Application of knowledge based fuzzy inference system on high voltage transmission line maintenance

APPLICATION OF KNOWLEDGE-BASED FUZZY INFERENCE SYSTEM
ON HIGH VOLTAGE TRANSMISSION LINE MAINTENANCE

A Thesis Submitted for the Degree of
Master of Engineering

By
Mohd Junaizee Mohd Noor, B.Sc. Elect. Eng. (Missouri)

School of Electrical & Electronic Systems Engineering
Queensland University of Technology

2004

1


KEYWORDS

High voltage transmission lines; insulators; tower structures; foundations; conductors;
maintenance management; visual inspection; artificial intelligence; fuzzy logic; fuzzy
inference systems; knowledge-based systems

2


ABSTRACT

A majority of utilities conduct maintenance of transmission line components based on the
results of routine visual inspection. The inspection is normally done by inspectors who
detect defects by visually checking transmission line components either from the air (in

helicopters), from the ground (by using high-powered binoculars) or from the top of the
structure (by climbing the structure).
The main problems with visual inspection of transmission lines are that the determination
of the defects varies depending on the inspectors’ knowledge and experience and that the
defects are often reported qualitatively using vague and linguistic terms such as “medium
crack”, “heavy rust”, “small deflection”. As a result of these drawbacks, there is a large
variance and inconsistency in defect reporting (which, in time, makes it difficult for the
utility to monitor the condition of the components) leading to ineffective or wrong
maintenance decisions. The use of inspection guides has not been able to fully address
these uncertainties.
This thesis reports on the application of a visual inspection methodology that is aimed at
addressing the above-mentioned problems. A knowledge-based Fuzzy Inference System
(FIS) is designed using Matlab’s Fuzzy Logic Toolbox as part of the methodology and its
application is demonstrated on utility visual inspection practice of porcelain cap and pin
insulators. The FIS consists of expert-specified input membership functions (representing
various insulator defect levels), output membership functions (indicating the overall
conditions of the insulator) and IF-THEN rules. Consistency in the inspection results is
achieved because the condition of the insulator is inferred using the same knowledge-base
in the FIS rather than by individual inspectors. The output of the FIS is also used in a
mathematical model that is developed to suggest appropriate component replacement
date.
It is hoped that the methodology that is introduced in this research will help utilities
achieve better maintenance management of transmission line assets.

3


CONTENTS

Keywords ....................................................................................................................................................... i

Abstract ......................................................................................................................................................... ii
Contents ....................................................................................................................................................... iii
List of Figures ............................................................................................................................................. vi
List of Tables............................................................................................................................................... ix
List of Abbreviations .................................................................................................................................. x
Statement of Original Authorship.......................................................................................................... xii
Acknowledgments ....................................................................................................................................xiii
Chapter 1: Introduction ....................................................................................................... 1
1.1 Justification for and Introduction to the Research ......................................................................... 1
1.2 Aims and Objectives of the Research ............................................................................................... 4
1.2 Organization of the Thesis.................................................................................................................. 4
Chapter 2: Components of Transmission Lines and their Failure Modes ...................... 7
2.1 Chapter Overview................................................................................................................................. 7
2.2 Towers and Structures ......................................................................................................................... 7
2.2.1 Functions of Transmission Towers ....................................................................................................8
2.2.2 Failure Modes of Steel Transmission Towers ................................................................................ 12

2.3 Foundations .........................................................................................................................................13
2.3.1 Functions of Foundations .................................................................................................................. 13
2.3.2 Failure Modes of Foundations .......................................................................................................... 16

2.4 Conductors and Earth Wires ............................................................................................................18
2.4.1 Function of Conductors and Earth Wires ...................................................................................... 18
2.4.2 Failure Modes of Conductors............................................................................................................ 21
2.4.2.1 Conductor Corrosion .......................................................................................................... 22
2.4.2.2 Conductor Vibration ........................................................................................................... 24

4

2.4.2.3 Conductor Annealing .......................................................................................................... 27

