Epri Longitudinal Load 5

Longitudinal Loading andCascading Failure Risk AssessmentWorkshopMarch 18-19, 1996

TR-1068842016-01

Final Report, June 1997

Prepared byJ.A. Jones Power Delivery, Inc.Post Office Box 187Haslet, Texas 76052

Principal InvestigatorsMark Ostendorp, Ph.D.Laura Marr

Prepared forElectric Power Research Institute3412 Hillview AvenuePalo Alto, California 94304

EPRI Project ManagerPaul F. Lyons

Power Delivery Group

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS REPORT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSOREDOR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OFEPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

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ORGANIZATION(S) THAT PREPARED THIS REPORT

J.A. Jones Power Delivery, Inc.

ORDERING INFORMATION

Requests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O. Box23205, Pleasant Hill, CA 94523, (510) 934-4212.

Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. POWERINGPROGRESS is a service mark of the Electric Power Research Institute, Inc.

Copyright © 1997 Electric Power Research Institute, Inc. All rights reserved.

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REPORT SUMMARY

This workshop is a step in EPRI's Load Optimization for Transmission Line Upgradesinitiative to develop state-of-the-art technology to determine system loads andassociated line reliability levels before and after upgrading/uprating. The workshopconstitutes a forum for utilities and industry representatives to discuss experiences,both good and bad. The workshop, sponsored by EPRI and the Bonneville PowerAdministration (BPA), also covered recent failure mitigation measures.

BackgroundEPRI, along with several utilities including BPA, has conducted cascade preventionresearch since the early seventies. EPRI's current research is to make identification oflines susceptible to cascading (multi-structure failures of more than three to fivestructures) easier by providing a tool that can be used efficiently and accurately. In thepast, identification of lines with high cascading potential has been time consuming andexpensive. Statistics indicate, however, that trigger events that can initiate a cascadewill eventually occur on most transmission lines. Although these events may berelatively infrequent, when significant enough to cause a cascade, they impose a largeeconomic loss for utilities. Consequently, utilities need an efficient and accurate tool toidentify effects of such extreme events on transmission lines in their service areas. Sucha tool would allow utilities to adequately plan and prepare for this potential sequenceof events, which by itself reduces the likelihood and extent of potential damage.

Objectives

• To review key aspects and experiences of current longitudinal loading technology todetermine system loads and reliability levels.

• To review current research in longitudinal loading of transmission lines.

ApproachThis EPRI- and BPA-sponsored workshop on Longitudinal Loading and CascadingFailure Risk Assessment—held March 18-19, 1996, in Fort Worth, Texas—was attendedby thirty-three participants. The two-day workshop included presentations on fieldfailures, design procedures, evaluation methodologies, and mitigation methods. Theworkshop closed with a group discussion of current test results, advantages and

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disadvantages of current design philosophy, and effects of upgrading on linereliability.

ResultsWorkshop recommendations indicated more detailed information is needed in theareas of longitudinal loading and cascading failure risk assessment. A large majority ofattendees indicated that the National Electrical Safety Code (NESC) should formallyrecognize this problem and provide either a cautionary statement or some guidelineson how to deal with it. Similarly, the group strongly favored issuing American Societyof Civil Engineers (ASCE) Manual 74 as a standard that would allow utilities toincorporate recommended analysis and design methods into their in-house procedures.Overall, the group felt that an educational process should be devised to increaseawareness for current longitudinal load prediction and cascading failure potentialissues. The consensus was that a better understanding of loads is required to developappropriate designs. Attendees also agreed that a scientific basis for load prediction isrequired to deal with the large variety of existing line systems.

EPRI PerspectiveThe Longitudinal Loading and Cascading Failure Risk Assessment workshop identifiedutility concern about current approaches used to deal with potential cascade failure oflines. It is evident that there is currently little common ground among utilities inlongitudinal load design procedure. As part of EPRI's Cascading Failure RiskAssessment (CASE) Project, tools are under development that can help provide acommon reference for evaluating longitudinal loading issues.

TR-106884

Interest CategoriesOverhead construction, O&MOverhead planning, analysis & design

KeywordsCascade failuresTransmission line failuresOverhead transmissionTransmission lines

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ABSTRACT

This report summarizes the minutes of the Electric Power Research Institute’sLongitudinal Loading and Cascading Failure Risk Assessment Workshop held onMarch 18-19, 1996 in Fort Worth, Texas. The report reviews the group discussionsessions and the presentations included in the meeting. Additionally, the presentationmaterials available for each presentation are included as appendices.

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CONTENTS

1 OVERVIEW............................................................................................................... 1-1

2 WORKSHOP PRESENTATIONS ............................................................................. 2-1

2.1 Introductions....................................................................................................... 2-1

2.2 A Brief Review of Experiences of the Last Near Century H. Brian White.......... 2-2

2.3 Past Analytical and Experimental Experience Alain Peyrot............................... 2-2

2.4 345 kV Line Damage Due to Gas Pipeline Explosion Dana Crissey................. 2-2

2.5 NPPD Cascading Failures Les Svatora ............................................................ 2-3

2.6 Design Load Development for Prevention and Containment of CascadesRon Carrington......................................................................................................... 2-3

2.7 Model Study Leon Kempner.............................................................................. 2-4

2.8 Design Procedures Charles Garcia................................................................... 2-4

2.9 Design Procedures Thien Do ............................................................................ 2-4

2.10 Fail-Safe Design Methods H. Brian White....................................................... 2-5

2.11 Strain Plates for Longitudinal Load Mitigation Goetz Schildt........................... 2-5

2.12 Benefits of a Transmission Line Load Limiter John Stoessel.......................... 2-5

2.13 Extreme Event Loading Long Shan................................................................. 2-6

2.14 EPRI Cascading Failure Risk Assessment Project Mark Ostendorp............... 2-7

2.15 Cascading Failure Mitigation Measures Mark Ostendorp ............................... 2-8

3 WORKSHOP DISCUSSIONS................................................................................... 3-1

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3.1 Utility Longitudinal Loading Design Methodologies............................................ 3-1

3.2 CASE Project and Test Results ......................................................................... 3-1

3.3 Advantages and Disadvantages of Current Design Philosophy......................... 3-2

3.4 Effects of Upgrading / Uprating on Line Reliability ............................................. 3-3

4 CONCLUSIONS ....................................................................................................... 4-1

5 APPENDIX A: WORKSHOP ATTENDEES.............................................................. 5-1

6 APPENDIX B: WORKSHOP AGENDA .................................................................... 6-1

7 APPENDIX C: CASE QUESTIONNAIRE ................................................................. 7-1

8 APPENDIX D: A BRIEF REVIEW OF THE EXPERIENCES OF THE LASTNEAR CENTURY......................................................................................................... 8-1

9 APPENDIX E: PAST ANALYTICAL AND EXPERIMENTAL EXPERIENCE ........... 9-1

10 APPENDIX F: 345 KV LINE DAMAGE DUE TO GAS PIPELINEEXPLOSION .............................................................................................................. 10-1

11 APPENDIX G: NPPD CASCADING FAILURES .................................................. 11-1

12 APPENDIX H: DESIGN LOAD DEVELOPMENT FOR PREVENTION ANDCONTAINMENT OF CASCADES ............................................................................. 12-1

13 APPENDIX I: BPA LONGITUDINAL IMPACT LOADING PROJECT.................. 13-1

14 APPENDIX J: WAPA DESIGN PROCEDURES................................................... 14-1

15 APPENDIX K: BPA DESIGN PROCEDURES...................................................... 15-1

16 APPENDIX L: BPA FAILURE DATABASE ......................................................... 16-1

17 APPENDIX M: USE OF STRAIN PLATES AND LONGITUDINAL LOADINGMITIGATION.............................................................................................................. 17-1

18 APPENDIX N: TRANSMISSION TOWER LOAD LIMITER.................................. 18-1

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19 APPENDIX O: EPRI ICE LOAD ASSESSMENT PROJECT............................... 19-1

20 APPENDIX P: EPRI WIND LOAD ASSESSMENT PROJECT............................ 20-1

21 APPENDIX Q: EPRI CASCADING FAILURE RISK ASSESSMENTPROJECT.................................................................................................................. 21-1

22 APPENDIX R: EPRI CASCADING FAILURE MITIGATION PROJECT.............. 22-1

23 APPENDIX S: UTILITY PRACTICE SUMMARY................................................. 23-1

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1 OVERVIEW

The workshop, sponsored by the Electric Power Research Institute (EPRI) and theBonneville Power Administration (BPA), commenced on the morning of Monday,March 18, 1996 in Fort Worth, Texas. The workshop was attended by thirty-threepersons. A list of attendees is included in Appendix A. The workshop agenda isprovided as Appendix B.

