Water tree growth in polyethylene cable insulation is discussed. The characteristics of water trees, the effect of aging parameters on water tree growth, and the possible mechanisms of growth are considered, emphasizing vented tree development in polyethylene insulating materials. The morphology of water trees, the characteristics of the tree-infested dielectric cable, and test methods and measures to reduce water treeing are discussed. >

Water treeing in polyethylene cables

The phenomenon of water-tree degradation of polyolefin insulating materials is reviewed with particular emphasis on the materials aspect of the problem. Specific topics include: methods of producing water trees in the laboratory, water-tree morphology, the nature of tree growth curves, and the effects of temperature, material, electrical variables, chemical factors and physical and mechanical changes on tree growth. Proposed mechanisms for tree initiation and growth, and contradictions in mechanisms and observations, are analyzed. Where possible, suggestions are made for experiments which would help clarify points of dispute. Appendices contain commonly accepted methods for staining treed materials, including filled rubber.

Water Treeing in Solid Dielectrics

Mechanisms of water treeing are reviewed. A definition of water trees is proposed on the basis of permanent the hydrophilic character and the reduced breakdown strength. Properties are discussed that are relevant to the water tree definition and to the inception and propagation mechanism. This includes the water content, the (sub)micrometer structure and the dielectric properties. The discussed propagation mechanisms include electro-mechanical forces, diffusion of hydrophilic species and two types of oxidation mechanisms. Also attention is paid to the correlations between processes and aging conditions. Inception mechanisms include injection of hydrophilic species and/or oxidation in damaged regions, electro-mechanical damage due to high electrical fields.

Inception and propagation mechanisms of water treeing

Recently, several diagnostic methods for measurement of permittivity and dielectric loss have been developed for non-destructive testing of water-tree degraded XLPE cables. Critical degradation is found to be associated with a more than proportional increase of the dielectric properties when increasing the applied test voltage above a certain level. In this paper an explanation for this nonlinearity will be presented, using a mechanical approach to the water treeing phenomenon. The water treed insulation is considered to consist of small water filled voids separated by crazing zones. At relatively low water content and low applied test voltage, several of these crazing zones are likely to be closed and insulating. When increasing the test voltage, Maxwell mechanical stresses will cause water to penetrate into the crazing zones, thus making electric contact between the elongated water droplets. Results from finite element method (FEM) calculations of electric fields and losses show that the effect of this will be to enhance the electric field at the tip of these conductive channels and to increase the dielectric losses of the water treed insulation.

Understanding water treeing mechanisms in the development of diagnostic test methods

The insulation from six 5 kV power cables, which has been in service underground for 6 to 8 years, was examined by infrared (IR) spectroscopy and oxidation induction time (OIT) analysis. Sections of insulation containing water trees were found to contain high levels of ionic contaminants. All insulation samples showed evidence of oxidative degradation in service and frequently there was a higher than average level of oxidation in the treed regions of the insulation. Sections of the insulation containing water trees had appreciably shorter OIT's than untreed regions, indicating that they were more prone to subsequent oxidative degradation. A model for water tree formation and electrical breakdown of the insulation is described where oxidative degradation during prolonged service reduces the ability of the insulation to withstand stress concentrations at defects, and water trees are initiated. Some localized oxidation may accompany the tree propagation step. Extensive localized oxidation then takes place in the treed regions, catalyzed by ionic contaminants, and insulation failure occurs.

Oxidation and Water Tree Formation in Service-Aged XLPE Cable Insulation

Treed insulation from field aged cable is more prone to oxidation than non- treed regions, as observed by shorter oxidation induction times. This could be due to the treed region being already partly oxidized or due to contaminants within the treed regions acting as catalysts to oxidation. It is not known if the oxidation induction times vary from tree to tree, or whether eventual preferential oxidation of a treed region might lead to cable failure.

Properties of water treed and non-treed XLPE cable insulation

The techniques of etching by carbon tetrachloride vapor and by permanganic acid are shown to be prone to artifacts, and so earlier conclusions based on these techniques, i.e. that XLPE cable insulation has a large scale (>> 10 ?m) spherulitic texture structure, need to be reexamined. A comparison with XLPE film samples, where spherulite size is readily determinable by small-angle light scattering and optical microscopy, indicates that typical spherulite dimensions are <5 ?m. Examination of freeze-fractured surfaces through XLPE insulation containing water-trees revealed cavities up to 10 pm diameter with no evidence of interconnecting channels. Freeze-fractured surfaces through electrical trees revealed channels several micrometers in diameter with evidence of extensive melting and polymer degradation.

Etching and the Morphology of Cross-Linked Polyethylene Cable Insulation

CCl 4 vapour produces a quilt-like surface texture on XLPE. This results from swelling of the surface and does not correspond to the original polymer morphology.

http://ieeexplore.ieee.org/iel7/7684613/7684614/07684669.pdf

An investigation into the morphology of XLPE cable insulation

It is well known that polymer insulated high-voltage cables experience a form of degradation in service referred to as water treeing. Small tree-like growths consisting of water-filled microvoids, slowly grow in an electrically stressed insulation exposed to moisture causing a reduction in the breakdown strength. Although the subject of water treeing has been studied extensively for at least ten years, the mechanisms of treeing and how the trees lead to breakdown are not well understood. The role of oxidative degradation on the initiation and growth of water trees and the final breakdown of the insulation is not clear. At this conference last year it was hypothesized that the decomposition of hydroperoxides formed during oxidation was a major factor in the treeing process [l]. However these authors did not measure polymer oxidation directly. Bernstein et at.[2] could not find any significant difference in the level of oxidation between treed and non-treed polyethylene taken from miniature cables. This paper describes some results of an investigation into the oxidation levels of XLPE cable insulation removed from field and laboratory-aged cables. The oxidation was measured by Fourier Transform Infrared Spectroscopy (FTIR) and Oxidation Induction Time analysis (OIT).

Oxidation and water treeing in XLPE cable insulation

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