Interfacial microstructure and insulation properties of 500 kV EHVDC XLPE cable factory joint under different roughness and degassing durations

Abstract This paper analyzes the microstructure and electrical insulation characteristics at the interface of 500 kV cross-linked polyethylene (XLPE) cable factory joints for extra high voltage direct current (EHVDC) application. The interface between two layers was polished by different grit sandpapers (80, 400, 1000, 2000, and 0000 (unpolished)). The samples (#1000) polished by 1000 grit mesh at the interface were degassed at 70°C for 0, 12, 36, 90, and 200 h, respectively. Physico-chemical and electrical experiments were performed on the prepared test samples. It is found that increasing the number of sandpaper grit meshes reduces the average lamellae thickness at the interface and increases the crystallinity. Moreover, microporous defects are observed at the interface. The parameters viz elongation at break, current density, and the number of microporous defects increase initially and then decrease. However, the space charge accumulation, threshold electric field strength, and DC breakdown decrease initially and then increase. Notably, the #1000 roughness is a turning point. On the other side, degassing treatment promotes the orderly folding and recrystallization of the molecular chains arranged the amorphous region and significantly reduces the microporous defects at the interface. The mechanical properties are affected to a certain extent. The improved microstructure reduces the space charge accumulation and the current density of the sample, and increases the DC breakdown strength. An interfacial microscopic model and a charge transport model are proposed to analyze the mechanism of material properties under different roughnesses and electric fields. The results indicate that the ultra-smooth interface and degassing treatment strengthen the molecular chain linking at the interface. Moreover, they significantly reduce the microporous defects at the interface and improve the insulation properties of the factory joint.