XLPE Cables Research Articles

운전 중인 고전력 XLPE 케이블의 절연저항과 습도의 측정 시스템 설계 및 방식층 절연저항과 습도의 상관관계

운전 중인 고전력 XLPE 케이블의 절연저항과 습도의 측정 시스템 설계 및 방식층 절연저항과 습도의 상관관계

Research on mechanical and dielectric properties of XLPE cable under accelerated electrical‐thermal aging

Research into the electrical‐thermal aging properties of cross‐linked polyethylene (XLPE) cable has great significance, because of its wide application. This study conducted accelerated electrical‐thermal aging tests on 10‐kV XLPE cable in order to assess the cable's mechanical and dielectric properties. After being aged by applying 34.8‐kV AC voltage at the four temperatures of 90, 103, 114, and 135°C, the cable samples were taken out in five stages according to the aging time and cut into slices. The slices were conducted experiments to test the breaking elongation, tensile strength, gel content, breakdown voltage, and frequency spectrums of the dielectric constant and dielectric loss. The results demonstrate that the mechanical strength and gel content of XLPE vary greatly under different aging temperatures, a finding that is associated with the crystallization characteristics of the material. The breakdown voltage shows a slight decreasing trend with aging time. The dielectric constant decreases with aging time in high‐frequency areas (103–106 Hz), while the dielectric loss factor increases with aging time at low frequencies (10−2–0 Hz). These two parameters can be used to characterize the degree of aging in cable. Copyright © 2016 John Wiley & Sons, Ltd.

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

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

A possible water tree initiation mechanism for service-aged XLPE cables: Conversion of electrical tree to water tree

In this paper, the conversion of electrical tree to water tree is found in a cross-linked polyethylene (XLPE) cable, and a possible conversion mechanism is presented. An electrical-tree initiation experiment was performed under AC voltages in XLPE cable specimens. After that, a 390-hour accelerated water-tree aging experiment was performed for the specimens, and water trees at the tips of the electrical tree were observed in the slices by an optical microscope. Also, to further understand and confirm the conversion, similar experiments were performed, and the same conversion phenomenon was observed in XLPE block specimens. Depending on the geometric size observed in water-tree cable slices, the 2D simulation model, including an electrical tree and a water tree, was constructed for electric field simulation. According to the results of electric field simulation, the electrical trees can be directly initiated at microscopic defects in service XLPE cables under the effect of transient over voltages. After initiation of the electrical tree, the forked branches can reduce the electric field around the tips of the electrical tree. When the over voltages disappear and moisture invades, the tips of the electrical tree can become the initiation sites of water trees because of stagnant propagation of the electrical tree. For this reason, the conversion of electrical tree to water tree is possible for service-aged cables, and these water trees even can make the electric field uniform at the tips of electrical tree.

Data recovery algorithm in space charge measurement by PEA method

Purpose The pulsed electro-acoustic (PEA) method is widely applied for space charge measurement in solid dielectrics. The signals, however, can be seriously distorted during transmission, especially in non-planar specimens. The aim of this work is to find an efficient algorithm to correctly recover the space charge profile for different types of specimens. Design/methodology/approach The distortion can be associated with both geometry and material (attenuation and dispersion). Hence the recovery algorithm consists of two parts respectively. The influences of geometries, causing the divergences of electric force and acoustic waveform, can be corrected by sets of factors. The attenuation and dispersion of the material can be suppressed based on the transfer function matrix in frequency domain, which could be obtained from calibration. Findings A general algorithm applicable to three kinds of specimens (single-layer, multi-layer and coaxial-geometry dielectrics) has been proposed. Compared with the other two algorithms in literature, the present one offers the most accurate solution while taking relatively shorter time. In addition, this algorithm is applied on signals measured from a planar LDPE sample and the results show that the new algorithm is fairly effective with excellent stability in a real system. Originality/value As one of the most accurate algorithms, the present one is theoretically one third quicker than the others. This algorithm would be helpful in applications calling for large calculations, i.e. 3-D imaging of space charge distribution in XLPE cable.

