In 1997, Entergy Corp. (New Orleans, Louisiana, U.S.) embarked on a concerted effort to overhaul its vegetation management program. The aim was to improve reliability and cost effectiveness. In 2001, Entergy saw the fruits of its efforts, with vegetation-related customer outages dropping about 50% since 1997. A close look at how this was done reveals valuable lessons for anyone seeking to achieve similar results.

The location of Entergy's service territory creates a special challenge. In the rich soil of the Mississippi River Delta and Gulf Coast — with near-tropical heat and rain for a good part of the year — the growing conditions are ideal for many trees, shrubs and vines. Added to that are the regular thunderstorms, occasional hurricanes and ice storms that create a utility vegetation manager's worst nightmare.

Entergy initially recognized the need to centralize the vegetation organization if it was to be successful in leveraging the scope of work and standardizing work practices. Therefore, in early 1997, Entergy formed a single vegetation department. Later that year, it commissioned a team to examine the current vegetation management program and practices, as well as to develop recommendations for a new business plan to include standard procedures, practices and contract strategies. In late 1997, the team presented its recommendations to management, which, in turn, implemented them in early 1998.

The typical progression of utility vegetation management practices occurs in three stages:

  • Stage 1: Get on cycle. Most utilities find it all too easy to defer tree trimming whenever revenue shortfalls or expense overruns cause earnings pressure. Yet tree-trimming specifications usually are designed to achieve a clearance that is likely to be effective in avoiding contact for a fixed number of years (such as a four-year cycle). Some fast-growth species may require more trimming or mid-cycle “hot spotting,” but most of the circuit should be relatively trouble-free from normal growth-caused contact for the given cycle. If funds are cut, trimming is deferred for another year and trouble begins. For the circuits experiencing trouble, future trimming will need to not only be restored to the cycle amount, but also increased to catch up what was missed. This, in turn, causes a built-in unevenness to future trimming schedules as well as the inefficiency of varying crews accordingly.

  • Stage 2: Optimize the cycle. Once a utility achieves consistent performance on a regular trimming cycle, it may try to step up to the next level of vegetation management to optimize the cycle and processes. This includes allowing the cycle to vary by circuit depending on factors that would cause one circuit to need a longer or shorter cycle.

    This is not the same as deferring trimming whenever the company needs more earnings. Instead, it is a carefully planned approach to doing a fixed amount of trimming on the system each year. This is similar to an approach that would target the worst-performing circuits first, but it combines it with the discipline of recognizing that there is a certain interval of time — different for different circuits — at which the circuit must be re-addressed.

    Typical optimizations include doing the backbone on a different cycle than the laterals because of the larger impact of backbone outages. Transmission trimming must be more aggressive than distribution trimming to the point where, for most utilities, transmission trimming means mowing and spraying a wide right-of-way under the towers, and side trimming plus danger-tree removal. Other adjustments may include 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. Included in this phase may be contracting improvements that typically include a move from time and materials (T&M) to unit price (or at least managing T&M as if it were unit-priced). Other enhancements may include smart use of herbicides to reduce stem growth and better work with communities to integrate utility trimming with urban forest aesthetics.

  • Stage 3: Target broken limb/fallen tree outages. Once a utility's growth-caused (or contact-caused) outages are less than 50% of its vegetation-caused outages, everyone will start asking the question, “We just trimmed those circuits, so why are they still having outages (especially in storms)?”

Even though most tree-trimming specifications will call for removal of “danger” trees that are dead and likely to hit the line, in practice the costs of such work would break the budget if done extensively. For example, if regular trimming costs US$2000 to US$4000 per mile, heavy removal of overhang above the normal amount or removal of trees or branches that are not dead but are structurally weak and likely to cause outages under stress could easily cost US$10,000 per mile.

The key to taking the next step is to carefully target the places where such work is done based upon impact on the system.

Many of these elements were part of Entergy's successful program, but Entergy did it all in one cycle. It is just finishing the fourth year of its four-year cycle with the satisfying results. Entergy's program included the following familiar elements:

  • Revamp the cycle plan. All circuits were put on a proactive cycle and have been trimmed accordingly (89% finished as of the end of 2001, and is on target to finish the rest this year and start the next cycle).

  • Switch those areas that were being trimmed by grid or area to trimming by circuit, so reliability can be monitored and accounted.

  • Vary the cycle by factors: voltage, species, clearance and impact (Entergy has not yet adopted different cycles for backbone and laterals).

  • Focus cost control on achieving the desired specification at a controlled cost per mile (in Entergy's case, it switched from T&M to unit pricing in its contracts).

  • Source tree trimming through multiple vendors in distinct market areas, allowing better vendors to increase their market share.

  • Increase use of the proper equipment for the work, including high-rise bucket trucks for high overhang and vertical saws where appropriate in rural roadsides.

  • Control reactive trimming by integrating it with cyclical trimming so cycle busters can be noted and skips can be charged back to the contractor.

  • Inspect work to ensure compliance and avoid callouts.

  • Continue use of low-volume basal spraying of selective herbicides to kill woody stems, but leave ground cover to reduce stem density and to reduce future trimming/removal costs.

  • Within the year, prioritize circuits that are due for trim this year to be trimmed earlier or later based on pre-inspection and determination of need.

  • Begin to target problem-prone overhang and danger trees, at least on the backbone and where circuits have been prone to broken limb/fallen tree trouble. (Entergy is still refining its efforts in this area, under a program they call Sky-Lining, not to be confused with ground-to-sky clearing, which is done in many transmission corridors at great expense.)

Entergy achieved a 30% reduction in the cost per mile to trim.

If there were a fourth stage in achieving excellence in vegetation management, it would be excelling at the community relations aspect of vegetation management. While some such effort may come during the “optimize the cycle” phase, for many utilities it may be the icing on the cake after all the other steps are taken. Entergy recognized this aspect when a very active tree-conservation group took it to task in the early 1990s for its trimming practices. Entergy quickly decided it was best to work with such groups and, as a result, helped to form a joint utility-community program to work with the community on the issue. In the process, Entergy learned how to make its vegetation practices arboricul-tural-friendly, including:

  • Pruning trees instead of trimming them

  • Helping customers understand when pruning actually helps a tree

  • Offering to replace selected trees with smaller ornamentals, or to plant new trees in suitable locations to offset removals under power lines

  • Ensuring that underground construction does not damage roots

  • Making wood chips and logs available to those who need them.

Entergy's success can be traced to the way it addressed the full spectrum of what must be done on several different reliability fronts, in its case, virtually simultaneously.

Danny Taylor has the BSEE degree from Mississippi State University and is a registered professional engineer in Mississippi. He has 23 years of experience with Entergy, primarily in distribution operations and engineering. He has been in management since 1986 and has held various positions in Mississippi and Texas.

Dan O'Neill is a director at Navigant Consulting, Inc. with the Electric & Natural Gas Distribution practice area. O'Neill has more than 16 years of industry experience, including four years as a utility financial executive and the remainder with major consulting firms that serve the industry. He has consulted on decision analysis, activity-based budgeting, work management, and information systems planning. He holds the Ph.D. degree in economics from MIT, taught at Georgia Tech's College of Industrial Management, and is a past president of the Atlanta Economics Club and the Atlanta Chapter of The Planning Forum.