# Surface Discharge Mechanism on Epoxy Resin in Electronegative Gases and Its Application
## 4 Discussion
### Surface Discharge Mechanism
Electron attachment is the only process that removes electrons in the discharge space between electrodes in the surface discharge mechanism. The removal of electrons suppresses the formation of the streamer and conductive channels. This suppression is the reason behind the high surface insulation performance of an electronegative gas. Therefore, the surface insulation performance of the insulating gas containing electronegative gases SF\({}_{6}\) and O\({}_{2}\) is superior to that of N\({}_{2}\). Although SF\({}_{6}\) and dry air are both electronegative gases, the disparity in the surface insulation performance between these gases can be explained by the difference in the electron attachment cross section, which determines the characteristics of the gases. Since the surface discharge grows on the surface of the epoxy dielectric, the capturing of electrons emitted from the surface appears to contribute to a significant improvement in the surface insulation performance. Electrons are generated from the epoxy surface during surface discharge by the SEEA and photoemission processes. The energy range of the electrons emitted from the surface by these processes was assumed to be 3.0-4.5 eV [16; 26]. Figure 4 shows the electron attachment cross sections of SF\({}_{6}\) and O\({}_{2}\) in the electron energy range. Evidently, the cross section of SF\({}_{6}\) is considerably larger than that of O\({}_{2}\). From this figure, it can be assumed that the ability of secondary electrons, emitted from the epoxy surface, to attach themselves is significantly better in SF\({}_{6}\) than in O\({}_{2}\). The progress of the streamer and the formation of a conductive channel on the epoxy surface can be suppressed by the capture (attachment) of the secondary electrons due to the large electron attachment cross sections. This could be the reason why SF\({}_{6}\), which has an excellent electron attachment ability in the electron energy range of 3.0-4.5 eV, exhibits a superior surface insulation performance compared to dry air [27]. Figure 4: Attachment cross sections of gases (\(\rightarrow\), SF\({}_{6}\) and \(\rightarrow\), O\({}_{2}\)). In addition, the noticeable saturation of surface flashover voltages with an increasing pressure, in the pressure range of 0.1 to 0.4 MPa in SF\({}_{6}\), may be related to electron detachment. During surface discharge development, negative ions collide with electrons, positive ions, and other negative ions in the medium. Electrons attached to electronegative gas are then separated and detached from the negative ions by these collisions. The separated electrons are imbibed into the SEEA or collision ionization, helping the formation of a conductive path along the epoxy surface between the electrodes. In other words, a large number of negative ions can improve the surface insulation performance, and at the same time, can function as electron sources [28]. The electron detachment of negative ions mainly occurs from collisions with ions rather than high electric fields [29]. Since the mean free path becomes shorter as the pressure increases, it can be assumed that the collision of positive and negative ions also occurs more frequently with an increasing pressure. Frequent collisions activate electron detachment, causing a noticeable saturation of the surface insulation performance with an increasing pressure. The rise of the surface flashover voltage of SF\({}_{6}\) at 0.6 MPa may be related to a high-pressure surface discharge mechanism, such as leader discharge. Insulation Design Method of an Insulating Gas Alternative to SF\({}_{6}\) Based on Electron Swarm Parameters  
An insulation design method that can ensure a good insulation performance in high-voltage power apparatus such as GIS can be proposed from our experimental results, derived from Figure 2. The surface insulation performance can be enhanced by the inclusion of electronegative gases and an increase in the gas pressure. Therefore, these factors must be considered during insulation design.