Growth of electrical trees in solid insulation

Arbab, Muhammad Naeem; Auckland, D.W.

doi:10.1049/ip-a-2.1989.0011

Cited by 25 publications

(6 citation statements)

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“…Based on proposed initiation and propagation mechanisms of electrical trees [10,[16][17][18][19][20][21][22] and the experimental results in this paper, it is suggested that similar initiation mechanisms exist among all of the trees in our experiments [14,15,23]. The initiating processes of electrical tree in the inner layer of 66 kV XLPE cable are: electron injection and extraction at the needle electrode due to high electric field, part of the insulation in the front of needle will be deteriorated by this process, this is followed by the intense partial discharge and photo-ionization due to charge recombination in the channel of the tree, the continuous micro-breakdown at the tree tips and the electron avalanche mechanisms, etc.…”

Section: Discussionmentioning

confidence: 99%

“…The necessary condition for the extension of tubular cracks is that the sum of both the electrostatic energy W es and the electric mechanical energy W em stored in tree channels must be larger than the sum of both the plastic deformation energy W fp and the surface energy W fs , their values can be approximately given by the following equations [16] (2) where is the surface tension, is a correction factor, y is the yield stress, and L and r are the length and radius of the cylindrical cavity of the electrical trees, respectively.…”

Section: The Electrical Trees With Residual Stress At Low Frequencymentioning

confidence: 99%

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Investigations of electrical trees in the inner layer of XLPE cable insulation using computer-aided image recording monitoring

Xie

Zheng

2010

IEEE Trans. Dielect. Electr. Insul.

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Using a computer-aided image recording monitoring system, extensive measurements have been performed in the inner layer of 66 kV cross-linked polyethylene (XLPE) cables. It has been found that there are three kinds of electrical trees in the samples, the branch-like tree, the bush-like tree and the mixed tree that is a mixture of the above two kinds. When the applied voltage frequency is less than or equal to 250 Hz, only the mixed tree appears in XLPE samples, when the frequency is greater than or equal to 500 Hz, only the dense branch-like tree develops, both of which are attributed to the coexistence of non-uniform crystallization and internal residual stress in semicrystalline XLPE cables during the process of manufacturing. Through the fractal analyses of these electrical trees, it has been found that both the propagation and structure characteristics can be described by fractal dimension directly or indirectly. It is suggested that the propagation and structural characteristics of electrical trees are closely related to the morphology and the residual stress in material at low frequency, i.e., the propagation characteristics of electrical trees depends upon not only the boundaries between big spherulites and amorphous region, but also the impurity, micropore concentration and the relative position of needle electrode tip with respect to spherulites or amorphous region in the low frequency range. However, at high frequency, it has nothing to do with the morphology of material. It is suggested that the injection and extraction process of charge from and to dielectrics via the needle electrode are more intense at high frequency than in low frequency. Thus, it can form relatively uniform dielectric weak region in front of needle electrode, which leads to similar initiation and propagation characteristics of electrical trees at high frequency.

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Section: Discussionmentioning

confidence: 99%

Section: The Electrical Trees With Residual Stress At Low Frequencymentioning

confidence: 99%

Investigations of electrical trees in the inner layer of XLPE cable insulation using computer-aided image recording monitoring

Xie

Zheng

2010

IEEE Trans. Dielect. Electr. Insul.

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“…Over a quarter of a century ago, Billing and Groves [1] and later Arbab and Auckland [2] were discussing the effect of externally applied mechanical stresses-for example by bending-on tree growth. In a simple bending experiment, it is immediately obvious that tree growth is accelerated in regions of tensile stress and retarded where compressive stress is present.…”

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confidence: 99%

Mechanical prestressing improves electrical strength

Varlow

2002

IEEE Electr. Insul. Mag.

