ML11349A091
| ML11349A091 | |
| Person / Time | |
|---|---|
| Site: | Indian Point  |
| Issue date: | 12/15/2011 |
| From: | Hampton N, Hartlein R, Lennartsson H, Orton H, Ramachandran R Dow Chemical Co, Neely Research Reactor, Institute of Electrical & Electronics Engineers |
| To: | Atomic Safety and Licensing Board Panel |
| SECY RAS | |
| References | |
| RAS 21545, 50-247-LR, 50-286-LR, ASLBP 07-858-03-LR-BD01 | |
| Download: ML11349A091 (6) | |
Text
NYS000144 Submitted: December 15, 2011 LONG-LiFE XLPE INSULATED POWER CABLE Nigel HAMPTON, NEETRAC, Georgia Tech, USA, nigel.hampton@neetrac.gatech.edu Rick HARTLEIN, NEETRAC, Georgia Tech, USA, rick.hartlein@neetrac.gatech.edu Hakan LENNARTSSON, Borouge Pty, Hong Kong, hakan.lennartsson@borouge.com Harry ORTON, OCEI, Vancouver, BC, Canada, h.orton@ieee.org Ram RAMACHANDRAN,The Dow Chemical Company, NJ, USA, ramachs@dow.com ABSTRACT microscopic cavities degraded the insulation over time, ultimately causing the cables to fail.
Crosslinked polyethylene (XLPE) has become the globally preferred insulation for power cables, both for distribution Today there are XLPE insulations that can be designed to and transmission system applications. This insulation inhibit the growth of water trees, allowing for even greater system provides cost efficiency in operation and reliability for distribution class cables. Semiconducting procurement, as well as lower environmental and screens that are free of excessive ionic contamination are maintenance requirements when compared to older also available. Manufacturers have also learned how to impregnated paper systems. produce cable with insulations that are free of voids and with smooth interfaces between the semiconducting screens and The purpose of this paper is to outline some of the the insulation.
developments that have led to this position. Understanding these developments will assist utilities to continue sourcing, Cables must also be specified, designed, manufactured, and installing, the reliable underground assets that they tested and installed such that the desired life is delivered. It require for the future. is clear that a high level of symbiosis is required by academics, cable manufacturers, compound suppliers and KEYWORDS utilities. This paper sets out to provide the foundations for this by identifying the critical developments and QUALITY, MEDIUM VOLTAGE, AGEING understanding. Many of the comments are relevant for all cable voltages (LV to EHV). However we will focus on the MV arena in this paper and address the higher voltage INTRODUCTION issues in a subsequent publication.
When medium voltage (MV) XLPE insulated cables were CABLE STRUCTURE AND MATERIALS first installed in the late 1960's, cable manufacturers and The structure of underground power cables appears electric utilities expected them to perform reliably for 20 or deceptively simple. However, each component has an even 30 years. History has shown that the service life of important purpose and must be selected carefully to assure some of these early cables was far shorter than expected. At that the composite cable structure will perform reliably in that time, cable engineers and material scientists were not service. The critical structural elements of underground aware that moisture, voltage stress, omitting jackets and power cables are discussed in the following sections.
imperfections within the cable structure would combine to accelerate the corrosion of neutral wires I tapes and cause 600..-------------------,
water trees. These defects degraded the cable performance so severely that many cables failed after only 10 to 15 years 500 in service.
400 The consequences of this lack of understanding were profound. It has been estimated that for every dollar that !& 300 utilities spent installing the cable, they had to spend at least
..:::o 10 dollars to replace it. Resources that could have been > 200 used to build new infrastructure were now diverted to replace cables that were less than 20 years old. This had an 100 impact on operating costs that electric utilities are still dealing with today [1].
1900 1925 1950 1975 2000 Engineers and scientists now know what went wrong. They Year discovered that voids and contamination in the insulation, combined with ionic contamination in the semiconducting Figure 1 Evolution of the Highest AC Cable Voltage shields, as well as other design and manufacturing Insulation materials used in MV power cables have long deficiencies, led to voltage stress concentrations within the included the mature technology of fluid-impregnated Kraft cables. These elevated voltage stresses, combined with paper. They have been successfully used for over 100 moisture ingress into the cable structure created what are years. Today, extruded crosslinked polymer insulations are known today as water trees. These dendritic growths of OAGI0001272_00001
the standard for all voltages (Figure 1). The service experience that led to the impact of crosslinked polymers on utility systems is provided in (Table 1). Crosslinked compounds provide a better reliability and higher operating temperature that the thermoplastic (un crosslinked) analogues. Thermoplastic materials will deform upon subsequent heating, whereas thermoset materials will tend to maintain their form at operating temperatures. This experience coupled with the interest in ever higher operating temperatures mean that this preference for crosslinked solutions will endure for the foreseeable future.
