Method and system for simulating dynamic evolution of needle plate electrode bubbles under multi-physics field of transformer

Abstract

The application provides a needle-plate electrode bubble dynamic evolution simulation method and system under a multi-physical field of a transformer, comprising the following steps: establishing a needle-plate electrode model, wherein the needle-plate electrode model comprises a container containing insulating oil, a needle electrode and a plate electrode are installed in the container in a relative arrangement, and an insulating paper is arranged on the upper surface of the plate electrode; setting the voltage of the surface of the needle electrode, collecting fluid data and material attribute parameters of the needle-plate electrode model in the discharge process of the needle electrode under the voltage; setting boundary conditions based on the structure of the needle-plate electrode model; selecting a calculation domain and performing grid division on the calculation domain; based on the fluid data, the material attribute parameters, the boundary conditions and the calculation domain, a multi-coupling field bubble dynamic model is constructed, which is used for the needle-plate electrode bubble dynamic evolution simulation under the multi-physical field of the transformer, can simulate the change of the speed component of a single suspended bubble in a power frequency cycle with the voltage amplitude and the change of the shape stretching, and can explain the influence of the bubbles in the transformer oil on the spatial evolution of the discharge under the multi-field coupling.

Description

Technical Field

[0001] This invention belongs to the field of power equipment technology, specifically relating to a method and system for simulating the dynamic evolution of bubbles in needle-plate electrodes under multi-physics fields in transformers. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Oil-paper composite insulation is commonly used in critical components of power transformers. Transformer oil plays a crucial role in insulation and cooling, and its quality significantly impacts the performance of the oil-paper insulation. Frequent partial discharges within the transformer oil cause it to gradually heat up through heat transfer, leading to decomposition and the generation of new gases. When the gas solubility in the insulating oil reaches saturation, microbubbles form. The presence of these bubbles accelerates the distortion of the spatial electric field, causing breakdown along the gas channels. This cyclical positive feedback effect results in an increasing number of bubbles in the oil, intensifying the discharge and causing irreversible and severe damage to the insulation, thus compromising the stability and reliability of operating equipment and the power system. Therefore, it is necessary to conduct in-depth research on the motion and deformation characteristics of suspended bubbles and analyze the gradual cumulative effect.

[0004] In recent years, scholars both domestically and internationally have conducted a series of studies on the generation mechanism, physical properties, and motion laws of various forms of bubbles through field experiments and simulation experiments, achieving relatively rich research results. For example, the Department of Thermal Fluid Flow at Tabriz University in Iran studied the influence of uniform and non-uniform DC electric fields on the rising motion and deformation of a single coarse bubble in a medium by digitally processing high-speed video images (600fps); Nick Lelekakis et al. at Monash University in Australia studied bubble formation in a vegetable oil-impregnated paper system under overload and compared the results with previous studies on mineral oil using similar testing equipment and methods; Tadashi Amakawa et al. at the Central Research Institute of Electric Power Industry in Japan analyzed the influence of bubble volume on arc energy, providing the bubble volume generated per unit energy, and studied the relationship between the bubbles generated by the arc in the insulating oil and the gas-liquid phase pressure in a two-phase closed container of air-insulating oil; Gaurav Tomar et al. at the Indian Institute of Science used a coupled level set and fluid volume method to study the coupled bubbles in two-phase flow. Numerical simulations were used to study the influence of AC electric field frequency on the bubble formation process at the pinhole and needle

[49] . Toshiaki Rokunohe et al. of Hitachi Group in Japan established a transformer insulation structure model and studied the interaction between partial discharge and bubble generation, as well as the influence of bubble generation on partial discharge after multiple lightning strikes. Korobeynikov et al. of the Fulianev Hydroelectric Research Institute conducted experimental research on the deformation of bubbles in transformer oil under uniform electric field. Based on the lattice Boltzmann equation method, they established a model of electrofluid flow in this process and studied the dynamic process of different single floating bubbles after partial discharge. DiMarco et al. of the Department of Energy and Systems Engineering at the University of Pisa Losa in Italy analyzed the original experiment of bubble separation and calculated the electric force of rising bubbles using the finite element method, clarifying the role of different mechanisms in enhancing the electrohydraulic dynamics of two-phase fluids.

[0005] In summary, current research on suspended bubbles in liquid phases primarily focuses on experiments and simulation modeling. However, in observational studies of bubbles, existing experiments often involve the generation of bubbles within the oil through a series of injections using probes or bubble generators. This artificially induced dynamic change in bubbles differs significantly from the natural movement of suspended bubbles under real-world conditions. Furthermore, experiments often emphasize the formation process of individual bubbles, with little description of their subsequent motion and deformation. Regarding bubble simulation model design, existing models typically use a single field as the finite element simulation environment for bubbles. However, in actual operating conditions, bubbles exist within a multi-physics field coupled with alternating electric fields, gravitational fields, and laminar liquid phase fields. Multiple bubbles frequently undergo coalescence and fusion reactions, the impact of which on partial discharge cannot be analyzed in existing experiments.

[0006] Therefore, the current research background on the dynamics of suspended bubbles in oil does not conform to the multi-coupled field environment in the actual transformer oil-paper insulation. The migration and deformation laws of more individual bubbles and complex bubbles are still in the preliminary exploration stage, and their close correlation with partial discharge breakdown has not been explained in detail. How to simulate the bubbles of the needle plate electrode under the multi-coupled physical field of the transformer to obtain the influence of the bubbles in the transformer oil on the spatial evolution of discharge under multi-field coupling is a technical problem that needs to be solved. Summary of the Invention

[0007] To address the aforementioned issues, this invention proposes a method and system for simulating the dynamic evolution of bubbles in a needle-plate electrode under multi-physics fields in a transformer. A dynamic model of bubbles in a needle-plate electrode under multiple coupled physical fields in a transformer is established. The migration trajectories of individual suspended bubbles obtained from experiments and simulations are compared to verify the reliability of the model. The invention simulates the velocity component variation and shape expansion / contraction changes of individual suspended bubbles within the power frequency cycle as a function of voltage amplitude, and analyzes the fluctuation law of pressure formed inside and outside the bubble by electrostrictive force during migration. Furthermore, the invention simulates the absorption process of multiple bubbles in a strong field region near the needle electrode, analyzes the changes in surrounding liquid phase pressure and spatial electric field distribution during gas channel formation, and interprets the influence of bubbles in transformer oil on the spatial evolution of discharge under multi-field coupling.

