A field-circuit coupled cable bridge cable core temperature calculation method

Abstract

This invention relates to a method for calculating the core temperature of cable groups in a cable bridge using field-circuit coupling. The method includes: establishing a finite element model of the cable bridge's electromagnetic field; calculating the core loss and sheath loss of each loop cable in the cable bridge; constructing a numerical model for direct burial of various cable types; calculating the thermal resistance of the insulation layer, water-blocking layer, and outer sheath to establish an equivalent thermal circuit model; analytically calculating the dielectric loss based on the cable's electrical parameters; constructing a thermal-fluid coupling model of the cable bridge that excludes the cable body and only includes the boundary of holes corresponding to the cable's outer diameter; applying the sum of core loss, sheath loss, and dielectric loss as a linear heat source to the hole boundary for field solving to obtain the cable sheath temperature; inversely deriving the sheath temperature and core temperature through the equivalent thermal circuit model; updating the loss and convective heat transfer coefficient based on the calculated temperature; iteratively executing this process until both the core temperature and the outer wall temperature converge, obtaining the final core temperature. This invention achieves high-precision calculation of the core temperature of cable groups in cable bridges.

Description

Technical Field

[0001] This invention relates to the field of power cable operation technology; and more particularly to a method for calculating the core temperature of cable groups in cable bridges with field-circuit coupling. Background Technology

[0002] Currently, power operation and maintenance departments require accurate assessment of cable thermal status to meet the demands of smart grids and modern power transmission and distribution systems for precise calculation of cable current-carrying capacity. Densely laid cable bridges contain multiple circuits, and accurate analysis of their temperature field is crucial for accurately assessing the current-carrying capacity of these circuits, thereby improving the utilization rate of power transmission and distribution systems and meeting the requirements of dynamic load adjustment.

[0003] Cable bridges are a special type of cable laying method. They contain two large air-sealed spaces and house 11 circuits of cables, including three circuits of three-core cables and eight circuits of 110kV and 220kV ultra-high voltage single-core cables. The single-core cables are arranged in a straight line, making them one of the main channels for urban power transmission and distribution lines. Due to the presence of two air-sealed spaces, the dense cable arrangement, and the fact that all four outer walls of the cable bridge rely on natural convection heat transfer, its heat transfer calculations differ significantly from existing methods such as direct burial, duct laying, and tunnels, which only rely on natural convection heat transfer at the ground level. Therefore, its temperature calculation model also differs considerably.

[0004] Due to the complex structure of cable bridges, the most effective method for temperature field analysis is finite element numerical analysis. However, because they contain multiple loop cables, have complex cable structures, and contain a large enclosed space with two holes, the temperature field numerical analysis is a complex electromagnetic-thermal-fluid-structure interaction calculation process. When using a desktop computer for temperature field analysis, problems arise such as either coarse mesh density leading to non-convergence and inaccurate results, or the inability to complete the meshing at a high mesh density, which greatly complicates the temperature field analysis.

[0005] Therefore, it is necessary to adopt new methods to analyze and calculate the core temperature of cable bridges. Summary of the Invention

[0006] Based on the above analysis, the present invention aims to disclose a method for calculating the core temperature of cable bridge cable groups in field-circuit coupling, thereby achieving high-precision calculation of the core temperature of cable bridge cable groups.

[0007] This invention discloses a method for calculating the core temperature of cable groups in a field-circuit coupled cable bridge, comprising: S1. Establish a finite element model of the electromagnetic field of the cable bridge, apply three-phase AC excitation to the cable core of each circuit, obtain the eddy current density by solving the vector magnetic potential equation, and then calculate the cable core loss and metal sheath loss of each circuit cable. S2. Construct numerical models for direct burial laying for each type of cable, apply a unit heat source and extract the temperature distribution using the finite element method, calculate the thermal resistance of the insulation layer, water-blocking layer and outer sheath to establish an equivalent thermal circuit model, and simultaneously calculate the dielectric loss based on the cable electrical parameters. S3. Construct a cable bridge thermal-fluid coupling model that does not include the cable body but only includes the boundary of the hole corresponding to the outer diameter of the cable. Initialize the convective heat transfer coefficient of the outer wall. Apply the sum of the cable core loss, metal sheath loss and dielectric loss as a linear heat source to the hole boundary to solve the field and obtain the cable sheath temperature. Based on the sheath temperature, inversely deduce the metal sheath temperature and cable core temperature through the equivalent thermal circuit model. Update the loss and convective heat transfer coefficient according to the calculated temperature. Iterate until the cable core temperature and the outer wall temperature converge to obtain the final cable core temperature.

