Combined type calandria single cable transient temperature rise acquisition method

Abstract

The application relates to a combined pipe single cable transient temperature rise acquisition method, which comprises the following steps: 1) cable core-skin models, skin-pipe wall models and pipe wall-environment models are respectively constructed, and parameter values related to the pipe single cable structure in the models are obtained through simulation solution; 2) at the current t moment, cable heat flow I1 is calculated according to cable current I, core-skin transient temperature rise Delta T1 and pipe wall transient temperature rise Delta T2 are respectively acquired according to the cable core-skin model and the pipe wall-environment model of the determined parameter values; 3) pipe wall temperature T 排管壁 at the current t moment is calculated according to the environment temperature, cable skin temperature T 外皮 is calculated according to the skin-pipe wall model, and cable core temperature at the current t moment is calculated according to the cable skin temperature T 外皮 . Compared with the prior art, the application has the advantages of rapid modeling and lean calculation.

Description

Technical Field

[0001] This invention relates to the field of temperature rise detection of cables in ductwork, and in particular to a method for obtaining transient temperature rise of a single cable in a combined ductwork system. Background Technology

[0002] Urban power grids, such as those in Beijing, Shanghai, Guangzhou, and Shenzhen, contain a large number of power cables. In areas with voltage levels of 110kV and below, especially within urban areas, direct burial and conduit laying are the primary methods of cable installation. Cable groups laid in conduits, due to their close proximity, experience significant heat transfer. Therefore, relatively conservative current-carrying capacity selection is generally adopted during the planning and design phase, with fixed values ​​given for different voltages and cross-sections, while the actual operating conditions of other cables within the conduit are rarely considered.

[0003] Due to the unique nature of power cable operation, it is generally impossible to obtain the core temperature of power cables through direct measurement. Therefore, technicians have proposed various methods to calculate the core temperature of power cables, all of which are engineering formulas or approximate formulas based on numerical solutions and experimental results. For example, the IEC 60287 standard is a classic method for calculating the core temperature of power cables, and there are also numerical calculation methods for determining current carrying capacity. However, similar studies mostly focus on single-circuit cables. For multi-circuit cables in actual operation, numerical methods are often used, but considering the complexity of the operating conditions, the required calculations are enormous, resulting in low efficiency in practical implementation. Therefore, the current of cables in duct groups is generally discounted uniformly based on the current of individual cables, without considering the actual load current of different cables, leading to a serious waste of cable and channel resources. The basis for rapid calculation of transient temperature rise of duct cable groups is rapid calculation of transient temperature rise of individual duct cables. From the research status of the above-mentioned cable steady-state temperature rise calculation analysis and actual operation needs, it can be seen that there is currently a lack of effective methods for rapid and lean calculation of transient temperature rise of individual cables adapted to duct laying to support the implementation of lean cable operation. Summary of the Invention

[0004] The purpose of this invention is to overcome the defects of the prior art and provide a method for obtaining the transient temperature rise of a single cable in a combined duct system.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] A method for obtaining transient temperature rise of a single cable in a combined duct system includes the following steps:

[0007] 1) Construct cable core-outer sheath model, outer sheath-pipe wall model, and pipe wall-environment model respectively, and obtain the parameter values ​​related to the structure of a single cable in the duct through simulation;

[0008] 2) At the current time t, calculate the cable heat flow I1 based on the cable current I, and obtain the transient temperature rise ∆T1 of the cable core-outer sheath and the transient temperature rise ∆T2 of the pipe wall based on the cable core-outer sheath model and the pipe wall-environment model with determined parameter values, respectively.

[0009] 3) Calculate the pipe wall temperature at time t based on the ambient temperature. The cable sheath temperature was calculated based on the sheath-tube wall model. And according to the cable sheath temperature Calculate the cable core temperature at time t.

[0010] In step 1), the cable core-outer sheath model includes four parallel sub-thermal circuits. The first sub-thermal circuit contains a cable heat flow I1, the second sub-thermal circuit contains a first thermal capacity C1, the third sub-thermal circuit contains a first thermal resistance R1 and a second thermal capacity C2 connected in series, and the fourth sub-thermal circuit contains a second thermal resistance R2 and a first thermal inductance L1 connected in series. The negative terminal of the cable heat flow I1 is the cable sheath temperature, which is grounded as a reference temperature. The positive electrode is the temperature of the cable core.

