Cellulose paper aging process polymerization degree prediction method based on molecular dynamics simulation
Through molecular dynamics simulation and multivariate regression model, the problem of difficult monitoring of the polymerization change of cellulose paper in the transformer is solved, and the accurate prediction of the aging process of cellulose paper is achieved, and the monitoring and design level of the transformer insulation system is improved. Extend the service life of the equipment.
Patent Information
- Application Number
- CN202411251869.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-09-09
- Publication Date
- 2025-01-07
AI Technical Summary
The prior art is difficult to monitor and predict the change in the degree of polymerization of cellulose paper in large oil-immersed power transformers in real time, resulting in a decline in the mechanical properties of the material and affecting the insulation effect and service life of the equipment.
Using a method based on molecular dynamics simulation, combined with molecular model, mechanical performance analysis and multivariate regression model, simulate the aging process of cellulose paper under different environmental conditions, predict the change of polymerization, and optimize the design and safety evaluation of transformer insulation system Provide theoretical basis.
Accurate prediction of the aging process of cellulose paper is achieved, the relationship between the change in polymerization degree and mechanical properties is revealed, the monitoring and maintenance level of transformer insulation system is improved, and scientific theoretical basis is provided to extend the service life of the equipment.
Smart Images
Figure CN119274667A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to the field of material science and power equipment insulation technology, and in particular to a method for predicting the degree of polymerization of cellulose paper during an aging process based on molecular dynamics simulation. Background Art
[0002] The oil-paper insulation system in large oil-immersed power transformers is a key component to ensure the long-term safe operation of the equipment. As the main insulating material, the performance of cellulose paper directly affects the insulation effect and service life of the transformer. During the operation of the transformer, cellulose paper will be affected by factors such as heat, oxygen, and moisture, and gradually age, which is manifested as a decrease in the degree of polymerization (DP). The degree of polymerization is an important indicator to measure the integrity of the cellulose chain. The decrease in the degree of polymerization is usually accompanied by a decrease in the mechanical properties of the material, such as a decrease in stiffness, elasticity, and shear strength.
[0003] In the prior art, the degree of polymerization of cellulose paper is usually measured by experimental methods, such as viscosity determination and chemical analysis. However, these methods have the disadvantages of complex operation, long time and high cost, and it is difficult to monitor the change of degree of polymerization in real time. Therefore, a method for predicting degree of polymerization based on computer simulation is proposed, which is of great significance for improving the monitoring and maintenance level of transformer insulation system. Summary of the invention
[0004] The present invention provides a method for predicting the degree of polymerization of cellulose paper during the aging process based on molecular dynamics simulation, which relates to the fields of material science and power equipment insulation technology, and in particular to the aging assessment and life prediction of transformer insulation materials. By combining molecular dynamics simulation, mechanical property analysis and multivariate regression model, the method can accurately predict the change in the degree of polymerization of cellulose paper during the aging process under the conditions of known temperature, oxygen concentration, moisture content and initial degree of polymerization, providing a theoretical basis for the optimization design and safety assessment of transformer insulation systems.
