High-temperature polarization analysis of polyethylene and polyethylene-semicon bilayers

Abstract

Crosslinked polyethylene (XLPE) is a key material for power cable insulation due to its superior electrical properties. It is co-extruded with a semiconducting “semicon” layer, which is significantly more conductive than XLPE due to the incorporation of carbon black. Understanding the electrical properties of the XLPE/semicon bilayer and the interface between them is critical due to their common use. Two techniques were used to study electrical properties of XLPE and XLPE/semicon bilayers. One was current-voltage measurements, which analyzed resistivity as a function of time. The other was high-voltage polarization, which analyzed polarizability and dielectric loss as a function of electrical field. Thin films of PE and semicon were made via melt pressing. Dicumyl peroxide was infused to the beads prior to melt pressing such that XLPE was formed. All samples were degassed to remove DCP byproducts. Semicon/polyethylene bilayers were made by pressing the two layers together. LDPE and LDPE/semicon were also studied. Bulk brass electrodes were used. Current-voltage measurements showed that adding the semicon layer increased conductivity by an order of magnitude. Polarization measurements revealed that significant enhancement in the dielectric loss occurred at electric fields of 15 kV/mm and above. Polyethylene by itself and semicon bilayer samples had distinct electrical properties, and thus a proper understanding of the performance of XLPE in power cables required analyzing both. Further work is suggested that focuses on correlating changes in measured conductivity to the nature of the material interface (e.g., via enhanced charge injection and/or introduction of charge carriers).