Simulation method for analyzing sulfur corrosivity of insulating oil by simulating electric field

Abstract

The invention discloses a simulation method for analyzing sulfur corrosivity of insulating oil by simulating an electric field. The simulation method comprises the following steps: S1, establishing a sulfur-containing insulating oil model; s2, constructing an alternating current electric field; s3, applying an alternating electric field to the sulfur-containing insulating oil model and performing molecular dynamics simulation; and S4, calculating mean square displacement and shear viscosity. Multiple sulfur corrosion simulation experiments are simulated on an insulating oil model containing an electric field, so that various factors influencing the sulfur corrosion degree, such as the concentration and the type of corrosive sulfur, the temperature and the action of the electric field, are further researched, and a data basis is provided for sulfur corrosion analysis in the working process of a transformer. Temperature rise can aggravate thermal motion of molecules, so that MSD is increased, and the shear viscosity of the insulating oil is reduced. The electric field polarizes oil molecules, and finally restricts the conversion of the molecules to vacancies and increases the shear viscosity of the insulating oil. In addition, the higher the field intensity is, the higher the shear viscosity is. As the electronegativity of sulfur atoms is low, sulfide is easy to polarize under the condition of an electric field.

Description

Technical Field

[0001] The invention relates to the technical field of molecular simulation, and in particular to a simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field. Background Art

[0002] Oil-immersed transformers are key components of power systems. Transformer accidents not only cause serious damage to equipment, but also interrupt power supply and cause considerable economic losses. At the same time, sulfur corrosion has a great impact on the safe and stable operation of transformers. It is reported that many power transformer failures are caused by corrosive sulfur, which reacts with copper windings. The resulting compounds can migrate to the insulating paper or insulating oil and then decompose to form copper sulfide. Copper sulfide increases the conductivity and dielectric loss of the oil-paper insulation system and eventually leads to insulation breakdown between copper windings. Sulfides also accelerate the thermal aging process of the oil-paper insulation system, thereby reducing the degree of polymerization and material strength of the insulating paper. Sulfides can increase the viscosity and furfural content of the insulating oil and accelerate the inflection point of the acid value growth rate.

[0003] Therefore, there is an urgent need for a simulation method to analyze the sulfur corrosion of insulating oil by simulating electric fields, so as to further study the various factors affecting the degree of sulfur corrosion, such as the concentration and type of corrosive sulfur, temperature, and the effect of electric fields, and provide a data basis for the analysis of sulfur corrosion during transformer operation. Summary of the invention

[0004] The purpose of the present invention is to propose a simulation method for analyzing the sulfur corrosion of insulating oil by simulating electric field, which can provide a certain reference for the microscopic mechanism of sulfide corrosion on insulating oil and has important significance for transformer insulation protection technology.

[0005] The objective of the present invention is achieved through the following technical solutions: A simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field, comprising the following steps: S1, establish a sulfur-containing insulating oil model; S2, build AC electric field; S3, applying an AC electric field to the sulfur-containing insulating oil model and performing a molecular dynamics simulation; S4, calculate the mean square displacement and shear viscosity.

[0006] Furthermore, the sulfur-containing insulating oil model includes: 27 C 20 H 42 Molecules, 18 C 20 H 38 Molecules and 9 Cs 20 H 26 The density of insulating oil is 0.86 g / cm 3 .

[0007] Furthermore, the sulfide of the sulfur-containing insulating oil model includes one or more combinations of DBDS, hexadecyl mercaptan and benzothiophene.

[0008] Furthermore, the period of the AC electric field is 100 ps, ​​running for 50 ps in the original electric field direction and then running for 50 ps in the opposite direction, and the AC electric field includes: 4 groups of electric field types; the 4 groups of electric field types are no electric field, a DC electric field of 0.5 V / Å, an AC electric field of 0.5 V / Å, and a DC electric field of 1 V / Å.

