Degraded insulation
Introduction
In electrical engineering, electrical arborescence is an electrical phenomenon prior to the dielectric breakdown of an insulating material. Sometimes, even in Spanish, the English term "electrical treeing" is often used as a synonym. It is a progressive and irreversible destructive process due to partial discharges, which advances within or on the surface of a dielectric when it is subjected to prolonged high voltage electrical stress. The name "arborescence" is due to the fact that it follows a pattern similar to the branches of a tree.
The phenomenon and its causes
Electrical treeing initiates and gradually propagates when a solid dielectric material is subjected to the stress of a high and divergent electric field for a long time. The characteristics of the arborescence will depend on the electrical voltage used, the dielectric material, and even variations in morphology produced by different mixing proportions or temperature variations.[1]
"Doping (semiconductors)") can also occur within doped semiconductor materials when they are polarized in the non-conducting direction, which will dramatically worsen their reverse peak voltage. It is observed that electrical trees begin at points in the insulator where impurities, gas bubbles, physical discontinuities, mechanical defects or conductive particles create small regions with high electrical stress on the dielectric material. This can ionize a gas within a discontinuity of the dielectric mass, and produce small electrical discharges between the walls of the discontinuity. An impurity or defect can even produce a partial fracture of the dielectric itself. The ultraviolet light and ozone produced by these partial discharges will react with and break down the neighboring dielectric material, degrading its ability as an insulator. Degraded dielectrics frequently release gases, thereby creating new physical discontinuities and fractures. These defects will at some point impoverish the insulating capacity of the material, worsen electrical stress and accelerate the partial discharge and degradation process.
Over time, a three-dimensional tree-like branching figure, partially conductive of electricity, will form within the dielectric. This arborescence can sometimes grow to the point of causing complete failure of the insulating material. This is a common long-term failure mechanism in polymer-insulated underground high-voltage cables. Sometimes this failure is initiated by the penetration of moisture into the insulating material, which will produce a phenomenon called , or , which will then be followed by an electric tree that will be seen superimposed on the image of the aqueous tree.[2][3].