Cross-linked Polyethylene Insulation Research Articles

Effect of micrometer bubble interfaces on the detection and measurement method of XLPE

Hidden air bubble defects in the XLPE (Cross-Linked Polyethylene) insulation will seriously affect its electrical performance, which needs to be detected timely to ensure the safe and stable operation of the power system, but the interfaces between bubbles and XLPE may affect the detection. This paper proposes a novel method for high-precision detection and measurement method of micron-sized bubbles in XLPE based on THz time-domain reflection technology. To analyze the influencing factors limiting the high-precision detection and measurement of micron-sized defects, firstly, the propagation law of THz wave in XLPE with and without defects is studied, and the unipolar and bipolar pulses reflected at the defects are analyzed by overlap, and then a new method of micron-sized defects detection and measurement is proposed based on the overlap property of the bipolar pulses, and the relationship between the minimum detecting frequency and the size of the defects in the various detecting methods is determined. And the finite element method is used to simulate the THz detection of XLPE samples with defect thicknesses of 100, 7, 5, and 3 μm, respectively, and the proposed method is validated in conjunction with experiments. The results show that micrometer defects can be detected and measured with high precision using this proposed method, and the error can be controlled within 4.39%, which is significantly better than the conventional detection method. The proposed method can provide a new idea for non-destructive testing of insulation defects.

A study of XLPE insulation failure in power cables under electromagnetic stress

Underground cables with cross-linked polyethylene (XLPE) insulation are integral to medium voltage (MV) power transmission systems, ensuring continuous electricity supply amidst operational challenges and environmental conditions. However, the reliability of these cables can be compromised over time due to aging and installation-related factors, particularly at joints and terminations. This study offers a comprehensive analysis of how various defects, including spherical air voids, pinholes, and irregularities in semiconducting layers, affect electric field and potential distributions within cable end terminations using COMSOL Multiphysics software. Through detailed simulations, the study identifies significant variations in electric field strength caused by these defects, highlighting critical stress concentration areas. In this study, it assumed that the cable and termination have the same cross-section. By analyzing these simulations, the study provides insights into optimizing cable design and installation practices to enhance the reliability and lifespan of underground power transmission systems. This study introduces a novel approach by combining advanced COMSOL Multiphysics simulations with a detailed analysis of defect impacts on electric field distributions, offering new insights into stress concentrations and degradation at cable terminations. Simulation outcomes reveal significant variations in electric field strengths due to air voids and pinholes in cables and terminations: for 2 mm voids, up to 2.45×106 V mm−1 near the conductor and 56.5 V mm−1 at termination. These findings enhance the understanding of XLPE-insulated cable behavior under electromagnetic stress, providing a basis for mitigating failures and improving overall system performance.

ЕЛЕКТРИЧНЕ ПОЛЕ НАПІВПРОВІДНОГО ЕКРАНА СИЛОВОГО КАБЕЛЮ З ПОЛІМЕРНОЮ ІЗОЛЯЦІЄЮ З УРАХУВАННЯМ СТРУКТУРНИХ НЕОДНОРІДНОСТЕЙ

The non-homogeneous electric field distributions in the cross-linked polyethylene insulation of 330 kV power cable with multi-conductor core depending on the electrical conductivity and thickness of the semiconducting screen as well as at different frequency of applied voltage (from 50 to 104 Hz) are studied by finite-element analysis. As shown, taking into account the periodic structure of core surface, the electrical conductivity of the material of internal semiconducting screen should exceed 10–6 S/m and its thickness should be at least 1.5 mm in order to obtain the uniform electric field in the basic insulation of the cable at the boundary with the screen. The paper also considers the effect of various types of the surface and volumetric defects in the semiconducting screen on the electric field inhomogeneity in cable insulation. It is shown that the defects in the screen can lead to a significant increase of the electric field in the local zones of their location (up to 5 times as compared to the field without defects) and therefore contribute to the formation and development of treeing structures in the insulation. The revealed results explain the reasons for the micro- and macro-level degradation of power cable insulation and confirm the need to ensure the uniform structure, smooth surface of both semiconducting screen and core during the technological process of manufacturing the power cables with cross-linked polyethylene insulation. References 23, figures 8, table 1.

