Rapid calculation method for low-frequency allowable current-carrying capacity of cable

Abstract

The invention discloses a quick calculation method for low-frequency allowable current-carrying capacity of a cable, which belongs to the technical field of alternating-current transmission cables, and comprises the following steps of: constructing a cable coupling simulation model, and performing simulation calculation on low-frequency and power-frequency allowable current-carrying capacity of most cables through the cable coupling simulation model; the correction coefficient of the current-carrying capacity correction equation is fitted according to the proportional relation between the low-frequency allowable current-carrying capacity and the power-frequency allowable current-carrying capacity of the same cable to obtain a low-frequency current-carrying capacity calculation equation, and the low-frequency allowable current-carrying capacity of the target cable can be directly obtained by inquiring the coupling parameter of the target cable and then according to the low-frequency current-carrying capacity calculation equation. According to the method, a corresponding coupling simulation model does not need to be constructed, tedious simulation steps are simplified, a required result can be obtained only by looking up a table and substituting a formula, and the calculation efficiency is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of AC transmission cables, and specifically, to a method for quickly calculating the allowable current-carrying capacity of cables at low frequencies. Background Art

[0002] The traditional AC transmission frequency is the industrial frequency of 50 Hz. As an innovative power transmission method, the low-frequency (10 - 30 Hz) transmission technology has broad application prospects in fields such as long-distance power transmission, new energy grid connection, and submarine cable power transmission. Currently, the current-carrying capacity calculation technology is constantly developing in the field of power transmission and distribution cables, with both achievements and challenges. Among them, the analytical calculation method is based on the IEC-60287 standard and the NM theory, and the numerical calculation method mainly uses the finite element simulation technology. A large amount of research work has been carried out on conventional industrial frequency cable lines, and the allowable current-carrying capacity tables for various types of cable products under typical laying methods have been established, which can be found in the product manuals of manufacturers and relevant technical manuals of cables. However, since the low-frequency cable system is still an emerging product, very little relevant research has been carried out, and there is no allowable current-carrying capacity table available for query.

[0003] Due to the difference in operating frequencies, the allowable current-carrying capacity of cables when operating at low frequencies is different from that at industrial frequencies. For users, it is very time-consuming and laborious to calculate the allowable current-carrying capacity for each type of cable in each application scenario, and currently, manufacturers are also unable to provide a reference allowable current-carrying capacity table for low-frequency cables, which brings great difficulties to the engineering application of low-frequency cable systems. Therefore, there is an urgent need to provide a convenient, effective, accurate, and engineering-applicable method for quickly calculating the allowable current-carrying capacity of low-frequency cable systems to meet the needs of industrial development.

[0004] Chinese Patent, Publication No.: CN117828915A, Publication Date: April 05, 2024, discloses a method for calculating the low-frequency current-carrying capacity of high-voltage land cables. By establishing a finite element multi-physical field coupling model of high-voltage land cables, performing model network meshing, then setting the model for double-end grounding, single-end grounding, and cross-bonding grounding respectively, and setting the initial transmission frequency and boundary conditions of the model, and calculating the electric field strength and magnetic field strength generated when the cable operates under this condition. By changing the core current, when the temperature of the land cable is equal to the set threshold, the core current at this time is used as the current-carrying capacity of the high-voltage land cable. Its calculation process involves multiple simulation steps and it is difficult to quickly obtain the calculation result, with low calculation efficiency. Summary of the Invention

[0005] In view of the problem of low calculation efficiency caused by the complex calculation steps for the allowable current-carrying capacity of cables operating under low-frequency voltages, the present invention provides a method for quickly calculating the low-frequency allowable current-carrying capacity of cables. By constructing a cable coupling simulation model, first, the allowable current-carrying capacities of low-frequency and power-frequency of most cables are simulated and calculated through the cable coupling simulation model. According to the proportional relationship between the low-frequency and power-frequency allowable current-carrying capacities of the same cable, the correction coefficient of the current-carrying capacity correction equation is fitted to obtain the low-frequency current-carrying capacity calculation equation. By querying the coupling parameters of the target cable and then according to the low-frequency current-carrying capacity calculation equation, the low-frequency allowable current-carrying capacity of the target cable can be directly obtained, without the need to construct a corresponding coupling simulation model, simplifying the cumbersome simulation steps. Only by looking up the table and substituting into the formula can the required results be obtained, significantly improving the calculation efficiency.

[0006] In a first aspect, a technical solution provided in an embodiment of the present invention is: a method for quickly calculating the low-frequency allowable current-carrying capacity of a cable, characterized by comprising the following steps: S1. Based on the test requirements, set the corresponding cable form to construct a cable coupling simulation model; S2. Set the coupling parameters in the cable coupling simulation model and perform simulation operation to obtain the standard allowable current-carrying capacities of the low-frequency and power-frequency of the cable; S3. Based on the ratio of the standard allowable current-carrying capacities of the low-frequency and power-frequency of the same cable and in combination with the current-carrying capacity correction equation, perform parameter fitting to obtain the correction coefficient; S4. Substitute the correction coefficient into the current-carrying capacity correction equation to obtain the low-frequency current-carrying capacity calculation equation, and substitute the coupling parameters of the target cable into the low-frequency current-carrying capacity calculation equation to obtain the low-frequency allowable current-carrying capacity of the target cable.

