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     

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     

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.     

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     

Accelerated life tests on a new water tree retardant insulation forpower cables

This paper describes the results of an investigation in which 15 kV rated cables insulated with a new water tree retardant cross-linked polyethylene (TR XLPE) were subjected to accelerated aging tests under a controlled voltage stress and thermal load cycle conditions. Cables insulated with conventional XLPE and a commercially available TR XLPE were used as reference test populations to affirm the test methodology. Under the chosen conditions, cable life of the new TR XLPE as calculated using Weibull and log normal statistical distributions, was more than twice that for the reference TR XLPE. Extensive diagnostic measurements (water content, dissipation factor, water tree analysis) were performed on failed cable samples to bring out the differences between the three insulations. Electron micrographic investigations revealed the size and distribution of micro voids in the new TR XLPE to be smaller supporting its extended life under these tests. The experimental details of the accelerated life tests are also documented in a clear manner facilitating any archival of the data for future analysis and comparison     

硫化工艺对抗水树交联聚乙烯绝缘电力电缆工频击穿性能的影响

为了研究不同硫化工艺对抗水树枝交联聚乙烯绝缘电缆击穿性能的影响,建立了相应的试验手段和评价程序。将使用相同导体屏蔽料、抗水树绝缘料、绝缘屏蔽料,并采用5种不同硫化工艺(A、B、C、D、E)生产的电压等级、型号规格相同且结构相似的电缆作为研究对象,每种电缆取6段作样品,共30段。分别对老化前和老化180d后的5种样品进行工频击穿试验,并观察击穿后样品切片的水树枝、界面微孔、突起和绝缘中的微孔、杂质。试验结果表明:经过180d的加速老化后,5种样品中均无微孔、界面光滑、有少量尺寸较小的杂质,不会导致击穿性能下降;不同硫化工艺生产的电缆工频击穿性能表现出明显差异,其中,A硫化工艺生产的电缆工频击穿强度下降了53.53%,击穿后的样品中观察到了水树枝,B、C硫化工艺生产的电缆工频击穿强度也有不同程度的下降,而D、E硫化工艺生产的电缆的工频击穿强度没有降低,说明硫化工艺对工频击穿强度有直接的影响,并建议实际生产中确定硫化工艺时,各区温度设定应逐渐降低,且初始硫化温度不应过低,生产线速度应适当。     

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。     

Evaluation of service aged 35 kV TR-XLPE URD cables

This paper provides information on the aging of URD power cable insulated with a tree-retardant crosslinked polyethylene (TR-XLPE) compound, installed in a typical utility environment, Numerous evaluations were performed on samples of power cables aged up to 7 years in-service. AC and impulse voltage breakdown data are compared with data for similar 35 kV ethylene propylene rubber (EPR) and crosslinked polyethylene (XLPE) insulated cables removed from the same utility system. The data show that, to date, the rate of degradation of TR-XLPE cables is less than that of the EPR and XLPE insulated cables. Extrapolation of the data, assuming the same rate, indicates TR-XLPE cable will have the longest life     

高压XLPE电缆绝缘V t特性研究综述

交联聚乙烯（cross linked polyethylene,XLPE）绝缘电力电缆是输电线路的重要电 力设备。针对高压交流和直流电缆系统的运行现状,介绍了运用V t特性（击穿电压与击穿时间的关系）曲线描述XLPE电缆绝缘的电老化寿命模型,分析了国内外高压交、直流XLPE电缆绝缘V t特性的研究方法及相关结果。已有的研究结果表明,交流XLPE电缆绝缘的电老化寿命指数n值在9~25之间,直流XLPE电缆绝缘的电老化寿命指数n值在13~20之间。国内目前尚未见有关直流电缆绝缘V t特性研究的文献报道。     

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     

Effect of Thermal Overload on the Voltage Breakdown Strength of Service-Aged URD Cables

Present industry specifications allow thermoset insulated polymeric cables to be subjected to emergency conductor temperatures of up to 130°C. The effect of the high temperatures on cable integrity has been questioned. This study shows that cyclic, long-term or fast-rise application of 130°C to service-aged, water treed underground residential distribution (URD), crosslinked polyethylene (XLPE) insulated cables, result in an increase in dielectric strength. Contrary to what happens in new cables, an increase in temperature from ambient to 130°C also results in an increase in voltage breakdown strength. It appears that at high temperature, moisture and some remnant by-products of the crosslinking reaction such as volatiles, diffuse from the insulation, contributing to the higher levels of dielectric strength. It is shown that thermoplastic insulation shields on XLPE service-aged cables are adversely affected by emergency temperatures.     

