Breakdown and voltage-time characteristics of turn-to-turn models for an HTS transformer

The breakdown and voltage-time characteristics of turn-to-turn models for point contact geometry and surface contact geometry using copper multiwrapped with polyimide film for a high-temperature superconducting transformer were investigated under ac and impulse voltage at 77 K. Polyimide film (Kapton) 0.025 mm thick is used for multiwrapping of the electrode. As expected, the breakdown voltages for the surface contact geometry are lower than that of the point contact geometry, because the contact area of the surface contact geometry is lager than that of the point contact geometry. The time to breakdown t(50) decreases as the applied voltage is increased, and the lifetime indexes increase slightly as the number of layers is increased. The electric field amplitude at the position where breakdown occurs is about 80% of the maximum electric field value.