Method for analyzing moisture migration in soil around buried cable

Abstract

The invention relates to the technical field of electric power system and soil heat and humidity coupling analysis, and discloses a method for analyzing moisture migration in soil around a buried cable, which considers mutual coupling of three physical fields, namely an electromagnetic field, a temperature field and a humidity field, and constructs an electromagnetic-heat-humidity multi-physical field coupling simulation model of the buried cable. And analyzing the influence of moisture migration in the soil on the running state of the buried cable. The problem that the running state of the buried cable is difficult to accurately reflect due to lack of dynamic analysis of water migration in the prior art is solved.

Description

Technical Field

[0001] The invention relates to the technical field of power system and soil thermal-humidity coupling analysis, and in particular to a method for analyzing moisture migration in soil around a direct-buried cable. Background Art

[0002] As the scale of new energy power stations continues to expand, the performance and stability of direct buried power cables, as key equipment for transmitting electric energy, have attracted much attention. In the direct buried laying method, a large amount of heat will be generated during the operation of the cable, and this heat will diffuse outward through the surrounding soil, thus affecting the temperature rise characteristics around the cable. At the same time, the moisture migration in the soil will significantly affect its thermal conductivity, which in turn has an important impact on the heat dissipation performance of the cable.

[0003] Studies have shown that the thermal conductivity, humidity and density of the soil have a direct impact on the current carrying capacity of the cable. During the operation of the direct buried cable, the moisture in the soil migrates due to the existence of temperature gradients and moisture gradients, mainly manifested as the diffusion of liquid water and water vapor. This moisture migration will cause the moisture content distribution of the soil around the cable to change, thereby affecting the thermal resistance characteristics of the soil and the heat dissipation capacity of the cable. However, the current research on the heat-humidity coupling effect during cable operation is relatively limited, especially the quantitative analysis method of the effect of soil moisture migration on the current carrying capacity and temperature rise of the cable still needs to be improved.

[0004] Existing technologies are mainly based on a single analysis method of soil thermal characteristics, ignoring the impact of the dynamic changes in soil moisture migration on the thermal conductivity of cables, which may lead to conservative or uneconomical cable laying designs. In addition, due to the lack of in-depth research on the phenomenon of soil moisture migration, the current consideration of humidity fields in cable current carrying capacity calculations and thermal circuit models is relatively simple, making it difficult to accurately reflect the complex environment in actual operation.

[0005] In the existing technology, there is insufficient research on the impact of moisture migration in the soil around direct buried cables on the cable's current-carrying capacity and heat dissipation performance. Specifically, there is a lack of dynamic analysis of moisture migration, and the impact of thermal-moisture coupling on soil properties and temperature rise is not fully considered. The conservative calculation method leads to an excessively large cable cross-section, making it difficult to achieve accurate cable design optimization, especially under fluctuating load scenarios. Summary of the invention

[0006] In order to overcome the shortcomings of the prior art, the present invention provides a method for analyzing moisture migration in the soil around a direct buried cable, which solves the problems existing in the prior art such as lack of dynamic analysis of moisture migration and thus difficulty in accurately reflecting the operating status of the direct buried cable.

[0007] The technical solution adopted by the present invention to solve the above problems is:

[0008] A method for analyzing moisture migration in the soil around a direct-buried cable considers the mutual coupling of the three physical fields of electromagnetic field, temperature field, and humidity field, constructs an electromagnetic-thermal-humidity multi-physical field coupling simulation model of the direct-buried cable, and analyzes the impact of moisture migration in the soil on the operating state of the direct-buried cable.

[0009] As a preferred technical solution, analyzing the influence of moisture migration in the soil on the operating state of the direct buried cable includes: calculating the temperature field distribution of the direct buried cable through a multi-physical field coupling simulation model.

