Method and system for simulating and analyzing bubbles in converter transformer oil

Abstract

The invention provides a method and system for simulating and analyzing bubbles in converter transformer oil, and belongs to the technical field of maintenance and diagnosis of high-voltage power transmission and transformation equipment. The method comprises the steps that transformer oil and insulating oil paper are selected as samples, and the samples are pretreated; building an electric-thermal combined aging experiment platform, and carrying out an aging experiment on the sample; placing the aged sample in a bubble dynamic behavior observation experiment platform under the partial discharge effect; collecting space position information of the bubbles; collecting form information of the bubbles; extracting characteristic parameters of a bubble migration process and characteristic parameters of a bubble deformation process; analyzing physical and chemical parameter changes of the transformer oil in different aging states; and associating the characteristic parameters of the migration process and the characteristic parameters of the deformation process with the physical and chemical parameters of the transformer oil in different aging states, and analyzing the migration characteristics and deformation characteristics of the bubbles in the converter transformer oil. According to the method, a new thought is provided for research on bubble dynamic behaviors with relatively high initial state randomness under complex working conditions.

Description

Technical Field

[0001] This application belongs to the field of high-voltage power transmission and transformation equipment maintenance and diagnosis technology, specifically relating to a method and system for simulating and analyzing bubbles in converter transformer oil. Background Technology

[0002] As a core component of ultra-high voltage direct current (UHVDC) transmission systems, converter transformers directly determine the safe and stable operation of the system. During long-term operation, the internal oil-paper insulation of the transformer undergoes irreversible degradation due to complex electric field distribution and excessively high local temperatures, leading to partial discharge at weak points in the insulation. The thermal effects, corona discharge, and acoustic cavitation effects caused by voltage surges resulting from partial discharge generate bubbles in the transformer oil. The dynamic behavior of these bubbles in strong electric field regions can easily lead to the accumulation of small bridges and even the breakdown of oil gaps, threatening the safe operation of the converter transformer.

[0003] Existing techniques use bubble generators or probes to introduce bubbles of the same shape, size, and velocity into transformer oil, then observe the effect of an applied electric field on their dynamic evolution. Compared to bubbles generated by artificial intervention, bubbles generated by partial discharge exhibit higher generation field strength (25 kV / mm), extremely short evolution time (1 ms), and extremely fast velocity (0.5 m / s). Furthermore, the location, size, and velocity of bubbles generated by partial discharge vary, leading to significant differences in the dynamic behavior of different bubbles within the same partial discharge process. Moreover, the degree of partial discharge varies with the aging state of the oil-paper insulation, resulting in even greater differences in the dynamic behavior of bubbles in the oil. Therefore, the research techniques for partial discharge and bubble development and evolution in oil are relatively independent, lacking research techniques for the dynamic behavior of bubbles in oil under the influence of partial discharge. Summary of the Invention

[0004] The purpose of this invention is to provide a method and system for simulating and analyzing bubbles in converter transformer oil, in order to solve the problem that existing methods cannot study the dynamic behavior of bubbles in oil under partial discharge.

[0005] In a first aspect, embodiments of this application provide a method for simulating and analyzing air bubbles in converter transformer oil, the method comprising the following steps: S1. Select transformer oil and insulating oil paper as samples, and pretreat the samples, including filtering, drying and degassing the transformer oil, and drying and oil immersion of the insulating oil paper to obtain pretreated samples. S2. Build an electro-thermal combined aging test platform, determine the aging temperature, aging time, AC / DC composite voltage ratio and AC / DC electric field strength, conduct aging tests on the pretreated samples, simulate the actual operating aging state of the converter transformer, and obtain the aged samples. S3. The aged sample was placed in an experimental platform for observing the dynamic behavior of bubbles under partial discharge to simulate the evolution of bubbles in oil when partial discharge occurs inside the converter transformer under the action of a non-uniform electric field. S4. Collect the spatial location information of the bubble and divide the bubble migration process into the approaching strong field stage, the moving away from the strong field stage, the descending stage, and the rising stage; S5. Collect bubble morphology information, calculate deformation coefficient, analyze bubble deformation process; and calculate surface tension to analyze the influence of surface tension on bubble deformation. S6. Extract the characteristic parameters of the bubble migration process, including the maximum displacement and migration time; extract the characteristic parameters of the bubble deformation process, including the maximum deformation coefficient, surface tension dominance time, and effective projected area of ​​the bubble. S7. Analyze the changes in the physicochemical parameters of transformer oil under different aging conditions. The physicochemical parameters include the acid value concentration, furfural content and volume resistivity of the oil. S8. The extracted characteristic parameters of the migration process and the characteristic parameters of the deformation process are correlated with the physicochemical parameters of transformer oil under different aging conditions to analyze the characteristics of bubble migration and deformation in converter transformer oil.

[0006] Further, in step S2, the electro-thermal combined aging test platform includes: Electric heating combined aging chamber, pressurization system and measurement system; The electrothermal combined aging chamber is used to couple the electric field and temperature field of the insulating oil paper, and to apply a temperature field to the oil paper insulation by controlling the ambient temperature. The pressurization system consists of AC-side and DC-side equipment. Both AC and DC voltages are applied to the high-voltage side of the plate electrodes, and the voltage across the plate is the sum of the AC and DC voltages. The AC-side equipment includes a high-voltage transformer TA1, a voltage regulating transformer TA2, and a 50kΩ resistor R. AC and a 2nF DC blocking capacitor C D-b The DC-side equipment includes a rectifier silicon stack D1, a DC-side high-voltage transformer TD1 and a voltage regulating transformer TD2, and a 5MΩ resistor R. DC and a coupling capacitor C of 0.1μF k1 The DC blocking capacitor C D-b Used to achieve parallel superposition of AC and DC components to protect the power supply; The measurement system consists of an oscilloscope and a high-voltage probe P for measuring AC voltage. AC High-voltage probe P for measuring DC voltage DC and the high-voltage probe P for measuring AC and DC voltage AC-DC composition.

[0007] Furthermore, the specific steps for determining the aging temperature, aging time, AC / DC composite voltage ratio, and AC / DC electric field strength in step S2 are as follows: S21. Determine the aging temperature and aging time based on the average operating temperature of the converter transformer windings, the normal lifespan of the transformer operating at this temperature, and the 6℃ rule. S22. Set the ratio of AC / DC composite voltage according to the actual wiring configuration of the transformer; S23. Construct a full-size finite element simulation model of the converter transformer and determine the applied AC and DC electric field strengths through finite element simulation.

