Cross-linked Polyethylene Cables Research Articles

Charge-Simulation-Based Electric Field Analysis and Electrical Tree Propagation Model with Defects in 10 kV XLPE Cable Joint

The most severe partial discharges and main insulation failures of 10 kV cross-linked polyethylene cables occur at the joint due to defects caused by various factors during the manufacturing and installation processes. The electric field distortion is analyzed as the indicator by the charge simulation method to identify four typical defects (air void, water film, metal debris, and metal needle). This charge simulation method is combined with random walk theory to describe the stochastic process of electrical tree growth around the defects with an analysis of the charge accumulation process. The results illustrate that the electrical trees around the metal debris and needle are more likely to approach the cable core and cause main insulation failure compared with other types of the defects because the vertical field vector to the cable core is significantly larger than the field vectors to other directions during the tree propagation process with conductive defects. The electric field was measured around the cable joint surface and compared with the simulation results to validate the calculation model and the measurement method. The air void and water film defects are difficult to detect when their sizes are less than 5 mm3 because the field distortions caused by the air void and water film are relatively small and might be concealed by interference. The proposed electric field analysis focuses on the electric field distortion in the cable joint, which is the original cause of the insulation material breakdown. This method identifies the defect and predicts the electrical tree growth in the cable joint simultaneously. It requires no directly attached or embedded sensors to impact the cable joint structure and maintains the power transmission during the detection process.

Influence of Different XLPE Cable Defects on the Initiation of Electric Trees

Defects in the insulation layer of cross-linked polyethylene (XLPE) cable are the leading cause of electrical tree. The mechanism that how the different insulation defects influence on the starting of electrical tree should be explored, and it will be utilized to improve cable insulation in the production and installation. In the paper, firstly the spike-air gap defect as the main defect characteristics of cable fault has been determined by analyzing the actual cable defect type and influence. Then, the needle-electrode short cable test system was built and the representative defect model was constructed, in order to simulate the influence of the spike-air gap defect of different sizes in the outer semiconductor layer on the initial voltage of electrical tree. Thirdly, the Physical model was created to analyze the influence of each defect parameters. The research showed that the electrical tree of XLPE cable was mainly caused by defects in the semiconductor layer. The maximum electric field distribution can be formulized by the maximum field strength Mason model. According to the formulation and the test results, the main parameters, including the spur curvature, the micro-pore pressure and the initial energy of generating electric tree, have a positive promotion on the initial voltage of the electrical tree and can hinder the tree developing. Therefore, utilizing the interface treatment method between the insulation layer and the semiconductor layer, the insulation heat treatment process and the filling of the inert gas are to improve the parameters and further resist electrical tree aging in the production and installation.

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.

An embedded wide-band sensor for PD on-line monitoring of XLPE cable joint

Monitoring partial discharge (PD) activities in the joint of cross-linked polyethylene (XLPE) cables has attracted much attention due to its extensive application and high fault rate. In this paper, we designed a wide-band sensor with simple configuration, which was embedded in the outer semi-conductive layer of the cable. Based on the analysis of circuit and antenna models, the structural parameters of sensor were determined, and it was found that the effective bandwidth reached up to 500 MHz. In addition, the sensor had a quick response to the pulse with a rising time longer than 2 ns by testing its square response, as well as a linear input-output characteristic and a sensitivity of 3 pC. At last, the sensor was applied to insulation defect type identification in the cable joint. As for three common defects, both the single PD pulse and PD patterns showed distinct. These indicated that the sensor had a potential application prospect in the PD online detection for cable joint.

