Electrical treeing
Encyclopedia
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
, treeing is an electrical pre-breakdown phenomenon. It is a damaging process due to partial discharge
Partial discharge
In electrical engineering, partial discharge is a localised dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress, which does not bridge the space between two conductors...
s and progresses through the stressed dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
insulation, whose path resembles the form of a tree.
Occurrence and causes
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, gasGas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
voids, mechanical defects, or conducting projections cause excessive electrical field stress within small regions of the dielectric. This can ionize
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
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
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
from these partial discharge
Partial discharge
In electrical engineering, partial discharge is a localised dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress, which does not bridge the space between two conductors...
s (PD) then react with the nearby dielectric, decomposing and further degrading its insulating capability. Gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es 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
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
-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 figure
Lichtenberg figure
Lichtenberg figures are branching electric discharges that sometimes appear on the surface or the interior of insulating materials. They are named after the German physicist Georg Christoph Lichtenberg, who originally discovered and studied them...
s.
Technical occurrence
Types of electrical trees
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.Bow-tie trees: Bow-tie trees are trees which start to grow from within the dielectric insulation and grow symmetrically outwards from the electrodes. As the trees start within the insulation, they have no free supply of air which will enable continuous support of partial discharges. Thus, these trees have discontinuous growth which is why the vented trees usually do not grow long enough to fully bridge the entire insulation between the electrodes, therefore causing no failure in the insulation.
Vented trees: Vented trees are trees which initiate at an electrode insulation interface and grow towards the opposite electrode. Having access to free air is a very important factor for the growth of the vented trees. These trees are able to grow continuously until they are long enough to bridge the electrodes, therefore causing failure in the insulation.
Detection and Location of electrical trees
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
Very low frequency
225px|thumb|right|A VLF receiving antenna at [[Palmer Station]], Antarctica, operated by Stanford UniversityVery low frequency or VLF refers to radio frequencies in the range of 3 kHz to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals...
) voltage is used, just as it is used with VLF-cable testing
VLF-cable testing
VLF cable testing is a technique for testing of medium voltage cables. The VLF test can be used in two ways: Apply VLF to measure insulation losses at different VLF frequencies that are typically in the range of 0.01 to 0.1 Hz. In this case, the IEEE Std...
.
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:
- Calibration Pulse (in accordance with IEC 60270) and end detection
- Background noise of the measurement arrangement
- Partial discharge inception voltage PDIV
- Partial discharge level at 1.7 Vo
- Partial discharge extinction voltage PDEV