A transformer fluorocarbon liquid replacement thermal stability evaluation method and system

Abstract

The application discloses a transformer fluorocarbon liquid replacement heat stability evaluation method and system, the method first acquires the structure parameters, operation condition parameters, insulation cooling medium parameters and internal cooling parameters of a transformer to be evaluated; then a transformer electromagnetic-thermal-flow coupling finite element model is established according to the above parameters, and the required data for transformer internal temperature field, velocity field, pressure field, heat source distribution and heat stability evaluation are obtained through simulation; further, a calculation relationship is constructed from the main heating area over-temperature risk, the insulation cooling medium natural circulation ability, the local high-temperature stagnation risk and the liquid area heat stratification; finally, the heat stability of the transformer insulation cooling medium and the internal cooling structure is evaluated according to the above calculation relationship. The application has the advantages that the shortcomings of the fluorocarbon liquid replacement effect evaluation only according to the highest temperature, the average flow rate or a single heat flow parameter can be avoided, and the heat stability adaptation condition between the fluorocarbon liquid and the transformer internal cooling structure can be judged.

Description

Technical Field

[0001] This invention belongs to the technical field of insulation cooling medium replacement evaluation for oil-immersed power transformers, specifically relating to a method and system for evaluating the thermal stability of transformer fluorocarbon liquid replacement. Background Technology

[0002] Oil-immersed power transformers rely on an internal insulating cooling medium to simultaneously achieve heat dissipation and temperature control of electrical insulation, windings, and core heating components. This medium is the core medium for the safe and long-term operation of transformers. Currently, the most widely used insulating medium in engineering applications is mineral insulating oil. It has advantages such as mature manufacturing process, low procurement cost, stable insulation dielectric properties, and fluidity and heat transfer characteristics that are compatible with traditional transformer cooling structures. It has long been used as the standard filling medium for conventional substation transformers.

[0003] However, mineral oil has inherent flammability. In applications with stringent fire safety requirements, such as urban underground utility tunnel substations, compact indoor substations in central urban areas, data center supporting substations, and high-rise building power distribution rooms, failures such as winding breakdown, enclosure leakage, or external fire can easily lead to oil spraying and combustion, causing a fire to spread and posing significant fire safety hazards. Furthermore, mineral oil leaks have a long degradation period in soil and water, resulting in high environmental remediation costs, further limiting its widespread use in high-density, high-safety-requirement areas.

[0004] Fluorocarbon insulating fluid, as a new type of flame-retardant insulating cooling medium, possesses outstanding advantages such as non-flammability or flame retardancy, strong chemical inertness, high breakdown voltage, aging resistance, low toxicity, and easy recyclability, making it a mainstream alternative to mineral oil and a key candidate for improving the fire safety level of transformers. However, fluorocarbon fluid differs significantly from traditional mineral oil in its basic physical properties: their density, dynamic viscosity, thermal conductivity, specific heat capacity at constant pressure, and coefficient of volume expansion all show orders of magnitude or significant numerical differences.

[0005] The existing transformer internal cooling channels, winding oil gap channels, heat sink channels, upper and lower confluence cavities of the tank, and bottom recirculation structure are all designed and finalized based on the heat transfer characteristics of mineral oil. Directly replacing the mineral oil in the tank with fluorocarbon liquid easily leads to incompatibility issues between the original cooling structure and the heat flow characteristics of the fluorocarbon liquid, resulting in multiple heat dissipation failure risks. Specifically, these include: 1) Abnormal temperature rise in local hot spots of the main heat-generating components such as windings and core, exceeding the insulation heat resistance limit and accelerating the aging of solid insulation; 2) Insufficient driving force for the natural circulation and buoyancy of the fluorocarbon liquid, resulting in a decrease in the overall circulation flow rate of the cooling medium and a reduction in overall heat transfer efficiency; 3) Low-speed stagnation dead zones appear in narrow winding channels, core corners, and the roots of heat sinks, causing continuous heat accumulation and forming localized high-temperature blind spots; 4) Severe vertical displacement of the liquid's thermal center of mass, widening the temperature difference between the upper and lower layers of the medium, exacerbating heat stratification, and further amplifying the risk of localized overheating.

[0006] Currently, the industry's evaluation methods for transformer cooling performance and dielectric compatibility have significant technical shortcomings. Existing assessment systems rely solely on single macroscopic parameters for qualitative or semi-quantitative judgments, resulting in a one-sided evaluation dimension and an inability to accurately quantify the multiple heat flow risks brought about by fluorocarbon liquid replacement. These shortcomings can be categorized into four types: 1. Relying solely on the highest temperature of a single hotspot is insufficient to distinguish between genuine and false risks. Existing solutions only extract the highest temperature value across the entire area to determine whether the temperature exceeds the limit, ignoring the differences in heat source density distribution: some single-point high temperatures are only instantaneous temperature peaks generated by local geometric sharp corners or wall contact, and the corresponding areas have extremely low losses and low risk of continuous heat; while if high heat source areas such as winding conductors and iron core laminations exceed the temperature slightly, the peak temperature is easily ignored because it does not become the maximum value across the entire area, making it impossible to quantify the long-term insulation degradation risk of the "high temperature + high loss" coupling superposition, and making it difficult to truly reflect the thermal safety level of the main heat-generating area.

[0007] 2. Relying solely on the average flow velocity of the medium to determine the circulating cooling capacity cannot identify localized stagnant areas. Traditional methods use the overall average flow velocity of the liquid inside the tank to characterize the medium's flow capacity. However, meeting the average flow velocity standard does not mean that there are no low-speed areas throughout the entire process. Fluorocarbon liquid has a large viscosity difference from mineral oil, which can easily form large-scale low-speed stagnant areas in the narrow gaps of the windings and the flow convergence area at the bottom of the heat sink. The average flow velocity index can mask these localized cooling blind spots and cannot quantify the risk of heat accumulation caused by the overlap of low-speed and high-temperature areas. After replacing the fluorocarbon liquid, a hidden fault of "overall flow velocity is qualified, but local overheating and burnout" may easily occur.

[0008] 3. Relying solely on the temperature difference between the upper and lower media within the enclosure to characterize heat stratification cannot quantify the degree of heat shift across the entire area. Current stratification evaluation methods only compare the temperature difference of the medium at two measuring points, one at the top and one at the bottom of the tank. This is a local, single-point comparison and cannot reflect the overall heat distribution characteristics of the entire liquid area. After the volume expansion coefficient of fluorocarbon liquid changes, the heat will shift upwards or downwards along the vertical direction. Relying solely on the temperature difference between the top and bottom points cannot characterize the magnitude of the shift of the overall thermal centroid of the liquid relative to its geometric center. The quantitative accuracy of stratification is insufficient, and it is impossible to predict the differential thermal aging problems of the upper and lower insulation and sealing components under long-term operation.

