Method and device for calculating electric-thermal field of high-voltage cable

Abstract

The invention provides a high-voltage cable electric-thermal field calculation method and device, and relates to the technical field of high-voltage cables. The method comprises the following steps: establishing a two-dimensional axisymmetric model of the high-voltage cable, setting material parameters and sizes of all structural layers of the two-dimensional axisymmetric model, and calculating the current density of a buffer layer according to a preset electric field control equation; the current density of the buffer layer is converted into heat source distribution, a convective heat transfer boundary is applied under a steady state condition, and temperature rise distribution caused by current in the buffer layer is simulated. According to the method, the specific reason of the ablation fault can be explained, the current density and the temperature rise distribution of the buffer layer are obtained, and the high-temperature hot spot and the current density concentration area of the buffer layer are obtained, so that the material structure of the buffer layer is matched with the electrical performance, and subsequent early fault diagnosis is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage cables, and particularly to a method and device for calculating the electro-thermal field of high-voltage cables. Background Art

[0002] In recent years, with the continuous improvement of the requirements for the transmission capacity and reliability of the power grid, the high-voltage cable transmission network has become an important means to build a green and environment-friendly power transmission system. Among them, cross-linked polyethylene (XLPE) cables are widely used due to their excellent electrical, heat-resistant and mechanical properties. High-voltage cables generally adopt a corrugated aluminum sheath structure, and a semiconductive and water-blocking buffer layer is provided between the metal sheath and the cable core. This buffer layer is usually composed of a fluffy cotton and a non-woven fabric made of polyester fiber, filled with water-blocking powder in the middle, and doped with carbon black to enhance the electrical performance. Its main functions are to provide good electrical contact, absorb thermal expansion and have a longitudinal water-blocking function.

[0003] However, in recent years, buffer layer ablation failures have occurred frequently in many places. Such failures are manifested as ablation marks and white powder in the internal buffer layer of the cable. In severe cases, the main insulation layer will be damaged, and the failures have strong concealment and serious consequences. At present, there is still a lack of efficient fault detection means, which has therefore attracted wide attention.

[0004] To reveal the ablation reasons, researchers have carried out simulation analysis work. The research shows that when the aluminum sheath is not closely attached to the buffer layer or the conductivity of the buffer layer decreases due to moisture, partial discharge is likely to occur, and the critical condition for fault triggering is reduced. At the same time, due to the asymmetry of the buffer layer structure, air gaps are easily formed in the upper part, further leading to current density concentration, significantly increasing the local temperature rise of the buffer layer, and thus triggering ablation.

[0005] However, the existing technology cannot explain the specific reasons for the ablation failure, and there are still deficiencies in the buffer layer material structure, electrical performance adaptation and early fault diagnosis. There is an urgent need to develop a more reliable structural design and effective monitoring methods to improve the safety and stability of the high-voltage cable system. Summary of the Invention

[0006] The purpose of the present invention is to provide a method and device for calculating the electro-thermal field of high-voltage cables, which are used to solve the problems that the existing technology cannot explain the specific reasons for the ablation failure, and there are still deficiencies in the buffer layer material structure, electrical performance adaptation and early fault diagnosis. The specific reasons for the ablation failure can be explained, the current density and temperature rise distribution of the buffer layer can be obtained, the high-temperature hot spots and current density concentration areas of the buffer layer can be obtained, so as to realize the adaptation of the buffer layer material structure and electrical performance, and is conducive to subsequent early fault diagnosis.

[0007] To achieve the above object, in a first aspect, the present invention provides a method for calculating the electro-thermal field of a high-voltage cable, comprising: Establish a two-dimensional axisymmetric model of the high-voltage cable; the high-voltage cable includes an aluminum sheath and a buffer layer. In the two-dimensional axisymmetric model, there are N contact positions between the peak or valley of the aluminum sheath and the buffer layer, where N is a positive integer, and at least one contact position is in good contact, and the remaining contact positions are in poor contact; good contact means that the buffer layer and the aluminum sheath are in an interference fit, and poor contact means that there is an air gap between the buffer layer and the aluminum sheath; Set the material parameters and dimensions of each structural layer of the two-dimensional axisymmetric model; Calculate the current density of the buffer layer according to a preset electric field control equation; Convert the current density of the buffer layer into a heat source distribution. Under steady-state conditions, apply a convective heat transfer boundary to simulate the temperature rise distribution in the buffer layer caused by the current.

