A Study of the Dielectric Characteristics of Gaseous Nitrogen With Respect to the Electrode Material for Developing a High Voltage Superconducting Fault Current Limiter

The current lead for a high-voltage superconducting fault-current limiter is made of copper or brass, and the cryostat chamber is often composed of a nonmagnetic material, such as stainless steel. Therefore, sparkover breakdown may occur at the current lead or the superconducting coil of such a superconducting coil system. The measurement of sparkover and partial discharge inception voltages is considered an important technique for diagnosing the dielectric conditions of a high-voltage superconducting instrument. In this paper, the sparkover voltage and the partial discharge inception voltage with respect to the electrode material were examined under an alternating-current voltage. Sphere-to-plane electrode systems made of stainless steel, copper, brass, and iron were tested in dielectric experiments to verify the sparkover and partial discharge inception voltages. It is observed that the sparkover and partial discharge inception voltages in gaseous nitrogen (GN₂) were strongly dependent on the electrode material. In addition, it is found that the sparkover and partial discharge inception characteristics of GN₂ conform to the work function of the electrode material.