AC Breakdown Characteristics of c-C4F8/N2 Gas Mixtures in an Extremely Non-Uniform Electric Field

1. Introduction

6) is widely used in power systems and electrical equipment owing to its high insulation level and arc-extinguishing capacity [6 during use [4,6 is a potent greenhouse gas whose global warming potential (GWP) is 23,900 times that of CO2, and whose life expectancy is 3200 years in the atmosphere, which causes a severe greenhouse effect [6 was listed as one of six gases requiring global regulation [6.

Sulfur hexafluoride (SF) is widely used in power systems and electrical equipment owing to its high insulation level and arc-extinguishing capacity [ 1 2 ]. However, the non-homogeneity of an electric field, the conducting particles, and the surface roughness of the electrode all have a significant influence on the dielectric strength of SFduring use [ 3 5 ]. Above all, SFis a potent greenhouse gas whose global warming potential (GWP) is 23,900 times that of CO, and whose life expectancy is 3200 years in the atmosphere, which causes a severe greenhouse effect [ 6 7 ]. Therefore, in the Kyoto protocol adopted by Japan on December 1997, SFwas listed as one of six gases requiring global regulation [ 8 ]. Considering the environment, it has become an inevitable tendency to find a new environment-friendly gas to replace SF

6, such as octurobutane (c-C4F8), trifluoroiodomethane(CF3I), heptafluorobutyronitrile (C4F7N) and trifluorovinyl ether (C5F10O). However, the boiling points of C4F7N and C5F10O are high, which restrict the use for low temperature environments. Although CF3I has low toxicity, the decomposition product of CF3I contains iodine, and the effect of iodine on breakdown is not clear [4F8 is an electronegative gas with stable chemical properties, and displays non-toxicity, non-combustibility, and non-ozone effects [4F8 is 1.3 times of SF6 under a uniform electric field, while the GWP of c-C4F8 is 8700, which is about 1/3 of SF6, so the impact on the environment is much smaller than that of SF6 [11,4F8 mixed gas was listed as an insulating gas for future long-term research at the 1997 technical conference of the National Association of Standards and Technology [4F8 gas, it cannot be directly used in electrical equipment, and buffer gases such as N2 or CO2 are often added in use. So far, researchers have conducted many studies on the electrical properties of c-C4F8 mixtures [4F8/N2 and c-C4F8/CO2 under a slightly non-uniform electric field, analyzed the breakdown products, and obtained the synergetic coefficients of mixed gases under different volume fractions. Results indicate that c-C4F8/N2 and c-C4F8/CO2 can be used as an alternative gas to SF6 under certain conditions [4F8/N2 mixed gas, and demonstrated the feasibility of substituting c-C4F8/N2 for SF6 in terms of the liquid temperature, environmental influence, and partial discharge characteristics [4F8/N2 mixed gas under typical faults such as local overheating, a partial discharge, spark, and arc discharges, and confirmed that the breakdown products of a c-C4F8/N2 mixture under several common faults are less toxic, thus c-C4F8/N2 can be used as an alternative to SF6 [

Thus, many gases with high insulating strength have been concentrated on by researchers to replace SF, such as octurobutane (c-C), trifluoroiodomethane(CFI), heptafluorobutyronitrile (CN) and trifluorovinyl ether (CO). However, the boiling points of CN and CO are high, which restrict the use for low temperature environments. Although CFI has low toxicity, the decomposition product of CFI contains iodine, and the effect of iodine on breakdown is not clear [ 9 ], while c-Cis an electronegative gas with stable chemical properties, and displays non-toxicity, non-combustibility, and non-ozone effects [ 10 ]. The breakdown voltage of c-Cis 1.3 times of SFunder a uniform electric field, while the GWP of c-Cis 8700, which is about 1/3 of SF, so the impact on the environment is much smaller than that of SF 12 ]. c-Cmixed gas was listed as an insulating gas for future long-term research at the 1997 technical conference of the National Association of Standards and Technology [ 13 ]. Owing to the high liquefaction temperature of c-Cgas, it cannot be directly used in electrical equipment, and buffer gases such as Nor COare often added in use. So far, researchers have conducted many studies on the electrical properties of c-Cmixtures [ 6 14 ]. Xiao et al. investigated the AC breakdown characteristics of c-C/Nand c-C/COunder a slightly non-uniform electric field, analyzed the breakdown products, and obtained the synergetic coefficients of mixed gases under different volume fractions. Results indicate that c-C/Nand c-C/COcan be used as an alternative gas to SFunder certain conditions [ 15 16 ]. Xing et al. studied the partial discharge characteristics of a c-C/Nmixed gas, and demonstrated the feasibility of substituting c-C/Nfor SFin terms of the liquid temperature, environmental influence, and partial discharge characteristics [ 17 ]. Li et al. analyzed the decomposition products of a c-C/Nmixed gas under typical faults such as local overheating, a partial discharge, spark, and arc discharges, and confirmed that the breakdown products of a c-C/Nmixture under several common faults are less toxic, thus c-C/Ncan be used as an alternative to SF 18 ].

Thus far, researchers have studied the insulation properties of a c-C4F8/N2 mixture under a uniform and slightly non-uniform electric field. However, in practical application, owing to the limitation of processing technology, burrs or suspended impurity particles are present in gas-insulated high-voltage equipment, which will lead to a distortion of their local electric field. Therefore, it is also particularly important to investigate the breakdown characteristics of the gas insulated under an extremely non-uniform electric field. In this paper, a discharge device was established and an AC test platform was set up. Based on the use conditions, the volume fraction of c-C4F8 in the test was determined, and the breakdown voltages of c-C4F8/N2 under different pressures, electrode distances, and volume fractions of c-C4F8 were tested. The trend of breakdown voltage changes with pressure, electrode distance and the volume fraction of c-C4F8 was then analyzed. The feasibility of replacing SF6 with c-C4F8/N2 is discussed.