[0007] In this solution, since traditional electro-thermal field coupling simulation requires setting complex physical field parameters, boundary conditions, and mesh division, while in this solution, by fitting the correction coefficient, the complex simulation process is simplified into a simple calculation equation. By constructing a cable coupling simulation model and fitting the correction coefficient, it is avoided that complex simulations are required every time the low-frequency allowable current-carrying capacity is calculated. The introduction of the correction coefficient enables the calculation of the low-frequency allowable current-carrying capacity to be directly completed through the formula, greatly reducing the calculation time and the demand for calculation resources; by fitting the correction coefficient, it can be applied to different types of cables (such as different materials, different structures, different laying environments, etc.). Only by substituting the coupling parameters of the target cable into the low-frequency current-carrying capacity calculation equation can the results be obtained, having high applicability.

[0008] Preferably, in S1, based on the test requirements, setting the corresponding cable form to construct a cable coupling simulation model includes the following steps: Construct a number of cable models based on different voltage levels and different conductor cross-sections of the cable during operation, construct corresponding environmental models based on different working environments of the cable, and perform modeling and simulation on the cable model and the environmental model based on the finite element electro-thermal field modeling and simulation method to obtain a cable coupling simulation model.

[0009] In this solution, since it is necessary to simulate and analyze the thermal effects generated by the cable under different voltages and currents, it is necessary to consider the characteristics of the cable itself and the influence of the environment on the cable during operation. Therefore, a cable model and an environmental model are constructed. Select appropriate cable models and environmental models according to the test requirements, and perform modeling and simulation on the cable according to the finite element electro-thermal field modeling and simulation method, providing a test basis for obtaining the standard allowable current-carrying capacity subsequently.

[0010] Preferably, in S2, set coupling parameters in the cable coupling simulation model and perform simulation operation to obtain the standard allowable current-carrying capacity of the cable at low frequency and power frequency, including the following steps: Set the structural parameters, material parameters, cable laying environment parameters and boundary conditions of the cable in the cable coupling simulation model; adjust the structural parameters, material parameters and cable laying environment parameters of the cable under the same voltage level; adjust the voltage level, cable laying environment parameters and cable material parameters under the condition of the same cable structural parameters; adjust the structural parameters, material parameters and voltage level of the cable under the same cable laying environment; adjust the voltage level, cable laying environment parameters and cable structural parameters under the condition of the same cable material parameters; Real-time collect the electrical effect parameters and thermal effect parameters around the cable, and solve the electrical effect parameters and thermal effect parameters to obtain the standard allowable current-carrying capacity of the cable at low frequency and power frequency.

[0011] In this solution, in order to analyze the relationship between the allowable current-carrying capacity of the cable at low frequency and power frequency, a large number of cables need to be simulated. The allowable current-carrying capacity is not only affected by the properties of the cable itself, but also related to the voltage magnitude and working environment when the cable is operating. Therefore, in order to improve the reliability and accuracy of the simulation results, it is necessary to comprehensively analyze and simulate the above influencing factors, and comprehensively consider various influencing factors by constructing different cable forms, providing a reliable data basis for calculating the allowable current-carrying capacity of any voltage level and any type of low-frequency cable system subsequently.

[0012] Preferably, solving the electrical effect parameters and thermal effect parameters to obtain the standard allowable current-carrying capacity of the cable at low frequency and power frequency includes the following steps: Set the operating frequency of the cable to low frequency, and adjust the magnitude of the current inside the cable under the boundary conditions until the cable temperature is equal to the maximum boundary value of the boundary conditions. The boundary conditions are the cable operating temperature range, and take the magnitude of the current inside the cable at this time as the standard allowable current-carrying capacity of the cable at low frequency; Set the operating frequency of the cable to the power frequency, and adjust the magnitude of the current inside the cable under boundary conditions until the cable temperature equals the maximum boundary value of the boundary conditions. Take the magnitude of the current inside the cable at this time as the standard allowable current-carrying capacity of the cable at power frequency.

[0013] In this solution, the allowable current-carrying capacity is the maximum current magnitude that can be allowed to pass through the inside of the cable. When the current passes through the cable, it will increase the cable temperature. The greater the current, the higher the cable temperature. And the cable needs to operate safely within a certain temperature range. If the temperature is too high, it may lead to operating failures and cause safety accidents. Therefore, when performing simulation on the cable, by keeping other conditions unchanged and continuously adjusting the current magnitude until the cable temperature is equal to the maximum value of the set temperature boundary, the magnitude of the current passing through the inside of the cable at this time is the allowable current-carrying capacity. Since the skin effect is weaker in the low-frequency operating mode of the cable and the current distribution is relatively uniform, while the skin effect and proximity effect in the power-frequency operating mode of the cable result in non-uniform current distribution, the low-frequency allowable current-carrying capacity of the cable is higher than the power-frequency allowable current-carrying capacity.

