XLPE Samples Research Articles

Space Charge Accumulation and Micro-Structure of Cross-linked Polyethylene

In this paper, laser induced pressure pulse (LIPP) method and electrostatic force microscopy (EFM) method are utilized to investigate the distribution of space charge in cross-linked polyethylene (XLPE), and the action on the groups of XLPE by the accumulative charge are investigated by infrared spectroscopy (IR) method. It was found that space charge in the sample has obvious influence on the vibration of chemical group, especially on group OH, group CH3 and group CH2. Group vibration affected considerably by space charge in XLPE sample locates on the interface between crystalline and amorphous domains. The experimental results also indicate that positive charge compared to negative charge has a different effect on bands.

Dielectric and viscoelastic properties of cross-linked polyethylene aged under multi stressing conditions

Different measurements were performed in cross-linked polyethylene (XLPE) employed as insulating material in coaxial cables that were field-aged and laboratory-aged under multistressing conditions at room temperature. Samples were peeled from the XLPE cable insulation in three different positions: just below the external semiconductor layer (outer layer), in the middle (middle layer) and just above the internal semiconductor layer of the cable (inner layer). The imaginary part of the electric susceptibility showed three peaks that obey the Dissado-Hill model. For laboratory-aged XLPE samples peeled from the inner and from the middle positions the peak at very low frequency region increased while in samples from the outer position a quasi-DC conduction process was observed. In medium frequency range a broadening of the peak was observed for all samples. Viscoelastic properties determined through dynamic mechanical analysis suggested that the aging generates processes that promoted changes of the crystallinity and the cross-linking degrees of the polymer. Fourier transform infrared spectroscopy (FTIR) measurements revealed an increase of oxidation products (esters), evidence of polar residues of the bow-tie tree and the presence of cross-linking by-products (acetophenone). Optical and scanning electronic microscope (SEM) measurements in aged samples revealed the existence of voids and bow-tie trees that were formed during aging in the middle region of the cable.

水トリー中の無機不純物の挙動に関する一考察

It is well known that water tree propagation in XLPE cable is significantly influenced by inorganic impurities in water. Therefore, we investigated both changes in concentration and deviation of isotopic content of inorganic elements in XLPE samples by water tree experiments in a clean environment. The concentration of several kinds of elements (e.g., Li, Na, Mg, Al, K, Ca, Fe, Ni, Pb, and Bi) in water-treed sample showed anomalous increase or decrease dependent on cation (K+, Na+, or Ag+) in water solution compared with blank or original samples. Furthermore, the isotopic content of Zn deviated over 6% from natural abundance. These results suggest that water tree propagation is related to unknown physical or electro-chemical reactions. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(2): 1–13, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20198

Application of the SSA Calorimetric Technique to Characterise an XLPE Insulator Aged under Multiple Stresses

In this work the thermal behaviour of crosslinked low density polyethylene (XLPE) used as an insulator for commercial underground high tension (15 kV) cables was studied. Three types of materials were selected: an uncrosslinked low density polyethylene (NXLPE) used as a control sample, an XLPE and an aged XLPE sample. The ageing conditions involved the application of multiple stresses: temperature, voltage and voltage impulses during 60 d under time and temperature cycles that are the most representative load of daily operation in Caracas, Venezuela. The effect of morphology segregation or thermal fractionation under multiple stresses conditions was analysed by measuring the percentage of crosslinking before and after the ageing tests were performed, and by investigating the thermal response of the material by conventional DSC and by the application of the successive self-nucleation and annealing (SSA) thermal fractionation technique. The degree of crosslinking was found to vary in the material depending on the distance from the conductor because a thermal gradient is generated radially during the curing reaction. Such differences did not significantly affect the usual DSC heating scans of the samples. However, when SSA was applied, a difference in the distribution of thermal fractions was detected as a function of the distance towards the conductor that could be correlated to the variations in the cross-linking degree. After the accelerated ageing the thermal response of XLPE changes as evidenced by the presence of multiple melting peaks in subsequent DSC heating scans. This multiple melting was interpreted, as a first approximation, as arising from thermal fractionation during ageing (ignoring the possible effects of the other stresses applied) and SSA was able to simulate a similar fractionation that was very accurate in the prediction of the exact temperatures of the melting peaks produced.