[0008] According to the method for calculating the electro-thermal field of a high-voltage cable provided by the present invention, the aluminum sheath is fitted with a sine function.

[0009] According to the method for calculating the electro-thermal field of a high-voltage cable provided by the present invention, the material parameters include relative permittivity, resistivity, thermal conductivity, constant pressure heat capacity, and density.

[0010] According to the method for calculating the electro-thermal field of a high-voltage cable provided by the present invention, the high-voltage cable is a high-voltage XLPE cable.

[0011] According to the method for calculating the electro-thermal field of a high-voltage cable provided by the present invention, the structural layers of the two-dimensional axisymmetric model include a conductor, a conductor shield, an XLPE insulation, an insulation shield, a buffer layer, an aluminum sheath, and an outer sheath.

[0012] According to the method for calculating the electro-thermal field of a high-voltage cable provided by the present invention, the resistivity of the buffer layer changes with the interference fit distance between the buffer layer and the aluminum sheath.

[0013] According to the method for calculating the electro-thermal field of a high-voltage cable provided by the present invention, the relationship curve between the resistivity of the buffer layer and the interference fit distance is a broken line.

[0014] According to the method for calculating the electro-thermal field of a high-voltage cable provided by the present invention, the preset electric field control equation is:

[0015] In the formula, J is the current density of the buffer layer; σ is the conductivity; E is the electric field strength; ω is the angular velocity of the current phasor; D is the electric displacement vector; Je is the conduction current density; ɛ r is the relative permittivity, ɛ and 0 is the permittivity of vacuum.

[0016] According to a method for calculating the electro-thermal field of a high-voltage cable provided by the present invention, it further includes: evaluating the local overheating risk and local current concentration risk of the buffer layer according to the current density and temperature rise distribution of the buffer layer.

[0017] In a second aspect, the present invention provides an electro-thermal field calculation device for a high-voltage cable, including: A modeling unit, configured to establish a two-dimensional axisymmetric model of the high-voltage cable; the high-voltage cable includes an aluminum sheath and a buffer layer. In the two-dimensional axisymmetric model, there are N contact positions between the peak or valley of the aluminum sheath and the buffer layer, N is a positive integer, at least one contact position is in good contact, and the remaining contact positions are in poor contact; being in good contact means that the buffer layer and the aluminum sheath are in an interference fit, and being in poor contact means that there is an air gap between the buffer layer and the aluminum sheath; A setting unit, configured to set the material parameters and dimensions of each structural layer of the two-dimensional axisymmetric model; A calculation unit, configured to calculate the current density of the buffer layer according to a preset electric field control equation; A simulation unit, configured to convert the current density of the buffer layer into a heat source distribution, and under steady-state conditions, apply a convective heat transfer boundary to simulate the temperature rise distribution caused by the current in the buffer layer.

[0018] The present invention has at least the following technical effects: A method and device for calculating the electro-thermal field of a high-voltage cable provided by the present invention. The method includes establishing a two-dimensional axisymmetric model of the high-voltage cable, setting the material parameters and dimensions of each structural layer of the two-dimensional axisymmetric model, and calculating the current density of the buffer layer according to a preset electric field control equation; converting the current density of the buffer layer into a heat source distribution, and under steady-state conditions, applying a convective heat transfer boundary to simulate the temperature rise distribution caused by the current in the buffer layer. It can explain the specific reasons for ablation failures, obtain the current density and temperature rise distribution of the buffer layer, and obtain the high-temperature hot spots and current density concentration areas of the buffer layer, so as to realize the adaptation of the buffer layer material structure and electrical performance, and is conducive to subsequent early fault diagnosis. Description of the Drawings

