A simulation method of streamer discharge in insulating oil based on finite element theory

Abstract

The present invention discloses a simulation method of streamer discharge in insulating oil based on finite element theory, which includes: combining the influence of the saturation velocity of electrons on streamer propagation, establishing an electrohydrodynamic model describing the physical process of discharge in insulating oil; constructing a needle-ball geometric model for simulating streamer discharge of transformer insulating oil under lightning impulse voltage, determining the shape and size of the discharge electrode, the electrode curvature radius, and setting the solution domain and boundary conditions; according to the solution domain and boundary conditions, combining finite element theory to calculate the coupled values ​​of the fluid field, electrostatic field and temperature field in the electrohydrodynamic model, and obtaining the electric field intensity distribution, spatial charge density distribution, streamer length and streamer propagation speed of the insulating oil streamer discharge process. The present invention can well describe the kinematic behavior of charged particles in insulating oil during the discharge process, and provide important data support and theoretical basis for promoting the interpretation of the mechanism of streamer discharge in insulating oil.

Description

Technical Field

[0001] The invention relates to the technical field of transformer insulating media, and in particular to a simulation method for streamer discharge in insulating oil based on finite element theory. Background Art

[0002] Transformer is one of the most important components in the power system. A single fault in the transformer can lead to the paralysis of the entire power system. Various studies on transformer failures have shown that about 33% of the total failures in distribution transformers occur due to failures in their insulation system, while for power transformers, this value is about 75%. Therefore, the operational reliability as well as the service life of any transformer mainly depends on its insulation system. Transformer oil is the main component of the transformer insulation system. Therefore, insulating oil is extremely necessary for the smooth operation of the transformer.

[0003] In order to obtain the method and mechanism for improving the breakdown voltage of insulating oil, it is necessary to conduct research on the formation and development mechanism of streamers in insulating oil. At present, most of the research on discharge characteristics is based on experimental observations; however, since the streamer discharge process is short and fast, it is very difficult to observe and record experimental data in the experiment, which makes it difficult to measure the electric field distribution and spatial charge distribution caused by the streamer during the discharge process; therefore, it is necessary to use streamer discharge simulation analysis for research; however, the existing model provided by the streamer discharge simulation method overestimates the electron velocity and ignores the influence of the electron saturation velocity on the streamer propagation, resulting in a large deviation between the simulation results and the actual experimental results. Therefore, improving the existing model is an important research direction at present. Summary of the invention

[0004] The purpose of the present invention is to overcome the deficiencies in the prior art and to provide a simulation method for streamer discharge in insulating oil based on finite element theory, which can better describe the kinematic behavior of charged particles in insulating oil during the discharge process, thereby providing important data support and theoretical basis for promoting the interpretation of the mechanism of streamer discharge in insulating oil.

[0005] To achieve the above object, the present invention is implemented by adopting the following technical solutions:

[0006] The present invention provides a simulation method for streamer discharge in insulating oil based on finite element theory, the method comprising:

[0007] Combined with the influence of the electron saturation velocity on the propagation of the streamer, an electrohydrodynamic model describing the physical process of discharge in the insulating oil is established. The electrohydrodynamic model includes the continuity equations of positive ions, electrons and negative ions in the fluid field of the insulating oil, the Poisson equation of the electrostatic field and the temperature field calculation equation.

[0008] Construct a needle-sphere geometric model for simulation of transformer insulating oil streamer discharge under lightning impulse voltage, determine the shape and size of the discharge electrode, the electrode curvature radius, and set the solution domain and boundary conditions;

[0009] According to the solution domain and boundary conditions, combined with finite element theory, the coupled values ​​of the fluid field, electrostatic field and temperature field in the electrohydrodynamic model are calculated to obtain the electric field intensity distribution, spatial charge density distribution, streamer length and streamer propagation velocity of the insulating oil streamer discharge process.