[0005] To achieve the above purpose, the present invention adopts the following technical route:
[0006] Step 1: Establish a molecular model of cellulose paper. Cellulose is composed of highly polymerized hydrocarbons, specifically a series of glucose group chains. The degree of polymerization (DP) of cellulose molecules is usually between 1000 and 1200, which represents the number of glucose monomers in each cellulose molecule. Since the degree of polymerization of cellulose will gradually decrease over time during the use of the transformer, a smaller degree of polymerization is usually selected when simulating the cellulose paper model to reduce the amount of calculation. Therefore, 50 cellobiose units are used instead of a complete cellulose chain to construct a cellulose insulation paper model. Cellobiose is the basic structural unit of cellulose and consists of two β-D-glucose molecules connected by a β(1→4) glycosidic bond. The size of the model is set to An amorphous periodic system was formed, and different concentrations of oxygen (O2) and water (H2O) molecules were added to simulate the aging process of cellulose paper under different environments;
[0007] Step 2: First, use the Forcite module in Material Studio (MS) software to optimize the geometry of the cellulose paper model to ensure that the system energy reaches the lowest state. Then, use the dynamic function (Dynamic) to relax and balance the structurally optimized model to obtain the most stable structural model. At 298K, the NPT system is used to relax the model for 30ps with a time step of 1fs. In this process, the total energy of the system is significantly reduced compared to the initial state, ensuring the stability of the model and providing an accurate basis for subsequent simulations;
[0008] Step three: Use LAMMPS software to perform reaction molecular dynamics simulation. This article selects the CHO.ff force field, which is suitable for the pyrolysis process of hydrocarbons. The simulated reaction temperature is set to 2700K, which only affects the reaction rate during the pyrolysis process, but does not change the final product or reaction mechanism. First, the model is relaxed at 300K for 10ps, and then the temperature is gradually increased to 2700K within 40ps, and a 50ps reaction simulation is performed at 2700K. The entire process adopts the NVT canonical ensemble, that is, the reaction process is simulated under constant temperature and constant volume conditions to ensure that there is no energy exchange with the outside world during the cracking process. The time step is set to 0.1fs, and the reaction state is output every 1000 steps during the simulation process, so that the pyrolysis process can be analyzed in detail later;
[0009] Step 4: Analyze the aging products of cellulose paper under different O2 and H2O concentrations. The main products studied include water (H2O), carbon monoxide (CO), carbon dioxide (CO2), formaldehyde (HCHO) and formic acid (HCOOH). The simulation shows that the role of oxygen in cellulose molecules is reflected in its ability to promote the formation of carbonyl (C=O) bonds and significantly increase the generation paths and types of aldehydes and acids. Water accelerates the reaction process by providing free hydrogen ions (H+) and hydroxyl groups (OH-), further promoting the generation of acids and aldehydes. During the reaction, oxygen not only increases the generation of acids and aldehydes through oxidation, but also significantly increases the possibility of oxidation reactions of these substances, thereby increasing the generation of CO and CO2;
[0010] Step 5: Study the synergistic mechanism of oxygen and water in the aging process of cellulose paper. Oxygen captures hydrogen atoms in two main ways: one is to directly capture hydrogen atoms from the carbon chain to form OOH bonds; the other is to replace hydrogen atoms by directly combining with the carbon chain. After adding water to the system, oxygen will also combine with hydrogen atoms in the water to form OOH bonds and hydroxyl groups (OH-). The increase in hydrogen ions (H+) in the system further accelerates the degradation process of cellulose molecules, resulting in a significant increase in the production of acid and aldehyde products;
[0011] Step 6: Compare and analyze the generation patterns of the main aging products in different systems, and preliminarily evaluate the changes in the degree of polymerization of cellulose paper based on the generation of aging products. The addition of oxygen and water not only increases the generation of acids and aldehydes, but also significantly increases the possibility of their oxidation to CO and CO2. Studies have shown that in systems with higher oxygen concentrations, the final concentrations of CO and CO2 are about 1.6 times that of systems containing only cellobiose. In systems containing water, water further promotes the generation of acids and aldehydes by providing free hydroxyls and hydrogen ions, thereby increasing the generation of CO and CO2.
[0012] Step 7: During the molecular dynamics simulation, as the degree of polymerization decreases, the mechanical properties of the cellulose chain gradually weaken. By applying tensile, compressive and shear stresses, the stress-strain relationship of the cellulose chain is calculated to obtain micromechanical parameters such as Young's modulus, elastic modulus and shear modulus.
[0013] Step 8: The change of DP is a process that takes place over time, so the influence of time factors (such as aging time) needs to be considered. Under the high temperature conditions of 2700K, the reaction molecular dynamics simulation is carried out. The rate of chemical reactions (such as chain breaking, oxidation, etc.) in the aging process is much faster than that at room temperature. Through the Arrhenius formula, the aging time at high temperature is converted to the time under actual working conditions to obtain an equivalent aging time scale.
[0014] Step 9: The aging process of cellulose paper is nonlinear, and many environmental factors (such as temperature, oxygen concentration, and moisture concentration) have nonlinear effects on DP changes. Use logarithmic, exponential, or other nonlinear forms to better capture these effects. Introduce Young's modulus, elastic modulus, and shear modulus to consider the effect of the macroscopic mechanical properties of the material on the change in degree of polymerization. The negative values of these coefficients indicate that higher mechanical properties generally slow down the decline in DP, reflecting that higher material stiffness and toughness help slow down the aging process.
