Thermal aging characteristics and mechanism of insulation layer and semi-conductive shielding layer of high voltage cable

The thermal aging characteristics of the insulation layer and the inner semi-conductive shielding layer directly affect whole the safe operating characteristics of the cables. In this work, the physical and chemical properties, the evolution of electrical insulation properties and the interface characteristics have been studied. Furthermore, the molecular simulation has been used to analyze the free volume change and molecular chain motion characteristics induced by the temperature to reveal the aging mechanism. Experimental results show that new functional groups are generated in both the insulation layer and the 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%. The volume resistivity decreases from 2.93×10 ¹⁷ Ω· cm to 2.48×10 ¹⁶ Ω· cm and the breakdown field decreases from 63.83 kV/mm to 51.53 kV/mm. Besides, both the 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. The 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 Å ² to 19.10 Å ² lead to the decrease of breakdown field. This work is instructive for the assessment of the aging state of high voltage cables.