A method for calculating stress distribution in dry-type cable terminals and evaluating their operational reliability

Abstract

The present invention belongs to the technical field of insulation of high-voltage electrical equipment, and discloses a method for calculating the internal stress distribution and evaluating the operational reliability of a dry-type cable terminal, comprising the following steps: obtaining the heat flow-temperature curve of epoxy resin at different heating rates through non-isothermal DSC measurement, and establishing a curing kinetic model of epoxy composite materials based on the KAS equation and the Sesták-Berggren model; simulating the distribution of residual stress and superimposed thermal stress of epoxy bushings at cable terminals based on finite element simulation methods; and using stress and strain safety factors to comprehensively evaluate the reliability of epoxy bushings under operating conditions at different temperatures. The present invention adopts the above-mentioned method for calculating the internal stress distribution and evaluating the operational reliability of a dry-type cable terminal, and obtains the overall superimposed stress distribution and operating temperature range of the epoxy bushing through experimental testing and simulation calculations, solving the current problems of overall strain distribution and operating temperature of epoxy bushings, and providing technical support for the stable operation of cable terminals under extreme temperature conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of insulation of high-voltage electrical equipment, and in particular to a method for calculating internal stress distribution and evaluating operation reliability of a dry-type cable terminal. Background Art

[0002] In the design and manufacturing process of cable terminals, the use of multi-media structures is common. This structure usually includes different material layers, such as insulation layer, stress cone layer and outer semiconductor layer. The thermodynamic parameters and mechanical properties of these different materials are usually different, resulting in internal stress during the curing process and under different working conditions. Among them, curing residual stress refers to the self-balanced stress remaining inside the material due to changes in temperature, pressure or other physical conditions during the curing process of the material. In the multi-media structure of the cable terminal, due to the different thermal expansion coefficients of each layer of material, an uneven stress distribution will be formed during the curing process, thereby generating thermal stress. As one of the key equipment in the high-voltage power transmission and distribution system, the gas insulated switchgear (GIS) cable terminal not only has to withstand a high temperature operating environment of 100°C or even higher, but also faces low temperature operating conditions of -40°C or even lower. The emergence of extreme temperature and temperature change environment makes the cable terminal under complex working conditions due to the mismatch of material thermal expansion coefficients. The thermal stress and curing residual stress are superimposed, threatening the safe and stable operation of the cable terminal.

[0003] Therefore, it is necessary to calculate the internal stress distribution of the cable terminal and evaluate the safety and reliability of its operation. Summary of the invention

[0004] The purpose of the present invention is to provide a method for calculating the internal stress distribution and evaluating the operation reliability of a dry-type cable terminal. The superimposed stress distribution and the operating temperature range of the epoxy casing as a whole are obtained through experimental testing and simulation calculations, which solves the current problems of the overall strain distribution and operating temperature of the epoxy casing, and provides technical support for the stable operation of cable terminals under different temperature environments, especially under extreme low temperature conditions.

[0005] To achieve the above object, the present invention provides a method for calculating stress distribution in a dry-type cable terminal and evaluating operation reliability, which is characterized by comprising the following steps:

[0006] S1. The heat flow-temperature curve of epoxy resin at different heating rates was obtained by non-isothermal DSC measurement. Based on the KAS equation and Sesták-Berggren model, the curing kinetic model of epoxy composites was established.

[0007] S2. Based on the finite element simulation method, the distribution of residual stress and thermal stress of the epoxy casing of the cable terminal is simulated;

[0008] S3. Use stress and strain safety factors to comprehensively evaluate the reliability of epoxy casing under different temperature operating conditions.

