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. Harry ORTON, OCEI, Vancouver, BC, Canada, h.orton@ieee
Ram RAMACHANDRAN,The Dow Chemical Company, NJ, USA,
ABSTRACT
Crosslinked polyethylene (XLPE) has become the globally preferred insulation for power cables, both for distribution and transmission system applications. This insulation system provides cost efficiency in
operation and procurement, as well as lower environmental and maintenance requirements when compared to older impregnated paper systems.
The purpo of this paper is to outline some of the developments that have led to this position. Understanding the developments will assist utilities to continue sourcing, and installing, the reliable underground asts that they require for the future.
KEYWORDS
QUALITY, MEDIUM VOLTAGE, AGEING巨蛙
INTRODUCTION
When medium voltage (MV) XLPE insulated cables were first installed in the late 1960’s, cable manufacturers and electric utilities expected them to perform reliably for 20 or even 30 years. History has shown that the rvice life of some of the early cables was far shorter than expected. At that time, cable engineers and material scientists were not aware that moisture, voltage stress, omitting jackets and imperfections within the cable structure would combine to accelerate the corrosion of neutral wires / tapes and cau water trees. The defects degraded the cable performance so verely that many cables failed after only 10 to 15 years in rvice.
The conquences of this lack of understanding were profound. It has been estimated that for every dollar that utilities spent installing the cable, they had to spend at least 10 dollars to replace it. Resources that could have been ud to build new infrastructure were now diverted to replace cables that were less than 20 years old. This had an impact on operating costs that electric utilities are still dealing with today [1].
Engineers and scientists now know what went wrong. They discovered that voids and contamination in the insulation, combined with ionic contamination in the miconducting shields, as well as other design and manufacturing deficiencies, led to voltage stress concentrations within the cables. The elevated voltage stress, combined with moisture ingress into the cable structure created what are
known today as water trees. The dendritic growths of
microscopic cavities degraded the insulation over time, ultimately causing the cables to fail.
Today there are XLPE insulations that can be designed to inhibit the growth of water trees, allowing for even greater reliability for distribution class cables. Semiconducting screens that are free of excessive ionic contamination are also available. Manufacturers have also learned how to produce cable with insulations that are free of voids and with smooth interfaces between the miconducting screens and the insulation.
Cables must also be specified, designed, manufactured, tested and installed such that the desired life is delivered. It is clear that a high level of symbiosis is required by academics, cable manufacturers, compound suppliers and utilities. This paper ts out to provide the foundations for this by identifying the critical developments and 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 issues in a subquent publication.
CABLE STRUCTURE AND MATERIALS
The structure of underground power cables appears deceptively simple. However, each component has an important purpo and must be lected carefully to assure that the composite cable structure will perform reliably in rvice. The critical structural elements of underground power cables are discusd in the following ctions.豹子怎么画
Figure 1 Evolution of the Highest AC Cable Voltage Insulation materials ud in MV power cables have long included the mature technology of fluid-impregnated Kraft paper. They have been successfully ud for over 100 years. Today, extruded crosslinked polymer insulations are
the standard for all voltages (Figure 1). The rvice 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 subquent heating, whereas thermot 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 foreeable future.
Table 1.MV Cable Service Failures in Europe (median failures/100 circuit. km/yr) – UNIPEDE 1995
Type 10 kV 20 kV 30
kV
XLPE
1979 – 1994 0.2 0.4 2.0
EPR
1979 – 1994 Crosslinked
2.3 1.4 2.0 LDPE
1979 – 1989 1.5 3.5 4.5
PVC
见利思义1979 – 1989 Thermoplastic
5.0 3.5 1
6.0
XLPE INSULATION
XLPE is a thermot material produced by the compounding
of LDPE with a crosslinking agent such as dicumyl peroxide. Al Gilbert and Frank Precopio invented XLPE in March 1963 in the GE Rearch Laboratory located in Niskayuna, New York [2]. In this process, the long-chain PE molecules “crosslink” during a curing (vulcanization) process to form a material that has electrical characteristics that are similar to thermoplastic PE, but with better mechanical properties, particularly at high temperatures. XLPE-insulated cables have a rated maximum conductor temperature of 90°C and
an emergency rating of up to 140°
C. Water Tree Retardant XLPE (WTR XLPE)
As noted earlier, the phenomenon of water treeing can reduce the rvice life of XLPE cables. Typical water trees are shown in Figure 3. Water trees grow relatively slowly over a period of months or years. As they grow, the electrical stress can increa to the point that an electrical tree is generated at the tip of the water tree [1,3-6]. Once initiated, electrical trees grow rapidly until the insu
lation is weakened to the point that it can no longer withstand the applied voltage and an electrical fault occurs at the water/electrical tree location. Many actions can be taken to
reduce water tree growth, but the approach that has been most widely adopted is the u of specially engineered insulating materials designed to limit water tree growth.
