Fiber Optic Cables in High Voltage Environments

1000444

Fiber Optic Cables in High Voltage Environments

1000444

Technical Progress, December 2000

EPRI Project Manager

B. Clairmont

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 • askepri@epri.com • www.epri.com

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EPRIsolutions

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CITATIONS This document was prepared by

EPRIsolutions 115 East New Lenox Road Lenox, Massachusetts 01240

Principal Investigator: B. Clairmont

Contributors: G. Gela T. McDonald R. Olsen

The publication is a corporate document that should be cited in the literature in the following manner:

Fiber Optic Cables in High Voltage Environments: EPRI, Palo Alto, CA: 2000. 1000444.

ACKNOWLEDGEMENTS EPRI gratefully acknowledges the efforts put forth in this important work by the utility Working Group members listed below. These individuals contribute to the project in many ways, including participation in project review meetings, technical contributions, gaining support from their respective organizations, and providing overall direction for the ongoing work.

Monty Tuominen (BPA) – Chairman Robert Emerson (APC) Bruce LaMar (NSP) Joe Graziano (TVA) Dan Piekarski (NIPSCO) Dan Sanders (NPPD) David Smith (Eskom) Tim Williamson (CPS) Jeff Wild (WAPA) William Torre (SDGE)

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ABSTRACT In recent years, it has become common for electric utilities to place fiber optic cables within their transmission rights-of-way. Overhead transmission power line corridors can provide the telecommunications industry with cost-effective alternative routes, and at the same time benefit the electric utilities by generating additional revenues using existing facilities. Also, within the power utility industry, reliable internal communications are vital to ensure protection and control of the power system. Such communications traditionally have been provided by methods such as power line carrier and microwave radio systems but are more recently being supplemented or replaced by fiber optics. However, with the advent of digital fault protection systems, integrated power system automation signal densities are increasing, and fiber optic communications can offer a unique solution to the ever-increasing demand in the future. Consequently, many electric utilities are installing high capacity fiber optic cables and wires on their high voltage lines to satisfy their own internal communication needs. The relatively new practice of integrating fiber optic cables into high-voltage corridors poses some technical and safety-related challenges. Since ADSS fiber optic cables are located in high electric fields, there is the threat of sheath damage and cable failure. Two mechanisms (dry band arcing, and corona discharges near hardware) are known to cause ADSS cable failures. Also, when ADSS cables become wet and/or polluted, they become semi-conductive and can be a safety hazard to workers who may come into contact with them. As for OPGW, a number of reported failures appear to be related to positive lightning strokes. This Interim Report was produced as a deliverable from the EPRI project Fiber Optic Cables in High Voltage Environments. The project is ongoing at the EPRI laboratory in Lenox, Massachusetts, and is overseen by an EPRI member Working Group.

CONTENTS

1 INTRODUCTION AND BACKGROUND...............................................1-1 Introduction ............................................................................................................. 1-1 Background............................................................................................................. 1-2

Accelerated Aging Tests.................................................................................... 1-3 Analytical Studies .............................................................................................. 1-3 Full-Scale Tests................................................................................................. 1-3 Live-Line Installation and Maintenance ............................................................. 1-4 Remote Inspection Tools................................................................................... 1-4 Icing Tests ......................................................................................................... 1-4 Failure Survey ................................................................................................... 1-4 Investigation of Failures in the Field .................................................................. 1-4 Lightning Characteristics ................................................................................... 1-5 Characterization of Cables ................................................................................ 1-5 References ........................................................................................................ 1-5

2 ACCELERATED AGING TESTS OF FIBER OPTIC CABLES.............2-1 Introduction and Background .................................................................................. 2-1 Aging Chamber Design and Construction............................................................... 2-1

High Voltage Power Supply............................................................................... 2-2 UV Light Source ................................................................................................ 2-5 Rain and Salt Spray System.............................................................................. 2-7 Heat and Ventilation .......................................................................................... 2-8 Environmental Monitoring and Control .............................................................. 2-8 Test Setup ......................................................................................................... 2-9 Cable Preparation and Installation................................................................... 2-11 Data Acquisition System.................................................................................. 2-13 Analysis of the Chamber Space Potential Profiles........................................... 2-14 Modification to the Capacitive Coupling........................................................... 2-15

The Aging Tests.................................................................................................... 2-17 Introduction...................................................................................................... 2-17 The ADSS Cable Test Samples - Overview .................................................... 2-17 The Aging Cycle .............................................................................................. 2-20 Salt Water Spray ............................................................................................. 2-21 The Beginning of the Aging Tests.................................................................... 2-21 Hydrophobicity and Leakage Current .............................................................. 2-22

The Class A Failures ....................................................................................... 2-35 The Class B Failures ....................................................................................... 2-40

Cable 8.1.................................................................................................... 2-40 Cable 6.3.................................................................................................... 2-41 Cable 3....................................................................................................... 2-45

Summary of Cable Samples............................................................................ 2-47 Observations......................................................................................................... 2-49

General Review of ADSS Cable Aging............................................................ 2-49 General Observations of Class A Cable Aging................................................ 2-50 General Observations of Class B Cable Aging................................................ 2-50 The Bumps ...................................................................................................... 2-51 Video Capture of Dry Band Arcing................................................................... 2-54 Failures of Class A Cables .............................................................................. 2-54 Performance of Class B Cables....................................................................... 2-55 Non-Lethal Jacket Changes ............................................................................ 2-55

Loss of Hydrophobicity............................................................................... 2-55 Powdery Deposits ...................................................................................... 2-55 The 'Bumps' ............................................................................................... 2-56

The Nitric Acid Test ......................................................................................... 2-56 Summary .............................................................................................................. 2-57

Performance of Class A Cables....................................................................... 2-57 Performance of Class B Cables....................................................................... 2-58 Non-Lethal Jacket Changes ............................................................................ 2-59

Powdery Deposits ...................................................................................... 2-59 The "Bumps" .............................................................................................. 2-59

Conclusions and Recommendations ............................................................... 2-59 References ...................................................................................................... 2-60

3 SURVEY OF ADSS AND OPGW CABLE FAILURES..........................3-1 Introduction ............................................................................................................. 3-1 Survey of Failures................................................................................................... 3-2

ADSS Damage and Failures.............................................................................. 3-2 OPGW Damage and Failures ................................................................................. 3-5

References ........................................................................................................ 3-9

4 ANALYSES OF TWO REPORTED FIELD FAILURES.........................4-1 Consolidated Edison's ADSS Failure...................................................................... 4-1 Site Visit.................................................................................................................. 4-2

Line Configurations................................................................................................. 4-6 Analysis of Samples of Failed ADSS Cable.......................................................... 4-11

Visual Inspection ............................................................................................. 4-11 Hydrophobicity................................................................................................. 4-13

Electrical Environment of the ADSS Cable ........................................................... 4-13 Space Potential, Induced Voltage, Induced Current ........................................ 4-13

Contamination Levels ........................................................................................... 4-21 Characterizing Contamination ......................................................................... 4-21 ADSS Cable on Staten Island.......................................................................... 4-21

Glass Insulators on Staten Island ......................................................................... 4-26 ADSS Cable in the Lenox Aging Chamber ...................................................... 4-28 Porcelain Insulators in the Lenox Aging Chamber........................................... 4-28

Summary and Conclusions ................................................................................... 4-28 ESKOM'S ADSS Failure ....................................................................................... 4-29

Background ..................................................................................................... 4-29 Observations ................................................................................................... 4-30

Weather Conditions.................................................................................... 4-30 Measurement of Cable Resistance ............................................................ 4-30 Calculation of Space Potential to which ADSS was Subjected .................. 4-30 Physical Appearance of Cable ................................................................... 4-30

Subsequent Actions......................................................................................... 4-32 Further Preliminary Information ....................................................................... 4-33 Surface Ring Growth on ADSS Cable Installed in Electric Fields.................... 4-33

5 MODELING (3-D) OF ADSS CABLES IN HIGH VOLTAGE TRANSMISSION CORRIDORS...............................................................5-1

Introduction ............................................................................................................. 5-1 The Model............................................................................................................... 5-3 Results.................................................................................................................... 5-5 How to Design ADSS Cable Installations Based on the 3D Model ......................... 5-9 References ........................................................................................................... 5-10

6 FULL-SCALE TESTS...........................................................................6-1 Introduction ............................................................................................................. 1-1 Design and Construction......................................................................................... 6-1 Measurement Techniques ...................................................................................... 6-8 Open-Circuit Voltage............................................................................................... 6-8 Induced Current .................................................................................................... 6-10

Short-Circuit Current to Ground ............................................................................ 6-13 Induced Voltage.................................................................................................... 6-17 Phasing Confirmation............................................................................................ 6-21

Full-Scale Tests−Measurements and Calculations ............................................... 6-22 Configuration 1 ................................................................................................ 6-23 Configuration 2 ................................................................................................ 6-29 Configuration 3 ................................................................................................ 6-35 Configuration 4 ................................................................................................ 6-42

Sensitivity Analysis ............................................................................................... 6-46 Summary .............................................................................................................. 6-51

7 LIVE LINE INSTALLATION, MAINTENANCE AND INSPECTION OF ADSS CABLE .........................................................................................7-1

Live Line Installation of ADSS Cable ...................................................................... 7-1 Equipment ......................................................................................................... 7-2 Tensioning and Pulling Requirements ............................................................... 7-2 Pulling Lengths.................................................................................................. 7-2 Installation Methods........................................................................................... 7-2

Live Line Maintenance of ADSS Cable ................................................................... 7-4 Types and Repair of Failures............................................................................. 7-5 Safety Related Issues........................................................................................ 7-5

Remote Inspection of ADSS Cable ......................................................................... 7-8 Tools ....................................................................................................................... 7-9 Visual Evidence of Damage.................................................................................. 7-10 Evidence of Electrical Activity ............................................................................... 7-10 Change in the Electrical Environment ................................................................... 7-11 Techniques ........................................................................................................... 7-11 References ........................................................................................................... 7-13

8 ICING TESTS OF ADSS CABLES.......................................................8-1 Introduction ............................................................................................................. 8-1 Test Plan and Setup ............................................................................................... 8-1 The Tests................................................................................................................ 8-4 Data and Catenary Calculations ............................................................................. 8-5 Observations......................................................................................................... 8-10 Conclusions .......................................................................................................... 8-13

9 LIGHTNING CHARACTERISTICS AND OPGW FAILURES................9-1 Introduction ............................................................................................................. 9-1 Lightning Characteristics......................................................................................... 9-2 Lightning Caused OPGW Failure in the United States............................................ 9-3

Nebraska Public Power District ......................................................................... 9-3 Georgia.............................................................................................................. 9-3 Northeast United States .................................................................................... 9-3 Other Failures in the Southeast United States Between 1988-1998.................. 9-3

Laboratory Lightning Tests ..................................................................................... 9-4 Interpretation of the Results.................................................................................... 9-4

Example ............................................................................................................ 9-8 Lightning Resistant OPGW ..................................................................................... 9-9 Recommendations for Further Research .............................................................. 9-10 Summary .............................................................................................................. 9-10 References ........................................................................................................... 9-11

10 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS ............10-1 Accelerated Aging Tests of ADSS Fiber Optic Cables.......................................... 10-1 Survey of ADSS and OPGW Cable Failures......................................................... 10-1 Analyses of Two Reported Field Failures ............................................................. 10-2 Modeling (3D) of ADSS Cables in High Voltage Transmission Corridors ............. 10-3 Full-Scale Tests .................................................................................................... 10-3 Live Line Installation, Maintenance and Inspection............................................... 10-3 Icing Tests of ADSS Cables.................................................................................. 10-4 Characterization of ADSS Fiber Optic Cables ...................................................... 10-4 Lightning Characteristics and OPGW Failures...................................................... 10-4

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1 INTRODUCTION AND BACKGROUND

Introduction

In recent years, it has become common for electric utilities to place fiber optic cables within their transmission right-of-ways. This use of overhead transmission power line corridors provides the telecommunications industry with cost-effective alternative routes for their cables while generating additional revenues for electric utilities from existing facilities.

In addition, because reliable internal communications are vital to ensure protection and control of utility systems, many electric utilities are installing high capacity fiber optic cables on their high voltage lines. Traditionally, such communications have been provided by power line carrier and microwave radio systems. However, with the advent of digital fault protection systems, signal densities are increasing, and fiber optic systems offer utilities the means to communicate effectively.

The advantage of fiber optic cables lies in their remarkably high capacity for carrying data. Also, because fiber optic cables are immune to electromagnetic interference, they are particularly well suited for use in power delivery systems (as long as care is taken to shield terminal and repeater stations). Further, there are no radiation or frequency assignment difficulties associated with fiber optics as is commonly the case with power line carrier, intra-bundle, and microwave communication systems. Fiber optics also increase the security of transmission systems since the technology virtually eliminates the unauthorized monitoring of vital communications, and because fiber optic cables do not require coupling devices or other specialized connectors, they can be easily and cost-effectively integrated into any digital network.

Currently, there are four different types of fiber optic cables that can be installed on high voltage overhead transmission structures:

• All-Dielectric-Self-Supporting cable (ADSS)

• Optical Ground Wire (OPGW - encased within the ground wire)

• Optical Phase Conductor (OPPC - encased in phase conductors)

• WRAPped (WRAP - wrapped around the phase conductor or the ground wire)

With the development of new materials with a high strength-to-weight ratio, ADSS cables are now suitable for use with overhead power lines with span lengths up to 1,000 m (3,284 ft.). Even under governing wind and ice loading conditions these cables can maintain the same midspan clearance to ground as the conductors without tower reinforcement. A major advantage

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of ADSS cables is that they can be regarded as separate from the power system and, therefore, attractive to third-party users such as telecommunication providers.

At present, there are a variety of OPGW cables that can be integrated directly into existing transmission right-of-ways as replacements for traditional shield wires. OPGW cables typically have the correct combination of flexibility and strength to approximate the sag and tension characteristics of existing shield wire installations for most span lengths. Consequently, the ease of integration into existing right-of-ways makes this alternative to underbuilt or wrapped cables very attractive to electric utilities.

OPPC cables have not been used extensively in North America but are in many ways very similar to OPGW cables. Because of the difficulty associated with the replacement and/or upgrade of phase conductors in OPPC installations, the technology appears not to have gained a foothold in American utility companies. Consequently, OPPC fiber optic cables have not been included in this ongoing EPRI research project.

WRAP cables eliminate the need for specially shielded transition hardware and typically can be installed or retrofitted economically on existing lines without structural modifications to the supports. Wind tunnel tests show that the wind load on the shield wire or conductor typically increases no more than 10% for most wrapped fiber optic cables resulting, at most, in a need for minor structural modifications. However, in order to limit the scope of the EPRI project, and because WRAP type cables are used far less extensively than ADSS or OPGW, the Working Group for the EPRI fiber optics project had decided to eliminate further work on WRAP cables at this time.

The relatively new practice of integrating fiber optic cables into high-voltage corridors does pose some technical and safety related challenges. Since ADSS fiber optic cables are located in high electric fields, there is a threat of cable jacket damage and catastrophic cable failure. Two mechanisms (dry band arcing and corona discharges near hardware) are known to cause ADSS cable failures. Also, when ADSS cables become wet and/or polluted, they tend to conduct surface leakage currents. This can cause rapid cable damage and may be a safety hazard to workers who come into contact with them. As for OPGW, a number of failures that appear related to positive lightning strokes have been reported.

Background

The National Grid (UK) had early experience with the use of ADSS cables within high voltage corridors and suffered the first reported jacket failure in 1987. The British also published some of the earliest analyses of applications and potential failure modes, including dry band arcing and corona discharges [1 – 11].

U.S. utilities became interested in fiber optic applications in high voltage environments in the early 1990s. As a result, EPRI sponsored a workshop in July of 1995 in Portland, Oregon, and a second workshop in December of 1996 in Tempe, Arizona. As a result of these workshops it was decided to develop a plan for EPRI-sponsored research and to form an EPRI member Working Group.

The Working Group was formed and had its first meeting at Arizona State University (ASU) in March of 1997. At this meeting priorities were set and a course of action was outlined. It was

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also decided that EPRI would publish a state-of-the-art report. This report, “Fiber Optic Cables in Overhead Transmission Corridors: A State of the Art Review,” was issued in November of 1997 (TR-108959).

Since that time Working Group meetings were held at the EPRI laboratory in Lenox, Massachusetts, at the rate of approximately one every six months. At these meetings the results of the ongoing research are reviewed, and discussions are held to direct the course of the work. An Interim Report was issued in December 1998.

The highlights of the fiber optics research project are briefly outlined below. Details of the work and results are presented throughout the report.

Accelerated Aging Tests

The purpose of this work was to test class B ADSS cables from different manufacturers in an electrical environment similar to that which they would experience in a real “25 kV space potential” application, and under stressful, but realistic, ambient conditions. The main goal was to rapidly age the cables and to observe any degradation, especially degradation due to dry band arcing. The tests took place in an accelerated aging chamber representing a coastal environment, and ran continuously for nearly two years.

Analytical Studies

Much work was performed on modeling the electrical environment in which fiber optic cables may reside, and the electrical parameters that characterize their behavior. The early work on this topic is presented in a 1998 Interim Report. The latest work includes the development, by Washington State University and BPA, of a spreadsheet-based, quasi 3-D model for ADSS cables strung on high voltage lines. The model is able to calculate the space potential, the induced voltage, the open-circuit voltage, and the induced current along cables.

Full-Scale Tests

A full-scale test line was constructed at the EPRI lab in Lenox, Massachusetts, for the purpose of performing specialized experiments with fiber optic cables. The test line is a single 600-ft. span with 3-phase energized conductors (up to 345 kV). The test line also includes an actual ADSS fiber optic cable, and an artificial fiber optic cable. Many line configurations can be simulated. The purpose of the test line includes the validation of analytical models, live-line installation and maintenance tests, tests of damage mitigation devices and strategies, tests of measurement devices, tests of inspection tools, and for other real-to-life observations.

Live-Line Installation and Maintenance

Even though ADSS cables are, by definition, dielectric, water and pollution on a cable’s jacket results in some conductivity which becomes a concern for worker safety during live-line installation and maintenance. Short-circuit currents to ground were measured during the installation of the ADSS cable on the full-scale test line, and were measured during other tests. In addition, methods of performing live-line work are considered part of the project.

Remote Inspection Tools

It is desirable to inspect remotely for damage, or ensuing damage due to the presence of arcing. Some work in this area has been performed, and much more is being planned.

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Icing Tests

At a Working Group (WG) meeting, it was decided that icing tests needed to be performed on an ADSS cable in a full-scale test setup. At issue was the question of whether ice can build up indefinitely on such a cable, or if a natural shedding process takes place. Specifically, there exists a hypothesis that as ice starts to form on an ADSS cable, and the cable begins to stretch under the additional weight, the bond between the cable and ice will break, and the ice will shed. Another hypothesis claims that this is not true. There are conflicting opinions coming from manufacturers and utilities. A special experiment was performed at the Lenox lab. An EPRI Tech Brief was issued on the tests, and the tests and results are presented here.

Failure Survey

An attempt is made to list, and document as much as practical, failures that occur anywhere in the world. This is very difficult for many reasons, and the list that is maintained in this project is most probably incomplete. First, there is no official way of obtaining this information. The researchers must rely on gathering information through informal contacts. Also, it is suspected that in some cases there is an unwillingness to attract attention to failures. It was also found in some cases that the utility involved was willing to share information about a failure, but the cable’s manufacturer was not.

Investigation of Failures in the Field

As discussed above, an attempt is being made at maintaining a catalogue of failures. In addition, whenever possible and practical, the EPRI researchers will perform investigations of failures on site in order to gather as much information as possible. One such investigation recently took place near New York City, and is documented in a section of this report.

Lightning Characteristics

Analysis is finding correlation between OPGW failures and lightning characteristics. This analysis is ongoing and is presented in the report. The main finding thus far is that it appears that failures are primarily due to positive strokes because of some unique characteristics.

Characterization of Cables

A project cosponsored by WAPA and EPRI is being performed at Arizona State University. The work is described in this report. The objective of this project is the following:

• Develop a new test method that will define and represent environmental conditions experienced by fiber optic cables strung along high-voltage networks experiencing different degrees of pollution levels, particularly in salty areas.

• Compare the performance of different fiber optic cable sheaths subjected to dry band arcing tests under controlled conditions.

• Study and compare the effects of current-limiting impedances used in exposing the cable’s sheath to high voltage.

• Provide the results of a methodology used to analyze the acquired leakage current data and consider its outcomes correlated to the failure characteristics of the fiber optic cables due to dry band arcing.

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References

1. Rowland, Craddock, Carter, Houghton, and Delme-Jones, “The Development of a Metal Free Self Supporting Optical Cable for Use on Long Span High Voltage Overhead Power Lines,” IWCS Proceedings, IWSC, pp. 449-456, 1987.

2. Bartlett, Carlton and Carter, “The Design and Application of Optical Cables into Overhead Lines up to 150 kV,” presented at the IEE Conference on Overhead Line Design and Construction: Theory and Practice, pp. 166-172, November 28-30, 1988.

3. Berkers, and Wetzer, “Electrical Stresses On A Self-Supporting Metal Free Cable in High Voltage Networks,” presented at the 5th International Conference on Dielectric Materials and Applications, Canterbury, U.K., pp. 69-72, 1988.

4. Wheeler, Lissenburg, Hinchliffe, and Slevin, “The Development and Testing of a Track Resistant Sheathing Material for Aerial Optical Fibre Cables,” presented at the 5th International Conference on Dielectric Materials and Applications, Canterbury, U.K., pp. 73-76, 1988.

5. Rowland and Carter, “The Evaluation of Sheathing Materials For an All-Dielectric Self-supporting Communication Cable for Use On Long Span Overhead Power Lines,” presented at the 5th International Conference on Dielectric Materials and Applications, Canterbury, U.K., pp. 77-80, 1988.

6. Carter, “Dry Band Electrical Activity on Optical Cables Separately Strung on Overhead Power Lines,” IWCS Proceedings, IWCS, pp. 117-121, 1988.

7. Bartlett, Carlton, Carter, and Peacock, “Self-supporting Metal Free Optical Cable for Long-Span Power Line Use,” CIRED, pp. 252-258, 1989.

8. Carter and Waldron, “Mathematical Model of Dry Band Arcing on Self-Supporting All-Dielectric Optical Cables Strung on Overhead Power Lines,” IEE Proceedings-A, IEE, Vol. 139, No 3, pp. 185-196, May 1992.

9. Carlton, Carter, Peacock, and Sutehall, “Monitoring Trials on All-Dielectric Self-supporting Optical Cable for Power Line Use,” IWCS Proceedings, IWCS, pp. 59-63, 1992.

10. Carter, “Arc Control Devices For Use On All- Dielectric Self Supporting Optical Cables,” IEE Proceedings-A, IEE, Vol. 140, No 5, pp. 357-361, September 1993.

11. Rowland and Easthope, “Electrical Aging and Testing of Dielectric Self-supporting Cables for Overhead Power Lines,” IEE Proceedings-A, IEE, Vol. 140, No 5, pp. 351-356, 1993.

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2 ACCELERATED AGING TESTS OF ADSS FIBER OPTIC CABLES Introduction and Background

The purpose of the accelerated aging project was to test class B cables from different manufacturers in an electrical environment similar to that which they would experience in a real “25 kV space potential” application, and under stressful, but realistic, ambient conditions. The main goal was to rapidly age the cables and to observe any degradation, especially degradation due to dry band arcing.

The EPRI Working Group that oversees this work decided that the initial tests would simulate a warm coastal environment similar that of the southeast US. A second set of tests is planned which will simulate a high altitude (more intense UV), dry environment.

The first set of tests, representing a coastal environment, took place in an accelerated aging chamber, and ran continuously for nearly two years.

Aging Chamber Design and Construction

The aging chamber is essentially a rectangular room approximately 8 ft x 6 ft x 30ft. There are three doors that allow access into the chamber with observation windows along one side (opposite the wall of UV lamps) and at the grounded end of the chamber. The floor has drains for the removal of water that is sprayed into the chamber. In addition, the chamber consists of the following elements:

• High voltage is brought into the chamber by a bushing passing through the ceiling at a far end (the East End).

• An electrode/capacitor arrangement for coupling the high voltage onto the ADSS cable test samples.

• An ADSS cable test sample mounting system.

• Fluorescent lamps to produce UV radiation.

• A rain and salt spray system.

• Heaters and fans.

• Environmental monitoring and control

• Data acquisition system.

These various elements are described below, and Figure 2-1 shows a photograph of the aging chamber from the outside.

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Figure 2-1 Photograph of the aging chamber.

High Voltage Power Supply

The high voltage is brought into the chamber by a 138 kV bushing that passes through the ceiling at one far end (the East End) of the chamber. This bushing is fed by a transformer, which in turn is fed by a controller, which is powered by 220 VAC. Figures 2-2, 2-3, and 2-4 provide photographs of this power system.

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Figure 2-2 The power supply controller.

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Figure 2-3 The power supply transformer.

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Figure 2-4 The high voltage bushing (outside chamber /inside chamber).

UV Light Source

The ultraviolet (UV) radiation system of the chamber duplicates exposure in that portion of the solar spectrum responsible for aging polymer materials [12]. Fluorescent lamps having the same low-end cutoff wavelength as sunlight are utilized (see Figure 2-5). If radiation having a lower wavelength cutoff were used, it would be possible to break organic bonds that would not be broken in natural sunlight. For example, the threshold wavelength below which carbon-hydrogen bonds break is 289 nm. Exposure to radiation containing wavelengths below this level would cause damage to material that would not occur in natural sunlight.

Figure 2-5 Spectrum of fluorescent lamps used in the NCI aging chamber.

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As seen in Figure 2-6, the fluorescent lamps are arranged in four vertical panels (two panels can be seen in the figure), each of which contains 32 lamps, for a total of 128 lamps. The number and spacing between the lamps were determined so that samples under test would experience the same light intensity as that measured during summer at noon in Florida with no cloud cover. To check the design of the UV light system in the chamber, measurements were made in the West Palm Beach area using a radiometer with a narrow band (10 nm) filter centered at 350 nm. The results are shown in Figure 2-7. For comparison, the same instrument was used to take measurements in the aging chamber. The chamber measurements of 1.0 mW/cm2 indicated good agreement with sunlight readings.

Figure 2-6 Arrangement of fluorescent lamps in the aging chamber.

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Figure 2-7 Solar UV radiation in West palm Beach, Florida, with a narrow band (10 nm) filter at 350 nm.

The system of lamps in the chamber is very efficient. Each lamp requires 40 W for a total array consumption of about 5 kW. Single xenon lamps may have energy dissipation in excess of this amount and would not give the uniform coverage provided by the 128 lamp array. In addition, much of the xenon lamp radiation is in the visible range, which causes no damage to organic materials, but can raise the temperature of the side of test samples facing the light. Surface temperature measurements of test samples indicated a temperature increase in the chamber of only 1 oC on the surface facing the light bulbs.

The UV light source consists of bulbs 1.2 m long. The bulb arrays are internally cooled to promote bulb life and preserve light intensity over the duration of the bulb life. Bulbs are replaced as needed to maintain intensity. The bulb arrays are turned on only during dry periods in the aging cycle.

Rain and Salt Spray System

The chamber utilizes a rain and salt spray system with all nozzles and hoses common within the chamber. Twelve nozzles located near the ceiling of the chamber discharge a fine mist horizontally over the samples. Care is given to avoid direct impingement on any samples that might locally affect the surface conditions. The nozzles are supplied with salt water composed of de-ionized water and 2.5 kg/m3 (0.25%) of salt during the salt cycle, or de-ionized water during the clean rain cycle.

Heat and Ventilation

Since temperature also affects the aging of polymer materials, provision was made in the accelerated aging chamber to elevate temperature by means of thermostatically and computer

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controlled heaters. Heat is supplied from two forced-air 5 kW electric heaters on the floor of the chamber. Heat is applied during the dry cycles in the chamber when temperatures are maintained at 41 oC to 45 oC. Enough air circulation is created by the fan to limit temperature variations along the length of the chamber to +/- 1 oC.

The ventilation system is a 300-cfm, or 8.5-cm/m (cubic meters/min.) blower ducted to a ceiling port at the East End of the chamber. Ventilation occurs at the end of the clean spray to clear the fog for the heat and sun cycle. Both the heat and the ventilation systems are computer controlled.

Environmental Monitoring and Control

The environment (rain, salt, UV light, temperature) in the chamber is controlled by an HP 385S computer. The chamber is subjected to an environmental cycle described in the next section. Sensors in the chamber are used to monitor temperature, humidity, and UV light intensity. Figure 2-8 shows a photograph of the housing that contains the temperature and humidity sensors. Figure 2-9 shows a photograph of the UVA intensity sensor.

Figure 2-8 Temperature and humidity sensor.

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Figure 2-9 UVA light intensity sensor.

Test Setup

The chamber was designed to accommodate eight ADSS cable test samples. The samples were about 23 ft long. The space potential was capacitively coupled to the samples at one end, and the other ends of the cables were grounded. The coupling was accomplished with an electrode arrangement at the East End of the chamber, just below the high voltage bushing. The electrode was in the configuration of a rectangular box open on two ends, and a bank of high voltage capacitors. The “box” electrode was made of a wire mesh. The cables entered the electrode through the open West End, and were mounted to the wall at the open East End via insulators. Figure 2-10 shows a photograph of this setup.

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Figure 2-10 Photograph showing ADSS cables entering the electrode. The UV light bank can be seen on the south wall (right side in photo), and the grading tube can be seen around the electrode.

The electrode was made of 304-stainless-steel welded-wire cloth (.063 in.), and the mesh density was 2x2 per in. The overall dimensions were 0.5 ft wide by 4 ft high by 4 ft long. The electrode was open at the ends where the cables enter it. The electrode was divided further by installing a 6-in.-wide x 45-in.-long wire cloth, of the same material as the rest of electrode, in between each cable. This produced a separate concentric electrode for each cable. There was an 8-in. metal tube (seen in Figure 2-10) that was attached around the West End of the box to grade the electric field.

The electrode was held in place by an insulator mounted to the ceiling and was hung at a 76-degree angle from horizontal. A steel rod was attached to the bottom plate on the insulator and to the middle bottom part of the electrode. This rod kept the electrode at an angle with the stack of ADSS cables to ensure that the higher cables did not drip on the cables below. The high voltage (25 kV) was applied to the electrode from the bushing with a 2-inch metal tube attached at the bottom plate on the insulator. To insure that water drops from the insulator were directed away from the cables, a piece of sheet metal was attached to the insulation. The electrode was tested to ensure that water drops rolled down the sides of the electrode and away from the cables passing through it.

The cables were also mounted such that they have a slight upward slope as they approach the grounded end to simulate the normal sag of a cable close to its tower attachment points. This could have an affect on performance because of the water drop movement along the cable.

Figure 2-11 provides an illustration of the test setup described above. Figure 2-12 provides an illustration of the electrode held at a 76 degree angle from horizontal, and shows dimensions of the electrode around the cables.

2-11

Figure 2-11 Layout of the test setup as viewed from the side.

Figure 2-12 Illustration of the electrode setup.

Cable Preparation and Installation

The cables were numbered 1 through 8 (top to bottom) and were about 23 ft. long. The cable ends were sealed with epoxy. A tapered rubber boot was cut at the smaller end to fit tightly around the cable. The boot was put on the ends of the cables to form a small cup to contain the liquid epoxy. The ends were then sealed with the epoxy, and the boot was left on the cables. Special tests were made with separate samples to assure that water could not pass through this

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seal. Nine-inch long steel cable grips were put on both ends of each cable. These grips, which allowed the cables to be anchored at each end of the chamber, were held in place with steal hose clamps to ensure that no slippage occurs.

