by Nicholas Abi-Samra
According to a 2008 Edison Electric Institute (EEI) Reliability Report, two-thirds of electrical outage minutes were weather-related, with a third of these attributed to vegetation contact with utility lines, poles and transformers.
Vegetation management is not only important during severe weather. Inadequate vegetation management-specifically tree trimming-and failure to manage tree growth in transmission rights of way (ROW) were primary causes of the 2003 Northeast blackout, one of the largest in U.S. history.
Vegetation management is fundamental to mitigating local outages for distribution systems, as well as widespread cascading outages and blackouts for transmissions systems.
For vegetation management programs to be effective, it's important to review the findings from wind and winter storms:
Line-outage frequency can be correlated with the number of trees-per-mile edge of the line. It is less dependent on variables such as line and tree heights and clearance between the trees and lines.
For distribution systems, a direct correlation exists between the proximity of trees to distribution lines and the vulnerability of the lines to severe wind and winter storms. Tree-related failures increase exponentially when wind speeds top 60 mph.
Distribution pole failures principally were caused by fallen trees-secondary failures-and not by the impact of the wind on the power delivery system directly-primary failures. This is because the risk from airborne debris and trees outside the ROW can exceed the risk of trees within the ROW by factors' sometimes exceeding a 3-to-1 ratio.
Increasing the intensity of the hazard-tree program did not necessarily produce noticeable electric system performance during major storm events.
Targeted vegetation management actions for neighborhoods and communities' experiencing greater than expected reliability challenges have been proven effective.
Undergrounding or Vegetation Management?
Vegetation management frequently competes for budget dollars and often faces strong public opposition-at least until major storm-related outages occur. Utilities might have several options to address reliability situations. Take, for example, undergrounding a distribution lateral through a new subdivision compared with increasing tree trimming on the main overhead trunk. If tree trimming results in higher reliability for lower cost, the loop could be built as a part of the construction budget and O&M funds can be boosted of vegetation management in that area. Progressive utilities routinely perform targeted reliability improvements that include a combination of vegetation management with selective undergrounding and other system enhancements across the distribution system for neighborhoods with reliability challenges. There is no universal solution to fixing reliability issues.
Where Do We Go From Here?
Utilities need to optimize reliability as they attempt to allocate capital and operational budgets based on well-defined reliability objectives. Revisiting in-place vegetation management programs periodically is vital to minimizing wind and winter storms' effects on power lines. Utilities should use the results of storm damage and subsequent restoration as part of their vegetation management assessments. In particular, application of storm data allows a critical review of damage and clearance standards. Finding the true causes of tree-related outages is critical to employing the necessary vegetation management. In addition, utilities should coordinate with property owners and local officials to plant and replace downed vegetation that is most conducive to system reliability. Just ensuring specific branch clearances from power lines is not as effective as targeted vegetation management. In the past, the public planted vegetation with little or no consideration of the impacts on surrounding utility systems. This has changed, but localities could assist in vegetation management by requiring that any new trees planted under power lines be those with shorter heights and longer life cycles.
On the technology side, improved vegetation management can be achieved through data analytics and business intelligence. Analytics can assist in tracking vegetation management practices at different regions and tree species at various times to maximize the return on investment as measured in system reliability. The ability to track all these metrics on geographic information systems with vegetation management databases and work histories, coupled with a data mining engine to develop the business intelligence, will help utilities tackle this major cause of storm-related outages. The smart grid's advanced distributed sensors, communications and data networks will enhance condition-based maintenance and enable intelligent vegetation management.
About the author: Nicholas Abi-Samra is a senior member of IEEE, a professional engineer and senior vice president at DNV GL. An expert in power systems, planning, operations, and maintenance, he served as the general chair and technical program coordinator for the 2012 IEEE Power & Energy Society (PES) General Meeting. He is the chair of PES and the Power Electronics Societies in San Diego. Reach him at firstname.lastname@example.org.