[0019] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0020] In the drawings: Figure 1 This is the flowchart of the electro-thermal field calculation method for the high-voltage cable of the present invention; Figure 2 This is the two-dimensional axisymmetric model structure diagram of the high-voltage cable of the present invention; Figure 3 This is the schematic diagram of the interference fit between the buffer layer and the aluminum sheath of the present invention; Figure 4 This is the relationship curve diagram between the resistivity of the buffer layer and the deformation distance of the present invention; Figure 5a , Figure 5b , Figure 5c and Figure 5d This is the buffer layer current density distribution diagram of the experimental group and the control group of the present invention; Figure 6 This is the relationship diagram between the maximum buffer layer current density of the experimental group and the control group of the present invention, the number of poor contact points, and the deformation distance; Figure 7 This is the relationship diagram between the maximum current density of the buffer layer of the experimental group of the present invention and the interference fit distance; Figure 8a , Figure 8b , Figure 8c and Figure 8d This is the buffer layer temperature distribution diagram of the experimental group and the control group of the present invention; Figure 9 This is the relationship diagram between the maximum buffer layer temperature rise and the number of poor contact points of the present invention; Figure 10 This is the relationship diagram between the maximum buffer layer temperature rise of the experimental group of the present invention and the interference fit distance. Specific embodiments

[0021] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without creative efforts shall fall within the protection scope of the present invention.

[0022] Below, some embodiments of the present invention will be described in detail in conjunction with the accompanying drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0023] Please refer to Figure 1 , the embodiment of the present invention provides a method for calculating the electro-thermal field of a high-voltage cable, including: Step 1: Establish a two-dimensional axisymmetric model of the high-voltage cable; Step 2: Set the material parameters and dimensions of each structural layer of the two-dimensional axisymmetric model; Specifically, the high-voltage cable is a high-voltage XLPE cable. As Figure 2 shown, the high-voltage cable from the inside out is successively a conductor, a conductor shield, XLPE insulation, an insulation shield, a buffer layer, a corrugated aluminum sheath, and an outer sheath. There is an air gap between the buffer layer and the aluminum sheath. In this embodiment, a two-dimensional axisymmetric model of a 110 kV high-voltage cable is established for electro-thermal field coupling simulation. The length of the 110 kV high-voltage cable is 1000 mm. The aluminum sheath contains 50 nodes (i.e., 50 corrugation peaks or valleys) along the length direction. The aluminum sheath is fitted with a sine function, and the corrugation depth is 5 mm and the pitch is 20 mm.

[0024] The dimensions (including outer diameter, etc.) and material parameters (including relative permittivity, resistivity, thermal conductivity, constant-pressure heat capacity, density, etc.) of each structural layer in the model are shown in Table 1.

[0025] Table 1. Material parameters and dimensions of the cable model

[0026] In this embodiment, the relative permittivity of the buffer layer is set to 500, but more critically, its resistivity will change with the interference fit distance (deformation distance) between the buffer layer and the aluminum sheath. The interference fit distance between the buffer layer and the aluminum sheath mentioned in the present invention refers to the radial distance between the lowest point after the buffer layer is depressed under pressure and its outer surface at a single contact position, as Figure 3 shown.

[0027] Specifically, there are N contact positions between the peak or valley of the aluminum sheath and the buffer layer, where N is a positive integer. At least one contact position is in good contact, and the remaining contact positions are in poor contact. Good contact means that the buffer layer and the aluminum sheath are in interference fit, and poor contact means that there is an air gap between the buffer layer and the aluminum sheath.

[0028] In this embodiment, in the cable model with a length of 1000 mm, 50 contact points (i.e., the contact positions between the peak or valley of the aluminum sheath and the buffer layer) are defined. The contact points may be in "good contact" or "poor contact", and the number of poor contact points can vary from 0 to 49. As Figure 3 shown, an air gap of 0.1 mm is set at the poor contact; at the good contact, it is considered that there is different degrees of compression and interference fit between the aluminum sheath and the buffer layer. When the proportion of poor contact points increases, the capacitive current will more concentrate and flow to the good contact area, resulting in local concentration of current density.

[0029] To simulate different compaction degrees between the buffer layer and the aluminum sheath, the interference fit distance of the buffer layer is set to [0.1 mm, 1 mm]; as the deformation distance decreases, the resistivity of the buffer layer decreases accordingly.

