Degradation behaviour of solid insulation under cryogenic conditions

The increasing applications of High Temperature Superconductors (HTS) require the understanding of dielectric behavior at cryogenic temperature. Liquid dielectrics such as liquid nitrogen (LN2) have the ability to recover after discharges provided that efficient cooling is available. However, partial discharge streamers during the pre-breakdown phenomena in LN2 can significantly erode solid insulation. This damage is irreversible and degrades the solid dielectric, therefore, affecting the reliability of insulation system. The paper investigates the erosion phenomena of cryogenic solid dielectrics caused by partial discharges in LN2 under AC applied voltages. The experiment set-up consists of a sharp tungsten point and a solid insulation barrier on top of a planar electrode. The materials investigated were polytetrafluoroethylene (PTFE) and glass fiber reinforced plastic (GRP). Partial discharges were measured during the tests following IEC 60720 standard and streamer channels were recorded using a high speed camera. Different erosion behavior was found for the two types of materials under study. Damage to the PTFE samples appeared only on the surface while erosion of GRP sample was found in form of a hole of which depth increased over time.