Experimental Research on DC Breakdown Characteristics of Liquid Nitrogen under Rod-plane Electrodes

With the rapid development of high temperature superconducting (HTS) apparatus, the importance of electrical insulation characteristic of liquid nitrogen (LN2) grows significantly. In the last two decades, as the R&D focus is on AC applications, DC electrical insulating performance of LN2 is not fully explored. In this research, DC breakdown (BD) characteristics of LN2 are investigated for different electrodes gap distances and polarity under rod-plane electrodes pair condition. To provide valuable reference for the insulation design of high voltage DC (HVDC) HTS power apparatus using LN2 as the independent major insulation such as resistive HTS fault current limiter, the gap ranges from 5 mm to 180 mm, which corresponds to BD voltage higher than 300 kV. Experimental results revealed that the DC BD strength of LN2 differs with electrode polarity. The positive rod condition has a higher BD strength when the gap is shorter than roughly 100 mm while negative rod withstands a higher voltage when the gap is bigger than that. Furthermore, positive rod condition reveals obvious saturation effect whereas the negative rod condition does not. The insulation design of HVDC HTS power apparatus should consider both the polarity and saturation effect of LN2 and take the pessimistic condition as the design basis.