Experimental Study on the Barrier Effects in Gaseous Helium for the Insulation Design of a High Voltage SFCL

It has been widely established that gaseous helium (GHe) can be injected into a sub-cooled liquid nitrogen (LN₂) cooling system to control the pressure of such a system because of its non-condensing characteristics. A sub-cooled LN₂ cooling system is known as one of the most promising method to develop a large scale superconducting machine. Unfortunately, the electrical breakdown voltage of GHe is not good enough for developing a high voltage electric machine such as a superconducting fault current limiter (SFCL). To enhance the dielectric characteristics of current leads placed in GHe, the use of solid barriers is regarded as an efficient method. In this study, experiments are made on the dielectric characteristics of GHe and the barrier effects and results are analyzed in order to improve the dielectric performance of a high voltage SFCL using a sub-cooled LN₂ cooling system. The dielectric experiments of the energized sphere and the grounded plane in GHe are examined according to various pressures with a solid insulating barrier. The dielectric experiments are performed by installing barriers made of glass fiber reinforced plastics (GFRP) between two electrodes and the electrical breakdown voltage according to the pressure of GHe and the position of a barrier is observed. As a result, it is found that the position of a solid barrier and the pressure of GHe are important in enhancing the dielectric characteristics of a sphere-to-plane electrode system with quasi-uniform field.