[0009] 4. The evaluation of the decoupled medium and cooling structure makes it impossible to quantitatively determine the coupling compatibility. Existing technologies separately evaluate the properties of the insulating medium and the transformer cooling structure, only testing the thermal conductivity and viscosity parameters of the medium individually, or checking the flow channel size resistance separately, without establishing a coupled quantitative relationship between the driving force of the medium's thermal buoyancy and the flow resistance of the cooling structure. For the scenario of directly replacing fluorocarbon liquid in the retrofit of old transformers, it is impossible to quantitatively determine whether the original flow channel and heat dissipation structure are suitable for the natural circulation characteristics of fluorocarbon liquid, making it difficult to predict in advance the global heat dissipation failure caused by insufficient circulation driving force and excessive flow channel resistance, and lacking a comprehensive evaluation method integrating the medium and structure.

[0010] In summary, existing single-parameter evaluation systems lack dimensionality and quantitative precision, making it difficult to accurately assess the compatibility of the medium-cooling structure after mineral oil transformers are replaced with fluorocarbon fluid. This fails to provide a reliable quantitative basis for fluorocarbon fluid retrofitting projects. Therefore, the industry urgently needs a multi-dimensional, coupled quantitative method for assessing the thermal stability of transformers after fluorocarbon fluid replacement, to comprehensively and accurately evaluate the overall thermal safety level of the transformer after replacement. Summary of the Invention

[0011] The problem this invention aims to solve is that there is a lack of a multi-dimensional, coupled quantitative method for evaluating the thermal stability of fluorocarbon liquid replacement in the current field of transformers.

[0012] The present invention solves the above-mentioned technical problems through the following technical means: A method for evaluating the thermal stability of transformer fluorocarbon liquid displacement includes the following steps: First, obtain the structural parameters, operating condition parameters, fluorocarbon liquid physical property parameters, and internal cooling structure parameters of the transformer to be evaluated. Secondly, an electromagnetic-thermal-fluid coupled finite element model of the transformer was built based on all parameters. The losses of windings, core, and structural components were solved by electromagnetic field and applied to the thermal flow field as a heat source. Next, the finite element model is solved to extract simulation data of the transformer's global temperature field, velocity field, pressure field, heat source distribution, and medium temperature rise. Then, based on simulation data, four types of quantitative calculation relationships were constructed: the risk of overheating in the main heating zone, the natural circulation capacity of fluorocarbon liquid, the risk of local high temperature stagnation, and the stratification of liquid heat. Finally, the thermal stability compatibility between the medium after fluorocarbon liquid replacement and the transformer cooling structure is comprehensively determined based on four types of quantitative indicators.

[0013] This invention overcomes the limitations of existing technologies that rely solely on single temperature and average flow rate to evaluate cooling performance through a complete process of "parameter acquisition—multi-physics coupling modeling—simulation solution—four-dimensional quantitative index construction—comprehensive adaptation judgment." It simultaneously covers the four major thermal failure risk dimensions unique to fluorocarbon liquid replacement, achieving integrated coupling and quantitative assessment of the fluorocarbon medium and the original transformer cooling structure. It can predict in advance hidden heat dissipation defects such as overheating, stagnation, insufficient circulation, and heat stratification caused by directly replacing fluorocarbon liquid with mineral oil, providing a standardized, full-process quantitative assessment method for transformer medium modification projects.

[0014] Preferably, the risk of overheating in the main heating zone is assessed using a heat source-weighted hotspot risk index. The characterization and calculation formula is as follows: ; In the formula, The main area of ​​fever, For local heat source density, For local temperature, For permissible temperature limits, For ambient temperature, As a risk amplification index, This represents the positive part of the function.

[0015] By coupling and weighting the local overheating amplitude with the regional heat source density, and introducing a positive part function to calculate the risk contribution only for the real overheating area, it can distinguish between instantaneous temperature peaks at geometric sharp corners and continuous hot spots with high losses such as windings and iron cores. This avoids the risk misjudgment caused by only looking at the highest single point temperature, accurately quantifies the long-term insulation aging risk of high heat source areas, and improves the authenticity and reliability of hot spot safety assessment after fluorocarbon liquid replacement.

[0016] Preferably, the natural circulation capability of the fluorocarbon liquid is determined using a natural circulation matching coefficient. The characterization and calculation formula is as follows: In the formula, For thermal floating pressure difference, The pressure difference represents the flow resistance of the cooling channel. To calculate the correction amount; thermal float pressure difference The flow resistance pressure difference is determined by the fluorocarbon liquid's physical properties and temperature rise distribution, and is jointly determined by the friction resistance along the cooling channel and the local resistance.

[0017] The thermal buoyancy driving force generated by the heating of fluorocarbon liquid is coupled with the flow resistance of the cooling channel. The ratio of the two is used to intuitively characterize the natural circulation capacity of fluorocarbon liquid in the original flow channel, and to quantitatively judge the flow compatibility of fluorocarbon liquid properties with the flow channel structure of winding and radiator. This solves the technical shortcomings of traditional solutions that cannot quantify whether the buoyancy driving force can overcome the flow channel resistance and predict the insufficient circulation flow rate in the whole domain.

[0018] Preferably, the risk of localized high-temperature stagnation is assessed using a high-temperature stagnation risk index. The characterization and calculation formula is as follows: In the formula, For fluorocarbon liquid medium region, This is a function for identifying low-speed regions. For localized temperature rise, This is the high-temperature amplification index.

[0019] By coupling the low-speed region identification function of the medium with the local temperature rise integral average, it can capture the narrow gap of the winding and the high-temperature stagnation blind zone at the bottom of the heat sink that cannot be reflected by the average flow rate. It can quantify the risk of local heat accumulation caused by the overlap of low-speed and high-temperature regions, accurately identify the hidden cooling dead zone caused by the viscosity difference of fluorocarbon liquid, and avoid the fault risk of "overall flow rate meets the standard, but local overheating and burnout" after replacement.

[0020] Preferably, the liquid thermal stratification adopts a thermal stratification coefficient. The characterization and calculation formula is as follows: In the formula, The height of the thermal centroid of the liquid region. The height of the geometric center of the liquid region. This represents the total height of the liquid region.