케이블 열회로의 전기적 등가회로 변환을 이용한 케이블 허용전류 검토 방법

Current rating of a power cable can be calculated by the maximum allowable temperature in an insulating material considering the heat transfer from cable conductor. Therefore, it is very important to calculate the current rating using electrical equivalent circuit by calculated cable thermal circuit parameters but, it has not been fully investigated yet. In this paper, in order to determine the current rating of power cable, conventional calculation method has been reviewed considering the conductor resistance, loss factor of sheath, dielectric losses and thermal resistances based on the maximum allowable temperature of 345 kV 2500 ㎟ XLPE cable. To confirm the calculation result of the current rating, the conductor temperature should be examined whether it reaches the maximum allowable temperature by the thermal equivalent circuit of the cable. Then, utilizing EMTP (Electro-Magnetic Transient Program) which is a conventional program for electrical circuit, the thermal equivalent circuit was transformed to an electric equivalent circuit using an analogous relationship between thermal circuit and electrical circuit, and temperature condition including cable conductor, sheath, cable jacket could be calculated by the current rating of 345 kV 2500 ㎟ XLPE cable.

Modeling dispersion of partial discharges due to propagation velocity variation in power cables

Existing models for partial discharge (PD) propagation based on a single attenuation constant are unable to explain how each frequency component travels with a different propagation velocity. This paper proposes a new model based on a complex propagation term whose real component does not depend on the frequency (f), and whose imaginary part is modeled with a second order polynomial in f. The proposed model explains how the PD is attenuated, delayed, and dispersed due to the fact that each frequency component is differently delayed.A closed-form expression is proposed for the PD peak value and width, and a method to derive the model parameters from a reference model existing in the bibliography. Simulation results show that the peak value and width of the propagated PD pulse are similar to those obtained with the proposed model. Additionally, the proposed model provides the velocity of each PD frequency component, which is crucial to get an accurate estimation of the PD source location.The parameters of the proposed model have been estimated using a vector network analyzer for a XLPE cable. These results have been compared to the measurement obtained in a medium voltage test bench where intentionally induced PDs have been captured and processed, confirming the results of attenuation, delay and dispersion predicted by the proposed model.

Research on Overvoltage for XLPE Cable in a Modular Multilevel Converter HVDC Transmission System

Due to the rapid development of the voltage-source converter (VSC), the demands for HVDC cables have increased significantly. Although more than ten VSC-HVDC projects are under construction and many more are in operations globally, the testing criteria for HVDC XLPE cables are mainly referenced to test guideline TB 496 recommended by CIGRE. Due to the large stray capacitance of the VSC-HVDC cables, the switching impulse overvoltage stress of the cables is the main electrical parameter concerned, a unified overvoltage factor of 2.1 p.u. being generally accepted. However, the switching impulse overvoltage stress of a VSC-HVDC cable system depends on not only the VSC topology but also the cable parameters and the volt-ampere characteristics of the dc cable surge arresters used. This paper studies the switching impulse overvoltage stresses for the dc cable system of a symmetrical monopole VSC-HVDC transmission system using the modular multilevel converter technology. The studies were performed according to the key technical parameters for a typical 1000-MW/320-kV VSC-HVDC transmission scheme, the XLPE dc cable links being assumed. The results show that a switching impulse overvoltage level of 2.3 p.u. should be applied during the type test of the overvoltage capability of the HVDC cables.

A new approach for the failure prediction in XLPE power cables using acoustic emission technique

Acoustic Emission (AE) is a Non destructive testing technique which has the unique potential for evaluating degradation and possibly failure prediction of XLPE cables as well. The authors have demonstrated its capability towards failure prediction on XLPE cables. In this study, five sets of fifty number of identical XLPE cables were taken up for the investigation. These tests were carried out on both damaged and undamaged conditions. The acoustic emission data acquired during their loading cycles is analyzed and a lucid empirical relation is developed to predict their failure performance. Using this approach one could appreciate that impending failure is significant even at 50–60% of maximum expected operating voltage (MEOV) with a reasonable error margin. In fact, till date there is no lucid method spelt out in the open literature for the failure prediction of XLPE Cables. Moreover, this methodology can also be applied to predict in real time the failure of similar XLPE cables made of other material systems.

Development of a partial discharge detection algorithm and verification test for extra‐high‐voltage cable system

Even though a lot of research has been devoted to find partial discharge (PD) sources based on the time difference of pulses, it has been reported that studies have been unable to obtain precise locations in joints due to the errors caused by the attenuation or distortion of PD pulses. In this study, in order to reduce the issues, ‘Precise PD finding’ is developed to obtain the accurate locations of PD sources by considering not only the time difference of pulses, but also pulse polarity. Accordingly, a PD detection algorithm is implemented. An attachable inductive sensor is designed and fabricated and then applied to a real scaled 154 kV XLPE cable with a joint. From the experimental results, the algorithm was verified to successfully find the locations of PD sources. The algorithm was then applied to an on-site, 345 kV XLPE cable. Consequently, PD sources were detected in a normal straight joint box and confirmed through a disassembly investigation.