[0008] According to some embodiments, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for simulating the dynamic evolution of bubbles in a needle-plate electrode under multi-physics fields of a transformer, comprising:

[0009] Establish a needle-plate electrode model, which includes a container containing insulating oil, a needle electrode and a plate electrode installed opposite to each other inside the container, and insulating paper on the upper surface of the plate electrode.

[0010] Set the voltage on the surface of the needle electrode, and collect fluid data and material property parameters during the discharge process of the needle electrode in the needle plate electrode model under this voltage;

[0011] Boundary conditions are set based on the structure of the needle plate electrode model;

[0012] Select a computational domain and mesh the computational domain;

[0013] Based on fluid data, material property parameters, boundary conditions, and computational domain, a multi-coupled field bubble dynamic model is constructed for simulating the dynamic evolution of bubbles in the needle plate electrode under multi-physics fields of a transformer.

[0014] Secondly, the present invention provides a simulation system for the dynamic evolution of bubbles in a needle-plate electrode under multi-physics fields of a transformer, comprising:

[0015] The needle-plate electrode module includes a container containing insulating oil, in which needle electrodes and plate electrodes are installed opposite each other, and insulating paper is provided on the upper surface of the plate electrodes.

[0016] The experimental data acquisition and observation module is used to set the voltage on the surface of the needle electrode and acquire fluid data and material property parameters during the discharge process of the needle electrode of the needle plate electrode model under this voltage.

[0017] The bubble dynamic evolution simulation module sets boundary conditions based on the structure of the needle plate electrode model; selects the computational domain and performs meshing on the computational domain; and constructs a multi-coupled field bubble dynamic model based on fluid data, material property parameters, boundary conditions, and computational domain for simulating the dynamic evolution of bubbles in the needle plate electrode under multi-physics fields of a transformer.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] 1. This invention establishes a needle-plate electrode model, which includes a container filled with insulating oil, with needle electrodes and plate electrodes installed opposite each other inside the container. Insulating paper is placed on the upper surface of the plate electrodes. Based on fluid data, material property parameters, boundary conditions, and computational domain, a multi-coupled-field bubble dynamic model is constructed to simulate the dynamic evolution of bubbles in the needle-plate electrode under multiple physical fields of a transformer. The migration trajectory of a single suspended bubble obtained from experiments and simulations is compared to verify the reliability of the model. The invention simulates the velocity component change and shape expansion and contraction of a single suspended bubble with voltage amplitude within the power frequency cycle, and analyzes the fluctuation law of electrostrictive force forming pressure inside and outside the bubble during migration. The invention also simulates the absorption process of multiple bubbles in the strong field region near the needle electrode, analyzes the changes in surrounding liquid phase pressure and spatial electric field distribution when the gas channel is formed, and interprets the influence of bubbles in transformer oil on the spatial evolution of discharge under multiple field coupling. This invention solves the technical problem of how to simulate the bubbles in the needle-plate electrode under multiple coupled physical fields of a transformer.

[0020] 2. To analyze the impact of the presence of microbubbles on the space surrounding the electrode, this invention establishes a longitudinally arranged three-bubble coalescing model. The three-bubble coalescing model is to set three circular bubbles with successively decreasing radii in a multi-coupled field bubble dynamics model and arrange them radially around the center of the sphere. By analyzing the changes in fluid volume fraction over a set time period, the impact of the presence of microbubbles on the space surrounding the electrode is analyzed.

[0021] 3. This invention provides a study on the complex dynamic process of bubbles under the combined effects of alternating electric fields, fluid phase fields, and gravitational fields. A gas-liquid two-phase flow model of the composite physical field is established. The changes in bubble velocity, expansion and contraction, pressure, and electric field strength are simulated from two aspects: the migration and deformation of individual bubbles and the interconnection and coalescence of multiple bubbles. The analysis shows that the initial displacement angle of a single suspended bubble located below the needle electrode increases with the increase of the AC voltage amplitude until it reaches the limit deflection angle; the change in bubble expansion and contraction is consistent with the change in bubble surface pressure and maximum pressure, and the periodic change of the volume force caused by field-induced expansion and contraction shows a decaying fluctuation; when multiple bubbles coalesce, a pressure difference is formed between the bubbles, and the central oil gap will gradually be repelled and isolated by the gas channel over time, resulting in a tendency to flow to both sides and a seepage phenomenon, forming a gas channel; as the voltage gradually increases, the field strength at the center of each suspended bubble is much higher than the field strength between the bubbles.

[0022] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

[0023] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0024] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0025] Figure 1 A flowchart illustrating the simulation method for the dynamic evolution of bubbles in a needle-plate electrode under multiphysics conditions in a transformer.

[0026] Figure 2 This is a diagram of a needle-plate electrode model;

[0027] Figure 3 Diagram of the experimental data acquisition and observation module;

[0028] Figure 4 This is a diagram of a bubble simulation model;

[0029] Figure 5a This is a spatial position map of a suspended bubble at different times under high-speed imaging.

[0030] Figure 5b This is a spatial position diagram of a suspended bubble at various times in the simulation environment;

[0031] Figure 6 A diagram showing the displacement characteristics of suspended bubbles during the observation experiment;

[0032] Figure 7This is a graph showing the displacement characteristics of a suspended bubble as a function of voltage in a simulation experiment.

[0033] Figure 8 The variation of the lateral velocity component of the suspended bubble in the simulation experiment;

[0034] Figure 9 This is a graph showing the variation of the longitudinal velocity component of the suspended bubble in the simulation experiment;

[0035] Figure 10 A classification diagram of the deformation of suspended bubbles;

[0036] Figure 11 A classification diagram of the deformation of suspended bubbles;

[0037] Figure 12 This is a diagram showing the periodic expansion and contraction deformation of a suspended bubble.

[0038] Figure 13 The distribution of the unit volume force of electrostriction on the bubble in the longitudinal and transverse directions;

[0039] Figure 14 This is a graph showing the periodic variation of the maximum pressure and surface pressure of a suspended bubble.

[0040] Figure 15 A diagram showing the setup for simulating multi-bubble coalescence;

[0041] Figure 16 The graph shows the volume fraction variation of multi-bubble aggregation.

[0042] Figure 17 This is a graph showing the absolute pressure changes as multiple bubbles coalesce;

[0043] Figure 18 A diagram showing the electric field distribution in the surrounding space caused by the existence of multiple bubbles and their dynamic deformation.

