A simulation method for the decomposition process of cable insulation materials coupled with a dynamic arc heat source

Abstract

The present invention relates to a method for simulating the decomposition process of cable insulation materials coupled with a dynamic arc heat source, comprising: constructing a cable dynamic arc heat source simulation model to calculate the electrical parameters of the arc; constructing an experimental platform for testing the physical parameters of carbonized products of the cable insulation materials to measure the relevant parameters of the thermal decomposition products of the cable insulation materials; constructing a cable insulation material combustion simulation model coupled with an arc heat source to calculate the temperature of the flame, the heat release rate over time, and the generation and flow of smoke during the combustion process; constructing a cable insulation material thermal decomposition process model coupled with an arc heat source and a flame heat source to simulate the decomposition of the cable insulation surface and the accumulation of carbonized materials under the high temperature of the arc; and constructing a cable thermal decomposition product distribution model under the action of airflow to simulate the movement and distribution of the cable thermal decomposition products under the airflow caused by the arc and flame. This method is advantageous for simulating the development of the cable arc and flame and the decomposition of the cable material.

Description

Technical Field

[0001] The present invention belongs to the field of power cables, and in particular relates to a method for simulating a decomposition process of cable insulation materials coupled with a dynamic arc heat source. Background Art

[0002] With the development of society, accelerated urbanization, and industrialization, the demand for electrical safety is becoming increasingly stringent. Power cables are a critical component of distribution networks and building electrical appliances. During operation, power cables are subject to hazards such as short circuits and overloads, which can potentially cause electrical fires. Currently, the insulation materials used in domestic cables are primarily hydrocarbons such as polyvinyl chloride (PVC) or cross-linked polyethylene (XLPE). These insulation materials are flammable. For example, PVC has a flash point of 120°C, generating 19,000-46,000 kJ of heat when burned. At temperatures exceeding 240°C, PVC undergoes significant chemical decomposition, releasing combustible gases such as HCl. At temperatures exceeding 470°C, PVC completely carbonizes. To increase the flash point of cables, flame retardants are often added to the insulation. However, prolonged exposure to high temperatures can still chemically decompose the cable insulation, losing its insulating properties and even producing noticeable flames.

[0003] When cables are energized for extended periods, arcing may occur at damaged insulation surfaces due to insulation breakdown, leakage current, and other factors. Arc temperatures typically exceed 3000°C, far exceeding the ignition point of the cable insulation. This arc can ignite the insulation, causing it to decompose and degrade, further exacerbating the arcing. In severe cases, this can lead to electrical fires and even endanger personnel safety. Furthermore, the decomposition of the insulation produces large amounts of harmful gases such as CO and HCl, as well as smoke, which can cause poisoning. Therefore, studying the decomposition of cable insulation materials under arc heat sources is crucial for preventing and fighting cable fires. Summary of the Invention

[0004] The object of the present invention is to provide a method for simulating the decomposition process of cable insulation materials coupled with a dynamic arc heat source, which is conducive to simulating the development of cable arcs and flames and the decomposition of cable materials.

[0005] To achieve the above object, the present invention adopts a technical solution: a method for simulating the decomposition process of cable insulation material coupled with a dynamic arc heat source, comprising the following steps:

[0006] Construct a cable dynamic arc heat source simulation model, simulate the arc development process based on magnetohydrodynamic theory, and calculate the dynamic characteristics of the arc's electrical, thermal, and airflow parameters;

[0007] Construct an experimental platform for testing the physical properties of carbonized products of cable insulation materials, and determine the electrical conductivity, thermal conductivity, heat release rate, and mass loss rate parameters of thermal decomposition products of cable insulation materials based on the tablet pressing method;

[0008] A cable insulation combustion simulation model coupled with an arc heat source was constructed. The arc temperature was used as the ignition source for the cable insulation surface combustion. A finite reaction rate combustion model was used to simulate the combustion process of the cable insulation surface. The flame temperature and heat release rate over time, as well as the generation and flow of smoke during the combustion process, were calculated.

