Study on the Mechanism of Reverse Surface Flashover of Insulators

It is very important to understand the mechanism of surface flashover characteristics of the insulator materials in vacuum in order to avoid the surface discharge of sensitive components. There are two models with dominant roles and highly reorganization in academic circle. One is secondary electron emission avalanche model (SEEA) and the other is electron triggered polarity relaxation model (EPTR). However these models are still not understood clearly and there is no model which can completely explain all the phenomenon of surface flashover in vacuum, especially the reverse surface flashover phenomenon. In this paper, DC surface flashover experiments of insulator are studied in vacuum to reveal the reverse surface flashover mechanism. Results showed that there is a phenomenon in which a reverse flashover current sometimes occurs in the surface flashover process. The reverse flashover current pulse may occur in both the initial pre-flashover phase and the flashover breakdown phase, that is, when the flashover current pulse is positive, the surface flashover voltage is negative. The experiments results can be explained as follows. The electrons injection phenomenon near the cathode and holes injection phenomenon simultaneously near the anode are formed in the high electric field intensity at the triple junction (cathode end, vacuum and dielectric surface, CTJ). The electron-hole pairs migrate to the anode and cathode in the applied electric field intensity, respectively. Some of the carriers in the migration are captured by the trap center, forming the positive and negative charge trapping center. The region between the trap center and the electrodes will form an additive electric field. The negative charge trapping center weaken the local region electric field between cathode and negative charge, forming a reverse electric field to limit further injection of charge. At the same time, the trapping center enhanced the additive electric field between the anode and the trapping center. The region of the space charge electric field formed by the center of the negative charge is completely opposite to the applied electric field. When the negative charge trapping center value is sufficiently deep, the reverse additive electric field intensity will reach a critical of flashover breakdown value in the region. The partial discharge usually cannot occur due to the weakening of the high external electric field intensity. However, the applied electric field intensity of the insulator is instantaneously reduced during the surface flashover breakdown. At the same time, the electrons in the trapping center are not detrapping, so the electric field intensity of the reverse in the cathode and trapping center will rise rapidly. As a consequence of the electric field intensity reaching the critical breakdown value, a reverse partial discharge is formed immediately. Unlike the usual case, the electrons will escape from trapping center and transfer towards cathode under the action of reverse electric field intensity in the process, forming a reverse surface flashover current pulse. The reverse flashover current pulse typically occurs at a time when the applied electric field intensity value is low in the flashover process. It is suggest that the traps will play a critical role in affecting the reverse surface flashover performance under DC voltage in vacuum.