An environmental protection insulating oil transformer cold flow characteristic evaluation method

Abstract

The application discloses an environmental protection insulating oil transformer cold flow characteristic evaluation method, belongs to the insulating oil cold flow characteristic evaluation field, and is characterized by the following steps: constructing an optimized molecular model, constructing an insulating oil system, calculating insulating oil density and free volume, utilizing molecular dynamics simulation, and combining density, free volume and related experimental tests to predict the physical and chemical properties of insulating oil which are difficult to be directly tested through experiments at extremely low ambient temperature, so that the shortcomings of studying the cold flow characteristics of insulating oil under cold environment are made up, a convenient and low-cost method for predicting the cold flow characteristics of natural ester insulating oil transformers is provided, and the natural ester insulating oil with excellent cold flow performance can be found. The application not only provides a theoretical approach and a guidance direction for improving the cold flow performance of natural ester insulating oil and transformers, but also helps to solve the technical bottleneck of the application of natural ester insulating oil and transformers in cold climate environment through comprehensive application of the evaluation method.

Description

Technical Field

[0001] This invention belongs to the fields of quantum chemical calculation, molecular dynamics simulation and model simulation technology, and specifically relates to a method for evaluating the cold flow characteristics of environmentally friendly insulating oil transformers. Background Technology

[0002] Mineral insulating oil is primarily used for transformer insulation. However, mineral insulating oil has a low flash point, making it prone to short circuits or core overheating under high transformer loads, potentially leading to transformer explosions or fires. Furthermore, its low biodegradability means leaks can severely damage the natural environment. Natural ester insulating oil, also known as plant-based insulating oil, is extracted primarily from plant flowers and seeds. It is a green and renewable energy source with sustainable development advantages. Natural ester insulating oil is a suitable alternative to mineral insulating oil. It is an environmentally friendly liquid dielectric with a high flash point and high ignition point. Suitable for applications with high environmental and fire safety requirements, it provides transformers with superior performance advantages such as short-term high overload capability, long lifespan, and high fire safety. Compared to mineral insulating oil, natural ester insulating oil is renewable, environmentally friendly, and has a high biodegradability rate. Its lifecycle carbon emissions are only 1 / 64th that of mineral insulating oil, enhancing the environmental performance of transformers.

[0003] The pour point is the lowest temperature at which a liquid can flow without stirring. It is an effective indicator of the cold flow properties of insulating fluids and affects the reliability of natural ester transformers operating at low temperatures. When the temperature of transformer insulating oil is below its pour point, the oil becomes viscous, like mud. This not only affects its fluidity and heat transfer performance, hindering the transfer and dissipation of heat from the transformer, but may also damage mechanical components immersed in the insulating oil, such as tap changers and circuit breakers, thus impairing normal operation. Furthermore, as the ambient temperature approaches the oil's pour point, the breakdown voltage of the natural ester insulating oil gradually decreases. Therefore, further research on the cold flow characteristics of natural ester insulating oil is necessary.

[0004] Measuring the performance of insulating oils in cold environments is more challenging than experiments at room temperature, and in some cases, almost impossible. Studying the cold flow properties of natural ester insulating oils is crucial for improving the safety and reliability of transformers operating in cold environments. Furthermore, raw plant oils extracted from plants cannot be directly used as natural ester insulating oils for transformers; chemical modification or additives are required to ensure that the natural ester insulating oils meet transformer insulating oil standards. This process increases time and financial costs. With the development of computer technology, calculating and analyzing the cold flow properties of natural ester insulating oils through molecular dynamics simulations is a convenient and effective method. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a method for evaluating the cold flow characteristics of environmentally friendly insulating oil transformers. By constructing an optimized molecular model, building an insulating oil system, calculating the density and free volume of the insulating oil, and using molecular dynamics simulation, combined with density and free volume, the physicochemical properties of the insulating oil that are difficult to test directly by experiments at extremely low ambient temperatures can be predicted. This method overcomes the shortcomings of studying the cold flow characteristics of insulating oil in cold environments and provides a convenient and low-cost method for predicting the cold flow characteristics of natural ester insulating oils and their transformers. It also helps to find natural ester insulating oils with excellent cold flow performance.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] A method for evaluating the cold flow characteristics of an environmentally friendly insulating oil transformer, comprising the following steps:

[0008] Step 1, construct and optimize molecular models: Based on density functional theory, quantum chemical calculation modules are used to construct and optimize triglyceride molecular models of tristearate, trioleate, trilinoleic acid and trilinolenic acid.

