Molecular Dynamics Simulation Of Electrothermal Performance Of Polyester Nanocomposite Coating Of DC GIL Insulator

Large-capacity and long-distance HVDC transmission technology is one of the major issues in my country's energy development in recent years.Compared with traditional overhead lines and cables,DC GIL has a simple structure,large transmission capacity,more flexible layout,and high reliability.In addition,it can isolate the adverse effects of high-voltage transmission lines on the external environment.However,during the operation of the DC GIL,the insulator is subjected to a unipolar electric field,and it is easy to accumulate charges on the surface area and cause local electric field distortion,which in turn causes surface flashover.Therefore,it is of great significance to study effective methods for regulating and suppressing the charge accumulation on the surface of DC GIL insulators for reliable design of DC GIL insulation.With the wide application of gas insulation equipment such as GIS/GIL,many optimization measures for the insulation problem of insulator interface have been tried in the field of DC GIL.At present,the application of a coating on the surface of the DC GIL insulator to improve the dielectric strength is still one of the main strategies to suppress the charge.In this paper,based on molecular dynamics methods,modeling and research are conducted on the optimization and design of the electrical and thermal properties of insulator coating materials.By coordinating microscopic and mesoscopic molecular simulation systems,the insulation failure mechanism and reaction path of polyester nanocomposite coating materials are analyzed,and the improvement law of thermal conductivity of PET nanocomposite materials is explored.The main research contents are as follows:(1)Based on ReaxFF reaction molecular dynamics method,the insulation failure mechanism of PET in DC GIL under high temperature is studied.Through the pyrolysis simulation of the multi-molecular PET system at different temperatures,the initial cracking mechanism of its reaction path is obtained,and the product distribution of the pyrolysis process and the formation mechanism of the main gas products are analyzed at the atomic level.The early cracking reaction mainly produces PTA,and the amount of production increases as the degree of polymerization decreases.The breaking of the C-O bond on the main chain is the main reason for the decrease in the degree of PET polymerization and insulation aging in the early stage.In the early stage of the PET cracking reaction,one of the main gas products,C2H4,is generated and accumulated in large amounts along with the breakage of the main chain,resulting in voids in the PET.In the late stage of the reaction,the cracking products are mainly CO2,mainly from the decarboxylation of PTA and PTA acid.(2)Based on the high-temperature cracking simulation results,PET molecular models with different degrees of polymerization are constructed,and molecular dynamics simulations under different field strengths are carried out.Comparing the different results under the electrolysis cracking simulation,the main action mechanism of high field strength in the cracking process is analyzed.There is no obvious difference in the cracking reaction path of PET under the action of electricity and heat,but there is a clear threshold for the resistance of PET to field strength,and it will decrease with the increase of aging.The increased degree of polymerization will obviously enhance the resistance of PET molecules to field strength.When the field strength exceeds the threshold,the cracking process will form positive feedback under the field strength and accelerate the cracking.At this time,the thermal cracking and electrical cracking of PET will promote each other,thereby accelerating the aging process of PET.(3)The coarse-grained models of polyester film composites with different doping concentrations of PET/BN and PET/AlN(lwt%,3wt%,5wt%,7wt%,10wt%,13wt%)are established.Simulate its thermal conductivity under the same conditions.The results show that the thermal conductivity of polyester film composites doped with nano-boron nitride and nano-aluminum nitride is improved,and the thermal conductivity of the composite film shows a rapid increase with the change of doping concentration.When the content of nanoparticles is low(lwt%,3wt%),the effect of different nanoparticles on the thermal conductivity of the composite material is not much different,and the improvement effect is not obvious:after increasing the concentration of nanoparticles,the thermal conductivity of the nanoparticles increases.The higher,the higher the thermal conductivity of the compound model mixed with it.At high doping concentration,the composite material doped with nano AIN has more obvious lifting effect.The molecular simulations used in this paper mainly revolve around the micro level and meso level,which is a step closer to the single micro dimension of traditional molecular simulation.By microscopically exploring the reaction mechanism of the pyrolysis process under the action of PET and the effect of the product on the aging process,the molecular model is coarse-grained,and the thermal conductivity of the PET composite model is studied.The coarse-grained force field is used to combine the results of the electrothermal cracking simulation and the results obtained from the PET composite material,so that the simulation results can be more restored to the actual running situation,and the multi-scale computer simulation study can be realized.It lays the foundation for more in-depth research and further optimization of the relevant properties of DC GIL insulator nanocomposite coating materials.