Correlations between structural changes and dielectric behavior of thermally aged XLPE

Abstract

This study aims at elucidating the correlations between structural changes and the variation of dielectric properties of cross-linked polyethylene (XLPE) during thermal aging. For this purpose, thermal aging experiments were carried out on XLPE samples at 80, 100, 120 and 140 °C for different aging times for a maximum total duration of 5040 hours. Chemical changes were assessed by Fourier Transform Infrared Spectrum (FTIR). Besides, the crystallinity ratio and the melting temperature were evaluated using Differential Scanning Calorimetry (DSC). Frequency domain dielectric spectroscopy was used to investigate the evolution of the dielectric constant and losses of XLPE during aging. The results show that aging at 80 and 100 °C could help to improve the crystalline state of XLPE which leads to the decrease of the dielectric constant. However, for higher thermal stress, the main aging mechanism of XLPE is thermal oxidation which leads to deteriorate the morphology and increase both the polarization and losses, mainly at low frequencies. It has been found also that in some cases, the non-homogeneity of the color of XLPE seems to indicate the increase of interfaces in the material which could lead to increase the dielectric losses due to the interfacial polarization.

Ageing state identification and analysis of AC 500 kV XLPE submarine cable based on high‐voltage frequency dielectric response

“…However, the intensity of absorption peak at 1726 and 3000–3550 cm −1 increased gradually. This shows that the polarity by‐products are produced after XLPE insulation degradation [22 ].…”

“…The strongest absorption peak at 2848 and 2913 cm −1 is relative to the symmetric and asymmetric stretching vibration of methylene groups (–CH 2 ) [20 ]. The absorption peak between 3000 and 3550 cm −1 corresponds to the hydroxyl group (–OH) [20–22 ]. From Fig.…”

“…11, from new, thermal ageing to electro‐thermal ageing, the intensity of absorption peak at 719, 1464, 2848 and 2913 cm −1 decreased gradually. This is caused by the break of the molecular chain inside XLPE insulation [22 ]. However, the intensity of absorption peak at 1726 and 3000–3550 cm −1 increased gradually.…”

“…At the same time, the polar groups such as carbonyl group, ketone, hydroxyl group etc. are produced, which leads to the enhancement of the polarisation strength and conductivity of the XLPE submarine cable insulation [21, 22 ]. The relaxation time of α relaxation polarisation is shortened.…”

Contact‐free dielectric response measurement based on limit fitting by neural network

“…x (k+1) = x (k) + h lm (15) The convergence condition set by the algorithm is E k < 10 −4 , which means at this time, x (k) stops iterating and the model is built.…”