The workshop included four group discussion sessions and presentations fromfourteen speakers. The first day of the meeting began with introductions, followed bythese presentations:

x A Brief Review of Experiences of the Last Near Century

x Past Analytical and Experimental Experience

x 345 kV Line Damage Due to Gas Pipeline Explosion

x NPPD Cascading Failures

x Design Load Development for Prevention and Containment of Cascades

x Model Study

x Design Procedures (Western Area Power Administration)

x Design Procedures (Bonneville Power Administration)

x Fail-Safe Design Methods

x Strain Plates for Longitudinal Load Mitigation

x Benefits of a Transmission Line Load Limiter

Additionally, the following topic was covered in group discussion:

x Utility Longitudinal Loading Design Methodologies

Overview

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The second day of the workshop included the following presentations:

x Extreme Event Loading

x EPRI’s Cascading Failure Risk Assessment (CASE) Project

x Cascading Failure Mitigation Measures

In addition, the following subjects were reviewed in group discussion sessions:

x CASE Project and Test Results

x Advantages and Disadvantages of Current Design Philosophy

x Effects of Upgrading / Uprating on Line Reliability

Slides or overheads accompanied the orations. In addition to documentation relative tothe meeting, available supporting materials for each presentation were provided to theworkshop participants. Questions were addressed at the conclusion of eachpresentation.

The meeting adjourned the afternoon of Tuesday, March 19, 1996. A tour of EPRI’sPower Delivery Center • Haslet followed the meeting for a group of workshopparticipants.

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2 WORKSHOP PRESENTATIONS

2.1 Introductions

Bob Nickerson, general manager of the EPRI Power Delivery Center • Haslet, openedthe meeting with a brief welcome and thank-you to the workshop guests for theirattendance.

Dr. Mark Ostendorp, the EPRI Power Delivery Center’s project manager and host ofthis meeting, then led introductions of meeting attendees and reviewed the meetingagenda. No changes to the agenda as defined were suggested. A request for completionof the meeting’s evaluation form and a questionnaire included in the attendees’documentation was also announced. The questionnaire (see Appendix C) is part ofEPRI’s Cascading Failure Risk Assessment (CASE) project to develop a simpleanalytical method that can be used to determine the cascading potential of atransmission line. The questionnaire will summarize the results provided byrepresentatives within the utility industry on the outcome of a specific test case. Theresults will provide valuable background information on current utility practicesrelative to longitudinal loads and cascading failures. A copy of the questionnaire hadrecently been mailed to several hundred utility representatives, including the meetingattendees. (Note: As of 5/31/96, thirty-six responses had been received).

Paul Lyons, the EPRI project manager, provided an overview of EPRI’s objective intheir research related to longitudinal loading and cascading failures. EPRI, along withseveral utilities including BPA, has conducted research relative to cascade preventionsince the early seventies. The basis of EPRI’s research is to make identification of linessusceptible to cascading (multi-structure failures of more than three to five structures)easier by providing a tool that can be used efficiently and accurately. In the past,identification of lines with high cascading potential has been time consuming andexpensive or not even possible. Statistics indicate, however, that trigger events that caninitiate a cascade will eventually occur on most transmission lines. Although theseevents may be relatively infrequent, when significant enough to cause a cascade, theyimpose a large economic loss for utilities. Consequently, utilities desire an efficient andaccurate tool that allows them to identify the effects of such extreme events on thetransmission lines in their service areas. Such a tool allows utilities to adequately plan

Workshop Presentations

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and prepare for this potential sequence of events, which by itself reduces the likelihoodand extent of the potential damage.

2.2 A Brief Review of Experiences of the Last Near CenturyH. Brian White

Mr. White, a consulting engineer with over forty years’ experience in transmission linedesign, delivered a presentation on the history of transmission line design in the lastnear century (see Appendix C). His discussion focused on the early years of electricaltransmission, the competition (as it applies to longitudinal loading) between copperconductors and ACSRs, the “Broken Wire Era,” the move to the “Residual Static Load”approach, etc. Mr. White concluded that in spite of great progress towardsunderstanding the phenomena of cascading, costly and lengthy cascades still occurfrequently enough to revive interest in the subject and call for a further look into theproblem.

2.3 Past Analytical and Experimental ExperienceAlain Peyrot

Dr. Alain Peyrot, a former professor at the University of Wisconsin and president ofPower Line® Systems, addressed and highlighted a variety of work performed in thepast twenty years, both analytically and experimentally (see Appendix D).Additionally, he elaborated on a number of items that he felt progress could be madeon and which would further the understanding of cascading failures. Dr. Peyrotreiterated Mr. White’s point that no scientific progress had been made in the lasttwenty years relating to the prediction or prevention of cascading failures. He alsomade several suggestions on what can be done to mitigate cascades, including use ofsimple models to understand the dynamic process and identify important parameters,run parametric studies, and correlate with practical experience (single phase failure,complete line failure).

2.4 345 kV Line Damage Due to Gas Pipeline ExplosionDana Crissey

Dana Crissey, a principal engineer with TU Electric (TU), provided a case study of anunusual trigger event that caused a cascade of a 345 kV transmission line in theirservice area in October 1993. The line damage experienced by TU was caused by a gaspipeline explosion. Mr. Crissey’s presentation included the “when, where, what, who,why, and recovery” associated with the failure (see Appendix E). The quickreconstruction of the line, due to the availability of materials diverted from otherprojects, was a point of interest to many. Most noticeably, this failure raised concerns


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with several attendees who had not considered the presence of a pipeline as a potentialtrigger event for a cascading failure of a transmission line.

2.5 NPPD Cascading FailuresLes Svatora

Les Svatora, a senior engineer with Nebraska Public Power District (NPPD), reviewedthe damage incurred on their transmission system during extensive ice storms thataffected a majority of NPPD’s service area. The presentation included thecharacteristics of the affected lines, the weather on the storm dates, a summary of thestorm damage, a summary of the extent and cost of each cascading failure, the criteriaused for the reconstruction of the storm damaged components, and an accounting ofthe reconstruction time required for each line (see Appendix F).

2.6 Design Load Development for Prevention and Containment ofCascades Ron Carrington

Design assistance and engineering support during the reconstruction of NPPD’sdamaged lines were provided by Power Engineers, a consulting firm. Ron Carringtonof Power Engineers provided an insight to the consulting engineer’s perception of theNPPD failures. Mr. Carrington’s presentation covered design load development forprevention and containment of future cascades for NPPD’s Pauline-Moore 345 kVtransmission line project (see Appendix G). He also provided the project objective,project description, load cases, implementation of the design philosophy, costs, etc.Questions following Mr. Carrington’s presentation addressed the subjects of stormguys, stop towers, shield wires, etc. in the NPPD system.

In a brief addition to his initial presentation, Mr. Carrington discussed a cascadingfailure that occurred south of Minneapolis in 1990. The cascading failure case studywas unique since the failures actually occurred over a time period of 12 to 24 hoursrather than a matter of minutes. Power Engineers determined that the cause of the slowmotion cascade was due to heavy ice loading on unequal adjacent spans facilitated byflexible horizontal-vee suspension assemblies. The unbalanced loads caused everincreasing bending moments in the support structures as a result of the increasing loadeccentricity (P-Delta Effect), ultimately failing each structure. Each failure caused afurther imbalance in the horizontal tensions which caused the next structure in the lineto fail in a similar manner.


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2.7 Model StudyLeon Kempner

Leon Kempner, a senior structural engineer with Bonneville Power Administration(BPA), reviewed a scale model study performed over an extended period of time thatexamined BPA’s cascading failure containment design concept and the longitudinalimpact loads that act on electrical transmission structures (see Appendix H). While theconcept followed was not developed by BPA, their study was unique in that they wereinvestigating failure containment rather than just a broken conductor. BPA expects thefinal report on this study to be completed by December, 1996 (tests were conductedfrom 1988 to 1990). Findings from the study indicate that conductor tension is the keyparameter, tower stiffness is a more moderate key, and the insulator to span ratio seemsto be an important driver. Essentially, BPA, for the sake of economy, is permittingfailure to a limited number of structures in each direction away from the initiatingevent as long as a cascade of the line is not initiated.

2.8 Design ProceduresCharles Garcia

Charles Garcia, a structural engineer at Western Area Power Administration (WAPA),elaborated on WAPA’s current design procedures that are used to quantifylongitudinal load cases and their containment philosophy (see Appendix I). Mr. Garciadescribed some of the more severe storms experienced in the WAPA service area andthe effects on WAPA’s transmission lines (no cascades were observed). As part of hispresentation, Mr. Garcia explained the loading criteria for wood pole and steelstructures from the 1950’s and 1960’s (when many of the affected lines were built).These criteria are still followed today by WAPA engineers.

2.9 Design ProceduresThien Do

Thien Do, a structural engineer at BPA, discussed BPA’s design procedures, addressingonly the longitudinal aspect of BPA’s loading philosophy in this presentation (seeAppendix J). He also provided a summary of broken conductor failures that haveoccurred on BPA’s transmission system. Additional commentary was provided by Mr.Kempner, who elaborated on BPA’s past failures (failure rate of approximately onefailure per 15,000 tower years). Mr. Kempner provided a complete summary of BPA’sfailure database. This summary is included in Appendix K (BPA Tower Failure andBroken Conductor Summary) of this document.


