Understanding mechanical and electrical risks allows utilities to focus resources cost-effectively.
Tree-conductor conflicts are a leading cause of unplanned power interruptions, causing both momentary and long-duration outages. Trees impact power-delivery systems to such a degree that vegetation management is often the largest expenditure in a utility's operating and maintenance budget. In fact, in 2010, U.S. electric utilities spent approximately $3 billion on vegetation management services.
Not only are vegetation management activities a significant maintenance expense, the results of these efforts typically are noticed only when unsuccessful. The annual cost of electric outages — for all outage causes — to the U.S. economy has been estimated at between $100 billion, according to Bank of America, and $119 billion, according to the Electric Power Research Institute. Trees are conservatively estimated as the cause of between one-quarter and one-third of all interruption events, or roughly $40 billion a year.
The following discussion identifies the conditions under which trees cause interruptions and ways to mitigate these risks, and suggests approaches for improving reliability. While the discussion applies to both electric distribution and transmission lines, there are some important differences. Reliability goals and the methods used to achieve them will vary depending on line type and priority, as well as regulatory requirements.
Measurements of success should be based on performance, defined in terms of reliability. The specific clearance distances being maintained between trees and conductors or the frequency in which vegetation maintenance is carried out are not accurate or direct indicators of reliability improvements. It also should be noted that, in addition to concerns related to service reliability, utilities perform vegetation maintenance to increase clearance distances, eliminate or reduce tree-conductor contact, enhance public safety and reduce the risk of wildfire.
Understanding How Trees Cause Interruptions
The way in which trees cause interruptions on the distribution system is not intuitively obvious. New shoots growing into contact with a conductor energized at common distribution voltages (35 kV) rarely cause interruptions. Rather, interruptions most frequently result from the structural failure of larger limbs or whole trees, often located well above the clearance zone or maintained corridor. Simply clearing vegetation from the immediate vicinity of conductors provides only a partial means of reducing the risk of tree-caused interruptions.
It is also important to recognize that while focusing on tree failures both within and beyond the maintained corridor right-of-way can improve reliability, it will not eliminate the risk of tree-caused interruptions. There is simply too much exposure and uncertainty associated with predicting tree failures. Some tree-caused interruptions will occur even with the best hazard identification and risk mitigation efforts in place. This is particularly true on the distribution system. The challenge for utility vegetation managers is to cost-effectively identify and mitigate the risk posed by trees that are most likely to fail.
Trees are living, growing, biological organisms that progress through their individual life cycles, responding to the environment in which they occur. The result is a dynamic system of interactions between trees and their surrounding environment, including factors such as storms, drought, soil type, exposure and site disturbances. Equally important are individual tree characteristics, including growth rate, decay, loss of structural integrity and mortality. In addition, there is a potential for improper practices carried out in the course of vegetation management operations to have an adverse impact on trees that remain in proximity to overhead lines, thereby increasing the risk they pose to the system.
Two basic modes of failure are associated with tree-initiated interruptions on the energy-delivery system:
- Mechanical mode of failure
This mode occurs when structural failure of a tree or parts of a tree (branches) cause physical damage to energy-delivery infrastructure. Mechanical damage results as a tree fails, taking down conductors, crossarms and poles as it falls. Mechanical damage can be extensive in major storm events.
Line-clearance tree pruning on distribution lines and vegetation-maintenance activities on transmission lines typically target trees with branches and trunks that may present a direct conflict with conductors. However, trees or branches from outside the clearance zone often represent the largest cause of interruptions. Hazard-tree-mitigation programs target high-risk trees, even if they are out of the normal clearance zone or maintained corridor.
- Electrical mode of failure
This mode occurs when a tree or parts of a tree provide a short-circuit fault pathway between areas of unequal electrical potential. An example would be a branch failing and providing a short-circuit fault pathway between energized phases of a distribution circuit. In this failure mode, the electric system infrastructure typically remains intact.
The majority of interruptions due to the electrical mode of failure are initiated by the sudden introduction of a suitable fault pathway coming into contact with conductors. This would occur in cases of branch failure or deflection, or as a result of changes in the position of conductors (sag and swing), rather than as a result of trees growing into contact with conductors. New vegetative growth growing into initial contact with distribution conductors is generally too small to provide a suitable fault pathway and represents very low risk to reliability.
Most tree-caused interruptions are initiated by sudden tree or branch failures, rather than by gradual growth into conductors. A well-designed vegetation management program includes preventive maintenance to maintain corridor vegetation as well as strategies to identify and mitigate the risk associated with both failure modes.
Which Trees Cause Interruptions?
Risk of structural failure varies by the species of tree, site, physiological characteristics and type of energy-delivery infrastructure involved. That said, some trends in tree-caused interruptions are worth noting:
- Tree failures and, therefore, the risk of tree-caused interruptions often occur in geographical clusters. They are not uniformly spatially distributed across a system. They are usually restricted to specific circuits and limited areas on those circuits.
- Tree-caused interruptions are often caused by a limited number of species (often one) of tree in a given area.
- The species that present the greatest risk often demonstrate a predominant failure characteristic that can be used in predictive risk assessment.
- On a well-maintained transmission or distribution system, the majority of the trees causing interruptions are located outside of the maintained corridor.
Historical information related to actual tree failures that have caused interruptions can be used to develop species and site-specific risk-assessment criteria to identify high-risk trees and sites. Armed with this system-specific information, utility arborists can identify high-risk sites and individual trees that pose the greatest risk of causing interruptions.
