Abstract � Severe weather events such as ice and tropical storms routinely cause extensive damage to electrical distribution systems. Much of the damage and service loss can be attributed to trees. Events where service restoration spans days or weeks are generally closely examined through regulator directed hearings. Even when no specific actions are subsequently ordered, the common theme is that utilities should find ways of reducing the impact of future storms. However, utility pruning that achieves the greatest service reliability may substantially impact tree form. These alterations of tree form often elicit a negative reaction from property owners and communities. While utilities justify their pruning as an effort to improve service, they have not had a quantifiable means of determining the extent of that improvement. The methodology outlined for calculating the increased risk of service interruption attributable to branches overhanging conductors should prove useful in communicating the impacts of both utility pruning and community restrictions placed on that pruning.
Index Terms - Power distribution reliability, power distribution maintenance, storms, power system restoration, prediction methods, vegetation overhanging
Hazard tree: a structurally unsound tree that could strike a target, such as utility lines, when it fails. Bole: that part of the trunk of a tree beneath the point where branching commences.
this paper presents a mathematical approach to determining the increased risk of electric distribution system service interruptions arising from maintenance that tolerates branches overhanging conductors. The next section provides a context by establishing that branches overhanging electrical conductors both pose a risk to the continuity of service and is a risk utilities wish to mitigate.
Electric utilities are faced with simultaneously assuring reliable service while appeasing public concern for tree preservation. When ice storms, hurricanes or other widespread high wind events occur, the electric system is often revealed to be vulnerable to extensive damage and extended service interruptions . Six reporting utilities faced 44 major storms between 1989 and 2003, affecting more than 12 million customers and causing almost 250 days of power outages . At a time when even momentary outages can cause substantial disruption to business , the average duration of these 1989 to 2003 power outages was 5.6 days . A subsequent Edison Electric Institute survey of 14 electric utilities identified 81 major storms between 1994 and 2004 costing the utilities over $2.7 billion in damages . While the cost to utilities may be catastrophic, exceeding all operating income, it is only a fraction of the regional economic losses associated with the loss of electric service .
In the aftermath of damaging weather events, the tone from politicians, regulators, media and public is often accusatory, laying blame for the service interruptions on the utilities  . For investor-owned utilities, which are not free to set their own electric rates but must apply to a regulator to have rates approved, including the recovery of storm remediation costs, the public relations aspects of service reliability must be managed . A public frustrated and angered by enduring the cost and inconvenience of loss of electrical service makes a hostile political climate for the recovery of storm restoration costs. Public relations would be better served if the expected performance of the electric system based on regulator, community, and public imposed conditions such as limiting tree to conductor clearances,  could be clearly communicated in advance of any stress on the system.
The majority of storm damage is the result of tree-conductor conflicts  . There has been very little quantitative work linking tree to conductor clearance, pruning types and total power line exposure to trees with electric system performance under storm stress loadings. While some work has been done to find the relationship between the frequency of maintenance activities and electric system performance, the work is focused on normal operating conditions . Consequently, the utility industry has difficulty articulating how trees and the electric system will interact under storm stress.
Media articles in the aftermath of storms causing extensive electric system damage reveal a commonly held belief that much of the damage could have been averted through a more current or aggressive tree trimming program  . This belief is erroneous. Electric system damage during major storms is predominantly a result of tree or branch failures  .
A normal component of a utility vegetation management program is the identification and removal of hazard trees. However, an Eastern Utilities study found only 44% of trees that failed under normal operating conditions (winds of 72 to 96 km per hour) had an indicator of structural weakness . The Eastern Utilities study reveals that as winds approach 96 km per hour the number of apparently fault-free trees that fail already exceeds the number of structurally weak trees that fail. Therefore, under severe storm stress loadings the majority of failed trees will have had no indicator of structural weakness. As apparently healthy trees or limbs that meet the tree to conductor clearance specifications are not removed during a maintenance event, a more current vegetation management program holds little potential to decrease storm damage.
To assess whether any type of pruning holds the potential to avert major storm related tree-caused outages it is necessary to consider the specific electric system vulnerabilities associated with different types of risk exposure.
IV. Tree and Stress Load Interaction
There are two types of stress loading trees experience that lead to tree-caused outages: wet snow or ice loading and wind loading. Wind loading causes branch or whole tree failures, resulting in tree parts falling into, across or through conductors. Tree windthrow has been found to increase with tree age and height, with softwoods being more susceptible to windthrow than hardwoods . Wet snow or ice loading leads to two modes of service interruption: trees or branches bending to lie on or across conductors and, branch or trunk failures with tree parts falling into, across or through conductors. The northeast ice storm in 1998 revealed damage was more extensive to hardwoods than softwoods; larger trees suffered more crown damage while smaller diameter trees (12.7 cm � 25.4 cm) were more susceptible to leaning (greater than 45� angle).
Tree and branch failures can cause electrical faults by bridging phases, pushing conductors into each other or by physical damage to equipment that disrupts the circuit. When branches bend or trees lean to lie on conductors, without pushing phases into such close proximity so as to cause a direct fault, whether a fault occurs is dependent on a number of variables including the voltage gradient, branch or tree diameter and tree species .
It emerges that there are a few pruning practices that will lead to reductions in tree-caused outages during severe storms and also, that there are conditions which can only be mitigated by managing the total tree exposure of power lines . Crown reduction pruning of the 12.7 cm � 25.4 cm diameter trees so that they cannot contact a conductor when they bend under load and the removal of overhanging branches that could either bend to lay on conductors or break and fall into or through conductors will reduce storm-caused interruptions. While pruning to remove overhangs does not preclude the possibility of windthrown branches from the crown causing service interruptions, it removes the most direct threat: that of a downward fall of a tree branch. The investigation of the December 2002 ice storm in the Carolinas found that the municipalities with the most restrictive tree-trimming ordinances and greatest amount of overhanging branches suffered the most electric system damage and customer outages .
Siegfried Guggenmoos is a Professional Agrologist, active in industry associations and reputable in the Vegetation Management Industry for knowledge, range of experience, financial accountability and environmental sensitivity. He contributes clarity to problem solving, innovation and new interpretations combining technical, environmental and financial domains.
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