Dielectric spectroscopy for diagnosis of water tree deteriorationin XLPE cables

An HV dielectric spectroscopy system has been developed for diagnostics of water tree deteriorated extruded medium voltage cables. The technique is based on the measurement of nonlinear dielectric response in the frequency domain. Today's commercially available systems are capable of resolving low loss and small variations of permittivity as a function of frequency and voltage. Experience from more than 200 field measurements was combined with laboratory investigations. Small samples were used in an accelerated aging test to elucidate the correlation between water tree growth and dielectric response. Furthermore, field aged cables were investigated in the laboratory. It has been shown that the dielectric response of water tree deteriorated crosslinked polyethylene (XLPE) cables can be recognized and classified into different types of responses related to the aging status and breakdown strength. The influence of termination and artifacts such as surface currents was investigated. The measurement method enables us to separate the response of the cable from the influence of accessories. Finally, two different field studies of the implementation of the diagnostic method are presented. The field studies show that the fault rate decreased significantly when replacement strategy was based on the diagnostic criteria formulated     

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     

Correlation between AC breakdown strength and low frequencydielectric loss of water tree aged XLPE cables

This paper describes experiments performed on laboratory aged 12 kV and service aged 24 kV XLPE cable samples. Results from measurements of residual AC breakdown strength, degree of water treeing and dissipation factors are presented. The dissipation factor at low frequencies was determined by both time and frequency domain measurements. The relationship between these methods are discussed and it is shown that both can be used for assessing the average ageing state of the cable, The results show that water treeing causes reduced residual AC breakdown strength and high and nonlinearly increasing low-frequency dielectric loss     

不同温度下退役高压电缆绝缘介电特性研究

为研究不同运行年限交联聚乙烯(XLPE)电缆绝缘层不同位置的介电特性变化,文中对2条退役和1条备用110 kV电缆绝缘层进行分层取样,在50～250℃之间选取8个温度点,进行介电谱试验。结果表明:XLPE电缆绝缘的复介电常数在高温下随频率变化明显;各试样的电导率和松弛峰峰值频率随运行年限的增大先减小后增大,能较好反映不同运行年限电缆的介电特性变化;同一电缆试样不同绝缘层位置的松弛峰峰值频率对应的活化能和Cole-Cole曲线对应的圆心角之间的差异随运行年限的增大先减小后增大,能较好反映电缆不同绝缘层位置之间的介电特性差异。各试样介电特性在高温下随运行年限的变化可作为评估XLPE电缆绝缘状况的有效手段。     

Long-term characteristics of XLPE cables

To study the long-term characteristics of XLPE cables installed in free air and in water, aging tests were conducted under various testing conditions using XLPE cables with both 3.5 mm and 6 mm insulation. From the Weibull plots of lifetime distribution under the voltage stress E_L as the minimum breakdown strength, the minimum value of time to breakdown t_L under the constant voltage was estimated. The results of accelerated aging tests of XLPE cables installed in free air demonstrated that the V-t characteristics of XLPE cables could not be described by the conventional inverse power law (t ∝ V⁻ⁿ) with a single constant life exponent n. Based on the microscopic observation of a sliced insulation removed from XLPE cables, it was concluded that bow-tie trees with longer tree length observed in cables tested in water were caused by the moisture from outside, whereas the trees in cables tested in free air were caused by the residual moisture originally existing in the insulation. The breakdown strength of the aged cables tested in water increases through cable drying. However, it does not recover to the original values.     

交联聚乙烯绝缘电缆的新应用

本文就ABB公司XLPE绝缘电缆的一系列新应用进行简单介绍,它的特点是采用电缆作为绕组,并就电缆做绕组产生的一系列与传统电磁装置的许多不同特性进行分析比较。通过比较,可以认为XLPE绝缘电缆绕组的新型电力设备将是21世纪电力的主流产品。     

高压交联聚乙烯绝缘电缆的选择

本文对高压交联聚乙烯绝缘电力电缆的有关问题进行了讨论 ,对电缆导体、金属屏蔽层或金属护套、外护套等选择进行了比较 ,也对金属屏蔽层的接地和敷设方式进行了分析和讨论。     

Comparative laboratory evaluation of TR-XLPE and XLPE cables with Super-smooth conductor shields

