High-voltage Cross-linked Polyethylene Research Articles

Maintenance Practices of High-Voltage Cross-Linked Polyethylene (XLPE) Cable Buffer Layers

As urbanization accelerates globally, the demand for electricity continues to rise, especially within high-voltage power transmission systems. Cross-linked polyethylene (XLPE) cables have become crucial due to their exceptional insulation properties and mechanical characteristics. However, the buffer layers of XLPE cables may undergo aging and faults caused by various factors, including electrical aging, thermal aging, chemical impacts, and improper design. This paper synthesizes existing literature to provide an in-depth analysis of the material properties, aging mechanisms, fault causes, design principles, and maintenance practices related to XLPE cable buffer layers. Through finite element method (FEM) simulations, it evaluates the electric field distribution across XLPE cables under different operating voltages and the contact state between the buffer layer and the metal sheath. Additionally, this study explores the recyclability and environmental impact of XLPE materials, as well as how improvements in material properties and detection technologies can enhance cable reliability and safety. The findings offer comprehensive theoretical and practical guidance to the power industry, aiding in optimizing cable design, improving system reliability, and reducing failure rates, while also suggesting future research directions.

Study on the ablation process and failure mechanism of the buffer layer in high-voltage XLPE cable

In recent years, the frequent occurrence of ablation faults in the buffer layers of high-voltage cross-linked polyethylene (XLPE) cables has significantly undermined the stability of power grid operations. However, existing research cannot fully explain the failure mechanism and transformation of related physical and chemical properties of high-voltage cable buffer layer. To address this, the paper have constructed an integrated analysis framework that includes multiphysics field simulations, an equivalent electrical network model, and simulated ablation experiments to comprehensively reveal the complete ablation failure mechanisms. Firstly, this study conducted a comparative analysis of the voltage distribution and temperature changes in the buffer layer using electro-thermal coupling simulations and an equivalent electrical network model. This analysis provides a detailed study of the dynamic development process of buffer layer ablation and identifies key electrical parameters. Subsequently, it verified the accuracy of the simulations through experiments on temperature variation characteristics, changes in hydrogen content, and other gas concentrations by constructing a simulated buffer layer ablation experimental platform. Finally, based on the study of the mechanisms of the buffer layer ablation failure process, this study divided it into three stages: initial electrochemical corrosion, appearance of white powder, and the onset of ablation discharge in the buffer layer. The comparison of experimental results with simulation data from the first two stages further validated the effectiveness and robustness of our analysis framework. In summary, this study provides a comprehensive analysis of the failure process in high-voltage cable buffer layers through simulation and experimental validation. This study provides a methodological basis for preventing cable ablation faults and detecting and evaluating the status of high-voltage XLPE cable buffer layers.

A Novel Condition Assessment Method for Corrugated Aluminum Sheathed XLPE Cables Based on Evolved Gas Analysis

Condition assessment is significant to the maintenance management of high-voltage cross-linked polyethylene (XLPE) power cables. This paper presents a condition assessment method for corrugated aluminum sheathed cables based on evolved gas analysis. First, simultaneous thermogravimetric and mass spectrometric analyses of the pyrolysis of each cable material were conducted to characterize the gas generation caused by thermal faults. Simulated experiments were designed to investigate gas generation caused by electrical faults. The results show that different cable faults (low and high-temperature pyrolysis of semiconductors, insulation, and buffer materials; low and high energy discharge activity across insulation materials and electrochemical corrosion of the buffer material) show distinct gas generation characteristics. Based on the evolved gases, a method of identifying faults from the key gas concentration is proposed in this study. Moreover, gas transport within the corrugated aluminum sheathed cable with a buffer layer was analyzed, based on which a gas sampling method for cables with a buffer layer is proposed. Finally, the developed method was validated by practical applications. The proposed method is comparable to conducting a blood test on a human body in that it can help discover various incipient cable faults. The fundamentals and data interpretation methods are adaptable to all XLPE cables.