[0008] The present invention has at least the following beneficial effects: The present invention discloses a method for calculating the core temperature of cable bridge cable groups using field-circuit coupling. This method separates the cable body from the complex field and calculates the core temperature using a thermal circuit model and solves the air temperature field using a thermal-fluid coupling model. This effectively solves the problems of difficulty and non-convergence in the numerical calculation of the entire field of cable bridges, and significantly reduces the computational resource requirements while ensuring calculation accuracy. This method can adapt to complex configuration scenarios of multi-type and multi-circuit cable groups, and achieves fast and accurate core temperature calculation, providing technical support for dynamic load adjustment of smart grids and accurate assessment of cable current carrying capacity. Attached Figure Description

[0009] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. Figure 1 This is a flowchart illustrating the method for calculating the core temperature of a cable bridge cable group in a field-circuit coupled embodiment of the present invention. Figure 2 This is a schematic diagram of the cable bridge structure in an embodiment of the present invention; Figure 3 This is a schematic diagram of the equivalent thermal circuit model of three types of cables in an embodiment of the present invention; Figure 4 This is a schematic diagram of the non-convergence results when performing full-field calculations using an equivalent cable structure model. Figure 5 This is a schematic diagram of the convergence results when using the field-circuit coupling method of the present invention; Figure 6 This is a schematic diagram showing the calculation results of the temperature distribution of the cable bridge in an embodiment of the present invention. Detailed Implementation

[0010] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and, together with the embodiments of the present invention, serve to illustrate the principles of the present invention.

[0011] One embodiment of the present invention discloses a method for calculating the core temperature of a cable bridge cable group in a field-circuit coupled manner, such as... Figure 1 As shown, it includes: S1. Establish a finite element model of the electromagnetic field of the cable bridge, apply three-phase AC excitation to the cable core of each circuit, obtain the eddy current density by solving the vector magnetic potential equation, and then calculate the cable core loss and metal sheath loss of each circuit cable. S2. Construct numerical models for direct burial laying for each type of cable, apply a unit heat source and extract the temperature distribution using the finite element method, calculate the thermal resistance of the insulation layer, water-blocking layer and outer sheath to establish an equivalent thermal circuit model, and simultaneously calculate the dielectric loss based on the cable electrical parameters. S3. Construct a cable bridge thermal-fluid coupling model that does not include the cable body but only includes the boundary of the hole corresponding to the outer diameter of the cable. Initialize the convective heat transfer coefficient of the outer wall. Apply the sum of the cable core loss, metal sheath loss and dielectric loss as a linear heat source to the hole boundary to solve the field and obtain the cable sheath temperature. Based on the sheath temperature, inversely deduce the metal sheath temperature and cable core temperature through the equivalent thermal circuit model. Update the loss and convective heat transfer coefficient according to the calculated temperature. Iterate until the cable core temperature and the outer wall temperature converge to obtain the final cable core temperature.

[0012] Specifically, S1 includes: S1-1. Establish a three-dimensional electromagnetic field finite element model of the cable bridge; The cable bridge is a multi-loop densely laid cable bridge containing two or more air layers and with all four walls being natural convection heat transfer boundaries. The cables in the cable bridge include 35kV three-core cables, 110kV ultra-high voltage single-core cross-linked cables, 220kV ultra-high voltage single-core cross-linked cables, and 220kV oil-filled cables.

[0013] like Figure 2 As shown, this is a specific cable bridge model given in this embodiment. The cable bridge has two holes and a total of 35kV three-core cable lines, 5 110kV cross-linked polyethylene cable lines, 1 220kV cross-linked polyethylene cable line, and 2 220kV oil-filled cable lines.

[0014] The cable bridge is 4400mm long, 1860mm high, and has a wall thickness of 240mm. Circuit 4 is 300mm from the bottom, and the vertical spacing between circuits is 300mm. The cable types and spacing of each circuit within the cable bridge are shown in Table 1. The distance from the wall is the distance from the center of the left or right cable within the circuit to the left or right bridge wall. The cable spacing within the circuit is the center-to-center distance between two adjacent phase cables within the circuit.

