Within the field of electrical engineering, treeing is an electrical pre-breakdown phenomenon. It is a damaging process due to partial discharges and progresses through the stressed dielectric insulation, whose path resembles the form of a tree.
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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.
Over time, a partially conductive, 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 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.
Electrical treeing or "Lichtenberg figures" also occur in high-voltage equipment just before breakdown. Following these Lichtenberg figures in the insulation during postmortem investigation of the broken down insulation can be most useful in finding the cause of breakdown. An experienced High-voltage engineer can see from the direction and the type of trees and their branches where the primary cause of the breakdown was situated and possibly find the cause. Broken-down transformers, High-voltage cables, bushings and other equipment can usefully be investigated in this way; the insulation is unrolled (in the case of paper insulation) or sliced in thin slices (in the case of solid insulations), the results are sketched and photographed and form a useful archive of the breakdown process.
The electrical trees can be further categorized depending on the different tree patterns. They are dendrites, branch type, bush type, spikes, strings, bow-ties and vented trees. The two most commonly found tree types are bow-tie trees and vented trees.[1]
Electrical trees can be detected and located by means of partial discharge measurement.
As the measurement values of this method allow no absolute interpretation, data collected during the procedure is compared to measurement values of the same cable gathered during the acceptance-test. This allows simple and quick classification of the dielectric condition (new, strongly aged, faulty) of the cable under test.
To measure the level of partial discharges, a sinusoidal 0.1 Hz VLF (very low frequency) voltage is used, just as it is used with VLF-cable testing.
Modern PD-detection systems employ a PC based software (for analysis and display of measurement results) and a PD-detector. By the so called time domain reflectometry (TDR) method the PD-detector actually measures partial discharge activity. The partial discharge intensity is measured in pico Coulomb (pC) and displayed versus time.
A fully automatic analysis of the reflectograms collected during the measurement allows the location of insulation irregularities and electrical trees. Usually they are displayed in a partial discharge mapping format. Additional useful information about the device under test can be derived from a phase related depiction of the partial discharges.
A sufficient measurement report contains:
Detection and location of electrical trees by partial discharge measurement