Cable insulation water treeing

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). 

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. 

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. 

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

Figure 4 FTIR microscopy of polyethylene cable insulation. (A) Water-tree, (B) undamaged area, and (C) the difference spectrum (A) (B). (Parker SF (1995) Industrial applications of vibrational spectroscopy and the role of the computer. In George WO and Steele D (eds.) Computing Applications in Molecular Spectroscopy, pp. 181-199. Cambridge The Royal Society of Chemistry reproduced by permission of The Royal Society of Chemistry.)...

HV cable insulation is commonly made of peroxide-cross-linked PE (XLPE). The cross-linking enhances the resistance to high temperatures of the PE, increasing its maximum service temperature from 70°C to 90°C. Hence, PE is more attractive than PP which cannot be cross-linked with peroxide. The weakness of the XLPE is that it undergoes water-treeing (tree-like cracks initiated by the combination of a high electric field and the presence of moisture and metal ions). The addition of a small percentage of EPR, based on 35 mol.% propylene, is successfully used as water-tree retardant. 

Keywords cable, sheathing, jacketing, insulation, telecommunication cable, power cable, electrical insulation, thermoplastic olefins, TPO, EPR, EPDM, dielectric properties, flammability, water-treeing. 

Figure 10.1 Bright-field transmission optical micrograph showing a methylene blue-stained water tree grown from a reamed hole in a sample of medium-voltage polyethylene cable insulation. From...

Figure 10.7 An etched surface within a sample of medium-voltage polyethylene cable insulation, as revealed in reflection using DIC optics. The dendritic structure of a water tree is evident, together with rectangular regions which correspond to beam damage as a result of prior examination in the SEM. From Olley et al. (1992) [7].

Water treeing arises with polyethylene insulated cables in wet conditions and at modest voltages where water diffuses into the insulation and, under particular conditions, forms fine chaimels. Wright (9) notes that this has also been seen in EPDM rubbers and he also cites a case of automotive cooling hose where a voltage was apparently generated by electrochemical processes between the coolant and metals in contact with the coolant. This makes a good example of a case where the most unexpected occurred. 

Water treeing failure of XPE high-voltage power cable insulation was found to occur. Failure occurred after seven years of service and was caused by the growth of vented water trees when exposed to a moist environment. XPE also undergoes also degradation when exposed to sunlight. 

Andrei, L., Vlad, I., Ciuprina, F. Electrical field distribution in power cable insulation affected by water tree. In 9th International S5mposium on Advanced Topics in Electrical Engineering , Bucharest, 7-9 May 2015... 

Meehanical stresses present in cable insulation can promote the initiation and propagation of water trees. Such stresses typically arise as a result of uneven cooling during extrusion or when cables are subjected to tight bends. Thick layers of insulation can develop internal stresses as high as 37 MN/m due to uneven... 

Other aspect to which the majority of researchers do not pay attention, especially in the studies on chemical weathering of insulating materials destined to electrical cables and wires is the evaluation of altered functional characteristics by electrical field. This aspect is quite important for the security of aerial and buried electric transport networks. The channels appeared as the result of the orientation of dipoles in oxidizing polymers die to electrical field allow the further penetration of water which forms electrical trees [39,40]. The length, the density and the size extension of these defects influence deeply the durability of electrical insulator. Moreover, the continuous action of electrical field amplifies the degradation, which increases proportionally the failure of outdoor operating cabled. 

The wide use of polymers in the electrical industry arises from their excellent electrical insulation and dielectric isolation properties. However, these are dependent upon the polymer permeation properties. For instance, the sheathing of cables and wires is sensitive to the presence of water in the polymer. It has been shown that the most common form of insulation breakdown arises from electrochemical treeing. This arises from the simultaneous effects of an electric field and moisture present in the shielding, either initially present or by permeation. 

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