Electrical treeing
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Within the field of electrical engineering, treeing is a process whereby branching channels, or chains of small fractures, are created within, or on the surface of, a solid dielectric material as a result of cumulative partial discharges.
[edit] Occurrence and causes
Treeing occurs when a dielectric material is subjected to excessive high voltage stress over a long period of time. Treeing is observed to originate at points where impurities, gas voids, or mechanical defects cause excessive electrical field stress within small regions of the dielectric. This can ionize gases within the void, creating small electrical discharges between the walls of the void. An impurity or defect may even result in the partial breakdown of the solid dielectric itself. Ultraviolet light and ozone from these discharges then react with the nearby dielectric, decomposing and degrading it. Gases are often liberated as the dielectric degrades, creating new voids and cracks. These weaken the dielectric strength of the material, enhance the electrical stress, and accelerate the PD process.
Over time, a branching 3D tree-like figure is formed within the dielectric. The tree can grow to the point that it eventually causes complete electrical failure of the dielectric. This has been a long-term failure mechanism for buried polymer-insulated high voltage power cables. In similar fashion, 2D trees can occur along the surface of a highly stressed dielectric, or across a dielectric surface that has been contaminated by a film of dust or mineral salts. Over time, the growing partially conductive trails can cause complete failure of the dielectric. Tracking, sometimes called dry banding, is a typical failure mechanism for electrical power insulators that are subjected to salt spray contamination along a coastline. Branching 2D and 3D figures are sometimes called Lichtenberg Figures.
Following are some examples of 2D and 3D electrical trees:
