# Surface Discharge Mechanism on Epoxy Resin in Electronegative Gases and Its Application
## 1 Introduction
Extensive research has been conducted on the surface discharge mechanism in different materials under different electric fields formed by AC, DC, and impulse voltage [10; 14; 15; 16]. [PERSON] et al. [10; 14; 15] presented the experimental characterization of discharge propagation over insulators such as Bakelite, epoxy, and glass immersed in a single gas or mixture gas, under lightning impulse, DC, and AC voltages. The shape and length of creeping discharge patterns were investigated to understand the capacitive charge effect and electric field influence. [PERSON] et al. [16] conducted experiments under a negative impulse voltage to clarify the surface flashover characteristics based on the existence of surface charges on an alumina insulator in a vacuum. The influence of surface charges on the electric field and the secondary electron emission avalanche were explained. These investigations have succeeded in revealing that the surface charges of insulators influence the surface flashover characteristic. In order to investigate the occurrence of surface flashover, the effects of the gas medium and its pressure, thickness and dielectric constant of a solid insulator, roughness and shape of the electrode, and applied voltage on gas breakdown have been further explored [3; 17; 18; 19; 20; 21]. These investigations have reported that surface flashover occurs at the triple junction and in its vicinities in several common and specified factors influencing the surface discharge [19]. [PERSON] et al. [3] compared the surface discharge characteristics of different gas media, such as SF\({}_{6}\), N\({}_{2}\)/O\({}_{2}\) mixture gases, and imitation air, and explained the important role of the O\({}_{2}\) concentration in the N\({}_{2}\)/O\({}_{2}\) mixed gas. [PERSON] et al. [19] conducted a comparative study on SF\({}_{6}\) alternative candidate gases for investigating the surface insulation performance of eco-friendly gas, including N\({}_{2}\)/O\({}_{2}\) mixture gas, dry air, and compressed air under AC voltage. Based on the experimental results and the comparative study, the effect of moisture contained in the candidate gases and the correlation among the electric filed intensity, collision-ionization coefficient, electron attachment coefficient, and gap length were discussed. [PERSON] et al. [20] presented the ignition of surface discharges and their propagation governed by repulsion due to electrostatic force and attraction caused by nonlocal photo-ionization. Before the occurrence of surface flashover, the solid dielectric causes the occurrence of physical phenomena, such as electron emission at a triple junction, surface charge accumulation, photoemission, and a secondary electron emission avalanche (SEEA). These phenomena contribute to electron avalanches and the formation of a conductive channel on the dielectric surface. There are several surface discharge mechanisms, based on electron generation and the high electric field as a physical phenomenon in a vacuum [16; 18; 22]. However, this physical phenomenon and the detailed progression of surface discharge have not yet been fully understood in compressed gases with various electronegative gases. This means that prior research dealing with the progression and suppression of surface discharge is rather limited, in terms of the detailed physical processes of electron generation and disappearance in compressed gas. Although the authors have studied the effect of the concentration of O\({}_{2}\) on surface discharge characteristics in compressed gas in previous works [3; 19], many studies on surface discharge seem to be carried out under vacuum or gases with a pressure of 0.3 MPa or less. This research trend has led to a gap in knowledge about the detailed mechanism by which an electronegative gas among high-pressure gases generates electrons during the surface discharge process. This knowledge gap means that the surface insulation design of SF\({}_{6}\)-free high-voltage equipment with high-pressure compressed gas is still a challenging problem.