Ultrasonic characteristics of XLPE/SIR insulation under thermal aging condition for high-voltage cables and their correlation with insulation performance

The aging state of insulating materials in power equipment directly affects the reliability and service life of its operation. Therefore, accurately identifying the degree of material aging is crucial for preventing equipment failures and ensuring the safety of the power grid. The electrical properties, physical and chemical properties, mechanical properties and ultrasonic characteristics of cable insulation Cross-linked polyethylene (XLPE) and silicone rubber (SIR) under thermal aging conditions were studied, and the correlation characteristics between the ultrasonic characteristics caused by aging and the electrical properties were explored. Furthermore, the intrinsic connection between the changes in the microstructure of materials and the degradation of their macroscopic properties was revealed through molecular dynamics simulation. Through molecular dynamics simulation, the intrinsic relationship between the microscopic structure changes of the material and the degradation of its macroscopic properties was revealed. The experimental results show that when XLPE is thermally aged at 135 °C (in air) and SIR at 175 °C (in air), the dielectric constant values of both materials gradually increase with the increase of aging time. After 1008 h, the dielectric constant of XLPE and SIR increased by 12.7 % and 31.7 % respectively. Under the same aging time, the volume resistivity of XLPE decreases from 1.8 × 10¹⁴ Ω·m to 2.1 × 10¹³ Ω·m, while the volume resistivity of SIR decreases from 1.71 × 10¹³ Ω·m to 6.79 × 10¹² Ω·m. The elongation at break and tensile strength of XLPE and SIR gradually decreased. The ultrasonic velocity of XLPE decreases from 2200 m/s to 1953.8 m/s, while that of SIR increases from 911.3 m/s to 1021.2 m/s. Simulation results show that the free volume and mean square displacement of XLPE increase after aging, while those of SIR decrease, which affect the change in sound velocity. The band gap width of the two materials decreases after aging, and the density of states changed, resulting in a decline in insulation performance. The ultrasonic sound velocity is significantly correlated with the aging degree of the material, and can be used as an effective indicator for evaluating the aging status of XLPE and SIR. This study establishes the correlation between ultrasonic characteristics and insulation performance during the aging process of materials through microstructure changes, providing a theoretical basis for on-site non-destructive testing of the thermal aging state of high-voltage cables.