[0014] Preferably, in S3, based on the ratio of the standard allowable current-carrying capacities of the low-frequency and power-frequency of the same cable and combined with the current-carrying capacity correction equation, parameter fitting is performed to obtain the correction coefficient, including the following steps: Construct a current-carrying capacity correction equation based on the cross-sectional area of the cable conductor and the rated voltage during cable operation, and add correction variables to the current-carrying capacity correction equation based on the influence degrees of the cross-sectional area of the cable conductor and the rated voltage on the current-carrying capacity; Perform fitting on the correction variables with the ratio of the standard allowable current-carrying capacities of the low-frequency and power-frequency of the same cable as the fitting target of the current-carrying capacity correction equation, and take the correction variable corresponding to the optimal fitting result as the correction coefficient.

[0015] In this solution, through performing simulation calculations on the allowable current-carrying capacity of a large number of low-frequency cables based on thermal field or thermal circuit modeling, and there is a certain relationship between the power-frequency and low-frequency allowable current-carrying capacities of the same cable. Therefore, by taking the ratio of the standard allowable current-carrying capacities of the low-frequency and power-frequency of the same cable as the fitting target of the current-carrying capacity correction equation, and taking the correction variable corresponding to the optimal fitting result as the correction coefficient, the correction coefficient is used to adjust the calculation direction of the current-carrying capacity correction equation, thereby obtaining the conversion relationship between the low-frequency allowable current-carrying capacity and the power-frequency allowable current-carrying capacity of the same cable. Subsequently, based on the existing power-frequency cable allowable current-carrying capacity table and combined with the current-carrying capacity correction equation, the low-frequency cable allowable current-carrying capacity can be quickly obtained. It is simple and practical, without the need to solve the allowable current-carrying capacity for each low-frequency cable based on thermal field or thermal circuit modeling, greatly saving the workload and time investment of engineering and technical personnel, and thus significantly improving the calculation efficiency.

[0016] Preferably, in S4, substitute the correction coefficient into the current-carrying capacity correction equation to obtain the low-frequency current-carrying capacity calculation equation, and the formula is expressed as follows: λ = aA con 2 + bA con + cU0 + d I low = λ·I 50 Where A con is the cross-sectional area of the conductor, and U0 is the rated voltage of the cable during operation; a, b, c, and d are correction factors; λ is the ratio of the allowable current-carrying capacity of the cable at low frequency and power frequency, and I 50 is the allowable current-carrying capacity of the cable at power frequency obtained from the cable product manual or technical manual.

[0017] In this solution, the correction factors obtained by fitting are used to further correct the current-carrying capacity correction equation, thereby quantifying the conversion relationship between the low-frequency current-carrying capacity and the power-frequency current-carrying capacity of the same cable, reducing the need for a large number of tests on actual cables, and thus reducing the test cost and time cost; the rapid calculation of the low-frequency allowable current-carrying capacity provides a technical inspiration for the rapid calculation of the allowable current-carrying capacity of the cable at other operating frequencies, and can be extended to evaluate the allowable current-carrying capacity of low-frequency AC cable systems at any operating frequency, with a wide range of application scenarios.

[0018] Preferably, in S4, substituting the coupling parameters of the target cable into the low-frequency current-carrying capacity calculation equation to obtain the low-frequency allowable current-carrying capacity of the target cable includes the following steps: Query the cross-sectional area of the conductor, the rated voltage during operation, and the allowable current-carrying capacity at power frequency of the target cable based on the product manual of the target cable and the relevant technical manual of the cable; Substitute the cross-sectional area of the conductor and the rated voltage during operation of the target cable into the low-frequency current-carrying capacity calculation equation to obtain the ratio of the allowable current-carrying capacity of the target cable at low frequency and power frequency, and multiply the ratio of the allowable current-carrying capacity of the target cable at low frequency and power frequency by the allowable current-carrying capacity at power frequency to obtain the low-frequency allowable current-carrying capacity of the target cable.

[0019] In this solution, through the low-frequency current-carrying capacity calculation equation, only the cross-sectional area of the conductor of the cable itself and the rated voltage during operation need to be queried, and then the power-frequency current-carrying capacity of the cable is queried through the manual, and the low-frequency current-carrying capacity of the cable can be quickly obtained. The calculation process is extremely convenient, significantly improving the work efficiency.

[0020] Preferably, the environmental model is an environmental simulation data model composed of weather, buildings, temperature, and humidity; the cable model is a cable simulation data model composed of cable models and cable structures.

