WAPA and DOE Field Test Composite 3M Conductor
North Dakota epitomizes the wide-open spaces of the prairie states in the central United States. With high winds, and extreme cold and ice, this is an ideal place to field-test a new transmission conductor — one that could meet growing loads and enhance transmission reliability as America's energy appetite grows.
Developed by the 3M Co. (St. Paul, Minnesota, U.S.), the composite conductor and associated hardware had been evaluated in several laboratory tests, but now it was time to test this conductor in selected field sites to validate its performance under operating conditions. Early promising laboratory results drew the interest of the U.S. Department of Energy (DOE), which funded field-testing. Western Area Power Administration (WAPA; Lakewood, Colorado, U.S.) provided one location for the field-testing program.
Known as aluminum conductor composite reinforced (ACCR), the 795-kcmil conductor's core consists of aluminum-matrix composite wires to carry high tensions with low sag characteristics, surrounded by aluminum zirconium wires that can withstand higher operating temperatures. This design allows the conductor to carry significantly more current than today's 795-kcmil aluminum conductor steel reinforced (ACSR) wire.
Actual Field Performance Test
The field test was performed to show how well the new cable could mechanically withstand the extreme winter and summer conditions prevalent in North Dakota. WAPA crews volunteered to install the conductor on a 1-mile (1.6-km) stretch of the agency's 230-kV transmission line that runs between Jamestown and Fargo, North Dakota. Four spans located between two deadends in an area known to experience strong winds and severe weather were selected.
WAPA crews installed a variety of in-line and deadend splices for testing so that the project sponsors could compare the performance. Crews also placed different hardware configurations on each of the three phases of the line. One phase had Alcoa compression deadends, another phase had an Alcoa deadend on one tower and a Preformed Line Products' Thermolign deadend on the other. The last phase had Thermolign deadends at both ends. Besides deadends, the crews also installed in-line tension splices by these same manufacturers to join two ends of the new conductor.
Other accessories evaluated in the field were PLP Thermolign suspension clamps and Alcoa Stockbridge-type vibration dampers. All accessories were designed and fully tested with the 795-kcmil ACCR prior to the field installation in Fargo.
Installation Process
Line installation began on Sept. 30, 2002, when three WAPA foremen and seven linemen gathered three manlifts and several other pieces of equipment in a soybean field southwest of Fargo. Because this was a field test, representatives from the various equipment manufacturers were on hand to observe their products being installed under real-world conditions.
After ensuring that the line was de-energized, the crews used standard industry methods to install the conductor. They set up a pulling trailer at one end of the span to reel in the old cable and a tensioning trailer at the other end to pay out the new line. They also installed travelers on each tower. Next, they disconnected the conductor from the insulator strings and lay it in the travelers. Using a sock, the crews pulled in the new cable by using the old conductor.
Finally, crews removed the travelers and hung the conductor on the insulator strings stretching from the three suspension towers between the two deadend structures that marked the test segment.
Monitoring System
The crews also installed CAT-1 monitoring equipment on two phases of the line. This system places a load cell in line with the conductor. Knowing the tension in the line, the line sag can be determined with input data, including effective solar radiation, line current and ambient temperature. This CAT-1-determined sag is then compared with the predicted sag, taking into account the loading history (creep) and temperature. The installed CAT-1 monitoring system includes two loads cells. The first load cell is set up to measure normal loading tensions on one phase of the line; the second load cell on a second phase is set up to measure ice loading. The monitoring equipment will be in place over a two-year period to evaluate the sag of the conductor.
Field measurements will enable evaluation of the characteristics of this ACCR. For example, test data will enable comparisons of predicted sag-tension values under known field conditions to those actually measured. Engineers can thus track the effect of changing temperatures on conductor sag. As stated earlier, both line current and ambient temperatures were monitored to infer the conductor temperature. An effective ambient conductor temperature was measured. This is the temperature that would occur in the absence of current. It takes into account solar radiation heating the line, and it was measured with a Net Radiation Sensor (NRS), developed by The Valley Group (Ridgefield, Connecticut, U.S.). This device, with thermal characteristics similar to that of the conductor, is placed at line height and is located parallel to the line.
| Designation | 795-T16 | metric | |
| Stranding | 26/19 | ||
| Diameter | 1.11 in | 28.1 mm | |
| Total area | 0.724 in |
467 mm |
|
| Aluminum area | 0.624 in |
403 mm |
|
| Weight | 0.896 lbs/linear ft | 1.333 kg/m | |
| Breaking strength | 31,134 lbs | 138.5 kn | |
| Thermal elongation | 16×10 |
16.3×10 |
|
| Ampacity @ 240°C & 2 ft/sec wind | 1775 amps | 1775 amps | |
| Resistance | |||
| AC @ 25°C AC @ 75°C |
0.1126 ohms/mile 0.1349 ohms/mile |
0.0700 ohms/km 0.0838 ohms/km |
|
Vibration monitors were also installed in two weeks in January 2003 to monitor the vibration performance of ACCR and accessories.
