Ring main unit elbow type head early fault simulation implementation method

Abstract

The invention discloses a simulation implementation method for an early fault of an elbow-shaped head of a ring main unit, and is applied to the technical field of arc simulation. Comprising the following steps: selecting a simulation dimension, calculating a used physical field and researching a solving mode; establishing a geometric model according to the actual operation state of the elbow-shaped head of the ring main unit; material physical property parameters, boundary conditions and initial conditions required by a physical field are set, and grids are divided; setting a time scale and a result output step length of simulation calculation; solving, calculating and judging whether a calculation result is reasonable or not; and analyzing a simulation calculation result. According to the method, the early fault of the elbow-shaped head of the ring main unit is simulated, the characteristic quantity of the elbow-shaped head in the early fault process is analyzed, the fault hidden danger is eliminated when the equipment operation state reaches the fault transition period, and the method has important significance for improving the operation reliability of a power distribution network.

Description

Technical Field

[0001] The invention relates to the technical field of arc simulation, and more particularly to a method for realizing early fault simulation of an elbow head of a ring network cabinet. Background Art

[0002] The 10kV power supply system is an important link between the power grid and power users, and is also the last link in the use of electric energy. Among the power outages that can be perceived by users, the proportion of accidents occurring in the distribution network is as high as 96%. Ensuring the reliable operation of the distribution network is a necessary condition for improving the quality of power supply and ensuring the safe and efficient use of electric energy. Compared with backbone networks of other voltage levels, the distribution network is widely distributed, uses a large number of equipment, and has a complex and changeable operating environment, resulting in more faults that affect its reliable operation.

[0003] The elbow head of the ring main unit works in the cable room, and the cable room is connected to the cable well. The internal humidity is generally high, and condensation is easy to form during normal operation. At the same time, the elbow head is affected by many factors, and it is easy to have interface separation between the elbow head and the casing due to aging, thereby forming air gap defects and reducing insulation performance. When condensation penetrates into the air gap, it will cause moisture creepage in the air gap between the elbow head and the casing, and even develop into surface flashover, causing fire and combustion, causing permanent failure, seriously damaging the power equipment, and having a bad impact on the operation reliability of the distribution network. From the perspective of the cable power supply system, faults caused by cable accessories account for more than 70% of cable line faults. Cable accessories are mainly divided into three categories: cable intermediate head, cable elbow head, and cable terminal head. The connection position between the elbow head and the casing is a high-incidence point of the ring network cabinet, and it is also one of the most prone to failure in the ring network cabinet. Its proportion in the failure of the ring network cabinet can even reach about 37%. The main factor causing the fire and combustion at the overlap of the elbow head and the casing is that after the air gap is generated between the elbow head and the casing, moisture penetrates into the air gap and approaches the high-voltage area inside the elbow head that contacts the cable terminal, thereby inducing an arc inside the air gap. The arc decomposes the insulating material at high temperature to produce carbon marks and creepage marks. After the arc occurs many times, the carbon marks and creepage marks enable the arc to directly penetrate the inside of the elbow head and the grounded shell, and cause surface flashover, causing the elbow head to catch fire and burn, and at the same time affecting the elbow heads installed in other phases, causing serious accidents and affecting production and life. The arc occurs inside the air gap between the elbow head and the bushing, which is difficult to detect from the outside. The fault is highly concealed. At the same time, the ring main unit itself is highly closed, and it is difficult to observe from the outside to the inside of the ring main unit. It is difficult to detect it in time by manual inspection and image detection. Therefore, how to provide a method for simulating the early fault of the elbow head of the ring main unit is a problem that technicians in this field need to solve urgently. Summary of the invention

[0004] In view of this, the present invention provides a method for simulating early faults of elbow heads of ring main units. Based on the theory of magnetohydrodynamics, a fault arc simulation model is built in the COMSOL simulation software to analyze the simulation results of the electric field, the morphology of the arc temperature field, the current density distribution, and the current waveform.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for simulating early faults of elbow heads of ring main units includes the following steps:

