Barrier Effect on the DC Breakdown Characteristics of Cryogenic Nitrogen Gas Evaporated From Liquid Nitrogen

Abstract

In superconducting power equipment, such as a resistive-type superconducting fault current limiter (R-SFCL), liquid nitrogen (LN ₂ ) and cryogenic nitrogen gas (CGN ₂ ) act as not only the cooling media but also the insulation media. In R-SFCLs, CGN ₂ will coexist with the LN ₂ . The objective of this article is to determine a barrier effect on the dc dielectric strength of the CGN2 evaporated from LN ₂ . The dc breakdown voltage V _B of a pair of rod-to-plane electrodes with an insulation barrier was measured. The gap distance b was 13.6 mm, in which the distance from the rod electrode to the insulation barrier was a and the distance from the rod electrode to the plane electrode was b. Three cases were measured, which were a/b = 1/7, 3/7, and 5/7, respectively. The materials of the insulation barrier were polytetrafluoroethylene (PTFE) and Kapton, respectively. The experimental results showed that, for both positive and negative polarities, when a/b was 1/7, the breakdown voltage was approximately two times higher than that without the barrier. When a/b was 3/7, the average breakdown voltage was approximately 1.5 times higher than that without the barrier. When a/b was 5/7, the breakdown voltage was nearly the same as that without the insulation barrier. Therefore, it suggested that the closer the insulation barrier was to the rod electrode side, the higher the breakdown voltage was. When a/b = 5/7 and the thickness of the insulation barrier was 0.10 mm, the breakdown voltage of the CGN ₂ /insulation composite insulation system with the Kapton film was higher than that with the PTFE film. A high-speed camera observation showed that the discharge started in the CGN ₂ , and then, a further breakdown occurred in the insulation barrier. An equivalent circuit model was built up to describe the breakdown process, which showed that the lower the electrical conductivity of the insulation barrier was, the higher the breakdown voltage was.