[0009] Furthermore, after the sulfur-containing insulating oil model was constructed, the model was subjected to a 20ps relaxation process.

[0010] Furthermore, the simulation temperature of the molecular dynamics simulation was selected to be 70°C and 140°C, and the pcff force field and the Ewald method of electrostatic interaction method were used in the simulation. The step size of each step was set to 1 fs, where a trajectory was output every 5000 steps, and each model finally obtained a 20-frame molecular trajectory file; Furthermore, the Analysis function in the Forcite module was used to calculate the mean square displacement of each frame in the 20-frame molecular trajectory file obtained for each model; Furthermore, the shear viscosity of the last frame of the model was calculated using the Analysis function in the Forcite module; Furthermore, the entire simulation process of molecular dynamics simulation uses the Forcite module in Materials Studio software, and the calculation force field uses the Compass III force field.

[0011] The beneficial effects of the present invention are as follows: multiple sulfur corrosion is simulated on an insulating oil model containing an electric field, and the mean square displacement and shear viscosity of the insulating oil under different electric field and thermal field combinations are studied. The research results can provide a certain reference for the microscopic mechanism of various corrosive sulfides and explore effective sulfur corrosion protection technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a flow chart of the present invention; Figure 2 The system energy change of the present invention after relaxation of 20ps; Figure 3 The variation rule of MSD of the insulating oil model of the present invention at different simulation times; Figure 4 The variation law of MSD of the insulating oil model of the present invention at different temperatures; Figure 5 The graph is a shear viscosity variation diagram of the insulating oil of the present invention under electrothermal conditions with various sulfur atoms. DETAILED DESCRIPTION

[0013] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0014] The present invention provides a method for simulating sulfur corrosion of insulating oil under the influence of electric field based on molecular dynamics, which can provide a certain reference for the microscopic mechanism of corrosion of insulating oil by sulfide.

[0015] The embodiment of the present invention provides a simulation method for analyzing the sulfur corrosion of insulating oil by simulating an electric field. In order to meet the corresponding parameters of an actual oil-immersed power transformer, the actual operation of the simulation experiment should take into account the actual operating conditions of the transformer, such as temperature, pressure, electric field, etc.

[0016] The mineral oil in the actual operation process of the transformer is a mixture of 60% alkanes, 10% to 40% cycloalkanes and 5% to 15% aromatics. The average number of carbon atoms in hydrocarbon molecules is between 15 and 23. Therefore, the insulating oil model in the simulation process of the present invention adopts 27 C 20 H 42 Molecules, 18 C 20 H 38 Molecules and 9 Cs 20 H 26 Molecular composition, the density of insulating oil is 0.86 g / cm 3 .

[0017] Since DBDS is the most corrosive sulfide. Hexadecyl mercaptan is highly active due to the sulfur atom at the end of the chain. Benzothiophene usually shows low activity, but long-term high temperature conditions may make it corrosive. Therefore, the sulfide is selected to be a combination of DBDS, hexadecyl mercaptan and benzothiophene.

[0018] like Figure 1 As shown, the specific steps of a simulation method for analyzing the sulfur corrosion of insulating oil by simulating an electric field are as follows: S1, establish a sulfur-containing insulating oil model; S2, build AC electric field; S3, applying an AC electric field to the sulfur-containing insulating oil model and performing a molecular dynamics simulation; S4, calculate the mean square displacement and shear viscosity.

[0019] Among them, establishing the sulfur-containing insulating oil model includes: using the "Amorphous cell" module in the MS software to add different sulfides of different concentrations to the established insulating oil model, using periodic boundary conditions in the simulation process, and ignoring the influence of boundary effects.

[0020] The AC electric field is simulated by changing the direction of the DC electric field. Specifically, during the simulation, an AC electric field with a period of 100 ps is applied, running for 50 ps in the original electric field direction and then running for 50 ps in the opposite direction.