The cables were strung through the electrode to an “S” hook on the backside of the chamber. The “S” hook was attached to a fiber glass rod, and the rod was attached to a metal plate. The metal plate provided the needed play to align the cables through the electrode and to the grounded end of the chamber. The metal plate was attached to the top of an insulator mounted to the back wall of the chamber. The insulator was grounded through the data acquisition system.

The grounded end of the cable’s grip was connected to a 12 kV insulator. This insulator ensured that all leakage current that flowed on the cable passed to ground through the CT. The other end of the insulator was connected to a steel cable. The steel cable ran to an eye bolt on the support structure of the chamber, and the other end of the steel cable was attached to five 10-lb. insulators (weights). Each cable was subjected to 100 lbs. of tension. This ensured that slack was picked up as the cables stretched. The tie point for the grounded (west) end of the ADSS cables was six inches higher than at the energized (east) end of the chamber. A close-up photo of the back of the electrode, and a photograph of the grounded ends of the cables and porcelain insulators used as weights, are shown in Figure 2-13.

Figure 2-13 A close-up photo of the back of the electrode, and a photograph of the grounded ends of the cables and porcelain insulators used as weights.

Data Acquisition System

The primary data acquisition system for the aging chamber was a system called OLCA. It has nine channels for monitoring leakage current, and has channels for measuring voltage, UVA light intensity, temperature, and humidity.

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The OLCA was configured to measure leakage current between .01 mA and 160 mA. The leakage currents along the surfaces of the eight ADSS cables were measured with the first eight channels of the system, and the insulator mounted on the back (east) wall of the chamber was monitored by the ninth channel to measure any leakage flowing in that direction.

Each ADSS cable was grounded through its ground-end cable grip. The ground wire was attached to the grip with a hose clamp. This wire was then routed through a current transformer (CT) to a ground strap in the chamber. A second wire from the ground strap was wrapped around the ground wire from the ADSS cable to reduce any noise that may be picked up. In addition to measuring the leakage currents on the cable sample surfaces, provision was made to monitor any current flowing through the inside of the cables. The HP computer monitored these currents. The setup is illustrated in Figure 2-14.

Figure 2-14 Layout of the data acquisition system.

Two other sensors mounted in the chamber collected data for the OLCA: A weather sensor measured temperature and humidity, and a light sensor measured the UVA light levels in the chamber. The OLCA also measured the bushing voltage (25 kV). All of this data was then used to make the calculations necessary for the different output reports the OLCA can generate. The OLCA stores data at a rate of one measurement per msec to an internal hard drive, and this data was downloaded to a PC once per day.

Analysis of the Chamber Space Potential Profile

It was important that the electrical environment along the individual cable samples was the same for each cable. The electrical environment in the chamber was modeled with a finite element software package, and the result of the simulation gave the unperturbed space potential and

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electric field along the paths through space in which the cables reside. These plots, shown in Figures 2-15 and 2-16, demonstrate that the cable samples were all exposed to approximately the same electrical stresses.

Figure 2-15 Plots of unperturbed space potential along the paths through space in which the cables resided.

Figure 2-16 Plots of unperturbed electric field along the paths through space in which the cables resided.

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Modification to the Capacitive Coupling

The short circuit current available to the ADSS cable samples was measured to be about 0.5 mA. During the course of the aging tests it was reported that a 1 mA short circuit current may be necessary to cause damage from dry band arcing on class B cables. As a result, it was decided to add capacitors in parallel with the electrode to increase the short circuit current available to the cables up to 10 mA. This corresponds to a capacitance of about 1 nF in series with each of the cables.

Custom built 25 kV/1 nF outdoor capacitors were ordered, and installed in the chamber. Schematically, each cable sample was approximately configured as illustrated in Figure 2-17. A photograph of the installed capacitors is shown in Figure 2-18.

Figure 2-17 Schematic of an ADSS cable sample in the test chamber.

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Figure 2-18 Photograph of the capacitors installed in the test chamber.

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The Aging Tests

Introduction

The basic idea of the tests was to mount ADSS cable samples in an environment which approximately simulates that which they would experience in a real-world application, and to age them at an accelerated rate from environmental factors. The accelerated aging was accomplished by using elevated temperatures, UV light, rain, and contamination, as described below.

Class B (track resistant) cables are specified by manufacturers to be able to operate in a “25 kV space potential”. This means that the unperturbed space potential near midspan in a high voltage transmission corridor can be as high as 25 kV. At higher space potential, the phenomenon known as dry band arcing can cause damage to the outer jacket, and ultimately result in cable failure.

In a real-world application, a cable resides in a high space potential away from the tower, and the space potential drops to zero as the tower is approached. The aging chamber was designed to represent the electrical environment in the region approaching the tower, with a 25 kV space potential at one end, and ground potential at the other. This difference in voltage drives currents over the surface of the cables when wet or contaminated, and causes a large voltage to be dropped over any dry bands that form. This results in dry band arcing, which can lead to cable damage [6, 8, 13].

The ADSS Cable Test Samples – Overview

Eight ADSS cable samples were mounted in the chamber as previously discussed. These cables are referred to as “cable 1” through “cable 8”. Cable 1 is the uppermost cable in the test setup, cable 2 is just below it, and cable 8 is the bottommost cable. Of the original eight test samples, seven were class B, and one was class A. The test samples were from four different cable manufacturers referred to as manufacturers a, b, c, and d. As the tests proceeded, different cables were replaced, but the samples were all from the four manufactures. Table 3-1A though Table 3-1G summarizes the history of the test samples’ installations and replacements.

Cable samples 1, 2, 3, 4, 5, and 7 were never replaced throughout the tests. The samples in cable positions 6 and 8 were changed at times and, therefore, the individual cable samples are labeled with a decimal point and an additional digit; namely, 6.1, 6.2, 6.3, 6.4, 8.1, and 8.2.

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Table 2-1A Summary of ADSS test samples as originally installed (7/27/98). The cable samples in positions 1-5 and 7 (in dark boxes below) did not change for the entire test period.

Cable number Manufacturer Class

1 a B

2 b B

3 c B

4 b B

5 a B

6.1 a A

7 c B

8.1 d B

Table 2-1B Summary of ADSS test samples for positions 6 and 8 as of 3/16/99 (cable in slot 6 was changed).

Cable number Manufacturer Class

6.2 a A

8.1 d B

Table 2-1C Summary of ADSS test samples for positions 6 and 8 as of 7/2/99 (cable in slot 6 was changed again).

Cable number Manufacturer Class

6.3 c B

8.1 d B

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Table 2-1D Summary of ADSS test samples a for positions 6 and 8 as of 8/16/99 (no cable in slot 8).

Cable number Manufacturer Class

6.3 c B

Table 2-1E Summary of ADSS test samples for positions 6 and 8 as of 11/12/99 (new class A cable in slot 8).

Cable number Manufacturer Class

6.3 c B

8.2 b A

Table 2-1F Summary of ADSS test samples for positions 6 and 8 as of 12/15/99 (no cable in slot 6).

Cable number Manufacturer Class

8.2 b A

Table 2-1G Summary of ADSS test samples for positions 6 and 8 as of 1/10/00 (new class B cables in slots 6 and 8).

Cable number Manufacturer Class

6.4 c B

8.3 d B

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The Aging Cycle

The environmental aging cycle implemented in the ADSS cable aging chamber was based on conditions, which exist in the Florida coastal region, as documented in the reference [12]. The sea-salt contamination levels in that environment are particularly heavy, especially in winter. The aging cycle was an updated version of an aging cycle used for a former EPRI aging project on non-ceramic post insulators.

The aging was based on an eight-hour cycle, for a total of three cycles per day. The components of a single cycle are outlined in Figure 2-19. The cycle started with 45 minutes of high-noon sunlight (UVA bulbs on), followed by salt water spray, clean water spray, heat, and a long drying period. In addition, the high voltage environment was maintained throughout the cycle.

It is extremely difficult to specify and verify an accelerated aging factor for any accelerated aging tests. However, it is estimated that for our tests, the factor was estimated to be 17. That means every month in the chamber is about equivalent to 17 months (1.4 years) in the field. This estimation is based on research results from long running non-ceramic insulator projects, and the basis for the estimation is documented in reference [12].

Figure 2-19 Illustration of the eight-hour aging cycle.

Salt Water Spray

The salt water for the contamination portion of the aging cycle consisted of de-ionized water with 0.25% dissolved salt. This concentration of salt was selected based on previous aging tests on non-ceramic insulators in a Florida coastal environment. The development of the aging cycle and the salt concentration level was discussed and agreed upon with NEETRAC prior to the initiation of the aging tests. Subsequent Equivalent Salt Deposit Density (ESDD) measurements on the porcelain insulators used as weights at the west end of the chamber indicated ESDD levels of up to 0.094 mg/cm2 on the bottom surfaces of the insulators during the salt period, which falls on the borderline of moderate and heavy contamination. Experience with non-ceramic insulators has shown [12] that this salt concentration is appropriate for the selected aging cycle and that, in

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particular, higher salt concentrations resulted in excessive discharge activities that were not observed in service.

The Beginning of the Aging Tests

The aging chamber was setup for tests during the early summer of 1998. Some initial tests were performed to verify the integrity of the chamber’s physical construction and capability of sustaining 25 kV on the electrode system. The environmental control and data acquisition systems were checked out, and the ADSS test cable samples were mounted. The chamber was first energized with the test samples in place on July 28, 1998, to 10 kV.

The chamber was kept operational at 10 kV for several days as a further checkout period. The voltage was then raised to 15 kV for several days, and then it was raised to the final level of 25 kV. During this checkout period the environmental aging cycle was operated continuously except for short maintenance periods. The tests were terminated on June 1, 2000.

Throughout the aging tests, leakage current was monitored on all eight test cables. A sample of the leakage current measurements during the early tests is shown in Figure 2-20. This shows the RMS current on cables 2 and 8 over a 10 day period as measured by the OLCA data acquisition system. Note that the time axis in Figure 2-20 shows the major marks in 2-day intervals. A 2-day interval covers six complete aging cycles and hence shows six current peaks. Each current peak corresponds to the salt portion of the aging cycle, and lowest currents correspond to the drying periods. The currents all remained well under 0.1 mA one month into the test. The current increased as the cables lost hydrophobicity.

Figure 2-20 RMS leakage currents on cables 2 and 8 near the beginning of the aging tests.

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Hydrophobicity and Leakage Current

Hydrophobicity is the property of a material to repel water. New polymer materials are generally very hydrophobic, which means that water will tend to bead (forms discreet drops), and not wet the surface. Hydrophobicity is quantified by comparing a sample’s surface with pictures from a chart, and is, therefore, somewhat subjective. This chart quantifies hydrophobicity over a range of six classes. Hydrophobicity of class 1 (HC1) indicates extreme hydrophobicity, class 6 indicates total loss of hydrophobicity. The chart is shown in Figure 2-21.

Figure 2-21 Hydrophobicity chart.

At the beginning of the tests the hydrophobicity was measured on all the cables. All the cable samples were very hydrophobic (HC 1).

Over time, the cables tended to lose their hydrophobicity. However, some cables lost hydrophobicity at a faster rate than others. Cables 1 and 5 lost most of their hydrophobicity during the first month in the chamber. These were both class B cables from manufacturer a.

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However, cable 6, which was a class A cable from the same manufacturer, lost hydrophobicity at a much lower rate. Figures 2-22 through 2-29 show these conditions for the eight cables after five months into the aging.

Cables 2 and 4 (manufacturer b) essentially have never lost hydrophobicity. Cables 3, 7, and 8 (manufacturers c and d), loss their hydrophobicity at about the same rate as the class A cable of manufacturer a.

Figure 2-22 Cable 1 after 5 months in the chamber. Note the total loss of hydrophobicity.

Figure 2-23 Cable 2 after 5 months in the chamber. Note that it is still very hydrophobic.

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Figure 2-24 Cable 3 after 5 months in the chamber. Note the near total loss of hydrophobicity.

Figure 2-25 Cable 4 after 5 months in the chamber. Note that it is still very hydrophobic.

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Figure 2-26 Cable 5 after 5 months in the chamber. Note the total loss of hydrophobicity.

Figure 2-27 Cable 6 after 5 months in the chamber. Note the near total loss of hydrophobicity.

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Figure 2-28 Cable 7 after 5 months in the chamber. Note the near total loss of hydrophobicity.

Figure 2-29 Cable 8 after 5 months in the chamber. Note that it loss some hydrophobicity.

After the aging tests were terminated (June 01, 2000) cable samples in positions 1, 2, and 3 were sprayed with water as a final hydrophobicity test. The three cables had been aged in the chamber for two years and represent class B cables from three different manufacturers. Figure 2-30 shows the results.

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Figure 2-30 Cables 3 (top), 1(middle), and 2(bottom) after 2 years in the chamber. Note that cable 2 has not lost any hydrophobicity.

Throughout the aging tests, the leakage current was monitored on all the cables. Leakage currents on all the cables started out at close to zero and after a full year of aging the highest currents were just over 1 mA. There is a hypothesis that leakage current would show a relatively rapid increase on a cable that was close to failing from dry band arcing, and this proved to be true. Also, as cables lost their hydrophobicity (see Table 3-2), the leakage currents generally increased.

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Table 2-2 Summary of hydrophobicity and leakage currents (approximated averages) over the first five months of aging.

Cable #

Man./Class

HC @ 1 month

Leak. @ 1 month (mA)

HC @ 5 months

Leak. @ 5 months

(mA)

HC @ 2 years

Leak. @ 2 years

(mA)

1 a/B 5 <.1 6 0.5 6 0.7

2 b/B 1 <.1 1 <.1 1 < 0.1

3 c/B 3 <.1 5 0.4 6 1.0

4 b/B 1 <.1 2 <.1 2 0.1

5 a/B 5 <.1 6 0.6 6 0.8

6 a/A 2 <.1 4 0.3 na na

7 c/B 3 <.1 5 0.3 6 >1.0

8 d/B 2 <.1 3 0.2 na na

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Figure 2-31 shows a five-day plot of rms leakage currents four months into the aging tests after most of the cables lost a significant portion of their hydrophobicity. Leakage currents are relatively high during the salt spray portions of the cycle, and quickly drop as soon as the clean water is applied.

Figure 2-31 Plots of leakage currents over a five day period after most cables showed loss of hydrophobicity.

As discussed in Section 2, the short circuit current available to the cables with the original electrode setup was measured to be about 0.5 mA. It was hypothesized that under these conditions there may not be enough energy in dry band arcing to sustain an arc and cause damage to the cables. It was recommended to make modifications such that the short circuit current would be above 1 mA. As a result, 1 nF capacitors were installed in series with each cable, and in parallel with the original electrode arrangement, to increase the available short circuit current for each cable to 10 mA. This modification was performed on February 26, 1999.

The leakage currents on the cables just prior to the electrode modification are shown in Figure 2-32 and the leakage currents shortly following the modification are shown in Figure 2-33. Applying ohm’s law to these current readings yields a net resistance of about 50 Mohm for the cables with the least resistance. This equates to 8.2 Mohm per meter.

Even though installation of the capacitors increases the available current to 10 mA for each cable, the measurements clearly indicate that the equivalent surface resistance of the cables, even during the salt period, does not drop to values that would require such high leakage currents to flow. The actual measured currents are less than 1.5 mA for all cables, and remained at this level throughout the first year of the project (see Figure 2-34). Also, it should be noted that the 1 mA leakage current “threshold” was not known or discussed at the initiation of the tests, and its significance is still not fully understood.

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Figures 2-35a through 2-35f show plots of the leakage currents as the project came to an end. (It is difficult to distinguish the individual cable plots on these leakage current graphs in black and white – the electronic file of this report provides these graphs in color).

Figure 2-32 Leakage currents prior to the addition of the 1 nF capacitors.

Figure 2-33 Leakage currents after the addition of the 1 nF capacitors.

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Figure 2-34 Leakage currents after a year of testing.

Figure 2-35a Cable 1 leakage current after two years of testing.

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Figure 2-35b Cable 2 leakage current after two years of testing (current level does not exceed .001 mA).

Figure 2-35c Cable 3 leakage current after two years of testing.

2-33

Figure 2-35d Cable 4 leakage current after two years of testing.

Figure 2-35e Cable 5 leakage current after two years of testing.

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Figure 2-35f Cable 7 leakage current after two years of testing.

The Class A Failures

There were three class A cable failures during the course of the project. The first class A cable that was installed at the beginning of the project failed in mid-March 1999. This cable was from manufacturer a. The cable texture did not change much until it was damaged. The cable had near total loss of hydrophobicity, and leakage current was at a medium level. The cable sample was replaced with another sample of the same class A cable, which ultimately failed in early July 1999. A cable was considered “failed” if its jacket became punctured through to the inside.

The third class A cable that failed was from manufacturer b. This cable failure was quite remarkable. The failure took place over a weekend and was discovered the following Monday morning. The cable completely disintegrated from end to end. There were only portions of kevlar that kept the cable from dropping to the ground. The glass fibers were even destroyed.

When there is no arcing taking place on a cable, the leakage current waveform is sinusoidal. When arcing takes place, the waveform deviates from a sinusoidal shape. When severe dry band arcing occurs, the waveform deviates significantly from a sinusoidal shape, and assumes the form shown in Figure 2-36. This is a plot of the waveform on the first class A cable the day before its failure. Figure 2-37 shows a similar plot of the waveform on the second class A cable that failed just prior to its failure.

2-35

Figure 2-36 Leakage current waveform on the first failed class A cable on the day before it failed.

Figure 2-37 Leakage current waveform on the second class A cable to fail the day before it failure.

About two weeks prior to the first class A failure, regular visual inspections of the cables found two very tiny “craters” (1 – 2 mm) along the bottom of the cable. The craters (as they will be called here) are indentations with raised walls (like craters), and have the appearance of melted plastic. The two craters were not located very close to each other.

2-36

Over the next two weeks, one of the craters grew in the longitudinal direction to about three times its original size. During this period, close visual inspections were made daily. During one of the inspections, a third large crater was discovered along the bottom that had fully punctured the cable, and had signs of melting all around it.

Figure 2-38 shows a picture of the bigger of the first two craters developed, and Figure 2-39 shows a photograph of the failed portion of the cable.

Dry band arcing damage

Figure 2-38 Dry band arcing damage on the bottom of a class A cable.

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Jacket punctured.Kevlar can be seen.

Figure 2-39 Dry band arcing failure on the bottom of a class A cable. Note the area that appears melted.

Following the first class A cable failure, a second class A sample was installed in its place (March 16, 1999). This sample came off the same reel as the first sample. In mid-June, a single small crater was found on the bottom of the cable. Over the next couple of weeks, more and more of these crater-like marks developed along the bottom of the cable. On July 2, 1999, punctures were found on at least five of these craters. Figure 2-40 shows a photograph of one of these failed regions.

Figure 2-40 One of many punctures along the bottom of the second class A cable.

The third class A cable failure occurred on a cable from manufacturer b. The cable was installed Nov 11, 1999. The failure was discovered on Monday morning Jan 10, 2000. The cable jacket had disintegrated along its entire length. . As mentioned above, even the glass fibers were completely destroyed. The remains left a trail along the floor of the test chamber under the intact kevlar fibers. Figure 2-38 shows photographs of the destroyed cable.

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Figure 2-41 Photographs of cable 8.2 following its destruction. The kevlar was the only part that remained.

The Class B Failures

There were three class B failures that occurred during the two year project (cables 8.1, 6.3, and 3). These are discussed below. Other surface peculiarities were observed, but they seemed to be only cosmetic in nature (such as discoloration), and are discussed in Section 4.

Throughout the project, waveforms on most of the cables frequently exhibited the characteristics of dry band arcing. Relatively early in the project the class A cables were experiencing the failures described above, but apparently, the class B cables were able to survive the arcing. Figure 2-39 shows a leakage current waveform from a class B cable. It clearly exhibits the deviation from a perfect sinusoid, caused by the non-linear effects of arcing.

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Figure 2-42 Leakage current waveform on a class B cable.

Cable 8.1

This cable (class B, manufacturer d) was installed in the chamber on 7/27/98. As with most of the cables, the cable gradually lost its hydrophobicity over the course of a few months, and its leakage current levels gradually increased to 1 mA. During the nighttime hours of 08/16/99, the leakage current on the cable significantly and rapidly increased, and the cable experienced a failure. The cable experienced major damage, presumably from dry band arcing. The damaged region was about a foot long, and appeared as gouges with tracking-like features, but did not have the melted-plastic look of the cable A failures. Most of the damage was along the bottom of the cable. Figure 2-40 shows photographs of the damaged region. The cable was removed from the chamber and analyzed (see Section 4).

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Figure 2-43 Photos of the damaged region of cable 8.1. The damaged area looked like gouges and had white material present.

Cable 6.3

This cable (class B, manufacturer c) was installed in the chamber on 7/2/99, and failed on 12/15/99. The cable had lost hydrophobicity, and leakage current increased to a relatively high level (see Figure 2-42). Just prior to identifying the failure, it was observed that the cable’s leakage current went very high (see Figure 2-42), and the cable near mid-point had become

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extremely hot to the touch. At this time, a detailed visual inspection was made of the cable. It was found that there was a burnt hole at the tip of the finger trap (see Figure 2-43), and a small crack on the bottom of the cable where the hot spot had been observed (see Figure 2-44) which did not have the appearance of being caused by arcing.

The cable was removed for further analysis. Dissection revealed signs of tracking inside the cable (see Figure 2-45). A hypothesis was made about what had transpired, and there are failure reports from the field, which support this. This is discussed in Section 4. Briefly, it appears that a crack developed in the cable, which led to water ingress, which led to arcing through the jacket at the finger trap.

Figure 2-44 Leakage current on cable 6.3 during the few days leading up to its failure. During the night of the 13-14th the leakage current was large and erratic. Over the course of the following day the chamber was shut down most of the time to make observations. During the times when the chamber was energized, leakage currents were as high as 9mA.

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Figure 2-45 The left image is the burnt hole at the finger trap of cable 6.3. It appears that an arc occurred through the jacket. The right image shows arcing damage inside the cable under the burnt hole. The kevlar fibers can be seen.

Figure 2-46 The right image shows the crack near midpoint on cable 6.3. The left image shows the inside of the cable at the crack following dissection. It appears that there was internal arcing damage.

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Figure 2-47 Photographs of the inside of cable 6.3 following dissection. The kevlar fibers can be seen, and they appear to have suffered arcing damage along their length.

Cable 3

Cable 3 (class B, manufacturer c) was installed in the chamber at the very beginning of the tests (7/27/98), and remained under test until the very end. In mid-May 1999, it developed a single feature which started off as a scratch-like feature, and over the following weeks evolved into a gouge-like feature. Figure 2-46 shows photographs of this feature near the completion of the project. The feature never punctured the jacket, but it did look similar to the gouges on the cable 8.1 failure (see above).

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Figure 2-48 Photos of the gouge-like feature on cable 3. This particular gouge never punctured the jacket.

During the final few weeks of the tests, the leakage current on cable 3 occasionally increased to 1.5 – 2.0 mA, but regular inspections did not reveal any damage other that the gouge-like feature. However, when the tests were terminated, and the cables removed, it was found that there was major damage along a foot-long section of the jacket. It was not previously observed because it was on the bottom of the cable, and out of sight well inside the electrode cage (no other cable sample had damage there).

The cable had clearly failed, but had not been damaged to the point where it fell. However, it is not precisely known when the failure occurred. It is presumed, on the basis of the leakage current evidence, that the failure was quite recent. If testing had continued, it is suspected that catastrophic damage would have ensued. Figures 2-47 and 2-48 show photographs of the damage.

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Figure 2-49 Photograph of the damage on cable 3 at the end of the project.

2-46

Figure 2-50 Photograph of the damage on cable 3 at the end of the project after the cable was cut up for inspection.

Summary of Cable Samples

The list below provides a brief summary of the history of each cable sample that was tested in the aging chamber project. Details are presented above.

Cable 1 (class B, manufacturer a): This cable was installed in the chamber on 7/27/98 and remained there until the tests were terminated (06/01/00). The cable had developed a surface roughened orange peel type texture with dark ring over different parts of the cable length. The cable lost hydrophobicity, and leakage current was relatively high at the end of the tests. The cable had developed “bumps”, but no sign of damage was found.

Cable 2 (class B, manufacturer b): This cable was installed in the chamber on 7/27/98 and remained there until the tests were terminated (06/01/00). The cable had developed a diamond pattern showing through the jacket. The cable remained hydrophobic and leakage current remained very low. The cable had developed “bumps”, but no sign of damage was found.

Cable 3 (class B, manufacturer c): This cable was installed in the chamber on 7/27/98 and remained there until the tests were terminated (06/01/00). The cable had developed dull black areas along its length, and lost some of the cable markings. The cable lost hydrophobicity, and leakage current was relatively high, especially near the end of the tests. The cable had developed “bumps”, and a single gouge-like feature grew over the course of the project. After the tests were terminated, and the cable samples removed for final inspections, this cable was found to have major damage. The damage appears to have occurred on Apr. 5, 2000. The cable had several arcing tracks located on the bottom of the cable. The damage was along the section inside the electrode arrangement.

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Cable 4 (class B, manufacturer b): This cable was installed in the chamber on 7/27/98 and remained there until the tests were terminated (06/01/00). The cable had developed a diamond pattern showing through the jacket. The cable remained hydrophobic and leakage current remained very low. The cable had developed “bumps”, but no sign of damage as of the end of the test.

Cable 5 (class B, manufacturer a): This cable was installed in the chamber on 7/27/98 and remained there until the tests were terminated (06/01/00). The cable had developed orange peel type texture with dark ring over different part of the cable length. The cable lost hydrophobicity and leakage current was high. The cable had developed “bumps”, but no sign of damage as of the end of the test.

Cable 6.1 (class A, manufacturer a): This cable was installed in the chamber on 7/27/98 and remained there until its failure on 03/16/99. The cable texture did not significantly change until the failure. The cable had near total loss of hydrophobicity and leakage current was at a medium level. The cable experienced major damage from dry band arcing and was replaced. The damage appeared as severe melted regions with hole puncturing the jacket on the bottom of the cable.

Cable 6.2 (class A, manufacturer a): This cable was installed in the chamber on 03/16/99 and remained there until its failure on 07/02/99. It was a replacement for cable 6.1, and was identical to the cable 6.1 sample. Its behavior in the chamber was identical to cable 6.1, as would be expected. The cable texture did not significantly change until the failure. The cable had near total loss of hydrophobicity and leakage current was at a medium level. The cable experienced major damage from dry band arcing and was replaced. The damage appeared as severe melted regions with hole puncturing the jacket on the bottom of the cable.

Cable 6.3 (class B, manufacturer c): This cable was installed in the chamber on 07/02/99 and remained there until its failure on 12/15/99. The cable had developed some dull black areas long its length, and lost some of its jacket markings. The cable had lost hydrophobicity, and leakage current had climbed to a very high level. The cable had developed “bumps.” The damage appeared as burn holes at both ends at the finger traps. The cable became hot to the touch in the middle section of the cable length just before the failure. Tracking was found under the outer jacket once it was cut open. There was a small crack found in the outer jacket at the middle section of the cable. This crack did not appear to be caused by dry band arcing.

Cable 6.4 (class B, manufacturer c): This cable was installed in the chamber on 01/10/00 and remained until the project was terminated (06/01/00). This cable was installed to perform a special test. A slit was purposely placed at the bottom of the cable near its midsection to try to duplicate the failure of cable 6.3. When the project was terminated, the cable had not failed. Dissection analysis indicated that water had not penetrated the slit. Perhaps the slit was too small. The cable had some loss of hydrophobicity and leakage current was high at the end of the tests. The cable had developed “bumps ”. However, a very small gouge showed signs of starting on the outer jacket, which seemed to be unrelated to the slit. This gouge looked similar to the one of cable 3 (also manufacturer c), but was smaller. It is suspected that this gouge would have grown if the tests had continued.

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Cable 7 (class B, manufacturer c): This cable was installed in the chamber on 7/27/98 and remained there until the tests were terminated (06/01/00). The cable had developed dull black areas long its length and lost of some of the jacket markings. The cable had lost hydrophobicity and leakage current was high. The cable had developed “bumps”, but no sign of damage was observed for the duration of the tests.

Cable 8.1 (class B, manufacturer d): This cable was installed in the chamber on 07/02/98 and remained there until its failure on 08/16/99. The cable had developed a “ribbing” look from the inside through the jacket. The cable had lost hydrophobicity and leakage current was high. The cable had developed “bumps.” The cable experienced major damage from dry band arcing and was replaced. The damage appeared as severe gouging along the damaged area. The damage appeared to start along the bottom of the cable.

Cable 8.2 (class A, manufacturer b): This cable was installed in the chamber on 11/12/99 and remained there until its failure on 01/10/00. The cable texture did not change much until its failure. The cable had lost some hydrophobicity and leakage current was at a medium level. The cable experienced catastrophic damage and was replaced. The cable’s jacket and glass fibers disintegrated along its entire length.

Cable 8.3 (class B, manufacturer d): This cable was installed in the chamber on 01/10/00 and remained until the project was terminated (06/01/00). This cable was installed to perform a special test. A slit was purposely placed at the bottom of the cable near its midsection to try to duplicate the failure of cable 6.3. When the project was terminated, the cable had not failed. Dissection analysis indicated that water had not penetrated the slit. Perhaps the slit was too small. The cable had some loss of hydrophobicity and leakage current was at a medium level at the end of the tests.

Observations

General Review of ADSS Cable Aging

The primary purpose of the project was to accelerate the aging effects of class B ADSS cables, from various manufacturers, in an electrical and climatic environment representative of real installations, and verify their survivability. As a benchmark, samples of class A cables were also tested in the same chamber. In addition, some short-term tests were performed on selected samples in order to focus on specific issues and phenomena.

The test results confirm some of the expected outcomes, for example, that class B cables are capable of withstanding severe electrical and climatic conditions which lead to failure of class A cables. However, the results also led to the discovery of some new and unexpected phenomena and outcomes that may need to be evaluated in more detail when applying ADSS cables in high voltage transmission environments.

Some of the most important and interesting results of the project are difficult to express in quantitative terms, and are simply observational. Below, some of the general observations made throughout the project are discussed. Some of these observations are also discussed in other sections of the report.

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General Observations of Class A Cable Aging

Below is a list of the general observations made of the class A cable test samples:

• It was noted that the outer jackets of class A cables were very shiny, and had the appearance of hard plastic, i.e. no subsurface texture could be seen (unlike class B cables).

• The class A cables lost hydrophobicity faster than some of the class B cables, but slower than most.

• The class A cables did not develop the “bumps” that the class B cables developed (discussed below).

• Arcing damage always had the appearance of tiny melted craters that grew over time. All these craters appeared along the bottom of the cables, and occurred anywhere along the test sample length, and seemed to be randomly spaced.

• The leakage current level for class A cables did not have to get much more than 0.5 ma for failures to occur.

• Failed cables in the chamber remained somewhat intact. This would probably not happen in real world applications due to the large tensions involved (1000lbs to 2000lbs or even more). The chamber’s cables were tensioned at about 100lbs.

General Observations of Class B Cable Aging

Below is a list of the general observations made of the class B cable test samples:

• All except one manufacturer’s class B cables lost hydrophobicity relatively early in the project (manufacturer b did not lose hydrophobicity throughout the entire project). Leakage currents remained extremely small on class B cables as long as they retained their hydrophobicity.

• About 6 weeks into the tests, the two cables from manufacturer a started to show some discoloration, turning slightly gray in some areas. This discoloration worsened with time, and then the cables turned to a dull black. At the time the cables started to turn dull black, they started to display ring-like patterns. These rings were not accompanied by any noticeable deformation, but were only due to coloration.

• Over the course of the project, the cables took on a roughened orange peel-like texture.

• The two cable samples from manufacturer b took on a diamond-like pattern on their surface over the course of the project. This pattern cannot be felt by touch, so it is not certain if it’s due to deformation. Apparently, the pattern is due to the cable’s underlayments.