[0009] A thermal decomposition model of cable insulation materials was constructed by coupling arc and flame heat sources. The temperatures generated by the coupled arc and flame heat sources were used to simulate the decomposition and carbonized material accumulation of the cable insulation surface under the action of high arc temperature through fluid-solid coupling, heat conduction, and the phased material changes of the cable insulation surface polymer material.

[0010] A distribution model of cable thermal decomposition products under airflow is constructed to simulate the movement and distribution of cable thermal decomposition products under the airflow caused by electric arc and flame.

[0011] Furthermore, the cable dynamic arc heat source simulation model is simulated as follows:

[0012] At the initial moment of the simulation, the temperature of the arc column area is initialized first, the conductivity σ of each arc unit is calculated according to the arc temperature, and then the current I in the loop is calculated;

[0013] Establish a cable electromagnetic field model, set the excitation source to a current excitation source with a magnitude of I, and solve the arc current density J at time 0;

[0014] A fluid field model of the cable is established to account for the fluid-solid coupling between the air, cable conductor, and cable insulation. Current density J and conductivity σ are loaded into the cable fluid field model, Joule heat W is calculated, and the momentum and energy conservation equations are solved to obtain the arc temperature T at time Δt.

[0015] Re-establish the cable electromagnetic field model, import the arc temperature T at time Δt into the cable electromagnetic field model, and calculate the arc current density J at time Δt;

[0016] Update the Joule heat W of the cable's fluid field model, solve the momentum conservation equation and the energy conservation equation, and obtain the arc temperature at time 2Δt;

[0017] Calculate according to the above logical process until the solution is completed; the cable arc temperature T at each moment in the solution process of step (1) is used as the heat source in steps (3) and (4), and is coupled to the models of steps (3) and (4) according to the time step.

[0018] Furthermore, the physical property parameter testing experimental platform of the cable insulation material carbonization product is measured according to the following method:

[0019] Obtain a black mixture powder of the cable pyrolysis product, mix it with PVP binder powder, grind it into a mass ratio of 1:2 to 1:3, place it in a tablet press, and press it into a disc-shaped sheet with a certain thickness;

[0020] The prepared disc-shaped sample is placed between flat-top electrodes with thermocouples inside to collect current, voltage and temperature data, and current is passed through them.

[0021] The samples were pretreated before the experiment, maintaining the set temperature for a certain period of time, which was lower than the pyrolysis temperature of PVC, to remove any moisture and volatile gases that may be present in the samples;

[0022] Then, multiple thermal cycles are performed. During the cycles, the current increases in equal values. Each set of current is maintained for a set time. The voltage and temperature of the sample corresponding to the current are recorded by the thermocouple.

[0023] Calculate the sample conductivity according to the following formula:

[0024]

[0025] Where σ is the conductivity, I is the current, h is the sample height, U is the voltage across the sample, and d is the sample diameter;

[0026] The thermal conductivity of the cable pyrolysis products is directly measured using a thermal conductivity meter;

[0027] The heat release rate and mass loss rate of cable insulation materials were measured using a thermogravimetric analyzer and a cone calorimeter.

[0028] Furthermore, the cable insulation material combustion simulation model coupled with the arc heat source is simulated as follows:

[0029] Based on computational fluid dynamics theory, a finite reaction rate combustion model is used to establish a model of the combustion process of cable insulation materials;

[0030] A geometric model of the cable was established based on its actual structure. The material parameters of the cable insulation layer were set according to the experimentally tested thermal conductivity, heat release rate, and mass loss rate of the insulation material as it changes with temperature. A cable dynamic arc heat source simulation model was used as the ignition source for the cable insulation material combustion process model. Based on heat transfer theory, the temperature distribution at each position of the power cable was calculated at each moment. If the temperature at a certain position was higher than the ignition point of the cable insulation material, a finite reaction rate combustion model was initiated.

