Hardening of Utility Lines — Implications for Utility Pole Design and Use

NORTH AMERICAN WOOD POLE COUNCIL TECHNICAL BULLETIN

The Hardening of Utility Lines — Implications for Utility Pole Design and Use

Prepared by: Martin Rollins, P.E. Abstract

The hurricanes of 2005 caused unprec- tential need and effectiveness of “harden- edented damage to critical infrastructure ing” the system to improve performance on the Gulf Coast, including electric util- in future storms. This paper provides a ity distribution and transmission systems. qualitative evaluation of the expected and Signifi cant damage has also been caused actual response of the electrical distribu- recently by ice storms and extreme wind tion system in these storms. In general, events in other parts of the country. The the system performed as expected and Public Service Commissions of several potential actions to “harden” the system states questioned the performance of the could be very expensive and the potential power delivery systems in these storms, benefi ts are unquantifi able. and initiated work to investigate the po-

2 Introduction

Hurricane Camille, which struck the Mis- In December of 2006, the Paciﬁ c North- sissippi Coast in 1969, had long been west experienced an extreme wind event considered the yardstick by which all other that caused more than 1.5 million custom- storms were measured. However, the dev- ers to lose power. Also in December of astation of Hurricane Katrina in 2005 made 2006, a severe winter storm resulted in all prior storms look tame in comparison. heavy icing in portions of the Midwest. Hurricane Katrina caused over 80 billion More than 500,000 public utility customers dollars in damage and easily became the lost power in Missouri and Illinois alone. most expensive natural disaster in history. The 2005 hurricane season was possibly These extreme weather events have the most active Atlantic hurricane season caused the public to question whether on record, with a total of 28 storms, 15 more can be done to prevent these out- hurricanes and 7 major hurricanes. Four ages or reduce time to restore power, of the major storms, Dennis, Katrina, Rita, and this has resulted in the Public Service and Wilma, struck the Gulf Coast with total Commissions (PSCs) of several states damages exceeding 120 billion dollars and starting formal inquiries into the need to more than 2,200 fatalities. harden the systems.

3 Discussion

The investigations by the various state and making 1.1 million overhead splices, PSCs have not focused only on hardening but only replaced 12,632 distribution poles the system as a means to improve extreme as a result of the 2005 hurricanes. Wood weather performance. Vegetation manage- and non-wood transmission lines also suf- ment, line inspection and maintenance, line fered damage in Wilma, including a section relocation, and undergrounding are among of a major 500 KV steel transmission line. the other opportunities identifi ed for im- The result of FP&L’s forensic analysis was proved storm performance. that the poles in its system performed as expected. (Florida Power and Light Presen- The basis for reaching decisions concerning tation at June 2006 Public Utility Research system improvement must be founded in Center Workshop) an accurate forensic analysis of actual failure mechanisms. The various states have In Hurricane Katrina, Mississippi Power found that this information is very diffi cult Company (MPC), which serves most of to determine with a reasonable degree of coastal Mississippi, bore the brunt of the certainty. In some cases, it has been pos- damage, but signifi cant damage occurred sible to develop qualitative data to confi rm as far north as Starkville and beyond. The or deny some early hypotheses. In the author’s observations in the area of his Gulf Coast region, initially the performance home, which is in Gulfport, Mississippi, and of wood poles was questioned. Both the about 15 miles from the coast, were that States of Texas and Florida considered 80% or more of the wood distribution poles eliminating the use of wood transmission were still standing, but virtually 100% of poles. the spans were on the ground, primarily as a result of fallen trees and other debris. The facts did not support these concerns. The author’s communication with the Cor- In the 2005 hurricanes, transmission and porate Information Manager with MPC con- distribution poles of all materials failed. fi rmed this phenomenon. Mississippi Power Information provided to the Florida PSC in- reportedly sustained damage to about 65% dicated proportionate failures of wood and of its transmission and distribution system concrete distribution poles. Forensic stud- which consists of approximately 6,000 miles ies of wood pole failures found few failures of line. There were approximately 26,500 associated with groundline decay. Florida spans down, but they replaced only about Power and Light (FP&L) reported that only 9,000 poles out of their standing inventory approximately 1% of distribution poles that of approximately 200,000 poles. experienced hurricane force winds in Hur- ricane Wilma failed, and that most failures In September of 2005, Hurricane Rita were associated with fallen trees or wind- struck western Louisiana and eastern Texas blown debris. FP&L reported installing 930 causing extensive damage in the coastal ar- miles of overhead distribution conductor, eas. Entergy reported 77% of its 372,891 570 miles of overhead service conductor, Texas customers were without power

