Method for evaluating cable temperature in a closed environment taking into account axial heat transfer attenuation

Abstract

The application provides a cable temperature evaluation method in a closed environment considering axial heat transfer attenuation, comprising the following steps: obtaining the structure, material and operation condition parameters of a cable joint; performing single-phase equivalent treatment on the cable, equivalent the cable heat transfer path to a circuit structure, and equivalent the heat loss of the cable to a voltage source; taking the midpoint of the cable joint as the center, segmenting the cable on the two sides of the cable joint within a distance range, and determining the radial conduction thermal resistance and the axial conduction thermal resistance of each segment; determining the DC resistance of the core of each cable segment, and determining the core heat loss based on the DC resistance of the core and the operation current of the wire; determining the heat loss of the crimping pipe segment of the cable joint and the heat loss of other segments outside the crimping pipe segment based on the core heat loss; inputting the heat loss of the crimping pipe segment of the cable joint, the heat loss of other segments outside the crimping pipe segment, the axial conduction thermal resistance and the radial conduction thermal resistance into a MATLAB / Simulink platform to obtain the temperature of the target position of the cable.

Description

Technical Field

[0001] This invention relates to a method for evaluating cable temperature, and more particularly to a method for evaluating cable temperature in a closed environment that takes into account axial heat transfer attenuation. Background Technology

[0002] Cable joints are key connection components in power cable systems. They have complex structures and are susceptible to localized heat accumulation in relatively enclosed environments such as tunnels due to factors such as ventilation, ambient temperature, and heat dissipation conditions, which can threaten the safe operation of the cable system.

[0003] In existing technologies, cable joint temperature calculation methods mainly include finite element models and transient thermal circuit models. Finite element models have high accuracy, but they are complex to model and have a large computational load, making it difficult to meet the needs of online evaluation and rapid diagnosis. Transient thermal circuit models have high computational efficiency and clear physical meaning of parameters, but traditional models are mostly based on the assumption of one-dimensional radial heat transfer or adopt a uniform axial layering method, which makes it difficult to accurately reflect the characteristics of heat transfer from the joint to both sides of the cable body along the axial direction and gradual attenuation.

[0004] Therefore, in order to solve the above-mentioned technical problems, it is urgent to propose a new technical approach. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a method for assessing cable temperature in a closed environment considering axial heat transfer attenuation. By using the structural parameters of the cable joint, the cable body is divided into corresponding segments. The heat loss of each segment is equivalent to a voltage source, and the radial thermal resistance, axial thermal resistance, and heat capacity of each segment are determined respectively, so that the equivalent structure of the cable joint matches the actual working conditions. Then, the calculated parameters are input into the MATLAB / Simulink platform for solving, thereby obtaining the temperature of each segment node. This can effectively ensure the accuracy of temperature prediction for each node of the cable, and provide accurate data support for cable temperature rise and fault diagnosis.

[0006] This invention provides a method for evaluating cable temperature in a closed environment considering axial heat transfer attenuation, comprising the following steps:

[0007] S1. Obtain the structure, materials, and operating parameters of the cable connector;

[0008] S2. Perform single-phase equivalent treatment on the cable, equating the cable heat transfer path to a circuit structure and the cable heat loss to a voltage source;

[0009] S3. With the midpoint of the cable joint as the center, the cable is axially segmented within a set distance range on both sides of the cable joint, and the radial and axial thermal resistance of each segment is determined.

[0010] S4. Determine the DC resistance of the conductor core for each cable segment, and determine the conductor core heat loss based on the DC resistance of the conductor core and the operating current of the conductor.

[0011] S5. Determine the heat loss of the cable joint crimping section and other sections outside the crimping section based on the conductor core heat loss.

[0012] S6. Input the heat loss of the cable joint crimping section, the heat loss of other sections outside the crimping section, the axial thermal resistance, and the radial thermal resistance into the MATLAB / Simulink platform to obtain the temperature at the target location of the cable.