[0010] Furthermore, the continuity equations of the positive ions, electrons and negative ions in the fluid field of the insulating oil are respectively:

[0011]

[0012]

[0013]

[0014] In the formula, is the Laplace operator; is the electric field strength vector; ρ p is the density of positive ion particles; ρ n is the density of negative ion particles; ρ e is the electron particle density; μ p is the positive ion mobility; μ n is the negative ion mobility; μ e is the electron mobility; G F1 is the charge density generation rate under the applied pulse electric field; G F2 G is the charge density generation rate under the pulse electric field in the gas phase streamer; F3 is the rate of change of charge density caused by the dissociation of ion pairs in insulating oil; R pn is the positive-negative ion recombination rate; R pe is the positive ion-electron recombination rate; τ a is the adsorption time constant.

[0015] Further, the electron mobility μ is determined e The steps for obtaining the value of include:

[0016] Considering the influence of the electron saturation velocity on streamer propagation, the electron velocity v is corrected, and the calculation formula is as follows:

[0017]

[0018] Where, E is the electric field strength; μ e_0 is the initial electron mobility, which is 1.0×10 -4 m 2 / (V·s); E0 and β are two different fitting parameters, E0=7.5×10 -7 km / s, β = 1;

[0019] According to the corrected electron velocity v, the electron mobility μ is determined e The calculation formula is:

[0020]

[0021] Furthermore, the electrostatic field Poisson equation is:

[0022]

[0023] Where: ε0 is the dielectric constant of vacuum; ε r is the relative dielectric constant of insulating oil.

[0024] Furthermore, the temperature field calculation equation is:

[0025]

[0026] Where: T is the temperature of the insulating oil, is the flow velocity, ρ oil is the oil ion density, c v is the specific heat capacity, k T is the thermal conductivity coefficient.

[0027] Furthermore, the step of constructing a simulated needle-ball geometric model for transformer insulating oil streamer discharge under lightning impulse voltage includes:

[0028] According to IEC60641 standard, the radius is set to 5×10 -4 m ball electrode, and set the needle electrode with a tip curvature of 50 μm;

[0029] The needle electrode is connected to the lightning impulse voltage terminal, and the ball electrode is grounded;

[0030] The standard lightning impulse voltage of two exponential functions subtracted from each other is applied to the needle electrode:

[0031]

[0032] Where V impulse is the standard lightning impulse voltage, K is the correction coefficient, V0 is the applied voltage amplitude, τ1 is the wave head time, τ2 is the wave tail time, and the rising and falling edge times of the impulse voltage are adjusted by adjusting τ1 and τ2.

[0033] Furthermore, the step of setting the solution domain and boundary conditions includes:

[0034] For the continuity equations of positive ions, electrons, and negative ions in the fluid field of the insulating oil, both the needle electrode and the spherical electrode take the convection diffusion condition The zero charge boundary condition n·N=0 is taken for each surface boundary of the calculation area, where n is the unit vector in the normal direction of the interface, N=ρμE, ρ is the charged particle density, μ is the charged particle mobility, and E is the field strength;

[0035] For the electrostatic field Poisson equation, the needle electrode is set to high voltage excitation, the voltage is in the form of lightning impulse voltage; the potential of the spherical electrode is set to zero; the zero charge boundary condition n·D=0 is taken for each surface boundary of the calculation area; wherein D is the field vector of the boundary;

[0036] For the temperature field calculation equation, all boundaries are set to be adiabatic.

[0037] Furthermore, it also includes meshing the solution domain and boundaries: setting mesh parameters for different solution domains and boundaries, including mesh shape, maximum cell size ratio adjustment coefficient, cell growth rate, mesh curvature coefficient, mesh cutoff curvature, narrow area relaxation and meshing method.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] The present invention establishes an electrohydrodynamic model for the streamer discharge process of insulating oil under lightning pulse voltage, which is used to describe the electric field intensity distribution, spatial charge density distribution, streamer length and streamer propagation speed of the insulating oil between electrodes. The model is based on the continuity equations of positive ions, electrons and negative ions in the fluid field, the Poisson equation of the electrostatic field and the temperature field calculation equation, and takes into account the influence of the electron saturation velocity on the streamer propagation. The model is solved through finite element theory to make the streamer propagation speed more consistent with the experimental results, thereby providing important data support and theoretical basis for improving the insulating performance of the insulating oil; at the same time, the generation mechanisms of three charged particles, namely field ionization, collision ionization and dissociation of ions in the oil, are taken into account to make the electrohydrodynamic model more perfect.