[0011] In the simulation to determine the parameter values ​​of the cable core-sheath model, based on the steady-state temperature rise between the core and sheath obtained from the simulation, the value of the second thermal resistance R2 is calculated, and then:

[0012] R2 = (Steady-state temperature of core - steady-state temperature of sheath) / heat flow of the fourth sub-circuit

[0013] L 1= C 2* R 1* R 2.

[0014] In step 1), the pipe wall-environment model includes four parallel sub-thermal paths. The fifth sub-thermal path contains a cable heat flow I1, the sixth sub-thermal path contains a third heat capacity C3, the seventh sub-thermal path contains a fourth thermal resistance R4 and a fourth heat capacity C4 connected in series, and the eighth sub-thermal path contains a fifth thermal resistance R5 and a second thermal inductor L2 connected in series. The negative terminal of the cable heat flow I1 is the ambient temperature, which is grounded as a reference temperature, and the positive terminal is the pipe wall temperature. .

[0015] In the simulation solution to determine the parameter values ​​of the pipe wall-environment model, based on the steady-state temperature rise between the pipe wall and the environment obtained from the simulation, the value of the fifth thermal resistance R5 is calculated, and then:

[0016] R5 = (Steady-state temperature of pipe wall - Ambient temperature) / Heat flow of the eighth sub-heat circuit

[0017] L 2= C 4*R 4* R 5.

[0018] In step 1), the internal thermal resistance of the pipe in the outer skin-pipe wall model is... The calculation formula is:

[0019]

[0020] in, The average temperature inside the conduit is the average of the conduit wall temperature and the cable sheath temperature. The outer diameter of the cable. U , V and Y These are the fixed parameters related to pipe laying.

[0021] In step 3), the pipe wall temperature at time t is... The transient temperature rise ∆T2 of the conduit wall is the sum of the ambient temperature, and the cable core temperature at time t is the cable sheath temperature. The sum of the transient temperature rise ∆T1 between the core and the outer sheath.

[0022] In step 3), the cable sheath temperature is calculated based on the sheath-tube wall model. Then we have:

[0023]

[0024] in, This is the heat flow passing through the outer skin.

[0025] In step 2), the formula for calculating the cable heat flux I1 is:

[0026] I1=I 2 *R

[0027] Where I is the cable current and R is the AC resistance related to the cable core temperature.

[0028] The method also includes the following steps:

[0029] 4) For time t+dt, calculate the AC resistance R at time t+dt based on the current cable core temperature at time t and update the cable heat flow I1 at time t+dt. Calculate the cable core temperature at time t+dt according to steps 2)-3) to complete the transient temperature rise of a single cable in the duct at each time.

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] To simplify the transient temperature rise modeling of cables in ducts, this invention divides the overall transient temperature rise model into three parts: "cable core-outer sheath," "outer sheath-duct wall," and "duct wall-environment," and models them separately. The "cable core-outer sheath" model is only related to the thermal characteristics of the cable itself, and each type of cable has a unique model. The "outer sheath-duct wall" model is only related to the outer diameter of the cable and the inner diameter of the duct. Similarly, the outer diameter of the cable and the inner diameter of the duct correspond to the same model. More importantly, since the air density is negligible, the heat capacity effect in this part is not obvious, and it only manifests as thermal resistance. This thermal resistance can be directly obtained by referring to relevant standards such as IEC, or it can be obtained through simulation calculation. The "duct wall-environment" model is only related to the thermal characteristics of the duct and the environment, and is relatively independent of the cable type, etc. Therefore, the modeling parameters are fewer and fixed, which can quickly and accurately obtain the transient temperature rise of the cables in ducts.

[0032] Second, through simulation examples, this invention has been verified that even with changes in cable type, cable current, thermal resistivity of the duct and soil, and burial depth of the duct, the model and modeling method still achieve good calculation results, with the maximum error being less than 1K, which can meet the needs of operation. Compared with previous modeling methods, the workload of this invention has been significantly reduced, making it more suitable for the promotion and application of the duct cable temperature rise model. Attached Figure Description

[0033] Figure 1 This is a lumped parameter model for the transient temperature rise of the pipe wall.

[0034] Figure 2 A lumped parameter model of transient temperature rise of cable core with wall temperature as the reference point.

[0035] Figure 3 The temperature rise model of a single cable in a duct is shown in Figure 3a, which is the temperature rise model of the core-outer sheath, Figure 3b is the temperature rise model of the outer sheath-duct wall, and Figure 3c is the temperature rise model of the duct wall-ambient environment.

[0036] Figure 4 This is the CYMCAP calculation model.

[0037] Figure 5 This refers to the transient temperature changes of the wire core, outer sheath, and pipe wall.

