Molecular Simulation Study On The Effect Of Moisture And Acids On Microscopic Properties Of PMIA Insulating Paper

As an important part of the transmission network system,the transformer plays a vital role in the safe operation of the power grid.With the continuous improvement of China's transmission voltage level,the electrical and mechanical properties of insulating paper,which plays a major role in the transformer,have higher requirements.As a new high-tech material,meta-aramid fiber?PMIA?has been widely used in transformer insulation systems.The oil-paper insulation system of the transformer will produce moisure,acid and other substances in actual operation.These products will reduce the mechanical properties of the insulation paper and accelerate its aging,which will cause damage to the safe operation of the transformer.Domestic and foreign scholars have conducted lots of research on how to extend the service life of transformer insulation systems.As an efficient and economical experimental method,molecular simulation technology has been widely used in the fields of physics,chemistry,biology,materials,etc.Based on molecular simulation technology,the law and mechanism of material change can be explained from the microscopic point of view,which will help us to understand more clearly the influence mechanism of water and acid substances on the mechanical properties and aging of PMIA insulation paper.It is important to study the influence of moisture and acid on the microscopic properties of PMIA insulation paper.Based on molecular simulation methods,the effects of moisture and acid on the microscopic properties of PMIA insulating paper amorphous regions were studied.First,an oil-paper composite system containing water and acid?formic acid,acetic acid,levulinic acid,naphthenic acid and stearic acid?was constructed.Based on molecular dynamics,the diffusion phenomenon and mechanism of moisture and acid were studied.On this basis,six different PMIA insulation paper composite systems with different water content and six different PMIA insulation paper composite systems were constructed.The mechanical properties of moisture and acid on PMIA insulation paper were analyzed from the microscopic point of view.The mechanism of thermal stability.Before studying the thermal decomposition of PMIA insulation paper,the balance of the simulation architecture was judged and the reaction force field was verified by the quantum mechanical method.Finally,the thermal decomposition process of PMIA insulation paper was simulated.At the same time,the thermal decomposition process of PMIA insulation paper under water and formic acid environment was simulated.The products of PMIA insulation paper thermal decomposition process were studied and analyzed.The pyrolysis kinetics of PMIA insulation paper in different environments was calculated and analyzed.The formation rules of the initial thermal decomposition intermediates and thermal decomposition products of PMIA under different environments were statistically analyzed.The main initial reaction processes and the main products were mainly generated.The path was tracked and analyzed.The main conclusions of this paper are as follows:?1?Under the normal temperature operating conditions of the transformer,the trajectories of water and acid in the oil-paper composite system were tracked,and the degree of diffusion of water and acid was calculated.The study found that the diffusion motion of water molecules is directional,and the tendency of moisture to diffuse from insulating oil to insulating paper.The strong intensity of acid diffusion from strong to weak is:formic acid,acetic acid,levulinic acid,naphthenic acid,stearic acid.Moisture and small molecular acids?mainly formic acid?in insulating oil easily diffuse into the interior of PMIA insulation paper,while macromolecular organic acids do not easily diffuse into PMIA insulation paper.?2?The microscopic mechanism of moisture reducing the mechanical properties and thermal stability of PMIA insulation paper is that a large number of hydrogen bonds formed between water molecules and PMIA fiber chains destroy the original hydrogen bond network of PMIA fibers.With the increase of water content,the number of hydrogen bonds between the molecular chains of PMIA fibers decreases with the increase of water content,and the interaction between PMIA fiber chains is weakened,which leads to the intensified chain motion of meta-aramid fibers.On the other hand,the degree of diffusion of moisture increases with the increase of its content,which accelerates the destruction rate of the original hydrogen bond network of PMIA fibers,resulting in a decrease in mechanical properties and a decrease in thermal stability of PMIA fibers.The mechanism of the effect of acid on the mechanical properties and thermal stability of PMIA fiber insulation paper is similar to that of moisture.The reason for the decline of mechanical properties of acid-containing PMIA insulating paper is that a large number of hydrogen bonds will form between acid and PMIA chain,which will destroy the hydrogen bonds between PMIA fiber chains,and aggravate the movement of PMIA cellulose chain and reduce the structural stability,leading to the decline of its mechanical properties.Formic acid enhances the ductility of meta-aramid fibers.?3?The analysis of the thermal decomposition process of PMIA fiber showed that the main initial cracking position of the meta-aramid fiber was N-H bond,C-N bond and C=O bond on the amide bond.As an important factor affecting the thermal decomposition of meta-aramid fiber,temperature rise will accelerate its thermal decomposition rate.The higher the temperature,the more complex the product.High temperature is conducive to the formation of stable gases.For example,HCN is the iconic gas for pyrolysis.The microscopic mechanism of moisture and acid accelerating the thermal decomposition of PMIA is that the moisture,formic acid and free radicals tend to redox with the N-H bond of PMIA,the C-N bond on the amide bond,and the C=O bond,which reduces the chemical stability of the PMIA fiber chain and makes the PMIA fiber chain prone to fracture and further decompose into smaller fragments.The main products?NH₃,H₂O,CO₂,etc.?in the initial aging stage of PMIA fibers were traced and analyzed.It was found that NH₂ group of PMIA is tend to capture adjacent H and form NH₃.The main formation route of H₂O formation is that the carbonyl group takes hydrogen to form a hydroxyl group,and then is formed by an intramolecular dehydration reaction or an intermolecular hydrogen abstraction reaction.The research show that formic acid can promote the production of CO,CO₂ and H₂O molecules in the initial stage of thermal decomposition of PMIA insulation paper,and the amount of CO produced in the acid environment is much larger than that of PMIA insulation paper in the other two environments.