[0040] In addition, experimental data show that the propagation speed and development pattern of the streamer in the electrohydrodynamic model established by the present invention are consistent with experimental observations; the streamer discharge models under different voltage levels are compared, verifying the hypothesis that the higher the voltage level, the longer the streamer propagation distance and the faster the speed; providing guidance for improving the performance of insulating oil and providing new ideas for subsequent researchers. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a flow chart of a simulation method for streamer discharge in insulating oil based on finite element theory provided by an embodiment of the present invention;

[0042] Figure 2It is a schematic diagram of a geometric model of a needle-ball electrode for partial discharge of transformer oil insulation under lightning impulse voltage provided by an embodiment of the present invention;

[0043] Figure 3 is a grid division diagram provided by an embodiment of the present invention;

[0044] Figure 4 is a voltage waveform diagram provided by an embodiment of the present invention;

[0045] Figure 5 is a schematic diagram of the speed of electrons at different field strengths at different electron saturation velocities provided by an embodiment of the present invention;

[0046] Figure 6 is a schematic diagram of electric field intensity distribution during streamer development provided by an embodiment of the present invention;

[0047] Figure 7 is a schematic diagram of spatial charge density distribution during the streamer development process provided by an embodiment of the present invention;

[0048] Figure 8 is a schematic diagram of the distribution of electric field strength under different voltage amplitudes provided by an embodiment of the present invention;

[0049] Fig. 9 is a schematic diagram of the change of the propagation length of the streamer along the z-axis with time provided by an embodiment of the present invention;

[0050] Fig.10 It is a schematic diagram comparing simulation and experimental results of streamer propagation speeds under different voltage amplitudes provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0051] The technical solution of the present invention is described in detail below through the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present application and the specific features in the embodiments are detailed descriptions of the technical solution of the present application, rather than limitations on the technical solution of the present application. In the absence of conflict, the embodiments of the present application and the technical features in the embodiments can be combined with each other.

[0052] The term "and / or" in this article is only a description of the association relationship of the associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0053] like Figure 1 As shown, this embodiment introduces a simulation method for streamer discharge in insulating oil based on finite element theory, which specifically includes the following steps:

[0054] Step S1: In combination with the influence of the saturation velocity of electrons on streamer propagation, an electrohydrodynamic model describing the physical process of discharge in insulating oil is established, wherein the electrohydrodynamic model includes continuity equations of positive ions, electrons, and negative ions in the fluid field of insulating oil, Poisson's equation of electrostatic field, and temperature field calculation equation;

[0055] Step S2: construct a needle-sphere geometric model for simulation of transformer insulating oil streamer discharge under lightning impulse voltage, determine the shape and size of the discharge electrode, the electrode curvature radius, and set the solution domain and boundary conditions;

[0056] Step S3: According to the solution domain and boundary conditions, combined with finite element theory, the coupled values ​​of the fluid field, electrostatic field and temperature field in the electrohydrodynamic model are calculated to obtain the electric field intensity distribution, spatial charge density distribution, streamer length and streamer propagation speed of the insulating oil streamer discharge process.

[0057] This embodiment specifically involves the following contents.

