S. Pelissou

Electrical conduction of polyethylene below and above its melting point

The electrical conduction of low-density polyethylene (LDPE) has been investigated between 20 degrees C and 150 degrees C and up to 1 MV/cm. Special molded specimens and a self-compensating test cell were used; particular care has been taken in the control of experimental conditions. Absorption currents show broad maxima, while resorption currents display polarity reversal. Steady-state currents increase with a power law of the electrical field, and they tend to saturate at high fields and temperatures. Results are discussed in the framework of space-charge-limited conduction and dispersive hopping transport models. >

Influence of insulation morphology, impurities and oxidation on some electric properties of cables

It is shown that the AC breakdown strength and dielectric loss of unaged and field-aged transmission and distribution cables vary with insulation crystallinity, contamination, and oxidation. It is also shown that the influence of these parameters is particularly important near the conductor shield and varies with aging. The influence of curing on cable properties is briefly discussed. The sample preparation, crystallinity, impurity, oxidation, AC breakdown, and dielectric loss measurements are described. >

Dielectric Breakdown in Polyethylene at Elevated Temperatures

A two-terminal dielectric test cell for the study of the dielectric properties of viscous polymers above their softening temperature was used to perform 60 Hz breakdown measurements in polyethylene between 23 and 150°C. Molded specimens of polyethylene with embedded electrodes of parallel-plane configurations were employed. The interelectrode spacing was kept constant over the entire temperature range by suitable compensation of differential thermal expansion effects in the test cell. Low density (0.919 g/cm3) polyethylene was used for this study. The results show a marked decrease of the breakdown strength as a function of temperature between room temperature and the crystalline melting temperature, Tm. Above Tm the breakdown strength appears to be only weakly dependent on temperature. The results are analyzed with the help of Weibull statistics. The temperature dependence of the breakdown strength is discussed in the light of Artbauers theory of free volume breakdown.

A critical evaluation of analytical. techniques for the characterization of extruded dielectric cables

Various analytical techniques used to characterize cables are reviewed. It is shown that cable morphology can be evaluated simply by density and differential-scanning-calorimetry (DSC) measurements and that oxidation is best evaluated by Fourier-transform-infrared (FTIR) spectroscopy. Elemental contamination is best evaluated by neutron activation analysis and proton-induced X-ray emission (PIXE) measurements, whereas ions can be detected by ionic chromatography. It is shown that the water content of cables can be reliably measured by commercial instruments relying on coulometry and Karl-Fischer titration. >

Impurities in XLPE cable resins

This paper presents results on an impurity analysis of various insulation and semiconducting shield resins made by four manufacturers and used for power cables. Three different strand-blocking materials used to fill the conductor interstices were also analyzed. The measurements were performed by neutron activation on many samples taken from several batches and for several elements. The resins are compared to identify and quantify cleaner materials, since it is well known that impurities play an important role in the initiation and growth of water trees. Cleaner insulation and conductor shield resins can be found nowadays, but insulation shield resins are still of low quality. Strand-blocking materials are found to be heavily contaminated, which is inconsistent with the trend to use cleaner components for extruded cables.<<ETX>>

Effect of water on the performance of XLPE insulation

Water-tree counting measurements, water and impurity content measurements, and AC breakdown tests were performed on several field-aged medium-voltage extruded cables. The results indicate that the water-tree density increases with the insulation contamination level, particularly for aluminium, sodium, and potassium contaminant. However, neither of these aging factors seems to be correlated with the residual dielectric strength, which, on the contrary, is clearly influenced by the water content. In fact, the results suggest that water is the major aging factor of XLPE (cross-linked polyethylene) cables.<<ETX>>

Preliminary Characterization of a Nanodielectric Material

This paper presents a preliminary electrical and thermal characterization carried out on an epoxy nanocomposite material. It consisted of an epoxy matrix containing micrometric quartz and a small amount of nanoclay. AC electrical breakdown (ACBD) was performed on sheet samples with and without nanomaterial. The thermal characterization of this composite material and its reference includes thermogravimetry (TGA) and differential scanning calorimetry (DSC). The results show that the ACBD strength is slightly affected by the presence of such concentration of nanomaterial in the epoxy: greatest ACBD voltage and less disperse data. The thermal analyses of the polymer composite showed that decomposition temperature remain almost unchanged with addition of such a small amount of nanoclay.

Measurement of organic and inorganic ions in cable insulation and shields

It was demonstrated that HPIC (high-performance ion chromatography) can detect inorganic and organic ions in PE (polyethylene) and XLPE (cross-linked PE) insulation and shields in concentrations as low as approximately 10 mu g/L in the extraction solutions. This analytical technique requires liquid extraction of the ions prior to their content evaluation. The limitations of the extraction technique used are discussed. The detected inorganic ions correspond to the elements determined by other analytical techniques, but the ionic fraction is small. The actual ionic fraction could be higher, since the extraction technique used is far from being optimized. Manufacturing and aging lead to increased SO/sup 2-//sub 4/ and Cl/sup -/ contamination in XLPE cables. Migration from the contaminated shields appears to be one of the causes for this increase. Use of the much cleaner acetylene black should improve the situation. Organic ions were also detected and appear to be essentially by-products of PE oxidation. It is possible that service aging induces the formation of more oxalate ions. Shields made with ethylene vinyl acetate may also generate some acetate ions.<<ETX>>

Final breakdown mechanism

Preliminary results on the final breakdown mechanism of XLPE (cross-linked polyethylene) insulated field-aged cables are presented. The main goal is to determine the conditions that lead to the growth of electrical trees from water trees. Samples peeled off from two 28 kV and well-characterized (density, impurity and water content, field-water-tree density, oxidation) cables are considered. Aging conditions were 10 kV/mm, 60 Hz, 0.05 M NaCl solution, and 22-23 degrees C. The results indicate that after 8000 h of aging the lab-water-tree length reaches 65-70% of the sample thickness and is not much affected by the presence of field water trees. The influence of the physico-chemical properties and AC breakdown strength is also reported.<<ETX>>

Oxidation dependence of breakdown strength of XLPE

It has been shown in a previous study (P. Rohl, 1982) that the breakdown strength of polyethylene increases with increasing oxidation levels. The purpose of the research reported here is to verify this somewhat surprising result. Unlike the earlier work, cross-linked polyethylene (XLPE), the principal insulation used in extruded cables, is utilized. The authors correlate dielectric breakdown measurements with oxidation, monitored by Fourier transform infrared (FTIR) spectroscopy, and the results are interpreted using the modified free-volume theory. It is found that the breakdown strength E/sub b/ indeed shows a slight tendency to increase with the oxidation level. In particular, the authors found a correlation between E/sub b/ and the degree of oxidation, expressed in terms of the carbonyl band (1741 cm/sup -1/) absorption. The exact reasons for this behavior are not clear at this time, but appear to be compatible with the modified free-volume theory breakdown.<<ETX>>