[0010] As a preferred technical solution, the steps of calculating the temperature field distribution of the direct buried cable include:

[0011] J1, set the initial conditions of the multi-physics field coupling simulation model; the initial conditions include: the conductivity of the direct buried cable, the thermal conductivity of the soil, and the specific heat capacity of the soil;

[0012] J2, input current value to the multi-physics coupling simulation model;

[0013] J3, calculates the electromagnetic loss of the direct buried cable according to the input current value;

[0014] J4, taking electromagnetic loss as heat source, temperature field calculation field temperature field distribution.

[0015] As a preferred technical solution, analyzing the impact of moisture migration in the soil on the operating state of the direct buried cable includes: calculating the current carrying capacity of the direct buried cable through a multi-physical field coupling simulation model.

[0016] As a preferred technical solution, calculating the current carrying capacity of the direct buried cable includes the following steps:

[0017] J5, obtain the temperature gradient according to the temperature field distribution, then calculate the humidity field, update the thermal conductivity of the soil and the specific heat capacity of the soil, and return to step J2 until both the temperature field and the humidity field meet the convergence conditions;

[0018] J6, correct the conductivity according to the conductor temperature value, return to step J3 to calculate the electromagnetic loss, and then repeat steps J3 to J5 until the electromagnetic loss, temperature field and humidity field all meet the convergence conditions, thereby obtaining the cable core temperature, and then execute step J7;

[0019] J7, determine whether the cable core temperature reaches the set threshold value. If it is less than the set threshold value, increase the input current; if it is greater than the set threshold value, reduce the input current, return to step J2, repeat steps J2 to J6 until the cable core temperature is equal to the set threshold value. The input current obtained when the cable core temperature is equal to the set threshold value is the current carrying capacity.

[0020] As a preferred technical solution, the steps of constructing an electromagnetic-thermal-humidity multi-physics field coupling simulation model of a direct buried cable include:

[0021] S1, Analysis of the relationship between soil moisture content and soil thermal conductivity: Analyze the relationship between soil moisture content and soil thermal conductivity;

[0022] S2, geometric model construction and material setting: setting the geometric shape of the multi-physics field coupling simulation model and setting the material property parameters of the multi-physics field coupling simulation model;

[0023] S3, physical field selection and setting: select the physical field and set the physical parameters of the physical field.

[0024] As a preferred technical solution, in step S2, the material property parameters of the cable include one or more of the following: thermal conductivity, density, heat capacity at constant pressure, electrical conductivity, relative dielectric constant, and relative magnetic permeability; the material property parameters of the soil include one or more of the following: thermal conductivity, density, and heat capacity at constant pressure.

[0025] As a preferred technical solution, in step S2, a physical model of the soil is set, including: a relative humidity model, an absolute temperature model, and an absolute pressure model.

[0026] As a preferred technical solution, in step S3, setting the physical parameters of the physical field includes: setting the coverage area, boundary conditions and initial values ​​of the electromagnetic field and the heat transfer field, setting the selection domain of building materials in the humidity field to soil, and setting the initial value and insulation conditions of the humidity field.

[0027] As a preferred technical solution, in step S3, when setting the physical parameters of the physical field, the heat transfer interface in the building material and the moisture transport interface in the building material are connected to the temperature field and the humidity field, and the calculation formula of the water vapor flux in the soil is:

[0028] g 0 =β p (p v.ext -p v )

[0029] Among them, g 0 represents the water vapor flux in the soil, β p represents the water migration coefficient, p v.ext represents the air pressure in the soil, p v Indicates air pressure;

[0030] The functional relationship of the distribution of water content in soil is:

[0031]

[0032] where θ(ψ) is the soil moisture content at a given water potential ψ; θ r is the residual moisture content; θ s is the saturated moisture content; α is the scale function; n is the slope characterization parameter of the soil moisture characteristic curve.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) The present invention focuses on the influence of soil moisture migration on the current carrying capacity of buried cables. A cable electromagnetic-thermal-humidity multi-physics field coupling simulation model is constructed to systematically simulate the influence of soil moisture migration on the cable operation status under different initial moisture contents; through simulation analysis, the potential influence mechanism of soil moisture migration on the temperature rise and current carrying capacity of cable conductors is revealed;