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

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

Detection of online PD signals in XLPE cables using the Bhattacharyya distance

Partial discharge (PD) signal detection can be used for insulation monitoring of power system equipment. Online PD detection as a main task of insulation monitoring for different power system equipment is a difficult procedure when environmental noise is taken into account. This paper proposes a new scheme based on the Bhattacharyya distance approach with the main goal of detecting the PD signals in cross-linked polyethylene cables. The proposed method, as a preprocessing tool, can be used for detection of PD signals in a noisy environment. The simplicity and the accuracy of the Bhattacharyya distance approach are the 2 main advantages that grab the attention of the authors in introducing this technique for detection of PD signals in different high-voltage power devices. In order to remove noises between intervals of measurement of PD signals, other simple signal processing techniques should be used as postprocessing tools. Detection of PD signals in a noisy environment, i.e. �nding the times of PD occurrences, is the most important and difficult stage in online PD signal monitoring. The Bhattacharyya distance method is a simple approach that can be used for detection of short duration time signals such as PDs.

基于核磁共振的XLPE电缆绝缘热老化试验研究

随着交联聚乙烯电缆在电力系统中的广泛应用，其老化行为成为电力系统长期安全运行的瓶颈，迫切需要我们研究其老化规律和机理。交联聚乙烯尽管具有较好的耐热性能和电性能。但在实际运行中长期处于高温下而发生化学变化，使得绝缘材料的物理性能和电气性能发生改变。本文按照IEC60287标准对XLPE绝缘材料进行热老化试验，利用核磁共振分析仪，选取共振峰积分面积和纵向弛豫时间为反应绝缘老化程度的特征量，对绝缘材料分子结构进行分析，得到更直观的老化程度结果；同时利用差示扫描量热分析技术对不同热老化程度的绝缘材料进行了热性能分析，提出了以玻璃转化温度、结晶度、结晶速率为指标的热性能分析技术。 XLPE cables are widely used in the power system; the aging behavior of XLPE cable insulation be-comes the bottleneck of long-term safe operation of the power system. The urgent need is to study the law and mechanism of aging. Although the XLPE has good heat resistance and electrical prop-erties, in actual long-term operations, cables are always under high temperature, physical and electrical properties of the insulating material are slowly changed. This article did the heat aging test of XLPE insulation material according to IEC 60287 Standard. With the help of nuclear magnetic resonance analyzer, the resonance peak integration area and longitudinal relaxation time are selected as the feature quantities to characterize the degree of aging of the insulation. The molecular structure of the insulation material is analyzed to obtain a more intuitive aging results while taking advantage of differential scanning calorimetry analysis of the different thermal aging of insulation material thermal performance analysis, determined the glass transition temperature, crystallinity and crystallization rate as index of thermal performance analysis techniques.

乾式架橋・三層同時押出（E-E方式）の6.6kV撤去CVケーブルの水トリー劣化調査

Dry-cured and extruded three-layer (E-E type) 6.6kV cross-linked polyethylene (XLPE) cables were introduced into electric power systems more than 30 years ago, but they do not experience failures because of water tree degradation. Also, the degradation index of water treeing for these cables has not been established. Therefore, investigating results of residual breakdown voltage and water tree degradation of these cables will help us plan for cable replacement and determine water tree degradation diagnosis scheduling, and will be fundamental data for cable lifetime evaluation. In this study, the authors measured the AC breakdown voltages of dry-cured and E-E type 6.6kV XLPE cables removed after 18-25 years of operation and observed the water trees in their XLPE insulation. As a result, it was observed that breakdown voltages were larger than the maximum operating voltage (6.9kV) and the AC voltage for the dielectric withstanding test (10.3kV). Water trees were mainly bow-tie water trees and their maximum length was approximately 1mm. Although the number of measured cables was limited, the lifetime of this type of cable was estimated to be approximately 40 years, even experiencing water immersion.