[0015] Table 1 Cable Laying Parameters

[0016] Set the electromagnetic parameters of the material: Cable core conductor (copper): conductivity γ = 5.8 × 10⁻⁶ 7 S / m, permeability μ=μ0=4π×10 -7 H / m; Metal sheath (lead): Conductivity γ = 4.55 × 10⁻⁶ 6 S / m, permeability μ=μ0; Insulation layer, outer sheath, air: electrical conductivity γ=0, magnetic permeability μ=μ0.

[0017] S1-2. Establish the vector magnetic potential equation of the cable field. Using vector magnetic potential as the independent variable, the governing equations for each region of the cable field are established, including: vector magnetic potential equations for conductive and non-conductive regions, as well as vector magnetic potential equations for each region of the cable field under the boundary condition of infinity.

[0018] Specifically, the vector magnetic potential equation is as follows: ; In the formula, is the Laplace operator, representing the second derivative in space; The imaginary unit; Angular frequency; Electrical conductivity; is the magnetic permeability.

[0019] S1-3, Apply three-phase alternating current excitation and solve. Three-phase power frequency AC excitation is applied to the cable core of each circuit. The three-phase current amplitudes are equal and the phases differ by 120 degrees. Coulomb's standard is introduced to establish a finite element matrix equation and solve it to obtain the vector magnetic potential of each node. Based on this, the eddy current density of the cable core conductor and the metal shielding layer region is derived.

[0020] The formula for calculating the eddy current density in the cable core region is as follows: ; In the formula, In three-phase alternating current, the first The vector magnetic potential of the phase.

[0021] S1-4: Calculate the electromagnetic loss of the unit; For the cable core region, the source current density and eddy current density are superimposed to calculate the unit Joule loss; for the metal shielding layer region, the unit eddy current loss is calculated based only on the eddy current density. The element loss in the cable core region is calculated from the eddy current density and the source current density: ; In the formula, , These are the source current density and eddy current density, respectively. The area of ​​the cable core region; The element loss in the metal sleeve region is calculated based on the eddy current density. ; In the formula, Let be the area of ​​the metal sleeve region.

[0022] S1-5, Integrate to obtain the total loop loss; By randomly inputting the current of a multi-circuit cable line, and using the unit loss calculation formula, domain probes are set in the cable core and metal shielding areas to calculate the integral value of the loss, thus obtaining the core loss of each cable circuit. and metal sleeve loss This serves as the heat source input for subsequent thermal circuit model construction and field-circuit coupling calculations.

[0023] Specifically, S2 includes: S2-1. For various types of cables, including three-core and single-core cables, construct finite element models for direct burial laying with complete layered structures. Since the cable bridge contains four types of cables: 35kV three-core cable, 110kV ultra-high voltage single-core cross-linked cable, 220kV ultra-high voltage single-core cross-linked cable, and 220kV oil-filled cable; among them, the thermal circuit model of the standard given three-core cable is relatively complex, the ultra-high voltage cross-linked cable contains a thick water-blocking layer, and the oil-filled cable contains oil channels. These factors have brought difficulties to the construction of the thermal circuit model using the standard given analytical method. Therefore, numerical methods can be used to extract the thermal resistance parameters.

[0024] S2-2. Apply a unit heat source to the core region of each cable model, solve the steady-state temperature field using the finite element method, and extract the temperature distribution of the core, insulation shield, metal sheath, and cable outer sheath. Apply a unit heat source to the core region of each cable model. =50W (total loss of three-phase cable), steady-state heat conduction is solved using the finite element method, with a solution accuracy of 10. -6 .

[0025] Set the following probe extraction temperature in the model: Core area probe: to obtain the average temperature of the cable core. ; Insulation layer outer surface boundary probe: to obtain the temperature of the insulation shielding layer ; Metal sleeve region probe: to obtain the average temperature of the metal sleeve ; Cable outer surface boundary probe: to obtain cable sheath temperature .

[0026] S2-3. Calculate the layered thermal resistance parameters of the insulation layer, water-blocking layer, and outer sheath based on the ratio of temperature difference to heat source power. Based on the ratio of temperature difference to heat source power, using the formula... Calculate the thermal resistance of the insulation layer, the thermal resistance of the water-blocking layer, and the thermal resistance of the outer sheath: The calculation results are shown in Table 2.