[0038] Figure 6 Comparison of the results of the temperature rise calculation using the "core-outer sheath" model and the CYMCAP calculation.

[0039] Figure 7 Comparison of the temperature rise calculation results of the "pipe wall-environment" model with those of CYMCAP.

[0040] Figure 8 Comparison of temperature rise calculation results from the "outer skin-pipe wall" model with those from CYMCAP.

[0041] Figure 9 Comparison of the results of transient temperature rise model calculation for cable core with those of CYMCAP calculation.

[0042] Figure 10 Comparison of the transient temperature rise model calculation results of the cable core and the CYMCAP calculation results in Example 1, which is applied to cable load changes.

[0043] Figure 11 Comparison of the transient temperature rise model calculation results of the cable core and the CYMCAP calculation results in Example 2, which is applied to cable load changes.

[0044] Figure 12 Comparison of the transient temperature rise model calculation results of cable cores and the CYMCAP calculation results in Example 1, which is used for the application of ductwork and environmental changes.

[0045] Figure 13 Comparison of the transient temperature rise model calculation results of cable cores and CYMCAP calculation results in Example 2, which is used for the application of ductwork and environmental changes.

[0046] Figure 14 This is a diagram showing the cable parameters.

[0047] Figure 15 Comparison of the transient temperature rise model calculation results of the cable core and the CYMCAP calculation results in Example 1, which is applied to the change of cable type.

[0048] Figure 16 Comparison of the transient temperature rise model calculation results of the cable core and the CYMCAP calculation results in Example 2, which is applied to the change of cable type.

[0049] Figure 17 This is a flowchart of the method of the present invention. Detailed Implementation

[0050] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0051] This invention provides a method for obtaining the transient temperature rise of a single cable in a combined ductwork system. By dividing the overall transient temperature rise model, it considers three parts for modeling: "cable core-outer sheath", "outer sheath-duct wall", and "duct wall-environment". It also realizes a calculation method for nonlinear correction of the transient influence of air convection in the ductwork at different temperatures, so as to complete the rapid modeling and refined calculation of the transient temperature rise of a single cable in the ductwork system.

[0052] The principle of this invention is as follows:

[0053] Currently, there are two types of temperature rise models for cables in ducts. One model integrates the cable core with the environment, where the thermal resistance between the cable sheath and the duct wall is represented by the average thermal resistance under multiple operating conditions. This model is characterized by its simplicity, low computational workload, and sufficient accuracy to meet operational requirements. The other model separately models the cable core-duct wall and the duct wall-environment relationship, where the thermal resistance between the cable sheath and the duct wall is represented by a variable thermal resistance. This model offers high computational accuracy but is more complex to implement. Figure 1 and 2 As shown.

[0054] However, regardless of the model used, the above modeling methods are all for specific "cable-conduit" combinations. For example, if there are 5 cable types, 4 conduit inner diameters, and 3 conduit forms, then 5*4*3=60 modeling operations are required. Figure 1 (The model shown) or 5*4+3=23 modeling iterations ( Figure 2 The model shown will require significantly more modeling work in practical applications, taking into account factors such as variations in soil thermal resistivity.

[0055] From the perspective of actual heat transfer, heat transfer consists of three parts: "cable core-outer sheath", "outer sheath-pipe wall", and "pipe wall-environment". The "cable core-outer sheath" model is only related to the thermal characteristics of the cable itself, and each type of cable corresponds to a unique model. The "outer sheath-pipe wall" model is only related to the outer diameter of the cable and the inner diameter of the pipe. Cables with the same outer diameter and pipe inner diameters correspond to the same model. More importantly, since the air density is negligible, the heat capacity effect in this part is not obvious, and it only manifests as thermal resistance. This thermal resistance can be directly obtained by referring to relevant standards such as IEC. The "pipe wall-environment" model is only related to the thermal characteristics of the pipe and the environment, and is relatively independent of the cable type.

[0056] The thermal resistance of the air portion inside the conduit changes with temperature, and needs to be corrected during lean calculations. The correction rule can be referred to the IEC standard, as shown in equation (1); it can also be obtained through numerical calculation. It should be noted that for the same combination of cable outer diameter and conduit inner diameter, the coefficients are fixed and are independent of parameters such as cable type, conduit structure, and thermal resistivity; compared with the IEC standard recommendation, numerical calculations can obtain better accuracy by solving for a single combination.

