A Detailed Investigation of Overhead Line Insulators

Name: Adam Junid Elective: ELEC9214 Report Topic: “A Detailed Investigation on Overhead Line Insulators”

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A DETAILED INVESTIGATION ON OVERHEAD LINE INSULATORS Author: Adam Junid Course: ELEC9214 (Power System Equipment) School: Electrical Engineering & Telecommunications Date: 3 June 2006

Disclaimer: Data and references sourced in this report are, as far as I know, without copyright. I have avoided, as far as possible, using photos or figures from the web where the webmaster or owner clearly indicated copyright presence. Abstract: This report is a relatively detailed investigation on overhead line insulators. Aspects covered in this report include insulator type, design and material; environmental and voltage stress effects, relevant standards, and condition monitoring techniques.


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Acknowledgements: I thank my course lecturer Dr Toan Phung for his efforts and explanations to my questions on specific practice and design during ELEC9214 (Power System Equipment) lectures. I also thank Azmi Md Yusof of Tenaga Nasional Berhad (TNB) for his helpful feedback on overhead line types, design, their condition monitoring and typical contractual specifications.


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Acronyms and Abbreviations: ANSI: American National Standards Institute (http://www.ansi.org/) AS: Australian Standard (http://www.standards.org.au) ASTM: American Society for Testing and Materials (http://en.wikipedia.org/wiki/ASTM_International) BS: British Standards (http://en.wikipedia.org/wiki/British_Standards) IEC: International Electrotechnical Commission (http://en.wikipedia.org/wiki/International_Electrotechnical_Commission)

JIS: Japanese Industrial Standard

(http://en.wikipedia.org/wiki/Japanese_Industrial_Standard) MDWL: Maximum Design Withstand Load of the line insulator, also referred to

as an insulator’s designed “holding tension” (IEEE Std 987, 2001) MAD: Minimum Approach Distance for live line work (IEEE Std 516, 2003) MAID: Minimum Air Insulation Distance for live line work (IEEE Std 516, 2003) SML: Specified Mechanical Load of the line insulator, also referred to as an

insulator’s “nominated conductor tension” (IEEE Std 987, 2001). An insulator’s SML will be much less than its MDWL

Std: Standard


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Glossary:

Capacitance grading: A method of balancing the voltage stress more evenly over a string of

insulator units by adjusting each units’ capacitance towards the line end (Yusof, 2006b).

Core: Fibre reinforced resin rod that forms the mechanical load-bearing

component of a synthetic composite line insulator. (IEEE Std 987, 2001)

Creep: “Permanent deformation under stress, time and temperature” (IEEE

Std 987, 2001). Creep behaviour of synthetic composites are different from metals.

Grading device: A device for controlling the potential gradient at insulator end fittings,

such as a metal ring or various semiconductive polymeric devices. (IEEE Std 987, 2001) Linemen: Service personnel for line construction or maintenance. Overhead line insulator: A structure designed specifically to electrically insulate live cables

from electricity pylons, towers or poles. Pylon: A structure that may resemble a pole or tower frame, from which

overhead lines are suspended via overhead line insulators . Hotstick: An electrically insulative rod or pole used by linemen to perform

measurements or tests on live lines while keeping a safe distance from it.

Housing: A synthetic composite insulator’s polymeric covering over its core

material. Hydrophobic: Water-repellent Phase to phase insulator: An insulator coupling two phases together as a means of controlling

conductor spacing during line galloping. It is designed to be loaded in tension, torsion, bending or compression.

(IEEE Std 987, 2001) Post insulator: A type of line insulator designed to be loaded in torsion, bending, or

compression. The most common applications are a horizontal line post where the post projects nearly horizontally from a pylon and is loaded in flexure by the conductor, and a station post insulator used


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as a bus support. When post insulators are used in disconnect switch applications, they may also be loaded in torsion. (IEEE Std 987, 2001)

Shed: The “fins” seen on many line insulators, also referred to as water-shed structures. Shed geometries are designed to prevent conductive rainwater paths and increase external surface flashover paths.

Static shielding: A method of reducing high potential gradients (voltage stress

densities) and reducing capacitance coupling. For line insulators, this is typically achieved by using Grading devices (Yusof, 2006b).

Strain insulator: Any line insulator intended primarily to carry tension loads. It includes

dead-end, V-string applications. Strain insulators may also be applied as suspension insulators, in which case they will carry less tension than in the strain application.

