Comparative tests of tape dielectrics impregnated with liquid nitrogen

Superconducting power cables with so-called "cold dielectric" insulation system normally use tape dielectrics impregnated with pressurized liquid nitrogen (LN2). The AC and lightning impulse (LI) dielectric strength of four commercially available dielectrics have been measured under nitrogen pressures in the range 0.15-0.35 MPa, absolute. A planar test cell is used, and slits were made in the dielectric sheets to simulate butt gaps. Polypropylene laminated paper (PPLPreg) and mica filled polyamide based paper (Nomexreg 418) show AC and LI breakdown field in the range 50-80 kV/mm and 130-160 kV/mm, respectively, whereas Kraft paper and polyethylene paper (Tyvekreg) are found to have a significantly lower dielectric strength. Most of the results are reasonably consistent with previous studies. However, as opposed to other investigations, the dielectric strength for PPLPreg and Nomexreg is not found to increase with increasing nitrogen pressure