Table 1. MV Cable Service Failures in Europe (median failures/100 circuit. km/yr) - UNIPEOE 1995 Figure 3 Water Trees Growing from the Inner (bottom) and Type 10 20 30 Outer (top) Semiconductive Screens kV kV kV Two approaches to insulation technology are in widespread XLPE use to retard the growth of water trees and each is a 0.2 0.4 2.0 1979 -1994 modification of the classic XLPE materials. These are:
Crosslinked EPR
- Modification of the polymer structure, "Polymer" WTR-2.3 1.4 2.0 1979 -1994 XLPE; sometimes termed copolymer - modified XLPE LOPE
- Modification of the additive package, "Additive" WTR-1.5 3.5 4.5 1979 -1989 XLPE; sometimes termed TR-XLPE The rmo plastic PVC In both instances, the compounds maintain the excellent 5.0 3.5 16.0 1979 -1989 electrical properties of XLPE (high dielectric strength and very low dielectric losses). WTR-XPLE insulations were commercialized in the early 1980's and have now been XLPE INSULATION performing reliably in service for over 20 years [3-7].
XLPE is a thermoset material produced by the compounding of LOPE with a crosslin king agent such as dicumyl peroxide. Productivity AI Gilbert and Frank Precopio invented XLPE in March In addition to the two basic technologies for retarding water 1963 in the GE Research Laboratory located in Niskayuna, tree growth a number of modifications in the basic polymer New York [2]. In this process, the long-chain PE molecules structure can be made to maximize productivity during the "crosslink" during a curing (vulcanization) process to form a cable manufacturing processes. In MV applications, the material that has electrical characteristics that are similar to reactivity can be boosted significantly. This results in higher thermoplastic PE, but with better mechanical properties, line speeds in the cases where there are limitations in either particularly at high temperatures. XLPE-insulated cables the curing or cooling processes within the continuous have a rated maximum conductor temperature of 90°C and vulcanization (CV) tubes used to crosslink the insulation.
an emergency rating of up to 140°C. XLPE insulations can also be modified to limit the amount of by-product gases that are generated during the crosslinking Water Tree Retardant XLPE (WTR XLPE) process. This is particularly useful for HV and EHV cable As noted earlier, the phenomenon of water treeing can applications, where degassing requirements can significantly reduce the service life of XLPE cables. Typical water trees lengthen the time required to manufacture the cable.
are shown in Figure 3. Water trees grow relatively slowly over a period of months or years. As they grow, the INSULATION CURING PROCESSES electrical stress can increase to the point that an electrical The crosslinking process begins with a carefully tree is generated at the tip of the water tree [1,3-6]. Once manufactured base polymer. A stabilizing package and initiated, electrical trees grow rapidly until the insulation is crosslin king package are then added to the polymer in a weakened to the point that it can no longer withstand the controlled mannerto form the compound. Crosslinking adds applied voltage and an electrical fault occurs at the tie points into the structure. Once crosslinked, the polymer water/electrical tree location. Many actions can be taken to chains retain flexibility but cannot be completely separated, reduce water tree growth, but the approach that has been for example, transformed into a free-flowing melt. There are most widely adopted is the use of specially engineered essentially two types of crosslin king processes that can be insulating materials designed to limit water tree growth. used for XLPE-insulated power cables:
These insulation materials are called WTR-XLPE. These insulation materials, combined with the use of clean semicon Peroxide cure - thermal decomposition of organic peroxide shields and sound manufacturing processes have dispelled after extrusion initiates the formation of crosslinks between the concems that many utilities had regarding the use of the molten polymer chains in the curing tube. This process cables with a polymeric insulation. can be used for XLPE or EPR insulations. The peroxide cure method is the most widely used crosslin king technology globally and is used to manufacture MV, HV and EHV insulated cables. The moisture-cure approach is almost universally used for making LV cables and is sometimes used to manufacture MV cables.
