Comparative Evaluation Between DC and AC Breakdown Characteristic of Dielectric Insulating Materials in Liquid Nitrogen

Due to the existence of AC loss in superconducting materials when an alternating voltage is applied, high cryogenic costs are inevitable to operate superconducting devices in AC networks. Therefore applications of superconducting devices in DC electric power networks could be regarded as the optimum choice for superconducting devices, because superconductors show exactly zero resistance to a DC source. Recently, DC superconducting devices such as DC cables have received noticeable attention as DC power transmission lines. In order to develop DC superconducting devices, the DC insulation characteristics in cryogenic liquids should be clarified. However, up to now, limited research 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 (polyimide) and Nomex (polyamide) papers have been prepared. Furthermore, a 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 sheets have shown 1.7-2.7 times higher dielectric strength than those of AC. Moreover, the Nomex and Kraft sheets have shown a remarkable increase in their dielectric strength in liquid nitrogen compared to air. However, the dielectric strength of the Kapton sheet did not show a remarkable increase in liquid nitrogen. From the turn-to-turn breakdown test, it was proved that the dielectric strength has been linearly increased according to the wrapping number of the sheets and that the DC breakdown voltage was 1.1-2.5 times higher than AC.