Impurities in electrical trees grown in field-aged cables

The micro-PIXE (proton-induced X-ray emission) facility of the University of Montreal was used to evaluate the impurity content in and out of electrical trees and the breakdown channels grown in a few field-aged cross-linked polyethylene cables. In a typical case, high amounts of Cl, Mg, Al, and K were detected in the electrical tree growing from a water tree. A wide dispersion of local impurity contents was observed in and out of the tree. In breakdown channels, traces of metallic conductor and concentric neutral as well as Si, Cl, K and Ca were observed in the carbonized walls of the channels. It is concluded that from this preliminary study it is difficult to determine whether some specific impurities may enhance the initiation of electrical tree, although very high amounts of chlorine, potassium, and calcium were detected in the studied trees