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Cross-linked polyethylene (XLPE) is often manufactured as pipe, film and foam products and used as electric insulation material. To improve the dielectric properties of XLPE film, CF4 dielectric barrier discharge (DBD) is applied as a surface modification method. The surface of XLPE film was modified at the low pressure of 1 kPa with different treatment time (15s, 30s, 60s, 120s, 240s and 480s). Electrical and optical characteristics are observed during DBD process, including applied voltage, measured current, photograph and emission spectra. Then, the surface properties, which are hydrophobicity, chemical composition, surface morphology, trap depth, dielectric loss and AC breakdown voltage, have been measured or observed by water contact angle test, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), isothermal relaxation current (IRC), frequency domain spectroscopy (FDS) and withstand voltage test. After the surface modification, the water contact angle, trap depth and breakdown voltage increase first and then decrease with treatment time extending. Meanwhile, the dielectric loss has an opposite trend, which represents better dielectric properties. The reason is that fluorine can reduce the surface energy, which can significantly influence the water contact angle. Better hydrophobicity can restrain the accumulation of outside pollution on the surface. Moreover, fluorine has strong electronegativity, so amorphous crystalline and impurities interface trap depth become deeper and the dielectric loss becomes lower. Therefore, appropriate DBD treatment conditions can apparently improve XLPE dielectric properties.

Dielectric response is closely related to the aging status of XLPE cables, and the isothermal relaxation current (IRC) has been applied to find out the change trend of cable insulation as a non-destructive method. In this paper, 4 groups of XLPE cable samples (16 cables in total) are artificially acceleratedly aged with different time interval at 120, 140 and 160°C, respectively, and their IRC curves are measured by a micro-current device. The results show that with the increasing aging time, IRC curves decrease slower and the absolute values of steady current become larger. In order to quantify the aging characteristics of XLPE cables, the trap parameters calculation theory is employed to work out the peak trap level of different polarization types. The experiment results show that the peak trap level of bulk polarization (type 1) remains stable and is around 0.76eV which is considered to be independent of aging degree. As the aging time increases, the peak trap level of amorphous crystalline relaxation (type 2), which is from 0.785eV to 0.82eV, increases at low temperature (120°C) due to the breaking of molecular chain and decreases at high temperature (more than 140°C) due to the over cross-linking. The peak trap level of impurities interface relaxation (type 3) is highly influenced by aging temperature, which is most sensitive to the aging degree of power cable. It is considered that IRC can be used as a practical way to judge cable insulation in the future.