[0057] Thermal resistance inside the pipe The formula for calculating the parameters is:

[0058] (1)

[0059] in, The average temperature inside the conduit is the average of the conduit wall temperature and the cable sheath temperature. The outer diameter of the cable; U , Vand Y These are the fixed parameters related to pipe laying; please refer to the relevant IEC standards for details.

[0060] Cable sheath temperature Pipe wall temperature With the heat flow through the outer skin The relationship between them is:

[0061] (2)

[0062] Based on the above ideas, the temperature rise model for a single cable combination in a ductwork structure proposed in this invention is as follows: Figure 3 As shown, where The relationship between the changes can be obtained from the standard. Therefore, for the 5 cable models, 4 duct inner diameters, and 3 duct forms, the modeling is simplified to: 5 cable body simulations and 3 duct simulations, for a total of 8 modelings. Whether comparing the first or the second model, the modeling workload is significantly reduced, making it suitable for the widespread application of the duct cable temperature rise model.

[0063] Based on the above principles, the method of the present invention includes the following steps:

[0064] (1) Based on the cable thermal load, the transient temperature rise model of a single cable and the transient temperature rise model of the pipe wall, calculate the transient temperature rise ∆T1 of the core-outer sheath, the transient temperature rise ∆T2 of the pipe wall and the real-time heat flow Q0 flowing through L1 in Figure (3a);

[0065] (2) Transient temperature rise of pipe wall ∆T2 + ambient temperature = pipe wall temperature ;

[0066] (3) The cable sheath temperature can be obtained by solving the implicit function using equation (2). ;

[0067] (4) Utilizing the transient temperature rise ∆T1 of the conductor-outer sheath + cable sheath temperature =Cable core temperature;

[0068] (5) For the working conditions to be solved in actual operation, the specific details are as follows:

[0069] 1) Use the model shown in Figure (3c) to calculate the transient temperature rise of the pipe wall at time t, then use the model shown in Figure (3b) to calculate the transient temperature rise of the cable sheath-pipe wall at that time, and finally use the model shown in Figure (3c) to calculate the transient temperature rise of the cable core-sheath at that time.

[0070] 2) The sum of these three factors constitutes the transient temperature rise of the cable core relative to the ambient temperature at that moment;

[0071] 3) Calculate the cable heat flux I1=I at this moment based on the temperature rise. 2*R, where I is the cable current and R is the AC resistance related to the cable core temperature, combined with Figure 3 The transient data of each component is substituted into the calculation of the transient temperature rise of the pipe wall, the transient temperature rise of the cable sheath to the pipe wall, and the transient temperature rise of the cable core to the sheath at time t+dt, to form the transient temperature rise of the cable core to the ambient temperature and the cable loss at time t+dt, which is then substituted into time t+2*dt.

[0072] 4) Repeat this process until the time calculation ends.

[0073] Example

[0074] I. Example Explanation

[0075] 1) Simulation calculation

[0076] Basic data calculations were performed using CYMCAP software. Piping and cable configurations, as well as parameter settings, are as follows: Figure 4 As shown. Main verification Figure 3 The accuracy of the model shown and the feasibility of changing the cable type.

[0077] With an initial ambient temperature of 20℃, a step current of 300A was applied to the cable, resulting in a calculated heat load of 20.31W / m. The steady-state temperatures of the core, sheath, and conduit wall were 47.5℃, 40.1℃, and 31.2℃, respectively.

[0078] The initial ambient temperature was 20℃. A step current of 300A was applied to the cable, and the transient temperatures of the core, sheath, and conduit wall were obtained as follows: Figure 5 As shown.

[0079] 2) Modeling

[0080] 21) “Core-Sheath” Model

[0081] Based on the steady-state temperature rise between the core and the sheath, R2 = (47.5-40.1) / 20.31 = 0.3685 in Figure (3a). On this basis, a genetic algorithm is used to solve for other transition parameters C1, C2, R1, and L1 in Figure (3c). L1 = C2 * R1 * R2, and the results are as follows: C1 = 4.1266, C2 = 24.6952, R1 = 0.2337, R2 = 0.3685, L1 = 2.1176. A comparison of the model calculation results with the CYMCAP calculation results is shown below. Figure 6 As shown.

[0082] 22) Pipe wall-environment model

[0083] Based on the steady-state temperature rise between the pipe wall and the environment, R5 = (31.2-20) / 20.31 = 0.5510 in Figure (3c). On this basis, a genetic algorithm is used to solve for other transition parameters C3, C4, R4, and L2 in Figure (3a). L2 = C4 * R4 * R5, and the results are as follows: C3 = 302.0438, C4 = 2084.8826, R4 = 0.2388, R5 = 0.5510, L2 = 273.1285. The model calculation and CYMCAP calculation results are compared as follows: Figure 7 As shown.

