Test method for mechanical properties of electrical-thermal-mechanical composite degradation of insulating oil-impregnated paper

Abstract

To provide a test method for mechanical characteristics of combined electrical, thermal and mechanical deterioration of insulating oil-paper.SOLUTION: The present invention comprises: constructing an AC and DC / thermal / mechanical stress combined deterioration experimental platform; preparing insulating oil-paper samples with different deterioration states; measuring a degree of polymerization and tensile strength test of the insulating oil-paper samples with different deterioration states; acquiring change rules of mechanical characteristics after deterioration; observing microscopic morphology diagrams and XRD spectra of the insulating oil-paper samples with different deterioration states; constructing an amorphous unit cell of a cellulose molecular chain; optimizing a structure; acquiring a cellulose model; constructing an oil-paper mixed model; performing molecular dynamics simulation on the oil-paper samples with different deterioration states based on the cellulose model and the oil-paper mixed model; and constructing the relation between the deterioration of macroscopic mechanical characteristics of insulating oil-paper and changes in microscopic physical and chemical characteristics, where a data basis and theoretical support for prediction of a life of insulating paper are provided.SELECTED DRAWING: Figure 19

Description

Technical Field

[0001] The present invention belongs to the technical field of insulation oil-paper deterioration, and particularly relates to a test method for mechanical properties of electrical-thermal-mechanical composite deterioration of insulation oil-paper.

Background Art

[0002] The description of this part only provides background technical information related to the present invention and does not necessarily constitute prior art.

[0003] The insulation oil-paper of a converter transformer will inevitably deteriorate due to moisture, oxygen, high temperature, AC and DC electric fields, and mechanical vibrations caused by DC bias magnetic fields during operation. Due to microscopic structural changes, the electrical and mechanical properties of the deteriorated insulating paper are greatly affected, and it can no longer function as an insulator for high-voltage equipment. In order to realize the prediction of the life and service time of insulating paper and ensure the stable operation of the power transmission network, scholars have proposed many physical and chemical properties of insulating paper as characteristic parameters of insulating paper deterioration, such as degree of polymerization, dielectric spectrum in the frequency domain, furfural in oil, and new characteristic evaluation quantities. Among them, the mechanical properties of insulating paper are easy to measure, and the life of insulating paper can be predicted more accurately. However, due to the complex working conditions during the operation of the converter transformer, it is difficult to realize its online monitoring and short-term inspection of mechanical performance. Therefore, studying the laws and mechanisms of the aging over time and the change of mechanical performance according to conditions of insulation oil-paper is of great significance for the stability and safety of converter transformers.

[0004] The cellulose insulating paper used in converter transformers is A-grade sulfate wood pulp paper mainly composed of α-cellulose, hemicellulose, and lignin, and the mass fractions of the three are 90%, 6% - 7%, and 3% - 4% respectively. α-Cellulose forms crystalline regions and amorphous regions (non-crystalline regions). The crystalline regions have stable physical and chemical properties, a uniform crystal grain size distribution, and many amorphous regions, and are prone to glass transition due to temperature changes. The deterioration of cellulose insulating paper is mainly caused by the cleavage and displacement of the cellulose main chain and the destruction of the orientation arrangement between cellulose chains. The main factors that may cause the deterioration and decomposition of cellulose during the operation of converter transformers are moisture, oxygen, and external stresses (such as electric fields, high temperatures, and mechanical stresses). Moisture and oxygen react directly with the functional groups of cellulose macromolecules, causing decomposition, hydrolysis, and oxidative decomposition respectively. External stresses decompose cellulose by enhancing the movement of cellulose molecules, changing the polarity of cellulose molecules, and stretching cellulose chains. Moreover, the action of the factor of external stress is particularly special. The deterioration of insulating paper caused by the three types of stresses, namely electrical stress, thermal stress, and mechanical stress, is not an "overlay" of "1 + 1", but rather a relationship of influencing, promoting, or suppressing each other. When the microscopic structure of cellulose changes, the physical and chemical properties such as macroscopic mechanical properties and electrical properties decrease. Therefore, by observing the rules and mechanisms of the changes in the mechanical properties of insulating paper, the degree of deterioration and the lifespan of insulating paper can be predicted.

[0005] Current research on the deterioration of cellulose insulating paper mainly includes the decomposition rate of cellulose, the effects of oxygen, moisture, etc. on the corresponding products, the life model of oil-paper insulation systems, the microscopic structural changes of insulating paper after deterioration, the modification of cellulose insulating paper, and the multi-field composite deterioration simulation of insulating paper introducing molecular dynamics. The above research plays an important role in clarifying the performance deterioration mechanism of insulating paper and predicting the life of insulation systems. However, the influence of the combined effects of electric field, thermal field, and mechanical stress field on the mechanical performance deterioration of insulating paper has not been comprehensively studied. Since the deterioration effects of external stresses on insulating paper interact with each other, research considering only one or both external stresses separately will deviate from the actual results.

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention provides a test method for the mechanical properties of the electrical-thermal-mechanical composite deterioration of insulating oil paper. First, a platform for the electrical-thermal-mechanical composite deterioration test of insulating paper is designed. Next, a multi-field composite deterioration test is carried out on the sample under different conditions. Furthermore, tests on the degree of polymerization and tensile strength are performed on the deteriorated sample to obtain the change rule of the mechanical properties of the sample after deterioration. Then, the cause of the deterioration of the mechanical properties is initially analyzed in combination with the XRD spectrum and microscopic morphology diagram; in order to further explore the microscopic mechanism of the deterioration of the mechanical properties of insulating paper under the action of multi-fields, molecular dynamics simulation is performed on insulating paper under the multi-physical field composite deterioration to establish the relationship between the deterioration of the macroscopic mechanical properties of insulating paper and the changes in microscopic physical and chemical properties, and to provide a data basis and theoretical support for the life prediction of insulating paper to solve the problems existing in the prior art.

