Still Going Strong: New Study Highlights Field Performance of Vintage TR-XLPE Cable

Electricity Today
Submitted January 27, 2004
By R. Reed and S. Ramachandran

A recent U.S. study on the field performance of early generation tree retardant crosslinked polyethylene (TR-XLPE) medium voltage (MV) power cables has provided some critical insights for utilities across North America, particularly those assessing underground distribution strategies for the first half of the 21st century.

In 1983, Dow Wire & Cable introduced HFDA-4202 NT EC as the first commercial TR-XLPE insulation specifically designed to retard water-tree growth while retaining the excellent electrical and physical properties of XLPE. Over the last 20 years, HFDA-4202 NT EC and the more recently introduced HFDB-4202 NT EC have become the leading MV cable insulation in the USA and Canada. Utilities began installing TR-XLPE in the mid-1980s expecting improved performance based on increased reliability and greater longevity than standard XLPE cables or cables insulated with ethylene propylene rubber (EPR). TR-XLPE is designed to resist water trees, which can be a significant cause of cable insulation degradation and affect the overall performance of the cable.

Despite the dominance of TR-XLPE in North America, very few comprehensive field-aging studies have been conducted on the performance of vintage TR-XLPE. This lends additional significance to a study by Alabama Power on 17-year-old TR-XLPE cable, insulated with Dow Wire & Cable’s HFDA-4202 NT EC TR power cable insulation compound. Seeking to evaluate the cable and assess its projected service life, while also comparing it to similarly aged EPR cable, Alabama Power removed about 1,200 feet of 35 kV TR-XLPE cable from a system serving The Galleria Mall in Birmingham, Alabama. The cable was then delivered to engineers at the Georgia Tech National Electric Energy Testing and Research & Application Center (NEETRAC) in Atlanta for extensive performance testing.

The range of tests performed on the TR-XLPE and EPR cables included treeing analysis, TR additive analysis, moisture analysis, stripping tension, dissipation factor, impulse breakdown, and AC breakdown.

Treeing Analysis

Tests were conducted to measure the extent of tree growth in both cable types. After 17 years of field operation, the cable insulated with HFDA-4202 NT EC came out favorably, with the longest bowtie trees detected being only 12 mils in length compared to trees of 26 mils for the EPR cable. Similarly, while there were no vented trees longer than 10 mils for the TR-XLPE cable, the EPR cable had a vented tree of 42 mils. These tests validated the TR characteristics of the HFDA-4202 NT EC compound manufactured by Dow Wire & Cable.

TR Additive Analysis

The uniformity of the TR additive in the base polymer was measured across the thickness of the cable insulation. Tests showed that the TR additive was indeed uniform across the insulation thickness and within the expected range for comparable new cable. This analysis demonstrated that the additive did not migrate out of the insulation or have non-uniform concentration during the 17 years that the cable was underground.

Moisture Analysis

Tests were conducted to determine the cables’ moisture content. Average values for the two cable types showed significantly greater moisture content for the EPR cable, a result that was expected due to the filler used in the EPR compound.

Stripping Tension

Problems for cables occur when adhesion between insulation and the insulation shield fails, leading to the development of voids between the two layers and the increased likelihood of partial discharges. Tests on the two cable types showed that stripping tension for both was comparable to new cable.

Dissipation Factor

The dissipation factor is a measure of electrical losses through dielectric instability in insulation compounds. The cable types were measured for losses at applied voltages of 20, 40, and 50 kV over a temperature range of ambient to 90ºC (194ºF). At each temperature the cable insulated with HFDA-4202 NT EC showed a dissipation factor of less than 0.1%, compared to the EPR cable which showed a dissipation factor of 0.4% at ambient temperature rising to over 0.7% at 90ºC.

Impulse Breakdown

MV cable must be able to withstand impulses from a lightning strike so it cannot exhibit marginal impulse breakdown characteristics. A cable damaged by lightning would have severely reduced longevity. In the Alabama Power study, when five TR-XLPE and EPR cable samples were subjected to The Association of Edison Illuminating Companies impulse test, the samples of cable made with HFDA-4202 NT EC showed higher impulse strength than did the EPR samples.

AC Breakdown

Five TR-XLPE and EPR cable samples were also tested for AC breakdown using five-minute time steps. These tests showed that the TR-XLPE samples had a higher AC breakdown strength than the EPR samples. This is a significant finding, in that after 17 years of continuous field operation the cable made with HFDA-4202 NT EC had very high electrical strength, attesting to its expected long life.

The Big Picture

After nearly 20 years of field service for Alabama Power, the TR-XLPE cables constructed with HFDA-4202 NT EC insulation compound manufactured by Dow Wire & Cable were found to be still providing excellent field performance. They showed essentially no loss in dielectric strength, no bowtie trees larger than 12 mils, and no vented trees larger than 10 mils. The TR additive level was maintained uniformly across the insulation thickness and the cables showed a stripping tension similar to that of fresh cable. This study should give utilities the confidence to expect a service life that could potentially be greater than 40 years for TR-XLPE cable made with insulation compounds manufactured by Dow Wire & Cable.

The Next Generation

The footnote to the Alabama Power study is that Dow Wire & Cable introduced a next-generation insulation material for TR-XLPE cables called HFDB-4202 NT EC in 1998. Offering all the benefits of its predecessor (HFDA-4202 NT EC), including its excellent dielectric properties, HFDB-4202 NT EC is formulated to minimize water tree growth and provides significant new performance advantages over other XLPE and TR-XLPE alternatives. Given the outcome of the Alabama Power tests, HFDB-4202 NT EC is arguably poised to become the market standard insulation compound for enhancing cable life in the 21st century, even in the most challenging environments.

