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.     

Effect of water on aging of XLPE cable insulation

Cables as elements of power distribution system have great influence on its reliable service and overall planning requirements. During last years, crosslinked polyethylene (XLPE) cables have been more and more used in power systems. This paper presents the results of an investigation of changing of (XLPE) cables insulation breakdown stress (AC BDS) due to water absorption. The paper deals with AC BDS of the following kinds of XLPE cable insulations: steam and dry cured with water tree retardant crosslinked polyethylene (TR-XLPE) and non-tree retardant crosslinked polyethylene (XLPE). During tests, the tap water was injected into, (1) conductor with cable ends closed; (2) into cable conductor with ends opened; and (3) into metallic screen with cable ends opened. The presence of water in XLPE cables was subjected to electrical stress and heating. AC BDS tests were performed as a function of aging time and water content in the cable insulation at different aging temperatures. Also, in this investigation, tests with the changing of AC BDS in the radial direction of unaged and aged XLPE cable insulations were carried out.     

Application of dielectric response measurement on power cable systems

Water treeing is one of the factors leading to failure of medium voltage XLPE cables in long-term service. Increased moisture content inside oil-paper insulated cable is not desirable. To identify water tree degraded XLPE cables or oil-paper cables with high moisture content, diagnostic tests based on dielectric response (DR) measurement in time and frequency domain are used. Review of individual DR measurement techniques in the time and frequency domains indicates that measurement of one parameter in either domain may not be sufficient to reveal the status of the cable insulation. But a combination of several DR parameters can improve diagnostic results with respect to water trees present in XLPE cables or increased moisture content in oil-paper cables. DR measurement is a very useful tool that reveals average condition of cable systems. However, it is unlikely that DR measurement will detect few, but long water trees. In addition, DR cannot locate the defect or water tree site within the cable system. Combination of DR and partial discharge (PD) measurements can improve diagnostic results with respect to global and local defects. However, it is doubtful whether PD test can identify the presence of water trees inside a cable in a nondestructive manner. Further research is needed for more detailed conclusions regarding the status of a particular insulation and for predicting the remaining life of the insulation system.     

Stress induced electrochemical degradation of the inner semiconlayer of XLPE-insulated cables and model samples

A degradation phenomenon of the inner semicon layer of extruded cable insulation occasionally has been observed in different service aged as well as in laboratory aged polymer insulated cables. To investigate this effect, accelerated aging tests on XLPE-insulated cable cores as well as on a new type of model sample (aluminum wires, semicon and XLPE insulation layer) were performed. It is shown that under the influence of an electrolyte and mechanical strain, channel-like structures develop in the semicon of both test objects, and that vented trees are initiated when these structures reach the interface to the insulation. Thus, this type of degradation also is of particular importance for the long time aging behavior and testing procedures of polymer insulated cables. It is demonstrated that the newly developed model sample test arrangement is a valuable tool for the study of aging of insulation systems     

交联聚乙烯电缆水树缺陷的修复方法研究

采用硅氧烷修复液修复交联聚乙烯电缆老化试样中的水树,进而分析修复效果及机理。将介质损耗因数为4%～6%,绝缘电阻7 500～10 000 MΩ的短电缆在7.5 kV 450 Hz交流电压下老化至介质损耗因数达到20%左右,绝缘电阻3 500～5 000 MΩ。然后用压力注入式修复装置把修复液注入缆芯对水树缺陷进行修复。以介质损耗因数、绝缘电阻和击穿电压为指标对修复效果进行评判;通过显微镜切片观察修复前后水树微观形态;通过仿真修复前后水树附近电场分布来分析和验证水树的修复机理。实验结果证明,修复液可以充分与电缆水树中的水发生反应生成胶状聚合物填充水树通道;修复后电缆介质损耗因数、绝缘电阻和击穿电压恢复到新电缆水平;改善了绝缘层电场分布;有效地抑制了水树生长。实验表明,该修复液可有效修复电缆中的水树缺陷,提高电缆绝缘水平。     

