A method for evaluating the molecular resistance of insulating oil to electric field based on microcosmic physical properties

Abstract

The present application relates to a kind of methods based on microcosmic physical property evaluation insulating oil molecule resistant electric field action, belong to transformer fault diagnosis field.The method includes the following steps: first step: constructing performance evaluation microcosmic physical property index parameter matrix;Second step: the numerical value of performance evaluation index parameter under different electric field action is calculated;Third step: the ability of different types of natural ester molecule resistant electric field action is compared and analyzed using radar chart;The present application constructs performance evaluation microcosmic physical property index parameter matrix, the numerical value of performance evaluation index parameter under different electric field action is calculated, the ability of different types of natural ester molecule resistant electric field action is compared and analyzed using radar chart;The present application can accurately, efficiently judge the ability of insulating oil molecule resistant electric field intensity and provide theoretical guidance.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of transformer fault diagnosis, and relates to a method for evaluating the tolerance of insulating oil molecules to electric field action based on microphysical properties. BACKGROUND

[0002] Mineral oil has been used in transformers for more than 100 years because of its good dielectric properties and economy. In recent years, accidents such as fires and explosions of mineral oil transformers have occurred frequently. Once the mineral oil leaks, it will cause great pollution to the environment. Natural esters have excellent anti-aging, environmental protection, fireproof performance, and are renewable, which meet the development needs of future dielectric liquids. In the past two decades, natural esters have been widely used as a substitute for mineral oil in oil-immersed transformers all over the world. At present, the number of natural ester insulating oil transformers in operation worldwide exceeds 1 million, with voltage levels from 10 kV to 420 kV and capacities from 6.25 MVA to 300 MVA. In the future, the number of natural ester transformers will continue to increase.

[0003] During the operation of the transformer, the oil-paper insulation system will be subjected to the action of electric field. The ability of insulating oil to withstand alternating current electric field and lightning impulse electric field is crucial to the structural design of the transformer. Studies have shown that the power frequency AC breakdown voltage of natural esters is higher than that of mineral oil. Wang Zhongdong et al. compared the lightning impulse breakdown characteristics of mineral oil and natural esters. It was found that the lightning impulse breakdown voltage of natural esters was about 10% and 15% lower than that of mineral oil under positive and negative polarity, respectively. During the lightning impulse discharge process, the streamer branches of mineral oil become brighter and brighter, while the streamer branches of natural esters are more dispersed. The AC breakdown voltage of insulating oil mainly depends on the impurity bridge, while the lightning impulse breakdown process of insulating oil is relatively short, mainly depending on the microstructure of the insulating oil molecules. At present, the research on the withstand power frequency and lightning impulse breakdown characteristics of insulating oil is mainly through experimental analysis. The construction and process of the test platform are complex and tedious, and there is a lack of evaluation system for the tolerance of insulating oil molecules to electric field strength from the molecular level.

[0004] In view of the existing technical defects, the purpose of the present application is to provide a method for evaluating the ability of insulating oil molecules to withstand electric field action by using microphysical properties, which includes three aspects: constructing a performance evaluation microphysical property index parameter matrix, calculating the numerical values of performance evaluation index parameters under different electric field actions, and comparing and analyzing the ability of different types of natural ester molecules to withstand electric field action by using a radar chart. The present application provides theoretical guidance for accurately and efficiently judging the ability of insulating oil molecules to withstand electric field strength. SUMMARY

[0005] Therefore, the present application aims to provide a method for evaluating the molecular resistance to electric field of insulating oil based on micro-physical properties, which first constructs a performance evaluation micro-physical property index parameter matrix: taking dipole moment, molecular polarity index, free volume fraction, ionization energy, excitation energy and energy gap as micro-physical property parameter characteristic quantities, a two-dimensional three-column characteristic quantity matrix is constructed. Then the numerical values of performance evaluation index parameters under different electric field actions are calculated: based on density functional theory and molecular dynamics theory, the characteristic quantities of dipole moment, molecular polarity index, free volume fraction, ionization energy, excitation energy and energy gap of mineral oil and natural ester molecules under different electric field strengths are calculated. Finally, radar chart is used to compare and analyze the resistance to electric field of different types of natural ester molecules. The present application can provide a theoretical basis for understanding the breakdown behavior of insulating oil under power frequency and lightning impulse, and provide a theoretical guidance for screening insulating oil molecules with excellent resistance to electric field.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0007] A method for evaluating the molecular resistance to electric field of insulating oil based on micro-physical properties, the method comprising the following steps:

[0008] First step: constructing a performance evaluation micro-physical property index parameter matrix; taking the dipole moment, molecular polarity index, free volume fraction, ionization energy, excitation energy and energy gap of insulating oil molecules as micro-physical property parameter characteristic quantities, a two-dimensional three-column characteristic quantity matrix is constructed; the first dimension of the matrix is the influencing factors of insulating oil power frequency breakdown characteristics and the secondary influencing factors of lightning impulse breakdown characteristics; the second dimension of the matrix is the influencing factors of insulating oil lightning impulse breakdown characteristics and the secondary influencing factors of power frequency breakdown characteristics;

[0009] Second step: calculating the numerical values of performance evaluation index parameters under different electric field actions; based on density functional theory, an insulating oil molecule model is constructed, and Gaussian software is used to calculate the dipole moment of the insulating oil molecule model under different electric field strengths using M062X\\6-311+G(d,p) method; M062X\\6-311G(d,p) method is used to calculate the values of molecular polarity index, ionization energy, excitation energy and energy gap of the insulating oil molecule model under different electric field strengths; based on molecular dynamics theory, MaterialStudios software is used to construct an insulating oil molecule system model, and the free volume fraction of the insulating oil molecule system under different electric field strengths is calculated;

[0010] Third step: using radar chart to compare and analyze the resistance to electric field of different types of natural ester molecules; based on the calculation process of performance evaluation index parameters in the second step, the six performance evaluation micro-physical properties of mineral oil molecules and natural ester molecules are plotted into a radar chart, and the resistance to electric field and the difference of mineral oil and natural ester molecules are comprehensively evaluated.

[0011] Optionally, the first step is specifically:

[0012] With the insulation oil molecular dipole moment μ, molecular polarity index MPI, free volume fraction FFV, ionization energy IP, excitation energy E exa and energy gap E gap As the micro-physical property parameter characteristic quantity, a two-dimensional three-column characteristic quantity matrix W is constructed;

[0013]

[0014] The dipole moment, molecular polarity index and free volume fraction of the first row of the matrix are the influencing factors of the power frequency breakdown characteristics of the insulation oil and the secondary influencing factors of the lightning impulse breakdown characteristics; the ionization, excitation energy and energy gap of the second row of the matrix are the influencing factors of the lightning impulse breakdown characteristics of the insulation oil and the secondary influencing factors of the power frequency breakdown characteristics.

[0015] Optionally, the second step is specifically:

[0016] Based on the density functional theory, the Gaussian software is used to perform geometric optimization on the constructed insulation oil molecular model by using the M062X functional with the 6-311G(d, p) basis set, and the y-axis of the insulation oil molecular model is subjected to an electric field with a strength of 0, 0.002 a.u., 0.004 a.u., 0.006 a.u. and 0.008 a.u., respectively, 1 a.u. = 5.1423 × 10 11 V / m, to obtain the optimal structure of the insulation oil molecule under different electric field strengths; on the basis of the optimized natural ester molecular model, the M062X functional with the 6-311+G(d, p) basis set is used to calculate the dipole moment of the insulation oil molecule according to formula (1):

[0017] μ = ql (1)

[0018] Where q is the charge quantity, and l is the distance between positive and negative charges;

[0019] The M062X functional with the 6-311G(d, p) basis set is used to calculate the polarity index of the insulation oil molecule according to formula (2):

[0020]

[0021] Where V is the molecular electrostatic potential, the integral is the integral over the molecular surface S, and A is the molecular surface area;

[0022] The M062X functional with the 6-311G(d, p) basis set is used to calculate the energy of the neutral molecule and the cation in the insulation oil, as shown in formula (3); the value of the ionization energy of the insulation oil molecule is the energy of the cationic molecule of the insulation oil minus the energy of the neutral molecule of the insulation oil;

[0023] IP = EM+ -E M (3)

[0024] In the formula, E M E is the energy calculated when the insulating oil molecule is neutral, E M+ E is the energy calculated when the insulating oil molecule is charged with +1; the M062X functional is used in combination with the 6-311G(d, p) basis set to calculate the excitation energy and the energy gap of the insulating oil molecule;

[0025] A molecular dynamics theory-based insulating oil molecule system model is constructed, the steepest gradient method is used to optimize the geometry of the insulating oil molecule system model, and the annealing treatment is performed on the optimized model; the NPT ensemble is used to balance the insulating oil molecule model for 500 ps, so that the density reaches a reasonable value; the NVT system is used to analyze the 500 ps dynamic simulation of the balanced model, and the free volume fraction of the insulating oil molecule system model is calculated.