[0007] To solve the above technical problems, the present invention is realized through the following technical solutions: The present invention is Construct an AC and DC, thermal, and mechanical stress combined degradation experiment platform to prepare insulating oil-paper samples in different degradation states, Measure the degree of polymerization and tensile strength of insulating oil-paper samples in different degradation states to obtain the change rules of mechanical properties after degradation, and observe the microscopic morphology diagrams and XRD spectra of insulating oil-paper samples in different degradation states to analyze the causes of mechanical property degradation, Construct the molecular chains of cellulose and the amorphous unit cells, optimize the structure, and obtain the cellulose model, Construct an oil-paper mixing model considering the influence of insulating oil on insulating oil-paper, Based on the cellulose model and the oil-paper mixing model, perform molecular dynamics simulations on oil-paper samples in different degradation states to establish the relationship between the macroscopic mechanical property degradation of insulating oil-paper and the changes in microscopic physical and chemical properties, It is a test method for the mechanical properties of the electrical, thermal, and mechanical composite degradation of insulating oil-paper including

[0008] Specifically, to construct an AC and DC, thermal, and mechanical stress combined degradation experiment platform, Construct a high-temperature degradation box, Install a vibration generator on the pedestal of the high-temperature degradation box to apply vibrations with a certain amplitude to the insulating oil-paper, Construct AC and DC circuits. The DC voltage and AC voltage are respectively supplied by two transformers, and the DC voltage and AC voltage are jointly applied to the insulating sleeve at the top of the degradation box, It includes putting the pre-treated insulating oil-paper into different degradation tanks respectively and arranging the degradation tanks inside the degradation box.

[0009] Specifically, to prepare insulating oil-paper samples in different degradation states, Using 140°C as the experimental temperature, 5 kV / mm as the electric field strength, 1:1 as the voltage ratio of alternating current to direct current, 100 Hz as the vibration frequency of the mechanical stress, 1 mm as the vibration amplitude, and combining different factors among the three external factors of temperature, electric field, and mechanical field, applying them to the insulating oil-paper sample, and conducting an accelerated degradation test on the insulating oil-paper sample for 0 to 360 hours is included.

[0010] Measure the degree of polymerization of the insulating oil-paper sample by the viscosity test method, fit the cumulative loss rate of the cellulose degree of polymerization with the first-order reaction rate equation, obtain the cumulative loss rate of cellulose under the influence of different factors, conduct a tensile strength test on the insulating oil-paper sample with vibration treatment and the insulating oil-paper sample without vibration treatment, draw the stress-strain relationship curves of the two insulating oil-paper samples, and obtain the elastic modulus and tensile strength under the action of different factors.

[0011] The addition of the alternating current electric field and the direct current electric field exerts a significant accelerating effect on the decrease in the degree of polymerization. The decrease in the degree of polymerization of the insulating paper after vibration is more significant compared to the insulating paper deteriorated by the alternating current electric field and the direct current electric field. That is, vibration is a more significant factor in promoting the decrease in the degree of polymerization of the insulating paper. The rigidity coefficient of the insulating paper after vibration decreases, and the insulating paper becomes more prone to breakage. That is, the mechanical properties of the insulating paper decrease due to vibration, and it becomes easier to decompose or break during the degradation process, and finally the insulating structure is destroyed.

[0012] As the degradation time becomes longer, the degradation of the insulating paper cellulose gradually becomes more intense. The reason is that under the action of stress, the molecular force between cellulose chains weakens, the arrangement loosens, the main chain of cellulose stretches or breaks, pores and cracks are generated on the surface of the fiber, and due to the action of mechanical force, the crystal of cellulose has a decrease in the content of the crystalline phase, the flexibility of the molecular chain increases, and it becomes more prone to deformation.

[0013] Construct a cellobiose molecular model, use the cellobiose molecular model as the basic repeating unit to construct two cellulose molecular chains with a degree of polymerization of 10, and use the AC module to have a density of 1.5 g / cm 3Construct an amorphous unit cell and use the Forcite module to optimize the structure to obtain a cellulose model.

[0014] Paraffin C in insulating oil 18 H 38 , cycloalkane C 18 H 32 and aromatic hydrocarbon C 18 H 18 are selected, and an oil-paper mixed model is constructed according to the molecular number ratio of 1:8:1.

[0015] Performing molecular dynamics simulations on samples in different degradation states specifically involves introducing the mean square displacement to explain the degree of particle diffusion, calculating the change in the mean square displacement of cellulose under different degradation conditions, obtaining the glass transition temperature of the cellulose model based on the mean square displacement of the cellulose model under different degradation conditions, calculating the number of hydrogen bonds contained in the cellulose model under different degradation conditions, counting the change in the number of hydrogen bonds under different degradation conditions, calculating the elastic modulus matrix of the degraded cellulose-oil paper mixed model, deriving the mechanical elastic modulus of cellulose molecules, and analyzing the change rule of the mechanical elastic modulus.

[0016] Insulating paper subjected to degradation by mechanical force is prone to glass transition, the cellulose chains in the glassy state are prone to deformation, the thermal field may break the intermolecular hydrogen bonds, the additional electric field breaks the hydrogen bonds in the later stage of degradation, but the mechanical field has little effect on the number of hydrogen bonds. In the X direction with the addition of the mechanical mechanics field, the Young's modulus increases compared to the case without the addition of the mechanical mechanics field, and the Young's modulus decreases in the Y-axis direction and Z-axis direction perpendicular to the X direction.

