A polypropylene cable insulation layer thickness design method based on multi-laying working condition thermal simulation

Abstract

The application discloses a kind of based on multiple laying condition thermal simulation polypropylene cable insulation layer thickness design method, comprising: obtaining the thermal conductivity of polypropylene insulating material, and determining engineering safety insulation thickness;Respectively establish the thermal simulation model of single polypropylene cable of multiple laying condition, and the experimental measured load flow and each layer temperature data under the same condition are verified and corrected using simulation model;From the nominal thickness of crosslinked polyethylene cable insulation of same voltage grade, the insulation thickness is iteratively reduced in feasible region, to simultaneously satisfy the constraint condition that the maximum temperature of insulation layer under three laying modes does not exceed the long-term allowable operating temperature of polypropylene material, and determine the optimal insulation thickness.Compared with prior art, under the premise of ensuring safety margin, only insulation thickness parameter needs to be changed to realize the quantitative thinning design of polypropylene cable insulation thickness by iterative calculation, effectively reduce cable outer diameter and material cost, shorten design cycle.

Description

Technical Field

[0001] This invention belongs to the field of power cable design technology, specifically relating to a method for designing the insulation layer thickness of polypropylene cables based on thermal simulation of multiple laying conditions. It is particularly applicable to the optimized design of the insulation layer thickness of 110kV polypropylene insulated power cables operating in three laying environments: direct burial in soil, duct laying, and tunnel laying. Background Technology

[0002] Existing insulation thickness design methods have the following shortcomings:

[0003] First, the current design of polypropylene cable insulation thickness mainly references empirical data from cross-linked polyethylene cables of the same voltage rating. According to Table 2 of the national standard GB / T 11017.2 "110kV Cross-linked Polyethylene Insulated Power Cables and Accessories Part 2: Cables," the nominal insulation thickness of 110kV cross-linked polyethylene cables is 16-19mm (specifically depending on the nominal conductor cross-section; for example, a cross-section in the range of 630mm²-1600mm² results in a nominal insulation thickness of 16mm). Existing polypropylene cables in their initial development also mostly adopted an insulation thickness design of around 16mm. This "reference design" approach does not fully utilize the thermal properties of polypropylene materials for targeted optimization. Chinese patent publication CN118520715A discloses a method and system for calculating the current-carrying capacity of polypropylene DC cables based on equivalent operating conditions. It directly calculates the insulation thickness using the electrical aging index and accumulated electrical damage of the polypropylene material. The insulation thickness is a fixed input parameter in subsequent temperature field and current-carrying capacity calculations, and the temperature field calculation results do not participate in determining the insulation thickness. Once the thickness is determined by electrical aging, thermal simulation only verifies it. If the calculated current carrying capacity fails to meet the standard, the only recourse is to passively increase the conductor cross-sectional area or replace the material; it is impossible to optimize heat dissipation by adjusting the thickness (because the thicker the conductor, the greater the thermal resistance and the higher the temperature rise). Furthermore, it only applies to a single typical operating condition (or a simple weighted average) and cannot cope with wide-range fluctuating loads.

[0004] Secondly, cables may be laid in various environments in actual engineering projects, such as direct burial in soil, duct laying, and tunnel laying. The heat dissipation conditions differ significantly among different laying methods—ranked by environmental thermal resistance, direct burial in soil generally has the worst heat dissipation conditions due to the low thermal conductivity of the soil and limited heat dissipation surface; tunnel laying has the best heat dissipation conditions due to superior air convection; duct laying falls in between. The temperature rise behavior of the same insulation thickness varies significantly under different laying methods. Existing design methods typically only verify the most stringent laying method, failing to fully utilize the heat dissipation margin under other laying methods, resulting in excessive redundancy in insulation design, wasted materials, and increased cable outer diameter.

[0005] Third, traditional insulation thickness optimization design often requires a large number of electrical breakdown and mechanical performance tests to determine the safety lower limit, which results in a long research and development cycle and high cost.

[0006] Therefore, there is an urgent need for an insulation layer thickness design method based on thermal simulation data of multiple laying conditions. This method should comprehensively consider the electrical properties of polypropylene materials, existing engineering verification results, and engineering conservatism principles to determine a sufficiently safe engineering insulation thickness as the design boundary. This would allow R&D resources to be focused on thermal optimization simulation, thereby achieving rapid and low-cost quantitative design. Summary of the Invention

[0007] Objective of the Invention: To address the aforementioned deficiencies of the prior art as pointed out in the background section, this invention provides a method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions. Starting from the reference thickness of cross-linked polyethylene, the insulation layer is gradually thinned within a feasible range defined by the engineering safety thickness. Each time the thickness is reduced, the maximum current carrying capacity at that thickness is determined by resimulating the insulation layer temperature reaching a certain limit as a unified benchmark. By comparing the increase in current carrying capacity at each thickness and the gain differences between different laying methods, the optimal insulation thickness is determined.