Research on Composition of White Powders in High-voltage XLPE Cable with Buffer Layer Failure

Recently, a new type of high-voltage cross-linked polyethylene (XLPE) cable failure called buffer layer failure occurs from time to time. The failure occurs at the cable body between the insulation screen and the aluminum sheath, which is different from the traditional one that occurs at the intermediate joint or cable accessory. The main feature is white corrosion marks on the inner surface of the aluminum sheath, dotted potholes, and ablation marks on the outer surface of the insulation screen. Especially there are dotted white powder and burnt holes on the surface of the water-blocking tape. Given the white powder in this new failure, imitating the actual structure of the cable, this paper builds a three-layer model of the buffer layer in the laboratory. The water-blocking tape is placed between the insulation screen and the aluminum sheath. The white powders were reproduced by injecting water into the tape and extruding it. The micro-morphology of white powder was observed by scanning electron microscope (SEM) and the element composition was measured by energy dispersive spectrometer (EDS). The results show that the white powder of water-blocking tape contains Al, which is the product of a chemical reaction with an aluminum sheath.

Terahertz Non-Destructive Testing and Imaging of High-Voltage Cables

The evaluation of the internal defects in high-voltage cross-linked polyethylene cables is highly crucial for the security and reliability of power transmission. Here, a terahertz (THz) frequency-modulated-continuous-wave non-destructive testing (NDT) imaging system is used to demonstrate the non-contact detection of a high-voltage cable (35 KV). Combined with linear scanning and axial rotation, the three-dimensional (3D) data of the columnar target is acquired, and a 3D perspective image is reconstructed. The correspondence between the target in the real space and image space is determined to accurately show the internal spiral structure and strand number of metal wires. The results indicate that prefabricated air and water holes in the polyethylene insulation layer can be displayed. Furthermore, feature defect signals are automatically classified and recognised by combining the principal component analysis dimension reduction method and the support vector machine classification method, which can predict abnormal defects more efficiently in mass data. These results provide technical guidance for the non-contact NDT and visual evaluation of the internal state of detected cable targets.

Experimental Study on Heat Distribution of High-Voltage Cable Terminals under Operating Conditions

The insulation failure of the outdoor terminal of the high-voltage cross-linked polyethylene (XLPE) cable has a significant impact on the operation stability of the power grid. Effective evaluation of its thermal state is of great significance to predicting its insulation failure. The paper uses infrared thermal imaging to analyse the thermal field distribution characteristics of the outdoor terminal surface under different ambient temperatures. Test results, theoretical calculations and simulation results all show that the thermal steady-state temperature of the inner and outer layers of the sheath shows a non-linear increase with the increase of the current carrying capacity of the cable, which verifies the correctness of the model, so this model can be used to predict And analyse the temperature distribution of the actual 10kV cable cold shrink terminal at the thermal steady state.

Assessment of Thermal Aging Degree of 10kV Cross-Linked Polyethylene Cable Based on Depolarization Current

During the long-term operation of high-voltage cross-linked polyethylene (XLPE) cables, the cable will gradually deteriorate under the effect of various aging factors. Thermal aging is one of the key factors directly related to the long-term operation reliability of the XLPE cables. In this paper, the thermal aging samples are prepared for PDC measurement. Based on the polymer trap theory and the extended Debye model, the shape of PDC curve, depolarization charge, parameters of the extended Debye model, aging factor (A), elongation at break retention rate (EB%) and their relationships under different thermal aging degrees are analyzed. The results show that the depolarization current curve and the depolarization charge can preliminarily judge the aging condition of the XLPE, but it is not accurate enough. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{3}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau _{3}$ </tex-math></inline-formula> of the branch, with the largest time constant must be the most sensitive to cable aging. With the increase of aging degree, the A shows an upward trend, and has a nonlinear relationship with EB%. Under the aging temperature of 140 °C, after 15–25 days of aging, the A suggests the samples are in the moderate to severe aging stage, and the PDC test should be performed on carried out for the cables with that have been used for 21 years of operation to evaluate the insulation aging condition. The study reveals that the PDC-based parameters can be serviced as an effective tool for the aging condition prediction of XLPE cable insulation.