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2.10 Fail-Safe Design MethodsH. Brian White

In this presentation, H. Brian White considered two methods to stop failures–the stoptower principle and containment built into every structure (consideration was paid tothe no ice load case only). Mr. White made comments related to cascading. Theseincluded the hazard of ground wires contributing to cascades, the lesser likelihood ofcross rope towers to cascade, the high incidence of transverse cascading with latticetowers (approximately 50% of cascading failures), the prevalence of wind relatedfailures, etc. Of particular interest were Mr. White’s comments relative to the use of the“new” fiber optic ground wires and their potentially negative impact on the cascadingpotential of transmission lines. Mr. White indicated that the increased strength of thefiber optic ground wire cross-sections is likely to increase the cascading potential ofcertain line configurations because additional slack can only be introduced by anincrease in the number of failed towers.

2.11 Strain Plates for Longitudinal Load MitigationGoetz Schildt

Goetz Schildt, of BC Hydro, discussed BC Hydro’s experiences relative to the use ofstrain plates to mitigate longitudinal loads (see Appendix L). Mr. Schildt reviewed therecent cross arm failures (1995) on two 500 kV transmission structures equipped withplate bracing designed to act as load fuses in case of longitudinal overload.Additionally, he provided an overview of the design history as well as climaticexperience of transmission lines in the BC Hydro service area. Mr. Schildt alsoindicated that cascading is not likely to occur on BC Hydro’s transmission lines becausetheir lines typically do not extend far between dead ends as a result of the mostlymountainous service area in British Columbia. BC Hydro’s most recent cascade failurewas initiated by an avalanche that swept away a single 500 kV lattice tower.

2.12 Benefits of a Transmission Line Load LimiterJohn Stoessel

John Stoessel of ANCO Engineers, Inc., covered the characteristics of ANCO’stransmission line load limiter (L3) (see Appendix M). Mr. Stoessel defined the needseen originally by ANCO for the device and described its development. Furthermore,he provided some test results from proof tests performed at the EPRI TLMRF in 1986.The ANCO representative indicated that the advantages of the L3 load limiter are theability of the device to absorb dynamic energy and the cost efficiency of the device (Mr.Stoessel estimated their cost at under $200 for each device). ANCO Engineers havereceived a patent on the transmission line load limiter L3 and are looking for


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opportunities to demonstrate its use in new and retrofit lines and to commercialize itsuse.

2.13 Extreme Event LoadingLong Shan

Dr. Long Shan, of the EPRI Power Delivery Center, addressed extreme event loading.His presentation covered two aspects of extreme event loading: ice loading and windloading (see Appendices N and O).

Initially, Dr. Shan addressed conductor ice loading. He discussed the need for anupdated United States ice map (the currently used map is based on a limited amount ofdata collected in the 1920’s and 1930’s, and concerns exist that the wire sizes on the sleetracks used in the study are not comparable to those on overhead transmission lines),and the lack of systematic ice data. Dr. Shan then described EPRI’s current project toprovide information on ice conditions in the United States and to develop new regionalice load maps. This research is anticipated to produce a national ice storm data base, iceseverity maps for the United States, and local area 50-year ice thickness maps andassociated wind speeds. The benefits of this research, he explained, will be lower costupgrades in capacity (with the knowledge of probable ice loads), lower costs for newline design, savings from avoiding costly failures, and more accurate prediction oftransmission line reliability.

Following this segment of the presentation, Dr. Shan addressed questions. Onequestion inquired about the lack of activity shown in the ice severity index map for theWestern states. Dr. Shan explained that this could be due to the smaller populationsand lack of weather reporting in these states. There were also indications fromattendees that the preliminary ice thickness levels shown for some regions appearedlow when compared to experiences by audience members. Additionally, someattendees expressed that, based on their experience, they have more confidence in theBennett map than the examples Dr. Shan presented. Dr. Shan explained that the resultswere accretion model dependent and that the methods of measuring ice thickness vary.However, he added that these are preliminary maps and changes are anticipated. Thereis even the possibility that another ice accretion model will be adopted. Paul Lyons(EPRI) also added that the ice accretion models under study need to be correlated withfield studies because field conditions must be just right for ice formation and very smallvariations in temperature can mean the difference between a severe ice event and acold, wet day. Additionally, accretion models need to be evaluated by sensitivitystudies that may facilitate smoothing of peaks and valleys to better represent measureddata.

A second aspect of extreme event loading, wind loading, is currently being researchedby EPRI. This project is expected to provide an improved understanding of wind load


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prediction methods and design parameter selection. Dr. Shan provided an overview ofcurrent wind load prediction methods including brief descriptions of ASCE Manual 74,NESC, examples used by three utilities and two international methods. He explainedthat EPRI’s research has already produced results in the areas of gust spectrum andturbulence scale definition, span effect, drag coefficient and air density determination,and the prediction of basic wind speed. Current efforts are directed towardsdeveloping the span - gust wind load relationship. EPRI activities in this area includepublishing a document for assessment of wind loads for line design and upgrade,conducting wind tunnel tests to generate a conductor drag coefficient data base that canbe used in design practice, writing a guideline for generating local area wind map byutilities, and generating local area wind maps for some utilities. Leon Kempnercommented on Dr. Shan’s use of this wind load method as a basis for comparison withother wind load methods. Dr. Shan explained that EPRI’s current approach to windload prediction constitutes the only wind load prediction model that is based onexperimental results.

2.14 EPRI Cascading Failure Risk Assessment ProjectMark Ostendorp

Dr. Mark Ostendorp furnished an overview of EPRI’s current Cascading Failure RiskAssessment (CASE) project, the basis for the workshop (see Appendix P). Dr.Ostendorp discussed the probability and effects of cascading failures on transmissionlines, especially if utilities neither design nor plan to contain failures. The objectives ofthe CASE project are to develop a method to economically determine the cascadingfailure risk of a line, determine the effects of upgrading or uprating on the cascadingpotential of lines, identify and evaluate remedial alternatives for restricted ROWupgrade / uprate configurations, and to develop an economical engineering tool tofacilitate cascading potential assessment.

Dr. Ostendorp discussed the potential losses resulting from a cascading failure and thebenefits, including a benefit / cost ratio, that the project results are expected to provide.Project tasks include: identifying cascading failure simulation tools, evaluating andvalidating simulation tools, identifying and classifying critical parameters, developingpreliminary CASE methodology, integrating preliminary CASE methodology,correlating predictions and observations, calibrating CASE methodology, anddeveloping an assessment tool. Dr. Ostendorp provided a mathematical definition ofthe containment philosophy inherent in the analytical model. The containmentphilosophy is based on the logarithmic decrement of the critical force acting on the first,second, and third structure. The logarithmic decrement equation provides anassessment of the transmission line security level on a scale of 0 to 10. While theanalytical model is relatively simple at this point, Dr. Ostendorp cited completedstudies that compared the model predictions to test data with very good results. Theproject is currently comparing full-scale test results with predictions made by the


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preliminary risk assessment methodology, comparing model study test results withpredictions made by the preliminary risk assessment methodology, comparing modelstudy test results with full-scale test results, and developing related projects with twomember utilities.

2.15 Cascading Failure Mitigation MeasuresMark Ostendorp

Dr. Ostendorp discussed a variety of cascading failure mitigation measures that arecurrently being researched by EPRI at the EPRI Power Delivery Center (see AppendixQ). The objectives of this project are to develop methods or devices to reduce loads bydissipating dynamic energy and to provide containment while maintaining conductorclearances. These methods or devices provide economic installation and low (or no)maintenance. Dr. Ostendorp described three mitigation philosophies: to providesufficient strength through the use of guys, stop structures, dead ends, or theindividual strength of each tower; to provide flexibility with materials, geometricconsiderations, components (cross arms, braces, poles), or the entire system (insulatorlength, span lengths, tensions and distance between dead ends); and through controlledfailure of cross arms, strain plates, connection details, shear bolts, or load fuses.

An economical mitigation option is energy dissipation. In this method, a line isdesigned to dispel energy in a failure so that the loads are diminished sufficiently andthe cascade stops by the time the failure reaches the third or fourth structure. Energydissipation, Dr. Ostendorp explained, can be achieved through deformation, friction, orslack. Energy dissipation devices include strain plates, shear bolts, ductile load fuses,friction load fuses, shock absorbers, and combinations of these components. Theadvantages of this mitigation method are containment, cost-effectiveness, no change inROW, improbable customer opposition, extension of existing asset life, and facilitationof upgrading / uprating.