[0009] Preferably, in step S1, a curing kinetics model of epoxy composite material is established based on KAS equation and Sesták-Berggren model in combination with heat flow-temperature curve, and the specific process is as follows:

[0010] S11. Based on the KAS equation and the Sesták-Berggren model, the activation energy, pre-exponential factor, and reaction order of the catalytic effect and concentration effect of the epoxy resin reaction were calculated, and the curing reaction equation of the epoxy composite material was established as shown below:

[0011]

[0012] Where R is the molar gas constant, T is the temperature, α is the degree of solidification; E a is the activation energy, based on the KAS equation for ln(β / T p 2 )-(1 / T p ) is linearly fitted to obtain; T p is the peak value of the heat flow-temperature curve of epoxy resin; A is the pre-exponential factor, The intercept of the linear fitting result is A; m and n represent the reaction order of catalytic effect and concentration effect respectively; among them, The slope of the linear fitting result is n, and m is given by m / n=α M (1-α M ) is calculated, α M The calculation method is as follows:

[0013]

[0014] S12, based on the KAS equation, linear fitting is performed on the equation terms to obtain the activation energy of the epoxy resin, and the activation energy is determined by combining the heat flow temperature curve. The relationship between α and the average value of the parameter under the heating rate is used to confirm the curing kinetic model of epoxy composite materials, as shown below:

[0015]

[0016] Wherein, R is the molar gas constant, T is the temperature, and α is the degree of solidification.

[0017] Preferably, in step S2, the cable terminal finite element simulation model includes: a curing dynamics module, a heat transfer module and a solid mechanics module;

[0018] During simulation, the curing dynamics module and the thermal field module perform bidirectional coupling through the electrothermal simulation interface to calculate the curing degree and temperature distribution; the solid mechanics module calculates the stress-strain field based on the material parameters at different curing degrees, thereby obtaining the superimposed distribution of residual stress and thermal stress during the epoxy resin curing process.

[0019] Preferably, the stress field simulation mainly considers the residual stress caused by the curing process of the epoxy resin and the thermal stress caused by the difference in thermal expansion coefficient between the epoxy resin and the metal insert under changes in ambient temperature; the stress field simulation mainly includes three heat transfer modes, namely: heat conduction between solid media, heat convection between gases, and heat radiation from solid media to the gas environment.

[0020] Preferably, the simulation verification method is mainly based on numerical calculation and finite element simulation; among them, in the verification method based on finite element simulation, the finite element simulation model of the cable terminal is designed according to the full-size cable terminal; the simulation physical field includes thermal field and stress field, and during simulation, the two are bidirectionally coupled through an electrothermal simulation interface to calculate the degree of solidification and temperature distribution.

[0021] Preferably, in step S3, the reliability of the epoxy sleeve under different temperature operating conditions is comprehensively evaluated using stress and strain safety factors, and the specific process is as follows:

[0022] Based on the stress calculation results of epoxy casing under different temperature conditions, the safety factor k is calculated as follows:

[0023]

[0024] Among them, σ max (T) is the maximum tensile strength at different temperatures, σ(T) is the thermal stress, ε max (T) is the elongation at break at different temperatures, α is the thermal expansion coefficient of epoxy resin, and T0 = 20°C is the reference temperature; the larger the k value, the higher the risk of defects. When k ≥ 1, the material will be damaged under the action of thermal stress. It is believed that the epoxy sleeve has a safety risk under the action of superimposed internal stress.

[0025] Therefore, the present invention adopts the above-mentioned method for calculating the internal stress distribution and evaluating the operation reliability of a dry-type cable terminal, and obtains the overall superimposed stress distribution and operating temperature range of the epoxy casing through experimental testing and simulation calculations, thereby solving the current problems of the overall strain distribution and operating temperature of the epoxy casing, and providing technical support for the stable operation of cable terminals under different temperature environments, especially under extreme low temperature conditions.

