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Whether it’s ice storms or wind storms, most of the damage to the electric system is due to tree failures. The only type of pruning that will avert these outages is the removal of overhangs (video for more detail on overhangs: Storm Hardening the Electric System). Tree-caused outages are correlated to the extent of electric system tree exposure (See 1, 2 & 3). Reducing tree exposure entails widening right-of-ways, increasing conductor height, drastically reducing tree height or undergrounding. We know what utilities need to do to decrease storm damage. However, utilities cannot implement the mitigation measures. The public wants undergrounding only if the utility bears the cost. The public wants no part of the other three means of decreasing tree exposure.

The current state of knowledge regarding tree-caused outages and their mitigation is detailed in Managing Tree-Caused Electric Service Interruptions. In Managing Tree-Caused Electric Service Interruptions I show it is likely during major storms that the number of hazard trees causing outages is not statistically significant. That is, storm outages are caused by the failure of apparently healthy trees, trees that are not targeted by the VM program for anything more than pruning. One could argue that as pruning reduces branch length and foliage surface area, failure rates will be lower for recently pruned trees. However, we have no data that quantifies the difference in failure rates.

In summary:

  • All trees that can interfere with electrical service are a liability; a risk to that service.
  • During major storms most damage is caused by the failure of healthy trees that are not targeted by typical VM programs.
  • Unless the current VM program maintenance cycle is and has been far too long, increasing the intensity of the hazard tree program will not reduce major storm damage.
  • The only way to reduce the number of storm related tree-caused outages is to reduce the electric system’s exposure to trees.
  • The benefit of the best-in-class VM program is expressed as greater reliability and less system damage during the more frequent, minor storms not during major storms.

Best-in-class utility VM programs have very few outages caused by trees growing into conductors. Thus successful outage mitigation strategies include removing branch overhangs, shortening the hazard tree cycle to the point of diminishing returns in reliability improvement and foremost, reducing the electric system’s exposure to trees. All of these strategies are resisted by the public as the trees involved are almost exclusively on private property. Regulator help in communicating to the public that electric reliability is not a utility issue but rather a social issue, as there are direct, scientifically proven relationships between the extent of tree exposure, overhangs and hazard trees and service reliability, would be greatly appreciated by VM professionals. Such support is often lacking within the utility as management declares a change of course when there is a public outcry against the initiatives undertaken by the VM group. That was one of the contributing factors to transmission rights of way, where there tend to be clear easement rights, being grown over and containing trees requiring pruning as was widely revealed with the adoption of NERC standard FAC 003-01.

Indeed for regulators with the courage of their convictions, tree-related service interruptions can be dramatically reduced even during major storm events. The process, ameliorating factors and avoided storm costs are detailed in The Neglected Option For Avoiding Electric System Storm Damage & Restoration Costs - Managing Tree Exposure.

In Part 6 we will wrap up this series, by providing some recommendations, questions and information requests regulators should put to their utilities that will ensure that senior utility management engages VM professionals rather than discounting or undervaluing their special expertise and that tree-related service interruptions are minimized for the community accepted standards.