Simulation method of continuous current pulse of negative polarity corona discharge

Abstract

The present invention discloses a method for simulating a continuous current pulse of a negative polarity corona discharge, specifically: step 1, according to the needle-plate electrode structure, a two-dimensional axisymmetric model is established in COMSOL software to determine the calculation area, and the two-dimensional axisymmetric model is meshed using triangular units; step 2, Bolsig+ software is used to solve the two-term Boltzmann equation to obtain the reduced ionization coefficient, reduced adsorption coefficient, electron mobility, and electron diffusion rate of the discharge gas; step 3, by establishing a continuous current pulse formation mechanism of a negative polarity corona discharge, and setting the boundary condition of the electron at the tip of the needle electrode as a secondary electron flux, the spatiotemporal distribution of electrons, positive ions, and negative ions is calculated; step 4, the discharge current is calculated. The present invention solves the problem that only a single discharge pulse can be obtained in the prior art.

Description

Technical Field

[0001] The invention belongs to the technical field of insulation monitoring and fault diagnosis of electric power equipment, and relates to a method for simulating continuous current pulses of negative polarity corona discharge. Background Art

[0002] SF6 gas has been widely used as an insulating medium in power equipment due to its excellent insulation and arc extinguishing properties. However, the greenhouse effect of SF6 gas is very serious, and its global warming potential is 23,900 times that of CO2. At present, the research on SF6 alternative gases mainly focuses on the mixture of SF6 with N2 and CF4, CF3I and its mixture, c_C4F8 and its mixture, C4F7N and its mixture, C5F 10 O and its mixed gas, C6F 12 O and its mixed gases, etc.

[0003] However, during the manufacturing, installation and operation of power equipment, burrs and metal particles are inevitably left behind, which will cause the internal electric field of the equipment to be distorted, triggering partial discharge or even flashover, and reducing the insulation performance of the insulating medium. Corona discharge is the most typical form of partial discharge in high-voltage electrical equipment, mostly caused by sharp points or protrusions on the conductor. Studying the corona discharge mechanism of SF6 and its alternative gases is of great significance to preventing sudden insulation failures in power systems and ensuring the safe and reliable operation of power systems.

[0004] Due to the lack of effective experimental means to detect the microscopic characteristics of corona discharge, the use of numerical simulation technology to study the pulse characteristics and discharge development process of corona discharge provides a new idea for revealing the mechanism of corona discharge. Most of the existing research focuses on air, N2, O2, etc. There are very few studies on the current pulse characteristics of SF6 and its alternative gas corona discharge, and most of them can only obtain a single discharge pulse. Summary of the invention

[0005] The purpose of the present invention is to provide a method for simulating a continuous current pulse of a negative polarity corona discharge, thereby solving the problem that only a single discharge pulse can be obtained in the prior art.

[0006] The technical solution adopted by the present invention is a method for simulating a continuous current pulse of a negative polarity corona discharge, which is specifically implemented according to the following steps:

[0007] Step 1, according to the needle-plate electrode structure, a two-dimensional axisymmetric model is established in COMSOL software to determine the calculation area, and the two-dimensional axisymmetric model is meshed using triangular elements;

[0008] Step 2: Use Bolsig+ software to solve the two-term Boltzmann equation to obtain the reduced ionization coefficient α / N, reduced adsorption coefficient η / N, and electron mobility μ of the discharge gas.e , electron diffusion rate D e ;

[0009] Among them, α is the ionization coefficient, η is the adsorption coefficient, and N is the total number density of particles;

[0010] Step 3, by establishing a continuous current pulse formation mechanism of negative polarity corona discharge and setting the boundary condition of electrons at the tip of the needle electrode as secondary electron flux, the spatiotemporal distribution of electrons, positive ions, and negative ions is calculated;

[0011] Step 4, calculate the discharge current.

[0012] The present invention is also characterized in that

[0013] In step 1, the calculation area includes the boundary of the needle electrode, the boundary of the plate electrode, and the discharge area of ​​the needle-plate gap.

[0014] In step 1, the diameter of the needle electrode in the needle-plate electrode is 4 mm, the radius of curvature of the needle electrode is 0.3 mm, the diameter of the plate electrode is 120 mm, the thickness of the plate electrode is 10 mm, and the gap between the needle and the plate is 8 mm.