[0035] (2) The present invention finds that in the calculation of the transient current carrying capacity of the cable, the influence of soil moisture migration on the temperature rise of the cable is extremely limited; compared with the traditional thermal circuit model, the contribution of soil moisture migration to the temperature rise of the cable is negligible; therefore, when optimizing the cable design and laying plan, the influence of moisture migration on the current carrying capacity can be ignored, thereby reducing costs;

[0036] (3) By comparing the simulation results under different scenarios with the prediction results of the traditional model, the present invention verifies that in conventional design, soil moisture migration has no significant effect on the current carrying capacity of the cable; this conclusion provides a reasonable simplified assumption for cable design, which helps to reduce the computational complexity in engineering practice and improve design efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic diagram of multi-physics field coupling;

[0038] Figure 2 It is the temperature variation curve;

[0039] Figure 3 is the steady-state humidity distribution diagram;

[0040] Figure 4 This is the relationship between humidity in the vertical direction of the cable and time;

[0041] Figure 5 This is a diagram showing the effect of moisture migration on temperature rise when a single-core cable is directly buried. DETAILED DESCRIPTION

[0042] The present invention will be further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0043] Example 1

[0044] like Figures 1 to 5As shown in the figure, it is of great significance to study the moisture migration characteristics of the soil around the direct buried power cable, establish a thermal-humidity coupling model, and explore the influence of moisture migration on the temperature rise and current carrying capacity of the cable. This can not only provide a scientific basis for the cable laying design, but also optimize the cable selection, improve the system operation efficiency, and reduce the construction and operation costs of the project.

[0045] In order to solve the problems of the prior art, the present invention proposes a soil direct buried cable analysis method taking into account the moisture migration characteristics, so as to improve the calculation accuracy of the cable current carrying capacity, optimize the design, reduce the engineering cost, and improve the system operation efficiency.

[0046] The present invention proposes a strategy for analyzing moisture migration in soil of directly buried power cables. In unsaturated porous media, moisture exists in the form of liquid and vapor. Therefore, only the migration of liquid water and water vapor in the soil is considered, and the influence of other nutrients, plants, stress, etc. is not considered. On this basis, the characteristic parameters of moisture migration are obtained.

[0047] This paper proposes a cable analysis strategy in a complex multi-physical field system. The three physical fields (electromagnetic field, temperature field, and humidity field) are coupled to each other, and the real environment of direct buried cable laying is simulated to the greatest extent. The research is carried out to obtain the accurate current carrying capacity and temperature field distribution of direct buried cables. On this basis, the electromagnetic, thermal, and humidity coupling calculation and the current carrying capacity calculation process under the corresponding scenarios are proposed.

[0048] The present invention proposes a cable multi-physical field coupling model. According to the thermal conductivity data and moisture content data of the soil sample, the relationship between the thermal conductivity and moisture content of the test soil is obtained by software fitting. The cable geometry model is established by COMSOL software, and the materials in the model are defined. When setting the soil parameters, the influence of the moisture content on its thermal conductivity is considered in combination with the fitting relationship. Finally, the electromagnetic thermal field and the building material heat transfer module (divided into two physical fields, the heat transfer field and the moisture transport field in the building material) are selected, and a total of three physical fields are coupled to establish a complete simulation model. Based on this model, the current carrying capacity of the direct buried cable under the action of moisture migration can be calculated.

[0049] Based on the characteristic that moisture can only affect the cable from the outside, the present invention establishes a moisture migration analysis strategy in the soil of directly buried power cables;

[0050] The present invention considers the electromagnetic-heat-humidity three-field coupling effect and provides a flow chart for calculating the current carrying capacity of direct buried cables.

[0051] The present invention establishes a multi-physical field coupling model based on the above analysis.