[0058] Reference Figure 2 As shown, a half model of the needle-ball geometric model for simulating transformer insulating oil streamer discharge under lightning impulse voltage provided by an embodiment of the present invention is shown; it is an axisymmetric geometric model with boundary 1 as the axis; the material of the ball electrode is brass, boundary 1 is selected as axisymmetric, needle electrode boundaries 6 and 7 are set to outflow, ball electrode boundary 5 is also set to outflow, and other boundaries 2, 3, and 4 are set to no flux; wherein boundary 1 is the oil gap width, and the width can be set as required; boundary 5 is the ball electrode boundary, and the radius is set to 5×10 according to the IEC 60641 standard -4 m; boundary 6 is the needle electrode tip, boundary 7 is the needle electrode end, and the needle tip curvature is set to 50μm according to IEC 60641; the needle electrode is connected to the lightning impulse voltage terminal, and the ball electrode is grounded;

[0059] For the lightning impulse voltage, the needle electrode uses the standard lightning impulse voltage which is the subtraction of two exponential functions:

[0060]

[0061] Where V impulse is the standard lightning impulse voltage, K is the correction coefficient, V0 is the applied voltage amplitude, τ1 is the wave head time, τ2 is the wave tail time, and the rising and falling edge times of the impulse voltage are adjusted by adjusting τ1 and τ2; the waveform of the lightning impulse voltage applied in this embodiment is as follows Figure 4 shown.

[0062] As an embodiment of the present invention, when establishing an electrohydrodynamic model describing the physical process of discharge in insulating oil, it should be noted that insulating oil is a liquid insulating medium and has fluid properties, and the generation, transport and disappearance of positive ions, electrons and negative ions in the insulating oil during streamer discharge can be described in the form of fluid motion; according to the principle of mass conservation, the fluid medium has continuity, and the convection-diffusion equation can be used to describe the relationship between the density of charged particles in the fluid and time;

[0063] Specifically, with respect to the generation of space charge, the carriers in the insulating oil can be divided into electrons, positive ions and negative ions. Under normal circumstances, the insulating oil contains only a small amount of electrons, positive ions and negative ions. When a strong electric field is applied to the insulating oil, when the molecular ionization energy is reached, many electrons, positive ions and negative ions will be generated, and streamer discharge begins to occur. The model established in the embodiment of the present invention considers three space charge generation mechanisms, namely field ionization, collision ionization and dissociation of oil molecules. The charge generation terms of the three mechanisms are:

[0064]

[0065]

[0066]

[0067]

[0068] In formula (2), G F1 is the charge density generation rate under the applied pulse electric field, q is the unit charge, n0 is the number of molecules contained in the unit volume of liquid dielectric, a is the molecular distance, is the absolute value of the electric field, m * is the effective electron mass, Δ0 is the electrical energy required for ionization of transformer oil molecules, γ is the electric field-related ionization potential coefficient, and h is Planck's constant; in formula (3), G F2 is the charge density generation rate under the pulse electric field in the gas phase streamer, A t is the collision ionization coefficient, B t is the collision coefficient, ρ e is the electron density, μ e is the electron migration rate; in formula (4), G F3 is the charge density change rate caused by the dissociation of ion pairs in the oil, I1 represents the first kind of modified Bessel function, ε0 is the vacuum dielectric constant, ε ro is the relative dielectric constant of transformer oil, σ is the conductivity of oil, μ p is the positive ion mobility and μ n is the negative ion mobility; in formula (5), k Bis the thermal conductivity of the oil, and T is the insulating oil temperature.

[0069] For electron speed: see Figure 5 is the speed of electrons at different field strengths at different electron saturation speeds; since the electron speed will reach the saturation speed under extremely high electric fields, and the existing models do not consider the influence of the electron saturation speed on the streamer propagation; therefore, in order to accurately describe the electron speed, the present invention combines the influence of the electron saturation speed on the streamer propagation and corrects the electron speed v, and its calculation formula is as follows:

[0070]

[0071] Where E is the electric field strength, and the value of E is 1.0×10 -4 m 2 / (V·s); E0 and β are two different fitting parameters, E0=7.5×10 -7 km / s, β=1; μ e_0 is the initial electron mobility;