2.5 Insulators ..............................................................................................................................................27
2.5.1 Functions of Insulators ....................................................................................................................... 27
2.5.2 Failure Modes of Insulators ............................................................................................................... 31
2.5.2.1 Mechanical Failures .............................................................................................................. 31
2.5.2.2 Electrical Failures ................................................................................................................. 34
2.5.2.3 Audible Noise and Radio Interference ............................................................................ 38

2.6 Chapter Summary ...............................................................................................................................39

Chapter 3: Inspection, Diagnosis and Maintenance of Transmission Line
Components ...................................................................................................................... 41
3.1 Chapter Overview...............................................................................................................................41
3.2 Review of Component Diagnosis Methods ..................................................................................41
3.2.1 Test for Tower Structural Strength .................................................................................................. 42
3.2.2 Diagnostic Tests on Tower Foundations ....................................................................................... 43
3.2.3 Diagnostic Tests on Conductors ...................................................................................................... 45
3.2.4 Insulator Diagnostic Tests ................................................................................................................. 46

3.3 Review of Inspection and Maintenance Methods ........................................................................53
3.3.1 McMahon Survey of Inspection Practice of Australian and New Zealand Utilities .............. 54
3.3.2 CIGRE Survey of Utility Assessment of Existing Transmission Lines ................................... 55

3.4 Visual Inspection ................................................................................................................................56
3.4.1 Ground-level and Climbing Inspection........................................................................................... 56
3.4.2 Aerial Inspection .................................................................................................................................. 58
3.4.3 Drawbacks of Visual Inspection ....................................................................................................... 60

3.5 Chapter Summary ...............................................................................................................................61

Chapter 4: Fuzzy Logic and Fuzzy Inference System .....................................................63
4.1 Chapter Overview...............................................................................................................................63
4.2 Fuzzy Logic ..........................................................................................................................................63
4.2.1 Membership Functions ....................................................................................................................... 64
4.2.2 Fuzzy IF-THEN Rules ....................................................................................................................... 67

4.3 Fuzzy Inference System .....................................................................................................................68
4.3.1 Inference and Defuzzification Techniques..................................................................................... 69

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4.3.2 Developing Fuzzy Inference Systems.............................................................................................. 72

4.4 Applications of Fuzzy Logic in Utility Environment ..................................................................73
4.5 Chapter Summary ...............................................................................................................................75

Chapter 5: Using Fuzzy Logic for Transmission Line Defect Assessment....................77
5.1 Chapter Overview...............................................................................................................................77
5.2 Application of the FIS on Porcelain Cap and Pin Insulators .....................................................78
5.2.1 Factors Affecting Pin Corrosion....................................................................................................... 79
5.2.2 Factors Affecting Chipped/Broken Porcelain Insulator Disc.................................................... 83
5.2.3 The Insulator Visual Inspection Process ........................................................................................ 89
5.2.4 The Fuzzy Inference System for Visual Inspection of Insulators ............................................. 91
5.2.4.1 Assessment of Single Insulators ........................................................................................ 91
5.2.4.2 Assessment of Multiple Insulators in a String .............................................................. 112
5.2.4.3 Assessment of Multiple Insulator Strings in a Transmission Line ........................... 114

5.3 Application of the FIS on Other Transmission Line Components ....................................... 118

5.4 Potential Savings due to Introduction of FIS in Tenaga Nasional Berhad’s
Transmission Line Inspection and Maintenance Practice .............................................................. 120
5.5 Chapter Summary ............................................................................................................................ 122

Chapter 6: Conclusions and Future Work ...................................................................... 124
6.1 Summary of the Research .............................................................................................................. 124
6.2 Major Research Contributions ...................................................................................................... 129
6.3 Future Work ..................................................................................................................................... 130
References ....................................................................................................................... 131
Appendix.......................................................................................................................... 138

