Aged XLPE Cables Research Articles

Thermal aged XLPE cables restoration by injecting hydrolyzable voltage stabilizer

Thermal aged XLPE cables restoration by injecting hydrolyzable voltage stabilizer

Correlations between physico-chemical properties and dielectric behavior of thermally aged XLPE cables insulation

The carried out investigations aim at clarifying the correlations between physico-chemical properties and the variation of dielectric losses of Cross-Linked Polyethylene (XLPE) as Medium Voltage cable insulation during thermal aging. The used cables were manufactured by ELSEWEDY CABLES. They have been installed in Algeria where they must resist high temperatures that could exceed 50 degrees in the south of the country. For this aim, thermal aging measurements were performed on XLPE cables samples at 80, 100 and 140 °C for aging time of 5000 hours. The chemical properties were evaluated through Fourier Transform Infrared Spectrum (FTIR). In addition, the effect of aging on the crystallinity ratio, the lamellar thickness and the melting peak temperature was assessed using Differential Scanning Calorimetry (DSC). Frequency domain dielectric spectroscopy has been conducted on the aged XLPE cables in order to study the evolution of the dielectric losses. The obtained results showed that the principal aging mechanism is thermal oxidation which heads to deteriorate the morphology and increase both the polarization and losses, principally at low frequencies. It has been found that the color changes caused by the chains scission indicate the increase of interfaces between the cable insulating and semi-conductive layers, which could head to increase the dielectric losses due to the interfacial polarization.

Study on Aging Characteristics of XLPE Cable Insulation Based on Quantum Chemical Calculation

In this paper, quantum chemical calculation and isothermal relaxation current (IRC) measurement are performed on thermally aged XLPE cables to investigate the effect of defects on dielectric properties and trap energy distribution of XLPE insulation. Samples of XLPE cables are thermally aged at 135 °C up to 45 days. Microscopic experiments including gel permeation chromatography, differential scanning calorimetry, and Fourier transform infrared spectrometer show that the molecular weight and crystallinity of XLPE decreases with aging time. The IRC results show that both the trap density and the trap depth increase with aging. To quantitatively calculate the trap depth variation due to physical and chemical defects by thermal aging, a quantum chemical calculation is carried out based on Gaussian 09. It is found that the broken cross-linking bonds between molecular chains weaken the stability of the molecular network structure, introducing the shallow physical trap (0.3 eV) in the aged XLPE. The molecular chain is thermally oxidized, the carbonyl group is introduced as a deep chemical trap (1.44 eV) in the XLPE. The chemical defect not only changes the localised energy state, but also significantly decreases the intrinsic breakdown strength of XLPE molecule from 0.62 to 0.51 V/Angstrom in quantum chemistry calculation.

A Techno-Economic Framework for Replacing Aged XLPE Cables in the Distribution Network

In this paper, a stochastic framework is proposed for optimizing replacement strategies of aged XLPE cables in the electric power distribution network. The proposed framework facilitates realizing an economic value for the risk of unplanned outages, thereby shifting the paradigm of replacing aged XLPE cable from a technical decision that is solely based on diagnostic measurements to a techno-economic strategy. The proposed framework constitutes of three main phases: (1) obtaining a distribution for the aging process of XLPE cable due to water-trees, (2) modeling the replacement of the aged XLPE cable as a Markov decision process, and (3) applying a linear programming algorithm in order to solve for the optimal replacement policy. Based on the proposed framework, a critical health index is defined for optimum replacement, depending on the installation costs of a new cable and the unplanned outage cost.

CVケーブルの設備形態ごとの劣化様相に関する調査

As XLPE cable laid in duct is mainly degraded by water-tree, water-tree growth and its affection to breakdown voltage of aged XLPE cable has been investigated. For XLPE cables laid under dry condition (substation, tunnel, etc.) or with water barrier, the growth rate of water-tree is considered to be lower than those laid in duct because of little water invading into the insulating layer. However, few reports related to this issue have been published and the degradation of aged XLPE cable laid under dry condition or with water barrier is not revealed. From these view-points, we investigated the breakdown voltage and the condition of water-tree degradation on removed 33kV XLPE cables laid under dry condition and with water barrier.

The study of semiconductor layer effect on underground cables with Time Domain Reflectometry (TDR)

Time Domain Reflectometry is a technique used for fault location and partial discharge analysis in underground electric distribution cables. The technique is based on the analysis of the propagation of short electric pulses. In underground cables, semiconductor layer properties and degradation cause perturbations in the signal wave shape. These perturbations complicate the interpretation of reflectograms and, as a consequence, fault location. This work presents a circuit model of distributed elements that includes the electrical resistance of the semiconductor layers. The results of computer simulations using the proposed model are compared with measurements in new and aged XLPE cables. This comparison shows that the proposed model can represent the phenomena satisfactorily.

Study of Siloxane Injection Technology on XLPE Cables for Water Tree Aging

This paper introduces one siloxane rejuvenation fluid used to rejuvenate aged XLPE cables and also studies the effects and the mechanism of the rejuvenation. Accelerated aging experiment was performed to obtain water tree aged samples. Inject the rejuvenation siloxane fluid into the aged cable samples with a pressure injection system. By comparing the dielectric loss factor and the breakdown voltage level of the samples before and after rejuvenation, inferred that the insulation level of the cable samples after rejuvenation is close to or higher than that of new cables. Additionally, through microscope, organic compounds created by chemical reaction were observed to fill the micro voids and eliminate the moisture in the XLPE layer of the samples. Finally, the electric field was simulated by Finite Element Method (FEM). Based on the results, it's further confirmed that the siloxane rejuvenation fluid can significantly enhance the aged cable insulation.

