Barrier Effect on the DC Breakdown Characteristics of Cryogenic Nitrogen Gas for the Designing of the Superconducting Fault Current Limiter

Resistive-type superconducting fault current limiter (R-SFCL) is one of the most promising superconducting power equipment. Liquid nitrogen (LN ₂ ) and cryogenic nitrogen gas (CGN ₂ ) are not only the cooling but also the insulation media for the R-SFCLs. In the R-SFCLs, CGN ₂ will coexist with the LN ₂ surface. The objective of this paper is to study the effect of barrier effect on DC dielectric strength of the CGN ₂ above the LN ₂ surface. In this paper, firstly the DC breakdown voltage V _B of rod-to-plane electrodes with insulation barrier were tested. The gap distance is 13.6 mm. The distance from the rod electrode to the insulation barrier is 1/7, 3/7 and 5/7 of the rod-to-plane gap distance. Secondly the discharge path was observed and analysed by the discharge traces on the insulation barrier. Finally, the discharge model and equivalent circuit were proposed in the paper. The results shown that when the a/b is 1/7, the VB is about two times larger than without the barrier. When the a/b is 5/7, the V _B is nearly the same as that without the barrier. The discharge traces shown that the discharge was started in the CGN ₂ , then the insulation barrier broke down. The equivalent circuit shown that the smaller the electrical conductivity of the insulation barrier is , the higher of the breakdown voltage is.