Treeing water

Initiation and propagation of water trees depend on a multitude of parameters electric field strength, mechanical stress, type and concentration of ions in the water, constitution of the polymer and its additives, etc. At this stage there is no satisfactory model that explains the process of water treeing. In a comprehensive review, Crine (1998) carefully considers the various types of possible mechanism in relation to a vast amount of experimental investigation, but is unable to come to any definite conclusion about primary mechanisms. Rather surprisingly, oxidation appears to play no major role (Bulinski et al., 1998). It seems only likely that ingress of water, aided by the presence of 

The profound effect of water on tree growth that is so widely reported may be expected with the electrokinetic model on the basis of three principal effects. Water filling the crazes as they develop will help prevent their collapse. Water, as a good solvent for ionic species, will be an excellent medium to facilitate entry of surface-active agents, which, by a process similar to that of environmental stress cracking, will advance the void and craze formation caused by the electric field. Water, with its high relative permittivity, will distort and locally enhance electric fields in the neighbourhood of the voids and crazes where it accumulates. Whether one or other of these effects dominate in a particular situation depends on the exact nature of the specimen and its environment. 

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Electrical trees consist of visible permanent hoUow channels, resulting from decomposition of the material, and show up clearly in polyethylene and other translucent soHd dielectrics when examined with an optical microscope. Eresh, unstained water trees appear diffuse and temporary. Water trees consist of very fine paths along which moisture has penetrated under the action of a voltage gradient. Considerable force is required to effect this... 

Electrochemical treeing is appHed in those cases of water treeing in which the water contains solute ions which move under the action of an electric field and are detected within the insulation layer, or at an electrode surface after having passed through the insulation. They are not encountered as often as the first two classes, for example, trees formed in a cable exposed to a hydrogen sulfide environment called sulfide trees. 

Tests on Cable Constructions. The Association of Edison Illumination Companies (AEIC) has approved an accelerated cable hfe test in which typical underground distribution power cables can be statistically compared based on their resistance to water treeing (number of days to fail). The comparison can be made by varying the type of insulation and/or other cable layers in an environment that contains hot water (90°C) under 8V/fi (200 V/mil) voltage stresses (four times the typical power cables operating voltages). 

Trees originating at a shield—insulation interface are mosdy due to the existence of protmsion from the shields. They are referred to as vented trees if moisture is present, they are called vented water trees. Particulate contaminants present in the insulation, and waterborne ionizable materials that find their way into the insulation, are also causes of tree formation. 

What to do Grow resistant cultivars. Mulch under trees to stop soil from drying out Water trees in dry weather. In winter, cut out infected shoots. In spring, remove infected leaves and shoots. Spray with seaweed extract to promote strong growth. Fungicide spray Sulfur, although it can harm some apples. Check the label before use. 

The degradation processes known as "water treeing" and "electrical treeing" are major problems causing premature failure of underground high voltage cables. 

Electrical trees are essentially breakdown channels whose size, typically 50 to 200 microns, together with the large variations in impurity concentrations in the surrounding polyethylene, makes the identification of the impurities associated with both kinds of trees very difficult by traditional techniques. The use of micro-PIXE for the location and analysis of trace elements in electrical and water trees found in the polyethylene insulation of high voltage cables will be described. 

To illustrate the use of PIXE and micro-PIXE in the study of breakdown phenomena in polyethylene high voltage cable insulation and other related topics we will describe a few typical measurements, first the study by standard PIXE of impurities in the organic semiconductor H2PC and in the carbon black semicon used in high voltage cables. Examples of the use of the microbeam to study some electrical and water trees as well as the diffusion of impurities from the semicon into polyethylene under typical electric field and humidity conditions will be given. 

More work remains to be done to better understand the role of impurities on water tree growth and we feel that micro-PIXE is a very powerful technique for such measurements since it has the required sensitivity and spatial resolution to provide detailed contour maps of the impurity concentrations, which can then be correlated with the visual tree. Our present micro-PIXE equipment with its 20 micron diameter beamspot is ideally suited for such measurements as it is very easy to use and provides online data. However, the use of only a few point measurements could miss essential components of the tree, and raster scans of the whole tree area would provide more complete information, and at the same time reduce beam induced damage. 

In many laboratories that have access to a nuclear reactor, neutron activation is used for the chemical analysis of rocks, minerals, petroleum, biological tissues, alloys, etc., and the technique is well suited for the determination of the concentrations of trace elements in polymers. Neutron activation analysis was used by Given et al. (1) in their studies of water tree growth in polymeric insulation and by Wu and Chen (2) in their studies of dopant-polymer interactions in MoCl5-dcped polyacetylene films. In this work the principles of the method are described and the possibilities are illustrated by means of measurements carried out on polyethylene. 

Seven of the twelve elements detected here Na, Al, Cl, K, Mn, Zn, and Br were also detected by Given et al. (1) in the polythene insulation of XlfE cable which failed in service. The other five were not detected, possibly because their neutron activation method for short-lived elements was not as sensitive as our method b. They also detected eleven other elements, probably because their material contained higher impurity concentrations, and they found that Na and Cl had migrated to regions containing water trees. 

Griffiths CL, Betteridge S (2000) The effect of environmental stress cracking on water tree growth. IEE Conference, 473 (Dielectric Materials, Measurements and Applications), pp 41-46... 

On trees, prune out the dead wood and the water sprouts. Avoid drought stress by watering trees during dry spells and keeping the root zone mulched. Gather up and destroy infected leaves. A dormant spray of bordeaux mix may provide some control. 

Boggs, S. Xu, J. Water treeing-filled versus unfilled cable insulation. IEEE Electrical Insulation Mag. 2001, 17 (1), 23-29. 

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