XLPE Material Research Articles

Improved DC performance of crosslinked polyethylene insulation depending on a higher purity

Super-clean crosslinked polyethylene (XLPE) has been widely used in DC power cables. In order to understand the nature of purity dependence of electrical insulation performance, two kinds of insulating XLPE materials used in AC and DC high voltage cables, whose abbreviation were AC-XLPE and DC-XLPE respectively, were investigated in this paper. The results showed that DC-XLPE had a relative higher DC electrical conductivity and relative lower thermal activation energy than AC-XLPE, which was governed by the trap characteristics. The thermal activation energies could match the energy depths of the bulk deep traps calculated by thermal stimulated current. Both positive and negative charges can be found in AC-XLPE while there were only positive charges in DC-XLPE. Meanwhile, DC-XLPE showed higher electric breakdown strength than AC-XLPE. It was proposed that the improved DC insulation performance of DC-XLPE was closely related to its higher purity. The Fourier transform infrared spectroscopy results showed AC-XLPE contained more polar groups such as carbonyl groups than DC-XPLE, which were caused by antioxidants. The oxidative induction times verified a higher content of antioxidant in AC-XLPE. X-ray diffraction showed little difference of crystallinity between two samples, but DC-XLPE exhibited a larger spherulite size. As a result, it is suggested that more efforts should be made to achieve higher purity of XLPE in HVDC insulation.

A Study on the Dielectric Characteristics of Non-Cross Linked Polyolefin for Eco-Friendly Power Cables

A cross linked polyethylene (XLPE) material has been widely used to develop a power cable due to its excellent electrical characteristics and mechanical strength. The XLPE material has several problems such as environmental disruption, electrical aging, thermosetting property, and impurities which cause degradation. Therefore, an eco-friendly dielectric material should be used for a power cable instead of the XLPE. In this paper, the electrical breakdown characteristics of seven types of non-cross-linked polyethylene materials which are eco-friendly are compared with those of the XPLE. All dielectric experiment methods comply with the standards of ASTM-D149a. In order to calculate the electric field intensity, the finite element method (FEM) is used.

Effect of Degassing Treatment on the Crystalline Morphology of XLPE Insulation Used for 110kV XLPE Cable

Cross-linked polyethylene (XLPE) is a main kind of insulation materials, used in medium and high voltage power cable. High voltage XLPE cable production mainly adopts peroxide crosslinking method, and the performance of XLPE cable will be affected by decomposing by-products of crosslinking agent (DCP, dicumyl peroxide).In this paper, the XLPE cable was degassed with different times. The XLPE samples were analyzed by infrared spectrum analysis, DSC analysis and electron microscopy, and the influences of degassing on XLPE crystalline morphology were analyzed. The results showed that the crystal structure of XLPE in the cable was affected by degassing treatment. With the degassing treatment time went on, the by-products kept releasing. The content of by-products in 110kV XLPE cable decreased obviously in 2-7 days’ degassing treatment, and changed slowly after 7 days’. The growth and perfection of the original crystal in XLPE material were obtained, and the new small grains were formed in the material.

Experimental set-up for determination of the large-strain tensile behaviour of polymers at low temperatures

In this study, we present a method to determine the large-strain tensile behaviour of polymers at low temperatures using a purpose-built temperature chamber made of polycarbonate (PC). This chamber allows for several cameras during testing. In our case, two digital cameras were utilized to monitor the two perpendicular surfaces of the test sample. Subsequently, the pictures were analysed with digital image correlation (DIC) software to determine the strain field on the surface of the specimen. In addition, a thermal camera was used to monitor self-heating during loading. It is demonstrated that the PC chamber does not influence the stress-strain curve as determined by DIC. Applying this set-up, a semi-crystalline cross-linked low-density polyethylene (XLPE) under quasi-static tensile loading has been successfully analysed using DIC at four different temperatures (25 °C, 0 °C, −15 °C, −30 °C). At the lower temperatures, the conventional method of applying a spray-paint speckle failed due to embrittlement and cracking of the spray-paint speckle when the tensile specimen deformed. An alternative method was developed utilising white grease with a black powder added as contrast. The results show a strong increase in both the Young’s modulus and the flow stress for decreasing temperatures within the experimental range. We also observe that although the XLPE material is practically incompressible at room temperature, the volumetric strains reach a value of about 0.1 at the lower temperatures.