[0030] In the present invention, based on the experimental measurement, the relationship curve between the resistivity of the buffer layer and the deformation distance is obtained. As shown Figure 4 in the figure, it is divided into a broken line, including the starting point A, the turning point B, and the ending point C, and is fitted linearly. There is a critical deformation distance (about 0.72 mm, at the turning point B) on this curve. When the deformation distance is less than this critical value, the resistivity of the buffer layer will decrease rapidly; when it is greater than this critical value, the degree of resistivity reduction is no longer significant.

[0031] Step 3: Calculate the current density of the buffer layer according to the preset electric field control equation; Specifically, considering the conduction current and displacement current existing in the buffer layer, the following preset electric field control equation is used for the calculation of the current field:

[0032] In the formula, J is the current density of the buffer layer, with the unit A / m 2 ; σ is the conductivity, with the unit S / m; E is the electric field strength, with the unit V / m; ω is the angular velocity of the current phasor, with the unit rad / s; D is the electric displacement vector, with the unit C / m 2 ; J e is the conduction current density, with the unit A / m 2 . In this embodiment, the conductivity σ is much larger than ωɛ 0 ɛ r , and the relative dielectric constant has little influence on the current density distribution of the buffer layer. To simplify the calculation, the influence of the deformation distance of the buffer layer on the relative dielectric constant ɛ r is ignored, and it is set to 500. ɛ 0 represents the vacuum permittivity.

[0033] Step 4: Convert the current density of the buffer layer into a heat source distribution. Under steady-state conditions, apply a convective heat transfer boundary to simulate the temperature rise distribution in the buffer layer caused by the current.

[0034] Specifically, Step 4 specifically includes: Convert the current density of the buffer layer obtained from the electric field calculation into a heat source distribution (Joule heat loss).

[0035] Under steady-state conditions, a convective heat transfer boundary is applied to the outer surface of the outer sheath, and the heat transfer coefficient is calculated and determined based on the actual geometric conditions.

[0036] The ambient temperature is set to 20 °C, and on this basis, the temperature rise distribution caused by the current in the buffer layer is simulated.

[0037] During the simulation process, the influence of the deformation of the buffer layer on its thermal parameters (thermal conductivity, constant-pressure heat capacity, etc.) is ignored, which belongs to a conservative (higher temperature rise) calculation.

[0038] During the simulation process of this embodiment, an effective phase voltage of 63.5 kV is applied to simulate the radial electric field during the normal operation of the high-voltage cable. Considering different numbers of poor contact points (0 - 49) and different interference fit distances (0.1 mm - 1 mm), the experimental group (the present invention takes into account the resistivity deformation characteristics of the buffer layer) and the control group (the resistivity of the buffer layer is fixed at 1000 Ω·m) are compared.

[0039] The buffer layer current density distributions of the experimental group and the control group are as Figure 5a , Figure 5b , Figure 5c and Figure 5d shown. Figure 5a is for the experimental group with an interference fit distance of 0.1 mm and 49 poor contact points; Figure 5b is for the experimental group with an interference fit distance of 0.5 mm and 49 poor contact points; Figure 5c is for the experimental group with an interference fit distance of 1 mm and 49 poor contact points; Figure 5d is for the control group with an interference fit distance of 0.1 mm and 49 poor contact points. The maximum value of the buffer layer current density is distributed at the intersection of the buffer layer and the valleys of the aluminum sheath and the air. This is because when the radial capacitive current concentrates at the well-contact points in the buffer layer, it is shunted on paths 1 and 2. Since the resistance of path 1 is smaller, the maximum value of the current density appears. In the experimental group compared with the control group, directly below the valley of the aluminum sheath, due to the smaller resistivity here, the phenomenon of current density concentration will also occur. As the interference fit distance increases, the proportion of the current flowing into the aluminum sheath through path 2 also increases. At the same time, the maximum value of the buffer layer current density in the experimental group is significantly greater than that in the control group. This is because when considering the deformation characteristics of the buffer layer, its resistivity will decrease significantly, so the overall resistance of the current path will be reduced, resulting in an increase in the maximum value of the current density. As the interference fit distance of the buffer layer increases, its overall current density decreases. This is because the radial capacitive current of each simulation model is almost the same. As the interference fit distance between the aluminum sheath and the buffer layer gradually increases, the contact area between the aluminum sheath and the buffer layer gradually increases, so the current density shows an overall downward trend.