[0013] Furthermore, the axial segmentation of the cable is specifically as follows:

[0014] ;

[0015] Where: N represents the total number of segments, This indicates the setting of a distance coefficient, where γ is the stratification weight coefficient. This represents the distance of the i-th segment node from the center of the cable joint. Indicates the length of the cable connector;

[0016] The length of the cable segment is: .

[0017] Furthermore, the radial thermal resistance is:

[0018] In the radial direction of the cable, the thermal resistance is:

[0019] ;

[0020] in: This represents the radial thermal resistance of the j-th layer. This represents the inner diameter of the (j+1)th layer. This represents the outer diameter of the j-th layer. Indicates the length of the cable segment. This represents the thermal conductivity.

[0021] Furthermore, the axial thermal resistance is:

[0022] The thermal resistance between the same radial layer in two adjacent axial segments is:

[0023] ;

[0024] in: This represents the length of the (i+1)th cable segment. This represents the outer diameter of the (j-1)th layer.

[0025] Furthermore, the heat loss of the conductor core is determined by the following method:

[0026] ;

[0027] Where: I represents the effective value of the cable's operating current, This indicates the core DC resistance at 293.15K. This indicates the temperature coefficient of resistance of the conductor core. Indicates the temperature of the cable conductor. Represents the proximity effect coefficient. This represents the skin effect coefficient.

[0028] Furthermore, the heat loss of the cable joint crimping section and the heat loss of other sections besides the crimping section are as follows:

[0029] ;

[0030] ;

[0031] in: This indicates the heat loss of the cable joint crimped pipe section. This indicates the heat loss in sections other than the pressurized pipe section. Indicates dielectric loss. denoted by , where represents the loss of the metal shielding layer, and k represents the contact coefficient.

[0032] The beneficial effects of this invention are as follows: By using the structural parameters of the cable joint, the cable body is divided into segments, and the heat loss of each segment is equivalent to a voltage source. The radial thermal resistance, axial thermal resistance, and heat capacity of each segment are determined, so that the equivalent structure of the cable joint matches the actual working conditions. Then, the calculated parameters are input into the MATLAB / Simulink platform for solving, thereby obtaining the temperature of each segment node. This can effectively ensure the accuracy of temperature prediction for each node of the cable, and provide accurate data support for cable temperature rise and fault diagnosis. Attached Figure Description

[0033] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0034] Figure 1 This is a schematic diagram of the process of the present invention.

[0035] Figure 2 This is the single-phase equivalent model under the three-phase symmetrical operating condition in this invention.

[0036] Figure 3 This is a transient thermal circuit model diagram of the cable connector of the present invention.

[0037] Figure 4 This is a schematic diagram of the segmented distribution curve of the cable under a piecewise function.

[0038] Figure 5 The temperature curves of the cable joint surface under different layering weight coefficients γ in the case of non-uniform layering.

[0039] Figure 6 The diagram shows the influence of the layer weighting coefficient γ and the number of layers N on the surface temperature prediction error and solution time. Detailed Implementation

[0040] The present invention will be further described in detail below:

[0041] This invention provides a method for evaluating cable temperature in a closed environment considering axial heat transfer attenuation, comprising the following steps:

[0042] S1. Obtain the structure, materials, and operating parameters of the cable joint; specifically: the structural parameters of the cable joint include the length of the cable, the radius of each layer of the cable joint and the cable body (including the outer diameter and inner diameter; for the cable core, only the outer diameter is considered); the material parameters are the thermal properties of the materials, including the thermal conductivity, density, and specific heat capacity at constant pressure of the core, insulation layer, shielding layer, filling layer, inner sheath, armor layer, outer sheath, and joint accessories; the operating parameters include the current and ambient temperature.

[0043] S2. Perform single-phase equivalent processing on the cable, equating the cable heat transfer path to a circuit structure and the cable heat loss to a voltage source; this equivalent processing is an existing technology, which will be briefly described below:

[0044] like Figure 2 and Figure 3 As shown: When performing equivalence, it is based on the three-phase symmetry condition, that is, the current magnitude of the three phases is equal, the phase is symmetrical, and the cable structure of each phase is the same.