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3 WORKSHOP DISCUSSIONS

3.1 Utility Longitudinal Loading Design Methodologies

A group discussion focusing on utility longitudinal loading design methodologiesproduced the following information:

x BC Hydro originally used ASCE Manual 74. However, their design philosophycontinues to evolve.

x Idaho Power does not have a written philosophy. However, they do include a safetyfactor of 1.5 in their design factors similar to square-based lattice structures, etc.

x Representatives from utilities located in the Northeast said they rarely have straight-aways of more than a few miles, so they have very little problem with cascadingfailures.

x A number of the utilities represented at the workshop indicated that they did notdesign wood transmission structures for longitudinal loading.

x Dr. Ostendorp asked whether anyone was using any specially-made devices toprevent or mitigate cascade failure. No one indicated their utility used any devicesfor this purpose.

x None of the utilities represented indicated that their utility used any specially-madedevices to prevent or mitigate cascade failure.

3.2 CASE Project and Test Results

Dr. Ostendorp clarified some aspects of the cascading simulation model of the CASEproject. The simulation model may ultimately be implemented as a software program.The information required from the user will be the typical variables–geometric layout,type of pole, cross-sectional parameters of braces, etc. He explained that the simulationmodel can be adjusted to fit a utility’s specific failure criterion and failure extentlimitations to reflect influential factors such as line age, importance, or potential

Workshop Discussions

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liability issues. The simulation model also has the capability to model the effects ofdifferent components such as guys, stronger cross-arms, horizontal-vees, etc.

Dr. Ostendorp responded to a suggestion by Alain Peyrot to allow for a suspendedsingle mass in the simulation model to provide more realistic results. Dr. Ostendorpnoted that the model had been attempted in a few test cases, but that these comparisonshad shown that the results derived from such a model did not compare as well as theCASE model. While Dr. Ostendorp acknowledged that the model is preliminary andwill go through refinements, he stated that he is hesitant to change some aspects of thesimulation model as long as it works well. Additionally, Dr. Peyrot inquired relative tothe use of the simulation model’s non-linear springs that release when somethingbreaks, and the definition of structural failure as defined in the model. Dr. Ostendorpexplained that failure is essentially simulated in the form of geometry changes in themodel by physically removing the stiffness and mass components of the member fromthe solution process. The analysis proceeds in distinctive steps: first, the formulation ofcomponent and global stiffness including the sagging of the conductor; second, theunbalanced loads caused by the failure of either one or multiple components on the linesystem. The failure modes considered by the simulation model are broken shield wire,broken insulator or cross-arm, broken conductor, or complete loss of a structure whilesubjected to any arbitrary loading condition. The model assumes energy is notdissipated in the loss of the first structure since the initiating event incorporates a highdegree of uncertainty.

3.3 Advantages and Disadvantages of Current Design Philosophy

A group discussion that focused on current design philosophy began with a remarkfrom Leon Kempner. Mr. Kempner commented on phrasing in the letter andquestionnaire sent by EPRI to utility members before the workshop to inquire abouttheir longitudinal loading practices. The statement in question said “. . . A number ofcatastrophic transmission line failures have occurred in the recent past (i.e., 1500+structures between 1990 and 1995). . . . The majority of the failed transmission lineshad been designed to resist some or all of the unbalanced longitudinal loads. . . .”Mr. Kempner felt these statements implied that these lines had been designed with aload case to specifically resist longitudinal loads, and that current practice regardinglongitudinal loading is inadequate. However, Dr. Ostendorp explained that the designof some of the lines that had experienced cascading failures had been investigated (asmuch as possible with the information available to him). The lines investigated werefound to have enough inherent strength to resist a single broken conductor or shieldwire load under service load conditions.

Mr. Kempner commented next on a statement in the same letter that he felt erroneouslyimplied ASCE Manual 74 does not provide a method to determine longitudinal loads.While he admits that the methods provided may not be the best, he maintains that the


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capability is there. Dr. Ostendorp explained that this statement was meant to imply thatASCE Manual 74 provided a “rule of thumb” (70% of RSL) based on somewhat limitedexperiences rather than an analytical approach that allowed the calculation of the actualloads as a function of critical parameters such as conductor tension, insulator length,structural rigidity, etc. Dr. Ostendorp proceeded to comment that the ASCE Manual 74“rule of thumb” provides only a yes-no decision comparison but does not give anindication on the severity of the consequences if a deviation from the rule cannot beavoided. Dr. Ostendorp assured the group that no criticism of ASCE Manual 74 wasintended by these statements.

Since cascading failures are typically “cleaned up” as soon as possible in an attempt torestore power, a thorough investigation of such failures is rarely performed and somespecifics about the failure may not be identified. It was suggested during thisdiscussion that failure investigation teams be formed to document failures before theyare cleaned up. However, no one volunteered to finance the effort. Alternatively, it wassuggested that an outline be prepared in advance to use during an investigation thatwould guide the team in the appropriate steps for documenting a failure event.

3.4 Effects of Upgrading / Uprating on Line Reliability

The final topic of discussion initially focused on the question of what is needed in thearea of cascading failure containment and longitudinal load prediction. The responsesincluded (not prioritized):

1. guidelines

2. an evaluation or simulation tool (possibly in the form of a software program)

3. advanced modeling

4. ASCE 74 guidelines to address ice, non-ice, and transverse cascading cases

5. an educational process to increase awareness of problem

6. knowledge of information required to quantify cascading potential

7. a better coordination with Dr. Ostendorp and the EPRI project

8. consideration and development of new load limiting devices

9. a case study of what has and hasn’t worked

10. ASCE 74 developed into a standard


3-4

11. NESC to recognize cascading problem

12. better knowledge of what the loads really are in order to design properly

13. scientific basis for decisions

Lastly, Mr. White reiterated his concern about the detrimental effects of the fiber opticshield wires that are now being installed on many systems. He cautioned attendees thatany changes to components of the system greatly influence the behavior of the overalltransmission line and that the effects of any changes made to the system must becarefully considered.

4-1

4 CONCLUSIONS

Recommendations made by the attendees of the workshop indicated that more effortshould be put forth to develop more detailed information in the areas of longitudinalloading and cascading failure risk assessment. A large majority of the attendeesindicated that NESC should provide some recognition of this problem and provideeither a cautionary statement or some guidelines on how to deal with the problem.Similarly, the group strongly favored the idea of issuing ASCE Manual 74 as a standardthat would allow utilities to incorporate recommended analysis and design methodsinto their in-house procedures.

Overall, the group felt that an educational process should be devised to increase thelevel of awareness relative to current longitudinal load prediction and cascading failurepotential issues. The rapid changes in utility staffs make this even more important.Additionally, they recommended that more information be collected from the industryto identify critical aspects in the line design process that increase or decrease expectedlongitudinal loads or the cascading potential. The detrimental effects of the fiber opticshield wires that are currently being installed on many systems was reiterated. Cautionwas expressed regarding any changes to components of the system because this couldgreatly influence the behavior of the overall transmission line.

Without exception, the group supported the objectives and goals of the current EPRICascading Failure Risk Assessment project. The group recommended industry supportfor the current efforts to develop a simulation and assessment tool and suggestedcoordination of industry groups with EPRI’s current effort in providing such a tool. Thegroup recognized that there are currently no tools available in the industry toaccurately predict the effects of unbalanced longitudinal loads at successive structuresalong the line or to assess the cascading potential. The consensus was that a betterunderstanding of the loads is required to develop appropriate designs and that ascientific basis is required to predict these loads to deal with the large variety ofexisting line systems.

The group’s consensus is also reflected in the responses observed in the CASEquestionnaires that have already been returned (Appendix R provides a summary ofthe responses). From the summary, it is evident that there exists little common groundin the longitudinal load design procedures among the utilities.

Conclusions

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While no dates were discussed, a majority of the attendees proposed to conduct anothermeeting in 1997 to continue discussion of progress in this area. Additional meetings onthe subject are anticipated to prove beneficial, but funding for such events have not yetbeen secured.

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5 APPENDIX A:

WORKSHOP ATTENDEES

Todd AdamsT & D DesignIdaho Power Company1221 West Idaho, P.O. Box 70Boise, Idaho 83707Phone: (208) 388-2740Fax: (208) 388-6906

Ron CarringtonProject Manager / Project EngineerPower Engineers, Inc.P.O. Box 1066Hailey, Idaho 83333Phone: (208) 788-0310Fax: (208) 788-2082

Dana R. CrisseyPrincipal EngineerTU Electric Co.P.O. Box 970Fort Worth, Texas 76101-0970Phone: (817) 882-6266Fax: (817) 882-6274

James R. DeenSenior EngineerTU ElectricP.O. Box 970, Suite 1105Fort Worth, Texas 76101-0970Phone: (817) 882-6259Fax: (817) 882-6274

Thien DoStructural EngineerBonneville Power AdministrationP.O. Box 3621 (TEDS)Portland, Oregon 97208Phone: (503) 230-5565Fax: (503) 230-3984

John EddingsTransmissionTri-State Generation & TransmissionAssn, Inc.P.O. Box 33695Denver, Colorado 80233Phone: (303) 452-6111Fax: (303) 254-6030

Jon FergusonMarketing ManagerJ.A. Jones Power Delivery Center100 Research DriveHaslet, Texas 76052Phone: (817) 234-8216Fax: (817) 439-1001

