Breakdown strength of ice-paper composite insulation in liquid nitrogen

This study was performed in order to clarify the feasibility of a newly proposed cryogenic insulation system used with liquid nitrogen. The proposed insulation system, being composed of multi-layer paper, is soaked with deionized water before it was impregnated into liquid nitrogen. The frozen water introduced into paper matrices is expected to suppress the generation of bubbles that may lead to the breakdown. As the result with three layer insulation, the insulation which had been soaked with water showed a higher AC breakdown strength than that without soaking. The existence of a punch hole, simulating a butt gap, reduced the breakdown strength. A punch hole at the second layer brought a lower breakdown strength than that in contact with the electrode. The ice, filling the punch hole at the second layer, would generate a larger crack than that in the water-soaked paper, and would fix a partial discharge to reduce the breakdown strength, as liquid nitrogen behind the paper could hardly fill this crack