6


LIST OF FIGURES

Figure 2-1: Example of a typical overhead transmission line.............................................................. 8
Figure 2-2: Monopole and steel lattice tower designs for 220 kV transmission lines ..................11
Figure 2-3: Structural failures of a transmission line due to an ice storm in Canada....................12
Figure 2-4: Uplift and compression forces on tower foundations...................................................14
Figure 2-5: Typical construction of transmission tower steel grillage foundation ........................15
Figure 2-6: Typical construction of transmission tower rock anchor foundation ........................16
Figure 2-7: Typical ACSR conductor stranding arrangements .........................................................18
Figure 2-8: Arrow showing evidence of rust on steel core of ACSR conductor ..........................22
Figure 2-9: Sectional view of ACSR conductor illustrating galvanic corrosion mechanism .......23
Figure 2-10: Arrow showing signs of corrosion at a conductor mid span joint............................24
Figure 2-11: Multiple fatigue cracks of outermost aluminum strands of an ACSR
conductor ....................................................................................................................................................25
Figure 2-12: Cross-sectional view of a cap and pin insulator............................................................29
Figure 2-13: Porcelain cap and pin insulator string.............................................................................29

Figure 2-14: Typical polymeric insulator construction.......................................................................30
Figure 2-15: Pin corrosion is most severe at cement interface .........................................................32
Figure 2-16: Brittle fractures on composite insulators .......................................................................33
Figure 2-17: ‘Doughnut’-like separation of the porcelain shell from its cap and pin ...................36
Figure 3-1: Sequence photograph of a transmission tower collapse during a structural insitu test.........................................................................................................................................................43
Figure 3-2: Diagram of the half-cell measurement method ..............................................................44
Figure 3-3: Electric field distribution across an insulator string with a punctured unit in the
middle ..........................................................................................................................................................49
Figure 3-4: Variation of electric field along an 18-unit insulator string which shows
defective insulator units at insulator number 7, 11, 14 and 15 .........................................................50
Figure 3-5: Aerial visual inspection of transmission line components using the helicopter .......59
Figure 3-6: Broken conductor strands due to gunshot ......................................................................59
Figure 4-1: Triangular membership function (10, 20, 30)..................................................................65
Figure 4-2: Intersection of 2 membership functions (A AND B) ...................................................66
Figure 4-3: Union of 2 membership functions (A OR B) .................................................................67
Figure 4-4: Components of a fuzzy inference system ........................................................................68

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Figure 4-5: Mamdani fuzzy inference method .....................................................................................70
Figure 4-6: Defuzzification schemes to derive a crisp output ..........................................................72
Figure 5-1: Voltage profile across a 132 kV insulator string .............................................................80
Figure 5-2: No rust insulator pin condition..........................................................................................81
Figure 5-3: Light rust insulator pin condition ......................................................................................82
Figure 5-4: Medium rust insulator pin condition ................................................................................82
Figure 5-5: Heavy rust insulator pin condition ....................................................................................82
Figure 5-6: P-F curve for rust condition of insulator pin ..................................................................83
Figure 5-7: Cutaway drawing of a normal type cap and pin insulator .............................................84
Figure 5-8: Cutaway drawing of an anti-fog type cap and pin insulator .........................................85

Figure 5-9: Chipped porcelain disc ........................................................................................................86
Figure 5-10: Small breakage of porcelain disc ......................................................................................86
Figure 5-11: Major radial breakage of porcelain disc ..........................................................................87
Figure 5-12: Total porcelain disc breakage ...........................................................................................87
Figure 5-13: Arrows show partially broken porcelain discs in an insulator string ........................87
Figure 5-14: Arrows show 2 totally broken porcelain discs in an insulator string ........................88
Figure 5-15: Structure of insulator inspection FIS ..............................................................................92
Figure 5-16: Input membership functions for pin rust conditions ..................................................93
Figure 5-17: Input membership functions for porcelain shell conditions ......................................94
Figure 5-18: Output membership functions for insulator condition...............................................94
Figure 5-19: A 3-dimensional plot of insulator inspection FIS showing the relationship
between the two inputs and output .......................................................................................................98
Figure 5-20: Insulator Sample 1 ..............................................................................................................99
Figure 5-21: Pin rust condition for insulator Sample 1 ................................................................... 100
Figure 5-22: Porcelain shell condition for insulator Sample 1 ....................................................... 100
Figure 5-23: Resultant of invoking Rule 6 ......................................................................................... 101
Figure 5-24: Resultant of invoking Rule 11 ....................................................................................... 101
Figure 5-25: Aggregation of Rules 6 and 11 resultants ................................................................... 102
Figure 5-26: Insulator Sample 2 ........................................................................................................... 103
Figure 5-27: Porcelain shell condition for insulator Sample 2 ....................................................... 104
Figure 5-28: Pin rust condition for insulator Sample 2 ................................................................... 104
Figure 5-29: Resultant of firing Rule 2 ............................................................................................... 105
Figure 5-30: Resultant of invoking Rule 3 ......................................................................................... 106
Figure 5-31: Aggregation of Rules 2 and 3 resultants...................................................................... 106