Dynamic Thermal Modeling of VoltageDivider Capacitive Coupling

This paper deals with the thermal model and compensation of the thermal drift in a capacitive voltage sensor installed on top of the semiconducting layer of cross-linked polyethylene (XLPE) cable. The analysis of the thermal phenomena underlying the system shows a high dependency on the relative permittivity of the XLPE material. The first compensation method consists of installing an external capacitor with the same temperature coefficient as the cable capacitance. This method is limited because of the nonlinear thermal behavior of the cable capacitance. The second method consists of measuring the conductor current and the outer XLPE temperature, and estimating the temperatures and properties of the cable by means of an offline-derived, built-in model. The simulations and tests show the obtained improvements of the different methods in the operational temperature range.

Analysis of Water Treeing Resistance Performance Characteristics of Nano Filled XLPE Cable Insulating Material

In recent times, high voltage secondary distribution network in urban areas is widely supported by XLPE cables because of its advantages over conventional overhead lines. Major problem faced in the usage of underground cables is the failure of insulation due to water treeing phenomena. It occurs mainly due to water diffusion in the base XLPE polymeric material leading to the breakdown of cable insulation over a period of time. As a result of water treeing, dielectric strength of the material reduces significantly. Recent advancements in nanotechnology shows that addition of nanofillers in the base polymeric material will enhance the thermal and electrical insulation characteristics of the material. However, very few reports are only available in the literature regarding the water treeing resistance characteristics of nano-modified XLPE material. Hence it is necessary to perform a variety of tests on nano-modified XLPE material in order to collect a large volume of data useful for electrical utilities. Considering these aspects, in this paper, important dielectric performance characteristics of nano SiO2 filled XLPE material such as dielectric constant, tan delta, volume resistivity and capacitance are studied at different %wt concentration of nano-filler. In addition, laboratory experiments were also carried out in order to understand the water treeing resistance characteristics of nano filled XLPE material under virgin and aged conditions.

Analysis of Electrical Tree Inception and Propagation in XLPE Nano-Composites

In this present work, electrical treeing analysis of the virgin and nano size filled Cross Linked Polyethylene (XLPE) material has been studied. Silicone Oxide (SiO2)nano-filler was used in the XLPE base material. Nano-fillers were added at 1% concentration by weight in the base material. Laboratory experiments were conducted as per IEC procedures at 12 kV AC voltage. Influence of filler concentration on the inception and propagation of electrical treeing were studied. Analysis of time to failure of XLPE materials at 1%nano-filler concentrations were carried out. Tensile strength were measured for understanding the mechanical properties of the nano filled XLPE material, Scanning Electron Microscope (SEM) carried out for understanding the dispersion of nano fillers in the nano filled XLPE material.

Correlating conductivity and space charge measurements in multi-dielectrics under various electrical and thermal stresses

The design of transmission systems requires electric field distribution estimation, which, in case of HVDC application is strongly sensitive to thermal and electrical configuration as well as to the nature of dielectric materials being used owing to the resistive field distribution. In this paper, the field distribution in a dielectric bilayer of XLPE and rubber materials, as representative of cable junctions, is estimated based on experimental data on field and temperature dependencies of conductivity. Through space charge measurements on bi-layer dielectrics, it is shown that the space charge density and electric field distributions are to a first order estimation consistent with data issued from conductivity measurements. Most notably, the interface charge building up between the two dielectrics changes sign, depending on field and temperature. However, in the high field range (order of 20 kV/mm), charge build-up in the bulk of dielectric materials introduces further distortion to field distribution.

A Fuzzy Logic Approach to Model and Predict HV Cable Insulation Behaviour under Thermal Aging

Given the non-linearity of changes in the dielectric and mechanical properties of insulation, it is difficult to find neither a theoretical nor an experimental model for the aging mechanism. Hence, the need to model the evolution of mechanical properties of the XLPE material under thermal aging using new methods is necessary. The use of fuzzy logic is one of these methods and presents an original idea in high voltage prediction problems. Several factors were considered, namely the aging time and the level of applied temperature. The study of the impact of these parameters on the quality of the fuzzy algorithm is carried out. The characteristics of the modelling function (trapezoidal or triangular) and the number of membership functions partitions was also taken into consideration. The obtained results were encouraging. It has been proved that fuzzy logic is a powerful tool that can be used in predicting the properties of insulation. Furthermore, the analytical findings were in good agreement with the experimental results with an acceptable error margin.