[0008] Technical solution: This invention discloses a method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions, including:

[0009] S1. Obtain the thermal conductivity of polypropylene insulation material within the operating temperature range and establish a model relating thermal conductivity to temperature;

[0010] S2. Establish thermal simulation models of single polypropylene cables under three laying methods: direct burial in soil, pipe laying, and tunnel laying. Validate and correct the simulation models using experimentally measured current carrying capacity and temperature data of each layer under the same working conditions, ensuring that the deviation between the simulation and experimental data under each laying method is within the preset threshold.

[0011] S3. Starting with the polyethylene reference insulation thickness, within the safe and feasible region defined by the engineering safety insulation thickness and the reference insulation thickness, the insulation layer is gradually thinned. For each insulation thickness, temperature-current iterative simulation is used to determine the maximum current carrying capacity that allows the insulation layer temperature to reach the limit temperature under each laying method.

[0012] S4. Based on the saturation trend of current carrying capacity gain as thickness decreases, determine the optimal insulation thickness when the current carrying capacity gain from thinning tends to saturate or the thickness reaches the engineering safety insulation thickness.

[0013] Furthermore, when determining the optimal insulation thickness, the maximum current carrying capacity, the current carrying capacity increase rate, and the insulation thickness reduction percentage corresponding to each laying method are also obtained.

[0014] Furthermore, the thickness of the engineering safety insulation ranges from 8.5mm to 16mm.

[0015] Furthermore, the engineering safety insulation thickness is 13mm.

[0016] Furthermore, the single polypropylene cable model structure, from the inside out, includes: a copper conductor, a semiconductive wrapping tape, a conductor shielding layer, a polypropylene insulation layer, an insulation shielding layer, a semiconductive resistive water expansion buffer layer, a corrugated aluminum sheath, and an outer sheath; the cable's heat source is Joule heat generated by current flowing through the conductor.

[0017] Furthermore, the specific operation procedure for step S3 is as follows:

[0018] S31: Set the initial insulation thickness t0. In the simulation model of the three laying methods, set the insulation layer thickness to t0.

[0019] S32: Through temperature-current iterative simulation, determine the current value that makes the insulation layer reach the limit temperature under each laying method, and record it as the maximum current carrying capacity under each laying method, thereby determining the unified definition of current carrying capacity under this thickness;

[0020] S33: Reduce the insulation layer thickness by one step Δt, set the corresponding insulation layer thickness in the simulation models of the three laying methods, re-execute the temperature-current iterative simulation, and determine the new current carrying capacity that makes the insulation layer temperature reach the limit temperature under each laying method;

[0021] S34: Gradually decrease the insulation layer thickness and record the current carrying capacity corresponding to each thickness;

[0022] S35: Calculate the current carrying capacity increment between every two adjacent thicknesses. When the decrease in current carrying capacity increment relative to the previous increment exceeds a preset ratio, or when the insulation thickness has reached the engineering safety insulation thickness, stop the iteration and determine the optimal insulation thickness t. opt .

[0023] Furthermore, when determining the uniform current carrying capacity under a given thickness, the minimum current value required to bring the insulation layer to a specified temperature under the three laying methods is taken as the uniform current carrying capacity under that thickness.

[0024] Furthermore, the optimal insulation thickness t opt The criterion is that at this thickness, the increase in current carrying capacity compared to the reference thickness t0 is 8% to 12%, and the increase in current carrying capacity due to further thinning is lower than the aforementioned increase.

[0025] Furthermore, during the temperature-current iterative simulation, in the simulation model with a given insulation thickness, starting from the initial current value, the conductor current load is gradually increased, and the steady-state temperature field is solved each time until the highest temperature of the insulation layer reaches the limit temperature, and the deviation is within ±1℃. The limit temperature is set to 110℃.

[0026] Furthermore, when the insulation layer thickness is reduced, only the thickness parameters of the polypropylene insulation layer are changed, while the thicknesses of the copper conductor, semiconductive wrapping tape, conductor shielding layer, insulating shielding layer, semiconductive resistive water expansion buffer layer, corrugated aluminum sheath, and outer sheath remain unchanged. Only the inner diameter of each layer is reduced accordingly to match the outer diameter of the thinned polypropylene insulation layer, so that each layer forms a tight contact without gaps.

