By Brian Trager, Hendrix Wire and Cable
Power distribution systems around the world face critical threats to their long-term reliability and performance. In 2010, the U.S. experienced a record number of major transmission outages, often the result of severe storms. In fact, 150 of that year’s 247 power failures were weather related. In 2013, the U.S. energy infrastructure received a barely passing grade of D+ from the American Society of Civil Engineers. Year after year, aging transmission and distribution equipment in dire need of updating is challenged to withstand intensifying storm activity. Areas of the country close to the ocean are at particular risk.
|ACS features four individual cables connected using spacer brackets. A heavy-gauge messenger wire that structurally supports the cable system and withstands environmental damage.|
In the early 2000s, the city of Natchitoches, Louisiana, was experiencing increasing loads on its power grid. To alleviate the strain, the city utility designed a ring bus that would encircle the city and enhance available power to residents. Much of the ring bus was built on new pole installations, but in an area that extended approximately one mile through historic sections of the city that were highly congested and heavily treed, it would have been almost impossible to install new poles to carry the associated three-cable feeder.
The small city of Natchitoches, population 18,000, is the oldest settlement in Louisiana, and is located just 120 miles from the Gulf of Mexico. Historic plantations from the 18th century are located on avenues lined with hundreds of magnificent, stately oaks. These imposing, ancient trees are the pride of the community and are fiercely protected by residents. Trimming or cutting the trees is a very sensitive topic, even if trimming would prevent weather related power outages. Cabling for the new ring bus feeder, therefore, had to run very close to, and even through, the ancient trees, a situation that virtually guaranteed power failures.
During storms, standard cable installed near trees can experience significant damage from falling limbs. When heavy limbs come down, the power is almost always disrupted. In high winds, bare wire cables spark and flash if they touch one another, which can set fire to trees and nearby structures. Coated cable also can flash in storms and experience outages when birds or rodents get caught in it. For all these reasons, the city decided to move away from bare wire and individual coated cables for this one-mile feeder. They instead looked for an alternative cable that was sturdy, durable and capable of withstanding damage caused by wind, trees and animals.
Because of the congestion in the area of the city where the feeder was being installed, city engineers knew installation of new poles would be cost-prohibitive. The new 69-kV cabling had to be installed on existing poles in the one-mile stretch. Existing poles stand approximately 100 feet tall. The feeder cable could run only where there was sufficient space-under the 138kV transmission line and above the lower voltage distribution lines that were already live and operating on the poles. In addition to placement constraints, the new 69-kV cabling had to extend across 600 to 800 foot expanses between one pole and the next. This was a complicated installation.
After an extensive search, engineers from the city of Natchitoches determined that a newly available 69-kV aerial cable system (ACS) from Hendrix could provide the reliability required for this challenging overhead conductor installation. Designed for strength and stability over long-spans, the ACS features a messenger supported primary distribution system that uses covered conductors in a close triangular configuration. The system has the mechanical strength to prevent faults due to phase-to-ground or phase-to-phase contact, tree contact or animal contact. The messenger cable is made of #8 aluminum, tensioned at 7,500 pounds at 60 F. The system is specifically engineered to withstand high winds, falling trees, damaging storms and long spans.
|ACS runs very close to, and even through, the city’s ancient trees, and is installed under the 138kV transmission line and above the lower voltage distribution lines.|
Hendrix was instrumental in the feeder’s design and installation. Company engineers consulted with the city of Natchitoches utility to custom design the feeder installation and provided project management services from start to close of the project. On behalf of the city, the team coordinated installation of the cable onto the existing poles at a height of approximately 70 to 75 feet above ground.
Installation of the feeder cable in the one-mile stretch was completed in about a week. The team accurately mapped the installation and created a customized kit of materials needed to complete the job. Engineers calculated the appropriate sag for the congested installation to ensure cables would not contact vegetation or other cables on or near the existing utility poles while exposed to high winds or under stress, and installed the ACS lines to meet these calculations.
“Once it was up and running, the Hendrix system worked flawlessly,” said Charles Brossette, operations manager for the city’s utility services. “Hendrix personnel were very helpful and extremely accurate during the installation phase of the project.”
The Atlantic hurricane season of 2005 started shortly after the installation was completed. This was the season that rewrote the record books. Three of the most intense Atlantic hurricanes ever recorded, all category 5 storms, developed in the Gulf of Mexico, and two of these storms made landfall in Louisiana. This record-breaking year of devastation included Katrina, the storm that gained notoriety for its powerful demolition of New Orleans and its threat to human life. Katrina is ranked among the five deadliest hurricanes on record.
While it devastated New Orleans, Katrina did not have any major impact on Natchitoches. But within a month of Katrina, Hurricane Rita, the fourth-most intense Atlantic hurricane ever recorded-stronger and potentially deadlier than Katrina-and the fifth major hurricane of the 2005 season, stormed into Louisiana. The storm surge devastated coastal communities and winds, rain and tornadoes caused fatalities and a wide swath of damage from eastern Texas to Alabama.
With sustained winds of 120 miles per hour (195 kilometers per hour), Rita knocked out hundreds of electrical lines, disrupting service in many areas of Texas and Louisiana for weeks. People living in parts of the country hardest hit by Hurricane Rita consider it “The Forgotten Storm,” because the devastation it caused got far less attention than Katrina, mostly because it struck less populated areas and was less of a threat to human life.
|Crews prepare to install spacers at 20 foot intervals by using string to measure the distance between each spacer, an easy and accurate method of installation.|
Many areas in the city of Natchitoches lost power during Rita, keeping the utility division busy for weeks. But according to Brossette, “The newly installed aerial cable stood up to Rita. We had no outages in that one mile of 69-kV feeder. Lines around it came down, huge branches cracked off the old oaks and the winds whipped the trees for hours during the storm. But there was no damage to the new cable. It proved its strength during that massive storm and many smaller ones that have come in the 10 years since.”
The ACS cable was the only possible solution for an installation with limited space, where the only place for the new feeder was under the existing 138-kV line. The feeder has reliably operated over the last 10 years without an outage or any damage despite multiple storms and long-term exposure to high temperatures and humidity. This specialty cable minimized both the potential for downed conductors and the need for expensive ongoing preventive maintenance activities.
“The cable is holding up great,” explained Brossette. “It’s like we just put it up. I am pretty sure from what I see of the ACS cable’s performance, we will not have to do any maintenance on it for the next 10 to 15 years.”
Brian Trager is director of technology for Hendrix Wire and Cable. He has held various positions in engineering, consulting and management at American Electric Power Co., Cooper Power Systems and Fisher-Pierce, as well as at Hendrix Wire and Cable He is a member of the IEEE and received his bachelor of science and master’s degrees in electric power engineering from Rensselaer Polytechnic Institute in New York. He also holds a master’s degree in business administration from the University of Pittsburgh. Mr. Trager has authored over 50 technical papers and articles for the IEEE and other national and international organizations.