[0015] In step 1, during the meshing process, the mesh density at the needle electrode 2 and the gap between the needle plate is greater than the density at a location far from the needle electrode 2 and the gap between the needle plate.

[0016] In step 2, the discharge gas SF6 and its substitute gas, the substitute gas includes SF6 and its mixed gas, CF3I and its mixed gas, c_C4F8 and its mixed gas, C4F7N and its mixed gas, C5F 10 O and its mixed gas, C6F 12 O and one of its mixed gases.

[0017] In step 3, the expression for calculating the spatiotemporal distribution of electrons, positive ions, and negative ions is:

[0018]

[0019]

[0020]

[0021]

[0022] In formulas (1)-(4), t is time, n is e is the number density of electrons, n p is the number density of positive ions, n n is the number density of negative ions, μ e is the electron mobility, μ pis the mobility of positive ions, μ n is the mobility of negative ions, D e is the electron diffusion rate, D p is the diffusion rate of positive ions, D n is the diffusion rate of negative ions, α is the ionization coefficient, η is the adsorption coefficient, is the electric field, K ep is the recombination coefficient of electrons and positive ions, K np is the recombination coefficient of positive and negative ions, V is the applied voltage, e is the electron charge, ε0 is the vacuum dielectric constant, ε r is the relative dielectric constant.

[0023] In step 3, the expression for the secondary electron flux is:

[0024]

[0025] In formula (5), Γ e is the secondary electron flux, and γ is the secondary electron emission coefficient.

[0026] In step 4, the expression for the discharge current is:

[0027]

[0028] In formula (6), I is the current, r is the radial coordinate, z is the axial coordinate, and E L is the Laplace electric field, and s is the calculation area.

[0029] The beneficial effects of the present invention are:

[0030] (1) The method for simulating the continuous current pulse of negative polarity corona discharge of the present invention can obtain the continuous current pulse and reveal the quasi-three-dimensional image of the discharge, reveal the formation mechanism of the continuous current pulse of negative polarity corona discharge and explain its internal mechanism, overcome the shortcomings of the prior art that the characteristic quantity of corona discharge cannot be measured and only a single discharge pulse can be obtained, and improve the corona discharge mechanism;

[0031] (2) The negative polarity corona discharge continuous current pulse simulation method of the present invention is not only for pure SF6, but also for SF6 and its mixed gas, CF3I and its mixed gas, c_C4F8 and its mixed gas, C4F7N and its mixed gas, C5F 10 O and its mixed gas, C6F 12 One of SF6 and its mixed gases, providing a theoretical basis for the application of SF6 and its alternative gases in power equipment and insulation optimization;

[0032] (3) The negative polarity corona discharge continuous current pulse simulation method of the present invention can save the cost and time required for the experiment, and the calculation amount is easily borne, which is of great significance in insulation monitoring and insulation optimization of power equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a structural schematic diagram of a needle-plate electrode in a negative polarity corona discharge continuous current pulse simulation method of the present invention;

[0034] Figure 2 It is a continuous current pulse diagram of negative polarity corona discharge of 20% SF6-80% CF4 mixed gas obtained by the negative polarity corona discharge continuous current pulse simulation method of the present invention;

[0035] Figure 3 It is a reduced field intensity diagram at the tip of the needle electrode of a 20% SF6-80% CF4 mixed gas negative polarity corona discharge obtained by the negative polarity corona discharge continuous current pulse simulation method of the present invention.

[0036] In the figure, 1. plate electrode, 2. needle electrode. DETAILED DESCRIPTION

[0037] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] The present invention provides a method for simulating a continuous current pulse of a negative polarity corona discharge, which is specifically implemented according to the following steps:

[0039] Step 1, using a needle-plate electrode to simulate the metal tip defect, establishing a two-dimensional axisymmetric model in COMSOL software according to the needle-plate electrode structure, and meshing the two-dimensional axisymmetric model using triangular elements, with a mesh quality greater than 0.95;

[0040] Since the entire discharge area is three-dimensional and radially isotropic, a quasi-three-dimensional image of the discharge can be obtained by selecting a two-dimensional axisymmetric plane. Therefore, a two-dimensional axisymmetric model is established to determine the calculation area, and the calculation area includes the boundary of the needle electrode, the boundary of the plate electrode, and the discharge area of ​​the needle-plate gap.