Advantages of the Invention

[0017] The present invention provides a test method for the mechanical properties of the electrical-thermal-mechanical composite degradation of insulating oil-paper. To study the mechanical properties of converter transformer insulating oil-paper under electrical-thermal-mechanical composite degradation conditions, the following insulating oil-paper degradation experiments with multi-physical field coupling and insulating oil-paper degradation simulations based on molecular dynamics were carried out. First, the present invention analyzes the degradation status of the macroscopic mechanical properties of the sample through the degree of polymerization and tensile strength of the degraded sample. Next, the microscopic mechanism of the degradation of the mechanical properties of the sample is initially analyzed using X-ray diffraction spectroscopy and SEM scanning electron microscopy. Finally, by combining with indicators such as the mean square displacement, interaction energy, and mechanical elastic modulus of insulating paper cellulose obtained by molecular dynamics simulation, the degradation mechanism of the mechanical properties of insulating paper under electrical-thermal-mechanical composite degradation conditions is clarified, the relationship between the macroscopic mechanical properties and the changes in microscopic physical and chemical properties of insulating paper is constructed, a data basis and theoretical support are provided for the life prediction of insulating paper. At the same time, an effective method is provided for studying the degradation mechanism of transformer oil-paper insulation at the atomic level.

[0018] Of course, any product implementing the present invention does not necessarily have to achieve all of the above advantages simultaneously.

Brief Description of the Drawings

[0019] To more clearly explain the technical solutions of the embodiments of the present invention, the drawings necessary for explaining the embodiments are briefly introduced below. Obviously, the drawings in the following description are only a part of the embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without creative efforts.

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Embodiments for Carrying Out the Invention

[0020] The technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the drawings of the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts are included within the protection scope of the present invention.

[0021] Example 1 Referring to FIGS. 1 to 19, the present invention is a test method for the mechanical properties of the electrical, thermal, and mechanical composite deterioration of insulating oil-paper, Constructing an AC and DC, thermal, and mechanical stress coupling deterioration experiment platform and preparing insulating oil-paper samples in different deterioration states; Measuring the degree of polymerization and tensile strength of insulating oil-paper samples in different deterioration states to obtain the change rule of the mechanical performance after deterioration, and observing the microscopic morphology diagram and XRD spectrum of insulating oil-paper samples in different deterioration states to analyze the cause of the deterioration of the mechanical performance; Constructing the molecular chain of cellulose and the amorphous unit cell, optimizing the structure, and obtaining a cellulose model; Constructing an oil-paper mixing model considering the influence of insulating oil on insulating oil-paper; Based on the cellulose model and the oiled paper mixed model, performing molecular dynamics simulations on oiled paper samples in different degradation states to establish the relationship between the degradation of the macroscopic mechanical properties and the changes in the microscopic physical and chemical properties of the insulating oiled paper, including the step of

[0022] (1) Degradation platform and simulation model (1) Construction of a thermal degradation platform under AC and DC electric fields Specifically, to construct an AC and DC - thermal - mechanical stress combined degradation experiment platform, Construct a high - temperature degradation box, Install a vibration generator on the pedestal of the high - temperature degradation box to apply vibrations with a certain amplitude to the insulating oiled paper, Construct AC and DC circuits. The DC voltage and AC voltage are respectively supplied by two transformers, and the DC voltage and AC voltage are jointly applied to the insulating sleeve at the top of the degradation box, It includes putting the pre - treated insulating oiled paper into different degradation tanks respectively and placing the degradation tanks inside the degradation box.

[0023] To study the influence of the combined AC and DC voltage inside the converter transformer, high temperature and mechanical stress on the degradation of oil - impregnated insulating paper, an AC and DC - thermal - mechanical stress combined degradation experiment platform is designed. As shown in Figure 1, the experimental platform is composed of a high - temperature degradation box, an AC circuit and a DC circuit connected externally, and a vibration generator.

[0024] In the degradation circuit shown in Fig. 1, the DC voltage and the AC voltage are respectively supplied by two transformers, and are applied together to the insulating sleeve at the top of the degradation box. To prevent insulation breakdown, the actually applied voltage value is monitored by a voltage divider and an oscilloscope. The pre-treated insulating oil paper is respectively placed in different degradation tanks, and the degradation tanks are arranged inside the degradation box. When conducting the test, the high-voltage side and the grounding end are respectively connected to the top sleeve and the right-side sleeve of the degradation box. In order to apply vibrations with a certain amplitude, a vibration generator is installed on the pedestal of the degradation box to simulate the core vibration caused by the DC bias magnetic field under the actual operating conditions of the converter transformer.

[0025] In the present invention, 140 °C is selected as the experimental temperature, and the acceleration degradation tests for 0 - 360 hours are respectively carried out on the samples. Thus, from the empirical rules, when the degradation temperature is 140 °C, compared with the case where the operating temperature is 80 °C, the degradation rate is accelerated by 1024 times, and it can be inferred that 360 hours corresponds to about 40 years in the actual degradation situation. Regarding the selection of the electric field strength, considering that in the actual operation of a ±800 kV converter transformer, the electric field strength of the insulating oil paper is between 0.3 - 2.8 kV / mm, in order to accelerate the degradation effect, in the present invention, the electric field strength is set to 5 kV / mm, and the ratio of the AC voltage to the DC voltage is 1:1. Since the winding vibration of the transformer mainly concentrates on the fundamental frequency of 100 Hz, in the present invention, 100 Hz is selected as the frequency of the mechanical stress. The vibration amplitude is set to 1 mm.

[0026] (2) Construction of the simulation model The main component of the insulating paper of the converter transformer is cellulose molecules composed of a large number of β-D-glucopyranosyl groups linked by (1-4)-β-glycosidic bonds, and its basic repeating unit is cellulose disaccharide with the chemical formula (C6H 10 O5) n (n is the degree of polymerization). Due to the internal rotation effect of the chemical bond and the existence of hydrogen bonds, the cellulose chain forms a C1 chair conformation with the bond angle and plane of H shown in Fig. 2. 4

[0027] According to research, it has been shown that there are no obvious differences in the molecular conformations and physical and chemical properties in molecular dynamics simulations for the degrees of polymerization of different cellulose chains. Moreover, in many documents, cellulose chains with a degree of polymerization of 10 were constructed to study their physical and chemical properties, insulation performance, and mechanical properties, and all obtained results that matched the actual data. In order to study the mechanical properties of cellulose chains under multi-field composite degradation, the present invention constructed cellulose chains with a degree of polymerization of 10. Considering the interactions between multiple molecular chains, in the present invention, two molecular chains are filled in each unit cell, and the average density is set to 1.5 g / cm 3 And set. The present invention constructs a cellobiose molecular model and uses it as the basic repeating unit to construct two cellulose molecular chains with a degree of polymerization of 10. Finally, an amorphous unit cell with a density of 1.5 g / cm 3 is constructed using the AC module, and the structure is optimized using the Forcite module. The construction process and model are shown in Figure 3.