Impedance Spectroscopy Behavior of Cross-Linked Polyethylene

Impedance spectroscopy (IS) was to analyze the dielectric behavior of cross-linked polyethylene (XLPE) and to produce an electric analog equivalent circuit. In the high-frequency range, a peak was observed, and the values of the resistance for this particular frequency were calculated for the XLPE samples with different thicknesses. Two ohmic regions with different slopes were observed, suggesting the existence of two regions in the crystalline phase with the same morphological characteristics, but different sizes (i.e., similar time constants). A reduced two-phase model was used to explain the observed behavior of samples with different thicknesses. It is suggested that thinner samples present a crystalline layer with lamellae perpendicular to the surface, whereas in thicker samples, a bulk spherulitic morphology is dominant.

Factors of hetero space charge generation in XLPE under dc electric field of 20 kV/mm

This report deals with the mechanism of space charge accumulation in cross-linked polyethylene (XLPE) under dc electric field. Space charge was measured by the pulsed-electroacoustic method with applying dc stress of 20 kV/mm. A large amount of hetero space charge accumulated in fresh XLPE samples. Factors influencing the space charge accumulation were analyzed in regard to cross-linking by-products and antioxidant. No space charge was seen when the fresh sample was degassed to remove cross-linking by-products. Introducing acetophenone, one of the cross-linking by-products, in a degassed sample produces no space charge, suggesting that acetophenone itself could not be the direct factor of space charge formation. However, heating this sample up to 150 °C results in formation of hetero space charges as in virgin samples. Hence, it is concluded that hetero space charges may be formed when impurities, such as an antioxidant, dissociate thermally with the help of acetophenone and that the dissociated products are attracted toward both electrodes under a dc field to form the hetero space charges. © 1999 Scripta Technica, Electr Eng Jpn, 129(2): 13–21, 1999

The temperature dependence of space charge accumulation in cross-linked polyethylene

This paper presents the results of laser-induced-pressure-pulse (LIPP) measurements of space charge accumulation in 0.5 mm thick planar XLPE samples with aluminium electrodes, over the temperature range 30–90°C. The applied field strength was 10 kV mm−1. The increase in transit time of the pressure pulse across the sample with increasing temperature indicated that Young's modulus decreased by 50% from 30 to 90°C. It was concluded that electrons injected at the cathode are transported to the anode at temperatures above 40°C, and that electron traps cause an accumulation of negative charge immediately adjacent to both cathode and anode. It appears that a concentration of impurity molecules accumulates close to the cathode, and field-assisted thermal ionisation of these molecules generates an immobile positive space charge adjoining the electronic space charge adjacent to the cathode.

Effects of Crystallization Temperature on Crystallinity and Melting Behavior of Photocross-linked Polyethylene.

The effect of crystallization temperature on the Crystallinity, melting point, and heat of fusion of photocross-linked high-density (HDPE) and low-density (LDPE) polyethylenes (XLPE) have been studied by using wide-angle X-ray diffractomer (WAXD), differential scanning calorimetry (DSC), and gel determinations. The WAXD measurements show that the crystallinity of polyethylene decreases with increasing irradiation time in the XLPE samples. However, comparing with photocross-linking the crystallization temperature has a crucial influence on the degree of crystallinity. The DSC data confirm that photocross-linking and different crystallization temperatures affect the regularity or size of crystals, which causes depression of the melting point (Tm) and decreases the heat of fusion.

Morphological study of aging phenomena in XLPE by TEM technique

The long-term dielectric performance of underground power cable XLPE (cross-linked polyethylene) insulation suffers from poorly understood aging phenomena. A study of the morphological modifications of XLPE due to electrical aging may provide insight for a better understanding of aging mechanisms. The TEM technique has been used to study the XLPE morphology of unaged, laboratory-aged, and field-aged cable samples. A suitable image contrast enhancing and a stabilization of radiation damage were both successfully achieved by staining the sections with ruthenium tetroxide (RuO4), as individual XLPE lamellae were neatly and reproducibly resolved. Image analysis was used to help in the determination of any aging-induced morphological changes. XLPE samples from an unaged cable (D1) has been exposed to a low-energy electron beam, in an attempt to simulate certain conditions thought to occur in electric field-stressed dielectrics. © 1996 John Wiley & Sons, Inc.