8


Figure 5-32: Insulator Sample 3 ........................................................................................................... 107
Figure 5-33: Porcelain shell condition for insulator sample 3........................................................ 107

Figure 5-34: Pin rust condition for insulator sample 3.................................................................... 108
Figure 5-35: Resultant of invoking Rule 12 ....................................................................................... 109
Figure 5-36: Resultant of invoking Rule 17 ....................................................................................... 109
Figure 5-37: Aggregation of Rule 12 and Rule 17 resultants.......................................................... 110
Figure 5-38: Proposed structure of tower inspection FIS .............................................................. 119
Figure 5-39: TNB transmission line forced outages 1997-2003 .................................................... 121

9


LIST OF TABLES

Table 2-1: Basic transmission structure types ........................................................................................ 9
Table 2-2: Requirement for zinc coating thickness as per BS EN ISO 1461 ................................11
Table 2-3: AN and RI limits for insulator at typical system voltages ..............................................39
Table 3-1: ASTM C867:1999 criteria for corrosion of steel in concrete.........................................44
Table 3-2: Emerging and available aerial inspection technologies ...................................................60
Table 4-1: Determination of membership function from -cut sets ...............................................66
Table 5-1: Category of in-service porcelain insulator disc defects ...................................................88
Table 5-2: Pin corrosion description used by Powerlink during visual inspection .......................90
Table 5-3: IF-THEN rules used in the insulator inspection FIS......................................................97
Table 5-4: Suggested insulator maintenance decision ........................................................................98
Table 5-5: Introduction of environmental coding for areas based on IEC’s pollution
severity classification .............................................................................................................................. 115
Table 5-6: Environmental coding for zinc loss ................................................................................ 116
Table 6-1: Summary of transmission line components, their functions and failure modes ..... 125
Table 6-2: Summary of TNB and Powerlink porcelain cap and pin insulator inspection
practice ..................................................................................................................................................... 127

10

LIST OF ABBREVIATIONS

AAAC

All Aluminum Alloy Conductor

AC

Alternating Current

ACSR

Aluminum Conductor Steel Reinforced

ACSS

Aluminum Conductor Steel Supported

AGNIR

Advisory Group on Non-Ionizing Radiation

AN

Audible Noise

ANSI

American National Standards Institute

ASTM

American Society for Testing and Materials

BS

British Standard

CIGRE

International Council on Large Electric Systems

DC

Direct Current

EMF

Electromagnetic Field

EPDM

Ethylene Propylene Diane Monomer

EPR

Ethylene Propylene Rubber

EPRI

Electric Power Research Institute

ESDD

Equivalent Salt Deposit Density

FIS

Fuzzy Inference System

FRP

Fiber Reinforced Plastic

GTACSR

Gap Built-in Heat Resistant Aluminum Alloy Conductor

IEC

International Electrotechnical Commission

IEEE

Institution of Electrical and Electronic Engineers

ISO

International Standard Organization

NRPB

National Radiological Protection Board

OPGW

Optical Fiber Ground Wire

RBS

Rated Breaking Strength

RCM

Reliability-centered Maintenance

RF

Radio Frequency

RIV

Radio Interference Voltage

ROW

Right-of-Way

RTV

Room Temperature Vulcanization

11


SiR

Silicon Rubber

TNB

Tenaga Nasional Berhad

UV

Ultra Violet

ZTACIR

Heat Resistant Aluminum Alloy Conductor Invar Reinforced

12


STATEMENT OF ORIGINAL AUTHORSHIP

“The work contained in this thesis has not been previously submitted for a master degree
at any other tertiary educational institution. To the best of my knowledge and belief, this

thesis contains no material previously published or written by another person, except
where due reference is made.”