[0021] In this solution, since the heat generated by the cable during operation is affected not only by the cable itself and the internal current, but also by the external environment. If the external environment is good, the allowable low-frequency current-carrying capacity of the cable may change. Therefore, in order to improve the calculation accuracy of the allowable low-frequency current-carrying capacity of the cable, the external environment during cable operation is also analyzed to make it closer to the actual situation during simulation, thereby improving the accuracy of the calculation results.

[0022] Preferably, the cable laying environment parameters at least include the burial depth, soil thermal resistance, air temperature, soil temperature, and soil surface heat transfer coefficient.

[0023] In this solution, when simulating the operation of the cable, in order to set the cable laying environment, it is set from aspects such as the burial depth, soil thermal resistance, air temperature, soil temperature, and soil surface heat transfer coefficient. The above parameters play a greater role in the heat conduction generated during cable operation, and other laying environment parameters have less impact on the heat generated by the cable itself. Therefore, in order to reduce the simulation steps without affecting the simulation results, only the parameters with greater influence need to be set, and there is no need to set other parameters, which can improve the simulation efficiency.

[0024] Preferably, the material parameters at least include the relative permittivity, conductivity, thermal conductivity, density, and specific heat capacity.

[0025] In this solution, by setting parameters such as the relative permittivity, conductivity, thermal conductivity, density, and specific heat capacity during simulation, there is no need to set other parameters that affect the allowable current-carrying capacity, which simplifies the parameter setting steps during simulation and thus improves the simulation efficiency.

[0026] The beneficial effects of the present invention are as follows: (1) By proposing a method for quickly obtaining the allowable low-frequency current-carrying capacity of a cable based on the existing allowable current-carrying capacity table of power-frequency cables and combining with the current-carrying capacity correction equation, the present invention is simple and practical, and there is no need to solve the allowable current-carrying capacity for each low-frequency cable based on thermal field or thermal circuit modeling, which greatly saves the workload and time investment of engineering and technical personnel; (2) The present invention determines the expression of the low-frequency current-carrying capacity calculation equation, considering two key influencing factors, namely the cable core cross-section and voltage level, and the result accuracy meets the requirements of engineering applications; (3) By proposing a method for determining the correlation coefficient in the current-carrying capacity correction equation, that is, obtaining the optimal solution by fitting the solution results of the power-frequency and low-frequency allowable current-carrying capacities of no less than 4 types of cables (including different voltage levels and different conductor cross-sections); (4) Once the low-frequency current-carrying capacity calculation equation of the present invention is obtained, it can be used to calculate the allowable current-carrying capacity of any voltage level and any type of low-frequency cable system, and it has strong engineering applicability; (5) The method proposed by the present invention can be extended to evaluate the allowable current-carrying capacity of low-frequency AC cable systems at any operating frequency, and has a wide range of application scenarios.

[0027] The above description of the invention content is only an overview of the technical solution of the present invention. In order to be able to understand the technical means of the present invention more clearly, it can be implemented according to the content of the specification. And in order to make the above and other purposes, features and advantages of the present invention more obvious and understandable, the specific embodiments of the present invention are specifically given below. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] By reading the detailed description of the non-limiting embodiments with reference to the following drawings, other features, purposes and advantages of the present invention will become more obvious. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present invention. Moreover, throughout the drawings, the same reference numerals are used to represent the same components.

[0029] Figure 1 It is a flow chart of a fast calculation method for the allowable current-carrying capacity of a cable at low frequency according to the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] In order to make the purpose, technical solution and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only the best embodiments of the present invention, which are only used to explain the present invention and do not limit the protection scope of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

[0031] Before discussing the exemplary embodiments in more detail, it should be mentioned that some exemplary embodiments are described as processes or methods depicted as flowcharts. Although the flowcharts describe the operations (or steps) as sequential processes, many of the operations (or steps) can be implemented in parallel, concurrently or simultaneously. In addition, the order of the operations can be rearranged. When the operations are completed, the process can be terminated, but there may also be additional steps not included in the drawings; the process can correspond to a method, function, procedure, subroutine, subprogram, etc.

[0032] Embodiment: As Figure 1 shown, in order to solve the problem of low calculation efficiency caused by the complex calculation steps for the allowable current-carrying capacity of cables operating under existing low-frequency voltages, this embodiment provides a fast calculation method for the allowable current-carrying capacity of cables at low frequency, including the following steps: S1: Based on the test requirements, set the corresponding cable form to construct a cable coupling simulation model.

[0033] In this embodiment, a cable coupling simulation model is constructed based on corresponding cable forms set according to test requirements, including the following steps: Based on different voltage levels during cable operation and different conductor cross-sections of the cable, several cable models are constructed. Based on different working environments of the cable, corresponding environment models are constructed. Based on the finite element electro-thermal field modeling and simulation method, the cable models and environment models are modeled and simulated to obtain a cable coupling simulation model.

[0034] In this embodiment, since it is necessary to simulate and analyze the thermal effects generated by the cable under different voltages and currents, it is necessary to consider the characteristics of the cable itself and the influence of the environment during cable operation on the cable. Therefore, a cable model and an environment model are constructed. Appropriate cable models and environment models are selected according to test requirements, and the cable is modeled and simulated according to the finite element electro-thermal field modeling and simulation method, providing a test basis for obtaining the standard allowable current-carrying capacity subsequently.