Performance of Cross-linked Polyethylene Insulated Cable based on Detection of High Voltage Electric Field

High voltage direct current transmission has been widely used in long-distance power transmission because of its advantages such as low loss and large throughput. The safety of cable is very important in the long-distance transmission process. Cross-linked polyethylene insulated cable has the advantages of simple manufacture, good heat resistance and easy installation; therefore it has been widely used in long-distance power transmission. In this study, conductance, dielectric and voltage withstanding performance of the insulating layers of cross-linked polyethylene cables with working voltage of 220 kV which was never used and has been used for 2, 4, 6 and 8 years were tested by various evaluation methods. The conductivity of the insulating layers was affected by temperature only under high and low electric field intensities and was affected by temperature and electric field strength under middle electric field intensity. Dielectric loss factor could reflect dielectric properties of materials; the larger the dielectric loss factor, the poorer the performance. The dielectric loss factor decreased with the increase of applied electric field frequency and increased with the increase of service years. Breakdown electric field strength could reflect voltage resistance of materials; the larger the breakdown electric field strength, the better the performance. The breakdown electric field strength was inversely proportional to the service years of the insulating layer, and the decrease amplitude increased significantly when the service time exceeded 2 years. In summary, cross-linked polyethylene insulated material satisfies the safety requirement of high voltage direct current transmission and can be used for manufacturing long-distance transmission cables.

Space charge suppression in environment-friendly PP nanocomposites by employing freeze-dried MgO with foam nanostructure for high-voltage power cable insulation

Space charge accumulation along insulation thickness is still regarded as a critical issue during the operation of high-voltage direct current transmission cables, which will result in the distortion of the partial electric field, seriously causing the degradation or aging of the main insulation. Polypropylene (PP) environment-friendly thermoplastic cables without cross-linking have shown great potential, which tend to gradually substitute the traditional unrecyclable extruded cross-linked polyethylene cables. Here, the styrene-ethylene-butylene-styrene block copolymer is selected to optimize the mechanical properties of PP. An extremely small amount of MgO with a foam nanostructure synthesized through a designed freeze-drying process is introduced to suppress the accumulation of space charge and further improve the electrical properties of PP insulation. With a very low concentration of foamed MgO (0.2 phr), it is found that the injection and accumulation of space charge could be largely suppressed under 60 kV/mm and the DC conductivity of PP composites remains as low as ∼1.6 × 10−15 S/m. Besides, the DC breakdown strength of the composites is up to 315 kV/mm. Therefore, the PP-matrix composites can be considered as potential high-performance insulating materials in the non-cross-linking environment-friendly thermoplastic high-voltage power cable.

Inhibition of Rejuvenation Liquid on Trees in XLPE Cables under Switching Impulse Voltages

In this paper, the inhibitory effect of preinjected rejuvenation liquid on trees in cross-linked polyethylene (XLPE) cables was investigated. Experimental samples were prepared by inserting needles into XLPE samples, and many equally-spaced holes existed in the outer semiconductive layer. All cable samples were divided into two groups. One sample group was treated with rejuvenation liquid, while the other group was the control group. A tree accelerated aging system was used to obtain trees in the XLPE cable samples. During the aging experiment, an impulse voltage was applied to the samples repeatedly. The micromorphologies of the two groups were observed. Based on the micromorphologies, two parameters were determined: the initiation rate of electrical trees and the average length of trees. Furthermore, the electric field distribution was simulated to analyze the initiation of electrical trees. The results indicate that an electrical tree is much harder to initiate in the pretreated XLPE cables than in the untreated cables. This phenomenon is likely attributed to the dielectrophoretic forces in the pretreated cables. Moreover, rejuvenation liquid deposited in XLPE causes a substantial reduction in the Maxwell stress of molecular chains. Rejuvenation liquid inhibits electrical tree initiation and water tree growth to a great extent.

Modeling of dielectric dissipation factor measurement for XLPE cable based on Davidson-Cole model

Dielectric response analysis is one approach for the condition assessment of high voltage insulation systems, and measurement of the dissipation factor or tan δ is widely used in practice. This article presents an experimental study of DF measurement over the frequency range from 1 mHz to 50 Hz. Three different short sections of service-aged 11-kV cross-linked polyethylene cable are used as test specimens. Low-voltage polarisation / depolarisation current and frequency domain spectroscopy techniques are employed in the investigation. Following the experimental work, numerical analysis is carried out utilizing the Davidson-Cole (D-C) model. To optimize the final parameters of the model, numerical simulation is performed using 3 or 4 R-C branches with different values of Jonscher and D-C constants. It is shown that results from numerical simulation agree well with the experimental results over the tested frequency range.

