Commonwealth Edison (ComEd; Chicago, Illinois), a unit of Exelon Corp., maintains 35,000 miles of overhead distribution power lines, providing reliable electric service to 3.8 million customers in northern Illinois. Yet one 5000-ft stretch along a tree-lined road in Sleepy Hollow, Illinois, experienced ongoing electric service interruptions.
This one overhead section had four to five times as many power interruptions as other parts of the system. It is routed through a section of town where trees line the streets and form a canopy over the main road. When high winds or severe weather hit the rural community, tree branches break off and topple on to the wires. As a result, the poles and wires incur significant damage, disrupting power not only to residents of Sleepy Hollow, but also to hundreds of residents in surrounding communities.
To prevent trees from damaging power lines, ComEd typically inspects and trims trees near a 12-kV feeder or circuit on a four-year cycle. A complete trim is conducted at the beginning of the cycle, then at the two-year midpoint, the utility inspects and spot trims. At the end of the fourth year, it does another complete trim.
While tree-trimming crews typically may not eliminate an entire canopy of trees, they will trim around the poles and wires in specific ways to support power quality and reliability. For example, for 12-kV circuits, the ComEd vegetation management program requires that trees be trimmed to clear up to 8 ft from the conductors horizontally on either side and up to 8 ft above the conductors. Depending upon the construction configuration and the tree growth, the trimming crews may have to trim more — for example, in those situations where they remove overhang that has potential to break and land on the wires.
However, the residents of Sleepy Hollow made it clear they wanted to maintain the aesthetics of their tree canopy and weren't open to extensive trimming. Their tree-lined road featured conventional open-wire construction, which made it imperative to keep vegetation off the power lines. This created a challenge for ComEd and its mission to maintain reliable service to all customers — unless the utility could devise a more tree-resistant construction.
While an effective vegetation management program is critical to reliable service, ComEd tries to be mindful of the other needs of its customers. For that reason, the utility set up meetings with the Village Board to explore ways to improve reliability while maintaining the tree aesthetics of the town.
One option considered was removing the overhead wires and converting to an underground installation, but to install the facilities underground would not have been cost effective and would have been impractical due to the area's terrain. It would have resulted in damage to many of the trees during cable installation. Relocating the facilities using conventional overhead construction also was not feasible. If the linemen relocated the power lines, they would have had to move the infrastructure from one heavily treed area to another. In addition, they would have disturbed some private property, which wasn't a viable option.
Because ComEd wanted to reduce the tree-related power interruptions, it researched products on the market that would support reliability, but yet be more aesthetically pleasing and require less tree trimming. In the end, ComEd opted to invest in a spacer-cable system from Hendrix Wire & Cable (Milford, New Hampshire).
The spacer cable offered ComEd many benefits. For example, the spacer cable's mechanical strength can weather severe storms, and its electrical strength can prevent faults due to phase-to-ground contact, phase-to-phase contact, tree contact or animal contact. The cable is constructed of high-strength messenger-supported covered conductors in a close triangular configuration. Its compact configuration and covered conductors can handle momentary tree contact, allowing trees to grow closer to the circuit before maintenance is required. Using this technology, ComEd could minimize the required amount of tree trimming and reduce the number of power outages in the area.
An additional benefit is related to the fact that this area of Sleepy Hollow featured winding roads with hard-to-access sections of line. By reducing the number of difficult and time-intensive repairs needed along the stretch, crews could be freed up for other critical work, especially during storm restoration.
Installation in the Field
ComEd engineers determined that the use of spacer cable would be the most effective means of reducing tree-related interruptions in Sleepy Hollow and began to prepare the appropriate least-cost design. This led ComEd and Hendrix to collaborate on a design to achieve the most cost-effective, efficient approach. This featured using much of the existing infrastructure and resulted in reduced material costs as well as labor hours.
After finalizing the design in April 2007, ComEd organized several meetings with the local community to educate them about the product and its benefits. The Sleepy Hollow Village Board agreed that the design would satisfy the community's desire to maintain the tree canopy, and the utility secured the necessary permits required to complete the cable installation.
ComEd's plan included staging construction materials at a local facility and making an arrangement with the municipality to periodically close a lane of traffic to gain construction efficiencies. Once ComEd implemented these measures, six journeymen linemen were assigned to install the product. During the installation process, which took about six weeks, the existing facilities remained in service.
The spacer-cable system consists of three-phase conductors in a compact configuration. This configuration is supported by a cable messenger suspended from hardware attached to the poles. The linemen installed the new poles, attached the supporting apparatus to the poles, installed the messenger and then pulled the phase conductors into position on the poles.
Training Field Crews
ComEd linemen learn how to install and maintain the spacer cable via a combination of classroom sessions, on-the-job training and training videos. Hendrix also visited the job site several times to provide hands-on instruction.
The majority of ComEd linemen are now trained in the installation and maintenance of the spacer-cable system. This ensures that if any cable damage occurs, the field crews are better prepared to make repairs. So far, however, the linemen haven't had to do much in the way of repair work. Tree limbs have come in contact with the spacer cable several times, but power hasn't been interrupted. For example, recently a storm affected hundreds of thousands of customers in ComEd's service territory. While the storm affected nearby lines, it didn't impact ComEd's redesigned circuit.
Use in Other Areas
As a result of ComEd's success with the spacer cable in Sleepy Hollow, it has installed the product in Chicago and surrounding suburbs. The company has discovered that this application works well in other heavily treed areas in which residents prefer not to have their trees trimmed.
By installing the spacer cable, service interruptions on ComEd's stretch of power line in Sleepy Hollow have been reduced. Best of all, the company implemented an engineering design solution that would not only keep the power on for its customers, but also strengthens customer satisfaction by addressing local environmental and aesthetic concerns.
Darryl Mitchell is a distribution, engineering and design manager for the West Region for Commonwealth Edison in Rockford, Illinois. He has been with the company for 28 years. Darryl.Mitchell@exeloncorp.com