Blake ForbesTransmission EngineerPublic Service Company of New MexicoAlvarado SquareAlbuquerque, New Mexico 87158Phone: (505) 241-2973Fax: (505) 241-2363

Appendix A:Workshop Attendees

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Charles T. GarciaStructural EngineerWestern Area Power Administration1627 Cole Blvd., Bldg. #18 A3920Golden, Colorado 80401Phone: (303) 275-2817Fax: (303) 275-1717

Bill GardnerSenior Field EngineerLincoln Electric SystemP.O. Box 80869Lincoln, Nebraska 68501Phone: (402) 467-7649Fax: (402) 467-7601

David GaskinsLeader, Civil EngineeringIllinois Power Company500 South 27th StreetDecatur, Illinois 62550Phone: (217) 424-7023Fax: (217) 362-7961

Chris HickmanEngineerPublic Service Company of New MexicoAlvarado Square, MS 0600Albuquerque, New Mexico 87158-0600Phone: (505) 241-4596Fax: (505) 241-2363

Kamran KhadivarChief Design EngineerFalcon Steel CompanyP.O. Box 162689Fort Worth, Texas 76161-2689Phone: (817) 581-9500Fax: (817) 581-6898

Leon Kempner, Jr.Senior Structural EngineerBonneville Power AdministrationP.O. Box 3621 (TED)Portland, Oregon 97208Phone: (503) 230-5563Fax: (503) 230-3984

Paul LyonsProject ManagerElectric Power Research Institute100 Research DriveHaslet, Texas 76052Phone: (817) 234-8200Fax: (817) 439-1001

Laura MarrResearch Engineer AssociateJ.A. Jones Power Delivery Center100 Research DriveHaslet, Texas 76052Phone: (817) 234-8219Fax: (817) 439-1001

James A. NelsonSupervising EngineerPublic Service Electric & Gas Company80 Park Plaza, MCT-14ANewark, New Jersey 07101Phone: (201) 430-7763Fax: (201) 623-2133

Robert E. NickersonGeneral ManagerJ.A. Jones Power Delivery Center100 Research DriveHaslet, Texas 76052Phone: (817) 234-8210Fax: (817) 439-1001


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G.J. (Jim) OberstEngineering SupervisorNew York State Electric and GasP.O. Box 5224Binghamton, New York 13902Phone: (607) 762-7610Fax: (607) 762-8502

Markus OstendorpResearch EngineerJ.A. Jones Power Delivery Center100 Research DriveHaslet, Texas 76052Phone: (817) 234-8213Fax: (817) 439-1001

Alain H. PeyrotPresidentPower Line® Systems918 University Bay DriveMadison, Wisconsin 53705Phone: (608) 238-2918Fax: (608) 238-9241

Steven C. RootDesign EngineerOklahoma Gas and Electric ServicesP.O. Box 321, MC 1005Oklahoma City, Oklahoma 73101Phone: (405) 553-3468Fax: (405) 553-3820

Goetz D. SchildtSenior Design EngineerB.C. Hydro6911 Southpoint Drive (A03)Burnaby, B.C. Canada V3N 4X8Phone: (604) 528-2193Fax: (604) 528-1883

Long ShanResearch EngineerJ.A. Jones Power Delivery Center100 Research DriveHaslet, Texas 76052Phone: (817) 234-8215Fax: (817) 439-1001

James G. SmithPrincipal EngineerCentral Louisiana Electric CompanyP.O. Box 5000Pineville, Louisiana 71360Phone: (318) 484-7529Fax: (318) 484-7394

John StoesselANCO Engineers, Inc.4826 Sterling DriveBoulder, Colorado 80301Phone: (303) 443-7580Fax: (303) 443-8034

Les SvatoraSenior EngineerNebraska Public Power DistrictP.O. Box 499Columbus, Nebraska 68602-0499Phone: (402) 563-5651Fax: (402) 563-5612

Sid ThompsonTransmission MaintenanceManager–EastTri-State Generation & TransmissionAssn, Inc.P.O. Box 33695Denver, Colorado 80233Phone: (303) 452-6111Fax: (303) 254-6030


4

Darel TracyPrincipal EngineerIdaho Power CompanyP.O. Box 70Boise, Idaho 83707Phone: (208) 388-2462Fax: (208) 388-6902

J.R. VinsonSenior DesignerOklahoma Gas and Electric CompanyP.O. Box 321Oklahoma City, Oklahoma 73101-0321Phone: (405) 553-3821Fax: (405) 553-3820

Marlon W. VogtSupervisor, Transmission Engineering& ConstructionCentral Iowa Power CooperativeP.O. Box 2517Cedar Rapids, Iowa 52406Phone: (319) 366-8011Fax: (319) 366-6328

Bob WardenCivil EngineerTennessee Valley Authority2A Lookout Place, MR-4BChattanooga, Tennessee 37402-2801Phone: (423) 751-7998Fax: (423) 751-6083

H. Brian WhiteTransmission Line ConsultantP.O. Box 939Hudson, Quebec Canada J0P 1H0Phone: (514) 458-4329Fax: (514) 458-4329

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6 APPENDIX B:

WORKSHOP AGENDA

Monday, March 18

8:00 am Continental Breakfast

8:30 am Welcome–Bob Nickerson, EPRI Power Delivery Centers

8:40 am Welcome and Introductions–Mark Ostendorp, EPRI Power Delivery Centers

8:50 am Overview of EPRI Project Objectives–Paul Lyons, EPRI

9:00 am A Brief Review of the Experiences of the Last Near Century–H. Brian White,Consultant

9:30 am Past Analytical and Experimental Experience –Alain Peyrot, Power Line®Systems

10:00 am Break

10:15 am 345 kV Line Damage Due to Gas Pipeline Explosion–Dana Crissey, TUElectric

10:45 am NPPD Cascading Failures–Les Svatora, Nebraska Public Power District

11:15 am Design Load Development for Prevention and Containment of Cascades–RonCarrington, Power Engineers

12:00 pm Lunch (provided)

1:00 pm Model Study–Leon Kempner, Bonneville Power Administration

1:30 pm Design Procedures–Charles Garcia, Western Area Power Administration

2:00 pm Design Procedures–Thien Do, Bonneville Power Administration

Appendix B:Workshop Agenda

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2:30 pm Discussion on Utility Longitudinal Loading Design Methodologies

3:30 pm Break

3:45 pm Fail-Safe Design Methods–H. Brian White, Consultant

4:15 pm Strain Plates for Longitudinal Load Mitigation–Goetz Schildt, BC Hydro

4:45 pm Benefits of a Transmission Line Load Limiter–John Stoessel, ANCOEngineers, Inc.

5:15 pm Adjourn

6:00 pm Reception (Juanita’s Restaurant, #17 on enclosed map)

Tuesday, March 19

8:00 am Continental Breakfast

8:30 am Extreme Event Loading–Long Shan, EPRI Power Delivery Centers

9:15 am EPRI Cascading Failure Risk Assessment Project–Mark Ostendorp, EPRIPower Delivery Centers

9:45 am Discussion of CASE Project and Test Results

10:15 am Break

10:30 am Cascading Failure Mitigation Measures–Mark Ostendorp, EPRI PowerDelivery Centers

11:00 am Design Load Development for Prevention and Containment of Cascades–RonCarrington, Power Engineers

11:15 am Discussion of Advantages and Disadvantages of Current Design Philosophy

12:00 pm Lunch (provided)

1:00 pm Discussion on the Effects of Upgrading/Uprating on Line Reliability

3:00 pm Adjourn

3:15 pm Tour of EPRI PDC • Haslet(optional, transportation provided, 2 hour duration)

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7 APPENDIX C:

CASE QUESTIONNAIRE

Appendix C:Case Questionnaire

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8 APPENDIX D:

A BRIEF REVIEW OF THE EXPERIENCES

OF THE LAST NEAR CENTURY

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9 APPENDIX E:

PAST ANALYTICAL AND EXPERIMENTAL

EXPERIENCE

Appendix E:Past Analytical and Experimental Experience

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10 APPENDIX F:

345 KV LINE DAMAGE DUE TO GAS PIPELINE

EXPLOSION

Appendix F:345 KV Line Damage Due to Gas Pipeline Explosion

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11-1

11 APPENDIX G:

NPPD CASCADING FAILURES

Appendix G:NPPD Cascading Failures

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12 APPENDIX H:

DESIGN LOAD DEVELOPMENT FOR PREVENTION

AND CONTAINMENT OF CASCADES

Appendix H:Design Load Development for Prevention and Containment of Cascades

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13 APPENDIX I:

BPA LONGITUDINAL IMPACT LOADING PROJECT

Appendix I:BPA Longitudinal Impact Loading Project

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14-1

14 APPENDIX J:

WAPA DESIGN PROCEDURES

EPRI Longitudinal Loading and Cascading FailureRisk Assessment WorkshopRadisson Plaza Hotel, Fort Worth, Texas, March 18-19, 1996