[0007] S1. Select the simulation dimension, the physical fields used in the calculation, and the research solution method;

[0008] S2. Establish a geometric model according to the actual operating state of the elbow head of the ring main unit;

[0009] S3. Set the material property parameters, boundary conditions, and initial conditions required by the physical fields, and divide the mesh;

[0010] S4. Set the time scale of the simulation calculation and the result output step size;

[0011] S5. Solve the calculation and judge whether the calculation result is reasonable. If it is not reasonable, return to S3. If it is reasonable, enter S6;

[0012] S6. Analyze the simulation calculation results.

[0013] Optionally, S1 is specifically:

[0014] S11. Considering that the simulation object is the early fault of the elbow head of the ring main unit and the arc simulation needs to couple the temperature field, the flow field, the electric field, and the magnetic field, two-dimensional modeling is used for the simulation;

[0015] S12. Set the temperature field, the flow field, the electric field, and the magnetic field to achieve the coupling of the temperature field and the flow field, the coupling of the temperature field and the electric field, and the coupling of the magnetic field and the flow field;

[0016] S13. Based on the purpose of studying the spatio-temporal evolution of the macroscopic state of the arc in a short time, the transient research solution is selected as the solution method.

[0017] Optionally, an external voltage is applied outside the geometric module through an external circuit in the simulation. The resistance of the external circuit is set to 2.53 Ω, the inductance is set to 3.233 mH, and the capacitance is set to 2.63 μF.

[0018] Optionally, S2 is specifically:

[0019] A geometric model is established based on the actual operating state of the elbow head of the ring main unit. The dimensions of the geometric structure of the simulation model are the same as those of the actual device, and each region corresponds to the actual device. The corresponding materials for each region include silicone rubber, epoxy resin, conductive rubber, water, and air arc.

[0020] Optionally, S3 is specifically as follows:

[0021] S31. Set different material physical property parameters and calculation regions of different physical fields in different regions of the geometric model. The material physical property parameters include conductivity, relative permittivity, dynamic viscosity, thermal conductivity, density, constant pressure heat capacity, and relative magnetic permeability.

[0022] S32. The simulation uses a system of differential equations to solve the changes in physical parameters of each point in the spatial domain over time. When solving the system of differential equations, initial conditions and boundary conditions must be provided for the system of differential equations to obtain a convergent result.

[0023] S33. Adopt free triangular meshes and divide the mesh density into three levels: the arc region is the densest, the density in the water band region is less than that in the arc region, and the remaining regions are sparse.

[0024] Optionally, S32 is specifically as follows:

[0025] Provide boundary conditions for the temperature field, flow field, electric field, and magnetic field respectively. The ambient temperature is set to 293.15K, the water band temperature is set to 373.15K, the flow field is set to a laminar flow including gravity, the air pressure is set to one atmosphere, and the air gap boundary is set to a no-slip boundary; set the initial phase of the B-phase power supply to 90°, set the air gap height between the elbow head of the ring main unit and the bushing to 1mm, set the air gap except the arc region to be filled with the water band, set the contact angle between the water band and silicone rubber and epoxy resin to 90°, set the initial length of the arc region in the approaching stage of the water band to 5mm, set the initial length of the arc region in the boiling stage of the water band to 3mm, and use the built-in dynamic mesh module to achieve the moving effect of the water band.

[0026] Optionally, S4 is specifically as follows:

[0027] Set the time scale of the simulation calculation to 80ms, and the output interval duration of the calculation results is 0.01ms.

[0028] Optionally, S6 is specifically as follows:

[0029] S61. Analyze the potential distribution and electric field intensity distribution when the external system voltage reaches the peak value in three cases where the elbow head of the ring main unit and the bushing are in full contact, there is only an air gap, and there is a water band in the air gap.

