Design of Current Leads for a High Voltage Superconducting Apparatus

In the Republic of Korea, it is widely thought that the subcooled liquid nitrogen cooling system is the most promising method to develop a high-voltage superconducting apparatus. This method involves injecting a gaseous insulation medium, such as He, Ne, or N₂, into the superconducting apparatus to control the system pressure [1]. Therefore, a study on the dielectric characteristics of gaseous and solid insulation materials must be conducted to design the current lead parts for a high-voltage superconducting apparatus, such as a superconducting fault current limiter, superconducting cable, and superconducting transformer with a subcooled liquid nitrogen cooling system. In this paper, a study on the electrical breakdown characteristics of gaseous insulation materials is conducted to estimate the thickness of the gaseous layer between the current lead and the metallic enclosure. An analysis on the dielectric characteristics of the GFRP (glass fiber reinforced plastic) is performed to determine the thickness of the GFRP sheath for a current lead. It is found that the electric field intensity of the gaseous insulation media at flashover can be represented as the function of field utilization factors at various pressures: 0.1, 0.2, 0.3, and 0.4 MPa. Also, it is revealed that the electric field intensity at sparkover with an electrical breakdown of the GFRP can be represented as a constant value regardless of system pressure, and a field utilization factor under the conditions of quasi-uniform electric fields. All dielectric experiments are conducted under liquid nitrogen at a temperature of 77 K. The conceptual insulation design of condenser-type current leads for a 22.9 kV and a 154 kV superconducting apparatus is undertaken.