Technical report: extrusion-molded joints for world's firstlong-distance XLPE cable line

The features of a long-distance underground line, using 275-kV cross-linked polyethylene (XLPE) cable with extrusion-type molded joints (EMJs) are described. The line has a transmission capacity of 440 MW/cct and a 9.5-km section length. The cable has a cross-sectional area of 1400 mm² and insulation thickness of 17 mm. The EMJ, which was developed to withstand the higher stresses of the 275-kV cable, has the following features: (1) the conductor is connected using a copper ferrule, (2) semiconducting heat-shrinkable tubes are used for both inner and outer semiconducting layers, and (3) the insulation unit, applied on top of the stress-relieving cone, is made of insulating and semiconducting XLPE parts cured together in the factory. The fabrication, of the joint is described     

Long-term testing and applications of high-stress MV EPR cables

High-stress design 35-kV ethylene propylene rubber (EPR)-insulated cables with 1.14-mm (45-mil) wall reduction have been in service for more than ten years. In the mid 1990s, an extensive cable testing program was initiated to demonstrate that high-stress designs up to 4 kV/mm (101.6 V/mil) exhibit a significant electrical and mechanical safety margin to operate on typical utility network systems. Investigations were also undertaken to demonstrate the suitability of accessories for high-stress designs. This paper will present the results of six-year, long-term wet electrical testing conducted to demonstrate that with proper design, the aging performance of standard wall and high-stress design cables provide satisfactory results and suitable performance for the intended application. Additionally, utility field experience is presented on 35- and 15-kV cables operating at high stress. The experience for these 15-kV medium-voltage EPR cable designs employ the highest recommended stress for cables of this class, as well as taking into account the complete system performance.     

Development of prefabricated joint for 275-kV XLPE cable

Application of XLPE cables to extra-high-voltage (EHV) transmission lines is expanding due to their excellent electrical properties and ease of maintenance, and 275-kV long-distance underground transmission lines are reaching the stage of practical utility. Extrusion molded joints (EMJs) are widely used as the XLPE cable joint for EHV transmission lines. However, EMJs have presented problems because of their insulation. They are formed on site through extruding and molding of crosslinkable PE like the cable, and need a long time for installation on site and higher jointing technique. The prefabricated joints (PJs), which are already applied to 154-kV transmission line, are installed on site by using the parts manufactured and tested in the factory, they require a shorter installation environment. This report introduces the results obtained regarding the improvement of the electrical property, workability and the performance of PJs, designed for 275-kV XLPE cable. These joints are planned to be used for 275-kV transmission lines from 1995     

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     

Successful testing of 345-kV XLPE cables and premolded joints atIREQ

Extruded polymeric cables and accessories are an alternative to self-contained fluid filled (SCFF) cables for extra-high-voltage (EHV) systems. Crosslinked polyethylene (XLPE) insulated cables have many advantages over the traditional fluid-filled pressurized cable system with the elimination of the hydraulic system and the associated equipment and complications during the installation and operation/maintenance of such systems. Other advantages of the extruded cable system arise from the new accessory technologies and the use of splices and terminations that are prefabricated and pretested in the factory and require less time to install on site. Concerns over the long term reliability of high voltage cable systems, in particular the accessories and the lack of service experience above 300 kV led to the decision of Hydro-Quebec to carry out a prequalification (long-term) test program to assess the reliability of the cable materials and to verify the cable and accessory installation methods to be employed. The cables were installed in duct banks and manholes simulating actual installation conditions used in Hydro-Quebec underground cable network. This paper describes the program and results of the prequalification tests of 345-kV XLPE cables and accessories performed at Hydro-Quebec's Research Institute (IREQ) in partnership with three international cable manufacturers, Alcatel, Fujikura and Pirelli     

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     

Service aged 69 and 115 kV XLPE cables

Five 69 kV XLPE cables, 6 to 23 years in service and two old vintage cables from storage plus a 115 kV XLPE cable from service together with a spare cable from the same production, kept in storage, were evaluated. All components of the cables were found to show little signs of deterioration except for AC breakdown. The 69 kV XLPE cable from service has breakdown levels ranging from 10.2 to 18.2 kV/mm, the spare cable 15.0 to 17.7 kV/mm compared to 27.2 kV/mm, the only previously reported value for a new cable. The 115 kV XLPE cable had a breakdown level of 16 kV/mm and the spare 10 kV/mm. The above cables are low-stress cables. Most modern cables operate at higher voltage stresses, taking advantage of cleaner insulations with smoother shields and moisture barriers. Caution is advised in using older, low-stress cables placed in storage     

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     

A contribution to the study of aging of XLPE insulated cables

The electrical, thermal, and multiple-stress (thermal and electrical) aging of cross-linked polyethylene (XLPE) cable models has been investigated in order to get information on aging effects and mechanisms. After endurance tests, XLPE cable models have been subjected to chemical, physical, electrical, and microstructural characterization. Thermal aging results reveal that bulk degradation occurs in the cables at temperatures higher than the melting point. Multiple-stress aging data emphasize a synergistic effect of electric field and temperature. Significant microstructural changes detected in the cables aged under multiple stress are evidence of this effect and can partly explain the time behavior of the electric strength     

Development of 500-kV XLPE cables and accessories for long distanceunderground transmission line. III. Electrical properties of 500-kVcables