[0026] Optionally, the third step specifically comprises:

[0027] The greater the dipole moment and the MPI value of the insulating oil molecule, the stronger the polarity, and the higher the interaction energy with water and acidic substances; the smaller the free volume fraction of the insulating oil molecule, the less the movement space of water and acidic substances; the greater the dipole moment and the MPI value of the insulating oil molecule and the smaller the free volume fraction in the process of the insulating oil power frequency breakdown, the more difficult the water and impurities to bridge, and the higher the power frequency breakdown voltage; the ionization energy of the molecule measures the ability of the molecule to lose electrons and ionize; the excitation energy of the molecule measures the difficulty of the excitation process of the molecule; the energy gap of the molecule is related to the conductivity of the molecule, and the greater the energy gap of the molecule, the weaker the conductivity; according to the discharge theory of natural esters under lightning impulse, the higher the lightning impulse breakdown voltage of the natural ester molecule with lower dipole moment, higher ionization energy, excitation energy and energy gap.

[0028] The beneficial effects of the present application are:

[0029] The present application constructs a performance evaluation microcosmic physical property index parameter matrix, calculates the values of the performance evaluation index parameters under the action of different electric fields, and compares and analyzes the electric field resistance of different types of natural ester molecules by using a radar chart. The present application provides theoretical guidance for accurately and efficiently judging the electric field resistance of insulating oil molecules.

[0030] Other advantages, objects, and features of the present application will be apparent to those skilled in the art from the following specification, and will be learned from the study of the following text, or will be taught from the practice of the present application. The objects and other advantages of the present application can be achieved and obtained by the following specification. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to make the objects, technical solutions and advantages of the present application clearer, the preferred embodiments of the present application will be described in detail below with reference to the drawings, in which:

[0032] Figure 1 is a whole flow chart of the present application;

[0033] Figure 2 is a radar plot of the ability of di-alkanes to withstand electric field action;

[0034] Figure 3 is a radar plot of the ability of different insulating oils to withstand electric field action; (a) is 0 a.u.; (b) is 0.004 a.u.; (c) is 0.008 a.u. DETAILED DESCRIPTION

[0035] The present application will be described in greater detail by way of specific embodiments, from which the skilled person will readily appreciate other advantages and utilities of the present application. The present application can also be implemented or applied in other different embodiments, and the details in the present description can be modified or changed based on different views and applications, without departing from the spirit of the present application. It should be noted that the figures provided in the following embodiments only illustrate the basic concepts of the present application in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0036] The drawings are only used for exemplary illustration, and the representation is only a schematic diagram, not a physical diagram, and should not be understood as a limitation of the present application; in order to better illustrate the embodiments of the present application, some components in the drawings can be omitted, enlarged or reduced, and do not represent the actual size of the product; it is understandable to those skilled in the art that some known structures and their descriptions in the drawings can be omitted.

[0037] The same or similar reference numerals in the drawings of the embodiments of the present application correspond to the same or similar components; in the description of the present application, it should be understood that the orientations or positional relationships indicated by the terms "upper", "lower", "left", "right", "front", "back" and the like are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore the terms describing the positional relationship in the drawings are only used for exemplary illustration, and should not be understood as a limitation of the present application, and for those skilled in the art, the specific meanings of the above terms can be understood according to the specific circumstances.

[0038] As Figure 1 shown, the present application comprises the following three steps:

[0039] The first step is constructing a performance evaluation micro-physical property index parameter matrix. Taking the molecular dipole moment, molecular polarity index, free volume fraction, ionization energy, excitation energy and energy gap of insulating oil as micro-physical property parameter characteristic quantities, a two-dimensional three-column characteristic quantity matrix is constructed. The first dimension of the matrix is the main influencing factor of the power frequency breakdown characteristic of insulating oil and the secondary influencing factor of the lightning impulse breakdown characteristic; the second dimension of the matrix is the main influencing factor of the lightning impulse breakdown characteristic of insulating oil and the secondary influencing factor of the power frequency breakdown characteristic.