[0031] According to the composition of the cable insulation material, calculate the components and mass fractions, and classify the combustible substances. The remaining substances are regarded as non-combustible substances;

[0032] According to the chemical equation of the combustion reaction, the combustion reaction products are determined, and the standard state molar formation enthalpy of the cable material combustion products is calculated according to the energy balance equation. The data of the flame temperature and heat release rate of the cable insulation layer changing with time during the combustion process are obtained by solving the equation;

[0033] Solve the momentum balance equation and calculate the flow of smoke during cable combustion.

[0034] Furthermore, the thermal decomposition process model of the cable insulation material coupled with the arc heat source and the flame heat source is simulated as follows:

[0035] The pyrolysis process of the cable insulation surface, whose main material is PVC polymer, is divided into three stages. The first stage is between 464K and 642K. The cable insulation surface material is decomposed by heat. The Cl on the PVC main chain is removed at high temperature and combines with H to release HCl gas, forming polyolefin chains, cycloalkanes and aromatic compounds. The second stage is between 691K and 808K, and HCl gas continues to be precipitated. The third stage is between 830K and 931K. The cable insulation surface is further decomposed into calcium carbonate and carbon black residues. The PVC material is divided into two parts: the Cl ion part and the other compound chains. part; the Cl ions are further divided into three parts: unstable Cl ion material, relatively stable Cl ion material and stable Cl ion material according to the three temperature stages of pyrolysis mentioned above; unstable Cl ions will precipitate HCl gas in the temperature stage of 464K-642K; relatively stable Cl ions will precipitate HCl gas in the temperature stage of 691K-808K; stable Cl ions will precipitate HCl gas in the temperature stage of 830K-931K; the unstable Cl ions, relatively stable Cl ions, stable Cl ions and other compound chains are evenly distributed on the grid of the cable insulation material according to percentage;

[0036] On the basis of step (1) and step (3), a thermal decomposition process model of cable insulation material is established; the temperature of the arc calculated by step (1) and the temperature of the flame calculated by step (3) are used as heat sources of the thermal decomposition process model of cable insulation material, and the heat conduction and temperature distribution inside the cable insulation skin under the action of the arc are calculated; when the temperature inside the insulation skin is between 464K and 642K in the first stage, it is considered that unstable Cl ions are removed and HCl gas is released at this time, and the mesh material allocated with unstable Cl ion material is replaced with air; when the temperature inside the insulation skin is between 691K and 808K in the second stage, the mesh material allocated with relatively stable Cl ion material is replaced with air; when the temperature inside the insulation skin is between 830K and 931K in the third stage, the mesh material allocated with stable Cl ion material is replaced with air; finally, the decomposition process of the insulating dielectric material and the accumulation of carbonized substances are obtained.

[0037] Furthermore, the distribution model of the thermal decomposition products of the cable under the action of the airflow is simulated as follows:

[0038] The Realizable k-ε turbulence model is used to establish a model according to the actual geometric structure of the cable; the arc in step (1) and the flame in step (3) are used as heat sources to solve the energy balance equation, momentum balance equation and mass balance equation of the gas in the cable gap, and the gas pressure and airflow velocity parameters in the cable gap are calculated;

[0039] The calculated gas pressure is used as the driving force to calculate the force acting on each pyrolysis product unit. Based on the dynamics theory, the acceleration, velocity and displacement motion parameters of the pyrolysis products under the action of airflow are obtained, thereby obtaining the distribution of the pyrolysis products under the action of airflow.

[0040] Compared with the existing technology, the present invention has the following beneficial effects: it provides a simulation method for the decomposition process of cable insulation materials coupled with a dynamic arc heat source. The simulation method can simulate the development of the cable arc and flame and the decomposition of the cable material, which serves as a basis for judging whether the fire caused by the arc in the cable will spread, and provides a reference basis for the selection of cable insulation materials, the layout of cables, the handling of cable fires and other issues. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Schematic diagram of the geometric structure of a cable sample according to an embodiment of the present invention.

[0042] Figure 2 2 is a schematic diagram of cable arc magnetohydrodynamic simulation in an embodiment of the present invention.

[0043] Figure 3 2 is a schematic diagram of cable arc simulation in an embodiment of the present invention.