Severe degradation of the conductor screen of service and laboratory aged medium voltage XLPE insulated cables

The main purpose of this paper is to show the strong correlation between corrosion of the metallic aluminum conductor and the formation of interconnected cracks / voids in the conductor screen, creating initiation sites for vented water trees in service aged medium voltage XLPE cables. The results show that porous structures in the conductor screen previously reported for laboratory aged insulation systems, also develop in the conductor screen in service aged medium voltage XLPE cables. These structures can bridge the screen and serve as path for contaminants and corrosion products from the aluminum conductor and initiate water trees. A prerequisite for the formation of such structures is the presence of liquid water at the interface between the conductor and conductor screen causing corrosion. The initiation site of such structures has been identified, and is likely caused by environmental stress cracking (ESC). Initiation sites were determined in all cables, but porous structures in the conductor screen were only observed in the cable suffered from service failure, where liquid water had entered the cable conductor between the strands. Severe degradation of the XLPE insulation was observed at the initiation sites for water trees growing from these structures.

Investigation of dielectric behavior of thermally aged XLPE cable in the high-frequency range

The widespread use of crosslinked polyethylene (XLPE) in the wire and cable industry may be attributed mainly to its outstanding physical and chemical properties. However, it has been recognized that degradation of XLPE insulation during service, such as thermo-oxidation and water treeing, may lead to premature electrical breakdown of the XLPE insulation cables. Diagnostic measurements would considerably assist to distinguish between bad and still good cables. Estimation of the degree of aging is at present the most important task of diagnostic methods. This paper presents a study on the thermo-oxidation-induced changes in the dielectric properties of the thermally aged 3.3 kV XLPE cables in the frequency range from 70 kHz to 10 MHz.

Analysis of failure of crosslinked polyethylene cables because of electrical treeing: A physicochemical approach

AbstractThe present work investigated the breakdown characteristics of high‐voltage crosslinked polyethylene (XLPE) cable by electrical trees under ac and composite voltages. The electrical trees resemble either a tree or a bushy structure. The importance of the Weibull parameters for the present study was emphasized. The failure zone of the XLPE cables was characterized by experimental techniques such as wide‐angle X‐ray diffraction, differential scanning calorimetry, and thermogravimetric–differential thermal analysis, to understand the phase constituents affected by electrical trees. The impact test and flexural test results indicate that material with high stiffness/toughness allows tree formation and causes early failure of the material. The characteristic variation of the aged XLPE cables was investigated by a dynamic mechanical analyzer (DMA). The activation energy values were calculated from the DMA data. The rate of tree propagation was found to be less for materials (XLPE) with high activation energy. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2169–2178, 2004

Correlation between space charge distribution and water-tree location in aged XLPE cable

The correlation between charge distribution in an aged crosslinked polyethylene (XLPE) cable and the location of water trees has been studied. The space charge radial distributions at different angle in the insulating wall of a service-aged XLPE coaxial cable, induced by application of a DC stress, were measured using the pulsed electroacoustic method. Furthermore, a cross section of sliced insulating wall was observed with an optical microscope to compare with the radial space charge distribution. The cable under investigation was electrically aged in service at 6.6 kV AC for 25 years in wet conditions. Water trees were found at the position in which space charge was observed. On the other hand, no water tree was found at the position in which the space charge was not observed. Judging from these results, the space charge measurement seems to be useful for the diagnosis of water trees in aged cables.

Correlation between AC breakdown strength and low frequency dielectric loss of water tree aged XLPE cables

This paper describes experiments performed on laboratory aged 12 kV and service aged 24 kV XLPE cable samples. Results from measurements of residual AC breakdown strength, degree of water treeing and dissipation factors are presented. The dissipation factor at low frequencies was determined by both time and frequency domain measurements. The relationship between these methods are discussed and it is shown that both can be used for assessing the average ageing state of the cable, The results show that water treeing causes reduced residual AC breakdown strength and high and nonlinearly increasing low-frequency dielectric loss.

Development of New Diagnostic Method for Hot-Line XLPE Cables with Water Trees

A new insulation diagnostic method for XLPE cables containing water trees is presented. A dc component in the ac charging current of these cables was found to be a significant sign of the existence of water trees. The dc component has good correlations with such insulation characteristics of aged XLPE cables as ac breakdown voltage and dc leakage current. Criteria for insulation diagnosis of 6.6kV XLPE cables have been established. An automatic insulation diagnostic device has been developed. It is now being applied to hot-line XLPF cables in distribution systems of TEPCO (The Tokyo Electric Power Co., Inc.).

The Impulse Strength of Naturally Aged XLPE Cables Containing Water Trees

The results of impulse-voltage tests on 15 kV crosslinked polyethylene (XLPE) cables with a taped conductor strand shield which have been in service for eight years are reported. The tests were performed with the insulation dry and also containing visible water trees at conductor temperatures of 25° and 90°C. The impulse strength is significantly lower at the higher temperature and with visible water trees.