[0009] Step 2, Constructing the insulating oil system: Construct a soybean-based natural ester insulating oil system composed of tristearate, trioleate, trilinoleic acid, and trilinolenic acid molecules in relative proportions.

[0010] Step 3: Calculate the density and free volume of the insulating oil: Based on the AMBER force field, the density of soybean-based natural ester insulating oil at different temperatures was calculated using molecular dynamics simulations, and the free volume of the soybean-based natural ester insulating oil system was calculated using quantum chemical calculation methods; the pour point of soybean-based natural ester insulating oil was predicted by combining the density and free volume.

[0011] Preferably, in step 1, the quantum chemical calculation module is the Gaussian module, and the constructed triglyceride molecular model consists of a glycerol backbone connecting three identical fatty acid groups, that is, a homoglyceride formed by the dehydration condensation of one glycerol molecule and three identical fatty acid molecules.

[0012] Preferably, in step 1, the triglyceride molecular model is structurally optimized using a B3LYP-D3(BJ) hybrid functional combined with a 6-311G(d,p) basis set, and DFT-D3 and Becke-Johnson damped dispersion correction are used to account for van der Waals interactions to obtain a structurally reasonable and stable molecular structure.

[0013] Preferably, in step 2, a soybean-based natural ester insulating oil system is constructed using the Packmol module, wherein the relative contents of the four triglyceride molecules—tristearate, trioleate, trilinoleic acid, and trilinolenic acid—are 16%, 24%, 50%, and 10%, respectively. The fatty acids in the soybean-based natural ester insulating oil include stearic acid, oleic acid, linoleic acid, and linolenic acid.

[0014] Preferably, in step 3, the energy minimization process and molecular dynamics simulations of the equilibrium phase and the generating phase of the soybean-based natural ester insulating oil system were performed using the GROMACS module combined with the AMBER force field. The steps are as follows:

[0015] 1) The energy of the soybean-based natural ester insulating oil system was minimized using a 30,000-step conjugate gradient method to obtain a stable system structure;

[0016] 2) Perform equilibrium phase simulation: The equilibrium phase adopts an isothermal and isobaric ensemble, that is, the number of particles, pressure and temperature are constant. The simulation time is 6 ns, the time step is 2 fs, the leapfrog method is used for molecular dynamics, and single annealing is used. Each temperature control group is set with 2 annealing points. The temperature is raised to the specified temperature after 30 ps. The pressure is controlled by a Berendsen pressure bath.

[0017] 3) Perform phase generation simulation: Use a canonical ensemble with a step size of 1fs and a simulation duration of 1ns, i.e., an isothermal and isovolute ensemble. Use the V-rescale method for the heat bath and keep the temperature setting consistent with the equilibrium phase.

[0018] Preferably, in step 3, the soybean-based natural ester insulating oil system is subjected to energy minimization and initial equilibrium molecular dynamics simulation using the conjugate gradient method, and the density of the soybean-based natural ester insulating oil system is adjusted according to the density of the soybean-based natural ester insulating oil at the actual temperature.

[0019] Preferably, in step 3, the main simulation parameters for molecular dynamics simulation include:

[0020] 1) Periodic boundary conditions were set in both the XYZ directions of the system;

[0021] 2) The van der Waals interactions were calculated using a truncation method with a cutoff value of 1.5 nm;

[0022] 3) Electrostatic interactions were handled using the Particle-Mesh Ewald method, with a short-range electrostatic interaction distance of 1.5 nm.

[0023] Preferably, in step 3, the density values ​​of the systems at different temperatures simulated under isothermal and isovolume ensembles are extracted using the built-in operation commands of the GROMACS module, and the free volume of the systems at different temperatures simulated under isothermal and isovolume ensembles is calculated using the Multiwfn module, thereby calculating the free volume fraction at different temperatures.

[0024] Preferably, in step 3, the density and free volume fraction of the soybean-based natural ester insulating oil system at different temperatures are fitted to obtain two straight lines with different slopes, and then it is found that the inflection point of the fitted straight line is the pour point of the soybean-based natural ester insulating oil system.