Design Procedures

Charles GarciaWestern Area Power Administration

Background

Western is responsible for over 16,700 miles of transmission lines in 15 Central andWestern states encompassing a 1.3-million square mile geographic area. Thetransmission line voltage ranges from 2.4-500kV. Our primary areas of concern forpotential storm damage to our transmission system are in the states of North Dakota,South Dakota, Minnesota, Iowa and Nebraska. Western s transmission system in theseAreas is primarily wood pole and lattice steel construction with over 6,800 miles oftransmission lines built in the 1950's and 1960's. Storms have caused numeroustransmission line failures, although most of Western s line failures are associated withtornadoes or high winds. Strong wind or ice storms normally damage from 1 to 4adjacent structures, at most, due to their localized nature. Extreme examples of stormdamage are an ice storm in 1965 that destroyed a 20 mile section of a 115kV line, and athunderstorm in 1995 that destroyed 12 miles of 115kV. Discussions with fieldmaintenance personal indicated that normally, damage is associated with hardwarecutting the overhead ground wire or conductor. Conductor arms have been damagedwhen the conductor dropped, but the structure groundwire peaks are generallyundamaged from losing the overhead groundwires. It is not uncommon, in these areas,for lineman to knock 5 inches of radial ice from the conductors and overheadgroundwire to reduce clearance problems. In general single wood poles are used for2.3-69kv, wood H-frames for 69-230kV and steel structures 230kV and above.

Appendix J:WAPA Design Procedures

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Design

Since we are primarily interested in cascading failures, the following will only considertangent structures; angle and dead end structures have been excluded. Also since amajority of the transmission lines located in this severe weather area were built in the1950's and 1960's a description of the loading criteria for that time period is included. Inthe 1950's and 1960's wood pole structures were designed by the electrical engineers,and the steel structures were designed by the structural engineers. Typically Westerndesigned its structures in accordance with NESC, grade B construction. The first sectionwill discuss the steel structure design, with wood pole structure design following.

Steel structures:

In the 1950's and 60's wire loadings shown in table 1 were used.

Table 1

Loading district

Heavy Medium Light

Horizontal wind pressure 4 4 9 (NESC-H30)

8 8 12 (Western)

(In the 1970's Western converted to current NESC horizontal wind pressure values.)

Table 2 compares NESC overload capacity factors used in the 1950's, 1960's and now, tothose used by Western.

Table 2

NESC Overload capacity factors

NESC H30, #81 Western NESC 1993 Western

Vertical strength 1.27 * 1.5 1.5

Transverse strength Wind 2.54 * 2.5 2.5

Longitudinal strength Atcrossings General

1.1 * 1.1 1.1

Elsewhere General 1.0 * 1.1 1.0

* See tables 3&4 for a sample design from the 1950's and 1960's; a value of 1.25 was used in the 1960's.


14-3

Normally only two loadcases were used to design the lattice steel structures built in the1950's. Table 3 shows an example of the design loads used in the 1950's for a singlecircuit, suspension, lattice steel tower.

Table 3

NESC Heavy230kVCond—954kcmil (54/7) max NESC tension 12,000 lbsOGW—½-in 3392 steel strand, max NESC tension 7,000 lbs

PART A PART B

Cond OGW Cond OGW

V 1650 950 4350 3550

T 650 400 1200 900

L 7100 5850 18900 13500

The tower was designed to support tower dead load, plus a transverse wind load of 12psf acting on 1-1/2 times the projected area of one face of the structure, plus the simultaneous application of theloading caused by any of the following conditions or combinations thereof, which produced themaximum stress in any member.

A. For part A1. All load groups on2. Any one conductor or any one ground wire broken

B. For part B1. All load groups on

An example of the design loads used in the 1960's for a single circuit, suspension, latticesteel tower is shown in table 4.


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Table 4

NESC Heavy230kVCond—954kcmil (54/7) max NESC tension 12,000lbsOGW—½-in 3392 steel strand, max NESC tension 7,000lbs

PART A PART B PART C

Cond OGW Cond OGW Cond OGW

V 2400 1400 4100 3000 2400 1400

T 1300 900 1300 900 1300 900

L 4500 3200 16700 10500 8500 7000

The tower was designed to support tower dead load, plus a transverse wind load of 13psf acting on 1-1/2 times the projected area of one face of the structure, plus the simultaneous application of theloading caused by any of the following conditions or combinations thereof, which will produced themaximum stress in any member.

A. For part A1. All load groups on2. Any one conductor or any one ground wire broken

B. For part B1. All load groups on

C. For part C1. All load groups on2. Any one conductor or any one ground wire broken

The structure shall be required to carry these design loads multiplied by 1.25.

Currently, Western analyzes and designs steel transmission structures for loadsresulting from NESC, High Wind, Broken wire, Rime ice (as needed), Stringing,Construction and Maintenance loadcases (and Camber loads for steel poles).

The broken conductor loadcase applies a longitudinal load of 2/3 NESC tensions, dueto insulator swing, for the conductors, and full NESC tensions for overhead groundwires, at any one conductor or ground wire location. These loads are multiplied by anoverload factor of 1.1 and applied to the structure. In designs prior to the 1980's fullNESC tension were used for both the conductors and overhead ground wires for theunbalanced longitudinal loads, with the overload factor of 1.1. (The overload factor inthe 1960's and 1970's was 1.25 for all loadcases, which was increased to 1.5 if thestructure had not been tested.)


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Wood pole structures:

Most of Western s wood pole structure designs were controlled by standard designsuntil the 1980's, ie. Class 2 poles with grade B construction, normally pole heights of 65-76 feet (which dictated spans of around 700 feet for clearance). Under this standard,from a strength standpoint , the cross arms were the controlling factor in the design.The wire loading shown in table 1 was used in wood structure designs in the 1950's and1960's. Currently all wood pole design is done in accordance with NESC, method B,grade B construction and Class 2 wood poles. Western has always placed a dead endstructures every 6 to 10 miles maximum. Current wood structure design includes abroken wire loadcase applied in the same manner as steel structures. All otherloadcases as listed above in the steel structure are also applied. Table 5 shows the overload factors used by Western for wood pole design compared to NESC H30(1949), and#81(1961).

Table 5

Min over load factors on materials, ultimate strength

Western(50's) Western(70's) NESC

Wood poles (L) 2.0 2.0 1.33

Wood poles (T) 2.0 2.0 4.0

Cross arms (V) 2.0 4.0 2.0

Cross arms (L) 2.0 4.0 1.0

Guys (T) 2.0 2.67 2.67

Guys (L) 2.0 2.0 1.0

(A wind pressure of 8psf was used on the wood pole design until the early 1970's, after which the NESC valueswere used.)

A typical 115kV, wood pole, tangent structure design from the 1950's and 1960'swould be:

Wood H-frame type with x-braceHeavy loading, ½-inch ice, 8 psf wind at 0 degrees.700 foot ruling spanClass 2 poles, Grade B constructionDouglas fir or Western red cedar polesDouglas fir cross arms(2-3/4x9-1/2-inches)Cond—397.5kcmil ACSR (26/7)


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Max tension under load 7,300 lbsFinal tension, 60 degrees, no load 2840 lbs

OGW—3/8 inch, 7-wire high strengthMax tension under load 4,631 lbsFinal tension, 60 degrees, no load 1,821 lbs

Special ice loading locations:

Western has in the past applied a special ice loading of 1-1/2 inches of rime ice insteadof the ½ inch of ice called for in the NESC heavy loading area. This loading has beenused as the result of experience or knowledge of icing in the location the transmissionline. A rime ice density of 40pcf was used for this special 1-1/2 inches of rime iceloading. The following example is of a special design for ice loading that was recentlycompleted. The ice loading was determined from actual ice loading on the existingtransmission line.

Summit-Watertown 115kV transmission line:

This line had a severe icing event in 1965 that destroyed 20 miles of the line. Over theyears the line has had maintenance problems with broken Cross arms, or damagedconductors. The hoarfrost on the conductors and overhead groundwires has beenmeasured at 10 inches on this line almost every year.

Original line information:115kV transmission lineWood pole H-frame constructionLine length = 30.7 milesOriginally built—1953 (partially replaced 1965)Designed—heavy, ½-inc ice, 8psf wind, 0 degrees

New design 1992: (Construction completed 1995)Wood pole H-frame construction with sixteen steel pole angles

and two steel pole deadends.Special loadcase consisting of 5-inch radial hoarfrost(12pcf), with a 2psf wind,with a design check for all other loadcases.