• The two cable samples from manufacturer c started out with a shiny black color (like all the cables), but over the course of the project they turned to a dull black over their entire length. In some areas, the white lettering disappeared. These are the only cables that lost their lettering.

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• The test sample from manufacturer d became slightly dull over the course of the project. The cable’s initial circular cross-section became somewhat irregular, and had the appearance of longitudinal ribs.

• On most of the cables (all but the class A cables and the class B cables from manufacturer b), a black substance could be rubbed off the cables with a finger. Manufacturer c had the greatest amount of this black substance; manufacturer d had more than manufacturer a.

• The class B cable arcing failures appear to look like gouges through the outer jacket with white material present inside the gouges. The gouges appear to start on the bottom of the cable, and tree out from there.

• The location of the class B failures occurred primarily in, or close to, the electrode, and no failure was found near the ground point in the chamber. This may be due to the fact that the leakage current is highest in that region because the current is injected at that end. As the cables leave the electrode arrangement, some current may be lost through capacitance to the chamber’s walls. This is unlike a true installation where leakage current is usually greatest at the grounded attachment points.

• A potentially new failure mode was discovered where a puncture of the outer jacket allows water to penetrate to the inner kevlar fibers. The water then wicks along the length of the cable and arcs through the jacket at a grounded attachment point. This is the suspected failure mode of cable 6.3. Arcing damage occurs at the grounded attachment point, but the true cause was a puncture of the cable at some other location. Reported failures from the field have indicated evidence consistent with phenomenon.

The Bumps

Several months into the project, surface blemishes that are referred to in this report as bumps started to appear on all the class B cables. The number of bumps per cable increased with time throughout the course of the project, but at a diminishing rate following early rapid formation.

During the project an EPRI member provided a reel of ADSS cable for possible future full-scale tests. After the bumps started to be noticed in the chamber samples, this reel was inspected, and some bumps were found. This cable had not been in-service, but its age and storage history is unknown.

As a special experiment, a short sample (2-ft) of each manufacturer’s cable was mounted in the chamber off to the side and away from the energized electrode arrangement. Half of each of these samples was shaded from the UV. It was found that the loss of hydrophobicity progressed at the same rate in the shaded and unshaded regions, and bumps formed at the same rate in the shaded and unshaded regions. Therefore, the loss of hydrophobicity and the formation of bumps do not appear to be correlated with electrical stress or exposure to UV. A photograph of these special test samples is shown in Figure 2-51.

2-51

Figure 2-51 Photograph of five special cable samples mounted in the chamber.

Figure 2-52 shows a photograph of the first bumps observed. The size, number, and shape of the bumps vary among the manufacturers. The ones shown in Figure 2-52 are relatively large.

Figure 2-53 shows a cross-section of a dissected bump. Below the bump is a void. The source of this void is unknown, but apparently some pressure from inside the void has pushed out the surface into a bump.

The bumps did not seem to be involved with any failure mechanism, and are probably only cosmetic features. It’s suspected that they are caused only by age and heat, and possibly moisture. Several representatives of cable manufacturers were conferred with about these bumps. None of these representatives had an explanation for the bumps, or were aware of their existence.

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Figure 2-52 Photograph of the first bumps that developed.

void

bump

Figure 2-53 Dissected bump showing the internal void.

Video Capture of Dry Band Arcing

Although there was evidence that dry band arcing was taking place throughout the project (including leakage current waveforms and failures on both class A and B cables) further direct evidence was desired. Special small video cameras were mounted in the chamber with the intent of recording the dry band arcing phenomenon. The cameras were used to record events during chamber operation. High quality video records are difficult to obtain during the salt period because visibility is near zero. To protect the cameras from damage, they were mounted in

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environmentally sealed housings equipped with windows that, unfortunately, further degrade the observed image. However, sufficient observations were made to confirm the occurrence arcing.

An interesting observation was made. It appears that on the class A cable, dry band arcing tended to repeat at the same locations over time. However, this was not the case with the class B cables. On class B cables, the locations of dry band arcing seemed random. It is hypothesized that the fact that the arcing does not tend to repeat at the same location is related to the class B’s ability to survive the dry band arcing phenomenon.

Other observation were as follows:

• The video cameras were moved to many different locations over a period of three weeks. Some dry band arcing was observed each time no matter where along a cable the camera was positioned. Not all cables were monitored, but all that have been viewed had some dry band arcing taking place.

• The video system was setup up several times to capture the chamber going from the dry cycle into the salt spray and then to fresh water spray. Every one of these recorded segments showed no dry band arcing taking until at lease 10 minutes into the salt cycle. The arcing would continue to some degree until the salt cycle came to an end. No dry band arcing was observed during the fresh water spray. This observation is consistent with special leakage current waveform measurements made with an oscilloscope. These measurements showed relatively pure sinusoids except during the periods where arcing was observed.

Failures of Class A cables

The three failures of class A cables, each occurring after a few months in the chamber, clearly indicated that class A cables are very likely to fail under service conditions represented in the test chamber (25 kV space potential, coastal environment). All three failures followed the expected failure mode, i.e., penetration and destruction of the outer jacket due to arcing.

Performance of Class B Cables

Some class B cables in the chamber suffered damage due to arcing, but only after relatively long periods of time in the aging chamber. However, assuming the aging acceleration factor of 17, as discussed in Chapter 3, one year of aging in the chamber is equivalent to 17 year of service. Hence, the aging tests have demonstrated that class B cables are, for practical purposes, capable of surviving in the environment represented in the chamber.

Non-Lethal Jacket Changes

Careful periodic observations of the cable jackets during the aging process indicated that some changes in the jacket external appearance, in addition to any erosion due to arcing, take place over time. These changes do not appear to be lethal, i.e.; they do not result in premature jacket damage and are not expected to lead to jacket penetration or cable failure. Three such apparently non-lethal changes were observed on all class B cables to various degrees are:

• Loss of hydrophobicity

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• Appearance of surface powdery deposits, usually white in color

• Appearance of “bumps” on the jacket surface

Loss of Hydrophobicity

Material is categorized as hydrophobic if it has the property that water beads on its surface instead of wetting the surface (i.e. the cohesive forces acting within a water droplet are greater than the adhesive forces acting between the droplet and the material’s surface). Materials that wet are called hydrophilic.

Most class B cables aged in the chamber (all except from manufacturer b) lost hydrophobicity over time. Leakage current levels on the cables appeared to be well correlated with hydrophobicity. In fact, observations have shown that leakage current levels on hydrophobic cables remain low even if there is a continuous stream of salt water being sprayed on them.

Powdery Deposits

Some class B cables in the chamber have shown signs of a powdery deposit formation over time, usually white in color, on their surfaces in the vicinity of arcing. The powdery deposit can be rubbed off with a finger. The nature and chemical composition of the powder have not been analyzed.

The “Bumps”

The bumps on the jackets have been observed on all class B cables aged in the chamber, and other class B cables not aged in the chamber. No damage, other than cosmetic distortion of the jacket, has been attributed to the bumps.

The Nitric Acid Test

There was some discussion among the researchers in the project that, perhaps, nitric acid formation, which is a byproduct of corona and arcing, could cause damage to the jacket surfaces. As a simple observational test, drops with 50% nitric acid were placed on the surfaces of four cables. The cable samples were taken from the original aging chamber rolls, but these samples had not been aged. A photograph of the test is shown in Figure 2-55. The results are outlined as follows:

Manufacturer a, Class A: After 5 minutes no damage was noted.

Manufacturer a, Class B: After 5 minutes no damage was noted.

Manufacturer b, Class B: After 5 minutes no damage was noted. There was a slight dulling on the cable surface.

Manufacturer d, Class B: After 5 minutes no damage was noted. There was a slight dulling on the cable surface.

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Figure 2-54 Drops of 50% nitric acid on the four samples.

Figure 2-55 Surface is dulled where the acid drop was applied on one of the cables.

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Summary

The primary purpose of the aging chamber project was to verify that class B ADSS fiber optic cables can survive in an electrical environment characterized by a 25 kV space potential, as specified by manufacturers, and to compare the performance of cables from several different manufacturers. The plan was to rapidly age class B ADSS cables from various manufacturers in an electrical and climatic environment representing real installations in a southeast U.S. coastal environment. As a benchmark, samples of class A cables were also tested in the chamber. In addition, some short-term tests were performed on selected samples in order to focus on specific issues and phenomena.

The test results confirm some of the expected outcomes, for example, that class B cables are capable of withstanding severe electrical and climatic conditions which lead to failure of class A cables. However, the results also led to the discovery of some new and unexpected phenomena and outcomes that may need to be evaluated in more detail when applying ADSS cables in high voltage transmission environments.

Performance of Class A cables

There were three failures of class A cables during the project, each occurring after a relatively short time in the chamber, clearly indicating that class A cables are very likely to fail under service conditions represented in the aging chamber (25 kV space potential and coastal environment). This result was expected because class A cables are specified by manufacturers for only up to 12 kV space potential applications.

Performance of Class B Cables

Overall, the class B cable samples in the chamber performed well. Even though several Class B cables failed, they all survived much longer than the class A samples. Taking into account the estimated accelerated aging factor of 17, they all would have performed satisfactorily in the field. The following summarizes the failures and survivors of the class B samples from the four manufacturers represented in the project.

Manufacturer a cable 1: Survived the entire 22-month chamber operation. cable 5: Survived the entire 22-month chamber operation.

Manufacturer b cable 2: Survived the entire 22-month chamber operation. cable 4: Survived the entire 22-month chamber operation.

Manufacturer c cable 3: Failed near the very end of the 22-month test.

cable 6.3: Failed after about 6 months of test.

cable 6.4: Survived for the 5-month period that it was in the chamber.

cable 7: Survived the entire 22-month chamber operation.

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Manufacturer d cable 8.1: Failed after 13.5 months of test.

The two cable samples from manufacturer a were in the chamber for the entire duration of tests. They both survived. Both cables had lost their hydrophobicity early in the tests, and both showed increased leakage current levels. Both cables had a very beaten appearance at the end, and a black residue could easily be wiped from their surfaces.

The two cable samples from manufacturer b were in the chamber for the entire duration of tests. They both survived. Neither cable ever lost hydrophobicity, and leakage current levels remained very low for the entire project. Except for salty deposits which were easily wiped off, both cables appeared to be in excellent condition.

Two of four cable samples from manufacturer c failed during the project. One of the samples that survived was only in the chamber for 5 months, and had a gouge that appeared to be heading toward failure.

The one cable sample from manufacturer d failed after 13.5 months in the chamber. However, with the estimated accelerated aging factor of 17, this corresponds to about 19 years in the field.

Non-Lethal Jacket Changes

Careful periodic observations of the cable jackets during the aging process had indicated that some changes in the jacket’s external appearance, in addition to any degradation due to arcing, takes place over time. These changes do not appear to be lethal, i.e., they did not result in jacket damage and are not expected to lead to jacket penetration or cable failure. Two such apparently non-lethal changes were observed on all the class B cables to various degrees:

• Appearance of surface powdery deposits, usually white in color

• Appearance of “bumps” on the jacket surface

Powdery Deposits

Some class B cables have shown signs of a powdery deposit, usually white in color, on their surfaces in the vicinity of (usually surrounding) areas where arcing had occurred. The powdery deposit can be rubbed off with a finger. The nature and chemical composition of the powder have not been analyzed. No damage was associated directly with the appearance of the deposit.

The “Bumps”

Bumps on the jackets have been observed on all the class B cables aged in the chamber, and on other class B cable samples that were inspected and were not part of this project. No damage, other than cosmetic distortion of the jacket, has been attributed to the bumps. The appearance of the bumps seem to be a normal characteristic of class B cables, and are not associated with any degradation or failure of the cables.

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Conclusions and Recommendations

The main conclusions and recommendations from the project are listed below.

• Cables from manufacturer b clearly performed the best in the tests of this project. Cables from manufacturer a performed nearly as well.

• The tests indicated that Class B cables from any manufacturer should perform reasonably well in the specified electrical environment, and in the climatic conditions represented in these tests.

• Class B cables can experience dry band arcing, and fail as a result. The failures are directly linked with loss of hydrophobicity and increased leakage current.

• A sudden relative increase in leakage current level is a good indication that a near-term failure is likely.

• Class B cables will develop surface blemishes, such as bumps and discoloration, which is not directly associated with any impending failure.

• Utilities can, with confidence, proceed with any plans they may have on installing class B cables in their high voltage transmission corridors.

• The electrical environment (space potential) and climatic conditions should be considered for installations of class B ADSS cables.

• When planning the installation of class B cables, the choice of manufacturer should take into consideration the results of this project.

• Inspections of existing installations should include measurements of hydrophobicity and leakage current levels.

• Class A cables will probably fail relatively fast in the electrical and climatic environment represented in the tests.

• Class A cables should definitely not be installed in electrically stressful environments.

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References

1. Rowland, Craddock, Carter, Houghton, and Delme-Jones, “The Development of a Metal Free Self Supporting Optical Cable for Use on Long Span High Voltage Overhead Power Lines”, IWCS Proceedings, IWSC, pp. 449-456, 1987.

2. Bartlett, Carlton and Carter, “The Design and Application of Optical Cables into Overhead Lines up to 150kV”, presented at the IEE Conference on Overhead Line Design and Construction: Theory and Practice, pp. 166-172, November 28-30, 1988.

3. Berkers, and Wetzer, “Electrical Stresses On A Self-Supporting Metal Free Cable in High Voltage Networks”, presented at the 5th International Conference on Dielectric Materials and Applications, Canterbury, U.K., pp. 69-72, 1988.

4. Wheeler, Lissenburg, Hinchliffe, and Slevin, “The Development and Testing of a Track Resistant Sheathing Material for Aerial Optical Fibre Cables”, presented at the 5th International Conference on Dielectric Materials and Applications, Canterbury, U.K., pp. 73-76, 1988.

5. Rowland and Carter, “The Evaluation of Sheathing Materials For an All Dielectric Self-supporting Communication Cable for Use On Long Span Overhead Power Lines”, presented at the 5th International Conference on Dielectric Materials and Applications, Canterbury, U.K., pp. 77-80, 1988.

6. Carter, “Dry Band Electrical Activity on Optical Cables Separately Strung on Overhead Power Lines”, IWCS Proceedings, IWCS, pp. 117-121, 1988.

7. Bartlett , Carlton, Carter, and Peacock, “Self-supporting Metal Free Optical Cable for Long-Span Power Line Use”, CIRED, pp. 252-258, 1989.

8. Carter and Waldron, “Mathematical Model of Dry Band Arcing on Self-Supporting All-Dielectric Optical Cables Strung on Overhead Power Lines”, IEE Proceedings-A, IEE, Vol 139, No 3, pp. 185-196, May 1992.

9. Carlton, Carter, Peacock, and Sutehall, “Monitoring Trials on All-Dielectric Self-supporting Optical Cable for Power Line Use”, IWCS Proceedings, IWCS, pp. 59-63, 1992.

10. Carter, “Arc Control Devices For Use On All- Dielectric Self Supporting Optical Cables”, IEE Proceedings-A, IEE, Vol 140, No 5, pp. 357-361, September 1993.

11. Rowland and Easthope, “Electrical Aging and Testing of Dielectric Self-supporting Cables for Overhead Power Lines”, IEE Proceedings-A, IEE, Vol 140, No 5, pp. 351-356, 1993.

12. H.M. Schneider et al, “Accelerated Aging and Flashover Tests on 138 kV Nonceramic Line Post Insulators”, IEEE Transactions on Power Delivery, Vol. 8, No. 1, January 1993, pp. 325-336.

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13. Kaidanov, Munteanu, Sheinfain, “Damages and Destruction of Fiber Optic Cables on 161 kV Overhead Transmission Lines”, IEEE Electrical Insulation Magazine, July/August 2000, Vol. 16, No. 4, pp. 16-23.

3-1

3 SURVEY OF ADSS AND OPGW CABLE FAILURES

Introduction

Utilities have had considerable success with optical ground wire (OPGW) applications in power transmission corridors at most system voltages. However, OPGW aerial cables are placed at the top of structures where they experience lightning strikes and short-circuit currents. It is assumed that the energy in these transients is dissipated safely due to the mass of the aluminum around the optical wire. Nevertheless, a number of reported failures appear to have been associated with positive lightning strokes as reported later in this section.

While the performance of OPGW cables has generally been very good, a number of utilities have opted for all-dielectric self-supporting (ADSS) cables because:

• ADSS cables are less expensive.

• Fault and lightning protection is not an issue. ADSS aerial cables are normally placed below the phase wires where they are usually not exposed to direct lightning strikes, and ADSS cables are free of metal components resulting in a fault current of negligible magnitude.

• ADSS are much easier to install and repair on energized circuits.

Operating experience with ADSS installations has been generally good [1]. Only one premature failure on a transmission line of less than 138 kV has been reported, and there is over ten years of successful operating experience with installations on transmission lines of up to 345 kV [2]. Finally, ADSS systems have been installed on 500 kV transmission lines and operated successfully for more than four years [3]. Nevertheless, problems have occurred with ADSS systems in the high electric field environments near transmission lines. In two cases in the United States, catastrophic failure of the ADSS cable occurred within one year of installation. In another, catastrophic failure occurred more than 10 years after installation. In two others, corona on hardware and tracking on PVC vibration damping devices caused fiber damage that could have resulted in cable failure if it had not been identified as soon as it was. Reports of similar problems from outside the United States are discussed later in this section.

At least two reasons for these failures are related to the fact that the cable is suspended in a strong electric field. First, the electrical potential of the cable (with respect to ground) at midspan is approximately that of the space potential at the cable position, but with the cable absent. At the tower, however, the cable is held at ground potential. This difference in potential causes currents to flow on the cable sheath that can become significant if pollutants have been deposited on the sheath, and especially if the sheath is wet. Due in part to the uneven flow of these currents along and around the sheath, dry bands can form on the sheath, and these may lead to dry band arcing and tracking [4]. This in turn can lead to thermally induced damage to the cable sheath, which may affect its long-term integrity. Second, metallic hardware is used to attach the cable to a tower. This hardware usually includes a number of long armor rods that are

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parallel to, and surround the cable and are bonded to the tower. If the structure is located in a strong electric field, corona may occur at the tips of the armor rods and/or microsparks may occur between them and the sheath [5,6]. As with dry band arcing, these phenomena can cause damage to the cable sheath and may affect its long-term integrity.

Since the degree of pollution is an important factor in the occurrence of dry band arcing, prevailing climactic conditions are likely to have a major effect on ADSS cable failures. More specifically, installations in polluted, wet climates may be more susceptible to dry band arcing while those in dry climates may be more susceptible to corona and microspark activity.

Survey of Failures

A survey of damage and failures was conducted as part of the EPRI project, and the results are listed below. Because there are many different sources of information, the accuracy, completeness and consistency of the reports cannot be guaranteed. Also, in all likelihood, utilities have experienced other instances of ADSS damage and failure of which the authors are not aware.

ADSS Damage and Failures

National Grid (UK) [1]

1987 - Fawley, England - 400 kV transmission line: The mounting position was at a space potential of 25 kV. The cable performed well for 2 ½ years but by end of third year had suffered severe sheath damage.

1989 - Hunterston, Scotland - 400 kV transmission line: The initial mounting position was at a space potential of 85 kV. Sheath degradation was noted very soon (within a matter of months) after installation.

1991 - Hunterston, Scotland - 400 kV transmission line: The initial mounting position was at a space potential of 25 kV. For three years the leakage currents did not exceed 1 mA, and no damage occurred. Leakage currents rose to 5 mA during extreme weather conditions (saline pollution), and damage was sustained.

BPA [3]

1996 - The Dalles, Oregon, USA - Slatt 500 kV Substation: On the east end of the substation the 2-D space potential was approximately 63 kV (electric field at tip of protruding armor rod = 51 kV/cm) while at the west end of the substation the 2-D space potential was 24 kV (electric field at tip of protruding armor rod = 25 kV/cm). Note that corona occurred when the electric field was approximately 20 - 30 kV/cm. Jacket damage on the East Side was observed after three months. After relocation of the cable to a space potential of less than 25 kV, there have been no further problems.

1998 - The Dalles, Oregon, USA - 345 kV flat configured: Corona damage was observed on a cable in 28 kV space potential. Cable was located on a wood pole adjacent to the line. The line has been retrofitted with a Corona Coil.

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Con Edison

1989 - Elmsford-Eastview - 345 kV double circuit line: The original mounting was in a space potential of 49.8 kV. The cable failed within a few months after installation and was relocated in a space potential of 16.5 kV. To date, there have been no cable failures.

1988 - Fresh Kills - Goethals line - 345 kV double circuit line, delta configuration: The cable was mounted in a 42 kV space potential for its entire life. Originally the PVC vibration dampers were located in too large an electric field, which resulted in corona degradation of the dampers. These were moved to a lower field region (i.e., more than 15 feet from grounded hardware) and no further damage occurred. The line also is protected from corona by grading rings on the armor rod. Annual inspections of the line (latest in 1998) indicate no damage to the cable.

1999 - Fresh Kills - Goethals line - 345 kV double circuit line, delta configuration: A failure occurred on this line in 1999. All evidence implicates corona since the “corona coil” was not installed on this span as it was on other spans. Details of an investigation into this failure are given in a later chapter of this report.

Southeast United States

1992-93 - 500 kV flat configuration: There was one reported failure, probably due to mechanical damage to the ADSS during installation.

Houston Lighting and Power

1994 - 138 kV double circuit line: The cable was originally located in a space potential of 34 kV, and failure occurred within two years of installation. The line was re-phased so that the new space potential was 15 kV, and there have been no reported problems since then. The cable jacket was non-track-resistant. Failure occurred at the armor rod, which might suggest corona failure, but the armor rod could also have been an arc root for a dry band arc.

Northeast U.S.

A very interesting failure was recently reported in New England. A utility crew that had been installing an ADSS cable used a rope to secure the cable for a weekend, but over the weekend the rope broke and dropped the ADSS cable onto a major highway. A visual inspection of the pulling rope indicated that dry band arcing occurred at the break point.

Israel Electric Corp. Ltd. [8]

1997 - 161 kV double circuit vertically configured line: The Israel Electric Corp. Ltd. installed ADSS cable on four 161 kV double circuit vertical transmission lines between 1993 and 1996 [8]. The cable used was reported to have a high-medium density polyethylene jacket with 4% carbon content. A considerable amount of pollution was found to accumulate during the year. Typical weather in this area consists of a long dry period with frequent dew and a short rainy period. Erosion and tracking on the ADSS cable were found on all transmission lines, within approximately 1 to 3 years after installation. Both corona and dry band arcing appeared to be responsible for the observed damage. All damage occurred near to the tower. In two cases, the damage was severe enough to cause catastrophic failure of the cable (i.e. breakage and falling).

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Using the tower geometry and voltages given in [8], the space potential and induced current and voltage across a dry band on the cable near the tower were calculated for a span length of 350 meters. For the first case, it was assumed that one of the circuits was grounded as shown, that the ADSS cable was sagged at 0.5 % and that the sag of the conductors varied between 1 – 2 % of the span length. The space potential varied from 4 to 6 kV and the induced current and dry band voltage were 0.8 – 1.0 mA and 4.8 – 5.7 kV respectively for a pollution level of 105 Ω/m. This appears to be enough to cause dry band arcing only if the pollution resistance is less than 105 Ω/m. If the grounded circuit was energized at the same voltage (ABC/ABC phasing), the space potential increased to between 12 and 16 kV and the induced current and dry band voltage were 2.4 – 2.8 mA and 13.7 – 16.3 kV for a pollution level of 105 Ω/m. In this case, dry band arcing might be expected if the surface resistance was as low as assumed. With ABC/CBA phasing, the space potential was reduced to between 7 and 12 kV and the induced current and dry band voltage were 1.4 – 1.6 mA and 8.2 – 10.5 kV. Dry band arcing might also be possible in this case.

Europe [9]

1995-97 – Europe: “several” failures were reported in space potentials less than 12 kV. This information comes from reference [9].

South Africa [10]

1998 – 88 kV line: A recent ADSS failure was reported in South Africa on an 88 kV line where the space potential was only 8 – 9 kV.

2000 – 132 kV single circuit flat line: ESKOM installed “A” type ADSS cable on a 132 kV single circuit horizontally configured transmission line in approximately 1996 [10]. After three years the cable was inspected and damage was found at all tower attachment points. In one case, the cable failed catastrophically (i.e. it broke and fell) before it could be inspected. The failure was at 50 meters from the tower and occurred during wind and rain following a long period of dry weather. Measurements of the contamination resistance on the cable after it was removed from the towers varied from approximately 104 Ω/m to 2.2 x 106 Ω/m.

Assuming a span length of 150 meters, ADSS cable sag of 0.5% and conductor sags from 1 – 4 % of span length, the space potential varied between 7 and 10 kV, low enough to believe that the cable was “safe” by the usual rule-of-thumb. However, using a contamination resistance of 6.0 x 104 Ω/m (roughly the average of the measured resistance values), the induced current at the tower was 1.6 – 1.9 mA and about 0.7 mA at the point of breakage. This is large enough to suspect dry band arcing at the tower but not at the point of breakage.

OPGW Damage and Failures

Nebraska Public Power District - September 2, 1997 1:06 AM CDT [11]

Transmission line: 345 kV H frame wood pole line with a horizontal configuration; two shield wires.

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Type of OPGW: FOCAS (0.547-in. diameter) with 12 strands of aluminum-clad steel wire and aluminum core with two embedded fiber tubes. The total number of fibers was 24 (two tubes of 12 each).

According to Alcoa, the total charge that a similar Alcoa OPGW can withstand is 48.5 Coulombs of transferred charge. Based on discussions with Global Atmospherics reported below, it is likely that the stroke that hit the OPGW exceeded this value.

Location of strike: The lightning strike was at midspan.

Nature of failure: OPGW failed catastrophically and fell between the phase conductors. The failure was not noticed immediately because, while the fibers were in service, communications automatically switched over to an alternate path and there was no outage on the line.

Lightning report from Global Atmospherics: + 63.8 kA peak (no information on total charge)

Note: In a discussion with Global Atmospherics, it was indicated that several facts about the report are indicative of a stroke large enough to cause this failure. It was further verified that all of the strokes on the report were positive. It was indicated that this is quite unusual.

• The fact that the time between strokes was large indicates that the storm was near its end and that the stroke would be positive and of large magnitude.

• The fact that the stroke is positive indicates that the probability of a continuing current is very high.

• Given the peak current of 63.8 kA, it is likely that the total transferred charge was 100 - 200 coulombs.

Note: there are a number of reasons why it is very difficult (if even possible) to measure the total charge of a lightning stroke from the normal electric and magnetic field measurements. These include an imperfect knowledge of the current distribution on the lightning channel and the dispersion inherent to propagation of the energy from the lightning source to the receiving antennas.

Georgia - February 16, 1998 10:31 PM EST

Transmission line: A 230 kV H-frame steel structure pole line with a horizontal configuration; two shield wires

Type of OPGW: FOCAS (0.557-in. diameter) with 12 strands of aluminum-clad steel wire and aluminum core with two embedded fiber tubes. The total number of fibers was 36.

Location of strike: The lightning strike was at midspan.

Nature of failure: The OPGW failed catastrophically, fell and made momentary contact with the phase conductor. At 10:31 PM two separate computer systems that used fibers for communications reported a loss of signal. This is suspected to be the time of the lightning strike. At 10:51 the SCADA system recorded a five-second-long outage. Apparently, wind caused the weakened fiber to fail at that time.

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Lightning report from Global Atmospherics: A fault-finder report was prepared. It was found that there were only two lightning strokes within five miles of the fault within +/- one hour of the fault. Both of these occurred after the fault and were not close enough to the fault to be suspected as responsible for the fault.

Northeast United States - Late Summer or Fall, 1997

Nature of failure: One or two strands of the OPGW failed, but the cable remained in place. The time was not known exactly since there was no failure.

Other Failures in the Southeast United States During the Period 1988 - 1998

The information in this subsection is based on a telephone interview with representatives of Fiber Optics Services, Clearwater, FL (813) 573-1310. Among other things, Fiber Optics Services repairs damaged OPGW. The company has been in business for more than 10 years.

It was reported that they have repaired approximately 12 OPGW failures in the last 10 years. Most of the failures have occurred near the tower attachment point. It was acknowledged that many failures might be due to lightning-induced phase-to-ground backflashes. It is believed that many failures begin with the breakage of a few strands. Over time the remaining strands are stretched and eventually also fail. A Fiber Optics Services representative even said he had seen a case where the fiber was holding up the OPGW.

Fiber Optics Services suggested that the “Utility Telecommunications Council” would be a good source of information about OPGW failures.

Fiber Optics Services was also asked about failures of WRAP cable. It was indicated that one would almost always find broken optical fibers in WRAP cable. This was attributed to high winds that carry debris, such as palm fronds (dead palm branches), that impacts the WRAP and causes fiber breakage. They also indicated that ADSS was not susceptible to the same problem because it can move in the wind and, hence, is less susceptible to impact from debris. Finally, they said that all utilities they know about are either removing WRAP or scheduling to do so.

Fiber Optics Services personnel also noted that ADSS jackets get nicked during construction, and in some cases bird talons penetrate the sheath (broken bird talons have been found embedded in fiber). They said that moisture penetrating the sheath by these methods or others have caused failure problems.

Note: If this failure mechanism occurs, it is different from those investigated so far.

Japan [12,13]

There is a photograph (Figure 2 of [12]) of lightning damage to a ground wire (not sure if this is OPGW or normal ground wire) in [12]. In the photo there appear to be several failed strands, although the ground wire did not fall.

The damage apparently occurred in an area of Japan where there is “winter lightning,” which carries large electrical charges. Charges transferred to ground by winter lightning have been measured up to 1000 Coulombs.

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M. Yokoya et described Japanese work on the development of lightning-resistant ground wire. al. in [13]. According to this paper, “We investigated actual damage to OPGW systems mainly in the region of Chuba Electric Power Co. (with a total length of approx. 2500 km) over 10 years from 1983 to 1992. The investigation results revealed that ground wires were damaged with their strands broken at 10 places in these 10 years. This can be expressed as a damage rate of 0.08 cases/year for every 100 km, which means strand breakage may occur at 2 places/year with the current length.” Note that this is the “damage rate” not the “failure rate.”

One of the results of this study was the development of an optical ground wire with zinc coating. This material disperses the thermal energy from a lightning strike over a wider area. Tests from the Sumitomo Electric Industry indicate that this type of cable can increase the current-handling capacity of a ground wire by a factor of two.

The most recent word from Japan is that they are not currently conducting a research project on lightning and OPGW at CRIEPI (National research laboratory).

Germany [14]

W. Zischank and J. Wiesinger have summarized recent German experience in [14]. They report, “…helicopter inspection of the Bavarian high voltage transmission lines reveal a number of strand breakage’s each year. The usual damage is about one or two broken strands, but also several cases with three or four broken strands were observed. The maximum damage found was seven broken strands of the outer layer of an overhead ground wire. After breakage of several strands of the outer layer, the inner layer, usually containing steel tubes with the optical fibers, may be damaged too, and the optical transmission may be disturbed or even interrupted.” Note that on the last point, no indication is given as to whether such outages actually occurred.

France [15]

J.P. Bonicel et discusses recent French experience with this subject. al. in [15]. No information about actual failures is given. The paper covers only laboratory tests of OPGW.

United Kingdom [1]

G. Carleton et provided much of the information for the UK. al. in [1]. The optical ground wires used in the UK are:

• 28.60 mm (1.12 in.) OD on the largest 400 kV lines

• 19.53 mm (0.77 in.) OD on the smaller 400 kV, 275 kV and the larger 132 kV lines

• 13.95 mm (0.55 in.) OD on the smaller 132 kV lines

The authors report that, “once installations have been completed, no fiber breaks or splice failures have occurred.” This may not be surprising, given that the diameter of the OPGW used on higher voltage lines in the U.K. is larger than the diameter of the OPGW on which failures have occurred in the U.S.