[0044] The components include: 1. Oil tank; 2. Adjusting rod; 3. Insulating paper; 4. Needle electrode; 5. Plate electrode; 6. Insulating oil; 101. Resistor; 102. AC power supply; 103. DC blocking capacitor; 104. Voltage divider; 105. Digital oscilloscope; 106. Flicker-free background light source; 107. CT; 108. Needle-plate discharge device; 109. High-speed camera. Detailed implementation method:

[0045] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0046] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0047] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0048] Example 1:

[0049] like Figure 1 As shown, this embodiment provides a method for simulating the dynamic evolution of bubbles in a needle-plate electrode under a multiphysics field of a transformer, including:

[0050] Establish a needle-plate electrode model, which includes a container containing insulating oil, a needle electrode and a plate electrode installed opposite to each other inside the container, and insulating paper on the upper surface of the plate electrode.

[0051] Set the voltage on the surface of the needle electrode, and collect fluid data and material property parameters during the discharge process of the needle electrode in the needle plate electrode model under this voltage;

[0052] Boundary conditions are set based on the structure of the needle plate electrode model;

[0053] Select a computational domain and mesh the computational domain;

[0054] Based on fluid data, material property parameters, boundary conditions, and computational domain, a multi-coupled field bubble dynamic model is constructed for simulating the dynamic evolution of bubbles in the needle plate electrode under multi-physics fields of a transformer.

[0055] The flow system of the multi-coupled field bubble dynamics model consists of insulating oil and bubble two-phase fluid substances. The fluid dynamics characteristics of the system are characterized by the Navier-Stokes equations (NS equations) in the domain of the two-phase flow-phase field interface.

[0056] The multi-coupled-field bubble dynamics model uses the Kahn-Hillard equation obtained by the phase field method to describe and distinguish the changes at the gas-liquid two-phase fluid interface.

[0057] The electric field coupled with the electrodynamic force of the two-phase flow-phase field in the multi-coupled field bubble dynamic model is represented by the Maxwell stress tensor in divergence form and must satisfy the electric field interface constraint condition.

[0058] The boundary conditions satisfy the charge conservation within the electrostatic field, and the voltage on the surface of the needle electrode is used to change the electric field distribution in space.

[0059] The needle electrode is positioned above the plate electrode. The needle-plate electrode model uses a container filled with insulating oil. The needle electrode and plate electrode are installed opposite to each other inside the container. Insulating paper is placed on the upper surface of the plate electrode, so that the insulating oil, needle-plate electrode, plate electrode and insulating paper form a multi-physics field coupled with alternating electric field, gravity field and laminar liquid phase field.

[0060] The multi-coupled field bubble dynamics model establishes a two-dimensional coordinate system with the needle tip as the origin. The boundary conditions include the laminar open boundary of the bubble seeping from the insulating oil surface, the two side edge boundaries of the insulating oil, the metal surface of the needle electrode, and the metal surface of the plate electrode.

[0061] The computational domain is selected by setting the vertical line between the metal surfaces of the needle tip and the needle plate electrode as the grid control edge, so that the grid on both sides can be customized in size according to the computational needs.

[0062] The needle electrode and the plate electrode are in insulating oil. During the experiment, the needle electrode is connected to an external voltage and the plate electrode is grounded.

[0063] The material property parameters include insulating oil density, insulating oil dynamic viscosity, insulating oil relative permittivity, gas density, gas dynamic viscosity, gas relative permittivity, and interface thickness.

[0064] The fluid data includes the velocity of each phase of the fluid within the system, the density of the fluid, the dynamic viscosity of the liquid, the pressure inside the fluid, and the volume fraction of gas and liquid in the fluid.

[0065] The formulas that characterize the fluid dynamics of a system using the Navier-Stokes equations (NS equations) are as follows:

[0066]

[0067]

[0068] Where u represents the velocity of each phase of the fluid within the system; ρ represents the density of the fluid; μ is the dynamic viscosity of the liquid (Pa·s); p represents the pressure inside the fluid; and g is the acceleration due to gravity. st and F e These represent the additional volume forces caused by fluid surface tension and electric field, respectively. is a vector differential operator; T represents transpose; t is time.

[0069] The Kahn-Hillard equation obtained using the phase-field method is as follows:

[0070]

[0071]

[0072] Ψ is the stream function; in the formula As phase field variables, insulating oil is assigned a value of 1 and gas a value of -1, meaning the region of stable phase field variable change is [-1, 1]. However, insulating oil and gas have different densities and dynamic viscosities, requiring further consideration based on... and F st Calculated separately. σ and ε represent the fluid surface tension coefficient and interface thickness parameter, respectively, while χ is the mobility of the control parameter adjustment interface. Since the initial charge of the model's electric field is 0, it conforms to the zero-charge potential Poisson equation as shown in equation (3-5).

[0073] The equation for the electromotive force coupled between the electric field and the two-phase flow-phase field is:

[0074]

[0075]

[0076]

[0077]

[0078] D=ε0ε r E (3-9)

[0079]

[0080] ε r =ε r1 V f1 +ε r2 V f2 (3-11)

[0081] In formulas (3-5)-(3-11), ε0, ε r Representing vacuum and relative permittivity respectively, V is electric potential; F and T are Maxwell's surface tension and tensor respectively, I is the identity matrix; E and D are electric field intensity and electric displacement vector respectively; in the second-order matrix, E x E y ε represents the electric field strength in the transverse and longitudinal directions of the fluid. r1 and ε r2 It is the relative permittivity of gases and liquids, V f1 With V f2 This refers to the fluid volume fraction corresponding to gas and liquid.

[0082] The values ​​of some material parameters in the formulas of the multi-coupled field bubble dynamic model are shown in Table 3-1:

[0083] Table 3-1 Values ​​of material property parameters in the simulation model

[0084] name Value Insulating oil density <![CDATA[885kg / cm 3 ]]> gas density <![CDATA[1.2kg / cm 3 ]]> Dynamic viscosity of insulating oil 0.0167 Pa·s Gas dynamic viscosity 1.83e-5 Pa·s Relative permittivity of insulating oil 2.4 Relative permittivity of gas 1 Interface thickness 9μm

[0085] To analyze the impact of the presence of microbubbles on the space surrounding the electrode, a three-bubble coalescing model arranged longitudinally was established. The three-bubble coalescing model consists of three circular bubbles with successively decreasing radii arranged radially around the center of the sphere in a multi-coupled field bubble dynamics model. The impact of the presence of microbubbles on the space surrounding the electrode is analyzed by the change in fluid volume fraction over a set time period.

[0086] As one implementation method, the three-bubble coalescing model involves setting up three circular bubbles, A, B, and C, with radii of 60μm, 45μm, and 40μm respectively, in the bubble dynamics model and arranging them radially from the center of the sphere. Six peripheral points are selected as special points for studying bubble-related variables. Figure 16 The volume fraction, pressure, and electric field intensity of the bubble at different locations were recorded at 0, 10, 20, 30, and 40 μs. The changes in fluid volume fraction within 40 μs were observed to analyze the influence of the presence of microbubbles on the space surrounding the electrode.

