Low-voltage line insulation damage early fault modeling method

Abstract

The invention discloses a low-voltage line insulation damage early fault modeling method, and relates to the technical field of low-voltage line fault simulation, and the method comprises the steps: S1, selecting a solving space dimension, a physical field interface and a research solver; s2, establishing a geometric model, setting material physical property parameters, boundary conditions and initial conditions, and dividing grids; s3, setting solving time and output step length; s4, solving, calculating and judging whether a solving result is reasonable or not, if not, returning to S2, and if yes, entering S5; and S5, analyzing a simulation calculation result. According to the invention, accurate positioning and intelligent diagnosis of early faults of equipment can be realized, and the reliability and stability of a power distribution network are improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of low-voltage line fault simulation, in particular to a low-voltage line insulation damage early fault modeling method. BACKGROUND

[0002] As the key hub of the "last mile" of the power system, the distribution equipment bears the core functions of power distribution, voltage regulation and safe transmission, and its operation state directly affects the power experience of end users and the stability of social and economic activities. Under the background of large-scale development and utilization of electric power, the rapid change of the topology structure of the distribution network brought by the high proportion of new energy access makes the "fault pre-warning-active intervention" full-cycle prevention and control system an important measure to improve power supply reliability, and an important breakthrough to expand the fault prevention system and realize active early warning.

[0003] The low-voltage line is a key component of the urban distribution network and bears the core task of power transmission. However, the low-voltage line in China has long been plagued by a lack of maintenance resources. As the operation period increases, it is continuously affected by mechanical stress compression, damp environment erosion, natural aging and other factors, and the insulation layer is easily damaged, thereby causing early fault arcs and developing into permanent faults after multiple discharges. Compared with other types of faults, insulation damage type faults have the characteristics of large current and high heat, which brings great safety hazards.

[0004] For low-voltage lines, traditional periodic inspection methods such as infrared thermal imaging, partial discharge detection and unmanned aerial vehicle inspection can effectively find fault hazards, but the operation and maintenance cost is high and it is difficult to cover all the time and all the areas. Moreover, the low-voltage line involves a large number of devices of various types and complex structures, and the environment, causes and mechanisms of early faults of different devices are very different, and the voltage and current signals generated have different characteristics.

[0005] Therefore, how to research the insulation damage type early fault arc in cable fault, realize accurate positioning and intelligent diagnosis of device early fault, and improve the reliability and stability of the distribution network is a technical problem that those skilled in the art need to solve. SUMMARY

[0006] Therefore, the application provides a low-voltage line insulation damage early fault modeling method, which solves the problems in the background art.

[0007] In order to achieve the above purpose, the application provides the following technical scheme: A low-voltage line insulation damage early fault modeling method, comprising the following steps: S1, selecting a solution space dimension, a physical field interface and a research solver; S2, a geometric model is established, material physical property parameters, boundary conditions and initial conditions are set, and a grid is divided; S3, a solving time and an output step length are set; S4, a calculation is solved, and whether the solving result is reasonable is judged, if not, returning to S2, if yes, entering S5; S5, a simulation calculation result is analyzed.

[0008] Optionally, S1 is specifically: Considering that the research object is an early fault arc of an insulation damaged low-voltage cable, the arc evolution process of the cable insulation damage fault arc is similar at different sections, and therefore, a two-dimensional simulation modeling is adopted; A temperature field, a flow field, an electric field and a magnetic field are set, a temperature field and a flow field are coupled through a non-isothermal flow module, a temperature field and an electric field are coupled by means of a balanced discharge heat source module and an electromagnetic heat module, and a magnetic field and a flow field are coupled by using a magneto-hydrodynamics module; Based on the arc simulation of the magneto-hydrodynamics theory, the macroscopic evolution and development process of the arc in a certain time scale is focused, and therefore, a transient mode is selected for solving, and a separation solving strategy is adopted.

[0009] Optionally, the temperature field, the flow field, the electric field and the magnetic field are set, and specifically: A current field calculation model is constructed through a current module, a spatial distribution characteristic of the current density in the arc region is solved, based on a Maxwell equation set, a conductivity parameter of the arc plasma is coupled with an electromagnetic field control equation, and a conduction path of the current in the cable conductor, the insulation defect gap and the air medium is quantitatively characterized; A magnetic field module is used to finely model and numerically calculate the magnetic field of the arc and the space around the arc, and a spatial distribution characteristic and a vector direction of the magnetic field strength are obtained; A laminar flow module is used to simulate and describe the laminar flow of the arc plasma, and a flow velocity, a flow direction and a velocity distribution of the plasma are obtained; A fluid heat transfer module is used to calculate a heat transfer process between the arc plasma and the surrounding environment, and a temperature distribution of the arc and a heat propagation in space are obtained.