[0028] In actual work, the insulating paper of the converter transformer is immersed in high-temperature insulating oil for a long time. However, organic compounds and trace moisture in the insulating oil are very easy to diffuse and form a stable composite material with the cellulose of the insulating paper. Therefore, considering the influence of the insulating oil on the physical and chemical properties and mechanical performance of the insulating paper, the present invention further constructs an oil-paper hybrid model. Currently, most of the insulating oils used in converter transformers are mineral oils composed of hydrocarbons. However, due to the complex refining process, it is difficult to measure all the various complex hydrocarbon substances contained in the mineral oil. Therefore, the present invention selects the most representative paraffin C 18 H 38 , cycloalkane C 18 H 32 and aromatic hydrocarbon C 18 H 18 , and their molecular number ratio is 1:8:1. The model is shown in Figure 4, and its mass fraction is shown in the table. An amorphous cell module is used to construct a unit cell with a cubic side length, and it is filled with insulating oil molecules and cellulose molecules, and its density is 1.5 g / cm3 It is, and its model diagram is shown in Fig. 5.

[0029] To make the hole distribution of the model closer to the actual material and overcome the potential wells between the local minima of the system, energy relaxation was performed on the model using the Anneal module. The initial energy of the system reached the lowest value after repeated annealing, and the model after annealing was consistent with the actual chair conformation of cellulose molecules.

[0030] The present invention studied the effects of three external factors, namely temperature, electric field, and mechanical field, on the mechanical properties when applied to insulating paper. The Forcite module and NPT ensemble were used for the simulation calculation and analysis of the whole process. Molecular dynamics calculations were performed every 50 K in the temperature range from 280 K to 680 K for 300 ps each. Here, the first 160 ps was used for the equilibrium of the system, the last 100 ps was used for data analysis, and the last 100 ps was set to collect molecular dynamics trajectories every 1 ps for calculation and analysis. The electric field strength was set to 0.01 V / A, that is, 100 kV / mm, considering the accelerating effect of degradation. In all simulation processes, the upgraded COMPASII mechanical field was used, and the Andersen method and Berendsen method were used to control the temperature and pressure respectively. The pressure during simulation was 0.5 GPa, the integration algorithm was Velocity Verlet, the electrostatic interaction was Eward, and the van der Waals interaction was Atom based. All simulations were completed by the Forcite module of MS8.0. After the simulation calculation was completed, analysis was performed using the Analysis module, and some data was exported for separate processing.

[0031] (2) Physical and Chemical Properties of Oil-Paper Insulation Degradation (1) Degree of Polymerization The degree of polymerization of the insulating paper is a basic parameter characterizing the degree of its deterioration and can intuitively reflect the mechanical strength of the insulating paper. In this experiment, the viscosity test method was used to measure the degree of polymerization of the insulating paper, and the degrees of polymerization of the insulating paper under thermal, electrical, and mechanical combined deterioration for 60 hours, 116 hours, 180 hours, 240 hours, 300 hours, and 360 hours were recorded to obtain the change of the degree of polymerization over time under each deterioration condition, as shown in Figure 6.

[0032] As can be seen from Figure 6, the degree of polymerization of all insulating papers before deterioration is about 1160. With the increase of the deterioration time, the degree of polymerization of all samples decreased significantly and finally became less than 500, indicating that the deterioration entered the middle and late stages. The addition of the AC electric field and DC electric field played a significant promoting role in the decrease of the degree of polymerization of the insulating paper. The decrease of the degree of polymerization of the insulating paper after vibration was significant compared with that of the insulating paper deteriorated by the AC electric field and DC electric field, that is, vibration is a more significant factor in promoting the decrease of the degree of polymerization of the insulating paper.

[0033] The main reason for the deterioration of the insulating paper is that under the influence of factors such as temperature, electric field, mechanical stress, and moisture, the chemical bonds of the cellulose chains constituting the insulating paper are broken or displaced, or the hydrogen bonds and van der Waals forces are destroyed, which are manifested as a decrease in parameters such as the degree of polymerization and tensile strength in macroscopic mechanical properties. Therefore, the cumulative loss rate of the degree of polymerization of cellulose can be fitted using the first-order reaction rate equation.

[0034]

Number

[0035] In the formula, ω DP is the cumulative loss rate of the degree of polymerization (when ω DP = 0, DP = DP0 indicates that the insulating paper is not decomposed; when ω DP = 1, DP = 0 means that the insulating paper is completely decomposed); ω * DP is the maximum value of ω DP and indicates the reduction rate of the degree of polymerization of cellulose when the decomposition of the insulating paper is completed. k DPrepresents the change rate of the cumulative loss rate.

[0036] The fitting curve of the cumulative loss rate of the cellulose degree of polymerization is shown in Figure 7, and its parameters and goodness of fit R 2 are shown in Table 1.

[0037] Table 1 Fitting parameters of the cumulative loss rate of the cellulose degree of polymerization

Table 1

[0038] As can be seen from Figure 7 and Table 1, the cumulative loss rate of the insulating paper cellulose in thermal - electrical - mechanical degradation and thermal - electrical degradation shows an upward trend. The cumulative loss rate increases faster under thermal - electrical - mechanical degradation, and its ratio, k DP increased from 0.01418 to 0.05996. This indicates that the degradation of the insulating paper is promoted by the application of stress. The reason is presumably that vibration promotes the loosening of the arrangement of cellulose chains, resulting in chain breakage and displacement, and a decrease in the degree of polymerization of cellulose.

