A possible water tree initiation mechanism for service-aged XLPE cables: Conversion of electrical tree to water tree

In this paper, the conversion of electrical tree to water tree is found in a cross-linked polyethylene (XLPE) cable, and a possible conversion mechanism is presented. An electrical-tree initiation experiment was performed under AC voltages in XLPE cable specimens. After that, a 390-hour accelerated water-tree aging experiment was performed for the specimens, and water trees at the tips of the electrical tree were observed in the slices by an optical microscope. Also, to further understand and confirm the conversion, similar experiments were performed, and the same conversion phenomenon was observed in XLPE block specimens. Depending on the geometric size observed in water-tree cable slices, the 2D simulation model, including an electrical tree and a water tree, was constructed for electric field simulation. According to the results of electric field simulation, the electrical trees can be directly initiated at microscopic defects in service XLPE cables under the effect of transient over voltages. After initiation of the electrical tree, the forked branches can reduce the electric field around the tips of the electrical tree. When the over voltages disappear and moisture invades, the tips of the electrical tree can become the initiation sites of water trees because of stagnant propagation of the electrical tree. For this reason, the conversion of electrical tree to water tree is possible for service-aged cables, and these water trees even can make the electric field uniform at the tips of electrical tree.