Any professional vegetation-management program should follow certain principles and practices. The focus is to maintain safe clearances between energized conductors and trees by adhering to natural forest processes. This approach to vegetation management has enabled Dixie Electric Membership Corp. (DEMCO; Baton Rouge, Louisiana, U.S.) to provide many benefits to its member customers as well as the Louisiana forests. While practicing professional vegetation management, DEMCO has lowered costs, reduced outages and improved wildlife habitat.
Since 1938, DEMCO has provided safe and reliable service to its member customers in seven parishes, known as counties in other states, located in and around Baton Rouge. The utility has employed various methods and techniques for rights-of-way (ROW) maintenance over the years. However, since 1993, the DEMCO forestry group has adopted a more professional approach to ROW maintenance that includes mechanical tree trimming, tree removal and replacement, selective use of herbicides, ROW reclamation and natural control methods.
Mechanical maintenance is scheduled and performed on a circuit basis, with an emphasis on removing trees located under conductors and pruning limbs from trees located on the sides of conductors. Shortly after the mechanical operation is performed, a follow-up herbicide application is used to control unwanted tree and brush species that pose safety and reliability hazards. Native grasses and wildflowers start to grow in the ROW and begin to naturally compete with unwanted tree and brush species.
This method has provided DEMCO the opportunity to meet its primary objective for ROW maintenance: safe clearances around energized conductors. Safe clearances must not only be achieved at the time of pruning, but must be maintained over the length of the maintenance cycle (usually four to five years). While providing safe clearances, DEMCO is able to promote native Louisiana plant communities that provide early successional habitat to many wildlife species.
Over the years, much of the prairie-type habitat has been lost because of invasive, non-native plants like privet (Ligustrum spp.) and Chinese tallow (Sapium sebiferum). These and other invasive plants have eliminated many acres that were formerly habitat for quail and turkey. The loss of this type of habitat is one of the biggest problems facing Southern forests. Nearly 99% of the original prairies in Texas, Louisiana, Florida, Mississippi and Arkansas have been lost or taken over by the invasive, non-native plants.
Through the use of professional vegetation management, utilities are reversing that trend. In addition to invasive plants, a lack of naturally occurring fire has negatively affected the prairie-type plants that once thrived in many of the Southern forests. The longleaf pine (Pinus palustris) timber, with prairie plants as an understory, once covered most of the Southern states. When you look at the diversity of species, longleaf pine forests may have as many as 400 different species in the ground cover; tropical rain forests average 300. These prairie-type acres were provided as a result of natural fire disturbances. Today, professional vegetation-management activities, through the use of selective herbicides, are helping to reinstate prairie-type acres that were once abundant in the South.
DEMCO's five-year mechanical pruning cycle target is based primarily on the provisions of the National Electrical Safety Code (NESC), Section 218. The NESC lists the following factors to be considered in determining the extent to which ROW must be acquired and maintained: normal tree growth, the combined movement of trees and conductors under adverse weather conditions, and the voltage and sagging of conductors at elevated temperatures. Of these, tree growth has been the subject of a recent study. DEMCO conducted its “Limb Re-Growth Study” to determine tree-growth characteristics and factors that affect those characteristics to provide support data for developing guidelines for ROW widths, determining re-trim cycles and calculating NESC horizontal and vertical clearance.
NESC, Section 218, was updated in 2007 to cover:
Species' growth rates and failure characteristics
Vegetation location in relation to the conductors
Potential combined movement of vegetation and conductors during routine winds
Sagging of conductors at elevated temperatures or icing.
When trees are pruned heavily to achieve required NESC clearances, regrowth is significant. The tree limbs that sprout as a result of heavy pruning can come into contact with energized conductors usually two to three years before the next scheduled mechanical pruning cycle. Without maintaining the proper clearances during this period, these trees present the possibility of a direct or indirect contact with high-voltage conductors by non-trained personnel.
