Investigation of the Flashover and Breakdown Voltage of Thin Polymer Films in Liquid Nitrogen

Resistive high-temperature superconducting fault current limiters (R-SFCL's) based on second-generation superconducting tapes (2G tapes) are successfully used in the medium voltage level. To develop R-SFCL's for high-voltage applications exceeding 100 kV, the dielectric properties of liquid nitrogen as insulation material in different arrangements have to be investigated more in detail. Considering R-SFCL's, the 2G tapes are typically arranged in bifilar coils. Adjacent tapes are insulated by a combination of thin polymer foils and liquid nitrogen resulting in a surface discharge arrangement, which dielectric properties were investigated. Specimens of foil material were inserted between the two electrodes and immersed in liquid nitrogen within an open isolated vessel. The applied ac voltage was raised until flashover occurred, and the shortest overhang of the foil material was recognized as flashover length. The investigations were performed for the polymer materials polyimide, PTFE, and PET and thicknesses of the foils between 75 and 250 μm in dependence on the specific properties of the electrode arrangement. The results reveal that the flashover lengths are rather small, so that in most cases, the breakdown of the polymer foil occurs before the flashover.