A few hours after quitting time, Friday, Dec. 23, 1994, an intense storm slammed into the southeastern coast of Massachusetts. By 10 p.m. it was raining sideways from 65-70 mph winds, and falling tree limbs were tearing down power lines everywhere. The timing couldn't have been worse for the 22,000 customers of Eastern Utilities Associates (EUA), West Bridgewater, Massachusetts, U.S. who were without electricity on Christmas Eve morning. Utility crews spent that day and Christmas day getting the lights back on.
The storm illustrated a need for EUA to implement a proactive tree-trimming program that would reduce the number of tree-related outages (Fig. 1).
To take a better look at the causes of tree-related outages, in 1995 EUA joined 13 other U.S. utilities to hire Environmental Consultants, Inc., South Hampton, Pennsylvania, U.S. to research "Tree-Caused Interruption." The study provided a database of the causes and frequency of tree-related outages, which are a driver of reliability statistics (Fig. 2).
Causes Are Dual The Tree-Caused Interruption project proved that tree failure is as much a driver of outage statistics as unmanaged growth. So, while the basis of a utility's tree-trimming program is to maintain clearance between trees and energized conductors, it is also important to mitigate falling trees and branches.
As a result, EUA designed a tree-management program that addressed both methods. Through systematic line-clearance tree trimming (SLCTT) and a hazardous-tree mitigation program (Fig. 3), EUA has successfully eliminated approximately 75% of its tree-failure outages during normal operating conditions in its Brockton service territory; and reduced its storm-caused tree outages up to 45% throughout portions of the system by selectively removing trees on the three-phase portions of the distribution system.
How Do Trees Cause Outages? Outages Caused By Tree Growth--The most widely acknowledged tree-related cause of outages is growth. While not yielding any fundamental surprises about growth-related outages, the Tree Caused Interruption research project showed that growth from the side, rather than from below the conductor appears to cause a proportionately greater percentage of outages.
Outages Caused By Tree And Branch Failure--Tree and branch failure outages are caused by trees that are close enough to distribution lines to pose problems if they break or fall. In EUA's Brockton service territory alone, there were more than 200,000 trees adjacent to the distribution system primary. At first glance, it may seem ludicrous to try preventing outages from trees or branches falling into power lines, but what if 45% of those outages could be prevented with only an incremental expense above the cost to trim them?
After the 1994 Christmas storm, a review of EUA's outage data for 1990-94 revealed that a disproportionately large percentage (57%) of tree-related 'Customer Outage Hours' were caused by less than 10% of tree-related outage events. Close to 50% of these interruptions were preventable because they were caused by trees that had an identifiable indicator of structural failure.
Of the more than 200,000 trees adjacent to primary circuits that must be managed, the number that are likely to fail and cause either significant damage to the distribution system or a major outage is relatively small. Thirty two percent of these hazardous trees are near the three-phase portion of the distribution system, and less than 4% have an indicator of failure. The end result: 12,000 hazardous trees to manage.
The tree-caused interruption research provided a statistical understanding of how hazardous trees might fail. The results for EUA indicated that 63% of outages were caused by broken branches and 11% from the structural failure of a tree trunk. Growth accounted for only 2% of outages because EUA's Line Clearance Tree Trimming (LCTT) program is on-cycle. (Dead limbs overhanging the lines and dead trees adjacent to primary circuits are routinely removed as part of this program.)
While preventing outages from branch or tree failure during storms is difficult, the research project results found that 20% of the failures occur on calm days. Most of these failures were avoidable.
What Do Tree-Caused Outages Cost? Systematic Line Clearance Tree Trimming - An examination of EUA's tree-program evolution since 1990 can serve as the basis for evaluating the costs of various tree-management alternatives.
Before 1992, EUA's tree-management activities consisted mainly of hot-spot trimming, which consists of tree crews being dispatched only when a pattern of outages occurs in an area.
This approach eliminates the immediate 'hot spot,' but does almost nothing to prevent an interruption further down the circuit.
During that time, responding to tree-related trouble calls accounted for approximately 66% of the tree budget. This inefficient method led to an increase of approximately 20% in the tree-trimming budget between 1988 and 1990 just to keep up with the escalation of trouble calls.
As a result of this cost inefficiency, EUA made a commitment to implement a SLCTT program in the Brockton service territory. From 1992 to 1994, EUA invested US$1.5 million in actual line-clearance tree trimming to get the system on a four-year cycle. Through that initial investment, the number of tree-caused outages was reduced by almost 55%; and by 1994, the amount of money spent on trouble calls was reduced to only 23% of the budget.
When devising a tree-management budget, remember that if US$100,000 is deferred from the SLCTT budget, it cannot simply be added back in the next year to achieve the same results since trees continue to grow. On average, for every dollar deferred in a year, it costs up to three times that amount the following year to regain clearance.
Figure 4 depicts the overall cost of tree management, including a full cost analysis of the establishment of a SLCTT program versus the incremental cost associated with hot-spot trimming and a resulting increase in trouble-call responses.
Hazardous-Tree Mitigation--Managing outages with SLCTT has no corollary effect on the tree damage incurred by a wind storm; and individual outages caused by the structural failure of trees and branches are more expensive than growth-caused outages. Tree failure usually damages conductors and sometimes poles, which requires time-consuming repairs and equipment replacement.
