Dielectric properties of cryogenic gas mixtures for superconducting power applications

Abstract

Dielectric breakdown strengths of ternary mixture of gaseous helium, hydrogen, and nitrogen at room temperature and cryogenic temperatures (77 K) have been investigated experimentally in an effort to identify a gaseous cryogen with higher dielectric strength than pure helium for high temperature superconducting (HTS) power applications. Building on our previous success in enhancing the dielectric strength of helium gas by 80% by using a binary mixture containing4 mol% hydrogen + 96 mol% helium, a ternary mixture with a composition of 4 mol% hydrogen + 88 mol% helium + 8 mol% nitrogen was investigated. The composition of the ternary mixture was the result of theoretical modelling studies which used the Boltzmann equation to calculate the dielectric strength of various ternary mixture ratios. The dielectric breakdown strength of the ternary mixture was measured at room temperature and at 77 K at several pressures up to 2 MPa. A 300 % enhancement in dielectric strength was observed for the ternary mixture compared to pure gaseous helium. The dielectric strength of the ternary mixture was measured to be 12 kV/mm at 1 MPa and 77 K, which is approaching the dielectric strength range of liquid nitrogen. The results on the new mixture are compared to the previous studies on helium-hydrogen binary mixtures and the implications of the observed enhancement for gas cooled HTS power devices is discussed. The challenges posed by the addition of nitrogen in terms of limited operating temperature and pressure due to the condensation of nitrogen at higher pressures and/or lower temperatures is also discussed.