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furnace black was 0.73 percent Today, a carbon black with Moisture cure - chemical (silane) species are inserted onto 0.01 percent ash content is available. Similarly, the total the polymer chain, these species form crosslinks when sulphur content has been reduced from 1.26 % to 0.01 %,
exposed to water. The curing process occurs in the solid while over the same period, the compound smoothness 2
phase, after extrusion. Moisture curing is most often based upon a contaminant count, has gone from 90 pip/cm 2
preferred for the manufacture of MV cables when many to 15 pip/cm However this improvement is not universal different cable designs are made on the same extrusion line and cannot be taken for granted. Table 2 shows the range of and/or when manufacturing lengths are relatively short. In cleanliness levels these situations, the separation of the extrusion and curing processes is attractive from a production standpoint. FREEDOM FROM DEFECTS - CLEANLINESS
& SMOOTHNESS CONDUCTOR AND INSULATION SCREEN The critical importance of cleanliness (of both the insulation COMPOUNDS and the semiconducting screens) and smoothness Semiconducting screens (sometimes called semicons or (insulation screen interface) has been a hard learned lesson semiconducting shields) are extruded over the conductor (Figure 2) [1, 6, 8, 9]. Improved cleanliness and interface and the insulation outer surface to maintain a uniformly smoothness increases operating stresses (important for HV divergent electric field, and to contain the electric field within & EHV) and delivered life. The cleanliness of all cable the cable core. These materials contain specially materials has improved significantly over the last 15 years.
engineered grades of carbon black to attain the correct level Cleaner raw materials, improved manufacturing technology, of stable conductivity for the cable semicon or screens. and handling techniques have all contributed to enhanced cleanliness. Out of these many initiatives, new generations Semiconducting screening materials are based on carbon of XLPE and WTR-XLPE materials have emerged. These black (manufactured by the complete and controlled are generally supplied with designations that define the combustion of hydrocarbons) that is dispersed within a cleanliness and voltage use levels.
polymer matrix. The concentration of carbon black needs to be sufficiently high to ensure an adequate and consistent conductivity. The incorporation must be optimized to provide a smooth interface between the conducting and insulating portions of the cable. The smooth surface is important as it decreases the occurrence of regions of high electrical stress. To provide the correct balance of these properties, it is essential that both the carbon black and polymer matrix be well engineered.
Table 2 Typical Impurity Analysis on Semiconductive Conductor Screen Compounds Manufactured with Selected Figure 2 Typical defects (contaminants - left & right, and Carbon Blacks - ICP data in ppm screen distortion - right) found in extruded cables Furnace Blacks Acetylene Cable manufacturers, in turn, have implemented material Blacks handling systems to prevent contamination during the Elements Low Standard High High course of manufacturing. One example is that clean rooms Quality Quality Quality Quality have been installed in most cable manufacturing plants and AI 15 5 6 3*
separate handling facilities for insulation and semiconductor Ca 160 3* 3* 3*
materials have been implemented.
Cr 2 3* 3* 3*
Fe 8 3* 3 3* Table 3 Relationship Between Voltage Class and the Ni 2 3* 3* 3* Generally Accepted Cleanliness Levels MQ 57 27 15 10* MV HV EHV S 3600 1900 100 3* 6 - 36 kV 36 -161 kV > 161 kV Si 47 10 4 3* Mean Electrical V 2 3* 3* 3* 2 6 10 Stress (kV/mm)
Zn 3* 3* 3* 3* Contaminants K 125 12 3* 3* 200 - 500 100 - 200 70 -100 Excluded (/.lm)
CI 105 13 11 3* Contaminants 100 - 200 70 -100 50 -70
- value at the detection limit of the ICP equipment. Controlled (/.lm)
It has long been recognised that the highest levels of The cleanliness of insulation materials (both peroxide and smoothness and cleanliness are achieved when Acetylene moisture cure) is often assessed by converting a based carbon black are used within the semiconducting representative sample of the polymer into a transparent matrix (Table 2) [8]. In recent years, fumace black chemical tape, then establishing the concentration of any impurities and ash content have been adjusted to achieve inhomogeneities. The inhomogeneities are detected by optimal levels required for semiconductive screen identifying variations in the transmission of light through the applications. In 1973 the ash content for a conventional tape. The data processing is carried out by a OAGI0001272_00003
microcomputer, which is able to produce size segregated concentration data for a number of selected levels of JACKETS obscuration [Table 3]. These cleaner XLPE insulation In most MV, HV and EHV cable applications, the metal materials lead to a much longer in-service life for cables. sheath/neutral is itself protected by a polymeric oversheath Utility acceptance of the cleaner compounds has been rapid or jacket. Due to the critical performance needed from the and widespread. oversheath, there are a number of properties that are required, such as good abrasion resistance, good CORE MANUFACTURE processability, reasonable moisture resistance properties, An extruded cable production line is a highly sophisticated and good stress cracking resistance. Experience has shown manufacturing process that must be run with great care to that the material with the best composite performance is a assure that the end product will perform reliably in service PE-based oversheath, though PVC, Chlorosulfonated for many years. It consists of many subprocesses that must Polyethylene and Nylon have been used as jacket materials.
