Breakdown Performance of CO₂, Technical Air, and Technical Air with 7.5% C5-Fluoroketone in a Weakly Nonuniform Field

In the search for alternatives to SF<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> in gas insulated switchgears, CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>, technical air (80% N2, 20% O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>), and technical air with small admixtures such as C5-fluoroketone (C5-FK) have emerged as some of the (more/most) potential options due to their low environmental impact. This article reports on the breakdown performance of these gases/gas mixtures under 50 Hz AC and 1.2/50 μs lightning impulse (LI) voltages at 1.3 bar absolute pressure in a 10 mm rod-plane gap with a weakly nonuniform field relevant to medium voltage applications. In general, the mixture of technical air with C5-FK was found to exhibit superior dielectric performance to CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> or technical air when stressed with both AC and LI voltages. For all the studied gases/gas mixtures, the breakdown voltage and its scatter were found to be higher under negative LI stress, indicating a lack of initial electrons. The results are also compared to those of SF6 and a mixture of 13.6% C5-FK in air, which were tested during the round-robin test campaign by the CIGRE D1.67 working group using similar methods and test configurations. Findings from this study will therefore aid in the development of SF<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf>-free switchgears as comparison of the results would provide the data required for design optimization.