Global Cable Aging Tests of TR-XLPE Cables Enhance Value for Utilities

It has been repeatedly proven that water treeing impacts underground cable performance over long periods of time. However, typical specifications such as IEC do not call for cable evaluations under conditions that aggressively model field conditions. These specifications are restricted to testing the cables under dry conditions.

The North American utility group, Association of Edison Illuminating Companies (AEIC), working in conjunction with cable manufacturers and the Insulated Cable Engineers Association (ICEA), developed a qualification test in the mid 1980s that has since been refined, and recently has gained recognition as the national standard, ICEA S-94-679-2004 (2). The primary focus of this test is to wet age up to one year, 15 kV, 53 sq. mm cable cores (no jacket) submersed in water in PVC conduits, while injecting water through cable strands. This test has been referred to as the Accelerated Water Treeing Test (AWTT).

Following dry thermal load cycling to drive off crosslinking reaction by-products, 12 cable samples are aged for 120, 180 and 360 days in water at 150V/mil (5.9 kV/mm) and load cycling. Each eight-hour "on" cycle must result in the insulation shield temperature of 45°C. At the end of each time period, samples are evaluated for AC breakdown (ACBD) as well as other diagnostic tests. The purpose is to evaluate cable performance under environmental conditions that are aggressively model field conditions, while also accelerating the process of induced degradation of the insulation, with water being forced to enter from both outside the cable core and from the conductor strands.

Typical AWTT Results

North America

In Figure 1, we have illustrated the typical AWTT test results after one year of wet aging. These results are illustrative of six or seven cable manufacturers' qualification tests and represent an independent body of results. The ACBD values are plotted as a function of time and it is evident that those TR-XLPE cables perform at a much higher aged electrical strength than standard XLPE cables.

Figure 1: AWTT Test Results

Europe

In contrast to North America, the earlier German specification called for a two-year aging test. Figure 2 compares typical results of TR-XLPE and standard XLPE. Once again, it is evident that TR-XLPE retains a higher electrical strength. In fact, regular XLPE may not consistently meet the minimum requirements.

Figure 2: German VDE Test: TR-XLPE vs. XLPE cables

China

Recently, Wuhan High Voltage Research Institute (WHVRI) undertook comparative AWTT performance evaluations of three commercial cables made in the People's Republic of China (PRC) with North American TR-XLPE, North American XLPE, and locally produced PRC XLPE compounds, respectively, all with corresponding conventional semiconductive shields. Figure 3 shows that TR-XLPE cables maintain higher electrical breakdown strength through the one-year wet aging test. Cables made with local XLPE compounds all electrically failed before 140 days and also displayed significant water treeing activity by that time, while TR-XLPE did not. These results demonstrate the importance of the quality and consistency of the materials that are used to manufacture cable where long cable life is the desired end result.

Figure 3: WHVRI Data on Chinese AWTT Test

The above information is from a new Dow Wire & Cable paper, "Proven insulation technology for enhancing distribution cable life and improving reliability" by S. Ramachandran, L. Du and H. Tan. To receive a full copy of the paper, please click here.