[0084] 23) “Outer skin-pipe wall” model

[0085] The thermal resistance of the air portion inside the pipe changes with temperature, and needs to be corrected during lean calculation. The correction rule can be referred to the IEC standard, as shown in equation (1). In this example, D e The value is 102.3 mm, U is 1.87, V is 0.28, and Y is 0.0036.

[0086] Based on the real-time heat flow through the outer skin and R3 obtained from Figure (3a), the temperature rise process of the "outer skin-pipe wall" can be obtained, and compared with the CYMCAP calculation results as follows: Figure 8 As shown.

[0087] 24) Integrated Model

[0088] The combination of the above three models can yield the transient temperature process of the cable core, and can be directly compared with CYMCAP calculations, such as... Figure 9 As shown in Table 1, the error statistics are as follows.

[0089] Table 1. Comparison of Error Statistics between Transient Temperature Rise Model Calculation and CYMCAP Calculation Results for Cable Cores

[0090]

[0091] 3) Model Application

[0092] 31) Application of cable load variation

[0093] Example 1: Ambient temperature 20℃, applied step current 500A

[0094] Figure 3 The model shown calculates the transient temperature process of the cable core and compares it with the direct calculation by CYMCAP, as follows: Figure 10 As shown.

[0095] Example 2: Ambient temperature 20℃, changing current over 24 hours

[0096] The current is shown in Table 2. Figure 3The model shown calculates the transient temperature process of the cable core and compares it with the direct calculation by CYMCAP, as follows: Figure 11 As shown.

[0097] Table 2 Applied Current Meter

[0098]

[0099] 32) Application of Pipeline Installations in Response to Environmental Changes

[0100] Example 1: Pipe thermal resistance coefficient = 0.6, soil thermal resistance coefficient = 0.6, burial depth = 2 meters, ambient temperature 20℃, applied step current 300A.

[0101] Based on the aforementioned pipe wall temperature rise modeling method, using the pipe wall temperature rise data calculated by CYMCAP, the model parameters shown in Figure (3c) can be obtained, where C3=385.690, C4=4435.807, R4=0.170, R5=0.355, and L2=268.169. Keeping Figures (3a) and (3b) unchanged, we can obtain... Figure 3 The model shown calculates the transient temperature process of the cable core and compares it with the direct calculation by CYMCAP, as follows: Figure 12 As shown.

[0102] Example 2: Pipe thermal resistance coefficient = 0.7, soil thermal resistance coefficient = 1.1, burial depth = 1.6 meters, ambient temperature 20℃, applied step current 500A.

[0103] Based on the aforementioned pipe wall temperature rise modeling method, using the pipe wall temperature rise data calculated by CYMCAP, the model parameters shown in Figure (3c) can be obtained, where C3=441.695, C4=2761.547, R4=0.181, R5=0.495, and L2=248.275. Keeping Figures (3a) and (3b) unchanged, we can obtain... Figure 3 The model shown calculates the transient temperature process of the cable core and compares it with the direct calculation by CYMCAP, as follows: Figure 13 As shown.

[0104] 33) Application of cable type changes

[0105] Replace the cable, such as Figure 14 As shown. Where D e The value is 76mm, and the values ​​of U, V and Y remain unchanged.

[0106] Example 1: Ambient temperature 20℃, applied step current 400A (equivalent heat load 36.48W / m)

[0107] Figure 3 The model shown calculates the transient temperature process of the cable core and compares it with the direct calculation by CYMCAP, as follows: Figure 15As shown.

[0108] Example 2: Ambient temperature 20℃, changing current over 24 hours

[0109] The current is shown in Table 3. Figure 3 The model shown calculates the transient temperature process of the cable core and compares it with the direct calculation by CYMCAP, as follows: Figure 16 As shown.

[0110] Table 3 Applied Current Meter

[0111]

[0112] 34) Verification conclusion

[0113] The comparison shows that, under different working conditions, varying cable types, environmental parameters, and operating conditions, the maximum deviation between the proposed method for core temperature rise and the direct calculation by CYMCAP is less than 1K. This demonstrates the accuracy of the method proposed in this invention, which can fully meet the requirements for rapid cable modeling and lean calculation of transient temperature rise of operating cables.

