A review of storm hardening for vegetation management
by Geoff Kempter and Kristin Wild, Asplundh Tree Expert Co.
An extreme weather event like Superstorm Sandy often causes utilities across the continent to question their investments in preventive maintenance and infrastructure improvements. What could have been done to prevent the loss of electricity to over 8 million homes and businesses across 17 states and parts of Canada? How could the restoration process have been sped up? Where would more spending have made a positive impact? Utilities and their customers and shareholders, as well as state governments and regulators all want to know the answers.
The reality is that the answers vary between utilities, and even between individual circuits. Regulatory mandates, geography, population density and construction materials are just some of the variables. One especially complicated variable is vegetation, in particular, trees. They represent the largest cause of outages in all types of weather, from routine to extreme. Any efforts to storm harden a utility’s system must address vegetation management.
The Wildest Weather in the World
Making an electrical system more reliable and resilient is no easy task, considering the fact that North America, and the United States in particular, gets more high-impact weather than any other country on the planet. The unimpeded collision of cold, dry arctic air with warmth and moisture from the Gulf of Mexico means that more tornadoes and severe thunderstorms hit the U.S. than any other nation on earth. Hurricanes, ice storms, drought, floods, wildfires, heat waves and cold snaps are also regular occurrences.
|FPL’s three-year average trim cycle on distribution feeder lines is supplemented by annual vegetation management on feeders that serve critical infrastructure facilities. In addition, FPL’s mid-cycle program includes patrolling and pruning feeders with tree conditions that may cause an interruption prior to the next planned cycle trim.|
Climate change has the potential to increase the frequency and intensity of these weather events. When combined with our expanding footprint as our population and economy grow, the damage caused by bad weather could grow exponentially. Another thing to consider is the fact that the tree canopy in the eastern U.S. is generally older, taller, denser and more widespread than it was 100 years ago. As these trees continue to age, and are impacted by changing climate and bad weather, the rate of failure, and the consequent effect on utility infrastructure, can only increase. And the ever increasing expectation of a reliable supply of electricity results in less patience when it’s not available. The threat of destructive weather is always pressuring utilities to manage the risk it poses to overhead and underground facilities and that pressure has reached the point where extreme weather has almost become the new normal.
Trees: A Moving Target?
Trees are a beautiful and valuable resource to our environment. Through shading and evapotranspiration, trees significantly cool urban environments, reducing peak demand for power. In addition, their ability to capture carbon, filter particulates, retain storm water and prevent erosion–all priorities for utilities and other infrastructure managers–are also a priority for the planet’s future. However, trees also pose a constant threat to overhead distribution and transmission lines, and even some underground lines when you consider root systems. The problem with trees is that they are biological systems that are constantly growing and reacting to changes in the environment.
While typical preventative maintenance clears lines and reduces some risk, no amount of vegetation management–except removing all trees within falling distance of lines– will eliminate the risk of a tree or limb falling into a line, especially in extreme weather events. Finding the ideal cost and benefit ratio for vegetation management efforts on any utility system requires combining many factors, including GIS data, historical tree-caused outage data, likely storm type, frequency and intensity, line priority and the age and species mix of the trees present. From this information, the most vulnerable or highest priority portions of the system can be identified. Then, the science and practice of tree risk assessment can be applied to identify trees and branches more likely to cause the most damage. By targeting these trees for mitigation, utilities can proactively reduce the risk posed by trees as part of accomplishing their storm hardening goals.
|Puget Sound Energy’s “line-to-sky” initiative targets certain species of trees along high-priority segments of low-performing circuits. These trees are known for limb failure in bad weather.|
Various Approaches to Manage Tree Risk
With aging forests and tree failures increasing due to natural and human causes, the bad news is that tree-related outages will trend upwards unless utility arborists intervene. The temptation is to cut down more trees, remove overhanging branches, increase clearances, and–in some cases–this is necessary. However, removing a tree or branch with a low likelihood of failure, even if it is close to or overhanging a line, may not be the best use of available funds. In some cases, widening a right-of-way and removing large, stable trees adjacent to a line will simply expose less stable trees that are not adapted to wind exposure. Using this approach is more costly and it ignores the good news: some trees are more resilient than others. A qualified arborist can identify trees that pose a higher risk of failure, and recommend mitigation measures, pruning or removal, to reduce service interruptions.