[0021] The degree of heat stratification is quantified based on the offset of the thermal centroid of the medium relative to the geometric center, which is different from the existing crude evaluation method that only relies on the temperature difference between the upper and lower points of the oil tank. It can completely characterize the overall degree of heat shift along the vertical direction after the physical properties of fluorocarbon liquid change, accurately predict the differential thermal aging problem of upper and lower insulation components during long-term operation, and has higher stratification quantification accuracy and covers the temperature distribution characteristics of the entire medium.

[0022] Preferably, the electromagnetic-thermal-fluid coupled finite element model includes an electromagnetic field calculation region, a solid heat transfer region, and an insulating cooling medium flow region.

[0023] The electromagnetic field, solid heat conduction, and fluorocarbon liquid convection heat transfer are solved in separate sections and coupled in a coordinated manner. The boundaries of each physical field are clear, avoiding the loss, temperature, and flow velocity calculation distortion caused by simplified simulation of a single field. It fully restores the real physical processes of electromagnetic heat generation, solid heat transfer, and medium flow heat transfer inside the transformer, providing high-precision simulation basic data for subsequent four-dimensional risk index calculation.

[0024] Preferably, the electromagnetic field calculation module solves for winding losses, core and structural component losses, and loads various losses as volumetric heat sources into the thermal-fluid finite element calculation domain.

[0025] Preferably, the extracted simulation data includes the temperature distribution in the winding region, the temperature distribution in the core region, the temperature distribution in the insulating cooling medium region, the velocity distribution in the insulating cooling medium region, the pressure distribution in the insulating cooling medium region, the heat source distribution in the main heat-generating region, and the temperature rise distribution in the liquid region.

[0026] Preferably, when comprehensively evaluating the thermal stability of the transformer's insulating cooling medium and internal cooling structure, a comprehensive judgment is made based on the degree of overheating in the main heat-generating areas, the degree of matching between natural circulation drive and flow resistance, the degree of local high-temperature stagnation, and the degree of heat stratification in the liquid area.

[0027] The present invention also discloses a transformer thermal stability assessment system using the above-mentioned transformer fluorocarbon liquid displacement thermal stability assessment method, including a parameter acquisition module, a multi-field coupling simulation module, a simulation data extraction module, a multi-dimensional index calculation module, and a comprehensive assessment output module; The parameter acquisition module is used to acquire the structural parameters, operating condition parameters, fluorocarbon liquid physical property parameters, and internal cooling structure parameters of the transformer to be evaluated. The multi-field coupling simulation module is used to construct and solve the electromagnetic-thermal-fluid coupling finite element model based on all collected parameters, and output global temperature field, velocity field, pressure field, heat source distribution and medium temperature rise data; The simulation data extraction module is used to extract temperature, flow rate, pressure, heat source, and temperature rise distribution data of the winding, core, and fluorocarbon liquid regions from the simulation results. The multi-dimensional index calculation module is used to calculate four types of quantitative indicators: heat source weighted hot spot risk index, natural circulation matching coefficient, high temperature stagnation risk index, and thermal stratification coefficient. The comprehensive evaluation output module is used to output the thermal stability compatibility judgment result between the medium and the cooling structure after fluorocarbon liquid replacement based on four types of quantitative indicators.

[0028] The advantages of this invention are: (1) Multi-physics coupling improves accuracy: This invention constructs an electromagnetic-thermal-fluid coupled finite element model that includes electromagnetic field, solid heat transfer, and fluorocarbon liquid flow region. It accurately calculates the real losses of winding, core, and structural components and loads them as heat sources to the heat flow field. It completely restores the entire process of transformer heat generation, heat conduction, and convective heat transfer. Compared with traditional simulation methods that only calculate temperature or flow rate separately, it eliminates the calculation deviation caused by heat source simplification and field fragmentation, and provides high-precision temperature, flow rate, pressure, and heat source basic data for thermal stability quantitative assessment. (2) Break through the shortcomings of the existing single parameter one-sided evaluation: abandon the crude evaluation mode that relies solely on the highest hot spot temperature, average oil velocity, and temperature difference between upper and lower layers, and innovatively construct four exclusive quantitative calculation formulas: heat source weighted hot spot risk, natural circulation matching coefficient, high temperature stagnation risk index, and thermal stratification coefficient; couple the heat source density to identify real high heat source hot spots and distinguish risk-free temperature peaks; combine the buoyancy driving force and flow channel resistance to quantitatively evaluate the adaptability of the medium's natural circulation; quantify the low-speed and high-temperature overlapping blind zone and identify the cooling dead zone covered by the average flow velocity; characterize the thermal stratification through the thermal centroid shift across the entire domain, which is more comprehensive than the two-point temperature difference evaluation and realizes the accurate quantification of the four types of core thermal failure risks brought about by fluorocarbon liquid replacement; (3) Achieve integrated coupling and adaptation assessment of fluorocarbon liquid medium and original cooling structure: This invention incorporates the physical properties of fluorocarbon liquid and the internal cooling channel structure of transformer into modeling and index calculation, quantitatively determines whether the original winding channel, radiator and junction cavity are adapted to the flow heat transfer characteristics of fluorocarbon liquid, supports comparative assessment of three working conditions: only changing medium, only changing structure, and simultaneous modification of medium and structure, and directly serves the pre-assessment of transformer fluorocarbon liquid modification project; (4) The assessment conclusions are objective and reliable and are in line with engineering decisions: This invention does not rely on a single indicator to draw conclusions. It integrates four dimensions—overheating risk, natural circulation matching degree, local stagnation degree, and heat stratification degree—to comprehensively evaluate thermal stability. It can distinguish between minor local heat dissipation defects and overall cooling system adaptation failures, and comprehensively weigh various thermal safety hazards after replacement. It provides a standardized, quantifiable, and reproducible basis for the fluorocarbon liquid modification of transformers in underground substations and indoor high fire protection scenarios, and avoids hidden faults such as winding overheating and accelerated insulation aging after replacement. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the overall process of a transformer fluorocarbon liquid replacement thermal stability assessment method according to Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of an electromagnetic-thermal-fluid coupled finite element model of a transformer fluorocarbon liquid displacement thermal stability assessment method according to Embodiment 1 of the present invention. Figure 3 This is a temperature cloud map of the transformer fluorocarbon liquid in a method for evaluating the thermal stability of transformer fluorocarbon liquid replacement according to Embodiment 1 of the present invention. Figure 4 This is a transformer mineral oil temperature cloud map of a transformer fluorocarbon liquid displacement thermal stability assessment method according to Embodiment 1 of the present invention. Figure 5 This is a mineral oil flow rate diagram for a method of evaluating the thermal stability of transformer fluorocarbon liquid replacement according to Embodiment 1 of the present invention. Figure 6 This is a fluorocarbon liquid flow rate diagram for a method of evaluating the thermal stability of transformer fluorocarbon liquid displacement according to Embodiment 1 of the present invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Example 1: The method for evaluating the thermal stability of transformer fluorocarbon liquid replacement according to the present invention is described in the following reference. Figure 1The process includes the following steps: S1. Obtain the structural parameters, operating condition parameters, insulating cooling medium property parameters, and internal cooling structure parameters of the transformer to be evaluated; S2. Establish an electromagnetic-thermal-fluid coupled finite element model of the transformer to be evaluated based on the structural parameters, operating condition parameters, insulating cooling medium property parameters, and internal cooling structure parameters; S3. Solve the finite element model to obtain the relevant data required for evaluating the thermal stability of the transformer; S4. Based on the finite element simulation results, construct corresponding characterization relationships from four aspects: over-temperature risk in the main heating area, natural circulation capacity of the insulating cooling medium, risk of local high-temperature stagnation, and heat stratification in the liquid area; S5. Conduct a comprehensive evaluation of the transformer's thermal stability based on the calculation relationships constructed in step S4.