[0025] The present invention can achieve the following beneficial effects:

[0026] This invention utilizes computer-based molecular dynamics simulations to rapidly, effectively, and cost-efficiently determine the cold flow characteristics of natural ester insulating oils. It can also calculate physicochemical properties that cannot be measured by macroscopic experimental methods, and the results show good agreement with experimental data. When the required experimental conditions cannot be met, an alternative method using molecular dynamics simulations is provided. This invention is applicable not only to natural ester insulating oils but also to various blends of insulating oils with different proportions. Attached Figure Description

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0028] Figure 1 The following are the molecular structures of four optimized triglycerides as presented in this invention;

[0029] Figure 2 This invention relates to a soybean-based natural ester insulating oil system.

[0030] Figure 3 This is a simulated density curve of the soybean-based natural ester insulating oil of the present invention at 15°C and -55°C.

[0031] Figure 4 This is a graph showing the relationship between the density of soybean-based natural ester insulating oil and its pour point, constructed according to the present invention.

[0032] Figure 5 This is a diagram showing the free volume of the soybean-based natural ester insulating oil of the present invention at 15°C and -55°C.

[0033] Figure 6 This is a graph showing the relationship between the free volume fraction of soybean-based natural ester insulating oil and the pour point, constructed according to the present invention.

[0034] In the picture: Figure 1The diagrams (a) through (d) are, in order, the molecular structures of trilinolenic acid glyceride (C57H92O6), trilinolenic acid glyceride (C57H98O6), triolenic acid glyceride (C57H104O6), and tristearate glyceride (C57H110O6). Detailed Implementation

[0035] Preferred solutions include Figures 1 to 6 As shown, a method for evaluating the cold flow characteristics of an environmentally friendly insulating oil transformer includes the following steps:

[0036] Step 1, Constructing and Optimizing the Molecular Model: Based on density functional theory, a quantum chemical calculation module was used to construct and optimize the molecular model of tristearate (C 57 H 110 O6), trioleic acid glycerides (C 57 H 104 O6), triglycerides of trioleate (C 57 H 98 O6) and trilinolenic acid glyceride (C 57 H 92 A molecular model of triglycerides (O6);

[0037] Preferably, in step 1, the quantum chemical calculation module is the Gaussian module, and the constructed triglyceride molecular model consists of a glycerol backbone connecting three identical fatty acid groups, that is, a homoglyceride formed by the dehydration condensation of one glycerol molecule and three identical fatty acid molecules.

[0038] Preferably, in step 1, the triglyceride molecular model is structurally optimized using a B3LYP-D3(BJ) hybrid functional combined with a 6-311G(d,p) basis set, and DFT-D3 and Becke-Johnson damped dispersion correction are used to account for van der Waals interactions to obtain a structurally reasonable and stable molecular structure.

[0039] Step 2, Constructing the insulating oil system: Constructing an insulating oil system based on glyceryl tristearate (C 57 H 110 O6), trioleic acid glycerides (C 57 H 104 O6), triglycerides of trioleate (C 57 H 98 O6) and trilinolenic acid glyceride (C 57 H 92 The soybean-based natural ester insulating oil system is composed of O6 molecules in a relative proportion; the fatty acids in the soybean-based natural ester insulating oil include stearic acid, oleic acid, linoleic acid, and linolenic acid.

[0040] Preferably, in step 2, a soybean-based natural ester insulating oil system was constructed using the Packmol module, wherein glyceryl tristearate (C 57 H 110 O6), trioleic acid glycerides (C 57 H 104 O6), triglycerides of trioleate (C 57 H 98 O6) and trilinolenic acid glyceride (C 57 H 92 The relative contents of the four triglyceride molecules (O6) were 16%, 24%, 50% and 10%, respectively.

[0041] Step 3: Calculate the density and free volume of the insulating oil: Based on the AMBER force field, the density of soybean-based natural ester insulating oil at different temperatures was calculated using molecular dynamics simulations, and the free volume of the soybean-based natural ester insulating oil system was calculated using quantum chemical calculation methods; the pour point of soybean-based natural ester insulating oil was predicted by combining the density and free volume.

[0042] Preferably, in step 3, the energy minimization process and molecular dynamics simulations of the equilibrium phase and the generating phase of the soybean-based natural ester insulating oil system were performed using the GROMACS module combined with the AMBER force field. The steps are as follows:

[0043] 1) The energy of the soybean-based natural ester insulating oil system was minimized using the 30,000-step conjugated gradient method (CG) to obtain a stable system structure;

[0044] 2) Perform equilibrium phase simulation: The equilibrium phase adopts an isothermal and isobaric ensemble, that is, the number of particles, pressure and temperature are constant. The simulation time is 6 ns, the time step is 2 fs, the leapfrog method is used for molecular dynamics, and single annealing is used. Each temperature control group is set with 2 annealing points. The temperature is raised to the specified temperature after 30 ps. The pressure is controlled by a Berendsen pressure bath.