This paper provides data on four commercial tree retardant crosslinked polyethylene (TR-XLPE) and one cross-linked polyethylene (XLPE) insulated 15 kV cables supplied by three manufacturers. The cables have "super-smooth" conductor shields and "extra-clean" insulation and insulation shields. AC and impulse voltage breakdown and selected other characterization data are presented for cables that were aged immersed in room temperature water (15-30/spl deg/C) up to 24 months of a planned 48 months aging program. The five cables have high ac voltage breakdown strength, three of the TR-XLPE cables, actually increased in breakdown strength during aging. The one TR-XLPE cable that had the lowest ac voltage breakdown had vented trees at the insulation shield and high dissipation factor, which the other cables did not have. The impulse voltage breakdown strength of all cables decreased during aging; the cable with the lowest ac voltage breakdown also has the lowest impulse voltage breakdown. The dissimilar performance of the TR-XLPE cables and the excellent performance of the XLPE cable indicates evaluations at longer times are required to differentiate between modern TR-XLPE and XLPE insulated cables.     

Development of 500-kV XLPE cables

A 500-kV XLPE insulated cable with an insulation thickness of 27 mm has been developed for long-distance transmission lines. Basic studies on the 500-kV XLPE cable have shown that contaminants in the insulation may determine electrical performance. This hypothesis is justified by the good correlation obtained between statistical estimations of the size of the largest contaminant in the insulation and electrical characteristics of full-size cables. Voltage-withstand and long-term tests have confirmed design values for minimum breakdown stress, ac and impulse voltage, and degradation coefficients. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (1): 28–40, 1997     

Retrofitting-new high voltage XLPE cables substitutingpaper-insulated power cables in steel pipes

Many utilities still have paper-insulated pipe type cables in service. In the near future they may have to replace some of these systems due to local increases in electric power demand or aged cable insulation. In many cases the steel pipes are well protected against corrosion and thus suitable for further use. For retrofitting in such installations, XLPE-insulated cables offer several advantages. Felten & Guilleaume Energietechnik AG has developed a new three-core XLPE-insulated 64/110 kV cable with reduced insulation thickness (10 mm), that can be pulled into a pipe. In co-operation with VEW ENERGIE AG, the cable performance is being investigated in a field test, that started in October 1996. This long-term loading cycle test will last for three years. The test voltage, applied between conductor and earth, amounts to 127 kV. The temperature is measured with the help of an optical-fiber measurement system. This device is computer controlled and works as a remote monitoring system     

Experience in PD site location in XLPE cables

A paper describing advances in electronic PD site location devices through a system comprised of an added purpose-built analog-to-digital converter, which has been added to a previously developed, PC-based, digital PD detector     

基于反射系数谱的XLPE电缆水树缺陷定位方法

地下电缆中的水树能够在不改变电缆性能的情况下持续生长数年,并最终导致无法预料的绝缘故障.为解决现有技术对电缆水树缺陷定位诊断时存在的不足,提出基于反射系数谱的水树定位方法.首先利用传输线理论构建了含水树缺陷的电缆反射系数谱的解析模型.然后以分布电容为对象分析了水树缺陷对反射系数谱的影响,并基于广义正交法构建了诊断函数D...     

交联聚乙烯电缆在线监测与检测

对我国交联聚乙烯(XLPE)电缆在线监测及检测技术的现状进行综述,涉及电缆主绝缘的在线监测(包括直流成分法、直流叠加法、低频叠加法、在线介质损耗角正切法和局部放电在线监测法等)、电缆护层的在线监测(包括接地容性电流监测、电缆护层综合监测和接地电流监测等)、电缆温度在线检测、电缆终端的红外和紫外在线检测等。对各监测手段的原理进行介绍,分析各种方法的优缺点,并对一些方法开展应用研究,提出了效果好、有实用价值的电缆在线检测方法。     

中压交联聚乙烯绝缘电力电缆鉴定试验

本文介绍了 5～ 4 6 k V交联聚乙烯绝缘电缆鉴定试验 (Qualification test)的试验程序、性能要求及试验方法。     

浅析交联电力电缆的交接和预防性试验

做好交联聚乙烯绝缘电缆的交接和预防性(重做终端和接头)试验是保证安全运行的重要工作。为此,阐述了交联聚乙烯绝缘电缆的交接和预防性试验有关问题,提出6～10kV和35kV交联聚乙烯绝缘电缆不同的试验方法和内容。并指出传统的直流耐压试验存在不足之处,对交联聚乙烯绝缘电缆的交接和预防性试验应优先采用变频率串联谐振试验。     