[0045] The independent layers of each phase, such as the conductor layer, insulation layer, and metal shielding layer, are connected in parallel and equivalently. The equivalent relationship is as follows:

[0046] ;

[0047] , and These are the equivalent single-phase thermal resistance, heat capacity, and heat source, respectively. , and These are the equivalent thermal resistance, heat capacity, and heat source, respectively; the thermal resistance and heat capacity remain unchanged for the phase-common layer in the filling layer, inner sheath, armor layer, outer sheath, and environmental heat exchange channel.

[0048] For the common layers such as the filler layer, inner sheath, armor layer, outer sheath, and ambient heat exchange channels, since their geometric dimensions and heat dissipation area do not change with the number of phases, their thermal resistance and heat capacity remain constant.

[0049] ;

[0050] Where Rshared,single and Rshared,eq are the thermal resistances of the shared layer before and after equivalence, respectively; Cshared,single and Cshared,eq are the thermal capacities of the shared layer before and after equivalence, respectively; under the above equivalence, while ensuring the consistency of the calculated temperature rise results of the connector core, insulation layer and sheath, the number of thermal network nodes is reduced and the model solution efficiency is improved.

[0051] S3. With the midpoint of the cable joint as the center, the cable is axially segmented within a set distance range on both sides of the cable joint, and the radial and axial thermal resistance of each segment is determined.

[0052] S4. Determine the DC resistance of the conductor core for each cable segment, and determine the conductor core heat loss based on the DC resistance of the conductor core and the operating current of the conductor.

[0053] S5. Determine the heat loss of the cable joint crimping section and other sections outside the crimping section based on the conductor core heat loss.

[0054] S6. Input the heat loss of the cable joint crimping section, the heat loss of other sections besides the crimping section, the axial conduction thermal resistance, and the radial conduction thermal resistance into the MATLAB / Simulink platform to obtain the temperature at the target location of the cable. Through this invention, the cable body is divided into corresponding segments by using the structural parameters of the cable joint. The heat loss of each segment is equivalent to a voltage source, and the radial conduction thermal resistance, axial conduction thermal resistance, and heat capacity of each segment are determined respectively. This makes the equivalent structure of the cable joint consistent with the actual working conditions. Then, the calculated parameters are input into the MATLAB / Simulink platform for solving, thereby obtaining the temperature of each segment node. This can effectively ensure the accuracy of temperature prediction at each node of the cable and provide accurate data support for cable temperature rise and fault diagnosis.

[0055] In this embodiment, the axial segmentation of the cable is specifically performed as follows:

[0056] ;

[0057] Where: N represents the total number of segments, This indicates the setting of a distance coefficient, where γ is the stratification weight coefficient. This represents the distance of the i-th segment node from the center of the cable joint. This indicates the length of the cable joint. It should be noted that the segmentation here only refers to the cable within a set distance range outside both ends of the cable structure. The cable joint itself also needs to be segmented. The segmentation depends on the structure of the cable joint. For the cable structure, the radial structure and material are different at different positions along the length. Therefore, the part with the same radial structure and material can be considered as one segment. The formula for calculating radial thermal resistance and thermal capacity is the same as the formula below.