[0027] Table 2 Thermal resistance calculation results

[0028] S2-4. Construct equivalent thermal circuit models for various cable types based on layered thermal resistance. Three-core cables adopt a single-node model, and single-core cables adopt a three-node series model. Based on the calculated thermal resistance parameters, equivalent thermal circuit models for various cable types are established: For example... Figure 3 As shown, like Figure 3 As shown in (a), the thermal path in the equivalent thermal path model of a 35kV three-core cross-linked cable is: core temperature node — equivalent thermal resistance — sheath temperature node; the losses of the three-phase cores work together on the equivalent thermal resistance; the equivalent thermal resistance is the cable thermal resistance of 0.07976 K·m / W calculated in Table 2; this model converts the multi-layer structure of the three-core cable into a single thermal resistance, which simplifies the calculation while meeting the engineering accuracy requirements. like Figure 3 As shown in (b), the thermal path of the equivalent thermal path model of the 110kV / 220kV ultra-high voltage single-core cross-linked cable is: core temperature node—insulation thermal resistance—water-blocking layer thermal resistance—metal sheath temperature node—outer sheath thermal resistance—surface temperature node. This path corresponds to the physical heat transfer structure of the cable insulation layer, water-blocking layer, and outer sheath in sequence. By setting the metal sheath temperature node, the temperature distribution and loss characteristics at the metal sheath can be accurately reflected, thereby improving the accuracy of temperature calculation for ultra-high voltage single-core cables.

[0029] like Figure 3 As shown in (c), the thermal path of the equivalent thermal model of a 220kV ultra-high voltage single-core oil-filled cable is: The temperature nodes are: core temperature node, insulation thermal resistance, metal sheath temperature node, water-blocking layer thermal resistance, copper wire layer temperature node, outer sheath thermal resistance, and surface temperature node. This path corresponds to the physical heat transfer structure of the cable insulation layer, water-blocking layer, and outer sheath in sequence. By setting the metal sheath temperature node, the heat transfer and loss distribution of the multi-layer structure of the oil-filled cable can be accurately characterized.

[0030] S2-5. Calculate the dielectric loss of each type of cable based on the operating voltage, insulation capacitance, and dielectric loss tangent, and use it as a fixed heat source item for subsequent calculations. The effect of insulation dielectric loss on cable core temperature can also be obtained using finite element numerical analysis. Since the dielectric loss of each cable type is fixed and only related to the operating voltage, dielectric loss tangent, dielectric constant, and cable insulation capacitance, it can be calculated in advance; the calculation formula is: ; In the formula, The angular frequency of the power supply; It is an insulating layer capacitor; Phase voltage; This is the tangent of the dielectric loss angle of the insulating layer.

[0031] Ignoring dielectric loss, the dielectric loss of the 35kV cable is 1.88 W / m for the 110kV cable, 4.79 W / m for the 220kV cross-linked cable, and 3.8 W / m for the 220kV oil-filled cable. The effects of dielectric loss on the core temperature of the three ultra-high voltage cables are shown in Table 3.

[0032] Table 3 Equivalent results of dielectric loss

[0033] Then, by utilizing the principle of thermal superposition, in actual calculations, the effect of thermal superposition on the core temperature can be directly summed with the calculated result of the core loss on the core temperature to obtain the core temperature.

[0034] Specifically, S3 includes: S3-1. Construct a thermal-fluid coupling calculation model for a cable bridge that does not contain the cable body but only includes the boundary of the holes corresponding to the outer diameter of the cable. Use an engineering turbulence numerical model to mesh the air domain. A cable bridge thermal-fluid coupling calculation model was constructed, which does not include the cable body but only contains the boundary of the holes corresponding to the outer diameter of the cable. Based on the three-dimensional electromagnetic field finite element model of the cable bridge established by S1, the cable bridge thermal-fluid coupling calculation model contains 27 cylindrical hole boundaries. The inner diameter of each hole is equal to the outer diameter of the corresponding cable model, which serves as the application position of the line heat source for subsequent thermal-fluid coupling calculation. Preferably, the air domain is meshed using a k-ε two-equation turbulence model; The surface convection heat transfer of a horizontal cylinder in a large space is laminar heat transfer, but the natural convection of the entire space is turbulent heat transfer. Direct simulation methods have too high computational resource requirements, and large eddy current simulation methods still require workstation-level configurations. The Reynolds time-averaged equation simulation method expresses unknown high-order time averages as functions of low-order deterministic quantities, which is the basic method for engineering turbulence calculation. Since the cables in the cable bridge are close to the bridge wall, the ε equation in the k-ε two-equation model has good adaptability. Therefore, this embodiment selects the k-ε two-equation turbulence model.

