In the mid 1980s, plans were underway to construct one of the largest shopping malls in the southeastern United States, just south of Birmingham, Alabama, in an area that Alabama Power designated for the extension of its newly developing 35-kV service area. The Galleria Mall was to encompass 2 million sq ft (185,806 sq meters) of retail space and have an estimated demand of approximately 20 MVA.
About this time, a modified cross-linked polyethylene (XLPE) cable insulation containing a tree-retardant additive (TR-XLPE) was introduced to the market. This new cable was expected to increase cable reliability, compared with standard XLPE that had been in use for many years. In addition, a new cable construction was introduced that used a viscous mastic-like material that was extruded within the interstices of the conductor strands to block water migration within the conductor. Seeking to provide the highest level of reliability and cable longevity for the mall, Alabama Power specified the use of this TR-XLPE cable design.
Since there were concerns about the possibility of excessive shrink back on the TR-XLPE cables, company engineers decided to use Ethylene Propylene Rubber (EPR)-insulated cables in half of the circuits serving the mall, based on many years of good experience with EPR cables. With this approach, Alabama Power had a system designed to allow either the TR-XLPE cables or the EPR cables to serve the mall if an emergency developed.
Now, after 17 years of service with no failures of either cable, the utility was interested in determining the remaining reliability in each cable type. Of secondary interest was to determine the performance of the TR-XLPE cable compared with the EPR cable. The Southern Co. Research Committee and the Dow Chemical Co. formed a partnership to find the answers. Southern Co. is the parent company of Alabama Power and Union Carbide, recently purchased by Dow Chemical, was the manufacturer of the TR-XLPE cable compound.
In the early 1970s, when the Association of Edison Illuminating Companies (AEIC; Birmingham) promulgated its specifications for electric utility cable, high on its list were the specifications for extruded medium-voltage insulated cable. These specs required all new cable designs to pass long-term tests to ensure reliability and longevity. The tests included high-voltage withstand, dissipation factor, thermal and mechanical characteristics, partial discharge and wet accelerated aging. These tests were important at a time when new materials and manufacturing processes were being introduced to the market and underground cable installations were rapidly increasing. By testing cables removed from service, engineers at Alabama Power could see if the cables that met the AEIC requirements really were performing well in service.
The testing project commenced with the removal of about 400 ft (122 m) of each cable type. The 1/0 AWG, three-phase, 35-kV cables with copper conductor were installed in a conduit manhole system serving the mall. The cables were wound on wooden reels and shipped to the Georgia Tech National Electric Energy Testing and Research & Application Center (NEETRAC) in Atlanta for testing.
Treeing Analysis. Water trees are a form of cable insulation degradation characterized by microchannels that develop as a tree-like structure in the insulation as a consequence of the interaction of water, electrical stress, impurities and manufacturing imperfections. Since water trees can degrade cable insulation over time, it was important to determine the extent of tree growth in these service-aged cables. The density of bow-tie trees was measured in size categories of 2 to 10 mils, 11 to 20 mils and 21 to 30 mils. The largest bow-tie trees detected for each material were 12 mils for TR-XLPE and 26 mils for EPR. The largest vented trees observed were 2 mils for TR-XLPE and 42 mils for EPR. Note that the 42-mil vented tree was found at a failure site in the EPR after an ac breakdown test.
Tree Retardant Additive Analysis. The TR-XLPE contains an additive to the base polymer, which was examined to ensure that the additive remained uniform across the insulation thickness. The tests confirmed that the TR additive was constant after 17 years of field service and was within the expected range for new cable. The results indicated that the additive does not migrate out of the insulation under normal-usage conditions.
Moisture Analysis was performed at 220°C (428°F) for 10 minutes on samples taken near the conductor shield and near the insulation shield. The average values indicated that moisture content in the EPR was significantly greater than the TR-XLPE due to the presence of the filler used in the EPR compound.
Stripping Tension. Good cable performance depends on good adhesion between insulation and the insulation shield to prevent voids between the two layers, where partial discharges could develop. The results of the tests indicated that stripping tension for both cables was comparable to that for new cables.