[0039] (2) Tensile strength To further study the influence of vibration on the insulating paper, a tensile strength test was conducted on the insulating paper with vibration treatment and the insulating paper without vibration treatment, and the stress - strain relationship curves of the two types of insulating paper are shown in Figure 8. As shown in Table 2, according to the stress - strain relationship, the elastic modulus and tensile strength of the insulating paper under the action of TE and TEM can be obtained.

[0040] Table 2 Mechanical performance table of the degraded insulating paper

Table 2

[0041] Observing Figure 8, it can be seen that the two curves show a similar trend of change and are mainly divided into three stages. First stage: The displacement is proportional to the tensile force, and the ratio of the tensile force to the displacement at this time is the stiffness coefficient of the material. The stiffness coefficient of the vibrated insulating paper is significantly lower than that of the non-vibrated insulating paper, that is, the insulating paper is more easily deformed by vibration.

[0042] Second stage: The displacement no longer is proportional to the tensile force. After passing through the inflection point, the displacement increases rapidly, and the tensile force basically does not change. This inflection point is the yield point of the material, and this phenomenon is called the yield phenomenon of the material. Due to the decrease in the stiffness coefficient, the vibrated insulating paper is more easily deformed, so the yield point is slightly delayed compared to the non-vibrated insulating paper, that is, the ductility of the insulating paper is improved by vibration.

[0043] Third stage: The displacement and the tensile force return to a proportional relationship. When the displacement exceeds a certain limit, the tensile force rapidly decreases to 0, and at this point the material breaks. The tensile force at which the vibrated insulating paper breaks is 300 N, and the tensile force at which the non-vibrated insulating paper breaks is 410 N, indicating that the breaking point of the insulating paper occurs earlier due to vibration.

[0044] Analyzing the above comprehensively, the vibrated insulating paper has a lower anti-deformation ability due to the decrease in the stiffness coefficient, and a stronger ductility of the material, so it has a higher yield point.

[0045] However, due to the destructive effect of vibration on the insulating paper, the breaking point occurs earlier, and the insulating paper is more likely to break. That is, vibration reduces the mechanical properties of the insulating paper, increasing the possibility of decomposition and breakage during the deterioration process, and ultimately destroying the insulation structure.

[0046] (3) Microscopic morphology As a result of observing the insulating paper that has undergone thermal, electrical, and mechanical composite deterioration with a SEM scanning electron microscope, Figure 9 was obtained. The deterioration status of the insulating paper can usually be confirmed by the arrangement status of cellulose, the surface smoothness, the average width, and the fracture status in the microscopic morphological diagram. As can be seen from Figure 9, the SEM insulating paper of the non-deteriorated insulating paper has a smooth surface, the fibers are densely arranged, the connection between the fibers is tight, the average width of the fibers is about 180 μm, and there are no obvious branches, pores, or breaks. As can be seen from Figure 9, after 120 hours of deterioration, the fiber width of the insulating paper has significantly narrowed to about 110 μm, and at the same time, some wrinkles and pores have occurred on the cellulose surface. When the deterioration time reaches 240 hours, as can be seen from Figure 9, the fiber arrangement becomes sparse, its average width decreases to about 60 μm / strip, the width of the thinnest cellulose is only 40 μm, the connection between the fibers becomes loose, the pores at the connection part increase significantly, and the protrusions and wrinkles on the surface become more prominent. After 360 hours of deterioration, as can be seen from Figure 9, the arrangement of the fibers becomes looser, the width becomes significantly narrower, the average is only about 50 μm, the number of pores has increased significantly compared with before, and multiple cracks on the fibers can be visually observed. Summing up the above, as the deterioration time passes, the deterioration of the insulating paper cellulose gradually progresses. The reason is that the intermolecular force between the cellulose chains becomes weak under stress, its arrangement becomes loose, elongation and breakage occur in the main chain of cellulose, resulting in pores and cracks on the fiber surface. This also somewhat supports the reasons for the decrease in the degree of polymerization and tensile strength of the insulating paper after deterioration described above from the perspective of the microscopic structure.

[0047] (4) X-ray diffraction spectrum In the polymer composed of cellulose, there are amorphous regions and crystalline regions. In the present invention, the crystal structure of the crystalline region of cellulose and the conversion between the crystalline region and the amorphous region are mainly analyzed by X-ray diffraction analysis. The diffraction intensity of the XRD diagram reflects the diffraction ability of the phase itself and the proportion in the mixture. The position of the diffraction peak is related to the type of crystal and represents the spacing between the diffraction planes. The full width at half maximum and the morphology are functions of the crystal size and strain. The XRD analysis results of the samples under different deterioration conditions are shown in Figure 10.

[0048] The diffraction peaks of the cellulose polymer appear near 2θ = 18.5° and near 2θ = 16°, which are the diffraction peaks representing the crystalline region and the "bulge peak" representing the amorphous region, respectively. From Figure 10, it can be seen that as the degradation time increases, the heights of the two diffraction peaks decrease, and the decrease is even greater under thermal, electrical, and thermo-electro-mechanical degradation conditions.

[0049] The calculated performance parameters of each XRD spectrum are shown in Table 2. In Table 2, I am is the diffraction peak intensity of the amorphous region (the diffraction intensity near 2θ = 16°, where θ is the Bragg angle), and I 002 is the diffraction peak intensity of the crystalline region (the diffraction intensity near 2θ = 18.5°).

[0050] From the table, it can be seen that under thermal, electrical, and thermo-electro-mechanical degradation conditions, both I am and I 002 decrease monotonically with the extension of the degradation time. Under different degradation times and conditions, the positions of all diffraction peaks are approximately 18.5° and 16°, showing no obvious change, indicating that the crystal type of the insulating paper does not change before and after degradation. As the degradation time increases, the diffraction intensity decreases, which indicates that as the degradation time increases, the crystal phase content in the cellulose crystal region decreases, and the diffraction peak intensity in TEM is lower than that in TE. This shows that the crystal phase content of cellulose crystals decreases due to the action of mechanical force.