Signed:
Mohd Junaizee Mohd Noor (Author)
Date:

13


ACKNOWLEDGEMENTS

In the name of God ALLAH, the Most Gracious the Most Merciful
First and foremost I would like to start by acknowledging, praising and thanking my Lord,
Allah, the Glorified and the Creator of all things, for blessing me with the good health and
wellbeing throughout my one-and-a-half year of studies and for making it a reality for me
to complete this thesis.
I would like to thank my academic supervisor, Associate Professor David Birtwhistle, for
his guidance and support during all the phases of this research degree. Throughout the
crests and troughs of the study, he was always there with invaluable technical advice and
assistance.
I would also like to state my sincere appreciation to my associate supervisor, Dr. Stewart
C. Bell of Powerlink Queensland, for sharing his expertise and thoughts as well as acting
as a sounding board for ideas, ensuring I knew exactly what I was talking about.
In addition, I would like to thank Ian Nichols and Maurice Donnelly from Powerlink
Queensland and Azmi Abdullah and Rafida Othman from TNB Malaysia for their
assistance during the data gathering phase of this study.
Thanks also to TNB Malaysia for providing me with the financial support and
opportunity to further my studies in this beautiful country of Australia.
Last and by no means the least, I would like to express my special gratitude to my

wonderful wife and colleague Dr. Farah Inaz Syed Abdullah, my lovely daughter
Esmeralda Noor, and my immediate family for their patience, compassion and
encouragements while pursuing this study. Hopefully they will get to see more of me, now
“it” is finished.

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CHAPTER 1: INTRODUCTION

1.1 Justification for and Introduction to the Research
High voltage transmission lines play a very important role in a power system. Apart from
their primary function of transferring power at high voltages from generating stations to
load centers, through various interconnections in the network, they provide the means for
effective, safe, economic and reliable operation of a power system. A transmission line
system consists of the following major components:


Tower or structure



Foundations



Conductors and earth wires



Insulators

Each of the components has its own functions in order for the transmission line to
operate. Failure of any of these components may render the transmission line inoperative.
Transmission line failures are highly undesirable in a power system because such failures
usually result in power interruption of a large area.
In order to ensure the healthy operation of transmission lines, power utilities conduct
periodic inspection and diagnosis of transmission line components. The main objective of
conducting inspection and diagnosis is to locate component defects so that appropriate
maintenance actions can be taken before the defects develop into catastrophic failures.
During the literature review, it was found that, apart from manual visual inspection, there
are other various methods of transmission line component inspection and diagnosis that
are currently available. Depending on the component, some of these methods utilize
equipments that detect characteristic parameters that are both electrical in nature, such as
voltage, current, corona and magnetic fields, and non-electrical in nature, such as vibration
and temperature. These equipments, however, were found to be either expensive, still in
development stages or highly affected by environmental factors during field operation.

15


However, in a recent survey on 90 utilities around the world conducted by CIGRE [1] and
a survey on Australian and New Zealand utilities conducted by McMahon [2], it was
discovered that the most common utility practice for locating component defects on
transmission lines is by visual inspection. Depending on which part of the tower that
needs to be assessed, visual inspection is conducted by line inspectors either from the
ground, from the top of the structure (by climbing the structure) or from the air (by
observing from fixed wing aircrafts or helicopters) [3].
Field experience indicates that there are drawbacks associated with visual assessment of
transmission line defects. These include:



Defect levels are expressed qualitatively using vague and linguistic terms such as
“medium crack”, “heavy rust”, “small deflection” and “large breakage”. These
statements vary from one inspector to another depending on the inspector’s
inherent knowledge, reasoning, experience, health and fitness levels, the
environment wherefrom the inspection is made and whether visual aids are used.