[0044] Figure 4 Schematic diagram of a test sample for testing the physical parameters of the carbonized product of the cable insulation material in an embodiment of the present invention.

[0045] Figure 5 Schematic diagram of material changes during the cable carbonization process in an embodiment of the present invention.

[0046] Figure 6 Schematic diagram of simulation of thermal decomposition products of cable insulation material in an embodiment of the present invention.

[0047] Figure 7 This is a flowchart of a method implementation according to an embodiment of the present invention. DETAILED DESCRIPTION

[0048] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0049] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0050] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0051] The geometric structure of the cable sample in this embodiment is as follows: Figure 1 As shown. Figure 7 As shown, this embodiment provides a method for simulating the decomposition process of cable insulation material coupled with a dynamic arc heat source, comprising the following steps:

[0052] (1) Construct a cable dynamic arc heat source simulation model, simulate the arc development process based on magnetohydrodynamics theory, and calculate the dynamic characteristics of the arc's electrical, thermal, and airflow parameters.

[0053] (2) Construct an experimental platform for testing the physical properties of the carbonized products of cable insulation materials, and determine the parameters such as the electrical conductivity, thermal conductivity, heat release rate and mass loss rate of the thermal decomposition products of cable insulation materials based on the tablet pressing method.

[0054] (3) Construct a cable insulation material combustion simulation model coupled with an arc heat source, use the arc temperature as the ignition source for the combustion of the cable insulation surface, and use the finite reaction rate combustion model to simulate the combustion process of the cable insulation surface. Calculate the temperature of the flame during the combustion process, the heat release rate data that changes with time, and the generation and flow of smoke.

[0055] (4) Construct a thermal decomposition process model of cable insulation materials by coupling arc heat source and flame heat source. The temperature generated by the coupled arc heat source and flame heat source is simulated through fluid-solid coupling, heat conduction and the phased material changes of the cable insulation surface polymer material under the action of arc high temperature and the accumulation of carbonized materials.

[0056] (5) Construct a distribution model of cable thermal decomposition products under the action of airflow to simulate the movement and distribution of cable thermal decomposition products under the airflow caused by electric arc and flame.

[0057] (1) Constructing a dynamic arc heat source simulation model for cables

[0058] The simulation model of cable arc is constructed based on the theory of magnetohydrodynamics. Figure 2 In this embodiment, the cable arc simulation (cutting off a section where the arc occurs) is shown as follows. Figure 3 shown.

[0059] At the initial simulation moment, the arc column temperature is initialized to 3000K. The conductivity σ of each arc unit is calculated based on the arc temperature, and the current I in the loop is then calculated. A cable electromagnetic field model is established, and the excitation source is set to a current excitation source with a magnitude of I. The arc current density J at time 0 is calculated. A cable fluid field model is established to account for the fluid-structure interaction between the air, cable conductor, and cable insulation. The current density J and conductivity σ are applied to the cable fluid field model, the Joule heat W is calculated, and the momentum and energy conservation equations are solved to obtain the arc temperature T at time Δt.

[0060] Re-establish the cable electromagnetic field model, import the arc temperature T at time Δt into the cable electromagnetic field model, and calculate the arc current density J at time Δt. Update the Joule heat W of the cable fluid field model, solve the momentum conservation equation and energy conservation equation, and obtain the arc temperature at time 2Δt.

[0061] Calculate according to the above logical process until the solution is completed; the cable arc temperature T at each moment in the solution process of step (1) is used as the heat source in steps (3) and (4), and is coupled to the models of steps (3) and (4) according to the time step.

[0062] (2) Constructing an experimental platform for testing the physical properties of carbonized products of cable insulation materials

[0063] The parameters based on the pellet pressing method are used to determine the electrical conductivity, thermal conductivity, heat release rate and mass loss rate of the thermal decomposition products of cable insulation materials.

[0064] The black mixture powder of the cable pyrolysis product is obtained, mixed and ground with PVP binder powder in a mass ratio of 1:2 to 1:3, placed in a tablet press, and pressed into a round sheet with a certain thickness.