退役高压XLPE电缆绝缘空间电荷行为研究

文中对两回110 k V退役高压交联电缆进行180 a预鉴定试验,目的在于研究不同运行年限的退役高压交联电缆老化前后的空间电荷行为差异,并评估电缆重新投入实际运行的可靠性。通过电声脉冲(PEA)法测量试验前后绝缘层交联聚乙烯(XLPE)试样的空间电荷分布,结合傅里叶红外光谱(FTIR)实验、X射线衍射(XRD)实验揭示试样的微观结构、聚集态结构状态变化与空间电荷的迁移、积聚和消散过程的潜在关系。实验结果表明:实际运行16 a的电缆绝缘由于存在大量杂质使得空间电荷的积聚严重,老化试验后,杂质的减少和淬火反应使得空间电荷的积聚降低且消散过程加快;实际运行32 a的电缆绝缘空间电荷的积聚和消散过程缓和,老化试验后,由于绝缘降解作用和晶态结构的破坏使得空间电荷的积聚加剧且消散过程变得缓慢。     

交联聚乙烯电缆生产过程中热应力的计算

本文围绕 XL PE电缆生产过程中 ,由于温差导致绝缘中残存热应力的问题展开讨论 ,并针对经典解法缺乏普遍性的问题 ,提出了使用有限元法求解热应力的平衡方程的思想 ,求解得到热应力和生产条件之间的关系 ,并提出了在生产过程中控制环境参数消除热应力的解决方案     

The behavior of water in XLPE and EPR cables and its influence onthe electric characteristics of insulation

This paper presents the latest results of continuous investigations of cable insulations degradation of crosslinked polyethylene (XPLE) and ethylene-propylene with rubber based formulation (EPR) when subjected to electric stress and heating in the presence of water or water vapour. The paper deals with water absorption and diffusion in two kinds of crosslinked polyethylene insulation-dry-cured and steam-cured, and steam-cured EPR insulation. The aim of this investigation is to present the results of the influence of changing of water or water vapour pressure in the conductors of XLPE and EPR cables in different service conditions on the electric characteristics of XLPE and EPR insulations-breakdown voltage (AC BDV), dissipation factor (tan δ) and rata of partial discharge (RPD). In this paper, RPD is defined as, the maximum electrical field when the beginning partial discharge in the cable insulation and partial discharge were measured in accordance with the IEC standard. This paper also shows the relation between AC BDV and water content, and AC BDV and tan δ in XLPE and EPR insulations. In this testing the tap water was put in the cable conductors and the ends were properly closed by terminal boxes. The results indicate that the combined effects of water or water vapour, pressure, moisture, electric field and temperature will greatly accelerate deterioration of XLPE and EPR insulations     

Study on the dielectric characteristics of DC XLPE cables

The DC characteristics of XLPE (crosslinked polyethylene) power cables were investigated. Cables with an insulation thickness of 2.5 mm, 9 mm, and 13 mm using either XLPE or conductive-inorganic-filled XLPE (XL-A) were manufactured, and four kinds of breakdown tests (DC, polarity reversal, impulse, and superposing opposite polarity impulse on DC prestress) were performed. A 250 kV, XL-A cable (20 mm thickness) was designed and manufactured using the results. The test results show that the XL-A cable possesses much better DC breakdown characteristics than the XLPE cable and is adequate for use in DC cables     

ESCA analysis of semiconducting layer materials in XLPE power cables

This paper describes an application of ESCA analysis to semiconducting materials in XLPE power cables in terms of the oxygen ratio (integrated intensity of oxygen spectrum/integrated intensity of carbon spectrum). Analyzed materials were carbon black, an additive mixed into semiconducting materials, and several semiconducting materials treated in various cross‐linking conditions. It was found that the additive increases the oxygen ratio of semiconducting materials as well as does the cross‐linking treatment. The higher the oxygen ratio of the semiconducting materials, the smaller is the average lamellar angle in the XLPE insulation at the semiconducting interface. The angle is thought to be a parameter which expresses the hyperstructure of polymer insulation and smaller angles correspond to a better state of the semiconducting interface in XLPE power cables. © 1998 Scripta Technica, Electr Eng Jpn, 126(2): 15–22, 1999