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Note that the authors mention a method for 275/400 kV double-circuit lines that allows ground wires to be changed while both lines are energized. As of 1995, however, it is required that when ground wires on 132 kV double-circuit lines are changed, one circuit must be de-energized.

Historically, ground wires have failed in the U.K. at a rate of 0.02 failure/100 km/year. This may appear lower than the Japanese rate because it is a measure of “failure” rather than “damage.” However, the authors note that with care in design, the reliability for OPGW could be better than this.

Brazil

Utilities in Brazil have installed OPGW on 138, 230, 345, 500 and 750 kV transmission lines. They have experienced damage (identified by visual inspection) at all voltage levels. On 300 miles of line, damage to the OPGW is observed approximately twice per year (0.4 cases/100 km/year). This rate is higher than that reported in Japan or England. It could be explained if either the OPGW was thinner than used elsewhere or if statistics only for the most vulnerable part of the system were reported. The latest report of damage was from February, 2000. All observed damage has been repaired and there have been no reports of catastrophic failures (i.e. broken and dropped cable). The problems are more likely to occur in the southern part of the country during January and February (summer in Brazil).

In one case there are two parallel lines with one using 266 kcmil ACSR shield wire and the other using OPGW. No damage has been observed on the shield wire, but was reported on the OPGW. While it is not clear why this occurs, it can be noted that OPGW has a pitch length 14 times the outer diameter, left hand wrap, per US standard while regular shield wire has pitch length about 22 times the outer diameter, right hand wrap, per IEC standard.

Laboratory tests on OPGW were done “according to the new standard with continuing current.” Test results showed “performance quite good up to 200 coulombs” of transferred charge for one layer of 3.6 mm diameter Alumoweld strand and two layers of 2.15 mm diameter aluminum alloy inner layer and 2.9 mm diameter Alumoweld outer layer. Both used an aluminum pipe inside for the optical fibers. The inner Alumoweld strand used in the test is larger than the 2.5 mm diameter strand presently used in the field.

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References

1. G. Carlton, A. Bartlett, C. Carter and T. Parkin, “UK Power Utilities’ Experience with Optical Telecommunications Cabling Systems,” Power Engineering Journal, Vol. 9, pp. 7-14, February 1995.

2. Electric Power Research Institute, “Fiber Optic Cables in High Voltage Environments” Interim Report, EPRI Energy Delivery and Utilization Center, Lenox, MA, December 1998

3. M.W. Tuominen, Bonneville Power Administration, private communication.

4. C.N. Carter and M.A. Waldron, “Mathematical model of Dry-Band Arcing on Self-Supporting, All-Dielectric, Optical Cables Strung on Overhead Power Lines,” IEE Proceedings-C, Vol. 139, No. 3, pp. 185-196, May 1992.

5. M.W. Tuominen, “ADSS Fiber Optic Cable in HV Electric Fields - Corona Considerations,” presentation made to the IEEE Corona Effects Working Group, February 1996.

6. G.G. Karady, M. Torgerson, D. Torgerson, J. Wild and M.W. Tuominen, “Evaluation of corona-caused aging of ADSS fiber-optic cables. Paper # TR08-080 presented at the 1999 IEEE Transmission and Distribution meeting.

7. G. Carlton, C. N. Carter and A. J. Peacock, “Progress in the Long Term Testing of an All dielectric Self supporting Cable for Power System Use,” contact C.N. Carter, National Grid, UK

8. F. Kaidanov, R. Munteanu and G. Sheinfain, “Damages and Destruction of Fiber Optic Cables on 161 kV Overhead Transmission Lines,” IEEE Insulation Magazine, Vol. 16, No. 4, pp. 16-23, July/August 2000,

9. “Continued Investigations of ADSS Designs and Reliability Considerations with Respect to Field Voltage Tracking, and Cable Installation Practices,” presented at the 1997 International Wire and Cable Symposium.

10. D.C. Smith, ESKOM, Private communication

11. R. W. Oswald, Nebraska Public Power District, Private Communication

12. “Development of Lightning-Resistant Overhead Ground Wire and Characteristics against Lightning Current Triggered by Rocket,” (in Japanese with English abstract), T. IEE Japan, Vol. 117-B, No. 4, pp. 464-471, 1997.

13. M. Yokoya et. al. , “Development of Lightning-Resistant Overhead Ground Wire,” IEEE Transactions on Power Delivery, Vol. 9, No. 3, pp. 1517-1523, July 1994.

14. W. Zischank and J. Wiesinger, “Damages to Optical Ground Wires Caused by Lightning,” 10th International Symposium on High Voltage Engineering, August 25-29, 1997, Montreal, Quebec, Canada.

15. J.P. Bonicel et. al. in the paper: “Lightning Strike Resistance of OPGW,” International Wire and Cable Symposium Proceedings 1995.

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4 ANALYSES OF TWO REPORTED FIELD FAILURES

Consolidated Edison’s ADSS Failure

Consolidated Edison has a well-known application of an ADSS fiber optic cable on a double-circuit 345 kV transmission line on Staten Island. This ADSS cable resides in a 46 kV space potential (measured in 1990), and in an assumed heavily contaminated environment. A large space potential, and a contaminated environment, are generally considered to be the ingredients for dry band arcing damage, yet this cable has been in operation for many years without suffering any known damage of this kind.

The space potential has been confirmed by both calculations and measurements. The measurements were performed by the research staff of the EPRI-Lenox lab, operated by EPRIsolutions and are provided in the report Measurements of Space Potential and Electric Field Along a Fiber Optic Cable submitted to Consolidated Edison, dated December 1990.

It is considered somewhat of an anomaly for this cable to have survived so long in such a stressful environment without any reported electrical damage. The research staff at the EPRI-Lenox lab was interested in this because of their recent work in the area of fiber optic applications in high voltage environments. They had been planning a site visit in order to make measurements of the contamination deposited on the cable when they received information that the cable had suffered a catastrophic failure (i.e. the cable suffered damage to the point that it fell to the ground).

The site visit was made a short time after the cable was repaired. Measurements of the cable’s ESDD were made by the EPRI staff with the support of a Consolidated Edison line crew. In addition, ESDD measurements of the transmission line’s insulators were made for comparison purposes, and other inspections were made. A report of the investigation is provided below.

Site Visit

A site visit was conducted at the location of the ADSS cable failure (towers M6 through M13 on Staten Island) on the 26th of February 2000. George Gela and Tom McDonald of the EPRI-Lenox Lab were invited by Consolidated Edison to inspect the failure while maintenance was being conducted on the line. The failure location was along the West Shore Expressway between exit 7 and 8 on Staten Island.

Consolidated Edison had one circuit of the double-circuit 345 kV line powered down during the site visit. The outage was to allow Consolidated Edison linemen to climb one side of the tower structure safely. The Consolidated Edison crew performed ADSS inspections at the armor rod tips and installed corona rings on the armor rods on the ADSS cable. They dead-ended the ADSS cable at towers M6 and M8, either side of the failure location, and also replaced a few sets of broken glass insulator strings on the transmission line. Figure 4-1 shows the area inspected by the EPRI team.

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CM10 M9

M8

M11

Figure 4-1 Area inspected by EPRIsolutions on February 26, 2000.

As an interim measure following the 1999 failure, Consolidated Edison crews simply cut out a section of the damaged ADSS cable. The ADSS cable ends were rolled up in a coil back to the towers. A section of replacement ADSS was attached to the poles of a nearby distribution line and spliced to the original ADSS cable. Both the existing cable and the replacement cable have Superior Optics markings. The replacement cable was manufactured in 1990, see Figure 4-2.

The failure occurred near Tower M7. Between Towers M6 and M7, the line goes over an inlet waterway or river. The cable came apart at the armor rod tip location on tower M7, however, the exact spot of the failure (i.e., the side and the distance from Tower M7) could not be determined. The cable fell to the ground and ended up lying in the river. The span of the 345 kV line between towers M8 and M9 crosses an overpass bridge (see Figure 4-1) over the railroad tracks and the river. The transmission lines and the ADSS cable make radical height changes as they cross the overpass. The ADSS cable moves from hanging under the conductors (middle arm of the tower) to hanging from attachments points on the top arm of the tower.

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Figure 4-2 The coil of original and replacement ADSS cables located on a wood pole near tower M8.

EPRIsolutions staff worked with the Consolidated Edison crews at tower M9. The crew had some problems with a rail crane/bucket truck they rented. The failure of the bucket truck required that all work be accomplished by climbing the tower. The EPRIsolutions personnel on site were not allowed to climb the towers. In order to get ESDD samples of the ADSS cable, we instructed the Consolidated Edison crew how to take the samples. The crew took ESDD samples from the ADSS cable on both sides of tower M9. The crew, also, took several pictures of the cable at the attachment point. The following Monday, Consolidated Edison delivered the removed glass insulator strings to the Lenox Center. ESDD samples were taken by EPRIsolutions personnel from the glass insulators once they arrived in Lenox.

Note: The ADSS cable manufacturer was present at some parts of the visit.

Many observations were made on site during the visit. The Consolidated Edison crew was very cooperative in answering questions about what happened at the time of the failure and what they were currently doing to repair and secure the ADSS cable at the time of the visit. The failure site and the configuration around the site were extensively photographed for later review.

The primary factor of concern is the change in the position (height) of the ADSS cable relative to the phase conductors in the span between Towers M7 and M8, and in the span between Towers

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M9 and M10. These changes in height could have a major impact on the induced voltage and induced current on the ADSS cable (see discussions later in this report).

As result of a 1990 failure of the ADSS cable (at tower M13), Consolidated Edison had a program to install corona rings on the ADSS cable armor rods (see Figure 4-3 below). The spans at the failure site did not have these rings installed. The two crews who originally installed the rings reportedly stopped short of completing this section.

Figure 4-3 Corona ring on the ADSS cable armor rod. The line crew working on the ADSS cable on February 26, 2000 reported that they previously performed inspections of the ADSS cables at the armor rod tips (see, for example, Figure 4-4). They lifted the tips of the armor rod and checked the cable for damage such as small holes. If holes are found, they used black electrician’s tape to wrap the ADSS cable and then put the armor rod tips back down. The crew reports that during such inspections noticeable arcs are seen as the armor rod tip is lifted off the ADSS cable. The arc appears to go into the ADSS cable. This phenomenon occurs at a number of different locations along the spans inspected.

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Figure 4-4 Armor rod on the ADSS cable. The plastic vibration dampers (manufactured by PLP) were also inspected and found to have damage by tracking. Figure 4-5 shows tracking on the vibration damper taken from Tower M9 on February 26, 2000.

Figure 4-5 Photograph showing tracking on the vibration dampers removed from Tower M9. The dampers were re-installed since replacement dampers were not available.

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The crew took EDSS samples from the ADSS cable on both sides of tower M9 (see Figure 4-6). The samples were taken by wiping a section of the cable extending from the edge of the armor rod to 3 feet way. These samples were returned to Lenox for processing and analysis.

Figure 4-6 Consolidated Edison crew taking ADSS measurements.

Line Configurations

Many previously unknown details regarding the actual line configurations were discovered during, and as a result of the site visit by EPRIsolutions personnel. Most importantly, the line configuration and the ADSS cable location on Tower M7 (location of the 1999 failure) are not the same as on Tower M13, where failure occurred in 1990.

Based on the information contained in the 1990 report, phases B & C of both circuits are located on the top arm of tower M13, while phase A is located on the middle arm, outboard sides, of the tower legs. See Figure 4-7 as an example. The ADSS cable is hung from the underside of the middle arm, midway between (and 15 feet below) the phase C conductors. This is the configuration of the majority of the 345 kV transmission line, and will be called the “normal” configuration for the purposes of this report. The location of the ADSS cable will be referred to as “Fiber Optic Low” in this report. Normal configuration is found at spans before structure M7, with V insulator hardware. Normal configuration is also found after structure M10, with Double Dead End insulator hardware and jumper loops.

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AA

B BCC

ADSS

Figure 4-7 “Normal” double-circuit configuration with “Fiber Optic Low” location of the ADSS cable (double deadended insulator assembly).

Between structures M8 and M9, an overpass bridge (Merideth Ave.) created an obstacle to “Fiber Optic Low” configuration (see Figure 4-1). This was overcome by changing to the “Fiber Optic High” configuration. In this configuration, the locations of phases A & B remain unchanged (i.e., middle and top arms, outboard side). However, phase C for both circuits is moved from the top arm to the middle arm. For this span only, the fiber optic cable is hung in the middle of the top arm, see Figure 4-8. Double Dead End insulator hardware with jumper loops hold each end of the span.

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AA

B B

C C

Figure 4-8 Photograph of tower M9 showing the “special” double-circuit configuration with “Fiber Optic High” location of the ADSS cable (dead-end insulator assembly) The transition from the “Fiber Optic Low” to the “Fiber Optic High” configuration occurs in spans M7 - M8 and M9 - M10. These spans are called “Transitional” in this report. Figure 4-9 shows a sketch of the towers of the “Transitional” spans. Figure 4-10 shows a sketch of the section of line near the ADSS cable failure in 1999.

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Figure 4-9 Sketch of the “Transitional” span towers.

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Figure 4-10 Diagram of the failure site. Note that the inboard phase C only of each circuit is drawn as well as the ADSS fiber optic cable

Details of the 600 foot spans between the consecutive structures M6 through M11 are included in Figure 4-10. The diagram is NOT to scale. The cross section diagram at the top of Figure 4-10 shows the phase arrangements of the 345 kV line, the fiber optic cable, and the insulator configuration on each structure. The plan view is shown in the middle of Figure 4-10. Only phase C of each circuit is drawn as well as the ADSS fiber optic cable. Billboard and bucket truck indicators are included to give the reader a point of reference with Figure 4-1. Vibration dampers are shown at their relative locations. There were no corona rings on ADSS cable armor rods on structures M6 through M9. The side view appears at the bottom of Figure 4-10, with only phase C of each circuit, the ADSS fiber optic cable, and the insulator configuration drawn. The “Transitional” spans M7-M8 and M9-M10 with the criss-cross of the ADSS fiber optic cable and the phase C conductor are evident in the side view (bottom of Figure 4-10).

The position of the C phase conductors of both 345kV circuits is also seen in Figure 4-1 as the phase C conductors pass over the overpass. Structure M10 is in front of the billboard. Structure M9 is behind the billboard, but in front of the overpass. Structure M8 can be seen behind the overpass.

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As explained by the Consolidated Edison crew, the cable failure occurred at tower M7 within the transitional span M7 - M8, i.e., at the end of the “Fiber Optic Low” span. It is on each of these spans that the phase C conductors are dropped from the top arm position to the middle arm position. Figure 4-11 shows the locations of the phase conductors and the ADSS cable within the transitional span M6-M7. The situation in span M8-M9 is the reverse of that in Figure 4-11.

Figure 4-11 “TRANSITION” span dimensional progression from fiber optic LOW (NORMAL) to fiber optic HIGH These three locations in the span shown in Figure 4-11 were subsequently used for calculation of space potential, induced current, and induced voltage along the transitional spans. The dimensions shown in Figures 9 and 11 were provided by Consolidated Edison for the 1990 report on the tower M13 fiber optic cable failure. It is assumed that all towers M6 through M11 have the same height and that their configurations horizontally symmetrical about their center line.

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Analysis of Samples of Failed ADSS Cable

Shortly after the 1999 cable failure, Consolidated Edison sent 8 pieces from the failed ADSS cable, each approximately 3 feet long, to Lenox. The samples were from the area of the attachment point on the tower as well as beyond the armor rod. The samples were examined for surface damage and internal damage or effects.

The cable was identified as SUPERIOR OPTICS – FREE-SPAN - 1989.

Visual Inspection

Visual inspection of the samples revealed several observations. One was that some portions of the outer surface of the jacket, not covered by armor rod had the cable markings missing, but other portions did not. This appears to correspond to what has been observed in the ADSS Aging Chamber in Lenox. Loss of cable marking is an indicator of dry band arcing activity.

A second observation was discoloration on the inside of the cable jacket. Portions of the outer jacket were removed from the cable pieces received from Consolidated Edison. The inside surface of the outer jacket was checked. All inspected inside surfaces appeared normal, i.e., smooth and shinny. There was one piece of cable that had a small line of roughness and brownish powder-like material running along the length of the inside of the outer jacket.

A third observation was that the cable surface was not hydrophobic (see discussion below).

From the distance markers on each cable sample, we were able to reconstruct the location and physical relationship of each sample to the others. The samples were taken from within 217 feet of cable, roughly centered around tower M7. Tower M7 is the site of the 1999 cable failure on the “Transitional” span side toward tower M8. Configuration toward tower M6 is “Normal”.

It is not known with certainty, which end of our sample-sequence was in the “Transitional” span, and which was in the “Normal” span. One end of the sequence at 112 feet away from tower M7 had extensive scarring, bumps, and gouges in the surface. The other end of the sequence had very modest surface markings at 105 feet away from the tower. The four samples located within 15 feet from both sides of the tower had no indication of electrical damage, as they were wrapped, or partially wrapped with armor rod. Both of the outboard samples with armor rod, located approximately 10 feet away from either side of tower M7, were entirely under the armor rod. It is guessed that the failure occurred just beyond one of those two samples, and wouldn’t have expected to find any evidence of damage in the samples themselves. At 20 feet away from both sides of the tower, both samples had markings that could have been from the vibration dampers. However, vibration dampers are reported to have been installed on only one side of each structure. Therefore, the spiral marking patterns are unexplained.

Except for the light impression of the distance marker, all cable identification is gone from the sample on the “scarred” sequence side of the tower. Missing cable markings have been an indicator of dry band arcing phenomenon. The “scarred” sequence most probably came from the “Transition” side of the span where the failure occurred.

The samples were dissected with very little discovery of electrical or unusual activity. As noted earlier, minimal brown residue was found inside the outer jacket on the presumed failure side of

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the sequence. However, there is no evidence of vaporized components (like a zip cord) or of damage to the non-metallic armor strands.

No significant conclusion can be drawn from the sample evidence.

Hydrophobicity

Hydrophobicity is the ability of a polymeric surface to bead up water. The cable samples supplied exhibited an HC5 classification on the STRI hydrophobicity scale. This demonstrates that moisture disperses on the surface, readily wetting it, see Figure 4-12.

Figure 4-12 Photograph showing loss of hydrophobicity on two of the ADSS cable samples.

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Electrical Environment of the ADSS Cable

Space Potential, Induced Voltage, Induced Current

Figures 13 through 17 present the data and curve sets generated using an analytical 3D model, currently under evaluation by EPRIsolutions. It is planned to be used to predict unperturbed space potential, induced currents, and induced voltages on ADSS fiber optic cables located in high electrical fields.

The following data sets are the predicted results of the Consolidated Edison double, 345 kV circuit configuration:

1. The normal, “Fiber Optic Low” – Figure 4-13;

2. The “Transition” configurations between structures at

3. Fiber Optic Low (¼ span) – Fig. 14,

4. Fiber Optic Middle (½ span) – Fig. 15,

5. Fiber Optic High (¾ span) – Fig. 16;

6. The “Fiber Optic High” (at the overpass) configuration – Figure 17.

Note that certain assumptions had to be made in order for the model to perform the calculations. The actual conductor sag and the ADSS fiber optic sag were not known. These values can readily affect all of the results.

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Figure 4-13 Data & curve set for “FIBER OPTIC LOW”, NORMAL configuration.

4-16

Figure 4-14 Data & curve set for “TRANSITION” between structures; with Fiber Optic at the LOW position (¼ span).

4-17

Figure 4-15 Data & curve set for “TRANSITION” between structures; with Fiber Optic at the MIDDLE position,

(½ span).

4-18

Figure 4-16 Data & curve set for “TRANSITION” between structures; with Fiber Optic at the HI position, (¾

span).

4-19

Figure 4-17 Data & Curve set for “FIBER OPTIC HIGH” between structures M8 - M9 (at the overpass).

4-20

The data contained in Figs. 13 – 17 are summarized in Table 4-1.

Table 4-1 Circuit space potential, induced current, and induced voltage predictions.

SPAN CONFIGURATION UNPERTURBED SPACE POTENTIAL (kV)

INDUCED CURRENT (mA)

INDUCED VOLTAGE (kV)

NORMAL (FO LOW)

All spans except below 57 kV max @ midspan 2.0 mA max @ structure 60 kV max @ midspan

FO LOW @ ¼ SPAN 62 kV 0.7 mA 62 kV

62 kV max avail @ ¼ span 3.0mA max avail @ structure 67 kV max avail @ ½ span

FO MID @ ½ SPAN 45 kV 0 mA 52 kV

62 kV max avail @ structure 3.0mA max avail @ structure 53 kV max avail @ ½ span

FO HIGH @ ¾ SPAN 26 kV 0.5 mA 26 kV

TR

AN

SIST

ION

(M

7-M

8 &

M9-

M10

)

29 kV max avail @ structure 1.0mA max avail @ structure 33 kV max avail @ ½ span

FO HIGH @ overpass (M8-M9) 28 kV max @ midspan 1.0 mA max @ structure 33 kV max @ midspan

On a “normal” Consolidated Edison span, our spreadsheet predicts 60 kV induced voltage on the fiber optic cable at midspan, with 2.0 mA current flowing to structure ground. This confirms what has been previously stated that the Consolidated Edison fiber optic ADSS cables are located in a high electrical field environment.

The span at and over the overpass, between structure M8 & M9, seems to be in a much more favorable electrical environment, even when taking the height of the bridge into account. Induced voltages and currents are about half of a “normal” span, and more in keeping with the magnitudes that we would like to see, at least to date in our studies.

The transition spans between M7 & M8 and M9 & M10 exhibit a disturbing combination of induced voltages and currents, due to their unique circuit configuration. At first glance at the above table, it would appear that the induced voltage and current trend toward the values of the adjacent NORMAL, or FO HIGH, spans. However, note that the induced voltage appears to remain high, or to actually increase to 67 kV, over a significant portion of the span. The predicted current flowing to the structure ground increases to 3.0 mA. The unperturbed space potential remains high, or actually increases to a high of 62 kV, in the proximity of the structure.

The severest electrical environment of Consolidated Edison’s three configurations is predicted to occur in the transition span, at the span-end where the fiber optic cable is low, under the power circuit. This was the location of the fiber optic cable burn-down.

4-21

It must be cautioned, however, that the calculations are set up to evaluate horizontal circuits. Because the Consolidated Edison transition is not horizontal, an estimate only is predicted by calculating 3 unique cases as though the conductors and fiber optic cables were horizontal for these ¼, ½, and ¾ span configurations. The “slices” are evaluated, looking for the resultant trends.

Contamination Levels

Characterizing Contamination

ESDD (Equivalent Salt-Deposit Density) is a measure of contamination level [IEEE Std 100]. The concept was developed for ceramic (porcelain and glass) insulators. Its application to and meaning for non-ceramic materials and surfaces has been debated for more than a decade, and the issues remains unresolved. However, in view of absence of an alternate method for measurement of contamination levels on ADSS cables, an attempt has been made to measure the ESDD on ADSS cables in the EPRIsolutions aging chamber, on the Consolidated Edison ADSS 345 kV installation on Staten Island, and on ceramic insulators at the same location. It is hoped that unique data presented in this section will be helpful in assessing the contamination severity of the installation.

It has been generally assumed that the contamination level on the Consolidated Edison installation is high since it is in a marshy area near the seacoast and is in an industrial neighborhood. However, contamination level is not constant. It varies with the occurrence of rain, since rain washes off some of the contaminants. Generally, then, ESDD is expected to be lower after rain than just prior to rain.

ESDD measurements were taken by Consolidated Edison personnel on the ADSS cable installed on Tower M9 of the 345 kV line on Staten Island. Glass dead-end insulator strings were removed from the same location and forwarded to Lenox for ESDD sampling.

It is unfortunate that the ESDD sampling of the ADSS cable and the removal of the glass insulators both occurred immediately after a rainy period in the area.

ADSS Cable on Staten Island

Consolidated Edison kindly invited Lenox personnel to visit the failure site and take ADSS samples on February 26, 2000. Unfortunately, due to problems with the bucket truck, repairs had to be performed by climbing the structure, for which Lenox personnel has not been trained. Therefore, Consolidated Edison linemen performed the ESDD sampling on the ADSS cable. Subsequently, Consolidated Edison removed several dead-end strings of glass insulators and shipped them (wrapped in plastic) to Lenox for ESDD analysis.

The ESDD data from the Staten Island ADSS cable are presented below. The contamination samples were also tested for NSDD (Non-Soluble Deposit Density) and chemical analysis was performed on selected samples.

4-22

Figure 4-18 shows Consolidated Edison linemen taking ESDD samples on the ADSS cable on Tower M9 after removal of the vibration damper. Using a sponge, the Consolidated Edison linemen wiped a 3’ long section of the ADSS cable. Care was taken to wipe the entire 3’ length and the entire circumference of the cable. The sponge was rinsed in a beaker of de-mineralized water, moistened again and the cable was wiped again. This procedure was repeated for a total of 5 wipings. The beaker was emptied into a bottle, the sponge was inserted into the bottle, and the bottle was sealed. The results of the ESDD, NSDD and chemical analysis are presented below.

The average (of two samples) ESDD level of the ADSS cable is 0.012 mg/cm2, which corresponds to VERY LIGHT contamination. The ESDD samples were taken after rain.

The NSDD level of the ADSS cable could not be obtained since the contamination samples were used up for chemical analysis.

Chemical analysis provided the following information (averages based on 2 samples):

Chloride: 14.5 mg/l

Sulfate: ND

Calcium: ND

Magnesium: 2.1 mg/l

Sodium: 13 mg/l

Figure 4-18 A Consolidated Edison lineman taking ESDD samples from the ADSS cable after removal of the vibration damper. View from bottom looking up.

4-23

Figures 19 – 24 are photographs of the ADSS cable taken by Consolidated Edison linemen on Tower M9.

tracking

Figure 4-19 The vibration damper on Tower 9. Tracking is visible.

Figure 4-20 Photograph of the armor rods, ADSS cable and vibration dampers.

4-24

Figure 4-21 The armor rods and ADSS cable with vibration dampers removed.

Figure 4-22 ADSS suspension hardware.

4-25

Figure 4-23 ADSS cable with vibration damper removed.

4-26

Figure 4-24 Another view of the ADSS cable with vibration damper removed.

Glass Insulators on Staten Island

Figures 4-25 and 4-26 show samples of glass insulators removed from Tower M11. Paint splashes are visible on some units.

4-27

Figure 4-25 Photograph of glass insulators removed from Tower M11. Broken unit is visible on the left side of the photograph.

Figure 4-26 Paint splashes on insulators removed from Tower M11.

4-28

The average (of 6 units) ESDD level of the glass insulators is 0.0016 mg/cm2, which corresponds to VERY LIGHT contamination. The ESDD samples were taken after rain.

The average (of 5 units) NSDD level of the glass insulators is 0.047 mg/cm2. The significance of this result is not clear.

ADSS Cable in the Lenox Aging Chamber

The ADSS cables were sampled to determined ESDD levels during a heat period (dry) and during a salt period (wet).

The average ESDD level (based on 3 samples) during heat (dry) period is 0.00345 mg/cm2, which corresponds to VERY LIGHT contamination.

The average ESDD level (based on 3 samples) during salt (wet) period is 0.0431 mg/cm2, which corresponds to LIGHT contamination.

It must be remembered, however, that the applicability of the ESDD concept to ADSS cables is under question.

Porcelain Insulators in the Lenox Aging Chamber

Porcelain insulators were suspended in the ADSS aging chamber as weights to tension the ADSS cables. These insulators were sampled to determined ESDD levels during a heat period (dry) and during a salt period (wet).

The average (top/bottom) ESDD level (based on 1 sample) during heat (dry) period is 0.0255 mg/cm2, which corresponds to VERY LIGHT contamination. The bottom surface registered at 0.0312 mg/cm2, which corresponds to LIGHT contamination.

The average (top/bottom) ESDD level (based on 1 sample) during salt (wet) period is 0.0552 mg/cm2, which corresponds to the borderline between LIGHT and MODERATE contamination. The bottom surface registered at 0.00.0944 mg/cm2, which corresponds to the borderline between MODERATE and HEAVY contamination.

4-29

Summary and Conclusions

A Class-B (track resistant) ADSS fiber optic cable on Consolidated Edison’s double circuit 345 kV line on Staten Island recently suffered a catastrophic failure. The cable was severed close to the supporting armor rod tip on Tower M7, and fell into a small river under the line. A section of the cable around the break was removed, and replaced with new cable.

Researchers from the EPRI laboratory in Lenox, Massachusetts visited the site to make observations. It was found that there was not a corona suppression device (corona ring) at the location of the cable break. Most (but not all) of the armor rod installations have these devices. The cause of the break is not known with certainty, but the absence of the corona ring is certainly a prime suspect.

It was observed that the failure occurred at a location where the transmission line goes through a configuration transition in order to negotiate a bridge. Analysis of the electrical environment in this region indicated that the electrical stresses (space potential, induced voltage, and induced current) experienced by the cable were significantly greater here than at typical spans. A high electrical-stress environment is certainly believed to have a significant impact on the probability of a cable failure

There is another failure mode that was recently discovered at the EPRI-Lenox laboratory that is consistent with the Staten Island failure. It was discovered that any puncture located anywhere along the cable can lead to water ingress. The water can move, by capillary action, relatively large distances through the cable, and can lead to spark flashovers at the grounded connections. These flashovers could cause severe damage. Because the damage appears at the tips of grounded hardware, it could be interpreted as corona damage.

Consolidated Edison’s line crew indicated that they had observed sparking at the armor rod tips when the tips were pulled away from the cable. This observation is consistent with the failure mode associated with water ingress.

It must be stressed that the cause of the failure is not known with certainty. However, if the failure was due to the water ingress-flashover failure mode, there is reason to believe that other failures are likely to occur, and the installation of corona suppression devices may not mitigate the problem.

4-30

ESKOM’s ADSS Failure

Background

Early in 2000, one of the EPRI Working Group members from ESKOM (David Smith) provided the Working Group with information about a recent failure of an ADSS cable strung on one of their transmission lines. The line in question (Plattekloof-Springfield) is 4km long, comprised of fourteen single-circuit, flat, horizontal 132kV steel structures fitted with 12 core ADSS cable. The ADSS cable was the “A” type with standard polyethylene sheath material.

After the cable had been in service for about 3 years, it was thought prudent to inspect the installation due to the fact that one of the local metropolitan council ADSS installations in the vicinity had suffered damage due to dry band arcing. This inspection was mostly done from ground level using binoculars – a bucket truck was only used for one or two of the towers. Damage was observed at all attachment points. It was interesting to note that the most severe cable sheath damage, due to dry band arcing and possibly corona, in the vicinity of the attachment hardware, was found on the southerly side of the cable, i.e. down-wind side of the prevailing wind that brings rain to the Western Cape Region (rain may wash pollutants off on the up-wind side). Signs of tracking were also found on some of the spiral vibration dampers.

Unfortunately, before any remedial action could be taken, the ADSS cable broke off about 50 meters from a structure on January 11, 2000.

Observations

Weather Conditions

Failure occurred when there was a strong southeasterly wind with rain after a prolonged period of hot dry weather.

Measurement of Cable Resistance

Resistance measurements were performed on a 9 meter sample of the failed fiber optic cable. These measurements were taken between failure points. Bare copper wire was tied around the cable at each failure point in order to make good contact with the cable. Table 4-2 summarizes the measurements.

4-31

Table 4-2 Summary of cable resistance measurements.

