真空中绝缘子闪络前表面带电现象的仿真研究

真空中绝缘子发生沿面闪络之前存在绝缘子表面的带电现象，该现象对闪络的发展具有重要影响，到目前为止对该现象进行实时测量还存在很大的难度。基于二次电子发射雪崩(secondary electron emission avalanche，SEEA)模型，利用Monte Carlo法研究了真空中圆柱型和圆台型绝缘子在闪络前表面电荷密度的二维分布。仿真中采用了氧化铝陶瓷、聚四氟乙烯(PTFE)、聚酰亚胺(PI)以及聚甲基丙烯酸甲酯(PMMA)等不同绝缘材料。考察了绝缘材料、施加电压以及圆锥绝缘子不同锥角对表面电荷密度和分布的影响。仿真结果表明，在靠近阴极处的绝缘子表面存在小区域的负电荷区，而后变为较大区域的正电荷区；二次电子发射系数较小的绝缘子表面的正电荷密度较小；随外施电压升高，负电荷的密度及区域减小，而正电荷的密度及区域增大，且正电荷区域的峰值向靠近阴极方向移动；圆台绝缘子的锥角为负时其表面正电荷密度大于锥角为正时的情况，当锥角在-22.5°~-30°之间时表面正电荷密度达到最大，而此时对应的闪络电压最低。仿真结果与实验结果有较好的对应关系。

Simulation of Surface Charging Phenomena on Insulator Prior to Flashover in Vacuum

Before flashover across an insulator in vacuum, there is charging phenomena on the insulator surface, which significantly affects the developing process of flashover. However, up to now it is still very difficult to measure the phenomena with applying voltage. Based on the secondary electron emission avalanche (SEEA) model, and by using the Monte Carlo method, a two-dimensional analysis of surface charge density on cylindrical and conical insulators prior to flashover has been performed in vacuum. Different materials are employed, i.e., alumina ceramic, Polytetrafluoroethylene (PTFE), Polyimide (PI) and Polymethyl Methacrylate (PMMA). The influences of different materials, applied voltages and conical angles on surface charge distribution are investigated. The results reveal that, negative charges exist in a small surface region near the cathode and then turn to positive charges in a large surface region away from cathode. And the surface positive charge density is lower for the insulator with the small secondary electron emission rate. While increasing the voltage applied on the insulator, the density and area of negative charges decreases, but the density and area of positive charges increases, and the peak of positive charge region moves towards the cathode. For the conical insulator with negative conical angle, its surface positive charge density is greater than that with positive conical angle, and while the angle is between -22.5° and -30°, the surface positive charge density reaches the crest, corresponding to a lowest flashover voltage. The simulation results are consistent with experiments.