[0030] S62. Analyze the geometric dimensions, current density distribution, and temperature distribution of the arc during the water belt approaching stage and the water belt boiling stage;

[0031] S63. Plot the arc current waveform curves for the water belt approaching stage and the water belt boiling stage respectively.

[0032] As can be seen from the above technical solutions, compared with the prior art, the present invention provides a method for simulating the early faults of the elbow head of a ring main unit, which has the following beneficial effects: By simulating the early faults of the elbow head of the ring main unit, the present invention obtains the characteristic signals unique to the elbow head of the ring main unit in the early faults, identifies and judges the early faults, so as to discover potential hazards before the equipment has permanent faults, complete replacement in time before it is completely damaged, reduce the losses caused by permanent faults, and is of great significance for improving the operation reliability of the distribution network. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained according to the provided drawings.

[0034] Figure 1 It is a flowchart of the method for simulating the early faults of the elbow head of the ring main unit of the present invention;

[0035] Figure 2 It is a schematic diagram of the external circuit in the embodiment of the present invention;

[0036] Figure 3 It is a schematic diagram of the elbow head geometric model in the embodiment of the present invention;

[0037] Figure 4 It is a schematic diagram of the potential distribution of the simulation results in the embodiment of the present invention;

[0038] Figure 5 It is a schematic diagram of the electric field strength distribution of the simulation results in the embodiment of the present invention;

[0039] Figure 6 It is an arc current waveform diagram during the water belt approaching period in the embodiment of the present invention;

[0040] Figure 7 It is an arc current waveform diagram during the water belt boiling period in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0041] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0042] An embodiment of the present invention discloses a method for simulating early faults of elbow heads of ring main units, as Figure 1 shown, including the following steps:

[0043] S1. Select the simulation dimension, the physical field used for calculation, and the research solution method;

[0044] S2. Establish a geometric model according to the actual operating state of the elbow head of the ring main unit;

[0045] S3. Set the material property parameters, boundary conditions, and initial conditions required by the physical field, and divide the grid;

[0046] S4. Set the time scale of the simulation calculation and the result output step size;

[0047] S5. Solve the calculation and determine whether the calculation result is reasonable. If it is not reasonable, return to S3. If it is reasonable, enter S6;

[0048] S6. Analyze the simulation calculation results.

[0049] Further, S1 is specifically:

[0050] S11. Considering that the simulation object is the early fault of the elbow head of the ring main unit, the arc simulation needs to couple the temperature field, flow field, electric field, and magnetic field, and two-dimensional modeling is used for simulation;

[0051] S12. Set the temperature field, flow field, electric field, and magnetic field to realize the coupling of the temperature field and the flow field, the coupling of the temperature field and the electric field, and the coupling of the magnetic field and the flow field;

[0052] S13. Based on the purpose of studying the spatio-temporal evolution of the macroscopic state of the arc in a short time, the transient research solution is selected as the solution method.

[0053] COMSOL provides five spatial dimensions for users to choose from: three-dimensional, two-dimensional axisymmetric, two-dimensional, one-dimensional axisymmetric, one-dimensional, and zero-dimensional. In the embodiments of the present invention, the simulation object is the early fault of the elbow head. The insulation interface between the elbow head and the bushing and its surrounding area should be modeled. Therefore, the number of dimensions should be selected as two-dimensional or three-dimensional. The bushing is an axisymmetric structure and two-dimensional axisymmetric modeling can be used. However, only part of the elbow head is an axisymmetric structure. Especially when considering that the arc occurs near the conductive rubber inside the elbow head, its structure cannot be completely regarded as an axisymmetric structure. Therefore, two-dimensional axisymmetric modeling cannot be selected. At the same time, arc simulation requires coupling the temperature field, flow field, electric field, and magnetic field. Considering the disadvantages of cumbersome and difficult three-dimensional modeling steps, large computational complexity, and difficulty in simulating three-dimensional arcs, as well as the fact that two-dimensional simulation can, to a certain extent, accurately obtain results that conform to the actual situation, the embodiments of the present invention use two-dimensional modeling to carry out the simulation work;