[0008] Furthermore, the experimental platform for observing the dynamic behavior of bubbles under partial discharge is based on a needle-plate electrode and includes: Experimental oil tank, image acquisition system, pressurization system, and measurement system; The experimental oil tank includes an oil passage, an oil pump, a temperature sensor, a heating element, and a flow meter. Partial discharge occurs in the oil passage. The oil pump is used to provide power for oil circulation, and the temperature sensor and heating element are used to control the oil temperature. The image acquisition system includes a high-speed camera, a cold light source, and an oscilloscope. The high-speed camera is mounted on a slide rail to adjust the distance between the camera and the oil tank. The oscilloscope is used to delay and trigger the high-speed camera to capture an image of the bubble movement under the action of partial discharge when the partial discharge pulse reaches a predetermined value. The pressurization system adopts an electric-thermal combined aging platform pressurization system; The measurement system includes a high-voltage probe containing the monitoring voltage and a partial discharge measurement device. The partial discharge measurement device includes a detection impedance Z... m Coupling capacitor C k2 The partial discharge instrument and oscilloscope are used. The high-voltage side of the coupling capacitor is connected to the needle plate electrode, and the low-voltage side is connected to the detection impedance. The partial discharge instrument is connected to the output terminal of the detection impedance and is used to measure and record the PRPD spectrum. The oscilloscope is used to record the pulse waveform of the partial discharge.

[0009] Furthermore, the specific steps of step S4 are as follows: S41. Based on Newton's second law, the dynamic equations of the movement of air bubbles in static transformer oil are obtained, and the electric field force, drag force, buoyancy force, and gravity of the air bubbles in the oil under the action of partial discharge are calculated; the electric field force includes dielectric force, electrostrictive force, and Coulomb force; the drag force is the resultant force of viscous resistance and oil flow drag force affected by the oil flow. S42. When the bubble moves along the negative x-axis and the dielectrophoretic force plays a dominant role, the absolute value of the bubble velocity gradually decreases to 0, which is then classified as the approaching field strength stage. S43. The bubble accelerates along the positive x-axis and reaches its maximum speed when the dielectrophoretic force and drag force are in balance; the bubble decelerates along the positive x-axis and the drag force becomes dominant until the net force is 0; the whole moves from the strong field region to the weak field region; this is divided into the stage of moving away from the field strength. S44. The bubble moves along the negative y-axis, while the dielectrophoretic force and drag force move along the positive y-axis; the drag force plays a dominant role, and the bubble velocity gradually decreases to 0. Due to inertia, dielectrophoretic force, and drag force, it moves from top to bottom; this is divided into a descending phase. S45. The bubble accelerates along the positive y-axis, and the drag force changes from the positive y-axis to the negative y-axis, with buoyancy playing a dominant role. When buoyancy, drag force, dielectrophoresis force, and gravity are in balance, the bubble velocity approaches a constant. Since buoyancy dominates the upward motion, this is divided into the rising phase.

[0010] Further, in step S5, the formula for calculating the deformation coefficient is:

[0011] in, a The effective diameter of the longitudinal axis of the air bubble in the oil. b The effective diameter of the horizontal axis of the gas in the oil; The formula for calculating the surface tension is:

[0012] in, n e It is the unit normal vector; For phase field variables, the transformer oil region Take 1, the bubble region Take as 1; σ It is the fluid surface tension coefficient; p b Let be the density of the gas inside the bubble. p o This refers to the density of the transformer oil.

[0013] Furthermore, the specific steps of step S6 are as follows: S61. Based on the four migration stages of the bubble—approaching the strong field, moving away from the strong field, falling, and rising—extract the maximum displacement and migration time of the bubble in each stage as characteristic parameters of the migration process. S62. Based on the deformation process of the bubble, extract the maximum deformation coefficient, the surface tension dominance time, and the effective projected area of ​​the bubble as characteristic parameters of the deformation process; S63. Extract n sets of bubble data and study the variation patterns of characteristic parameters of the migration process and characteristic parameters of the deformation process under different aging times.

[0014] Furthermore, the specific steps of step S7 are as follows: S71. Measure the acid value concentration, furfural content and volume resistivity of transformer oil at different aging times, and obtain numerical curves showing their changes with aging time; S72. Analyze the variation patterns of the physicochemical parameters. Among them, the acid value concentration increases slowly in the early stage of combined aging, grows rapidly in an exponential manner in the middle stage, and decreases in the later stage; the furfural content increases slowly in the early and middle stages of combined aging, and grows rapidly in the later stage; the volume resistivity decreases rapidly in the early stage of combined aging, and the rate of decrease gradually slows down in the later stage. S73. Analyze the influence of changes in physicochemical parameters on the forces acting on bubbles. Among them, the increase in acid value concentration and furfural content increases the dynamic viscosity coefficient of transformer oil, making the drag force on bubbles more obvious; the decrease in volume resistivity increases the influence of the electric field in the oil on bubbles, making the dielectric force on bubbles more obvious.

[0015] Furthermore, the specific steps of step S8 are as follows: S81. Using the force conditions of the bubble as the correlation medium, the forces on the bubble include drag force, dielectric force, buoyancy, and electrostrictive force. Combining the influence of the aging time of the oil paper insulation on the forces, establish the correlation logic between the aging state and the dynamic behavior characteristic parameters of the bubble. S82. Correlate the characteristic parameters and aging state of the migration process near the strong field stage; correlate the characteristic parameters and aging state of the migration process far from the strong field stage; correlate the characteristic parameters and aging state of the migration process in the descent stage; correlate the characteristic parameters and aging state of the migration process in the ascent stage; correlate the characteristic parameters and aging state of the deformation process. S83. For the logic after association, analyze the characteristics of bubble migration and deformation in the converter transformer oil.

[0016] Secondly, embodiments of this application also provide a system for simulating and analyzing air bubbles in converter transformer oil, the system being used to implement the method described in the first aspect, the system comprising: The oil-paper insulation sample selection and pretreatment module is used to select transformer oil and insulating oil paper as samples and pretreat the samples, including filtering, drying and degassing the transformer oil, and drying and oil immersion of the insulating oil paper. The electro-thermal combined aging platform construction and aging test module is used to build an electro-thermal combined aging test platform, determine the aging temperature, aging time, AC / DC composite voltage ratio and AC / DC electric field strength, and conduct aging tests on the samples to simulate the actual aging state of the converter transformer. The partial discharge bubble observation platform construction and evolution simulation module is used to place aged samples in an experimental platform for observing the dynamic behavior of bubbles under partial discharge, simulating the evolution behavior of bubbles in oil when partial discharge occurs inside a converter transformer under the action of a non-uniform electric field. The bubble position acquisition and migration stage division module is used to acquire the spatial position information of the bubble and divide the bubble migration process into the approaching strong field stage, the moving away from the strong field stage, the descending stage, and the rising stage. The bubble morphology acquisition and deformation process analysis module is used to acquire bubble morphology information, calculate the deformation coefficient, analyze the bubble deformation process, and calculate surface tension to analyze the influence of surface tension on bubble deformation. The bubble dynamic behavior feature parameter extraction module is used to extract feature parameters of the bubble migration process, including the maximum displacement and migration time; and to extract feature parameters of the bubble deformation process, including the maximum deformation coefficient, surface tension dominance time, and effective projected area of ​​the bubble. The physicochemical parameter monitoring and change analysis module is used to analyze the changes in the physicochemical parameters of transformer oil under different aging conditions. The physicochemical parameters include the acid value concentration, furfural content and volume resistivity in the oil. The bubble parameter and physicochemical parameter correlation analysis module is used to correlate the extracted characteristic parameters of the migration process and the characteristic parameters of the deformation process with the physicochemical parameters of transformer oil under different aging conditions, and to analyze the bubble migration characteristics and deformation characteristics in converter transformer oil.