交联聚乙烯热老化与绝缘性能的关联关系

为研究交联聚乙烯（cross-linked polyethylene,XLPE）绝缘材料的热分解活化能、电气特性和力学特性随热老化程度变化的规律,对交流电力电缆绝缘用XLPE材料在110 ℃下开展加速热老化实验。采用热失重（thermogravimtric analyzer, TGA）测试手段,对XLPE在20~600 ℃的热分解行为进行研究;采用交流击穿测试、宽频介电谱测试及体积电阻率测试,研究老化后XLPE试样的电气特性;采用拉伸实验测试,研究老化后XLPE试样的力学特性。结果表明：热老化使得XLPE的交联结构和结晶状态被破坏,XLPE活化能呈减小趋势。由于氧化反应快速进行,使得XLPE分子链发生断裂,交联结构变弱,导致XLPE绝缘材料严重劣化,其活化能、击穿强度、体积电阻率、弹性模量和断裂伸长率随老化时间增长呈下降趋势,而介电常数、介电损耗和电导率呈增加趋势。     

交联聚乙烯热老化与绝缘性能的关联关系

为研究交联聚乙烯（cross-linked polyethylene,XLPE）绝缘材料的热分解活化能、电气特性和力学特性随热老化程度变化的规律,对交流电力电缆绝缘用XLPE材料在110 ℃下开展加速热老化实验。采用热失重（thermogravimtric analyzer, TGA）测试手段,对XLPE在20~600 ℃的热分解行为进行研究;采用交流击穿测试、宽频介电谱测试及体积电阻率测试,研究老化后XLPE试样的电气特性;采用拉伸实验测试,研究老化后XLPE试样的力学特性。结果表明：热老化使得XLPE的交联结构和结晶状态被破坏,XLPE活化能呈减小趋势。由于氧化反应快速进行,使得XLPE分子链发生断裂,交联结构变弱,导致XLPE绝缘材料严重劣化,其活化能、击穿强度、体积电阻率、弹性模量和断裂伸长率随老化时间增长呈下降趋势,而介电常数、介电损耗和电导率呈增加趋势。     

The world's first use of 500 kV XLPE insulated aluminium sheathedpower cables at the Shimogo and Imaichi power stations

The world's first practical applications of 500 kV XLPE (cross-linked polyethylene) cables took place in 1988 at the Shimogo power station of the Electric Power Development Co. Ltd., and the Imaichi power station of the Tokyo Electric Power Co. Inc.. Research and development work on the cables had been conducted since 1982. Efforts were concentrated on quality control to remove contaminants from the insulation and to extrude the very thick-walled insulation without any defects. Insulation performance tests, including long-term aging, confirmed that the 500 kV XLPE cables and the terminations were highly reliable with superior initial and long-term performance. The installation was completed successfully, and the cables are now in satisfactory operation at both power stations. The cable routes, design of the cable and termination, cable manufacture, results of the insulation performance tests, installation work, and site test are discussed     

EPRI report-characterization of cable insulation materials bydynamic mechanical spectroscopy

A project that is part of a broad effort to apply nonelectrical diagnostics to the study of aging-induced changes in solid dielectric cables is described. The technique of dynamic mechanical spectroscopy is explained. The goal of the work described was to determine whether aging in high-molecular-weight polyethylene (HMWPE) or crosslinked polyethylene (XLPE) insulated distribution cable insulation could be detected by solid-state rheological measurements. Insulation from seven HMWPE and five XLPE cables were studied. The results and their interpretation are summarized. They suggest that a more detailed study of a number of service-aged and laboratory aged XLPE's, tree-resistant XLPEs, and EPR would be highly beneficial     

Polymer insulated cable systems up to 220 kV

The state-of-the-art of polymer insulated cable systems in Switzerland is presented. The construction and the features of high-voltage power cables with crosslinked polyethylene (XLPE) or ethylene-propylene rubber (EPR) insulation, accessories such as joints and terminations, and some special laying techniques are discussed. Results of type tests, long duration tests, and field tests are presented     

Application of oxidation induction time and compensation effect tothe diagnosis of HV polymeric cable insulation

Oxidative stability tests were performed on field and laboratory-aged crosslinked polyethylene (XLPE), ethylene-propylene rubber (EPR) and polypropylene (PP) insulations using differential scanning calorimetry. Flat films and miniature cables aged in the laboratory were subjected to a wide range of aging conditions that included thermal, electrical and a combination of thermal-electrical aging, in dry and wet environments. The results were analyzed using the Eyring rate theory. It is shown that for a given material the oxidative stability data can be described by a single linear relationship between the activation entropy ΔS and the activation enthalpy ΔH of the oxidative process, and thus are governed by the so called compensation effect. It is argued that the position of a data point representing a certain operating condition of an insulation on the ΔS vs. ΔH compensation plot is a measure of the degree of degradation and can be used as a diagnostic indicator of the operating conditions of the insulation     

A Review of Possible Methods for Defining Tree Retardant Crosslinked Polyethylene (TRXLPE)

This article documents the activities and discussions of the ICC discussion group A4D comparing the performance of TRXLPE and XLPE insulation material used in medium voltage underground power cables. It was not possible at the time to use water tree growth tests, carried out according to ASTM 6097-97, to provide a definition of a water tree retardant material. This was due to the relatively small number of tests carried out, the large scatter in the data of repeat tests on the same material, and the overlap in the data observed between XLPE and TRXLPE insulations.     

Performance characteristics of XLPE versus EPR as insulation forhigh voltage cables

The physical and electrical properties of crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR) are compared in the context of their use in transmission class cables. Results indicate that the 138-kV XLPE cable has AC withstand/breakdown strength at least 25% higher than the 150-kV EPR cable. The XLPE cable exhibits about 70% higher impulse strength than the EPR cable. The loss factor of the XLPE cable is at least 20 times lower than that of EPR cable. Thus with XLPE cables, the yearly energy savings can be on the order of 15 MWh/cct. km for a 69-kV system, 52 MWh/cct. km for a 138-kV system and 127 MHh/cct. km for a 230-kV system