Spatial distribution of crystalline impurities in degraded high voltage XLPE cable screens using synchrotron wide-angle x-ray scattering

Degradation of cross-linked polyethylene (XLPE) insulations by vented water treeing is a phenomenon that can limit the lifetime and reliability of subsea power cables, as well as their voltage rating. Recent studies have shown that inorganic impurities embedded in the bulk of the semi-conductive (SC) screens can be responsible for inception and growth of vented water trees through channel-like nanostructured tracks. Characterization of the entire region of interest, stretching from the contaminant to the vented water tree, has proven challenging with conventional techniques. Here we have developed a qualitative methodology based on synchrotron wide-angle x-ray scattering to spatially locate crystalline impurities in the cable insulation system, enabling detection of very small impurities in a large bulk sample. NaCl was the dominant crystalline impurity and was present in the voids, along the nanostructured tracks in the SC screen and the vented water trees. Trace amounts of NaCl were also detected within a large volume of an unaged cable screen, indicating that impurities are present prior to exposure of the cables to standardized tests including elevated water temperature. These results provide crucial information about the chemical prerequisites for the formation of the nanostructured track degradation causing inception of long vented water trees at the SC screen/XLPE interface.