[0054] In the embodiments of the present invention, the spatio-temporal evolution of the macroscopic state of the arc can be obtained through the coupled calculation of the temperature field, flow field, electric field, and magnetic field. The temperature field is set using the fluid heat transfer module built into COMSOL; the flow field is set using the laminar flow module; the electric field is set using the current module and the circuit module; the magnetic field is set using the magnetic field module. At the same time, in the multi-physics field module, the non-isothermal flow module is used to couple the temperature field and the flow field; the equilibrium discharge heat source module and the electromagnetic heat module are used to couple the temperature field and the electric field; the Lorentz force module is used to couple the magnetic field and the flow field;

[0055] In the embodiments of the present invention, COMSOL generally provides three research and solution methods for users to choose from: frequency domain, steady state, and transient state. The embodiments of the present invention mainly calculate the spatio-temporal evolution of the macroscopic state of the arc in a short time, rather than studying the frequency domain state of the signal or the steady-state development of the arc.

[0056] Furthermore, in the simulation, an external voltage is applied outside the geometric module through an external circuit. The resistance of the external circuit is set to 2.53 Ω, the inductance is set to 3.233 mH, and the capacitance is set to 2.63 μF.

[0057] In the embodiments of the present invention, the external circuit is as Figure 2 shown, E A 、E B 、E C represent the three-phase voltages of the 10 kV power frequency system of the distribution network, and their amplitudes are 8165 V; R ∑ 、L ∑ 、C ∑ correspond to the lumped resistance, inductance, and capacitance values of the cable line between the power supply and the faulty elbow head respectively. Phase B is set as the faulty phase, that is, the water hose enters the elbow head of Phase B, causing an early fault. The geometric model of the elbow head is as Figure 3As shown in the figure, the boundaries ac and af of the geometric model in the figure correspond to node 1 in the external circuit, and de is set to correspond to node 2, realizing the coupling of the external circuit and the geometric model in the simulation model.

[0058] Furthermore, S2 specifically is:

[0059] Establish a geometric model according to the actual operating state of the elbow head of the ring main unit. The dimensions of the geometric structure of the simulation model are the same as those of the prototype device, and each region corresponds to the prototype device. The corresponding materials for each region include silicone rubber, epoxy resin, conductive rubber, water, and air arc.

[0060] Furthermore, S3 specifically is:

[0061] S31. Set different material physical property parameters and calculation regions of different physical fields in different regions of the geometric model. The material physical property parameters include conductivity, relative permittivity, dynamic viscosity, thermal conductivity, density, constant pressure heat capacity, and relative magnetic permeability;

[0062] S32. The simulation uses a system of differential equations to solve the change of physical parameters at each point in the spatial domain within the time domain. When solving the system of differential equations, initial conditions and boundary conditions must be provided for the system of differential equations to obtain a convergent result;

[0063] S33. Adopt free triangular meshes and divide the mesh density into three levels: the arc region is the densest, the density of the water band region is less than that of the arc region, and the remaining regions are sparse.

[0064] The main mesh generation methods provided by COMSOL for users include free triangular meshes and free quadrilateral meshes. In addition, it also provides a variety of mesh processing operations. The simulation model established in the embodiment of the present invention does not require a special mesh generation method, and the commonly used free triangular meshes can be adopted.