[0051] Under the same degradation conditions, the full width at half maximum FwHM 002 increased monotonically with the degradation time. The reason is considered to be that as the degradation time extends, the lattice distortion of cellulose and the refinement of crystal blocks progress, resulting in an increase in the selective reflection region in the reciprocal lattice space and a broadening of the diffraction peak.

[0052] The relative crystallinity CrI in Table 2 indicates the proportion of the crystalline region in the whole cellulose microfibril having a crystalline region and an amorphous region (non-crystalline region), and the crystallite size D indicates the size of the crystallites. They are obtained according to the Segal formula and the Scherrer formula, respectively.

[0053] [Number]

[0054] Here, D is the average thickness (Å) in the direction perpendicular to the crystal plane of the crystallites, K is the Scherrer constant, B is the half-width of the diffraction peak, so K = 0.89, θ is the Bragg diffraction angle, γ is the wavelength of the X-ray, usually 1.54056 Å.

[0055] According to the fibrillar micelle structure theory, the crystal structure of cellulose is composed of regularly arranged crystalline micelle cells and fibrillar whiskers extending from the unit cells. Since it is a continuous transition from a highly crystallized and ordered region to a completely amorphous disordered region, there is no clear boundary. As a result, the crystalline region and the amorphous region of cellulose are mutually converted, and the crystallinity increases or decreases with the passage of the degradation time. The mechanical force promoted the refinement of the crystallites, but the cellulose had a significantly higher crystallinity under the TE condition with the same degradation days than that under the TEM. This indicates that a part of the crystalline region of the insulating cardboard with mechanical stress applied has changed to the amorphous region. This increases the flexibility of the molecular chain and makes it easier to deform.

[0056] Table 3 XRD parameters [Table 3]

[0057] (III) Molecular dynamics study on the degradation of the insulation of oiled paper (1) Mean square displacement The performance degradation of insulating paper is closely related to the molecular motion of cellulose. To study the influence of the coupling degradation of multi-physical fields on the molecular motion of insulating paper, the present invention introduces the mean square displacement to describe the degree of particle diffusion. The mean square displacement (MSD) is an index indicating the deviation of the position of a particle after moving over time from a reference position, and its definition formula is as follows.

[0058]

Equation

[0059] N represents the total number of atoms calculated, TIFF0007692632000007.tif53 represents the position vector of an atom, and t represents the current moment.

[0060] The larger the MSD value of the system, the more intense the motion of the atoms in the system, that is, the worse the stability of the model. To investigate the influence of changes in external conditions on the degree of cellulose molecular motion, in the present invention, the change situation of the MSD of cellulose under multiple conditions is calculated and shown in FIGS. 11 and 12. As can be seen from FIGS. 11 and 12, under both conditions, the MSD increases with the increase in temperature, which indicates that the temperature field strengthens the motion of cellulose molecules, and this is consistent with the theorem of molecular thermal motion. When a temperature field between 300K and 500K is applied, the change in the MSD value of cellulose is not obvious, and the fluctuation range does not exceed 16 Å 2 When the temperature exceeds 600K, the MSD begins to increase significantly with the passage of time, and its fluctuation range is up to 36 Å 2 and 80 Å 2It can reach this value. This indicates that between 500K and 600K, the cellulose molecules are highly likely to undergo some state transition, the intermolecular force becomes weaker, the condensation structure of the polymer changes, and relative displacement between molecular chains becomes more likely to occur. Also, when the temperature is between 300K and 500K, there is no significant difference in the MSD of cellulose molecules before and after applying mechanical force. However, when the temperature is between 600K and 800K, the MSD of cellulose molecules after applying the mechanical field can reach twice that without applying the mechanical field, which indicates that as the temperature rises, the influence of mechanical force on the movement of molecular chains becomes stronger. Analyzing the reason, it is considered that as the temperature rises, the polymer expands, the free volume for segment movement increases, and the influence of mechanical force on the movement of molecular chains also increases.

[0061] The glass transition temperature (T g ) refers to the temperature at which a polymer changes from a glassy state to a highly elastic state. In the highly elastic state, the molecules have a stronger ability to move. Since the glass transition temperature is caused by the movement of molecular chains, the glass transition temperature of cellulose insulating paper can be determined by examining the movement changes of molecular chains. The MSD values of the cellulose model at 10 ps under each degradation condition were taken as representative values at that temperature, and a scatter plot was created. By fitting the scatter plot, two fitting curves were obtained, and the point on the horizontal axis corresponding to the intersection of them is the glass transition temperature. The state of the polymer changes with the change of temperature, and the process is shown in the figure.

[0062] From Figure 13, it can be seen that the glass transition temperature under the action of T+E is 519.32K, and the glass transition temperature under the action of T+E+M is 516.86K. The glass transition temperature of cellulose decreased due to mechanical force. This indicates that the insulating paper subjected to the degradation action of mechanical force is more likely to undergo glass transition, and the cellulose chains in the glassy state are more likely to deform. The glass transition temperature is mainly related to the flexibility of cellulose chain segments. The softer the chain segments, the lower the glass transition temperature. This shows that mechanical force increases the flexibility of cellulose molecular chains and makes the insulating paper more likely to deform.

[0063] (2) Hydrogen bond Numerous hydroxyl groups and glycosidic bonds within cellulose provided oxygen atoms sufficient to interact with hydrogen atoms to form hydrogen bonds. The hydrogen bonds in cellulose exist both intermolecularly and intramolecularly, with a binding energy of approximately 12 - 40 kcal / mol, a bond length of approximately 0.24 - 0.32 nm, and possessing orientation, saturation, and additivity. Hydrogen bonds bring about a certain restraint effect on each particle and can inhibit the movement of particles and the cleavage of chemical bonds. However, since the total number of hydrogen bonds is limited and its ability to restrain particles is equal to the total binding energy, there is a strong correlation between hydrogen bond energy and the number and length of hydrogen bonds. The hydrogen bonds within the cellulose model are as shown in Figure 18.