[0065] The prepared disc sample was placed between flat-top electrodes equipped with thermocouples that collected current, voltage, and temperature data. Current was then applied. The sample was preconditioned before the experiment by maintaining a temperature of 150°C for 10 minutes, below the pyrolysis temperature of PVC, to remove any moisture and volatile gases that might be present. The thermal cycle was then repeated five times, with the current increasing at equal values ​​during each cycle. Each current was maintained for 5 minutes, and the voltage and temperature of the sample corresponding to the current were recorded by the thermocouples.

[0066] Calculate the sample conductivity according to the following formula:

[0067]

[0068] Where σ is the conductivity, I is the current, h is the sample height, U is the voltage across the sample, and d is the sample diameter.

[0069] The thermal conductivity of the cable pyrolysis products was directly measured using a thermal conductivity meter. The sample preparation specifications were standard specimens with a diameter of 20 mm and a thickness of 2.8 mm.

[0070] The heat release rate and mass loss rate of cable insulation materials were measured using a thermogravimetric analyzer and cone calorimeter, according to ISO 5660 and ASTM E 1354. Six 10cm long cable sections were selected and arranged horizontally. The bottoms were filled with several layers of ceramic fiberboard for thermal insulation and surrounded by tin foil to form a single-sided heated specimen. The overall specimen dimensions were 10cm x 10cm x 9-10cm. The heat release rate and mass loss rate parameters of the cable materials were then determined.

[0071] The test samples of the physical parameters of the carbonized product of the cable insulation material in this embodiment are as follows: Figure 4 shown.

[0072] (3) Constructing a simulation model of cable insulation material combustion coupled with arc heat source

[0073] Based on the computational fluid dynamics theory, the finite reaction rate combustion model is adopted to establish a model of the combustion process of cable insulation materials.

[0074] A geometric model of the cable was established based on its actual structure. The material parameters of the cable insulation layer were set based on experimentally tested parameters such as the thermal conductivity, heat release rate, and mass loss rate of the insulation material as it changes with temperature. A dynamic arc heat source simulation model was used as the ignition source for the cable insulation material combustion process model. Based on heat transfer theory, the temperature distribution at each location on the power cable was calculated at each moment. If the temperature at a specific location exceeded the ignition point of the cable insulation material, a finite reaction rate combustion model was initiated.

[0075] According to the composition of the cable insulation material, calculate the components and mass fractions, and classify the combustible substances. The remaining substances are regarded as non-combustible substances.

[0076] Based on the chemical equation for the combustion reaction, the combustion reaction products are determined. The standard molar enthalpy of formation of the cable material combustion products is calculated based on the energy balance equation. The solution then obtains the time-varying data for the flame temperature and heat release rate of the cable insulation during combustion. The momentum balance equation is then solved to calculate the flow of smoke during cable combustion.

[0077] (4) Constructing a thermal decomposition process model of cable insulation materials with coupled arc heat source and flame heat source

[0078] The pyrolysis process of the cable insulation surface (mainly made of PVC polymer) is divided into three stages. The first stage is between 464K and 642K, during which the cable insulation surface material is thermally decomposed. The Cl on the PVC main chain is removed at high temperature and combines with H to release gases such as HCl, forming polyolefin chains, cycloalkanes and aromatic compounds. The second stage is between 691K and 808K, during which HCl gas continues to be precipitated. The third stage is between 830K and 931K, during which the cable insulation surface is further thermally decomposed into residues such as calcium carbonate and carbon black.

[0079] Considering the pyrolysis mechanism of PVC, the material is divided into two parts: Cl ion and other compound chains. Cl ions are further divided into unstable Cl ion material, relatively stable Cl ion material, and stable Cl ion material according to the three pyrolysis temperature stages mentioned above. Unstable Cl ions will release HCl gas in the 464K-642K temperature stage; relatively stable Cl ions will release HCl gas in the 691K-808K temperature stage; and stable Cl ions will release HCl gas in the 830K-931K temperature stage. Unstable Cl ions, relatively stable Cl ions, stable Cl ions, and other compound chains are evenly distributed on the grid of the cable insulation material according to percentage.