The assessment of the defects is based solely on the inspector’s personal judgment
making the inspection results highly subjective. Many a time the maintenance
engineer requires a second inspection by a more experienced inspector to confirm
the extent of the defect.



As a result, there is a large variance and inconsistency in defect level reporting
which, in time, makes it difficult to monitor the condition of the components.



This leads to the utility making ineffective or wrong maintenance decisions and
inefficient control of maintenance and repair expenditures.

The aim of this research is to develop a decision support tool that will primarily address
the problem of inherent human bias and subjective judgment that prevail during
transmission line inspection and assessment. The hypothesis that provides the motivation
for the study is that if such a tool is able to reduce or remove the uncertainty associated to
inspector visual assessment during field inspection, then the information gathered from

field inspection can be used effectively by the utility to manage its maintenance actions.
The tool is in the form of a standardized inference system utilizing fuzzy logic. Fuzzy
logic, as introduced by Zadeh in 1965 [4], is a form of artificial intelligence technique that

16


was specifically developed to handle the intrinsically fuzzy human thinking, reasoning,
cognition and perception process.
This thesis presents the development, application and results of a knowledge-based fuzzy
inference system (FIS) on utility inspection practice of assessing one of the most
important transmission line components, porcelain cap and pin insulators. The FIS is
designed using Matlab’s proprietary Fuzzy Logic module. Data regarding insulator
inspection practice and sample defective insulators on which the system is tested are
obtained from TNB Malaysia and Powerlink, the company responsible for the operation
and maintenance of transmission network in Queensland, Australia. The results of
applying the FIS show that not only can it reduce the subjectivity associated with visual
inspection of insulators but it can also be used to assist engineers in making strategic
decisions such as ‘replace immediately’, ‘flag for next maintenance cycle’ or ‘do nothing’.
The success of applying the FIS on insulator visual inspection provides the impetus for
applying the same design principle of the FIS on other components of the transmission
line which are also normally subjected to visual inspection.
The major contribution of the research is the presentation of a methodology that
improves the visual inspection of transmission line components. The methodology uses a
knowledge–based decision support tool. Such a tool can be programmed into mobile
electronic devices such as laptop computers or handheld personal digital assistants
(PDAs) for field use. The novel methodology can facilitate the visual inspection process
by:


Making available expert knowledge in the field



Reducing the inherent uncertainty faced by field inspectors



Providing a facility to store defect data in a computerized system for defect
analysis and condition monitoring



Enabling effective use of field data for use with modern maintenance policies
such as Condition-based Maintenance or Reliability-centered Maintenance

Another contribution of the research is the development of a mathematical model that
utilizes the output of the inspection program to plan for bulk maintenance of transmission
line components. To the best of the author’s knowledge, such a methodology has not

17


been investigated before and it is hoped that the research will provide utilities with a better
method of maintaining its transmission line assets.

1.2 Aims and Objectives of the Research
The research was conducted to achieve the following objectives:


To identify the main components of transmission lines, their various designs, and
their functions



To understand their failure modes that lead to loss of functions including factors
that affect their operation



To review the inspection and maintenance practices of utilities around the world



To identify the problems associated with utility practice of manual visual
inspection



To study the principles of fuzzy logic and fuzzy inference system



To design, apply and test a knowledge-based fuzzy inference system applied on
the visual inspection of porcelain cap and pin insulators



To investigate the use of the system as a decision-support tool for maintenance
management of transmission lines

1.3 Organization of the Thesis
Chapter two provides an introductory account of the components of transmission lines,
their functions and failure modes. Factors that can affect the operation of transmission
lines in a power system are discussed. Mitigation steps that are currently taken by utilities
to address these problems are also deliberated. Once the failure mechanisms are
identified, it is necessary to appreciate the available methods of inspection and
maintenance as practiced by the utilities. This is presented in the following chapter.
Chapter three is divided into two parts. The first part of the chapter looks into the
available methods of inspection and diagnosis of transmission line components that are
reported in various literatures. The advantages and disadvantages of these methods, some
of which utilize equipments that detect parameters such as voltage, current, noise,
vibration and temperature, are discussed. The second part of the chapter reviews the