[0027] Beneficial effects:

[0028] 1. This invention uses the insulation layer temperature reaching a certain temperature (110℃ in this embodiment) as a unified benchmark, and successively thins the insulation layer and re-solves the maximum current carrying capacity. It incorporates the complete optimization chain of "thinning insulation → reducing thermal resistance → increasing current carrying capacity" into the iterative design process, providing a quantitative evaluation basis for the insulation thinning of polypropylene cables.

[0029] 2. This invention establishes thermal simulation models for three laying methods and verifies them bidirectionally using experimental data under identical working conditions. All subsequent iterative calculations are based on these experimentally verified reliable models, and the results have high engineering reliability.

[0030] 3. This invention takes the temperature constraints of three laying methods as a condition to simultaneously meet them. When determining the maximum current carrying capacity, the minimum value among the laying methods is taken as the unified current carrying capacity under the insulation thickness, so as to ensure that the cable can operate safely in any laying environment.

[0031] 4. The successive reduction of insulation thickness and the determination of the maximum current carrying capacity at each thickness are completed entirely in the simulation environment. There is no need to manufacture cable samples of various thicknesses and conduct current carrying capacity temperature rise experiments separately, which greatly reduces the research and development cost and test cycle.

[0032] 5. Through multi-condition comparative simulation, this invention can quantitatively reveal the significant differences in the improvement of current carrying capacity due to insulation thinning under different laying methods. The improvement is greatest under direct burial in soil with the worst heat dissipation conditions. This discovery can provide a more accurate solution to the bottleneck of power transmission capacity for long-distance lines with mixed laying, and has stronger engineering adaptability. Attached Figure Description

[0033] Figure 1 This is an overall flowchart of the method of the present invention;

[0034] Figure 2This is a graph showing the relationship between insulation thickness and maximum current carrying capacity under various laying methods.

[0035] Figure 3 This is a graph showing the trend of current carrying capacity increment as insulation thickness decreases;

[0036] Figure 4 The graph shows a comparison and verification of simulation and experimental temperature data under three laying methods.

[0037] Figure 5 This is a schematic diagram of the cable structure of the present invention. Detailed Implementation

[0038] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0039] Example: Optimization of insulation thinning for 110kV polypropylene insulated power cables

[0040] S1: Obtain the thermal property parameters of polypropylene insulation material

[0041] The thermal conductivity of the polypropylene insulation material was experimentally determined at multiple temperature points within the range of 30℃ to 110℃ to obtain the thermal conductivity of the polypropylene insulation material within the operating temperature range. This embodiment employs the steady-state heat flow method, conducting tests at 30℃, 50℃, 70℃, 90℃, and 110℃, and obtaining the thermal conductivity-temperature characteristic curve through data fitting. Simultaneously, considering the electrical insulation properties of the polypropylene material, the withstand voltage requirements of the 110kV voltage level, and the reliability requirements of engineering applications, the engineering safety insulation thickness T of this design method was determined. min This serves as an insurmountable lower limit for thickness in subsequent thermal optimization iterations.

[0042] Engineering safety insulation thickness T min = 13mm. The determination is based on the following: Existing literature indicates that 110kV polypropylene cables with 16mm insulation thickness have passed power frequency margin and impulse margin voltage tests, and have a large electrical margin, allowing for thinning. In the medium-voltage field, the electrical reliability of polypropylene cable insulation thickness reduced to 8.5mm has been verified, providing a reference for high-voltage thinning. However, the safety margin systems for medium-voltage and 110kV high-voltage are different and cannot be directly transferred equivalently. Considering the higher withstand voltage requirements of high-voltage levels, batch differences in materials, manufacturing process deviations, and long-term operational aging uncertainties, this method preferentially selects T. min = 13mm. This value falls between the verified value for medium voltage and the current nominal thickness of XLPE, utilizing the performance advantages of polypropylene to achieve an insulation reduction of approximately 18.75% while leaving sufficient engineering safety margin. Simultaneously, this thickness allows for a transition from a 16mm base thickness to T...min The iterative process has sufficient simulation data points (3 iteration points in 1mm increments), which can fully demonstrate the decreasing law and saturation trend of current carrying capacity gain.

[0043] S2: Establish thermal simulation models for three laying methods and conduct experimental verification.