[0010] Optionally, in S2, the geometric model is established, and specifically: An insulation damage type fault cable is prepared, and a basic shape is created by using a parameterized geometry tool; Based on the relative relationship between the damage position and the cable body, the insulation damage type is divided into a center insulation layer damage and a side insulation layer damage; The insulation material region of the geometric structure is integrated, regarded as a unified physical region, and the outer insulation tape is ignored, a separate region is set in the main arc occurrence area, and a simplified geometric structure of the insulation damage type fault cable is obtained.

[0011] Optionally, in S2, the material physical property parameters include specific heat capacity, electrical conductivity, density, and thermal conductivity; wherein the physical property parameters of copper and polyvinyl chloride are set as fixed constants, and the physical property parameters of air are derived from a gas discharge plasma basic database.

[0012] Optionally, in S2, the setting of the boundary conditions and the initial conditions is specifically as follows: The boundary of air is an open boundary, the gas pressure is set as one standard atmospheric pressure, the environmental temperature is set as 293.15 K, the boundary is set as a non-slip boundary, and the initial values of the pressure and the velocity in the laminar flow region are both set as 0; There is a stable arc at the start of simulation, and an initial arc region is set in the air between the cable conductors; The initial phase of the power frequency alternating voltage source is set at-90°, so as to ensure that the voltage value output by the voltage source at the start of simulation is at a peak value, thereby matching the conductive characteristics of the existing stable arc plasma.

[0013] Optionally, in S2, the network is divided, and the division is specifically as follows: The free triangular mesh division method is adopted, and in the setting of the division parameters, the maximum unit size is set as 0.36, and the narrow region resolution is set as 1.0; The narrow region is divided into finer meshes, the maximum unit size is set as 0.104, and the narrow region resolution is set as 1.0.

[0014] Optionally, in S3, the solution time is set as 0-100 ms, and the output step length is set as 0.1 ms.

[0015] Optionally, S5 is specifically as follows: The Joule heat generated during arc combustion forms a non-uniform temperature field in space, the temperature in the edge region of the arc is lower than that in the center region due to fluid heat transfer, the arc temperature acts on the development of the arc, the medium conductivity in the high-temperature center region is high, a local low-impedance channel is formed, the driving current is concentrated to the center region, and the heat-electricity two-way coupling process makes the fault arc current density and the temperature field present a dynamic coupling relationship; In the center insulation layer damage simulation model, the arc mainly occurs in the center region of the two cables, and the temperature field presents an axisymmetric distribution characteristic; In the side insulation layer damage simulation model, the arc occurs in the side insulation damage region, and the temperature field presents a fan-shaped distribution.

[0016] According to the technical solution, compared with the prior art, the present disclosure provides a low-voltage line insulation damage early fault modeling method, which has the following beneficial effects: The present application takes magnetohydrodynamic theory as the core, and constructs a multi-physical field coupling model of low-voltage line insulation damage fault arc based on a COMSOL simulation platform, and through scientific setting of model physical property parameters, initial conditions, boundary conditions and physical field control equations, the model can ensure that the dynamic process of the fault arc can be truly restored; on this basis, the simulation results of the obtained arc temperature field shape, current density distribution and fault signal waveform are analyzed, which provides a theoretical basis for insulation damage fault identification, and has important significance for ensuring the safe and stable operation of the distribution network. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only a part of the embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the provided drawings.

[0018] Figure 1 The flow chart of the low-voltage line insulation damage early fault modeling method provided by the present application is shown in the figure. Figure 2 The multi-physical field setting detail schematic diagram provided by the present application is shown in the figure. Figure 3 The simulation model external circuit diagram provided by the present application is shown in the figure. Figure 4 The arc current density distribution and temperature distribution schematic diagram provided by the present application is shown in the figure. (a) is a center insulation damage type at 13.6 ms, and (b) is a side edge insulation damage type at 7.3 ms. Figure 5 The arc maximum temperature curve diagram provided by the present application is shown in the figure. (a) is a center insulation damage fault, and (b) is a side edge insulation damage fault. Figure 6 The side edge insulation damage fault signal provided by the present application is shown in the figure. Figure 7 The center insulation damage fault signal provided by the present application is shown in the figure. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0020] The embodiments of the present application disclose a low-voltage line insulation damage early fault modeling method, as shown in the figure. Figure 1As shown, comprising the following steps: S1, selecting a solution space dimension, a physical field interface, and a research solver; S2, establishing a geometric model, setting material property parameters, boundary conditions and initial conditions, and dividing the grid; S3, setting the solution time and output step; S4, solving the calculation and judging whether the solution result is reasonable, if not, returning to S2, if reasonable, entering S5; S5, analyzing the simulation calculation result.