[0035] In this embodiment, the environment model is an environmental simulation data model composed of weather, buildings, temperature, and humidity; the cable model is a cable simulation data model composed of cable models and cable structures.

[0036] In this embodiment, since the heat generated by the cable during operation is not only affected by the cable itself and the internal current, but also by the external environment. If the external environment is good, the low-frequency allowable current-carrying capacity of the cable may change. Therefore, in order to improve the calculation accuracy of the low-frequency allowable current-carrying capacity of the cable, the external environment during cable operation is also analyzed to make it closer to the actual situation during simulation, thereby improving the accuracy of the calculation results.

[0037] S2: Set coupling parameters in the cable coupling simulation model and perform simulation operations to obtain the standard allowable current-carrying capacity of the cable at low frequency and power frequency.

[0038] In this embodiment, setting coupling parameters in the cable coupling simulation model and performing simulation operations to obtain the standard allowable current-carrying capacity of the cable at low frequency and power frequency includes the following steps: Set the structural parameters, material parameters, cable laying environment parameters, and boundary conditions of the cable in the cable coupling simulation model; adjust the structural parameters, material parameters, and cable laying environment parameters of the cable under the same voltage level; adjust the voltage level, cable laying environment parameters, and cable material parameters under the same cable structural parameter conditions; adjust the structural parameters, material parameters, and voltage level of the cable under the same cable laying environment; adjust the voltage level, cable laying environment parameters, and cable structural parameters under the same cable material parameter conditions; Real-time collect the electro-effect parameters and thermal-effect parameters around the cable, and solve the electro-effect parameters and thermal-effect parameters to obtain the standard allowable current-carrying capacity of the cable at low frequency and power frequency.

[0039] In this embodiment, in order to analyze the relationship between the allowable current-carrying capacity of the cable at low frequency and power frequency, a large number of cables need to be simulated. The allowable current-carrying capacity is not only affected by the properties of the cable itself, but also related to the voltage magnitude and working environment when the cable is operating. Therefore, in order to improve the reliability and accuracy of the simulation results, it is necessary to comprehensively analyze and simulate the above influencing factors. By constructing different cable forms, various influencing factors are comprehensively considered, providing a reliable data basis for calculating the allowable current-carrying capacity of low-frequency cable systems with any voltage level and any model subsequently.

[0040] In this embodiment, the standard allowable current-carrying capacity of the cable at low frequency and power frequency is obtained by solving the electrical effect parameters and thermal effect parameters, including the following steps: Set the operating frequency of the cable to low frequency, and adjust the magnitude of the current inside the cable under the boundary conditions until the cable temperature is equal to the maximum boundary value of the boundary conditions. The boundary conditions are the operating temperature range of the cable. Take the magnitude of the current inside the cable at this time as the standard allowable current-carrying capacity of the cable at low frequency; Set the operating frequency of the cable to power frequency, and adjust the magnitude of the current inside the cable under the boundary conditions until the cable temperature is equal to the maximum boundary value of the boundary conditions. Take the magnitude of the current inside the cable at this time as the standard allowable current-carrying capacity of the cable at power frequency.

[0041] The allowable current-carrying capacity in this embodiment is the maximum current magnitude that can be allowed to pass through inside the cable. When the current passes through the cable, it will make the cable temperature rise. The greater the current, the higher the cable temperature. And the cable needs to operate safely within a certain temperature range. If the temperature is too high, it may lead to operating failures and cause safety accidents. Therefore, when simulating the cable, by keeping other conditions unchanged and continuously adjusting the current magnitude until the cable temperature is equal to the maximum value of the set temperature boundary, the magnitude of the current passing through inside the cable at this time is the allowable current-carrying capacity. And because the skin effect is weak in the low-frequency operating mode of the cable and the current distribution is relatively uniform, while the skin effect and proximity effect in the power-frequency operating mode of the cable lead to non-uniform current distribution, the low-frequency allowable current-carrying capacity of the cable is higher than the power-frequency allowable current-carrying capacity.

[0042] S3: Based on the ratio of the standard allowable current-carrying capacity of the same cable at low frequency and power frequency and combined with the current-carrying capacity correction equation, parameter fitting is carried out to obtain the correction coefficient.

