The Micromechanism On The Doping Modification Of Nano SiO₂ To The Oil-immersed Cellulose Insulating Paper

The oil immersed power transformer has been widely used in power grid because of its advantages of low loss,good heat dissipation and superior insulation performance.The internal insulation system of oil immersed power transformer is composed of insulating oil and insulating paper.The performance of the insulation system directly determines the service life of the transformer.With the continuous increase of transmission voltage level in China,the performance requirements of oil paper insulation inside the transformer are also increasing.The traditional oil paper insulation system has been difficult to meet the requirements of today's high-voltage transmission.In the transformer,the insulating paper can't be replaced during the operation of the transformer,and the insulating oil can be replaced by the way of replacing the insulation performance.So,improving the insulation paper's performance is the key step to improve the performance of the oil paper insulation system.In recent years,the research on the modification of nanoparticles for polymer insulating materials has become a research hotspot.Nanoparticles have also been applied in the field of cellulose insulating paper,but only from the macro test point of view,it is difficult to obtain the micro mechanism of the interaction between the nanoparticles and the cellulose insulating paper.In addition,moisture is the biggest“enemy”of insulating paper aging except for temperature.At present,the study of using nano particles to modify insulating paper has not yet considered how to better inhibit the influence of moisture on the performance of insulating paper.Therefore,it is of great practical significance and academic value to explore a method which can not only improve the performance of insulating paper but also restrain the effect of moisture on the performance of insulating paper and analyze its micro mechanism.The micromechanism on the doping modification of nano SiO₂ to the Insulating cellulose paper immersed by transformer oil was studied in this paper.Firstly,molecular dynamics and quantum chemistry methods were used to study the cellulose amorphous model of different degree of polymerization.From the aspects of physical and chemical properties,the minimum degree of stability of the model was obtained.Then,the molecular dynamics method was used to study the effect of nano SiO₂ with different shapes and sizes on the modification of cellulose,and the better shape and size of the nanoparticles were obtained.Finally,the interaction between the nano SiO₂with the surface grafting of melamine and the oil immersed cellulose insulating paper was studied under the conditions of heat,moisture and electric field from the microcosmic angle of molecular dynamics and quantum chemistry.The main conclusions of this paper are as follows:?1?Based on the steady state minimum polymerization degree method to establish the amorphous model of cellulose:The mechanical properties and chemical properties of cellulose amorphous models with different degrees of polymerization were studied in two aspects.The results show that the mechanical properties of the established amorphous models tend to be stable when the degree of polymerization of cellulose is greater than 10.The minimum degree of polymerization is 8 when the solubility parameters tend to be stable,and the minimum degree of polymerization is 2 when the front orbital energy levels tend to be stable.Finally,the steady-state minimum polymerization degree is 10 when the cellulose amorphous model is established.The established model takes account of the physical and chemical properties of the material and represents the real performance of the insulating paper cellulose.Moreover,the efficiency of the simulation study can be improved by selecting the minimum steady-state degree of polymerization for modeling and calculation.?2?Research on the influence of the shape and size of nano SiO₂ on the modification of cellulose:The study of molecular dynamics simulation shows that the increase of the size of the nanoparticles leads to the decrease of mechanical properties of composite models and surface area bond energy of nano particles and cohesive energy density gradually,the increase of binding energy.When the nano volume is523.61?³,the solubility parameter of the composite model is close to that of cellulose.Especially,the solubility parameter of the composite model doped with spherical nanoparticles is most close to that of cellulose.In summary,when spherical nano SiO₂particle radius is 5?,the doping effect is better of modified cellulose insulating paper.?3?Research on the effect of nano SiO₂ on the thermal stability of oil immersed cellulose insulating paper:The results of molecular dynamics simulation show that the mechanical propertiy of the model is better when the content of nano SiO₂ particles is5%.The nano SiO₂ particles of surface unsaturated Si atoms are treated with-OH,and unsaturated O atoms are treated with H are closer to the real situation.The apparent jump temperature of the mean square displacement and free volume fraction in the modified model is about 50 K higher than that of the unmodified model.Under the condition of water,melamine grafted nano SiO₂ modified cellulose model has the relative optimum mechanical properties and the glass transition temperature is increased by 69 K compared with the unmodified model.With the increase of temperature,the number of hydrogen bonds in the model is decreasing,but adding nano SiO₂ can protect the cellulose chain better,and make the connections between cellulose and inside cellulose are not destroyed by water molecules.The nano SiO₂particles grafted with melamine on the surface have a better effect on the mechanical properties of oil-cellulose under water environment.When the temperature is 650 K,the modulus of elasticity and the shear modulus of the modified model are increased by53.3%and 71.6%,respectively,compared with those of the unmodified model.The mean square displacement and the trajectories area of the water molecules in the modified model show the first obvious jump temperature and the obvious change temperature increased by 50 K compared with the unmodified model.The apparent jump temperature of free volume fraction in the modified model is about 60 K higher than that of the unmodified model.The comparison between the simulation results and the experimental results indicate that the thermal stability of modified insulating paper is better when the content of nano SiO₂ grafted on melamine is 5%.?4?Research on the effect of nano SiO₂ on the electrical properties of oil immersed cellulose insulating paper:The microscopic breakdown characteristics of cellulose-oil model?CY?,cellulose-oil-water model?CYH?and the nano SiO₂ with the surface grafting of melamine modified cellulose-oil-water model?CYHN?were studied by quantum chemistry.The results show that the electric field strength of HOMO energy difference and energy difference appear to increase obviously in the model is CYHN>CY>CYH.In the initial state,the energy gap of the CYHN model is2.24 times and 2.57 times as much as that of the CY and CYH model respectively.As the strength of the electric field increasing,the valence electrons begin to become free electrons and the transition phenomenon of free electrons appears,and eventually the conduction band and the valence band are joined together to lead to complete breakdown.However,the electric field intensity of the energy gap decrease obviously of the CYHN model is still the greatest.The comparison between the simulation results and the experimental results show that when the nano SiO₂ particles content is 5%,the breakdown voltage of melamine grafted nano SiO₂ particles modified cellulose insulating paper is the highest.From the macroscopic and microcosmic point of view,the performance of the nano SiO₂ with the surface grafting of melamine on the insulating paper is improved to improve the breakdown properties.