[0064] After completing the molecular dynamics simulation calculations for the cellulose model under the influence of a thermal field, electro-thermal coupling, and electro-thermal-mechanical coupling, the trajectory file for the last 100 ps was obtained, and a script was used to calculate the number of hydrogen bonds contained in the cellulose model within each trajectory file. To reduce errors, the average of the number of hydrogen bonds in 100 trajectory files was taken, and the changes in the number of hydrogen bonds under different conditions were statistically obtained, as shown in Figure 15. From Figure 15, it can be seen that the higher the temperature, the fewer the number of hydrogen bonds; the addition of an electric field reduces the number of intermolecular hydrogen bonds; and the force field has little effect on hydrogen bonds. As the temperature rises, the number of hydrogen bonds continues to decrease, which indicates that the higher the temperature and the longer the degradation time, the lower the binding energy of the intermolecular hydrogen bonds within the insulating paper, the worse the intermolecular bonding, and the more intense the movement of the molecules. This is consistent with the experimental results showing that the decomposition products of the insulating paper increase with the degradation time and the degree of polymerization decreases with the degradation time, indicating that the thermal force field may destroy hydrogen bonds and cause the degradation of the insulating paper.

[0065] After applying the force field effect, the hydrogen bonds have not changed significantly compared to the case without applying the force field, indicating that the effect of the force field on hydrogen bonds is not significant. When comparing the change in the amount of hydrogen bonds after applying an electric field between 300K and 480K, the amount of hydrogen bonds in cellulose is higher when the electric field is applied than when it is not, but the difference is not large. After 500K in the later stage, under the action of the electric field, the amount of hydrogen bonds in the cellulose model decreases rapidly and becomes even lower than in the normal state without applying the electric field. From this, it is speculated that in the initial stage of degradation, the intermolecular distance of cellulose molecules is relatively close and the degradation does not have a profound impact. At this time, since the intermolecular force is strong, the electric field does not play an obvious role. As the degradation progresses, the degree of intermolecular bonding decreases. At this time, the influence of the electric field force becomes significant and the number of hydrogen bonds also decreases. This may cause the degradation effect to intensify because the electrothermal bonding degradation is not simply a cumulative degradation of 1+1 but includes a complex composite mechanism.

[0066] As can be seen from Figure 15, as the temperature rises, the length of the hydrogen bond also increases, and the proportion of hydrogen bonds with a length exceeding 2.0 Å gradually increases from 300K to 800K. This indicates that under the action of TEM, even as the temperature rises, the proportion of relatively long hydrogen bonds increases. The binding energy of the hydrogen bond is inversely proportional to its length, and the longer the length, the weaker the binding energy. This shows that as the intensity of the thermal field increases, the binding energy of the hydrogen bonds between cellulose weakens and is more easily broken.

[0067] The change in the number of hydrogen bonds can affect the mechanical properties of polymers. When the hydrogen bonds in cellulose decrease, the intermolecular force between cellulose molecules weakens, making it easier for the molecular chains to orient under external stress. At the same time, the flexibility of the cellulose chain increases and the tensile strength decreases. Analyzing the above comprehensively, the thermal field destroys the intermolecular hydrogen bonds, and the additional electric field destroys the hydrogen bonds in the later stage of degradation, but the force field has little effect on the number of hydrogen bonds. Therefore, the thermal field and the electric field reduce the tensile strength of cellulose and make orientation more likely to occur under the action of external forces.

[0068] (3) Mechanical properties To verify the accuracy of the change in the tensile strength of the insulating paper after deterioration and predict the mechanical elastic modulus of cellulose, the elastic modulus matrix of the deteriorated cellulose-oil mixed model was calculated by molecular dynamics simulation, the mechanical elastic modulus of cellulose molecules was derived, and its changing rules were analyzed.

[0069] In material mechanics, the stress applied to a material and the resulting strain are expressed as follows by Hooke's law.

[0070]

Number

[0071] C in the formula ij is a sixth-order elastic coefficient matrix, and all mechanical properties of the material can be derived from this matrix. TIFF0007692632000009.tif44 is the stress. TIFF0007692632000010.tif43 is the strain.

[0072] The effective bulk elastic modulus (B) and the effective shear elastic modulus (C) can be obtained by the RUSS averaging method.

[0073]

Number

[0074]

Number

[0075] Since the cellulose model is isotropic, the relationship between the elastic moduli can be obtained as follows.

[0076]

Number

[0077] Among the elastic moduli, Young's modulus represents the relationship between stress and strain when a tensile stress is applied to a material, and has a corresponding relationship with the concept of tensile strength. The elastic modulus of a material can accurately represent the strain generated by the material when it is subjected to an external stress. The study of the change rule of the elastic modulus of the material after deterioration is the basis for evaluating the deterioration state and predicting the life of insulating paper.

[0078] In the present invention, as shown in the figure, the Young's modulus and the shear modulus were calculated using Mechanical Properties of the Forcite module of MS software. As can be seen from Figure 18, the Young's modulus in the X direction with an applied mechanical force field increased compared to the case without an applied mechanical force field, but the Young's modulus decreased in the Y-axis and Z-axis directions perpendicular to it. This is because the molecular chains are uniaxially oriented by the action of the mechanical stress in the X-axis, the mechanical elastic modulus of cellulose in the X-axis increases, and the Young's modulus in the Y-axis and Z-axis directions perpendicular to the X direction decreases. This is consistent with the previous experimental fact that the elastic modulus decreases in the direction perpendicular to the applied force.