[0087] Specifically, this embodiment addresses existing problems by establishing a dynamic model of bubble in a needle-plate electrode under multiple coupled physical fields of a transformer. The migration trajectories of individual suspended bubbles obtained from experiments and simulations were compared to verify the model's reliability. The velocity component variation and shape expansion / contraction of individual suspended bubbles within the power frequency cycle were simulated with respect to voltage amplitude, and the fluctuation law of pressure generated inside and outside the bubble by electrostrictive force during migration was analyzed. The absorption process of multiple bubbles in the strong field region near the needle electrode was simulated, and the changes in surrounding liquid phase pressure and spatial electric field distribution during gas channel formation were analyzed, illustrating the influence of bubbles in transformer oil on the spatial evolution of discharge under multiple field coupling.

[0088] A test apparatus for suspended air bubbles in oil is used in large power transformers. During manufacturing and use, the oil-paper insulation structure of the windings undergoes physical and chemical corrosion, producing products such as copper sulfide and iron oxide. When these products adhere to the metal surface, they form defects such as burrs. Charge accumulates in areas with small radii of curvature, gradually generating a highly non-uniform electric field in space, leading to partial discharge. Therefore, to more closely approximate the actual operating conditions of partial discharge in the transformer windings, an oil-paper insulation model with pin-plate electrodes is used, such as... Figure 2 As shown.

[0089] Figure 2 In the needle-plate electrode model, the radius of curvature of the needle electrode tip is 50 μm, the thickness of the oil-impregnated cardboard, and the needle-plate spacing are all set to 1.0 mm. The needle electrode is made of carbon steel, while its support and plate electrode are made of brass. During partial discharge experiments, the needle electrode is connected to an externally applied voltage, and the plate electrode is directly grounded. The integrated experimental observation circuit connected to the needle-plate discharge model is as follows: Figure 3 As shown.

[0090] Figure 3The experimental data acquisition and observation module consists of an electrical section and an observation section. The electrical section includes an AC power supply obtained by boosting the voltage from a power frequency AC voltage generator, a current-limiting protective resistor, and a DC blocking capacitor to eliminate DC interference. The observation section includes a voltage divider to assist in measuring the voltage and current of the discharge device, current transformers (CTs) connected to the voltage and current signal channels of a digital oscilloscope, a flicker-free supplementary light, and a high-speed imaging system.

[0091] Table 1. Main components and equipment parameters of the experimental measurement platform:

[0092]

[0093]

[0094] This embodiment provides simulation modeling and experimental verification of bubble two-phase flow under multi-field coupling. First, a bubble two-phase flow simulation model is performed. The discharge device used in the experiment is a cylindrical glass container. Since the high-speed camera only captures a 4000×2000 (μm) area of ​​the discharge environment during actual observation, the rectangular frame selected for the simulation model also uses the same dimensions. Figure 4 As shown. The radius of curvature of the needle electrode is set to 50 μm according to the actual experimental size, and a two-dimensional coordinate system is established with the needle tip as the origin. B1 is the laminar open boundary from which bubbles seep out of the insulating oil surface; B2, B3, and B4 represent the two side edge boundaries of the oil and the metal surface of the needle plate electrode, respectively, both of which are set as the laminar no-slip wall and the phase field wetting wall. Phase field variable fluid 1 is specified as bubble, and phase field variable fluid 2 is selected as transformer oil. The entire rectangular region satisfies the charge conservation in the electrostatic field. The electric field distribution in the space is changed by the voltage set on the needle electrode surface B3 (the potential of the ground surface below B4 is 0). Finally, the vertical line between the needle tip and B4 is set as the grid control edge, so that the grid on both sides is customized according to the calculation needs.

[0095] This embodiment provides experimental verification of the bubble simulation model. To eliminate the influence of bubble clusters, fiber impurities, and discharge impacts on the selected bubble, the experiment selected a recovery and stabilization period with a relatively long discharge interval for filming. During this time period, the motion trajectory of the suspended bubble near the needle electrode was recorded. Taking the start-up moment of the suspended bubble as time zero, the spatial position changes of a single suspended bubble at 0ms, 2ms, 4ms, 6ms, 8ms, and 10ms were filmed under a 25kV AC voltage. Figure 5a As shown.

[0096] Figure 5a The spatial position of the suspended bubble at various times under high-speed imaging is given by... Figure 5aIt can be seen that within a 10ms timeframe (half a cycle of the power frequency), the bubble mainly undergoes initial hovering, initial acceleration, longitudinal stretching, near-circular bottom sliding, offset upward floating, and natural floating after moving away from the strong field region. Based on the force conditions of the gas-liquid two-phase flow model under multi-coupled fields, it can be seen that the bubble migration process is affected by the combined forces of field-induced traction, buoyancy, flow velocity viscous drag, and gravity. The corresponding spatial positions in the simulation environment and at various experimental moments are as follows: Figure 5b As shown.

[0097] Figure 5b The spatial position of the suspended bubble at various moments in the simulation environment is determined by... Figure 5b It can be seen that the bubble accelerates under the field-induced traction force, undergoes longitudinal stretching deformation, and gradually becomes dominant as it moves to the right, overcoming the viscous resistance of the insulating oil and smoothly rising from its lowest point in the longitudinal direction. Figure 5a In comparison, it can be seen that the trends in the spatial position changes of the bubble in the simulation and the experiment are basically consistent. According to... Figure 4 A rectangular coordinate system was established as shown. The complete change process of a single bubble's spatial position from hovering to floating within two cycles (40ms) of the power frequency voltage was statistically analyzed. Its movement pattern is shown as curve. Figure 6 As shown.

[0098] Figure 6 To observe the displacement characteristics of suspended bubbles in the experiment, by Figure 6 It can be seen that the bubble located obliquely below the tip of the needle electrode moves in an approximately "U"-shaped trajectory under the combined effects of the alternating electric field, gravitational field, and laminar phase field. Referring to ideal experimental conditions, in the simulation settings, AC voltages of 5kV, 10kV, 15kV, 20kV, and 25kV were applied to the needle electrode. The centers of bubbles with different shapes (the center of a sphere as the center, and the center of an ellipsoid as the intersection of the major and minor axes) were used as the basic points of motion to study the influence of different voltage amplitudes on the fluid motion of the bubble. Figure 7 As shown.

