| The aging of electrical tree of high-voltage power cable insulation is an important factor leading to the failure of the insulation.Under the combined effect of mechanical stress electric field,the probability of failure of cable insulation will be further increased.Small deformations such as sheet crystal slip may exist in semi-crystalline polymers under the effect of mechanical stress,and the change of microstructure will also have certain effects on the electrical properties of polymer insulation materials.In this thesis,the effects of mechanical stress on the AC electrical tree properties in the insulation layer,which are common in cable manufacturing,and the effects of composite insulation materials modified by nano-SiO2 filling on the AC electrical tree properties under stress were investigated to explore the mechanism of tensile stress on the AC electrical tree action of LDPE and SiO2/LDPE and the improvement mechanism.The results show that the unstretched LDPE has an obvious spherical crystal structure with a spherical crystal diameter of about 3～5μm,and the SiO2/LDPE composite has a loose spherical crystal structure with a size of about 1～2μm,and the spherical crystal structure of the LDPE material after the stress has an obvious orientation The spherical crystal structure of SiO2/LDPE composites is more stable than that of SiO2/LDPE composites,with the long-axis diameter reaching 8μm along the stress direction.The crystalline structure of the materials before and after the tensile stress treatment was characterized by differential scanning calorimetry（DSC）and X-ray diffraction（XRD）,and it was found that the tensile stress caused a slight increase in the crystallinity of LDPE and a slight decrease in the crystallinity of SiO2/LDPE.The AC electrical tree properties of parallel stress pins and vertical stress pins were tested,and the results showed that the direction of electrical tree growth and the direction of stress action showed an orthogonal relationship,and the transverse extension ratio of the electrical tree before and after modification with nano-SiO2 was about 11.5:7 for parallel stress pins,and the transverse extension range of the electrical tree was significantly reduced.The electrical tree structure of the LDPE samples with vertical stress pins showed an obvious tree structure,while the SiO2/LDPE samples with vertical stress pins showed a clumped dendritic structure along the electric field direction.The results of the combined local discharge test showed that the hysteresis period of the SiO2/LDPE samples was significantly longer than that of the LDPE samples,and the analysis suggested that the nano-SiO2 changed the microscopic crystalline structure of the LDPE matrix and had the ability to homogenize the electric field.Compared with the LDPE samples,the modified SiO2/LDPE samples have significantly improved the local discharge resistance. |
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