Insulation Characteristics and Failure Mechanism of High-Voltage Cables under Different Thermal Aging Temperatures

Cross-linked polyethylene (XLPE) high-voltage cables are affected by environmental factors such as temperature, voltage and humidity during operation, resulting in the degradation of the insulation properties of high-voltage cables and the occurrence of aging problems. Temperature is the main factor causing cable aging. When a short circuit occurs in the cable, the temperature of the cable can reach up to 250℃, which seriously affects the insulation characteristics of the cable. At present, the aging problem of high-voltage cables is mainly studied for the insulation state at individual temperatures, and there is a lack of corresponding research reports on the long-term operating characteristics of high-voltage cables at different temperatures. In this paper, the physicochemical and electrical properties of high-voltage cable insulation at different thermal aging temperatures were studied, and the changes of molecular dynamics and internal temperature field of XLPE high-voltage cables were analyzed by combining experimental results with molecular simulation and finite element simulation analysis. Firstly, commercial high-voltage cable insulation material was used to make XLPE insulation layer by melt preparation. The XLPE was placed in ovens with different thermal aging temperatures, and the aging temperatures were set to 140℃, 160℃ and 180℃, respectively. The change patterns of functional groups, dielectric constant, dielectric loss and breakdown strength of XLPE were tested under different aging times, respectively. Secondly, the thermal-oxidative aging mechanism of high-voltage cables was analyzed according to the change pattern of physicochemical and electrical properties of XLPE under different thermal aging temperatures. The results show the deterioration of XLPE molecular chains accelerates the generation of polar functional groups, mainly including carbonyl and carbon-carbon double bonds, with the increase of aging temperature. The insulation performance reaches a critical value when the aging time of the high-voltage cable insulation layer exceeds the failure point of the life, the content of polar groups increases rapidly. The change of microscopic molecular structure leads to the rapid decline of macroscopic electrical properties. The higher the temperature, the faster the decrease of breakdown field strength, and the breakdown field strength decreased to 53.73 kV/mm after aging at 180℃ for 24 h. Finally, the molecular dynamics characteristics of the high-voltage cable are analyzed in combination with molecular simulation and finite element simulation with respect to the variation of the internal temperature field. The free volume rate of XLPE is 12.53%, 13.40%, and 14.07% when the temperature is 140℃, 160℃, and 180℃, respectively. The higher the aging temperature, the higher the free volume of XLPE molecules. The increase in the free volume of XLPE molecules leads to an increase in the space available for the free movement of electrons, a decrease in the resistance to electron movement and an increase in the kinetic energy of the electrons, which exacerbates the degree of damage to the molecular chain. For example, in 2 000 ps, the mean square displacements are 22.43 Å2, 26.57 Å2, and 44.77 Å2 at temperatures of 140℃, 160℃, and 180℃, respectively. According to the variation rules of dielectric constant and dielectric loss at different aging temperatures, the internal temperature of high-voltage cables increases with the increase of dielectric constant and dielectric loss, which plays a positive feedback role in the thermal aging of high-voltage cables. © 2024 Chinese Machine Press. All rights reserved.