[0017] As can be seen from the above technical solutions, the present invention has the following advantages: This application discloses a method and system for simulating and analyzing bubbles in converter transformer oil. Through standardized sample pretreatment, an electro-thermal combined aging platform, and a needle-plate electrode observation platform, it simulates the special operating conditions of partial discharge bubbles, characterized by high generation field strength, short evolution time, and rapid movement speed, overcoming the research difficulties caused by the randomness of bubble parameters. By dividing bubble migration into four stages, calculating deformation coefficients and surface tension, extracting characteristic parameters, and correlating them with physicochemical parameters such as acid value concentration, furfural content, and volume resistivity in the transformer oil, it establishes a mechanical correlation between oil paper aging and bubble dynamic behavior, quantifying the influence of different aging states on bubble migration and deformation. This provides accurate data support for early warning of insulation faults in converter transformers, enabling early identification of the risk of oil gap breakdown caused by bubble accumulation, effectively reducing the probability of insulation faults caused by bubble development, and ensuring the safe and stable operation of the UHVDC transmission system. Attached Figure Description

[0018] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a flowchart illustrating the method for simulating and analyzing bubbles in transformer oil according to this application.

[0020] Figure 2 This is a schematic diagram of the pretreatment process for the oil-paper insulation sample of this application.

[0021] Figure 3 This is a schematic diagram of the geometric model of the slightly non-uniform electrode in this application.

[0022] Figure 4 This is a schematic diagram of the electro-thermal combined aging test platform of this application.

[0023] Figure 5 This is a schematic diagram of the electric field distribution inside the transformer of this application.

[0024] Figure 6 This is the highly non-uniform electrode model used in this application.

[0025] Figure 7 This is the bubble observation experimental platform for this application.

[0026] Figure 8 This is the fuel tank used for the observation experiment in this application.

[0027] Figure 9 This is the image acquisition device described in this application.

[0028] Figure 10 For the bubble edge of this application x Distribution diagram of migration displacement, velocity and acceleration in the direction.

[0029] Figure 11 For the bubble edge of this application y Distribution diagram of migration displacement, velocity and acceleration in the direction.

[0030] Figure 12 The effective diameters of the bubble's longitudinal and transverse axes in this application are denoted as .

[0031] Figure 13 This refers to the change in the deformation coefficient of the bubble in this application.

[0032] Figure 14 This represents the maximum displacement of the bubble in this application when it approaches a strong field.

[0033] Figure 15This represents the maximum displacement of the bubble in this application during the stage away from the strong field.

[0034] Figure 16 This represents the maximum displacement of the bubble during the descent phase in this application.

[0035] Figure 17 This represents the maximum displacement of the bubble during the rising phase of this application.

[0036] Figure 18 This is the maximum value of the bubble deformation coefficient in this application. Q M Surface tension dominance time △t And the effective projected area of ​​the bubble at 1ms S The changes.

[0037] Figure 19 This is a numerical curve showing the change of typical physicochemical characteristic parameters in the oil of this application with aging time.

[0038] Figure 20 A schematic diagram of a system for simulating and analyzing bubbles in transformer oil. Detailed Implementation

[0039] Various embodiments of the invention will be described more fully in the detailed steps of the method for simulating and analyzing bubbles in converter transformer oil, which will be described in detail below. The invention may have various embodiments, and adjustments and changes may be made therein. However, it should be understood that there is no intention to limit the various embodiments of the invention to the specific embodiments disclosed herein, but rather the invention should be understood to cover all adjustments, equivalents, and / or alternatives falling within the spirit and scope of the various embodiments of the invention.

[0040] To make the objectives, features, and advantages of this invention more apparent and understandable, specific embodiments and accompanying drawings will be used to clearly and completely describe the technical solutions protected by this invention. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0041] Please see Figure 1 The diagram shows a flowchart of a method for simulating and analyzing bubbles in converter transformer oil. The method includes the following steps: S1. Select transformer oil and insulating oil paper as samples, and pretreat the samples, including filtering, drying and degassing the transformer oil, and drying and oil immersion of the insulating oil paper to obtain pretreated samples. S2. Build an electro-thermal combined aging test platform, determine the aging temperature, aging time, AC / DC composite voltage ratio and AC / DC electric field strength, conduct aging tests on the pretreated samples, simulate the actual operating aging state of the converter transformer, and obtain the aged samples. S3. The aged sample was placed in an experimental platform for observing the dynamic behavior of bubbles under partial discharge to simulate the evolution of bubbles in oil when partial discharge occurs inside the converter transformer under the action of a non-uniform electric field. S4. Collect the spatial location information of the bubble and divide the bubble migration process into the approaching strong field stage, the moving away from the strong field stage, the descending stage, and the rising stage; S5. Collect bubble morphology information, calculate deformation coefficient, analyze bubble deformation process; and calculate surface tension to analyze the influence of surface tension on bubble deformation. S6. Extract the characteristic parameters of the bubble migration process, including the maximum displacement and migration time; extract the characteristic parameters of the bubble deformation process, including the maximum deformation coefficient, surface tension dominance time, and effective projected area of ​​the bubble. S7. Analyze the changes in the physicochemical parameters of transformer oil under different aging conditions. The physicochemical parameters include the acid value concentration, furfural content and volume resistivity of the oil. S8. The extracted characteristic parameters of the migration process and the characteristic parameters of the deformation process are correlated with the physicochemical parameters of transformer oil under different aging conditions to analyze the characteristics of bubble migration and deformation in converter transformer oil.

[0042] It should be noted that by pre-treating the samples and simulating actual working conditions on dual experimental platforms, the bubble migration stage is divided, characteristic parameters are extracted, and the physicochemical parameters of the transformer oil are correlated, providing support for early warning of insulation faults in converter transformers, reducing the risk of faults caused by bubbles, and ensuring the stability of the UHVDC transmission system.

[0043] Furthermore, as a refinement and extension of the specific implementation methods of the above embodiments, and to fully illustrate the specific implementation process in this embodiment, another method for simulating and analyzing bubbles in converter transformer oil is provided, including: S1. Transformer oil and insulating paper were selected as samples. The samples were pretreated, including filtering, drying, and degassing the transformer oil, and drying and oil-immersing the insulating paper, to obtain pretreated samples. The pretreatment process is as follows: Figure 2 As shown.

[0044] The experimental samples were Kunlun brand 25# transformer oil and Weidmann brand insulating oil paper with a thickness of 1 mm. The transformer oil was filtered, dried and degassed using a vacuum oil filter (ZY-10) at 65℃ and 50 Pa. The insulating paper was dried in a vacuum chamber at 110℃ and 50 Pa for 72 hours, and then immersed in oil at 80℃ and 50 Pa for 48 hours.