[0099] Figure 7 To simulate the displacement characteristics of suspended bubbles as voltage changes in the experiment, Figure 7Using the initial coordinates (200, -250) of the bubble as a reference point, it can be seen that when the bubble moves diagonally downwards and to the right in the early stage, its trajectory is close to a straight line. Furthermore, the larger the AC voltage amplitude, the larger the clockwise angle between the displacement vector and the x-axis. When the voltage increase reaches its limit, its displacement deflection angle will also approach the initial direction angle of 53.14°. Similarly, as the voltage amplitude gradually decreases, the vertical component of the electrodynamic force on the bubble also decreases accordingly. When the resultant force of this component and gravity gradually weakens to below the bubble's own buoyancy, its displacement deflection angle will gradually decrease counterclockwise until it crosses the horizontal direction and reaches the first quadrant. Finally, when the voltage is 0, the bubble's movement direction coincides with the positive direction of the vertical axis, achieving a natural floating state. In addition, comparing the five sets of displacement curves in the figure, when the voltage amplitude increases, the arc of the "U"-shaped trajectory becomes larger, and the lateral and longitudinal displacement distances also increase relatively. This indicates that the increase in the lateral and longitudinal components of the electrodynamic force before floating directly imparts more kinetic energy to the bubble through work.

[0100] The randomly observed bubble size in the experiment was approximately 150 μm, with initial coordinates around (200, -360); in contrast, the simulated bubble size was 70 μm, with initial coordinates at (200, -250). From the perspective of interference in the experiment itself, various impurities in the insulating oil have a certain adsorption effect on the bubbles. Furthermore, due to fluctuations caused by weak discharges (such as corona discharge), the insulating oil experiences small oscillations within it. These factors all contribute to the bubbles deviating from their theoretical trajectory. However, the control group... Figure 6 and Figure 7 It was found that although the initial conditions of the bubbles were different, the bubble displacement laws obtained by the experiment and simulation were roughly consistent within the allowable error range. However, the larger the bubble size and the farther the longitudinal distance from the needle tip, the greater the arc of the trajectory and the longer the time spent at the bottom. This reflects that the bubble dynamics model in the simulation experiment can describe the various dynamic behaviors of bubbles in multiple coupled physical fields relatively accurately.

[0101] This embodiment provides an analysis of the migration and deformation characteristics of a single bubble. Velocity, as an important vector in bubble dynamics, can further analyze the speed and direction of movement of the suspended bubble. The velocity vector of the bubble is decomposed along the horizontal and vertical axes to describe the velocity changes along the horizontal and vertical axes of the bubble within two power frequency cycles, such as... Figure 8 and Figure 9 As shown.

[0102] Figure 8 To observe the overall trend of the transverse velocity component variation of suspended bubbles in the simulation experiment. Figure 8The transverse velocity of the bubble was observed to first increase to a peak, then decrease to a trough, and then rise back to a peak over time, undergoing four cycles within 40 ms (frequency 100 Hz). Observing the timing of the peaks and troughs, it was found that the moments of maximum and minimum velocity values ​​almost correspond one-to-one with the moments of maximum and minimum (zero) values ​​of the absolute value function of the AC voltage at power frequency. When the AC voltage is at the rising edge of the positive half-cycle or the falling edge of the negative half-cycle, the higher the voltage change rate, the stronger the electric field distortion rate inside the bubble, and the greater the electromotive force formed by the Maxwell tensor and projected onto the transverse axis. Therefore, in the stage where the voltage change rate is greater than zero, the transverse electromotive force overcomes the viscous resistance of the oil and does positive work on the bubble, increasing the kinetic energy gained by the bubble. Since the bubble mass remains constant, its velocity increment exhibits a quadratic change. In the first acceleration stage, the bubble will reach its maximum kinetic energy, reaching the peak velocity. Similarly, when the voltage is at the falling edge of the positive half-cycle or the rising edge of the negative half-cycle, the rate of change of voltage is less than zero. The transverse electrodynamic force on the bubble is in the opposite direction. However, because the bubble acquires a large velocity during the acceleration phase of the electrodynamic force, the reverse electrodynamic force and the viscous friction of the insulating oil together do insufficient negative work on the bubble to offset the previously stored kinetic energy. Moreover, at this time, the displacement of the bubble is far beyond the strong field region of the electric field, and the net transverse external force decreases as the transverse displacement increases. Ultimately, the bubble still maintains a positive velocity and continues to move away from the needle electrode, thus explaining... Figure 8 The phenomenon that the amplitude of the peaks and troughs of each velocity curve decreases sequentially with the increase of the number of cycles.

[0103] Comparing the transverse velocity curves of five voltage levels and analyzing their decay trends based on velocity changes, it can be concluded that the higher the voltage amplitude (e.g., 20kV, 25kV), the faster the extreme decay rate of the transverse velocity, meaning the velocity fluctuation variance is larger than at lower voltages. Similarly, the transverse velocity at lower voltages (e.g., 5kV) is closer to a 100Hz quasi-sine curve, with extremely slow convergence. Combined with... Figure 7 It is easy to see that as time goes on, the lateral distance between the bubble and the needle electrode increases, and the electrodynamic force will gradually lose its dominant role in driving the lateral speed change of the bubble. Therefore, by Figure 8 The later time shows that the lateral velocity of the bubble is very close to zero due to the resistance of natural viscous friction.

[0104] Figure 9 To simulate the longitudinal velocity component change of the suspended bubble in the experiment, and also from... Figure 9Observing the overall trend of the longitudinal velocity of the bubble, it can be divided into two stages. Stage 1: The longitudinal velocity of the bubble is negative, indicating that the bubble is moving downwards. Initially, the voltage rise rate is highest, and the longitudinal component of the electrodynamic force of the bubble closest to the needle tip is also at its maximum, resulting in the largest initial acceleration. However, as the bubble's longitudinal distance increases, the longitudinal component of the electrodynamic force gradually weakens, while the viscous drag, proportional to the flow velocity, gradually increases. When the rate of change of the alternating electric field decreases, causing the longitudinal component of the resultant force of gravity, electrodynamic force, buoyancy, and viscous force on the bubble to reach 0 (approximately t = 2.3 ms), the velocity will reach its first peak. At this point, the longitudinal electrodynamic modulus of the bubble will be less than the buoyancy under the expanded volume, and the resultant force acceleration will be upwards. When the voltage enters the falling edge of the positive half-cycle, the voltage change rate becomes negative. Simultaneously, the electrodynamic force, in the opposite direction, does negative work on the bubble along with buoyancy and viscous drag, until the bubble's kinetic energy is exhausted and it reaches its deepest longitudinal point. Stage 2: The longitudinal velocity of the bubble is positive, indicating that the bubble begins to float. After the voltage changes for 10ms, it can be seen that the acceleration period of the longitudinal velocity value corresponds to the rising edge of the positive half-cycle or the falling edge of the negative half-cycle of the sinusoidal voltage, while the deceleration period of the longitudinal velocity value corresponds to the falling edge of the positive half-cycle or the rising edge of the negative half-cycle of the AC voltage. Its basic dynamic principle is similar to... Figure 8 The change in lateral velocity is similar. Essentially, the longitudinal component of the electrodynamic force, which changes periodically by 10ms, alters the kinetic energy of the bubble through work, thus giving the longitudinal component of velocity the characteristic of periodic change. The only difference from the lateral component of velocity is the influence of buoyancy on the longitudinal acceleration.

