Annealing effect on the crystallinity characteristics of XLPE from High-voltage Cables

Abstract

The thermal effects during the cable operation brought various changes in cable insulation, strongly affecting the cable’s long-term behavior. In this study, a new and seven-year serviced 110 kV XLPE cable was taken, and the insulation layer was peeled and thermally aged at 115 for various 60, 120, and 180 days. Then, they were annealed at 90 to 115°C with a gap of 5°C, respectively. The gel content and differential scanning calorimetry (DSC) were measured. The results show that the 60 and 120-day aging tests promote the crystallization of XLPE and show a higher recrystallization point and crystallinity, while the 180 days aging test brought an opposite change. After the annealing process, melting point and crystallinity increase with the increasing annealing temperature. Before they reached the highest values, 60-day-aged peels showed higher the two factors than the fresh peels, with a slight difference to 120-day-aged peels, but 180-day aged showed the lowest values. However, the annealing process results in a lower melting point and crystallinity when the temperature exceeds the optimum point. Finally, it was found that the crystallization characteristics of the aged XLPE peels could be improved if the annealing was done at proper temperatures.