work in concert with each other. If any part of the line fails to Tests on XLPE cables retrieved after 10 years of operation function properly, it can create problems that will lead to show that the mean breakdown strength falls by almost 50%
poorly made cable and will potentially generate many metres (from 20 to 11 kV/mm - HOPE & PVC, respectively) when of scrap cable [1]. PVC is used as a jacket material. Many utilities now specify robust PE based jackets as a result. The hardness of PE is 37.5.,..---------------------, also an advantage when protection is required from termite 35.0 damage.
EXTRLEION I 32.5 HEAD Jackets extend cable life by retarding the ingress of water
~ 30.0 TRIPLE and soluble ions from the ground, minimizing cable
~ 27.5 installation damage and mitigating neutral corrosion. Ninety-
~ three percent of investor-owned utilities in the USA specify a F:
- ...... protective jacket. The semiconductive jacket or oversheath c*****
....~.
is recommended for high lightning incident areas or joint-use trenches where telecommunications cables co-exist with
~ 20.0 15 kV Cables
... power cables.
Aged at 3 lb, 75C Conductor, SOC Water 150 200 300 400 500 600 The selection of the oversheath material and the cable llME(DAYS) design including water-swellable tapes or powders, has a strong influence on the water ingression rate from the Figure 3 Influence Of Extrusion Head Configuration on outside of the cable to the conductor. A comparison of the Cable Aging, As Measured By Breakdown Strength [7] physical properties of most of the most common jacket materials is given in Table 4.
The process begins when pellets of insulating and semiconducting compounds are melted within the extruder. Table 4 Physical Properties of Jacket Compounds.
The melt is pressurised and this conveys material to the Moisture Vapor crosshead where the respective cable layers are formed. Base Resin Hardness Transmission Between the end of the screw and the start of the crosshead Compound Density ATSM E 96 it is possible to place meshes or screens, which act as (g/cm 3) (Shore D)
(g/day/m2) filters. The purpose of these screens was, in the earliest days of cable extrusion, to remove particles, or LOPE 0.92 43 1.16 contaminants that might be present within the melt. While LLDPE 0.92 45-48 0.74 still used today, the clean characteristics oftoday's materials MOPE 0.93 53-54 0.51 minimize the need for this type of filter. In fact, if these HDPE-1 0.941 - 0.58 screens are too tight, they themselves can generate HDPE-2 0.948 57-61 0.32 contaminants in the form of scorch or precrosslinking. PVC 1.4-1.5 35-43 10 Nevertheless, appropriately sized (100 to 200-micron hole size) filters are helpful to stabilize the melt and protect the PRODUCTION TESTS cable from large foreign particles that most often enter from Production tests are conducted to assure that cables are the materials handling system. good quality and made according to required specifications.
Cable manufacturers conduct these tests before the cable The most current technology uses a method called a true leaves the factory. Most of the widely used cable standards triple extrusion process where the conductor shield, [10,11] include production test procedures and insulation and insulation shield are coextruded requirements. However, it should be recognized that these simultaneously. The cables produced in this way have been tests represent the minimum requirements. Experienced shown to have better longevity (Figure 3) [7]. cable makers will very often complement these minimum requirements with extra or extended tests (Figure 4) to After the structure of the core is formed the cable is provide additional assurance that the cable is well made.
crosslinked to impart the high temperature performance. When considering production test programs the frequency of When a CV tube is used fine control of the temperature and tests are equally as important as the tests themselves, residence time (Iinespeed) is required to ensure that the especially when the periodic nature of the typical defects are core is crosslinked to the correct level. considered.