[0052] Analysis of moisture migration in soil for direct buried power cables:

[0053] Since the outer surface of the cable has insulation and water-proof capabilities, water cannot pass through the cable skin and enter the cable interior. The humidity field only acts on the environment where the direct buried cable is located, that is, there is water flow in the unsaturated soil. In unsaturated porous media, water exists in the form of liquid and steam. Therefore, only the migration of liquid water and water vapor in the soil is considered, and the effects of other nutrients, plants, stress, etc. are not considered.

[0054] The moisture transfer phenomenon is for liquid water diffusion and water vapor diffusion. For liquid water and water vapor diffusion, the moisture content in the soil can be obtained as θ. The Whitaker volume average method is used to integrate the experimental soil, and the mass conservation law of water is:

[0055]

[0056] Where t represents time; V represents the volume of the experimental soil; <·> represents taking the average value of the variable; ρ 1 Indicates water density in kg / m 3 ;v 1 represents the water migration speed, the unit is m / s; n is the slope characteristic parameter of the soil moisture characteristic curve; A represents the cross-sectional area of ​​the control volume, the unit is m 2 ; m represents the amount of water evaporated per unit volume, in kg / m 3 ξ 1 It indicates the volume percentage of liquid water in the soil, with the unit being 1.

[0057] v 1 =v l +v vd (2)

[0058] In the formula, v l represents the migration velocity of liquid water, v l represents the moisture content in the soil, v vd Indicates the migration speed of water vapor.

[0059]

[0060] Where D Tv It represents the water vapor diffusion coefficient caused by temperature gradient, in m 2 s -1 K -1 ; represents differential; T represents temperature; θ represents water content; D lv It represents the diffusion coefficient of water vapor caused by the moisture content gradient, in m 2 s -1 .

[0061] Electromagnetic, thermal and wet field coupling of direct buried cables:

[0062] For directly buried underground power cables, the coupled analysis among the electromagnetic, thermal, and moisture fields needs to be considered. These three physical fields are coupled and interact with each other, constituting a complex multi-physical field system, as Figure 1 shown. The electromagnetic field refers to the alternating magnetic field generated inside the cable when an alternating current is passed through the directly buried cable. At the same time, there are core losses, dielectric losses, metal sheath losses, armor losses, etc.; the temperature field characterizes the heat transfer inside the cable, the heat transfer between the surrounding soil layers of the cable, the heat transfer between the cable and the soil, and the heat exchange between the surface of the covering soil layer and the atmosphere; the humidity field refers to the process of water transfer and diffusion in the soil layer. Considering comprehensively, the coupling of these three physical fields can simulate the real environment of directly buried cable laying to the greatest extent, so as to conduct research on this and obtain the accurate ampacity and temperature field distribution of the directly buried cable.

[0063] Steps for electromagnetic, thermal, and moisture coupling calculation and ampacity calculation:

[0064] (1) Set initial conditions: the conductivity of the directly buried cable, the thermal conductivity of the soil, and the specific heat capacity of the soil;

[0065] (2) Input the current value;

[0066] (3) Using the IEC standard, calculate the electromagnetic losses of the cable according to the given current;

[0067] (4) Taking the electromagnetic losses as the heat source, conduct temperature field calculation to obtain the temperature field distribution of the field domain;

[0068] (5) According to the temperature gradient obtained from the temperature field distribution, conduct humidity field calculation, update the thermal conductivity and specific heat capacity of each part of the soil, and return to step (2) of the calculation until both the temperature field and the humidity field meet the convergence conditions, thereby obtaining the core temperature;

[0069] (6) According to the conductor temperature value, correct the conductivity, return to step (3), calculate the electromagnetic losses, and repeat steps (3), (4), and (5) until the electromagnetic losses, temperature field, and humidity field all meet the convergence conditions, and then execute step (7);

[0070] (7) Judge whether the core temperature reaches 90 °C. If it is less than 90 °C, increase the input current; if it is greater than 90 °C, decrease the input current, return to step (2), and repeat steps (2)-(6) until the core temperature is equal to 90 °C. At this time, the input current obtained is the ampacity.