[0021] Furthermore, in order to overcome the influence of atomic thermal motion in vacuum, it is necessary to select a strong electric field much larger than the experimental value to shorten the simulation time. Therefore, the electric field is set to 0.5V / Å and 1V / Å, and 4 types of electric fields are obtained: no electric field, 0.5V / Å DC electric field, 0.5V / Å AC electric field, and 1V / Å DC electric field.

[0022] Furthermore, the spatial coordinates of a given molecule are not necessarily located in the most stable position of the molecular force field. Therefore, each atom is not in equilibrium, which leads to a relatively high energy of the system. In order to prevent very large mechanical effects from occurring in the early stages of the simulation, the starting positions of the atoms should be kept as low as possible. Therefore, for the complex system composed of large molecules that appears in this simulation, an energy minimization step is required to keep the system energy at the lowest point.

[0023] Relaxation is the process of keeping the system under certain conditions, allowing the particles in it to move sufficiently to maintain the relative equilibrium state in the system. Therefore, all models in this simulation process were subjected to a 20ps relaxation process to balance the system energy. Figure 2 The total energy change pattern of the model after 20 ps relaxation is shown, and the system energy tends to equilibrium after 20 ps relaxation.

[0024] The molecular dynamics simulation was performed for 100 ps in the constructed model. Specifically, the step size of each step was set to 1 fs, and a trajectory was output every 5000 steps. Each model finally obtained a 20-frame molecular trajectory file.

[0025] Furthermore, the ensemble used in the simulation is the canonical ensemble (NVT). The NVT ensemble is characterized by the fact that the number of atoms (N), volume (V), and temperature (T) of the system remain constant. Since there is no coupling with pressure, the NVT ensemble can provide the advantage of less trajectory perturbations.

[0026] Furthermore, during the molecular dynamics simulation, the Andersen method was selected to control the temperature, randomize the velocities of all atoms within a predefined collision period, and couple the system to a heat bath through random forces that change the kinetic energy of atoms or molecules. In order to ensure the simulation efficiency at the same time under the specified temperature, 70°C and 140°C were selected as the simulation temperatures.

[0027] Furthermore, the electrostatic interaction method of the pcff force field of organic matter and the Ewald method was adopted.

[0028] Furthermore, the mean square displacement (MSD) represents the average value of the offset between the positions of all particles and their initial positions at a certain moment, which can accurately indicate the intensity of the chain motion. The stronger the molecular chain motion, the worse the thermal stability.

[0029] Furthermore, the Analysis function in the Forcite module was used to calculate the mean square displacement of each frame in the 20-frame molecular trajectory file obtained for each model.

[0030] Furthermore, molecular dynamics simulations of the model with AC cycles of 20, 100, and 500 ps were performed in the NVT ensemble at 343 K, and the AC periods were 10, 50, and 250 ps, ​​respectively. Figure 3 As shown, the simulation results show the same trend. The AC frequency only speeds up the simulation response and has limited impact on the result trend.

[0031] Further, Figure 4 is the MSD of the model at 343 and 423 K. Figure 4 It can be seen that at 343K, the magnitude and direction of the electric field have limited effects on the MSD. When the temperature rises to 423K, when the MSD of the oil is lower than 1V / Å, the MSD under DC conditions becomes much larger than in other cases. Therefore, the magnitude of the electric field strength will affect the MSD. The greater the field strength, the greater the MSD, and the greater the chain motion of the insulating oil. This result can be attributed to the fact that under the action of a strong electric field, the molecular polarization is affected by the electric field force, causing the oil molecules to shift. Therefore, the increase in MSD is greater than that in the absence of an electric field. AC and DC electric fields of the same amplitude show similar effects on MSD. Therefore, the direction of the electric field has limited effect on the chain motion of the oil.

[0032] Furthermore, the shear viscosity of the last frame of the model was calculated using the Analysis function in the Forcite module. For the 20-frame model file obtained after the molecular dynamics calculation, the last frame was selected as the final calculation model.

