Influence of Bubble Formation on the Dielectric Behavior of Liquid Nitrogen

This paper presents the results of the investigation of liquid nitrogen as an insulation liquid. Thereby the influence of sub-cooled liquid nitrogen and the presence of bubbles between Rogowski electrodes stressed with lightning surge impulses are investigated. These bubbles were produced directly on the electrode surface without a damage of the electrode surface. The results show that the sub-cooled liquid nitrogen has a positive effect on the breakdown strength. However the presence of bubbles reduces the breakdown strength significantly regardless if the liquid nitrogen is sub-cooled or not. Furthermore the breakdowns occur earlier if bubbles are present between the electrodes.