[0041] The needle-plate electrode is a typical discharge structure in corona discharge, such as Figure 1 As shown, it includes a plate electrode 1, a needle electrode 2 is arranged above the plate electrode 1, and the needle electrode 2 is perpendicular to the plate electrode 1, and a needle-plate gap is arranged between the plate electrode 1 and the needle electrode 2, wherein the diameter of the needle electrode 2 is 4 mm, the diameter of the plate electrode 1 is 120 mm, the thickness of the plate electrode 1 is 10 mm, the needle-plate gap is 8 mm, and the curvature radius of the needle electrode is 0.3 mm;

[0042] During the meshing process, the mesh density at the gap between the needle electrode 2 and the needle plate is greater than the density far away from the gap between the needle electrode 2 and the needle plate, that is, the mesh at the gap between the needle electrode 2 and the needle plate is denser, and the mesh far away from the gap between the needle electrode 2 and the needle plate is sparser.

[0043] Step 2: Take the discharge gas ratio and the collision cross section data of the discharge gas found in the lxcat database or literature as input, and use Bolsig+ software to solve the two-term Boltzmann equation (take the first two terms after expanding the Boltzmann equation) to obtain the reduced ionization coefficient α / N, reduced adsorption coefficient η / N, and electron mobility μ of the discharge gas. e , electron diffusion rate D e ;

[0044] Among them, α is the ionization coefficient, η is the adsorption coefficient, and N is the total number density of particles;

[0045] The discharge gas is SF6 and its substitute gas, and the substitute gas includes SF6 and its mixed gas, CF3I and its mixed gas, c_C4F8 and its mixed gas, C4F7N and its mixed gas, C5F 10 O and its mixed gas, C6F 12 O and one of its mixed gases. The specific discharge gas is determined according to the mixed gases and their ratios commonly used in the actual operation of GIS. The gas pressure is the typical gas pressure in GIS, i.e., 0.4-0.6 MPa.

[0046] Step 3, by establishing a continuous current pulse formation mechanism of negative polarity corona discharge, calculate the spatiotemporal distribution of electrons, positive ions, and negative ions;

[0047] The corona discharge process of the discharge gas includes a large number of particles and chemical reactions. However, it is easy to miss some chemical reactions when summarizing the chemical reactions and the amount of calculation is unbearable. This patent uses the coupling of the migration-diffusion equation of electrons, positive ions and negative ions and the Poisson equation to study the dynamic behavior of corona discharge and the formation mechanism of continuous current pulses, such as equations (1) to (4). The source terms of the equations include ionization reactions, adsorption reactions, recombination reactions of electrons and positive ions, and recombination reactions of positive ions and negative ions. By solving equations (1) to (4) together, the spatiotemporal distribution of electrons, positive ions and negative ions is obtained.

[0048]

[0049]

[0050]

[0051]

[0052] In formulas (1)-(4), t is time, n is e is the number density of electrons, n p is the number density of positive ions, n n is the number density of negative ions, μ e is the electron mobility, μ p is the mobility of positive ions, μ n is the mobility of negative ions, D e is the electron diffusion rate, D p is the diffusion rate of positive ions, D n is the diffusion rate of negative ions, α is the ionization coefficient, η is the adsorption coefficient, is the electric field, K ep is the recombination coefficient of electrons and positive ions, K np is the recombination coefficient of positive and negative ions, V is the applied voltage, e is the electron charge, ε0 is the vacuum dielectric constant, ε r is the relative dielectric constant;

[0053] Since the negative polarity corona discharge is a self-sustaining discharge, it is maintained by the secondary electrons generated by the collision of positive ions with the needle electrode. Therefore, it is necessary to set the electron boundary condition at the tip of the needle electrode as the secondary electron flux. The expression of the secondary electron flux is:

[0054]

[0055] In formula (5), Γ e is the secondary electron flux, γ is the secondary electron emission coefficient;

[0056] Step 4, calculate the discharge current, the expression is:

[0057]

[0058] In formula (6), I is the current, r is the radial coordinate, z is the axial coordinate, and E L is the Laplace electric field, and s is the calculation area.