High Voltage Cross-Linked Polyethylene Insulator Characteristics Improvement Using Functionalized ZnO Nanoparticles

This study aims to develop large-scale commercial Cross-Linked Polyethylene (XLPE) nanocomposites used for industrial applications with power cables insulations. To achieve this goal, XLPE/zinc oxide nanocomposites were prepared with four unique loadings of Zinc Oxid (ZnO) nanoparticles: 0.5, 2, 3.5 and 5 wt.%, in the presence of appropriate coupling agent which used to reduce the agglomeration of nanoparticles and to improve the compatibility inside the polymer matrix. Amino silane was the coupling agent used in this study and the nanocomposites preparation was done using the melt blending method. The morphology and the distribution of nanoparticles inside XLPE polymer were studied using Field Emission Scanning Electron Microscopy (FE-SEM). The mechanical properties were also evaluated. Using Differential Scanning Calorimetry (DSC) and Thermo Gravimetric Analysis (TGA), the thermal properties for nanocomposites were investigated. The dielectric properties of these prepared XLPE/ZnO nanocomposites were studied by measuring the loss tangent (tan δ) and the dielectric constant (ɛr) under frequencies ranging from 1 Hz to 1 MHz. The AC Breakdown Voltage (AC-BDV) was also measured using a controlled high voltage testing transformer (50 Hz) under the sphere-to-sphere field then; the AC dielectric strength was calculated using the Finite Element Method (FEM) technique.

Thermal Effect of Short-term Overload on High-voltage Cross-linked Polyethylene (XLPE) Cable Insulation

This paper describes changes on micro-structure and aggregation structure of cross-linked polyethylene (XLPE) with different thermal histories after different times of overheating treatment to analogize the short-term fault or emergency overload in actual operating cables. 5, 10 and 20 times overheating treatment were conducted on a spare cable and a retired cable with service years of 32, whose insulation were analyzed by the diagnostic measurements of Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and thermogravimetry (TG). The results show that the spare cable has presented a trend of improvement on the insulation properties after overheating treatment below 20 times, but slight deterioration occurs after 20 times overheating treatment; the retired cable with service years of 32 has presented a trend of degradation on the insulation properties at the first 5 times overheating treatment, and a severe deterioration occurs after 20 times overheating treatment.

Design of Smooth Aluminum Bonded Sheaths in HV XLPE Cables Based on the Bending Performance Research

In China, recent ablation failures of water-blocking buffer layer occurred in high voltage (HV) cross-linked polyethylene (XLPE) cables with corrugated aluminum sheath have stimulated the product development of cables with smooth aluminum bonded sheath. Despite the many advantages, bending performance is one of the few weaknesses of smooth aluminum bonded sheath. In this paper, a three-dimensional model of a cable with smooth aluminum bonded sheath in a four-point bending test was set up, where the cohesive zone model was applied to simulate the mechanical behavior of the adhesive layer (bonding interface). The influence on the bending performance of the bonded sheath by the thickness of aluminum layer, the parameters of hot melt adhesive, the thickness and material parameters of outer jacket, as well as the cable specification was investigated, together with the stress distribution and damage analysis of the adhesive layer. Based on the research, the material selection and structural design of smooth aluminum bonded sheath were recommended for HV XLPE cables, including an additional requirement for the shear strength of hot melt adhesive besides those already in IEC and IEEE standards. Finally, a 110 kV XLPE cable with smooth aluminum bonded sheath was manufactured and type tested.

Amplitude Distribution of Partial Discharge Signals on Tunnel-Installed High-Voltage Cables

For 110 kV and above tunnel-installed high-voltage (HV) cross-linked poly-ethylene (XLPE) cable systems, it is a normal procedure to adopt a cross-bonding scheme. The high-frequency current method is frequently used in the cross-bonded cable systems for on-site or online partial discharge (PD) detection by monitoring the signals on the cross-bonding wires. To further study the amplitude distribution characteristics of the PD signals, a parametric characteristic admittance model of a three-phase cable system in a tunnel is established based on Tylavsky’s formulas. The model is used to calculate the amplitude distribution formula of the PD pulse current on the cross-bonding wires. In addition, the influence of cable laying and tunnel environment on the amplitude distribution is also studied. Finally, the correctness of the model and the conclusion are verified by simulation experiments and on-site tests. The results show that the signal amplitude distribution is determined by the ratio of the characteristic admittances. As the distance between the cables and the distance from the inner wall of the tunnel increase, the amplitude difference gradually decreases.

