Study On Enhancement Effect Of Alumina Filler Morphology On Performance Of Aramid Nanofiber Insulating Paper

Transformers play a vital role in the high-speed development of UHV transmission in China.Nowadays,with the development of UHV transmission technology,the demand for transformers in safe and stable operation is increasingly strong.The most important part of transformer insulation is insulating paper.Because of the poor insulation strength of traditional insulating paper,it cannot meet the requirements of UHV transmission technology.Because of its good insulation performance and high temperature stability,aramid insulation paper has been widely concerned,but its performance still needs to be improved,so aramid composite insulating paper has become one of the research hotspots.Most of the current research has focused on the enhancement of insulation properties by different filler types,but little attention has been paid to the influence of the filler shape itself on insulation properties,and the mechanism of the influence of different filler shapes on insulation properties is not yet clear.In this thesis,aramid fibres with excellent overall performance and different shapes of alumina are selected as raw materials to prepare aramid composite insulating materials with excellent insulation performance.The effect of filler shape on insulation performance is determined by dielectric strength and volume resistivity,and finally the effect of different shapes of alumina on insulation performance is explained by simulation.Firstly,the aramid fibres were nano-fused by alkali fusion,and the aramid-based composite insulating paper was prepared by vacuum-assisted filtration after mixing the aramid nanofibres and spherical alumina,and the effect of spherical alumina on the insulation properties of the aramid paper and the high-temperature ageing performance of the composite insulating paper was analysed.The results showed that the addition of spherical alumina helped to improve the insulation properties,as well as the high temperature resistance and high temperature ageing of the aramid paper.The highest dielectric strength（172.3 k V/mm）,volume resistivity(3.53×10¹³Ω·cm)and tensile strength（145.87 MPa）were obtained when the proportion of spherical alumina was 5%.The use of silane coupling agent（KH-550）treatment to improve the dispersion of the spherical alumina can further improve the insulation properties of the aramid paper.Secondly,αandγphase alumina flakes were prepared by high-temperature calcination.α-Al₂O₃ was stable at high temperatures,whileγ-Al₂O₃ was wear-resistant and easily dispersed,and was blended with aramid nanofibres to produce aramid composite insulation paper to investigate the effect of the flake alumina on the insulation properties.The results show that the addition of flaked alumina helps to improve the insulation properties of the aramid paper.Compared to spherical alumina,flaked alumina has a more significant effect on the insulation properties,with the optimum addition ratio of 5%,corresponding to a maximum dielectric strength of 188.8 k V/mm（α-Al₂O₃）and 182.6 k V/mm（γ-Al₂O₃）,and a maximum volume resistivity of 3.90×10¹³Ω·cm（α-Al₂O₃）and 3.74×10¹³Ω·cm（γ-Al₂O₃）,and the maximum tensile strength is 155.76 MPa（α-Al₂O₃）and 149.42MPa（γ-Al₂O₃）.The flake alumina was treated with silane coupling agent（KH-550）to improve both the dispersion and the insulation properties.Finally,Comsol software was used to simulate the improvement of insulation performance of arylon matrix composites with different morphologies of alumina.Two models of arylon insulating paper with different morphologies of alumina were established to explain the improvement of insulation performance caused by different morphologies of alumina from three aspects:electric field strength,leakage current density and energy density.The breakdown paths of the two morphologies of alumina were analyzed.The results show that,compared with spherical alumina,flake alumina has great advantages in homogenizing electric field,reducing leakage current density and system energy density,and can better improve the penetration resistance of aramid insulating paper.