Connection for transfer of Liquid Nitrogen from High Voltage to ground potential

In order to operate a superconducting cable conductor it must be kept at a cryogenic temperature (e.g. using liquid nitrogen). The superconducting cable conductor is at high voltage and the cooling equipment is kept at ground potential. This requires a thermally insulating connection that is also electrically insulating. Here, the design, construction and test results of such a device are described. It consists of 2 coaxial glass fibre reinforced polymer (GFRP) tubes with insulating foam between them. Foams are generally not recommended for use in high electrical fields, due to low electrical breakdown strength. However, samples of ExpancelÒ (polymer foam) have recently proved to withstand large electrical fields at room temperature as well as at cryogenic temperatures. In this work, two prototype devices have been tested with respect to the partial discharge inception voltage, thermal insulation properties and withstand towards high-pressure liquid nitrogen. The length per joint is approximately 900 mm, including a Johnstoncoupling. The joints are tested in a closed liquid nitrogen circuit, with a pressure of up to 10 bars. The rated voltage of the cable system is 36 kV (phase-phase).