[0015] Step 10: Based on multivariate regression analysis, a model including temperature, oxygen content, moisture content and material mechanical properties is established. Considering the effects of initial degree of polymerization (DP), temperature (T), oxygen concentration (O2), moisture concentration (H2O), time (t), and Young's modulus (E), elastic modulus (G), and shear modulus (S) on DP changes, the change of DP of cellulose insulation paper follows an attenuation model:
[0016] DP(t)=DP 0 ×exp(-k×t)
[0017] The attenuation coefficient k is defined as:
[0018]
[0019] By performing multiple data fittings, the coefficients in the above formula are fitted using existing experimental data or simulation data to obtain:
[0020]
[0021] The model accurately predicts the trend of polymerization degree changes under different conditions by fitting the least squares method in the experimental data, and is then used to evaluate the changes in the mechanical properties of cellulose paper during the aging process.
[0022] The method provided by the present invention can study the aging behavior of cellulose paper under different oxygen and moisture conditions in detail, reveal the relationship between the change in degree of polymerization and macroscopic mechanical properties, and provide an important theoretical basis for the prediction of the service life of transformer insulation materials, material selection and system design optimization. This method can not only reveal the changing rules of the molecular structure of cellulose paper during the aging process at a microscopic scale, but also provide scientific support for improving the safety and reliability of transformer insulation systems. BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Figure 1 Flowchart of the DP change model during cellulose paper aging based on molecular dynamics simulation.
[0024] Figure 2 This is a cellulose paper model diagram.
[0025] Figure 3 This is a graph showing the changes in the content of main products during the aging process of cellulose paper.
[0026] Figure 4 This is the DP change diagram of cellulose paper after setting the initial parameters. DETAILED DESCRIPTION
[0027] The present invention provides a method for predicting the degree of polymerization of cellulose paper during aging based on molecular dynamics simulation, and the specific implementation methods are as follows:
[0028] Step 1: Establish a cellulose paper model. Cellulose is composed of highly polymerized hydrocarbons, specifically a series of glucose group chains. The degree of polymerization (DP) of cellulose molecules is usually between 1000 and 1200, which represents the number of glucose monomers in each cellulose molecule. Since the degree of polymerization of cellulose will gradually decrease over time during the use of the transformer, a smaller degree of polymerization is usually selected when simulating the cellulose paper model to reduce the amount of calculation. Therefore, 50 cellobiose units are used instead of a complete cellulose chain to construct a cellulose insulation paper model. Cellobiose is the basic structural unit of cellulose and consists of two β-D-glucose molecules connected by a β(1→4) glycosidic bond. The size of the model is set to An amorphous periodic system was formed, and different concentrations of oxygen (O2) and water (H2O) molecules were added to simulate the aging process of cellulose paper under different environments;
[0029] Step 2: In order to ensure the stability and accuracy of the molecular model during the simulation, the initial model needs to be optimized. First, the Forcite module in the Material Studio (MS) software is used to geometrically optimize the cellulose paper model to ensure that the total energy of the model is reduced to the lowest state. This step is crucial because it determines whether the initial state of the model is close to the actual physical conditions. Next, at a temperature of 298K, the NPT (constant pressure and constant temperature) ensemble is used to perform a 30ps kinetic relaxation calculation on the model, and the time step is set to 1fs. The relaxation process adjusts the intermolecular interactions so that the system reaches a thermodynamic equilibrium state, thereby ensuring the stability of the model in subsequent high-temperature simulations. The optimized model can more realistically simulate the physical and chemical properties of cellulose paper, thereby improving the accuracy of subsequent simulations;
[0030] Step 3: Use LAMMPS software to perform molecular dynamics simulation of the reaction. Select the CHO.ff force field, and set the simulation temperature to 2700K. The simulation process includes three stages: first, a 10ps relaxation at 300K, then gradually increase the temperature to 2700K within 40ps, and finally a 50ps reaction simulation at 2700K. The entire process uses the NVT canonical ensemble to ensure the energy stability of the simulation process. The time step is 0.1fs, and the reaction state is output once every 1000 steps. The entire simulation process uses the NVT (constant temperature and constant volume) canonical ensemble to ensure energy stability during the simulation process. The time step is 0.1fs, and the reaction state is output once every 1000 steps during the simulation process for detailed subsequent analysis. The simulation results will provide detailed molecular dynamics data for subsequent analysis;