[0079] Conclusion: Based on the thermal, electrical, and mechanical composite deterioration experiments carried out according to the present invention, the degree of polymerization, tensile strength tests on deteriorated samples, X-ray diffraction patterns, scanning electron micrographs, and the results of molecular dynamics simulations, the following conclusions were obtained: 1) Due to the thermal, electrical, and mechanical effects, the degree of polymerization of the deteriorated insulating paper decreases, and when vibration is applied, the rising rate of the cumulative loss rate increases. The cumulative loss rate of cellulose reaches a steady state faster, indicating that mechanical vibration can accelerate the deterioration of the mechanical properties of insulating paper. 2) The rigidity of the insulating paper subjected to vibration decreases, the yield point occurs earlier, and it becomes more prone to breakage than before vibration. This is because the intermolecular force is destroyed by the mechanical force, the crystallinity decreases, the cellulose is oriented, and the tensile strength in the direction perpendicular to the orientation direction decreases. 3) The thermal, electrical, and mechanical effects do not change the type of microcrystals but reduce the relative crystallinity and grain size of the insulating paper fibers. At the same time, the thermal, electrical, and mechanical actions promoted the refinement of the fibers and the appearance and development of pores. The occurrence of vibration promoted the refinement, branching, and breaking of the fibers, loosened the arrangement of cellulose, increased and enlarged the pores, and promoted the deterioration of the mechanical properties of the insulating paper.

[0080] In the description of this specification, the descriptions of terms such as "one embodiment", "example", "specific example", etc. mean that the specific features, structures, materials, or characteristics described in relation to the said embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described can be combined in an appropriate manner in any one or more embodiments or examples.

[0081] The preferred embodiments of the present invention disclosed above are only intended to assist in the description of the present invention. The preferred embodiments do not describe all details comprehensively and do not limit the present invention to only the specific embodiments described. Obviously, many modifications and variations can be made in accordance with this specification. These embodiments are selected and specifically described in this specification in order to better explain the principles and practical applications of the present invention so that those skilled in the technical field to which the present invention pertains can understand and utilize the present invention well. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. constructing an AC and DC, thermal, and mechanical stress combined degradation experiment platform to prepare insulating oil-paper samples in different degradation states; measuring the degree of polymerization and tensile strength of insulating oil-paper samples in different degradation states to obtain the variation rule of the mechanical performance after degradation, and observing the morphological diagram and XRD spectrum showing the changes in the microscopic physical and chemical characteristics of insulating oil-paper samples in different degradation states to analyze the cause of mechanical performance degradation; constructing the molecular chain of cellulose and the amorphous unit cell, optimizing the structure, and obtaining a cellulose model; selecting paraffin C18H38, cycloalkane C18H32, and aromatic hydrocarbon C18H18 in insulating oil, and constructing an oil-paper mixing model according to a molecular number ratio of 1:8:1; based on the cellulose model and the oil-paper mixing model, performing molecular dynamics simulations on insulating oil-paper samples in different degradation states to establish the relationship between the macroscopic mechanical performance degradation and the microscopic physical and chemical property changes of insulating oil-paper. A test method for the mechanical properties of the electrical, thermal, and mechanical combined degradation of insulating oil-paper, characterized by including the above steps.

2. Specifically, to construct an AC and DC, thermal, and mechanical stress combined degradation experiment platform: constructing a high-temperature degradation box; installing a vibration generator on the pedestal of the high-temperature degradation box to apply vibrations with a certain amplitude to the insulating oil-paper; constructing AC and DC circuits, where the DC voltage and AC voltage are respectively supplied by two transformers, and the DC voltage and AC voltage are jointly applied to the insulating sleeve at the top of the high-temperature degradation box; including putting the pre-treated insulating oil-paper into different degradation tanks respectively and placing the degradation tanks inside the high-temperature degradation box. The test method for the mechanical properties of the electrical, thermal, and mechanical combined degradation of insulating oil-paper according to Claim 1, characterized by the above.

3. Specifically, to prepare insulating oil-paper samples in different degradation states: Using 140°C as the experimental temperature, 5 kV / mm as the electric field strength, 1:1 as the voltage ratio of alternating current to direct current, 100 Hz as the vibration frequency of the mechanical stress, 1 mm as the vibration amplitude, and combining different factors among the three external factors of temperature, electric field, and mechanical field, applying them to the insulating oil-paper sample, and including performing an accelerated degradation test on the insulating oil-paper sample for 0 to 360 hours. The test method for the mechanical properties of the electrical-thermal-mechanical composite degradation of the insulating oil-paper according to claim 1.

4. Measuring the degree of polymerization of the insulating oil-paper sample by the viscosity test method, fitting the cumulative loss rate of the cellulose degree of polymerization with the first-order reaction rate equation, obtaining the cumulative loss rate of cellulose under the influence of different factors, performing a tensile strength test on the insulating oil-paper sample with vibration treatment and the insulating oil-paper sample without vibration treatment, drawing the stress-strain relationship curves of the two insulating oil-paper samples, and obtaining the elastic modulus and tensile strength under the action of different factors. The test method for the mechanical properties of the electrical-thermal-mechanical composite degradation of the insulating oil-paper according to claim 1.

5. Constructing a cellobiose molecular model, using the cellobiose molecular model as the basic repeating unit to construct two cellulose molecular chains with a degree of polymerization of 10, using the AC module to construct an amorphous unit cell with a density of 1.5 g / cm ３ and using the Forcite module to optimize the structure to obtain a cellulose model. The test method for the mechanical properties of the electrical-thermal-mechanical composite degradation of the insulating oil-paper according to claim 1.

6. Performing molecular dynamics simulations on samples in different degradation states specifically includes Introducing the mean square displacement to explain the degree of particle diffusion, calculating the change situation of the mean square displacement of cellulose under different degradation conditions, obtaining the glass transition temperature of the cellulose model based on the mean square displacement of the cellulose model under different degradation conditions, calculating the number of hydrogen bonds contained in the cellulose model under different degradation conditions, and counting the change in the number of hydrogen bonds under different degradation conditions, The test method for the mechanical properties of the electrical, thermal, and mechanical composite deterioration of insulating oil paper according to claim 1, characterized in that it is to calculate the elastic modulus matrix of the deteriorated cellulose-oil paper hybrid model, derive the mechanical elastic modulus of cellulose molecules, and analyze the change rule of the mechanical elastic modulus.

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