[0044] Using finite element simulation software, thermal simulation models of single polypropylene cables were established for three laying methods: direct burial in soil, pipe laying, and tunnel laying. The cable model structure is shown in [reference needed]. Figure 5 From the inside out, the structure consists of: a copper conductor (outer diameter 48.5 mm), a semi-conductive wrapping tape (0.64 mm), a conductor shielding layer (thickness 1.5 mm), a polypropylene insulation layer (initial thickness 16 mm), an insulating shielding layer (thickness 1.0 mm), a semi-conductive resistive water layer (thickness 5.0 mm), a metallic shielding layer (thickness 2.3 mm), and an outer sheath (thickness 5.0 mm), which is completely consistent with the experimental cable sample. See Table 1 below:

[0045] Table 1 Cable Model Structure and Parameter Settings

[0046] 1 copper conductor 48.5±0.8 2 Semiconducting wrapping tape 0.64 49.8 3 Conductor shielding layer 1.5 52.8 4 Polypropylene insulation layer 16.0 84.8 5 Insulating shielding layer 1.0 86.8±1.5 6 Semiconductor resistive water expansion buffer layer 5.0 96.8 7 Wrinkled aluminum sheath 2.3 110.2 8 outer sheath 5.0 120.6±3.0

[0047] Environmental boundary conditions for each laying method: For direct burial in soil, the soil thermal conductivity is 1 W / (m²·K), the deep ground temperature is 15℃, and the burial depth is 1.0m; for duct laying, polyvinyl chloride ducts are added, the duct material has a thermal conductivity of 0.16 W / (m²·K), the heat transfer coefficient of the outer sheath surface and the inner surface of the duct is 3 W / (m²·K), and the surface is provided with radiative heat conduction with an emissivity of 0.95; for tunnel laying, the tunnel cross-section is 3.0m×3.0m, the cable is laid horizontally on the support, under natural convection conditions, and the inflow air temperature is 25℃.

[0048] The simulation model was verified and corrected using experimentally measured current-carrying capacity and temperature data of each floor under the same operating conditions, ensuring that the deviation between the simulation and experimental data for each laying method was within a preset threshold. (See [link to relevant documentation]). Figure 4 By comparing the experimentally measured temperatures of each layer under the same working conditions with the simulation results, the deviations under the three laying methods were all within 5%, and the model verification was qualified.

[0049] S3: Successively thin the insulation layer and iteratively solve for the maximum current carrying capacity at each thickness.

[0050] S31: Set reference thickness

[0051] Starting from the cross-linked polyethylene (XLPE) reference insulation thickness, the initial insulation thickness t0 = 16mm. According to national standard GB / T 11017.2, for 110kV XLPE cables with a conductor nominal cross-section in the range of 800mm² to 2000mm², the nominal insulation thickness is 16mm.

[0052] S32: Solve for the maximum current carrying capacity at the reference thickness.

[0053] In the simulation model with an insulation thickness of 16mm, temperature-current iterative simulations were performed for three different laying methods: starting from the initial current value, the conductor current was gradually increased, and the steady-state temperature field was solved each time until the highest temperature of the insulation layer reached the limit temperature of 110℃ (within ±1℃).

[0054] Through iterative calculations, the current values ​​required to raise the insulation layer temperature to 110℃ under various laying methods are as follows:

[0055] Direct burial in soil: 2200A

[0056] Pipe laying: 2050A

[0057] • Tunnel laying: 2295A

[0058] The minimum value among the three, the unified definition of current carrying capacity I corresponding to an insulation thickness of 16mm. max (16mm) = 2050A. This value means that the cable, with an insulation thickness of 16mm, can safely carry a current of 2050A regardless of the laying environment.

[0059] S33-S35: Successively thin the material and solve for the current carrying capacity corresponding to each thickness.

[0060] The insulation thickness was gradually reduced in increments of Δt = 1 mm (during thinning, only the thickness of the polypropylene insulation layer was changed, while the thicknesses of other layers remained unchanged; only the inner diameter of each layer was reduced accordingly to match the outer diameter of the thinned polypropylene insulation layer, ensuring tight contact between layers without gaps). Temperature-current iterative simulations were re-executed for each thickness to obtain the maximum current carrying capacity under each laying method. The minimum value among the three laying methods was taken as the unified current carrying capacity corresponding to that thickness. The iterative data is recorded in Table 2 below:

[0061] Table 2. Uniformly defined current carrying capacity for different thicknesses.