[0040] The relationships between the maximum buffer layer current density of the experimental group and the control group, the number of poor contact points, and the deformation distance are as Figure 6 shown. The maximum current density of both the experimental group and the control group is positively correlated with the number of poor contact points. When the number of poor contact points exceeds 40, the growth rate of the maximum current density significantly accelerates. At the same time, the maximum buffer layer current density is negatively correlated with the deformation distance.

[0041] The calculation results between different deformation distances of the control group have less dispersion. The relationship between the maximum buffer layer current density of the experimental group and the interference fit distance is as Figure 7 shown. When the deformation distance is less than the critical deformation distance (0.72 mm), the growth rate of the maximum current density significantly accelerates; when the deformation distance is less than the critical deformation distance, the growth rate of the maximum current density is slower, indicating that there is good electrical contact between the buffer layer and the aluminum sheath at this time. Through the comparative analysis of the experimental group and the control group, it can be obtained that when considering the influence of the interference fit distance between the buffer layer and the aluminum sheath on the resistivity of the buffer layer, the calculated result of the current density of the buffer layer will increase, and as the working conditions gradually deteriorate (the number of poor contact points increases or the interference fit distance decreases), the increase in the current density becomes larger. Therefore, the buffer layer current density under actual working conditions will be greater than the calculated result of the existing research model.

[0042] The buffer layer temperature distribution of the experimental group and the control group is as Figure 8a , Figure 8b , Figure 8c and Figure 8d shown. Figure 8a For the experimental group, the interference fit distance is 0.1 mm and the number of poor contact points is 49; Figure 8b For the experimental group, the interference fit distance is 0.5 mm and the number of poor contact points is 49; Figure 8c For the experimental group, the interference fit distance is 1 mm and the number of poor contact points is 49; Figure 8d For the control group, the interference fit distance is 0.1 mm and the number of poor contact points is 49. The maximum temperature values are all distributed directly below the trough of the aluminum sheath, and the experimental group is significantly greater than the control group. At the same time, as the interference fit distance increases, the maximum buffer layer temperature significantly decreases. This is because, after considering the deformation characteristics of the buffer layer, the resistance of the current loop decreases as the interference fit distance increases, the current increases, resulting in increased heat generation here.

[0043] The relationship between the maximum buffer layer temperature rise and the number of poor contact points is as Figure 9 shown. Under the same conditions of poor contact and deformation distance, the maximum buffer layer temperature rise of the experimental group is significantly greater than that of the control group. The relationship between the maximum buffer layer temperature rise of the experimental group and the interference fit distance is as Figure 10As shown, when the deformation distance of the buffer layer is less than the critical deformation distance (0.72 mm), the maximum temperature rise of the buffer layer increases significantly; when the deformation distance of the buffer layer is greater than the critical deformation distance, the change in the maximum temperature rise is relatively small.

[0044] Under the above electro-thermal field calculation method, the following results were mainly concerned in this embodiment: the distribution and maximum value of the current density in the buffer layer, as well as the temperature distribution and maximum temperature rise in the buffer layer area. The calculation effects can be mainly summarized as follows: 1. Distribution and maximum value of the buffer layer current density Phenomenon of current density concentration. Due to the large air gap and high impedance at the poor contact points, the radial capacitive current will concentrate in the low-resistance areas with good contact. Especially below the trough of the aluminum sheath, obvious concentration of current density is likely to occur. After considering the deformation characteristics of the buffer layer, its resistivity decreases significantly with the decrease of deformation, resulting in a significantly higher maximum value of the current density obtained by the experimental group than that of the control group under the same working conditions.

[0045] 2. Influence of the number of poor contact points and the interference fit distance The more the number of poor contact points, the greater the current that the low-resistance area needs to carry; when the number of poor contact points exceeds 40, the growth of the current density shows an accelerating trend.

