A Method for Analysing and Characterizing the Arc Cooling Effect of Different Gases in Strong Axial Flow With SF₆ and Air as Examples

A method of quantifying the abilities of different gases to cool an axis-symmetric arc in strong axial flow is established. It is able to quantitatively account for the contributions of different energy exchange processes, such as convection, conduction, radiation and turbulent mixing, towards arc cooling (i.e., increase of arc resistance) during the current zero period of the interruption process. Applying the method to a decaying SF₆ arc and an air arc in a converging-diverging nozzle with the arc current ramped down linearly from 1 kA to zero at a rate of 13.5 A/μs, it is shown that the arc cooling effects of turbulence and radial convection, in terms of the reciprocal of their arc cooling characteristic time (1/τ_k), keep increasing in SF₆ in the last 2 μs before current zero, but remain effectively unchanged in air. The arc cooling index (ACI) defined at 1 μs before current zero is found to be 2.59/μs for SF₆ and 0.96/μs for air. The SF₆ arc resistance at current zero is approximately 4 times that of air under the arcing conditions used in this study.