Keeping Cool Under Pressure: Dielectric Strength of Sheet Insulation in Boiling Liquid Nitrogen

According to our previous research on the dielectric strength of Nomex T410 paper under a horizontal turn-to-turn electrode system, a significant decrease by approximately 38% of the dielectric strength was found after the introduction of film boiling. In this paper, a wide range of sheet insulation samples has been tested under a turn-to-turn electrode system in vertical arrangement, more practical to the real design of high-temperature superconducting power transformers. Samples from three different types of paper (NMN, Nomex, and Kapton) have been investigated under a static situation with no boiling introduced. In addition, their dynamic dielectric characteristics under different levels of electric stress have also been tested by introducing a transient thermal stress. The experiment has been conducted in open bath liquid nitrogen (77 K) as well as in subcooled liquid nitrogen (67 K). Both of the tests are conducted under one atmospheric pressure. The result shows that all samples have enhanced dielectric performance when immersed in the subcooled liquid nitrogen. Moreover, the Kapton film is found to always have the best dielectric performance. However, its poor impregnation of liquid nitrogen largely affects its dielectric strength, especially at 77 K. When compared to Nomex paper with a similar thickness, the NMN paper is better due to its nonporous structure.