Changes on structural and electrical properties of retired cross-linked polyethylene (XLPE) cable insulation under electro-thermal aging test

In order to study the insulation performance of two retired high voltage cross-linked polyethylene (XLPE) cables with service years of 16 and 32 and evaluate the reliability of reusing these cables for actual operation, a 180-day electro-thermal aging test was exerted on a section of these two cables to inspect the changes on structural and electrical properties. Several diagnostic measurements including fourier infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and breakdown field strength were carried out on the untreated and treated samples. The measured results show that the crystalline structure of the 16 years cable has been improved after the accelerated aging test probably because of the reduction of the impurities and the annealing effect, which further improve the electrical property. Even though the molecular chains of the 32 years cable insulation has been damaged and a slight degradation happens on the crystalline structure under the intensive aging test condition, which is responsible for the decline in the electrical property, the cable still have the potential to resist the stresses properly under the actual operation condition for a long time from the perspective of the XLPE. Therefore, it can be expected that the rest of these two retired cables still have the ability to reuse in the long-term actual operation condition.

Thermal Effect of Different Laying Modes on Cross-Linked Polyethylene (XLPE) Insulation and a New Estimation on Cable Ampacity

This paper verifies the fluctuation on thermal parameters and ampacity of the high-voltage cross-linked polyethylene (XLPE) cables with different insulation conditions and describes the results of a thermal aging experiment on the XLPE insulation with different operating years in different laying modes guided by Comsol Multiphysics modeling software. The thermal parameters of the cables applied on the models are detected by thermal parameter detection control platform and differential scanning calorimetry (DSC) measurement to assure the effectivity of the simulation. Several diagnostic measurements including Fourier infrared spectroscopy (FTIR), DSC, X-ray diffraction (XRD), and breakdown field strength were conducted on the treated and untreated specimens in order to reveal the changes of properties and the relationship between the thermal effect and the cable ampacity. Moreover, a new estimation on cable ampacity from the perspective on XLPE insulation itself has been proposed in this paper, which is also a possible way to judge the insulation condition of the cable with specific aging degree in specific laying mode for a period of time.

Testing and Diagnosis of Extra High Voltage Power Cables Using Damped AC Voltages Combined with Distributed Partial Discharge Measurement

State assessment method of cables with extruded insulation and their accessories, DAC withstand test combined with a diagnostic test (such as PD measurement) has two shortcomings, the output voltage is not high enough for EHV cables, it cannot detect and locate PD effectively when defects are far away from signal acquisition point. This paper focus on DAC voltages up to 250kV and a kind of distributed measurement of PD. Defects existing in arbitrary of tested cable can be accurately measured in theory by installing sensor at intervals along the tested cable. This method has been validated on a long extra high voltage cross-linked polyethylene (XLPE) insulated underground cable circuit and the filed PD test has been completed successfully.

Inhibiting Effect of Nonlinear Shielding Layer on Electrical Tree Propagation inside Insulation Layer of High-Voltage Cable

Nonlinear insulated materials can uniform electric field distribution in non-uniform electric field. In order to inhibit the electric tree initiation and propagation inside high-voltage cross-linked polyethylene (XLPE) insulated cable, a kind of 220kV high-voltage XLPE insulated cable with new structure is designed by embedding nonlinear shielding layer into XLPE insulation layer of high-voltage cable with traditional structure in this study. Experimental and simulation results indicate that the nonlinear shielding layer can effectively inhibit electrical tree propagation inside the XLPE specimens, and obviously extend the breakdown time caused by electric tree propagation. When the electrical tree propagates into the nonlinear shielding layer sandwiched between insulation layers of cable, the electric field distribution near the tip of electrical tree is obviously improved. These findings prove the feasibility and the effectivity of inhibiting electrical tree propagation inside high-voltage cable by adding nonlinear shielding layer into the insulation layer.

Inhibition of Electrical Tree Initiation inside High-Voltage Cross-Linked Polyethylene Cable with Nonlinear Shielding Layer

Electrical tree aging is one of the leading factors causing degradation and breakdown of insulation performance of high-voltage cross-linked polyethylene (XLPE) cable. A lot of attention has been paid to inhibiting electric tree initiation and propagation inside high-voltage cable. In this study, insulation composites with nonlinear electrical conductivity are prepared. Experimental results show that these composites are helpful for enhancing the apparent breakdown electric field strength of XLPE in the pole electrode system. A kind of high-voltage XLPE insulated cable with new structure is designed by using these composites as nonlinear shielding layer. Numerical simulation results indicate that nonlinear shielding layer can obviously improve the distribution of electrical field at the defect of insulation surface of high-voltage XLPE cable, and decrease the maximum electrical field strength at the defect. This new structure is potential for effectively inhibiting electrical tree initiation inside high-voltage XLPE insulation cable, thus increasing life span and operational reliability of the cable.