[0045] 3) Perform phase generation simulation: Use a canonical ensemble with a step size of 1fs and a simulation duration of 1ns, i.e., an isothermal and isovolute ensemble. Use the V-rescale method for the heat bath and keep the temperature setting consistent with the equilibrium phase.

[0046] Preferably, in step 3, the soybean-based natural ester insulating oil system is subjected to energy minimization and initial equilibrium molecular dynamics simulation using the conjugate gradient method, and the density of the soybean-based natural ester insulating oil system is adjusted according to the density of the soybean-based natural ester insulating oil at the actual temperature.

[0047] Preferably, in step 3, the main simulation parameters for molecular dynamics simulation include:

[0048] 1) Periodic boundary conditions were set in both the XYZ directions of the system;

[0049] 2) Van der Waals interactions were calculated using a cut-off method with a cutoff value of 1.5 nm (0.85 nm in the equilibrium phase).

[0050] 3) Electrostatic interactions were handled using the Particle-Mesh Ewald (PME) method, with a short-range electrostatic interaction distance of 1.5 nm (the distance of the medium-range electrostatic interaction in the equilibrium phase is 0.85 nm).

[0051] Preferably, in step 3, the density values ​​of the systems at different temperatures simulated under the isothermal isovolume ensemble (NVT) are extracted using the built-in operation instructions of the GROMACS module, and the free volume of the systems at different temperatures simulated under the isothermal isovolume ensemble (NVT) is calculated using the Multiwfn module, thereby calculating the free volume fraction at different temperatures.

[0052] Preferably, in step 3, the density and free volume fraction of the soybean-based natural ester insulating oil system at different temperatures are fitted to obtain two straight lines with different slopes, and then it is found that the inflection point of the fitted straight line is the pour point of the soybean-based natural ester insulating oil system.

[0053] Example 1:

[0054] First, some data on the physicochemical properties of soybean-based natural ester insulating oil, such as density and pour point, were obtained through experiments.

[0055] Constructing optimized tristearate (C 57 H 110 O6), trioleic acid glycerides (C 57 H 104 O6), triglycerides of trioleate (C 57 H 98 O6) and trilinolenic acid glyceride (C 57 H 92 A molecular model of triglycerides (O6).

[0056] In practice, an initial molecule is constructed within the GaussView module, or an initial molecular model provided by the ChemSpider or PubChem database is optimized using density functional theory in the Gaussian quantum chemistry calculation module. The triglyceride molecular model is structurally optimized using a B3LYP-D3(BJ) hybrid functional combined with a 6-311G(d,p) basis set. DFT-D3 and Becke-Johnson damped dispersion correction are used to account for van der Waals interactions, resulting in a structurally sound and stable molecular structure. Figure 1 As shown. Oxygen atoms are red, carbon atoms are blue, and hydrogen atoms are white. A soybean-based natural ester insulating oil system was constructed using the Packmol module, in which glyceryl tristearate (C...) was used. 57 H 110 O6), trioleic acid glycerides (C 57 H 104 O6), triglycerides of trioleate (C 57 H 98 O6) and trilinolenic acid glyceride (C 57 H 92 The relative contents of the four triglyceride molecules (O6) were 16%, 24%, 50%, and 10%, respectively. Figure 2 As shown.

[0057] In this work, molecular dynamics simulations were performed using the molecular dynamics program GROMACS (single-precision version: 2018.8). The force field used in the simulation was the Generation Amber Force Field.

[0058] In practice, the main parameters for molecular dynamics simulation include: periodic boundary conditions are set in the XYZ directions of the system; van der Waals interactions are calculated using a cut-off method with a cutoff value of 1.5 nm (0.85 nm in the equilibrium phase); and electrostatic interactions are handled using the Particle-Mesh Ewald (PME) method, with a short-range electrostatic interaction distance of 1.5 nm (0.85 nm in the equilibrium phase).

[0059] The first step is to use the 30,000-step conjugated gradient method (CG) to minimize the energy of the soybean-based natural ester insulating oil system and obtain a stable system structure.

[0060] The second step is to perform equilibrium phase simulation. The equilibrium phase adopts the isothermal-isobaric ensemble (NPT), that is, the number of particles, pressure and temperature are constant. The simulation time is 6 ns and the time step is 2 fs. The leapfrog method is used for molecular dynamics. Single annealing is used. Each temperature control group is set with 2 annealing points. The temperature is raised to the specified temperature after 30 ps. The pressure is controlled by the Berendsen pressure bath.