Modeling of dielectric dissipation factor measurement for XLPE cable based on Davidson-Cole model

Dielectric response analysis is one approach for the condition assessment of high voltage insulation systems, and measurement of the dissipation factor or tan δ is widely used in practice. This article presents an experimental study of DF measurement over the frequency range from 1 mHz to 50 Hz. Three different short sections of service-aged 11-kV cross-linked polyethylene cable are used as test specimens. Low-voltage polarisation / depolarisation current and frequency domain spectroscopy techniques are employed in the investigation. Following the experimental work, numerical analysis is carried out utilizing the Davidson-Cole (D-C) model. To optimize the final parameters of the model, numerical simulation is performed using 3 or 4 R-C branches with different values of Jonscher and D-C constants. It is shown that results from numerical simulation agree well with the experimental results over the tested frequency range.

Numerical modelling, simulation and experimental validation of partial discharge in cross‐linked polyethylene cables

A clear understanding of the partial discharge (PD) phenomena occurring in cavities at various radial locations in the insulation is essential for proper monitoring of cables. This work presents a comparison of finite element method (FEM) model and capacitive model for simulation of PD in the insulation and an attempt to identify reliable parameters for monitoring the condition of cable insulation is done. A three-dimensional FEM-based model of the cable is developed in ANSYS software by including the effects of surface charge accumulation and propagation along with the phenomena of charge decay at the void surface. The induced charge concept which is correlated with changes in capacitance is employed to model a capacitive model for simulation of PD by considering the free electron availability for simulation of PDs in cables in EMTP software. The experimental validation of developed models is done by PD measurement on 11 kV cross-linked polyethylene cables. Analysis of the simulation results recommends the relevance of the capacitive model to be employed for condition assessment of cable insulation in view of its simplicity and appropriateness in the estimation of model parameters from actual cable measurements.

A Study on the Space Charge Characteristics of AC Sliced XLPE Cables

Cross-linked polyethylene (XLPE) cables are widely used in power systems due to their excellent electrical and mechanical properties. Space charge is an important factor in cable aging and breakdown. At present, XLPE cables in the power system are mainly ac XLPE cables, so it is necessary to study the space charge characteristics under ac voltage. In this paper, an electro-thermal aging platform for XLPE cable was built and a 10-kV cable was aged. XLPE slices were obtained from the aging cable and ac space charge experiments were carried out on the XLPE slices at different stages of aging. The trap energy levels were calculated and the effects of different electric fields on the ac space charge characteristics were compared. The experimental results show that the trap energy level increases with the aging process and the non-averaged electric field is a necessary condition for charge migration.

Performance of Cross-Linked Polyethylene Insulated Cable Based on Detection of High Voltage Electric Field

High voltage direct current transmission has been widely used in long-distance power transmission because of its advantages such as low loss and large throughput. The safety of cable is very important in the long-distance transmission process. Cross-linked polyethylene insulated cable has the advantages of simple manufacture, good heat resistance and easy installation; therefore it has been widely used in long-distance power transmission. In this study, conductance, dielectric and voltage withstanding performance of the insulating layers of cross-linked polyethylene cables with working voltage of 220 kV which was never used and has been used for 2, 4, 6 and 8 years were tested by various evaluation methods. The conductivity of the insulating layers was affected by temperature only under high and low electric field intensities and was affected by temperature and electric field strength under middle electric field intensity. Dielectric loss factor could reflect dielectric properties of materials; the larger the dielectric loss factor, the poorer the performance. The dielectric loss factor decreased with the increase of applied electric field frequency and increased with the increase of service years. Breakdown electric field strength could reflect voltage resistance of materials; the larger the breakdown electric field strength, the better the performance. The breakdown electric field strength was inversely proportional to the service years of the insulating layer, and the decrease amplitude increased significantly when the service time exceeded 2 years. In summary, cross-linked polyethylene insulated material satisfies the safety requirement of high voltage direct current transmission and can be used for manufacturing long-distance transmission cables.