[0021] In this embodiment, the arc is simulated and calculated using the multi-physical field simulation analysis software COMSOL, which is widely used in the modeling and simulation of complex physical phenomena in engineering, physics, chemistry, biology and other fields. The core advantage of COMSOL is that it can couple the analysis of multiple physical fields such as electromagnetism, heat, structural mechanics, fluid, and acoustics, accurately simulate the interaction mechanism of multiple physical processes under real working conditions, and help researchers reveal the nature of physical phenomena, optimize system design and simulate system evolution trend. Next, referring to Figure 1 , the complete link from model establishment, parameter setting, grid division to solution analysis is introduced in detail, which provides clear guidance for the standardization and repeatability of simulation work.

[0022] I. Selecting a solution space dimension, a physical field interface, and a research solver

[0023] 1) COMSOL provides one-dimensional, two-dimensional, and three-dimensional space dimensions. The research object of this embodiment is the early fault arc of the insulation damaged low-voltage cable, which needs to model the cable cross section and the surrounding air area, so it needs to use two-dimensional or three-dimensional modeling method. Three-dimensional simulation can completely present the cable and arc in the real space, fully and accurately reflect the physical process, but the calculation amount is extremely large, the hardware requirement is high, the calculation time is long, and the model construction and setting are complex. Two-dimensional consideration of cable cross section can analyze the physical phenomena in cross section and certain axial plane, such as electric field and magnetic field distribution, heat conduction, etc., balancing between calculation efficiency and accuracy, facilitating visualization of results, better balancing the analysis needs of arc physical process and calculation resources, efficiency, and can effectively study the interaction between arc and cable in cross section and selected axial plane. Because the arc evolution process of cable insulation damage fault arc is similar in different cross sections, therefore two-dimensional simulation can more fully reflect its dynamic evolution process, finally this embodiment selects to use two-dimensional simulation modeling

[0024] 2) The macroscopic state of the arc evolves over time and space, which can be calculated by the mutual coupling of temperature field, flow field, electric field and magnetic field. Therefore, the core of the simulation is to realize the setting and coupling of the four physical fields. COMSOL provides various physical fields required for simulation. The temperature field is set using the fluid heat transfer module; the gas flow field is set using the laminar flow module; the electric field and external circuit are constructed using the current module and circuit module; and the magnetic field is set using the magnetic field module. In the multi-physical field module, the coupling of temperature field and flow field is completed by the non-isothermal flow module; the coupling of temperature field and electric field is completed by the balance discharge heat source module and electromagnetic heat module; and the coupling of magnetic field and flow field is completed by the magnetohydrodynamics module.

[0025] A. Single physical field setting

[0026] In this embodiment, a current module is added to the simulation model to construct a current field calculation model, focusing on solving the spatial distribution characteristics of the current density in the arc region. Based on Maxwell's equations, the conductivity parameters of the arc plasma are coupled with the electromagnetic field control equation to quantitatively characterize the conduction path of the current in the cable conductor, the insulation defect gap and the air medium. The simulation model strictly limits the calculation domain boundary, defines the current field action range as the cable metal layer, the polyvinyl chloride insulation layer and the air domain, and thus accurately reflects the non-uniform characteristics of the current density distribution under the insulation damage scenario. The calculation result provides a physical constraint condition for subsequent arc heat source term loading and temperature field iterative calculation.

[0027] In this embodiment, a high-precision magnetic field module is introduced into the simulation model to precisely model and numerically calculate the magnetic field of the arc and its surrounding space, and accurately obtain the spatial distribution characteristics and vector direction of the magnetic field strength. Through the solution of the magnetic field distribution, input parameters are provided for the electromagnetic heat and magnetohydrodynamics modules, and thus the constraint and action of the magnetic field on the arc plasma in three different media are realized.

[0028] In this embodiment, a laminar flow module is added to the simulation model to accurately simulate and describe the laminar flow of the arc plasma. In the arc environment, the flow state of the plasma has an important influence on the characteristics and behavior of the arc. Through the laminar flow module, the flow velocity, flow direction and velocity distribution of the plasma can be accurately calculated, providing a basis for in-depth study of the transmission process and energy distribution of the arc. According to the material properties, the action range of the laminar flow field is set to air, and the temperature output by the non-isothermal flow field and the pressure output by the laminar flow field are used as input parameters for the model.

[0029] This embodiment accurately calculates the heat transfer process between the arc plasma and the surrounding environment by adding a fluid heat transfer module to the simulation model. It considers multiple heat transfer mechanisms, including thermal conduction, thermal convection, and thermal radiation. By calculating these heat transfer processes, the temperature distribution of the arc and the propagation of heat in space can be obtained, which helps to analyze the heating effect of the arc on the surrounding medium and the cooling mechanism of the arc itself. According to the material properties, the range of the fluid heat transfer field is set to air, cable, and insulation layer. Air is set as a fluid, and cable and insulation layer are set as solids. The temperature output by the temperature field and the absolute pressure output by the non-isothermal flow field are used as input parameters for the model.