[0031] Step 4: Measure and analyze the generation pathways and change patterns of the main products (such as H2O, CO, CO2, HCHO and HCOOH) generated during the aging of cellulose paper, and study the effects of oxygen and moisture on the changes of the main products during the aging of cellulose paper. The simulation shows that the role of oxygen in cellulose molecules is reflected in its ability to promote the formation of carbonyl (C=O) bonds and significantly increase the generation pathways and types of aldehydes and acids. Moisture accelerates the reaction process by providing free hydrogen ions (H+) and hydroxyl groups (OH-), further promoting the generation of acids and aldehydes. During the reaction, oxygen not only increases the generation of acids and aldehydes through oxidation, but also significantly increases the possibility of oxidation reactions of these substances, thereby increasing the generation of CO and CO2;
[0032] Step 5: Study the synergistic mechanism of oxygen and water in the aging process of cellulose paper. Oxygen captures hydrogen atoms in two main ways: one is to directly capture hydrogen atoms from the carbon chain to form OOH bonds; the other is to replace hydrogen atoms by directly combining with the carbon chain. After adding water to the system, oxygen will also combine with hydrogen atoms in the water to form OOH bonds and hydroxyl groups (OH-). The increase in hydrogen ions (H+) in the system further accelerates the degradation process of cellulose molecules, resulting in a significant increase in the production of acid and aldehyde products;
[0033] Step 6: Compare and analyze the generation patterns of the main aging products in different systems, and preliminarily evaluate the changes in the degree of polymerization of cellulose paper based on the generation of aging products. The addition of oxygen and water not only increases the generation of acids and aldehydes, but also significantly increases the possibility of their oxidation to CO and CO2. Studies have shown that in systems with higher oxygen concentrations, the final concentrations of CO and CO2 are about 1.6 times that of systems containing only cellobiose. In systems containing water, water further promotes the generation of acids and aldehydes by providing free hydroxyls and hydrogen ions, thereby increasing the generation of CO and CO2.
[0034] Step 7: During the molecular dynamics simulation, as the degree of polymerization decreases, the mechanical properties of the cellulose chain gradually weaken. By applying tensile, compressive and shear stresses, the stress-strain relationship of the cellulose chain is calculated to obtain micromechanical parameters such as Young's modulus, elastic modulus and shear modulus.
[0035] Step 8: The change of DP is a process that takes place over time, so the influence of time factors (such as aging time) needs to be considered. Under the high temperature conditions of 2700K, the reaction molecular dynamics simulation is carried out. The rate of chemical reactions (such as chain breaking, oxidation, etc.) during the aging process is much faster than that at room temperature. The Arrhenius formula is used to convert the simulation time to the actual aging time:
[0036]
[0037] Among them, ts is the simulation time, Ts is the simulation temperature (such as 2700K), Tr is the temperature under actual working conditions (such as 300K), Ea is the activation energy of cellulose paper, and R is the gas constant. Through this conversion formula, the simulation time under 2700K conditions can be converted to the equivalent aging time under actual working conditions;
[0038] Step 9: The aging process of cellulose paper is nonlinear, and many environmental factors (such as temperature, oxygen concentration, and moisture concentration) have nonlinear effects on DP changes. Use logarithmic, exponential, or other nonlinear forms to better capture these effects. Introduce Young's modulus, elastic modulus, and shear modulus to consider the effect of the macroscopic mechanical properties of the material on the change in degree of polymerization. The negative values of these coefficients indicate that higher mechanical properties generally slow down the decline in DP, reflecting that higher material stiffness and toughness help slow down the aging process.
[0039] Step 10: Based on multivariate regression analysis, a model including temperature, oxygen content, moisture content and material mechanical properties is established. Considering the effects of initial degree of polymerization (DP), temperature (T), oxygen concentration ([O2]), moisture concentration ([H2O]), time (t), and Young's modulus (E), elastic modulus (G), and shear modulus (S) on DP changes, the change in DP of cellulose insulation paper follows an attenuation model:
[0040] DP(t)=DP 0 ×exp(-k×t)
[0041] The attenuation coefficient k is defined as:
[0042]
[0043] By performing multiple data fittings, the coefficients in the above formula are fitted using existing experimental data or simulation data to obtain:
[0044]
[0045] The model accurately predicts the trend of polymerization degree changes under different conditions by fitting the least squares method in the experimental data, and is then used to evaluate the changes in the mechanical properties of cellulose paper during the aging process.
