Breakdown Field Strength of Lapped Kraft Paper, PPLP or Kapton in Liquid Nitrogen

Superconducting cables are good alternatives to transport electric power over long distances insofar as the insulation operates correctly at cryogenic temperatures. The breakdown electric field strength of various materials were tested in liquid nitrogen in a lapped configuration corresponding to a realistic geometry for the high voltage superconducting cable insulation. A specific sample holder was designed to perform several breakdown tests on the same sample without breaking testing conditions. It is shown that breakdown value is improved by increasing hydrostatic pressure and by reducing the gap between strips. It is also shown that the insulating structure still operates with little degradation after breakdowns although perforations are more destructive for PPLP and Kapton than for Kraft paper. Kapton presents a higher breakdown strength than PPLP itself higher than the one of Kraft paper though this latter material exhibits an interesting behavior under pressure due to its permeability to liquid nitrogen.