[0030] B. Multi-physical field setting

[0031] Given that the development of an arc is essentially a complex coupled system involving the interaction of multiple physical processes, this embodiment is based on actual research needs and has constructed a simulation system containing four core modules: non-isothermal flow, equilibrium discharge heat source, electromagnetic heat effect, and magnetohydrodynamics. Through the coordinated action and data interaction between multiple modules, the fine control of the coupling process of multiple physical fields such as flow field, electric field, magnetic field, and temperature field in the plasma arc is realized, so as to more truly restore the physical mechanism of the dynamic development of the arc.

[0032] Since the arc is a high-temperature plasma, the internal temperature distribution is extremely uneven. The temperature in the central region of the arc is as high as several thousand or even tens of thousands of Kelvin, but the temperature in the edge region is relatively low. The uneven temperature will significantly affect various physical parameters of the plasma arc, such as specific heat capacity, electrical conductivity, density, thermal conductivity, etc. Therefore, a non-isothermal flow module needs to be added to the arc simulation model to consider the changes in fluid parameters caused by temperature changes. At the same time, the development process of the arc is also accompanied by complex heat convection and heat conduction. The non-isothermal flow module can consider both heat transfer mechanisms, accurately simulate the heat transfer process in the arc by calculating the coupling effect of temperature field and flow field. In this embodiment, the non-isothermal flow module couples the laminar flow and fluid heat transfer physical fields by inputting temperature and pressure parameters.

[0033] In the actual arc discharge process, the power supply continuously inputs energy to the arc to maintain the existence and combustion of the arc. This embodiment uses the equilibrium discharge heat source and electromagnetic heat modules to couple the current and fluid heat transfer interfaces to simulate this stable energy input process, so that the energy state of the arc in the simulation is consistent with the actual situation, thereby ensuring that the arc can exist and develop stably in the simulation environment, which helps to accurately simulate various characteristics of the arc, such as temperature distribution.

[0034] The arc plasma can be regarded as a conductive fluid, which will be affected by Lorentz force in a magnetic field. The magneto-hydrodynamic module is added in this embodiment to calculate the Lorentz force and electromotive force, couple the laminar flow field and the magnetic field, and then simulate the fluid flow driven by the electromagnetic force. The details of the related physical field settings are shown in FIGS. 8A and 8B. Figure 2

[0035] 3) COMSOL provides three solving methods of frequency domain, steady state and transient state. The transient state solving method can accurately capture the dynamic changes of the physical field over time, is suitable for analyzing the transient process of evolution and development, but requires setting parameters such as time step, has large amount of calculation, long time consumption, and high requirement for hardware performance. The arc simulation based on the magneto-hydrodynamic theory mainly focuses on the macro evolution and development process of the arc in a certain time scale, and therefore the transient state solving method is selected. In terms of solving method, the separation solving strategy is adopted. The problem of multi-physical field coupling is usually complex, involving the interaction of multiple physical processes. The separation solving method separates these complex physical processes and solves each physical field respectively. This method has significant advantages. On the one hand, it can effectively reduce the amount of calculation and avoid the excessive consumption of computing resources caused by simultaneously solving multiple highly coupled equations, thereby improving the calculation efficiency. On the other hand, it provides convenience for separately controlling and optimizing the solving process of each physical field.

[0036] II. Establishing a geometric model, setting material physical property parameters, boundary conditions and initial conditions, and dividing grids

[0037] 1) Geometric model

[0038] An insulation damage type fault cable is prepared, and a basic shape is created using a parameterized geometric tool. Specifically, the insulation damage type fault cable is prepared based on the national standard GB / T31143-2014 as follows: two conductors with a cross-sectional area of 1.5 mm 2 are tightly bundled using an insulating tape, the cable sample is cut to a minimum length of 200 mm, the insulation layer in the middle of the cable is cut, the cutting depth should be able to expose the conductor core without damaging the metal conductor, and finally the cutting part is wrapped with two layers of insulating tape; Due to the influence of the actual operating environment, there are differences in the degree of damage of the cable insulation layer, and these factors have important influence on the development of the arc. Therefore, the relative relationship between the damage position and the cable body is considered, and the insulation damage type is divided into central insulation layer damage and side insulation layer damage. The insulation material region of the geometric structure is integrated and regarded as a unified physical region. The outer insulating tape is rapidly pyrolyzed at the initial stage of arc burning, has weak influence on the dynamic development process of the arc, and is therefore ignored. A separate region is set in the main arc occurrence area, which is only used for dividing a more dense network, and a simplified geometric structure of the insulation damage type fault cable is obtained to ensure the convergence of the model. ​