[0032] This embodiment takes a 110 kV oil-immersed power transformer as an example. Keeping the transformer core, windings, oil tank and heat dissipation structure unchanged, mineral oil and fluorocarbon liquid are used as insulating cooling media respectively, and the thermal stability after fluorocarbon liquid replacement is evaluated.

[0033] The specific process is as follows: S1. Obtain the structural parameters, operating condition parameters, insulation and cooling medium properties, and internal cooling structure parameters of the transformer to be evaluated.

[0034] The specific process of step S1 includes: The object to be evaluated is a 110kV oil-immersed power transformer. This evaluation maintains the core, windings, tank, and heat dissipation structure unchanged. Two sets of insulation cooling medium parameters—mineral oil and fluorocarbon liquid—were recorded to compare thermal stability performance before and after the replacement. The specific data collected for the four types of parameters are as follows: Structural parameters: The dimensions of the windings, core, oil tank, heat sink, and liquid flow channels were collected. The high-voltage winding has 590 turns, and the low-voltage winding has 98 turns. The inner diameter of the low-voltage winding is 705mm and the outer diameter is 828mm. The inner diameter of the high-voltage winding is 920mm and the outer diameter is 1102mm. The effective height of the winding is 900mm. The outer shape of the core, the overall dimensions of the oil tank, the geometric dimensions of the heat sink, and the internal flow channel dimensions of the windings were collected simultaneously.

[0035] Operating parameters: Under rated load conditions, input rated voltage, ambient temperature, and rated loss parameters, and set the heat dissipation boundary conditions for natural air convection outside the oil tank and heat sink.

[0036] Insulating cooling medium physical properties: Two sets of medium physical properties were entered for mineral oil and fluorocarbon liquid respectively. Each set included density, dynamic viscosity, thermal conductivity, specific heat capacity at constant pressure, and volume expansion coefficient to distinguish the differences in thermal flow characteristics between the two media.

[0037] Internal cooling structure parameters: Collect the geometric dimensions of the high and low voltage winding liquid channels, the outer periphery of the iron core flow channels, the upper and lower confluence areas of the housing, and the internal flow channels of the heat sink.

[0038] S2. Based on the structural parameters, operating condition parameters, insulating cooling medium property parameters, and internal cooling structure parameters, establish an electromagnetic-thermal-fluid coupled finite element model of the transformer to be evaluated.

[0039] The specific process of step S2 includes: See Figure 2 Based on the transformer structural parameters, operating condition parameters, insulating cooling medium property parameters, and internal cooling structure parameters obtained from S1, a three-dimensional finite element model of the electromagnetic-thermal-fluid coupled 110kV oil-immersed power transformer was constructed. Various losses generated by the windings and core were solved using the electromagnetic field module, and these losses were converted into volumetric heat sources and applied to the thermal-fluid coupled computational domain. The specific implementation process consisted of four stages: geometric modeling, multiphysics region partitioning, electromagnetic field solution configuration, and coupling boundary and medium assignment. ① 3D geometric solid modeling Based on the precise dimensions obtained from step S1, a full-size 3D geometric model was constructed to completely recreate all core components of the transformer: the laminated core structure, high and low voltage windings, metal tank, and external heat sink assembly. The winding parameters were modeled according to the parameters given in step S1: 590 turns for the high-voltage winding and 98 turns for the low-voltage winding; the inner diameter of the low-voltage winding was 705 mm and the outer diameter was 828 mm; the inner diameter of the high-voltage winding was 920 mm and the outer diameter was 1102 mm; the effective height of the winding was 900 mm. To reduce the complexity of mesh calculations, the layered high and low voltage windings were treated as a homogeneous cylindrical heat-generating entity. Simultaneously, all internal cooling structures, including the internal cooling oil gap channels, the flow channels surrounding the core, the upper and lower confluence cavities of the tank, and the internal flow channels of the heat sink, were reproduced, strictly matching the internal cooling structure dimensional parameters obtained from step S1 to ensure that the flow channel geometry is consistent with the actual transformer.

[0040] ② Divide into three independent coupled computational regions After completing the geometric modeling, three types of computational domains are defined for the model partitioning, realizing the coupling of the solutions for the electromagnetic, solid heat transfer, and fluid flow fields: a. Electromagnetic field calculation area: Covers the conductive and magnetic entities of the iron core, high-voltage winding, and low-voltage winding, used to solve various power losses generated under alternating electromagnetic excitation; b. Solid heat transfer region: includes all solid components such as iron core, high and low voltage winding metal conductors, winding insulation layer, oil tank steel plate, heat sink metal substrate, etc., used to calculate the internal heat conduction process of solids; c. Insulating cooling medium flow region: Filled in the winding gap, the periphery of the iron core, the inner cavity of the oil tank, and the heat sink channel, corresponding to mineral oil and fluorocarbon liquid respectively, used to solve the natural convection flow and convection heat transfer process of the medium.

[0041] ③ Electromagnetic field simulation solution, extracting losses as coupled heat sources. The rated voltage, rated load, and other operating condition parameters obtained from S1 are imported into the electromagnetic field calculation domain. Corresponding power frequency electromagnetic excitation is applied, and steady-state electromagnetic field finite element solution is carried out. The high-voltage winding copper loss, low-voltage winding copper loss, core hysteresis and eddy current loss are calculated independently. The above three types of losses are uniformly converted into unit volume heat generation power (volume heat source density). Then, the heat source data is used as a coupling loading term and transferred to the solid heat transfer region and the insulating cooling medium flow region as the only internal heat generation condition for the heat-flow field calculation. This accurately restores the heat source spatial distribution of the transformer during actual operation and avoids the simulation distortion caused by using a uniform heat source throughout the entire domain.

