Identification of dipolar relaxations in dielectric spectra of mid--voltage cross--linked polyethylene cables

TL;DR

This study combines TSDC and IDC techniques to identify dipolar relaxations in XLPE cable insulation, revealing that molecular dipole relaxation from manufacturing additives causes a broad dielectric peak around 95°C.

Contribution

It introduces a combined TSDC and IDC approach to analyze dielectric relaxations in XLPE cables, clarifying the origin of broad peaks in spectra.

Findings

Broad peak at 95°C in TSDC spectra attributed to dipolar relaxation.

IDC data fitted with KWW model showing Arrhenius behavior.

Main relaxation linked to additives' molecular dipoles.

Abstract

Medium-voltage cross-linked polyethylene (MV-XLPE) cables have an important role in the electrical power distribution system. For this reason, the study of XLPE insulation is crucial to improve cable features and lifetime. Although a relaxational analysis using Thermally Stimulated Depolarization Currents (TSDC) can yield a lot of information about XLPE properties, sometimes its results are difficult to interpret. In previous works it was found that the TSDC spectrum of cables is dominated by a broad heteropolar peak, that appears just before an homopolar inversion, but the analysis of the cause of the peak was not conclusive. We have used a combination of TSDC and Isothermal Depolarization Currents (IDC) techniques to investigate further this issue. In order to discard spurious effects from the semiconductor interfaces, samples have been prepared in certain configurations and preliminary measurements have been done. Then, TSDC experiments have been performed using conventional polarization between 140C and 40C. Also, IDC measurements have been carried out between 90C and 110C in 2C steps. The TSDC spectra show the broad peak at 95C. On the other hand, IDC show a combination of power and exponential charge currents. Exponential currents are fitted to a Kohlrausch--Williams--Watts (KWW) model. The parameters obtained present approximately an Arrhenius behavior with E_a=1.32eV, tau_0=3.29e-16s, with a KWW parameter beta=0.8. The depolarization current calculated from the obtained parameters turns out to match the dominant peak of TSDC spectra rather well. From the results and given the partially molten state of the material, we conclude that the most likely cause of the exponential IDC and the main TSDC peak is the relaxation of molecular dipoles from additives incorporated during the manufacturing process.