Long-Term Permissible Load Currents of Single-Core Cables with a Voltage of 10 kV with Cross-Linked Polyethylene Insulation of Various Designs

Long-Term Permissible Load Currents of Single-Core Cables with a Voltage of 10 kV with Cross-Linked Polyethylene Insulation of Various Designs

СИСТЕМИ МОНІТОРИНГУ СУЧАСНИХ КАБЕЛЬНИХ ЛІНІЙ ЕЛЕКТРОПЕРЕДАЧІ

The overview of fiber-optic technologies for real-time monitoring of the technical condition and operational characteristics of modern power cables with cross-linked polyethylene insulation and with the fiber-optic module integrated into their structure is presented. In particular, the new intelligent means for monitoring of the temperature, load current, deformation (mechanical damages), partial discharges in the insulation, moisture penetration inside the cables and control over sheath voltage limitation are described. The topicality of using and developing the technologies of smart diagnostics for power cable lines in order to achieve the effective and safe operation of cables as well as the high reliability of networks is shown. Ref. 47, fig. 7. Key words: power cables, power cable lines, cross-linked polyethylene insulation, integrated fiber-optic modules, power networks, smart monitoring.

DEVELOPMENT OF A TESTER FOR DIAGNOSTICS OF CABLE LINES STATE WITH XLPE INSULATION

Link for citation: Konev A.A., Khakimyanov M.I. Development of a tester for diagnostics of cable lines state with XLPE insulation. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 9, рр. 216-223. In Rus. Relevance. The moral and physical deterioration of cable lines voltage of 6–10 kV in our country ranges from 40 to 80 %. At the same time, up to 63 % of all accidents in electrical equipment are due to cable line failures. Recently, cables with cross-linked polyethylene insulation have been actively used, for the diagnosis of which special approaches are required. Therefore, the issues of developing high-voltage test facilities for diagnosing the state of insulation of electrical equipment are quite relevant. The authors proposed an original design of a high-voltage test facility that allows generating both direct voltage and alternating voltages of industrial and ultra-low frequencies. A feature of the device is the use of a multifunctional integrated electromagnetic component in it, which combines the functions of both an inductor and a capacitor. The use of this component allows reducing the dimensions and weight of the test facility. The main aims of the research are to develop a design of a high-voltage test facility and to study its characteristics; build a mathematical model and get dependency graphs. Objects: cables with XLPE insulation, high voltage test facilities. Methods: theory of electrical circuits, mathematical modeling, program for mathematical and engineering calculations Mathcad, MATLAB Simulink. Results. The authors proposed the original design of a high-voltage test facility, which makes it possible to generate both direct voltage and alternating voltages of industrial and ultra-low frequencies. The simulation of the installation was carried out. A method for calculating high-voltage test facilities is proposed.

Перспективы рециклинга в строительстве с целью создания инновационных композиционных материалов

The article classifies construction wastes and assesses the possibility of using them to produce new construction materials. The authors present examples of obtaining composites on the basis of glass and brick waste, reinforced concrete debris, gypsum plasterboard, drywall, wood, asbestos cement, polyvinyl chloride, and cross-linked polyethylene. The analysis of literature sources reveals the manufacturing of building materials and products on the basis of construction waste. Their using is a promising direction of production development, allowing us to reduce costs, conserve natural resources, and minimize the negative impact on the environment. The wastes of cross-linked polyethylene (insulation of cables and pipelines) belong to the wastes, recycling of which is poorly studied or complicated. The article also provides the results of research aimed at the synthesis of oil-resistant concrete on the basis of cross-linked polyethylene waste and crushed bricks, indicating the prospects of using cross-linked polyethylene as an aggregate for the production of concrete.

