# Green Gas for Grid as an Eco-Friendly Alternative Insulation Gas to SF\({}_{6}\): A Review
## 1 Introduction  
Sulfur hexafluoride (SF\({}_{6}\)) has been used as the most popular insulation gas in gas-insulated power facilities, such as switchgear (GIS), circuit breaker (GCB), transmission line (GIL), and transformer (GIT), since 1947. This is because SF\({}_{6}\) has an exceptional combination of physical and chemical properties for high-voltage applications, including high-dielectric strength (three times that of air at atmospheric pressure), arc-quenching capability, thermal conductivity, thermal stability, nonflammability, nontoxicity, nonexplosive, and chemical inertness [1; 2; 3; 4]. As a result, SF\({}_{6}\)-insulated equipment can be designed with reduced size, high reliability, and economic efficiency [5; 6]. It is widely employed in space-limited locations such as city substations, offshore plants, or wind-power plants [7].  
However, SF\({}_{6}\) was labeled as one of the greenhouse gases in the Kyoto Protocol and Paris Agreements owing to its excessive size, radiative effect, and atmospheric lifetime [8; 9; 10; 11; 12]. The global warming potential (GWP) of SF\({}_{6}\) is 23,500 times that of carbon dioxide (CO\({}_{2}\)) over a 100-year time horizon and is 32,600 times that of CO\({}_{2}\) over a 500-year time horizon. The atmospheric lifetime of SF\({}_{6}\) is 3200 years, whereas that of CO\({}_{2}\) is only 30-90 years [1]. It was reported that 1 kg of SF\({}_{6}\) released into the atmosphere has the equivalent global warming impact as 23.5 tons of CO\({}_{2}\)[13]. Therefore, emission and consumption of SF\({}_{6}\) must be controlled by the year 2020 and regulations have been implemented to reduce the environmental impact caused by SF\({}_{6}\)[14; 15; 16]. In addition, SF\({}_{6}\) decomposes into lower fluorides of sulfur when an electrical discharge such as an arc, spark, or corona occurs. Some decomposition byproducts, including disulfur decafluoride (S\({}_{2}\)F\({}_{10}\)), sulfur tetrafluoride (SF\({}_{4}\)), and hydrogen fluoride (HF), are known to be highly reactive, corrosive, and toxic, which cause great concerns regarding equipment lifetime and personnel safety [17; 18].  
Negative impacts have led the power industry to seek an eco-friendly alternative insulation gas to partially or completely replace SF\({}_{6}\). Studies have been carried out to investigate the potential of natural gases such as CO\({}_{2}\)[19] and nitrogen (N\({}_{2}\)) [20], and dry air, vacuum, SF\({}_{6}\) gas mixture [20; 21; 22], and fluorinated gases with reduced GWP such as trifluoroiodomethane (CF\({}_{3}\)I) [4; 13; 14; 23; 24], perfluorocarbons (PFCs) [1; 25], as well as hydrofluoroolefins (HFOs) [7; 26]. Unfortunately, all of them have respective drawbacks, making them unsuitable for application in high-voltage gas-insulated equipment.  
Recently, green gas for grid (g3), which was developed in collaboration by General Electric and 3M Company, has been regarded as the latest generation insulation gas and is the most promising candidate for replacing SF\({}_{6}\). G3 is a gas mixture that is composed of C\({}_{4}\)F\({}_{7}\)N (NovecTM 4710) and CO\({}_{2}\) or N\({}_{2}\), which aims to reduce the environmental impact and to find a compromise between the dielectric performance and the minimum operating temperature of the apparatus [26; 27; 28]. Properties of g3 gas have been individually studied in the past five years, whereas a comprehensive overview of g3 gas for applications in high-voltage gas-insulated equipment has not been presented until now. This paper firstly discusses the required physical and chemical properties of gas for high-voltage insulation applications and then carries out a review on the state-of-the-art performance investigations of g3 gas.