액체질소에서 절연지의 AC와 DC 절연파괴 특성 연구

Abstract

Due to the existence of ac loss in superconducting material when alternating voltage was applied, high cryogenic costs is inevitable to operate superconducting devices in ac networks. Therefore applications of superconducting devices into dc electric power networks would be regarded as optimum choice for superconducting devices, because superconductors show exactly zero resistance for dc source. Recently, dc superconducting devices such as dc cable, dc fault current limiters have been received noticeable attractions for dc power transmission line. In order to develop dc superconducting devices, dc insulation characteristics in cryogenic liquids should be clarified. But up to now, there's only few research works has been reported in this field. In this paper, to clarify the different breakdown characteristics of dc and ac applications, various kinds of cryogenic dielectric sheets including Kraft, Kapton(polymide) and Nomex(polyamide) papers have been prepared. And penetrating breakdown test for three kinds of sheets and turn-to-turn breakdown tests have been performed in liquid nitrogen(LN2). Consequently, it was found that the prepared have shown 1.7-2.7 times higher breakdown strength than those of ac. And especially, Nomex and Kraft sheets have shown remarkable increase in breakdown strength in liquid nitrogen than in air. But breakdown strength of Kapton sheet did not show remarkable increase in liquid nitrogen. From turn-to-turn breakdown test, it was proved that breakdown strength has been linearly increased according to the wrapping number of the sheets and dc breakdown voltage was 1.1-2.5 times higher than ac.