[0045] S2: Construct an electro-thermal combined aging test platform based on slightly non-uniform electrodes, determine the aging temperature, aging time, AC / DC composite voltage ratio and AC / DC electric field strength, conduct aging tests on the pretreated samples, simulate the actual operating aging state of the converter transformer, and obtain the aged samples. The slightly non-uniform electrode geometry model and the electro-thermal combined aging test platform are respectively as follows: Figure 3 , Figure 4 As shown.

[0046] The aging platform consists of an electrothermal combined aging chamber, a pressurization system, and a measurement system. The electrothermal combined aging chamber is used to couple the electric field and temperature field of the insulating oil paper, and to apply a temperature field to the oil paper insulation by controlling the ambient temperature. The pressurization system consists of AC-side and DC-side equipment. Both AC and DC voltages are applied to the high-voltage side of the plate electrodes, and the voltage borne is the sum of the AC and DC voltages. The AC-side equipment includes a high-voltage transformer T on the AC side. A1 and voltage regulating transformer T A2 50kΩ resistor R AC and a 2nF DC blocking capacitor C D-b The DC-side equipment includes a rectifier silicon stack D1, a DC-side high-voltage transformer TD1 and a voltage regulating transformer TD2, and a 5MΩ resistor R. DC and a coupling capacitor C of 0.1μF k1 The DC blocking capacitor C D-b Used to achieve parallel superposition of AC and DC components to protect the power supply; The measurement system consists of an oscilloscope and a high-voltage probe P for measuring AC voltage. AC High-voltage probe P for measuring DC voltage DC and the high-voltage probe P for measuring AC and DC voltage AC-DC composition.

[0047] Select the aging stress and aging time for oil-paper insulation: The aging time is determined by the applied temperature. The average operating temperature of converter transformer windings is 85℃, and the normal lifespan of a transformer operating at this temperature is 20 years. Accelerated aging tests were conducted on oil-paper insulation at high temperatures according to the 6℃ rule proposed by the International Electrotechnical Commission (IEC). When the aging temperature reaches above 145℃, the aging mechanism inside the insulation changes, and the 6℃ rule no longer applies. Placing it at 140℃ for 15 days is equivalent to approximately 20 years of actual operation for oil-paper insulation. Therefore, the experimental aging temperature was set at 140℃, and the aging time was set at 60 hours.

[0048] ±800kV UHVDC projects generally adopt a dual 12-pulse bipolar system connection, with the converter transformer using a single-phase dual-winding configuration and connection groups of Y / Y and Y / Δ. The AC / DC ratio of the voltage across the valve-side windings of the converter transformer varies depending on the connection type in the high- and low-end valve halls. The ratios of the effective AC value to the DC component are 1:7, 1:5, 1:3, and 1:1, respectively. The ratios are 1:7 for the high-end valve hall Y / Y connection, 1:5 for the high-end valve hall Y / Δ connection, 1:3 for the low-end valve hall Y / Y connection, and 1:1 for the low-end valve hall Y / Δ connection. Taking the low-end valve hall Y / Δ connection as an example, the AC / DC composite voltage ratio is set to 1:1 in the aging test.

[0049] A full-scale finite element simulation model of a converter transformer was built using the drawings of an actual converter transformer (model: ZZDFPZ-415000 / 500-800). Under the Y / Δ connection of the low-end valve hall, the effective value of the AC voltage across the valve-side winding is 105kV, and the DC voltage is 101kV. Based on this, the simulation model was set up with a 1:1 composite high voltage applied to the valve-side winding, and the grid-side winding grounded. The electric field distribution cloud diagram inside the transformer is shown below. Figure 5 As shown in the figure. Finite element simulation revealed that the average electric field strength of the converter transformer's oil-paper insulation is approximately 5 kV / mm. Therefore, the effective value of the combined electric field strength in the aging test was controlled at 5 kV / mm. Finally, the applied AC and DC electric field strengths in the aging test were both controlled at approximately 3.5 kV / mm.

[0050] S3: Construct an experimental platform for observing the dynamic behavior of bubbles under partial discharge, based on a needle-plate electrode. Place aged samples in the platform to simulate the evolution of bubbles in oil under partial discharge in a converter transformer. The highly non-uniform electrode model and the bubble observation platform are described below. Figure 6 , Figure 7 As shown.

[0051] The observation experimental platform consists of an experimental fuel tank, an image acquisition system, a pressurization system, and a measurement system. The experimental oil tank consists of an oil passage, an oil pump, a temperature sensor, a heating element, and a flow meter, such as... Figure 8 As shown. Partial discharge occurs within the oil channel. The oil pump provides power for oil circulation, and a temperature sensor and heating element work together to control the oil temperature. The main oil channel of the oil circulation device is 51cm long, 26cm wide, and 19cm high, assembled using a 3cm thick transparent acrylic sheet. Both ends of the oil channel are connected to the oil pump via stainless steel pipes with an inner diameter of 42mm, forming a circulation loop. The pipes at both ends of the device are made of expandable corrugated tubing. The total volume of the oil channel and pipes is approximately 18L. Adjustable needle-plate electrodes are installed at the upper and lower ends of the oil channel, and oil paper is laid flat on top of the plate motor. The oil pump output is set based on the average oil flow velocity within the transformer. A frequency converter controls the pump speed, and an ultrasonic flow meter monitors the oil flow velocity. A 500W heating element, controlled by a temperature controller, ensures that the oil temperature in the device increases by 1℃ every 63.6 seconds.

[0052] Image acquisition devices consist of a high-speed camera, a cold light source, and an oscilloscope, such as Figure 9 As shown. A high-speed camera is mounted on a slide rail to adjust the distance between the camera and the fuel tank. A cold light source is used to compensate for insufficient incoming light, and the brightness can be adjusted via a controller. An oscilloscope triggers the high-speed camera with a delay, capturing an image of the bubble movement under partial discharge when the partial discharge pulse reaches a predetermined value.

[0053] The pressurization system employs a pressurization system combined with an electrothermal aging platform. The measurement system, in addition to a high-voltage probe for monitoring voltage, also includes a partial discharge measurement device. The partial discharge measurement device consists of a detection impedance (…). Z m ), coupling capacitor (C) k2 It consists of a partial discharge analyzer and an oscilloscope. The high-voltage side of the coupling capacitor is connected to the needle electrode, and the low-voltage side is connected to the sensing impedance. The partial discharge analyzer is connected to the output terminal of the sensing impedance and is used to measure and record the PRPD spectrum. The oscilloscope records the pulse waveform of the partial discharge.

[0054] S4. Collect the spatial location information of the bubble and divide the bubble migration process into the approaching strong field stage, the moving away from the strong field stage, the descending stage, and the rising stage; Bubble migration characteristics treat a bubble as a point mass and examine the spatiotemporal distribution of its spatial displacement and migration velocity under the influence of volume forces, typically neglecting the effect of bubble deformation. Based on Newton's second law, the dynamic equations for the movement of bubbles in stationary transformer oil are derived as follows:

[0055] m For bubble mass; v The velocity of the bubble; t For time; F EThe electric force acting on the bubble; F D This is the resultant force of the viscous resistance and the drag force of the oil flow affecting the air bubble; F B This is the resultant force of buoyancy and gravity acting on the bubble.