[0043] In this embodiment, based on the ratio of the standard allowable current-carrying capacity of the same cable at low frequency and power frequency and combined with the current-carrying capacity correction equation, parameter fitting is carried out to obtain the correction coefficient, including the following steps: Construct a current-carrying capacity correction equation based on the cross-sectional area of the cable conductor and the rated voltage when the cable is operating. Add correction variables to the current-carrying capacity correction equation based on the influence degrees of the cross-sectional area of the cable conductor and the rated voltage on the current-carrying capacity; Taking the ratio of the standard allowable current-carrying capacity of the low-frequency and power-frequency of the same cable as the fitting target of the current-carrying capacity correction equation to fit the correction variables, and taking the correction variables corresponding to the optimal fitting result as the correction coefficients. The formula for the ratio of the standard allowable current-carrying capacity of the low-frequency and power-frequency of the cable is as follows: where λ i为 the ratio of the allowable current-carrying capacity of the low-frequency and power-frequency of the cable (i = 1…n, n is the number of cables for simulation in the cable coupling model), I i-low is the allowable current-carrying capacity of the low-frequency of the i-th cable for simulation in the cable coupling model, I i-50 is the allowable current-carrying capacity of the power-frequency of the i-th cable for simulation in the cable coupling model.

[0044] In this embodiment, by performing simulation calculations on the allowable current-carrying capacity based on the thermal field or thermal circuit modeling of a large number of low-frequency cables, and there is a certain relationship between the power-frequency and low-frequency allowable current-carrying capacities of the same cable. Therefore, taking the ratio of the standard allowable current-carrying capacity of the low-frequency and power-frequency of the same cable as the fitting target of the current-carrying capacity correction equation, and taking the correction variables corresponding to the optimal fitting result as the correction coefficients, using the correction coefficients to adjust the calculation direction of the current-carrying capacity correction equation, thus obtaining the conversion relationship between the low-frequency allowable current-carrying capacity and the power-frequency allowable current-carrying capacity of the same cable. Subsequently, according to the existing power-frequency cable allowable current-carrying capacity table, combined with the current-carrying capacity correction equation, the low-frequency cable allowable current-carrying capacity can be quickly obtained, which is simple and practical, without the need to solve the allowable current-carrying capacity for each low-frequency cable based on the thermal field or thermal circuit modeling, greatly saving the workload and time investment of engineering and technical personnel, thus significantly improving the calculation efficiency.

[0045] S4: Substitute the correction coefficient into the current-carrying capacity correction equation to obtain the low-frequency current-carrying capacity calculation equation, and substitute the coupling parameters of the target cable into the low-frequency current-carrying capacity calculation equation to obtain the low-frequency allowable current-carrying capacity of the target cable.

[0046] In this embodiment, substituting the correction coefficient into the current-carrying capacity correction equation to obtain the low-frequency current-carrying capacity calculation equation, the formula is expressed as follows: λ = aA con 2 + bA con + cU0 + d I low = λ·I 50 where A con is the cross-sectional area of the conductor, U0 is the rated voltage when the cable is working; a, b, c, and d are correction coefficients; λ is the ratio of the allowable current-carrying capacity of the low-frequency and power-frequency of the cable, I 50 is the power-frequency allowable current-carrying capacity of this cable obtained from the cable product manual or technical manual.

[0047] In this embodiment, the correction coefficient obtained by fitting is used to further correct the ampacity correction equation, thereby quantifying the conversion relationship between the low-frequency ampacity and the power-frequency ampacity of the same cable, reducing the need for a large number of tests on actual cables, and thus reducing the test cost and time cost. The rapid calculation of the low-frequency allowable ampacity provides a technical inspiration for the rapid calculation of the allowable ampacity of the cable at other operating frequencies, and can be extended to evaluate the allowable ampacity of low-frequency AC cable systems at any operating frequency, with a wide range of application scenarios.

[0048] In this embodiment, substituting the coupling parameters of the target cable into the low-frequency ampacity calculation equation to obtain the low-frequency allowable ampacity of the target cable includes the following steps: Query the conductor cross-sectional area, rated voltage during operation, and power-frequency allowable ampacity of the target cable based on the product manual of the target cable and the relevant technical manuals of the cable. Substitute the conductor cross-sectional area and rated voltage during operation of the target cable into the low-frequency ampacity calculation equation to obtain the ratio of the low-frequency and power-frequency allowable ampacities of the target cable, and multiply the ratio of the low-frequency and power-frequency allowable ampacities of the target cable by the power-frequency allowable ampacity to obtain the low-frequency allowable ampacity of the target cable.

[0049] In this embodiment, through the low-frequency ampacity calculation equation, only the conductor cross-sectional area and rated voltage during operation of the cable itself need to be queried, and then the power-frequency ampacity of the cable is queried through the manual, and the low-frequency ampacity of the cable can be quickly obtained. The calculation process is extremely convenient, significantly improving the work efficiency.

