High-Temperature Polarization Analysis of Polyethylene and Polyethylene- Semicon Bilayers

Abstract

Crosslinked polyethylene (XLPE) is a key material for power cables due to its superior performance as electrical insulation. It is co-extruded with a carbon black-filled semiconducting “semicon” polymer layer, which is significantly more conductive than XLPE. Understanding the electrical properties of the XLPE/semicon bilayer and the interface between the layers is critical due to their common use in high-voltage cables, and so, XLPE/semicon bilayers were developed, examined, and compared to XLPE in isolation. These materials were examined using current-voltage and dielectric displacement-voltage [D(P)–E] loop measurements. Current-voltage measurements were meant to examine the changes in leakage current while D(P)–E loops were used to examine the changes in dielectric loss, in both cases due to the addition of the semicon interface. Both techniques were used to analyze the charge transport response and development of space charge polarization in the bulk polymer and across the bilayer interface as a function of polarity at $90~^{\circ }$ C. An increase in the apparent conductivity of the XLPE was measured when layered with the semicon, attributed to the increase in charge injection at the XLPE/semicon interface. Additionally, increases in the conductivity were observed with the application of higher electric fields. The addition of the semicon layer resulted in an increase in both the dielectric constant and the dielectric loss, and greater increases in both as the field is increased. Thus, it led to an enhancement in space charge polarization. Based on the experimental measurements from high-voltage D(P)–E loop measurement, a nonlinear circuit model was developed to fit the data and provide a close match to experimental values for resistivity and capacitance. The high-field circuit model was based on forward and reverse biased resistor and diode pathways in parallel with each other and with the bulk damped capacitor, and it predicts space charge limited conduction with a semicon electrode and Poole-Frenkel conduction with a metal electrode, highlighting the importance of interfaces on XLPE insulation performance.