Claims
1. A method for predicting the degree of polymerization of cellulose paper during aging based on molecular dynamics simulation, characterized in that: The following steps are involved: Step 1: A molecular model of cellulose paper was constructed by selecting 50 cellobiose units as basic structural units; this model was used to simulate the aging process of transformer insulation paper under different oxygen (O2) and water (H2O) concentrations, and the accuracy and representativeness of the simulation results were ensured by considering the microstructural characteristics of the cellulose chain and its dynamic changes under the influence of the external environment; Step 2: Use the Forcite module in Material Studio (MS) software to optimize the geometry of the cellulose paper model to ensure that the model energy reaches the lowest state; then, use the NPT ensemble to perform a 30ps relaxation calculation on the model at 298K with a time step of 1fs to ensure the stability of the model; Step 3: Use LAMMPS software to perform reaction molecular dynamics simulation, select CHO.ff force field, and set the simulation temperature to 2700K; relax the model at 300K for 10ps, then gradually increase the temperature to 2700K within 40ps, and perform a 50ps reaction simulation at 2700K. The whole process uses the NVT canonical ensemble with a time step of 0.1fs; Step 4: Analyze the aging products of cellulose paper under different O2 and H2O concentrations, and study the generation paths and change patterns of the main products such as water (H2O), carbon monoxide (CO), carbon dioxide (CO2), formaldehyde (HCHO) and formic acid (HCOOH); Step 5: Study the synergistic mechanism of oxygen and water in the aging process of cellulose paper, combine the effects of O2 and H2O on the change of polymerization degree, and evaluate their effects on the aging behavior of cellulose paper; Step 6: Convert the simulation time under high temperature conditions to the time under actual working conditions through the Arrhenius formula, and calculate the equivalent time scale of the aging process; Step 7: During the molecular dynamics simulation, tensile, compressive and shear stresses are applied to calculate the stress-strain relationship of the cellulose chain and obtain micromechanical parameters such as Young's modulus, elastic modulus and shear modulus; Step 8: Based on multivariate regression analysis, a model including temperature, oxygen concentration, moisture concentration and material mechanical properties was established to predict the change in degree of polymerization of cellulose paper during aging. The model formula is as follows: DP(t)=DP0×exp(-k×t) The calculation formula of the attenuation coefficient k is:
2. The method for predicting the degree of polymerization of cellulose paper during aging based on molecular dynamics simulation according to claim 1, characterized in that: In the step of constructing the cellulose paper model, the long-chain structure and intermolecular interactions of the cellulose chain are accurately simulated by adjusting the number of cellobiose units and the cross-linking density between chain segments, so as to better reflect the mechanical properties of the actual material.
3. The method for predicting the degree of polymerization of cellulose paper during aging based on molecular dynamics simulation according to claim 1, characterized in that: In the molecular dynamics simulation step, the simulation process includes a multi-stage heating cycle to simulate the thermal cycle aging behavior of transformer insulation paper under actual working conditions and analyze the degradation mode of cellulose chains under different temperature gradients and thermal stress conditions.
4. The method for predicting the degree of polymerization of cellulose paper during aging based on molecular dynamics simulation according to claim 1, characterized in that: In the step of calculating the micromechanical parameters, different types of stress are applied during the molecular dynamics simulation to calculate micromechanical parameters such as Young's modulus, elastic modulus and shear modulus of the cellulose chain, revealing the micromechanical response of cellulose paper during the aging process and providing a more accurate prediction of the change in degree of polymerization.
5. The method for predicting the degree of polymerization of cellulose paper during aging based on molecular dynamics simulation according to claim 1, characterized in that: In the step of modeling the association between degree of polymerization and mechanical properties, the model takes into account the effects of environmental factors such as temperature, humidity and oxygen concentration on cellulose DP, and establishes a universal degree of polymerization prediction model by comparing simulation results under different environmental conditions.