Study On Pulsed Flashover Characteristic Of Epoxy Composites In Vacuum

In the field of high power pulse technique, surface flashover typically takes place on the surface region of solid insulating materials at applied electric stress much lower than the bulk breakdown strength of the material. Due to the requirement of higher power, higher capacity and miniaturization for the pulse power devices in future, surface flashover in vacuum insulation material becomes an important problem in the development of pulse power technology. Therefore, the vacuum insulation material needs to be improved for the pulse power equipment, and the effects of dielectric property, filler scale and applied electric field on the flashover characteristic of epoxy composites need to be investigated. From different microscopic and macroscopic views, the behaviors of charge injection, transportaion and distribution of epoxy-based material for pulsed flashover were investigated. The deep and shallow trap model based on the multi-layer structure of interface was proposed to explain the vacuum flashover properties of epoxy-based material. The formula describing the relationship between flashover voltage and pulse steepess base on the electron emission effect was also proposed. The main results are shown as follow:Different flashover characteristics of epoxy composite filled different micro fillers, such as Al₂O₃·3H₂O, Al₂O₃, BaTiO₃ and TiO₂ micro particles, were studied, and the flashover mechanism was investigated. Considering to the electron emission process of electrode material, the influence of filler type on the charge injection and the influence of fillers'dielectric property on the triple-joint electrical field, the rule of charge generation was understood. Charge transportation process was studied by the measurements of secondary electron emission coefficients and thermal stimulated depolarization current. The surface charge distribution on different materials after flashover was measured, and the influence on the flashover voltage due to surface charge distribution was analyzed. Furthermore, a deep and shallow trap model is proposed, which is able to explain the charge generation and transportation process during the different stages of flashover.The microcomposites, nanocomposites and nano-mico-composites loaded with the filler of Al₂O₃·3H₂O particles show different flashover performance, and the effect of filler size on the flashover performance of epoxy-based material was discussed based on the size influnence. Firstly, the typical interface structure was described, and the coupling agent effect was emphasized. Secondly, the parameters of interface volume proportion and specific surface area were calculated based on the simplified cubic model. The influence of filler size and content on the interface structure, interface layer distribution and interface volume proportion was discussed. The distribution of trap energy level in the interface of particles was analyzed. The relationship between particle size influence and flashover voltage was analyzed. At last, the deep and shallow trap model based on the multi-layer model was proposed. Considering the size influnence of fillers, the flashover characteristic is able to be explained.Considering the effect of different pulse steepness, the flashover characteristic of pure epoxy under different pulse steepness was investigated. The relationship of flashover voltage and pulse steepness could hardly to explain the physical process based on the simple math fitting equation. When the electron emission process, charge generation and charge transportation were considered, the flashover time delay, flashover voltage and pulse steepness were discussed, and then a new fitting formula based on the electron emission equation was proposed to describe the relationship between the pulse steepnees and flashover voltage. The formula is able to reflect the change with the applied voltage and the real physical process. Finally, the flashover voltage can be evaluated approximately.