Test point/ Burn mark

Cable length Between test Points (mm)

Resistance Measurements with a Fluke Multimeter (kΩΩΩΩ)

5kV Megger: Voltage at which 500kΩΩΩΩ was measured (Vdc)

Voltage at which the resistance fell to practically zero (Vdc)

End to 1 1900 720 X X 1 to 2 570 74 X X 2 to 3 500 1100 500 1000 3 to 4 1650 1200 500 1000 4 to 5 1040 10 250 300 5 to 6 1050 60 200 250 6 to 7 150 * 100 150 7 to 8 730 * 760 1000 8 to 9 1260 * 260 400 9 to end 2 170 * 125 200 6 to end 2 2310 X 400 550 End 1 to End 2 9020 X 2000 X

* = From point 6 to end 120kΩ was measured X = not measured

Calculation of Space Potential to which ADSS was Subjected

The unperturbed space potential along the cable’s path through space was calculated using the line parameters provided below, and using the quasi 3D model described in a later chapter of this report. This indicated an 8.6kV space potential at midspan. Calculations of the induced current, assuming a cable resistance of 60kΩ/m, gave 2mA at the tower, and about 2/3 of this at the failure point 50m from the tower.

Physical Appearance of Cable

See sample pictures below.

4-32

Figure 4-27 Photograph of damaged cable.

Figure 4-28 Photograph of damaged cable.

4-33

Figure 4-29 Photograph of damaged cable. In addition to the clearly visible damage suffered by the ADSS cable as shown it the pictures, an interesting phenomenon of alternating light and dark bands, running longitudinally along the outer sheath of the cable, was also observed – unfortunately there was insufficient contrast to successfully take photographs of this effect.

The following results are measurements taken on a sample of the failed cable:

• Ring Width: Approximately 30mm to 60mm (the majority of them are 50mm wide).

• Distance between Rings: The average distance, based on 22 measurements, is 160mm. The measurements varied between 45mm and 320mm. In the area where actual tracking is evident, ring spacings were closer compared to the rest of the cable.

A possible explanation for this unusual ring formation is discussed below.

Subsequent Actions

The entire ADSS cable was replaced, free of charge by the supplier, with a new cable with a track resistant sheath material. As a further safeguard, the new cable has been installed at a different, “safer “ position on the towers, with X,Y attachment co-ordinates, in meters, of (+1.2, +11.8), resulting in a midspan space potential of about 3.5kV and an expected maximum induced current of 0.3mA at the attachment points under severe pollution conditions. The intention is to perform regular inspections on the installation.

The new ADSS position is almost at the limit of statutory clearance to ground and hopefully vandalism will not be a deciding factor. The area over which the line passes is one in which no bush fires are likely to occur.

4-34

Further Preliminary Information

Failure (mechanical breakage) occurred 50m from the nearest tower, after the ADSS had been in service for about 3 years. The line on which the ADSS was installed is situated about 10km from the sea and hence the cable was subjected to “mild” (?) maritime pollution. Evidence of dry band arcing was also clearly visible at all the tower attachment points.

Line Parameters

Table 4-3 summarizes the line parameters, which were used for the calculations.

Table 4-3 Voltage and Phase Arrangement.

Phase or Earth

“X” Attachment point of Conductor (m)

“Y” Attachment point of Conductor (m)

Line to ground Voltage (kV)

Phase angle (deg)

Phase 1 -6.72 17.875 76.2 0 Phase 2 0.0 17.875 76.2 240 Phase 3 +6.72 17.875 76.2 120 Earth -4.2 21.7 0 0 Earth +4.2 21.7 0 0 ADSS +2.88 15.5 0 0

Phase Conductor: Single “Goat”, diam. 25.97mm

Earthwire: Single 19/2.65 steel, diam. 13.25mm

ADSS: standard “A” type with polyethylene sheath, diam. 13mm

Table 4-4 provides the details of the line sags.

Table 4-4 Line Sag Details

Temperature (deg C) Conductor Sag (m) Earthwire Sag (m) 15 5.85 5.85 75 7.83 7.19 -5 5.16 5.37

Span: 294.5m

ADSS sag had not been defined by the installer, who stated that they “normally” tension the ADSS so the resultant sag is about the same as that of the earthwires.

Surface Ring Growth on ADSS Cable Installed in Electric Fields

The following explanation about the growth of surface ring marks was provided to ESKOM by Dr. Reinhard Engel of Siemens R&D Department in Erfurt, Germany.

If an ADSS cable gets wet, a thin conductive water film will cover the cable surface. The electrical potential gradient along the ADSS cable generates a surface current flow. In the

4-35

beginning the current is equally distributed around the cable, heating the complete surface up. If one spot of the cable surface gets dry and therefore isolating, the current is forced to flow around the spot, generating more heat in the radial vicinity of the dry spot. Caused by this additional heating the dry spot starts to grow radially, becoming a dry ring. Immediately after the ring is completed, dry band arcing is generated at the thinnest place of the dry ring zone.

The heat of the arc causes a chemical alteration of the surface, for example splitting the Aluminum hydroxide of the jacket compound into aluminum oxide, and leaves a tiny injury. This injury is the germ for a dry spot after the next rain or mist, causing a new ring in the same place as before, and so on. After a series of rain-dry periods you will then see a gray ring of aluminum oxide on the ADSS cable.

Computer simulations of this process were made. The figures below show how the rings can develop.

Figure 4-30 Randomly distributed irregularities on surface of sheath (5%).

4-36

Figure 4-31 Beginning of ring growth due to the irregularities.

Figure 4-32 Ring growth continues.

4-37

Figure 4-33 Ring growth continues.

Figure 4-34 Partially completed ring development.

5-1

5 MODELING (3-D) OF ADSS CABLES IN HIGH VOLTAGE TRANSMISSION CORRIDORS

Introduction

As discussed earlier in this report, all-dielectric self-supporting (ADSS) optical fiber cable on high voltage structures is exposed to electric fields of sufficient strength to cause dry band arcing [1]. A brief review of this phenomenon will provide the background needed to understand how the model to be discussed here can be used to predict dry band arcing.

When first installed, the outer jacket of an ADSS cable is hydrophobic and non-conductive. As a result, its resistance is very high even when wet. Over time, however, it usually becomes hydrophilic and, in some environments, significant contamination may accumulate. As shown in Figure 5-1, the contamination layer can become conductive during wet conditions, and capacitively coupled currents from adjacent energized conductors flow in the layer. As the contamination dries, narrow dry bands form. These bands can have voltages across them high enough to cause arcs. If the current available to the arcs is also large enough, arc heating can degrade the ADSS jacket and cause cable failure [1,2].

Dry Band Voltage

WetWet Dry

Induced currentin wet contaminationArc location

Figure 5-1 Dry band arc on ADSS.

Here, a 3-D analytical model is described that can be used to predict voltages across dry bands and currents available to dry band arcs on ADSS cable [3]. The voltage is needed to initiate an arc and the current is needed to sustain it. As shown later, these calculations can be used to determine whether a particular ADSS cable is susceptible to damaging dry band arcing. The model accounts for transmission line cross-section geometry and phasing, ADSS cable tower

5-2

attachment point, contamination resistance, span length, phase conductor sag, and ADSS cable sag. The model is also useful for studying the safety of workers touching ADSS cables near energized conductors and for developing laboratory tests for ADSS cable that properly simulate field conditions [4].

Here, a conservative estimate of dry band voltage (the available dry band voltage”) is obtained by assuming the dry band to be an open circuit. For this to be true the dry band resistance must be much larger than the characteristic impedance of the contaminated cable over earth [4]. The current available to the arc is identical to the current induced in the ADSS contamination layer prior to dry-band formation. This “available arc current” is equivalent to the “short-circuit current” in references [3, 4]. The larger this current, the greater the potential for damaging arcs. Note that the dry band voltage is not equivalent to the space potential of the transmission line in which the ADSS is located. However, space potential is often a useful, but very rough estimate, of the dry band voltage [4].

The parameter that has the most influence on available dry band voltage and current is the ADSS cable contamination level. This parameter is usually quantified by the resistance-per-length of the cable under wet conditions (assuming that the cable is hydrophilic since hydrophobic cables have significantly higher resistance levels when wet). The resistance is very dependent upon local climatic conditions and pollution sources and should be measured if possible [5]. In the absence of such information, a range of typical contamination levels are assumed. Three levels generally assumed are heavy, medium, and light corresponding to 105, 106, and 107 Ω/meter respectively [6]. Other parameters that can affect the available dry band voltage and current are span length and the relative phase conductor and ADSS cable sags.

Manufacturers usually offer ADSS cable with either a “standard” or “track resistant” jacket. Early studies in Great Britain suggest that available dry band voltages greater than several 10’s of kV and available arc currents of approximately 1 mA, or greater, are required to sustain arcs which cause cable jacket damage on “standard” cable jackets [1, 2]. All tests and experience indicate that cables with “track resistant” jackets perform better than this [7]. However, the specific degree of improvement is unknown at this time. Nevertheless, a proposed standard test for ADSS cable would characterize a given cable by a curve of available dry band voltage vs. available arc current that separates safe from dangerous operating conditions [8]. Such a proposed curve is shown in Figure 5-2.

5-3

AVAILABLEDRY BAND VOLTAGE

AVAILABLE ARC CURRENT

“DANGER OF DAMAGING ARCS”

“SAFEOPERATION”

Figure 5-2 Typical curve that separates “safe” from “dangerous” operating conditions for a given cable.

The Model

Transmission line electrical characteristics are often described by distributed inductance, capacitance, and resistance-per-length [9]. Inductance can be ignored for calculation of induced voltage and current on ADSS cable because the resistance-per-length of the ADSS cable is much greater than its inductive reactance per unit length. Figure 5-3 depicts a lumped parameter ADSS cable circuit model. In this model, the span 0 ≤ z ≤ S, has been divided into N sections of length ∆z where ∆z = S/N. Each section consists of the total contamination resistance within the section (R01, R12, R23, etc.), the capacitance between each phase and the ADSS cable (C1A , C1B , C1C , C2A , etc.), and the capacitance between the cable and ground (C1g , C2g , etc.). In each section Rn-1,n = rn-1,n∆z, Cnk = cnk∆z

R

R

R

R

C

C

C

C

C

C C

C

C

C

C

C

V

V

VV

V

V

Span - Tower to towerI01

A

B

C

1C

2C

3C

1g

2g

3g

01

12

23

341A

1B 2A

2B 3A

3B

1

2

312

23

34

I

I

I

Figure 5-3 Distributed parameters of the lumped circuit ADSS model (tower-tower span length = S).

5-4

where rn-1.n and cnk (k = A,B,C,g) are respectively the resistance per unit length of the ADSS cable and the capacitance per unit length between the ADSS cable and the phase conductors and ground. Details of how these parameters can be calculated can be found in [10]. Note that there are also capacitances between phase conductors, and between each phase conductor and ground. However, these do not affect the induced currents or voltages in the ADSS cable, and are therefore not included in the model. VA, VB, and VC are the conductor voltages. V1 , V2 , etc. and I01, , I12 , I23 , etc. are the induced voltages and currents on the ADSS cable.

In this report, a simplified two-dimensional calculation [10] is used as an approximation for calculating the capacitances. For this calculation the towers are ignored and the energized conductors treated as infinitely long straight conductors. Note, however, that sag causes the cross sectional locations of the phase conductors and the ADSS cable to change with distance from the tower. This problem is resolved by repeating the two-dimensional capacitance calculation (or alternatively the space potential calculation) N times, once for the cross section at each segment of the path between the towers. The resulting capacitance is called quasi-three-dimensional (Q3D). Finally, it should be noted that phase conductor sag will change with load and weather conditions. Thus, a range of sags should be considered in any study used for locating ADSS cable.

Even though the Q3D space potential does not approach zero at the grounded tower as required by the physics of the problem, it has been shown in [3] that, for contamination resistances less than 106 Ω/m, the use of the Q3D space potential is sufficient. Basically, the tower has negligible effect and can be ignored for low contamination resistance. Experimental studies at the Bonneville Power Administration have confirmed this conclusion.

Using node analysis, the following set of equations can be written for the unknown voltages in the circuit of Figure 5-3.

[ ][ ] [ ]Y V I= (5-1)

where

( )

( )

YR

j C C C CR

Y YR

I j V C V C V C

nnn n

nA nB nC ngn n

nm mnmn

n A nA B nB C nC

= + + + + +

= =−

= + +

− +

1 1

11 1, ,

ω

ω

While a solution to this equation could be obtained using general Gaussian elimination, the fact that the matrix is symmetric and tri-diagonal leads to a much more economic solution. Row n-1 is multiplied by Ynn and row n by Yn-1,n. The two resulting equations are subtracted and row n-1 replaced by the difference. The new row n-1 has a 0 in the spot occupied by Yn-1,n. This process is repeated with row n-2 and row n-1, and so on, until the matrix becomes lower diagonal. This equation can be easily solved by back substitution.

5-5

Note that the available dry band voltage at some point along the ADSS cable can be determined by replacing the nearest resistor with an open circuit. The current available to an arc can be determined by replacing the same resistor by a short circuit. In practice, however, this current is approximately the same as the current through the resistor without replacing it by a short circuit. The “induced voltage” is the voltage between any point on the ADSS cable and ground. It is useful for safety analyses.

Results

In this section, results are given based on the algorithm described in the above. Figure 5-4 shows an example tower configuration. A 304.8 meter (1000 ft.) span was selected as typical for a 500 kV line, conductor sags were 2%, and the ADSS cable was sagged at 0.5%, 1.25%, and 2 %. The phasing was CAB as illustrated in the figure.

8.84m (29')

(30.7')

Conductors: 3x33.07mm (1.302") @ 43.3cm (17.04") spacing

18.57m (60.9')

A

BC

19.82m (65') to ADSS

1.23m

9.36m(4')

Figure 5-4 Dimensions of example tower with ADSS cable.

The effect of sag on available dry band voltage and arc current is illustrated in Figures 5-5 and 5-6. In Figure 5-5 the available arc current magnitude is plotted as a function of location along the span with ADSS cable sag as a parameter. It is assumed that r = 105 Ω/m (heavy contamination). In this case, the effect of ADSS cable sag on the induced current at the tower is modest; the maximum induced current varies from just above 6 mA to just above 8 mA.

5-6

Induced Current at r = 105 ohm/m - Phasing CAB

0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250 300Span Distance - Meters

Cu

rren

t -

ma

2%

1.25%

0.50%

Sag Legend

Figure 5-5 Magnitude of available arc current on ADSS cable for various sags and conductor phasing.

A similar effect can be observed for the available dry band voltage which is plotted in Figure 5-6. This is not surprising since the available arc current equals the available dry band voltage divided by an impedance which is essentially independent of conductor phasing or sag [4].

Dry Band Voltage at r = 105 ohm/m - Phasing CAB

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300

Span Distance - Meters

Vo

ltag

e -

kV

2.00%

1.25%

0.50%

Sag Legend

Figure 5-6 Magnitude of available dry band voltage on an ADSS cable.

5-7

It is useful to note that the space potential at points along the ADSS cable for the three sag levels studied here ranges from just above 20 kV to almost 70 kV. Clearly the proper application of space potential to this problem is ambiguous and thus the use of space potential as a parameter is not encouraged.

Figures 5-7 and 5-8 illustrate the effect of changing the contamination level. They are a replication of Figures 5-5 and 5-6 but for r = 106 Ω/m. It is clear that the available arc currents are considerably smaller that those for r = 105 Ω/m case. In fact, the available arc current is sufficient (1 to 2 mA) to support damaging arcing only very close to the tower. The available dry band voltage, however, is on the same order of magnitude as those for the case r = 105 Ω/m. The only notable difference is that the largest voltages are somewhat smaller than those in the r = 105 Ω/m. Clearly, increasing r further to 107 Ω/m will result in dry band arcing that either does not exist at all or is not dangerous. It can be concluded that a knowledge of contamination levels is very important for estimating the possibility of ADSS cable damage.

Induced Current at r = 106 ohm/m - Phasing CAB

0

1

2

3

0 50 100 150 200 250 300

Span Distance - Meters

curr

ent

- m

a

2%1.25%0.5%

Sag Legend

Figure 5-7 Magnitude of available arc current on an ADSS cable for r = 106 ΩΩΩΩ/m.

5-8

Dry B and Voltage at r = 106

ohm/m - Phasing CAB

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300

S pan Dis tance - Meters

Vo

ltag

e k

V2%1.25%0.5%

S ag L egend

Figure 5-8 Magnitude of available arc voltage on an ADSS cable for r = 106 ΩΩΩΩ/m.

How to Design ADSS Cable Installations Based on the 3D Model

In this subsection, a procedure for designing ADSS cable installations on high voltage transmission lines between identical towers above level ground is discussed. The extension to the case for non-identical towers and non-level ground is straightforward.

The first information that must be supplied is the cross sectional geometry of the transmission line towers on which the ADSS cable is to be mounted. Specifically needed are the tower attachment points for each phase conductor and ground wire. For each phase conductor, it is also necessary to specify its voltage (amplitude and phase). Ground wires are assumed to be at zero potential. The second piece of information needed is the range of span lengths for the transmission line. The next information needed is the range of sags expected for the phase conductors. These will be determined by the range of ambient temperatures and load currents and expressed as a percentage of the span length. If such data are not available, the range of sag between flat conductors and conductors sagged to the minimum NESC level should be considered.

If anything is known about the level of contamination expected near the transmission line, this may be useful. However, it is unlikely that anything will be known about the resistance per unit length of the ADSS cable unless measurements have been made of ADSS cables in the area. In the absence of this information, it is recommended that the range of expected contamination be assumed as 105 to 107 Ω/m along the ADSS cable.

5-9

Given this information, a tentative tower attachment point and sag is chosen for the ADSS cable. With this information, it is possible to calculate the available dry band arc voltage and current using the method outlined earlier in this subsection. This should be done for all possible values of ADSS resistance per unit length (i.e. contamination level), span length and phase, and ground conductor sags. All combinations of available dry band voltage and current should be recorded.

The staff at the EPRI/Lenox facility have examined all known data on the failure of ADSS cables. Based on their conservative evaluation, they suggest that the available arc voltage and current data obtained from the analysis described above be used to select ADSS cable in the following way. If the largest available arc voltage is less than 25 kV and the available arc current is less than 1 mA, then Class B may be safely used. If the largest available arc voltage is less than 12 kV and the available arc current is less than 0.5 mA, then Class A may be safely used. Continuing investigations may lead to refinements on these criteria.

This process can be repeated until an appropriate combination of tower attachment point, ADSS cable sag and cable jacket type is identified.

5-10

References

1. G. Carlton, A. Bartlett, C. Carter and T. Parkin, “UK power utilities’ experience with optical telecommunications cabling systems,” Power Engineering Journal, Vol. 9, pp. 7-14, Feb. 1995.

2. C. N. Carter, J. Deas, N. R. Haigh, and S. M. Rowland, “Applicability of all-dielectric self supporting cable systems to very high voltage overhead power lines”, 46th Proc. Intl. Wire and Cable Symposium, pp. 622-631, 1997.

3. M.W. Tuominen and R.G. Olsen, “Electrical Design on All-Dielectric Self-Supporting Optical Fiber Cable,” IEEE Transactions on Power Delivery – to appear

4. R.G. Olsen, “An improved model for the electromagnetic compatibility of all-dielectric self-supporting fiber optic cable and high voltage power lines,” IEEE Transactions on Electromagnetic Compatibility, Vol. 41, No. 3, August, 1999, pp. 180 - 192

5. K.S. Edwards, P.D. Pedrow, and R.G. Olsen, “Portable ADSS Surface contamination Meter,”Proceedings of the 1999 IEEE Conference on Electrical Insulation and Dielectric Phenomena, Volume I, 1999, pp. 158-161.

6. C. N. Carter and M. A. Waldron, “Mathematical model of dry band arcing on self-supporting all dielectric optical cables strung on overhead power lines’, IEE Proceedings - C. Vol. 139, No.3, pp.185-196, May 1992.

7. M. D. Johnson and J. O. Lo, “ADSS jacket arc resistant material tests”, Laboratory Services Report #TNF(M)-98-24a, Bonneville Power Admin., US Dept. of Energy, January 21, 1999.

8. R.G. Olsen, “Laboratory Simulation of Dry Band Arcing on All-Dielectric Self-Supporting Fiber Optic Cable Near High Voltage Power Lines, presented at the 1999 IEEE EMC Society Symposium, Seattle, WA.

9. EPRI, Transmission Lines/ 345 kV and Above, Electric Power Research Institute, Palo Alto, CA

10. M.W. Tuominen, “3 Phase Circuit Model for ADSS Optical Fiber Contamination Currents”, Engineering Report TNL3-99-1, Bonneville Power Admin., US Dept. of Energy, May, 1999.

6-1

6 FULL-SCALE TESTS

Introduction

During late 1999, there was discussion at EPRI Working Group and other meetings, about the need for some full-scale test capability. There would be several purposes for such capability, including the following:

• Validation of the quasi-3D analytical model.

• Live-line installation and maintenance measurements and demonstrations.

• Testing mitigation devices and strategies for dry band arcing, corona, and icing damage.

• Testing instrumentation for making measurements of electrical parameters, especially pollution level.

• Testing, evaluating, and demonstrating remote inspection tools.

• Making real-to-life observations of dry band arcing, corona, etc.

• Others to be identified.

It was decided to construct a single full-scale test span at the EPRI laboratory in Lenox, Massachusetts. The first application of this tool was to verify the analytical model discussed previously in this report. Also, during the stringing of an actual fiber optic cable, the opportunity was taken to address live-line installation concerns.

Design and Construction

The full-scale test line was designed as a single span supported by wood structures at each end. The structures were designed with several crossarms at different heights in order to have the capability of mounting phase conductors and fiber optic cables at various positions.

For the first set of tests, the phase conductors were installed in a flat configuration with a phase spacing and ground clearance approximating typical 230 kV installations. An ADSS fiber optic cable was mounted below the phase conductors between two of the phases. Figure 6-1 shows an illustration of the structure design and the initial placement of phase conductors and ADSS cable at the structure. The test span runs north-to-south, and Figure 6-1 shows the north structure, looking north.

6-2

46’-

6”

26’-

6”

Ground Level

Phase A Phase B Phase C

Cross Arm

36 foot long

22’-0” 22’-0”

North Tower

(looking north)

Cross Arm

36 foot long

Cross Arm

55 foot long

ADSS

Cable

11’-0”

65’-

0”

Figure 6-1 Illustration of structure design and initial placement of phase conductors and ADSS cable.

Referring to Figure 6-1, it can be seen that the structure is supported by two vertical wood poles with three horizontal crossarms. Crossarms are located at heights above ground of 26.5 ft., 46.5 ft., and 65 ft., and are of lengths 36 ft., 55 ft., and 36 ft., respectively.

Phase conductors were initially installed on the middle crossarm with a phase spacing of 22 ft. The phase conductors are 1.3-inch ACSR, and are insulated from the structures with non-ceramic insulators (NCI) designed for operation up to 345 kV.

A real ADSS fiber optic cable was initially mounted on the lowest crossarm midway between the two phase conductors to the east (phases labeled “Phase B” and “Phase C” in Figure 6-1). The ADSS cable is a 0.61-inch, Alcoa, Class B cable.

A side view of the test span is shown in Figure 6-2. The span is 600 ft. long. As indicated in the figure, the ground is not perfectly flat along the span’s length, therefore, the heights above ground of the attachment points on the two structures are different. Also, the crossarms on the two structures are not quite level with each other, the ones on the south structure being about 2 ft. lower than their counterparts on the north structure. The phase conductors and ADSS cable were initially strung with sags of 3.67% and 1.97%, respectively.

6-3

Profile View of Full-Scale Test Line46

’-6”

26’-

6”

37’-

6” 57’-

6”

13’-

0”

24’-

6”

16’-

0”

Ground Level

Nor

th T

ower

Sout

h T

ower

Cross Arm

600’-0”

Phase Conductors (sag=3.67%)ADSS Cable (sag=1.97%)

Figure 6-2 Side view illustration of the test span.

Figures 6-3 through 6-5 below show photographs of the test line.

6-4

Figure 6-3 Photograph of line crew working on north structure of test line.

Figure 6-4 Installing NCI on the north structure.

6-5

Figure 6-5 Photograph looking down the test line toward the south structure.

When ADSS cables are dry, virtually no current is induced along their lengths, even if the cables are old and polluted. It is when a cable accumulates surface pollution, such as salt deposits, fertilizer, etc., and then becomes wet that the cable starts to experience significant levels of induced current. The amount of pollution is characterized by a cable’s resistance-per-length.

The ADSS cable on the full-scale test line was “artificially polluted” with a 12-gauge copper wire which ran along the cable’s length with a resistor placed in series with the copper wire every two meters. In this way, the resistance-per-meter is well known, and constant over length and time. This is needed in order to properly compare measurements to calculations from the analytical model.

For the first series of tests, 100k-ohm resistors were placed every two meters for a resistance per-meter of 50k-ohms. Careful measurements of the resistors indicated that the actual resistance was closer to 53.8 k-ohms per meter. This is believed to correspond to very heavy pollution levels.

Figure 6-6 shows a close-up photograph of one of the resistors. Figure 6-7 shows a photograph taken from the north structure looking down the line toward the south structure. The ADSS cable, its attachment hardware, and some of the resistors can be seen.

6-6

Figure 6-6 Photograph of one of the resistors mounted along the test line’s ADSS cable.

6-7

Figure 6-7 Photograph from the north structure looking south along the ADSS cable.

6-8

Measurement Techniques

Measurements were made of open-circuit voltage (Voc), induced voltage (V), induced current (I), and short-circuit current to ground (Isc) along the ADSS cable. All four of these parameters are functions of the distance z along the ADSS cable. The instrumentation and techniques used are discussed below.

Open-Circuit Voltage

The open-circuit voltage along the ADSS cable was measured by breaking the continuity of the copper wire, and inserting a voltmeter. The break was done at resistor connections, and the voltmeter was hung from the ADSS cable. It was important to shield the voltmeter from the electric fields produced by the overhead phase conductors. Figure 6-8 shows a simple illustration of the technique.

ADSS Cable

Voltmeter

High Voltage Probe

Shield

Figure 6-8 Illustration of making open-circuit voltage measurements.

The meter used for these measurements was a 383274 EXTECH Datalogger Multimeter. Shields for both the meter and the high voltage probe were fabricated out of copper foil. Figure 6-9 shows a photograph of the meter’s shield, the meter, and the shielded high voltage probe.

The meter’s shield is essentially a copper foil case with openings for the probe connections and LCD display. A strap was attached to suspend the meter from the ADSS cable. The high voltage probe was similarly shielded.

6-9

Figure 6-9 The meter’s copper foil shield case with strap, the EXTECH Multimeter, and the shielded high voltage probe (bottom to top, respectively).

Some special tests were performed to verify the effectiveness of the shields and the validity of the technique. Measurements of known voltage were made high up off the ground near the energized phase conductors. These tests confirmed the necessity of shields, and the performance of the shields.

The procedure for making the measurements was for a technician to ride in a bucket truck up close to the ADSS cable, insert the meter, and then move back down to the ground. The presence of the bucket perturbed the measurements if the bucket was near to the cable. Measurements were made with binoculars from the ground.

This procedure was repeated over the length of the cable. For some line configurations it was necessary, for safety reasons, to de-energize the phase conductors while the meter was being placed, then re-energize.

Figure 6-10 shows a more detailed illustration of the measurement setup.

6-10

Measurement of Open-Circuit Voltage

2 meters

3.283.283.283.28

Simulated Pollutionon ADSS Cable

Resistor100kΩ

Fluke Probe Lead

ADSS Cable

2 meters

Tie-Wrap

Velcro Strap

Extech Digital Multimeter(encased in Copper-FoilFaraday Cage)

Fluke HighVoltage Probe

Fluke Ground Wire

(Copper-Foil Faraday Cage)

Alligator Clips

Figure 6-10 Illustration of the technique used for measuring open-circuit voltage.

Induced Current

The technique for measuring induced current (I) was very similar to the measurements of open-circuit voltage described above. The same EXTECH Multimeter, operating as an ammeter, was placed in series with the copper wire and resistors as was done for Voc measurements. Figure 6-11 shows a simple illustration of the technique.

6-11

ADSS Cable

AmmeterShield

Figure 6-11 Simple illustration of the technique used to measure induced current along the ADSS cable.

The probe for these measurements was a standard coaxial-type probe. The technique was validated by injecting a known current while the overhead phase conductors were energized.

Figure 6-12 shows a more detailed illustration of the setup, and Figure 6-13 shows a photograph of the meter in place on the ADSS cable.

Measurement of Induced Current

2 meters

3.283.283.283.28

Simulated Pollutionon ADSS Cable

Resistor100kΩ

Coaxial Cable Probe Lead

Resistor 100kΩ(connected to shieldof coaxial cable)

Alligator ClipsADSS Cable

2 meters

Tie-Wrap

Velcro Strap

Extech Digital Multimeter(encased in Copper-FoilFaraday Cage)

Figure 6-12 More detailed illustration of the technique for measuring induced current.

6-12

Figure 6-13 Shielded EXTECH Multimeter suspended from the ADSS cable for measuring induced current.

6-13

Short-Circuit Current to Ground

Short-circuit current to ground was measured by connecting a ground-shielded probe directly to the copper wire on the ADSS cable and measuring the current to ground through an ammeter. The ammeter (the same EXTECH Multimeter used in the above measurements) was located close to the ground in its shielded copper foil case. A “ground” was established by running a copper wire along the ground under the entire length of the test line with driven ground rods along its length, including connections to ground plates at the base of the test line’s supporting poles. A simplified illustration of the arrangement is shown in Figure 6-14.

A

GroundedShield

ADSS Cable

..

Figure 6-14 Simplified illustration of the setup for making measurements of short-circuit current to ground.

The presence of the grounded shield could have some effect on the distribution of induced voltage along the cable, which affects Isc. However, this is not believed to undermine the measurements since in order to have a short-circuit current to ground, “ground” must be brought up and made to come in contact with the cable, such as a live-line worker in a bucket truck.

Because it was expected that Isc levels could be very small, a special experiment was setup to test the accuracy of the current measurements with the EXTECH Multimeter. A resistance R was placed in series with the ammeter and a 60 Hz power supply. The resistance and the voltage of the power supply were varied, and the “actual current” was measured with high precision voltage measurements across the resistor (VR). The data is summarized in Table 6-1 below. It was concluded from these data that the ammeter could confidently read on the order of microamps.

6-14

Table 6-1 Data validating accuracy of ammeter.

VR (volts) R (ohms) VR/R (µµµµA) Iammeter (µµµµA)

9.61 9.57k 1000 1010

9.8 96.8k 97.7 100

9.75 1M 9.75 10.8

1.0 1M 1.0 1.0

0.487 1M 0.487 0.4

In addition, a special experiment was performed to test the validity of the technique. In this experiment, a grounded copper sphere of known radius was held well above the ground at a known height under the energized test line. Such a sphere will have a current induced in it given by

I = 4 π ω ε r Vsp (6-1)

where: 4 π ω ε are well known constants r = sphere radius Vsp = unperturbed space potential at sphere

The copper sphere used for the experiment was 6 inches in diameter, and the space potential was calculated with EPRI’s Transmission Line Workstation software. The sphere was held 12 ft. above the ground with a hotstick, in a location where the space potential was calculated to be 21 kV. According to the formula above, 56 µA should be induced in the sphere. Measurements with the EXTECH Multimeter indicated 76 µA with an unshielded lead, and 55.6 µA with a grounded-shield lead. It can be concluded that the measurement technique is reliable, as long as a shielded lead is used. Figure 6-15 provides a simple illustration of the setup.

6-15

230 kV Phase Conductor

r

A

I = 4 π ω ε π ω ε π ω ε π ω ε r Vsp

Figure 6-15 Illustration of setup to test measurement technique for short-circuit current to ground.