[0039] 2) Material properties

[0040] Material properties include specific heat capacity, electrical conductivity, density, thermal conductivity, etc. Among them, electrical conductivity directly determines the conduction ability of current in the plasma. According to Ohm's law, under a given electric field, the higher the electrical conductivity, the greater the current density, and the more Joule heat generated. The specific heat capacity determines the amplitude of temperature change when the plasma absorbs or releases heat. The Joule heat makes the plasma temperature rise, and the specific heat capacity affects the establishment and change process of the temperature field, and further affects the state and properties of the plasma. Density affects the mechanical properties and thermophysical processes of the plasma. In the flow field analysis, density is a key parameter in the computational fluid dynamics equation, which interacts with pressure, velocity, etc. variables to determine the flow characteristics of the plasma. Thermal conductivity describes the ability of heat conduction in the plasma. High thermal conductivity means fast heat conduction within the plasma, which helps to make the temperature field more uniform. In the arc plasma, thermal conductivity determines the rate of heat transfer from the arc center to the electrode, which is crucial for analyzing the thermal load of the electrode, the heat exchange between the plasma and the surrounding medium, and the stable distribution of the temperature field.

[0041] The simulation model of this embodiment involves three materials: air, copper, and polyvinyl chloride. In comparison, the changes in the physical properties of copper and polyvinyl chloride materials under the action of the arc have less impact on the development of the arc. To balance the calculation accuracy and efficiency, the physical property parameters of copper and polyvinyl chloride are set as fixed constants in this simulation process, and the physical property parameters of air are derived from the gas discharge plasma basic database.

[0042] 3) Boundary conditions and initial conditions

[0043] When simulating arc failure caused by insulation damage, setting reasonable initial conditions and boundary conditions is the core to ensure the authenticity and reliability of the simulation results. The initial conditions define the trigger starting point and initial physical state of the arc, and the boundary conditions affect the development of the arc by constraining the energy exchange and physical field evolution of the system. The multi-physical field arc simulation model based on magnetohydrodynamics theory needs to set initial conditions and boundary conditions that meet the actual working conditions for current, magnetic field, temperature field, and flow field.

[0044] Low-voltage distribution lines often occur in open atmospheric environments, so the air boundary is an open boundary, the gas pressure is set to one standard atmosphere, the environmental temperature is set to 293.15 K, the boundary is set to a no-slip boundary, and the initial values of pressure and velocity in the laminar flow region are both set to 0; There is a stable arc at the beginning of the simulation, and an initial arc region is set in the air between the cable conductors; since the arc has been burning stably, the temperature is usually several thousand Kelvin, and the initial temperature in this model is set to 5000 K; In the construction of the simulation model, the voltage and current are determined by the external circuit shown in Figure 3 The external circuit contains a 220V power frequency AC voltage source, a 50Ω current-limiting resistor R, and a fault arc model. The arc terminal is connected in series with the line resistance and grounded. At the same time, an ammeter and a voltmeter are added to the circuit to detect the voltage and current changes during the arc occurrence. The initial phase of the power frequency AC voltage source is set at-90° to ensure that the voltage output by the voltage source at the start of the simulation is at the peak value, thereby matching the conductive characteristics of the existing stable arc plasma and avoiding sudden changes in the arc state at the start of the simulation due to improper voltage phase.

[0045] 4) Mesh division

[0046] In the use of COMSOL software to develop low-voltage line insulation damage type fault arc magnetohydrodynamic multi-physical field simulation, finite element mesh division as the key link between the upper and lower, not only directly affects the precision and efficiency of simulation calculation, but also is the key basis to ensure the reliability of multi-physical field coupling analysis. Its implementation process needs to strictly follow the scientific process and method, through reasonable planning of mesh type, density distribution and boundary conditions, so as to realize accurate modeling and efficient solution of complex arc physical phenomena.

[0047] In this simulation process, the free triangular mesh division method is adopted. In the setting of division parameters, the maximum unit size is set to 0.36 (which limits the maximum size of the mesh unit, and can ensure that in the whole model, the unit will not be too large to cause the loss of key physical information), and the narrow area resolution is set to 1.0. The narrow area is divided into finer mesh, the maximum unit size is set to 0.104, and the narrow area resolution is set to 1.0, which ensures that there is enough grid resolution in these areas to accurately simulate the complex physical process and avoid simulation distortion caused by sparse grid.

[0048] The above division method has high flexibility and can better adapt to the complex geometry of low-voltage line insulation damage. Whether the damaged area is regular or irregular, the free triangular mesh division method can effectively fill the model and ensure accurate discretization. Through the above parameter setting, the finite element mesh division of the low-voltage line insulation damage type fault arc model is completed, laying a solid foundation for subsequent accurate multi-physical field simulation calculation.