[0065] Furthermore, S32 specifically is:

[0066] Provide boundary conditions for the temperature field, flow field, electric field, and magnetic field respectively. The ambient temperature is set to 293.15K, the water band temperature is set to 373.15K, the flow field is set to a laminar flow including gravity, the air pressure is set to one atmosphere, and the air gap boundary is set to a no-slip boundary; set the initial phase of the B-phase power supply to 90°, set the air gap height between the elbow head of the ring main unit and the bushing to 1mm, set the air gap except the arc region to be filled with the water band, set the contact angle between the water band and silicone rubber and epoxy resin to 90°, set the initial length of the arc region in the water band approaching stage to 5mm, set the initial length of the arc region in the water band boiling stage to 3mm, and use the built-in dynamic mesh module to achieve the moving effect of the water band.

[0067] Furthermore, S4 specifically is:

[0068] Set the time scale of the simulation calculation to 80 ms, and the output interval of the calculation results is 0.01 ms.

[0069] In the embodiment of the present invention, S5 needs to judge the rationality of the calculation results. COMSOL has two built-in solution methods for transient solvers for users: fully coupled and segregated; the fully coupled solver combines all variables to be solved for calculation, which takes a longer time but requires less memory; the segregated solver solves the variables step by step to achieve the coupling relationship, which takes a shorter time but requires more memory. In the embodiment of the present invention, any one of them can be selected for calculation. If the calculation results are unreasonable or no convergent results can be obtained, the geometric model, material physical property parameters, boundary conditions and initial condition settings, and mesh division can be checked and changed, such as adjusting the geometric sharp corners to rounded corners, avoiding extreme values in the solution matrix, setting conditions reasonably, adjusting the mesh size, etc., and recalculating to obtain reasonable results.

[0070] When performing calculations, the control equations adopted by the simulation model include: mass conservation equation, momentum conservation equation, energy conservation equation, Maxwell equation and Ohm's law equation. In addition to the above equations, it also includes the electric field force on the water belt:

[0071]

[0072] In the formula, F e is the electric field force, D = εE, ε is the dielectric constant, and E is the electric field strength;

[0073] Power per unit volume of the arc:

[0074] P = JE

[0075] In the formula, P is the power per unit volume of the arc, and J is the current density;

[0076] Energy required for the vaporization of liquid water:

[0077] W = ρ0Vc(T2 - T1) + r

[0078] In the formula, W is the energy required for the vaporization of liquid water, ρ0 is the density of liquid water, V is the volume of vaporized liquid water, c is the specific heat capacity of liquid water, T2 is the boiling point of liquid water, T1 is the initial temperature of liquid water, and r is the latent heat of vaporization required for the vaporization of liquid water.

[0079] Furthermore, S6 is specifically:

[0080] S61. Analyze the potential distribution and electric field strength distribution when the external system voltage reaches the peak value in three cases where the elbow head of the ring main unit and the bushing are in full contact, only have an air gap, and there is a water belt in the air gap.

[0081] S62. Analyze the geometric dimensions, current density distribution, and temperature distribution of the arc during the water belt approaching stage and the water belt boiling stage;

[0082] S63. Draw the arc current waveform curves for the water belt approaching stage and the water belt boiling stage respectively.

[0083] In an embodiment of the present invention, the potential distribution and electric field intensity distribution in three cases obtained from the simulation result analysis are as Figure 4 and Figure 5 shown. It can be seen that when the elbow head and the bushing are in a state of full contact and only an air gap exists, the potential distribution is almost the same, indicating that only the existence of an air gap between the elbow head and the bushing is not much different from the normal operating state, and only the existence of an air gap is not sufficient to cause a failure of the elbow head. When moisture enters the air gap between the elbow head and the bushing, since the water belt is connected to the grounded outer surface of the elbow head, the entire water belt is at the ground potential, and the air gap between the conductive rubber inside the elbow head and the water belt bears the potential difference between the conductive rubber and the water belt, that is, the distribution network voltage. The presence of the water belt significantly increases the voltage borne by the air part in the air gap.

