Cristina Stancu

Computation of the Electric Field in Cable Insulation in the Presence of Water Trees and Space Charge

The presence of space charge changes locally the electric field distribution in power cable insulation and may play an important role in tree development, thus accelerating the dielectric breakdown. This paper is concerned with the computation of the electric field in polyethylene-insulated power cables affected by water trees which grow from the following: 1) the inner semiconducting layer; 2) the outer semiconducting layer; and 3) the inner and outer semiconducting layers, taking into account the space charge corresponding to the ions present in the treeing area. Space charge in plane samples where trees have been developed in an accelerated manner was estimated using the thermal step method. Average charge values given by space charge measurements were then used for the electric field computation in cable insulation with continuous or/and individual water trees. For the calculation of the electric field, an analytical and a numerical method have been used. This paper shows that the space charge changes the electric field distribution inside and outside the trees (the field increases in some areas and decreases in others) and that the field variations depend on the magnitude and on the polarity of the space charge, as well as on the dimensions of the water trees developed in the cable insulation. The obtained results show that, in the presence of water trees and space charge, the initiation of electric trees is more probable in the case of individual water trees than in the case of continuous water trees.

Electrical conductivity of polyethylene-neodymium composites

In this paper, a study regarding the influence of the filler content on the electrical conductivity of low density polyethylene (LDPE)-Neodymium (Nd) composites is presented. Flat samples having the dimensions 100 · 100 · 0.5 mm3 were manufactured from low density polyethylene (LDPE) as polymeric matrix and Nd spherical particles (purity - 99 %) as filler by a melt mixing process using a compatibility agent. The distribution of the particles in the polymeric matrix was analyzed by Scanning Electron Microscopy (SEM) and optical microscopy. The variations of the DC conductivity and AC conductivity with temperature, frequency and filler content were studied. The symmetric hopping model for disordered solids was used to explain the results. Finally, the possibility to use these materials in electrical engineering field is analyzed.

Dielectric losses in polyethylene/neodymium composites

Polymer/metal composite materials have good rheological properties due to the polymer matrix and superior electrical or magnetic properties through the conductive or magnetic filler. For this reason, they can be used in a number of industrial applications as electromagnetic shields or permanent magnets. This paper deals with the results of an experimental study regarding the manufacturing and characterization of composite materials containing low density polyethylene (LDPE) as matrix and neodymium (Nd) as filler, obtained by melt processing. The structure of composites, the loss factor measured in harmonic fields at frequencies between 1 mHz and 1 MHz and temperature between 30 and 80 oC and the relative and specific losses are presented and discussed in this paper. Both loss factor and specific losses vary with the frequency of the electric field, temperature and filler content. For certain values of the frequency, the loss factor and relative losses show maximum values which increase of the temperature and filler content. The specific losses decrease with temperature but increase with the filler content and frequency of the electric field. For frequency values close to 1 MHz the specific losses overlap the ones in copper conductors.

Lifetime estimation method of polyethylene power cables insulation subjected to electric field in the presence of water

Lifetime estimation of power cables insulations is a very important issue for development of a preventive maintenance policy of cables in order to avoid their premature breakdown and interruption of power supply to consumers. In this paper a lifetime estimation method of a low density polyethylene insulated cable is presented. For this purpose, accelerated tests on model cable samples immersed in a NaCl solution (having a concentration of 0.1 mole/l) and subjected to a sinusoidal voltage (U = 5 kV and variable frequencies between 1 and 5 kHz) for different ageing times τ (τ = 24...500 h) were performed. The water trees lengths were measured and the variation curves of maximum lengths 1wtm with the ageing time (1wtm (τ)) for three frequencies (1, 2 and 3 kHz) were drawn. Considering three end of life criteria (according to current standards), the lifetime lines were determined and the lifetime corresponding to an electric field of frequency 50 Hz was computed. Finally, it is shown that the tested polyethylene insulation is not recommended to be used for medium voltage underground cables.

Influence of 50 Hz electromagnetic field on the yeast (saccharomyces cerevisiae) metabolism

In this paper an experimental study regarding the behavior of the yeast subjected to the electric field action at a frequency of 50 Hz is presented. The aqueous suspension was prepared from distilled water - 94 ml, dry yeast - 5 g and sugar - 1 g. The dielectric spectroscopy method was used in order to characterize the contributions of proteins to the dielectric properties of the suspension. The complex permittivity and conductivity were measured at 50 Hz at room temperature. The applied electric field varies between 1 V/cm and 6 V/cm (effective value). Conductivity and permittivity are found to increase, resulting in variations in magnitude of the applied field.