[0058] The length of the cable segment is: The cable body is segmented, typically within a set range around the cable joint. For example, if the set value is 3m, then the cable body is segmented within a 3m range on both sides of the cable joint. Because the distance is too great, the axial heat transfer effect of the cable joint becomes negligible. After segmentation, the axial heat transfer of each segment is considered. The layer weighting coefficient γ, the total number of segments, and the set distance coefficient need to be determined through prior simulation experiments. That is, in the simulation, a simulation model identical to the target cable is constructed. Then, during the simulation, the error between the actual temperature of each layer of the cable and the temperature calculated under the layer weighting coefficient γ, the total number of segments, and the set distance coefficient is determined. It is then determined whether the error is less than the set value. If not, the layer weighting coefficient γ, the total number of segments, and the set distance coefficient are adjusted until the error is less than the set value. The current layer weighting coefficient γ, the total number of segments, and the set distance coefficient are then used as the segmentation parameters for the target cable. In practice, to ensure accuracy, adjustments can be made multiple times until the error no longer decreases, and the corresponding parameters are used as the segmentation parameters. However, in this case, the computational load is relatively large. Figure 5 As shown, when the stratification weighting coefficient γ changes, the predicted results of non-uniform stratification node distribution and cable joint surface temperature also change accordingly; for example... Figure 6 As shown, the number of strata N and the stratification weighting coefficient γ both affect the temperature prediction error and the solution time. Therefore, by comparing the temperature prediction error and solution time under different parameter combinations, the optimal number of strata N and the optimal stratification weighting coefficient γ that balances accuracy and efficiency are determined. opt By determining the weighting coefficients, the node spacing near the center of the cable joint is smaller, while the node spacing gradually increases further away from the center. Because the temperature gradient is larger near the center of the joint, denser layered nodes are used; the temperature change is slower further away from the joint, so sparser nodes are used, resulting in a decreasing temperature trend along the axial direction.

[0059] In this embodiment, the radial thermal resistance is:

[0060] In the radial direction of the cable, the thermal resistance is:

[0061] ;

[0062] in: This represents the radial thermal resistance of the j-th layer. This represents the inner diameter of the (j+1)th layer. This represents the outer diameter of the j-th layer. Indicates the length of the cable segment. This represents the thermal conductivity.

[0063] Among them, Figure 3 In the middle, C th R represents the geothermal capacity of the node. a Axial thermal resistance; radial thermal resistance includes: R il R is the radial thermal resistance of the insulating layer. cb Radial thermal resistance of copper braided tape; R fl The radial thermal resistance of the filler layer; R os For the radial thermal resistance of the outer sheath; R cs R is the radial thermal resistance of the stress cone. sl The radial thermal resistance of the shield; R is For the radial thermal resistance of the inner sheath; R ar For the radial thermal resistance of the armor layer; R env For environmental heat transfer thermal resistance, T env The ambient temperature is the source. For each segment node, each thermal resistance is equivalent to a resistor, and the heat loss is equivalent to a voltage source. The voltage across each resistor is the heat power corresponding to the current node temperature, and the capacitance of each node is a heat capacity. The formula for calculating the heat capacity of a node is:

[0064] The node heat capacity is:

[0065] ;

[0066] Among them, C th Here, ρ is the nodal heat capacity, ρ is the material density, and c is the density of the material. p Let V be the specific heat capacity under isobaric pressure of the material, and V be the material volume corresponding to the node.

[0067] The ambient heat transfer thermal resistance Renv is:

[0068] ;

[0069] Where h is the overall heat transfer coefficient and As is the heat transfer area of ​​the outer surface.

[0070] The axial thermal resistance is:

[0071] The thermal resistance between the same radial layer in two adjacent axial segments is:

[0072] ;

[0073] in: This represents the length of the (i+1)th cable segment. This represents the outer diameter of the (j-1)th layer.

[0074] The heat loss of the conductor core is determined by the following method:

[0075] ;

[0076] Where: I represents the effective value of the cable's operating current, This indicates the core DC resistance at 293.15K. This indicates the temperature coefficient of resistance of the conductor core. Indicates the temperature of the cable conductor. Represents the proximity effect coefficient. This represents the skin effect coefficient.

[0077] The heat loss of the cable joint crimping section and the heat loss of other sections are as follows:

[0078] ;

[0079] ;

[0080] in: This indicates the heat loss of the cable joint crimped pipe section. This indicates the heat loss in sections other than the pressurized pipe section. Indicates dielectric loss. denoted by , where represents the loss of the metal shielding layer, and k represents the contact coefficient.