[0026] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The simulation geometric model diagram of the present invention;

[0028] Figure 2 : is the influence diagram of different thermal expansion coefficients on the residual strain at the metal insert / epoxy resin interface; wherein, (a) is the influence diagram of different thermal expansion coefficients on the thermal strain at the metal insert / epoxy resin interface; (b) is the influence diagram of different thermal expansion coefficients on the total strain superimposed by thermal strain and chemical shrinkage strain at the metal insert / epoxy resin interface;

[0029] Figure 3 This is the effect of different thermal expansion coefficients on the thermal stress of the epoxy casing interface;

[0030] Figure 4 This is a graph showing the influence of different thermal expansion coefficients on the low-temperature applicable range of epoxy casing. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0032] The present invention provides a method for calculating the internal stress distribution of a dry-type cable terminal and evaluating its operation reliability, comprising the following steps:

[0033] S1. The heat flow-temperature curves of epoxy resin at different heating rates were obtained by non-isothermal DSC measurement. Based on the KAS equation and Sesták-Berggren model, a curing kinetic model of epoxy composites was established.

[0034] S11. Based on the KAS equation and the Sesták-Berggren model, the activation energy, pre-exponential factor, and reaction order of the catalytic effect and concentration effect of the epoxy resin reaction were calculated, and the curing reaction equation of the epoxy composite material was established as shown below:

[0035]

[0036] Where R is the molar gas constant, T is the temperature, α is the degree of solidification; E a is the activation energy, based on the KAS equation for ln(β / T p 2 )-(1 / T p ) is linearly fitted to obtain; T p is the peak value of the heat flow-temperature curve of epoxy resin; A is the pre-exponential factor, The intercept of the linear fitting result is A; m and n represent the reaction order of catalytic effect and concentration effect respectively; among them, The slope of the linear fitting result is n, and m is given by m / n=α M (1-α M ) is calculated, α M The calculation method is as follows:

[0037]

[0038] S12, based on the KAS equation, linear fitting is performed on the equation terms to obtain the activation energy of the epoxy resin, and the activation energy is determined by combining the heat flow temperature curve. The relationship between α and the average value of the parameter under the heating rate is used to confirm the curing kinetic model of epoxy composite materials, as shown below:

[0039]

[0040] Wherein, R is the molar gas constant, T is the temperature, and α is the degree of solidification.

[0041] S2. Based on the finite element simulation method, the distribution of residual stress and thermal stress of the cable terminal epoxy casing is simulated.

[0042] like Figure 1 As shown, the finite element simulation model of the cable terminal includes: a solidification dynamics module, a heat transfer module and a solid mechanics module.

[0043] During simulation, the curing dynamics module and the thermal field module perform bidirectional coupling calculations of the curing degree and temperature distribution through the electrothermal simulation interface. The solid mechanics module calculates the stress-strain field based on the material parameters at different curing degrees, thereby obtaining the superposition distribution of residual stress and thermal stress during the epoxy resin curing process.

[0044] Stress field simulation mainly considers the residual stress caused by the curing process of epoxy resin and the thermal stress caused by the difference in thermal expansion coefficient between epoxy resin and metal insert under ambient temperature changes. Stress field simulation mainly includes three heat transfer modes: heat conduction between solid media, heat convection between gases, and heat radiation from solid media to gas environment.

[0045] The simulation verification method is mainly based on numerical calculation and finite element simulation. In particular, in the verification method based on finite element simulation, the finite element simulation model of the cable terminal is designed according to the full-size cable terminal.

[0046] The simulated physical fields include thermal field and stress field. During simulation, the two fields are bidirectionally coupled through the electrothermal simulation interface to calculate the curing degree and temperature distribution.

[0047] S3. Use stress and strain safety factors to comprehensively evaluate the reliability of epoxy casing under different temperature operating conditions.

[0048] Based on the stress calculation results of epoxy casing under different temperature conditions, the safety factor k is calculated as follows:

[0049]

[0050] Among them, σ max (T) is the maximum tensile strength at different temperatures, σ(T) is the thermal stress, ε max (T) is the elongation at break at different temperatures, α is the thermal expansion coefficient of epoxy resin, and T0 = 20°C is the reference temperature.

[0051] The larger the k value, the higher the risk of defects. When k≥1, the material will be damaged under thermal stress. It is believed that the epoxy sleeve has safety risks under superimposed internal stress. The applicable working temperature range is determined by the safety factor k. When the safety margin requirement is 1 times, the temperature range of k≤1 is the applicable working temperature range. Similarly, when the safety margin requirement is 2 times, the temperature range of k≤0.5 is the applicable working temperature range.

