Effect of degassing treatment durations on physico-chemical and electrical properties of 500 kV extra HVDC XLPE cable insulation

An optimal degassing treatment duration for the preparation of commercial 500 kV extra high voltage direct current (EHVDC) cross-linked polyethylene (XLPE) insulation can improve its overall properties, reduce the manufacturing cost and time, which is urgently needed for high voltage cables suppliers. This paper analyzes the effect of different degassing duration on the content of cross-linking by-products, morphological structure and insulation properties of 500 kV EHVDC XLPE insulation. The raw pellets of commercial 500 kV EHVDC XLPE cable material are used to prepare the test samples by the hot-pressing method. Five samples with the different degassing durations of 0 h, 12 h, 36 h, 90 h, and 200 h were at 70°C. Physico-chemical and electrical methods are used to characterize the overall properties of the material under the different degassing durations. The results show that the measured content of the crosslinking by-products is consistent with the theoretical values calculated by Fick's second law. The oxidation induction time of the samples is reduced and tensile properties are improved with the increase of the degassing time. Furthermore, degassing treatment increased the lamellae thickness in the early stages and reduced the difference in the internal aggregation structure of the material. It was found that the degassing treatment significantly suppressed the space charge accumulation inside the material. Moreover, the degassing treatment of 36 h can increase the dielectric breakdown strength and reduce the current density of the samples.