Reflection characteristics of water treeing insulation in medium voltage cables

This paper presents an analytical model based on wave reflection of dual interfaces for studying the reflection due to water treeing in the cross-linked polyethylene (XLPE) insulation of medium voltage cables. The model is expected to predict the reflection characteristics more accurately than that only considering reflection of a single interface. Experiments on a cable sample with mimic water trees are conducted to confirm the effectiveness of the presented model. The model can potentially be used to relate time-domain reflectometry (TDR) measurement results with characteristics of water treeing insulation in the cables.

Quantitative Evaluation of Thermal Ageing State of Cross-Linked Polyethylene Insulation Based on Polarization and Depolarization Current

The widespread use of cross-linked polyethylene (XLPE) as insulation in cables may be attributed to its outstanding mechanical and dielectric properties. In order to quantitatively evaluate the insulation status of XLPE after thermal ageing, an accelerated thermal ageing experimental platform is established. Polarization and depolarization current (PDC) as well as elongation at break of XLPE insulation under different ageing durations are measured. XLPE insulation state is determined by the elongation at break retention rate (ER%). Based on the extended Debye model, the paper proposed the stable relaxation charge quantity and the dissipation factor at 0.1 Hz to evaluate the insulation state of XLPE. The results show that the ER% of XLPE insulation decreases with the growth of ageing degree. The polarization and depolarization current of XLPE insulation will increase obviously with thermal ageing. Conductivity and trap level density will also increase. The number of branches of the extended Debye model increases, and new polarization types appear. Stable relaxation charge quantity and dissipation factor at 0.1 Hz proposed in this paper have a good fitting relationship with ER% of XLPE insulation, which can evaluate the thermal ageing state of XLPE insulation effectively.

Effect of thermo-oxidative aging on thermal elongation performance of XLPE insulation for high-voltage cables

The thermal elongation of cross-linked polyethylene (XLPE) insulation for high-voltage cables is an important factor for evaluating the high-temperature mechanical performance, which is an important parameter included in the testing standards of the power cable insulation. Previous studies have found that the thermal elongation of XLPE insulation for high-voltage cable depends on the cross-linking degree, but have not dealt with the effect of the thermo-oxidative aging during the thermal elongation test. In this paper, the thermal elongation experiments of the XLPE samples compounded with different kinds of antioxidants were carried out under both air and nitrogen atmospheres. The molecular structure changes of the XLPE samples after thermal elongation under air atmosphere were examined by gel content, FTIR spectra and X-ray diffraction. The oxidation induction periods were also measured to analyze the antioxidation performance of the phenolic antioxidants and study its contribution to the thermal elongation performance. The results showed that the XLPE samples underwent thermo-oxidative aging in the thermal elongation test under air atmosphere at 200 °C. It caused that the thermal elongation properties can not accurately reflect the cross-linking degree of the original XLPE samples, but has a strong correlation with the gel content of the aged samples. Antioxidants can inhibit the thermo-oxidative aging effect on the network structure, leading to a slower uptrend of elongation under the air atmosphere. This indicates that the thermal elongation performance of XLPE insulation is determined by the synergistic effect of the network structure and the antioxidation performance, and this can provide reference significance for optimizing the evaluation methods of high-voltage cable XLPE insulation performance.

Remote Condition Monitoring of HVDC Cable Insulation Using Deep Learning-Aided Space Charge Classification

The homo-charges accumulated inside the polymeric insulation of high voltage dc (HVdc) cables increase the electrical field beyond the uniform value, whereas the hetero-charges reduce it. Such uneven electrical field distribution is detrimental to HVdc cables’ longevity and thus, needs to be identified accurately. To this end, this article proposes a deep learning (DL) framework for the automated detection of space charges in HVdc cable insulation. The space charges inside the cross-linked polyethylene (XLPE) insulation samples were measured under varying electric fields (10–50 kV/mm) as well as at different temperatures (30 °C–70 °C) conditions. The obtained space charge profiles corresponding to no charge, hetero-charge, and homo-charge categories were fed to two benchmark convolutional neural network (CNN) models, i.e., AlexNet and VGGNet16, for automated feature extraction and classification purpose. The convolutional blocks of the two CNN models were sequentially fine-tuned using transfer learning (TL) to observe the variation in classification accuracies. The proposed approach delivers appreciably high classification accuracies irrespective of varying electric field and temperature conditions with reduced computational time toward distinguishing different categories of space charge accumulations compared to other CNN models. Hence, it can be potentially used for real-time HVdc insulation diagnostic purposes.