[0071] Cable multi-physical field coupling model:

[0072] (1) Analysis of the relationship between soil moisture content and thermal conductivity:

[0073] Direct buried cables are buried directly underground and in direct contact with the soil. During the operation of the cable, there will be heat transfer, and the soil will become a heat transfer medium. The main factor affecting soil heat transfer is the change in soil thermal conductivity, which is also called thermal conductivity. The larger the thermal conductivity, the stronger the heat dissipation capacity of the soil, and the faster the soil temperature drops; the smaller the thermal conductivity, the worse the heat dissipation capacity of the soil, and the slower the temperature drops, so that the heat around the cable cannot be dissipated, and the cable core temperature gradually increases to the maximum allowable value (90°C). Therefore, measuring the thermal conductivity of the soil in the cable laying environment is an important link.

[0074] There are two main methods for measuring soil thermal conductivity: the steady-state method and the transient method. The steady-state method is a classic method for measuring soil thermal conductivity. Its principle is to use the Fourier heat conduction model to calculate the thermal conductivity of the soil based on the heat passing through the heat transfer plate in the stable heat transfer process, where the heat transfer rate is equal to the heat dissipation rate. See Table 1.

[0075] Table 1 Relationship between moisture content and thermal conductivity of soil samples

[0076] Serial number Moisture content / % Thermal conductivity / W / (m·K) 1 1 0.508 2 10 0.83 3 16.3 1.029 4 19.9 1.19 5 20.7 1.22 6 21.3 1.242 7 23.9 1.282 8 25.1 1.412 9 27.2 1.4 10 31 1.515 11 34 1.517

[0077] According to the thermal conductivity data and moisture content data of the soil samples, the relationship between the thermal conductivity and moisture content of the test soil can be obtained by software fitting:

[0078] λ=-0.0003θ 2 +0.0416θ+0.4792 (4-4)

[0079] (2) Geometry model construction and material setting:

[0080] The cable model uses the aluminum alloy cable selected in the previous article as the research object. The cable is buried 1m below the soil. The size parameters of the aluminum alloy cable are shown in Table 2.

[0081] Table 2 Aluminum alloy cable size parameters

[0082] Structure Name Thickness / mm Outer diameter / mm conductor - 13 Conductor shield 0.3 13.6 XLPE shield 10.5 34.6 Insulation shield 0.5 35.6 Copper shield 0.2 36 Bag strap 0.4 78.1 Inner sheath 1.5 81.1 Armor layer 1.35 83.8 Outer sheath 4.1 92

[0083] After the direct buried cable model is established, the next step is to define the composition of the power cable materials and soil. Among the material properties of the cable, the main properties are thermal conductivity, density, constant pressure heat capacity, electrical conductivity, relative dielectric constant and relative magnetic permeability. These material properties involve heat transfer, magnetic field analysis, and temperature field analysis calculations.

[0084] The phenomenon of moisture migration only occurs in the soil of directly buried cables. When adding material property parameters to the soil, it is necessary to pay attention to the relationship between various physical parameters. The material property parameters in the soil mainly include: thermal conductivity, density, and constant pressure heat capacity. Considering the thermal-humidity coupling effect of moisture migration and heat transfer in the soil, the main property parameters in the soil cannot be simply regarded as fixed constants. The relationship between thermal conductivity and water content should be considered comprehensively. When adding, the specific functional relationship needs to be clarified to ensure the subsequent simulation calculation and reduce the calculation error. The thermal conductivity (λ) of the soil is a key variable for understanding the heat transfer process of the cable in the soil. It has been regarded as a constant in previous finite element analysis. Many scholars have shown that the moisture content of the soil has the greatest influence on the thermal conductivity, and the influence of the moisture content on the thermal conductivity should be considered.

[0085] In the study, the thermal conductivity is regarded as a time-varying number related to the moisture content. The relationship between the thermal conductivity of soil and the thermal conductivity of soil can be obtained from formula (4-4).

