Effect of Thermo-Mechanical Stresses on Insulating Properties of 500 kV EHVDC XLPE Cables Under Multi-Field Ageing

This study used a self-constructed multi-field ageing experimental setup to ageing experiments on ring-cut XLPE cable samples. Crystalline properties, DC conductivity and DC breakdown strength were analyzed. A degradation model incorporating electric field, temperature, and mechanical stress was established and fitted using the Levenberg–Marquardt algorithm. Experimental results showed that under electro-thermal-mechanical ageing, the crystallinity of XLPE decreased 29.4%, and lamellar thickness reduced from 5.51 nm to 4.94 nm. The threshold field dropped significantly to 5.54 kV/mm, and the DC breakdown strength declined to 220.88 kV/mm, indicating degradation under electric field stress. In contrast, samples aged without electric field retained higher crystallinity and lamellar thickness, with threshold field and breakdown strength reaching 11.40 kV/mm and 298.46 kV/mm, respectively. The model achieved high accuracy, with a fitting error below 2%. These findings highlight the relationship between microstructural changes and insulation performance, offering insights for evaluating and predicting the long-term reliability of XLPE insulation in multi-field environments.