Dielectric Characteristics of Solid Insulation Materials With Respect to Surface Roughness

Three kinds of solid insulation materials, such as glass-fiber-reinforced plastic (GFRP), Bakelite, and MC Nylon, are often used in high-voltage superconducting apparatuses as formers and insulation barriers to improve dielectric characteristics. GFRP, Bakelite, and MC Nylon are considered promising candidates as cryogenic solid insulation materials for developing a high-voltage superconducting apparatus due to their excellent dielectric characteristics and robust mechanical strength under cryogenic conditions. The surface roughness of solid insulation materials could differently vary depending on the machining processes. These effects may result in reduced dielectric characteristics. Currently, the details of any relationship between surface roughness and dielectric characteristics under cryogenic conditions have not been analyzed, and there are only a few reports. Thus, a study of the dielectric characteristics of solid insulation materials with respect to surface roughness in cryogenic conditions should be conducted when designing high-voltage superconducting apparatuses with high reliability. In this paper, we present a study on the dielectric characteristics of several materials, such as GFRP, Bakelite, and MC Nylon, used as formers and insulation barriers as a function of surface roughness to assess the effects of changes in surface roughness. Experiments with respect to surface roughness were performed with alternating-current voltages in gaseous nitrogen (GN 2 ) and liquid nitrogen (LN 2 ). A barrier-type solid insulation material was installed between a rod-to-plane electrode system. Penetrating electrical breakdown, creep discharge, and sparkover voltage with a barrier system were observed in the experiments. It was found that the penetrating electrical breakdown, the creep discharge, and the sparkover voltage with a barrier were influenced by the surface roughness of the solid insulation barriers.