[0056] The electric force experienced by air bubbles in oil under partial discharge is:

[0057] F dp The dielectric force acting on the bubble; F es The electrostrictive force acting on the bubble; F q The bubble is subjected to Coulomb force. F dp , F es and F q The following formulas are shown respectively:

[0058]

[0059] e b Let be the relative permittivity of the bubble; e o The relative permittivity of the transformer oil; r The effective radius of the bubble is denoted as . F dp This causes the bubbles to move towards regions with lower electric field strength in a non-uniform electric field. F es This causes a change in the shape of the bubbles.

[0060] The resultant force (i.e., drag force) of the viscous resistance and the drag force of the oil flow affecting the bubble is:

[0061] v The velocity of the bubble; v 0 represents the oil flow velocity; L 0 represents the characteristic length of the fluid; m This refers to the dynamic viscosity of the transformer oil.

[0062] The resultant force of buoyancy and gravity acting on the bubble is:

[0063] p o The density of transformer oil;p b The density of the gas inside the bubble; Taking a bubble as an example, let's analyze its migration process from a mechanical perspective. The bubble travels along... x and y The distribution diagrams of displacement, velocity, and acceleration in the direction of migration are shown below. Figure 10 , Figure 11 As shown. Due to the deformation of the bubble during migration, five sets of coordinate data were taken from the bubble at different times (one set each from the left, right, top, bottom, and centroid of the bubble). The standard deviation curve of this curve represents the coordinates of the bubble at a certain moment to reduce error. The standard deviation curve of the bubble is used as its migration displacement, and its velocity and acceleration are obtained by fitting the migration displacement.

[0064] exist x Along the axial direction, the bubble is subjected to the combined effects of drag force and dielectric force.

[0065] Figure 10 In stage ①, the bubble moves along x Moving in the negative direction of the axis, the force analysis of the bubble at this time is shown in the figure. As the bubble continues to move towards the needle tip, the dielectrophoretic force on the bubble gradually increases. During this stage, the dielectrophoretic force plays a dominant role, so the absolute value of its velocity gradually decreases, and the drag force also gradually decreases. When the absolute value of the velocity gradually decreases to 0 (t = 0.17 ms), the bubble will begin to move along the negative axis. x The bubble moves in the positive direction of the axis. During stage ①, the bubble approaches the strong field region under the influence of inertia, so stage ① is defined as the stage in which the bubble approaches the strong field.

[0066] exist Figure 10 The force analysis of the bubble in stage ② is shown in the figure. Under the action of dielectrophoretic force, the bubble moves along... x The bubble velocity in the positive direction of the axis increases sharply, and the bubble rapidly moves away from the strong field region. However, as the bubble velocity increases and its position changes, the drag force gradually increases, while the dielectrophoretic force gradually decreases. When the dielectrophoretic force and drag force reach equilibrium... t =0.5ms), the bubble along x The velocity in the positive direction of the axis reaches its maximum, and the acceleration drops to zero. Figure 9 The force analysis of the bubble in stage ③ is shown in the figure. Because the bubble velocity is... t The maximum value is reached at 0.5ms, and the bubble has already migrated to the weak field region. Therefore, the drag force on the bubble plays a dominant role in stage ③. x In the negative direction of the axis, the bubble moves along xThe velocity in the positive direction of the axis will gradually decrease, and the bubble will slowly move away from the strong field region. As the bubble moves further away from the strong field region, the net force on the bubble will decrease to zero again, and the bubble velocity will gradually approach uniformity. In stages ② and ③, the bubble moves from the strong field region to the weak field region under the action of dielectrophoresis force and drag force; therefore, ② and ③ are defined as the stage in which the bubble moves away from the strong field.

[0067] exist y Along the axial direction, the bubble is subjected to the combined effects of dielectric force, drag force, gravity and buoyancy.

[0068] Figure 11 The force analysis of the bubble in stage I is shown in the figure. During stage I, the directions of the drag force and dielectrophoretic force acting on the bubble are both along... y In the positive direction of the axis, the bubble continuously moves towards the needle tip, and the electric force on the bubble gradually increases, reaching its maximum at the end of time period I. t =0.07ms), the direction of the dielectric force on the bubble is reversed and the value of the dielectric force reaches its maximum at this time. Figure 10 The force analysis of the bubble in stage II is shown in the figure. Because the bubble... y The velocity in the negative direction of the axis is still very large, so along y The drag force in the positive axis direction still plays a dominant role, and the direction of the resultant force remains the same. y In the positive direction of the axis, the bubble continues to decelerate and descend. During stage II (0.07ms-0.47ms), the drag force and dielectrophoretic force on the bubble gradually decrease, and the velocity gradually decreases to 0. During stages I and II, the bubble moves from top to bottom under the dominant influence of inertia, dielectrophoretic force, and drag force; therefore, stages I and II are defined as the bubble descent stages.

[0069] Figure 11 The force analysis of the bubble in stage III is shown in the figure. The bubble moves along... y The velocity in the positive direction of the axis will gradually increase from 0, and the direction of the drag force on the bubble will change from... y Rotation in the positive direction of the axis y In the negative direction of the axis, the buoyancy of the bubble plays a dominant role, with the resultant force upward, causing the bubble to gradually move away from the oil paper and accelerate towards the oil surface. As the bubble's velocity increases, the drag force it experiences also gradually increases. When the buoyancy, drag force, dielectric force, and gravity of the bubble reach equilibrium (…),… t =0.86ms), the bubble migration enters stage IV. The force analysis of the bubble in stage IV is shown in the figure. During this stage (after 0.86m), the bubble acceleration is 0, along... y As the bubble velocity along the axial direction gradually approaches a constant, the bubble rises towards the oil level at a near-uniform speed. During stages I and II, the bubble moves upward under the dominant influence of buoyancy; therefore, stages III and IV are defined as the rising stages of the bubble.

[0070] S5. Collect bubble morphology information, calculate deformation coefficient, analyze bubble deformation process; and calculate surface tension to analyze the influence of surface tension on bubble deformation. The deformation coefficient of the bubble is:

[0071] a , b These represent the effective diameters of the vertical and horizontal axes of the air bubbles in the oil, respectively. Figure 12 As shown.

[0072] Deformation is the structural change that occurs at the interface of a bubble after it is subjected to surface forces. Liquid molecules at the gas-liquid interface are attracted by the liquid side, causing the bubble to tend to maintain a standard spherical shape; therefore, surface tension... F et Its function is to suppress the deformation of the bubble. The surface tension on the bubble is:

[0073] n e It is the unit normal vector; f For phase field variables, the transformer oil region f Take 1, the bubble region f Take as 1; s is the fluid surface tension coefficient.

[0074] Taking the bubble in step S4 as an example, the deformation process of the bubble is analyzed from a mechanical perspective. The change in the deformation coefficient of the bubble is as follows: Figure 13 As shown.

