A study funded by the Public Utility Commission of Texas investigates the costs and benefits of practices to strengthen the distribution grid.
When Hurricane Ike made landfall in Galveston, Texas, U.S., on Sept. 13, 2008, its Category 2 winds extended 275 miles (443 km) from the center. Behind Andrew in 1992 and Katrina in 2005, Ike was the third-costliest U.S. hurricane of all time, causing more than 13 million businesses and homes to lose power.
Typically, distribution systems are not designed to survive major weather events like hurricanes, including direct damage from wind and storm surges, and indirect damage from falling trees and flying debris. Many are beginning to wonder whether it would be beneficial for utilities to harden their systems so that they incur less storm damage and have faster restoration times.
In December 2008, a few months after Hurricane Ike, the Public Utility Commission of Texas hired Quanta Technology to investigate the costs and benefits of a variety of potential storm-hardening projects. Part of this investigation included a utility survey on best practices for distribution hardening.
A Dozen Best Practices
Based on the overall survey results, a list of 12 best practices was assembled to ensure distribution hardening is pursued through a process that is cost effective, consistent, transparent and data driven. The full survey report discusses an additional six recommendations for utilities that would like to be more aggressive.
The best practices are organized into two stages. In the first stage, the best practices are either inexpensive or good practices regardless of hardening considerations. They are also relatively simple to implement. In addition to being potential quick wins, they set the foundation for more ambitious actions. The best practices in the second stage are designed to be implemented in the intermediate term and generally require more utility effort, investment and potential changes.
First-Stage Best Practices
The first-stage best practices are no-brainers:
- Pole test and treat
Wood poles are susceptible to decay, causing a reduction in strength and a corresponding increase in failure probability during a major storm. As such, utilities should establish and maintain a test-and-treat cycle for wood poles. This program should focus on, but not necessarily be limited to, decay at the ground line since this is typically the failure point for distribution structures under wind loading and the part of the pole most susceptible to decay. The goals of the test-and-treat program are to ensure no pole has lost more than one-third of its original strength and no pole is likely to have lost more than one-third of its original strength before its next scheduled inspection. This program should ensure deficient poles are reinforced or replaced in a timely manner.
- Feeder inspections
Second-Stage Best Practices
Utilities should have a formal feeder inspection program that periodically examines feeders for problems that will likely lead to an outage during normal and/or storm conditions. At a minimum, all three-phase main feeder trunks should be inspected every five years, although more aggressive programs are encouraged. Inspectors should be trained to watch for specific issues such as broken crossarms, cracked insulators, pole-top decay and so forth. The feeder inspection program is separate from the test-and-treat program. Information from these inspections should be kept in a common database, facilitating the analyses of trends and backlog.
- Attachment audits
Attachments are a source of wind loading on poles. Therefore, it is important that utilities have a good understanding of the number and size of third-party attachments on their distribution poles. Third-party attachment audits should occur, at a minimum, every five years for all three-phase main feeder trunks. The attachment audit can be combined with feeder inspections if desired. Processes should be in place to identify new attachments, to determine whether the new attachments have overloaded the distribution poles and to mitigate overloaded poles.
- Foreign-owned poles
Hardening Road Map
Not all utilities own all of the poles on which they have equipment. Major storms do not distinguish between pole ownership when inflicting damage. Electric utilities should try their best to ensure foreign-owned poles are in as good of shape as their own poles in terms of remaining strength and loading. The processes addressing foreign poles can vary widely, ranging from the electric utility performing all inspections and maintenance to the electric utility ensuring the foreign owner is doing an adequate job.
- Setting depths
A strong pole is of no use if its foundation is insufficient. Therefore, each utility should develop standards and processes to ensure the foundation of a distribution pole will not fail before the pole. These standards should have setting depth tables for poles of different heights and classes and for different soil conditions. Tables also should be made for very strong poles, including non-wood poles, which may be used for hardening purposes. The standards should describe how a setting depth calculation should be performed when none of the tables apply.
- Loading calculations
The ability of a distribution pole to withstand extreme loads, such as wind and ice, is a direct function of its loading. A utility should have systems and processes in place to ensure poles do not become overloaded after they are initially installed. At a minimum, this should include a loading assessment whenever an additional piece of equipment is placed on the pole, a loading assessment whenever a new attachment is discovered on the pole and mitigation actions as appropriate.