[0061] The third step is to perform a phase generation simulation using a canonical ensemble with a step size of 1 fs and a simulation duration of 1 ns, namely an isothermal isovolume ensemble (NVT). The V-rescale method is used for the heat bath, and the temperature setting is consistent with that of the equilibrium phase.

[0062] In step 3, the density values ​​of the system at different temperatures simulated under the NPT ensemble are extracted using the built-in operation commands of the GROMACS module, and the free volume of the system at different temperatures simulated under the NPT ensemble is calculated using the Multiwfn module, thereby calculating the free volume fraction at different temperatures.

[0063] Simulations were performed on a soybean-based natural ester insulating oil system at eight temperature points ranging from 15 to -55℃, with 10℃ intervals, and the density of the generated phase was investigated. The density simulation results at 15℃ and -55℃ are used as examples. Figure 3 The table shows the density versus time curves during NPT ensemble simulations at 15℃ and -55℃. To make the simulated density values ​​more realistic, we used the average density from 4000 to 6000 ps at different temperatures to represent the density at those temperatures, and compared the results with experimental values ​​(Table 1). Since the density of soybean-based natural ester insulating oil cannot be directly measured experimentally at temperatures below -15℃, experimental density data for this section are not included in the table. The highest relative error between the simulated and experimental densities is only 0.42%, reflecting the applicability of using the AMBER force field for molecular dynamics simulations of the soybean-based natural ester insulating oil system.

[0064] Table 1. Simulated and experimental densities of soybean-based natural ester insulating oils at different temperatures.

[0065]

[0066] The pour point is also the solid-liquid phase transition point of natural ester insulating oil; therefore, the density of natural ester insulating oil undergoes a sudden change at the pour point. This invention performs linear fitting on the data points, obtaining two straight lines with different slopes. The intersection of these two lines is the pour point, as shown below. Figure 4As shown, the pour point of soybean-based natural ester insulating oil, as determined by molecular dynamics simulation, is 252.7 K, or -20.4 °C. This is close to the experimentally measured pour point data of soybean-based natural ester insulating oil, which is -18 to -21 °C. Therefore, this invention can accurately predict the pour point of soybean-based natural ester insulating oil.

[0067] Based on the good simulation of the density and pour point of soybean-based natural ester insulating oil by the equilibrium phase NPT ensemble, this invention further simulates the generation phase NVT ensemble. Free volume analysis of the soybean-based natural ester insulating oil system in the generation phase was performed using the Multiwfn module, and the results are shown in Table 2.

[0068] Table 2. Total volume and free volume of soybean-based natural ester insulating oil system at different temperatures.

[0069]

[0070] To provide a more intuitive demonstration, this invention uses soybean-based natural ester insulating oil system models at two temperatures: 15℃ and -55℃, to illustrate its free volume, as shown below. Figure 5 (a) and Figure 5 As shown in (b), the gold area represents the free volume of soybean-based natural ester insulating oil molecules. The gold area at -55°C is significantly smaller than that at 15°C, indicating that the kinematic viscosity of soybean-based natural ester insulating oil is increasing and its fluidity is decreasing. The lower the pour point, the stronger the low-temperature fluidity of the insulating oil.

[0071] This invention correlates free volume fraction with pour point, fitting the free volume fraction at eight temperature points, such as... Figure 6 As shown, the pour point of soybean-based natural ester insulating oil, predicted by the relationship between free volume fraction and temperature, is -21.2℃, which is close to the experimentally measured pour point data of -18 to -21℃. In summary, this invention establishes a method for predicting the cold flow characteristics of natural ester insulating oil.

[0072] The above embodiments are merely preferred technical solutions of the present invention and should not be considered as limitations on the present invention. The scope of protection of the present invention should be limited to the technical solutions described in the claims, including equivalent substitutions of the technical features described in the claims. That is, equivalent substitutions and improvements within this scope are also within the scope of protection of the present invention.

