Surface flashover and breakdown characteristics of glass fiber reinforced plastic and transformer board materials in liquid nitrogen

Abstract:
Transformer board material is investigated as cost-effective alternative to high-priced glass fiber reinforced plastic (GFRP) typically used as insulating support in superconducting devices based on high temperature superconductors cooled with liquid nitrogen (LN2). Permittivity, dielectric loss factor, short-time breakdown voltages, withstand voltages and flashover voltages of a cellulose based transformer board and an epoxy based GFRP material with thicknesses of 1 mm, 2 mm and 3 mm, respectively, are measured in LN2 at atmospheric pressure. The results reveal that the transformer boards have nearly the same breakdown performance in LN2 in comparison to the values impregnated in insulating oil. In comparison to GFRP the values of breakdown, withstand and flashover voltages are roughly 20 % to 30 % higher, which can be explained partly by the lower permittivity of the cellulose material in comparison to GFRP. In the frame of the investigations the performance of the transformer board materials in LN2 is not influenced by the water content of the materials.