Research on Water Content and Physicochemical Properties of Insulating Materials for Water-Soaked XLPE Cable

The buried environment of cross-linked polyethylene (XPLE) cable is relatively severe, and some ones are even immersed in water directly. In this paper, an XLPE cable was soaked in foul water to simulate the cable’s actual water area environment, lasting for three years. The water content in the insulation layers of water-soaked cable was measured by infrared spectrometer and moisture analyzer, and the crystal morphology and physicochemical properties of the insulation layers were investigated by differential Scanning Calorimeter (DSC), scanning electron microscopy (SEM) and tensile testing technology. The results showed that a small amount of water did enter into the cable insulation owing to the increasing of hydroxyl (-OH) content, and followed being verified by moisture analyzer. The degree of crystallinity of water-soaked cable insulating material is also increased, and the mechanical properties were deteriorated.

Research on the discharge characteristics for water tree in crosslinked polyethylene cable based on plasma-chemical model

Water tree is the predominant defect in high-voltage crosslinked polyethylene cables. The microscopic mechanism in the discharge process is not fully understood; hence, a drawback is created towards an effective method to evaluate the insulation status. In order to investigate the growth of water tree, a plasma-chemical model is developed. The dynamic characteristics of the discharge process including voltage waveform, current waveform, electron density, electric potential, and electric field intensity are analyzed. Our results show that the distorted electric field is the predominant contributing factor of electron avalanche formation, which inevitably leads to the formation of pulse current. In addition, it is found that characteristic parameters such as the pulse width and pulse number have a great relevance to the length of water tree. Accordingly, the growth of water tree can be divided into the initial stage, development stage, and pre-breakdown stage, which provides a reference for evaluating the deteriorated stages of crosslinked polyethylene cables.

Current integrated technique for insulation diagnosis of water-tree degraded cable

The diagnosis of insulation performance in the high-voltage alternating current (HVAC) and high-voltage direct current (HVDC) systems has always attracted many researchers in the field of electrical engineering. New diagnostic technology and condition monitoring apparatus are of prime importance for HVAC and HVDC developments. In this study, we introduce a valuable technique called direct current integrated charge (DCIC-Q(t)) for determining the degradation of insulating materials by means of charge accumulation. This work demonstrates the typical results of full-size cross-linked polyethylene (XLPE) cables by using this effective method. Firstly, we describe the basic measurement principle of DCIC-Q(t) technique and the evaluation details of the degradation. Then, the validity of this technique is confirmed by comparing with the pulsed electroacoustic (PEA) results. The similar charge behaviors of polystyrene and low-density polyethylene are presented via the DCIC-Q(t) and PEA methods, which indicates the good availability of this technique. Three underground, full-size, 6.6-kV XLPE cables were diagnosed using the DCIC-Q(t) method. Two cables were degraded by water and electric stress for a different time under controlled conditions. The degradation extent can be evaluated easily by the charge accumulation from the DCIC-Q(t) results. Furthermore, based on the results of charge dynamics related to the applied voltage, the properties of the degraded cable by water tree are discussed.

A Strategy to Determine Partial Discharge in XLPE Power Cables Using Acoustic Emission Detection Technique

Abstract Acoustic emission (AE) is a nondestructive technique that has unique potential for evaluating degradation and possibly for failure prediction, as well, of cross-linked polyethylene (XLPE) cables. This technique is more suited in high-voltage tests because of its external antielectromagnetism interference. The authors have demonstrated its capability toward failure prediction on XLPE cables. In this study, five sets of fifty identical XLPE cables were taken up for the investigation. These tests were carried out on both damaged and undamaged conditions. The AE data acquired during loading cycles is analyzed, and a lucid empirical relation is developed to predict their failure performance. Even though there is no pinpoint method spelled out in the open literature for the failure prediction of XLPE cables, one could appreciate the approach that impending failure is significant even at 50–60 % of maximum expected operating voltage with a reasonable error margin. Moreover, this methodology can also be applied to predict in real time the failure of similar XLPE cables made of other material systems.