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FIELD PERFORMANCE OF MV CABLES High quality cables are required to assure that cable systems deliver the required reliability. Therefore having addressed various ways and means of ensuring quality (consistency, design, materials), we briefly review some relevant information from utility experience.
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Test, When The Insulation Is Rendered Transparent. 05: I Production tests are vitally important and must be taken 2 3 10 seriously by the cable manufacturer and the user. These Breakdown Strength (lIo) tests are the last chance to assure that the cable is made Figure 5 AC Step (Uo/5min) Breakdown Tests On XLPE correctly and avoid the consequence of premature field Unjacketed Cables Identified As Poor Performers. Vertical failure. A user may specify high-quality, high-performance Dashed Lines Show The Weibull Scale Parameter.
materials for use in the manufacturing processes. However, if problems occur during manufacture the cable performance Figure 5 shows results from a study carried out in Sweden may be severely compromised, leading to high replacement and Norway, to assess the condition of unjacketed cables costs in the future. Some utilities require that a factory tests made with Classic XLP E [9]. Diagnostic tests showed that a are supplemented testing at independent laboratories. significant percentage of these cables had degraded dielectric characteristics. As a result, two 12 kV cables were Cable standards and specifications prepared by the IEC, removed from the Swedish network and subjected to ramp ANSIIICEA, JEC and CENELEC [10,11] include a variety of AC breakdown tests. The Wei bull characteristic AC production test requirements, as well as established long breakdown strength for these cables was between four and term aging test protocols for type or qualification of cables. five times Uo, the operating service voltage. When they Electrical production tests are performed on the entire were new, these cables had AC breakdown strengths production length, often by testing every shipping length. between 15 and 20 Uo. The breakdown strengths indicated that the cable insulations had deteriorated significantly as PLANT AUDITS dissection, showed trees bridging the whole of the insulation.
Cable users often find great value in visiting the It is interesting to note that 1980 vintage cable has a higher manufacturing plant that produces their cable. This confirms dielectric strength than the 1988 vintage cable. This is an the purchaser's genuine interest in purchasing and installing important observation which has been confirmed by many a high-quality cable. It provides the opportunity for the different utilities. Cables do not fail simply as a function of purchaser to provide feedback to the cable manufacturer. their age, but rather as function of their age, loading and The primary purpose is to better understand the complete quality.
manufacturing process and assure that the manufacturer is operating in the expected manner (conducting all required In Germany, a great deal of cable failure data has been tests and has organized, uniform procedures, and that the gathered in an attempt to understand cable performance.
plant is clean and well organized). Figure 6 shows the in-service cable (insulation) failure rate for Germany as a function of the year of installation [1,13].
QUALIFICATION TESTS Early designs used poor-quality extrusion technology, taped Qualification tests is a very large subject and their details semicon screens and XLPE insulations that were not nearly are the subject of many technical papers. Thus their detailed as clean as today's insulations. The data clearly shows the discussion is outside the scope of this paper [10 -13]. improvement in system reliability from the mid-1980's. This However they are the bedrock of high quality cables; they improvement has primarily come from the move to "Polymer" provide the proof that the cable complies to with the WTR-XLPE insulations.
requirements. Consequently it is important that a user statisfies themselves that cables are suitably qualified. Of equal importance is the need for users to verify that they remain valid in the light of the minor changes that can often occur in designs, manufacture, test methods and uses.
OAGI0001272_00005
Figure 6 & Figure 7 provide dramatic and practical examples of how cable performance has improved for each new
'j.."' 0.500 1.000 generation of cable, from the original thermoplastic cables to the Classic XLPE, to more modem WTR-XLPE cables.
!~
These performance improvements are responsible for the 0.100 current trend of installing cables with WTR-XLPE insulations
.! 0.050 in preference to Classic XLPE - shown by Figure 6 after "Polymer" WTR XLPE introduced from 1980
&! 1990.
E
.i!