硅氧化焼注入后水树老化交联聚乙烯电缆电气特性和微观结构（英文）

To evaluate the effect of using siloxane liquid to rejuvenate water tree defects in cross-linked polyethylene(XLPE)cables,we investigated the electrical properties and micro-structures of water-tree aged XLPE cables after siloxane liquid injection treatment.The water-tree aged samples were prepared by performing accelerated aging experiment using water-needle electrodes,and the siloxane liquid is injected into the aged cable through a pressurized injection system.Dielectric loss factors of the samples before and after the rejuvenation were compared.The water trees and the internal filler were observed using scanning electron microscope(SEM).Electrical properties of the reactants are measured.Electric field simulation is conducted to verify the rejuvenation effect by finite element method.The results show that the siloxane liquid diffused into the insulation layer in a short time and reacted with water in the water trees.The electrical properties of the formed organic filler are in accord with that of XLPE.Therefore,the action between siloxane and water can inhibit the growth of water trees and reduce electric field distortion of the water tree areas.As a result,insulation performance of the cable is enhanced.A 70 m long cable was aged and rejuvenated in laboratory and an on-site rejuvenation experiment was conducted,and in both cases the dielectric loss factor and leakage current halved after rejuvenation.     

Development of New Diagnostic Method for Hot-Line XLPE Cables with Water Trees

A new insulation diagnostic method for XLPE cables containing water trees is presented. A dc component in the ac charging current of these cables was found to be a significant sign of the existence of water trees. The dc component has good correlations with such insulation characteristics of aged XLPE cables as ac breakdown voltage and dc leakage current. Criteria for insulation diagnosis of 6.6kV XLPE cables have been established. An automatic insulation diagnostic device has been developed. It is now being applied to hot-line XLPF cables in distribution systems of TEPCO (The Tokyo Electric Power Co., Inc.).     

XLPE电缆绝缘中的电树枝结构及其生长特性

为研究不同频率下半结晶XLPE电缆绝缘材料中的电树枝引发、生长及结构特征,系统的归纳了XLPE电缆绝缘中可能出现的电树枝特征及其与材料聚集态和残存应力的关系;采用变频高电压发生器、实时显微数字摄像技术及专用试样加工工艺,进行了大量的电树枝培养实验。实验研究发现,由于半结晶高聚物的不均匀结晶和电缆生产过程中在绝缘层中产生的残存应力的影响,使得50Hz施压频率下XLPE电缆绝缘试样中会生成枝状、枝状与丛林混合状及纯丛林状3类电树枝,而>500Hz高频下则只能生成稠密枝状电树枝,它们分别对应于不同的生长机理。低频下电树枝生长特性和电树枝结构与材料的聚集态密切相关,而高频下则关系不大。分形分析这些电树枝的结构后发现,电树枝的生长特性与其分形维数及其分形维数的变化有对应关系,故可用分形维数分类和定量描述这些电树枝.最后探讨了几种不同结构电树枝的生长机理。     

The influence of the water on water absorption and density of XLPEcable insulation

This paper presents the results of a continuing investigation into effect of water on water absorption and density of crosslinked polyethylene (XLPE). The experimental set up was made for the following XLPE cable insulations: steam and dry cured with water tree retardant crosslinked polyethylene (TR-XLPE) and without water tree retardant crosslinked polyethylene (natural XLPE). During tests, the tap water was injected (1) into the cable conductor with cable ends closed, (2) into the cable conductor with cable ends opened, and (3) into the metallic screen with cable ends opened. The XLPE cable insulation together with the water present in the cable was subjected to electrical stress and heating. The results were analyzed theoretically and experimentally. The aim of this paper is to present the results of the influence of the water on water absorption and density of various kinds of XLPE cable insulation in different service conditions     

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.     

A study of treeing phenomena in the development of insulation for500 kV XLPE cables

This review summarizes research on treeing phenomena, i.e. the formation of electrical trees and water trees, that has been undertaken in Japan for the development of 500 kV XLPE cable. Section 1 presents the results of factors affecting XLPE cable insulation breakdown under commercial ac and lightning impulse voltages. Section 2 verifies the phenomena of electrical tree formation in XLPE cable insulation using block samples and model cables, and gives the results of studies to determine the level electrical field stress initiation for such trees. Section 3 summarizes the results of studies on long-term aging characteristics, which is a particular problem under commercial ac voltages, while Section 4 explains how this research influenced the design of 500 kV XLPE cable insulation. All authors were members of `The investigation committee of fundamental process of treeing degradation' under IEEJ     

Investigation of Water Tree Degradation in Dry‐Cured and Extruded Three‐Layer (E‐E Type) 6.6‐kV Removed XLPE Cables

Dry‐cured and extruded three‐layer (E‐E type) 6.6‐kV 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 6.6‐kV XLPE cables removed after 18 to 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 (6.9 kV) and the ac voltage for the dielectric withstanding test (10.3 kV). Water trees were mainly bow‐tie water trees and their maximum length was approximately 1 mm. 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.     