[0059] Figure 2 This is a continuous current pulse diagram of negative polarity corona discharge of 20% SF6-80% CF4 mixed gas obtained by the present invention. Figure 3 The reduced field intensity diagram at the tip of the negative polarity corona discharge needle electrode of the 20% SF6-80% CF4 mixed gas obtained by the present invention, wherein the reduced field intensity is E / N, E is the electric field, N is the total number density of particles, and the calculation time is 3μs. Three continuous pulses can be clearly seen, and the amplitude of the pulses gradually decreases with time. This is mainly caused by the strong electronegativity of SF6. After being formed, electrons are easily adsorbed by molecules to form negative ions; Figure 3The reduced field intensity at the tip of the middle needle electrode has a maximum value, a minimum value and a stable value, indicating that the electric field will experience a process of space charge (i.e., negative ions) suppression and electric field recovery during the discharge process. The pulse characteristics of the continuous current pulse obtained by the present invention, including parameters such as pulse amplitude, pulse frequency, pulse interval time, pulse rise time, pulse fall time, etc., can be used to evaluate the degree of corona discharge and the degree of insulation degradation. These pulse characteristics and electric field change parameters such as pulse amplitude, pulse frequency, interval time between pulses, rise time, fall time, etc. can be used for insulation monitoring and fault prediction of power equipment, thereby reducing sudden faults.

Claims

1. A method for simulating a continuous current pulse of a negative polarity corona discharge, characterized in that: Follow the steps below to implement it: Step 1, according to the needle-plate electrode structure, a two-dimensional axisymmetric model is established in COMSOL software to determine the calculation area, and the two-dimensional axisymmetric model is meshed using triangular elements; Step 2: Use Bolsig+ software to solve the two-term Boltzmann equation to obtain the reduced ionization coefficient α / N, reduced adsorption coefficient η / N, and electron mobility μ of the discharge gas. e , electron diffusion rate D e ; Among them, α is the ionization coefficient, η is the adsorption coefficient, and N is the total number density of particles; Step 3, by establishing a continuous current pulse formation mechanism of negative polarity corona discharge and setting the boundary condition of electrons at the tip of the needle electrode as secondary electron flux, the spatiotemporal distribution of electrons, positive ions, and negative ions is calculated; In step 3, the expression for calculating the spatiotemporal distribution of electrons, positive ions, and negative ions is: In formulas (1)-(4), t is time, n is e is the number density of electrons, n p is the number density of positive ions, n n is the number density of negative ions, μ e is the electron mobility, μ p is the mobility of positive ions, μ n is the mobility of negative ions, D e is the electron diffusion rate, D p is the diffusion rate of positive ions, D n is the diffusion rate of negative ions, α is the ionization coefficient, η is the adsorption coefficient, is the electric field, K ep is the recombination coefficient of electrons and positive ions, K np is the recombination coefficient of positive and negative ions, V is the applied voltage, e is the electron charge, ε0 is the vacuum dielectric constant, ε r is the relative dielectric constant; In step 3, the expression for the secondary electron flux is: In formula (5), Γ e is the secondary electron flux, γ is the secondary electron emission coefficient; Step 4, calculating the discharge current; In step 4, the expression for the discharge current is: In formula (6), I is the current, r is the radial coordinate, z is the axial coordinate, and E L is the Laplace electric field, and s is the calculation area.

2. The negative polarity corona discharge continuous current pulse simulation method according to claim 1, characterized in that: In step 1, the calculation area includes the boundary of the needle electrode, the boundary of the plate electrode, and the discharge area of ​​the needle-plate gap.

3. The negative polarity corona discharge continuous current pulse simulation method according to claim 1, characterized in that: In step 1, the diameter of the needle electrode in the needle-plate electrode is 4 mm, the radius of curvature of the needle electrode is 0.3 mm, the diameter of the plate electrode is 120 mm, the thickness of the plate electrode is 10 mm, and the gap between the needle and the plate is 8 mm.

4. The method for simulating a continuous current pulse of a negative polarity corona discharge according to claim 1, characterized in that: In step 1, during meshing, the mesh density at the gap between the needle electrode and the needle plate is greater than the density far from the gap between the needle electrode and the needle plate.

5. The method for simulating a continuous current pulse of a negative polarity corona discharge according to claim 1, characterized in that: In step 2, the discharge gas SF6 and its substitute gas, the substitute gas includes SF6 and its mixed gas, CF3I and its mixed gas, c_C4F8 and its mixed gas, C4F7N and its mixed gas, C5F 10 O and its mixed gas, C6F 12 O and one of its mixed gases.

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