While an experienced inspector using visual criteria and historic data can identify many of the risk factors that predispose a tree to failure, not all trees that fail exhibit the apparent symptoms that would suggest they are predisposed to fail. And, vice versa, trees that otherwise appear to be healthy and sound have been known to fail. Experience suggests that more than one-third of all tree failures that have resulted in an outage cannot be easily explained. Clearly, the industry has some work to do in this area.
The most effective means for a vegetation management program to improve reliability involves a systematic approach and focus on identification and mitigation of risk. The basic concept is simple: To achieve the greatest return, apply resources to circuits and sites that have the greatest risk exposure and where vegetation management practices will have the greatest effect on risk mitigation. While the concept is easily stated, things get complicated in the execution.
The industry recognizes that the best return on an investment in vegetation management comes from periodic inspections and a regular program of preventive-maintenance actions. Irregular funding and reactive maintenance is ineffective at reducing the impact of trees on system reliability. The challenge in improving reliability beyond what historically has been achieved is in being able to identify trees and sites that present the greatest risk of causing interruptions in the future. This requires development of effective risk-assessment criteria and risk-mitigation practices.
Systematic inspections can be conducted applying these criteria and identifying the need for pruning and tree removal. An effective way to do this is by separating inspections from the performance of the actual line-clearance work. In this model, inspections form the basis of a site-specific vegetation-maintenance prescription. This allows an experienced utility arborist to evaluate situations in more detail and identify which trees, whether within or outside the traditionally maintained corridor, have the greatest potential to cause interruptions. Vegetation management maintenance work tasks are then completed with a focus on risk mitigation and, in some cases, risk elimination.
A relatively short inspection cycle followed by periodic scheduling of preventive vegetation-maintenance activities is effective. In this model, the vegetation management work performed is intended to provide adequate risk mitigation for a period significantly longer than the inspection interval. The result is active assessment of risk exposure that focuses on managing trees that pose high risk while maintaining other vegetation on a preventive basis based on an economic cycle. A common variant is the use of pre-inspection prior to scheduled maintenance, and then subsequently scheduled as an interim mid-cycle risk inspection.
It is important to recognize that the risk trees represent to the overhead energy-delivery system is not consistent across all voltage classes and line types. Risk levels vary in terms of susceptibility to tree-initiated faults, the extent of potential damage and the implication for connected load. When these considerations are taken with species and other site variables, it becomes clear that a one-size-fits-all approach to vegetation is suboptimal.
Contemporary Vegetation Management
The utility industry has embraced an integrated approach to maintaining vegetation and managing for system reliability. Integrated vegetation management (IVM) refers to an approach to program optimization that is based on a comprehensive suite of maintenance and management practices. Contemporary IVM-based programs also recognize differences in electric-delivery infrastructure as related to the risk and consequences of tree-initiated faults.
The basic premise of IVM is the creation of site-specific vegetation-maintenance prescriptions based on the conditions, species, electric system and risk present. A variety of vegetation management tools are used to mitigate and manage risk:
- Cultural methods including arboriculturally correct pruning practices
- Mechanical methods including mechanical clearing and mowing of right-of-way vegetation
- Chemical methods including the selective use of herbicides and tree-growth regulators
- Biological methods including tree-replacement planting and selective management of stable, low-growing plant communities
- Engineering methods including consideration of overcurrent protection system coordination, as well as alterations and additions to existing infrastructure.
While they are not considered part of the classical definition of IVM, the inclusion of engineering methods is increasingly becoming recognized as an important option. There are sites on every system where, from a practical or economic standpoint, the risk of tree-caused interruptions cannot be adequately mitigated by traditional vegetation management practices. Examples of engineering methods may include biasing to either preserve or sacrifice fuses, adding fuses, using coated conductors (also known as tree wire), and reconfiguring or relocating line.
The success of a utility's IVM program depends greatly on the participation of qualified staff and capabilities of the field forces available as well as on adequate and sustained funding. Successful IVM programs also include active communication and engagement with stakeholders, and make use of data analysis in a continuous improvement process to assess and refine the effectiveness of the vegetation-maintenance practices being used. A key requirement is that an IVM program be given time to work.
The real benefits from an IVM-based approach typically require a long-term commitment to active management of the population of trees and right-of-way vegetation as an ecosystem, with the objective of establishing stability and reduced risk to reliability. It can take several years to establish a relatively stable population of properly pruned trees and compatible plant communities. Disruptions in funding or other resource limitations can have a serious impact on achieving long-term goals.
Achieving the Greatest Improvements in Reliability
Vegetation management programs that achieve the greatest improvements in reliability include a component that focuses on identifying and targeting trees with the highest probability of causing interruptions. They are based on IVM constructs and are adequately and sustainably funded. These programs make use of performance-based measures to guide the processes and include the involvement of qualified personnel in managing for results.
The performance metrics used typically set targets that recognize that some level of tree-caused interruptions will still occur. The actual vegetation management program, including the work processes and practices, will vary by power system requirements, site types and tree species.
John W. Goodfellow (firstname.lastname@example.org) is a principal consultant for BioCompliance Consulting Inc. and a vegetation management researcher with more than 30 years of experience in the electric utility industry. He has held positions of responsibility for vegetation management, engineering and field services at three large investor-owned electric and gas utilities. He has also managed T&D services for a major contracting organization. He has bachelor's degrees in forestry and natural resources management from Syracuse University and the SUNY College of Environmental Science and Forestry.
Ward Peterson (email@example.com) is manager of Utility and Urban Resources with the Davey Resource Group, a division of Davey Tree. He is past president of the Utility Arborist Association (UAA), chair of the UAA research committee and serves on the board of directors for the International Society of Arboriculture. He was named the Utility Arborist of the Year for 2011 by the UAA.