[0086] In addition to setting the above-mentioned material property parameters, you also need to set the general physical model needed in the subsequent physical field calculation. The specific operation is: in the basic property setting interface of the soil, select Add physical model and enter the physical model to be added. The general models that need to be set for the soil where the direct buried cable is buried are relative humidity, absolute temperature and absolute pressure.

[0087] (3)Physical field selection and setting:

[0088] After setting the geometry of the model and adding the model material properties, you need to add a physical field to divide the effects of each area in the model and smoothly carry out the subsequent simulation operations and simulations. From the above analysis, it can be seen that the direct buried cable model studied in this project mainly involves electromagnetic fields, heat transfer fields and humidity fields, which are related to the coupling of multiple physical fields. Selecting the electromagnetic thermal field and building material heat transfer module (divided into two physical fields: the heat transfer field and the moisture transport field in the building materials), a total of three physical fields are coupled, which meets the requirements of computational simulation and simplifies the operation process.

[0089] The coverage area, boundary conditions and initial values ​​of the electromagnetic field and heat transfer field are set, and the humidity field is added and set. Similar to the heat transfer field, the physical field of water transport in the soil also needs to set some physical parameters, including building materials, initial values, insulation, etc.

[0090] The water transport occurs in the soil, and other locations such as cables and pipes do not have the effect of water migration, so the selection domain of building materials is soil. Other inputs such as the physical model are not much different from the heat transfer field. The general physical model input is used, and the absolute pressure input is a standard atmospheric pressure.

[0091] The initial value of the humidity field refers to the initial moisture content of the area where the physical field occurs. Based on multiple large-scale measurements of soil moisture by a humidity meter, the initial value of the soil moisture content can be set to 0.31. If there is no influence from other effects, such as heat transfer, the overall moisture content in the soil should be maintained around 0.31.

[0092] Insulation conditions in humidity fields: Since moisture migration only exists in the soil, the effect of moisture inside the cable is not considered.

[0093] Water vapor flux in soil: Water vapor flux occurs at the convective water vapor flux between soil and air, and the reason for this is the difference in air pressure between soil and air. The specific formula for water vapor flux is:

[0094] g 0 =β p (p v.ext -p v ) (4-5)

[0095] In the formula, β p The water migration coefficient represents the movement of water in the soil. The unit is s / m. Determine the water migration coefficient β p The value of p is 0.0005, v.ext represents the air pressure in the soil, p v Indicates air pressure.

[0096] The distribution of water content in the soil also follows certain rules, and the specific functional relationship is:

[0097]

[0098] where θ(ψ) is the soil moisture content at a given water potential ψ; θ r is the residual moisture content, that is, the moisture content in the soil that cannot be fully utilized by plants; θ s is the saturated moisture content, that is, the moisture content when the soil is completely saturated with water; α is the scale function, which is a parameter related to the soil type and affects the shape of the curve; n is the slope characterization parameter of the soil moisture characteristic curve, which is another parameter related to the soil type and affects the shape of the curve.

[0099] In addition to the moisture content distribution inside the soil, there is also humidity outside the soil, and because there is convective water vapor flux at the boundary, the humidity inside and outside the soil must not be equal. The external humidity is set to 0.38 after testing. After setting up the heat transfer field and moisture transport field, the joint effect of multiple physical fields should be considered. The heat transfer interface in the building material and the moisture transport interface in the building material are simultaneously connected to the thermal and wet physical field (i.e., temperature field, humidity field).

[0100] As Figures 1 to 5 shown, as a further optimization of Example 1, on the basis of Example 1, this example further includes the following technical features:

[0101] Through the research and calculation of the simulated multi-physical fields, distribution maps of magnetic fields, temperature distribution maps, humidity distribution maps, and related curves will be obtained.

[0102] According to the experimental requirements, when considering moisture migration, from the simulation results (the process of the cable core temperature changing with time) as Figure 2 , it can be obtained that the temperatures of the cable core and the outer skin rise relatively fast at the beginning when the cable is energized, and gradually, after 25 hours, they tend to be flat and the rising speed slows down.