[0075] Within 0-0.26 ms, the bubble stretches at both ends and compresses inward in the middle along the electric field lines under the influence of electrostrictive force. Simultaneously, the direction of the drag force on the bubble gradually becomes perpendicular. Under the dominant influence of the perpendicular component of the drag force and the electrostrictive force, the bubble's longitudinal stretching increases. At 0.26 ms, the longitudinal stretching reaches its maximum, at which point the bubble's deformation coefficient also reaches its maximum value.

[0076] Within 0.26 ms to 1 ms, the bubble morphology gradually changes from a longitudinally stretched state to a spherical shape. During this period, the electrostrictive force on the bubble gradually decreases, and its influence on the bubble deformation also gradually diminishes. Simultaneously, the direction of the drag force on the bubble gradually becomes horizontal. Under the dominant influence of the horizontal component of the drag force and surface tension, the bubble gradually transforms from a longitudinally stretched state to a spherical shape. And at approximately 1 ms, the bubble returns to its spherical shape.

[0077] S6. Extract the characteristic parameters of the bubble migration process, including the maximum displacement and migration time; extract the characteristic parameters of the bubble deformation process, including the maximum deformation coefficient, surface tension dominance time, and effective projected area of ​​the bubble. Based on the four bubble migration stages proposed in step S4, the maximum displacement and migration time of the bubble in the four stages are extracted as characteristic parameters of the migration process; based on the bubble deformation process, the maximum deformation coefficient, surface tension dominance time, and effective projected area of ​​the bubble are extracted as characteristic parameters of the deformation process.

[0078] 105 sets of bubble data were extracted to study the changes in bubble characteristic parameters under different aging times. The maximum displacement of bubbles in the stages of approaching, moving away from, descending, and rising is as follows: Figure 14 , Figure 15 , Figure 16 , Figure 17 As shown. Maximum value of bubble deformation coefficient. Q M Surface tension dominance time △t And the effective projected area of ​​the bubble at 1ms S Changes such as Figure 18 As shown.

[0079] As aging time increases, the maximum displacement of the bubble near the strong field gradually decreases, making it increasingly difficult for the bubble to approach the strong field region. The maximum displacement of the bubble away from the strong field initially increases and then decreases. The maximum displacement of the bubble during the descent phase also initially increases and then decreases. The maximum displacement of the bubble during the ascent phase increases slowly. (The maximum value of the bubble deformation coefficient is also mentioned.) Q M Surface tension dominance time △t And the effective projected area of ​​the bubble at 1ms S All of these increase with increasing aging time.

[0080] S7. Analyze the changes in the physicochemical parameters of transformer oil under different aging conditions. The physicochemical parameters include the acid value concentration, furfural content and volume resistivity of the oil. Numerical curves showing the changes in typical physicochemical parameters of oil, such as acid value concentration, furfural content, and volume resistivity, over aging time are shown below. Figure 19 As shown.

[0081] In the early stages of combined aging, the acid value concentration increases slowly. However, with increasing aging time, the acid value concentration increases rapidly in an exponential manner. In the later stages of combined aging, the acid value concentration in the oil decreases. In the early to mid-stages of combined aging, the furfural concentration in the oil increases slowly, but as aging progresses, the furfural content increases rapidly. As the content of dissolved impurities in the oil continuously increases, the kinetic viscosity coefficient of the transformer oil continuously increases, and the drag force on the bubbles caused by this becomes increasingly significant.

[0082] In the early stages of combined aging, the volume resistivity of transformer oil decreases rapidly, but the rate of decrease gradually slows down as aging progresses. Therefore, as the combined aging process continues, the influence of the electric field on the bubbles in the oil increases, and the dielectric force exerted on the bubbles by this field becomes more pronounced.

[0083] S8. The extracted characteristic parameters of the migration process and the characteristic parameters of the deformation process are correlated with the physicochemical parameters of transformer oil under different aging conditions to analyze the characteristics of bubble migration and deformation in converter transformer oil.

[0084] by Figure 14 , Figure 15 , Figure 16 , Figure 17 , Figure 18 , Figure 19 As shown, during the phase when the bubble approaches a strong field, the directions of the drag force and dielectrophoresis force acting on the bubble are both... x The direction of the axis is positive, so the movement of bubbles towards the strong field region is suppressed. Furthermore, as the aging time increases, the drag force and dielectrophoretic force on the bubbles gradually increase, so the maximum displacement of the bubbles moving from the weak field region to the strong field region gradually decreases. x It will become increasingly difficult to approach strong field regions along the axial direction.

[0085] In stage ②, which is further away from the strong field, the dielectric force on the bubbles dominates. As the aging time increases, the dielectric force on the bubbles continuously increases, so the displacement in stage ② increases rapidly. In stage ③, which is further away from the strong field, the drag force on the bubbles gradually becomes dominant. As the drag force continues to increase during the aging process, the displacement in stage ③ gradually decreases. That is, the influence of the field strength distribution on bubble movement gradually weakens, while the influence of the kinematic viscosity of the transformer oil on bubble movement gradually increases. Therefore, after 300 hours of aging, the maximum displacement in the stage far from the strong field shows a decreasing trend.

[0086] As the aging process progresses, the drag force and dielectric force on the bubbles continuously increase. The velocity of the bubbles gradually decreases upon completing stage I, and significantly decreases upon entering stage II. The dielectric force gradually becomes dominant in stage II, leading to a rapid increase in the maximum displacement of the bubbles during stage II under its influence. With further aging, the impact of the kinematic viscosity of the transformer oil on bubble motion gradually increases compared to the electric field distribution. Therefore, the drag force on the bubbles gradually restricts their descent, resulting in a decrease in the maximum displacement during stage II. Thus, with increasing aging time, the maximum displacement of the bubbles during the descent phase will first increase and then decrease.

[0087] As aging time increases, the maximum displacement of the bubbles during the rising phase increases slowly. During this phase, the bubbles gradually rise to the oil surface under the dominant influence of buoyancy. As aging time increases, due to changes in bubble volume, the buoyancy experienced by the bubbles gradually increases, and the total time spent in the descent phase decreases, thus the maximum displacement of the bubbles during the rising phase increases slowly.

[0088] The electric field strength in transformer oil increases continuously as the aging process progresses. The influence of the electric field on air bubbles in the oil increases, and the electrostrictive force on these bubbles becomes more pronounced. Therefore, the stretching of the bubble morphology increases, and the maximum value of the bubble deformation coefficient increases. Q M The surface tension coefficient of transformer oil will gradually increase. As the combined aging time increases, the surface tension coefficient will continuously decrease, making it increasingly difficult for bubbles to return to a spherical shape during the surface tension-dominated time. Therefore, the surface tension-dominated time... △t As the bubble grows, its effective projected area will also increase.