Douglas fir, class 2 poles, laminated cross arms, grade B construction.Cond—795kcmil ACSR (30/19)

Max tension under load 23,000 lbsNESC tension 13,400 lbs

OGW—½-inch EHSMax tension under load 15,270 lbsNESC tension 6,710 lbs

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15 APPENDIX K:

BPA DESIGN PROCEDURES

Appendix K:BPA Design Procedures

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Appendix K:BPA Design Procedures

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16 APPENDIX L:

BPA FAILURE DATABASE

Appendix L:BPA Failure Database

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Appendix L:BPA Failure Database

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17 APPENDIX M:

USE OF STRAIN PLATES AND LONGITUDINAL

LOADING MITIGATION

Appendix M:Use of Strain Plates and Longitudinal Loading Mitigation

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18 APPENDIX N:

TRANSMISSION TOWER LOAD LIMITER

Appendix N:Transmission Tower Load Limiter

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19 APPENDIX O:

EPRI ICE LOAD ASSESSMENT PROJECT

Recent Development inImproving Ice Load Prediction

Long Shan, Ph.D.

J.A. Jones Power Delivery, Inc.EPRI PDC @ Haslet, TX

Longitudinal Loading and Cascading Failure Risk Assessment WorkshopFort Worth, Texas, March 1996

Conductor Ice Loading

Research Need:Q Systematic ice data not availableQ Current U.S. ice map unsatisfactory

Project Objectives:Q Provide info. on ice condition in U.S.Q Develop new ice load maps

Current Research

Q National ice storm data baseQ Ice Severity Maps for the U.S.Q Local area 50-year ice thickness

maps and associated wind speeds(Using ice accretion models andbased on meteorological data - windspeed, precipitation rate, temperature,and present weather code)

Benefits

Q Upgrades in capacity, knowing mostprobable ice loads

Q Potential lower cost for new linedesign

Q Saving from avoiding costly failuresQ More accurate prediction of

transmission line integrity

Appendix O:EPRI Ice Load Assessment Project

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20 APPENDIX P:

EPRI WIND LOAD ASSESSMENT PROJECT

Recent Development inImproving Wind Load Prediction

Long Shan, Ph.D.

J.A. Jones Power Delivery, Inc.EPRI PDC @ Haslet, TX

Longitudinal Loading and Cascading Failure Risk Assessment WorkshopFort Worth, Texas, March 1996

Introduction

Background:Q Deficiency in current wind load modelsQ Design parameters not well defined

Development Goals:Q Improve design wind load methodQ Improve design parameter selection

Overview of Wind Load Methods

Q ASCE Manual 74Q NESCQ Companies A and BQ Company CQ Others

ASCE Manual 74

ASCE 74 basic wind load equation is written as follows:

F = Q (ZvVfm)2 Gw Cf d L

where

Q = air density factor (0.00256, at 60°F at sea level)Vfm = basic wind speed (fastest mile at 33 ft)Gw = gust response factorCf = force coefficient/drag coefficient of 1.0d = conductor diameterL = span length

ASCE 74 Gust Response Factor

∈ = approximate coefficientBw = dimensionless quasi-static response termRw = dimensionless resonant response termE = exposure factorgs = statistical peak factorKv = wind speed conversion factor

Gw simplified E Bw( ) = . + . 0 7 19

G full formg E B R

Kw

s w w

v( - ) =

+ + 12

∈

NESC

Wind pressure equation for NESC Extreme Wind LoadCase is given as follows:

p = 0.00256 (Vfm)2

where

0.00256 = air density factor at 60 °F at sea levelVfm = basic wind speed (fastest mile at 33 ft)

Note: NESC uses specific overload factors for differentmaterial types and load cases

Appendix P:EPRI Wind Load Assessment Project

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Companies A and B Method

Problem:Q NESC extreme wind load is inadequate

for applications in many areas

Solution:Q Use a large overload factor, e.g., 1.5

vs. 1.0 for steel and prestressedconcrete structures

Company C Method

Step 1:Q Determine a height factor, (h/30)2/7,

applied to wind pressure

Step 2:Q Multiply fastest mile wind pressure by

the height factor and an overload factorof 1.1

Other Methods

IEC 826 (International):Q IEC 826 is simpler than ASCE 74, but

the history of the IEC combined windfactor is not clear

JEC 127 (Japan):Q JEC 127 provides a unique set of gust

factors, drag coefficients, and spanreduction factors

Results of EPRI Research

Q Gust Spectrum and Turbulence ScaleQ Span EffectQ Drag Coefficient and Air DensityQ Basic Wind SpeedQ Span Gust Wind Load Approach

Gust Spectrum,Turbulence Scale

Q Gust spectrum and turbulence scaleare used in deriving ASCE 74 method

Results of Field Data Analysis:Q Gust spectrum “constants” higher than

the ones used by ASCE and vary a lotQ Actual turbulence scale values have a

much wider range than that in ASCEQ Influenced by the type of wind storm

Span Effect

Q Span effect may be described as “thelonger the span length, the less theeffective span gust speed”

Q The existence of span effect wasconfirmed by EPRI experiments

Q Span reduction factor (from field data):

where S is span length in feet

SS

p =+

−

1

1 213 5 12340

2

. .x

Drag Coefficient,Air Density

Q Drag coefficient and air density aretwo important parameters in wind loadcalculation

Q Typically, nominal values of 1.0 and0.076 lb/ft3 are used in design

Q EPRI experiments showed thatcomputed wind loads using actualvalues of drag coefficient and airdensity correlate well with field data

Basic Wind Speed for Design

Q In U.S., basic wind speed is fastestmile wind speed (average time of 60seconds at 60 mph)

Q Gust wind speed (average time of 2 or3 seconds) has become important inrecent years

Q EPRI experiments showed that gustwind speed is more closely related tospan gust load than fastest mile wind


20-3

Span-gust Wind Load Approach

Q Span-gust approach is based on thefield-derived quantitative relationshipsbetween reference wind speeds andspan gust speeds

Q These relationships are:

where S is span length in feet

VS

Vg spanmean− − −=

++11773

1 3 022

6 14858 1.

. . minx

mph

VS

Vg spanmean− − −=

+1

1 213 5 12340 3. . sec

x

Span-gust Wind Load Approach

Q Span-gust wind load calculation:

wherefa = conductor response factorρ = air densityCd = wind- tunnel drag coefficientA = wind area = 3-second mean span-gust speed

P f C A Vc a d g spanmean= −

12

2ρ ( )

Vg spanmean−

Examples

Q Wind Loads on a 500-ft SpanQ Wind Loads on a 1250-ft SpanQ Wind Loads for Lines at High

ElevationQ NESC District Loads vs. NESC

Extreme Wind LoadsQ Estimation of Local Extreme Wind

Speeds for Design

Example 1

Q Span length: 500 ft.Q Conductor: Chukar (has drag data)Q Air density: 0.076 lb./ft.3 at 0-ft. & 60°FQ Terrain exposure: C (Open country)Q For ASCE 74 Method:Q Gradient height: 900 ft.Q Surface drag coefficient: 0.005Q Turbulence scale: 220 ft.

Example 3

Line 1:Q same as Example 1 except that the

line is at a higher elevation (5280 ft.)Line 2:Q same as Example 2 except that the

line is at a higher elevation (5280 ft.)Actual Air Density:Q 0.063 lb./ft.3 at 5280-ft. & 60°F

Example 4

Q NESC Extreme Wind may be ignoredif the extreme wind speed is less than90 mph in NESC Light District

Is this true for Medium & Heavy Districtloads?

Q NESC Medium and Heavy arecombined wind and ice loads

Q Example 4 compares structuralweights using the line in Example 2

Example 5

Q Current ASCE 74 wind map generallyneglects local variation of windclimate, and can give unrealisticdesign wind speeds in some areas

Q Reliable design wind speed for aspecific area can be established usinghistoric wind data

Q Example 5 estimates design windspeeds using actual wind data fromone unnamed weather station


20-4

Discussion- Codes and Standards

Q In U.S., no loading standards directlyapplicable to power line design

Q NESC is only a safety codeQ ASCE 74 is a guide and has problems

that may not be easily solvedQ The span-gust approach (based on

field data) can help utilities improvetheir line design, and code committeesimprove their wind load provisions

Discussion- Reliability of Lines

Q Reliability of power lines is anincreasingly important issue to utilities

Q It is difficult to perform a full reliabilityassessment of a line

Q It is possible to define partial reliabilityof a line based on wind load or others

Q A line designed for 50-year windspeed may not be designed for 50-year wind load if the model is not good

Current EPRI Activities

Q Publish a document for assessment ofwind loads for line design and upgrade

Q Conduct wind tunnel tests to generatea conductor drag coefficient data basethat can be used in design practice

Q Write a guideline for generating localarea wind map by utilities

Q Generate local area wind maps forutility clients


20-5


20-6


20-7


20-8


20-9

21-1

21 APPENDIX Q:

EPRI CASCADING FAILURE RISK ASSESSMENT

PROJECT

$6&( 0$18$/ ��$6&( 0$18$/ ��$6&( 0$18$/ ��

QQ ‘A line design should be based on the anticipation that such mishaps (i.e.,‘A line design should be based on the anticipation that such mishaps (i.e.,structure or component failure) will occur and therefore should include strengthsstructure or component failure) will occur and therefore should include strengthsthat will ensure that damage is limited to within a few structures...’that will ensure that damage is limited to within a few structures...’