(IEEE Std 987, 2001) Support Insulator: Line insulators such as post and pin types designed to support

cantilevered weight and come with a support (compression) rating. Tracking: Irreversible deterioration of surface material due to formation of

conductive carbonized paths (IEEE Std 987, 2001). Treeing: Irreversible internal deterioration by the formation of conductive or

nonconductive filamentary channels (IEEE Std 987, 2001).


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Contents:

Title 1 Abstract 1 Acknowledgements 2 Acronyms and Abbreviations 3 Glossary 4 Contents 6 List of Figures 7 Chapter 1: What is an overhead line insulator? 9 1.1: Types 1.2: Design 1.3: Material 1.4 Testing 1.4.1 Porcelain and glass insulators 1.4.2 Composite synthetic insulators 1.4.3 Design tests 1.4.4 Type tests 1.4.5 Tests to simulate lifespan and wearing

1.5 Other relevant standards Chapter 2: Customer requirements, selection, installation and wearing 25

2.1 Load bearing requirements 2.2 Electrical withstand requirements 2.3 Creepage distance requirements 2.4 Grading requirements and string efficiency 2.5 Handling requirements 2.6 Installation practice 2.7 Voltage stress effects 2.8 Mechanical stress effects 2.9 Environmental effects 2.10 Vandalism

Chapter 3: Condition monitoring and maintenance 34

3.1 Condition monitoring techniques 3.2 Maintenance 3.3 Replacement 3.4 Reducing bird risk

References and Citations 41 Appendices

A: Insulation Design for Overhead Lines (Narain, 2006) 45 B: Sample design and type test report (Hubbell, 1995)


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List of Figures:

Figure 1: Electrical pylon with disc and pin type overhead line insulators 9 Figure 2: Disc-type insulators applied as a strain (tension) string. 9 Figure 3: Disc insulators. Note the mixture of ball-socket and clevis-tongue

interlocking mechanisms. 10 Figure 4: Longrod insulators with clevis and tongue interlocking mechanisms. 11 Figure 5: Post insulator unit in a suspension application (foreground) and three 11kV

disc insulator units insulating 33kV lines in a strain (tension) application (background) 12

Figure 6: Pin type insulators. 13 Figure 7: Disc type (foreground) and Pin type (background) insulators. 13 Figure 8: Shackle type insulators. 14 Figure 9: Shackle type insulators in operation. 14 Figure 10: Post type insulators. 15 Figure 11: Hewlett type insulator. 16 Figure 12: Hewlett type insulator in operation with “corona rings” used as grading devices to help distribute electrical stress at the line bend. 16 Figure 13: Pot type insulators. 17 Figure 14: Synthetic composite insulator end-fitting, silicone rubber housing and

FRP rod core. 19

Figure 15: Suspended glass insulator undergoing HV discharge during accelerated aging test. 23 Figs. 16,17: Line insulator assemblies for strain (left) and suspension applications. 25 Figure 18: Line insulators taking strain (tension) at change of transmission line

direction. 26 Figure 19: Line insulators assembled prior to pole erection. 28 Figs. 20,21: Overhead lines strung and clipped to insulators after pole erection. 29 Figure 22: Surface tracking seen on a contaminated insulator surface. 30


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Figure 23: Arcing horns used to protect a Hewlett-type insulator from surface

tracking. 31 Figure 24: Disc insulator pin corroded by airborne saltwater. 32 Figure 26: Line insulator polluted by bird droppings. 33 Figs. 26,27: Birds electrocuted at pin-type (upright) insulators. 34 Figure 28: A vandalised disc insulator suspension string. Note the cement

integrity and adhesion to remaining glass despite disc destruction. Note also the grading device rings below the suspension string. 34

Figs. 29-31: Radio Direction Finding (RDF) methods used to pinpoint RFI sources. 35 Figs. 32,33: Infrared (IR) and Ultraviolet (UV) photography methods used to locate and monitor insulator defects. 36 Figure 34: Line insulator washing via helicopter. 37 Figure 35: Live line insulator replacement. 38 Figure 36: Application of safe bird perches and bird guards to protect line

insulators. 39 Figs 37,38: Application of bird guards over line insulators. 39 Figure 39: A bird nesting over line insulators. 40 Figure 40: Preparing to hoist a bird nesting platform beneath line insulators. 40


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1.0 What is an overhead line insulator? Overhead line insulators, as the name suggests, are used to electrically insulate pylons (see Figure 1) from live electrical cables (RTE, 2006).

Figure 1 Electrical pylon with disc and pin type overhead line insulators

(Source: Teleramics, 2006) Overhead line insulators may consist of a string of insulator units (see Figure 2), depending on insulator type and application. The higher the line voltage insulated, the more insulator units used in the string (RTE, 2006).