[0080] On the basis of step (1) and step (3), a thermal decomposition process model of cable insulation material is established; the temperature of the arc calculated by step (1) and the temperature of the flame calculated by step (3) are used as heat sources of the thermal decomposition process model of cable insulation material, and the heat conduction and temperature distribution inside the cable insulation skin under the action of the arc are calculated; when the temperature inside the insulation skin is between 464K-642K in the first stage, it is considered that the unstable Cl ions are removed and gases such as HCl are released, and the mesh material allocated with unstable Cl ion materials is replaced with air; when the temperature inside the insulation skin is between 691K-808K in the second stage, the mesh material allocated with relatively stable Cl ion materials is replaced with air; when the temperature inside the insulation skin is between 830K-931K in the third stage, the mesh material allocated with stable Cl ion materials is replaced with air; finally, the decomposition process of the insulating dielectric material and the accumulation of carbonized substances are obtained. The material changes in the cable carbonization process in this embodiment are as follows: Figure 5 In this embodiment, the thermal decomposition product of the cable insulation material is simulated as shown in FIG. Figure 6 shown.

[0081] (5) Construct a distribution model of cable thermal decomposition products under airflow

[0082] The thermal decomposition products of cable insulation are loose, porous substances. The high temperatures of the arc and flame create significant pressure within the gaps in the cable, intensifying the flow of gas within these gaps. Strong airflow can also move the thermal decomposition products of the cable insulation, changing their distribution.

[0083] The Realizable k-ε turbulence model is used to establish a model according to the actual geometric structure of the cable. The arc in step (1) and the flame in step (3) are used as heat sources to solve the energy balance equation, momentum balance equation and mass balance equation of the gas in the cable gap, and the gas pressure and airflow velocity parameters in the cable gap are calculated.

[0084] The calculated gas pressure is used as the driving force to calculate the force acting on each pyrolysis product unit. Based on the dynamics theory, the motion parameters such as acceleration, velocity and displacement of the pyrolysis products under the action of airflow are obtained, thereby obtaining the distribution of the pyrolysis products under the action of airflow.

[0085] The method of the present invention proposes a hybrid model of the cable insulation material combustion process that couples arc heat sources and flame heat sources. The finite reaction rate combustion model is used to establish the cable insulation material combustion process model, and the arc temperature is coupled to the cable insulation material combustion process model as an ignition source. According to the combustion reaction chemical equation, the temperature and heat release rate of the cable insulation layer during the combustion process are calculated as a function of time, as well as the generation and flow of smoke, thereby providing heat source calculation for the thermal decomposition simulation of the cable insulation medium.

[0086] This method constructs a model for the thermal decomposition of cable insulation. By assigning materials to the cable insulation mesh based on percentages, it combines the complex polymeric material of the cable insulation with the chemical reaction process. Coupled with arc and flame heat sources, the method simulates the dynamic changes in the cable insulation material and the accumulation of carbonized materials at different pyrolysis stages. The type and accumulation of thermal decomposition products can reflect the severity of a cable fire.

[0087] The method of the present invention proposes a distribution model of cable thermal decomposition products under an airflow field. The method adopts a Realizable k-ε turbulence model, couples arc heat sources and flame heat sources, and calculates parameters such as gas pressure and airflow velocity in the cable gap. Based on dynamic theory, the motion parameters and distribution of thermal decomposition products of cable insulation materials driven by airflow are solved. Based on the distribution of the thermal decomposition products of the cable, it is determined whether the arc is persistent, and thus whether the fire caused by the arc will spread. This method provides a reference basis for the selection of cable insulation materials, cable layout, and cable fire handling, laying a theoretical foundation for cable-related electrical fires.

