# Surface Discharge Mechanism on Epoxy Resin in Electronegative Gases and Its Application
## 3 Experimental Results  
Figure 2 illustrates the surface flashover voltages of SF\({}_{6}\), dry air, and N\({}_{2}\), each at a gas pressure ranging from 0.1 to 0.6 MPa. From this, the effect of pressure on the surface flashover voltages and the breakdown voltages can be analyzed. These voltages increase monotonically with an increasing pressure of SF\({}_{6}\), dry air, and N\({}_{2}\). This monotonous rise is attributed to a shorter mean free path with an increasing gas pressure. The short mean free path limits the acceleration of electrons and reduces the number of electrons with high kinetic energy between the electrodes. The limitation and reduction suppress collision ionization and the SEEA, and such suppression results in an increase in the breakdown voltage and surface flashover voltage. An interesting observation in Figure 2 is the surface discharge characteristic of SF\({}_{6}\). The insulation characteristics of electronegative gases such as SF\({}_{6}\), under a non-uniform electric field, are a clear \(N\)-shaped breakdown voltage. This \(N\)-shape rises after the breakdown voltage decreases with an increase in pressure [23; 24]. The \(N\)-characteristic has also been observed in N\({}_{2}\)/SF\({}_{6}\) mixed gas containing 0.1% SF\({}_{6}\)[24]. However, the \(N\) characteristics were not revealed in this study. The corona stabilization effect due to positive space charges leads to \(N\)-shaped properties. It is presumed that the unobserved \(N\)-characteristic in this study is due to the weak corona stabilization effect, because of the different experimental conditions employed by other researchers [23; 24]. The most evident difference between the references [23; 24] and our experiment  
Figure 1: Experimental setup: (**a**) Scheme of the electrode arrangement and (**b**) the experimental GIS chamber. is the presence of a solid dielectric. The existence of the solid dielectric may strongly influence the distribution of positive ions between electrodes. The mechanism of surface discharge in our electrode system with a solid dielectric would be different from the mechanism of breakdown in an electrode system without any dielectric. The dominant mechanism of surface discharge is the SEEA from the solid dielectric, while that of breakdown manifests as additional avalanches due to positive space charges. The solid dielectric placed between electrodes plays an important role as an electron source that emits electrons through electron collision and photon irradiation on the surface of the dielectric. Therefore, the dielectric leads the electron generation mechanism, including SEEA and photoemission, while developing surface discharge over the solid dielectric. In contrast, this electron generation mechanism does not occur in the breakdown mechanism under the electrode system without any solid dielectric between electrodes. Figure 2 shows that the surface flashover voltage measured with a solid dielectric is higher than the breakdown voltage of the electrode system without any solid dielectric. In general, as the solid dielectric forms a triple junction point with a high electric field and electron sources, it is known that the surface flashover voltage is lower than the breakdown voltage. However, our experimental results showed the opposite. This is because the diameter of the solid dielectric is considerably larger than that of the plane electrode in our electrode system. The surface insulation performance and the rate of increase in surface flashover voltages with an increasing pressure are noticeably different for electronegative gases (i.e., SF\({}_{6}\) and dry air) and the non-electronegative gas (i.e., N\({}_{2}\)). The surface insulation performance of SF\({}_{6}\) and dry air was better than that of N\({}_{2}\) in the gas pressure range of this experiment. SF\({}_{6}\), being an electronegative gas, demonstrated a higher surface insulation performance than that of dry air.