T&D World Magazine

Managing System Performance Expectations

Last month I wrote about the need for utilities to prove the effect that the regulators’ limiting of funding and the public rejection of effective tree-caused outage management practices have on the extent of system outages. In doing so, realistic electric system performance expectations could be set. However, the topic is broader than that. Regarding VM, utilities should articulate what the key message is they wish to communicate. All communication going outside the organization should be checked to ensure it is aligned with the key message. Managing expectations is difficult.

Sara Sankowich at Unitil brought the following to my attention and thought it would provide a good springboard to discuss the need to manage expectations.

How Aggressive Trimming Minimized Outages
Since more than 90% of all power outages during a storm with high winds or heavy snow are caused by trees, PSNH pursues cyclical tree trimming in all of its service communities to help ensure electric service reliability in the worst of weather conditions. Vegetation management conducted by PSNH and its contractors during the six to 12 months previous to Irene’s arrival helped substantially to reduce outages once the storm hit.

Chances are high that PSNH was criticized for implementing a more aggressive VM program. It may have lacked regulatory support for its VM program. In that context, the above statement may be viewed as a justification and a solicitation for support. However, the information contained in this release contradicts the current state of knowledge regarding storm caused tree-related outages. As such, there is no guarantee that the next major storm to come along won’t devastate the electric system. Indeed, along came Snowtober and took out about half the system.

What is the state of knowledge regarding tree-caused outages? For an in depth look see Managing Tree-Caused Electric Service Interruptions.

During storms, wind, snow or ice loading results in branch or whole tree failures. These failures cause phase-to-phase faults, knock the conductor to the ground or break the conductor. It’s possible that when pruning has fallen behind, that some outages may arise from swaying trees slapping phases together. However, if this were a primary cause of outages, restoration would be much quicker and cheaper. There would not be much requiring repair. There wouldn’t be much need for tree crews once the wind stopped blowing. Even an optimal pruning cycle that does not eliminate but economically manages hot spots will contribute little to avoiding storm damage.

Generally, utilities do not know their total tree exposure nor the local natural tree mortality rate. As a consequence, there is no expectation for the number of hazard trees forming each year. However, it need be recognized that simply due to competition between trees for light, water and nutrients, hazard trees are continually developing. Due to the lack of tree exposure and mortality data and the fact that it is not economically feasible to inspect for hazard trees 20, 30 or more feet inside the forest edge, I believe utilities greatly underestimate their hazard tree exposure. This is an area where shorter inspection cycles and more rigorous efforts to identify failure risks may pay dividends in reduced tree-related outages.

However, I have often heard it said after a storm that more than 90% of the tree failures arose from trees without apparent faults. Considering a hazard tree is, by definition, a tree that will tolerate less stress loading before failure, it is more likely that the benefit of a good hazard tree program will be realized during minor storms that fall within normal operating conditions. Due to pressure to operate in a fiscally prudent manner, utilities are left to choose the maintenance cycle that delivers a reasonable balance between expenditures and reliability. Typically the maintenance cycles fall in the three- to eight-year range. Hazard trees are dispersed across the electric system and because it is impossible to monitor the whole system in real time for the development of hazard trees, there is necessarily a residual hazard tree population.

While data I have is limited based on the inspection of trees that caused outages on Eastern Utilities, National Grid Transmission and Puget Sound Energy, 56-68% of these trees have no discernible fault that would see them classified as hazard trees. While the information would be extremely useful, I know of no utility that investigates and records detailed outage causes in the aftermath of a major storm. Consequently, if up to 68% of trees that caused outages during normal operating conditions are apparently healthy trees, then it follows, the hazard tree population being limited, that this percentage of healthy tree failures will be even higher during major storms.

In Managing Tree-Caused Electric Service Interruptions I show it is likely during a major storm 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 light of this information what is/are the key message(s) I would suggest for utilities wishing to manage system performance expectations?

What then do you say when your system escaped a storm with substantially less damage than neighbouring utilities? Try this.

“Because tree-related outages during major storms are predominantly due to the failure of apparently healthy trees that neither we nor our communities want us to remove, and given our system has considerable tree exposure, simply put, we were lucky. Let’s count our blessings.”

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