[0035] Set the air physical properties as follows: density ρ = 1.165 kg / m³ 3 Specific heat capacity =1.005 kJ / (kg·K), thermal conductivity λ=0.0267 W / (m·K), kinematic viscosity v=1.6×10 -5 m² / s, coefficient of volume expansion =0.0033 K -1 The air zone partition density is set to 2mm, and the cable bridge wall partition density is set to 5mm.

[0036] S3-2. Based on the ambient temperature and the set initial temperature of the outer wall, initialize the convective heat transfer coefficients of the four outer walls of the cable bridge; The convective heat transfer coefficient The formula is: ; in, It is an empirical constant. The thermal conductivity of the fluid, For characteristic length, It is the acceleration due to gravity. The coefficient of volume expansion is 1. Kinematic viscosity, For isobaric specific heat capacity, For dynamic viscosity, It is an exponential constant. The temperature difference between the wall and the fluid. , The temperature of the outer wall of the cable bridge. The ambient temperature is used. The convective heat transfer coefficient is updated based on the temperature of the cable bridge's outer wall.

[0037] Set ambient temperature =30℃, set the initial temperature of the four outer walls of the cable bridge. =35℃, calculate the temperature difference. =5℃; Calculate the characteristic length based on the cable bridge geometry. Calculate the Grashof numbers for the upper surface, lower surface, and left and right sidewalls respectively. And Prandtl ,according to Range of selection of empirical constants and : The upper boundary of the cable bridge can be equivalent to a flat plate with the hot side facing upwards. hour, , , hour, , ; The lower boundary of the cable bridge can be equivalent to a flat plate with the hot side facing down. hour, , ; The left and right sides of the cable bridge can be considered equivalent to vertical flat plates. hour, , ;when hour, , ; when hour, , .

[0038] S3-3. The sum of the core loss, metal sheath loss and dielectric loss of each cable is converted into heat flux density and applied as a line heat source to the corresponding hole boundary. The k-ε two-equation turbulence model is used for heat-fluid coupling solution. Boundary probes are set to obtain the temperature of each hole boundary as the cable sheath temperature and the temperature of the four outer walls. Specifically, it includes: 1) Calculate the total loss and heat flux density; add the core loss, sheath loss and dielectric loss of a single cable to get the total heat generation power, and then divide by the perimeter of the cable sheath to get the heat flux density. 2) Apply boundary conditions; apply the heat flux density as the second type of boundary condition to the corresponding hole boundary, and apply the initial convective heat transfer coefficient as the third type of boundary condition to the four outer walls of the cable bridge to establish the convective heat transfer relationship between the outer walls and the environment. The expression for the third type of boundary condition is: , in Let be the temperature gradient along the boundary normal direction. Indicates at the boundary Take the value above; 3) Perform a thermal-fluid coupled solution: Simultaneously solve the continuity equation, momentum equation, energy equation, and k-ε turbulence equations, setting the residual convergence accuracy to 10. -5 The temperature distribution and air velocity distribution of the entire field are obtained; boundary probes are set at the boundaries of each hole and the outer wall to extract the average temperature as the cable sheath temperature and the outer wall temperature.

[0039] S3-4. Based on the cable sheath temperature obtained from the solution, combined with the equivalent thermal circuit model of the corresponding cable model and the pre-calculated thermal resistance parameters, the metal sheath temperature and cable core temperature are obtained by reverse calculation. in, 35kV three-core cable: ; K·m / W; 110kV / 220kV ultra-high voltage single-core cross-linked cable: ; ; 220kV ultra-high voltage single-core oil-filled cable: ; ; In the formula, For cable core temperature, For the temperature of the metal sleeve, The cable sheath temperature, , , These are respectively the thermal resistance of the insulation layer, water-blocking layer, and outer sheath; These are cable core loss, metal sheath loss, and dielectric loss, respectively. The temperature difference between the cable core and the metal sheath caused by dielectric loss is 1.07℃ for 110kV cables, 1.8℃ for 220kV cross-linked cables, and 0.813℃ for 220kV oil-filled cables.