Also, Figure 6-16 provides a more detailed illustration of the measurement technique.

6-16

Measurement of Current to Ground

3.283.283.283.28

Simulated Pollutionon ADSS Cable

Resistor100kΩ

Coaxial CableProbe Lead

Alligator Clip

ADSS Cable

2 meters

Tie-Wrap

Extech Digital Multimeter(encased in Copper-FoilFaraday Cage)

Coaxial CableShield (grounded)

Coaxial CableCenter Conductor(carrying signal)

App

roxi

mat

ely

16’

to 2

6’

Ground Wire(laying on gnd.)

Figure 6-16 Illustration of technique used to measure short-circuit current to ground.

6-17

Induced Voltage

Induced voltage (reference being ground) was measured by connecting a ground-shielded probe directly to the copper wire on the ADSS cable and measuring the voltage with a voltage divider. The voltmeter (the same EXTECH Multimeter used in the above measurements) was located close to the ground in its shielded copper foil case. Figure 6-17 shows a simplified illustration of the measurement technique.

ADSS Cable

V

1000 M Ω

1M Ω

Shield

Figure 6-17 Simplified illustration of the measurement technique for induced voltage on the ADSS cable.

Figure 6-18 shows a more detailed illustration of the measurement technique.

6-18

Measurement of Induced Voltage

3.283.283.283.28

Simulated Pollutionon ADSS Cable

Resistor100kΩ

FlukeProbe Lead

Alligator Clip

ADSS Cable

2 meters

Tie-Wrap

Extech Digital Multimeter(encased in Copper-FoilFaraday Cage)

Coaxial CableShield (grounded)

Coaxial CableCenter Conductor(carrying signal)

App

roxi

mat

ely

16’

to 2

6’

Ground Wire(laying on gnd.)

Fluke HighVoltage Probe

Fluke GroundWire

Figure 6-18 Detailed illustration of the technique used to measure induced voltage on the ADSS cable.

6-19

The technique used to measure induced voltage was similar to the technique used to measure short-circuit current to ground. However, it was argued above that the presence of the grounded shield around the lead did not have an adverse effect on measurements of short-circuit current to ground because “ground” must be brought up and made to come in contact with the cable, such as a live-line worker in a bucket truck. However, there was concern that the grounded shield could perturb the induced voltage distribution along the cable, resulting in erroneous measurements of unperturbed induced voltage.

It was suspected that perhaps the perturbation would be insignificant. To test this hypothesis, the following special experiment was performed. A ground wire, similar to the grounded shield of the multimeter’s lead, was brought up close to the ADSS cable while simultaneously monitoring the induced current on the cable a short distance away. It was hypothesized that if the presence of the ground wire significantly perturbed the voltage distribution along the cable, it would do so more on the side of the ammeter adjacent to the ground wire location, and to a lesser extent on the side of the ammeter further from the ground wire location. Since the current through the ammeter is proportional to this voltage difference, a change in current would be noted if the presence of the ground wire results in significant perturbation. Figures 6-19 and 6-20 illustrate the setup of the experiment, and the results.

Effect of Ground Wire on CurrentThrough Simulated ADSS Cable

ADigitalMultimeter

Ground Wire

SimulatedADSS Cable

1 cm1 m

2 meters

Measurement taken approximately 50 feet from the north tower.

With no ground wire present, current measured 2.14mA.

With a ground wire present, current measured 2.12mA.

(Test 1)

Resistor100k-ohm

Figure 6-19 Illustration of special measurement to determine if measurements of induced voltage are perturbed by presence of grounded shield.

6-20

Effect of Ground Wire on CurrentThrough Simulated ADSS Cable

ADigitalMultimeter

Ground Wire

SimulatedADSS Cable

1 cm1 m

2 meters

Measurement taken approximately 50 feet from the north tower.

With no ground wire present, current measured 2.16mA.

With a ground wire present, current measured 2.14mA.

(Test 2)

Resistor100k-ohm

Figure 6-20 Illustration of 2nd special measurement to determine if measurements of induced voltage are perturbed by presence of grounded shield.

In the first experiment (see Figure 6-19) the ammeter was placed one meter from the ground wire, and the ground wire and ammeter were between the same two resistors. With no ground wire present the induced current at the ammeter was measured to be 2.14 mA, and with the ground wire present the current was 2.12 mA.

In the second experiment (see Figure 6-20) the ammeter was placed one meter from the ground wire, and the ground wire and ammeter had a resistor between them. With no ground wire present the induced current at the ammeter was measured to be 2.16 mA, and with the ground wire present the current was 2.14 mA.

Because the change in current was so small with and without the presence of the ground wire, it was inferred that the presence of a thin ground wire very near the ADSS cable passing down to ground (i.e. the grounded shield on a multimeter lead) would not significantly perturb the voltage distribution along the cable. However, this conclusion remains suspect. This is discussed further in the subsection on measurements and calculations of induced voltage.

6-21

Phasing Confirmation

As discussed elsewhere in this report, the phasing of the phase conductors can, in some cases, make a difference in the electrical profiles along the ADSS cable (Voc, V, I, Isc). An A-B-C phasing can produce different results than A-C-B phasing. These are the only possible phasings. Therefore, it was important to verify the phasing on the test line.

Referring to Figure 6-21, an observer standing under the line looking north at the north structure will either “observe” phasing A-B-C or A-C-B, going west to east. Since phase A is arbitrarily chosen, the west-most phase is assigned the phase “A”. The question then becomes “does the center phase lead or lag phase A in voltage?” If the center phase leads phase A, then it is phase C, if it lags it is phase B, according to standard convention.

The phasing was verified by measuring the induced voltage, with a dual trace oscilloscope, on copper spheres held high above the ground. The induced voltage is about in phase with the voltage on the overhead phase conductor, if the sphere is placed relatively close to the conductor. A verification of phasing was performed each time some change was made to the test line.

One sphere was held at a fixed location under phase A, and a second one was held under the center phase. Then, the second sphere would be moved to under the east-most phase, and the shift of phase would be observed. The test is illustrated in Figure 6-21, and a photograph of the test being made is shown in Figure 6-22.

Id en tify in g P h ase R e la tio n sh ip o f O v erh ead C o n d u c to rs2 3 0 k V L in e V o ltag e

45

’-0

”

G ro un d L ev e l

C o n d uc to r C o n d uc to r C o n d uc to r

2 2 ’-0 ” 2 2 ’-0 ”

N o rth T o w er

( lo o k in g so u th )

O sc illo sc o pe

C h a nn e l A

C h a nn e l B (A lte rn a te )

C h a nn e l B

H o t S tic kC o a x ia l C ab le(S h ie ld G ro un d e d)

S p h ere

Figure 6-21 Illustration of phasing verification test.

6-22

Figure 6-22 Photograph of phasing verification measurement.

Full-Scale Tests – Measurements and Calculations

One of the main purposes of the full-scale test line is to verify the quasi-3D analytical model discussed in Section 5. A set of four tests was performed during the fall of 2000, with some mixed results. These tests are presented in this subsection.

The four tests are referred to below as Configurations 1, 2, 3, and 4. For all of them the resistance of the ADSS cable was fixed at 53.8 kΩ/m, corresponding to a very heavily polluted cable, and the phase voltage of the line was 230 kV. The Configurations are summarized in Table 6-2.

6-23

Table 6-2 Summary of the tests performed.

Configuration Basic Phase Geometry

Phase Spacing

ADSS Position

Phase Cond. Sag

ADSS Sag

1 Flat (A-C-B) 22 ft. Between phases C and B, 20 ft. below

3.67% 1.97%

2 Same Same Same Same 0.67%

3 Delta (A-C-B)

18 ft. 2 ft. off center, 2 ft. higher than

bottom phase conductors

3.75% 2.86%

4 Delta (A-B-C)

Same Same Same Same

Configuration 1

Figure 6-23 shows the test line’s attachment point geometry at the north structure, looking north. The phase conductors were arranged in a flat configuration with 22 ft. phase spacing, and were attached 46.5 ft. above the ground. The applied voltage was 230 kV, phasing A-C-B, west to east. The ADSS fiber optic cable was attached midway between phases C and B, 20 ft. lower (26.5 ft. above the ground).

Figure 6-24 shows a view of the test line from the side. Note that the ground is not perfectly level under the test line.

6-24

Section View of Configuration 146

’-6”

26’-

6”

Ground Level

Phase A

230kV Line Voltage

Phase C Phase B

Cross Arm

36 foot long

22’-0” 22’-0”

North Tower

(looking north)

Cross Arm

36 foot long

Cross Arm

55 foot long

ADSS

Cable

11’-0”

Figure 6-23 Configuration 1, view of north structure, facing north

6-25

Profile View of Configuration 146

’-6”

26’-

6”

39’-

6” 59’-

6”

13’-

0”

24’-

6”

16’-

0”

Ground Level

Nor

th T

ower

Sout

h T

ower

Cross Arm

600’-0”

Phase Conductors (sag=3.67%)ADSS Cable (sag=1.97%)

Midspan

Figure 6-24 Configuration 1, view from the side.

Figures 6-25 and 6-26 show plots of the measured and calculated open-circuit voltage and induced current along the ADSS cable’s length, respectively. Note that there are two calculated curves on each plot, corresponding to two different ground clearances because the ground clearance at the north end of the line is different than the south end, and the analytical model presently can only handle a level ground. Therefore, a calculated curve is shown for the two ground clearances.

Figure 6-27 shows a plot of the measured short-circuit current to ground along the ADSS cable. Calculations are not presently available for this parameter. Measurements of induced voltage were not made for this configuration.

Referring to Figures 6-25 and 6-26, it is clear that the analytical model does a very good job in predicting the measured values. On both plots there is a slight dip in measured magnitude close to the north tower. It is suspected that this may be due to the presence of a loop of phase conductor on the opposite side of the north structure which feeds the line. These loops can be seen in the photograph of Figure 6-3.

Referring to Figure 6-27, the short-circuit current to ground peaks at the middle of the line at a magnitude of about 3.5 mA. The threshold for human perception is about 1 mA, and an

6-26

uncomfortable sensation (let-go level) occurs at about 10 mA. Therefore, the level measured would be considered safe for live line work.

Configuration 1: Open-Circuit Voltage (53.8kΩΩΩΩ/m)

0

2

4

6

8

10

12

14

16

0

100

200

300

400

500

600

Distance from North Tower (feet)

Vo

ltag

e (k

V)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-25 Plots of measured and calculated open-circuit voltage along the ADSS cable for Configuration 1.

6-27

Configuration 1: Induced Current (53.8kΩΩΩΩ/m)

0

0.5

1

1.5

2

2.5

30

100

200

300

400

500

600

Distance from North Tower (feet)

Cu

rren

t (m

A)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-26 Plots of measured and calculated induced current along the ADSS cable for Configuration 1.

6-28

Configuration 1: Current to Ground (53.8kΩΩΩΩ/m)

0

0.5

1

1.5

2

2.5

3

3.5

40

32.8

65.6

98.4

131

164

197

230

262

295

328

361

394

427

459

492

525

558

591

Distance from North Tower (feet)

Cu

rren

t (m

A)

Figure 6-27 Plot of measured short-circuit current to ground along the ADSS cable for Configuration 1.

6-29

Configuration 2

Configuration 2 is the same as Configuration 1, except that the ADSS cable was raised at midspan by increasing its tension, thereby reducing its sag to 0.67%. Figure 6-28 shows a side view of the line’s configuration.

Profile View of Configuration 2

Phase Conductors (sag=3.67%)ADSS Cable (sag=0.67%)

46’-

6”

26’-

6”

39’-

6” 59’-

6”

13’-

0”

24’-

6”

24’-

0”

Ground Level

Nor

th T

ower

Sout

h T

ower

Cross Arm

600’-0”

Midspan

Figure 6-28 Configuration 2, View from the side.

Figures 6-29, 6-30, and 6-31 show plots of measured and calculated open-circuit voltage, induced current, and induced voltage (ground reference) along the ADSS cable’s length, respectively. Figure 6-32 shows a plot of measured short-circuit current to ground.

Referring to Figures 6-29 and 6-30, it is clear again that the analytical model does a very good job in predicting the measured levels of open-circuit voltage and induced current. However, in Figure 6-31, measured and predicted values are significantly different over most of the cable; measured values being significantly lower than calculations. The reasons for this have not yet been resolved, however, it is suspected that the error may be in the measurements. This will be investigated further in the future.

6-30

Referring to Figure 6-32, the measured short-circuit current to ground peaks at midspan at about 4.2 mA. This is well within safe limits for performing live-line work.

Figure 6-33 shows a plot of the calculated unperturbed space potential along the path through space that the ADSS cable resides. At midspan, the space potential is about 19 kV. When manufacturers specify an upper limit on the “space potential” for their cable (e.g. 25 kV for Class B), this is the quantity they are referring to. As can be seen by comparing the plots of electrical parameters for Configuration 2, or for most any configuration, this quantity diminishes in relevancy when cables become polluted.

Configuration 2: Open-Circuit Voltage (53.8kΩΩΩΩ/m)

0

2

4

6

8

10

12

14

16

18

20

0

100

200

300

400

500

600

Distance from North Tower (feet)

Vo

ltag

e (k

V)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-29 Plots of measured and calculated open-circuit voltage along the ADSS cable for Configuration 2.

6-31

Configuration 2: Induced Current (53.8kΩΩΩΩ/m)

0

0.5

1

1.5

2

2.5

3

3.50

100

200

300

400

500

600

Distance from North Tower (feet)

Cu

rren

t (m

A)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-30 Plots of measured and calculated induced current along the ADSS cable for Configuration 2.

6-32

Configuration 2: Induced Voltage (53.8kΩΩΩΩ/m)

0

1

2

3

4

5

6

7

8

9

100

100

200

300

400

500

600

Distance from North Tower (feet)

Vo

ltag

e (k

V)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-31 Plots of measured and calculated induced voltage along the ADSS cable for Configuration 2.

6-33

Configuration 2: Current to Ground (53.8kΩΩΩΩ /m)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.50

13.1

52.5

91.9

131

171

210

249

289

308

348

387

427

466

505

545

584

597

Distance from North Tower (feet)

Cu

rren

t (m

A)

Figure 6-32 Plot of measured short-circuit current to ground along the ADSS cable for Configuration 2.

6-34

Configuration 2: Unperturbed Space Potential (53.8kΩΩΩΩ/m)

0

2

4

6

8

10

12

14

16

18

20

0

100

200

300

400

500

600

Distance from North Tower (feet)

Vo

ltag

e (k

V)

Figure 6-33 Plot of calculated unperturbed space potential along the ADSS cable for Configuration 2.

6-35

Configuration 3

As discussed above and in Section 5, the phasing of the overhead energized conductors can make a significant difference on the profiles of electrical parameters along the length of an ADSS cable in some situations. This implies that phase arrangement A-B-C versus A-C-B could make a difference in the life expectancy of an ADSS fiber optic cable. Note also that this difference is equivalent to placing an ADSS cable in its mirror-image position. In other words, it may be that the side of a structure an ADSS cable is mounted makes a difference in its life expectancy. Obviously, this would be an extremely important conclusion.

For Configuration 3, the researchers searched, via the 3D analytical model, for a test line configuration that would exhibit this feature (as it will be called here). Standard flat configurations, such as for Configurations 1 and 2, do not exhibit this feature. As discussed in Section 5, this feature can be very pronounced for certain delta configurations (which corresponds to an actual situation on a 500 kV line in the northwest).

The results of this search indicated that for the 600 ft. test line, the phase configuration had to be a relatively compact arrangement to observe the feature. Also, it was found that the feature was extremely sensitive to the line’s exact configuration, including relative sags of the phase conductors and the ADSS cable.

Figure 6-34 provides an illustration of the test configuration at the structure attachment point for Configuration 3; north structure, looking north. The phase conductors were in a delta configuration with 18 ft. separating phases A and B along the bottom of the delta, and phase C being 18.5 ft. higher, at the apex of the delta. The ADSS cable was mounted 2 ft. off center, and 2 ft. higher than the bottom phase conductors. Tests were performed at 230 kV.

Figure 6-35 shows a side view of Configuration 3. The phase conductors were sagged at 3.75%, and the ADSS cable was sagged at 2.86%. However, in a real-life situation it is difficult to establish a given geometry, such as this one, with an extreme degree of accuracy. This is especially true when its realized that temperature changes of the conductors and ADSS cable can result in sag changes, especially with the black ADSS cable when there is, or isn’t, direct sunlight. Also, as mentioned above, the calculated results for this configuration are very sensitive to the geometry of the line. This is discussed in more detail below.

6-36

Section View of Configuration 346

’-6”

18’-

6”

Ground Level

ADSS Cable

Phase C

230kV Line Voltage

Phase A Phase B

Cross Arm

36 foot long

11’-0” 7’-0”

North Tower

(looking north)

Cross Arm

36 foot long

Cross Arm

55 foot long

2’-0”

2’-0

”

Figure 6-34 View of Configuration 3 at the north structure, looking north.

6-37

Profile View of Configuration 348

’-6”

46’-

6”

13’-

0”

Ground Level

Nor

th T

ower

Sou

th T

ower

Upper Cross Arm

600’-0”

Two Phase Conductors (sag=3.75%)

ADSS Cable (sag=2.86%)

One Phase Conductor (sag=3.75%)

65’-

0”

Middle Cross Arm

24’-

0”

31’-

4” 42’-

6”

18’-

6”

Midspan

(North TowerGround Level)

Figure 6-35 Side view of Configuration 3.

Figures 6-36, 6-37, and 6-38 show plots of measured and calculated open-circuit voltage, induced current, and induced voltage, respectively, along the ADSS cable. Figure 6-39 shows a plot of measured short-circuit current to ground.

Referring to Figure 6-36, it appears that the analytical model does a reasonably good job at predicting measured values of open-circuit voltage, however, not as good as was done in the flat line geometries of Configurations 1 and 2. Figure 6-37 shows a very good correlation between calculated and measured induced current.

Referring to Figure 6-38, it can be seen that there is a significant difference between the measured and predicted values of induced voltage, especially near the middle of the test span. This is consistent with the results shown above for Configuration 2. As was mentioned above, the reasons for this discrepancy are unknown at this time, but it is suspected to be a flaw with the measurement process. This will be investigated further in the future.

Referring to Figure 6-39, the measured short-circuit current to ground peaks at just over 2 mA, which is well within safety limits for live-line work.

6-38

Configuration 3: Open-Circuit Voltage (53.8kΩΩΩΩ/m)

0

1

2

3

4

5

6

7

8

9

100

100

200

300

400

500

600

Distance from North Tower (feet)

Vo

ltag

e (k

V)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-36 Plots of measured and calculated open-circuit voltage along the ADSS cable for Configuration 3.

6-39

Configuration 3: Induced Current (53.8kΩΩΩΩ/m)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

20

100

200

300

400

500

600

Distance from North Tower (feet)

Cu

rren

t (m

A)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-37 Plots of measured and calculated induced current along the ADSS cable for Configuration 3.

6-40

Configuration 3: Voltage to Ground (53.8kΩΩΩΩ/m)

0

0.5

1

1.5

2

2.5

3

3.5

40

100

200

300

400

500

600

Distance from North Tower (feet)

Vo

ltag

e (k

V)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-38 Plots of measured and calculated induced voltage along the ADSS cable for Configuration 3.

6-41

Configuration 3: Short-Circuit Current (53.8kΩΩΩΩ/m)

0

0.5

1

1.5

2

2.50

100

200

300

400

500

600

Distance from North Tower (feet)

Cu

rren

t (m

A)

Figure 6-39 Plot of measured short-circuit current to ground along the ADSS cable for Configuration 3.

6-42

Configuration 4

The geometry for Configuration 4 was not changed from Configuration 3, but the phasing was changed from A-C-B to A-B-C. Figures 6-40, 6-41, and 6-42 show plots of measured and calculated open-circuit voltage, induced current, and induced voltage, respectively. Figure 6-43 shows a plot of measured short-circuit current to ground.

Configuration 4: Open-Circuit Voltage (53.8kΩΩΩΩ/m)

0

2

4

6

8

10

12

14

0

100

200

300

400

500

600

Distance from North Tower (feet)

Vo

ltag

e (k

V)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-40 Plots of measured and calculated open-circuit voltage along the ADSS cable for Configuration 4.

6-43

Configuration 4: Induced Current (53.8kΩΩΩΩ/m)

0

0.5

1

1.5

2

2.50

100

200

300

400

500

600

Distance from North Tower (feet)

Cu

rren

t (m

A)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-41 Plots of measured and calculated induced current along the ADSS cable for Configuration 4.

6-44

Configuration 4: Voltage to Ground (53.8kΩΩΩΩ/m)

0

0.5

1

1.5

2

2.5

3

3.5

40

100

200

300

400

500

600

Distance from North Tower (feet)

Vo

ltag

e (k

V)

Measured Calculated (using North gnd. Clearance) Calculated (using South gnd. Clearance)

Figure 6-42 Plots of measured and calculated induced voltage along the ADSS cable for Configuration 4.

6-45

Configuration 4: Short-Circuit Current (53.8kΩΩΩΩ/m)

0

0.5

1

1.5

2

2.5

30

100

200

300

400

500

600

Distance from North Tower (feet)

Cu

rren

t (m

A)

Figure 6-43 Plots of measured short-circuit current to ground along the ADSS cable for Configuration 4.

Comparing the calculated values of open-circuit voltage between Configurations 3 and 4 (Figures 6-40 and 6-36), it can be seen that the change of phasing was expected to give a significant difference. The same is true for induced current (see Figures 6-41 and 6-37). However, referring to Figures 6-40 and 6-41, it can be seen that the measured and calculated values of open-circuit voltage and induced current compare very poorly for Configuration 4. The reason for this is believed to be due to changes in geometry over the course of time due to changes in temperature, and also probably due to the flexing of poles at the structures and stretching of the conductors and ADSS cable because of tension.

Careful measurements of the line’s geometry indicated that this was true. In fact, it was found that the geometry could change throughout the course of a single day due to sunlight and other ambient conditions, i.e. the geometry could change during the measurement process. For most configurations, such as the flat ones of Configurations 1 and 2, these relatively small changes in

6-46

geometry make little difference. However, as mentioned above, for the compact delta geometries of Configurations 3 and 4, the results are extremely sensitive to the geometry. This is demonstrated in a sensitivity analysis below. Considering this, it is concluded that the results of Configuration 4 are not reliable.

Sensitivity Analysis

It is suspected that errors in the measured geometry of Configuration 4 resulted in calculated values of electrical parameters that are not true to the actual geometry that existed at the time of measurement. For instance, it was observed that ground clearances could easily change by +/- 1 ft. between the beginning and end of a given workday. It is not really known how much change could have occurred over a several day period. All the measurements for a given configuration took about a week of time.

A sensitivity analysis was performed with Configuration 4 to demonstrate the sensitivity. Figures 6-44 through 6-51 show plots of calculated open-circuit voltage and induced current for various scenarios.

Figure 6-44 shows calculated open-circuit voltage when the phase conductors are fixed with a sag of 3.50%, and the ADSS cable varies +/- 0.5% in sag from 2.75% (corresponding to a sag of 16.5 ft., +/- 3 ft.). As can be seen, the open-circuit voltage profile is very dependent on the exact value of ADSS sag. Figure 6-45 shows similar plots when the phase conductor sag is held fixed at 3.75%.

Figures 6-46 and 4-47 show similar plots where the ADSS cable’s sag is held fixed at 2.75% and 3.00%, respectively, and the phase conductor sag is allowed to vary.

Figures 6-48 through 6-51 show similar plots for induced current. Conclusions are the same.

6-47

Sensitivity of Full-Scale Test Line(phase conductor sag=3.50%, ADSS sag variable)

0

2

4

6

8

10

12

14

0 100 200 300 400 500 600

Distance from North Tower (feet)

Op

en-C

ircu

it V

olt

age

(kV

)

Sag=2.25%

Sag=2.50%

Sag=2.75%

Sag=3.00%

Sag=3.25%

Figure 6-44 Plots of open-circuit voltage for various ADSS cable sags; phase conductor sag held fixed.

Sensitivity of Full-Scale Test Line(phase conductor sag=3.75%, ADSS sag variable)

0

2

4

6

8

10

12

14

0 100 200 300 400 500 600

Distance from North Tower (feet)

Op

en-C

ircu

it V

olt

age

(kV

)

Sag=2.25%

Sag=2.50%

Sag=2.75%

Sag=3.00%

Sag=3.25%

Figure 6-45 Plots of open-circuit voltage for various ADSS cable sags; phase conductor sag held fixed.

6-48

Sensitivity of Full-Scale Test Line(phase conductor sag variable, ADSS sag=2.75%)

0

2

4

6

8

10

12

14

0 100 200 300 400 500 600

Distance from North Tower (feet)

Op

en-C

ircu

it V

olt

age

(kV

)

Sag=3.00%

Sag=3.25%

Sag=3.50%

Sag=3.75%

Sag=4.00%

Figure 6-46 Plots of open-circuit voltage for various phase conductor sags; ADSS cable sag held fixed.

Sensitivity of Full-Scale Test Line(phase conductor sag variable, ADSS sag=3.00%)

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500 600

Distance from North Tower (feet)

Op

en-C

ircu

it V

olt

age

(kV

)

Sag=3.00%

Sag=3.25%

Sag=3.50%

Sag=3.75%

Sag=4.00%

Figure 6-47 Plots of open-circuit voltage for various phase conductor sags; ADSS cable sag held fixed.

6-49

Sensitivity of Full-Scale Test Line(phase conductor sag=3.50%, ADSS sag variable)

0

0.5

1

1.5

2

2.5

3

3.5

0 100 200 300 400 500 600

Distance from North Tower (feet)

Ind

uce

d C

urr

ent

(mA

)

Sag=2.25%

Sag=2.50%

Sag=2.75%

Sag=3.00%

Sag=3.25%

Figure 6-48 Plots of induced current for various ADSS cable sags; phase conductor sag held fixed.

Sensitivity of Full-Scale Test Line(phase conductor sag=3.75%, ADSS sag variable)

0

0.5

1

1.5

2

2.5

0 100 200 300 400 500 600

Distance from North Tower (feet)

Ind

uce

d C

urr

ent

(mA

)

Sag=2.25%

Sag=2.50%

Sag=2.75%

Sag=3.00%

Sag=3.25%

Figure 6-49 Plots of induced current for various ADSS cable sags; phase conductor sag held fixed.

6-50

Sensitivity of Full-Scale Test Line(phase conductor sag variable, ADSS sag=2.75%)

0

0.5

1

1.5

2

2.5

3

3.5

0 100 200 300 400 500 600

Distance from North Tower (feet)

Ind

uce

d C

urr

ent

(mA

)

Sag=3.00%

Sag=3.25%

Sag=3.50%

Sag=3.75%

Sag=4.00%

Figure 6-50 Plots of induced current for various phase conductor sags; ADSS cable sag held fixed.

Sensitivity of Full-Scale Test Line(phase conductor sag variable, ADSS sag=3.00%)

0

0.5

1

1.5

2

2.5

3

3.5

4

0 100 200 300 400 500 600

Distance from North Tower (feet)

Ind

uce

d C

urr

ent

(mA

)

Sag=3.00%

Sag=3.25%

Sag=3.50%

Sag=3.75%

Sag=4.00%

Figure 6-51 Plots of induced current for various phase conductor sags; ADSS cable sag held fixed.

6-51

Summary

In summary, a full-scale 3-phase test line was successfully constructed which included an actual ADSS fiber optic cable. The ADSS cable was “artificially” contaminated with a copper wire and fixed resistors.

Calculations and measurements of open-circuit voltage, induced current, induced voltage, and measurements of short-circuit current to ground were made. The measurement technique for all these parameters was relatively basic and considered reliable, except for the measurement of induced voltage.

There was some uncertainty from the very beginning about the effect of the grounded shield of the multimeter’s leads on the induced voltage. There was also some uncertainty about the loading effect of the multimeter’s voltage divider on the induced voltage, although this is not believed to be the cause of any significant error for an ADSS cable which was “very heavily polluted”.

However, of the four electrical parameters considered, induced voltage is the least relevant. Open circuit voltage and induced current are the governing factors for dry band arcing, and short-circuit current to ground is important for live-line worker safety considerations.

The comparisons of calculated and measured parameters indicate that the quasi-3D analytical model is a viable tool for making predictions of those electrical parameters (except for induced voltage, where there is some uncertainty). For Configuration 4 calculations and measurements did not agree. However, a sensitivity analysis proved that the calculated results are extremely sensitive to the line’s geometry. The line’s geometry had some natural fluctuations due to temperature changes, etc., and there are also natural errors in the geometry measurements. Therefore, the poor results from Configuration 4 are believed to be inconclusive at this time.

Further tests in the future are expected to resolve the few remaining uncertainties discussed above.

7-1

7 LIVE INSTALLATION, MAINTENANCE AND INSPECTION OF ADSS CABLE

Introduction

Installation (stringing) of all-dielectric self-supporting (ADSS) cable can be carried out live (i.e., without de-energizing the line) since new ADSS cable is non-conductive, i.e., it can be considered an insulator. The cable can be installed anywhere on transmission line structures, subject to the constraints of electric field and space potential discussed elsewhere in this report. Normally, ADSS cable is installed either below the energized phases or, in the case of a “delta” phase configuration, between the phases. These locations are preferred in order to reduce the mechanical loading on the structures. Invariably, however, ADSS cable is installed fairly close to the energized phases. Since it would not be economical to de-energize the transmission line in order to install the ADSS cable, installation is performed with the line energized and mandates the use of live working installation (stringing) procedures.

Live maintenance of ADSS cables was originally thought to be a relatively simple matter since new cable is considered to be non-conductive [1]. However, experience developed in the Lenox aging chamber [2] and field data collected by several utilities have shown that ADSS cables, after exposure to the elements (rain, contamination) can develop a certain degree of surface conduction. In fact, a casual survey of utilities indicates approximately 50-50 division of opinion: about half of surveyed utilities considers ADSS cables in service to be non-conductive and not requiring live work procedures, while the other half considers the cable to be conductive, thus requiring live work procedures for maintenance.

Also, regardless of the age of the cable, caution during both stringing and maintenance is advised to avoid potential hazards of induced voltage on metallic hardware that is used to support the ADSS cable, unless such hardware is shown to be well grounded.

While there are no general industry guidelines and approved procedures at this time for stringing and maintenance of ADSS cables, a great amount of practical experience has been collected over the last years and a proposal (from United Kingdom) to develop such guidelines has been recently circulated through IEC TC78 “Live working” [3].

Live Installation of ADSS Cable

The primary consideration during live work is the complete safety of the workers. A secondary concern is the comfort level of the workers and the avoidance of low-level electric shock, both of which could distract the workers and result in inadvertent reaction, dropping of tools and loss of precision in executing high-skill tasks.

Safety, proper level of comfort and avoidance of shock are achieved by maintaining appropriate minimum distances to the energized and/or grounded parts, the use of suitable tools and

7-2

insistence on following established safety work rules. A viable code of practice includes the following elements:

• The ADSS cable and the pulling rope connection should be light, non-metallic, flexible and suitable for use near energized transmission line components.

• Installation equipment, such as tensioning gears, should be calibrated and adjusted for tensions suitable for ADSS cable.

• Adequate working distances (the Minimum Approach Distances, or MAD) to energized parts should be kept at all times.

• All equipment should be installed securely.

• Temporary ground leads should be used as needed.

• Installation crews should have suitable general training and should be adequately trained in the specific tasks to be performed

Tests were conducted on a full-scale mockup of a 345 kV line at the EPRI Engineering and Test Center – Lenox to study the conditions (voltages and currents) on the ADSS cable during live stringing. It was found that, when the cable is new and clean (as it would be during stringing), the current that could be available to flow through a worker is practically insignificant. Tests on installed cable show that the current is still small even after several months of exposure of the ADSS cable to the elements. Further studies need to be carried out to determine under what conditions (length of exposure, for example) the current could reach significant levels.