[0072] According to the corrected electron velocity v, the electron mobility μ is determined e The calculation formula is:

[0073]

[0074] Based on the calculations of the above formulas (2) to (7), the positive ion density, negative ion density and electron density are coupled through the Poisson equation, and the heat conduction equation is introduced to simulate the temperature change in the liquid and the generation of oil and gas. The electrohydrodynamic model describing the physical process of discharge in insulating oil is established as shown in the following equation group:

[0075]

[0076]

[0077]

[0078]

[0079]

[0080] In the formula, is the Laplace operator; is the electric field strength vector; ρ p is the density of positive ion particles; ρ n is the density of negative ion particles; ρ e is the electron particle density; μ p is the positive ion mobility; μ n is the negative ion mobility; μe is the electron mobility; G F1 is the charge density generation rate under the applied pulse electric field; G F2 G is the charge density generation rate under the pulse electric field in the gas phase streamer; F3 is the rate of change of charge density caused by the dissociation of ion pairs in insulating oil; R pn is the positive-negative ion recombination rate; R pe is the positive ion-electron recombination rate; τ a is the adsorption time constant; ε0 is the vacuum dielectric constant; ε r is the relative dielectric constant of the insulating oil; T is the temperature of the insulating oil, is the flow velocity, ρ oil is the oil ion density, c v is the specific heat capacity, k T is the thermal conductivity coefficient; where formula (8) is the Poisson equation for the electrostatic field; formulas (9), (10) and (11) are the continuity equations for positive ions, electrons and negative ions in the fluid field of the insulating oil respectively; formula (12) is the temperature field calculation equation.

[0081] As an embodiment of the present invention, before calculating the coupled values ​​of the fluid field, electrostatic field and temperature field in the electrohydrodynamic model, it is necessary to further explain that: since this model has four equations with strong coupling properties, four equations need to be solved at each grid point. Therefore, the amount of calculation required to solve the model increases exponentially with the increase in grid points. Excessive amount of calculation may exceed the computing power of the computer and make the equations unable to be solved. Therefore, it is necessary to optimize the grid division in combination with the physical characteristics of the flow development. When targeting the area near the main flow channel, a more detailed grid division is used. Figure 3 As shown, when the mesh is divided in the embodiment of the present invention, the needle-ball electrode connecting line is used as the symmetry axis, and a rectangular auxiliary surface is made within 50 microns on the left and right of the symmetry axis. The maximum mesh side length in the auxiliary surface is set to 5 microns; in other areas, the method of controlling the mesh variables is adopted so that the mesh size gradually increases from the rectangular auxiliary surface outward; the mesh control variables are shown in Table 1:

[0082] Table 1

[0083] Parameter name Parameter selection Grid Shape triangle Maximum cell size scaling factor 1 Unit growth rate 1.3 Mesh curvature coefficient 0.3 Mesh cutoff curvature 0.001 Narrow zone relaxation 1 The best quality yes

[0084] In practice, from the expressions (2), (3), and (4) of the three carrier generation mechanisms of on-site ionization, impact ionization, and dissociation of oil molecules, it can be seen that the carrier generation rate is the highest at the peak of the electric field, so a large number of carriers are generated in the area near the peak; after the carriers are generated, the electrons move rapidly to the anode under the action of the electric field force; and because the mobility of positive ions is small, the displacement is not large within the time scale of the streamer development, so the net space charge near the needle electrode is positive. In addition, when the initial local field strength exceeds the streamer starting field strength (2×10 8 V / m), a large number of carriers will be generated in these local areas through impact ionization and move in the insulating oil; due to the different migration rates of electrons and ions, the different spatial distributions of positive ions and electrons, the total space charge is not zero, which will cause the electric field to be distorted; when the space charge is large enough, the electric field at the head of the streamer is significantly enhanced, promoting the further development of the streamer.

