Polarity Effect on Standard Lightning Impulse Surface Flashover Under Extremely Nonuniform Electric Field in Liquid Nitrogen

With the improving voltage grade for high-temperature superconducting (HTS) power devices, the surface flashover phenomenon in liquid nitrogen has become an important factor that makes the HTS devices break down. So, the study on surface flashover characteristics in liquid nitrogen has become more important. This paper studied the polarity effect of standard lightning impulse surface flashover under the extremely nonuniform electric field in liquid nitrogen by experiment. It is found that flashover voltages under positive impulses are generally higher than that under negative polarity, which is opposite to the results in gas gaps breakdown and on solid-SF
₆
gas and solid-transformer oil interface. By the analysis of flashover traces and electric field distribution between the triangle–plane electrodes, combining the related surface flashover theory in liquid nitrogen, it is assumed that surface flashover in liquid nitrogen is started from the electron emission from cathode triple junction. Because of the nonuniformity of electric field distribution, the initial voltages of the electron emission from cathode triple junction and the forces suffered on the head of flashover streamers have differed. It is the main reason that the polarity effect in liquid nitrogen is formed. The results could provide a reference for the dielectric design of HTS devices.