Experimental investigation on the cause of harmfulness of the blue water tree to XLPE cable insulation

Through investigations made on crosslinked polyethylene (XLPE) insulated po wer cables that had experienced failures, blue-colored water trees were obs erved in addition to ordinary white water trees. The blue water tree was fo und to be much more harmful than the white water tree, since more electrica l trees started at the blue trees. Elemental analysis revealed that iron an d sulfur exist in the blue tree. In the case of this blue water tree, space charge exists abundantly but only at the tree tip, while no charge signal is observed on the electrode from which the tree starts. This indicates tha t the conductivity of the blue tree is very high. This is considered to be the main reason of the harmfulness of this tree.