CONCLUSIONS
...~ 0.010 a 0.005 Achieving a long cable life using XLPE compounds is not difficult. However it does require attention to details that 1970 1975 1980 1985 1990 1995 2000 may, on the surface, appearto bring little immediate benefit.
Year of Cable Installation The purpose of this paper is to outline some of the most Figure 6 In-service Cable (Insulation) Failure Rate for common practices that can help industry and electric utilities Germany as a Function of the Year of Installation [1,13]. obtain a cost- effective XLPE-insulated cable with long, reliable service life. The most critical element within the One element that is missing from Figure 6 and most whole process is the awareness of Quality Issues and its analyses in the literature is a representation of the ultimate value within the Utilities.
relationship to the amount of cables installed in each year.
Much more cable was installed in 1995 than in 1985. The REFERENCES importance of this concept can be seen in a USA study. At 1. Orton, H.E. and Hartlein R., "Long Life XLPE Insulated TXU Electric, a large utility located in Texas, an extensive Power Cables", October 2006.
analysis was conducted on their MV cables to understand 2. Precopio, F."The Invention of Chemically Cross Linked how they have performed [1,14]. These cables were Polyethylene". IEEE Electrical Insulation Magazine, installed in the 1970's and early 1980's. One method of January/February 1999, Vol. 15, Issue 1, pp. 23-25.
analyzing the data, which allows for the amount of cable 3. Campus, A. & Ulrich, M. (2003) 20 Years Experience installed as well as its age, is provided in Figure 7. In this with Copolymer Power Cable Insulation. Jicable03, graph, the x axis is the cumulative product of the amount of Paper B1.6, pp. 350-356.
cable installed in a given year and the number of years that 4. Person, T., Shattuck, G.,& Hartlein, R. (2003).
the cable was in service (l: cable length x cable age for each Evaluation of Tree Retardant XLPE- (TR-XLPE) and year) [1]. A constant gradient shows a constant failure rate, EPR-insulated 35 kV Cables after 17 Years of Field however a downward curvature (lower gradient) shows a service. Jicable03.
lower failure rate. 5. Banks V, Faremo H & Steenis EF; An accelerated ageing test on the basis of 500Hz for water treeing in 1000 Cables; Jicable95, pp 347 -- 356
~
- 6. Dissado LA & Fothergill JC Electrical Degradation and 500 j 7.
Breakdown in Polymers; lEE Materials and Devices.
Sarma, H., Cometa, E. & Densley, J. (2002,
'0
~
. 100 March/April). Accelerated Aging Tests on Polymeric
- i!
.~
.1i 50
-- 8.
Cables Using Water-filled Tanks - a Critical Review.
Electrical Ins Magazine, IEEE, Vol 18, No 2, pp 15-26.
Nilsson UH; The use of model cables for the evaluation S
...a 10 of the electrical performance of polymeric power cable materials; Nord-IS05 Trondheim pp 101-106 j 9. Holmgren, B. & Hvidsten S; Status of Condition III Assessment of Water Treed 12 & 24 kV XLPE Cables in Norway & Sweden,NORDIS05, pp 71-75.
- 10. CENELEC HD620; Distribution cables with extruded insulation for rated voltages from 3,6/6 (7,2)kV to Figure 7 Failure Data for Three generations of MV Cables 20,8/36 (42)kV Installed at TXU. 11. ICEA Publication S-97-682 (1999). Standard for Utility The figure is best understood by examining the km-year Power Cables Rated 5 through 46 kV.
value for a given number of failures for each cable design, 12. IEEE 1407-1998. Trial-use Guide for Accelerated Aging for example 30 failures occurred after only 9,000 km years Tests for Medium Voltage Extruded Electric Power for HMWPE. It took 22,000 km years for the XLPE cable to Cables using Water-filled Tanks.
experience 30 failures. Furthermore, the graph enables 13. Fischer M., (1997). VDEW Umfrage zu Schaden an predictions to be made, for instance it will likely take over VPE Kabeln. Elektrizitatswirtschaft, Jg 96 pp 1154-100,000 km years before 30 failures will occur on the 1158.
"Additive" WTR-XLPE cable. 14. Harp, R., & Smith III, J; Cable Failure Statistics &
Analysis at TXU Electric Delivery Co, IEEE Insulated Conductors Committee Education Session (2004, fall).
OAGI0001272_00006