Equipment

ADSS cable is normally supplied on either wooden or metal reels. The type and construction of the reel stand determine the methods and tools required for handling. Reels must be supported by either an axle or from above with the help of a spreader bar. The stand should have a tensioner to supply the required back tension to the cable. The stand should be selected to accommodate the cable reel dimensions and gross weight. ADSS cable may be pulled directly from the reel stand if the installation process involves slack stringing methods that allow only minimal tension to be applied.

Tensioning and Pulling Requirements

Mechanical strength and bending behavior are extremely important considerations in the installation of ADSS cable. To ensure the integrity of the cable, i.e., to avoid the breaking of cable fibers under normal operating conditions, prior to installation the cable must pass a quality control screening test of at least 5 N (1.1 lbs.). This means that the whole fiber has been exposed to a force of 5 N (1.1 lbs.) so that small flaws and defects in the primary coating are detected by a breakage of the fiber.

Pulling rates of 1.5 to 8 km/hr. (1 to 5 miles/hr.) allow the cable to pass through the sheaves smoothly. Once the cable has started to move, progress of the cable should be maintained at a

7-3

constant rate until the whole cable segment has been pulled into place. At all times during the pull, the operator of the tensioner should monitor the tension in the cable to ensure that the maximum pulling tension is not exceeded. Generally, the pulling tension of the cable should not exceed 50% of the maximum initial sagging tension.

Pulling Lengths

The selection of pull, tension, anchor, and splicing sites involves many factors ranging from system design issues to logistics and equipment capabilities. Segment lengths are governed by the allowable number of splices, accessibility, obstacles in the right-of-way, and cable reel lengths. Other critical factors include the maximum load the cable can handle, the maximum permissible structural loads, and the availability of adequate grounding systems wherever necessary. Typical pulling lengths of ADSS cable range from 2 to 5 km (1 to 3 miles) depending on the weight and diameter of the cable to be installed.

Installation Methods

The installation of ADSS cable normally requires ground access along the power line (similar to stringing of conductors), even though tension stringing techniques are generally employed. Suitable running blocks, which are usually recommended by the ADSS cable manufacturer, are suspended from the attachment points. A pilot rope is then manually threaded through all the running blocks in the section to be strung. During installation the puller and tensioner should be positioned so as to prevent the cable from coming in contact with the ground (thus potentially being scratched) as it moves between the equipment and the first tower. Additionally, it is recommended that a large block be used at the first structure to minimize bending stresses in the cable during installation.

The pulling winch and back tensioner are positioned at the first and last tower, respectively. As the pilot rope is pulled, it draws the cable through the running blocks, and the ADSS drum (which is located at the back tensioner) provides sufficient back tension to keep the cable clear of the ground and other obstructions. Once the cable has been pulled in place, the tension is adjusted to achieve the correct sag-tension parameters. The cable is then clamped off at the end of the first tension section, and the process is repeated down the line.

Electric utilities have also developed methods for installing ADSS cables on multiple-circuit overhead power lines without taking an outage on adjacent circuits. These methods usually employ helicopters and cradle blocks. Typically, the cable is pulled using a previously installed light rope at low tension. With this method of installation there is no need for circuit outages, and the effects of roads, railways, low voltage lines and other obstructions are minimized.

Previous experience with stringing ground and phase wires indicates that significant induced currents (as high as 500 A) can occur when stringing is performed near other energized lines [4, 5, 6]. Although the ADSS cable is non-conducting and will not experience high induced currents, the pulling rope, especially when wet, may burn up even in the presence of small currents.

Hence, suitable precautions need to be taken. These normally include the use of “running grounds,” i.e., roller assemblies that allow the cable (and rope) being pulled to pass through well-grounded rollers under sufficient tension to maintain good continuous contact. The running

7-4

ground assemblies are grounded to structure steel and/or driven ground rods. These procedures are based on conductor stringing guidelines [7 – 10] and the assumption that the ADSS cable is treated as a conductor.

On the other hand, one utility recently experienced a serious failure during stringing when the metallic pulleys were grounded. Initially the utility used regular metallic hardware that was grounded to the poles. Running grounds were not used. The insulating rope was pulled through, and the ADSS cable was pulled part way when work was stopped for the weekend. When the rope became wet because of rain, it burned through due to induced current and the ADSS cable dropped to the ground. To solve this problem, all hardware was insulated from the poles, and all other possible connections to ground were eliminated. Installation of the ADSS cable was completed successfully.

Installation of ADSS cable presents two other issues:

• Handling and installation of metallic hardware in the vicinity of live lines

• Training of workers in fiber optics and live working These two issues require totally dissimilar skills and training, but they are brought together during installation of ADSS cable on energized lines. At this time, workers are typically trained in one or the other, but not both. Often, in fact, the optical aspect of the work is the specialty of workers in communications-related departments, while live installation skills are available in the operations and maintenance departments.

To resolve this dichotomy, utilities may wish to invest in training special crews to perform both functions or to control very strictly the activities and access of the two groups. For example, one utility utilized maintenance crews skilled in live working to install the cables on structures under live conditions while restricting the communications workers from approaching the structures. Once the structure work was completed, the communications workers could make the necessary connections to the cables on the ground. As mentioned earlier, live installation guidelines are now under development [3].

Live Maintenance of ADSS Cable

Fiber optic system problems are typically dealt with on two different levels. A lowering of the transmission quality of fiber optic communication is a minor problem, while the loss of transmission capability requires immediate remedial action by the system operator. The following transmission characteristics of optical fiber and terminal equipment are usually monitored:

• End-to-end power loss

• Attenuation in optical fiber

• Attenuation at splices and/or connectors

• Multiplexer characteristics

7-5

Many of the electrical safety issues that are of concern during maintenance and repair of ADSS cables are related to the fact that the operations often bring together two previously unrelated technologies and skill sets:

• Fiber optic technology

• Live working technology

• As mentioned in connection with installation of the ADSS cables, communications workers are typically not trained in live working procedures, and live working crews are not skilled in handling delicate fiber optic cables, equipment or components. As mentioned earlier, live installation guidelines are now under development [3].

Types and Repair of Failures

In general, operating experience for all types of fiber optic cable and any associated hardware has been good. However, some problems have been reported with all cable types and fittings. Most of the major problems associated with ADSS cable installed on lower voltage systems appear to be related to galloping, damage of the cable sheath from bird pecking and bird claws, and broken fibers. Most of the major problems associated with ADSS cable at higher circuit voltages are related to corona cutting, tracking and arcing.

Usually, an alarm indication of a broken fiber or change of light signature is required to identify a defect in any of the fibers. Consequently, an interruption (i.e., potential mechanical failure) of optical transmission in the whole cable is usually indicated by a large number of fibers detected to be defective. However, as long as the number of undamaged fibers exceeds the number of used fibers, the repair can be planned but does not have to be performed immediately upon recognition of the problem. The repair can therefore be delayed until other maintenance operations are required. On the other hand, if the communication system is using all available fibers, repairs must usually be performed without delay. Consequently, it is a good practice to install ADSS cables with considerably fiber redundancy to allow re-assignment of fiber in case of failure.

If the defect is located at a splice, the repair can usually be done with the remaining part of the optical cable in service. However, if the defect is located along the power line, the cable usually must be taken out of service to allow for the repair. Recently, manufacturers have developed a few midspan joints, but they are not commonly used. Typically, a midspan repair requires both significant resources and a circuit outage, which makes the cost of this technology prohibitive. Under these circumstances it is usually more effective to replace a section of cable at locations where splicing can be facilitated.

Safety Related Issues

ADSS cables require special safety precautions as a result of their tendency to conduct leakage currents when their surface is contaminated. Reference 11 presents a simplified method of calculating the range of currents that could be experienced by a worker touching an ADSS cable installed on an energized transmission line. This question can be addressed by considering the geometry shown in Figure 7-1.

7-6

ADSSV=0

PHASE CONDUCTORS

TOWER

V=0

TOWER

Figure 7-1 Calculating the current a worker experiences when touching an ADSS cable with the power line energized.

Short-circuit contact current - calculations

The contact current through the worker in Figure 7-1 can be calculated by identifying a Thevenin equivalent for the terminals between the worker’s hand and ground. The open circuit voltage Vw

oc(z) for this equivalent circuit is the voltage induced at that point along the cable prior to the contact. The Thevenin impedance is the parallel combination of the input impedance to the left and that to the right. Details of the calculations are included in Reference 11.

The low frequency impedance of an uninsulated worker is typically 500 - 1500 Ω, which is small compared to typical Thevenin impedances for the conditions in Figure 7-1. Thus, the short-circuit current is a realistic estimate of the current flowing through a worker in contact with the ADSS cable.

It should be recalled that the level at which current can be perceived by a person is approximately 1 mA [12, 13]. Although there are no guidelines for permissible current level for live work on ADSS installations, it is reasonable, as a first attempt, to propose to limit the contact current to the level of 5 mA, which is the National Electrical Safety Code (NESC) limit for currents induced on vehicles near energized overhead transmission lines. Figure 7-2 shows the calculated variation of the short-circuit contact current as a function of distance from the structure for a set of typical ADSS cable jacket surface resistance values for a long span (300 m). For a smaller span length, the maximum short-circuit current is smaller, as shown in Figure 7-3.

7-7

0

2

4

6

8

0 50 100 150

NESC LIMIT - 5 mA

PERCEPTION THRESHOLD - 1 mA

R = 105

R = 106

R = 107

DISTANCE ALONG CABLE (m)

SH

OR

T C

IRC

UIT

CO

NT

AC

T C

UR

RE

NT

MA

GN

ITU

DE

(m

A)

Figure 7-2 The short-circuit contact current on an ADSS cable (2L = 300 m).

0

1

2

3

4

5

0 15 30 45 60 75

PERCEPTION THRESHOLD - 1 mA

R = 105

R = 106

R = 107

DISTANCE ALONG CABLE (m)

SH

OR

T C

IRC

UIT

CO

NT

AC

T C

UR

RE

NT

MA

GN

ITU

CE

(m

A)

Figure 7-3 The short-circuit contact current on an ADSS cable (2L = 150 m).

Note that these calculations are for a space potential of 20 kV. For other space potentials, the results can (roughly) be scaled by the factor VSP(2D)/20 kV. Also shown on these figures are the 1 mA current level (at which current can be perceived) and the 5 mA maximum level taken from the NESC. Note first that the contact current near the towers is in reasonable agreement with that calculated using equation (32) of Reference 11, which predicts 3.3, 1.0 and 0.33 mA respectively for R = 105, 106 and 107 Ω/m.

Note also that there is a slight increase in these values as the tower spacing becomes smaller. For R ≥ 107 Ω/m, it is clear that the contact current anywhere along the cable will not be

7-8

perceptible unless the space potential is increased to 30 kV. The fact that the contact current distribution is flat over most of its range is consistent with the upper bound discussed in connection with equation (20) of Reference 11. For R = 106 Ω/m, the contact current is everywhere less than 5 mA. However, it can exceed 1 mA for larger tower spacings and points away from the towers.

For R = 105 Ω/m, the situation is a bit more complex. For the shorter distances between towers, the contact current will not exceed the NESC threshold anywhere along the cable. However, for 2L = 300 m, the contact current exceeds 5 mA but only for distances greater than 50 m from the tower. If R is larger than 105 Ω/m and if work is performed within 50 m of the towers (which would normally be the case), then the NESC value is not violated.

Short-circuit contact current – measurements on full-scale mock-ups

Tests were conducted on a full-scale mockup of a 345 kV line at the EPRI Engineering and Test Center – Lenox to study the conditions (voltages and currents) on the ADSS cable during live stringing. The full-scale mock-up is described elsewhere in this report and is shown also in Figure 7-4. A wooden reel of ADSS cable was used on a makeshift tensioner shown in Figure 7-5.

Figure 7-4 General view of the full-scale mock-up for measuring short-circuit contact current during live stringing of ADSS cable.

7-9

Typical stringing blocks and running grounds were used, as shown in Figures 7-6 and 7-7. The running grounds were connected to a driven ground rod along with grounds of other equipment (the tensioner and vehicles), see Figure 7-8. A non-conductive pulling rope was attached to the ADSS cable using callum grips, see Figure 7-9.

Figure 7-5 View of the ADSS cable reel on a makeshift tensioned used for live stringing of the cable.

7-10

Figure 7-6 Typical stringing blocks used for live stringing of the ADSS cable.

7-11

Figure 7-7 Typical stringing running grounds used for live stringing of the ADSS cable.

7-12

Figure 7-8 Photograph showing the connection of running grounds and other equipment grounds to a driven ground rod for live stringing of the ADSS cable.

7-13

Figure 7-9 Non-conductive pulling rope attached to the ADSS cable using callum grips for live stringing of the ADSS cable.

The short-circuit contact current was measured near the wooden H-frame structure (which was covered with a metal mesh to represent a steel structure). The current measuring instrument (Extech 383274 multimeter with data logging capability) was inserted into a metal sphere for shielded from electric field, see Figure 7-10. The opening in the sphere facilitated reading the meter indication from the ground.

7-14

Figure 7-10 Extech 383274 multimeter in a shielding metal sphere for measuring the short-circuit contact current from the ADSS cable

The connection of the meter lead to the ADSS cable consisted of two or three turns of #14 copper wire, or of a layer of aluminum foil about 10 cm (4”) long wrapped around the cable to represent the grip of a worker’s hand, see Figure 7-11. Measurements of the current in the ground cable of the running ground were also made.

7-15

Figure 7-11 Aluminum foil wrapped around the ADSS cable to represent the worker’s hand.

Current measurements were made for the following stringing conditions:

• ADSS cable pulled ¼ of the span length (i.e., ¾ of the span length covered by the non-conducting rope)

• ADSS cable pulled ½ of the span length (i.e., ½ of the span length covered by the non-conducting rope)

• ADSS cable pulled ¾ of the span length (i.e., ¼ of the span length covered by the non-conducting rope)

• ADSS cable pulled to full span length (i.e., no non-conducting rope in the span)

In addition, current measurements were made with the ADSS cable pulled to full span length and deflected horizontally by about 1 m (3’) to one side. This represents the effect of wind swinging the ADSS cable out of its intended location and moving the cable closer to one of the energized phase conductors. Although live work such as stringing would not be performed in high wind conditions, maintenance or inspection may still need to be done in such cases.

7-16

Results of short-circuit contact current measurements on full-scale mock-ups

Essentially, all measured currents during stringing were very, very small, in the order of several tens of µA at most. This is well below the normal perception level for adults of about 1 mA. Considering that workers (both linemen and communications) would wear gloves (often insulating gloves), the danger to the worker from the available short-circuit contact current is minimal during stringing. The reason for the very low current values measured during the stringing exercise in Lenox is that the ADSS cable used for stringing is new and not contaminated (surface resistance R = 107 Ω/m or higher). This finding supports the calculated current values for high R values predicted in Ref. 11. Calculations of the short-circuit current were also performed based on the space potential values at the cable location, using the method described in the EPRI “Red Book” [12]. Good agreement between measured and calculated values was obtained.

During maintenance and inspection of installed cable, however, the surface resistance can be significantly lower and the short-circuit contact current would be correspondingly higher, as predicted by calculations. In this case, care must be taken during work to avoid hazards to the worker. As predicted by calculations, current values can reach a few mA, which is above the perception level and approaches the NESC value of 5 mA. It is not known whether or not the let-go level of about 10 mA for adults can be reached on ADSS cables after several years of service (note, however, the a rough rule of thumb suggests that surface current of 1 mA may be sufficient to damage the cable jacket). Tests on the installed cable in Lenox have shown that the surface current is still small even after several months of exposure of the ADSS cable to the elements. Further studies need to be carried out to determine under what conditions (length of exposure, for example) the current could reach significant levels.

The tests on the full-scale mock-up in Lenox also indicates that the current is very sensitive to capacitive pickup by the measuring instrumentation. Hence, measurement of the short-circuit contact current on actual ADSS installations needs to be performed with great care and detailed understanding the possible measurement problems.

Inspection of ADSS Cable

The ADSS cable, when installed in a high-voltage environment, can suffer damage due to corona and dry band arcing. Such damage is usually very destructive and eventually catastrophic if allowed to persist for an extended period of time. However, initial indication of damage or destructive activities may be detectable prior to catastrophic failure, and it is possible that damage could be arrested and corrected if detected sufficiently early. Hence, inspection of the ADSS cable in service and detection of incipient damage are important topics in the operation of fiber optic installations.

To be economically viable, inspection and incipient failure detection should be performed with the power system energized. Furthermore, remote inspection methods are invariably less costly and intrusive that other approaches such as contact or climbing inspection.

Some types of defects cannot be detected since they do not produce external symptoms or evidence that can be seen easily, especially using remote inspection methods. For example, moisture ingress into the ADSS cable through a jacket puncture (due to bird claw damage) may

7-17

not produce any external evidence. Internally to the cable, however, current flow can occur and lead to complete destruction (burn through) of the cable.

On the other hand, corona activities on armor rod tips or dry band arcing on the cable jacket can produce jacket damage (discoloration, pitting) that is very difficult to detect remotely. One utility has detected significant cable jacket damage at armor rod tips during climbing inspection. The configuration of armor rod tips is shown in Figure 7-12. Repair was attempted by lifting the tips of the armor rod using a screwdriver and then covering the damaged area with black electrician’s tape. Sparking between the armor rod tips and the screwdriver was observed during this operation, confirming presence of significant potential differences between the grounded armor rod and the ADSS cable jacket, as predicted by theoretical models.

Recently developed technology to view corona in daylight shows promise for detecting the presence of corona on armor rods and also possibly the occurrence of dry band arcing. The feasibility of applying this new technology to ADSS cable installations needs to be investigated.

Figure 7-12 Typical configuration of armor rod tips that can result in corona at the tips.

7-18

Inspection Tools

A large variety of tools, mainly used or developed for inspection of transmission line insulators [14, 15], can be used for inspection of ADSS cable installations, including:

• Binoculars with high-quality optics

• Spotting scope (20X – 60X)

• 35 mm camera

• Image intensifier (night vision technology)

• Ultrasonic listening device

• Infrared imaging system

• Corona viewing device in daytime (DayCor)

• Audible noise (AN) measurement equipment

• RIV measurement equipment

• Continuous coverage radio receiver (30 kHz to 200 MHz)

• Power frequency electric field measurement equipment

• Power frequency magnetic field measurement equipment

It is useful to group the above-listed tools according to the objective of the detection function, as shown in Table 4-1.

Table 4-1 Classification of inspection tools.

Symptom Examples Arsenal of tools

Visual evidence of damage Jacket puncture, discoloration

Binoculars, spotting scope, camera

Evidence of resulting electrical activities

Corona, RIV, AN, localized heating

Binoculars, spotting scope, camera, image intensifier, ultrasonic device, infrared imaging, corona viewing device (DayCor), AN equipment, RIV equipment, radio receiver

Change in the electrical environment

Power frequency magnetic field due to leakage current

Electric field meter, magnetic field meter

A thorough study of these devices should be undertaken.

7-19

Inspection techniques

Essentially two techniques for using the above inspection tools are worthy of consideration:

• Totally remote contact-free. This includes walking, driving, airborne inspection, and climbing the structures without making contact with the parts under inspection. The simplest example of this technique is the use of binoculars. Climbing has the potential for greatest accuracy but is quite costly and slow, and requires ground access to the ROW along the entire length of the line. The fixed-wing airborne method is fast, but considerable development is needed to ensure proper accuracy and sensitivity for detecting localized damage. The use of helicopters allows the observer to stop (hover) and concentrate on suspected trouble spots, but at significantly increased cost. Ground-based methods such as walking and driving are both slow and offer low sensitivity (due to the large distance and limited observation positions) and require ground access to the ROW along the entire length of the line.

• Close approach and contact method. This includes close-up inspection and installation of detecting devices such as a clamp-on ammeter. Depending on the particular tools used, high accuracy and sensitivity is achievable, but these techniques require a close approach to and contact with the ADSS cable, which should be considered an energized part based on earlier discussions. Hence, live working procedures need to be followed, and the safety of the worker must be assured in all situations, especially under wet conditions (for example, morning dew) when dry band arcing is likely to occur. Further study of these techniques is required before they can be considered viable inspection options.

7-20

References

1. Fiber Optic Cables in Overhead Transmission Corridors: A State-of-the-Art Review, EPRI, Palo Alto, CA: November 1997. Report TR-108959.

2. Accelerated Aging Tests of ADSS Optic Cables, EPRI, Palo Alto, CA: August 2000.

3. IEC document 78/348/NP, “Live-working – Guidelines for the installation and maintenance of optical cabling – Part 1: Overhead high voltage power supply lines”

4. Jurdens, Haag, and Buchwald, “Experience with Optical Fibre Aerial Cables on High Voltage Power Lines,” CIGRÉ, TC 22-11, pp. 1-10, 1988.

5. Optical Fibre Planning Guide for Power Utilities, CIGRÉ WG 35.04, 1995, Report.

6. Kuffel, and Zaengl, High-Voltage Engineering, Pergamon Press, New York, NY, 1984, ISBN-0-08-024213-8.

7. IEEE, Guide to the Installation of Overhead Transmission Line Conductors, IEEE-SA 524-1992, Institute of Electrical and Electronics Engineers.

8. IEEE, Guide to Grounding during the Installation of Overhead Transmission Line Conductors: Supplement to IEEE Guide to the Installation of Overhead Transmission Line Conductors, IEEE-SA 524a-1993, Institute of Electrical and Electronics Engineers.

9. IEC 61328, Live Working – Installation of Transmission Line Conductors and Earthwires – Stringing Equipment and Accessory items, International Electrotechnical Commission.

10. IEC 61911, Live Working – Installation of Distribution Line Conductors – Stringing Equipment and Accessory items, International Electrotechnical Commission

11. R.G. Olsen, “An Improved Model for the Electromagnetic Compatibility of All-Di-electric Self-Supporting Fiber-Optic Cable and High-Voltage Power Lines,” IEEE Transactions on Electromagnetic Compatibility. Vol. 41, No. 3, p. 180. August 1999.

12. EPRI, “Transmission Line Reference Book - 345 kV and Above,” EPRI, 3412 Hillview Ave., Palo Alto, CA, 1982.

13. J.P. Reilly, Principal Author, “Electric and Magnetic Field Coupling from High voltage AC Power Transmission Lines - Classification of Short-Term Effects on People,” IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, pp. 2243-2252, Nov./Dec. 1978.

14. EPRI, “Inspection and Detection Techniques: Defects in Porcelain Insulator Strings,” Final Report, TR-109451-V2, December 1997.

15. EPRI, “Application Guide for Transmission Line Non-Ceramic Insulators,” Final Report, TR-111566, November 1998.

8-1

8 ICING TESTS OF ADSS CABLES

Introduction

The work described in this test report was performed as part of the EPRI base funded project entitled Fiber Optic Cables in High Voltage Environments. The project is managed, and most of the work is being performed, at the EPRI laboratory in Lenox, Massachusetts.

At a recent Working Group (WG) meeting, it was decided that icing tests needed to be performed on an ADSS cable in a full-scale test setup. At issue is the question of whether ice can build up indefinitely on such a cable, or if a natural shedding process takes place. Specifically, there exists a hypothesis that as ice starts to form on an ADSS cable, and the cable begins to stretch under the additional weight, the bond between the cable and ice will break, and the ice will shed. Another hypothesis claims that this is not true. There are conflicting opinions coming from manufacturers and utilities.

Such tests can also provide additional data on the stress-strain relationship for ADSS cables.

Test Plan and Setup

The plan was to install an ADSS cable on a full-scale test line at the EPRI-Lenox laboratory, and wait for extended cold weather (a common occurrence at this location in the winter). The cable would be sprayed with water to form ice. The weight of the ice and the sag of the ADSS cable would be monitored.

The full-scale test span consisted of two 40 ft.-high wood poles separated by 494 ft. The test sample was a 0.708-inch diameter, 204 lb/1000 ft, 96 fiber, class B (25 kV) ADSS cable. The ADSS cable was fixed at both ends at the poles (25 ft. above ground at each pole – the 2 attachment points being very near to the same elevation) with cable pulls - hardware commonly referred to as “steel finger traps”. These cable pulls were 30 in. long and were attached to the poles as dead-ends at a height of 25 ft. from the ground (see Figure 8-1). The initial stringing tension was about 1300 lb. The specific termination hardware is irrelevant for the tests at hand, as long as the cable ends were held at fixed locations in space. Markers were placed on the cable close to the terminations so that any slippage would be detected. Also, a transit scope viewed one pole to detect any pole movement.

8-2

Figure 8-1 Photograph of a cable termination.

According to manufacturer’s specifications, with a 494 ft. span, the specified initial installed sag is 4.75 ft. This corresponds to 0.98% sag, and a tension of about 1300 lb. Figure 8-2 provides an illustration of the setup.

Figure 8-2 Illustration of the basic test setup.

8-3

To ice the cable, workers on a lift truck sprayed water through a hose nozzle, which was adjusted to a medium spray, onto the cable as the truck was driven back and forth between the two poles. The truck was kept at constant speed while the cable was being sprayed.

In order to monitor the weight of the ice formed on the cable, a separate short (107.5 inch) sample of the cable was suspended from a digital scale in a test fixture. This fixture was positioned to the side of the full-scale test setup, and was sprayed in an equivalent manner on each pass of the truck. This is illustrated in Figures 8-3 and 8-4.

Figure 8-3 The cable setup with lift truck and test fixture.

Figure 8-4 Illustration of the test fixture for monitoring weight of ice.

8-4

The Tests

Plans were made to perform the tests sometime in mid-winter when cold weather was quite certain to occur. Since the ADSS cable would be installed at well below 68 oF, the cable manufacturer was contacted for proper installation specifications. The manufacturing engineer stated that the sag and tension specifications recommended for installations at 68 oF should be used, even though the temperature was just 34 oF at the time of installation.

An ADSS cable sample was not received and strung until mid February. There was concern that the northeast would not experience consistently cold weather for the remainder of the winter. Fortunately, a cold weather pattern moved through the region during the week of February 21, 1999. This provided ideal conditions necessary to conduct the tests. The temperature remained well below freezing 24 hours per day for several days, and was not accompanied by heavy wind or precipitation as confounding factors. The test started late afternoon of February 21, and was completed mid-day of February 24.

Before the test began, the cable was cleaned with alcohol to remove any oil film that may have been present from the manufacturing process (the test sample was a brand new cable).

Figure 8-5 Spraying the cable with water.

8-5

Figure 8- 6 Spraying the ADSS cable at dusk.

The icing procedure consisted of a 3-man team passing back and forth with the lift truck along the entire length of the cable. One person operated the truck, and one performed the spraying. A third person assisted from the ground with moving the hose, taking pictures, data recording, tending the short test sample, operating lights, etc. The spray nozzle was moved slowly by hand back and forth as the lift truck slowly moved along the cable. The spray was directed well above the cable so that small water drops fell onto the cable from above.

This procedure continued almost 24 hours per day for almost 3 full days. Working conditions were quite severe – the crew having to bear extremely cold temperatures and moderate winds throughout the night while spraying a large volume of water into the air. Hot beverages and food were provided throughout the night to the test crew. Figures 8-5 and 8-6 show photographs of the spraying operation.

Data and Catenary Calculations

During each pass of the lift-truck-spraying-system along the ADSS cable, a short length of cable mounted in a test fixture was equivalently sprayed. This cable sample was 107.5 inches in length, and was suspended from a digital scale. The scale was zeroed such that only the weight of the ice was measured. The purpose was to provide an estimate of the weight-per-length of ice along the full-scale cable sample. Figures 8-7 and 8-8 show photographs of the test fixture.

8-6

Figure 8-7 Test fixture for determining the weight-per-length of ice on the cable.

Figure 8-8 Digital scale mounted in test fixture.

Throughout the tests, ambient temperature, weight-per-length of ice on the cable, cable sag, and other variables were continually monitored. These data are provided in Table 8-1. Also, the density of the ice was measured. The density of the ice was determined by dividing the mass of a small ice sample by its volume. The ice sample’s mass was 165g, as determined by its weight.

8-7

The ice sample’s volume was 200 cm3 , as determined by the volume of water displaced in a graduated cylinder. This yields a density of 0.825 g/cm3.

Table 8-1 Test Data.

Date/time deg F Sag pounds ice / ft

02/21/1999 16:54 28.45 4.75

02/21/1999 18:02 21.55

02/21/1999 18:30 20.64 0.0134

02/21/1999 18:46 20.64

02/21/1999 19:02 19.73 0.0201

02/21/1999 19:46 19.73

02/21/1999 20:02 18.8 0.0424

02/21/1999 20:06 18.8

02/21/1999 20:18 18.8

02/21/1999 20:22 18.8

02/21/1999 20:34 17.86 0.0580

02/21/1999 20:42 17.86 0.0781

02/21/1999 21:54 16.92 0.1563

02/21/1999 22:30 14.98 5.58

02/21/1999 23:54 12.99 0.2255

02/22/1999 1:54 10.93 6.67

02/22/1999 1:58 9.88

02/22/1999 3:02 8.81

02/22/1999 4:18 6.62

02/22/1999 4:38 6.62 7.75 0.2344

02/22/1999 5:26 4.34

02/22/1999 9:46 12.99

02/22/1999 9:58 12.99 12.00

02/22/1999 11:18 16.92 14.00 0.7479

02/22/1999 13:38 21.55 15.33

02/22/1999 14:18 22.44 16.42

02/22/1999 16:38 15.95 1.3284

02/22/1999 18:26 10.93 18.50 1.4266

02/22/1999 19:26 9.88

02/22/1999 20:34 8.81

8-8

02/22/1999 20:38 8.81

02/24/1999 9:30 23.33

02/24/1999 11:30 35.66 21.83 1.8664 equivalent to an ice layer of 0.92 inches

The stress-strain relationship for the ADSS sample was obtained from the manufacturer, and the sag was calculated with SAG-10 using the measured weight-per-length of ice and assuming the iced cable forms a true catenary. Sources of error in the calculations include using the measured ambient temperature as the temperature of the cable, errors in the measured weight of the ice, and perhaps slight flexing of the poles and hardware. Nevertheless, calculations and measurements agreed very well, even through the extreme sag produced in these tests. Comparisons are shown in Figure 8-9.

Note that it is possible for a manufacturer to modify the stress-strain relationship of a cable without modifying the optical fibers or jacket material. This can be accomplished my modifying the internal load carrying materials (such as reinforcing yarns, internal polymer tubes, etc.).

Two weeks after the ice had melted off the cable, the sag returned to 7.67 ft (it was 4.75 ft. at the start). This would be due to a combination of warmer temperatures, and possible elongation of the cable.

05

10152025

0 0.5 1 1.5 2

measured lb-ice/ft

Sag

(ft

.)

Measured Calculated

Figure 8-9 Comparison of measured and calculated cable sag.

8-9

Observations

At the beginning of the tests, it was difficult to get any ice accretion on the cable, even though the temperature was very low. The ice formation started with small icicles attached just to the underside of the cable. Once ice started to form on the cable, however, it was able to grow at an increasing rate and covered the entire cable’s circumference. As the ice load grew, it formed a solid cylindrical-like shell around the cable with long icicles hanging below.

Figure 8-10 shows an illustration of the approximate cross section of the iced cable at the end of the tests. The cylinder of ice around the cable was approximately 2.0 inches in diameter, and a typical icicle was approximately equivalent to a 0.5-in. x 10-in. rectangle of ice hanging below. Also, it is estimated that the spaces in between the individual icicles correspond to half of the volume of ice that would be there if there were no spaces (i.e. if the rectangle of ice representing the icicles in Figure 8-10 was a solid continuous block running the length of the cable).

Figure 8-10 A representation of the approximate cross-section of the iced ADSS cable at the end of the tests.

