Pressurizing and sub-cooling effects on electrical breakdown of LN₂ in modeled HTS coils

Pressurizing and sub-cooling of liquid nitrogen are considered to have advantages aspects in electrical insulation and the critical current in high temperature superconducting (HTS) coils. This paper deals with experimental studies on bubble behavior and partial breakdown (PBD) characteristics in a simulated electrode system of HTS coils immersed in pressurized liquid nitrogen following our previous reports with saturated liquid nitrogen at atmospheric pressure. The tested electrode system consists of a coaxial coil layer to cylindrical electrode with an insulation barrier and spacers. A heater is mounted inside the coil electrode to generate boiling which will appear on quenched superconducting coils. Liquid nitrogen is pressurized to 0.2 MPa under constant temperature of ∼77 K. The experimental results showed that partial discharge (PD) voltage is affected markedly by the bubble behavior which depends on the applied pressure on the liquid nitrogen. Pressurization suppresses bubble growth in the insulation space and increases the inception voltage of PD. The charge quantity associated with a single PD at a given applied voltage increases with the applied pressure, and deterioration of solid insulations such as spacer and barrier is accelerated compared to that at atmospheric pressure if the PBD occurred.