Su Jingang, Wei Liqiang, Zheng Jingquan, Liu Jiahao, Zhang Peng, Pang Xianhai, Xing Yunqi
Electric Power Research Institute of State Grid Hebei Electric Power Supply Co., Ltd., Shijiazhuang 050021, China.
State Key Laboratory of Reliability and Intelligence of Electrical Equipment, Hebei University of Technology, Tianjin 300123, China.
Polymers (Basel). 2022 Jul 20;14(14):2927. doi: 10.3390/polym14142927.
Mechanical stresses generated during manufacturing and laying process of high voltage cables can result in degradation of insulation properties, affecting the stable operation of the transmission system. Traditional test methods for testing the effect of mechanical stress on the insulation properties of polyethylene still have some shortcomings to be explored and it is able to explain the changes of the insulation properties of polyethylene under mechanical stress from a microscopic perspective. In order to further study the effect of stress on the insulation properties of polyethylene, microstructural changes, the breakdown field strength, conductivity and charge distribution of polyethylene at different elongation rates are investigated by a combination of experimental and molecular dynamics simulations. The results show that the increase in stress leads to a decrease in crystallinity and microcrystalline size of the material decrease. The untwisting and orientation of the polyethylene molecular chains during the stretching process can create cavities, resulting in an uneven sample distribution and thickness reduction, leading to a reduction in the breakdown field strength. Meanwhile, some crystal regions are transformed into amorphous regions. The loose amorphous regions facilitate the directional migration of carriers, resulting in the increase of conductivity. When the elongation ratio is smaller, the distance between the molecular chains increases and the trap depth of the specimen becomes shallower. This facilitates the migration of ions and electrons and increases the rate of decay of the surface potential. When the stretch is further increased, new traps are created by broken molecular chains to limit the movement of charges, decreasing the decay rate of the surface potential and reducing the insulation properties of the polyethylene. Meanwhile, the molecular dynamics model of semi-crystalline polyethylene was developed to observe the microstructure and energy changes during the stretching process. The conclusions in terms of tensile tests were verified from a microscopic perspective.
高压电缆制造和铺设过程中产生的机械应力会导致绝缘性能下降，影响输电系统的稳定运行。传统的测试机械应力对聚乙烯绝缘性能影响的方法仍存在一些有待探索的缺点，且无法从微观角度解释聚乙烯在机械应力作用下绝缘性能的变化。为了进一步研究应力对聚乙烯绝缘性能的影响，通过实验和分子动力学模拟相结合的方法，研究了不同伸长率下聚乙烯的微观结构变化、击穿场强、电导率和电荷分布。结果表明，应力的增加导致材料的结晶度降低，微晶尺寸减小。拉伸过程中聚乙烯分子链的解缠和取向会产生空洞，导致样品分布不均匀和厚度减小，从而导致击穿场强降低。同时，一些结晶区域转变为非晶区域。疏松的非晶区域有利于载流子的定向迁移，导致电导率增加。当伸长率较小时，分子链间距增大，样品的陷阱深度变浅。这有利于离子和电子的迁移，增加表面电位的衰减速率。当拉伸进一步增加时，断裂的分子链会产生新的陷阱，限制电荷的移动，降低表面电位的衰减速率，降低聚乙烯的绝缘性能。同时，建立了半结晶聚乙烯的分子动力学模型，以观察拉伸过程中的微观结构和能量变化。从微观角度验证了拉伸试验的结论。
