Terahertz Time-Domain Spectroscopy Characterization of Aged XLPE Cable Insulation

Cross-linked polyethylene (XLPE) has been widely used in high voltage power cables as insulation material due to its superior mechanical and electrical performance. Thermal aging causes irreversible chemical and morphological change that may strongly reduce the dielectric strength of XLPE, producing a limitation to the effective service life of the power cables. Therefore, it is worthwhile to monitor the aging condition of XLPE insulation so that the safety of the power system can be ensured. Terahertz technology has attracted much attention because many molecular structure information is involved in terahertz frequency. In this paper, terahertz time-domain spectroscopy (THz-TDS) was employed to study the effect of thermal aging on XLPE power cable insulation. XLPE sheets were aged at 130 °C for different periods in an air forced oven. The pristine and aged samples were studied by differential scanning calorimeter (DSC) and the crystallinity were determined. The extent of oxidation of the insulations were assessed by Fourier transform infrared (FTIR) spectroscopy. Our experimental results show that the index of refraction and relative dielectric constant in the frequency range from 0.5–2 THz increase with aging time. The aged samples show an increase in oxidation products and a decrease in the crystallinity, which indicates the molecular chain scission and oxidation. This result suggests that THz-TDS has potential to obtain information about the cable insulation condition.