[0046] When the interference fit distance between the buffer layer and the aluminum sheath increases (greater than the critical value of 0.72 mm), the resistivity of the buffer layer remains at a relatively high level, and the overall maximum value of the current density shows a downward trend. On the contrary, when the deformation distance is much less than 0.72 mm, the resistivity decreases significantly, and the current is more likely to concentrate at some trough positions.

[0047] Under the most severe working conditions (49 poor contact points, deformation distance of 0.1 mm), the maximum value of the current density of the experimental group is about 22 A / m², while the maximum value of the current density of the control group is only 17 A / m², with a difference of about 1.3 times.

[0048] 3. Temperature distribution and temperature rise of the buffer layer Characteristics of temperature distribution. The maximum temperature rise also appears below the trough of the aluminum sheath. Since the experimental group generates a larger current after the resistivity decreases, the overall temperature distribution is relatively high.

[0049] The smaller the interference fit distance (the smaller the deformation distance), the greater the current density, and the temperature rise of the buffer layer in this local area also increases accordingly.

[0050] Under the most severe working conditions, the maximum temperature rise of the control group is only 0.3 °C; while that of the experimental group reaches 8 °C, with a difference of up to 26.6 times. It can be seen that ignoring the resistivity inhomogeneity of the buffer layer in actual working conditions will significantly underestimate the risk of local temperature rise.

[0051] As the deformation distance is greater than the critical value (about 0.72 mm), the increase rate of temperature rise begins to tend to be gentle, indicating that the contact between the aluminum sheath and the buffer layer is relatively good at this time, and a small number of poor contact points will not significantly raise the temperature.

[0052] In some embodiments, the method for calculating the electro-thermal field of the high-voltage cable of the present invention further includes: Step 5, evaluating the local overheating risk and local current concentration risk of the buffer layer according to the current density and temperature rise distribution of the buffer layer.

[0053] Therefore, the method for calculating the electro-thermal field of the high-voltage cable provided by the present invention has the following characteristics: 1. A more practical calculation model In the present invention, by introducing the resistivity deformation characteristics of the buffer layer after being pressed, the calculation of the electro-thermal field is closer to the actual working conditions, and can more explain the phenomenon that "ablation faults mostly occur below the trough of the aluminum sheath".

[0054] Under the most severe working conditions, the current density of the buffer layer can be increased by 1.3 times compared with the traditional uniform resistivity model, and the temperature rise can be increased by 26.6 times, indicating that there is an obvious underestimation in the traditional model (without considering the deformation characteristics of the buffer layer).

[0055] 2. The critical deformation distance has guiding significance The measured critical deformation distance (about 0.72 mm) can be used as a reference for whether the electrical contact between the buffer layer and the aluminum sheath is good: If the interference fit distance is greater than this value, the local temperature and current density will not rise excessively with the increase of poor contact points; If it is less than this value, when the number of poor contact points increases, the temperature rise and current density will increase significantly; In engineering design, 0.72 mm can be used as the recommended value of the interference fit distance of the buffer layer.

[0056] 3. Hint for actual engineering design When the cable length is relatively long (up to several kilometers), the radial capacitive current increases with the increase of the length; in addition, due to the spiral corrugation structure of the aluminum sheath and the gravity influence caused by the operating environment, a large number of poor contact points are more likely to appear in practice. If the resistivity of the buffer layer drops rapidly with deformation, the risk of excessive concentration of current and temperature rise in some local areas is more likely to occur. This model and result have high reference value for cable design and fault protection.

[0057] Based on the same inventive concept, another embodiment of the present invention provides a device for calculating the electro-thermal field of a high-voltage cable. This device corresponds to the method of the foregoing embodiment, and this device includes: A building unit, configured to build a two-dimensional axisymmetric model of a high-voltage cable; the high-voltage cable includes an aluminum sheath and a buffer layer. In the two-dimensional axisymmetric model, there are N contact positions between the wave crest or wave trough of the aluminum sheath and the buffer layer, where N is a positive integer, and at least one contact position has good contact, while the remaining contact positions have poor contact. Good contact means that the buffer layer and the aluminum sheath are in interference fit, and poor contact means that there is an air gap between the buffer layer and the aluminum sheath; A setting unit, configured to set the material parameters and dimensions of each structural layer of the two-dimensional axisymmetric model; A calculation unit, configured to calculate the current density of the buffer layer according to a preset electric field control equation; A simulation unit, configured to convert the current density of the buffer layer into a heat source distribution, and under steady-state conditions, apply a convective heat transfer boundary to simulate the temperature rise distribution caused by current in the buffer layer.

