] The imaginary part of the complex dielectric function is:

[0068] ε″(ω)=2n(ω)κ(ω) (6)

[0069] The dielectric loss tangent value is used to characterize the dielectric loss of the sample, which is:

[0070]

[0071] Relevant parameters are obtained through terahertz time-domain and frequency-domain spectra, and the refractive index, dielectric constant, and dielectric loss are calculated according to the above formula.

[0072] Furthermore, in step S2.3, the voltage regulation method is manual voltage regulation, the voltage regulation rate is 0.5 KV / S, and the initial voltage level is 10 KV.

[0073] Furthermore, after the sample is subjected to terahertz detection in step S2.4, relevant parameters are obtained through terahertz time domain and frequency domain spectra according to equations (1)-(7), and the refractive index, dielectric constant, and dielectric loss are calculated.

[0074] The beneficial effects of the present invention are as follows:

[0075] (1) The electric thermal aging box of the present invention can set the temperature inside the box to simulate the operating environment of the EPR cable terminal at high temperature. Torsional vibration is carried out simultaneously with the box. The torsional vibration table can provide three options: torsion, vibration, and torsion and vibration to simulate the mechanical stress that the EPR cable terminal is subjected to during actual operation. The intensity and number of torsions (i.e., frequency) of the torsional vibration, as well as the time of the torsional vibration are set, and the experiment is accelerated until defects appear in the cable terminal test sample, and the sample is further analyzed and tested. The high-voltage breakdown device has high safety, low manufacturing cost, can be used multiple times, and can accurately reflect the breakdown voltage of the EPR cable terminal test sample.

[0076] (2) The terahertz detection and imaging system detects the defects of the EPR cable terminal samples in two states, obtains the time domain spectrum and frequency domain spectrum of the samples, determines the defect conditions of the samples, and extracts relevant parameter information for further analysis and conversion.

[0077] (3) The experimental data including the intensity and number of torsional vibrations (i.e., frequency), the time of torsional vibrations, the voltage level at breakdown, and the refractive index of terahertz waves are introduced into the MATLAB software. The dielectric constant and dielectric loss are calculated. The data are further analyzed and the normal distribution diagram of the data is obtained. The stress-strength interference model is established based on this. Finally, the model can be used to input any parameters and the required other parameter values ​​can be calculated according to the model. That is, the safety threshold is evaluated according to the operating environment of the EPR cable terminal.

[0078] (4) The present invention studies the breakdown voltage of cables under different temperatures and different torsional vibration stresses, and uses terahertz detection technology to characterize the terahertz spectrum of cable terminal defects, traces the breakdown phenomenon, explores the essential relationship between the breakdown of the cable terminal and the operating state of the cable, and designs a stress-strength interference model based on the obtained test data to achieve an accurate assessment of the safety threshold of the EPR cable terminal. It has great practical significance and reference value in the assessment of the EPR cable terminal of high-speed rail. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Figure 1 The flowchart of the stress strength model evaluation method of EPR cable terminal based on terahertz detection;

[0080] Figure 2 It is a schematic diagram of the structure of the temperature stress and mechanical stress test equipment used in the present invention;

[0081] Figure 3 It is a schematic structural diagram of a sample electrical breakdown device in the electrical stress test equipment used in the present invention;

[0082] Figure 4 It is a structural schematic diagram of a high voltage circuit in the electric stress test equipment used in the present invention;

[0083] The reference numerals are as follows:

[0084] 1-data acquisition and processing system; 2-terahertz detection and imaging system; 3-torsion vibration table; 4-electrical thermal aging box; 5-control console; 7-temperature monitoring device; 8-sample; 9-plate-plate electrode; 10-visible insulation cover; 11-insulating support; 12-bottom plate; 13-conductor rod; 14-high-voltage line; 15-transformer; 16-electric breakdown device. DETAILED DESCRIPTION

[0085] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are 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 creative work are within the scope of protection of the present invention.

[0086] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed but is merely representative of selected embodiments of the present invention.

[0087] Example 1

[0088] 1. An EPR cable terminal stress strength model evaluation method based on terahertz detection, the method comprises the following steps:

[0089] S1. Preparation of samples and testing equipment;

[0090] S1.1, sample preparation;

[0091] The EPR cable terminal samples were cut into annular sheets and polished with sandpaper to control the thickness to 1mm±0.05mm with uniform thickness and smooth surface.

