Road to System Resiliency Varies, but all Benefit From Taking a Closer Look
BY NELSON G. BINGEL III, NATIONAL ELECTRICAL SAFETY CODE
Reliability metrics for electric systems came about as demand for more reliable power grew; especially with the evolution of computers. Service interruptions caused during major storms, however, often were not included in the reliability indices. There was no measure of system performance in major storms.
The devastation and service interruptions caused by Hurricane Sandy in 2012 brought national attention to the resiliency of the electric grid and telecommunication systems. State public utility commissions, the U.S. Department of Energy (DOE) and the Office of Electric Reliability, among others, are developing metrics for system performance in major weather events.
Data available on the structural resiliency of overhead lines might motivate regulators and utilities to reassess pole management practices that can save money, reduce outages, shorten restoration time and costs and keep the power on for the customers who pay the bills.
Though I raise the issue as chairman of the National Electrical Safety Code (NESC), action on this issue will fall to regulators and utilities, as the NESC is neither a complete design guide nor a how-to manual.
First, a review of the concept of resiliency and the role of pole management in structural resiliency which prevents a reduction in system resiliency is needed. This will provide context for a look at data that underscores the documented value of comprehensive pole management programs.
System resiliency for overhead electric and telecom lines is described as the ability of a system to withstand a major storm to minimize service interruptions along with how quickly service is restored. Many factors contribute to system resiliency, including system monitoring and communications, sectionalizing, redundancy and overall utility storm preparedness, among others. Another factor often overlooked is the structural resiliency of the wood poles that support overhead lines.
The Role of Wood Poles
The NESC’s design criteria are applied by utilities as construction criteria for distribution systems, and in large part for transmission systems. The NESC ice and wind loading plus the strength and load factors for the construction grade are primary factors in providing the lines’ original “structural resiliency.” Once poles are installed, however, subsequent structural resiliency depends on how well utility companies maintain the original structural resiliency.
Wood pole manufacturing includes full-length treatment with a preservative that helps prevent decay deterioration for many years. At some point, however, in-service wood poles may decay in the section from groundline to 18 inches below, which is, of course, out of sight. There usually is not enough oxygen in the soil to support decay deeper than that.
Decay in the groundline zone directly reduces structural capacity and resiliency. The NESC allows up to a one-third reduction in the required bending strength before a pole is deemed a “reject” and must be restored or replaced.
Pole Management Practices: Superficial or Comprehensive?
Most utilities inspect some portion of their poles during an annual program, but their approaches vary and the results related to structural resiliency are widely different. Some utilities simply require sounding a pole with a hammer, above groundline. This method identifies less than half of current “reject” poles (finding only the worst of the worst) and few of the poles with earlier stages of decay that require maintenance or restoration or both.
In contrast, a comprehensive pole inspection program uses multiple traditional inspection methods and includes excavation below groundline for greater accuracy in assessing a pole’s condition. Such a program also includes removing early decay and applying supplemental preservatives to control the decay process through the next cycle.
Several cycles of a comprehensive pole management program will maintain the structural resiliency and significantly extend a pole’s service life. Data shows that the national average pole life without pole management is approximately 45 years. The average life occurs when half of the poles are likely to have a remaining bending strength that is below code requirements.
In contrast, the national average life of poles within an effective pole management program is 73 years. This life extension is accomplished through multiple inspection and treatment cycles that help prevent the reduction in structural resiliency. The total cost over the extended life of the pole is likely to be $250 or less. This total cost represents 5 percent to 8 percent of pole replacement costs, ranging $3,000 to $5,000. In addition, the cost of applying preservative treatment can be considered for capitalization due to the significant betterment of an asset through life extension.
|FIGURE 1: Neighboring Utility Companies Exposed to the Same Hurricane|
|Comprehensive wood pole management maintains grid structural resiliency, reduces costs: Utility programs vary but data points to benefits of taking a closer look|
A/B Comparison of Structural Resiliency Affecting System Resiliency
There have been many instances where one major storm event impacted neighboring utilities. Structural and system resilience occurred at the different utilities, but the extent of the system resiliency and structural impact was different. Data taken from public reports on how two neighboring utilities fared following a hurricane is presented in Figure 1. The numbers for Utility B were adjusted to account for having 60 percent more poles.
The comprehensive groundline management program of Utility A included excavation and supplemental treatment and achieved a 98 percent efficacy for retaining structural resiliency. Utility B had a less rigorous program that omitted effective supplemental preservative treatments to control decay and retain structural resilience. The efficacy of this program is rated significantly lower because many more weakened poles remain in-service and continue to decay.
As Figure 1 illustrates, Utility A replaced 152 poles following the event while Utility B replaced 2,790; different by a factor of 18. The restoration costs for Utility B were 16 times higher than the restoration costs of Utility A; $310 million vs. $20 million, resulting in a factor of 16.
Figure 1 also shows that Utility A had fewer weakened poles resulting in fewer outages, faster restoration time and a far lower cost of restoration.
It’s important to point out that, while hurricanes provide a dramatic example, weather phenomena such as ice storms, which are nearly ubiquitous if unpredictable across the nation, can be just as devastating. Such storms have wreaked havoc in many states, leading state commissions to require improved pole management programs.
Action on findings such as those discussed above fall to state regulators and the utilities. In the wake of various major weather events, regulators in California, Florida, Kentucky, Missouri and other states have established more prescriptive pole management programs. Utility commissions in additional states might find it beneficial to review aspects of pole owner wood pole management programs. They could find answers to questions like: What kind of supplemental preservatives are incorporated? Are the costs of preservative application capitalized? And, Does the program result in lower overall costs to consumers and retention of structural resiliency?
It’s important to emphasize the new mantra: “Structural resiliency affects system resiliency,” and “Data analysis on effective pole management programs point to a positive cost-benefit ratio. A more comprehensive, proactive approach goes to the heart of utilities’ traditional mission: to provide electric power or communications that is safe, affordable, reliable and resilient.
To learn more about the NESC and related products, visit: standards.ieee.org/nesc/.
Nelson Bingel III has 30 years of industry experience focused around structural issues related to overhead lines. In addition to being NESC’s chairman and Chairman of the ASC O5 Committee, which develops specifications for new wood poles, Bingel is vice president or product strategy for Osmose Utilities Services Inc. In this capacity, Bingel overseas research and development of improved structure inspection processes, preservatives and restoration systems. He also serves on several IEEE technical committees for overhead lines. He received a BSME degree from Purdue University