| In alternative energy source power system,meta-aramid has been regarded as an ideal material for novel insulating paper due to its excellent insulating,mechanical and thermodynamic properties,and shows broad prospects in transformer,bushing and other essential transmission and distribution devices.However,mainstream meta-aramid products on the market are still monopolized by US and Japan,which imposes a heavy burden on the national economy.In recent years,under the leading of our Party’s policies,domestic enterprises rise mushrooming,with fiber production and paper making industry climbing rapidly.However,the acceptance degree of domestic aramid papers in civil power system is not ideal.Conventional devices cannot afford the high cost,and high-end devices worry about the risk of insulation substitute.It does be a dilemma faced by enterprises and our scientific researchers of meta-aramid materials that domestic aramid paper is still not accepted by power grid.Recently,domestic aramid products target for the high-end imported ones such as Nomex and Cornex series,showing obvious traces of process migration and formula iteration guided by limited production experience.However,previous researches on aramid paper mainly focused on the structural,thermal and mechanical properties,and rarely concerned about the electrical insulation,not to mention effective strategies for insulation improvement.Accordingly,this thesis conducted the research of "Study on Electrical Properties and Modification Methods of Meta-Aramid Paper for High Voltage Insulation",and the main contents are as follows:(1)The insulating performance and material damage mechanism of metaaramid paper in typical insulating structure were studied.Firstly,the breakdown strength of aramid paper under dry-type,oil-immersed type,single-layer and laminated structure was tested.It was found that the breakdown strength of aramid paper first increased and then decreased with the increase of paper thickness,and the difference between domestic paper and the Nomex increased with the thickness.The breakdown strength of the laminated paper decreased monotonically with the layer number increasing.Furthermore,it appeared that in the dry-type structure,the breakdown strength of an one-layer thick paper was much higher than that of a multi-layer thin paper structure with the same overall thickness,and the relative value relation reversed when there came the oil-immersed condition.The difference between domestic paper and the Nomex became more significant in the oil-immersed laminated structure.Then,the partial discharge(PD)characteristics of the laminated oil-immersed aramid paper were studied,and it was found that the domestic paper with low PD resistance showed the air-gap discharge symbol.The insulation performance of domestic aramid paper was considered to be affected by its serious interface defects of air gap.When the paper thickness is large or it is immersed with oil,the domestic paper will show a significant breakdown decline than the Nomex.Finally,based on reactive molecular dynamics,the damage of meta-aramid under discharge temperature was simulated,and the product statistical law together with the decomposition path of the typical groups were analyzed,the effect of gap components as O2 and H2O on promoting damage was analyzed,and the insulation deterioration of meta-aramid paper caused by air gap was explained.(2)The key factors and influencing laws of insulation performance of wet-laid meta-aramid paper were studied.Firstly,the wet-laid paper making platform and the electrical insulation test platform were built.Then,the effect of chopped fiber parameters on paper properties was studied.Within the common parameter range,the insulation strength of paper showed a decreasing trend with the increase of fiber diameter and fiber length or the decrease of crystallization temperature,and the max decrease of breakdown strength was 64%.The breakdown strength increased first and then decreased with the increase of the proportion of chopped fiber.In testing,it was observed that the breakdown decreasing related to paper flaws as fiber interface debonding,precipitated fibird hollowing,and chopped fiber warping.Then,the effect of pulp composition was studied,and it was found that PEO decomposed more thoroughly than functional cellulose in hot pressing,and thus was more suitable as the dispersing agent for aramid paper.The optimal dosage of PEO was 0.4 wt%.Decreasing or doubling the dosage would lead to loose interface and PEO residue,respectively.The ions and insoluble impurities in the pulp showed mutual retaining,resulting in the increase of conductivity and shallow traps in the aramid,thus reducing the paper insulation.Then,the influence of hot pressing parameter was studied,and it was found that 270℃ and 14 MPa of hot pressing parameters could produce aramid paper with dense structure and high breakdown strength.Increasing the temperature would melt the chopped fiber and cause the embrittleness of the precipitated fibrid,while decreasing temperature or pressure would form loose interface defects.Through the factor traversal in this part,it can be considered that the fiber interface status plays a decisive role in the electrical insulation performance of paper.It is an anticipated effective method to improve the insulation performance of aramid paper by modifying the defect structure of fiber interface.