Breakdown Characteristics of a Fluoronitrile Mixture Gas According to Oxygen Content for High-Voltage Power Equipment Application

Fluoronitrile <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathrm{C}_{4}\mathrm{F}_{7}\mathrm{N})$</tex> has many advantages, such as high dielectric strength, low global warming potential (GWP), and short atmospheric lifetime compared to sulfur hexafluoride <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{SF}_{6})$</tex>. In particular, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{C}_{4}\mathrm{F}_{7}\mathrm{N}/\text{CO}_{2}$</tex> mixture gas has excellent applicability in high-pressure environments and is gaining attention as an insulating media for eco-friendly high-voltage gas insulated switchgear (GIS). Recently, a method of adding oxygen (O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>) to fluorinated mixture gas has been proposed to suppress the formation of carbon soot in insulating gases. However, research on the influence of adding more than 10% O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> to the dielectric strength of fluorinated mixture gas has been insufficient. Therefore, in this paper, the influence of a high-proportion O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> addition on the dielectric characteristics of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{C}_{4}\mathrm{F}_{7}\mathrm{N}$</tex> 5%/CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>was investigated. The breakdown tests for power frequency and negative lightning impulse voltage under weakly non-uniform electric field were conducted by increasing the O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> content in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{C}_{4}\mathrm{F}_{7}\mathrm{N}$</tex> 5%/CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>to 5%, 10%, and 20%. From these experiments, it was confirmed that the addition of 5% O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> to the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{C}_{4}\mathrm{F}_{7}\mathrm{N}$</tex> 5%/CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>resulted in an improvement of about 10% in dielectric strength at pressure below 0.4 MPa. However, the dielectric strength decreased when the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{O}_{2}$</tex> contents were 10% and 20%. Furthermore, the dielectric strength at pressures above 0.6 MPa decreased by more than 10% due to the addition of O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>, regardless of the proportion.