[0088] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other manner. Any person skilled in the art may utilize the above-disclosed technical content to modify or modify the present invention into equivalent embodiments. However, any simple modifications, equivalent variations, and modifications to the above embodiments that do not depart from the technical content of the present invention and are based on the technical essence of the present invention remain within the scope of protection of the present invention.

Claims

1. A method for simulating the decomposition process of cable insulation material coupled with a dynamic arc heat source, characterized in that: The following steps are involved: (1) Construct a cable dynamic arc heat source simulation model, simulate the arc development process based on magnetohydrodynamic theory, and calculate the dynamic characteristics of the arc's electrical, thermal, and airflow parameters; (2) Construct an experimental platform for testing the physical properties of carbonized products of cable insulation materials, and determine the electrical conductivity, thermal conductivity, heat release rate, and mass loss rate parameters of thermal decomposition products of cable insulation materials based on the tablet pressing method; (3) Construct a cable insulation material combustion simulation model coupled with an arc heat source, use the arc temperature as the ignition source for the cable insulation surface combustion, and use a finite reaction rate combustion model to simulate the combustion process of the cable insulation surface. Calculate the temperature of the flame, the heat release rate over time, and the generation and flow of smoke during the combustion process. (4) Construct a thermal decomposition process model of cable insulation materials by coupling arc heat source and flame heat source. The temperature generated by the coupled arc heat source and flame heat source is used to simulate the decomposition and carbonized material accumulation process of the cable insulation surface under the action of arc high temperature through fluid-solid coupling, heat conduction and staged material changes of the cable insulation surface polymer material. (5) Construct a distribution model of cable thermal decomposition products under the action of airflow to simulate the movement and distribution of cable thermal decomposition products under the airflow caused by arc and flame; The cable dynamic arc heat source simulation model is simulated as follows: At the initial moment of the simulation, the temperature of the arc column area is initialized first, the conductivity σ of each arc unit is calculated according to the arc temperature, and then the current I in the loop is calculated; Establish a cable electromagnetic field model, set the excitation source to a current excitation source with a magnitude of I, and solve the arc current density J at time 0; Establish a fluid field model of the cable to consider the fluid-solid coupling between air, cable conductor and cable insulation surface; Load the current density J and conductivity σ into the fluid field model of the cable, calculate the Joule heat W, solve the momentum conservation equation and the energy conservation equation, and obtain the arc temperature T at time Δt; Re-establish the cable electromagnetic field model, import the arc temperature T at time Δt into the cable electromagnetic field model, and calculate the arc current density J at time Δt; Update the Joule heat W of the cable's fluid field model, solve the momentum conservation equation and energy conservation equation, and obtain the arc temperature at time 2Δt; Calculate according to the above logical process until the solution is completed; the cable arc temperature T at each moment in the solution process of step (1) is used as the heat source in steps (3) and (4), and is coupled to the models of steps (3) and (4) according to the time step; The physical parameter testing experimental platform for the carbonized product of the cable insulation material is measured in the following manner: Obtain a black mixture powder of the cable pyrolysis product, mix and grind it with PVP binder powder at a mass ratio of 1:2 to 1:3, place it in a tablet press, and press it into a disc-shaped sheet with a certain thickness; The prepared disc-shaped sample is placed between flat-top electrodes with thermocouples inside to collect current, voltage and temperature data, and current is passed through them. Before the experiment, the samples were pretreated and maintained at a set temperature for a certain period of time, which was lower than the pyrolysis temperature of PVC, to remove any moisture and volatile gases that may be present in the samples; Then, multiple thermal cycles are performed. During the cycles, the current increases in equal values. Each set of current is maintained for a set time. The voltage and temperature of the sample corresponding to the current are recorded by the thermocouple. Calculate the sample conductivity according to the following formula: Where σ is the conductivity, I is the current, h is the sample height, U is the voltage across the sample, and d is the sample diameter; The thermal conductivity of the cable pyrolysis products is directly measured using a thermal conductivity meter; The heat release rate and mass loss rate of cable insulation materials are measured using a thermogravimetric analyzer