Claims

1. A method for obtaining transient temperature rise of a single cable in a combined duct system, characterized in that, Includes the following steps: 1) Construct cable core-outer sheath model, outer sheath-pipe wall model, and pipe wall-environment model respectively, and obtain the parameter values ​​related to the structure of a single cable in the duct through simulation; 2) At the current time t, calculate the cable heat flow I1 based on the cable current I, and obtain the transient temperature rise ΔT1 of the cable core-outer sheath and the transient temperature rise ΔT2 of the pipe wall based on the cable core-outer sheath model and the pipe wall-environment model with determined parameter values, respectively. 3) Calculate the pipe wall temperature T at time t based on the ambient temperature. 排管壁 The cable sheath temperature T is calculated based on the sheath-tube wall model. 外皮 And based on the cable sheath temperature T 外皮 Calculate the cable core temperature at time t. In step 1), the cable core-outer sheath model includes four parallel sub-thermal circuits. The first sub-thermal circuit contains a cable heat flow I1, the second sub-thermal circuit contains a first thermal capacity C1, the third sub-thermal circuit contains a first thermal resistance R1 and a second thermal capacity C2 connected in series, and the fourth sub-thermal circuit contains a second thermal resistance R2 and a first thermal inductance L1 connected in series. The negative terminal of the cable heat flow I1 is the cable sheath temperature T, which is grounded as a reference temperature. 外皮 The positive electrode is the temperature of the cable core. In step 1), the pipe wall-environment model includes four parallel sub-thermal paths. The fifth sub-thermal path contains a cable heat flow I1, the sixth sub-thermal path contains a third heat capacity C3, the seventh sub-thermal path contains a fourth thermal resistance R4 and a fourth heat capacity C4 connected in series, and the eighth sub-thermal path contains a fifth thermal resistance R5 and a second thermal inductor L2 connected in series. The negative terminal of the cable heat flow I1 is the ambient temperature, which is grounded as a reference temperature, and the positive terminal is the pipe wall temperature T. 排管壁 ; In step 3), the current pipe wall temperature T at time t is... 排管壁 The transient temperature rise ΔT2 of the conduit wall is the sum of the ambient temperature, and the cable core temperature at time t is the cable sheath temperature T. 外皮 The sum of the transient temperature rise ΔT1 between the core and the outer sheath; In step 3), the cable sheath temperature T is calculated based on the sheath-tube wall model. 外皮 Then we have: Where Q0 is the heat flow through the outer skin.

2. The method for obtaining transient temperature rise of a single cable in a combined ductwork system according to claim 1, characterized in that, In the simulation to determine the parameter values ​​of the cable core-sheath model, based on the steady-state temperature rise between the core and sheath obtained from the simulation, the value of the second thermal resistance R2 is calculated, and then: L1 = C2 * R1 * R2 In the formula, T x T is the steady-state temperature of the wire core. w T4 represents the steady-state temperature of the outer skin, and T4 represents the heat flow of the fourth sub-thermal path.

3. The method for obtaining transient temperature rise of a single cable in a combined ductwork according to claim 1, characterized in that, In the simulation solution to determine the parameter values ​​of the pipe wall-environment model, based on the steady-state temperature rise between the pipe wall and the environment obtained from the simulation, the value of the fifth thermal resistance R5 is calculated, and then: L2 = C4 * R4 * R5 In the formula, T p T is the steady-state temperature of the pipe wall. h T1 represents the ambient temperature, and T8 represents the heat flow of the eighth sub-thermal circuit.

4. The method for obtaining transient temperature rise of a single cable in a combined ductwork according to claim 1, characterized in that, In step 1), the formula for calculating the internal thermal resistance R3 of the pipe in the outer skin-pipe wall model is as follows: Where, θ m The average temperature inside the conduit is the average of the conduit wall temperature and the cable sheath temperature, D. e U represents the outer diameter of the cable, while U, V, and Y are fixed parameters related to the ductwork.

5. The method for obtaining transient temperature rise of a single cable in a combined ductwork system according to claim 1, characterized in that, In step 2), the formula for calculating the cable heat flux I1 is: I1=I 2 *R Where I is the cable current and R is the AC resistance related to the cable core temperature.

6. The method for obtaining transient temperature rise of a single cable in a combined ductwork according to claim 1, characterized in that, The method also includes the following steps: 4) For time t+dt, calculate the AC resistance R at time t+dt based on the current cable core temperature at time t and update the cable heat flow I1 at time t+dt. Calculate the cable core temperature at time t+dt based on steps 2)-3) to complete the transient temperature rise of a single cable in the duct at each time.

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