On the conducting and non-conducting electrical trees in XLPE cable insulation specimens

The conducting and non-conducting properties of electrical trees in cross-linked polyethylene (XLPE) cable insulation are studied by means of partial discharge (PD) measurement, optical microscope, confocal Raman microprobe spectroscopy and scanning electron microscopy (SEM). Specimens equipped with an embedded electrode system and constituted of a needle separated by approximately 2 mm from the counter electrode were utilized. Various types of electrical trees were produced at a range of voltage levels from 9 to 15 kV. It was found that the electrical trees grown at low voltage levels exhibited very different electrical properties from those grown at higher voltage levels. Branch-pine trees were formed at the low voltage conditions, displaying buildup of conducting main channels composed of disordered graphitic carbon deposited on the tree sidewalls. The average domain of the graphitic residues was estimated at the range of 8-8.4 nm in size, which resulted in a sufficient channel conductivity to suppress the PD activity within the main tree channels and caused growth of the pine structure. The branch trees, grown at higher voltage levels, showed typical characteristics of non-conducting trees, in which continuous discharges eroded the tree sidewalls and the observed intensity of fluorescence decreased evidently with the distance from the needle electrode to the tree tip. The bush trees at high voltage levels also showed the non-conducting characteristics. However, some carbonized residues were formed locally in them near the needle electrode, which can be connected to the long and intensive discharge activity during the tree growth.