[0103] The initial moisture content of the soil in the entire model is uniform, all being 0.45 (i.e., 45%), and after energizing the single-core cable, the moisture content in the soil changes, as Figure 3 can be seen. During the transient process, moisture migration occurs in the soil, and the final migration state is: the moisture content of the soil near the cable is the lowest, about 0.2; as the soil is farther away from the heat source, the moisture content gradually increases. It can be seen from the cross-sectional view that at approximately 0.5 meters away from the center of the cable, the moisture content of the soil no longer changes and maintains the original moisture content of 45%. This is consistent with what the senior Ying Qiliang from Shanghai Cable Research Institute described in the article: when considering moisture migration in the soil of directly buried cables, moisture migration occurs within 0.5 meters from the heat source, and the soil more than 0.5 meters away from the heat source is not greatly affected by the heat source, that is, moisture migration is not likely to occur in the deep soil.

[0104] Figure 4 shows the relationship between the moisture content at two positions in the vertical direction of the cable changing with time. It can be seen that the initial moisture content is near 45% at both positions. At the same initial time point, the closer the soil is to the cable direction, the greater the moisture content. It is known that the buried depth of the cable is 20 cm, and the change in the moisture content of the deep soil is slower; but as the cable temperature rises, the change in the moisture content in the area near the cable is the most obvious. Eventually, when the transient process ends, the moisture content of the soil near the cable is the lowest, and the moisture content of the soil near the ground is higher because the cable temperature has little impact on the soil far away from the cable, almost no impact. The longer the cable is energized, the more obvious the change in humidity in the vertical direction is compared to the change in humidity in the horizontal direction. This shows that in actual soil, the lateral propagation ability of humidity is weaker than the longitudinal propagation ability. Taking the same time point, it can be known that the closer to the cable, the greater the change in the moisture content of the soil during the transient process, indicating that the soil closer to the cable is drier, that is, it is less conducive to heat conduction.

[0105] From Figure 5It can be seen that when the power-on time is less than 7h, the temperature rise of the cable core considering moisture migration is almost the same as that of the cable core not considering moisture migration. When the power-on time is greater than 7h, the temperature rise without considering moisture migration is slightly greater than the temperature rise considering moisture migration. At 40h, the temperature difference between the two is about 1°C, and at 120h, the temperature difference between the two is about 2°C. It can be seen that the effect of moisture migration on the transient temperature rise of the cable is small and can be ignored.

[0106] The present invention constructs a corresponding simulation model based on the multi-physical field formed by the mutual coupling of electromagnetic, thermal and wet fields to analyze the influence of moisture migration on direct buried cables. At the same time, the relationship between soil thermal conductivity and soil moisture is obtained through data fitting.

[0107] The present invention verifies that: in conventional design, soil moisture migration has no significant effect on the current carrying capacity of cables. This conclusion provides a reasonable simplified assumption for cable design, which helps to reduce calculation complexity and improve design efficiency in engineering practice.

[0108] As described above, the present invention can be preferably implemented.

[0109] All features disclosed in all embodiments in this specification, or steps in all methods or processes implicitly disclosed, except for mutually exclusive features and / or steps, can be combined and / or expanded or replaced in any manner.

[0110] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. According to the technical essence of the present invention, within the spirit and principles of the present invention, any simple modification, equivalent replacement and improvement made to the above embodiment still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for analyzing water migration in soil around a direct buried cable, characterized in that: Considering the mutual coupling of electromagnetic field, temperature field and humidity field, an electromagnetic-thermal-humidity multi-physical field coupling simulation model of direct buried cables is constructed to analyze the influence of moisture migration in the soil on the operating state of direct buried cables.

2. A method for analyzing moisture migration in soil around a direct buried cable according to claim 1, characterized in that: Analyzing the impact of moisture migration in the soil on the operating status of the direct buried cable includes: calculating the temperature field distribution of the direct buried cable through a multi-physics field coupling simulation model.

