Arc Conductance and Contact Erosion in Liquid Nitrogen under DC Current Interruption

Abstract

Previous Studies showed that liquid nitrogen (LN2) has good insulating properties and reseasonable DC current breaking performance. However, there is still no research on arc conductance of liquid nitrogen and on contact erosion in DC current interruption. The objective of this paper is to understand the arc conductance of LN2 and the contact erosion in DC current interruption. Air arc was also studied as a benchmark. DC Current of 110-210 A were applied, which is initiated by a C-L-R discharging. Butt type contacts were chosen. The contact diameter was 25 mm and the contact gaps were 20 mm. Two kinds of contact materials were chosen, which were pure copper and tungsten copper, respectively. A drawn arc was initiated. The average opening velocity was 0.6 m/s. Experimental results showed the arc conductance during arcing in LN2 and in air. In successful breaking, the arc conductance declined to zero quickly without a "plateau" before current zero. While in failed breaking, there was a "plateau" after a quick decline of the arc conductance. The plateau of the arc conductance increased from 0.3 S to 1.3 S when the current increased from 185 to 210 A. Arc conductance in LN2 was approximatley two-thirds of that in air.The contact erosion area in LN2 was nearly half of that in air after arcing, while depth of erosion was nearly the same. SEM photos revealed that the droplet-like metal balls on the contact surfaces in LN2 were much higher than that in air, which implied the contact surface after arcing in LN2 was rougher than in air.