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T here is nothing original in the proposition that mechanical stress has a controlling influence on the propagation of electrical trees in insulating materials. Over a quarter of a century ago, Billing and Groves [1] and later Arbab and Auckland [2] were discussing the effect of externally applied mechanical stresses-for example by bending-on tree growth. In a simple bending experiment, it is immediately obvious that tree growth is accelerated in regions of tensile stress and retarded where compressive stress is present. The effect of internal stresses on electrical tree growth, determined by photo-elastic techniques, was also reported by Champion, et al. [3] and David, et al. [4] in 1992 and 1994, respectively. Whereas the above were concerned with laboratory specimens, similar conclusions have also been drawn from more practical applications, such as in cables, in numerous publications from Densley in 1979 [5] through to Ildstad and Hagen in 1992 [6].The theoretical analysis of the mechanical aspects of tree growth began with Zeller, et al. [7], [8], who attributed the growth of filamentary cracks to electrostatic forces, a process they named electrofracture. This theoretical approach has been developed subsequently by Hikata, et al. [9] and Fothergill [10].On the basis of this approach to an understanding of the physical processes, there is a strong temptation to interpret electrical tree growth in terms of fracture mechanics, but the analogy must not be pushed too far. Certainly, it is possible to draw a direct correlation between treeing characteristics and the mechanical properties of insulating materials. A preliminary study of tree inception and growth in materials of greatly differing mechanical properties [11] revealed that the mechanical characteristics of the material were paramount. It was proposed that the residual mechanical stress developed under alternating voltage application produced fatigue cracking in the inception phase. Since crack formation is determined by, among other things, tensile strength, it followed that this parameter would influence the tree initiation time. This view was supported by measurements of the tree inception times of polyester resin and an elastomer (PL-3). The tree inception time in the elastomer was less than 30% that of the polyester resin, which had a tensile strength five times greater than that of the elastomer.The buildup of internal tensile stress due to the alternating electrostatic forces, as well as leading to crack formation and tree initiation, will also increase the material's propensity for tree growth. It follows that tree growth in elastic materials may be less rapid than in inelastic materials, since the former will not suffer from long-term mechanical stress buildup. Thus, a material's modulus of elasticity could provide a measure of its resistance to tree growth. This was confirmed by a comparison between PL-3 and polyester resin, which demonstrated an inverse dependence of the time for a tree to cross the interelectrode spacing on the modulus of...

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“…The mechanical stress can be either compression or tension stresses in the insulation. The effect of mechanical stress on treeing has been studied [4,5], however, further study is required.…”

Section: Introductionmentioning

confidence: 99%

The influence of survival mechanical stress and voltage frequency on electrical tree in XLPE

Zheng

Chen²,

Davies³

et al.

Annual Report Conference on Electrical Insulation and Dielectric Phenomena

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The initiation and growth of electrical trees under high electric stress have been studied in XLPE to explore the effects of the internal mechanical stress. Treeing tests were carried out under an applied voltage of 7 kVrms and both tree initiation and growth were monitored. The needle with a tip radius around 5 pm was inserted into XLPE block at elevated temperature that was cut from a XLPE power cable. Two types of samples have been prepared i.e. one with a significant internal mechanical stress around the tip of the needle and the other without. It was found that in the presence of the mechanical stress around the tip the tree initiation time is significantly shorter and growth rate is faster compared with those without. The combining effects of the internal mechanical stress and the frequency of the applied voltage on the structure of electrical tree during the growth stage have also been investigated. For example, in the presence of the internal mechanical stress, the results showed that the tree has a mixture of branch-like and bush-like shllcture at low frequency. In the absence of the mechanical stless, the results indicated that the tree at the early stage has many thicker branches at high frequency and thinner branches at low frequency.

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Growth of electrical trees in solid insulation

Cited by 25 publications

References 0 publications

Investigations of electrical trees in the inner layer of XLPE cable insulation using computer-aided image recording monitoring

Investigations of electrical trees in the inner layer of XLPE cable insulation using computer-aided image recording monitoring

Mechanical prestressing improves electrical strength

The influence of survival mechanical stress and voltage frequency on electrical tree in XLPE

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