From the assumption that water trees influence the AC electric field distribution in the insulation due to their permittivity higher than that of the surrounding medium, calculations of electric field in the vicinity of model water trees are presented. The electric field is locally amplified and reaches values high enough to be considered as a danger for the cables.
[Show abstract][Hide abstract] ABSTRACT: Under the action of an electric field and in the presence of water, water trees develop inside power cables insulations. Regardless of their type, shape or dimension, water trees contribute to the worsening of electrical properties, especially lowering the inception voltages of partial discharges and electrical trees leading to breakdown . In this paper an experimental study regarding the variation of permittivity and loss factor of the polyethylene with the frequency (f<sub>a</sub>) and the duration (τ) of applied electrical field is presented. The experiments were made on flat XLPE samples sliced from a power cables insulation. The samples were subjected to an electrical field with the intensity E = 4 kV/mm and frequencies f<sub>a</sub> = 3 and 5 kHz for τ = 48, 72 and 96 hours. After ageing the water trees dimensions (length l<sub>wt</sub> and diameter D) and concentration c<sub>wt</sub> were measured. The permittivity (ε<sub>r</sub>) and the loss factor (tgδ) were measured (using a NOVOCONTROL dielectric spectrometer) at the temperature T = 30°C and frequency f<sub>m</sub> = 10<sup>3-</sup>-10<sup>6</sup> Hz. The results show that with the increase of ageing time or the ageing frequency all the quantities c<sub>wt</sub> l<sub>wt</sub> D, ε<sub>r</sub> and tgδ increase, too.
Advanced Topics in Electrical Engineering (ATEE), 2011 7th International Symposium on; 06/2011
[Show abstract][Hide abstract] ABSTRACT: A study on the influence of water trees in polymeric insulations for a point–point geometry is presented. Using an adequate numerical method we were able to calculate the electric field distribution: the variation of the maximum electric field is considered as a function of the water tree length and permittivity. In laboratory, we obtained the partial-discharge inception voltage (PDIV) as well as the pre-breakdown voltage (PV) and we observed the alteration of the breakdown paths induced by the PE degradation. We also found a good correlation between these experimental results and the calculated distribution of electric field. Both experimental and computational results provide interesting knowledge about the effect of water treeing in polymeric insulations.
Journal of Electrostatics 03/2000; 48(3-4-48):165-178. DOI:10.1016/S0304-3886(99)00057-1 · 0.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In order to explain the development of different types of water trees and the related dielectric breakdowns in extruded power cables, it is necessary to analyse the dielectric properties of the corresponding treed regions and their influence on the distribution of electric field. The study presented in this paper is both experimental and theoretical. Experimentally, we performed the laboratory ageing of a power cable for accelerated conditions of applied voltage and frequency: we inspected the different shapes, orientations and dimensions of vented, bow-tie, single-bow-tie and global water trees produced in the polymeric insulation. From a theoretical point of view we proposed adequate models for obtaining the field distribution analytically and using a suitable finite element method. A local electric field amplification is calculated according to the different characteristics of the water trees: length, shape, permittivity and law of variation for permittivity. From our results, a compared analysis of the risk of electric breakdown is performed for the different types of water trees according to their configurations.
Journal of Electrostatics 10/2001; 53(4-53):267-294. DOI:10.1016/S0304-3886(01)00164-4 · 0.86 Impact Factor
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