[0105] Comparing the changes in the longitudinal velocity component under different voltage levels, it was found that in the first stage, the higher the voltage level, the larger the negative peak value of the longitudinal velocity component, and the shorter the time to reach the peak value. In the second stage, the first few velocity extreme values ​​(maximum and minimum values) during the bubble's ascent increase sequentially with the increase of voltage level, and the first positive peak velocity decreases as the voltage level decreases, and the corresponding peak time is gradually delayed. The overall downward trend is intensified with the increase of voltage level. Finally, when buoyancy gradually replaces electrodynamics as the dominant factor for acceleration, the longitudinal velocity of the bubble will infinitely approach the natural upward floating law.

[0106] When a bubble migrates within a multiphysics field consisting of an alternating electric field, a gravitational field, and a laminar phase field, it undergoes deformation under the forces of these fields. To accurately and reasonably describe the morphological changes of the bubble during its motion, six typical deformation images within 0-10ms were captured from the bubble simulation model to observe the changes in the bubble's shape, such as... Figure 10 As shown.

[0107] Figure 10 To classify the deformation of suspended bubbles, by Figure 10It can be seen that the bubble begins to stretch obliquely along the longitudinal axis from a perfect spherical shape. At 2 ms, its shape becomes more like a pebble, narrower at the top and wider at the bottom. At 4 ms, the lateral distance is extremely narrow, approximately half the initial diameter. Subsequently, the longitudinal distance gradually shortens, while the lateral distance slowly lengthens. When 8 ms arrives, the bubble becomes a short-distance ellipsoid, very close to a perfect sphere. It returns to its initial shape after half a cycle, but with a slightly reduced radius. From the bubble deformation within 10 ms, it is easy to infer that its various shapes are close to an ellipsoid, and therefore can be classified into the following types of ellipsoids, such as... Figure 11 As shown.

[0108] Figure 11 Classification of deformation of suspended bubbles, Figure 11 Three types of ellipsoids are defined: longitudinally stretched, laterally compressed, and obliquely stretched. Here, a and b represent the effective diameters of the ellipsoid along the longitudinal and transverse axes, respectively, and the degree of bubble deformation is measured by the scaling factor Q, as shown in equation (4-1).

[0109]

[0110] When b is greater than a, the Q value is less than zero, indicating that the bubble is in a state of lateral stretching or oblique stretching with a larger effective diameter in lateral projection. Similarly, when b is less than a, the Q value is greater than zero, indicating that the bubble is in a state of longitudinal stretching or oblique stretching with a larger effective diameter in longitudinal projection. The last special case is when a and b are equal, in which case the bubble is in an ideal circular state. This state only exists under the initial hovering conditions set in the simulation experiment. The periodic expansion and contraction of a single suspended bubble is as follows: Figure 12 As shown.

[0111] Figure 12 For the periodic expansion and contraction deformation of suspended bubbles, such as Figure 12 As shown, the expansion coefficient of the bubble over two periods (40 ms) was statistically analyzed. The overall curve shows that the Q value is always greater than or equal to zero, indicating that the bubble exists in a longitudinally stretched state for the majority of the time in the physical field. Since the initial position of the bubble is located to the lower right of the origin, its deformation during motion is an inclined stretching shape with a larger effective longitudinal projection diameter. The Q value's gradual change over time oscillates with a period of 10 ms. Its maximum value corresponds to the peak value of the absolute value function of the power frequency AC voltage, while its minimum value corresponds to the zero point of the absolute value function. The higher the voltage level, the more pronounced the weakening of the periodic oscillation characteristic. Under sufficiently high insulating oil levels, the transverse electrostrictive force within the bubble gradually weakens over time, with the longitudinal forces of buoyancy and viscosity becoming dominant. Buoyancy causes the bubble to accelerate upwards, while viscous resistance compresses the bubble longitudinally, causing the Q value to gradually decrease to less than zero. The distribution of the unit volume force of electrostriction on the bubble in the longitudinal and transverse directions is shown in the figure. Figure 13 As shown.

[0112] Figure 13 The red arrows represent volume force vectors at a certain point, and the size of the arrow represents the magnitude. The positive and negative values ​​of the longitudinal volume force represent the positive and negative directions along the y-axis, respectively; similarly, the positive and negative values ​​of the transverse volume force represent the positive and negative directions along the x-axis, respectively. From the direction and distribution of the arrows in the figure, the volume force near the outer surface of the bubble is relatively small, while the volume force reaches its maximum as it gradually enters the inner crescent layer of the bubble along the direction of the arrows. All components of the volume force point towards the vicinity of the bubble center, therefore the center is compressed, and its internal pressure also reaches its maximum. This phenomenon can be verified by statistically analyzing the changes in the surface pressure of the bubble and the maximum pressure at the bubble center over two periods, such as... Figure 14 .

[0113] Figure 14 This represents the periodic changes in the maximum pressure and surface pressure of the suspended bubble. Figure 14 In this model, the dependent variable is the absolute pressure of the bubble, which is the difference between the actual pressure of the bubble and the standard atmospheric pressure. The overall variation law of the bubble pressure and the central pressure fully illustrates the direct influence of the bubble pressure on the deformation. Its extreme value decays periodically and increases with voltage increase, which is consistent with the variation law of the Q value. This also proves from another perspective that the electrostrictive force inside the bubble changes the bubble shape through periodic oscillation.

[0114] This embodiment provides an analysis of the mechanism by which multiple bubble fusion channels affect discharge breakdown. Based on the bubble distribution before oil gap breakdown discharge as captured in experimental images, it can be seen that the greater the number of bubbles and the closer they are to the needle electrode, the easier it is to induce insulating oil breakdown discharge. Therefore, to analyze the impact of the presence of microbubbles on the space surrounding the electrode, it is necessary to simulate the dynamic changes of multiple bubbles near the electrode through simulation experiments, and study the influence of changes in the physical quantities of the surrounding space during their movement and deformation on the mechanism of discharge breakdown. To more closely resemble the images of bubble breakdown discharge in actual experiments, a simulation model is established... Figure 15 The model shown is a vertically arranged three-bubble coalescing model.