[0040] S3-5. Based on the cable core temperature and metal sheath temperature obtained by reverse calculation, update the cable core loss and metal sheath loss using the resistivity temperature coefficient, and update the convective heat transfer coefficient based on the outer wall temperature. Update cable core loss based on cable core temperature: ; Update metal sleeve wear based on metal sleeve temperature: ; in, The temperature coefficient of resistivity of the copper core is 0.00393℃. - ¹, The resistivity temperature coefficient of the lead-clad metal is 0.00376℃. - ¹.

[0041] S3-6. Determine whether the iterative changes in the core temperature and the outer wall temperature are both less than the set threshold. If they are, output the final core temperature; otherwise, return to S3-3 to continue iterating.

[0042] The set threshold is 1 degree Celsius.

[0043] Implementation effect verification In this embodiment, for Figure 2 The cable bridge model was used to calculate the core temperature of the cable group; Comparison of simplified model computational capabilities: The computer configuration is: a DELL-R750 server with dual CPUs (model 6144, 16 cores, 3.5GHz), 64GB of RAM, and a 4TB hard drive. The model, constructed entirely using cable structures, could not be meshed when the air domain mesh size was set to 5mm. Setting the mesh size for both cables and air domains too large will directly affect the accuracy of the calculation results.

[0044] Since the metal sheath is a good thermal conductor, assuming the entire internal structure is equivalent to metal, the cable structure is equivalent to a two-layer structure of an inner metal sheath + an outer sheath. Setting the air domain partition density to 2mm and the cable domain partition density to 1mm, partitioning can be completed, but the calculation convergence performance is poor. Figure 4 To calculate the residual convergence plot and the cable core temperature tracking plot, the residual convergence curve of the full-structure equivalent model is shown in Figure 4(a). In the figure, the residual curves of the continuity equation, the x-direction velocity equation, the y-direction velocity equation, the energy equation, the turbulent kinetic energy (k) equation, and the turbulent dissipation rate (ε) equation all exhibit periodic and violent fluctuations throughout the entire process, without a continuous decay trend, and never reach the preset 1e. -5 Convergence accuracy; As shown in Figure 4(b), the average temperature curves of the 11 regions failed to converge throughout the entire iteration process. The core temperature in some regions still fluctuated significantly and did not reach a steady state. Therefore, the calculation results were not suitable for engineering applications.

[0045] The cable body domain and the air domain are separated. The cable body domain is solved using a thermal circuit model, while the air domain is solved using field analysis in a coupled manner. Figure 5 The residual convergence process of the field-circuit coupling model and the cable sheath temperature tracking diagram are shown in Figure 5(a). The residual convergence curve of the field-circuit coupling model is shown in Figure 5(a). The residuals of each governing equation decay rapidly and converge stably to 1e. -5 Below the order of magnitude, it meets the preset convergence accuracy requirements; the cable sheath temperature tracking curve is as follows: Figure 5 As shown in (b), the average temperature curves of the 11 regions quickly approach a steady state, the temperature distribution pattern is clear, and the results are highly deterministic, which can be directly used for subsequent engineering analysis.

[0046] Calculation results: When the air temperature is 30 degrees Celsius, its thermophysical parameters are as follows: =1.005kJ / (kg·K), thermal conductivity =0.0267 W / (m·K), kinematic viscosity =1.6×10⁻⁵ m / s2 ; =0.0033K -1 ; =18.6×10 -6 First, taking the temperature as 35°C, the convective heat transfer coefficient of the upper surface of the cable bridge can be calculated to be 2.26 W / (m²). 2 The convective heat transfer coefficients on the left and right sides are 3.0623 W / (m²). 2 The lower surface convective heat transfer coefficient is 1.13 W / (m²). 2 ·K).

[0047] The input currents for circuits 1-11 are: 107.83A; 62.58A; 213.75A; 288.57A; 301.83A; 259.8A; 164.18A; 212.49A; 244.44A; 283.64A; 125.8A.

[0048] After two iterations, the maximum error in the calculation result was 0.58℃, and the impact of the loss was less than 0.001, so the iteration can be considered to have converged. At this point, the temperatures of the four outer walls were 31.41, 31.12, 31.12, and 31.15℃, respectively, and the convective heat transfer coefficient was updated to 0.8848 W / (m²) for the upper surface. 2 The convective heat transfer coefficients on the left and right sides are 1.1319 W / (m²). 2 The convective heat transfer coefficient of the lower surface is 0.4424 W / (m²). 2 ·K).