[0042] ④ Unified configuration of multi-field coupling boundary conditions and medium properties a. Basic mechanical boundary: A vertical downward gravitational acceleration of 9.81 m / s² is applied within the fluid computational domain to simulate the natural buoyancy driving force of fluorocarbon liquid and mineral oil rising when heated; b. Solid-liquid interface boundary: All contact surfaces between the insulating cooling medium and the winding, iron core, inner wall of the oil tank, and inner wall of the heat sink are uniformly set with non-slip wall boundaries, which conform to the near-wall flow law of viscous fluid; c. External heat dissipation boundary: The outer wall of the oil tank and the outer surface of all heat sinks are set with natural air convection heat transfer boundary. The boundary heat transfer coefficient is set according to the heat dissipation boundary working condition parameters entered in S1 to simulate the heat dissipation process of the equipment to the environment. d. Dual-medium operating condition assignment: Two sets of insulating medium physical property parameters are loaded into the fluid region respectively. The first set is the density, dynamic viscosity, thermal conductivity, specific heat capacity at constant pressure, and volume expansion coefficient of traditional mineral oil. The second set is the complete set of physical property parameters of the fluorocarbon liquid to be replaced. This generates two independent electromagnetic-thermal-fluid coupled finite element models with only different cooling media. The heat flow differences between the original mineral oil operating condition and the fluorocarbon liquid replacement operating condition can be directly compared in the future.

[0043] S3. Solve the finite element model to obtain the relevant data required for evaluating the thermal stability of the transformer under test.

[0044] The specific process of step S3 includes: Steady-state numerical solutions were performed on the two electromagnetic-thermal-fluid coupled finite element models constructed in step S2, one for mineral oil and the other for fluorocarbon liquid. After the calculations were completed, all simulation data required for thermal stability evaluation, including the transformer's global temperature field, velocity field, pressure field, heat source distribution, and liquid temperature rise distribution, were extracted in batches. The entire process includes three parts: model solution setup, simulation calculation, and multi-dimensional field data extraction. ① Finite element model solution parameter configuration A unified steady-state solver was set for both models, and a coupled solution algorithm was used to synchronously and iteratively solve the governing equations of the electromagnetic field, solid heat transfer field, and fluid flow field. A convergence threshold was set to ensure that the iterative residuals of temperature, flow velocity, pressure, and heat source variables met the accuracy requirements of engineering simulation. A turbulent flow model suitable for low-velocity natural convection was selected for the fluid domain, adapting to the natural circulation flow characteristics of mineral oil and fluorocarbon liquid. The mesh sizes of the solid heat transfer domain and fluid domain were kept completely consistent between the two models to eliminate the interference of mesh differences on the comparison results. Only the insulating medium properties of the two models were distinguished.

[0045] ② Dual-condition independent simulation calculation Numerical calculations were performed sequentially on the mineral oil baseline model and the fluorocarbon liquid replacement model, and the complete iteration was performed until all physical quantities converged. Except for the different properties of the cooling medium, the two models were completely identical in terms of winding, core, oil tank, heat dissipation structure geometry, electromagnetic excitation, external heat dissipation boundary, gravity conditions, wall constraints, etc., to ensure the uniqueness of variables and the simulation results can be directly compared horizontally.

[0046] After completing the solution, two sets of simulation results for the global continuous distribution under different working conditions are obtained, and typical macroscopic results are output. (See reference...) Figures 3-6 In this embodiment, the average winding temperature under fluorocarbon liquid operation is 1.60K lower than that under mineral oil operation, and the average core temperature is 1.73K lower; the average flow velocity of the mineral oil medium across the entire domain is... The global average flow velocity of the fluorocarbon liquid medium is The average temperature difference between the upper and lower liquids of mineral oil is 2.60K, and the average temperature difference between the upper and lower liquids of fluorocarbon liquid is 2.00K.

[0047] ③ Extract all simulation field data required for evaluation by region From the converged simulation results file, the full set of basic data for thermal stability assessment is extracted by region classification, covering two major categories: solid heating components and insulating liquid media. a. Temperature data of solid heating areas: Extract the temperature distribution of nodes across the entire high-voltage winding and low-voltage winding, as well as the temperature distribution of each lamination area of ​​the iron core, and record the local temperature at each location in a complete manner. ; b. Global field data of insulating cooling medium: Extract temperature distribution, flow velocity vector distribution, hydrostatic pressure distribution, and local temperature rise of all grid nodes in the fluorocarbon liquid or mineral oil filled area. ; c. Heat source distribution data: Derive the volumetric heat source density of the entire region obtained from the electromagnetic field solution. It includes the differential loss values ​​at different locations of the winding and the iron core.

[0048] ④ Data classification and storage The temperature, flow rate, pressure, heat source, and temperature rise distribution data extracted from mineral oil and fluorocarbon liquid operating conditions are stored separately and used as a unified input data source for the calculation formulas of the four types of quantitative indicators in step S4, supporting the quantitative calculation of the four indicators of overheating risk in the main heating zone, natural circulation capacity, high temperature stagnation risk, and heat stratification.

[0049] S4. Based on the finite element simulation results, corresponding characterization relationships are constructed from four aspects: the risk of overheating in the main heating area, the natural circulation capacity of the insulating cooling medium, the risk of local high temperature stagnation, and the heat stratification in the liquid area.

[0050] The specific process of step S4 includes: Based on two sets of full-domain simulation field data extracted from S3 for mineral oil and fluorocarbon liquid conditions, corresponding quantitative calculation relationships were constructed in four dimensions. The heat source weighted hot spot risk index, natural circulation matching coefficient, high-temperature stagnation risk index, and thermal stratification coefficient were solved to achieve quantitative characterization of various heat flow risks after fluorocarbon liquid replacement. The calculation process for the four dimensions was carried out independently, as detailed below: ① Calculation of overheating risk in main heat-generating areas Extract input data and import global heat source density. Local temperature of each grid Preset uniform allowable temperature limits Ambient temperature Delineate the main heat-generating integration area composed of windings and iron core. Set a fixed risk amplification index .