[0052] Example

[0053] Based on the curing dynamics of polymer materials and the finite element simulation model of cable terminals, the curing residual stress of the epoxy casing in the cable terminal and the thermal stress distribution under different operating temperatures are superimposed and evaluated. The stress and strain safety factors are used to comprehensively evaluate the reliability of the epoxy casing under different temperature operating conditions, and then the operating temperature range of the cable terminal is obtained, providing technical support for the status evaluation of the cable terminal under complex operating conditions.

[0054] The curing kinetics equation of epoxy resin based on the Sesták-Berggren model was used to calculate the equation ln(β / T p 2 )-(1 / T p ) is linearly fitted to obtain the activation energy E of epoxy resin a By integrating and differentiating the heat flow-temperature curve, we can obtain The relationship between α and A linear fit is performed, and the slope and intercept of the fitting results can respectively determine the reaction order and pre-exponential factor of the concentration effect. The reaction order of the catalytic effect can then be determined by the ratio of the reaction order of the catalytic effect to that of the concentration effect. Finally, the curing kinetic model of the epoxy composite material is confirmed based on the average values ​​of the parameters under four groups of heating rates.

[0055] Based on the curing dynamics equation, a finite element simulation model of a high-voltage GIS cable terminal (taking the 110kV model as an example) is built. The model includes three modules: heat transfer module, curing dynamics module and solid mechanics module. The heat transfer module and the curing dynamics module form a bidirectional coupling to calculate the curing degree and temperature distribution. The solid mechanics module calculates the stress-strain field based on the material parameters at different curing degrees, thereby obtaining the residual stress distribution during the epoxy resin curing process.

[0056] The curing reaction process is simulated only for the epoxy casing, while the impact of ambient temperature changes is simulated for the entire GIS terminal. After curing, the safety factor k is used to evaluate the adaptability of the epoxy casing to low-temperature conditions. The larger the value, the higher the risk of defects. When k ≥ 1, the material is damaged under thermal stress.

[0057] Through the above process, the distribution of the overall curing residual stress and thermal stress of the epoxy resin casing is obtained, and the working temperature range is determined, which solves the current problems of overall strain distribution and working temperature of the epoxy casing, and provides technical support for the stable operation of cable terminals under different temperature environments, especially under extreme low temperature conditions.

[0058] like Figure 2 As shown in the figure, after epoxy resin is modified with fillers with negative expansion coefficients, the strain of epoxy resin during the curing process mainly includes thermal strain and chemical shrinkage strain, which are superimposed as the total strain on a macro scale. The change in the thermal expansion coefficient of the material will only affect the thermal strain during the curing process, but will have no effect on the chemical shrinkage strain. In stage II, the reduction in the thermal expansion coefficient makes the chemical shrinkage strain dominant. In stage III, the curing reaction is basically completed, the chemical shrinkage strain tends to be constant, and the thermal strain tends to be dominant. The reduction in the thermal expansion coefficient will significantly reduce the thermal strain at the metal insert / epoxy resin interface, which significantly reduces the total residual stress inside the epoxy sleeve after curing.

[0059] like Figure 3 As shown in the figure, with the decrease of thermal expansion coefficient, the residual stress in the curing reaction process shows a trend of first decreasing and then increasing. However, in the cooling stage of the curing process, the decrease of thermal expansion coefficient will significantly reduce the thermal strain in the cooling process, making the residual stress of epoxy sleeve curing show a monotonous decreasing trend with the decrease of thermal expansion coefficient.

[0060] like Figure 4 As shown in the figure, with the decrease of thermal expansion coefficient, the maximum thermal stress of epoxy sleeve gradually decreases, and the applicable working temperature is wider. -5 When the thermal expansion coefficient is reduced to 3.52×10 -5 When the temperature is below, it can operate at -40℃ without cracking.