18


inspection practices and maintenance strategies as conducted by utilities worldwide. The
results of recent industry surveys conducted by CIGRE and McMahon, which indicate
that worldwide utility practice of locating transmission line defects is by visual inspection,
are elaborated on. The chapter finally discusses the difficulties faced by the inspectors
when making visual inspection and the problems faced by utilities that use visual
inspection to manage its maintenance actions. This leads to the need for a decision
support tool that can be used to address the uncertainties that are associated with visual
inspection. This type of uncertainty can be best handled by the artificial intelligence
technique of fuzzy logic.
The next chapter provides background information on fuzzy logic which will be used later
in the proposed decision support tool to assist visual inspection of transmission line
components. The main elements of a FIS, namely membership functions, fuzzy IFTHEN rules, and defuzzification methods, are described here. Several example
applications of fuzzy logic in the utility environment that are available in the literature are

also discussed and commented on.
Having understood the principles of fuzzy logic and its applications, chapter five details
primarily the design, development and application of a knowledge-based FIS applied on
utility visual inspection practice to locate defects on porcelain cap and pin insulators. A
closer look at two types of porcelain insulator failure mechanisms – corrosion of the pin
and breakage of porcelain shells – are firstly presented. Current insulator visual inspection
practices of the two utilities (TNB and Powerlink) to locate the above defects are then
discussed, highlighting the associated problems. The chapter then proceeds to explain the
design of the insulator inspection FIS and its simulated application on actual in-service
insulators taken from both the utilities. The results of this application are then discussed,
indicating how the FIS assists in achieving consistency in assessing insulator defects. It is
shown next how the output of the FIS can be used to assess multiple insulators in a string
and also multiple insulator strings in a transmission line. It is also shown how the output
of the FIS can be further used in a proposed mathematical model to estimate the
appropriate date for bulk replacements of insulators. The chapter finally discusses the
application of the knowledge-based FIS on the visual assessment exercise of other
components of the transmission line.

19


Lastly, chapter six details the conclusions derived from the research contained in this
thesis. Future work, which the author feels may prove particularly beneficial, is also
elaborated on.

20


CHAPTER 2: COMPONENTS OF TRANSMISSION LINES AND THEIR
FAILURE MODES

2.1 Chapter Overview
This chapter presents an overview of the functions of transmission line components and
their failure modes relevant to the research described later in this thesis. The major
components of high voltage transmission lines as covered in this chapter are as follows:


Towers and structures



Foundations



Conductors and earth wire (including joints)



Insulators

Each of these components has its own functions that enable power to be transmitted
safely and reliably through transmission lines. Failure of a transmission line component as
outlined in this thesis is defined by the loss of its functions. This chapter discusses the
design of the components, their features and, most importantly, how they fail in
operation.
It is highlighted in this chapter that functional failures of transmission line components
are generally gradual and can be of electrical, structural or mechanical in nature. It is also
highlighted that these degradations are primarily influenced by environmental factors such
as local climate, weather, elevation and ambience wherein the transmission line is built.

Mitigation steps that utilities take to address these problems are also deliberated in this
chapter.
2.2 Towers and structures
Figure 2-1 shows a typical single circuit 765 kV extra high voltage transmission line and its
components commonly used in North America. Notice that the transmission line has two
earth wires (denoted by “shield conductors” in the figure) strung at the top of the
structure and the bundle conductors are held by v-string insulators.

