Surface Dielectric Characteristics of GFRP and PTFE in Cryogenic Environment Under the Switching Impulse Superimposed on DC Voltage

For insulation materials utilized in DC superconducting fault current limiter, surface discharge along the interface is more damaging to the insulation relative to penetration breakdown. Therefore, for the reliable and economical insulation design of DC superconducting fault current limiter, it is very important to investigate an insulation materials and study the surface dielectric characteristics. Additionally, DC superimposed switching impulse can cause critical stress to the insulation of DC superconducting fault current limiter and can potentially result in breakdown. Therefore, the insulation materials of DC superconducting fault current limiter must have sufficient surface distance to prevent surface discharge under DC superimposed switching impulse. In this paper, the surface breakdown tests were performed by applying DC superimposed switching impulse to glass fiber reinforced polymer (GFRP) and polytetrafluoroethylene (PTFE) specimen in cryogenic environment. Surface dielectric characteristics according to positive and negative polarities of DC and switching impulse were analyzed. In addition, the surface dielectric characteristics were analyzed by selecting the gap distance. From the experimental results, the surface dielectric strength of GFRP was higher than that of PTFE, but the difference in surface dielectric strength between GFRP and PTFE decreased as the gap distance increased. The surface breakdown voltage of PTFE was lowest at the superposition of positive DC and positive switching impulse. However, GFRP has the lowest surface breakdown voltage when negative DC superimposed positive switching impulse. Consequently, it is possible to design the optimal surface distance of solid insulation materials in DC superconducting current limiter by considering the superposition of DC and switching impulse polarity.