After the devastation caused by the 2004 and 2005 hurricane seasons, Florida Power & Light (FPL) began investing in a storm hardening program that includes electrical system and vegetation management improvements. In addition to strengthening poles, lines and other electrical components, FPL uses contractors to inspect and perform preventive maintenance like tree pruning and removal and herbicide applications. The program is comprised of multiple initiatives designed to reduce the average time customers are without electricity as a result of vegetation-related interruptions.
FPL clears vegetation from its distribution feeder lines on a three-year average trim cycle because it offers the optimal balance of reliability performance and vegetation clearing cost. In addition, FPL’s mid-cycle program encompasses patrolling and trimming feeders between planned maintenance cycles to address tree conditions that may cause an interruption prior to the next planned cycle trim. Those feeders that serve critical infrastructure facilities are maintained before the height of storm season every year, which is frequent for vegetation management cycles. These lines serve hospitals, emergency service facilities, water treatment plants and other key facilities that help a community get back on its feet after a major storm. By shortening the preventive maintenance cycle for critical infrastructure lines, the post-storm restoration goes faster.
FPL also sets budgeted funds aside to address customer concerns, removal of high risk trees and maintenance of fast-growing species.
Another approach in improving reliability through vegetation management is to identify and remove high-risk trees. National Grid in the northeastern U.S. has developed a rigorous method for selecting the right circuits for its hazard tree budget in order to get the biggest reliability bang for the buck.
Taking into account the number of customers served on the circuit, miles of bare-wire three-phase construction, miles of actual tree exposure, the three-year average total of tree-related customer interruptions and the three-year average of customers interrupted per tree event, National Grid is able to identify the circuits that have the greatest need for hazard tree removal. The method can be more precise by partitioning the circuit according to the location of protective devices and targeting the segment that has the poorest performance. Using this methodology, National Grid reports a 52 percent reduction in average tree-related customer interruption three years after the high-risk tree work was performed. This targeted expense goes a long way in reducing storm-related outages and improving overall reliability.
Puget Sound Energy (PSE) in the state of Washington took a slightly different approach to managing high-risk trees when it first established its Tree Watch program in the late 1990s. With mean tree heights that are up to 30 feet taller than those found on National Grid’s system in the East, it may come as no surprise that 80 percent of tree-related outages on the PSE system are caused by trees outside the utility’s right-of-way. As a result, PSE designates 15 percent of its vegetation management budget to target off right-of-way hazard trees for removal. The utility’s poorest performing circuits are identified for this effort and then PSE uses contract utility foresters and notification personnel trained in tree risk assessment methods and customer relations to obtain approval from property owners. This nationally-recognized effort has reduced the number of tree-caused power outages by 22 percent.
PSE is also piloting enhanced tree pruning programs (line-to-sky) on high priority segments of poor performing circuits where limb failure of certain tree species is a cause for flickers and outages. This line-to-sky work is applied to Douglas-fir, red alder, black cottonwood and big leaf maple trees which have proven to pose the greatest threat of branch failure.
Connecticut Light & Power (CL&P), in response to the tree-related outages caused by Hurricane Irene and a late fall snow storm in 2011, initiated an enhanced tree pruning program in March 2012. This involved removing all overhanging branches within 8 ft. of wires from the ground up on high priority lines with poor performing segments. High-risk trees that could fail and fall on CL&P lines were also identified and removed. Eight months into the program, Superstorm Sandy struck Connecticut and CL&P reported that the number of outages decreased by 50 percent where enhancing tree pruning had been performed.
The new American National Standards Institute (ANSI) Standard for Tree Risk Assessment, published in late 2011, acknowledges that successfully managing vegetation for electric reliability cannot be done with a one-size-fits-all approach. Professional utility arborists must employ a variety of tactics that take into account the age, species and condition of trees and their sites, as well as line voltage, fusing, switching and outage history on the circuit. Tree risk assessment, conducted by qualified arborists, is becoming an essential tool in the storm hardening effort, making it possible to identify and mitigate the trees that pose the highest risk to the system. This investment can deliver positive results in reliability and resiliency after a storm.
Geoff Kempter, manager of technical services for Asplundh Tree Expert Co. is the past chair of the International Society of Arboriculture (ISA) Certification Board and has served on the ANSI A300 Tree Care Standards Committee since 1996. He is the author of the ISA’s “Best Management Practices Guide for Utility Pruning of Trees” and is a respected presenter for many vegetation management topics. Contact him at email@example.com.
Kristin Wild, writer and editor, works in Asplundh Tree Expert Co.’s corporate communications department producing content for corporate and industry publications. A member of the Utility Arborist Association editorial committee, she is also former trustee for the Tree Research and Education Endowment Fund. Reach her at firstname.lastname@example.org.