[0051] Substitute into the calculation formula: ; The positive part of the formula Only retain local temperature The region participates in the integral, and the risk contribution of the region that does not exceed the temperature limit is recorded as 0; by weighting by heat source density, the safety risk caused by overheating in high heat source regions is amplified, and the invalid interference of lossless instantaneous temperature peaks at geometric sharp corners is eliminated. The calculation results of this embodiment show that, under mineral oil conditions for Fluorocarbon liquid operating conditions Only .

[0052] ② Calculation of the natural circulation capacity of the insulating cooling medium Extract input data and calculate the global thermal float pressure difference based on the medium temperature rise distribution and the physical properties of fluorocarbon liquid or mineral oil. Combining the geometric dimensions of the winding flow channel and heat sink channel collected by S1, the friction resistance and local resistance of the cooling channel are calculated to obtain the flow resistance pressure difference. Configure fixed calculation correction amount .

[0053] Substitute into the calculation formula: ; This index, representing the ratio of buoyancy driving force to flow channel resistance, characterizes the degree of natural circulation adaptation. A higher value indicates a stronger ability of the medium to overcome flow channel resistance and form a stable circulation. The calculation results in this embodiment show that, under mineral oil operating conditions… Fluorocarbon liquid operating conditions .

[0054] ③ Calculation of risk of local high temperature stagnation Extract the input data and delineate the entire insulating liquid integration region. Set a low-velocity threshold (50% of the average flow velocity of the corresponding medium) and generate a low-velocity region identification function. Localized temperature rise at various locations where the introduced medium is introduced. Set the high temperature amplification index .

[0055] Substitute into the calculation formula: ; The overlap between low-velocity and high-temperature regions is quantified by global integral averaging, compensating for the inability of average flow velocity to identify local cooling dead zones. Higher index values ​​indicate a greater risk of heat accumulation and stagnation. The calculation results of this embodiment show that, under mineral oil conditions… Fluorocarbon liquid operating conditions .

[0056] ④ Calculation of heat stratification in the liquid region Extract the input data, based on the vertical coordinates of each liquid region and the corresponding local temperature rise. Calculate the global thermal centroid height Read the height of the geometric center of the liquid-filled area. Total vertical height of liquid .

[0057] Substitute into the calculation formula: ; The degree of vertical heat stratification is quantified by the offset of the thermal centroid relative to the geometric center. A value closer to 0 indicates a more uniform heat distribution in the liquid and a weaker stratification phenomenon. The calculation results in this embodiment show that, under mineral oil conditions… Fluorocarbon liquid operating conditions .

[0058] Finally, the four quantitative indicators under the two operating conditions of mineral oil and fluorocarbon liquid were archived in a unified manner to form a four-dimensional comparison dataset, which serves as the complete quantitative basis for the comprehensive thermal stability evaluation of S5.

[0059] S5. Based on the calculation relationship established in step S4, a comprehensive evaluation of the thermal stability of the transformer is conducted.

[0060] The specific process of step S5 includes: Combining the four quantitative indicators corresponding to the mineral oil baseline operating condition and the fluorocarbon liquid replacement operating condition calculated by S4, a horizontal comparison is conducted from four dimensions: the degree of overheating in the main heat-generating area, the degree of matching between natural circulation drive and flow resistance, the degree of local high-temperature stagnation, and the degree of heat stratification in the liquid area. This comprehensive evaluation assesses the overall thermal stability compatibility of the fluorocarbon medium with the original internal cooling structure of the transformer after replacing mineral oil with fluorocarbon liquid. The complete judgment process is as follows: ① Compare the risk indicators of the two types of media item by item from different dimensions. a. Comparison of overheating levels in major heat-generating areas. Comparison of heat source-weighted hotspot risk indicators. Mineral oil operating conditions Fluorocarbon liquid operating conditions The fluorocarbon liquid index values ​​are much lower than those of mineral oil, indicating that the weighted risk of overheating in high-loss heat-generating areas such as windings and iron cores is lower, and there is no safety hazard caused by the superposition of high heat sources and local overheating.

[0061] b. Comparison of natural cycle matching degree. (This refers to the comparison of natural cycle matching coefficients.) Mineral oil operating conditions Fluorocarbon liquid operating conditions A matching coefficient greater than 1 for fluorocarbon liquid means that the thermal buoyancy pressure difference generated by the fluorocarbon liquid when heated can completely overcome the flow resistance brought by the winding flow channel and heat sink channel, and can form a continuous and sufficient natural circulation flow. On the other hand, a matching coefficient less than 1 for mineral oil means that the buoyancy driving force is insufficient to offset the flow channel resistance, and the overall circulation capacity is weak.

[0062] c. Comparison of high-temperature flow stagnation levels. (Regarding high-temperature flow stagnation risk indicators) Mineral oil operating conditions Fluorocarbon liquid operating conditions In fluorocarbon liquid operation, the overlap between the low-speed fluid region and the high-temperature region is smaller, and there are no large-scale cooling stagnation dead zones in places such as the narrow gap of the winding and the root of the heat sink, thus reducing the risk of local heat accumulation.

[0063] d. Comparison of thermal stratification: Comparison of thermal stratification coefficients Mineral oil operating conditions Fluorocarbon liquid operating conditions The thermal centroid of the fluorocarbon liquid is closer to the geometric center of the liquid, resulting in a more uniform vertical heat distribution and a smaller temperature difference between the upper and lower layers. This prevents heat from concentrating and rising, thus avoiding delamination issues that could exacerbate insulation aging in the upper layer.

[0064] ② Unified Judgment Rules for Dimensional Indicators A comprehensive judgment standard is established by simultaneously weighing the advantages and disadvantages of four indicators. If the medium to be evaluated meets the four conditions of low heat source weighted hot spot risk index, natural circulation matching coefficient ≥1, low high temperature stagnation risk index, and low thermal stratification coefficient, then the medium is judged to have good thermal stability compatibility with the original cooling structure of the transformer. If any one or more of the following conditions are present: high hot spot risk, natural circulation matching coefficient <1, high stagnation risk, and significant thermal stratification, then the medium is judged to have poor compatibility with the cooling structure, and there is a risk of heat dissipation failure after replacement. In this embodiment, all four indicators of the fluorocarbon liquid are better than those of the original mineral oil condition, and it fully meets the judgment conditions of good compatibility.

[0065] ③ Output comprehensive evaluation conclusions and engineering guidance suggestions The final evaluation conclusion is output based on the comparison results of four-dimensional indicators. In this embodiment, under rated operating conditions, replacing the mineral oil in the existing core, windings, tank, and heat dissipation structure of the 110kV transformer with fluorocarbon liquid improves the overall thermal stability of the transformer. The fluorocarbon liquid has good heat flow compatibility with the original internal cooling channels and heat dissipation structure, allowing for direct fluorocarbon liquid replacement modification. Simultaneously, a risk assessment can be output, clearly defining the improvement in heat dissipation performance in each dimension after replacement, providing quantitative data support for transformer fire prevention modification projects and operation and maintenance plan formulation. If a certain indicator fails to meet the standard, targeted improvement directions such as cooling structure optimization and medium parameter adjustment can be provided.