Influence of internal mechanical stress and strain on electrical performance of polyethylene electrical treeing resistance

Crosslinked polyethylene (XLPE) and low-density polyethylene (LDPE) insulations used in HV cables are not only subjected to electrical and thermal stresses, but also exposed to mechanical stresses, whether residual internal stresses created during the cooling process of the fabrication, external forces when cables are bent during installation or thermomechanical stresses caused by differential thermal expansion between the conductor and the polymeric material. In order to investigate the possible influence of mechanical stresses on dielectric properties of polyethylene, measurements were conducted on pin-plane XLPE and LDPE samples with various magnitudes of residual mechanical stresses around the embedded electrode. The time to inception, the growing rate and the shape of the electrical trees under different voltages are reported in this paper. Specimens with the highest values of residual stresses were found to have the shortest inception times and the longest trees after one hour of aging under different voltages. When the mechanical stress was allowed to relax, the treeing resistance was measured to be significantly improved.

Photocross‐linking of low‐density polyethylene. II. Structure and morphology

AbstractMorphological structure and other properties of polyethylene photocross‐linked in the melt (XLPE) have been studied by wide‐angle X‐ray diffraction (WAXD), differential scanning calorimetry (DSC), density, and swelling measurements. The WAXD measurements show that the crystallinity of polyethylene decreases with increasing gel content in the XLPE samples and that the regularity and the size of crystals are affected by photocross‐linking. DSC data confirm that photocross‐linking occurs preferentially in the amorphous and interfacial regions and causes depression of the melting point and decreases the heat of fusion. The density of XLPE apparently decreases with increasing gel content, which is due to decreased crystallinity and lattice expansion. The cross‐link density determined by the swelling measurements and evaluated using the Flory–Rehner equation correlates well with the changes in crystallinity and melting point. © 1993 John Wiley & Sons, Inc.

Modification of XLPE exposed to partial discharges at elevated temperature

Extensions are presented of research on partial-discharge-induced surface modifications of XLPE (crosslinked polyethylene) wherein the XLPE sample surface during corona exposure is maintained at an elevated temperature ranging up to 160 degrees C. As for the ambient temperature case, circular degradation patterns are found, but their appearance and composition depend strongly on temperature. Furthermore, oxidation within a shallow (several micrometers) surface layer is found to increase sharply with rising temperature, an effect attributed to inward diffusion of corona-produced oxidizing species, particularly ozone and nitrogen oxides. >

Backscattered electron imaging and energy-dispersive X-ray studies of water-treed polymeric insulation

The backscattered-electron imaging mode in a scanning electron microscope has been used to detect and image water trees in electrically stressed cross-linked polyethylene (XLPE) insulation. The location and distribution of inorganic contaminants, such as Cu or Cl, within the water-treed regions of the insulation were revealed using backscattered-electron imaging. Energy-dispersive X-ray (EDX) spectroscopy and mapping were subsequently used to ascertain the chemical nature of the contaminants. Contaminants were only detected by EDX inside the treed areas delineated by the backscattered-electron images. A careful comparison of backscattered-electron and optical images of thin microtomed XLPE samples did not show any contaminant penetration beyond the visible-treed region. Results from a variety of service-aged and laboratory-stressed samples are presented, as well as a preliminary attempt to extend backscattered-electron imaging to optically opaque ethylene-propylene-rubber insulation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of Some Chemical and Mechanicaleffects on XLPE Degradation

It is shown that the relatively large concentration of impurities in cables is even higher in water trees. Gas bubbles observed in cavities filled with different salt solutions generate very high pressures able to fracture XLPE samples. It was also observed that fracture in XLPE under a cyclic mechanical load is frequency-dependent. The influence of oxidation on dielectric properties is also discussed.

Electrical Performance of Oven-Dried XLPE Cable

Accelerated aging tests conducted on miniature XLPE insulated cable have demonstrated that the oven-drying of steam-cuted XLPE cable insulation at 70°C does not impart any improvement in the initial breakdown voltage or in long-term electrical performance of the insulation over the as-received XLPE samples. Therefore, the oven-drying of steam-cured XLPE cable at 70°C is not considered as an effective method of improving cable quality.