[0062] 16mm 2200A 2050A 2295A 2050A benchmark — 15mm 2210A 2062A 2303A 2062A +12A 0.59% 14mm 2219A 2073A 2310A 2073A +11A 1.12% 13mm 2227A 2083A 2316A 2083A +10A 1.61%

[0063] S36: Determine the termination condition of the iteration

[0064] A comprehensive analysis was conducted on the gain saturation trend of current carrying capacity as thickness decreases, as well as the differences in gain amplitude between different laying methods, until the current carrying capacity gain brought about by thickness reduction tends to saturate or the thickness reaches T. min The optimal insulation thickness is determined. An optimized insulation thinning scheme is output, including the optimal insulation thickness value, the maximum current-carrying capacity value corresponding to each laying method, the current-carrying capacity increase rate, and the percentage of insulation thickness reduction.

[0065] Calculate the current carrying capacity increment ΔI between every two adjacent thicknesses. max (t i When ΔI max (t i The iteration stops when the attenuation relative to the previous increment exceeds a preset proportion (e.g., the increment drops below 20% of the initial increment), or when the insulation thickness reaches the minimum thickness specified for electrical safety. Optimal insulation thickness t opt The criterion is that at this thickness, the current carrying capacity is significantly increased compared to the reference thickness t0 (usually by 8% to 12%), and the increase in current carrying capacity due to further thinning is no longer significant.

[0066] When the insulation thickness is reduced to 13.0 mm, the engineering safety insulation thickness T determined in step S1 has been achieved. min As shown in the table, within the thinning range of 16mm to 13mm, the increase in current carrying capacity per step exhibits a stable decreasing trend: the initial thinning (16→15mm) increment is 12A, gradually decreasing to 11A and 10A thereafter. Each increment is greater than 20% of the initial 12A increment (i.e., 2.4A), indicating that the thermal gain has not yet fully saturated, and 13mm remains within the effective range for thinning benefits. Continuing to thin may still yield a slight gain, but the safety margin will be continuously compressed. Stopping iteration at 13mm achieves a quantifiable increase in current carrying capacity while preserving sufficient engineering safety boundaries.

[0067] S37: Determine the optimal insulation thickness

[0068] Analysis of the simulation data yields two key conclusions:

[0069] First, the gain reduction pattern under a single operating condition. Within the safe and feasible range of [13mm, 16mm], the current carrying capacity increases monotonically with decreasing insulation thickness. The current carrying capacity is uniformly defined as increasing from 2050A to 2083A, with a total increase of approximately 1.61%. The gain magnitude shows a regular decreasing trend (12A→11A→10A per step), without a precipitous drop, indicating that 13mm is still within the effective range for thermal improvement.

[0070] Second, the gain varies significantly under different operating conditions. At a safety thickness of 13mm, compared to the 16mm baseline, the current carrying capacity increase for each laying method is as follows:

[0071] • Piping installation: Upgraded from 2050A to 2083A, an increase of 1.61%.

[0072] • Direct burial in soil: Upgraded from 2200A to 2227A, an increase of 1.23%.

[0073] • Tunnel laying: Upgraded from 2295A to 2316A, an increase of 0.92%.

[0074] It is evident that the gain is most pronounced in the duct laying environment with the worst heat dissipation conditions, while the gain is smallest in the tunnel environment with the best heat dissipation conditions. This pattern confirms that insulation thinning has engineering value in improving power transmission bottlenecks in harsh laying environments.

[0075] Based on comprehensive judgment, the optimal insulation thickness t is determined. opt = 13mm. This thickness is the engineering safety thickness T. min The optimal solution within the safe and feasible domain simultaneously satisfies all of the following conditions: ① The current carrying capacity is increased by 1.61% compared to the 16mm baseline, the gain is quantifiable and has engineering significance; ② The amount of insulation material used is reduced by about 18.75%, and the cable outer diameter is significantly reduced; ③ Sufficient electrical and mechanical safety margins are provided to fully cover engineering uncertainties such as batch differences in materials, manufacturing process deviations, and long-term aging.

[0076] It should be noted that this embodiment uses a large cross-section conductor of 1800mm², where the thermal resistance of the insulation layer accounts for a relatively small proportion of the total heat dissipation path, thus the current carrying capacity gain is relatively limited (1.61%). For smaller cross-section cables such as 800mm², the thermal resistance of the insulation layer accounts for a larger proportion, and the benefits of thinning will be more significant. This method is universal and applicable to the optimization design of cable insulation thickness for various cross-section specifications.