Figure 2 Disc-type insulators applied as a strain (tension) string.

(Source: Yusof, 2006a)


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1.1 Types Different types of line insulators are used, depending on voltage and mechanical strain (tension) requirements. The more widely used types are as follows (UNSW, 2006a): 1) Disc type (Figure 3 below), where insulation discs (also called insulation units) are

strung together depending on the insulation level desired.

Figure 3

Disc insulators. Note the mixture of ball-socket and clevis-tongue interlocking mechanisms. (Source: Adapt Australia P/L, 2006)

Each disc is typically rated at 10-12kV, with a capacitance of 30-40pF (UNSW, 2006b). Discs are strung together via their caps and pins. Locking mechanisms may be ball-socket or clevis-tongue type. The cap is insulated form the pin via the porcelain (or glass) disc which adheres to the cap and pin via adhesive cement.


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2) Longrod type (Figure 4 below). These may also be strung together for higher

insulation and may have similar ball-socket and clevis-tongue locking mechanisms used among the disc types (UNSW, 2006a). Their longer length makes them applicable for phase-to-phase insulation to reduce line galloping during strong winds (IEEE Std 987, 2001).

Figure 4 Longrod insulators with clevis and tongue interlocking mechanisms.

(Source: Adapt Australia P/L, 2006)


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Both disc and longrod-type insulators are commonly used in suspension or strain (tension) insulator applications (Figure 5 below).

Figure 5 Post insulator unit in a suspension application (foreground) and three 11kV disc insulator units insulating

33kV lines in a strain (tension) application (background) (Source: Teleramics, 2006)


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3) Pin type (Figure 6). Pin types are screwed onto a bolt shank secured on the cross-

arm of the transmission pole or pylon.

Figure 6

Pin type insulators. (Source: Adapt Australia P/L, 2006)

As seen in the below Figure 7 background, the pin type does not take main transmission line strain (tension) (Cotton, 1958), and functions as a jumper line insulator.

Figure 7 Disc type (foreground) and Pin type (background) insulators.

(Source: Teleramics, 2006)


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4) Shackle type insulators (Figures 8 and 9 below). These are mostly applied to

support line strain (tension), such as at changes of transmission line direction (Cotton, 1958).

Figure 8 Shackle type insulators.

(Source: Utilex, 2006)

Figure 9 Shackle type insulators in operation.



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5) Post type (Figure 10 below). These may have thicker insulation and more discs than pin types and can be mounted via clamp (Dulmison-Tyco, 2006) or pin method. They may be applied as a pin or strain type insulator, but rarely as a suspension type (IEEE Std 987, 2001).

Figure 10

Post type insulators. (Source: Adapt Australia P/L, 2006)

Since post-type insulators may also also act as a cantilever to support line weight,

post-type insulators normally have a Maximum Design Cantilever Load (MDCL) rating1 (IEEE Std 987, 2001).

Note: 1. IEEE Std 987 (2001) allows full utilisation of the MDCL rating for cantilever work loadings on post-type applications.


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6) Hewlett type (Figures 11, 12). A variation of the disc type, but can take more

mechanical strain due to internally insulated steel bolt interlocks holding discs together instead of cement. On the other hand, the Hewlett type has higher internal electrical stress due to its internal steel bolts.

Figure 11 Hewlett type insulator. (Source: Gish, 2006)

Figure 12 Hewlett type insulator in operation with “corona rings” used as grading devices to help distribute electrical

stress at the line bend. (Source: Gish, 2006)


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7) Pot type (see Figure 13), which are usually pin mounted and often used with telephone lines.

Figure 13 Pot type insulators.


Telephone utilities may use various types of insulators other than the above pot types (Teleramics, 2006). However, most telephone line insulator design variations appear very similar to the pot type.


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1.2 Design Overhead line insulators are designed to have both electrical insulation and mechanical strength. Highly insulative material is used (see section 1.3) and a recurring design theme are the “watershed” fins that discourage conductive water paths during rain and provides the required electrical leakage insulation distance (IEEE Std 987, 2001). The following design and manufacturing care is taken to ensure smooth electrical stress loading and mechanical integrity of line insulators: 1) Insulation materials may only be drilled or cored parallel sided, and may only be

hot-punched at forging temperatures (AS1154.1, 2004). 2) Sharp radii of curvature shall be avoided to reduce electrical stress (AS1154.1,

2004). In practice, this also means that porcelain insulators will be glazed and free from rough particles and unevenness (TNB, 2006). Glass insulators will be free from internal bubbles (TNB, 2006) that cause partial discharge and put the insulator at risk of explosion and failure

3) Dimensions such as shed and creepage distances may be adjusted for service in

high pollution environments (with or without rainwashing), areas of airborne sea salts, icing (IEEE Std 987, 2001) and bird risk areas (AS2947.2, 2002). Extra creepage distances are used to avoid inadvertent flashover in such highly ionised atmospheres or areas with large bird sizes (e.g. Sudan, North America).

