Evaluation of Uniform and Non-uniform Breakdown Characteristics of Liquid Nitrogen With Different Electrode Materials

High temperature superconducting devices utilize liquid nitrogen (LN₂) both for cryogenic liquids and insulating media. Regarding the insulating performance of LN₂, it shows excellent characteristics comparable to insulating oil which is widely used for conventional electric equipment. But, detailed complex insulating structure in LN₂ composed of cryogenic liquids, solid insulating material and conductor material has not been fully considered yet. In order to commercialize superconducting devices, insulating characteristics of LN₂ should be analysed and confirmed to the level of insulating oil. In this work, we focused on the breakdown characteristics of LN₂ especially for different electrode materials. Aluminum, Stainless steel, and Copper electrode systems were fabricated and tested in LN₂ to verify uniform and non-uniform characteristics of LN₂ according to the electrode materials. AC withstand voltage test and lightning impulse test has been performed. From the test results, it was revealed that the breakdown voltages were significantly changed according to the electrode materials. Especially for Stainless steel, it shows prominent enhancement of breakdown voltage compared to copper and aluminum electrode in LN₂. Consequently, it was deduced that optimum choice of conductor materials in LN₂ is another critical factor to determine complex insulating performance of superconducting devices.