Enhanced Vegetation Management-Impact on Electric System Reliability

Electric Utilities typically evolve through three stages with respect to vegetation management (commonly referred to as tree trimming):

Get on Cycle

Tree trimming specifications normally establish a periodic (commonly a four- or five year) cycle to ensure proper clearances are maintained between vegetation and electric distribution lines. However, electric utilities have commonly deferred tree trimming activities and related expenditures when revenue shortfalls or expense overruns produce earnings pressure. Consequently, for utilities that have not maintained adequate vegetation management, this initial stage of vegetation management addresses both situations by:

• Restoring the cycle on all circuits, and
• Augmenting the normal cycle on fast-growth species (referred to as mid-cycle “hot spotting”).

It will take several cycles to regain control of vegetation growth (to the point where the specified clearances are maintained system wide), but once achieved, the foundation has been laid for a more cost-effective approach.

Optimize the Vegetation Management Cycle

Once an electric utility achieves consistency related to getting on cycle (stage 1), it is now ready to optimize its cycle (allowing it to vary by circuit depending on factors of growth and density of vegetation and customers). This obviously requires careful planning to ensure a pre-specified amount of trimming occurs each year. The priorities are based on vegetation growth-rate and customer densities and all circuits are maintained in a manner consistent with the clearance requirements defined in the tree trimming specifications.Typical optimizations include:

• Addressing the feeder backbone (and other parts of the system with large concentrations of customers) on a different cycle than the laterals because of the larger impact of these categories on outages.
• Trimming lower voltages on a longer cycle and trimming urban areas, where easements may be narrower and clearances harder to obtain, on a shorter cycle.
• Using herbicides to reduce stem growth and better work with communities to integrate utility trimming with urban forest aesthetics.

Target Broken Limb/Fallen-Tree Outages

Once a utility’s growth-caused (or contact-caused) outages are less than 50 percent of its vegetation-caused outages, the focus should then be expanded to include the removal of overhang (i.e. trees and limbs outside the clearance required by the tree trimming specification) and “danger” trees in their entirety (those trees that are most susceptible to falling on a line). Recognizing that the costs of such work can be 2 to 5 times the cost of “normal” tree trimming and that often (particularly in the case of overhang) there can be significant resistance from the customers, it is important that this level of tree trimming is targeted in locations where outage prevention will have the greatest impact in eliminating customer interruptions.

Electric utilities typically identify those feeders that exhibit the worst experience with respect to tree-caused backbone outages. With few exceptions, these outages are normally categorized as “tree non-preventable” (i.e. the specified clearances have been maintained and the cause is usually the result of a fallen tree or severe overhang). In instances where these outages are on the taps (and often serve two-lane suburban streets), the cost in terms of dollars and customer resistance and the typically small impact on system reliability performance may not warrant removal or “ground to sky” trimming.

However, in the event these outages occur on the feeder backbone, implementation of stage 3 makes sense:

• The trees tend to be adjacent to the major thoroughfares and commercial areas (removal and trimming less costly)
• Commercial customers are less emotional about tree trimming (fewer customer complaints)
• The number of affected customers equate to a significant reduction of customer interruptions (improved SAIFI)