# Surface Discharge Mechanism on Epoxy Resin in Electronegative Gases and Its Application
## 1 Introduction  
Sulfur hexafluoride (SF\({}_{6}\)) plays an important role as an insulating gas in high-voltage power equipment. More than 80% of SF\({}_{6}\) produced worldwide is supplied for use in high-voltage gas circuit breakers, gas-insulated switchgears, and gas-insulated lines, due to its exceptional physical and chemical properties, including its high dielectric strength, current interruption performance, thermal conductivity, thermal stability, nonflammability, nontoxicity, and non-explosive characteristics [1; 2; 3; 4]. Despite the outperformance of SF\({}_{6}\), its use in power equipment is becoming increasingly dangerous owing to its high global warming effect, its long atmospheric lifetime, and the high toxicity of decomposed byproducts that result in many environmental problems. The global warming potential (GWP) of SF\({}_{6}\), relative to CO\({}_{2}\), over a 100-year time horizon is 23,500. This means that the global warming effect of 1 kg of SF\({}_{6}\) gas is equivalent to that of 23.5 tons of CO\({}_{2}\) in the atmosphere. Considering that the global annual emission of SF\({}_{6}\) was 8100 tons in 2012, which is equivalent to annual greenhouse gas emissions of approximately 185 million tons of CO\({}_{2}\), its overall GWP impact is not marginal compared to that of CO\({}_{2}\)[5]. Owing to its high greenhouse effect, the European Union intends to reduce the SF\({}_{6}\) sales of 2014 to one-fifth by 2030 [6]. The California Air Resources Board (CARB) has proposed stricter requirements governing the use of SF\({}_{6}\), including phasing out its usage in gas-insulated equipment and further reducing allowable greenhouse gas (GHG) emissions from such equipment from 2020 onwards [7]. Moreover, the decomposition products are produced by the electrical and thermal decomposition of SF\({}_{6}\) in the presence of other molecules, such as H\({}_{2}\)O, SiO\({}_{2}\), N\({}_{2}\), O\({}_{2}\), H\({}_{2}\), and Ar. It should be noted that the products containing fluorine and/or sulfur in SF\({}_{6}\) decomposition, including as S\({}_{2}\)F\({}_{10}\), SF\({}_{4}\), and hydrofluoric acid (HF), are highly reactive, corrosive, and toxic. These are also potent GHGs [8]. Therefore, to reduce its use, many replacements for SF\({}_{6}\) have been investigated for application in high-voltage power equipment. These investigations have focused on using common gases (CO\({}_{2}\), N\({}_{2}\), and dry air), halogenated gases (CF\({}_{3}\)I, hydrofluoroolefins (HFOs), and perfluorinated compounds (PFCs)), and mixture gases as substitutes for SF\({}_{6}\). Their dielectric strength and characteristics have been compared with those of SF\({}_{6}\)[9]. In addition to gas insulation, composite insulation has been applied to SF\({}_{6}\)-free gas-insulated switchgear (GIS), instead of simple dielectric insulations by vacuum, gases, or solid dielectrics [10; 11]. Composite insulation is a method of using a solid dielectric and an insulating gas together. However, composite insulation comes with problems of deterioration in the insulation performance of the equipment, due to the use of a solid dielectric. The insulation performance deteriorates owing to the formation of a triple junction or a surface discharge. The triple junction induces a high electric field in the region where an electrode is in contact with both the insulating gas and the solid dielectric. Surface discharge is undesired gas breakdown that occurs on a surface at a high voltage, lower than the breakdown voltage of the insulating gas. This breakdown is attributed to the weak point of the dielectrics used and their interactions within the power equipment [10; 12; 13]. To design a composite insulation for SF\({}_{6}\)-free power equipment, it is necessary to investigate the fundamental surface discharge characteristics of the composite insulation and identify its weaknesses.