[0049] Finally, the field calculation results are as follows: Figure 6 As shown in Table 5, the internal temperature field of the cable bridge is uniformly distributed with no abnormally high temperature areas. The core temperature calculated by the thermal circuit model is shown in Table 5, and the calculation results are stable and reliable.

[0050] Table 5. Results of cable core temperature calculation based on thermal circuit.

[0051] In summary, the field-circuit coupled cable bridge cable group core temperature calculation method disclosed in this embodiment separates the cable body from the complex field, calculates the core temperature using a thermal circuit model, and solves the air temperature field using a thermal-fluid coupling model. This effectively solves the problems of difficulty and non-convergence in the full-field numerical calculation of cable bridges, and significantly reduces the computational resource requirements while ensuring calculation accuracy. This method can adapt to complex configuration scenarios of multi-type, multi-circuit cable groups, and achieves fast and accurate core temperature calculation, providing technical support for dynamic load adjustment of smart grids and accurate assessment of cable current carrying capacity.

[0052] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for calculating the core temperature of a cable bridge cable group in a field-circuit coupled system, characterized in that, include: S1. Establish a finite element model of the electromagnetic field of the cable bridge, apply three-phase AC excitation to the cable core of each circuit, obtain the eddy current density by solving the vector magnetic potential equation, and then calculate the cable core loss and metal sheath loss of each circuit cable. S2. For each type of cable in the cable bridge, a numerical model for direct burial is constructed. A unit heat source is applied and the temperature distribution is extracted by the finite element method. The thermal resistance of the insulation layer, water-blocking layer and outer sheath is calculated to establish an equivalent thermal circuit model. At the same time, the dielectric loss is calculated based on the electrical parameters of the cable. S3. Construct a cable bridge thermal-fluid coupling model that does not include the cable body but only includes the boundary of the hole corresponding to the outer diameter of the cable. Initialize the convective heat transfer coefficient of the outer wall. Apply the sum of the cable core loss, metal sheath loss and dielectric loss as a linear heat source to the hole boundary to solve the field and obtain the cable sheath temperature. Based on the sheath temperature, inversely deduce the metal sheath temperature and cable core temperature through the equivalent thermal circuit model. Update the loss and convective heat transfer coefficient according to the calculated temperature. Iterate until the cable core temperature and the outer wall temperature converge to obtain the final cable core temperature.

2. The method for calculating the core temperature of cable groups in a field-circuit coupled cable bridge according to claim 1, characterized in that, The cable bridge is a multi-loop densely laid cable bridge containing two or more air layers and with all four walls being natural convection heat transfer boundaries. The cables in the cable bridge include 35kV three-core cables, 110kV ultra-high voltage single-core cross-linked cables, 220kV ultra-high voltage single-core cross-linked cables, and 220kV oil-filled cables.

3. The method for calculating the core temperature of a cable bridge cable group in field-circuit coupling according to claim 2, characterized in that, The thermal path in the equivalent thermal circuit model of a 35kV three-core cross-linked cable is: core temperature node — equivalent thermal resistance — sheath temperature node; the three-phase core losses work together on the equivalent thermal resistance. The thermal path of the equivalent thermal circuit model of a 110kV / 220kV ultra-high voltage single-core cross-linked cable is: core temperature node — insulation thermal resistance — water-blocking layer thermal resistance — metal sheath temperature node — outer sheath thermal resistance — surface temperature node. The thermal path of the equivalent thermal model of a 220kV ultra-high voltage single-core oil-filled cable is as follows: Cable core temperature node—insulation layer thermal resistance—metal sheath temperature node—water-blocking layer thermal resistance—copper wire layer temperature node—outer sheath thermal resistance—surface temperature node.

4. The method for calculating the core temperature of a cable bridge cable group in field-circuit coupling according to claim 1, characterized in that, Dielectric loss is calculated based on cable operating voltage, dielectric loss tangent, dielectric constant, and insulation layer geometric parameters. The calculation formula is: ； In the formula, The angular frequency of the power supply; It is an insulating layer capacitor; Phase voltage; This is the tangent of the dielectric loss angle of the insulating layer.