QQ ‘Failure containment provisions should be taken to prevent cascading beyond‘Failure containment provisions should be taken to prevent cascading beyondacceptable limits.’acceptable limits.’

QQ ‘This inability to quantify the dynamic energy or impact component that might be‘This inability to quantify the dynamic energy or impact component that might beimposed on the structures adjacent to the initial failure has directed attention toimposed on the structures adjacent to the initial failure has directed attention tothe security of the second or third structure away from the failure.’the security of the second or third structure away from the failure.’

QQ ‘...utilities that did not include longitudinal strength requirements had extensive‘...utilities that did not include longitudinal strength requirements had extensivecascades.’cascades.’

QQ ‘...underlying theory that the ground wires...would provide longitudinal‘...underlying theory that the ground wires...would provide longitudinalrestraint...some of the longest cascades occurred on these flexible ground wirerestraint...some of the longest cascades occurred on these flexible ground wiresupported lines...’supported lines...’

QQ ‘...there is a need for more precise definition of the necessary and sufficient‘...there is a need for more precise definition of the necessary and sufficientfailure containment loads...’failure containment loads...’

QQ ‘There is need for more information on failure containment experience...’‘There is need for more information on failure containment experience...’

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Appendix Q:EPRI Cascading Failure Risk Assessment Project

21-2

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22-1

22 APPENDIX R:

EPRI CASCADING FAILURE MITIGATION PROJECT

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Appendix R:EPRI Cascading Failure Mitigation Project

22-2

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23-1

23 APPENDIX S:

UTILITY PRACTICE SUMMARY

1) Does the structure adhere to NESC structural design requirements?

Yes 61%No 0%Other 39%

Comments:N/AWe use GO 95 in CA.We do not install wood H-frame for transmission structures,existing structures are being replaced with steel or concretestructures, therefore, we do not have the expertise to answerquestions 1-8.We do not use NESC, instead we use CSA 22.3.We do not build transmission lines using Douglas Fir poles; weuse Western Red Cedar for all transmission lines.

a) Is the ruling span less than the calculated allowable span?

Yes 57%Comments:1790' max. allow. span (pole is weak link)

No 0%Other 43%

Comments:N/AInsufficient data

b) Is the cross-arm sized adequately to support NESC design loads?

Yes 71%Comments:4400' max. allow. NESC span (outside brace is weak link)

No 0%Other 29%

Comments:

Appendix S:Utility Practice Summary

23-2

N/AInsufficient data

c) Is the shield wire arm sized adequately to support NESC design loads?



d) Are the V-braces sized adequately to support NESC design loads?



e) In your opinion, are there any characteristics that make this H-frame differentfrom other designs?

Yes 29%Comments:The double wood shield wire arm.

No 53%Other 18%

Comments:N/A

2) Can the longitudinal strength of the structure be determined with reasonableaccuracy?


Comments:N/A

a) If 'Yes' in 1) provide allowable overturning moment and base shear.

Moment ; ShearMoment - 325; I have a problem determining the relationship ofthe conductor load to the shieldwire load.109 (G.L.); 26 (G.L. Not applicable, how about shear @ X armheight)215; 37


23-3

b) If 'No' in 1) provide a short justification

Broken wire calculation not defined.No simple analytical method exists.Too many different parameters of input to accurately affectoutput (which phase fails, how many phases fail, etc.?)The entire system (flexibility of the structure, insulator ?, slack)must be considered.

3) Can the longitudinal stiffness of the structure be determined with reasonableaccuracy?


Comments:N/A

a) Should the structure be considered flexible or rigid if loaded in the longitudinaldirection?

Very Flexible 21%Flexible 64%Neither 0%Rigid 14%Very Rigid 0%

b) Should the cross-arm be considered flexible or rigid if loaded in the longitudinaldirection?


c) Should the insulator strain rods be considered flexible or rigid if loaded in thelongitudinal direction?



23-4

4) Can the contribution of individual components to the longitudinal strength bedefined?


Comments:N/A

a) Does the interior V-brace (wood brace) contribute to the longitudinal strength?


Comments:N/A

b) Does the exterior V-brace (fiberglass rod) contribute to the longitudinal strength?


Comments:N/A

c) Does the shield wire cross-arm (GluLam) contribute to the longitudinal strength?


Comments:N/A

d) Does the conductor cross-arm (GluLam) contribute to the longitudinal strength?


Comments:N/A

e) Does the X-brace (GluLam) contribute to the longitudinal strength?

Yes 7%Comments:Yes, if an outside phase breaks.

No 87%Other 7%

Comments:


23-5

N/A

5) Can unbalanced loads from a conductor failure be determined with reasonableaccuracy?

Yes 25%No 69%

Comments:No, since cond. insulator length is not shown.

Other 6%Comments:N/A

a) Can unbalanced loads be determined on the 1st structure from the conductorfailure?If 'Yes' please indicate load magnitudes for shield wire 1 and 2, and conductors 1,2, 3, respectively.

Yes 38%Comments:Insufficient data.Magnitudes of shield wire 1, shield wire 2, conductor 1,conductor 2, conductor 3OK, OK, 7.8, 7.8, 7.8

No 46%Other 15%

Comments:N/A

b) Can unbalanced loads be determined on the 2nd structure from the conductorfailure?If 'Yes' please indicate load magnitudes for shield wire 1 and 2, and conductors 1,2, 3 , respectively.

Yes 7%Magnitudes of shield wire 1, shield wire 2, conductor 1,conductor 2, conductor 3none provided

No 79%Other 14%

Comments:Not really sure what would happen to subsequent structures.N/A

c) Can unbalanced loads be determined on the 3rd structure from the conductorfailure?


23-6

If 'Yes' please indicate load magnitudes for shield wire 1 and 2, and conductors 1,2, 3 , respectively.


No 71%Other 21%

Comments:N/ANot really sure what would happen to subsequent structures.

6) Can unbalanced loads from a shield wire failure be determined with reasonableaccuracy?

Yes 31%No 56%

Comments:No, since conductor insulator length is not shown.

Other 13%Comments:N/A

a) Can unbalanced loads be determined on the 1st structure from the shield wirefailure?If 'Yes' please indicate load magnitudes for shield wire 1 and 2, and conductors 1,2, 3 , respectively.

Yes 33%Magnitudes of shield wire 1, shield wire 2, conductor 1,conductor 2, conductor 33.8, 3.8, OK, OK, OK

No 53%Other 13%

Comments:N/A

b) Can unbalanced loads be determined on the 2nd structure from the shield wirefailure?If 'Yes' please indicate load magnitudes for shield wire 1 and 2, and conductors 1,2, 3 , respectively.



23-7

No 67%Other 13%

Comments:Not really sure what would happen to subsequent structures.N/A

c) Can unbalanced loads be determined on the 3rd structure from the shield wirefailure?If 'Yes' please indicate load magnitudes for shield wire 1 and 2, and conductors 1,2, 3 , respectively.


No 60%Other 20%

Comments:N/ANot really sure what would happen to subsequent structures.

7) Will the loss of 1 structure cause a cascading failure under service load conditions?

Very Likely 14%Likely 14%Undecided 14%Not Likely 43%Very Unlikely 14%

a) Will the loss of 1 shield wire cause a cascading failure under service loadconditions?


b) Will the loss of both shield wires cause a cascading failure under service loadconditions?



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c) Will the loss of 1 shield wire and 1 conductor cause a cascading failure underservice load conditions?


d) Will the loss of 1 conductor cause a cascading failure under service loadconditions?


e) Will the loss of 2 conductors cause a cascading failure under service loadconditions?


8) Will the loss of 1 structure cause a cascading failure under NESC ice loadingconditions?


a) Will the loss of 1 shield wire cause a cascading failure under NESC ice loadconditions?


b) Will the loss of both shield wires cause a cascading failure under NESC ice loadconditions?

Very Likely 8%


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Likely 31%Undecided 38%Not Likely 23%Very Unlikely 0%

c) Will the loss of 1 shield wire and 1 conductor cause a cascading failure underNESC ice load conditions?


d) Will the loss of 1 conductor cause a cascading failure under NESC ice loadconditions?


e) Will the loss of 2 conductors cause a cascading failure under NESC ice loadconditions?


9) Is this subject of interest to you?

Very Much 28%Much 33%Undecided 0%Not Much 11%No 28%

a) Has your company experienced any cascading failures between 1980 and 1995?

Yes 39%No 61%

b) How many structures has your company lost due to cascading failures between1980 and 1995?

>345kV 415 Structures<345kV 276 Structures


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c) Do you believe that there is a lack of guidance in the industry on how to determinelongitudinal loads?


d) Do you believe that a more accurate longitudinal load prediction method willbenefit your company?


e) Does longitudinal strength constitute an evaluation criteria to determine a line'supgrade potential?


f) Would you be interested to serve the current EPRI project in an advisory function?


Epri Longitudinal Load 5

Documents

epri representative

jones power delivery

final report

epri project managerpaul

epri distribution center

epris current research

identification of lines

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