[0089] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0090] Please see Figure 20 The diagram shows a system for simulating and analyzing air bubbles in converter transformer oil. The system includes: The oil-paper insulation sample selection and pretreatment module is used to select transformer oil and insulating oil paper as samples and pretreat the samples, including filtering, drying and degassing the transformer oil, and drying and oil immersion of the insulating oil paper. The electro-thermal combined aging platform construction and aging test module is used to build an electro-thermal combined aging test platform, determine the aging temperature, aging time, AC / DC composite voltage ratio and AC / DC electric field strength, and conduct aging tests on the samples to simulate the actual aging state of the converter transformer. The partial discharge bubble observation platform construction and evolution simulation module is used to place aged samples in an experimental platform for observing the dynamic behavior of bubbles under partial discharge, simulating the evolution behavior of bubbles in oil when partial discharge occurs inside a converter transformer under the action of a non-uniform electric field. The bubble position acquisition and migration stage division module is used to acquire the spatial position information of the bubble and divide the bubble migration process into the approaching strong field stage, the moving away from the strong field stage, the descending stage, and the rising stage. The bubble morphology acquisition and deformation process analysis module is used to acquire bubble morphology information, calculate the deformation coefficient, analyze the bubble deformation process, and calculate surface tension to analyze the influence of surface tension on bubble deformation. The bubble dynamic behavior feature parameter extraction module is used to extract feature parameters of the bubble migration process, including the maximum displacement and migration time; and to extract feature parameters of the bubble deformation process, including the maximum deformation coefficient, surface tension dominance time, and effective projected area of ​​the bubble. The physicochemical parameter monitoring and change analysis module is used to analyze the changes in the physicochemical parameters of transformer oil under different aging conditions. The physicochemical parameters include the acid value concentration, furfural content and volume resistivity in the oil. The bubble parameter and physicochemical parameter correlation analysis module is used to correlate the extracted characteristic parameters of the migration process and the characteristic parameters of the deformation process with the physicochemical parameters of transformer oil under different aging conditions, and to analyze the bubble migration characteristics and deformation characteristics in converter transformer oil.

[0091] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method of simulation and analysis of gas bubbles in a converter transformer oil, characterized by, The method comprises the following steps: S1. Selecting transformer oil and insulating oil paper as samples, and pre-treating the samples, including filtering, drying and degassing treatment for the transformer oil, and drying and oil immersion treatment for the insulating oil paper, to obtain pre-treated samples; S2. Building an electro-thermal combined aging experiment platform, determining the aging temperature, aging time, AC-DC composite voltage ratio and AC-DC electric field intensity, and performing aging experiments on the pre-treated samples to simulate the actual operation aging state of the converter transformer, to obtain aged samples; S3. Placing the aged samples in a bubble dynamics behavior observation experiment platform under the action of partial discharge, simulating the evolution behavior of bubbles in the oil when partial discharge occurs in the converter transformer under the action of non-uniform electric field; S4. Collecting the spatial position information of the bubbles, and dividing the bubble migration process into a stage of approaching the strong field, a stage of moving away from the strong field, a descending stage and an ascending stage; S5. Collecting the morphological information of the bubbles, calculating the deformation coefficient, and analyzing the bubble deformation process; and calculating the surface tension, and analyzing the influence of the surface tension on the bubble deformation; S6. Extracting the characteristic parameters of the bubble migration process, including the maximum displacement and migration time of the migration process; extracting the characteristic parameters of the bubble deformation process, including the maximum deformation coefficient, the surface tension dominant time and the effective projection area of the bubble; S7. Analyzing the changes of the physicochemical parameters of the transformer oil under different aging states, the physicochemical parameters including the acid value concentration, furfural content and volume resistivity in the oil; S8. Correlating the extracted characteristic parameters of the migration process and the deformation process with the physicochemical parameters of the transformer oil under different aging states, and analyzing the bubble migration characteristics and deformation characteristics in the converter transformer oil.

2. The method of gas bubble simulation and analysis in a converter transformer oil according to claim 1, characterized in that, In step S2, the electro-thermal combined aging experiment platform comprises: an electro-thermal combined aging box, a pressurizing system and a measuring system; the electro-thermal combined aging box is used for coupling the electric field and temperature field suffered by the insulating oil paper, and applying a temperature field to the oil paper insulation by controlling the environmental temperature; The pressurizing system is composed of AC side equipment and DC side equipment, wherein AC and DC voltages are applied to the high voltage side of the plate electrode together, and the voltage borne is the sum of the AC voltage and the DC voltage; the AC side equipment includes AC side high voltage transformer TA1 and voltage regulating transformer TA2, 50 kΩ resistor R AC and 2 nF DC blocking capacitor C D-b ; the DC side equipment includes rectifier silicon stack D1, DC side high voltage transformer TD1 and voltage regulating transformer TD2, 5 MΩ resistor R DC and 0.1 μF coupling capacitor C k1 ; the DC blocking capacitor C D-b is used to realize parallel superposition of AC and DC components and protect the power supply; The measuring system consists of an oscilloscope and high-voltage probes P for measuring AC voltage AC , high-voltage probes P for measuring DC voltage DC and high-voltage probes P for measuring AC and DC voltage AC-DC .

3. The method of gas bubble simulation and analysis in a converter transformer oil according to claim 1, characterized in that, The specific steps for determining the aging temperature, aging time, AC-DC composite voltage ratio and AC-DC electric field intensity in step S2 are as follows: S21. Determining the aging temperature and aging time according to the average operating temperature of the converter transformer winding, the normal service life of the transformer operating at the temperature and the 6℃ rule; S22. Setting the ratio of AC-DC composite voltage according to the actual wiring form of the transformer; S23. Constructing a full-size finite element simulation model of the converter transformer, and determining the applied AC-DC electric field intensity through finite element simulation.

4. The method of gas bubble simulation and analysis in a converter transformer oil according to claim 1, characterized in that, In step S3, the bubble dynamics behavior observation experiment platform under the action of partial discharge is based on a needle-plate electrode, and comprises: an experimental oil tank, an image acquisition system, a pressurizing system and a measuring system; the experimental oil tank comprises an oil channel, an oil pump, a temperature sensor, a heating pipe and a flowmeter, partial discharge occurs in the oil channel, the oil pump is used to provide power for oil circulation, and the temperature sensor and the heating pipe are used to cooperate in controlling the oil temperature; The image acquisition system comprises a high-speed camera, a cold light source and an oscilloscope, the high-speed camera is installed on a slide rail and is used for adjusting the distance between the camera and the oil tank, and the oscilloscope is used for delaying triggering the high-speed camera to shoot the bubble movement image under the partial discharge action when the partial discharge pulse reaches a predetermined value; The pressurizing system adopts an electric-thermal combined aging platform pressurizing system; The measuring system comprises a high-voltage probe containing a monitoring voltage and a partial discharge measuring device, the partial discharge measuring device comprising a detection impedance Z m , a coupling capacitor C k2 , a partial discharge detector and an oscilloscope, the high-voltage side of the coupling capacitor being connected to the needle-plate electrode, and the low-voltage side being connected to the detection impedance; the partial discharge detector is connected to the output end of the detection impedance, for measuring and recording the PRPD spectrum; and the oscilloscope is used for recording the pulse waveform of the partial discharge.

