The danger of water trees in polymer insulated power cables evaluated from calculations of electric field in the presence of water trees of different shapes and permittivity distributions
ABSTRACT 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.
Conference Proceeding: Electrical ageing of polyethylene power cables insulation subjected to an electric field in the presence of water[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