and a cone calorimeter. The cable insulation material combustion simulation model coupled with the arc heat source is simulated as follows: Based on computational fluid dynamics theory, a finite reaction rate combustion model is used to establish a model of the combustion process of cable insulation materials; A geometric model of the cable is established according to the actual structure of the cable; the material parameters of the cable insulation layer are set according to the thermal conductivity, heat release rate and mass loss rate parameters of the insulation material as the temperature changes obtained from the experimental test in step (2); the cable dynamic arc heat source simulation model of step (1) is used as the ignition source of the cable insulation material combustion process model, and the temperature distribution of each position of the power cable at each moment is calculated based on the heat transfer theory. If the temperature at a certain position is higher than the ignition point of the cable insulation material, the finite reaction rate combustion model is started; According to the composition of the cable insulation material, calculate the components and mass fractions, and classify the combustible substances. The remaining substances are regarded as non-combustible substances; According to the chemical equation of the combustion reaction, the combustion reaction products are determined, and the standard state molar formation enthalpy of the cable material combustion products is calculated according to the energy balance equation. The data of the flame temperature and heat release rate of the cable insulation layer changing with time during the combustion process are obtained by solving the equation; Solve the momentum balance equation and calculate the flow of smoke during cable combustion; The thermal decomposition process model of the cable insulation material coupled with the arc heat source and the flame heat source is simulated as follows: The pyrolysis process of the cable insulation surface, whose main material is PVC polymer, is divided into three stages. The first stage is between 464K and 642K. The cable insulation surface material is decomposed by heat. The Cl on the PVC main chain is removed at high temperature and combines with H to release HCl gas, forming polyolefin chains, cycloalkanes and aromatic compounds. The second stage is between 691K and 808K, and HCl gas continues to be precipitated. The third stage is between 830K and 931K. The cable insulation surface is further decomposed into calcium carbonate and carbon black residues. The PVC material is divided into two parts: the Cl ion part and the other compound chains. part; the Cl ions are further divided into three parts: unstable Cl ion material, relatively stable Cl ion material and stable Cl ion material according to the three temperature stages of pyrolysis mentioned above; unstable Cl ions will precipitate HCl gas in the temperature stage of 464K-642K; relatively stable Cl ions will precipitate HCl gas in the temperature stage of 691K-808K; stable Cl ions will precipitate HCl gas in the temperature stage of 830K-931K; the unstable Cl ions, relatively stable Cl ions, stable Cl ions and other compound chains are evenly distributed on the grid of the cable insulation material according to percentage; On the basis of step (1) and step (3), a thermal decomposition process model of cable insulation material is established; the temperature of the arc calculated by step (1) and the temperature of the flame calculated by step (3) are used as heat sources of the thermal decomposition process model of cable insulation material, and the heat conduction and temperature distribution inside the cable insulation skin under the action of the arc are calculated; when the temperature inside the insulation skin is between 464K and 642K in the first stage, it is considered that unstable Cl ions are removed and HCl gas is released at this time, and the grid material allocated with unstable Cl ion material is replaced with air; when the temperature inside the insulation skin is between 691K and 808K in the second stage, the grid material allocated with relatively stable Cl ion material is replaced with air; when the temperature inside the insulation skin is between 830K and 931K in the third stage, the grid material allocated with stable Cl ion material is replaced with air; finally, the decomposition process of the insulating dielectric material and the accumulation of carbonized substances are obtained; The distribution model of cable thermal decomposition products under the action of the airflow is simulated as follows: The Realizable k-ε turbulence model is used to establish a model according to the actual geometric structure of the cable; the arc in step (1) and the flame in step (3) are used as heat sources to solve the energy balance equation, momentum balance equation and mass balance equation of the gas in the cable gap, and the gas pressure and airflow velocity parameters in the cable gap are calculated; The calculated gas pressure is used as the driving force to calculate the force acting on each pyrolysis product unit; Based on the dynamics theory, the acceleration, velocity and displacement motion parameters of the thermal decomposition products under the action of airflow are obtained, thereby obtaining the distribution of the thermal decomposition products under the action of airflow.

Images