XLPE电缆绝缘中水树的形成机理和抑制方法分析

叙述了交联聚乙烯电缆中的水树对中高压XLPE电缆的危害性;介绍了水树的本质、水树生长特性,引发水树的电-机械理论和化学反应理论;分析了影响水树生长的因素和国内外抗水树电缆料的研究情况。     

Condition assessment of 12- and 24-kV XLPE cables installed during the 80s. Results from a joint Norwegian/Swedish research project

This paper reports the results from the condition assessment of 12- and 24-kV cross-linked polyethylene (XPLE) cables using a technique based on dielectric spectroscopy initially developed at KTH in Sweden. The work aims to examine whether the method could detect water tree degradation for the second generation medium voltage (MV) cables with long, but not bridging, water trees. While the overall cable condition was better than expected for second generation XPLE cables, water trees were found in most of the selected cables. The diagnostic method based on the measurement of the dielectric response could only detect water tree degradation in the examined second generation cables when the water trees bridged the insulation wall. Condition assessment above service stress may, in some cases, be required to detect bridging water trees. The results indicate that there is a correlation between the voltage level and the breakdown voltage of the cable. This can be used as a diagnostic criterion for this group of cables.     

XLPE电缆交流工况下雷电冲击作用的起始放电特性实验研究

应用针—板电极系统模拟高压交联聚乙烯（以下简称ＸＬＰＥ）电缆绝缘中的杂质和半导体层中的凸起物等因素引起的电场不均匀现象,通过实验探讨了在交流电压和冲击电压共同作用下ＸＬＰＥ电缆电树枝老化时起始放电电压特性。     

XLPE电缆电树枝二次生长特性分析(英文)

In order to study the growth characteristics of electrical trees in XLPE cable under secondary applied voltage, a short cable metal needle defect test device is adopted to study the growth characteristics of the new trees after the electrical trees in XLPE cable under the action of the voltage of 12 kV are influenced by secondary applied voltage (15 kV). The research results show that influenced by secondary applied voltage and voltage increase rate, there will be a peculiar "bush-branch" electrical tree in XLPE cable insulation layer and the new trees under secondary applied voltage have the characteristics of short initiation time, fast growth rate and narrow discharge channel, etc, which shows that secondary applied voltage has a great effect on the secondary initiation and growth of electrical trees in XLPE cable and it is an important factor of accelerating cable aging and breakdown.     

Bow-tie-tree in EPR cables are accelerated water treeing test

The authors describe the results of an accelerated water treeing test after a period of approximately one year with two recently manufactured (1988) varieties of commercial EPR (ethylene-propylene rubber) cables, produced by different manufacturers and possessing different cable structures, as well as water impervious XLPE cable for comparison. In the test, bow-tie trees were found in the EPR insulation, which had previously been thought to develop no or few bow-tie trees. The analysis results of bow-tie in EPR cables are described     

交联聚乙烯电缆中的水树

水树对交联聚乙烯电缆运行寿命有害。本文通过在交联聚乙烯绝缘料试片和原尺寸交联聚乙烯电缆上进行的加速水树引发试验,观察研究水树枝的生长特性,比较二种不同交联方法对水树生长的影响。最后探讨了形成水树的可能机理。     

Electrical stress enhancement of contaminants in XLPE insulation used for power cables

The use of XLPE as the insulation for power cables has grown steadily since it first introduction more than 30 years ago. Today XLPE is rapidly becoming the preferred insulation system for even the highest transmission voltages. This preference is due to the high reliability, low dielectric losses, and low environmental impact that can be achieved with XLPE. The positive effects of high quality insulation materials on improved cable performance have been well known since the start of cable making. The purpose of this paper is to investigate the technical background for the cleanliness levels and to quantify the level of performance required from clean materials. The advantages of clean insulation materials are seen at all voltages. However, this work focuses on the technical basis for the benefits for HV and EHV cables, which typically are designed with a water impervious layer to ensure that the cable remains dry throughout its entire lifetime. The presence of metallic contaminants in MV cable is known to enhance the growth of trees by raising the electric stress level locally. The singular impact of cleanliness on the performance of MV cables is somewhat more complicated as it is influenced both by the cleanliness of the insulation and the ability of the insulation material to resist the growth of water trees.     

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