Synergistic Effect of TNPP and Carbon Black in Weathered XLPE Materials

Carbon black is one of the most widely used and most effective ultraviolet (UV) light stabilizers for plastics applications. Several important segments of the plastics industry rely on carbon black for UV stabilization of weather-resistant products, including telecommunications, power cable jacketing, and plastic pipes. In this research work a combination of Trisnonylphospate (TNPP) antioxidant and different size carbon black were applied in crosslinked polyethylene (XLPE) to improve its wetherability. The primary reason for cross-linking polyethylene (PE) is to raise the thermal stability of the material under load. This substantially improves environmental stress crack resistance and resistance to slow crack growth. The results achieved of this additive package combination show a synergism effect and improved weatherability of electrical cable. Increased weathering lifetime was also achieved. Further, we were able to confirm in this work, that the size and quality of the carbon black dispersion in a XLPE samples is an important component of both the UV-resistance and mechanical properties of the finished plastic article. Incremental improvements of carbon black dispersion can positively influence the expected life of plastic articles. Mechanical testing and FTIR were used to detect degradation of the accelerated weathered XLPE samples. The morphological considerations of UV energy absorption and presents laboratory data demonstrating the link between dispersion and weatherability as well as between morphology and weatherability

A Field-circuit Coupled Method to Accurately Determine Intrinsic Complex Permittivity of XLPE Insulation Material

To identify the location of partial discharge occurring in a power cable, it is essential to obtain the intrinsic values of permittivity of cross-linked polyethylene (XLPE) material. This paper reveals that when the conventional two-parallel-electrode method is used, and due to the fringing effect, the measured permittivity of XLPE material pronouncedly differs from its intrinsic value, consequently significant measurement error could occur. An effective method based on the field-circuit coupled theory is presented in this paper to investigate the difference between the measured and intrinsic permittivity of XLPE insulation material. A capacitor constructed from a concentrically cylindrical XLPE sample is used. By assuming a certain value of the intrinsic permittivity for XLPE material, the electric field distribution in the XLPE material and the ambient is calculated. Then the total energy is determined, as well as the measurable permittivity. By assuming different values of intrinsic permittivity, corresponding measurable values of permittivity are determined, and a curve of measurable permittivity against its intrinsic value is then obtained, from which the intrinsic value can be found once its measurable value is obtained from experiment. The influence factors of the correction curve are analyzed, i.e. the sizes of sample and electrode. A measurement was carried out to verify this method.

EFFECTS OF ORGANIC COLORANTS ON PHOTO-INITIATED CROSSLINKING AND PHOTO-OXIDATION DEGRADATION OF POLYETHYLENE AND RELATED MECHANISM

The effects of three organic colorants on photo-initiated crosslinking and photo-oxidation degradation of polyethylene (PE) samples irradiated by microwave excited (MWE) UV lamp in the melt and the related mechanism have been studied by gel content and thermal extension rate determinations, X-ray photoelectron spectroscopy (XPS), mechanical property tests, UV spectroscopy, and light microscope. The data from the gel content and thermal extension rate determinations of photo-crosslinked polyethylene (XLPE) samples show that the three colorants can decrease the efficiency of photo-initiated crosslinking of polyethylene to some different degree, in which the effect of red colorant is the largest among the three colorants. The colorized samples of 1 mm thickness are easily to be crosslinked to a satisfactory gel content of about 70% by the MWE lamp and optimized reaction conditions, such as the concentration of colorant, irradiation time, and so on. The XPS results give the evidence that the colorants can accelerate the surface photo-oxidation during the photo-crosslinking of polyethylene. The photo-oxidation products such as -CH2-O- and -C(C=O)- groups on the surface of XLPE samples with the colorants apparently increase with increasing the irradiation time. The data from the mechanical tests show that the colorants reduce the tensile strength and improve the elongation at break of XLPE samples. All the above results show that the effects of the three colorants on photo-initiated crosslinking and photo-oxidative degradation decrease with the order of red > blue > green colorants. The light microscope photos show that the colorant can disperse well in PE resin. The mechanism of the colorant effects can be elucidated by comparison of the UV absorption spectra of photo-initiator and colorants. This is because the colorants absorb the same UV wavelength regions as photo-initiator, and thus decrease the photo-crosslinking efficiency of photo-initiator and accelerate the surface photo-oxidation of XLPE materials.