[0061] Therefore, the present invention adopts the above-mentioned method for calculating the internal stress distribution and evaluating the operation reliability of a dry-type cable terminal, and obtains the overall superimposed stress distribution and operating temperature range of the epoxy casing through experimental testing and simulation calculations, thereby solving the current problems of the overall strain distribution and operating temperature of the epoxy casing, and providing technical support for the stable operation of cable terminals under different temperature environments, especially under extreme low temperature conditions.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for calculating stress distribution in a dry cable terminal and evaluating operation reliability, characterized in that: The following steps are involved: S1. The heat flow-temperature curves of epoxy resin at different heating rates were obtained by non-isothermal DSC measurement. Based on the KAS equation and Sesták-Berggren model, the curing kinetic model of epoxy composite materials was established. The specific process is as follows: S11. Based on the KAS equation and the Sesták-Berggren model, the activation energy, pre-exponential factor, and reaction order of the catalytic effect and concentration effect of the epoxy resin reaction were calculated, and the curing reaction equation of the epoxy composite material was established as shown below: ； in, R is the molar gas constant, T is the temperature, α is the degree of curing; E a is the activation energy, based on the KAS equation Perform linear fitting to obtain; T p It is the peak value of heat flow-temperature curve of epoxy resin; A is the pre-exponential factor, The intercept of the linear fitting result is A ; m , n represent the reaction orders of catalytic effect and concentration effect respectively; among them, The slope of the linear fitting result is n , m Depend on Calculated, α M The calculation method is as follows: ； S12, based on the KAS equation, linear fitting is performed on the equation terms to obtain the activation energy of the epoxy resin, and the activation energy is determined by combining the heat flow temperature curve. and α According to the average value of the parameters under the heating rate, the curing kinetic model of epoxy composite materials is confirmed as follows: ； S2. Based on the finite element simulation method, the distribution of residual stress and thermal stress of the epoxy casing of the cable terminal is simulated; S3. Use stress and strain safety factors to comprehensively evaluate the reliability of epoxy casing under different temperature operating conditions. The specific process is as follows: Calculate the safety factor based on the stress calculation results of epoxy casing under different temperature conditions k , as shown below: ； in, σ max ( T ) is the maximum tensile strength at different temperatures, σ ( T ) is the magnitude of thermal stress, ε max ( T ) is the elongation at break at different temperatures, λ is the thermal expansion coefficient of epoxy resin, is the reference temperature; k The larger the value, the higher the risk of defects. When k ≥ 1, the material will be damaged under the action of thermal stress. It is believed that there is a safety risk in the epoxy casing under the action of superimposed internal stress.

2. A method for calculating stress distribution in a dry-type cable terminal and evaluating operation reliability according to claim 1, characterized in that: In step S2, the cable terminal finite element simulation model includes: a curing dynamics module, a heat transfer module and a solid mechanics module; During simulation, the curing dynamics module and the thermal field module perform bidirectional coupling through the electrothermal simulation interface to calculate the curing degree and temperature distribution; the solid mechanics module calculates the stress-strain field based on the material parameters at different curing degrees, thereby obtaining the superimposed distribution of residual stress and thermal stress during the epoxy resin curing process.

3. A method for calculating stress distribution in a dry-type cable terminal and evaluating operation reliability according to claim 2, characterized in that: The stress field simulation mainly considers the residual stress caused by the curing process of epoxy resin and the thermal stress caused by the difference in thermal expansion coefficient between epoxy resin and metal insert under changing ambient temperature; the stress field simulation mainly includes three heat transfer modes, namely: heat conduction between solid media, heat convection between gases, and heat radiation from solid media to the gas environment.

4. A method for calculating stress distribution in a dry-type cable terminal and evaluating operation reliability according to claim 3, characterized in that: The simulation verification method is mainly based on numerical calculation and finite element simulation; among them, in the verification method based on finite element simulation, the finite element simulation model of the cable terminal is designed according to the full-size cable terminal; the simulation physical field includes thermal field and stress field, and during simulation, the two are bidirectionally coupled through the electrothermal simulation interface to calculate the degree of solidification and temperature distribution.

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