4 for some period of time. Approximately design loads or increasing extreme weather 8,870 distribution poles, or approximately design loads to higher values. The dif- 2.8% of its standing distribution inventory, fi culty is not in the design or construction were replaced. Approximately 5.6% of of a hardened system, the diffi culty is in its 142,358 distribution transformers were the ability to quantitate the expected per- replaced. In addition, approximately 981, formance improvement so that rational or 2.8%, of the 34,600 wood transmission decisions can be made regarding increased poles were replaced, and 26, or 2.8%, of costs, which must be paid by the consumer, the 940 steel lattice transmission towers versus an anticipated future benefi t. The required replacement. (Entergy presenta- actual loads imposed by extreme weather tion fi led with Texas PUC on January 30, events are diffi cult to quantify. The load 2006) of a 40 mph wind on a conductor covered with 1/2 inch of radial ice is quantifi able, On November 30 - December 1, 2006, but the load of an ice-covered tree blown an ice storm hit portions of the Midwest. into a line, or a section of mobile home AmerenEU reported to the Missouri PSC blown into a line is not readily quantifi able. that approximately 270,000 customers The Florida PSC considered requiring that experienced a power outage. At the peak all distribution lines be designed to the of the restoration effort, 4,391 personnel NESC extreme wind loading criteria. The were involved. A total of 214 miles of new current NESC exempts structures 60 feet conductor were installed and many miles and less in height from the extreme wind of downed conductor were reinstalled, yet loading criteria of Section 250C of the AmerenUE reported replacing only 392 code. For the last several code cycles, poles in Missouri. proposals have been made to remove the 60-foot exemption. The overwhelming Based on these fi eld reports, it appears comments received on the proposals were that wood poles performed well and that that most storm failures are associated with the failure rate of poles was much lower secondary damage effects and that design- than the failure rate of other system com- ing to extreme wind would have little effect ponents. on observed damage. Based on these fi eld comments, the NESC has retained the 60- Even though it is apparent that wood foot exemption. poles and other system components have performed as expected in these extreme Greatly reducing span length and greatly weather events, the various PSCs continue increasing the number of poles per mile is to evaluate means to improve system one way to meet the extreme wind criteria. performance. From a system design and If secondary damage effects are indeed construction perspective, there are some responsible for most pole failures, then this obvious avenues to investigate. Overhead approach could result in more pole failures systems can be made stronger by using rather than fewer and the time to restore stronger poles and other system compo- service could be longer rather than shorter. nents, reducing spans, increasing guying, The utilities in Florida indicated that requir- increasing safety factors using present ing distribution systems to be designed to

5 extreme wind criteria would double to qua- Edison Electric Institute (EEI) in 2006 (Out druple the cost. Based on this feedback of Sight, Out of Mind?, July 2006) reports from utilities, the Florida PSC dropped its that overhead lines in the State of Virginia demand that all distribution lines be de- in 2003 experienced outages at rates 4 to signed to the NESC extreme wind load cri- 5 times that of underground, but the aver- teria. Instead, they have asked the utilities age duration of the outage for underground to harden lines serving critical infrastruc- systems was about 2.5 times that of over- ture such as hospitals, police stations, and head systems. A study by the North Caro- fuel terminals, and to complete forensic lina Utilities Commission (The Feasibility of analyses in future storms to evaluate the Placing Electric Distribution Facilities Un- effectiveness of the targeted hardening. In derground, November 2003) for the years addition, Florida has mandated inspections 1998 - 2002 showed that underground at set frequencies for all lines and improved systems experienced about half the out- vegetation management for all lines. There ages of overhead, but the duration of the is a vigorous debate going on in Florida underground outages was 1.6 times longer. over how the costs of hardening will be dis- Therefore, much of the reduced frequency tributed between the utilities and the third advantage of underground is offset by the party attachers. increased duration of the outages.

Another way of potentially improving The EEI report also contains some informa- storm performance is to move overhead tion that suggests that the outage advan- systems to underground. This approach is tage of underground diminishes as the generally popular with the public until the system ages. One utility reported better potential impact on rates is understood. It reliability in 40-year-old overhead lines than is true that underground lines are gener- 20-year-old underground lines. Maryland ally less exposed to the large physical loads utilities reported underground systems associated with extreme weather events. were becoming unreliable after 15 to 20 However, they are not totally immune to years and that they were nearing the end failure in storms. Pad-mounted trans- of their service life in 25 to 35 years. formers are subject to storm surge and other secondary damage effects. On the Although a signiﬁ cant portion of new dis- Alabama coast in Hurricane Ivan in 2004, tribution lines are going underground, cost many miles of underground lines were remains a signiﬁ cant deterrent to replacing physically uncovered and destroyed due to existing lines with underground. The in- wave action and storm surge. Some loca- creased cost for installation of new lines is tions in this area were without power for typically paid by the developer who simply more than a year as a result. rolls those costs into the price of the lots be- ing sold. The rate-payer would have to bear Although underground lines may be less the cost for undergrounding existing lines. likely to have an outage, the time to lo- The EEI report and the North Carolina Utili- cate and repair the outage is much longer ties Commission study indicate that the cost than for overhead. A study published by of underground construction may be about 10 times the cost of overhead. North Caro-

6 lina concluded that embarking on a state- this data, North Carolina concluded that it wide program to replace overhead lines was not ﬁ nancially feasible to pursue. The with underground would require a 122% EEI study found that the cost to under- increase in utility bills, and it would require ground was typically about 10 times more 25 years to complete the work. Based on than consumers were willing to pay.

Summary and Conclusions

The desire to improve the performance of Considering these diffi culties, the prudent utility systems in extreme weather events approach would appear to be one which is shared by consumers, utilities, and state provides targeted hardening of certain regulatory commissions. Given that it is lines or line segments and comparing the impossible to design and build a system response of these lines to lines of normal that can not be damaged by catastrophic construction located in the same area and natural events, the question becomes one subjected to similar conditions in terms of of balancing increased costs and potential wind exposure and secondary damage. benefi ts. While increased costs can be cal- culated for any incremental level of system Only through forensic analysis of the hardening, the potential benefi ts are large- performance of various designs and con- ly unquantifi able because of the diffi culty struction methods can rational decisions in determining the frequency distribution be made regarding whether present de- of actual loads that may be placed on the signs and construction are optional from a system in an extreme weather event. storm response perspective.

7 Disclaimer The North American Wood Pole Council and its members believe the information contained herein to be based on up-to-date sci- entiﬁ c information. In furnishing this information, the NAWPC and the author make no warranty or representation, either expressed or implied, as to the reliability or accuracy of such information; nor do NAWPC and the author assume any liability resulting from use of or reliance upon the information by any party. This information should not be construed as a recommendation to violate any federal, provincial, state, or municipal law, rule or regulation, and any party using poles should review all such laws, rules, or regulations prior to doing so.

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