As part of its mechanical maintenance program, DEMCO has in place a tree-replacement program that promotes the placement of native trees outside the limits of the ROW. By placing the trees outside the ROW, they can grow to assume their natural shape with little or no impact to overhead electric lines. This also allows the trees to provide maximum benefits to the Louisiana forest community and to provide customers with energy savings. DEMCO promotes native tree replacements such as Florida maple (Acer barbatum), parsley hawthorn (Crataegus marshallii), longleaf pine (Pinus palustris) and bald cypress (Taxodium distichum).
All electric utilities have an obligation to provide safe clearances around trees and energized conductors. In addition to clearances for safety, reliability has become more important since the 2003 Northeast Blackout. There were four major causes for the blackout, with poor vegetation management being one of them.
In conjunction with the Northeast Blackout, the Energy Policy Act of 2005 gave the Federal Energy Regulatory Commission jurisdiction over a newly created electric reliability organization. Utilities are now held responsible for reliability according to certain vegetation-management standards that are being developed by the North American Electric Reliability Council. While these standards primarily will affect utilities with transmission voltages over 200 kV, it is possible that state regulating agencies or utility commissions will create reliability requirements for distribution. This is another factor that promotes being more proactive about clearances over the entire length of the maintenance cycle.
BENEFITS AND RECOMMENDATIONS
Long-term partnerships with professional contractors that center on providing practical solutions for safe clearances by adhering to natural forest principles have resulted in financial benefits for both mechanical and herbicide contracts. Asplundh Tree Expert Co.'s (Willow Grove, Pennsylvania, U.S.) professional mechanical practices have enabled DEMCO to reduce costs by 41.8% in 2005, 24.8% in 2006 and 24.5% in 2007, compared to previous cycle costs. And Superior Forestry Service's (Tilly, Arkansas) professional herbicide practices have enabled DEMCO to reduce herbicide costs by more than 50% since the vegetation-management program began.
In addition to reducing costs, the number of outages and the duration of those outages have improved greatly. When Hurricane Andrew hit DEMCO's system in 1992, the utility lost every single customer and a tremendous amount of ROW had to be cleared before service could be restored. In 2005, when hurricanes Katrina and Rita hit DEMCO's system, service was restored to approximately one-half of the affected customers in the first 24 hours after each storm. This progress can be directly attributed to the improvements made in the vegetation-management program.
To achieve success with a vegetation-management program:
Remove trees located directly under the conductors and replace them with native trees planted outside the ROW
Use herbicides selectively to provide prairie-type habitat that was once abundantly provided by natural fire disturbances
Respect natural forest principles
Explain your objectives to employees, political entities and the public
Develop partnerships and long-term contracts with professionals
Reclaim existing ROW and obtain proper easement agreements before new lines are constructed
Be willing to change your prescription and practices based on what you see occurring in nature.
A professional approach to vegetation management has provided safe clearances between energized conductors and trees. In addition, forest-management activities that consider the Louisiana forest as a whole have encouraged prairie-type habitats that benefit native wildlife. While placing an emphasis on safe clearances and professional forest management, long-term financial benefits have been realized.
Transmission vegetation-management standards have been developed, and it is possible that vegetation-management standards and reliability requirements will be created for distribution also. The father of North American Forestry, Bernhard E. Fernow, said, “Now is the time for abandoning the politician's method, who works for the day and to please his party, and to substitute a statesman's broad view, who works for all time and to please the country now and forever.”
DEMCO's system was tested in 2005 with hurricanes Katrina and Rita. When a test presents itself, it shows again that forestry in words and forestry in the woods are two entirely different things. Consider a long-term and professional approach to vegetation management that focuses on working with the forest the way it was created.
Greg Lindsly is vice president of engineering and operations at DEMCO. He has 23 years of experience in this position at DEMCO and 11 years as a consulting engineer in the utility industry. He is a licensed professional engineer in Louisiana and a licensed utility arborist. LGregL@demco.org
Frederick L. Dabney Sr. is the operations assistant and has been in the right-of-way business at DEMCO for 25 years. He was integral in the Forestry Group to reformulating the ROW management program in 1993, after Hurricane Andrew hit DEMCO in August 1992. email@example.com