For the past two years, EUA has focused on finding a way to reduce tree-caused outages in adverse weather. The Hazardous-Tree Mitigation project consists of removing trees that have an indicator of structural weakness and storm-proof pruning overhanging portions of trees adjacent to the three-phase primary circuits. Although removing trees can be costly, many of the outages they cause can be more costly. Figure 4 includes a cost comparison of the expense of hazardous-tree mitigation and the measurable costs of damage caused by tree and branch failure. Clearly, the average cost of removing or storm-proof pruning trees along the three-phase portion of the system is less expensive than the expense of emergency restoration.
Cost justification for the project is based on four factors that cumulatively support the investment:
1. Smaller trees are less expensive to remove than trim, especially if the cost analysis used is estimated over the potential life of the tree.
2. The cost to repair the damage from outages caused by tree or branch failures on three-phase lines is generally more expensive than on single-phase lines. This is the single most significant direct-cost consideration.
3. During outages, the meters stop turning and revenues stop. Even though this is the most obvious factor, it had the least influence because the percentage of the total revenue stream affected in a localized storm is minimal.
4. Structurally weak trees are a mutual hazard to power lines and public roadways. If a publicly owned tree is a mutual hazard, it is in the community's best interest to have it removed. Therefore, most communities provide some logistical support, labor, and equipment to work alongside EUA's tree-contractor crews. This had not been a readily accepted practice before, but has become so in most service territory communities due to a substantial commitment by the utility to communicate the concept of 'mutual hazards.'
In addition to the above financial benefits of a hazardous-tree mitigation project, there are associated hidden costs that are saved as well. A record consisting of frequent, long, and inexplicable outages can cost a utility dearly in customer trust and loyalty, which eventually costs real dollars to correct.
Trees and Reliability Most of the Christmas Eve storm outages were from downed limbs and trees. At the time, this was particularly unsettling because EUA had made a substantial investment in its LCTT program. It did pay-off though, since the overall number of outages from tree growth declined by about 89% in 1994.
At EUA, two indices measure reliability. System Average Interruption Frequency Index (SAIFI) measures the number of times the average customer is interrupted during a specific time period. System Average Interruption Duration Index (SAIDI) measures the total length of time in minutes that the average interrupted customer is without service during a specific time period. Even though both indices measure reliability for the system, data is collected so that individual circuits can be evaluated.
Figure 5 is a time-series analysis that shows how outages from growth drive tree-related SAIFI. The SAIFI in 1990 and 1991 represents SAIFI from hot spotting. Ultimately, hot spotting results in growth-related outages that account for 25 to 35% of the total SAIFI, which could become the primary factor in frequency-based reliability. The slope of the tree SAIFI from 1992-1994 illustrates the rate of improvement from getting a third of the territory on-cycle each year.
A reasonable expectation for a systematic trimming program on 80-90% of the system is between 5-8% of the total SAIFI. It is an indisputable fact that reliance on hot-spot trimming escalates and accelerates outages from growth. Also, outages from tree and branch failure have little effect on SAIFI.
Tree-caused SAIDI, on the other hand, is driven by tree failure. When a tree or branch falls, it usually breaks something, which can take a long time to fix, and trees usually fail in adverse weather conditions. These are well-established facts. But to what degree are they true? Prior to implementation of EUA's Hazardous-Tree Mitigation program, every storm increased the total SAIDI by 15-25% and accounted for almost 50% of tree-related SAIDI (Fig. 6). Hazardous-tree mitigation reduced tree-caused SAIDI by 62% per storm. A more disturbing statistic was that as much as 7% of SAIDI was caused by trees that fell when there was no storm. Hazardous-tree mitigation, though designed to reduce tree failure in storms, also reduced tree-related SAIDI in normal operating conditions to less than 2%. (Storm intensity and its impact across a service territory varies widely, so the data found in Figs. 5 & 6 were 'normalized' for wind speed and the percentage of distribution feeders that were involved.)
Conclusion Line-clearance tree trimming is the industry norm for managing tree-caused outages, but alone it is not the most cost-effective program, since trees not only cause outages from growth, but also from failure.
Reliability and cost are the two main factors that drive a tree-management program. EUA took a careful look at both, and came up with a program that addresses each. First, the actual tree-trimming practices were modified by changing the shape of the clearance. Less extensive trimming is done to growth below the conductors, but clearance is still maintained on a four-year cycle. This resulted in a cost savings that partially funded the Hazardous-Tree Mitigation program.
Second, the Hazardous-Tree Mitigation program and storm-proof pruning were incorporated into the tree management program. The reduction in the number of storm-caused tree outages pays for hazardous-tree mitigation. It only takes the avoided costs of one storm per year to pay for that program. No additional budget dollars have been allocated for hazardous-tree mitigation.
Implementing the new EUA tree management program required a great deal of energy initially, and was only possible with the full support of EUA management. But it has paid off. The cooperation between the utility and the communities it serves has not only saved money, but has dramatically improved reliability. A portion of EUA's cost savings is being reinvested in the communities through the sponsorship of tree-planting programs--a win-win situation for all.
Peter Simpson is manager of vegetation management for Eastern Utilities and has 20 years of experience in arboroculture. He has the BS degree in Urban Forestry from the University of Massachusetts and the MBA degree from Boston University. He was an arborist for several eastern U.S. firms, owned a tree service company and most recently was tree warden for the City of Newport. Through Simpson's efforts, Eastern Utilities has instituted a system-wide Tree Removal-Planting program, subscribed to the EPS's Pesticide Environmental Stewardship Program and achieved Tree-Line USA designation for three consecutive years.