Investigations of electrical trees in the inner layer of XLPE cable insulation

It is experimentally found that there are three kinds of electrical trees, the branch-like tree, the bush-like tree and the mixed tree combining the above two types in insulation inner layer of a 66 kV cross linked polyethylene cables. The mixed tree will appear in XLPE sample when voltage frequency is less or equal to 250 Hz, yet only one kind of electrical tree, i.e., the dense branch-like tree, will develop when the frequency is equal to or higher than 500 Hz. The reason is that both non-uniform crystallization and internal residual stress will exist in semi-crystalline cross linked polyethylene cable main insulation in the course of cable manufacturing. Based on analyses of fractal properties of these electrical trees, it is found that the propagation characteristics of electrical trees can be depicted by the growth time dependence of fractal dimension and the structure of electrical trees can also be classified by means of fractal dimension. Finally the explanation for these experimental results is given in the paper.