[0058] In some embodiments, the electro-thermal field calculation device for a high-voltage cable of the present invention further includes an evaluation unit, configured to evaluate the local overheating risk and local current concentration risk of the buffer layer according to the current density and temperature rise distribution of the buffer layer.

[0059] In summary, the electro-thermal field calculation method and device for a high-voltage cable provided by the present invention make a more accurate evaluation of the current density and temperature rise of a high-voltage XLPE cable under the actual working condition where the resistivity of the buffer layer is related to the interference fit distance: 1. Calculation method of the model: A two-dimensional axisymmetric model that considers the actual waveform of the aluminum sheath; Taking into account both conduction current and displacement current; Performing a coupled analysis of the steady-state thermal field and the electric field.

[0060] 2. Key technical points: Regarding the resistivity of the buffer layer as a variable related to the deformation distance, rather than a constant; Proposing the concept of critical deformation distance and giving a quantitative recommended value (0.72 mm).

[0061] 3. Main technical effects: The current density and temperature rise increase significantly, and a "concentration" phenomenon appears below the wave trough; Under the most severe working conditions, the maximum temperature rise is approximately 26 times higher than that of the model without considering the deformation characteristics, and the maximum current density is increased by 1.3 times; It has direct guiding significance for "strengthening the interference fit between the buffer layer and the aluminum sheath and reducing the number of poor contact points" in engineering design.

[0062] Through the technical solution of the present invention, in the design, operation and fault diagnosis of actual high-voltage cables, the local overheating risk and local current concentration risk of the buffer layer can be more accurately evaluated, thereby improving the reliability and safety of high-voltage cables.

[0063] Most traditional models regard the resistivity of the buffer layer as a fixed value, which is insufficient to explain the phenomenon that "ablation mostly occurs below the trough of the aluminum sheath". By taking into account the deformation characteristics of the buffer layer, the present invention model points out that the resistivity below the trough will decrease sharply as the interference fit distance decreases, resulting in current concentration here and causing local overheating, successfully explaining the mechanism of uneven distribution of ablation positions.

[0064] Under the most severe working conditions (a large number of poor contact points and a small deformation distance), the maximum temperature rise of the buffer layer can reach 26.6 times that of the traditional model, and the maximum current density can be increased by about 1.3 times; this shows that the existing simplified model is prone to significantly underestimate the overheating risk inside the cable, while the solution of the present invention can more accurately reflect the actual state of the cable in the operating environment.

[0065] The "critical deformation distance" (about 0.72 mm) of the buffer layer measured in the present invention can be used as an important basis for judging whether the contact between the aluminum sheath and the buffer layer is good; if it is ensured that the interference fit distance is greater than this critical value during design or operation, even if there are some poor contact points, the increase in current density and temperature rise will be relatively limited; thus providing clear engineering design guidance for cable manufacturing, installation and subsequent maintenance.

[0066] The high-temperature hot spots and current density concentration areas obtained through fine simulation can prompt to focus on these parts during operation and maintenance, and take detection or reinforcement measures in a timely manner; it can also conduct more practical comparison and analysis on the monitoring data such as temperature and leakage current collected during subsequent fault diagnosis.

[0067] High-voltage cables often reach several kilometers in length and may be affected by multiple factors such as gravity, corrugated structure, and environmental vibration during operation; because the present invention considers the non-uniformity and variability of the resistivity of the buffer layer, it can effectively evaluate the local fault risk in a larger range, improving the safety and reliability of engineering applications.

[0068] In short, the solution of the present invention not only quantitatively reveals the amplification effect of the buffer layer deformation on the internal current distribution and temperature rise of the cable, but also can further put forward important conclusions such as "critical deformation distance" and "key hot spot areas" that can guide actual engineering, significantly improving the safety margin and evaluation accuracy of the cable in the dimensions of design, manufacturing and operation.

