Diagnosis of moisture in oil/paper distribution cables - Part I: Estimation of moisture content using frequency-domain spectroscopy

Dielectric spectroscopy (dielectric response measurements) has been applied for nondestructive estimation of humidity in oil-paper cable insulation. The experiments have been based upon two field-aged cables, 20 and 50 years old. Paper samples from these cables have been characterized and subjected to environments with different relative humidity. Dielectric loss and capacitance have been measured in a frequency range 1 MHz to 1 kHz and related to the moisture content determined by Karl Fisher titration. A method has been verified where the moisture content is correlated to the minimum value of loss tangent (tan/spl delta/). A number of field measurements have been performed where the method has been applied to estimate the moisture content in the distribution cables.     

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.     

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.     

Performance of reduced wall EPR insulated medium voltage powercables. I. Electrical characteristics

Paper-insulated lead covered power cables (PILC) have had a long and successful heritage. After almost 100 years, this design of cable is still in operation and continues to be manufactured. However, electric utilities are now looking for a reliable replacement for PILC cables. This is due to two primary reasons: (1) the difficulty in installing and maintaining this type of cable; and (2) increasing pressure to replace these cables due to environmental concerns. To date, diameter limitations of conventional extruded dielectric cables has impeded their replacement in existing PILC conduits. This paper describes a study for the evaluation for reliably reducing the insulation thickness to achieve a lower diameter cable to effectively replace PILC cable in existing conduits. Part I of the investigation reviews the theory of insulation wall determination and the test program carried out to evaluate electrical performance of reduced wall EPR cables. Additionally, cable design concepts and constructions are discussed     

Advanced solution for on-site diagnosis of distribution power cables

This paper describes important aspects of partial discharge (PD) diagnostics of distribution power cables. PD parameter were discussed under consideration of attenuation and dispersive effects of cable characteristics. Field experiences with PD diagnosis on paper insulated cables (PILC) and cables with cross linked polyethylen insulation (XLPE) by energizing with Damped AC Voltage demonstrate relevant demands and the effective outcome for asset management decision support. The main difficulty is to evaluate the risc of PD occurrences on the reliability of the cable system. If an sufficient amount of PD diagnostic data for the cable components is available statistical methods can be used for determing threshold levels and relevant condition indexes for the asset management.     

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     

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     

Evaluation of an in-service aged ethylene-propylene rubber (EPR)insulated distribution cable

An 18-year-old EPR-type compound insulated cable was removed from service for the purpose of evaluating its condition. AC breakdown test, microscopic examination, moisture analysis, tensile/elongation, and a U-bend plate test were performed on various pieces of the cable. The results, together with the trouble-free performance of other cables of this type on Northeast Utilities' system, suggest that cables of this particular type will provide trouble-free service over their expected lifetime (at least forty years)     

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.     

Diagnostic dielectric spectroscopy methods applied to water-treedcable

Considerable effort has gone into developing polymer formulations and cable designs to minimize failures through water tree growth. However, diagnostic techniques still are required to enable the estimation of the level of damage present within a service cable. This paper reports on progress regarding the application of dielectric spectroscopy to cable diagnostics. A 40 kV, crosslinked polyethylene (XLPE) insulated coaxial cable was used as a model power cable. Sample lengths were immersed in a potassium chloride solution and some of these were subjected to AC electrical stress. After an 8 week duration, a high density of tress was found in the electrically stressed cable. Dielectric spectra have been measured for both sample types in the frequency range of 10^-5 to 10⁵ Hz. Insertion loss measurements were also carried out in the frequency range of 3×10 ⁵ to 3×10⁹ Hz. From both types of measurement, it was possible to distinguish between the cables containing water trees and those that were free from water tree structures. These approaches could therefore be developed in order to provide diagnostics for the detection of water tree damage in electrical power cables     

Application of the AC Superposition Method to Degradation Diagnosis of 22/33‐kV Class XLPE Cables

Water trees in the insulator of XLPE cable may considerably reduce the dielectric breakdown voltage, thus being an important lifetime‐governing factor. The ac superposition method we have investigated is a new technique for hot‐line diagnosis of 6.6‐kV XLPE cables, and the diagnostic apparatus using this technique is now widely used in the distribution line field. In order to study the application of the ac superposition method to degradation diagnosis of 22/33‐kV class XLPE cables, we measured deterioration signals of the cables with water trees by a modification of the above diagnostic apparatus. The deterioration signals, hereafter called “ac superposition current”, were generated by an amplitude modulation effect due to the nonlinear resistance of water trees. Moreover, we evaluated the relationship between the ac superposition current and the ac breakdown voltage. It is difficult to judge whether a correlation between then is present or not because of the uneven distribution of the measured data. However, the ac superposition current tends to increase linearly with a decrease in the residual thickness of the insulator. For example, the ac superposition current was about 40 nA when the residual thickness decreased to 3 mm. Thus, we consider that the ac superposition method is effective for degradation diagnosis of 22/33‐kV class XLPE cables.     