A Modified Algorithm for the Simulation of Charge Behavior in Water Tree Aged Cross-Linked Polyethylene Cable

In this paper, a modified algorithm is introduced to explain the process of charge formation in water tree aged cable insulation with respect to the dielectric properties of water tree areas and the cylindrical geometries of cable insulation. The algorithm combined with the classic bipolar charge transport model represents a novel attempt to simulate charge behavior in insulation with water tree. The finite-difference method is employed to solve for the electric field in the presence of space charge, and the finite-volume method is employed to solve the charge continuity equation. The dielectric parameters in this paper are not instantaneous near the interface but are functions of the radius. To overcome the Courant–Friedrichs–Lewy condition, the Crank–Nicolson method is used to separate the governing equations in the time domain. To validate the accuracy of the model, this paper also presents the space charge behavior in water tree aged cross-linked polyethylene cable samples measured by the full-scale pulsed electro-acoustic method. The homocharge is injected under a high electric field. During the depolarization process, the shallow trap charge gradually disappears, whereas the homocharge continues to form a positive peak signal. Simulations show that the homocharge accumulates in the tip of the water tree because the dielectric characteristics of the material change during the water-aging process. The model describes the behavior of space charge in the water tree aged insulation of cylindrical geometries. Improvements will be needed in the following research.

On-line monitoring of relative dielectric losses in cross-bonded cables using sheath currents

Cross-bonded three-phase single-core Cross-linked Polyethylene (XLPE) cables are widely used in power transmission and distribution systems. A great deal has been published on on-line monitoring of HV cable systems to prevent failure. Relative Dielectric Losses (DL) monitoring reveals the general condition of the cable insulation. Generally, DL measurement relies on measuring operating voltage and insulation leakage current. The main challenge to on-line monitoring of DL in Cross-bonded High voltage (CB HV) cables is the interconnectedness of metal sheath. Measured sheath currents are a combination of the leakage currents of each section and the imbalanced circulating current. Differentiating the leakage current of each interconnected section from the measured data is complex. The second challenge is that co-axial cables are used to connect cable sheaths to cable link boxes, for ease of installation and protection against moisture. As a result, any measured value contains currents from the two sheaths connecting at a joint. The third challenge is that it is hard to obtain the reference voltage synchronously and with high accuracy. Consequently, this paper investigates separation of leakage current components and analysis of DL based on leakage currents without reference voltage. A circuit model is proposed to describe the leakage current separation method for CB HV cables to allow on-line monitoring of DL, and the criterion required for assessing relative change of DL among different phases or sections of the cable for insulation deterioration are presented. Operational measurements from a cable system are analyzed and discussed. Results show that there is no significant deterioration in this monitored major section of a CB HV cable system.

Understanding Electrical Performance and Microstructure of Water Tree Aged Cables after Silicone Injection during Electrical-Thermal Accelerated Aging

To further understand the long-term effect of rejuvenation fluid on water tree aged cross-linked polyethylene (XLPE) cables after silicone injection, electrical performance and microstructure of water tree aged cables were investigated during an electrical-thermal accelerated aging experiment. Two groups of treated and untreated water tree aged cable samples were subjected to electrical-thermal aging for 2 weeks. The results of the dielectric loss factor showed that the electrical performance of the treated samples was significantly better than that of untreated samples after electrical-thermal aging. Microobservation results showed that the overall water tree sizes of the treated samples were much smaller than those of the untreated samples after electrical-thermal aging. Furthermore, there was a second growth of water trees in the original water tree region during electrical-thermal aging in both groups of samples. Using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), rejuvenation fillers were observed tightly embedded in water tree voids after electrical-thermal aging. The infrared (IR) spectroscopy analysis results showed that the concentration of rejuvenation fluid increased from the outer layer to the inner layer of insulation. Based on the results, due to the improvement of the electric field at the water tree tip and the diffusion of silicone fluid in the insulation layer, both the second growth in the original water tree region and continuing growth at the water tree tip can be effectively inhibited.