Comparison Between the DC and AC Breakdown Characteristics of Dielectric Sheets in Liquid Nitrogen

With the emergence of superconductor technology, high-temperature superconducting (HTS) power devices are quickly being developed. HTS power devices are receiving considerable attention due to their high capacity and low loss. Compared with ac HTS power devices, dc superconducting power devices have the advantage of zero resistance. To aid in the development of superconducting power devices, the ac and dc dielectric breakdown characteristics of dielectric sheets in liquid nitrogen ( LN2) should be better understood. In this paper, results are reported for ac and dc breakdown tests conducted in air (room temperature) on six types of sheets, including polypropylene laminated paper (PPLP), Nomex (polyamide) T410 fiber paper, Kapton (polyimide) 100HN, Kapton 100CR, Kapton 150FN019, and Kapton 150FCR019, and in LN2 on seven types of sheets, including PPLP, Kapton 100HN, Kapton 100CR, Kapton 150FN019, Kapton 150FCR019, Nomex T410, and Nomex T418 fiber paper. The insulation dielectric properties in air (room temperature) were compared with the insulation dielectric properties at a low temperature. At room temperature, the dc breakdown strength at 63.2% probability is 1.12-1.72 times the ac breakdown strength at 63.2% probability, and at a low temperature, the 63.2% probability of the dc breakdown strength is 1.52-2.14 times the 63.2% probability of the ac breakdown strength. Compared with the dielectric sheets of Kapton 150FCR019, Kapton 100CR, Kapton 150FN019, Kapton 100HN, and PPLP, Nomex T410 sheets have shown a remarkable increase in their dielectric strength in liquid nitrogen compared with air.