(3)The methodology for improving the insulation performance of meta-aramid paper based on fiber interface modification was proposed.Firstly,the plasma discharge treatment platform and chemical solvent treatment platform of aramid chopped fiber were built.Then,a method of hydrogen bond coupling modification of fiber interface based on co-treatment with plasma discharge and coupling agent was proposed,and the effect of fiber modification on the paper performance was studied.It was found that the plasma activated the fiber surface through addition reaction of C=O,which was conducive to the coupling agent grafting.The increase of breakdown and tensile strength of aramid paper reached 25%and 13%,respectively after using terminal amino coupling agent AP.The molecular simulation showed that the interface modification effect of the coupling agent depended on the hydrogen bond donor property of its terminal group.The stronger the donor property of the amino group,the easier for it to form hydrogen bond with the carbonyl O atoms of aramid,which will also reflect the optimal modification effect of paper.In view of the fact that the interface coupling grafting modification causes damage to the fiber body and reduces the improvement effect of mechanical properties,we aim to realize a more low-loss modification path.Thus,a method of regulating fiber polarity based on polycation PDDA treatment was proposed,and the effect of fiber polarity on the aramid paper performance was studied.It was found that polycations could be adsorbed on the fiber surface by electrostatic force,to promote the adhesion of precipitated fibrids and chopped fibers.The increase of the breakdown and tensile strength of aramid paper reached 32%and 35%,respectively.The effect of electric polarity regulation originates from the electrostatic attraction between the N atoms of different PDDA units and the carbonyl O atoms both in precipitated and chopped fibers,which forms a non-covalent interaction across the interface,can form more stable interfacial interactions than interfacial grafting(4)The methodology for improving insulating and thermal-conductive properties of meta-aramid paper based on BN composition was proposed.Firstly,the compatibility between BN composite and wet process was studied,the relationship between mesh number and filler size was clarified,and the adverse effect of high mesh number on the breakdown strength of aramid paper was determined.Then,the effect of the coupling agent on the properties of the composite paper was studied.It was found that without the use of coupling agent,although the thermal conductivity of the composite paper increased from 0.22 Wm-1K-1 to 0.39 Wm-1K-1,the breakdown strength decreased by 34%.After coupling agent modification,the insulation performance of composite paper was improved.The terminal amino coupling agent AP and the terminal fluoroalkyl coupling agent TFO showed relatively optimal effects,and the breakdown increase reached up to 30%compared with pure aramid paper.Then,based on the molecular dynamics analysis,the influence of the interface components of filler and aramid on the composite system was analyzed,and the screening strategy of coupling agent in the composite modification was further obtained.Finally,the effect of filler size compounding on the properties of the composite paper was investigated,and it was found that the optimal values of particle size compounding in terms of thermal conductivity and breakdown field strength did not coincide,which was attributed to the additional requirement of nanofillers for defect-containing wet-laid aramid paper.(5)The methodology for improving insulating and thermal conductive properties of meta-aramid paper based on AlN composition was proposed.Firstly,the effect of AlN modified by thermal oxidation ang coupling agent on the aramid paper properties was studied.It was found that after doping 15 wt%modified AlN,the thermal conductivity of the composite paper increased to 0.32 Wm-1K-1,but the insulation strength increased by only 3%,which reflected the limitation of the effect of anti-hydrolysis modification of AlN.Based on this,the matrix nano splitting and AlN composition method were combined,so that AlN and aramid nanofibers could be dispersed and composited in an organic environment,and the interface thermal resistance could be reduced on the premise of avoiding AlN hydrolysis.An aramid composite paper with high compactness,thermal conductivity(1.96 Wm-1K-1)and high breakdown strength(69 kV/mm)was prepared.The maximum thermal conductivity and breakdown strength were reached at the same particle size ratio.Finally,the representative volume element method was used to analyze the synergistic regulation mechanism of the filler size grading on the electrical and heating flow field in the composite system.Academically,this study provides theoretical guidance for the comprehensive modification of meta-aramid insulating paper,providing a universal method for the modification of nonwoven paper products.In terms of engineering aspects,this study provides experimental support for the development of high performance domestic meta-aramid paper,providing referable experience for improvement and application monitoring of high-performance insulating paper. |
PDF Full Text Request