Dissipation factor, a measure of electrical losses, was an important parameter to determine if changes were occurring that would indicate dielectric instability in the insulation compounds. Measurements were made at applied voltages of 20, 40 and 50 kV over a temperature range of ambient to 90°C (194°F). The TR-XLPE cable exhibited a dissipation factor below 0.1% at all measured temperatures, and the EPR displayed a dissipation factor of about 0.4% at ambient temperature, increasing to above 0.7% as temperature was increased to 90°C.
Volume Resistivity of the Shields. Since the conductor and insulation shields must maintain a minimum level of conductance to ensure uniform voltage stress distribution at the interfaces, volume resistivity was measured at ambient temperature and at 45°C (113°F). While the shields for both materials were well below the commonly specified limits in the range of 500 to 1000 ohm-m, the EPR cable, which employed EPR-based semiconductive compounds, was more conductive than the XLPE semiconductive compounds.
Impulse Breakdown. Impulses due to lightning endanger the cable if its impulse-breakdown characteristics are marginal. For this reason, it is important to determine these characteristics in assessing the cable's projected longevity. These tests were made on five cable samples from each cable using the AEIC impulse test procedure. A log-normal distribution function was found to be the best fit for describing the failure data. The TR-XLPE demonstrated a higher impulse breakdown strength than EPR, as illustrated by the statistical differentiation that showed an absence of overlap of the 90% confidence intervals at the 50% characteristic level.
AC Breakdown. Five samples from each of the cables were subjected to a standard AEIC ac breakdown test using five-minute time steps. A two-parameter Weibull distribution function was the best fit for the failure stresses, revealing that the TR-XLPE had a higher ac breakdown strength than the EPR. It should be noted that the 380 V/mil breakdown stress from one of the TR-XLPE samples was treated as a “suspension” in the analysis. Failure site examination revealed that this particular sample contained a manufacturing defect, which resulted in incomplete coverage of the conductor by the conductor shield. However, the cable had performed well for 17 years in service with no failures.
Published Data Comparisons
Although long-term field aging has not been systematically pursued by investigators, cable tests were made in the mid- and late-1990s on 35-kV cables insulated with 345 mils of the same TR-XLPE and EPR compounds after nine years of field service.
The results of this work suggested that the impulse breakdown strength of EPR was greater than the TR-XLPE, which contrasted with results discussed above where TR-XLPE showed superior results. Both sets of data, however, revealed that the impulse breakdown strength of both compounds exhibited good relative stability, as did their ac breakdown strength.
Confirmations and Conclusions
The present study on field-aged 35-kV cables after 17 years of service supports the expectation that either cable will provide service life greater than 40 years. Overall, the TR-XLPE cables exhibited higher impulse breakdown strength and higher ac breakdown strength than the EPR cables.
A comparison to other field-aged cable evaluations at shorter aging times indicated that both cables exhibited good stability, relative to breakdown strength. Water-treeing analysis indicated that longer bow-tie trees and vented trees were present within the EPR than in TR-XLPE indicating that the TR additive in the TR-XLPE insulation continues to perform as designed. The dissipation factor for the TR-XLPE material was between four and seven times lower than that of the EPR. While there are some differences in dielectric strength, in dielectric losses and in water tree growth, both insulation materials can be used with the assurance of long cable life.
The authors would like to acknowledge the assistance of Dr. Timothy Person, Research Specialist with the research and development group of the Wire & Cable Compound department of the Dow Chemical Co., who assisted in the analysis of the data and in the writing of this article.
G. Bruce Shattuck graduated from the University of Alabama in 1971 with a BSEE degree. He began his career as a student engineer with the company in 1969, progressing through several levels of responsibility to his present position as principal engineer in the Power Delivery-Distribution Engineering Services Group.
Rick Hartlein spent 25 years at the Georgia Power Research Center evaluating transmission and distribution materials, developing specifications and industry standards, managing research and testing programs and providing engineering services. He joined Georgia Tech in 1996 to help establish NEETRAC, where he has been Underground Systems Program Manager. Hartlein is a past chair of the IEEE Insulator Conductors Committee and serves as a technical consultant to the AEIC Cable Engineering Committee.