Gray intensity image feature extraction of partial discharge in high-voltage cross-linked polyethylene power cable joint

SUMMARY A feature extraction method for gray intensity image of partial discharge (PD) is applied to recognize the insulating defects in high-voltage cross-linked polyethylene power cable joint. The method is based on a two-directional and two-dimensional (2-D) maximum margin criterion (MMC). A 2-D orthogonal projection of gray intensity image of PD was performed in horizontal and vertical directions. Projected image data were taken as discriminant vector of different gray intensity images to solve the high dimensional and small sample size of PD gray intensity image. The nearest neighbor classifier was used to classify the PD gray intensity image to recognize different insulating defects in the joint. The recongnition results of four typical insulating defects in the laboratory indicated that the extraction speed of the gray intensity image feature and recognition rate of insulating defects are superior compared with the common principal component analysis and Fisher discriminant analysis with MMC. Copyright © 2012 John Wiley & Sons, Ltd.

Water ingress in high-voltage cross-linked polyethylene (XLPE) cable terminations

In Norway, there are 6000 to 7000 oil-filled high-voltage cable terminations in the transmission network, most of which are constructed following the old design shown in Figure 1. Although the number of reported failures of medium-voltage cross-linked polyethylene (XLPE) cables and cable accessories is low, during the last few decades some severe explosive failures with potential for personnel injury have been reported [1]. Examination of some of the failed cable terminations showed that the most likely cause was the presence of water in the housing. Water can enter the housing through defective O-rings or through an air ventilation screw on the top cover of the termination housing. As part of their regular maintenance programs, some Norwegian utilities measure the water content of the filling oil. Liquid water was found in some installations, and in one case, the stress cone was found to be covered in ice when the porcelain housing was removed [2] (Figure 2). Similar breakdowns have been reported in other European countries [3], [4]. The main purpose of the work described in this article was to investigate the effect of water ingress in oil-filled high-voltage cable terminations, in particular the associated breakdown mechanisms. The work was performed using full-size and model cable terminations.

Failures, Monitoring and New Trends of Power Transformers

This article presents a survey on failures, monitoring, and new trends of power transformers. There are three main types of power transformers, namely, oil immersed, gas-insulated, and dry-type transformers with or without cast coil insulation system. Operating stresses of power transformers have increased due to the load growth and the increased bulk power transactions, where recent and imminent alternating current (AC) systems are rated 1,100 kV and 1,200 kV, respectively. Failures of windings, onload tap changers (OLTC), and bushings are the main defective components as they represent about 84% of the failure statistics. Online and offline diagnostic monitoring of power transformers can be used to detect faults at an early stage, prevent degeneration into catastrophic phenomena, and monitor the aging process of the insulating systems. Many of the well-known preventive maintenance techniques are discussed. New trends of power transformers have recently taken many steps forward in different dimensions, e.g., Powerformer, Dryformer and Windformer utilizing high-voltage (HV) cross-linked polyethylene (XLPE) cables, gas-insulated transformers (GIT), converter transformers rated ±800 kV, and high-temperature super conducting (HTS) transformers. The latter type represents a key technology for future power systems engineering as they offer many advantages over the others.

Dimensional Effects on the Measurement of Permittivity of Semiconducting Materials Used in High Voltage XLPE Cables

The intrinsic permittivity, not apparent permittivity, of semiconducting layers of high voltage cross-linked polyethylene (XLPE) cables imposes a significant influence on the design of partial discharge detecting sensors. It has extremely high permittivity, resulting in a dimensional effect, an embodiment of the difference between the intrinsic permittivity and apparent or measurable permittivity. To investigate this dimensional effect in semiconducting material, a mathematical model is set up in this paper for a capacitor with two rectangular-shaped electrodes in parallel, between which is inserted a semi-conducting sample. First, the expression of the electric field in the semiconducting material is worked out theoretically. Then, the measurable or apparent complex permittivity is expressed as a function of intrinsic permittivity, dimensions of the sample and frequency. Next, five blocks with different dimensions are introduced to study the dimensional effect. The numerical analysis demonstrates that above 10 MHz, samples with different dimensions result in different apparent permittivity or measurable permittivity if experiments are carried out for the samples with the assumed dimensions. This implies that dimensional effects should be considered when accurate intrinsic permittivity of the semiconducting materials is needed.