[0050] As a further supplement to this embodiment, the following scenario is taken as an example to further illustrate this solution: This embodiment takes a three-core submarine cable as an example to establish a cable coupling simulation model, and the cable structure parameters are shown in Table 1: Table 1. Cable structure parameters The cable laying environment parameters are shown in Table 2: Table 2. Cable laying environment parameters Laying conditions Parameters Burial depth 1m Soil thermal resistance 1.2 K / W Air temperature 30℃ Soil temperature 20℃ Soil surface heat transfer coefficient <![CDATA[6.5 W / (m 2 ·K)]]> The cable material parameters are shown in Table 3: Table 3. Cable material parameters According to the parameters in Tables 1 - 3, set the cable coupling simulation model and perform simulation runs to obtain the low-frequency 20Hz and power-frequency 50Hz allowable ampacities of different cables as shown in Table 4: Table 4. Cable allowable ampacity table Voltage level / kV <![CDATA[Conductor cross-sectional area / mm 2 > Power frequency current-carrying capacity / A Low-frequency current-carrying capacity / A 35 50 204 205 35 400 575 623 35 800 717 835 220 800 691 897 220 1200 738 1031 According to the cable allowable current-carrying capacity table, the calculated ratios of the allowable current-carrying capacities of each group of cables at low frequency 20Hz and power frequency 50Hz are 1.005, 1.083, 1.165, 1.298, and 1.397 respectively. Combining the core cross-section and the rated voltage level during operation of each group of cables, parameter fitting is performed on the current-carrying capacity correction equation, and the current-carrying capacity correction equation with correction parameters is obtained as follows: λ = 2.26×10 -8 A con 2 + 0.0002A con + 0.00075U0 + 0.966 Taking the solution of the low-frequency allowable current-carrying capacities of 35kV 3×300mm2 and 220kV 3×400mm2 cables as an example, under the same conditions as above, their power-frequency 50Hz current-carrying capacities are 493A and 549A respectively; the corresponding current-carrying capacity correction factors determined by the correction formula are 1.05 and 1.21. Then, the low-frequency 20Hz current-carrying capacities of these two types of cables are 517A and 664A respectively. Only by querying the core cross-section area, the rated voltage level during operation, and the power-frequency allowable current-carrying capacity of the cable to be calculated, and combining the current-carrying capacity correction equation, the corresponding low-frequency allowable current-carrying capacity can be quickly solved, and the calculation process is very simple and convenient.

[0051] It can be seen from the above embodiments that at least the following substantial effects are achieved: (1) The present invention proposes a method for quickly obtaining the low-frequency cable allowable current-carrying capacity by combining the current-carrying capacity correction equation based on the existing power-frequency cable allowable current-carrying capacity table. It is simple and practical, and there is no need to solve the allowable current-carrying capacity for each low-frequency cable based on thermal field or thermal circuit modeling, which greatly saves the workload and time investment of engineering and technical personnel; (2) The present invention determines the expression of the low-frequency current-carrying capacity calculation equation, considering two key influencing factors, namely the core cross-section of the cable and the voltage level, and the result accuracy meets the requirements of engineering applications; (3) The present invention proposes a method for determining the relevant coefficients in the current-carrying capacity correction equation, that is, by solving the power-frequency and low-frequency allowable current-carrying capacities of no less than 4 types of cable forms (including different voltage levels and different conductor cross-sections), and obtaining the optimal solution by fitting means; (4) Once the low-frequency current-carrying capacity calculation equation of the present invention is obtained, it can be used to calculate the allowable current-carrying capacity of any voltage level and any type of low-frequency cable system, and has strong engineering applicability; (5) The method proposed by the present invention can be extended to evaluate the allowable current-carrying capacity of low-frequency AC cable systems at any operating frequency, and has a wide range of application scenarios.

[0052] The above-described specific embodiments are the preferred embodiments of a method for quickly calculating the low-frequency allowable current-carrying capacity of a cable according to the present invention, and do not limit the specific implementation scope of the present invention. The scope of the present invention includes but is not limited to this specific embodiment. Any equivalent changes made in accordance with the shape and structure of the present invention are within the protection scope of the present invention.

Claims

1. A method for quickly calculating the allowable low-frequency current-carrying capacity of a cable, characterized in that: It includes the following steps: S1. Based on the test requirements, set the corresponding cable form to construct a cable coupling simulation model; S2. Set the coupling parameters in the cable coupling simulation model and conduct simulation runs to obtain the standard allowable current-carrying capacities of the cable at low frequency and power frequency; S3. Based on the ratio of the standard allowable current-carrying capacities of the same cable at low frequency and power frequency and combined with the current-carrying capacity correction equation, conduct parameter fitting to obtain the correction coefficient; S4. Substitute the correction coefficient into the current-carrying capacity correction equation to obtain the low-frequency current-carrying capacity calculation equation, and substitute the coupling parameters of the target cable into the low-frequency current-carrying capacity calculation equation to obtain the low-frequency allowable current-carrying capacity of the target cable.

2. A method for quickly calculating the low-frequency allowable current-carrying capacity of a cable according to claim 1, characterized in that: In S1, based on the test requirements, set the corresponding cable form to construct a cable coupling simulation model, including the following steps: Construct a number of cable models based on different voltage levels during cable operation and different conductor cross-sections of the cable, construct corresponding environmental models based on different cable working environments, and use the finite element electro-thermal field modeling and simulation method to conduct modeling and simulation of the cable model and the environmental model to obtain the cable coupling simulation model.

