Influence of thermal treatment and residues on space charge accumulation in XLPE for DC power cable application

The effects of cross-linking by-products (residues) quantity and thermal treatment on space charge accumulation and decay in manufacturer modified XLPEs for DC power cable application have been investigated using the pulsed electro-acoustic technique. The threshold stress for space charge generation among the modified and reference XLPEs during voltage-ramping was found to show considerable variation and to depend upon the material and the amount of residue present. However, the modified XLPE material was found to exhibit a higher threshold for space charge accumulation than the reference XLPE whatever the conditions. De-gassed samples were found to exhibit the highest threshold stress, with that of the modified de-gassed XLPE accumulating no space charge at all even after 24 h stressing at 70 kV. In general heterocharge regions were formed when the residues were present and homocharge or no charge was formed when the residues were removed by degassing. Differences were also found in the space charge decay following short-circuit (volts-off), with the decay of heterocharge being rapid, whereas that of homocharge being slow. The charge accumulations have been tentatively explained by the mechanisms of ion-pair separation when residues are present, and interfacial injection when residues are absent. Decay of the heterocharge is governed by ion-pair displacement just as the generation and will have a similar time scale. In contrast homocharge decay will be governed by charge de-trapping and extend to time scales well beyond that of injection for charge in the deepest traps