Pulse response measurement for locating water tree degradation in XLPE cables

Water treeing is a degradation mode of power cable with polymeric insulation. A water tree is composed of small droplets filled with water. As the conductivity in a water tree is very high, it leads to dielectric breakdown when it grows. Because the inside of the water tree is filled with trap sites, it is polarized with a certain distribution of relaxation time when a DC poling voltage is applied. Although its depolarization process after removing the poling voltage depends on the ambient temperature, applying a “depolarizing voltage” with the opposite polarity can accelerate the process. If a short pulse propagating through the cable is employed as a depolarization voltage, we may locate the water tree by looking at the time‐resolved pulse response. This would lead to a diagnosis method with spatial resolution. In order to retain 100‐m spatial resolution, the response should be as sharp as 1 μs. As a preliminary study, a coaxial communication cable was aged to form water trees. A DC poling voltage was applied followed by a pulse voltage with opposite polarity. The rise time of the pulse was several hundreds of microseconds. A sharp pulse current response 50 μs wide was observed, suggesting that rapid depolarization took place. No such response was seen when the cable specimen was not aged. We concluded that the technique is quite feasible. Because the response was found to be as quick as several microseconds, an experiment using a cable 405 m long with 5 m of degraded length in the middle was performed. It was shown that the degraded point was successfully located. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 180(3): 18–24, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21294     

陷阱参数及其与交联聚乙烯电缆绝缘状态

Space charge issues have raised many attentions in recent years,especially in high voltage direct current(HVDC)application.Space charge accumulation in insulation system will give rise to acceleration of ageing and even cause premature failure of the material.However,from another angle,space charge might be also considered as a diagnostic tool of ageing for insulation materials.In this paper,a trapping-detrapping model has been developed to estimate trapping parameters of cross-linked polyethylene(XLPE)cable sections,which were taken from different HVAC operation conditions of 12 years and 8 years.The results reveal that,for both cable sections,samples from the inner location have the greatest trap density and the deepest trap depth.Additionally,breakdown strength tests and FTIR(Fourier-transform infrared)measurements on those samples have been carried out.From FTIR measurement results,the degree of oxidation among three layers could be found by the carbonyl index values.The oxidation degree of aged cable at the outer layer is higher than that at the other two layers probably because of the most sufficient contact with oxygen.Also,it has been noticed that the results from these measurements show some correlations with the estimated trapping parameters,especially for breakdown strength.     

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

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

Investigation of water effects on degradation of crosslinkedpolyethylene (XLPE) insulation

This paper presents the results of the study of the influence of moisture on the electrical characteristics of XLPE power cable insulation under various service conditions. Tap water was put into the cable conductors and the ends were properly closed by terminal boxes in the first case, and opened in the second case. The samples of cables were subjected to electric stress and heating. Results from the accelerated aging tests of XLPE cables in these conditions are reported with reference to the changing of the XLPE's electrical characteristics. On the basis of the compared performances of XLPE cables given by this investigation, the lifetime of XLPE cables was estimated in the case of service under these conditions. Results of testing indicate that the combined effects of pressure of water or water vapour, electric field and temperature will greatly accelerate the deterioration of XLPE insulation     

交联聚乙烯电缆空间电荷与理化性能的关系

针对高压交联XLPE电缆,通过对未老化、加速老化1年和实际运行30年的电缆绝缘的空间电荷特性、力学性能及理化性能进行研究,分析了老化过程中电缆绝缘空间电荷分布与理化性能之间的关系。结果表明:沿电缆径向由内向外,未老化电缆电荷积累量增加,加速老化1年的电缆电荷积累量呈下降趋势,实际运行30年的电缆电荷积累量上升。结合力学性能及理化分析认为,加速老化电缆绝缘老化起始于绝缘内侧,并且影响到绝缘中间部位;而实际运行30年的电缆绝缘老化起始于绝缘外侧。     

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

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

检测XLPE电缆绝缘水树老化状态的超低频方波-工频叠加法研究

本文提出了能有效检测ＸＬＰＥ电缆绝缘水树老化状态的超低频方波－工频电压叠加法新技术,对含水树ＸＬＰＥ电缆分别放加单极性方波和正负对称方波电压叠加工频电压所产生的响应电流特性进行了实验研究,找出了最佳叠加工频电压值,最后对产生超低频方波－工频电压叠加电汉响应的机理及其检测的影响因素进行了探讨。     

10kV油纸绝缘电缆与XLPE电缆热缩中间接头安装

介绍了 10 k V油浸纸绝缘电缆与交联聚乙烯电缆通过热收缩中间接头对接的新工艺 ,新工艺施工方便并可节省投资。     

A study of the actual failure situation of XLPE cable and the order of priority of degradation diagnosis

We analyzed statistical data on failures due to dielectric breakdowns caused by water tree degradation in 6‐kV XLPE cables using the Hazard Analysis. The data used in this report reflect actual conditions of XLPE cables. The data are classified by the conductor size of the cables. Calculated failure rates were different for cables having different conductor sizes. In this report, the failure rates were calculated based on the information regarding conductor size, cable length, and cable age. The results obtained make it possible to quantitatively establish the order of the priority of degradation diagnosis. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(4): 50–58, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10301     