[0066] Example 2: This embodiment further describes the system of the present invention based on Embodiment 1. This embodiment is a transformer thermal stability assessment system, which is mounted on a high-performance simulation computing workstation and includes five major functional modules: parameter acquisition module, multi-field coupling simulation module, simulation data extraction module, multi-dimensional index calculation module, and comprehensive assessment output module. It can be adapted to various oil-immersed power transformers and complete the energy-based comparative assessment of thermal stability before and after mineral oil is replaced with fluorocarbon insulating liquid.

[0067] 1. Parameter acquisition module This module corresponds to step S1 of the evaluation method and features visual parameter input, data verification, and categorized storage functions. It is divided into four parameter input sections: Transformer structural parameters are divided into sections: enter winding dimensions, core dimensions, overall tank dimensions, heat sink dimensions, and internal fluid flow channel dimensions of the windings; Operating condition parameters partition: Enter the equipment's rated load, rated voltage, ambient temperature, rated loss, and external air convection heat transfer boundary conditions; Insulating cooling medium property zoning: Supports parallel input of two sets of medium parameters, one set of original mineral oil properties and the other set of fluorocarbon liquid properties to be replaced. The input parameters include medium density, dynamic viscosity, thermal conductivity, specific heat capacity at constant pressure, and volume expansion coefficient. Internal cooling structure parameter partitioning: Enter the geometric dimensions of the winding cooling channel, the outer periphery of the iron core flow channel, the upper and lower confluence areas of the oil tank, and the internal flow channel of the heat sink.

[0068] After the module completes the integrity verification of all parameters, removes missing and abnormal parameters, generates a standardized parameter data package, and transmits it to the multi-field coupled simulation module.

[0069] 2. Multi-field coupling simulation module Receive the parameter data packet sent by the parameter acquisition module and execute all modeling and simulation configuration work in step S2 of the evaluation method.

[0070] The system automatically generates a three-dimensional electromagnetic-thermal-fluid coupled finite element geometric model of a transformer, fully restoring the solid structure of the core, windings, oil tank, and heat sink. The windings are equivalent to a homogeneous cylindrical heating region. The model is automatically divided into three major computational domains: electromagnetic field calculation region, solid heat transfer region, and insulating cooling medium flow region. A rated electromagnetic excitation is applied in the electromagnetic field calculation domain to solve for winding loss, core loss and structural stray loss. Various losses are converted into volume heat sources and coupled to the solid heat transfer and medium flow calculation domain. Unified configuration of global simulation boundaries: Gravitational acceleration is applied to the fluid domain, a non-slip wall is set at the solid-liquid contact surface, and natural convection heat dissipation boundaries are set on the outer surfaces of the oil tank and heat sink; two sets of medium properties, mineral oil and fluorocarbon liquid, are loaded respectively to generate two independent finite element calculation models.

[0071] After completing the model initialization configuration, push both models to the solver to perform numerical calculations.

[0072] 3. Simulation data extraction module In the corresponding evaluation method step S3, after the two sets of finite element models have completed iterative convergence calculations, the simulation result file is automatically read, and all field data required for thermal stability evaluation are extracted by partition.

[0073] Solid heating zone data: Extract the global temperature distribution of the winding, the global temperature distribution of the iron core, and the global heat source density distribution; Insulating medium region data: Extract the global temperature field, flow velocity vector field, pressure field, and local temperature rise data of each grid. All field data extracted from mineral oil and fluorocarbon liquid operating conditions are stored in separate folders and then packaged and transmitted to the multi-dimensional index calculation module.

[0074] 4. Multi-dimensional indicator calculation module In step S4 of the corresponding evaluation method, after receiving the simulation data, the built-in calculation formula is automatically substituted into four dimensions to solve the four quantitative indicators under two sets of working conditions in batches: The formula for the weighted hotspot risk index of heat sources is used to calculate the risk of overheating in the main heat-generating area; By applying the natural circulation matching coefficient formula and combining the medium buoyancy pressure difference and the flow channel resistance pressure difference, the natural circulation adaptability index of the medium is solved. The high-temperature stagnation risk index formula is called, and combined with the low-speed identification function and local temperature rise, the risk of local high-temperature stagnation accumulation is quantified. The thermal stratification coefficient formula is used to calculate the degree of vertical thermal stratification based on the height of the liquid's thermal centroid and the height of its geometric center.

[0075] The module automatically saves the values ​​of four indicators corresponding to the two sets of media, generates an indicator comparison data table, and sends it to the comprehensive evaluation output module.

[0076] 5. Comprehensive evaluation output module In the corresponding evaluation method step S5, the four-dimensional quantitative index data is received, and the horizontal comparison judgment is automatically completed according to the preset comprehensive evaluation logic: ① Compare the values ​​of four indicators under mineral oil and fluorocarbon liquid conditions: hot spot risk, natural circulation matching degree, stagnation risk, and heat stratification; ② Comprehensively judge the thermal stability compatibility between fluorocarbon liquid and the original cooling structure of the transformer according to the judgment rules; ③ Visualize and output the evaluation report, which includes basic model information, two sets of medium simulation cloud maps, comparison tables of the four indicators, and comprehensive compatibility conclusions; if the evaluation finds that the thermal risk in a certain dimension is too high, provide reference suggestions for cooling structure optimization and medium selection adjustment.

[0077] The entire system's modules work in sequence, with data flowing automatically. There is no need for manual data transfer between modules. The entire process is automated, completing "parameter input - multi-physics modeling and simulation - field data extraction - four-dimensional index quantification calculation - comprehensive adaptation judgment". It standardizes the pre-assessment of thermal stability of any type of transformer before mineral oil is replaced with fluorocarbon liquid.

