Lukas Graber (Florida State University)
Past efforts of cooling high temperature superconducting (HTS) power cables by gaseous cryogens focused exclusively on helium . We are working on exploring the benefits of gas mixtures containing helium with small amounts of hydrogen and neon gas to mitigate the limited dielectric strength of pure helium gas. This could potentially improve dielectric characteristics while maintaining the thermal, non-flammable and non-corrosive properties of pure helium gas. From the dielectric point of view, hydrogen gas is far superior to helium and neon. Hydrogen gas has approximately 50% of the dielectric strength of nitrogen gas at room temperature . The noble gases helium and neon exhibit dielectric strength 15% and 25% respectively that of nitrogen gas at room temperature. It is known from other gas mixtures studies that even a small amount of a superior gas can considerably improve the dielectric strength of the mixture more than the ratio would suggest . The dielectric properties of pure helium gas at temperatures in the range of 50 K to 77 K have been studied in past HTS cable projects at Florida State University. The challenges with low breakdown voltage have led to the investigation of potential alternative cooling gases. The dielectric strength of helium gas mixtures containing 2% and 4% of neon and hydrogen have been measured and compared to that of pure helium. The limit of 4% of hydrogen was to ensure to keep the gas mixture non-flammable in air. All experiments were performed at 77 K at pressures between 0.1 and 2.0 MPa.  S. Pamidi et al., 11th EPRI Superconductivity Conference, Houston, TX, October 28-30, 2013.  A. K. Vijh, IEEE Trans. on Electrical Insulation, 12, 313, 1997.  N. H. Malik et al., IEEE Trans. Electr. Insul., Vol. 14, pp. 1-13, 1979.
Mr Peter Cheetham (Center for Advanced Power Systems, Florida State University)