8-10

With these approximations, the volume of ice on the cable was 1032 cm3/ft. With a density of 0.825 g/cm3, this corresponds to 852 g/ft, which is equivalent to 1.876 lb/ft. This is reasonably close to the 1.866 lb/ft estimated from weighing the short test cable (see above).

An important observation and conclusion is that ice can, for all practical purposes, grow on an ADSS cable indefinitely. Clearly, the hypotheses that ice would self-shed on such a cable before it accumulated to significant levels is false. In fact, at the end of the tests, the researchers purposely shook the cable (see shake.avi), and it was not apparent that the ice would shed even under these extreme conditions. This was discussed with the manufacturer, and they were quite surprised.

Close visual inspection of the iced cable indicated that the ice formed a very strong solid structure around the cable. It is the opinion of the researchers, based on these inspections, that any differences between the test method described here, and the accumulation of ice during actual precipitation during cold weather, is meaningless, and the conclusion that ice can grow indefinitely on an ADSS cable is true. Below are several figures showing photographs taken during the tests.

Figure 8-11 Researcher inspects iced cable during the night at about midpoint of the tests

8-11

Figure 8-12 ADSS Cable near the end of the test.

Figure 8-13 The iced ADSS Cable.

8-12

Figure 8-14 The iced ADSS Cable

8-13

Conclusions

An extensive ADSS icing test was successfully performed at the EPRI-Lenox laboratory. Measured ice density, approximated volume of ice on the cable, measured ice weight, measured cable sag, and calculated cable sag gave results that were mutually consistent.

The stress-strain relationship provided by the manufacturer provided experimentally confirmed results over a range of modest sag to extreme sag.

Most importantly, it was discovered that ice can build up indefinitely on ADSS cables, and does not self-shed as hypothesized by some industry experts.

9-1

9 LIGHTNING CHARACTERISTICS AND OPGW FAILURES

Introduction

In recent years it has become common for utilities to locate optical fiber communication systems on their transmission line towers. The need for internal utility communications and the desire for revenue from leased fiber optic cables have driven this activity, which generally is based on one of the following types of installations: Optical ground wires (OPGW), in which the fibers are installed at the center of shield wires normally used for lightning protection; WRAP cable, which is an all-dielectric cable that is wrapped around phase conductors on lower voltage lines or shield wires on higher voltage lines; or all-dielectric self-supporting (ADSS) cable, which is mounted below the phase conductors.

OPGW cable is the most frequent choice of utilities. If ground wire is part of the design for lightning protection on a new installation, a utility can incorporate OPGW simply by specifying a different type of ground wire. With both new installations and retrofits, OPGW offers the benefit of being less susceptible to vandalism. Since the primary purpose of ground wires is lightning protection, these wires are subject to lightning strikes and must be designed to withstand them.

The ability of ground wires without optical fibers to withstand lightning strikes is an important but not a critical issue. If such a ground wire fails, it can be repaired in the course of normal maintenance with little sacrifice in the reliability of the transmission line. With the addition of optical fibers to ground wires, however, utilities often must guarantee the availability of communication circuits, and the reliability of OPGW becomes a critical issue.

Estimates of ground wire failure or damage caused by lightning have been published for Japan and the U.K. An investigation into actual damage to OPGW systems in the region of Chuba Electric Power Co. in western Japan revealed a damage rate of 0.08 cases/100 km/year [1]. In the U.K., ground wires (not necessarily OPGW) have failed at a rate of 0.02 cases/100 km/year [2]. This may be lower than the Japanese rate either because it is a measure of “failure” rather than “damage,” as in the Japanese study, because the sizing of ground wires in the U.K. is different than in western Japan, or because the Chuba Electric Co. is located in a region of higher lightning activity. The failure/damage rates in the U.K. and Japan are reasonably consistent with statistics from the southeastern U.S., which suggests one OPGW failure per year during the last 10 years. No information was available on the installed length of cable that would allow calculating a directly comparable number of cases/100 km/year.

This report summarizes studies of OPGW field failures and laboratory lightning tests and pays special attention to the conditions under which either failure or significant cable damage occurs. The report also recommends additional experiments needed for the design of reliable OPGW

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installations and offers suggestions for specifying OPGW based on the best information available at this time.

Lightning Characteristics

Lightning can be divided into two types:

• Negative lightning, which is the most common, transfers negative charges from a cloud to ground.

• Positive lightning, which represents 5% to 30% of all lightning, transfers positive charges from a cloud to ground.

The duration of a typical negative lightning pulse is significantly less than 1 millisecond. Its rise time is on the order of 1 µs, and its fall time is on the order of 100-500 µs. Its peak amplitude varies widely between approximately 5 kA and 100 kA, while the total transferred charge is typically less than 50 Coulombs [3].

Positive lightning has several characteristics that distinguish it from negative lightning and are relevant to the question of lightning damage to overhead ground wires [4]. First, it generally has larger peak currents than negative lightning. These may exceed 200 kA. Second, in almost all cases there is a single return stroke followed by a long (up to 500 milliseconds) but relatively small (usually < 1 kA) amplitude continuing current after the peak. The charge transferred by these continuing currents, however, can exceed 400 Coulombs and, in Japan, has been measured up to 1000 Coulombs. Third, the geographic distributions of positive lightning are different from those of negative lightning [5, 6]. The U.S. Midwest is exposed to more positive lightning than any other section of the country [6]. In contrast, the southeastern part of the United States is exposed to more negative lightning than any other area of the country. Finally, positive lightning is more common in the winter than in the summer.

Because positive lightning strokes have larger peak currents and transferred charges, it is expected that damage caused to OPGW by positive lightning will be greater than that caused by negative lightning. This expectation is corroborated by several fairly recent laboratory tests of OPGW [1, 7, 8]. In [7] it was shown that (even when 85 Coulombs of charge was transferred) a simulated negative lightning pulse (i. e., total duration < 1 millisecond) caused little damage to OPGW of relatively small diameter. The addition of a continuing current (characteristic of positive lightning) to the lightning pulse, however, resulted in significant damage. Further, in the most well documented case of OPGW failure to date at the Nebraska Public Power District, the stroke was determined to be positive [9]. This experience is consistent with Japanese experience with “winter” lightning [1], which is positive lightning. Finally, it is interesting to note that failures of OPGW have generally occurred in the high positive lightning activity areas of both the U.S. and Japan.

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Lightning Caused OPGW Failures in the United States

Nebraska Public Power District

On September 2, 1997, at 1:06 a.m. CDT, a 0.547-in. diameter OPGW consisting of 12 strands of aluminum-clad steel wire with an aluminum core with two embedded fiber tubes was struck by lightning at midspan and failed immediately. The OPGW was mounted on a 345 kV H-frame horizontally configured wood pole line with two ground wires.

According to a faultfinder report from Global Atmospherics, the stroke was positive with a peak current of +63.8 kA. Given this, it is likely that the positive stroke had a total transferred charge of 100 - 200 Coulombs. According to one manufacturer, OPGW of this size can withstand approximately 50 Coulombs of transferred charge. Therefore, it is no surprise that the OPGW failed.

While the lightning detection network (LDN) can distinguish between positive and negative lightning, it would be very helpful if it could also measure the total charge transferred by a lightning stroke. Unfortunately, there are a number of reasons why it is essentially impossible to do this from electric and magnetic field measurements of the LDN. These include an imperfect knowledge of the current distribution on the lightning channel and the dispersion inherent to propagation of the energy from the lightning source to the receiving antennas.

Georgia

On February 16, 1998, at 10:31 p.m. EST, a 0.557-in. diameter OPGW consisting of 12 strands of aluminum-clad steel wire with an aluminum core with two embedded fiber tubes failed at midspan. While the failure was determined to result from a lightning strike, the strike apparently occurred some time earlier and only weakened the cable. Normal in-service stresses caused the failure at a later time.

Northeast United States

In the late summer or fall of 1997 one or two strands of an OPGW failed, but the cable remained in place. The time was not known exactly since there was no failure.

Other Failures in the Southeast United States Between 1988 - 1998

According to one company that repairs broken OPGW and has been in business for more than 10 years, approximately 12 OPGW failures have occurred in the southeast U.S. in the last 10 years [10]. Most of the failures have occurred near a tower attachment point. Most apparently begin with the breakage of a few strands and, over time, the remaining strands are stretched and eventually fail. It should be noted that some of the failures might be due to lightning-induced phase-to-ground backflashes.

Laboratory Lightning Tests

The standard lightning pulse is a periodic voltage impulse that rises to its peak in 1.2 µs and reduces to half its peak value in 50 µs without passing through zero [11, 12]. This pulse is

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representative of a negative lightning stroke as described above. In some cases a damped sinusoidal waveform that contains the same energy is used in laboratory lightning tests.

Many OPGW tests have used relatively short impulses such as those described in the last paragraph. These simulate a negative lightning strike. Only recently has the continuing current also been included in OPGW tests. Both types of tests are summarized in Table 9-1, as are the two most well documented failures in the U.S.

Interpretation of the Results

It is clear that when OPGW tests only involve short impulses representative of negative lightning, there is little or no damage to the OPGW. This is even true when a charge of 85 Coulombs is transferred to a small-diameter OPGW that is known to fail under similar transferred charge levels from continuing currents [7]. Typical damage reported in these cases is burns, marking and some erosion. While this damage is slight, it may not be insignificant. It has been reported that such damage might eventually result in exposure of the steel to the atmosphere, which eventually causes rust. Given this, the remaining life of the OPGW may be reduced, although estimates of this reduction are not available.

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Table 9-1 Summary of OPGW Lightning Tests and Failures. (The capital letters by the Alcatel tests refer to the specific types of cable in that reference [8])

Title [ref.]

Test (T) or Field (F)

OPGWdia.(in.)

OPGW area

(mm2)

short (<1 ms) current

peak

long (>1 ms) current

charge/(duration)

Damage

NPPD[13] F 0.547 49 64 kA 100-200 C est. (?) fail

GA F 0.557 ? ? ? fail

AEP [14] T 0.524 84 150 kA - arc burns

“ T 0.598 105 150 kA - “

“ T 0.614 91 150 kA - “

“ T 0.662 125 150 kA - “

Sumitomo [15]

T ? 80 8 kA 320 C (40ms) melted Al.

Barthrop [16] T 0.586 ? 100 kA - marking

BICC [17] T 0.480 ? 150 kA - none

Carter[18] T 0.51-1.1 ? 175 kA - arc erosion

Germany [8]

T ? 149 - 200 C (500ms)a,b 5 bkn. str.

Alcatel [7](A)

T 0.440 44 220 kA 200 C (500ms) 7 bkn. str.

“ (B) T 0.484 59 220 kA “ 2 bkn. str.

“ (C) T 0.535 82 220 kA “ 2 bkn. str.

“ (D) T 0.602 106 220 kA “ 8 bkn. str.

“ (E) T 0.756 157 220 kA “ 6 bkn. str.

“ (F) T 0.939 280 220 kA “ 9 bkn. str.

“ (G) T 0.939 272 220 kA “ 5 bkn. str.

Japan[1] T ? 260 - 500 C (50ms)c 5 bkn. str.

Notes for Table 9-1:

1. Experiments with only a short impulse current produced no damage to the wires.

2. Experiments were also done with 30 and 100 Coulombs.

3. Experiments were done between 200 and 800 Coulombs.

When a long-duration continuing current component is added, however, significant damage to the OPGW occurs. Even when the duration of the continuing current is only 40 milliseconds, there is significant melting of the aluminum [15]. When longer-duration currents are used with 200 Coulombs of transferred charge, broken strands (and the accompanying loss of strength) are common [7, 8].

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0

20

40

60

80

100

0 100 200 300

200 COULOMBS100 COULOMBS 30 COULOMBS200 COULOMBS100 COULOMBS 30 COULOMBS

TOTAL CHARGE TRANSFERRED BY THE CONTINUING CURRENT

TOTAL CROSS-SECTIONAL AREA OF OPGW CONDUCTORS (MM2)

RE

MA

ININ

G S

TR

EN

GT

H A

FT

ER

ST

RIK

E (

%)

Figure 9-1 Remaining strength of OPGW after a positive lightning strike as a function of OPGW aluminum/steel cross-sectional area with transferred charge as a parameter [7]. The solid and dashed lines are curves that best fit the data.

It is clear from Table 9-1 that the smaller the transferred charge and the larger the shield wire, the smaller the damage. These data are consistent with the quote from one manufacturer that a 0.55-in.-diameter OPGW (a small diameter) could be expected to withstand only 50 Coulombs of transferred charge.

The data shown in Table 9-1 are presented in a different way in Figure 9-1. Here the strength remaining in the OPGW after application of a simulated lightning continuing current is plotted as a function of total OPGW aluminum/steel wire cross section. The remaining strength ratio is calculated as the ratio of unbroken strands after the test to the original total number of strands (an actual strength measurement is used in [1]). The total charge applied is a parameter. In addition to the data that are plotted as data marks, a best-fit curve for each transferred charge has been computed and plotted.

It is clear from these plots that 30 Coulombs of charge transferred by the continuing current will have little effect on the strength of the OPGW. However, the strength remaining after application of either 100 or 200 Coulombs can be significantly smaller than the original strength. It is somewhat surprising that the data suggest only a small difference between the effect of 100- and 200-Coulomb transferred charges.

The data from Figure 9-1 can be used to determine the probability of a failure in any given part of the U.S. To do this the data must be supplemented by:

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• The data of [6]

• A formula from which the number of discharges to the OPGW by positive lightning can be calculated

• A curve that shows the probability that a given positive lightning stroke transfers more than a specified charge

0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400

TOTAL TRANSFERRED CHARGE (COULOMBS)

PR

OB

AB

ILIT

Y T

HA

T C

HA

RG

E E

XC

EE

DS

A G

IVE

RN

VA

LU

E

Figure 9-2 Probability that the total charge transferred by a lightning discharge exceeds a given value.

Several formulas have been used to estimate the number of positive lightning discharges to an OPGW [19, 20]. Two alternatives are given in (1). Here, the formula used in [20] will be used, although substitution of the formula from [19] is straightforward.

NN only OPGW

N one OPGW and one ground wire

N N

N N b h

N N b h

opgw

ground

ground

ground line

line positive

line positive

=

=

= +

= +

10

0 5

0 4

01 28 19

01 4 20

0 6

1 09

. ( )

. ( )

.

. ( ) [ ]

. ( ) [ ]

.

.

(1)

where

b = spacing between ground wire and OPGW (= 0 if only OPGW) meters

h = hgt - 2/3(hgt-hgm) meters

hgt is the height of the OPGW at the tower attachment.

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hgm is the height of the OPGW at midspan.

Nopgw is the number of positive lightning strikes/100 km/year to the OPGW.

Nground is the number of positive lightning strikes/100 km/year to any ground wire.

Nline is the number of positive lightning strikes/100 km/year to the line (including towers).

Npositive is the number of positive lightning strikes/ km2/year.

The probability that a positive lightning stroke transfers a charge greater than a given value can obtained from [3] and is shown in Figure 9-2. It is assumed here that this distribution is representative of any location in the world.

From (1) and [6] and Figure 9-2 it is possible to calculate the number of strikes to the OPGW/100 km/year that will transfer a charge (via the continuing current) of more than a specified amount. Figure 9-1 can then be used to assess the damage to the OPGW.

Example

An example can be used to illustrate how the lightning risk to OPGW can be evaluated. Consider an OPGW with a cross-sectional area of strength members of 100 mm2, which is typical of many installations in the U.S. According to Figure 9-2, this OPGW (if struck by a positive stroke with transferred charge of greater than 200 Coulombs) will suffer a 20% loss in strength. Consider a calculation of the probability that the wire will be struck by lightning and suffer a loss of strength greater than 20%.

Suppose this OPGW is installed on a 230 kV line located in the center of Indiana. This line has one regular ground wire and one OPGW spaced (b) 10 meters apart at an average height (h) above the earth of 20 meters.

From [6], an average value for Npositive in Indiana (using 1992, 1994 and 1995 data) is 0.23 flashes/km2/year. Using Equation (1) with the formula from [20], Nopgw can be calculated to be

Nopgw = 0.53 strikes/100 km/year (2)

Had the formula from [19] been used, Nopgw would have been 0.83 strikes/100 km/year. From Figure 9-2, the probability that a positive stroke will transfer more than 200 Coulombs of charge is approximately 0.2. Therefore, the probability that an OPGW will be struck and retain less than 80% of its strength is:

P<80% remaining strength = 0.105/100 km/year (3)

If this remaining strength is interpreted as the threshold for ultimate failure, then (3) represents the number of failures /100km/year on the power system in Central Indiana.

This result (for an area of the U.S. that is not susceptible to much positive lightning) is slightly higher than the damage rate for ground wires in a high lightning area of Japan [1]. It is also five times higher than the reported failure rate of ground wires in the U.K. It is possible that that the reason for this surprising result is that the assumed ground wire is smaller than that generally used in Japan and is more susceptible to lightning damage.

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Lightning Resistant OPGW

Because of extremely high positive lightning activity in western Japan, CRIEPI has developed a specially designed OPGW called “lightning resistant OPGW” [1]. This design incorporates strands that are of a different shape and/or partially made of zinc to reduce the susceptibility of OPGW to a lightning strike. For transferred charges above 200 Coulombs, this design results in significantly fewer broken strands and a commensurately higher residual strength. At this time, “lightning resistant OPGW” has been supplied by the Sumitomo Co. for one 500 kV line in Japan, but it is not generally available because Sumitomo does not have the ability to mass- produce it.

The standard ground wire used in western Japan has a cross-sectional area of 260 mm2, which is very large. This is used because transferred charges in this region can exceed 1000 Coulombs and can damage wire even of this size. As a result, the “lightning resistant OPGW” has the same diameter. To date no tests have been done on smaller-diameter versions of “lightning resistant OPGW,” which might be of interest to U.S. utilities.

Recommendations for Further Research

The method outlined above is only approximate for a variety of reasons. Consider some of the assumptions:

1. The capability of OPGW to withstand a lightning strike is completely determined by its diameter or the cross-sectional area of the aluminum/steel wires that provide its strength. The peak current of the stroke is not assumed to be relevant.

2. The distribution of transferred charge in Figure 9-2 is representative of positive discharges anywhere in the world. There is reason to suspect that the maximum transferred charge by positive lightning may be different in different regions of the world.

3. Equation (1) is useful, for example, even when the local terrain is not flat.

4. The number of strikes to the shield/OPGW is 40% of the total strokes hitting the line. The remainder strikes the tower.

5. A 20% loss in strength is enough to ensure eventual failure of the OPGW.

In order to evaluate the potential for lightning damage of OPGW more quantitatively, it will be necessary to evaluate these assumptions. This could be the basis for further research. Another possible research project would be the design and testing of lightning resistant OPGW with diameters smaller than those developed for use in western Japan.

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Summary

1. Published values of OPGW damage/failure rates range from 0.02 - 0.08 cases/100 km/year.

2. Damage to ground wires is caused by low-amplitude ( ≅ 1 kA) long-duration ( ≅ 500 ms.) currents, not short impulsive currents. These continuing currents can transfer charges of up to 1000 Coulombs to ground.

3. When considering OPGW damage, positive lightning is more important than negative lightning because it is commonly associated with a continuing current.

4. Geographic distributions of positive lightning (number of positive strokes to ground/km2/year) in the U.S. are now available. The highest levels are found in the Midwest.

5. Data on the probability distribution for transferred charge by positive strokes has been reported.

6. A survey of known OPGW failures in the U.S. has been reported. 7. Appropriate laboratory tests for OPGW lightning sensitivity are described. 8. A model for estimating the number of OPGW failures given the number of positive strokes to

ground/km2/year, OPGW size and line geometry has been developed. Initial results appear to exceed the actual failure statistics by as much as a factor of 5.

9. Suggestions for research to resolve the difference between estimates and actual failure statistics have been made.

10. A description of “lightning resistant OPGW” developed in Japan has been given along with a suggestion for research on similar cable of smaller diameter.

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References

1. Yokoya, Yukio Katsuragi, Y. Goda, Y. Nagata and Y. Asano, “Development of Lightning-resistant Overhead Ground Wire,” IEEE Transactions on Power Delivery, Vol. 9, No. 3, pp. 1517-1523, July 1994.

2. Carleton, A. Bartlett, C. Carter and T. Parkin, “UK Power Utilities Experience with Optical Telecommunications Cabling Systems,” Power Engineering Journal, Vol. 9, No. 1, pp. 7-14, February 1995.

3. Berger et. al., “Parameters of Lightning Flashes,” CIGRÉ, Electra, No. 41.

4. M.A. Uman and E.P. Krider, “Natural and Artificially Initiated Lightning,” Science, Vol. 246, pp. 457-464, 27 October 1989.

5. R.E. Orville, “Cloud-to-Ground Lightning Flash Characteristics in the Contiguous United States: 1989-1991,” Journal of Geophysical Research, Vol. 99, No. D5, pp. 10,833-10,841, May 20, 1994.

6. R.E. Orville, “Lightning Ground Flash Density in the Contiguous United States: 1992-1995,” Monthly Weather Review, Vol. 125, No. 4, pp. 631-638, April 1997.

7. Bonicel, O. Tatat, U. Jansen, and G. Couvrie, “Lightning Strike Resistance of OPGW,” 1995 International Wire and Cable Symposium Proceedings, pp. 800-806.

8. Zischank and J. Wiesinger, “Damages to Optical Ground Wires Caused by Lightning,” Proceedings of the 10th International Symposium on High Voltage Engineering, Montreal, Quebec, Canada, 1997.

9. Global Atmospherics Inc., Tucson, “AZ Lightning Faultfinder Report to the Nebraska Public Power District,” Columbus, NE, September 4, 1997.

10. Private Communication with Mr. Rod Garry, President, Fiber Optics Services, Clearwater, FL (813) 573-1310.

11. IEEE, IEEE Standard Techniques for High Voltage Testing, IEEE Std. 4-1978, Distributed by Wiley Interscience, New York, pp. 42-47.

12. Kuffel and W.S. Zaengl, High Voltage Engineering Fundamentals, Pergamon Press, Oxford, pp. 470-473.

13. Private Communication with Mr. Robert Oswald, Nebraska Public Power District, Columbus, NE.

14. Nourai, “Simulated Lightning Tests on Optical Groundwires,” American Electric Power Service Corp. Reports 92-01,92-02, February 24, 1992.

15. Kuboto et. al., “Field Trial of Composite Fiber Optic Overhead Groundwire,” 1983 International Wire and Cable Symposium.

16. “Simulated Lightning Test on FIBRAL Conductor,” Report No. BHPL830201, Barthorp EMP Limited, England.

17. K.A. Austin, et. al., “Optical Communications Using Overhead Power Transmission Lines,” CIGRÉ Paper No. 35-04, 1984.

18. C.N. Carter, et. al., “Lightning Simulation Tests on Power Transmission Conductors Carrying Embedded Optical Communication Cable,” International Conference on Lightning and Power Systems, pp. 207-209, 1984.

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19. A.J. Eriksson, “The Incidence of Lightning Strikes to Power Lines,” IEEE Transactions on Power Delivery, Vol. PWRD-2, No. 3, pp. 859-870, July 1987.

20. EPRI, Transmission Lines/ 345 kV and Above, EPRI, Palo Alto, CA.

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10 SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS

Accelerated Aging Tests of ADSS Fiber Optic Cables

The first set of tests in the Lenox accelerated aging chamber was completed. These tests ran continuously for about 2 years with an environment simulating a warm, wet coastal environment similar to the southeast US. The accelerated aging factor was estimated to be about 17.

Cable samples (Class B) from four different manufacturers were represented in the tests. Several cable failures occurred during the course of the two years that the tests ran, with the first failure occurring after about one year. However, considering the accelerated aging factor of 17, this corresponds to about 17 years of operation in the field. Therefore, based on the results of these tests, it can be concluded that Class B cables should generally survive a reasonable lifetime in operation.

Nevertheless, one manufacturer’s cables clearly performed the best. It was found that failures were generally directly correlated with loss of hydrophobicity and increased leakage current. This manufacturer’s cables never lost hydrophobicity, never experienced the higher leakage currents associated with arcing failures, and experienced no failures in the chamber. It is recommended that a hydrophobicity measurement be made on an operating cable from this manufacturer. An installation has been identified, and plans are being made to perform on-site measurements.

Higher leakage currents (~1 mA) and/or a sudden increase in leakage current may indicate imminent cable failure. It is recommended that an instrument be developed to monitor leakage currents of ADSS cable installations.

Survey of ADSS and OPGW Cable Failures

The overall operating experience with ADSS and OPGW cables has been very good. However, several failures have occurred and they have caused considerable concern about the long-term performance of ADSS cable and the lightning performance of OPGW cables.

ADSS failures have been attributed to two factors:

• Corona on hardware, especially in situations where an armor rod tip protrudes beyond the neighboring tips and experiences corona activities due the locally enhanced electric field.

• Dry band arcing on contaminated ADSS cables when they are placed in regions of high space potential near transmission line conductors.

The hardware corona problem can be eliminated with the use of grading rings or specially designed “corona coils” that are placed on the armor rod. The dry band arcing problem is more

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difficult to analyze, understand, and prevent. Some remedies, including insulating the ADSS cable from the grounded support structures to eliminate the leakage current responsible for dry band arcing and the use of long semi-conductive grading rods have been proposed on experimental basis. Their effectiveness and long-term performance still need to be evaluated.

OPGW failures have been attributed to relatively rare positive lightning strokes. The frequency of these damaging strokes is small, but their effect is usually totally destructive. While “lightning resistant OPGW” cables have been developed in Japan, further work is needed to confirm their effectiveness and to evaluate their economic value.

Analyses of Two Reported Field Failures

A Class-B (track resistant) ADSS fiber optic cable on a Consolidated Edison double circuit 345 kV line on Staten Island recently suffered a catastrophic failure. The cable was severed close to the supporting armor rod tip on Tower M7, and fell into a small river under the line. A section of the cable around the break was removed, and replaced with new cable.

Researchers from the EPRI laboratory in Lenox, Massachusetts visited the site to make observations. It was found that there was not a corona suppression device (corona ring) at the location of the cable break. Most (but not all) of the armor rod installations have these devices. The cause of the break is not known with certainty, but the absence of the corona ring is certainly a prime suspect.

It was observed that the failure occurred at a location where the transmission line goes through a configuration transition in order to negotiate a bridge. Analysis of the electrical environment in this region indicated that the electrical stresses (space potential, induced voltage, and induced current) experienced by the cable were significantly greater here than at typical spans. A high electrical-stress environment is certainly believed to have a significant impact on the probability of a cable failure.

There is another failure mode that was recently discovered at the EPRI-Lenox laboratory that is consistent with the Staten Island failure. It was discovered that any puncture located anywhere along the cable can lead to water ingress. The water can move, by capillary action, relatively large distances through the cable, and can lead to spark flashovers at the grounded connections. These flashovers could cause severe damage. Because the damage appears at the tips of grounded hardware, it could be interpreted as corona damage.

Consolidated Edison’s line crew indicated that they had observed sparking at the armor rod tips when the tips were pulled away from the cable. This observation is consistent with the failure mode associated with water ingress.

It must be stressed that the cause of the failure is not known with certainty. However, if the failure was due to the water ingress-flashover failure mode, there is reason to believe that other failures are likely to occur, and the installation of corona suppression devices may not mitigate the problem.

Also, Eskom (South Africa) recently experienced a catastrophic failure of a Class A ADSS fiber optic cable. Eskom provided EPRI with a summary about this failure, however, neither a site visit nor a detailed physical analysis was made.

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Modeling (3D) of ADSS Cables in HV Transmission Corridors

An algorithm for modeling the electrical performance of ADSS fiber optic cables in high voltage transmission corridors has been developed. The open-circuit voltage, induced current, and induced voltage along a cable can be calculated. The model accounts for the sags of the ADSS cable and phase conductors. The model is being verified with full-scale tests.

Work needs to continue on the development of the model into a versatile tool that can be used by utilities. Also, more work needs to be performed to better understand the relationship between the calculated electrical parameters and failure modes.

Full-Scale Tests

A full-scale 3-phase test line was constructed at the EPRI laboratory in Lenox, Massachusetts, which included an actual ADSS fiber optic cable. The test line has many purposes, the first being to verify the analytical model for predicting the electrical performance of ADSS cables. Calculations and measurements of open-circuit voltage, induced current, induced voltage, and measurements of short-circuit current to ground were made for several line configurations.

The comparisons of calculated and measured parameters indicate that the quasi-3D analytical model is a viable tool for making predictions of those electrical parameters (except for induced voltage, where there is some uncertainty. Further tests in the future are expected to resolve remaining uncertainties.

Live-Line Installation, Maintenance, and Inspection

For safety considerations, when a worker comes in contact with an ADSS cable, the amount of current that could go through his body must remain below acceptable levels. This current is characterized by the level of short-circuit current to ground.

A worker can just barely perceive a current through his body of about 1 mA. At 10 mA, a worker would experience significant discomfort, and would not be able to hold on to an ADSS cable providing this much current through the hand to ground. This is sometimes referred to as the let-go level. A current of 80 mA can be lethal.

For all the tests performed with the full-scale test line, the short-circuit current to ground remained well under 10 mA, even with the simulated heavy pollution level. Also, during the construction short-circuit currents to ground were made on a new, clean, dry ADSS cable. Short-circuit currents to ground were negligible.

Icing Tests of ADSS Cables

There exists a hypothesis that ice cannot build up to large extents on ADSS fiber optic cables. The hypothesis is that as an ADSS cable stretches under the weight of ice, the bond between the cable’s surface and the ice will break, and the ice will shed (the “ice cube tray effect”). A full-scale test was performed at the request of the EPRI Working Group to test this hypothesis.

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The test has shown that the hypothesis is incorrect, and ice can build up to extensive levels without self-shedding. This finding should be considered when designing ADSS cable installations in ice prone regions.

Characterization of ADSS Fiber Optic Cables

This work is being performed at Arizona State University and being cosponsored by EPRI and WAPA. One goal is to develop a standard acceptance test for ADSS cables. The work is ongoing.

Lightning Characteristics and OPGW Failures

It appears that OPGW failures are primarily due to positive stroke lightning. Positive lightning has several characteristics that distinguish it from negative lightning and are relevant to the question of lightning damage to overhead ground wires. First, it generally has larger peak currents than negative lightning. These may exceed 200 kA. Second, in almost all cases there is a single return stroke followed by a long (up to 500 milliseconds) but relatively small (usually < 1 kA) amplitude continuing current after the peak. The charge transferred by these continuing currents, however, can exceed 400 Coulombs and, in Japan, has been measured up to 1000 Coulombs. Third, the geographic distributions of positive lightning are different from those of negative lightning. The U.S. Midwest is exposed to more positive lightning than any other section of the country. In contrast, the southeastern part of the United States is exposed to more negative lightning than any other area of the country. Finally, positive lightning is more common in the winter than in the summer.

Because positive lightning strokes have larger peak currents and transferred charges, it is expected that damage caused to OPGW by positive lightning will be greater than that caused by negative lightning. This expectation is corroborated by several fairly recent laboratory tests of OPGW. In one it was shown that (even when 85 Coulombs of charge was transferred) a simulated negative lightning pulse (i.e., total duration < 1 millisecond) caused little damage to OPGW of relatively small diameter. The addition of a continuing current (characteristic of positive lightning) to the lightning pulse, however, resulted in significant damage. Further, in the most well documented case of OPGW failure to date at the Nebraska Public Power District, the stroke was determined to be positive. This experience is consistent with Japanese experience with “winter” lightning, which is positive lightning. Finally, it is interesting to note that failures of OPGW have generally occurred in the high positive lightning activity areas of both the U.S. and Japan.

The EPRI project documented in this report has focused primarily on ADSS cable. However, there is definitely an interest amid EPRI members in OPGW cable. More work is expected to focus on this technology in the future.

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