AC Flashover Voltages Along Epoxy Surfaces in Gaseous Helium Compared to Liquid Nitrogen and Transformer Oil

Gaseous helium at high pressure is regarded as a potential coolant for superconducting cables. The dielectric aspects of cryogenic helium gas are both complex and demanding. In this experimental study, we looked at the interface between a smooth epoxy surface and high pressure helium gas in a homogeneous electric field. The ac flashover voltages of epoxy samples whose heights are 3 and 5 mm of diameters 9.5, 12.7, and 19 mm are presented. The results have been analyzed using Weibull statistics. In addition to the behavior of the epoxy in gaseous helium as a function of pressure and temperature, we also present data of the characteristics of the epoxy surface in transformer oil at 293 K and in liquid nitrogen (77 K). The breakdown characteristics of a uniform field gap in gaseous helium as a function of pressure and temperature under ac, dc, and lightning impulse voltages are also given. Field calculations have been made for one of the experimental geometries in an attempt to explain some of the anomalies in the experimental results. The results show that cold compressed helium gas is a viable dielectric but requires different insulator designs compared to insulators for liquid nitrogen.