XLPE high voltage insulation; A link between DC conductivity and microstructure

Abstract

Crosslinked low-density polyethylene (XLPE) with its simple chemical composition is currently used as the most promising material for the insulation layer in extruded power cables. The processability of low-density polyethylene during extrusion followed by its crosslinking results in an infusible network that withstands high operating temperatures. One of the most important electrical properties that determine the insulation performance is the high voltage DC conductivity (direct-current conductivity under a high electric field). A lower DC conductivity at elevated temperatures decreases the losses in the insulation and hence lowers the risk of thermal runaway. The DC conductivity in pure polyethylene (PE) is predominantly due to electronic transport, and in the case of XLPE the DC conductivity is also affected by the combined effects of the concentration of crosslinking byproducts and the semicrystalline microstructure. The semicrystalline microstructure is complex, and it includes the lamellar morphology, the micrometer-sized superstructure, and the degree of crystallinity. These parameters inter-play to affect the final DC conductivity. This article first describes an overview of XLPE insulation of extruded HVDC cable and then addresses the less-known link among polymer processing, microstructure, and macroscale property (DC conductivity) of XLPE. The revealing of this link leads to a deep understanding of XLPE microstructure and possible ways to improve XLPE insulating properties during polymer processing.