[0081] A transient thermal path model has been constructed in the MATLAB / Simulink platform, specifically as follows:

[0082] ;

[0083] Cth(i, j) is the heat capacity of the node located at axial position i and radial layer j, in J / K; T(i, j) is the node temperature; Q(i, j) is the heat source power injected into the node; T(x) is the temperature of the adjacent nodes connected to the node, including the axially adjacent nodes i−1 and i+1 and the radially adjacent nodes j−1 and j+1; Rth(i, j→x) is the thermal resistance between node (i, j) and the adjacent node x; the heat source power Q(i, j) injected into the node is the heat power transferred through the axial and radial thermal resistances. By adjusting the value of T(i, j), Q can be adjusted. cThe value of each heat loss power is then adjusted. After adjustment, the heat transfer between each node can be determined, and it can be judged whether the above thermal circuit model has reached equilibrium. If so, the value of the current node T(i, j) can be determined. Of course, in the process of determining the above segment parameters, the temperature value is also calculated according to the above dust prevention.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for evaluating cable temperature in a closed environment considering axial heat transfer attenuation, characterized in that: Includes the following steps: S1. Obtain the structure, materials, and operating parameters of the cable connector; S2. Perform single-phase equivalent treatment on the cable, equating the cable heat transfer path to a circuit structure and the cable heat loss to a voltage source; S3. With the midpoint of the cable joint as the center, the cable is axially segmented within a set distance range on both sides of the cable joint, and the radial and axial thermal resistance of each segment is determined. S4. Determine the DC resistance of the conductor core for each cable segment, and determine the conductor core heat loss based on the DC resistance of the conductor core and the operating current of the conductor. S5. Determine the heat loss of the cable joint crimping section and other sections outside the crimping section based on the conductor core heat loss. S6. Input the heat loss of the cable joint crimping section, the heat loss of other sections outside the crimping section, the axial thermal resistance, and the radial thermal resistance into the MATLAB / Simulink platform to obtain the temperature at the target location of the cable.

2. The cable temperature assessment method in a closed environment considering axial heat transfer attenuation according to claim 1, characterized in that: The axial segmentation of the cable is specifically as follows: ； Where: N represents the total number of segments, This indicates the setting of a distance coefficient, where γ is the stratification weight coefficient. This represents the distance from the i-th segment node to the center of the cable joint. Indicates the length of the cable connector; The length of the cable segment is: .

3. The cable temperature assessment method in a closed environment considering axial heat transfer attenuation according to claim 2, characterized in that: The radial thermal resistance is: In the radial direction of the cable, the thermal resistance is: ； in: This represents the radial thermal resistance of the j-th layer. This represents the inner diameter of the (j+1)th layer. This represents the outer diameter of the j-th layer. Indicates the length of the cable segment. This represents the thermal conductivity.

4. The cable temperature assessment method in a closed environment considering axial heat transfer attenuation according to claim 2, characterized in that: The axial thermal resistance is: The thermal resistance between the same radial layer in two adjacent axial segments is: ； in: This represents the length of the (i+1)th cable segment. This represents the outer diameter of the (j-1)th layer.

5. The cable temperature assessment method in a closed environment considering axial heat transfer attenuation according to claim 1, characterized in that: The heat loss of the conductor core is determined by the following method: ； Where: I represents the effective value of the cable's operating current, This indicates the core DC resistance at 293.15K. This indicates the temperature coefficient of resistance of the conductor core. Indicates the temperature of the cable conductor. Represents the proximity effect coefficient. This represents the skin effect coefficient.

6. The cable temperature assessment method in a closed environment considering axial heat transfer attenuation according to claim 5, characterized in that: The heat loss of the cable joint crimping section and the heat loss of other sections are as follows: ； ； in: This indicates the heat loss of the cable joint crimped pipe section. This indicates the heat loss in sections other than the pressurized pipe section. Indicates dielectric loss. denoted by , where represents the loss of the metal shielding layer, and k represents the contact coefficient.

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