5. The method for calculating the core temperature of a cable bridge cable group in field-circuit coupling according to claim 1, characterized in that, S3 includes: S3-1. Construct a thermal-fluid coupling calculation model for a cable bridge that does not contain the cable body but only includes the boundary of the holes corresponding to the outer diameter of the cable. Use an engineering turbulence numerical model to mesh the air domain. S3-2. Based on the ambient temperature and the set initial temperature of the outer wall, initialize the convective heat transfer coefficients of the four outer walls of the cable bridge; S3-3. Convert the sum of the core loss, metal sheath loss and dielectric loss of each cable into heat flux density, apply it as a line heat source to the corresponding hole boundary, use the engineering turbulence numerical model to solve the heat-fluid coupling, set boundary probes to obtain the temperature of each hole boundary as the cable sheath temperature and the temperature of the four outer walls. S3-4. Based on the cable sheath temperature obtained from the solution, combined with the equivalent thermal circuit model of the corresponding cable model and the pre-calculated thermal resistance parameters, the metal sheath temperature and cable core temperature are obtained by reverse calculation. S3-5. Based on the core temperature and metal sheath temperature obtained by reverse calculation, update the core loss and metal sheath loss using the resistivity temperature coefficient, and update the convective heat transfer coefficient based on the outer wall temperature. S3-6. Determine whether the iterative changes in the core temperature and the outer wall temperature are both less than the set threshold. If they are, output the final core temperature; otherwise, return to S3-3 to continue iterating.

6. The method for calculating the core temperature of a cable bridge cable group in field-circuit coupling according to claim 5, characterized in that, The numerical model for engineering turbulence is a k-ε two-equation turbulence model.

7. The method for calculating the core temperature of a cable bridge cable group in field-circuit coupling according to claim 5, characterized in that, The convective heat transfer coefficient The formula is: ； in, It is an empirical constant. The thermal conductivity of the fluid, For characteristic length, It is the acceleration due to gravity. The coefficient of volume expansion is 1. Kinematic viscosity, For isobaric specific heat capacity, For dynamic viscosity, It is an exponential constant. The temperature difference between the wall and the fluid. , The temperature of the outer wall of the cable bridge. The ambient temperature is used; the convective heat transfer coefficient is updated based on the temperature of the outer wall of the cable bridge.

8. The method for calculating the core temperature of a cable bridge cable group in field-circuit coupling according to claim 5, characterized in that, S3-3 includes: 1) Calculate the total loss and heat flux density; add the core loss, sheath loss and dielectric loss of a single cable to get the total heat generation power, and then divide by the perimeter of the cable sheath to get the heat flux density. 2) Apply boundary conditions; apply the heat flux density as the second type of boundary condition to the corresponding hole boundary, and apply the initial convective heat transfer coefficient as the third type of boundary condition to the four outer walls of the cable bridge to establish the convective heat transfer relationship between the outer walls and the environment. 3) Perform thermal-fluid coupling solution: Combine the continuity equation, momentum equation, energy equation and k-ε turbulence equation, set the residual convergence accuracy, and obtain the temperature distribution and air velocity distribution in the whole field; set boundary probes at the boundaries of each hole and the outer wall, and extract the average temperature as the cable sheath temperature and the outer wall temperature.

9. The method for calculating the core temperature of a cable bridge cable group in field-circuit coupling according to claim 5, characterized in that, In S3-4, the temperature of the metal sheath and the cable core are obtained by reverse calculation, including: 35kV three-core cable: ; K·m / W; 110kV / 220kV ultra-high voltage single-core cross-linked cable: ； ； 220kV ultra-high voltage single-core oil-filled cable: ； ； In the formula, For cable core temperature, For the temperature of the metal sleeve, The cable sheath temperature, , , These are respectively the thermal resistance of the insulation layer, water-blocking layer, and outer sheath; These are cable core loss, metal sheath loss, and dielectric loss, respectively. This refers to the temperature difference between the cable core and the metal sheath caused by dielectric loss.

10. The method for calculating the core temperature of a cable bridge cable group in field-circuit coupling according to claim 5, characterized in that, S3-5, updating cable core loss and sheath loss using resistivity temperature coefficient, including: Update cable core loss based on cable core temperature: ; Update metal sleeve wear based on metal sleeve temperature: ; in, The temperature coefficient of resistivity of copper in the cable core. The resistivity temperature coefficient of the metal-clad lead.

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