[0092] S1.2. Preparation of temperature stress and mechanical stress test equipment;

[0093] The temperature stress and mechanical stress test equipment includes: a torsional vibration table 3 and a temperature monitoring device 7 installed in an electric thermal aging box 4. The temperature monitoring device 7 is connected to a control console 5, and the control console 5 is arranged on the outer wall of the electric thermal aging box 4; the control console 5 can collect and record all temperature data in real time and transmit them to a computer through a data line.

[0094] in:

[0095] The electric thermal aging box 4 is an electric thermal aging box produced by Xiamen Easy Instrument Co., Ltd., model SF-401A, serial number L21-10281, which can be heated to a maximum of 350°C, has a timing function, and does not require full-time supervision.

[0096] Torsion vibration table 3 is a uT6820 model torsional vibration table, which consists of ABB variable frequency motor, variable frequency drive, controller, 10-inch touch screen, 10-meter diameter and 1-meter length slender shaft, balance plate, torque speed sensor, DC motor, DC motor drive, DC motor controller, etc. It can provide two options of linear vibration and torsional vibration. The equipment can provide 10-100Hz vibration frequency and 10cm maximum displacement. The equipment has low noise during operation and does not need to be guarded throughout the process.

[0097] S1.3, preparation of electrical stress test equipment;

[0098] The electrical stress test equipment includes: a high voltage circuit and an electrical breakdown device 16;

[0099] The high-voltage circuit includes a loop consisting of a three-phase power supply, a voltage regulator, a transformer 15, a current-limiting resistor, and a test sample; a compensation capacitor is provided between the voltage regulator and the transformer 15 in the loop; and the test sample is provided in an electrical breakdown device 16;

[0100] The sample electrical breakdown device 16 includes: a transformer oil loading container composed of a bottom plate 12 and a visible insulating cover 10 arranged thereon, wherein the loaded transformer oil covers the sample 8; an insulating support 11 is arranged on the bottom plate 12, and a plate-plate electrode 9 clamped in a group of conductor rods 13 is arranged above the insulating support 11, and the sample 8 of the EPR cable terminal is clamped in the plate-plate electrode 9, the top of the upper conductor rod 13 is connected to the high-voltage line 14, the high-voltage line 14 is connected to the transformer 15, and the lower conductor rod 13 is connected to the bottom of the bottom plate 11, the bottom plate 12 is stainless steel, and the plate-plate electrode 9 is copper. The stress-strength interference model of the EPR cable terminal is established to evaluate the health status of the EPR cable terminal and determine the safety threshold.

[0101] Among them, transformer model 15 is GY1013-50KVA / 100KV non-partial discharge test transformer produced by Jiangsu Yice Electric Power Equipment Co., Ltd., with a rated capacity of 50KVA and a high voltage of 100KV.

[0102] S1.4, terahertz detection and imaging system preparation;

[0103] The terahertz detection and imaging system 2 selects the terahertz time-domain spectrometer CCT-1800 in the prior art, whose terahertz emitter is a LT-GaAs photoconductive semiconductor emitter, the spectral range is from 0.06THz to 4THz, the laser wavelength is 780nm, and the laser power range is 50mW≤Ps≤80mW. It can effectively analyze and deeply study the spectral characteristics, composition structure, and molecular vibration and rotation characteristics of materials, and can perform qualitative analysis on samples.

[0104] S2, sample simulation test and data collection and processing;

[0105] S2.1, temperature stress and mechanical stress test data collection;

[0106] The samples prepared in S1.1 are fixed on the torsional vibration table 3 in the electric heat aging box 4 and reinforced with insulating supports. The number of samples is 100, and the test is carried out in 10 groups. The number of samples that can be fixed at one time is 10.

[0107] The temperature of the electric heat aging box 4 is set, and the intensity and the number of torsional vibrations (ie, the frequency) of the torsional vibration table 3, as well as the time of the torsional vibration, are set.

[0108] The initial setting of the torsional vibration continuous operation time is 800 hours. After the torsional vibration table stops working for the first time, the continuous operation time is set to decrease by 100 hours each time starting from the second time. Each time the torsional vibration table stops working, the samples are observed, the defective samples are taken out, and the normal samples are retained until all samples have defects.

[0109] There are two types of sample defects: delamination defects under mechanical stress and temperature stress that occur in this step, and electrical breakdown under electrical stress in step S2.3.

