Dielectric spectroscopy in the time or frequency domain offers new opportunities for an off-line, on-site insulation condition assessment of HV electric power equipment and its predictive maintenance.

Applications of dielectric spectroscopy in time and frequency domain for HV power equipment

This paper describes briefly the main ageing and failure mechanisms and will indicate the advantages and limitations of the diagnostic tests available for the different insulation systems used in distribution and transmission cable systems. SC21 of CIGRE has addressed the subject of ageing factors and diagnostics of cable systems and has published three reports covering both fluid-filled and extruded cables. The reports describe ageing factors and several diagnostic methods, their purpose, and guidelines for use.

Ageing mechanisms and diagnostics for power cables - an overview

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

A sensor and guide wire assembly includes a core wire having a distal end, a proximal end, and a plurality of sections of different cross sections and thereby different flexibilities. At least one of the core wire sections has an enlarged portion with a sensor receptacle therein. The assembly also includes a tube that encloses the core wire over at least a fraction of its length such that the core wire extends out from a distal end of the tube. The tube is configured to enable the sensor and guide wire assembly to be inserted into an artery and to be passed to a measurement site inside a patient's body. The assembly further includes a sensor mounted in the sensor receptacle of the enlarged portion of the core wire. A first coil is arranged to enclose a first portion of the core wire extending out from the distal end of the tube, and the first coil is located nearer to the proximal end of the core wire than the sensor. A second coil is arranged to enclose a second portion of the core wire extending out from the distal end of the tube. The second coil is located nearer to the distal end of the core wire than the sensor.

Sensor and guide wire assembly

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

Polymeric dielectrics which are exposed to highly divergent electric fields break down more quickly than expected because of the generation of ultra-violet radiation at defect points which degrades the polymer locally and leads to the formation of electrical trees in a polymeric insulation The use of ultra-violet stabilizers, preferably in combination with reduced concentration of oxygen in the polymer, significantly extends the time to initiation of electrical treeing by preventing photodegradation of the polymer

Electrical tree suppression in high-voltage polymeric insulation

Water treeing tests were performed on low density polyethylene (LDPE) and four different binary blends of sharp linear polyethylene (LPE) fractions (M/sub w/=2500 and 76000), which were either quenched in air from the melt or isothermally crystallised at 123/spl deg/C. Although the morphology and initial mechanical properties of the materials tested were significantly different, the vented tree growth characteristics were similar for all of them. This is in disagreement with the electromechanical models of water treeing, which correlate water tree growth with the fracture toughness of the material. Time to breakdown distributions were also similar for both LDPE and the binary LPE blends, which indicates that, regardless of the initial material morphology and the actual structure of water trees, the length of water trees is one of the controlling factors in insulation failure. The visible light image of water trees in LPE blends did not disappear upon drying as it usually does in LDPE and crosslinked polyethylene insulation. >

Water treeing in binary linear polyethylene blends: the mechanical aspect

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

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