Effect of Polycyclic Aromatic Compounds on Electrical Treeing Growth in XLPE Insulation

This article focuses on the effect of polycyclic aromatic compounds (PACs) on the electrical tree of cross-linked polyethylene (XLPE) insulation for high-voltage (HV) cables. The XLPE insulation samples with or without PACs are prepared for electrical treeing degradation at constant and cyclic temperatures. The results show that the growth of the electrical tree within the XLPE/PACs insulation is retarded. The partial discharge (PD) amplitudes of XLPE/PACs insulation are significantly decreased during the treeing process compared to XLPE insulation. Furthermore, under cyclic temperature conditions, the tree growth of XLPE/PACs composites stagnates as the temperature decreases from 90 °C to 25 °C. Even when it rebounds to 90 °C, the treeing degradation of XLPE/PACs insulation is still retarded, resulting from a significant weakness of PD. The results show that the XLPE/PACs composites exhibit a greater electrical treeing resistance. Based on density functional theory (DFT), analysis of the excitation characteristics and density of states (DoS) reveals that the PACs absorb photon energy to prevent damage to insulation. In contrast, the excited PACs result in a cross-linking reaction to XLPE and introduce electron and hole traps, which capture hot electrons. The cross-linking product acts as localized states of electron traps and hole traps, which capture the high energy carriers during treeing. In summary, the PACs retard the growth of electrical trees and improve the insulation performance.

ЕЛЕКТРОФІЗИЧНІ ПРОЦЕСИ ДЕГРАДАЦІЇ ЗШИТОЇ ПОЛІЕТИЛЕНОВОЇ ІЗОЛЯЦІЇ СИЛОВИХ КАБЕЛІВ І САМОУТРИМНИХ ІЗОЛЬОВАНИХ ПРОВОДІВ ЗА НЕСИНУСОЇДНИХ НАПРУГАХ І СТРУМАХ

The features of electro-physical processes of degradation of cross-linked polyethylene (XLPE) insulation of power cables and self-carrying insulated wires (SIW) are studied, taking into account water microinclusions and dendrite branches that appear in such insulation during manufacture and operation. It is substantiated that the appearance of non-sinusoidal voltages and cur-rents in power transmission lines (TL) using modern power cables and SIW with solid XLPE insulation causes an increase in electro-physical factors aimed at increasing the pulsating electromechanical pressure on the surface of solid XLPE insulation by liquid conductive microinclusions and dendrite branches and an increase in over time, their sizes along the strength of the exist-ing electric field (EF). The paper defines the patterns of change over time in the magnitude of the stressed volume and the maxi-mum pressure in the XLPE insulation and the additional impact on such patterns of higher harmonic components of electrical voltages and currents during non-sinusoidal processes in such insulation. It is indicated that such electro-physical effects on solid polymer insulation lead to an increase in the intensity of both threshold mechanisms and stochastic ones of destruction of the microstructure of the solid XLPE insulation of power cables and self-carrying insulated wires and require the development of additional regulations for standard conditions for their manufacture and operation. References 10, figures 3.