[0078] In summary, the present invention has the following advantages: The transformer fluorocarbon liquid displacement thermal stability assessment method and system proposed in this application overcomes the limitations of existing methods that rely solely on single temperatures and average flow rates for cooling performance evaluation. It utilizes electromagnetic-thermal-fluid coupled finite element simulation to accurately recreate the entire process of electromagnetic heat generation, solid-state heat conduction, and medium convection heat transfer in the transformer, comprehensively acquiring high-precision simulation data on temperature, flow rate, pressure, and heat source distribution across the entire domain. Furthermore, it innovatively constructs a four-dimensional quantitative calculation system encompassing heat source weighted hotspot risk, natural circulation matching coefficient, high-temperature stagnation risk index, and thermal stratification coefficient. This system can distinguish between geometric temperature peaks and persistent hotspots with high heat sources, quantitatively characterizing the driving force of fluorocarbon liquid buoyancy. The system identifies high-temperature stagnation blind spots that cannot be exposed by average flow velocity by matching the resistance of cooling channels and accurately measures the degree of heat stratification across the entire region through liquid thermal centroid shift. This enables quantitative evaluation of the integrated coupling and adaptation between the fluorocarbon medium and the original cooling structure. The entire solution can cover various engineering conditions, including only medium replacement, cooling structure modification, and simultaneous adjustment of both. The modular evaluation system enables fully automated calculation and comprehensive judgment throughout the process. The evaluation dimensions are comprehensive and the quantitative accuracy is high. It can predict various heat dissipation failure hazards caused by fluorocarbon liquid replacement in advance, providing a standardized and reliable quantitative decision-making basis for transformer medium modification in high fire-resistant scenarios.

[0079] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Terms such as "upper," "lower," "left," "right," "front," and "rear" used in the invention are merely for clarity of description and are not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0080] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for evaluating the thermal stability of transformer fluorocarbon liquid displacement, characterized in that, First, obtain the structural parameters, operating condition parameters, fluorocarbon liquid physical property parameters, and internal cooling structure parameters of the transformer to be evaluated. Secondly, an electromagnetic-thermal-fluid coupled finite element model of the transformer was built based on all parameters. The losses of windings, core, and structural components were solved by electromagnetic field and applied to the thermal flow field as a heat source. Next, the finite element model is solved to extract simulation data of the transformer's global temperature field, velocity field, pressure field, heat source distribution, and medium temperature rise. Then, based on simulation data, four types of quantitative calculation relationships were constructed: the risk of overheating in the main heating zone, the natural circulation capacity of fluorocarbon liquid, the risk of local high temperature stagnation, and the stratification of liquid heat. Finally, the thermal stability compatibility between the medium after fluorocarbon liquid replacement and the transformer cooling structure is comprehensively determined based on four types of quantitative indicators.

2. The method for evaluating the thermal stability of transformer fluorocarbon liquid displacement according to claim 1, characterized in that, The risk of overheating in the main heating zone is assessed using a heat source-weighted hotspot risk index. The characterization and calculation formula is as follows: ； In the formula, The main area of ​​fever, For local heat source density, For local temperature, For permissible temperature limits, For ambient temperature, As a risk amplification index, This represents the positive part of the function.

3. The method for evaluating the thermal stability of transformer fluorocarbon liquid displacement according to claim 1, characterized in that, The natural circulation capability of the fluorocarbon liquid is determined by a natural circulation matching coefficient. The characterization and calculation formula is as follows: In the formula, For thermal floating pressure difference, The pressure difference represents the flow resistance of the cooling channel. To calculate the correction amount; thermal float pressure difference The flow resistance pressure difference is determined by the fluorocarbon liquid's physical properties and temperature rise distribution, and is jointly determined by the friction resistance along the cooling channel and the local resistance.

4. The method for evaluating the thermal stability of transformer fluorocarbon liquid displacement according to claim 1, characterized in that, The risk of localized high-temperature stagnation is described using the high-temperature stagnation risk index. The characterization and calculation formula is as follows: In the formula, For fluorocarbon liquid medium region, This is a function for identifying low-speed regions. For localized temperature rise, This is the high-temperature amplification index.

5. The method for evaluating the thermal stability of transformer fluorocarbon liquid displacement according to claim 1, characterized in that, The liquid thermal stratification uses a thermal stratification coefficient. The characterization and calculation formula is as follows: In the formula, The height of the thermal centroid of the liquid region. The height of the geometric center of the liquid region. This represents the total height of the liquid region.

6. The method for evaluating the thermal stability of transformer fluorocarbon liquid displacement according to claim 1, characterized in that, The electromagnetic-thermal-fluid coupled finite element model includes an electromagnetic field calculation region, a solid heat transfer region, and an insulating cooling medium flow region.

7. The method for evaluating the thermal stability of transformer fluorocarbon liquid displacement according to claim 6, characterized in that, The electromagnetic field calculation module solves for winding losses, core and structural component losses, and loads various losses as volumetric heat sources into the thermal-fluid finite element calculation domain.

8. The method for evaluating the thermal stability of transformer fluorocarbon liquid displacement according to claim 1, characterized in that, The extracted simulation data includes temperature distribution in the winding region, temperature distribution in the core region, temperature distribution in the insulating cooling medium region, velocity distribution in the insulating cooling medium region, pressure distribution in the insulating cooling medium region, heat source distribution in the main heat-generating region, and temperature rise distribution in the liquid region.

9. The method for evaluating the thermal stability of transformer fluorocarbon liquid displacement according to claim 1, characterized in that, When comprehensively evaluating the thermal stability of the transformer's insulating cooling medium and internal cooling structure, a comprehensive judgment is made based on the degree of overheating in the main heat-generating areas, the degree of matching between natural circulation drive and flow resistance, the degree of local high-temperature stagnation, and the degree of heat stratification in the liquid area.

10. A transformer thermal stability assessment system, characterized in that, It includes a parameter acquisition module, a multi-field coupling simulation module, a simulation data extraction module, a multi-dimensional index calculation module, and a comprehensive evaluation output module; The parameter acquisition module is used to acquire the structural parameters, operating condition parameters, fluorocarbon liquid physical property parameters, and internal cooling structure parameters of the transformer to be evaluated. The multi-field coupling simulation module is used to construct and solve the electromagnetic-thermal-fluid coupling finite element model based on all collected parameters, and output global temperature field, velocity field, pressure field, heat source distribution and medium temperature rise data; The simulation data extraction module is used to extract temperature, flow rate, pressure, heat source, and temperature rise distribution data of the winding, core, and fluorocarbon liquid regions from the simulation results. The multi-dimensional index calculation module is used to calculate four types of quantitative indicators: heat source weighted hot spot risk index, natural circulation matching coefficient, high temperature stagnation risk index, and thermal stratification coefficient. The comprehensive evaluation output module is used to output the thermal stability compatibility judgment result between the medium and the cooling structure after fluorocarbon liquid replacement based on four types of quantitative indicators. The system performs the transformer fluorocarbon liquid displacement thermal stability assessment method as described in any one of claims 1 to 9.

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