Structures and propagation characteristics of electrical trees in XLPE cable insulation

The initiation, propagation and structures feature of electrical trees under sine high voltage of 50-2000Hz in semi-crystalline XLPE cable insulation were studied, and the structures feature of electrical trees and the relation between the structures feature of electrical trees and the aggregation state and residual mechanical stress of material in XLPE cable insulation samples were summarized systematically. The main experiment setups included a high voltage generator with adjustable frequency, a living digital camera and a microscope computer system. A special needle electrode and sample processing technology were used in the experiment. It is experimentally found that there are three kinds of electrical trees, the branch-like tree, mixed tree combining branch with jungle, and single jungle-like trees, which will appear in the sample at the frequency of 50Hz due to non-uniform crystallization and micro-defects concentrated in XLPE cable insulation. Only one kind of electrical tree, the dense branch-like tree, will develop at the frequency equal to or higher than 500Hz. The propagation mechanisms of these kinds of electrical trees mainly depend on crystalline state of the dielectric and the frequency of applying voltage. The structures and propagation characteristics of electrical trees are not related to the aggregation state of material at high frequency (≥500Hz) but closely related to aggregation state of material in low frequency (50Hz). The propagation characteristics of electrical trees not only depend upon the crystal-boundary and microspore in amorphous interface, but also depend upon the impurity concentration and the relative position between electrode tip and sphere-crystals or amorphous region in low frequency. Because the processes of injection and initiation charge from and to dielectric by electrode are more violent at high frequency than in low frequency and it can form relatively uniform dielectric weak region at the front of needle electrode, so the initiation and propagation law of electrical trees at high frequency are similar to each other. Based on the analyses of the fractal properties of these electrical trees, it is found that the propagation characteristics of electrical trees can be characterized by the growth time dependence of fractal dimension, and that these electrical trees can be classified and quantized by means of fractal dimension. Finally the explanation for these experimental results is given.