Automated Space Charge Classification Inside ±500-kV HVDC Cable Insulation Using Fusion of Superpixel and Deep Features for Remote Condition Assessment

The accumulated space charges cause electrical field distortion, which is fatal to the safe and reliable operation of polymeric high-voltage direct current (HVDC) cables. Hence, this paper aims to detect and classify the space charges to ensure reliability and a longer operating life of HVDC cables. To achieve this, experiments were carried out on cross-linked polyethylene (XLPE) insulation samples and space charge distributions were recorded under altering electric fields (10–50 kV/mm) and at different temperatures (30–70 °C). Subsequently, super-pixel color features were extracted from the space charge images using the simple linear iterative clustering (SLIC) algorithm. In addition, deep features were extracted using the AlexNet convolutional neural network (CNN) model. The fusion of the handcrafted and deep features was fed to three benchmark machine-learning classifiers for the recognition of different space charge accumulation categories. The method delivered high recognition performance in spite of altering electric fields and varying temperatures. As a result, the proposed framework can detect space charges in HVDC cable insulation in real time.

FEM-based Arrhenius modeling of the thermal effects of a heating pipeline and pavements on underground power cables

It is well-known that, in thermal aging management of underground power cables, analytical Arrhenius models can include the thermal effect of a heating pipeline on the lifespan of power cables through a rise in the soil temperature and an appropriate ampacity de-rating factor. However, the thermal effect of a pavement on the lifespan of power cables installed below that pavement cannot be taken into account using any de-rating factor. Accordingly, this paper proposes FEM-based Arrhenius models to analyze the thermal effects of one heating pipeline and several different types of pavements on the thermal lifespan of an underground line of 110 kV consisting of cables with cross-linked polyethylene (XLPE) insulation. Each proposed Arrhenius model is created using both IEC 60287 and IEC TR 62095, as well as combining the traditional Arrhenius model with an appropriate FEM-based steady-state thermal model. An experimental background consisting of the existing thermal aging tests with XLPE insulation is also used. Different pavements are represented by different values of thermal and surface radiation properties, namely: thermal conductivity, solar absorptivity, and thermal emissivity. The pavement surface radiation properties are specified within the frames of the most unfavorable summer conditions, and the most common winter conditions. Moreover, different distances between the 110 kV cables and the heating pipeline are considered. Finally, it is found that: the considered thermal effects can shorten the expected total service lifespan of the 110 kV cables in the area adjacent to the heating pipeline by up to 297.5 h; the pavement surface radiation properties do not affect the consumption of total thermal lifespan; as well as the largest consumption of total thermal lifespan occurs when the 110 kV cables are at the smallest distance from the heating pipeline.

Multiscale Analysis of Naturally Weathered High-Voltage XLPE Cable Insulation in Two Extreme Environments

Crosslinked polyethylene (XLPE) insulation is widely used in power cables because of its excellent properties. Due to the remoteness of end users, the energy is generally transported over long distances. These cables cross various areas being sometimes exposed to different environmental stresses that affect their performance and increase their degradation. While most published literatures are limited to laboratory scales, this article uniquely deals with a multiscale analysis of a 30 months naturally weathered XLPE samples in two harsh environments. The first site is a marine environment with high humidity level, sea salt content in the air, seasonal variation of temperature, and concomitant radiation doses. The second one is a desert environment characterized by extremely high temperatures, which reached 50 °C during summer, coupled with thermal cycling and large solar radiation doses with UV rays of high energy wavelengths. Modifications in the XLPE insulation characteristics due to weathering were studied using different experimental techniques. The testing matrices included dielectric and mechanical characterization, scanning electron microscopy (SEM), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, and differential scanning calorimetry (DSC) measurements on fresh and weathered XLPE insulation samples. It is found that the degradation rate was faster in the arid desert climate compared with the coastal one. Furthermore, correlation mechanisms between the obtained results are discussed in order to better understand the weathering degradations.

Performance Evaluation of Cross-Linked Polyethylene Insulation of Operating 110 kV Power Cables.

