Abstract
In this paper, quantum chemical calculation and isothermal relaxation current (IRC) measurement are performed on thermally aged XLPE cables to investigate the effect of defects on dielectric properties and trap energy distribution of XLPE insulation. Samples of XLPE cables are thermally aged at 135 °C up to 45 days. Microscopic experiments including gel permeation chromatography, differential scanning calorimetry, and Fourier transform infrared spectrometer show that the molecular weight and crystallinity of XLPE decreases with aging time. The IRC results show that both the trap density and the trap depth increase with aging. To quantitatively calculate the trap depth variation due to physical and chemical defects by thermal aging, a quantum chemical calculation is carried out based on Gaussian 09. It is found that the broken cross-linking bonds between molecular chains weaken the stability of the molecular network structure, introducing the shallow physical trap (0.3 eV) in the aged XLPE. The molecular chain is thermally oxidized, the carbonyl group is introduced as a deep chemical trap (1.44 eV) in the XLPE. The chemical defect not only changes the localised energy state, but also significantly decreases the intrinsic breakdown strength of XLPE molecule from 0.62 to 0.51 V/Angstrom in quantum chemistry calculation.
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