Effects of Thermal Aging on the Crystal Structures of the XLPE Cable Insulating Material at Different Temperatures

The cross-linked polyethylene (XLPE) has been extensively used as an electrical insulating material for medium and high voltage power distribution cables for its outstanding physical, chemical and electrical properties. In order to study the influence of thermal aging at different temperatures on the crystal structures of XLPE cable insulation, thermal aging was conducted on 110 kV XLPE cable insulation at 100℃ and 160℃ respectively. The crystal structures of the aged samples were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), and the concentration of antioxidants in different aged samples was quantitatively measured by DSC method. It is deduced by the experiment results that the thermal aging process can be divided into the physical aging stage and the chemical aging stage depending on the concentration of antioxidants in XLPE. At the physical aging stage, the re-crystallization process lead to the improvement of crystal structure and the increase in crystallinity. At the chemical aging stage, drastic chain scission lead to sharp breakage of intramolecular and intermolecular bonds, leading to the decrease in crystallinity. At 100℃, the thermal aging mainly affects the amorphous regions because of the much denser structures of crystalline regions, which may make the oxygen diffusion more difficult. At 160℃, the thermal aging affects the molten spherulites, resulting in decreased lamellae, increased interlamellar spacing and destruction of spherulites.