The insulation materials are called WTR-XLPE. The insulation materials, combined with the u of clean micon shields and sound manufacturing process have dispelled the concerns that many utilities had regarding the u of cables with a polymeric insulation.
Figure 3 Water Trees Growing from the Inner (bottom) and Outer (top) Semiconductive Screens
节寰袁公传Two approaches to insulation technology are in widespread u to retard the growth of water trees and each is a modification of the classic XLPE materials. The are: • Modification of the polymer structure, “Polymer” WTR-XLPE; sometimes termed copolymer - modified XLPE • Modification of the additive package, “Additive” WTR-XLPE; sometimes termed TR-XLPE
In both instances, the compounds maintain the excellent electrical properties of XLPE (high dielectric strength and very low dielectric loss). WTR-XPLE insulations were commercialized in the early 1980’s and have now been performing reliably in rvice for over 20 years [3-7].
Productivity
In addition to the two basic technologies for retarding water tree growth a number of modifications in the basic polymer structure can be made to maximize productivity during the cable manufacturing process. In MV applications, the reactivity can be boosted significantly. This results in higher line speeds in the cas where there are limitations in either the curing or cooling process within the continuous vulcanization (CV) tubes ud to crosslink the insulation. XLPE insulations can also be modified to limit the amount of by-product gas that are generated during the crosslinking process. This is particularly uful for HV and EHV cable applications, where degassing requirements can significantly lengthen the time required to manufacture the cable.
INSULATION CURING PROCESSES
The crosslinking process begins with a carefully manufactured ba polymer. A stabilizing package and crosslinking package are then added to the polymer in a controlled manner to form the compound. Crosslinking adds tie points into the structure. Once crosslinked, the polymer chains retain flexibility but cannot be completely parated, for example, transformed into a free-flowing melt. There are esntially two types of crosslinking process that can be ud for XLPE-insulated power cables:
Peroxide cure – thermal decomposition of organic peroxide after extrusion initiates the formation of crosslinks between the molten polymer chains in the curing tube. This process can be ud for XLPE or EPR insulations. The peroxide cure method is the most widely ud crosslinking technology globally and is ud to manufacture MV, HV and EHV insulated cables. The moisture-cure approach is almost universally ud for making LV cables and is sometimes ud to manufacture MV cables.
Moisture cure – chemical (silane) species are inrted onto the polymer chain, the species form crosslinks when expod to water. The curing process occurs in the solid pha, after extrusion. Moisture curing is most often preferred for the manufacture of MV cables when many different cable designs are made on the same extrusion line and/or when manufacturing lengths are relatively short. In the situations, the paration of the extrusion and curing process is attractive from a production standpoint.
CONDUCTOR AND INSULATION SCREEN COMPOUNDS
Semiconducting screens (sometimes called micons or miconducting shields) are extruded over the conductor and the insulation outer surface to maintain a uniformly divergent electric field, and to contain the electric field within the cable core. The materials contain specially engineered grades of carbon black to attain the correct level of stable conductivity for the cable micon or screens.
Semiconducting screening materials are bad on carbon black (manufactured by the complete and controlled combustion of hydrocarbons) that is disperd within a 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 decreas the occurrence of regions of high electrical stress. To provide the correct balance of the properties, it is esntial that both the carbon black and polymer matrix be well engineered.
Table 2 Typical Impurity Analysis on Semiconductive
Conductor Screen Compounds Manufactured with Selected Carbon Blacks - ICP data in ppm
Furnace Blacks
Acetylene Blacks
Elements Low Quality Standard
Quality
High Quality
High Quality
Al 15 5 6 3*
Ca 160 3* 3* 3* Cr 2 3* 3* 3* Fe 8 3* 3 3* Ni 2 3* 3* 3* Mg 57 27 15 10* S 3600 1900 100 3* Si 47 10 4 3* V 2 3*
3* 3* Zn 3* 3* 3* 3* K 125 12 3* 3* Cl 105 13 11 3* * value at the detection limit of the ICP equipment.