The ageing characteristic of XLPE insulation of operating a 110 kV power cable with different service time is studied in this paper. The microscopic morphology of XLPE films from different cables were characterized by using Differential Scanning Calorimetry (DSC), X-ray Diffraction method (XRD), and Fourier Transform Infrared Spectroscopy (FTIR) methods, and the dielectric, mechanical, and electrical properties of XLPE were also measured. The relationship of several typical property parameters with the cable service time were established, and the ageing mechanism of XLPE insulation of the operating cable was also analyzed. It was found that XLPE insulation would endure a recrystallization process in the initial operation stage during which the microscopic morphology would become more perfect with higher crystallinity and denser crystal structure. Then, the thermal oxidation would dominate the ageing process of XLPE with the molecular chains broken and more micromolecular products generated after the cable had operated for more than 10–15 years. The AC breakdown strength decreases with the increase of cable service time, with lower decreasing rate in the initial operation stage and a larger rate after 10–15 years. The Pearson correlation coefficient between the cable service time with the characteristic parameters were calculated, and some of them were found to be effective to be used as indicators for operation state detection of operating power cables.

Estimation of Moisture Content in XLPE Cable Insulation Using Electric Modulus

In this article, a novel approach is proposed for accurate moisture content (MC) estimation of cross-linked polyethylene (XLPE) insulation employing electric modulus. Additionally, the variation of real and imaginary part of electric modulus in the argand plane is fitted using Cole-Cole (C-C) model for understanding of distributed relaxation behavior of XLPE insulation. Based on the frequency variation of dielectric parameters, five moisture sensitive parameters are identified and using these parameters, five empirical relationships are proposed for quantitative assessment of MC in XLPE insulation. It is observed that parameters obtained from the complex modulus spectrum yields better accuracy in estimation of MC compared to the parameters obtained from traditional frequency domain spectroscopy (FDS).

Investigation of XLPE Cable Insulation Using Electrical, Thermal and Mechanical Properties, and Aging Level Adopting Machine Learning Techniques.

Hydrothermal and chemical aging tests on a 230 kV cross-linked polyethylene (XLPE) insulation cable were carried out in the present study to evaluate the degradation and aging levels qualitatively. The samples were subjected to water aging at a temperature of 80 °C, and in an aqueous ionic solution of CuSO4 at room temperature. The diffusion coefficient results indicated that the ion migration was not at the same rate in the aging conditions. The diffusion coefficient–D–of the sample immersed in an aqueous CuSO4 solution was lower than the hydrothermally aged specimens. The hydrophobicity of aged specimens decreased considerably compared to unaged samples. The distribution of trapped charges was quantitatively characterized. The presence of shallow trap energy states were observed in unaged XLPE, whereas the deep trap sites were noticed in aged specimens. In addition, the charge trap characteristics were different for positive and negative charge deposition. Various material characterization techniques, viz. dynamic mechanical analysis (DMA), tensile, contact angle, and LIBS, were further employed on the aged and virgin specimens. The tensile behavior of the hydrothermally aged specimen was degraded due to the oxidised regions, which had formed a weak spot against the mechanical stress. Reduced glass transition temperature and increased loss tangent measurements were noticed for aged specimens over their unaged counterparts. Machine learning techniques, such as the principal component analysis (PCA) and the artificial neural network (ANN) analysis, were performed on LIBS spectral data of the samples to classify the aging mechanisms qualitatively.

Improvement of cable defect identification for power distribution networks

Power distribution networks are critical infrastructures that secure delivery of electricity in a power system. Since electricity is mainly distributed through underground 11 kV and 22 kV power cables on Hong Kong Island, the maintenance of underground power cables is one of the most challenging jobs that The Hongkong Electric Co., Ltd. (HK Electric) has to face. This is because excavation for repair and replacement is difficult and time-consuming due to busy traffic and congested underground conditions. In order to improve maintenance efficacy, this study puts forward a new method to screen and locate potential defective component(s) of underground power cables by collectively using online and offline partial discharge (PD) diagnosing methods. The study also introduces new evaluation metrics (standard deviation of mean tan delta, and delta of mean tan delta) to identify cables at risk of developing water trees inside the cross-linked polyethylene (XLPE) insulation. The effectiveness of these two proposed methods has been validated on its distribution network. These improved identification methods for detecting cable defects could further enhance the reliability and security of HK Electric’s power distribution system.