Investigation of dielectric and mechanical properties of undegassed XLPE cable with thermal treatment

In order to investigate the influence of cross-linking byproducts on dielectric and mechanical properties of 110 kV cross-linked polyethylene (XLPE) cable insulation, we selected the undegassed XLPE cable insulation which is heated at 70℃ in laboratory for different duration in terms of heating time, to prepare XLPE samples with different mass loss ratio. We carried out Fourier transform infrared spectroscopy (FTIR) test, electric breakdown test at 50 Hz, pulsed electro-acoustic (PEA) measurement and tensile test for XLPE samples. In FTIR spectrogram, the characteristic absorption peaks were observed to appear at 1680 cm^-1 and 1600 cm^-1, which can be related to two kinds of cross-linking byproducts including α-methylstyrene and acetophenone. The intensity of these absorption peaks decreases with the increase of mass loss ratio. It can be proposed that the mass loss of XLPE samples in the process of thermal treatment is caused by volatiliztion of α-methylstyrene and acetophenone. Therefore, the change of mass loss ratio can reflect the change of content of α-methylstyrene and acetophenone in the process of thermal treatment. The results of electric breakdown test showed that the reduction of α-methylstyrene and acetophenone leads to the enhancement of electric breakdown strength at 50 Hz. PEA measurement results showed that heterocharge is produced and accumulated when cross-linking byproducts are present, whereas no heterocharge is found when α-methylstyrene and acetophenone are absent. The formation of heterocharge is attributed to ionization of cross-linking byproducts under electric stress. Based on these results, it can be suggested that when AC electric field is applied on XLPE samples, the occurrence of ionization of cross-linking byproducts results in the increase of free carriers in XLPE insulation, leading to a lower electric breakdown strength. The results of tensile test showed that tensile strength is proportional to the mass loss ratio of XLPE samples, which suggests that cross-linking byproducts can reduce the tensile strength of XLPE insulation.