Thermal Aging Characteristics and Mechanism of Insulation Layer and Semi-conductive Shielding Layer of High Voltage Cable

Thermal aging characteristics of insulation layer and inner semi-conductive shielding layer directly affect the safe operating characteristics of cables. In this work, physical and chemical properties, the evolution of electrical insulation properties, and interface characteristics are studied. Furthermore, molecular simulation has been used to analyze free volume change and molecular chain motion characteristics induced by temperature to reveal the ageing mechanism. Experimental results show new functional groups are generated in both insulation layer and inner semi-conductive layer. The carbonyl index of insulation layer increases from 0.10 to 0.29, while the degree of crystallinity decreases from 35% to 29.5%. Volume resistivity decreases from 2.93 × 10¹⁷ Ω·cm to 2.48 × 10¹⁶ Ω·cm, and the breakdown field decreases from 63.83 kV/mm to 51.53 kV/mm. Besides, both dielectric constant and dielectric loss of specimen increase after thermal aging. The degree of crystallinity of semi-conductive shielding layer drops from 9.00% to 3.68% and its volume resistivity also decreases. Simulation results show that the free volume fraction of XLPE molecules increases from 15.3% to 16.5%, and mean square displacement increases from 4.29 Ų to 19.10 Ų lead to the decrease of breakdown field. This work is instructive for the assessment of the aging state of high-voltage cables.