5. The method of gas bubble simulation and analysis in a converter transformer oil according to claim 1, characterized in that, The specific steps of step S4 are: S41. Based on Newton's second law, a kinetic equation of the bubble movement process in the static transformer oil is obtained, and the electric field force, the drag force, the buoyancy and the gravity that the bubble receives under the action of the partial discharge are calculated; the electric field force comprises dielectrophoresis force, electrostrictive force and Coulomb force; the drag force is the combined force of the viscous resistance and the oil flow drag affected by the oil flow; S42. When the dielectrophoresis force plays a leading role when the bubble moves along the negative direction of the x axis, the absolute value of the bubble speed gradually decreases to 0, and then the bubble is divided into the stage of approaching the field strength; S43. When the dielectrophoresis force and the drag force are balanced, the speed reaches the maximum when the bubble accelerates along the positive direction of the x axis, and the drag force plays a leading role until the combined force is 0 when the bubble decelerates along the positive direction of the x axis; the whole moves from the strong field region to the weak field region; then the stage of moving away from the field strength is divided; S44. When the bubble moves along the negative direction of the y axis, the dielectrophoresis force and the drag force are along the positive direction of the y axis; the drag force plays a leading role, and the bubble speed gradually decreases to 0 due to the inertia, the dielectrophoresis force and the drag force moving from top to bottom; the stage of falling is divided; S45. When the bubble accelerates along the positive direction of the y axis, the drag force direction changes from the positive direction of the y axis to the negative direction of the y axis, and the buoyancy plays a leading role; the buoyancy, the drag force, the dielectrophoresis force and the gravity are balanced, and the bubble speed tends to be constant due to the leading role of the buoyancy and the movement from bottom to top; then the stage of rising is divided.

6. The method of gas bubble simulation and analysis in a converter transformer oil according to claim 1, characterized in that, In step S5, the calculation formula of the deformation coefficient is: wherein, a D is the effective diameter of the longitudinal axis of the gas bubble in the oil, b D is the effective diameter of the transverse axis of the gas bubble in the oil; The calculation formula of the surface tension is: wherein n e is the unit normal vector; is a phase field variable, wherein the transformer oil region has takes the value 1, the bubble region has takes the value 1; σ is the fluid surface tension coefficient; ρ b is the density of the gas within the bubble, ρ o is the density of the transformer oil.

7. The method of gas bubble simulation and analysis in a converter transformer oil according to claim 1, characterized in that, The specific steps of step S6 are: S61. According to the four migration stages of the bubble, namely, the stage of approaching the strong field, the stage of moving away from the strong field, the stage of falling and the stage of rising, the maximum displacement and the migration time of the bubble in each stage are extracted as the characteristic parameters of the migration process; S62. According to the deformation process of the bubble, the maximum deformation coefficient, the surface tension dominant time and the effective projection area of the bubble are extracted as the characteristic parameters of the deformation process; S63. Extracting n groups of bubble data, the change law of the characteristic parameters of the migration process and the deformation process characteristic parameters under different aging times is studied.

8. The method of gas bubble simulation and analysis in a converter transformer oil according to claim 1, characterized in that, The specific steps of step S7 are: S71. The acid value concentration, the furfural content and the volume resistivity of the transformer oil under different aging times are measured, and the numerical curve of the change of the acid value concentration with the aging time is obtained; S72. The change law of the physicochemical parameters is analyzed, wherein the acid value concentration increases slowly in the early stage of the combined aging, increases rapidly in the form of an exponential in the middle stage, and decreases in the later stage; the furfural content increases slowly in the early and middle stages of the combined aging, and increases rapidly in the later stage; the volume resistivity decreases rapidly in the early stage of the combined aging, and the decreasing speed gradually slows down in the later stage; S73. Analyzing the influence of the changes of physicochemical parameters on the force acting on the bubble, wherein the increase of acid value concentration and furfural content increases the dynamic viscosity coefficient of the transformer oil, and the drag force acting on the bubble is more obvious; the decrease of volume resistivity increases the influence of the electric field in the oil on the bubble, and the dielectrophoresis force acting on the bubble is more obvious.

9. The method of gas bubble simulation and analysis in a converter transformer oil according to claim 1, characterized in that, The specific steps of step S8 are: S81. Taking the force acting on the bubble as the correlation medium, wherein the force acting on the bubble includes drag force, dielectrophoresis force, buoyancy force and electrostriction force, and combining the influence of the aging time change of oil-paper insulation on the force, a correlation logic between the aging state and the characteristic parameters of the dynamic behavior of the bubble is established; S82. Correlating the characteristic parameters of the migration process in the stage of approaching the strong field with the aging state; correlating the characteristic parameters of the migration process in the stage of moving away from the strong field with the aging state; Correlating the characteristic parameters of the migration process in the descending stage with the aging state; Correlating the characteristic parameters of the migration process in the ascending stage with the aging state; correlating the characteristic parameters of the deformation process with the aging state; S83. For the correlated logic, the migration characteristics and deformation characteristics of the bubble in the transformer oil are analyzed.

10. A system for simulation and analysis of gas bubbles in a converter transformer oil, characterized by The system is used to implement the method according to any one of claims 1-9, and the system comprises: An oil-paper insulation sample selection and pretreatment module, which is used to select transformer oil and insulation oil paper as samples, and to pretreat the samples, including filtering, drying and degassing treatment of the transformer oil, and drying and oil immersion treatment of the insulation oil paper, to obtain pretreated samples; An electric-thermal combined aging platform building and aging experiment module, which is used to build an electric-thermal combined aging experiment platform, to determine the aging temperature, aging time, AC-DC composite voltage ratio and AC-DC electric field strength, and to perform an aging experiment on the pretreated samples to simulate the actual operation aging state of the converter transformer, to obtain aged samples; A partial discharge bubble observation platform building and evolution simulation module, which is used to place the aged samples in a bubble dynamic behavior observation experiment platform under the action of partial discharge, to simulate the evolution behavior of bubbles in the oil when partial discharge occurs in the converter transformer under the action of a non-uniform electric field; A bubble position collection and migration stage division module, which is used to collect the spatial position information of the bubbles, and to divide the migration process of the bubbles into the stages of approaching the strong field, moving away from the strong field, descending and ascending; A bubble shape collection and deformation process analysis module, which is used to collect the shape information of the bubbles, to calculate the deformation coefficient, to analyze the deformation process of the bubbles, to calculate the surface tension, and to analyze the influence of the surface tension on the deformation of the bubbles; A bubble dynamic behavior characteristic parameter extraction module, which is used to extract the characteristic parameters of the migration process of the bubbles, including the maximum displacement and migration time of the migration process; and to extract the characteristic parameters of the deformation process of the bubbles, including the maximum deformation coefficient, surface tension dominant time and effective projection area of the bubbles; A physicochemical parameter monitoring and change analysis module, which is used to analyze the changes of the physicochemical parameters of the transformer oil under different aging states, wherein the physicochemical parameters include the acid value concentration, furfural content and volume resistivity in the oil; The bubble parameter and physicochemical parameter correlation analysis module is used for correlating the extracted characteristic parameters of the migration process and the characteristic parameters of the deformation process with physicochemical parameters of the transformer oil under different aging states, and analyzing the bubble migration characteristics and deformation characteristics in the converter transformer oil.

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