AFTER SAFETY, ELECTRIC RELIABILITY IS THE MOST IMPORTANT RESPONSIBILITY for line clearance contractors and right-of-way vegetation managers. Responsible arborists want to accomplish this goal without damaging valuable trees, and operations managers want to do it as efficiently and cost-effectively as possible. These two goals do not have to be at odds, however, and implementing a few changes in a vegetation management program can result in significant savings for utilities.
Although necessary for safety issues, line clearance efforts have been shown to have little effect on outages. Just pruning trees away from lines will not stop all the outages. Several studies conducted by experienced right-of-way managers across the continent have shown that trees growing into power lines actually caused less than 14% of the outages for all utilities contacted.
The data further showed that trees that fall into the lines — often from outside of the rights of way — cause 66% to 94% of the outages.
Current research suggests that the trees causing outages are in clusters in specific areas along specific circuits. They are not uniformly spread across the system. Outages are often initiated from events that occur off the rights of way. This is probably because problem trees in the rights of way have already been pruned or removed. Outages are most often caused from a few species of trees (usually one) that often has a primary failure characteristic within that species, such as included bark or poor root systems. In a recent study of several circuits in California, four species caused 72% of the outages. On any one circuit, usually only one species was causing 80% of the outages.
MANAGING OUTAGES EFFECTIVELY
Keeping this information in mind, Ken Finch and Craig Allen at Niagara Mohawk, a National Grid company, developed a special program to reduce outages with outstanding results. They developed the Tree Outage Reduction Operation (TORO), which focused on specific areas along problem circuits.
They treated three-phase 13.2-kV feeders from the substation to the first protective device. The treatment included hazard tree removal, enhanced overhead work and, of course, normal specification trimming. By targeting the areas that had the most outages and the trees that could cause future outages, they were able to reduce customer outages by 94% on average.
DATA ANALYSIS IS KEY
Right-of-way managers also can decrease their outages by collecting and analyzing data on their own systems. Suggestions include:
Evaluate and analyze current and historical data. Use good records to identify the locations and trends. If this information is unavailable or inaccurate, it is necessary to begin collecting data on outages.
Look for problem areas (circuits and clusters) by tracking the locations of the outages. With current GIS and GPS tools, this is inexpensive and relatively simple. It does not have to be overly precise because the general location will indicate clusters.
Identify trends in species and failure types. The remedies applied will depend on the species and failure types, but whatever the remedy, it only has to be applied to specific areas or clusters.
Evaluate current electric system configurations that will have a significant effect on tree-caused outages. The remedy may be an engineering solution.
Consider a feasibility analysis to demonstrate the need and results. Decisions must have a financial basis and provide value. By targeting fund-generating projects, managers can show that the money is being used efficiently and is well invested in outage prevention.
Perform ongoing monitoring or at least a statistical review to continue to improve the system over time. As clusters are treated and controlled, new areas can be targeted.
Use a dedicated, accurate database system. There is not that much data to analyze, so large computer systems are not necessary. It is important that the data are readily available and can be easily evaluated.
Have educated servicemen or foresters collect the data. Species identification and failure causes are sometimes difficult for linemen or troubleshooters to determine. Utilities have found up to a 65% reduction in the listed tree-caused outages just by having a forester examine the outages. At times, “tree caused” can become a catchall for difficult-to-identify outages.
|Number of Customers Interrupted||Number of Customers Interrupted||Percent Improved|
|Circuit||Circuit Number||Date Completed||Pre-project||Post-project||Pre/Year||Post/Year|
|Brunswick||26453||Sept. 1, 2000||1814||55||680.3||41.3||94%|
|Boyntonville||33351||Aug. 15, 2000||4303||108||1639.2||78.5||95%|
|Curry Road||36555||May 15, 2000||4114||299||1673.5||193.9||88%|
|Grooms||34555||May 15, 2000||2803||71||2038.5||43.7||98%|
|Interruption Data from 1998 to 2001 year-end. Maintaining clearances on circuits shows immediate reliability improvements.|
WHAT DATA TO COLLECT
If the utility does not already have records on outages, a recent outage study by Davey Resource Group found that useful data include:
Location: It does not have to be very precise; span is close enough. The goal is to find the general area or cluster along a circuit, not the detailed location of each outage.
Species: This is critical. It will help identify trees that could cause outages in the future.
Type of failure: This helps guide what type of work and what crews will be needed to correct the problems. For example, large removals can be handled more efficiently with dedicated removal crews.
Condition: Listing the condition of the tree that contributed to failure (such as conks [the fruiting bodies of a decay fungus] and poor mechanical structure) allows the work planners to better target trees in jeopardy.
Tree density: This often helps identify clusters. A disproportionate number of outages come from areas with high tree density. It can also indicate the concentration of work in a specific area.
Proximity: Proximity to lines tells how far the trees are from the lines. Often it is off the right of way and can be some distance from the lines.
Weather: Weather can be used to determine if the outage was preventable, or if the storm was so severe that normally good trees were knocked down.
Soil-condition data (grading cuts, erosion, compaction and saturation): These are primarily used to explain and later predict root-caused tree failures. Again, if it can help target the trees that need to be investigated, it can help focus the work planning.
Aspect: Finally, this may have possible impact. The prevailing winds may stress trees on one side of the lines more than the other. This is not totally clear, but it is being considered in current studies.
Analyze the outage data collected by location (cluster areas), species, species by failure type and location by species. By following these steps, corrective programs can focus on treating probable outage-causing trees. An experienced arborist can identify the trees efficiently, and the most effective treatment method for the problem can be implemented in a planned, systematic manner.
The long-term savings to utility companies from such a program could be significant. Consider that an estimate from EPRI places the cost to society from outages at US$119 billion per year, and the estimated cost for rights-of-way line clearance and vegetation management is estimated at just over $2 billion a year. Investing more money in improving these vegetation management programs could be money well invested.
Ward Peterson is a utility resources manager for the Davey Resource Group. Peterson provides technical advice for utilities across North America. He has a bachelor's degree in urban forestry and a master's in business administration. firstname.lastname@example.org
HOW DO TREES CAUSE OUTAGES?
Trees cause outages in two general ways: mechanically and electrically. The first way, mechanically, is simple and readily identified. The tree (or branch) falls, breaks the conductor, interrupts the circuit and causes an outage. The conductor is on the ground in pieces, and part of a tree is lying on the ground as well.
Electrical outages, however, are more involved. When a branch lies across two conductors, the electricity will seek the path of least resistance and try to complete the circuit through the branch. The consulting firm ECI came to the following conclusions:
The larger in diameter the branch, the greater the risk.
Variation in bark physiology and physical characteristics may explain differences in the outage potential among species.
Live branches present a greater risk, especially at lower voltage gradients.
Moisture content is less a factor than one might guess, at least in the range tested.