It has long been recognid that the highest levels of smoothness and cleanliness are achieved when Acetylene bad carbon black are ud within the miconducting matrix (Table 2) [8]. In recent years, furnace black chemical impurities and ash content have been adjusted to achieve optimal levels required for miconductive screen applications. In 1973 the ash content for a conventional
furnace black was 0.73 percent Today, a carbon black with 0.01 percent ash content is available. Similarly, the total sulphur content has been reduced from 1.26 % to 0.01 %, while over the same period, the compound smoothness花茶的冲泡方法
bad upon a contaminant count, has gone from 90 pip/cm 2
to 15 pip/cm 2
. However this improvement is not universal and cannot be taken for granted. Table 2 shows the range of cleanliness levels
金蛙FREEDOM FROM DEFECTS – CLEANLINESS & SMOOTHNESS
The critical importance of cleanliness (of both the insulation and the miconducting screens) and smoothness (insulation screen interface) has been a hard learned lesson (Figure 2) [1, 6, 8, 9]. Improved cleanliness and interface smoothness increas operating stress (important for HV & EHV) and delivered life. The cleanliness of all cable materials has improved significantly over the last 15 years. Cleaner raw materials, improved manufacturing technology, and handling techniques have all contributed to enhanced cleanliness. Out of the many initiatives, new generations of XLPE and WTR-XLPE materials have emerged. The are generally supplied with designations that define the cleanliness and voltage u levels.
Figure 2 Typical defects (contaminants – left & right, and screen distortion – right) found in extruded cables
Cable manufacturers, in turn, have implemented material handling systems to prevent contamination during the cour of manufacturing. One example is that clean rooms have been installed in most cable manufacturing plants and parate handling facilities for insulation and miconductor materials have been implemented.表格统计
Table 3 Relationship Between Voltage Class and the Generally Accepted Cleanliness Levels
MV
6 – 36 kV
HV
36 – 161 kV
EHV
> 161 kV
Mean Electrical
Stress (kV/mm) 2 6 10
Contaminants
Excluded (µm) 200 – 500 100 – 200 70 – 100
Contaminants
Controlled (µm) 100 – 200 70 – 100 50 – 70
The cleanliness of insulation materials (both peroxide and moisture cure) is often assd by converting a reprentative sample of the polymer into a transparent tape, then establishing the concentration of any inhomogeneities. The inhomogeneities are detected by identifying variations in the transmission of light through the tape. The data processing is carried out by a
microcomputer, which is able to produce size gregated concentration data for a number of lected levels of obscuration [Table 3]. The cleaner XLPE insulation materials lead to a much longer in-rvice life for cables. Utility acceptance of the cleaner compounds has been rapid and widespread.
CORE MANUFACTURE
An extruded cable production line is a highly sophisticated manufacturing process that must be run with great care to assure that the end product will perform reliably in rvice for many years. It consists of many subprocess that must work in concert with each other. If any part of the line fails to function properly, it can create problems that will lead to poorly made cable and will potentially generate many metres
of scrap cable [1].
Figure 3 Influence Of Extrusion Head Configuration on Cable Aging, As Measured By Breakdown Strength [7]
安妮宝贝作品The process begins when pellets of insulating and miconducting compounds are melted within the extruder. The melt is pressurid and this conveys material to the crosshead where the respective cable layers are formed. Between the end of the screw and the start of the crosshead
it is possible to place meshes or screens, which act as filters. The purpo of the screens was, in the earliest days of cable extrusion, to remove particles, or contaminants that might be prent within the melt. While still ud today, the clean characteristics of today’s materials minimize the need for this type of filter. In fact, if the screens are too tight, they themlves can generate contaminants in the form of scorch or precrosslinking. Nevertheless, appropriately sized (100 to 200-micron hole size) filters are helpful to stabilize the melt and protect the cable from large foreign particles that most often enter from the materials handling system.
The most current technology us a method called a true triple extrusion process where the conductor shield, insulation and insulation shield are coextruded simultaneously. The cables produced in this way have been shown to have better longevity (Figure 3) [7].
After the structure of the core is formed the cable is crosslinked to impart the high temperature performance. When a CV tube is ud fine control of the temperature and residence time (linespeed) is required to ensure that the core is crosslinked to the correct level. JACKETS
In most MV, HV and EHV cable applications, the metal sheath/neutral is itlf protected by a polymeric oversheath or jacket. Due to the critical performance needed from the oversheath, there are a number of properties that are required, such as good abrasion resistance, good processability, reasonable moisture resistance properties, and good stress cracking resistance. Experience has shown that the material with the best composite performance is a PE-bad oversheath, though PVC, Chlorosulfonated Polyethylene and Nylon have been ud as jacket materials. Tests on XLPE cables retrieved after 10 years of operation show that the mean breakdown strength falls by almost 50% (from 20 to 11 kV/mm – HDPE & PVC, respectively) when PVC is ud as a jacket material. Many utilities now specify robust PE bad jackets as a result. The hardness of PE is also an advantage when protection is required from termite damage.
Jackets extend cable life by retarding the ingress of water and soluble ions from the ground, minimizing cable installation damage and mitigating neutral corrosion. Ninety-three percent of investor-owned utilities in the USA specify a protective jacket. The miconductive jacket or oversheath
is recommended for high lightning incident areas or joint-u trenches where telecommunications cables co-exist with power cables.