[0040] The second step is calculating the values of performance evaluation index parameters under the action of different electric fields. Based on the density functional theory, the molecular model of insulating oil is constructed, and the Gaussian software is used to calculate the dipole moment of the insulating oil molecular model under different electric field strengths by using the M062X\\6-311+G(d,p) method; the M062X\\6-311G(d,p) method is used to calculate the values of the molecular polarity index, ionization energy, excitation energy and energy gap of the insulating oil molecular model under different electric field strengths. Based on the molecular dynamics theory, the molecular system model of insulating oil is constructed using MaterialStudios software, and the free volume fraction of the insulating oil molecular system under different electric field strengths is calculated.

[0041] The third step is to compare and analyze the electric field resistance ability of different types of natural ester molecules by radar chart. Based on the calculation process of the performance evaluation index parameters in the second step, the six performance evaluation micro-physical property indexes of mineral oil molecules and natural ester molecules are drawn into a radar chart, and the electric field resistance ability and differences of mineral oil and natural ester molecules are comprehensively evaluated.

[0042] 1 Constructing a performance evaluation micro-physical property index parameter matrix

[0043] Taking the molecular dipole moment (μ), molecular polarity index (MPI), free volume fraction (FFV), ionization energy (IP), excitation energy (E exa ) and energy gap (E gap ) of insulating oil as micro-physical property parameter characteristic quantities, a two-dimensional three-column characteristic quantity matrix W is constructed.

[0044]

[0045] The dipole moment, molecular polarity index and free volume fraction of the insulating oil molecule in the first row of the matrix are the main influencing factors of the power frequency breakdown characteristic of insulating oil and the secondary influencing factors of the lightning impulse breakdown characteristic; the ionization, excitation energy and energy gap in the second row of the matrix are the main influencing factors of the lightning impulse breakdown characteristic of insulating oil and the secondary influencing factors of the power frequency breakdown characteristic.

[0046] 2 Calculate the values of performance evaluation index parameters under the action of different electric fields

[0047] Based on density functional theory, the Gaussian software was used to optimize the geometry of the insulating oil molecular model using the M062X functional with the 6-311G(d, p) basis set. The insulating oil molecular model was subjected to an electric field with a strength of 0, 0.002 a.u., 0.004 a.u., 0.006 a.u., and 0.008 a.u. along the y-axis (1 a.u. = 5.1423 x 10 11 V / m), and the optimal structure of the insulating oil molecule under different electric field strengths was obtained. Based on the optimized natural ester molecular model, the M062X functional with the 6-311+G(d, p) basis set was used to calculate the dipole moment of the insulating oil molecule using formula (1):

[0048] μ = ql (1)

[0049] where q is the charge quantity and l is the distance between positive and negative charges.

[0050] The M062X functional with the 6-311G(d, p) basis set was used to calculate the polarity index of the insulating oil molecule using formula (2):

[0051]

[0052] where V is the molecular electrostatic potential, the integral is the integral over the surface S of the molecule, and A is the surface area of the molecule.

[0053] The M062X functional with the 6-311G(d, p) basis set was used to calculate the energy of the neutral molecule and the cation in the insulating oil, as shown in formula (3); the value of the ionization energy of the insulating oil molecule is the energy of the cationic molecule minus the energy of the neutral molecule.

[0054] IP = E M+ -E M (3)

[0055] where E M is the calculated energy when the insulating oil molecule is neutral, and E M+ is the calculated energy when the charge of the insulating oil molecule is +1. The M062X functional with the 6-311G(d, p) basis set was used to calculate the excitation energy and energy gap of the insulating oil molecule.

[0056] Based on the molecular dynamics theory, the insulating oil molecular system model was constructed, and the steepest descent method was used to optimize the geometry of the insulating oil molecular system model. The optimized model was subjected to annealing treatment. The NPT ensemble was used to balance the insulating oil molecular model for 500 ps to achieve a reasonable density value. The NVT system was used to analyze the balanced model for 500 ps of dynamics simulation, and the free volume fraction of the insulating oil molecular system model was calculated.

[0057] 3 Radar chart was used to compare the ability of different types of natural ester molecules to resist electric field

[0058] The greater the dipole moment and MPI value of the insulating oil molecules, the stronger the polarity, and the higher the interaction with water and acidic substances; the smaller the free volume fraction of the insulating oil molecules, the less the movement space of water and acidic substances. In the process of power frequency breakdown of insulating oil, the greater the dipole moment and MPI value of the insulating oil molecules and the smaller the free volume fraction, the more difficult it is for water and impurities to form a bridge, and the higher the power frequency breakdown voltage. The ionization energy of the molecule can measure the ability of the molecule to lose electrons and ionize; the excitation energy of the molecule can measure the difficulty of the excitation process of the molecule; the energy gap of the molecule is related to the conductivity of the molecule, and the greater the energy gap of the molecule, the weaker the conductivity. According to the discharge theory of natural ester under lightning impulse, the higher the lightning impulse breakdown voltage of the natural ester molecule with lower dipole moment, higher ionization energy, excitation energy and energy gap.