3. A method for analyzing water migration in soil around a direct buried cable according to claim 2, characterized in that: The steps to calculate the temperature field distribution of direct buried cables include: J1, set the initial conditions of the multi-physics field coupling simulation model; the initial conditions include: the conductivity of the direct buried cable, the thermal conductivity of the soil, and the specific heat capacity of the soil; J2, input current value to the multi-physics coupling simulation model; J3, calculates the electromagnetic loss of the direct buried cable according to the input current value; J4, taking electromagnetic loss as heat source, temperature field calculation field temperature field distribution.

4. A method for analyzing water migration in soil around a direct buried cable according to claim 3, characterized in that: Analyzing the impact of moisture migration in the soil on the operating status of direct buried cables includes: calculating the current carrying capacity of direct buried cables through a multi-physics field coupling simulation model.

5. A method for analyzing water migration in soil around a direct buried cable according to claim 4, characterized in that: Calculating the ampacity of direct buried cable involves the following steps: J5, obtain the temperature gradient according to the temperature field distribution, then calculate the humidity field, update the thermal conductivity of the soil and the specific heat capacity of the soil, and return to step J2 until both the temperature field and the humidity field meet the convergence conditions; J6, correct the conductivity according to the conductor temperature value, return to step J3 to calculate the electromagnetic loss, and then repeat steps J3 to J5 until the electromagnetic loss, temperature field and humidity field all meet the convergence conditions, thereby obtaining the cable core temperature, and then execute step J7; J7, determine whether the cable core temperature reaches the set threshold value. If it is less than the set threshold value, increase the input current; if it is greater than the set threshold value, reduce the input current, return to step J2, repeat steps J2 to J6 until the cable core temperature is equal to the set threshold value. The input current obtained when the cable core temperature is equal to the set threshold value is the current carrying capacity.

6. A method for analyzing water migration in soil around a direct buried cable according to any one of claims 1 to 5, characterized in that: The steps to build an electromagnetic-thermal-moisture multiphysics coupled simulation model for direct buried cables include: S1, Analysis of the relationship between soil moisture content and soil thermal conductivity: Analyze the relationship between soil moisture content and soil thermal conductivity; S2, geometric model construction and material setting: setting the geometric shape of the multi-physics field coupling simulation model and setting the material property parameters of the multi-physics field coupling simulation model; S3, physical field selection and setting: select the physical field and set the physical parameters of the physical field.

7. A method for analyzing water migration in soil around a direct buried cable according to claim 6, characterized in that: In step S2, the material property parameters of the cable include one or more of the following: thermal conductivity, density, heat capacity at constant pressure, electrical conductivity, relative dielectric constant, and relative magnetic permeability; the material property parameters of the soil include one or more of the following: thermal conductivity, density, and heat capacity at constant pressure.

8. A method for analyzing water migration in soil around a direct buried cable according to claim 6, characterized in that: In step S2, a physical model of the soil is set, including: a relative humidity model, an absolute temperature model, and an absolute pressure model.

9. A method for analyzing water migration in soil around a direct buried cable according to claim 6, characterized in that: In step S3, setting the physical parameters of the physical field includes: setting the coverage area, boundary conditions and initial values ​​of the electromagnetic field and the heat transfer field, setting the selection domain of building materials in the humidity field to soil, and setting the initial value and insulation conditions of the humidity field.

10. A method for analyzing water migration in soil around a direct buried cable according to claim 6, characterized in that: In step S3, when setting the physical parameters of the physical field, the heat transfer interface in the building material and the moisture transport interface in the building material are connected to the temperature field and the humidity field. The calculation formula for the water vapor flux in the soil is: g0=β p (p v.ext -p v ) Where g0 represents the water vapor flux in the soil, β p represents the water migration coefficient, p v.ext represents the air pressure in the soil, p v Indicates air pressure; The functional relationship of the distribution of water content in soil is: where θ(ψ) is the soil moisture content at a given water potential ψ; θ r is the residual moisture content; θ s is the saturated moisture content; α is the scale function; n is the slope characterization parameter of the soil moisture characteristic curve.

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