[0049] III. Set the solution time and output step

[0050] The development of a fault arc triggered by insulation failure is quite complex. The complete cycle from arc generation to extinction is the core of its dynamic evolution, and the continuous alternation of dozens of cycles throughout the combustion process demonstrates the complexity and regularity of the arc phenomenon. The entire process can last for a considerable time; therefore, setting the solution time to 0-100ms allows for detailed analysis of the periodic changes in the fault arc. Both the maximum simulation step size and the output step size are set to 0.1ms. A smaller maximum step size helps improve the accuracy of the simulation results and ensures the convergence of the simulation model. A smaller output step size can more meticulously depict the changes in the physical characteristics of the arc at various instants, such as the arc's temperature distribution, current density changes, and fault signal changes.

[0051] IV. Solve the problem and determine whether the solution is reasonable.

[0052] V. Simulation Result Analysis

[0053] 1) Evolution of fault arc

[0054] Taking the 13.6 ms of central insulation failure and the 7.3 ms of lateral insulation failure as examples, plot the arc current density and arc temperature distribution, as follows. Figure 4 As shown, the arc current density and temperature field exhibit a dynamic coupling relationship. This is because the Joule heat generated during arc combustion creates a non-uniform temperature field in space. The temperature in the center of the arc can reach over 6000K, while the temperature in the edge region is significantly lower due to fluid heat transfer. Simultaneously, the arc temperature influences arc development; the high conductivity of the medium in the high-temperature central region creates a localized low-impedance channel, driving the current to concentrate in the central region. Ultimately, this thermo-electrical bidirectional coupling process results in a highly nonlinear correlation between the arc current density distribution and the temperature field.

[0055] Overall, during the evolution of the arc temperature field, the arcs generated by center insulation failure faults and side insulation failure faults exhibit similar changing patterns, yet each displays its own unique temperature distribution characteristics and evolutionary properties due to differences in their respective conditions. Radially, the center of the arc column is the hottest region, with the arc temperature gradually decreasing outwards from the center. Axially, the arc root is the hottest region because it is in close contact with the electrode. Since the metal conductor itself is a solid, its heat transfer capacity is limited, resulting in relatively slow heat exchange between the arc root region and the surrounding environment.

[0056] Compared with the center insulation breakdown fault, the shape of the gap in the side insulation breakdown fault significantly changes the arc discharge space. Because one side of the insulation layer is still intact, the fault arc can only develop on the single side of the damaged insulation and cannot freely develop between the cable gaps. This asymmetric arc morphology makes the temperature field exhibit obvious differences on both sides of the cable, forming an asymmetric temperature distribution characteristic, which is in sharp contrast to the relatively symmetric temperature field under the center insulation breakdown fault. At the same time, due to the presence of the intact insulation layer on the other side, the side insulation breakdown fault arc appears a certain degree of distortion at the initial stage of arcing, and the discharge channel is longer.

[0057] The minimum temperature of the side insulation breakdown fault gap is 2906.6 K, which is higher than the 2594.6 K of the center insulation breakdown type. This difference is due to the significant impact of cable insulation breakdown type on the intensity of gas thermal convection: the side insulation breakdown fault is limited by the single-sided damage structure, and thermal convection can only occur on the damaged side, resulting in a weaker thermal convection intensity than the center insulation breakdown fault. Since the thermal convection intensity is directly related to the heat dissipation efficiency of the arc, the weaker thermal convection makes the side fault arc heat dissipate slowly, thus maintaining a higher minimum temperature. In contrast, the center insulation breakdown fault, with relatively sufficient thermal convection, accelerates heat diffusion, making its temperature drop rate significantly faster than that of the side insulation breakdown fault during the arc extinction phase.

[0058] Figure 5 The arc maximum temperature curve changes with time, which overall corresponds to the current waveform and voltage waveform, and the arc temperature change rate during the arcing and extinction instants is particularly prominent. This is because at the moment of arc state switching, the energy supply that maintains its combustion changes abruptly: when the arc is extinguished, the energy input is quickly interrupted, and the original energy balance in the arc is broken, and the high-temperature region rapidly cools down as it cannot continuously obtain energy supply; when the arc is burning, the large increase in energy input promotes the rapid rise of the arc temperature. This dramatic change in energy supply causes the arc to produce a very high temperature change rate at the state transition moment.

[0059] In both insulation breakdown conditions, the time of the highest temperature peak of the arc is earlier than the peak of the current waveform. This phenomenon shows that, compared with the current change, the arc temperature presents a more rapid response characteristic in the fault process. The reason is that the arc will continuously transfer heat to the surrounding air during the combustion process. When the current gradually rises, although the energy input to the arc is increasing, a large amount of energy is also rapidly diffused to the surrounding air through heat conduction, heat convection and heat radiation. In the process of the current approaching the peak, the energy increment obtained by the arc and the energy lost to the air reach a critical state, at which the energy used by the arc to raise its temperature is limited. When the current approaches the peak, the energy provided to the arc for combustion tends to be stable, while the arc continues to transfer heat to the surrounding air, and as the arc temperature rises, the energy transferred to the surrounding air is still increasing, resulting in a slight decrease in the temperature of the arc due to continuous heat loss when the current waveform reaches the peak.