[0115] Figure 15 For multi-bubble coalescence simulation settings, such as Figure 15 As shown, in the bubble dynamics model, three circular bubbles, A, B, and C, with radii of 60μm, 45μm, and 40μm respectively, are set and arranged radially from the center of the sphere. Six peripheral points in the figure are taken as special points for studying bubble-related variables. Figure 16 The volume fraction, pressure, and electric field strength of the bubble at different locations were recorded at 0, 10, 20, 30, and 40 μs.

[0116] Figure 16 To observe the volume fraction change of multi-bubble aggregation, observe Figure 16The fluid volume fraction changes within 40 μs. At position 1, when bubble A is close enough to the needle tip, the bubble surface and the metal interface generate a normal-direction pulling force. When the viscous resistance of the thin insulating oil layer in the short gap is insufficient to counteract this tendency, the bubble undergoes an elongation deformation along the direction of the pulling force and finally adheres to the electrode surface. This process takes less than 10 μs. At this time, near positions 2, 3, 4, and 5, the bubble surfaces gradually stretch and converge, forming a gas channel and causing dialysis. At this point, the three bubbles become one, but the volume fraction at the connecting channel is still smaller than the volume fraction corresponding to the original three sphere centers. This indicates that the distribution of flowing gas molecules at the channel is more sparse. Only when the gas pressure reaches equilibrium can the channel expand into a new narrow bubble wall, and the three bubbles become a whole.

[0117] Figure 17 The absolute pressure change for the coalescence of multiple bubbles, in Figure 17 In the initial stage, the absolute pressure values ​​(difference between actual pressure and standard atmospheric pressure) of bubbles A, B, and C decrease sequentially. At this time, the bubbles are mainly affected by the hydraulic pressure of the insulating oil. Therefore, the greater the vertical distance from the oil surface, the greater the bubble pressure. At position 1, due to the adsorption tension of gas molecules on the electrode surface at the thin wall of the bubble, a pressure higher than that of the insulating oil film is generated. Positions 2, 3, 4, and 5 are located on both sides of the bubble coalescing channel. When a pressure difference is formed between the bubbles, the central oil gap will gradually be repelled and isolated by the gas channel over time, resulting in a tendency to flow to both sides. The maximum pressure is reached at the point where a new bubble wall is formed. During the gradual coalescing of the bubbles, it can be seen that the pressure at position 6 at the bottom of bubble C is always negative. This indicates that the pressure at this point is always less than the standard atmospheric pressure. This is because the bubble contraction effect is obvious at this point, and the gas concentrates and flows towards the connecting channel. The edge tension generated by the flowing gas on the surface drags the gas along the flow direction. The faster the flow rate, the lower the pressure. The normal tension component along the surface decreases, and the deformation becomes more severe, thus forming a negative pressure environment in the surrounding space.

[0118] Figure 18This demonstrates how the presence of multiple bubbles and their dynamic deformation alter the electric field distribution in the surrounding space. In the absence of bubbles, the electric field contour lines radiate outwards in an elliptical ring shape from the needle electrode, decreasing progressively and symmetrically distributed, without overlap or closure between them. However, as time progresses, the voltage amplitude initially increases at 10 μs, while the electric field at position 1 undergoes a significant shift. The strong field region initially present at the electrode tip gradually extends into the interior of bubble A near the electrode, forming annular weak field regions at the central boundaries of bubbles B and C, and bubbles 2, 3, 4, and 5. Within the weak electric field regions of the bubbles, the field strength is highest at the center and gradually decreases outwards. Within the bubble's condensation channel, the field strength is lowest in the central region and gradually increases outwards until it opens into the boundary electric field of the liquid phase layer. This phenomenon is related to the polarization and distribution of gas molecules. The air inside the bubble, acting as a dielectric, is polarized by the initial electric field. The polarized molecules follow a Boltzmann distribution and, under the influence of Coulomb forces, continuously surge towards areas of high electric field strength. Simultaneously, an internal electric field, opposite to the external electric field, is generated within the bubble, weakening the overall electric field strength. This field-directional movement alters the concentration of dielectric molecules within the gas channel, resulting in denser molecules in strong-field regions and sparser molecules in weak-field regions. Consequently, during the merging of multiple bubbles, the molecular density within the gas channel remains lower than the molecular density at the bubble's center.

[0119] This embodiment studies the complex dynamic process of bubbles under the combined effects of alternating electric field, fluid phase field, and gravitational field. A gas-liquid two-phase flow model with a composite physical field was established. The changes in bubble velocity, expansion and contraction, pressure, and electric field strength were simulated from two aspects: the migration and deformation of a single bubble and the interconnection and merger of multiple bubbles. The conclusions obtained from the analysis are as follows:

[0120] 1) The initial displacement angle of the single suspended bubble located below the needle electrode increases with the increase of AC voltage amplitude until it reaches the limit deflection angle; as the voltage amplitude gradually decreases, its displacement deflection angle will gradually decrease counterclockwise until it crosses the horizontal direction and reaches the first quadrant, reaching a natural floating state when the voltage is 0. Its displacement characteristic curve is U-shaped, and the higher the voltage, the greater the arc of the trajectory, and the greater the lateral and longitudinal displacement distances.

[0121] 2) The velocity of a single bubble increases to a peak, decreases to a trough, and then rises back to a peak over time, with a period equal to half the period of the AC voltage at 100Hz. The peak and trough times of the velocity approximately correspond to the maximum and minimum (zero) times of the absolute value function of the AC voltage. The electric field force does positive work on the bubble during the rising edge of the positive half-cycle or the falling edge of the negative half-cycle, increasing the bubble's kinetic energy; and does negative work during the falling edge of the positive half-cycle or the rising edge of the negative half-cycle, consuming the bubble's kinetic energy. The change in bubble expansion and contraction is consistent with the change in surface pressure and maximum pressure, and the periodic change of the volumetric force caused by the electric field exhibits a decaying fluctuation.

[0122] 3) When multiple bubbles coalesce, a pressure difference is formed between them. The central oil gap is gradually repelled and isolated by the gas channel over time, causing a tendency to flow to both sides, resulting in dialysis and the formation of gas channels. The volume fraction at the connecting channel is smaller than that at the center of the bubble, and the distribution of flowing gas molecules at the channel is more sparse. However, the gas molecules flow faster at the channel, generating the maximum pressure at the point where new bubble walls are formed, creating a negative pressure environment around the edge of the tail bubble.