橡胶材料与XLPE绝缘界面电气特性研究

为了深入了解电缆附件采用的橡胶绝缘材料与电缆的交联聚乙烯(XLPE)绝缘之间形成的绝缘界面的电气性能,及其与橡胶材料的厚度、过盈量的配合等相互之间的关系,进行了系统研究和试验,获得了一系列的结果,为今后的电缆附件的设计与制造提供了可靠的依据。     

国产与进口CCV交联聚乙烯绝缘电缆工艺装备技术差异及未来的发展

通过对国内运行的部分国内外CCV交联聚乙烯绝缘电缆生产装备技术差异的分析,提出我国交联电缆工艺装备在技术、环保等方面改进与持续发展的思考和建议。期望国产工艺装备在“形”似的基础上达到“神”似,为我国电线电缆行业提供更优质更先进的工艺装备,促进行业技术进步。     

XLPE电缆绕组芯线与外半导电层间的暂态耦合特性

为了解XLPE电缆外半导电层与芯线的暂态电压耦合特性,测取了冲击电压下外半导电层及芯线的电压分布;给出了单匝电缆的等效电路模型。结果表明,冲击电压下芯线与外半导电层间为电容耦合。试验和计算结果吻合。     

Electrical treeing characteristics in XLPE power cable insulation in frequency range between 20 and 500 Hz

Electrical treeing is one of the main reasons for long term degradation of polymeric materials used in high voltage ac applications. In this paper we report on an investigation of electrical tree growth characteristics in XLPE samples from a commercial XLPE power cable. Electrical trees have been grown over a frequency range from 20 Hz to 500 Hz and images of trees were taken using CCD camera without interrupting the application of voltage. The fractal dimension of electric tree is obtained using a simple box-counting technique. Contrary to our expectation it has been found that the fractal dimension prior to the breakdown shows no significant change when frequency of the applied voltage increases. Instead, the frequency accelerates tree growth rate and reduces the time to breakdown. A new approach for investigating the frequency effect on trees has been devised. In addition to looking into the fractal analysis of tree as a whole, regions of growth are being sectioned to reveal differences in terms of growth rate, accumulated damage and fractal dimension.     

光与健康的关系

1　光线影响精神状态据国外科学研究发现 ,人的精神状态与接受的光照息息相关 ,而精神状态又影响着饮食量。冬季易肥胖 ,与光照不足关系微妙。人们知道 ,一年四季 ,日出时间有长短 ,深秋和冬季相对于夏季 ,最长的可相差 4～ 5h。而且 ,秋冬季不仅日照时间短、光线也较弱。太阳光照不足 ,会导致人大脑中心的松果体分泌的褪黑激素相对增多 ,于是人就容易出现精神不振、注意力不集中、郁郁寡欢等精神症状。知觉上称此为SAD(季节性情感障碍 )。如在美国南部的佛罗里达地区 ,冬季阳光明媚 ,呈现精神症状的人仅为 4 % ,而在冬季时间漫长、天气阴…     

交联电缆绝缘的热延伸差异及其影响因素

通过对过氧化物化学交联、硅烷温水交联及电子辐照交联电缆绝缘的交联机理及分子结构的分析 ,解释它们热延伸测试值的统计性差异 ,并进一步指出了影响热延伸的综合因素     

Thermoluminescence in XLPE cable insulation

Crosslinked polyethylene (XLPE) has been employed in underground transmission and distribution cables because of its excellent electrical and mechanical properties, such as low permittivity and dielectric loss, high degree of toughness, and good flexibility. An underground power cable operates at temperatures above ambient and the polymeric insulation is usually crosslinked to provide mechanical strength to withstand the high temperatures. Chemical crosslinking is commonly employed; however, chemical crosslinking creates byproducts such as acetophenone, α-methylene styrene, and cumyl alcohol. The general practice is to decrease the concentration of the volatile crosslinking byproducts from the newly manufactured transmission class cables before they are commissioned into service. The concentration of the byproducts is decreased by treating the cables at a high temperature in a vacuum oven. At present, to determine the residual concentration of the byproducts, the treatment has to be stopped, a sample of the polymer has to be cut from the treated cable and the byproducts have to be extracted for several hours from the polymer before they can be analyzed. This paper describes a novel, non-destructive optical method for determining the concentration of the byproducts in XLPE prior to cable installation. The method involves in situ detection and measurement of thermoluminescence emitted by the crosslinking byproducts during the pretreatment of the cable, It is shown that the measurement of the intensity of thermoluminescence provides a direct indication of the concentration of the byproducts and that it is more sensitive than mass spectrometry     

上海市电力需求与经济政策关系研究

经济发展的周期波动必然引起电力需求的波动。采用上海市经济和电量数据,根据经济变量周期波动理论,结合经济指标与电力需求间的关联分析,对上海市电力需求变化规律进行捕捉,得出各周期内电力需求受经济政策的影响情况,为预测电力需求增长趋势提供理论依据。     

变压器负载系数与损耗曲线的关系

对两台并运行变压器在自动投切中负载系数与损耗曲线的关系进行了探讨，找出了最佳的运行方案。