[0085] Correspondingly, Figure 6 to Figure 10 The simulation effect of the experiment using the simulation method of streamer discharge in insulating oil based on finite element theory provided by the embodiment of the present invention is demonstrated.

[0086] For details, see Figure 6 As shown in the figure, it shows the simulation of the initiation and development of the electric field intensity of the streamer in the oil gap at 30ns, 60ns, 90ns, 120ns, 220ns and 250ns under the action of a pulse voltage with a peak value of 120kV and a rising edge of 100ns, using a two-dimensional axisymmetric needle-ball electrode model; it can be seen from the figure that with the extension of time, the streamer extends away from the needle electrode to the ball, and the streamer begins to develop from about 30ns, the streamer extends away from the needle electrode to the ball, and the volume of the streamer head gradually increases; at 250ns, the streamer reaches the ball electrode, and the time of the entire breakdown process is about 250ns. Within 30ns, the concentration of space charge has basically not changed, indicating that the molecular ionization energy has not been reached in the insulating oil, and the electric field strength at this time cannot ionize the neutral molecules in the insulating oil. At 90ns, an obvious streamer is generated, which gradually develops away from the needle electrode and flows toward the ball electrode. At about 250ns, the streamer reaches the ball electrode, and the time of the entire breakdown process is about 250ns.

[0087] See also Figure 7As shown in the figure, under the action of a pulse voltage with a peak value of 120kV and a rising edge of 100ns, a two-dimensional axisymmetric needle-ball electrode model is used to simulate the initiation and development of the spatial electric field intensity of the streamer in the oil gap at 90ns, 120ns, 220ns and 250ns. It can be seen from the figure that as time goes on, the streamer extends away from the needle electrode to the ball. The streamer starts to develop from about 90ns, the streamer extends away from the needle electrode to the ball, and the volume of the streamer head gradually increases. At 250ns, the streamer reaches the ball electrode, and the entire breakdown process takes about 250ns.

[0088] See also Figure 8 As shown in the figure, the influence of voltage amplitude on the electric field strength and space charge density distribution in the streamer discharge process in insulating oil was investigated, and positive polarity pulse voltages with applied voltage amplitudes of 80kV, 100kV, 120kV and 140kV were applied respectively. It was found that a higher applied voltage will lead to a higher initial field strength. Under the influence of a higher initial field strength, more space charges are generated, and the further development of the streamer is promoted, which means that a higher applied voltage is more conducive to promoting the development of the streamer, and the coverage area of ​​the streamer development is larger.

[0089] See also Fig. 9 As shown, the lengths of the streamers under different applied voltage amplitudes are compared; it can be clearly seen from the figure that the streamer length of the model of the present invention is smaller than the streamer length of the existing model; this is because the existing model overestimates the electron speed, so the streamer length of the existing model is longer; therefore, the simulation results of the improved model provided by the embodiment of the present invention are more consistent with the actual results.

[0090] See also Fig.10 As shown, the comparison between the model of the present invention and the experimental results under different applied voltage amplitudes is further shown; since the applied voltage amplitude ranges from 50kV to 90kV, the streamer velocity ranges from 2.08km / s to 4.53km / s, and the deviation from the experimental results is less than 3%; therefore, the present invention more accurately simulates the propagation velocity of the streamer.