Influence of thermal treatment and residues on space charge accumulation in XLPE for DC power cable application

The effects of cross-linking by-products (residues) quantity and thermal treatment on space charge accumulation and decay in manufacturer modified XLPEs for DC power cable application have been investigated using the pulsed electro-acoustic technique. The threshold stress for space charge generation among the modified and reference XLPEs during voltage-ramping was found to show considerable variation and to depend upon the material and the amount of residue present. However, the modified XLPE material was found to exhibit a higher threshold for space charge accumulation than the reference XLPE whatever the conditions. De-gassed samples were found to exhibit the highest threshold stress, with that of the modified de-gassed XLPE accumulating no space charge at all even after 24 h stressing at 70 kV. In general heterocharge regions were formed when the residues were present and homocharge or no charge was formed when the residues were removed by degassing. Differences were also found in the space charge decay following short-circuit (volts-off), with the decay of heterocharge being rapid, whereas that of homocharge being slow. The charge accumulations have been tentatively explained by the mechanisms of ion-pair separation when residues are present, and interfacial injection when residues are absent. Decay of the heterocharge is governed by ion-pair displacement just as the generation and will have a similar time scale. In contrast homocharge decay will be governed by charge de-trapping and extend to time scales well beyond that of injection for charge in the deepest traps

A New Method of Evaluating the Resistance of Cross-Linked Polyethylene Compounds to Water Treeing

In this paper, a new method of evaluating the resistance to water trees of various cross-linked polyethylene (XLPE) insulation compounds is presented. The new method is that water trees are grown in XLPE materials which are immersed in AgNO3 solution used as needle electrode. Water trees grown by this method have several characteristic properties such as spontaneous colouring and fast growth during the formation of water trees, as well as retention of their original structures after removal of the electric field. Several XLPE insulation compounds, whose water-tree-resistant properties are known, have been used to test and verify that the growth of water trees in AgNO3 solution is a very effective test method for evaluating the water-tree-resistant properties of different XLPE compounds.

Water treeing and dielectric loss of WTR-XLPE cable insulation

Abstract: In the investigation, the authors have tested three commercially available water-tree retardant XLPE (WTR) materials and compared some of their properties with conventional XLPE insulation. They have focused on measurements of the initiation and growth of water trees and dielectric loss at power frequency (50 Hz). Water- tree tests were performed on Rogowski-type test objects kept at 20°C and energised at 10 kV/mm for up to nine weeks of aging. Measurements of the dissipation factor tan 6 were performed on 12 kV cable samples in the temperature range 20- 130C, up to a maximum voltage of 96 kV,,, . The results show that water trees are also readily initi- ated in WTR materials, and that their growth rates are approximately as for conventional XLPE. In all the materials tested, introduction of mechanical strain strongly enhanced the initiation and growth of water trees. The WTR-cable samples examined showed different temperature- and voltage-dependent dissipation factors. Typic- ally, the dissipation factor of degassed cable samples at 20°C was about two times higher than that of conventional XLPE, and at 90°C it was six times higher. This difference was found to increase with increasing voltage. Prior to the degassing of the by-products from the curing process, some of the WTR cables showed high dissipation factors even at 20C, and their values were found to increase nearly proportionally with applied voltage.

Effect of Antioxidants on Radiation Stability of Polymers Used for Electric Cable Insulation

The effect of various antioxidants and antioxidant concentrations on the room-temperature radiation stability of ethylene propylene diene monomer (EPDM) and cross-linked polyethylene (XLPE) materials used as insulation for electric cable in nuclear power plants is measured. Relative elongation to rupture between aged and unaged material is used as the measure of stability. The materials are irradiated to doses up to 2.0 MGy (200 Mrad) at a dose rate of 300 Gy/h (3 x 104 rad/h) in the Cobalt-60 Gamma Irradiation Facility at the University of Virginia. Several of the antioxidants in EPDM perform well to the maximum doses for which reliable data were obtained of either 1.0 or 1.6 MGy. An XLPE material with Agerite™ MA antioxidant performs well to the maximum dose for which reliable data were obtained of 1.0 MGy. Stabilization at 1.0 MGy increases with increasing antioxidant concentration, though not in proportion to the concentration.

HPLC and DSC Analysis of Crosslinked PE from HV Cables

High performance liquid chromatography (HPLC) has beenused to simultaneously determine the amounts of antioxidant(Santonox(R) R), peroxide, acetophenone and ?-methylstyrenepresent in crosslinked-polyethylene cable samples andmolded specimens, as well as to detect some other peroxidedecomposition products.It was found that the antioxidant is grafted to thepolymer chains after curing (even after partial curing).Results also seem to suggest that a reduction of the phenoliccharacter and perhaps of the effectiveness of thentiantioxidant might result from this grafting process. Peroxidedecomposition products were found to depend on the curingconditions used and to affect the dielectric behavior insome cases.Various XLPE and EPR materials have also been examined bydifferential scanning calorimetry (DSC), and experimentalprocedures were developed for evaluating the degree ofcrosslinking by DSC.This work was part of a program supported by the Electric Power Research Institute.