Claims

1. A method for evaluating the cold flow characteristics of an environmentally friendly insulating oil transformer, characterized in that... Includes the following steps: Step 1, constructing and optimizing molecular models: Based on density functional theory, quantum chemical calculation modules are used to construct and optimize triglyceride molecular models of tristearate, trioleate, trilinoleic acid, and trilinolenic acid. The triglyceride molecular models consist of a glycerol backbone connecting three identical fatty acid groups, that is, homoglycerides formed by the dehydration condensation of one glycerol molecule with three identical fatty acid molecules. Step 2, Constructing the insulating oil system: Construct a soybean-based natural ester insulating oil system composed of tristearate, trioleate, trilinoleic acid, and trilinolenic acid molecules in relative proportions. Step 3, Calculate the density and free volume of the insulating oil: Based on the AMBER force field, the density of soybean-based natural ester insulating oil at different temperatures was calculated using molecular dynamics simulation, and the free volume of the soybean-based natural ester insulating oil system was calculated using quantum chemical calculation methods; the pour point of soybean-based natural ester insulating oil was predicted by combining the density and free volume. In step 2, a soybean-based natural ester insulating oil system was constructed using the Packmol module, wherein the relative contents of four triglyceride molecules—tristearate, trioleate, trilinoleic acid, and trilinolenic acid—were 16%, 24%, 50%, and 10%, respectively; the fatty acids in the soybean-based natural ester insulating oil included stearic acid, oleic acid, linoleic acid, and linolenic acid. In step 3, the density and free volume fraction of the soybean-based natural ester insulating oil system at different temperatures were fitted, resulting in two straight lines with different slopes. It was then discovered that the inflection point of the fitted straight line is the pour point of the soybean-based natural ester insulating oil system.

2. The method for evaluating the cold flow characteristics of environmentally friendly insulating oil transformers according to claim 1, characterized in that: In step 1, the quantum chemical calculation module is the Gaussian module.

3. The method for evaluating the cold flow characteristics of environmentally friendly insulating oil transformers according to claim 2, characterized in that: In step 1, the triglyceride molecular model is based on B3LYP. D3(BJ) hybrid functional combination 6 The structure was optimized using the 311G(d,p) basis set. DFT-D3 and Becke-Johnson damped dispersion correction were used to account for van der Waals interactions, resulting in a structurally sound and stable molecular structure.

4. The method for evaluating the cold flow characteristics of environmentally friendly insulating oil transformers according to claim 1, characterized in that: In step 3, the energy minimization and molecular dynamics simulations of the equilibrium and generating phases of the soybean-based natural ester insulating oil system were performed using the GROMACS module combined with the AMBER force field. The steps are as follows: 1) The energy of the soybean-based natural ester insulating oil system was minimized using a 30,000-step conjugate gradient method to obtain a stable system structure; 2) Perform equilibrium phase simulation: The equilibrium phase adopts an isothermal and isobaric ensemble, that is, the number of particles, pressure and temperature are constant. The simulation time is 6 ns, the time step is 2 fs, the leapfrog method is used for molecular dynamics, and single annealing is used. Each temperature control group is set with 2 annealing points. The temperature is raised to the specified temperature after 30 ps. The pressure is controlled by a Berendsen pressure bath. 3) Perform phase generation simulation: Use a canonical ensemble with a step size of 1fs and a simulation duration of 1ns, i.e., an isothermal and isovolute ensemble. Use the V-rescale method for the heat bath and keep the temperature setting consistent with the equilibrium phase.

5. The method for evaluating the cold flow characteristics of environmentally friendly insulating oil transformers according to claim 4, characterized in that: In step 3, the soybean-based natural ester insulating oil system is subjected to energy minimization and initial equilibrium molecular dynamics simulation using the conjugate gradient method. The density of the soybean-based natural ester insulating oil system is adjusted according to the density of the soybean-based natural ester insulating oil at the actual temperature.

6. The method for evaluating the cold flow characteristics of environmentally friendly insulating oil transformers according to claim 5, characterized in that: In step 3, the main simulation parameters for molecular dynamics simulation include: 1) Periodic boundary conditions were set in both the XYZ directions of the system; 2) The van der Waals interactions were calculated using a truncation method with a cutoff value of 1.5 nm; 3) Electrostatic interactions were handled using the Particle-Mesh Ewald method, with a short-range electrostatic interaction distance of 1.5 nm.

7. The method for evaluating the cold flow characteristics of environmentally friendly insulating oil transformers according to claim 6, characterized in that: Preferably, in step 3, the density values ​​of the systems at different temperatures simulated under isothermal and isovolume ensembles are extracted using the built-in operation commands of the GROMACS module, and the free volume of the systems at different temperatures simulated under isothermal and isovolume ensembles is calculated using the Multiwfn module, thereby calculating the free volume fraction at different temperatures.

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