About the Authors:
R. Reed is end-use marketing manager, and S. Ramachandran is senior program manager at Dow Wire & Cable, a global market-facing business unit of The Dow Chemical Company.


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Reed and S. Ramachandran A recent U.S. study on the field performance of early generation tree retardant crosslinked polyethylene (TR-XLPE) medium voltage (MV) power cables has provided some critical insights for utilities across North America, particularly those assessing underground distribution strategies for the first half of the 21st century. In 1983, Dow Wire & Cable introduced HFDA-4202 NT EC as the first commercial TR-XLPE insulation specifically designed to retard water-tree growth while retaining the excellent electrical and physical properties of XLPE. Over the last 20 years, HFDA-4202 NT EC and the more recently introduced HFDB-4202 NT EC have become the leading MV cable insulation in the USA and Canada. Utilities began installing TR-XLPE in the mid-1980s expecting improved performance based on increased reliability and greater longevity than standard XLPE cables or cables insulated with ethylene propylene rubber (EPR). TR-XLPE is designed to resist water trees, which can be a significant cause of cable insulation degradation and affect the overall performance of the cable. Despite the dominance of TR-XLPE in North America, very few comprehensive field-aging studies have been conducted on the performance of vintage TR-XLPE. This lends additional significance to a study by Alabama Power on 17-year-old TR-XLPE cable, insulated with Dow Wire & Cable’s HFDA-4202 NT EC TR power cable insulation compound. Seeking to evaluate the cable and assess its projected service life, while also comparing it to similarly aged EPR cable, Alabama Power removed about 1,200 feet of 35 kV TR-XLPE cable from a system serving The Galleria Mall in Birmingham, Alabama. The cable was then delivered to engineers at the Georgia Tech National Electric Energy Testing and Research & Application Center (NEETRAC) in Atlanta for extensive performance testing. The range of tests performed on the TR-XLPE and EPR cables included treeing analysis, TR additive analysis, moisture analysis, stripping tension, dissipation factor, impulse breakdown, and AC breakdown. Treeing Analysis Tests were conducted to measure the extent of tree growth in both cable types. After 17 years of field operation, the cable insulated with HFDA-4202 NT EC came out favorably, with the longest bowtie trees detected being only 12 mils in length compared to trees of 26 mils for the EPR cable. Similarly, while there were no vented trees longer than 10 mils for the TR-XLPE cable, the EPR cable had a vented tree of 42 mils. These tests validated the TR characteristics of the HFDA-4202 NT EC compound manufactured by Dow Wire & Cable. TR Additive Analysis The uniformity of the TR additive in the base polymer was measured across the thickness of the cable insulation. Tests showed that the TR additive was indeed uniform across the insulation thickness and within the expected range for comparable new cable. This analysis demonstrated that the additive did not migrate out of the insulation or have non-uniform concentration during the 17 years that the cable was underground. Moisture Analysis Tests were conducted to determine the cables’ moisture content. Average values for the two cable types showed significantly greater moisture content for the EPR cable, a result that was expected due to the filler used in the EPR compound. Stripping Tension Problems for cables occur when adhesion between insulation and the insulation shield fails, leading to the development of voids between the two layers and the increased likelihood of partial discharges. Tests on the two cable types showed that stripping tension for both was comparable to new cable. Dissipation Factor The dissipation factor is a measure of electrical losses through dielectric instability in insulation compounds. The cable types were measured for losses at applied voltages of 20, 40, and 50 kV over a temperature range of ambient to 90ºC (194ºF). At each temperature the cable insulated with HFDA-4202 NT EC showed a dissipation factor of less than 0.1%, compared to the EPR cable which showed a dissipation factor of 0.4% at ambient temperature rising to over 0.7% at 90ºC. Impulse Breakdown MV cable must be able to withstand impulses from a lightning strike so it cannot exhibit marginal impulse breakdown characteristics. A cable damaged by lightning would have severely reduced longevity. In the Alabama Power study, when five TR-XLPE and EPR cable samples were subjected to The Association of Edison Illuminating Companies impulse test, the samples of cable made with HFDA-4202 NT EC showed higher impulse strength than did the EPR samples. AC Breakdown Five TR-XLPE and EPR cable samples were also tested for AC breakdown using five-minute time steps. These tests showed that the TR-XLPE samples had a higher AC breakdown strength than the EPR samples. This is a significant finding, in that after 17 years of continuous field operation the cable made with HFDA-4202 NT EC had very high electrical strength, attesting to its expected long life. The Big Picture After nearly 20 years of field service for Alabama Power, the TR-XLPE cables constructed with HFDA-4202 NT EC insulation compound manufactured by Dow Wire & Cable were found to be still providing excellent field performance. They showed essentially no loss in dielectric strength, no bowtie trees larger than 12 mils, and no vented trees larger than 10 mils. The TR additive level was maintained uniformly across the insulation thickness and the cables showed a stripping tension similar to that of fresh cable. This study should give utilities the confidence to expect a service life that could potentially be greater than 40 years for TR-XLPE cable made with insulation compounds manufactured by Dow Wire & Cable. The Next Generation The footnote to the Alabama Power study is that Dow Wire & Cable introduced a next-generation insulation material for TR-XLPE cables called HFDB-4202 NT EC in 1998. Offering all the benefits of its predecessor (HFDA-4202 NT EC), including its excellent dielectric properties, HFDB-4202 NT EC is formulated to minimize water tree growth and provides significant new performance advantages over other XLPE and TR-XLPE alternatives. Given the outcome of the Alabama Power tests, HFDB-4202 NT EC is arguably poised to become the market standard insulation compound for enhancing cable life in the 21st century, even in the most challenging environments. About the Authors: R. Reed is end-use marketing manager, and S. Ramachandran is senior program manager at Dow Wire & Cable, a global market-facing business unit of The Dow Chemical Company.
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