Mechanisms of water treeing are reviewed. A definition of water trees is proposed on the basis of permanent the hydrophilic character and the reduced breakdown strength. Properties are discussed that are relevant to the water tree definition and to the inception and propagation mechanism. This includes the water content, the (sub)micrometer structure and the dielectric properties. The discussed propagation mechanisms include electro-mechanical forces, diffusion of hydrophilic species and two types of oxidation mechanisms. Also attention is paid to the correlations between processes and aging conditions. Inception mechanisms include injection of hydrophilic species and/or oxidation in damaged regions, electro-mechanical damage due to high electrical fields.

Inception and propagation mechanisms of water treeing

A literature survey on the mechanisms of water-treeing electrical cables is presented The survey provides an introduction, some background, and examples of current research on water-treeing mechanisms The mechanisms of water-treeing have been broken into three categories: chemical, electrical, and mechanical Along with discussion on the types of mechanisms, methods are discussed for reducing tree growth by fillers, additives, or structural changes in polyethylene Some interesting new theories have been proposed which may help to explain the appearance and growth of water trees in electric cables

Water Treeing in Polyethylene - A Review of Mechanisms

Electron injection into polymers from field stress concentration sites is now generally accepted as important for the initiation of electrical trees, although very few published studieshave so far provided direct evidence for this. By means of the thermally stimulated discharge (TSD) technique in low density polyethylene (LDPE) double needle samples, electronic space charge densities of 2.7×10-3 Ccm-3 have been shown to exist in shallow (~3 ?m) regions surrounding the needle tips following ac stressing at a maximum field of 1.2×108 Vm-1. The present TSD results agree well with ?conventional? TSD data (planar samples, dc polarization) in the literature.

Thermally Stimulated Discharge of Polyethylene Following Ac Stressing

Water trees, which grow in all organic dielectrics under the action of electrical stress in a wet environment, have the characteristic of vanishing when the stress is removed and the material containing them is permitted to become dry. This behavior is quite distinctive in comparison to electrical trees which are actually permanent hollow channels, and electrochemical trees which are permanently stained or colored during growth by chemical agents from their environment.

Method for Visualization of Water Trees by Staining

A new water-tree-retardant XLPE has been developed for power cables after intensive studies of various materials. The new XLPE is filled with an organic polymer that acts as a barrier against water-tree propagation. Cables having this new insulation material generate far fewer and smaller water trees and have a voltage life in water at least four to ten times longer than normal XLPE cables.

Development of Water-Tree-Retardant XLPE Cables

Many organic insulating materials have voltage lives but the mechanism determining the voltage lives has not been established. In the case of cross-linked polyethylene (XLPE), by using a new experimental method, we observed that visible unhomogeneous micro-deterioration “microtrees” occurred in the insulation. We concluded that the voltage lives may be determined by these microtrees.

H. Matsubara

Papers

The mechanism determining voltage lives of cross-linked PE insulation