Development Of Aramid/Natural Fiber Three-layer Crosslinked Composite Insulating Paper

As a classical composite insulation structure,oil paper composite system is widely applied in oil immersed transformer.Reducing the dielectric constant of insulating paper can improve the dielectric coordination of oil paper composite system,so as to improve the insulation performance of oil paper composite system;enhancing the thermal stability of insulating paper can prolong the service life of transformer.Therefore,reducing the dielectric constant and improving the thermal stability have always been the research hotspot of insulating paper modification.In engineering,natural fiber and synthetic fiber are combined to prepare composite insulating paper with excellent performance.However,the composite structure and interface characteristics of two-phase fiber are not fully considered in the existing research,and most of them sacrifice the mechanical properties of insulating paper,so they are not widely used.In addition,the existing insulating paper research and development still adopts the method of"trial"repeated experiment,and the direct scientific theory or simulation guidance is insufficient.Therefore,it is of great engineering value and theoretical significance to design and prepare a new type of natural fiber based composite insulating paper with low dielectric constant and high thermal stability by using scientific material research and development methods,starting from the molecular binding mode and composite structure.In this paper,a comprehensive quantitative evaluation method of fiber properties is constructed by using the parameters obtained from molecular dynamics simulation.The synthetic polymer as composite reinforcement phase is selected.Then,the effects of molecular combination mode,composite proportion and structure on the polarity and mechanical properties of fiber composite system are studied.The synthetic polymer/natural polymer with low dielectric constant and high thermal stability is formulated design scheme of fiber composite system;according to the design scheme,a new type of composite insulating paper was prepared,which was consistent with the composition and structure of the design scheme.The insulation,discharge characteristics and operation stability of the paper after oil immersion were studied,and the excellent performance of the paper as oil immersion insulation material in transformer was further evaluated.The main research results are as follows:(1)Molecular dynamics simulation results were used to characterize the fiber polarity,compatibility,mechanical properties and thermal stability,and then the comprehensive quantitative evaluation method of fiber properties was established.The method was applied to guide the composition and microstructure design of new composite insulating paper.The polarization and dielectric constant obtained by molecular dynamics simulation were used to characterize the polarity of the fiber system;Flory-Huggins interaction parameters,tensile modulus and K/G value were used to characterize the compatibility and mechanical properties of the fiber system;glass transition temperature,molecular mean square displacement,hydrogen bond and cohesive energy density were used to characterize the thermal stability of the fiber system.Based on the above three properties obtained by molecular dynamics simulation,a comprehensive quantitative evaluation method of fiber properties is established,which can be used to guide the design of the composition,molecular binding mode and composite structure of new composite insulating paper.(2)The molecular structure of PPTA/natural fiber composite with high performance and low dielectric constant was designed.By using the quantitative evaluation method of fiber performance based on molecular dynamics simulation,the type of polymer reinforcement phase,the combination mode with natural fiber molecules,the composite proportion and structure of the composite were optimized.The molecular structure characteristics of the new composite fiber material were determined:the upper and lower layers were composed by 10 wt.%PPTA+23.3 wt.%cellulose,and the middle layer was 33.3 wt.%cellulose.And the cross-linking"network"is formed by the esterification of butane tetracarboxylic acid(BTCA).(3)The process of preparing PPTA and natural fiber molecular crosslinking and three-layer composite insulating paper was put forward:the PPTA pulp was grafted with hydroxyl group,in accordance with the proportion of 20 wt.%of the total mass and mixed with wood pulp to construct a three-layer structure with 10 wt.%PPTA+23.3wt.%cellulose in the outer two layers and 33.3 wt.%pure wood fiber in the middle layer;then treated with 0.2 mol/L BTCA/Na H₂PO₂ solution at 180?for 15 min.A new type of PPTA/natural fiber composite insulating paper was prepared in the laboratory.The dielectric constant and dielectric loss of the new oil impregnated composite insulating paper are 3.21 and 0.392%,respectively,which are 19.4%and 33.2%lower than those of the pure wood fiber insulating paper.The tensile strength retention rate of the new oil impregnated composite insulating paper is 94.3%of that of the pure wood fiber insulating paper.(4)The electrical properties and accelerated thermal aging tests of PPTA/natural fiber composite insulating paper and mineral insulating oil were carried out.The results show that the new oil paper insulation has excellent electrical strength,partial discharge characteristics and electrical and thermal life,and has good compatibility with mineral insulating oil.In the oil paper composite structure with oil gap width of 0.5?3 mm,the short-time breakdown strength of oil new composite insulating paper is 24.6%higher than that of oil pure wood fiber insulating paper,and the initial field strength of partial discharge is 62.5%higher than that of oil pure wood fiber insulating paper.Compared with pure wood fiber insulating paper,the electrical life index of oil impregnated new composite insulating paper increased by 42.3%under the electric field of 60?90 k V/mm.During the accelerated thermal aging process at 130?,the thermal life of oil impregnated new composite insulating paper increased to 2.13 times of pure wood fiber insulating paper;during the aging process,the performance degradation of the new composite insulating paper and the aging process of mineral insulating oil is slower.The new composite insulating paper has good compatibility with mineral insulating oil.