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 model is proposed to describe all experimental results on electrical aging of cables reported in Part 1. It is based on simple thermodynamics concepts in the Eyring theory, includes the concept of submicrocavity formation proposed by Zhurkov, and supposes that the first step in electrical aging is essentially a molecular process, as in Crine and Vijh's model. Our model of electrical aging under ac fields supposes that molecular-chain deformation is essentially a fatigue process and, therefore, that high frequencies generate more defects and thus reduce cable life, as indeed demonstrated by others. An original feature of the model is the submicrocavity formation above a critical field F/sub c/, whose value can be approximately predicted knowing the energy of cohesion of the polymer. This leads to a simple lifetime equation depending on just two physical parameters /spl Delta/G/sub 0/ (energy of activation of the chain deformation process) and /spl lambda//sub max/ (the maximum size of submicrocavities) with no adjustable unknowns. Above F/sub c/, there is an exponential relation between time and field, whereas below F/sub c/, the breakdown strength of the insulation varies very little with time; in other words, there is very limited (if any) aging. The slope of the exponential regime gives the value of /spl lambda//sub max/ directly whereas the intercept gives the value of /spl Delta/G/sub 0/. The predictions made by the model are discussed in correlation with existing experimental data. In addition to these basic assumptions, the model confirms that there is a relation between cable endurance and insulation morphology. Actually, the size of submicrocavities is ultimately limited by the amorphous-phase thickness. The max values deduced from the slopes of the exponential regime between F and log t for polyethylene (PE) (Part 1), XLPE and EPR insulation are in excellent agreement with the size of the amorphous phase of these samples, as measured by X-ray spectroscopy. It is also shown that the presence of water results in a lower /spl Delta/G/sub 0/ value, i.e. a shorter life. The precise relation between /spl Delta/G/sub 0/ and the nature and concentration of the impurity (including water) needs more work. The impact of these conclusions on the experimental limits of a reliable accelerated aging test and on the final breakdown process are discussed in a subsequent paper.

Electrical aging of extruded dielectric cables. A physical model

This paper reports on space charge evolution in crosslinked polyethylene (XLPE) planar samples approximately 1.20 mm thick subjected to electric stress level of 30 kVdc/mm at four temperatures 25, 50, 70 and 90 degC for 24 h. Space charge profiles in both as-received and degassed samples were measured using the laser induced pressure pulse (LIPP) technique. The DC threshold stresses at which space charge initiates are greatly affected by testing temperatures. The results suggest that testing temperature has numerous effects on space charge dynamics such as enhancement of ionic dissociation of polar crosslinked by-products, charge injection, charge mobility and electrical conductivity. Space charge distributions of very different nature were seen at lower temperatures when comparing the results of as-received samples with degassed samples. However at higher temperature, the space charge distribution took the same form, although of lower concentration in degassed samples. Space charge distributions are dominated by positive charge when tested at high temperatures regardless of sample treatment and positive charge propagation enhances as testing temperature increases. This can be a major cause of concern as positive charge propagation has been reported to be related to insulation breakdown

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Temperature effect on space charge dynamics in XLPE insulation

The electrical, thermal, and multiple-stress (thermal and electrical) aging of cross-linked polyethylene (XLPE) cable models has been investigated in order to get information on aging effects and mechanisms. After endurance tests, XLPE cable models have been subjected to chemical, physical, electrical, and microstructural characterization. Thermal aging results reveal that bulk degradation occurs in the cables at temperatures higher than the melting point. Multiple-stress aging data emphasize a synergistic effect of electric field and temperature. Significant microstructural changes detected in the cables aged under multiple stress are evidence of this effect and can partly explain the time behavior of the electric strength. >

A contribution to the study of aging of XLPE insulated cables

On Additives, Morphological Evolution and Dielectric Breakdown in Low Density Polyethylene

This paper describes fundamental studies into the effects of (1) the thermal properties, and (2) the morphology of cross-linked polyethylene on its impulse voltage breakdown stress, as determined by the use of miniature cables. Nine grades of high pressure polyethylene, characterized by molecular structure, are used in the studies. The degree of crystallinity of XLPE is found to be closely related to impulse voltage breakdown stress, which increases when crystallinity is increased. Therefore, the fact that impulse voltage breakdown stress decreases with temperature can be associated with a decrease in crystallinity. Likewise the thickness of the amorphous zone is found to increase with temperature, although the crystal thickness has changed onlyslightly. From this point of view, the amorphous zone appears to play an important role in the occurrence of impulse breakdown in that the breakdown seems to originate from the amorphous zone.

The Effect of Morphology on the Impulse Voltage Breakdown in XLPE Cable Insulation

This paper describes fundamental studies into the effects of 1) the thermal properties, and 2) the solid state structure of cross-linked polyethylene on its impulse voltage breakdown stress, as determined by the use of miniaturized cable models. Nine grades of high pressure polyethylene, clarified by molecular structure, are used in the studies. The degree of crystallinity of XLPE is found to be closely related to impulse voltage breakdown stress, which increases when crystallinity is increased. Therefore, that impulse voltage breakdown stress decreases with temperature can be attributed to a decrease in crystallinity. Likewise the thickness of the amorphous zone is found to increase with temperature, although the crystal thickness is little changeable. From this point of view, the amorphous zone is considered to play an important role in the occurrence of impulse breakdown. That is, breakdown seems to originate from the amorphous zone.

Masahiro Maeda

Papers

The Effect of Morphology on the Impulse Voltage Breakdown in XLPE Cable Insulation

Factors governing the voltage breakdown of insulating materials for cross-linked polyethylene (XLPE) insulated power cables