[0084] When the elbow head and the bushing are in a state of full contact, the electric field intensity is concentrated near the conductive rubber inside the elbow head. However, during the manufacturing process of the elbow head, the conductive rubber used to improve the electric field distribution inside the elbow head and the silicone rubber serving as the insulating main body of the elbow head are pressed together, and the conductive rubber and the silicone rubber are tightly combined. The high electric field intensity near the conductive rubber is not sufficient to break down the solid silicone rubber insulation. Therefore, the elbow head can operate stably under normal circumstances. When an air gap appears between the elbow head and the bushing, the electric field intensity is still concentrated near the conductive rubber inside the elbow head, but its electric field intensity is not sufficient to break down the entire air gap and trigger surface flashover, and the elbow head can still operate safely. When moisture enters the air gap between the elbow head and the bushing, especially when the water belt has approached the conductive rubber, the electric field concentration point appears at the conductive rubber and the water belt near the conductive rubber, which provides conditions for the air between the conductive rubber and the water belt to be broken down. When the water belt is close enough to the conductive rubber, the air between the conductive rubber and the water belt is broken down, and an arc is formed. The electric field intensity near the water belt on the side close to the conductive rubber is relatively large, and the electric field force acting on the water belt mainly acts on the water belt at this place, causing it to extend inward.

[0085] In an embodiment of the present invention, the geometric dimensions, current density distribution, and temperature distribution of the arc during the water belt approaching stage and the water belt boiling stage are analyzed. From the perspective of the arc morphology, the arcs in the water belt approaching stage and the water belt boiling stage have common characteristics, as well as special characteristics formed due to different respective conditions. Generally speaking, the temperature field in the arc region is basically symmetrically distributed. The center of the arc column is the region with the highest radial temperature. The temperature of the arc in the radial direction gradually decreases from the center of the arc column outwards, which conforms to the characteristics of a general arc. The high temperature in the axial direction of the arc is concentrated at the contact between the arc column and the water belt, that is, the root of the arc has the highest temperature. The reason is that there is a significant difference in the conductivity between the arc in the high-temperature state and the water belt. The resistance at their contact is high, so the heat generation is large. During the water belt approaching stage, due to the approach of the water belt and the influence of thermal buoyancy, the contact between the arc column and the water belt shows a certain degree of upward floating. In the water belt boiling stage, the movement trend of the water belt is away from the arc region, and the space of the arc is not occupied and compressed by the water belt. Therefore, there is no obvious upward floating at the contact position between the arc column and the water belt.

[0086] During the water belt approaching stage, only one side of the arc is the water belt. Compared with the situation where both sides of the arc are the water belt in the water belt boiling stage, the arc in the water belt approaching stage is easier to retain heat. At the same time, the water belt approaches the arc direction, compressing the arc region and the high-temperature air, reducing the heat transfer area of the arc to the solid insulating medium. The water belt can also absorb heat from the solid insulation where the temperature has risen, reducing the energy absorbed from the arc. Overall, it relatively promotes the re-ignition of the arc. However, the environment of the arc in the water belt boiling stage is exactly the opposite of that in the water belt approaching stage, and the overall re-ignition difficulty continuously increases. The arc in the water belt approaching stage is basically full of the air gap in height, while the arc in the water belt boiling stage is slender after ignition. The two are basically unchanged relative to the radial width of their own arc columns. However, due to the easier heat retention of the arc in the water belt approaching stage, its temperature field distribution is also basically unchanged, and the temperature of the arc column even increases slightly. In the water belt boiling stage, due to the gradually increasing energy dissipation of the arc, the temperature of its arc column slightly decreases, and it is difficult to maintain the temperature at the lower temperature part of the arc region.

[0087] The distribution shape and numerical value of the current density are basically unchanged during the water belt approaching stage. Although the shape is basically unchanged in the water belt boiling stage, its numerical value significantly increases. The reason is that during the water belt approaching stage, the arc is compressed by the approaching water belt, and the arc shortens, which is beneficial to the accumulation of temperature. In the water belt boiling stage, the arc is elongated by the away water belt, exacerbating the heat dissipation in the arc region, increasing the resistance of the outer arc column in the radial direction of the arc to a certain extent, concentrating the current density towards the center of the arc column, and causing the increase in the current density.