500-kV XLPE-insulated cable with an insulation thickness of 27 mm has been developed for long-distance transmission lines. Basic studies on 500-kV XLPE cable have shown that contaminants in the insulation may be the factor determining 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 the electrical characteristics of full-sized cables. Voltage-withstand and long-term tests have confirmed the design values for minimum breakdown stress, for AC and impulse voltage, and for the degradation coefficients     

±320kV直流电缆交联聚乙烯/三元乙丙橡胶附件击穿特性

高压直流电缆接头与终端为电缆系统故障的多发点,其击穿强度为直流输电系统安全稳定运行的重要基础。文中以±320 kV高压直流海底电缆中交联聚乙烯(cross linked polyethylene,XLPE)/三元乙丙橡胶(ethylene propylene diene monomer,EPDM)附件为研究对象。首先,研究电缆及附件负荷循环耐压试验,发现附件界面为击穿薄弱环节;其次,研究绝缘材料电导率随温度变化特性对电场分布的影响规律,通过有限元仿真模拟电缆空载和满载运行时附件的温度分布与电场分布,发现最大电场出现在电缆绝缘靠近附件应力锥一侧,为29.5 kV/mm,低于附件材料的击穿场强;最后,研究界面在直流电场下空间电荷特性对电场分布规律的影响,通过电声脉冲法测试复合叠层片状样品介质界面的空间电荷及其电场分布,发现场强畸变率约为100%~200%。同材料本征绝缘匹配相比,界面空间电荷积聚对附件内部电场造成的畸变程度更严重,在后续附件提升中应更注重开发抑制空间电荷的绝缘材料。     

A New Approach to the Statistical Enlargement Law for Comparing the Breakdown Performance of Power Cables. 1. Theory

This paper, the first of two companion papers, describes a new approach to the application of the statistical "enlargement law" to power cable insulation. The results of this work allow for selecting better insulation for medium or high voltage cables. The selection is based on the results of dielectric strength tests performed on cable models or full size cables of a limited length. The relevance of the so-called crossing length, where one insulating compound performs better below it and another compound above this length, is discussed. The proposed theory was used to analyze the performances of two ethylene-propylene rubber (EPR) compounds and two cross-linked polyethylene (XLPE) compounds. The results are presented in the companion paper.     

Technological progress in high-voltage XLPE power cables in Japan.II

For pt.I see ibid., vol.4, no.5, p.9-16 (1988). Cables designed for particular applications and improved performance are described. The products discussed are: (1) water-impervious cable; (2) composite XLPE/optical fiber cable; (3) 66-kV submarine cable; (4) commercial long-distance 275-kV line; and (5) 500-kV short-distance line. The discussion is confined to the Japanese cable industry     

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.     

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     

场强的提高对局部放电测试的影响

局部放电 (PD)是检测 XL PE电缆缺陷的一种重要手段 ,缺陷会造成电缆的局部场强的不均匀 ,在施加外部电压后引起局部放电的现象。本文提出 :在 XL PE电缆承受耐压的过程中 ,采用提高场强的方法 ,能更有效地发现 PD缺陷 ,从而降低 XL PE电缆在运行中的击穿事故。     

Electrical aging of extruded dielectric cables: review of existingtheories and data

Despite the huge amount of data on so-called electrical aging of extruded HV cables, the fundamental phenomena responsible for it or evolving with aging time, are still far from well understood. It is therefore not surprising why it is so difficult to predict reliable cable lifetimes in service from accelerated aging experiments in the laboratory. The objective of this paper is to review critically the existing theories of electrical aging of solid dielectric materials. A relatively large number of models and theories exist but none of the most often used is known to yield reliable life predictions. One conclusion is that there is a need for a more comprehensive model of electrical aging of extruded dielectric cables. In order to develop this model, an extensive review of existing literature data was undertaken. This paper summarizes the data collected from more than 200 papers on aging of PE, XLPE and EPR cables. It appears that cable breakdown strength should not be plotted on log field vs. log time graphs to yield long-time (i.e. low-field) values, since results obtained over a long time period do not obey an inverse power law. In fact, high-field results are better described by an exponential relation between time and field. The models of Simoni, Montanari and Crine seem to give the best fit to experimental results obtained under a wide variety of experimental conditions. It is also shown that the lower field limit for the exponential regime with XLPE cable is in the 8 to 15 kV/mm range, which corresponds to the onset of strong charge injection. The influence of environment, insulation nature and morphology, and testing temperature are discussed     

The world's first long-distance 500 kV-XLPE cable line. 2. Jointsand after-installation test

For pt. 1 see ibid., vol. 18, no. 2, p. 65-67 (2002). In this second part of a three part series introducing the Shin-Keiyo/Toyosu underground transmission line using a 500-kV XLPE cable, the authors describe the specially developed extrusion-molded joints and after-installation test procedures. Based on the results of basic research extensively done to design the 500-kV extrusion-molded joints, the insulation thickness was determined to be 36 mm through careful considerations of various design parameters obtained. As in the case of the cable, the main factors that determine the performance of the joints are contaminants and voids, and their permissible levels are determined. Unlike cables, it is impossible to conduct sampling inspections for the extrusion-molded joints after they were assembled. Therefore, particular emphasis has to be placed on supervision and inspection of conditions during the assembly process     

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.