[0110] The temperature acquisition frequency in the electric heat aging box 4 is 5 minutes / time;

[0111] At the same time, the intensity and number of torsional vibrations (ie, frequency) of defects in each sample, as well as the duration of torsional vibrations were recorded.

[0112] S2.2, data collection of terahertz detection test;

[0113] The sample with defects in S2.1 is tested by terahertz. The terahertz transmission and reflection coefficients can be expressed by the Fresnel formula as follows:

[0114]

[0115] In formula (1), ω is the angular frequency of the THz wave; is the complex refractive index of THz wave propagating in air; is the complex refractive index of the THz wave propagating in the sample; θ and β are the incident angle and refraction angle respectively; a and s are the air medium and the sample medium respectively.

[0116] The refractive index of the sample is:

[0117]

[0118] in

[0119]

[0120] κ s (ω) is the absorption coefficient.

[0121] The complex dielectric constant of the sample is:

[0122]

[0123] The real part of the complex dielectric function is:

[0124] ε′(ω)=n 2 (ω)-κ 2 (ω) (5)

[0125] The imaginary part of the complex dielectric function is:

[0126] ε″(ω)=2n(ω)κ(ω) (6)

[0127] The dielectric loss tangent value is used to characterize the dielectric loss of the sample, which is:

[0128]

[0129] Relevant parameters are obtained through terahertz time domain and frequency domain spectra. According to the above formula, the refractive index, dielectric constant and dielectric loss can be calculated.

[0130] S2.3, electrical stress test data collection;

[0131] The sample with defects in S2.1 is clamped between the plate-plate electrodes 9 and pressed Figure 3 and Figure 4 After wiring, the transformer 15 adjusts the voltage level through the voltage regulator, gradually increasing it until electrical breakdown occurs, and the voltage level of each sample when it breaks down is recorded.

[0132] The voltage regulation method is manual voltage regulation, the voltage regulation rate is 0.5KV / S, and the initial voltage level is 10KV.

[0133] S2.4, data collection of terahertz detection experiment 2;

[0134] Referring to the test steps of S2.2, the same terahertz test is performed on the sample that was broken down in S2.3. Also according to equations (1)-(7), the relevant parameters are obtained through the terahertz time domain and frequency domain spectra, and the refractive index, dielectric constant, and dielectric loss are calculated.

[0135] S3, establish a stress-strength interference model to characterize the health status of the EPR cable terminal and set a reasonable safety threshold;

[0136] The stress-strength interference model is essentially based on fatigue theory, involving basic theories such as matrix theory, mathematical statistics, fatigue mechanics, and material mechanics. The purpose is to meet the requirements of reliability and safety, cope with various conditions under normal operation, complete the established functional tasks, and achieve the required expected life. At the same time, during the specified service life, the probability of failure of the EPR cable terminal under temperature stress, mechanical stress, and electrical stress is minimized. Therefore, it is feasible to establish the mechanical theory of the stress-strength interference model under the condition of clear fatigue mechanism.

[0137] Summarizing the data of S2.1-S2.4, its distribution satisfies the normal distribution, that is, it is concentrated in a certain interval, or satisfies the exponential distribution, or satisfies the logarithmic distribution characteristics. Based on this rule, we can summarize the relevant parameters of the data, such as: mean, variance and mode, and use them as a reference to establish the following model.

[0138] S3.1 General formula of one-dimensional stress-strength interference model:

[0139] Known fatigue stress amplitude s a The fatigue strength amplitude S a The one-dimensional probability density functions are f(s a ) and g(Sa), then the general formula of the one-dimensional stress-intensity interference model is:

[0140]

[0141] In most cases, fatigue stress and fatigue limit are two independent normal variables. When they satisfy the normal distribution, the calculation formula of the stress-strength interference model can be obtained:

[0142]

[0143] In formula (9), μ and σ are the mean and variance of the matrix, μ1 and σ1 are the mean and variance of stress, and μ2 and σ2 are the mean and variance of strength.

[0144] Formula (9) comprehensively considers the fatigue limit of stress and strength, and uses reliability as the basis for judgment. Therefore, this formula can also be defined as a fatigue reliability formula.

[0145] Different samples have different reliability requirements. If the reliability is less than 0, the specimen is damaged, and if the reliability is greater than 0, it may also be damaged.