[0060] The simulation data show that the peak time of the arc of the center insulation breakdown type fault is significantly earlier than that of the side insulation breakdown type fault. This phenomenon reveals that the center breakdown has a larger fault area, resulting in higher heat transfer efficiency and more rapid heat diffusion. In the short stage before and after insulation breakdown, its temperature shows a sharp rising trend; and when the arc enters the stable combustion stage, the temperature immediately starts to decline due to the relatively good heat dissipation condition. In sharp contrast, the side insulation breakdown type fault is hindered by heat dissipation, and in the process of current rising, the temperature always maintains a synchronous increasing trend, and the time interval between the temperature peak and the current peak is significantly shortened, which reflects the significant influence of different breakdown locations on the thermal characteristics of the fault arc.

[0061] From the above analysis, it can be seen that under the excitation of 220V alternating voltage, the arc at the insulation breakdown of the low-voltage line evolves along the periodic trajectory of "arcing-combustion-extinction-reignition", and each time the arc is extinguished before the voltage zero crossing point, and when the voltage reverses and rises to the breakdown threshold, the arc will reignite at the breakdown, and this cycle will continue, showing the dynamic characteristics of the alternating current fault arc.

[0062] 2) Fault arc waveform analysis

[0063] During the development of the arc, the voltage and current signals are affected by the power supply characteristics, arc conductivity, surrounding medium, electrode material and shape, etc. The power supply voltage determines the basic power supply conditions of the arc, the change of arc properties will change the conductivity and affect the voltage and current, the properties and pressure of the surrounding medium will affect the heat dissipation and ionization degree of the arc, and the electrode material and shape will affect the arc striking and stability. At the same time, the voltage provides energy for the arc to maintain the ionization state of the plasma, the current size determines the energy input and temperature of the arc, and then affects the shape, brightness and stability of the arc. According to the simulation results, the arc voltage and current waveform signal curves under two insulation damage conditions are drawn respectively.

[0064] It takes a certain voltage level to break down the air. The voltage borne by the arc region at the time of breakdown is related to the difficulty of arc reignition. The higher the voltage required to break down the air, the later the time of sudden current increase, and the longer the duration of zero-hold shoulder in the current waveform. As shown in Figure 6 , the fault current peak is 5.2A, the voltage peak is 272.32V, the average duration of single arc is 5.5ms, and the zero-hold duration is stable at about 4.5ms. As shown in Figure 7 , the fault current peak is 5.4A, the voltage peak is 211.7V, the average duration of single arc is 6.8ms, and the zero-hold duration is stable at about 3.2ms. Since the fault arc and the temperature field distribution are coupled with each other, the fault signal characteristics and the temperature field distribution change process maintain a corresponding relationship.

[0065] Considering the inherent periodicity of the fault signal, and the dynamic process of the fault arc completing the complete cycle of "stable arc - arc extinction - reignition - stable arc" needs to last for 0.5 cycles. Therefore, this embodiment takes the signal of insulation damage fault in the 10ms-20ms interval as an example to analyze the waveform characteristics of the fault arc, and further explores the evolution law of the electric signal in the transition process of the arc at different stages.

[0066] Overall, the current and voltage waveform trends of the two types of insulation breakdown fault arcs are basically consistent. When the fault arc is in the "stable arc" stage, the arc generated by the Joule heat and the heat dissipation of heat transfer are close to each other, the arc conductivity is relatively stable, the fault current is close to the standard sine waveform. When the fault arc enters the "arc extinction" stage, the heat dissipation of heat transfer dissipates a large amount of heat, the electric conductivity decreases sharply, the fault current decreases to below 0.3A, and the fault voltage rises to close to the power supply voltage. When the fault arc is in the "re-arc" stage, the electric conductivity increases sharply, the fault voltage drops rapidly, and the fault current rises rapidly. After a short arcing process, the fault arc enters the stable arc state again, forming a "stable arc-extinction-re-arc-stable arc" periodic cycle. During this process, the dynamic changes of the current and voltage waveform clearly reflect the electrical characteristic evolution law of the fault arc at different development stages.

[0067] Although the current and voltage waveforms of the two types of insulation breakdown fault arcs show similarity in the overall evolution trend, there are still significant differences in key characteristic parameters such as zero-hold time, current instantaneous value and signal amplitude. The re-arc time of the side insulation breakdown fault is significantly lagging behind the center insulation breakdown fault arc, resulting in a zero-hold time of 4.5ms for the side insulation breakdown fault arc, which is significantly longer than the 3.2ms of the side insulation breakdown fault. The reason is that the side insulation breakdown fault arc is distorted to some extent, the fault arc length is longer, and the field strength required for breakdown of air is higher, making it more difficult to re-arc after zero crossing.