[0059] Taking the dialkane in mineral oil as an example, the values of six kinds of micro-physical properties of dialkane under an electric field intensity of 0.008 a.u. were extracted, and a radar chart was drawn. By comparing the differences of lightning charts of different insulating oil molecules, the ability of insulating oil molecules to resist electric field can be evaluated. Figure 2 Radar chart of the ability of dialkane to resist electric field.

[0060] Taking dialkane as the mineral oil molecule and glycerol trioleate as the natural ester molecule, the values of dipole moment (μ), molecular polarity index (MPI), free volume fraction (FFV), ionization energy (IP), excitation energy (E exa ) and energy gap (E gap ) parameters of the two kinds of insulating oil molecules under different electric field intensities were calculated, as shown in Tables 1 and 2. The data values of electric field intensity of 0, 0.004 a.u. and 0.008 a.u. were selected to draw the radar chart of the micro-physical property indexes of mineral oil molecules and natural ester molecules, and the differences of the micro-physical property index values of mineral oil molecules and natural ester molecules were compared. Figure 3 In the figure, (a) is 0 a.u.; (b) is 0.004 a.u.; (c) is 0.008 a.u. Figure 3 As can be seen from the results, the dipole moment and molecular polarity index of mineral oil molecules are smaller than those of natural ester molecules, the free volume fraction of mineral oil molecules is greater than that of natural ester molecules, so the power frequency breakdown voltage of mineral oil should be lower than that of vegetable oil; the ionization energy, excitation energy and energy gap values of mineral oil molecules are higher than those of natural ester molecules, so the lightning impulse breakdown voltage of mineral oil should be higher than that of natural ester. Table 3 shows the experimental values of the breakdown voltage of natural ester and mineral oil.

[0061] In order to verify the result of the method for evaluating the ability of molecular of insulating oil to resist electric field effect by using microscopic physical parameters, the power frequency breakdown voltage and lightning impulse breakdown voltage tests of mineral oil and natural ester molecules are carried out. The lightning impulse breakdown voltage test of insulating oil is carried out by using step-by-step voltage rising method. The electrode is needle-plate electrode, and the electrode spacing is 10 mm. The power frequency breakdown voltage of insulating oil is tested according to IEC60156 standard. The experimental results show that the power frequency breakdown voltage of natural ester is higher than that of mineral oil, and the lightning impulse voltage under positive and negative polarity is lower than that of mineral oil, which is consistent with the result of the method for evaluating the ability of molecular of insulating oil to resist electric field effect by using microscopic physical parameters.

[0062] Table 1: Microscopic physical index values of mineral oil molecules under different electric field intensities

[0063]

[0064] Table 2: Microscopic physical index values of natural ester molecules under different electric field intensities

[0065]

[0066] Table 3: Breakdown voltage experimental values (kV) of natural ester and mineral oil

[0067]

[0068] Finally, it should be pointed out that the above examples are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the purpose and scope of the present application, and they should be included in the scope of the claims of the present application.