The conductor will continue to be monitored for another year. After the field trial is over, the conductor will remain in service. Another part of the test is to evaluate the cost-benefit of using 3M's new conductor to reconductor existing lines. WAPA staff is now working with 3M to compare the costs of performing a conventional reconductoring job to a line uprated using the same towers with this cable.
Measured and Predicted Sag
The sag was predicted with two methods: the Strain Summation Method to account for the full loading history, and the more commonly used Graphic Method, using software such as Alcoa SAG-10.
The Strain Summation Method of Sag-Tension Calculation took into account creep as a function of time. A conductor data file was created based on stress strain tests performed by NEETRAC (Atlanta, Georgia, U.S.) on 795-kcmil ACCR. The Strain Summation Method accounts for creep, ice and wind loads daily because it can examine any number of conductor states in sequence. This differs from the Graphic Method, which considers only two states: initial and final.
The model began with stringing the conductor on Oct. 3, 2002, matching the CAT-1 measured tension at the NRS temperature also provided by the CAT-1 system. CAT-1 load and NRS temperatures and times were used as input, and the model provided sags and tensions as the corresponding output.
The calculated and measured tensions, both adjusted to 0°C (32°F), are shown in Fig. 3. The measured tensions were about 12 lb (5.5 kg) lower (about 0.2% of total load) than calculated in October 2002, but did not decrease quite as much as the calculated values thereafter. By the end of March 2003, the measured tensions were about 5 lb (2 kg) higher (0.1% of total load) than calculated.
In the first two months, creep caused the tension to drop about 50 lb (22 kg), while in December 2002 and January 2003, the main cause of permanent elongation was the high tension caused by the low temperatures, resulting in a further tension drop of 15 to 20 lb (7 to 9 kg). In April 2002, an electrical load drop from about 230 A to 150 A increased tensions.
The predicted data match calculated data within a precision of 0.1 to 0.2% on tension when accounting for the conductor stress-strain, thermal elongation, temperature, current and creep history. The corresponding measured sag matches the predictions within ±1 inch (2.54 cm).
The Strain Summation Method used above can predict the behavior with remarkable accuracy because it accounts for the actual loading history of the conductor. It is also important to verify that the sag-temperature data are predicted by the Graphic Method (SAG-10).
Figure 4 shows the midnight data for October 2002 and April 2003. It is compared to the predicted “Initial,” “10-Year Creep” and “Final Curve.” The sags are computed from the CAT-1 tensions, with significant difference between the measured maximum tension and the horizontal component of tension.
The total tension drop of 65 to 70 lb (29 to 32 kg) is shown in Fig. 4. This corresponds to a sag increase of 6.5 inches (16.5 cm). The measured data fall in between the predicted Initial and 10-Year Creep curves generated by the Graphic Method.
The “Initial” line indicates the sag-temperature relation for a newly strung conductor. This is based on stringing to 4247 lb (193 kg) maximum tension at 8.7°C (48°F) at approximately noon on Oct. 3, 2002.
The 10-Year Creep line indicates the sag-temperature relationship after 10 years of creep at room temperature, which is typically considered a “final” condition in the Graphic Method.
The “Ice & Wind” line indicates the sag-temperature relationship after NESC heavy loading, which is 0°F (-17.8 °C), 0.5-inch (1.3-cm) ice with 4 lb/ft
Potential Technology Benefits
One year of operation at the field site has been completed. So far, the ACCR has behaved as predicted under a wide range of weather conditions. The new technology could offer many benefits for utilities. Perhaps most significantly, installation of the smaller 3M ACCR could help relieve transmission bottlenecks that prevent lower-cost energy from being dispatched to where it is needed. This conductor could also be installed in locations where utilities could uprate lines without increasing the width of existing rights-of-way. The conductor's high strength-to-weight ratio also could offer a solution for long-span applications.
Acknowledgments
This material is based on work supported by the U.S. Department of Energy. The authors thank Bob Whapham (PLP), Wayne Quesnel (Alcoa Conductor Accessories), Brian Morris (Western), Rob Mohr (The Valley Group), Todd Staffaroni and Herve Deve (3M) for their contributions.
Ross Clark received a BSEE degree from Colorado State University and an MBA from the University of Phoenix. Clark worked six years at the Bureau of Reclamation before transferring to Western Area Power Administration, where he has held various engineering positions. With 25 years experience in the industry, he is currently the electrical engineering manager. He is a registered professional engineer in Colorado.
Dr. Stephen Barrett obtained an honors BS degree and a Ph.D. in physics from McMaster University in Hamilton, Ontario. Barrett worked for 20 years in the Research Division of Ontario Hydro (now Kinetrics) where his main activity was the modeling of the electrical, mechanical and thermal properties of conductors and insulators. This was followed by two years in asset management at Hydro One. Since 2000, Barrett Research has provided consulting services to the industry.
barrett.s@sympatico.com
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