[0069] Other embodiments of the present invention will be readily apparent to those skilled in the art upon consideration of the specification and practice of the embodiments disclosed herein. The present invention is intended to cover any variations, uses, or adaptations of the invention following the general principles of the invention and including known common knowledge or conventional technical means in the technical field not disclosed by the present invention. It should be understood that the present invention is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is limited only by the appended claims.

Claims

1. A method for calculating the electro-thermal field of a high-voltage cable, characterized in that, Including: Establish a two-dimensional axisymmetric model of a high-voltage cable; the high-voltage cable includes an aluminum sheath and a buffer layer. In the two-dimensional axisymmetric model, there are N contact positions between the crest or trough of the aluminum sheath and the buffer layer, where N is a positive integer, at least one contact position is in good contact, and the remaining contact positions are in poor contact; being in good contact means that the buffer layer and the aluminum sheath are in an interference fit, and being in poor contact means that there is an air gap between the buffer layer and the aluminum sheath; Set the material parameters and dimensions of each structural layer of the two-dimensional axisymmetric model; Calculate the current density of the buffer layer according to a preset electric field control equation; Convert the current density of the buffer layer into a heat source distribution. Under steady-state conditions, apply a convective heat transfer boundary to simulate the temperature rise distribution caused by current in the buffer layer.

2. The electro-thermal field calculation method for high-voltage cables according to claim 1, characterized in that, The aluminum sheath is fitted with a sine function.

3. The electro-thermal field calculation method for high-voltage cables according to claim 1, characterized in that The material parameters include relative permittivity, resistivity, thermal conductivity, constant-pressure heat capacity, and density.

4. The electro-thermal field calculation method of the high-voltage cable according to claim 3, characterized in that, The high-voltage cable is a high-voltage XLPE cable.

5. The electro-thermal field calculation method for high-voltage cables according to claim 4, wherein, The structural layers of the two-dimensional axisymmetric model include a conductor, a conductor shield, XLPE insulation, an insulation shield, a buffer layer, an aluminum sheath, and an outer sheath.

6. The electro-thermal field calculation method for high-voltage cables according to claim 5, characterized in that The resistivity of the buffer layer changes with the interference fit distance between the buffer layer and the aluminum sheath.

7. The electro-thermal field calculation method for high-voltage cables according to claim 6, characterized in that The relationship curve between the resistivity of the buffer layer and the interference fit distance is a broken line.

8. The electro-thermal field calculation method for high-voltage cables according to claim 1, characterized in that, The preset electric field control equation is: Wherein, J is the current density of the buffer layer; σ is the conductivity; E is the electric field strength; ω is the angular velocity of the current phasor; D is the electric displacement vector; J e is the conduction current density; ɛ r is the relative permittivity, ɛ and 0 is the permittivity of vacuum.

9. The electro-thermal field calculation method for high-voltage cables according to claim 1, characterized in that, Also including: Evaluate the local overheating risk and local current concentration risk of the buffer layer according to the current density and temperature rise distribution of the buffer layer.

10. A high-voltage cable electro-thermal field calculation device, characterized in that, Including: A building unit for establishing a two-dimensional axisymmetric model of a high-voltage cable; the high-voltage cable includes an aluminum sheath and a buffer layer. In the two-dimensional axisymmetric model, there are N contact positions between the crest or trough of the aluminum sheath and the buffer layer, where N is a positive integer, at least one contact position is in good contact, and the remaining contact positions are in poor contact; being in good contact means that the buffer layer and the aluminum sheath are in an interference fit, and being in poor contact means that there is an air gap between the buffer layer and the aluminum sheath; A setting unit for setting the material parameters and dimensions of each structural layer of the two-dimensional axisymmetric model; A calculation unit for calculating the current density of the buffer layer according to a preset electric field control equation; A simulation unit for converting the current density of the buffer layer into a heat source distribution. Under steady-state conditions, apply a convective heat transfer boundary to simulate the temperature rise distribution caused by current in the buffer layer.

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