Claims

1. A method for evaluating the molecular resistance of insulating oil to the action of an electric field based on microscopic properties, characterized by: The method comprises the following steps: The first step is to construct a performance evaluation microphysical property index parameter matrix; a two-dimensional three-column characteristic quantity matrix is constructed by taking the dipole moment, molecular polarity index, free volume fraction, ionization energy, excitation energy and energy gap of the insulating oil molecules as the microphysical property parameter characteristic quantities; the first dimension of the matrix is the influencing factors of the power frequency breakdown characteristics of the insulating oil and the secondary influencing factors of the lightning impulse breakdown characteristics; the second dimension of the matrix is the influencing factors of the lightning impulse breakdown characteristics of the insulating oil and the secondary influencing factors of the power frequency breakdown characteristics; specifically: The micro-physical property parameters characteristic quantities are molecular dipole moment μ, molecular polarity index MPI, free volume fraction FFV, ionization energy IP, and excitation energy E exa and energy gap E gap A two-dimensional three-column characteristic quantity matrix W is constructed. The dipole moment, molecular polarity index and free volume fraction of the insulating oil molecules in the first row of the matrix are the influencing factors of the power frequency breakdown characteristics of the insulating oil and the secondary influencing factors of the lightning impulse breakdown characteristics; the ionization, excitation energy and energy gap in the second row of the matrix are the influencing factors of the lightning impulse breakdown characteristics of the insulating oil and the secondary influencing factors of the power frequency breakdown characteristics; The second step is to calculate the values of the performance evaluation index parameters under the action of different electric fields; based on the density functional theory, an insulating oil molecule model is constructed, and the dipole moment of the insulating oil molecule model under different electric field strengths is calculated by using the Gaussian software and the M062X\\6-311+G(d, p) method; the values of the molecular polarity index, ionization energy, excitation energy and energy gap of the insulating oil molecule model under different electric field strengths are calculated by using the M062X\\6-311G(d, p) method; based on the molecular dynamics theory, an insulating oil molecule system model is constructed by using the Material Studios software, and the free volume fraction of the insulating oil molecule system under different electric field strengths is calculated; The third step is to compare and analyze the electric field resistance ability of different types of natural ester molecules by using a radar chart; based on the calculation process of the performance evaluation index parameters in the second step, the six performance evaluation microphysical property indexes of the mineral oil molecules and the natural ester molecules are drawn into a radar chart, and the electric field resistance ability and the difference of the mineral oil molecules and the natural ester molecules are comprehensively evaluated.

2. The method for evaluating the molecular resistance of insulating oil to electric field according to claim 1, characterized by: The second step is specifically: Based on density functional theory, the Gaussian software was used to optimize the geometry of the insulating oil molecular model using the M062X functional with the 6-311G(d, p) basis set. The insulating oil molecular model was subjected to an electric field with a strength of 0, 0.002 a.u., 0.004 a.u., 0.006 a.u., and 0.008 a.u. along the y-axis, respectively, 1 a.u. = 5.1423 x 10 11 V / m, and the optimal structure of the insulating oil molecule under different electric field strengths was obtained; based on the optimized natural ester molecular model, the M062X functional with the 6-311+G(d, p) basis set was used to calculate the dipole moment of the insulating oil molecule according to formula (1): μ=ql (1) In the formula, q is the charge quantity, and l is the distance between positive and negative charges; The polarity index of the insulating oil molecules is calculated by using the M062X functional with the 6-311G(d, p) basis set according to formula (2): In the formula, V is the molecular electrostatic potential, the integral is the integral on the surface S of the molecule, and A is the surface area of the molecule; The energy of the neutral molecules and cations in the insulating oil is calculated by using the M062X functional with the 6-311G(d, p) basis set, as shown in formula (3); the value of the ionization energy of the insulating oil molecules is the energy of the cation molecules of the insulating oil minus the energy of the neutral molecules of the insulating oil; IP = E M+ -E M (3) where E M E is the energy calculated for the neutral insulating oil molecule M+ E is the energy calculated for the insulating oil molecule with a charge of +1; the M062X functional with 6-311G(d, p) basis set was used for the calculation of the excitation energy and the energy gap of the insulating oil molecule; Based on the molecular dynamics theory, an insulating oil molecule system model is constructed, the steepest gradient method is used for geometric optimization of the insulating oil molecule system model, and the optimized model is subjected to annealing treatment; the NPT ensemble is used for 500ps balance treatment of the insulating oil molecule model, so that the density reaches a reasonable value; the NVT system is used for 500ps dynamic simulation analysis of the balanced model, and the free volume fraction of the insulating oil molecule system model is calculated.

3. The method for evaluating the molecular resistance of insulating oil to electric field according to claim 2, characterized by: The third step is specifically: The greater the value of the dipole moment and MPI of the insulating oil molecules, the stronger the polarity, the higher the interaction with water and acidic substances; the smaller the free volume fraction of the insulating oil molecules, the less the movement space of water and acidic substances; the greater the value of the dipole moment and MPI of the insulating oil molecules and the smaller the free volume fraction, the more difficult the bridging of water and impurities in the process of power frequency breakdown of the insulating oil, and the higher the power frequency breakdown voltage; the ionization energy of the molecules measures the ability of the molecules to lose electrons and ionize; the excitation energy of the molecules measures the difficulty of the excitation process of the molecules; the energy gap of the molecules is related to the conductivity of the molecules, and the greater the energy gap of the molecules, the weaker the conductivity; according to the discharge theory of natural esters under lightning impulse, the higher the lightning impulse breakdown voltage of the natural ester molecules with lower dipole moment, higher ionization energy, excitation energy and energy gap.

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