[0146] S3.2 General formula of two-dimensional stress-strength interference model:

[0147]

[0148] In formula (10), g is the probability density function of fatigue limit amplitude, Sa and Sm are two-dimensional fatigue extreme data (Sa, Sm).

[0149] According to the two stress-strength interference models established above, the fatigue mechanism and working threshold of the EPR cable terminal sample 8 are characterized.

[0150] S4. Application of results;

[0151] Taking the two stress-strength interference models established in S3 as reference, according to the specific requirements in the actual project, the parameters can be set at will, and the required other parameter values ​​can be calculated according to the model.

[0152] That is, the safety threshold is evaluated according to the operating environment of the EPR cable terminal.

[0153] Based on the above steps, the present invention has great practical significance and reference value in the evaluation of EPR cable terminals of high-speed railways.

Claims

1. An EPR cable terminal stress strength model evaluation method based on terahertz detection is characterized by: The method is as follows: S1. Preparation of samples and testing equipment; S1.1, sample preparation; Cut the EPR cable terminal sample into annular sheets and polish them with sandpaper to control the thickness to 1mm±0.05mm with uniform thickness and smooth surface; S1.

2. Preparation of temperature stress and mechanical stress test equipment; The temperature stress and mechanical stress test equipment comprises: a torsional vibration table (3) and a temperature monitoring device (7) installed in an electric heat aging box (4); the temperature monitoring device (7) is connected to a control console (5), and the control console (5) is arranged on the outer wall of the electric heat aging box (4); the control console (5) can collect and record all temperature data in real time and transmit them to a computer through a data line; S1.3, preparation of electrical stress test equipment; The electrical stress test equipment comprises: a high voltage circuit and an electrical breakdown device (16); The high-voltage circuit comprises a loop consisting of a three-phase power supply, a voltage regulator, a transformer (15), a current-limiting resistor, and a test sample; a compensation capacitor is provided between the voltage regulator and the transformer (15) in the loop; and the test sample is provided in an electrical breakdown device (16); The sample electrical breakdown device (16) comprises: a transformer oil loading container consisting of a bottom plate (12) and a visible insulating cover (10) arranged thereon, wherein the loaded transformer oil covers the sample (8); an insulating support (11) is arranged on the bottom plate (12), a plate-plate electrode (9) clamped in a group of conductor rods (13) is arranged above the insulating support (11), the sample (8) of the EPR cable terminal is clamped in the plate-plate electrode (9), the top end of the conductor rod (13) is connected to a high-voltage line (14), the high-voltage line (14) is connected to a transformer (15), and the conductor rod (13) below is connected to the bottom of the bottom plate (11); S1.4, terahertz detection and imaging system preparation; Select any existing terahertz detection and imaging system and complete debugging; S2, sample simulation test and data collection and processing; S2.1, temperature stress and mechanical stress test data collection; The sample prepared in S1.1 is fixed on a torsional vibration table (3) in an electric heat aging box (4), and is reinforced with an insulating support before testing. The intensity and number of torsional vibrations (i.e., frequency) of defects in each sample, as well as the duration of torsional vibration, are recorded at the same time; S2.2, data collection of terahertz detection test; The samples with defects in S2.1 are tested by terahertz, and the relevant parameters are obtained through terahertz time domain and frequency domain spectrograms to calculate the refractive index, dielectric constant, and dielectric loss; S2.3, electrical stress test data collection; The defective sample in S2.1 is clamped between the plate-plate electrodes (9), and connected according to Figures 3 and 4. The transformer (15) adjusts the voltage level through the voltage regulator and gradually increases it until electrical breakdown occurs. The voltage level of each sample when breakdown occurs is recorded; S2.4, data collection of terahertz detection experiment 2; Refer to the test steps in S2.2, and perform the same terahertz test on the sample broken down in S2.

