J. Densley

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

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 OITs 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.

Water treeing in binary linear polyethylene blends: the mechanical aspect

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. >

Etching and the Morphology of Cross-Linked Polyethylene 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.

Properties of water treed and non-treed 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.

Backscattered electron imaging and energy-dispersive X-ray studies of water-treed polymeric insulation

The backscattered-electron imaging mode in a scanning electron microscope has been used to detect and image water trees in electrically stressed cross-linked polyethylene (XLPE) insulation. The location and distribution of inorganic contaminants, such as Cu or Cl, within the water-treed regions of the insulation were revealed using backscattered-electron imaging. Energy-dispersive X-ray (EDX) spectroscopy and mapping were subsequently used to ascertain the chemical nature of the contaminants. Contaminants were only detected by EDX inside the treed areas delineated by the backscattered-electron images. A careful comparison of backscattered-electron and optical images of thin microtomed XLPE samples did not show any contaminant penetration beyond the visible-treed region. Results from a variety of service-aged and laboratory-stressed samples are presented, as well as a preliminary attempt to extend backscattered-electron imaging to optically opaque ethylene-propylene-rubber insulation. >

Application of oxidation induction time and compensation effect to the diagnosis of HV polymeric cable insulation

Oxidative stability tests were performed on field and laboratory-aged crosslinked polyethylene (XLPE), ethylene-propylene rubber (EPR) and polypropylene (PP) insulations using differential scanning calorimetry. Flat films and miniature cables aged in the laboratory were subjected to a wide range of aging conditions that included thermal, electrical and a combination of thermal-electrical aging, in dry and wet environments. The results were analyzed using the Eyring rate theory. It is shown that for a given material the oxidative stability data can be described by a single linear relationship between the activation entropy /spl Delta/S and the activation enthalpy /spl Delta/H of the oxidative process, and thus are governed by the so called compensation effect. It is argued that the position of a data point representing a certain operating condition of an insulation on the /spl Delta/S vs. /spl Delta/H compensation plot is a measure of the degree of degradation and can be used as a diagnostic indicator of the operating conditions of the insulation.

Surge-induced temperature rise in water-containing defects in XLPE: mechanism for conversion of water trees to electrical trees

Circumstantial evidence suggests that large water trees convert to electrical trees as a result of lightning-induced surges. Computations employing transient nonlinear finite element analysis suggest that such conversion involves boiling of the water which results in creation of a cavity which can support partial discharge.

Water treeing degradation under combined mechanical and electrical stresses

The authors present results of experiments in which LDPE (low-density polyethylene) and/or XLPE (cross-linked polyethylene) specimens were subjected to mechanical vibrations in water or to mechanical vibrations and electric stress combined. Water tree growth experiments in stretched films of XLPE cable insulation are also described. Attempts to grow water trees by application of vibrational mechanical stresses to polyethylene immersed in water or containing cavities filled with water were unsuccessful. Vibrational mechanical stresses of moderate amplitude applied to the insulation together with electric stress do not change the water tree growth rates as compared with the insulation subjected to electric stress only. Static tensile stresses enhance the growth of vented water trees but have no effect on bow-tie trees. However, the effect is significant up to 30% strain only.<<ETX>>

High voltage insulation for power cables utilizing high temperature superconductivity

This paper discusses several properties of HV tape-type insulation impregnated with LN/sub 2/ for cryogenic cables. Fibrous, film and laminated materials have been tested for their short and long-term properties under AC and impulse voltages. The effect of partial discharges and the relationship between the AC and the transient characteristics of the insulation are discussed.

Wavelength and energy dispersive X-ray studies of contaminants in water treed insulation

The authors describe the results of an investigation to locate the contaminants within water trees grown in polymeric insulation using energy- and wavelength-dispersive X-ray (EDX and WDX) spectroscopy. Trees were grown in low-density polyethylene cross-linked polyethylene and dry-cured exposed to solutions of NaCl or CuSO/sub 4/ at stresses up to 8 kV/mm. Some specimens examined semiconducting electrodes and also a semiconducting layer in the middle of the insulation, to form a sandwich-type specimen. It is shown that water trees contain contaminants in sufficient concentration to be detected by EDX and WDX X-ray spectroscopy. In steam-cured cables WDX has shown that the contaminants appear to concentrate in the cavities of the treed regions within these cables. A scanning electron microscopy backscattered electron image was used to reveal the location of water trees and their contaminants distribution. Contaminants appear to be distributed throughout the entire water tree and do not seem to concentrate in any specific region of the tree. It is also shown that particles at the semiconductor/insulation interface result in the rapid growth of water trees in the presence of moisture and electric stress.<<ETX>>