The lection of the oversheath material and the cable design including water-swellable tapes or powders, has a strong influence on the water ingression rate from the outside of the cable to the conductor. A comparison of the physical properties of most of the most common jacket materials is given in Table 4.
Table 4 Physical Properties of Jacket Compounds. Compound
Ba Resin
Density
(g/cm3)
Hardness
(Shore D)
Moisture Vapor
Transmission
ATSM E 96
(g/day/m2) LDPE 0.92 43 1.16 LLDPE 0.92 45-48 0.74 MDPE 0.93 53-54 0.51 HDPE-1 0.941 - 0.58 HDPE-2 0.948 57-61 0.32 PVC 1.4-1.5 35-43 10 PRODUCTION TESTS
Production tests are conducted to assure that cables are good quality and made according to required specifications. Cable manufacturers conduct the tests before the cable leaves the factory. Most of the widely ud cable standards [10,11] include production test procedures and requirements. However, it should be recognized that the tests reprent the minimum requirements. Experienced cable makers will very often complement the minimum requirements with extra or extended tests (Figure 4) to provide additional assurance that the cable is well made. When considering production test programs the frequency of tests are equally as important as the tests themlves, especially when the periodic nature of the typical defects are considered.
Figure 4 Conductor Shield Defect Revealed In A ”Hot Oil” Test, When The Insulation Is Rendered Transparent. Production tests are vitally important and must be taken riously by the cable manufacturer and the ur. The tests are the last chance to assure that the cable is made correctly and avoid the conquence of premature field failure. A ur may specify high-quality, high-
performance materials for u in the manufacturing process. However, if problems occur during manufacture the cable performance may be verely compromid, leading to high replacement costs in the future. Some utilities require that a factory tests are supplemented testing at independent laboratories. Cable standards and specifications prepared by the IEC, ANSI/ICEA, JEC and CENELEC [10,11] include a variety of production test requirements, as well as established long term aging test protocols for type or qualification of cables. Electrical production tests are performed on the entire production length, often by testing every shipping length. PLANT AUDITS
Cable urs often find great value in visiting the manufacturing plant that produces their cable. This confirms the purchar’s genuine interest in purchasing and installing a high-quality cable. It provides the opportunity for the purchar to provide feedback to the cable manufacturer. The primary purpo is to better understand the complete manufacturing process and assure that the manufacturer is operating in the expected manner (conducting all required tests and has organized, uniform procedures, and that the plant is clean and well organized).
QUALIFICATION TESTS
Qualification tests is a very large subject and their details are the subject of many technical papers.
Thus their detailed discussion is outside the scope of this paper [10 -13]. However they are the bedrock of high quality cables ; they provide the proof that the cable complies to with the requirements. Conquently it is important that a ur statisfies themlves that cables are suitably qualified. Of equal importance is the need for urs to verify that they remain valid in the light of the minor changes that can often occur in designs, manufacture, test methods and us. FIELD PERFORMANCE OF MV CABLES
High quality cables are required to assure that cable systems deliver the required reliability. Therefore having addresd various ways and means of ensuring quality (consistency, design, materials), we briefly review some relevant information from utility experience.
Unjacketed Cables Identified As Poor Performers. Vertical Dashed Lines Show The Weibull Scale Parameter. Figure 5 shows results from a study carried out in Sweden and Norway, to asss the c
ondition of unjacketed cables made with Classic XLPE [9]. Diagnostic tests showed that a significant percentage of the cables had degraded dielectric characteristics. As a result, two 12 kV cables were removed from the Swedish network and subjected to ramp AC breakdown tests. The Weibull characteristic AC breakdown strength for the cables was between four and five times Uo, the operating rvice voltage. When they were new, the cables had AC breakdown strengths between 15 and 20 Uo. The breakdown strengths indicated that the cable insulations had deteriorated significantly as disction, showed trees bridging the whole of the insulation. It is interesting to note that 1980 vintage cable has a higher dielectric strength than the 1988 vintage cable. This is an important obrvation which has been confirmed by many different utilities. Cables do not fail simply as a function of their age, but rather as function of their age, loading and quality.
In Germany, a great deal of cable failure data has been gathered in an attempt to understand cable performance. Figure 6 shows the in-rvice cable (insulation) failure rate for Germany as a function of the year of installation [1,13]. Early designs ud poor-quality extrusion technology, taped micon screens and XLPE insulations that were not nearly as clean as today’s insulations. The data clearly shows the improvement in system reliability from the mid-1980’s. This improvement has primarily come from the move to “Polymer” WTR-XLPE insulations.