Performance of reduced wall EPR insulated medium voltage powercables. II. Mechanical characteristics

For the replacement of paper-insulated lead covered power cables (PILC), reduced insulation wall designs have been developed. They provide a reliable lower diameter cable design for installation in existing PILC conduits. This paper presents the results of mechanical tests conducted on both reduced- and full-wall EPR insulated power cables. Both jacketed and nonjacketed cable designs have been subjected to mechanical pulling forces which greatly exceed recommended industry limitations, followed by electrical testing to ascertain cable performance. The results of this study conclude that reduced-wall EPR insulated cables can safely withstand the same pulling forces as recommended for conventional walls and can be designed for installation under the same pulling limitations     

Approach for wide use of diagnostic method for XLPE cables using harmonics in AC loss current

A water tree is one aspect of the degradation of XLPE cables used for underground distribution or transmission lines. We have developed the loss current method using the third harmonic in AC loss current for cable diagnosis. The harmonic components in loss current arise as a result of the nonlinear voltage– current characteristics of water trees. We confirmed that the third harmonic in the AC loss current has good correlation with respect to water tree growth and breakdown strength. After that, we applied this method to the actual 66‐kV XLPE cable lines. Up to now, results on over 130 lines have been obtained. In the case of cable lines terminated at gas‐insulated switchgear (GIS), we have to remove the lightning arrestor (LA) and the potential transformer (PT) from the test circuit. The reason is that we are afraid that each LA and PT disturbs the degradation signal from cable lines. It requires extra time (1 or 2 days) and costs more to remove the LA and PT in GIS from a circuit. In order to achieve easy and reasonable diagnosis, we have developed a new method for cable lines terminated at GIS by utilizing a technique that enables one to cancel the signal of the LA and PT from disturbed signal of the cable lines. We confirmed the effect of the new method through experiments with actual cables. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(4): 52–59, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20510     

The rise and decline in the United States of impregnated paperinsulated metallic sheathed cable, solid-type. I. The rise

Paper-insulated lead cable (PILC) has had an impressive history of service. Many of the cables installed earlier in this century are still performing well. Because of this and because of tradition, PILC is still being manufactured and sold today. This article outlines a history of the development and the improvements in the technology of manufacturing lead/paper cables as well as the history of the companies involved. A future article will discuss some of the reasons for the decline in the installation of lead/paper cable     

Selectivity of damped AC (DAC) and VLF voltages in after-laying tests of extruded MV cable systems

The purpose of HV after-laying tests on cable systems on-site is to check the quality of installation. The test on extruded MV cable systems is usually a voltage test. However, in order to enhance the quality of after installation many researchers have proposed performance of diagnosis tests such as detection, location and identification of partial discharges (PD) and tan /spl delta/ measurements. Damped AC voltage (DAC) also called oscillating voltage waves (OVW) is used for PD measurement in after-laying tests of new cables and in diagnostic test of old cables. Continuous AC voltage of very low frequency (VLF) is used for withstand voltage tests as well as for diagnostic tests with PD and tan /spl delta/ measurements. Review on the DAC and VLF tests to detect defects during on-site after-laying tests of extruded MV cable systems is presented. Selectivity of DAC and VLF voltages in after-laying testing depends on different test parameters. PD process depends on type and frequency of the test voltage and hence, the breakdown voltage is different. The withstand voltage of XLPE cable insulation decreases linearly with increasing frequency in log scale. Experimental studies with artificial XLPE cable model indicate that detection of defects with DAC or VLF voltage can be done at a lower voltage than with DC. DAC voltage is sensitive in detecting defects that cause a breakdown due to void discharge, while VLF is sensitive in detecting defects that cause breakdown directly led by inception of electrical trees.     

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。     

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 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.     

Long term accelerated aging tests on distribution cables under wetconditions

Two accelerated aging test procedures are proposed for use on solid dielectric extruded distribution cables under wet conditions. One test method is a fixed time duration test in which the degree of cable specimen aging is assessed in terms of breakdown tests, while the other test method is a time to breakdown test in which the cable specimens are voltage stressed until failure ensues     

我国发展直流海底电力电缆的前景

例举世界重要的海底电缆工程,表明其中大部分为直流海底电缆工程。叙述直流输电特点,着重以不同类型的直流和交流海缆载流量计算,证明直流海底电缆在输电容量、输电损耗和电缆线路长度限制方面显著优于交流海底电缆。肯定了我国发展直流海底电缆的必要性。有些运行条件下直流海底电缆会是优先的方案,甚至是唯一的选择。提出以发展直流交联聚乙烯(XLPE)海底电缆作为主要目标,并在比较不同类型海缆绝缘中的空间电荷对电场分布和电气绝缘性能影响和深入研究空间电荷积聚、抑制和移去机理基础上,积极研发抑制空间电荷积聚的XLPE绝缘料,作为关键技术突破,用于开发直流XLPE绝缘海底电缆,推进我国直流海底电力电缆的技术发展。