[0088] In an embodiment of the present invention, the arc current waveform curves in the water belt approaching stage and the water belt boiling stage are plotted as Figure 6 and Figure 7As shown, it can be seen that the dry area between the conductive rubber inside the elbow head and the water belt during the water belt approaching stage and the dry area between the water belts during the water belt boiling stage need the voltage to reach a certain value to be broken down. The voltage borne by the arc area during breakdown is related to the difficulty of arc reignition. The higher the voltage required to break down the dry area, the later the occurrence time of the sudden increase in current in the arc current waveform, and it also shows that the zero rest duration of the current waveform is longer;

[0089] The zero rest durations of the current waveforms during the water belt approaching stage are 3.52 ms, 3.14 ms, 2.81 ms, 2.62 ms, 2.24 ms, and 1.91 ms respectively, and the zero rest durations of the current waveforms during the water belt boiling stage are 2.95 ms, 3.46 ms, 3.93 ms, 4.19 ms, 4.35 ms, and 4.52 ms respectively. Through the analysis of the arc shape simulation results, it can be seen that during the water belt approaching stage, the water belt is affected by polarized charges and extends towards the conductive rubber inside the elbow head under the action of the electric field force, compressing the length of the arc area, reducing the dissipation of arc energy, facilitating the accumulation of arc area temperature, providing favorable conditions for arc reignition, and reducing the difficulty of arc reignition. Therefore, during the alternation of arc extinction and reignition, the voltage required to break down the dry area between the conductive rubber and the water belt decreases, the current sudden increase point advances continuously, and the zero rest flat shoulder duration decreases. During the water belt boiling stage, the water belt is rapidly evaporated under the influence of the high arc temperature and retreats in the direction away from the arc. The arc is elongated due to the movement of the water belt, increasing the energy loss of the arc. A higher voltage needs to be applied to the dry area for arc reignition, and the external power supply takes a longer time to reach the breakdown voltage, so the zero rest flat shoulder duration increases.

[0090] A large number of bubbles will be randomly generated in the water body when the water belt is boiling. The expansion, movement, combination, and rupture of the bubbles will generally increase the resistance of the water belt. During the "approaching" stage of the water belt, although the dry area between the conductive rubber and the water belt has been broken down and an arc has been generated, the water belt quickly extends towards the conductive rubber and comes into contact with it. The arc action time is very short. The energy generated by the arc and the Joule heat generated by the current flowing through the water belt in this stage are not enough to make the water belt reach the boiling state, and the water belt is still in a normal liquid state. In the water belt boiling stage, the water belt connecting the conductive rubber inside the elbow head and the grounded shell quickly boils under the combined action of the energy accumulated in the previous stage and the current Joule heat, and continues until the water belt dries up. During this period, the presence of bubbles significantly increases the resistance of the water belt. Therefore, the current amplitude in the water belt approaching stage is significantly greater than that in the water belt boiling stage. The arc in the water belt approaching stage obtains more energy from the system, and the arc intensity is significantly higher than that in the water belt boiling stage.

[0091] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same or similar parts among the various embodiments, reference can be made to each other.

[0092] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the broadest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for simulating the early faults of elbow joints of ring main units, characterized in that It includes the following steps: S1. Select the simulation dimension, the physical fields used in the calculation, and the research solution method; S2. Establish a geometric model according to the actual operating state of the elbow head of the ring main unit; S3. Set the material property parameters, boundary conditions, and initial conditions required by the physical fields, and divide the grid; S4. Set the time scale of the simulation calculation and the result output step size; S5. Solve the calculation and determine whether the calculation result is reasonable. If it is not reasonable, return to S3. If it is reasonable, enter S6; S6. Analyze the simulation calculation results.

