## 1 Introduction  
Insulation gases are used in electrical power apparatus such as gas-insulated switchgears (GIS), circuit breakers, and transmission lines to provide a high dielectric strength and to avoid electrical breakdown. Sulfur hexafluoride (SF\({}_{6}\)) has been used as the most popular insulation gas since the 1960s owing to its excellent dielectric strength (approximately three times greater than air) and remarkable arc-quenching ability, making the gas-insulated facilities available at a compact size with a high reliability. In addition, SF\({}_{6}\) has general features of gas for high voltage insulation applications, including high heat dissipation, a low condensation temperature, low toxicity, non-flammability, and chemical stability [1; 2; 3; 4]. However, SF\({}_{6}\) has significant environmental impacts owing to its high global warming potential (GWP) that is 23,500 times of that of carbon dioxide (CO\({}_{2}\)) and its long lifetime of 3200 years in the atmosphere, whereas the lifetime of CO\({}_{2}\) is about 30-95 years [5]. 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\({}_{2}\)[6]. As a result, the Kyoto Protocol designated SF\({}_{6}\) as one of the six greenhouse gases whose emission should be limited and must be gradually reduced by the year of 2020 [7]. It is therefore necessary to find an alternative insulation gas for green manufacturing and the application of gas-insulated power facilities. Researchers and manufactures have carried out studies to investigate an alternative gas or gas mixture for substituting SF\({}_{6}\). The desirable gas or mixture is one that has the required properties for electrical insulation applications as advantages of SF\({}_{6}\), while having little impact on the environment. Natural gases such as dry air, CO\({}_{2}\), and Nitrogen have a low GWP, but a very limited dielectric strength that is only 40% of that of SF\({}_{6}\). The use of such gases results in a significant increase of the pressure and size of electrical facilities [8; 9]. Trifluoriodomethane (CF\({}_{3}\)I) has been introduced as an emerging candidate that combines a high dielectric strength and lower environmental concerns. It was verified that 30% CF\({}_{3}\)I/70% CO\({}_{2}\) has the best performance to replace SF\({}_{6}\) for GIS [10; 11]. However, CF\({}_{3}\)I is classified as a carcinogenic and mutagenic gas and is therefore not suitable for industrial applications [12]. Hydrofluoroolefins (HFOs), whose molecular formula is C\({}_{\text{n}}\)(H,F)\({}_{2\text{n}}\), has a GWP lower than 9 and a good insulation performance; however, its dielectric strength is highly dependent on the gas pressure. In addition, the liquefaction temperature of HFOs is so low that gas liquefies at 0.42 MPa at room temperature. When a flashover occurs in HFOs, a carbon deposit may appear in the equipment [13; 14]. Perfluorinated ketones, such as C\({}_{5}\)F\({}_{10}\)O, have a low GWP of 1 and a high dielectric strength. However, the boiling point of C\({}_{5}\)F\({}_{10}\)O is as high as 24 \({}^{\circ}\)C at 0.1 MPa and it has a higher minimum operating temperature compared with SF\({}_{6}\)[15]. The green gas employed for a grid (g3) is an SF\({}_{6}\)-free gas mixture introduced by Alstom and GE in the last three years as a promising candidate to replace SF\({}_{6}\) for high voltage applications. Its insulation performances have been investigated by analyzing the breakdown voltage under AC power frequency and lightning impulse voltage [6; 14]. However, few studies have been carried out to investigate the partial discharge (PD) characteristics in g3 gas, whereas PD is at an early stage and an indicator of insulation breakdown [3; 16; 17; 18]. This paper studied the insulation performance of the g3 gas mixture from the perspective of PD.