外电场下交联聚乙烯电介质材料分子结构变化及其电老化微观机理研究

为了从微观角度分析交联聚乙烯(XLPE)材料的电树枝老化,本文采用分子模拟方法计算并优化得到了XLPE分子结构.沿着聚乙烯链施加不同大小电场强度,分析交联聚乙烯分子的几何结构、偶极矩、极化率、电荷分布、前线轨道能量和红外光谱变化规律.计算结果表明,随着外电场的增大,交联聚乙烯分子红外光谱发生较大变化;当外施电场达到0.026a.u.后,红外光谱图中出现虚频,表明分子空间结构不再稳定,易发生断键;另外从前线轨道图的变化可以看出断键现象最先发生在交联聚乙烯链端部;沿着电场方向,原子所带电荷量由交联处向端部转移,当外施电场达到0.029a.u.后,链端部的C-H和C-C键断裂产生H·和CH_3·自由基.游离的自由基会形成空间电荷并发生积聚,产生局部较大场强,从而进一步影响交联聚乙烯链的空间结构.而电介质内部微观特性的变化必定会导致交联聚乙烯材料绝缘性能的下降,这些变化对揭示交联聚乙烯电缆电树枝形成的微观规律具有重要研究意义.

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 dis-tribution, frontier orbital energy and infrared spectrum of the cross-linked polyethylene molecule were analyzed. The cal-culation results show that infrared spectrum of cross-linked polyethylene molecular changes greatly with the increase of the external electric field. When the external electric field intensity reaches 0.026a.u., virtual frequency appears in in-frared spectrum. This indicates that the molecular spatial structure is no longer stable and prone to bond breaking. In addi-tion, from the change of frontier orbital diagram, it is found that the bond breaking occurs firstly at the ends of polyeth-ylene 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 0.029a.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 char-acteristics 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.