2. The simulation implementation method for early faults of the elbow head of a ring main unit according to claim 1, wherein, Specifically, S1 is as follows: S11. Considering that the simulation object is the early failure of the elbow head of the ring main unit, the arc simulation needs to couple the temperature field, flow field, electric field, and magnetic field, and two-dimensional modeling is used for simulation; S12. Set the temperature field, flow field, electric field, and magnetic field to achieve the coupling of the temperature field and the flow field, the coupling of the temperature field and the electric field, and the coupling of the magnetic field and the flow field; S13. Based on the purpose of studying the spatio-temporal evolution of the macroscopic state of the arc in a short time, the transient research solution is selected as the solution method.

3. A method for simulating the early failure of an elbow head of a ring main unit according to claim 2, characterized in that In the simulation, an external voltage is applied outside the geometric module through an external circuit. The resistance of the external circuit is set to 2.53 Ω, the inductance is set to 3.233 mH, and the capacitance is set to 2.63 μF.

4. A method for simulating the early faults of an elbow head of a ring main unit according to claim 1, characterized in that, Specifically, S2 is as follows: Establish a geometric model according to the actual operating state of the elbow head of the ring main unit. The size of the geometric structure of the simulation model is the same as that of the actual device, and each region corresponds to the actual device. The materials corresponding to each region include silicone rubber, epoxy resin, conductive rubber, water, and air arc.

5. A method for simulating the early faults of an elbow head of a ring main unit according to claim 1, characterized in that, Specifically, S3 is as follows: S31. Set different material property parameters and the calculation action regions of different physical fields in different regions of the geometric model. The material property parameters include conductivity, relative permittivity, dynamic viscosity, thermal conductivity, density, constant pressure heat capacity, and relative magnetic permeability; S32. The simulation uses a system of differential equations to solve the change of physical parameters at each point in the spatial domain within the time domain. When solving the system of differential equations, initial conditions and boundary conditions must be provided for the system of differential equations to obtain a convergent result; S33. Use free triangular meshes and divide the mesh density into three levels: the arc region is the densest, the density in the water band region is less than that in the arc region, and the rest of the regions are sparse.

6. The method for simulating and realizing the early fault of the elbow head of the ring main unit according to claim 5, characterized in that, Specifically, S32 is as follows: Provide boundary conditions for the temperature field, flow field, electric field, and magnetic field respectively. The ambient temperature is set to 293.15 K, the water band temperature is set to 373.15 K, the flow field is set to laminar flow including gravity, the air pressure is set to one atmosphere, and the air gap boundary is set to a no-slip boundary; set the initial phase of the B-phase power supply to 90°, the air gap height between the elbow head of the ring main unit and the bushing is set to 1 mm, set the air gap except the arc region to be filled with a water band, the contact angle between the water band and silicone rubber and epoxy resin is set to 90°, set the initial length of the arc region in the water band approaching stage to 5 mm, set the initial length of the arc region in the water band boiling stage to 3 mm, and use the built-in dynamic mesh module to achieve the movement effect of the water band.

7. A method for simulating the early faults of an elbow head of a ring main unit according to claim 1, characterized in that, Specifically, S4 is as follows: Set the time scale of the simulation calculation to 80 ms, and the calculation result output interval duration is 0.01 ms.

8. A method for simulating and realizing early faults of elbow heads in ring main units according to claim 1, characterized in that, Specifically, S6 is as follows: S61. Analyze the potential distribution and electric field intensity distribution when the external system voltage reaches the peak value under three conditions: the elbow head of the ring main unit is in full contact with the bushing, there is only an air gap, and there is a water band in the air gap; S62. Analyze the geometric dimensions, current density distribution, and temperature distribution of the arc during the water band approaching stage and the water band boiling stage; S63. Draw the arc current waveform curves during the water band approaching stage and the water band boiling stage respectively.

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