[0033] Furthermore, shear viscosity can be expressed as the ability to limit the relative flow of a fluid. Insulating oil should have a strong heat dissipation capacity, so the shear viscosity should not be too high to avoid affecting its flow and heat transfer. Figure 5 The results of the influence of electric field and thermal field on the shear viscosity of insulating oil containing various sulfur atoms are shown. It can be seen that the shear viscosity of the model decreases with increasing temperature.

[0034] This phenomenon indicates that the thermal motion of molecules increases and the molecular gap becomes larger, which leads to a decrease in shear viscosity. At the same time, the comparison of various electric field strengths shows that the greater the electric field strength, the higher the shear viscosity. Strong electric fields polarize oil molecules. Polarized molecules are affected by the electric field force, and the frequency of the molecules is transformed into vacancies. The increase in the energy barrier height hinders the transformation of molecules to vacancies, thereby increasing the shear viscosity of the liquid. The polarization loss caused by the opposite direction of the AC electric field is not significant, and with the introduction of sulfides, the sulfur atoms with less electronegativity are easily polarized by the electric field. The more sulfur atoms there are, the stronger the polarization effect and the greater the increase in shear viscosity.

[0035] In summary, the increase in temperature intensifies the thermal motion of molecules, resulting in an increase in MSD and a decrease in the shear viscosity of the insulating oil. The electric field polarizes the oil molecules, ultimately limiting the transition of the molecules to vacancies and increasing the shear viscosity of the insulating oil. In addition, the greater the field strength, the greater the shear viscosity. Due to the low electronegativity of sulfur atoms, sulfides are easily polarized under electric field conditions. Therefore, the higher the sulfide concentration, the greater the shear viscosity.

Claims

1. A simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field, characterized in that: The following steps are involved: S1, establish a sulfur-containing insulating oil model; S2, build AC electric field; S3, applying an AC electric field to the sulfur-containing insulating oil model and performing a molecular dynamics simulation; S4, calculate the mean square displacement and shear viscosity.

2. The simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field according to claim 1, characterized in that: The sulfur-containing insulating oil model includes: 27 C20H42 molecules, 18 C20H38 molecules and 9 C20H26 molecules. The density of the insulating oil is 0.86 g / cm 3 .

3. The simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field according to claim 1, characterized in that: The sulfides used to establish the sulfur-containing insulating oil model include one or more combinations of DBDS, hexadecyl mercaptan and benzothiophene.

4. The simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field according to claim 1, characterized in that: The period of the AC electric field is 100 ps, ​​running for 50 ps in the original electric field direction and then running for 50 ps in the opposite direction; The AC electric field includes 4 types of electric field: no electric field, a DC electric field of 0.5 V / Å, an AC electric field of 0.5 V / Å, and a DC electric field of 1 V / Å.

5. The simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field according to claim 1, characterized in that: After the sulfur-containing insulating oil model was constructed, the model was subjected to a 20ps relaxation process.

6. The simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field according to claim 1, characterized in that: The simulation conditions of molecular dynamics simulation were as follows: the temperature was selected as 70°C and 140°C, the pcff force field and the Ewald method of electrostatic interaction were used, the step size of each step was set to 1 fs, a trajectory was output every 5000 steps, and a 20-frame molecular trajectory file was finally obtained for each model.

7. The simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field according to claim 1, characterized in that: Calculate mean square displacement and shear viscosity, including: The Analysis function in the Forcite module was used to calculate the mean square displacement of each frame in the 20-frame molecular trajectory file obtained for each model.

8. The simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field according to claim 1, characterized in that: Calculates mean square displacement and shear viscosity, also includes: The shear viscosity of the last frame of the model was calculated using the Analysis function in the Forcite module.

9. The simulation method for analyzing sulfur corrosion of insulating oil by simulating electric field according to any one of claims 1 to 8, characterized in that: The entire simulation process of molecular dynamics simulation uses the Forcite module in Materials Studio software, and the calculation force field uses the Compass III force field.

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