Electrical treeing

In electrical engineering, treeing is an electrical pre-breakdown phenomenon in solid insulation. It is a damaging process due to partial discharges and progresses through the stressed dielectric insulation, in a path resembling the branches of a tree. Treeing of solid high-voltage cable insulation is a common breakdown mechanism and source of electrical faults in underground power cables.

Electrical treeing first occurs and propagates when a dry dielectric material is subjected to high and divergent electrical field stress over a long period of time. Electrical treeing is observed to originate at points where impurities, gas voids, mechanical defects, or conducting projections cause excessive electrical field stress within small regions of the dielectric. This can ionize gases within voids inside the bulk dielectric, 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 partial discharges (PD) then react with the nearby dielectric, decomposing and further degrading its insulating capability. Gases are often liberated as the dielectric degrades, creating new voids and cracks. These defects further weaken the dielectric strength of the material, enhance the electrical stress, and accelerate the PD process.

In the presence of water, a diffuse, partially conductive 3D plume-like structure, called a water tree, may form within the polyethylene dielectric used in buried or water-immersed high voltage cables. The plume is known to consist of a dense network of extremely small water-filled channels which are defined by the native crystalline structure of the polymer. Individual channels are extremely difficult to see using optical magnification, so their study usually requires using a scanning electron microscope (SEM). Water trees begin as a microscopic region near a defect. They then grow under the continued presence of a high electrical field and water. Water trees may eventually grow to the point where they bridge the outer ground layer to the center high voltage conductor, at which point the stress redistributes across the insulation. Water trees are not generally a reliability concern unless they are able to initiate an electrical tree. Another type of tree-like structure can form with or without the presence of water is called an electrical tree. It also forms within a polyethylene dielectric (as well as many other solid dielectrics). Electrical trees also originate where bulk or surface stress enhancements initiate dielectric breakdown in a small region of the insulation. This permanently damages the insulating material in that region. Further tree growth then occurs through as additional small electrical breakdown events (called partial discharges). Electrical tree growth may be accelerated by rapid voltage changes, such as utility switching operations. Also, cables injected with high voltage DC may also develop electrical trees over time as electrical charges migrate into the dielectric nearest the HV conductor. The region of injected charge (called a space charge) amplifies the electrical field in the dielectric, stimulating further stress enhancement and the initiation of electrical trees as the site of pre-existing stress enhancements. Since the electrical tree itself is typically partially conducting, its presence also increases the electrical stress in the region between the tree and the opposite conductor. Unlike water trees, the individual channels of electrical trees are larger and more easily seen. Treeing has been a long-term failure mechanism for buried polymer-insulated high voltage power cables, first reported in 1969. In a similar fashion, 2D trees can occur along the surface of a highly stressed dielectric, or across a dielectric surface that has been contaminated by dust or mineral salts. Over time, these partially conductive trails can grow until they cause complete failure of the dielectric. Electrical tracking, sometimes called dry banding, is a typical failure mechanism for electrical power insulators that are subjected to salt spray contamination along coastlines. The branching 2D and 3D patterns are sometimes called Lichtenberg figures.

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