21


Figure 2-1: Example of a typical overhead transmission line (Source: [5])

2.2.1 Functions of transmission towers
The main purpose of the transmission tower is to carry the overhead transmission line
conductors and earth wires above the ground. In fulfilling this role, it has to:


withstand all the variety of forces that it is exposed to with regards to the
environment it is located. These forces include normal still air loads, extreme wind
loads, ice loads, loads during erection and maintenance, and the changing of
conductor sag when the conductor expands and contracts with normal daily
current loads [5]. In certain instances, tower designs must withstand loads
imposed by extreme conditions such as in earthquake, cyclone and tornado areas
[6].



maintain electrical clearances between live conductors and any earthed body in the
vicinity of the tower such that the energized lines do not induce any hazardous

voltage that could render the operation of the transmission lines dangerous to the
environment and the public.

22




provide a path to earth fault and/or lightning current so that any danger to the
environment as a result of the two occurrences is reduced. Hence, the tower must
also exhibit low resistance to ground during transient lightning over voltages.

Depending on network requirements of a power system, the tower may be designed to
cater for a single 3-phase circuit, double 3-phase circuits, multiple voltage circuits, and,
with direct current transmission, either monopolar or bipolar construction. In certain
countries, due to land constraints, new transmission lines are always built on double
circuit towers and old single circuit lines are upgraded to double circuits to optimize the
use of land easements.
There are basically six different types of transmission line structures in common use
worldwide with many design variants based upon them. Table 2-1 lists the basic structure
types [7]:

Structure Type

Description

Self-supporting

Most common design style, easiest to work on but

lattice steel tower

makes the largest visual impact. Relative medium cost.

Lattice guyed steel Simple foundation but requires large easement width to
tower

accommodate guys. Lowest capital cost.

Steel/concrete

Slim appearance, low maintenance for galvanized steel

monopole

and concrete. Small easement width. Applicable in
urban areas where easement acquisition is expensive
and aesthetic visual impact is required. Highest capital
cost.

Compact

Special adaptations of the foregoing types to meet

structures

specific space or environmental limitations

H-frame in either Traditional rectilinear design, best suited to single
wood

pole

or circuit use. Requires wide easement.

concrete

Table 2-1: Basic transmission structure types
The selection of the type of structures to be used in a transmission line is mainly affected
by the environment where the structures are located. These factors are [7]:

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

Pollution level: whether the route of the transmission line is going to cross highly
polluted and corrosive area or environmentally sensitive country which requires
specific designs or surface treatment of the structures on part or all of the route



Terrain: whether there is difficult terrain where the line trespasses which makes
the structures susceptible to problems of land slides, floods or tidal inundations,
high soil resistivity, ‘bottomless’ sand, exposure to wind, lightning or other natural
phenomena



Aesthetic: whether the visual impact of the transmission line can be improved to

make it more acceptable to the public eye by using either a compact design or a
monopole



Climatic: whether the transmission line has to operate in extreme climatic
conditions such as regular high winds, extremes of ambient temperature
(including snow and ice) or marine salt sprays



Current load: whether the transmission line is meant to be operated during
emergency loading conditions within the transmission network



Maintenance requirements: whether maintenance on the transmission line can be
done live or de-energized, with climbing, using elevated platform vehicles or
helicopter

Figure 2-2 shows an example of 2 types of tower structures which carry 220 kV
transmission lines located side-by-side.

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Figure 2-2: Monopole and steel lattice tower designs
for 220 kV transmission lines (Source: [8])
Under normal circumstances, utilities worldwide use steel lattice transmission tower as the
structure of choice due to its cheap installation cost and relatively low operational

maintenance cost compared to the other types. To make the steel tower resistant to the
corrosive effects of the environment, the steel lattice members are treated in a process
called ‘hot-dip galvanisation’ which is to provide a layer of zinc by dipping the members
into a zinc bath. The zinc coating acts as a sacrificial element to protect the steel from
being oxidized or developing rust. One of the international reference standards for
coating thickness is based on BS EN ISO 1461:1999 [9] which requires coating thickness
of the steel members as per Table 2 below:

Steel thickness

Minimum average coating
weight, G/M2

Coating thickness,
M

1 mm up to 2 mm

335

47

2 mm up to 5 mm

460

65

Table 2-2: Requirement for zinc coating thickness as per BS EN ISO 1461 [9]

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