Study on molecular structure change and micro-mechanism of electrical aging of XLPE dielectric materials under external electric fields

In order to analyze the electrical tree aging of cross-linked polyethylene (XLPE) material from the micro views, this paper adopts molecular simulation method to calculate and optimize the XLPE molecular structure. Exerting different grades electric field intensities along the polyethylene chain, the geometrical structure, dipole moment, polarizability, charge distribution, frontier orbital energy and infrared spectrum of the cross-linked polyethylene molecule were analyzed. The calculation results show that infrared spectrum of cross-linked polyethylene molecular changes greatly with the increase of the external electric fields. When the external electric field intensity reaches 0.026 a. u., virtual frequency appears in infrared spectrum. This indicates that the molecular spatial structure is no longer stable and prone to bond breaking. In addition, from the change of frontier orbital diagram, it is found that the bond breaking occurs firstly at the ends of polyethylene chains. Along the direction of the electric field, the atomic charge amount is transferred from the junction to the end. When the exerted electric field reaches 0.029 a. u., the C-H and C-C bonds at the end of the chain break to produce H · and CH₃ · radicals. The free radicals form the space charge and accumulate, resulting in a larger local electric field, which further influences the spatial structure of the cross-linked polyethylene chain. The change of dielectric micro characteristics will lead to the decrease of insulation property of XLPE material. These changes have important significance for revealing the microscopic rules of electrical tree formation. (authors)