直流漏れ電流測定による水トリー劣化位置標定

In general, the degradation span of 6.6kV XLPE cable with plural connections is determined by DC leakage current measurement. This method needs to disconnect and reconnect the shielding layer and the outer semi-conducting layer at the cable connecting part, and hence it takes a lot of time and cost. We investigated a new method to locate the water tree degradation by DC leakage current. In 6.6kV XLPE cables the pulse-like leakage current called “kick” due to water tree degradation is often observed. Consequently, we detected the currents at the DC applying terminal and another terminal, and transformed them to the signals like a sine-wave having the phase difference exactly proportional to the time difference of arrival (TDOA) by using ceramic filters at 455kHz. The position of the water tree is calculated from TDOA obtained by the phase difference. In the experiments to locate the water tree using 6.6kV XLPE cable samples up to 85m long, the mean squared error was 2.2m. From the results, it is possible to locate the water tree in 6.6kV XLPE cable 2km long with an error of about 20m by optimizing the center frequency of the ceramic filter.

Diagnosis of insulation Performance of XLPE Cables

Diagnosis of insulation Performance of XLPE Cables

電力ケーブル接続部に有効な周波数加速劣化試験の基礎的検討

電力ケーブル接続部に有効な周波数加速劣化試験の基礎的検討

Application of the AC Superposition Method to Degradation Diagnosis of 22/33‐kV Class XLPE Cables

SUMMARYWater trees in the insulator of XLPE cable may considerably reduce the dielectric breakdown voltage, thus being an important lifetime‐governing factor. The ac superposition method we have investigated is a new technique for hot‐line diagnosis of 6.6‐kV XLPE cables, and the diagnostic apparatus using this technique is now widely used in the distribution line field. In order to study the application of the ac superposition method to degradation diagnosis of 22/33‐kV class XLPE cables, we measured deterioration signals of the cables with water trees by a modification of the above diagnostic apparatus. The deterioration signals, hereafter called “ac superposition current”, were generated by an amplitude modulation effect due to the nonlinear resistance of water trees. Moreover, we evaluated the relationship between the ac superposition current and the ac breakdown voltage. It is difficult to judge whether a correlation between then is present or not because of the uneven distribution of the measured data. However, the ac superposition current tends to increase linearly with a decrease in the residual thickness of the insulator. For example, the ac superposition current was about 40 nA when the residual thickness decreased to 3 mm. Thus, we consider that the ac superposition method is effective for degradation diagnosis of 22/33‐kV class XLPE cables.

Accelerated aging study on 15 kV XLPE and EPR cables insulation caused by switching impulses

Cross-linked polyethylene (XLPE) and Ethylene Propylene Rubber (EPR) are the common polymeric materials used for power cable insulation in the USA. While in service, the main threats to the cable insulation include electrical stress, thermal stress, and mechanical stress. The electrical stress, from either operation voltages or abnormal over-voltages during transients, has negative impacts to the power cables insulation. The excessive electrical stress will trigger the aging of insulation materials, which will lead to the failures of cables, resulting in outages of power systems. The aging phenomena of polymeric power cables have been extensively studied during the past decades. Although the study of aging mechanisms was done on molded samples, little is known about the aging phenomena of the XLPE and EPR cables by switching impulses. In this experiment, the 15 kV XLPE and EPR cable samples were aged up to 10,000 switching impulses. The study helped to assess the reliability of the XLPE and EPR cables serving in electrical power systems.

Comparison between Parameter of Single Core Unarmoured Aluminium Conductor 6.35/11 kV Underground Cable for Partial Discharge Measurement Using EMTP-ATP Software

Partial Discharge (PD) is one of a symptom in the underground cable before the fault happens.This paper concerned with estimation and detection problems related to PD phenomena in buried underground cables. The Rogowski coil has been implemented to analyze this symptom. The various sizes of nominal cross-sectional area cross-linked polyethylene (XLPE) cable with 6.35/11 KV single core unarmored aluminum conductors that follows Nexans manufacturer datasheet based on design standard IEC 60502 has been used in the simulation using Electromagnetic Transient Program-Alternative Transient Program (EMTP-ATP) software. The Lumped parameter is used for the Rogowski coil sensor model. Meanwhile, for Double-end measurement using J-Marti model is designed to evaluate the parameter changing in the measurement.Based on the measurement, the value of amplitude voltage and current is proportional increase depends on the size of cross-sectional area of conductor XLPE cable.