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. Further-more

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 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17Ω·cm to 2.48 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16Ω·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 Å<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2to 19.10 Å<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2 lead to the decrease of breakdown field. This work is instructive for the assessment of the aging state of high-voltage cables.