[0123] 4) As the voltage gradually increases, the electric field strength at the center of each suspended bubble is much higher than that between the bubbles. As the electric field strength continues to increase, the electric field strength at the center of the bubble closest to the needle electrode's strong field region will reach the breakdown critical value first. The discharge will first break down along the bubble with the strongest electric field at the closest distance, and then continue to extend along the channel generated by the gas flow, eventually penetrating the entire bubble string.

[0124] Example 2

[0125] This embodiment provides a simulation system for the dynamic evolution of bubbles in a needle-plate electrode under a multiphysics field of a transformer, including:

[0126] The needle-plate electrode module includes a container containing insulating oil, in which needle electrodes and plate electrodes are installed opposite each other, and insulating paper is provided on the upper surface of the plate electrodes.

[0127] The experimental data acquisition and observation module is used to set the voltage on the surface of the needle electrode and acquire fluid data and material property parameters during the discharge process of the needle electrode of the needle plate electrode model under this voltage.

[0128] The bubble dynamic evolution simulation module sets boundary conditions based on the structure of the needle plate electrode model; selects the computational domain and performs meshing on the computational domain; and constructs a multi-coupled field bubble dynamic model based on fluid data, material property parameters, boundary conditions, and computational domain for simulating the dynamic evolution of bubbles in the needle plate electrode under multi-physics fields of a transformer.

[0129] The needle-plate electrode module, experimental data acquisition and observation module, and bubble dynamic evolution simulation module correspond to the specific layout scheme of the needle-plate electrode bubble dynamic evolution simulation method under transformer multi-physics field.

[0130] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for simulating the dynamic evolution of bubbles in a needle-plate electrode under multiphysics conditions in a transformer, characterized in that, include: Establish a needle-plate electrode model, which includes a container containing insulating oil, a needle electrode and a plate electrode installed opposite to each other inside the container, and insulating paper on the upper surface of the plate electrode. Set the voltage on the surface of the needle electrode, and collect fluid data and material property parameters during the discharge process of the needle electrode in the needle plate electrode model under this voltage; Boundary conditions are set based on the structure of the needle plate electrode model; Select a computational domain and mesh the computational domain; Based on fluid data, material property parameters, boundary conditions, and computational domain, a multi-coupled field bubble dynamic model is constructed for simulating the dynamic evolution of bubbles in the needle plate electrode under multi-physics fields of a transformer. A three-bubble coalescing model with longitudinal arrangement is established. The three-bubble coalescing model is set in a multi-coupled field bubble dynamic model with three circular bubbles with successively decreasing radii and arranged radially around the center of the sphere. The influence of the presence of microbubbles on the space around the electrode is analyzed by the change of fluid volume fraction within a set time. The experimental data acquisition and observation module includes electrical components and observation components; Electrical components: These include an AC power supply obtained by stepping up the AC voltage from a power frequency AC voltage generator, a protective resistor that limits current, and a DC blocking capacitor that eliminates DC interference. Observation components: These include a voltage divider to assist in measuring the voltage and current of the discharge device, a current transformer (CT), a flicker-free supplementary light, and a high-speed imaging system.

2. The method for simulating the dynamic evolution of bubbles in a needle-plate electrode under multiphysics conditions in a transformer as described in claim 1, characterized in that, The flow system of the multi-coupled field bubble dynamics model consists of insulating oil and bubble two-phase fluid substances. The fluid dynamics characteristics of the system are characterized by the Navier-Stokes equations in the domain of the two-phase flow-phase field interface.

3. The method for simulating the dynamic evolution of bubbles in a needle-plate electrode under multiphysics conditions in a transformer as described in claim 1, characterized in that, The multi-coupled-field bubble dynamics model uses the Kahn-Hillard equation obtained by the phase field method to describe and distinguish the changes in the gas-liquid two-phase fluid interface.

4. The method for simulating the dynamic evolution of bubbles in a needle-plate electrode under multiphysics conditions in a transformer as described in claim 1, characterized in that, The electric field coupled with the electrodynamic force of the two-phase flow-phase field in the multi-coupled field bubble dynamic model is represented by the Maxwell stress tensor in divergence form.

5. The method for simulating the dynamic evolution of bubbles in a needle-plate electrode under multiphysics conditions in a transformer as described in claim 1, characterized in that, The computational domain is selected by using the vertical line between the metal surfaces of the needle tip and the needle plate electrode as the grid control edge.

6. The method for simulating the dynamic evolution of bubbles in a needle-plate electrode under multiphysics conditions in a transformer as described in claim 1, characterized in that, The multi-coupled field bubble dynamics model establishes a two-dimensional coordinate system with the needle tip as the origin. The boundary conditions include the laminar open boundary of the bubble seeping from the insulating oil surface, the two side edge boundaries of the insulating oil, the metal surface of the needle electrode, and the metal surface of the plate electrode.

7. The method for simulating the dynamic evolution of bubbles in a needle-plate electrode under multiphysics conditions in a transformer as described in claim 1, characterized in that, The material property parameters include insulating oil density, insulating oil dynamic viscosity, insulating oil relative permittivity, gas density, gas dynamic viscosity, gas relative permittivity, and interface thickness.

8. The method for simulating the dynamic evolution of bubbles in a needle-plate electrode under multiphysics conditions in a transformer as described in claim 1, characterized in that, The fluid data includes the velocity of each phase of the fluid within the system, the density of the fluid, the dynamic viscosity of the liquid, the pressure inside the fluid, and the volume fraction of gas and liquid in the fluid.

9. A simulation system for dynamic evolution of needle-plate electrode bubbles under multiphysics conditions in a transformer, employing the simulation method for dynamic evolution of needle-plate electrode bubbles under multiphysics conditions in a transformer as described in any one of claims 1-8, characterized in that, include: The needle-plate electrode module includes a container containing insulating oil, in which needle electrodes and plate electrodes are installed opposite each other, and insulating paper is provided on the upper surface of the plate electrodes. The experimental data acquisition and observation module is used to set the voltage on the surface of the needle electrode and acquire fluid data and material property parameters during the discharge process of the needle electrode of the needle plate electrode model under this voltage. The bubble dynamic evolution simulation module sets boundary conditions based on the structure of the needle plate electrode model; Select a computational domain and mesh the computational domain; Based on fluid data, material property parameters, boundary conditions, and computational domain, a multi-coupled field bubble dynamic model is constructed for simulating the dynamic evolution of bubbles in the needle plate electrode under multi-physics fields of a transformer.

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