The second-stage best practices generally require more from the utility:
- Grade B construction
Most utilities already have a stronger distribution grade of construction they use in special situations, such as those used for railway and highway crossings. For most U.S. utilities, the stronger standard corresponds to National Electrical Safety Code (NESC) Grade B. Based on the utility survey, the use of Grade B for storm hardening is popular, effective and easy to implement. This recommendation calls for utilities to have an explicit process to review new construction and rebuilds to decide whether the system should be built to Grade B, or equivalent, rather than a weaker standard. The intent is not to have utilities build all-new distribution construction or upgrade all existing distribution structures to Grade B. Rather, its intent is to have utilities review distribution construction projects and decide whether Grade B is appropriate or not. For example, for a line-relocation project necessitated by the widening of a coastal road, it may be worthwhile to consider Grade B construction if the exposure to extreme winds is high enough.
- Non-wood poles
There are many reasons, including hardening, why a utility may, in certain cases, wish to use a non-wood distribution pole. Most are not susceptible to decay and most can be very strong without requiring heavy cranes for installation. At a minimum, utilities should have standards for at least one type of non-wood distribution pole, and they should install some on their system to gain field experience. The intent is not to have utilities build new distribution construction or upgrade existing distribution structures with non-wood poles. Rather, the intent is for utilities to have a viable alternative to wood should this be necessary in certain hardening situations.
- Post-storm data collection
A lot of distribution damage occurs during major storms. This data is invaluable when trying to address system hardening in a manner that is most beneficial during major storms. Therefore, a utility should have a plan that has trained staff collect data on distribution damage sites immediately after a storm subsides. This data should be collected in a way that is statistically representative of the entire system.
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The intent of this recommendation is not to have a large number of data collectors who otherwise could be helping with storm restoration. Rather, a utility should train a few data collection teams (for example, three teams of two engineers) and have these teams spend the first two days of storm restoration collecting data.
- Hardening tool kit
Utilities that intend to harden portions of their system should develop a hardening tool kit that consists of a set of approved approaches to hardening and an application guide for their use. Utilities should ensure all of the appropriate standards are in place for each element of the hardening tool kit and install pilot applications for each unfamiliar element to gain field experience.
- Like-for-unlike replacement
Utilities are continuously inspecting, repairing, replacing and generally working on the distribution system. When a utility identifies a cost-effective approach to storm hardening, it should enact systems and processes that allow the system to be gradually hardened through normal work processes.
For example, a utility might identify that porcelain insulators have a tendency to break during storms. Therefore, a like-for-unlike approach would ensure any porcelain insulators needing replacement were replaced with a composite insulator instead of another porcelain insulator. Similarly, a utility might decide it wants to upgrade certain parts of its system to Grade B. When a pole in these areas fail, the utility would ensure it is replaced with a larger pole that results in Grade B construction.
- Strengthen critical poles
A good way for a utility to gain experience in hardening is to identify critical poles that are highly undesirable to fail during a major storm. This could be because the pole is very difficult to restore (freeway crossing), expensive to restore (automation equipment) or critical during restoration (communications repeater). After identifying the critical poles, the utility should take targeted actions to strengthen these poles, such as upgrading them to Grade B or stronger, and monitor their performance during future major storms.
There are four primary motivations for storm hardening:
- To keep high-priority customers' power on
- To keep important structures standing
- To keep economic centers on
- To strengthen structures that are likely to fail.
Ideally, a utility can compute the expected damage that will occur in future storms, compute the cost of various hardening options and determine the expected damage reduction and societal benefits that will result from each of these options. This process allows for decisions to be made based on quantifiable costs and benefits, and results in a multi-year hardening road map.
Not all utilities are ready to commit to a full hardening road map. Regardless, utilities can get a good start by first focusing on about 0.l% of their critical distribution poles, followed by a more aggressive initiative addressing an additional 1%. The goal is to increase structural wind ratings and should be coordinated with parallel efforts on vegetation management; a similar best practices survey was performed in this area as well.
Implementing the 12 best practices for distribution hardening will result in a well-managed distribution system infrastructure and will establish good credibility for a utility's current and planned hardening activities. These best practices will result in modest reductions in overall storm damage, significant reductions in critical pole failures, faster restoration times and lower restoration costs.
Richard E. Brown (firstname.lastname@example.org) is the senior vice president of consulting for Quanta Technology. He has authored the books Electric Power Distribution Reliability and Business Essentials for Utility Engineers. Dr. Brown is an IEEE fellow and a professional engineer.
Quanta Technology www.quanta-technology.com