[0091] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0092] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0093] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0094] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0095] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A simulation method for streamer discharge in insulating oil based on finite element theory, characterized in that: The method comprises: Combined with the influence of the electron saturation velocity on the propagation of the streamer, an electrohydrodynamic model describing the physical process of discharge in the insulating oil is established. The electrohydrodynamic model includes the continuity equations of positive ions, electrons and negative ions in the fluid field of the insulating oil, the Poisson equation of the electrostatic field and the temperature field calculation equation. Construct a needle-sphere geometric model for simulation of transformer insulating oil streamer discharge under lightning impulse voltage, determine the shape and size of the discharge electrode, the electrode curvature radius, and set the solution domain and boundary conditions; According to the solution domain and boundary conditions, the coupled values ​​of the fluid field, electrostatic field and temperature field in the electrohydrodynamic model are calculated in combination with finite element theory to obtain the electric field intensity distribution, spatial charge density distribution, streamer length and streamer propagation speed of the insulating oil streamer discharge process; Among them, the continuity equations of the positive ions, electrons and negative ions in the fluid field of the insulating oil are respectively: ； ； ； In the formula, is the Laplace operator; is the electric field strength vector; is the density of positive ion particles; is the density of negative ion particles; is the electron particle density; is the positive ion mobility; is the negative ion mobility; is the electron mobility; is the charge density generation rate under the applied pulse electric field; is the charge density generation rate under the pulsed electric field in the gas phase streamer; is the rate of change of charge density caused by the dissociation of ion pairs in the insulating oil; is the positive-negative ion recombination rate; is the positive ion-electron recombination rate; is the adsorption time constant.

2. The simulation method of streamer discharge in insulating oil based on finite element theory according to claim 1, characterized in that: Determine the electron mobility The steps for obtaining the value of include: Combined with the effect of electron saturation velocity on streamer propagation, the electron velocity The calculation formula is as follows: ； In the formula, is the electric field strength; is the initial electron mobility, which is 1.0×10 -4 m 2 / (V·s); E0 and β are two different fitting parameters, E0=7.5×10 -7 km / s, β=1; According to the corrected electron velocity , determines the electron mobility The calculation formula is: 。 3. The simulation method of streamer discharge in insulating oil based on finite element theory according to claim 2, characterized in that: The Poisson equation for the electrostatic field is: ； Where: is the dielectric constant of vacuum; is the relative dielectric constant of insulating oil.

4. The simulation method of streamer discharge in insulating oil based on finite element theory according to claim 3 is characterized in that: The temperature field calculation equation is: ； Where: is the temperature of the insulating oil, is the flow velocity, is the oil ion density, is the specific heat capacity, is the thermal conductivity coefficient.

5. The simulation method of streamer discharge in insulating oil based on finite element theory according to any one of claims 1 to 4, characterized in that: The steps of constructing a simulated needle-ball geometric model of transformer insulating oil streamer discharge under lightning impulse voltage include: According to IEC60641 standard, the radius is set to A ball electrode and a needle electrode with a tip curvature of 50 μm are provided; The needle electrode is connected to the lightning impulse voltage terminal, and the ball electrode is grounded; The standard lightning impulse voltage of two exponential functions subtracted from each other is applied to the needle electrode: ； In the formula, is the standard lightning impulse voltage, is the correction factor, is the applied voltage amplitude, is the wave head time, is the wave tail time, by adjusting and Adjust the rising and falling edge time of the impulse voltage.

6. The simulation method of streamer discharge in insulating oil based on finite element theory according to claim 5, characterized in that: The steps of setting the solution domain and boundary conditions include: For the continuity equations of positive ions, electrons, and negative ions in the fluid field of the insulating oil, both the needle electrode and the spherical electrode take the convection diffusion condition ; The boundary conditions of each surface in the calculation area are zero charge boundary conditions ,in, is the unit vector in the normal direction of the interface, , is the charged particle density, is the charged particle mobility, is the field strength; For the electrostatic field Poisson equation, the needle electrode is set to high voltage excitation, the voltage form is lightning impulse voltage; the potential of the spherical electrode is set to zero; the boundaries of each surface in the calculation area take zero charge boundary conditions ;in, is the field vector of the boundary; For the temperature field calculation equation, all boundaries are set to be adiabatic.

7. The simulation method of streamer discharge in insulating oil based on finite element theory according to claim 1 or 6, characterized in that: It also includes meshing the solution domain and boundaries: setting mesh parameters for different solution domains and boundaries, including mesh shape, maximum cell size scaling factor, cell growth rate, mesh curvature factor, mesh cutoff curvature, narrow area relaxation, and meshing method.

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