Claims

1. A method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions, characterized in that, include: S1. Obtain the thermal conductivity of polypropylene insulation material within the operating temperature range and establish a model relating thermal conductivity to temperature; S2. Establish thermal simulation models of single polypropylene cables under three laying methods: direct burial in soil, pipe laying, and tunnel laying. Validate and correct the simulation models using experimentally measured current carrying capacity and temperature data of each layer under the same working conditions, ensuring that the deviation between the simulation and experimental data under each laying method is within the preset threshold. S3. Starting with the polyethylene reference insulation thickness, within the safe and feasible region defined by the engineering safety insulation thickness and the reference insulation thickness, the insulation layer is gradually thinned. For each insulation thickness, temperature-current iterative simulation is used to determine the maximum current carrying capacity that allows the insulation layer temperature to reach the limit temperature under each laying method. S4. Based on the saturation trend of current carrying capacity gain as thickness decreases, determine the optimal insulation thickness when the current carrying capacity gain from thinning tends to saturate or the thickness reaches the engineering safety insulation thickness.

2. The method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions according to claim 1, characterized in that, When determining the optimal insulation thickness, output an insulation thinning optimization scheme, including the maximum current carrying capacity value, current carrying capacity increase rate, and insulation thickness reduction percentage for each laying method.

3. The method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions according to claim 1, characterized in that, The thickness of the safety insulation in the project ranges from 8.5mm to 16mm.

4. The method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions according to claim 3, characterized in that, The engineering safety insulation thickness is 13mm.

5. The method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions according to claim 1, characterized in that, The single polypropylene cable model structure, from the inside out, includes: a copper conductor, a semi-conductive wrapping tape, a conductor shielding layer, a polypropylene insulation layer, an insulation shielding layer, a semi-conductive resistive water expansion buffer layer, a corrugated aluminum sheath, and an outer sheath; the cable's heat source is Joule heat generated by current flowing through the conductor.

6. The method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions according to claim 5, characterized in that, The specific operation procedure for step S3 is as follows: S31: Set the initial insulation thickness t0. In the simulation model of the three laying methods, set the insulation layer thickness to t0. S32: Through temperature-current iterative simulation, determine the current value that makes the insulation layer reach the limit temperature under each laying method, and record it as the maximum current carrying capacity under each laying method, thereby determining the unified definition of current carrying capacity under this thickness; S33: Reduce the insulation layer thickness by one step Δt, set the corresponding insulation layer thickness in the simulation models of the three laying methods, re-execute the temperature-current iterative simulation, and determine the new current carrying capacity that makes the insulation layer temperature reach the limit temperature under each laying method; S34: Gradually decrease the insulation layer thickness and record the current carrying capacity corresponding to each thickness; S35: Calculate the current carrying capacity increment between every two adjacent thicknesses. When the decrease in current carrying capacity increment relative to the previous increment exceeds a preset ratio, or when the insulation thickness has reached the engineering safety insulation thickness, stop the iteration and determine the optimal insulation thickness t. opt .

7. The method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions according to claim 6, characterized in that, When determining the uniform current carrying capacity for a given thickness, the minimum current value required to bring the insulation layer to the specified temperature under the three laying methods is taken as the uniform current carrying capacity for that thickness.

8. The method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions according to claim 6, characterized in that, The optimal insulation thickness t opt The criterion is that at this thickness, the increase in current carrying capacity compared to the reference thickness t0 is 8% to 12%, and the increase in current carrying capacity due to further thinning is lower than the aforementioned increase.

9. The method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions according to claim 6, characterized in that, During the temperature-current iterative simulation, in the simulation model with a given insulation thickness, starting from the initial current value, the conductor current load is gradually increased, and the steady-state temperature field is solved each time until the highest temperature of the insulation layer reaches the limit temperature, and the deviation is within ±1℃. The limit temperature is set to 110℃.

10. The method for designing the insulation layer thickness of polypropylene cables based on thermal simulation under multiple laying conditions according to claim 6, characterized in that, When the insulation layer thickness is reduced, only the polypropylene insulation layer thickness parameter is changed. The thicknesses of the copper conductor, semiconductive wrapping tape, conductor shielding layer, insulating shielding layer, semiconductive resistive water expansion buffer layer, corrugated aluminum sheath, and outer sheath remain unchanged. Only the inner diameter of each layer is reduced accordingly to match the outer diameter of the thinned polypropylene insulation layer, so that each layer forms a tight contact without gaps.

Images