3. Obtain relevant parameters through terahertz time domain and frequency domain spectra, and calculate the refractive index, dielectric constant, and dielectric loss; S3, establish a stress-strength interference model to characterize the health status of the EPR cable terminal and set a reasonable safety threshold; The data of S2.1-S2.4 are summarized, and their distribution satisfies the normal distribution, that is, they are concentrated in a certain interval, or satisfy the exponential distribution, or satisfy the logarithmic distribution characteristics. Based on this law, three relevant parameters of the data are summarized: mean, variance and mode, and they are used as references in turn to establish the stress-strength interference model; S3.1 one-dimensional stress-strength interference model universal formula: Known fatigue stress amplitude s a The fatigue strength amplitude S a The one-dimensional probability density functions are f(s a ) and g(Sa), then the general formula of the one-dimensional stress-intensity interference model is: Fatigue stress and fatigue limit are two independent normal variables. When they satisfy the normal distribution, the calculation formula of the stress-strength interference model can be obtained: In formula (9), μ and σ are the mean and variance of the matrix, μ1 and σ1 are the mean and variance of stress, and μ2 and σ2 are the mean and variance of strength; Formula (9) comprehensively considers the fatigue limit of stress and strength, and uses reliability as the basis for judgment, so this formula can also be defined as the fatigue reliability formula; S3.2 General formula of two-dimensional stress-strength interference model: In formula (10), g is the probability density function of fatigue limit amplitude, Sa and Sm are two-dimensional fatigue extreme data (Sa, Sm); According to the two stress-strength interference models established above, the fatigue mechanism and working threshold of sample (8) are characterized; S4. Application of results; Taking the stress-strength interference model established in S3 as a reference, according to the specific requirements of the actual project, the parameters can be set at will, and the required other parameter values ​​can be calculated according to the model; That is, the safety threshold of the EPR cable terminal is evaluated according to its operating environment.

2. The method for evaluating the EPR cable terminal stress strength model based on terahertz detection according to claim 1, characterized in that: The specific test process in step S2.1 is: The samples prepared in S1.1 are fixed on the torsional vibration table (3) in the electric heat aging box (4) and reinforced with insulating supports. The number of samples is 100, and the test is carried out in 10 groups. The number of samples that can be fixed at one time is 10; Setting the temperature of the electric heat aging box (4), setting the torsional vibration intensity and torsional frequency of the torsional vibration table (3), and the torsional vibration time; The first setting of the torsional vibration time is 800 hours. After the torsional vibration table stops working for the first time, the continuous operation time is set to decrease by 100 hours each time starting from the second time. Each time the torsional vibration table stops working, the samples are observed, and the defective samples are taken out and the normal samples are retained until all samples have defects. The temperature collection frequency in the electric heat aging box (4) is 5 minutes / time; At the same time, the intensity and number of torsional vibrations, i.e., the frequency, and the duration of torsional vibrations of each sample where defects occur are recorded.

3. The method for evaluating the EPR cable terminal stress strength model based on terahertz detection according to claim 1, characterized in that: The specific calculation process in step S2.2 is as follows: The sample with defects in S2.1 is tested by terahertz. The terahertz transmission and reflection coefficients can be expressed by the Fresnel formula as follows: In formula (1), ω is the angular frequency of the THz wave; is the complex refractive index of THz wave propagating in air; is the complex refractive index of the THz wave propagating in the sample; θ and β are the incident angle and refraction angle respectively; a and s are the air medium and the sample medium respectively; The refractive index of the sample is: in κ s (ω) is the absorption coefficient; The complex dielectric constant of the sample is: The real part of the complex dielectric function is: e′(ω)=n 2 (ω)-k 2 (oh)(5) The imaginary part of the complex dielectric function is: ε″(ω)=2n(ω)κ(ω)(6) The dielectric loss tangent value is used to characterize the dielectric loss of the sample, which is: Relevant parameters are obtained through terahertz time-domain and frequency-domain spectra, and the refractive index, dielectric constant, and dielectric loss are calculated according to the above formula.

4. The method for evaluating the EPR cable terminal stress strength model based on terahertz detection according to claim 1, characterized in that: In step S2.3, the voltage regulation method is manual voltage regulation, the voltage regulation rate is 0.5KV / S, and the initial voltage level is 10KV.

5. The method for evaluating the EPR cable terminal stress intensity model based on terahertz detection according to claim 4, characterized in that: After the sample is subjected to terahertz detection in step S2.4, the relevant parameters are obtained through the terahertz time domain and frequency domain spectra according to equations (1)-(7), and the refractive index, dielectric constant, and dielectric loss are calculated.

6. The method for evaluating the EPR cable terminal stress intensity model based on terahertz detection according to claim 1, characterized in that: In step S1.3, the bottom plate (12) is made of stainless steel; the plate-plate electrode (9) is made of copper.

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