Analysis of the energy transport processes in DC air and SF₆ nozzle arcs

As a naturally existing gas of abundant supply, the insulation and interruption capability of air has been extensively investigated. Research has also recently been done to use air as a dilution gas of Fluoroketone for SF₆ free interruption in medium voltage switchgear. Much evidence has shown that air gas-blast circuit breakers have far inferior interruption capability than SF₆ breakers. However, no clear explanation has so far been given regarding why an air switching arc has much larger time constant than an SF₆ arc, and thus exhibiting far inferior interruption capability than the latter. Based on reliable experimental measurements, the present work first calibrated the turbulence parameter in a two-dimensional axisymmetrical turbulent arc model which was then used for computational investigation of the air arc behaviour and analysis of the dominant energy transport processes. Through comparative study, it has been found that, for air nozzle arc at low current, the radial and axial cooling mechanisms play almost equal roles, while for SF₆ arc the radial cooling mechanism is dominant. The difference in cooling mechanisms for air and SF₆ arcs is directly related to the difference in radial temperature profiles which is controlled by the material property, ρC_p, through turbulence enhanced thermal conduction. The present work therefore provides clear direction on the explanation of the differing interruption performance of different SF₆ replacement gases under transient discharge conditions.