Florin Ciuprina

Polyethylene crosslinking and water treeing

Water tree resistance of crosslinked polyethylene (XLPE) and of low density polyethylene (LDPE) were compared in order to elucidate whether the crosslinking influences or not, the water tree propagation in polymer insulation. To avoid possible errors due to by-products from chemical crosslinking, this study used irradiation crosslinked polyethylene. Irradiation conditions (high dose rate, vacuum) were carefully chosen in order to minimize the errors due to concomitant oxidation. The results obtained indicate that there is no significant difference caused by crosslinking, between water treeing lengths in XLPE and LDPE.

Dielectric Properties of Nanodielectrics with Inorganic Fillers

One of the main targets of the research in the field of polymer nanocomposite dielectrics is to obtain new materials with improved dielectric properties (resistivity, dielectric strength, permittivity and dielectric losses). In this paper the variation of the real part of the permittivity and of the loss tangent with the frequency are investigated for three formulations of nanocomposites obtained from polyethylene filled with nanoparticles of SiO2, TiO2 and Al2O3, respectively. The influence of the filler concentration (between 2 and 10 wt%) on the dielectric behavior of the nanocomposite is analyzed as well. To simulate the electrical behavior of the polymer-filler interface which might explain the experimental results a 3D electrostatic model proposed on the basis of Tanakas multi-core model is discussed. This model allows one to study the influence of several parameters such as the nanoparticle diameter, thickness of the interface layers, concentration and permittivity of the nanoparticles or the permittivities of the interface layers, on the effective permittivity of a plane nanodielectric sample.

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.

Electric field computation in water treed polyethylene with space charge accumulation

Electrical degradation of the insulating systems of energy cables is close related to the water treeing and space charge development. In this paper the results of the calculations of the electric field repartition in the insulation of a medium voltage cable affected by water trees and their related space charge are presented. Continuous and individual water trees, growing from outer and/or inner semiconductor have been considered. The space charge density has been determined by measurements performed on plane samples of polyethylene by using the thermal step method. The repartition of the space charge in the water treed regions has been estimated from FTIR analysis on the repartition of the ions associated to water trees. The results show that the presence of water trees and of space charge determines an important increase of the electric field strength near to the semiconductor layers, which facilitate the inception of electrical trees in insulations and consequently the breakdown at lower voltage

Dielectric properties of LDPE-SiO 2 nanocomposites

The dielectric behaviour of nano-SiO2 filled low density polyethylene is investigated over a frequency range of 10 mHz–10 MHz and for different temperatures from 250 K to 350 K. It is shown that the presence of nanoparticles change significantly the dielectric behaviour of the polymer system. The frequency variations of the permittivity and of the tan delta emphasize a α-relaxation process, for each of the nanocomposite samples. The relaxation is more important and occurs at higher frequencies with the increase of the filler content. The increase of the temperature leads to a shift of the relaxation frequency to higher values. Results from chemiluminescence measurements and from X-ray diffraction analysis are discussed in connection with the dielectric behaviour.

Electrical properties of polyolefin blends under /spl gamma/-radiation exposure

Polymer dielectrics: low density polyethylene (LDPE), high density polyethylene (HDPE), previously degraded polypropylene (dPP) and ethylene-propylene terpolymer (EPDM) were investigated after /spl gamma/-irradiation in air. Changes in electrical properties: volume resistivity, dielectric constant and dielectric loss, were determined for ethylene-propylene terpolymer and degraded polypropylene blends containing 0, 20, 40, 60, 80 and 100 phr of polypropylene. The contribution of each component to the evolution of electrical parameters is discussed. While virgin polyolefin blend samples present a steady state in electrical resistance on time, the irradiated compounds containing degraded polypropylene trend to a continuous increasing values of resistivity. The presence of oxygenated compounds in the radiation modified samples leads to the decrease in volume resistivity. The comparative behaviour of polyolefin blends shows that the blend containing ethylene-propylene elastomer and low-density polyethylene has electrical features between the other two mixtures.

Chemical crosslinking of polyethylene and its effect on water tree initiation and propagation

The water tree resistance of chemically crosslinked polyethylene and of low density polyethylene were compared in order to elucidate whether the crosslinking itself influences or not the water tree characteristics of polymeric cable insulation materials. For this purpose, water trees were grown in compression moulded disks, obtained from pellets of either thermoplastic or chemically crosslinked polyethylene. Three types of crosslinked polyethylene were evaluated: one containing only peroxide (XLA) and two others having, besides peroxide, a tree retarding additive system (XLB and XLC), and the results were compared with those obtained on their thermoplastic correspondents (A, B and C). Results were determined for both Water Tree Propagation and Water Tree Initiation. The results obtained for Water Tree Propagation indicate that there is generally a large difference between the types (A, B & C) yet only small difference between thermoplastic and crosslinked samples. However for two cases (A & B) these small differences between crosslinking are statistically significant. The complementary Water Tree Initiation results show both large and significant differences between material types and the crosslinking.

Electric field distribution in power cable insulation affected by water trees

Electrostatic numerical models of cable insulation affected by cavities, vented and bow-tie water trees are proposed in this study. The influence of the vented tree lengths on the electric field in the untreed polymer, in a cavity in front of the tree, and in a bow-tie tree growing toward the vented tree is analyzed. The values and spatial distribution of relative permittivity and space charge in the subdomains of the 3D computational domain are set by scaling experimental results from literature. The models are implemented in the finite element method based on the software package COMSOL Multiphysics.

Electric field distribution in power cable insulation affected by various defects

Electrostatic numerical models of cable insulation affected by cavities/impurities and water trees are proposed in this study. The individual influence of cavities and the simultaneous effects of cavities and vented water trees, and of solid impurities and bow-tie water trees are analyzed. The values and spatial distribution of relative permittivity and space charge in the subdomains of the 3D computational domain are set by scaling experimental results found in literature. The models are implemented in the finite element method based on the software package COMSOL Multiphysics. The results show the influence on the electric field distribution of the cavity position inside the insulation, both in the absence and in the presence of water trees, as well as of a bow-tie water tree grown in the insulation from a dielectric impurity. The role of the cavity diameter and of the permittivity of the impurity is also discussed.

AC breakdown strength and dielectric response of water-treed low density polyethylene

Results on AC breakdown strength and on dielectric behavior of water-treed plane samples of low density polyethylene (LDPE) are presented in this paper. Water trees were grown at room temperature, under an electric field of 4 kV/mm, 5 kHz, and having a 0.1 M NaCl solution as electrolyte in contact with a face of the sample having needle-like defects created by pressing a sheet of abrasive paper. The breakdown tests have been carried out under an AC voltage ramp, at 50 Hz. The dielectric spectroscopy was the tool used to analyze the dielectric behavior of LDPE samples with and without water trees, over a frequency range of 10 mHz-1 MHz, at room temperature. The results show a good correlation of the breakdown strength decrease with the increase of water tree average length and relative volume. The dielectric spectra of water-treed samples emphasize an increase of the sample permittivity with the water tree growth, as well as three relaxation peaks in the loss tangent frequency variation obeying the Havriliak-Negami law. Possible electrical equivalent circuits for treed and non-treed samples are also discussed.