3. A method for quickly calculating the low-frequency allowable current-carrying capacity of a cable according to claim 1, characterized in that: In S2, set the coupling parameters in the cable coupling simulation model and conduct simulation runs to obtain the standard allowable current-carrying capacities of the cable at low frequency and power frequency, including the following steps: Set the structural parameters, material parameters, cable laying environment parameters and boundary conditions of the cable in the cable coupling simulation model; adjust the structural parameters, material parameters and cable laying environment parameters of the cable under the same voltage level; adjust the voltage level, cable laying environment parameters and cable material parameters under the condition of the same cable structural parameters; adjust the structural parameters, material parameters and voltage level of the cable under the same cable laying environment; adjust the voltage level, cable laying environment parameters and cable structural parameters under the condition of the same cable material parameters; Real-time collect the electrical effect parameters and thermal effect parameters around the cable, and solve the electrical effect parameters and thermal effect parameters to obtain the standard allowable current-carrying capacities of the cable at low frequency and power frequency.

4. A method for quickly calculating the low-frequency allowable current-carrying capacity of a cable according to claim 3, characterized in that: Solving the electrical effect parameters and thermal effect parameters to obtain the standard allowable current-carrying capacities of the cable at low frequency and power frequency includes the following steps: Set the cable operating frequency to low frequency, adjust the magnitude of the current inside the cable under the boundary conditions until the cable temperature is equal to the maximum boundary value of the boundary conditions, and the boundary conditions are the cable operating temperature range. Take the magnitude of the current inside the cable at this time as the standard allowable current-carrying capacity of the cable at low frequency; Set the cable operating frequency to power frequency, adjust the magnitude of the current inside the cable under the boundary conditions until the cable temperature is equal to the maximum boundary value of the boundary conditions, and take the magnitude of the current inside the cable at this time as the standard allowable current-carrying capacity of the cable at power frequency.

5. A method for quickly calculating the low-frequency allowable current-carrying capacity of a cable according to claim 1, characterized in that: In S3, based on the ratio of the standard allowable current-carrying capacities of the low-frequency and power-frequency of the same cable and combined with the current-carrying capacity correction equation, parameter fitting is performed to obtain the correction coefficient, including the following steps: Construct a current-carrying capacity correction equation based on the cross-sectional area of the cable conductor and the rated voltage during the operation of the cable, and add correction variables to the current-carrying capacity correction equation based on the influence degrees of the cross-sectional area of the cable conductor and the rated voltage on the current-carrying capacity; Use the ratio of the standard allowable current-carrying capacities of the low-frequency and power-frequency of the same cable as the fitting target of the current-carrying capacity correction equation to fit the correction variables, and use the correction variables corresponding to the optimal fitting result as the correction coefficient.

6. The rapid calculation method for the low-frequency allowable current-carrying capacity of a cable according to claim 1, wherein: In S4, substitute the correction coefficient into the current-carrying capacity correction equation to obtain the low-frequency current-carrying capacity calculation equation, and the formula is expressed as follows: λ = aA con 2 + bA con + cU0 + d I low = λ·I 50 where A con is the cross-sectional area of the conductor, U0 is the rated voltage during cable operation; a, b, c, and d are correction factors; λ is the ratio of the allowable current-carrying capacity of the cable at low frequency and power frequency, and I 50 is the allowable current-carrying capacity of the cable at power frequency obtained from the cable product manual or technical manual.

7. The rapid calculation method for the low-frequency allowable current-carrying capacity of a cable according to claim 1, wherein: In S4, substitute the coupling parameters of the target cable into the low-frequency current-carrying capacity calculation equation to obtain the low-frequency allowable current-carrying capacity of the target cable, including the following steps: Query the cross-sectional area of the cable conductor, the rated voltage during operation, and the power-frequency allowable current-carrying capacity of the target cable based on the product manual of the target cable and the relevant technical manual of the cable; Substitute the cross-sectional area of the cable conductor and the rated voltage during operation of the target cable into the low-frequency current-carrying capacity calculation equation to obtain the ratio of the low-frequency and power-frequency allowable current-carrying capacities of the target cable, and multiply the ratio of the low-frequency and power-frequency allowable current-carrying capacities of the target cable by the power-frequency allowable current-carrying capacity to obtain the low-frequency allowable current-carrying capacity of the target cable.

8. The rapid calculation method for the low-frequency allowable current-carrying capacity of a cable according to claim 2, wherein: The environment model is an environmental simulation data model composed of weather, buildings, temperature, and humidity; the cable model is a cable simulation data model composed of the cable model and the cable structure.

9. The rapid calculation method for the low-frequency allowable current-carrying capacity of a cable according to claim 3, wherein: The cable laying environment parameters at least include the burial depth, soil thermal resistance, air temperature, soil temperature, and soil surface heat transfer coefficient.

10. The rapid calculation method for the low-frequency allowable current-carrying capacity of a cable according to claim 3, wherein: The material parameters at least include relative permittivity, conductivity, thermal conductivity, density, and specific heat capacity.

Images