[0068] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0069] The above description of the disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method of modeling early fault of insulation breakdown of low voltage line, characterized in that, The method comprises the following steps: S1, selecting a solution space dimension, a physical field interface and a research solver; S2, establishing a geometric model, setting material physical property parameters, boundary conditions and initial conditions, and dividing a grid; S3, setting a solution time and an output step length; S4, solving calculation and judging whether the solution result is reasonable, if not, returning to S2, if yes, entering S5; S5, analyzing the simulation calculation result.

2. The method of claim 1, wherein, S1 is specifically: Considering that the research object is an early fault arc of an insulation damaged low-voltage cable, the cable insulation damage fault arc has similarity in the evolution process of different sections, therefore, a two-dimensional simulation modeling is adopted; temperature field, flow field, electric field and magnetic field are set, the coupling of the temperature field and the flow field is completed through a non-isothermal flow module, the coupling of the temperature field and the electric field is completed by means of a balanced discharge heat source module and an electromagnetic heat module, and the coupling of the magnetic field and the flow field is completed by using a magnetohydrodynamics module; based on the arc simulation of the magnetohydrodynamics theory, the evolution and development process of the arc macroscopically in a certain time scale is focused, therefore, a transient mode is selected for solving, and a separation solving strategy is adopted.

3. The method of claim 2, wherein, temperature field, flow field, electric field and magnetic field are set, specifically: a current field calculation model is constructed through a current module, the spatial distribution characteristics of the current density in the arc region are solved; based on the Maxwell equation set, the conductivity parameters of the arc plasma are coupled with the electromagnetic field control equation, and the conduction path of the current in the cable conductor, the insulation defect gap and the air medium is quantitatively characterized; the magnetic field of the arc and the space around the arc is finely modeled and numerically calculated by using a magnetic field module, so as to obtain the spatial distribution characteristics and the vector direction of the magnetic field strength; the laminar flow of the arc plasma is simulated and described by using a laminar flow module, so as to obtain the flow velocity, flow direction and velocity distribution of the plasma; the heat transfer process between the arc plasma and the surrounding environment is calculated by using a fluid heat transfer module, so as to obtain the temperature distribution of the arc and the heat propagation in space.

4. The method of claim 1, wherein, In S2, the geometric model is established, specifically: an insulation damage type fault cable is prepared, and a parameterized geometry tool is used to create a basic shape; based on the relative relationship between the damage position and the cable body, the insulation damage type is divided into central insulation layer damage and side insulation layer damage; the insulation material region of the geometric structure is integrated and regarded as a unified physical region, the outer insulation tape is ignored, a separate region is set in the main arc occurrence area, and the simplified geometric structure of the insulation damage type fault cable is obtained.

5. The method of claim 1, wherein, In S2, the material physical property parameters include specific heat capacity, electrical conductivity, density and thermal conductivity; wherein, the physical property parameters of copper and polyvinyl chloride are set as fixed constants, and the physical property parameters of air are derived from a gas discharge plasma basic database.

6. The method of claim 1, wherein, In S2, the setting of the boundary conditions and the initial conditions is specifically: the boundary of the air is an open boundary, the air pressure is set to one standard atmosphere, the environmental temperature is set to 293.15K, the boundary is set to a no-slip boundary, and the initial values of the pressure and the velocity of the laminar flow region are both set to 0; there is a stable arc at the beginning of the simulation, and an initial arc region is set in the air between the cable conductors; The initial phase of the power frequency AC voltage source is set at-90° to ensure that the voltage output by the voltage source at the start of the simulation is at its peak value, matching the conductive characteristics of the existing stable arc plasma.

7. The method of claim 1, wherein, In S2, the network is divided, specifically: The free triangular mesh division method is adopted, and in the setting of the division parameters, the maximum element size is set to 0.36, and the narrow area resolution is set to 1.0; The narrow area is divided into finer meshes, with the maximum element size set to 0.104 and the narrow area resolution set to 1.

0.

8. The method of claim 1, wherein, In S3, the solution time is set to 0-100 ms, and the output step is set to 0.1 ms.

9. The method of claim 1, wherein, S5 is specifically: The joule heat generated by the arc combustion forms a non-uniform temperature field in space, and the temperature in the edge area of the arc is lower than that in the center area due to fluid heat transfer. The arc temperature reacts on the development of the arc, the high-temperature area in the center has high conductivity, forming a local low-impedance channel, and the driving current is concentrated in the center area. This heat-electricity two-way coupling process makes the fault arc current density and the temperature field present a dynamic coupling relationship; In the simulation model of the damaged center insulating layer, the arc mainly occurs in the center area of the two cables, and the temperature field presents an axisymmetric distribution characteristic; In the simulation model of the damaged side insulating layer, the arc occurs in the damaged area of the side insulating layer, and the temperature field presents a fan-shaped distribution.

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