Electrical trees

Electrical trees consist of visible permanent hoUow channels, resulting from decomposition of the material, and show up clearly in polyethylene and other translucent soHd dielectrics when examined with an optical microscope. Eresh, unstained water trees appear diffuse and temporary. Water trees consist of very fine paths along which moisture has penetrated under the action of a voltage gradient. Considerable force is required to effect this... 

The degradation processes known as "water treeing" and "electrical treeing" are major problems causing premature failure of underground high voltage cables. 

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. 

Micro-PIXE with a 20 micron beam spot has proved to be a versatile tool for investigations of water and electrical trees, as well as a variety of other samples. This system is simple to operate and does not require complex target preparation, so that multi-element surveys of a variety of samples can be rapidly performed. 

Sarathi, R. Das, S. Kumar, C.R.A. Velmurugan, R. Analysis of failure of crosslinked polyethylene cables because of electrical treeing a physicochemical approach. J. Appl. Polym. Sci. 2004, 92, 2169-2178. 

Rg. 10,3,4 Semi cry stall tie structure of poly(ethylene), (a) Rigid crystalline and mobile amorphous domains form sandwich structures with a period of the order of lOnm. They are separated by narrow interfaciai regions with intermediate mobility. Adapted from BluK with permission from Dr, Dietrich SteinkopffVerlag. (b) Photo of an electrical tree in PE. The tree consists of a root-like system from about 10 cavities with typical diameters of 5 xm which are produced by electrical discharges. The pcncil-likc tip of the needle electrode is recognized above the tree [Salll,... 

Electrochemical trees form at relatively low stress levels, propagate slowly but do cause deterioration. Even if not immediately causing the failure they may weaken the insulation and may sire electrical trees which in turn cause failures. The increasing rate of... 

In 1978, it was found that the use of a HDPE/LDPE blend enhanced the electrical treeing inception voltage [Nitta and Funayama, 1978]. This approach to electrical treeing inhibition and some of the experimental results on the use of SB/LDPE and HDPE/LDPE blends are given in Table 12.35 [Wu and Chen, 1983]. 

Diffusion-Limited Aggregation (DLA) Clustering of particles in random due to Brownian motion. DLA is prevalent in diffusion systems. Lich-tenberg figures (or electric trees) are a common example of DLA. 

Dodd SJ, Champion JV, Zhao Y, Vaughan AS, Sutton SJ, Swingler SG (2003) Influence of morphology on electrical treeing in polyethylene blends. lEE Proc Sci Meas Technol 150 58-64... 

Figure 10.8 SEM micrograph showing the internal appearance of an electrical tree channel (see Figure 10.4). The tree was grown in an epoxy resin block which was then fractured open to reveal...

AT Bulinski, SS Bamji, RJ Densley. Factors affecting the transition from a water tree to an electrical tree. 

Y Sekii, K Manila, T Okada. The effect of inclusions in XLPE insulators on electrical trees generated by ac and grounded dc voltage. Proceedings of IEEE International Conference on Conduction and Breakdown in Solid Dielectrics, Vasteras, 1998, pp 317-321. 

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. 

Ahmad, M. H., Jamil, A. A. A., Ahmad, H., Piah, M. A. M., Dams, A., Arief, Y. Z., Bashir, N. Oil palm empty fruit bunch as a new organic filler for electrical tree inhibition. International Journal of Electronics and Electrical Engineering 2012, 6(6), 213-218. 

The theoretical basis for electric breakdown in solids is still limited. Several types of breakdown in solids are recognized intrinsic breakdown, breakdown dependent on physical defect, thermal breakdown, discharge-dependent breakdown, failure by electrical treeing, and failure by water (electrochemical) treeing. Conditions and material characteristics determine the type of breakdown which takes place. Actual electrical breakdown may involve two or more types simultaneously or sequentially. 

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