Aging phenomenon by water treeing and its mechanism in high voltage transmission XLPE cables

Investigations were made on 66 kV XLPE insulated power transmission cables removed from 7 lines that experienced failures after 10 or more years in service. It was found by pre-breakdown detection test that the weakest point of each cables corresponded to vented water trees from conductor shielding layer. Such vented trees were classified into two types: “blur” water trees appearing blue in color and ordinary “white” water trees. Elemental analyses revealed that iron and sulfur were contained peculiarly at the nuclei and the paths of “blue tree”. Reproduction of blue trees was attempted, and it turned out that blue trees were developed by the presence of both of iron sulfate solution and aromatic amine additive. It turned out that blue trees became harmful due to the intrusion of such conductive impurities into the paths of blue trees. Based on these investigations, the mechanisms for the formation of the blue trees and the premature breakdown of the cables were proposed