4) Dimensions of insulator couplings are material strength dependent and guidelines

are specified in national standards such as AS2947.3 (2004) and IEC 120. 5) Voltage and waveform tests, including test control parameters such as water and

dampness are also specified in standards such as AS2947.4 ‘Insulators – Porcelain and glass for overhead power lines (Voltages greater than 1000Vac) – Part 4: Test methods – Insulator strings and insulator sets’ (2004).

6) For disc-type insulators, the cement adhesion strength should be strong enough to

hold the contacted disc insulator material to the cap and pin even when the exposed disc perimeter is shattered or vandalised (Figure 28, section 2.9).


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1.3 Materials Overhead line insulators are mostly made of the following materials (UNSW, 2006a): 1) Porcelain, which is widely used for all the abovementioned overhead line insulator

types. 2) Glass, which may be used for disc and pin types (Figure 7). It’s thermal stability is

consistent up to 538 degrees C (UNSW, 2006a). 3) Composite synthetics, which may be a combination of fibreglass, plastic and resin.

These are sometimes used for the longrod and post type insulators and have been in service for more than 25 years (IEEE Std 987, 2001). When modern composite synthetics are used, often the insulative core consists of glass fibers in a resin-based matrix to achieve maximum tensile strength (AS4435.1, 1996).

4) Plasticised wood, also referred to as Polymer Concrete (Gunasekaran, 1992) is

sometimes used for post type insulators. Polymer Concrete has demonstrated thermal stability in excess of 300 degrees C. Since both these designs utlise organic material, there have been concerns about material lifespan and lack of UV resistance (Kobayashi et al, 1999).

Figure 14 Synthetic composite insulator end-fitting, silicone rubber housing and FRP rod core.

(Source: Kobayashi et al, 2000)

The housing (Figure 14) that encloses a composite synthetic or plasticized wood also forms the water-sheds (Kobayashi et al, 1999) and may be hydrophobic (water repellent), which helps reduce leakage current. Some housings are designed to remain hydrophobic when polluted, giving composite synthetics a distinct advantage over porcelain types (Trench, 2000). A rule of thumb operating temperature range spec for housing is -50 to 50 degrees Celsius (IEEE Std 987, 2001).

5) Coupling fittings for overhead line insulators (i.e. the ball-socket and clevis-tongue

interlocks are normally galvanised cast iron and forged or mild steel. Clevis and pins may be specified with a coating of hot-dipped galvanised zinc to protect the base metal against severe corrosion (IEEE Std C135.20, 1998 and TNB, 2006).

6) For ease of load specification identification, each insulator is marked with its specified

Electromechanical Failing Load in conformance with IEC 60383(1993). Each insulator is also marked with the name or trademark of the manufacturer in accordance with IEC Publication 60383 (1993) (TNB, 2006).


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1.4 Testing 1.4.1 Porcelain and glass insulators In Australia, two types of tests are specified (AS1154.1, 2004) for porcelain and glass insulators: type tests and batch tests. For type tests, AS1154.1 (2004) requires three representative samples from each factory production specification run tested. A single factory production specification run is a factory production run using a consistent set of manufacturing specifications. Type tests for porcelain and glass insulators are mainly voltage withstand tests (Seeing Corona, 2004). For batch tests, AS1154.3 (2004) specifies both voltage and mechanical loading tests, based on the amount of units per batch of customer orders (IEEE C135.61, 1997). 1.4.2 Composite synthetic insulators Due to greater manufacturing variability in electrical and mechanical properties of composite synthetic insulators and difficulty in predicting polymer insulation performance (IEEE Std 987, 2001), two additional tests are required to ensure design viability and quality consistency: (1) design tests and (2) routine tests (AS4435.1, 2006). The complete family of design, type, batch and routine tests for composite synthetic insulators specified in AS4435.1 (2006) appears more comprehensive and stringent than the tests specified for porcelain and glass in AS1154.3 (2004). The tests detailed in AS4435.1 (2006) are: 1) Design tests, which include sudden release loading, thermal-mechanical loading,

water immersion, mechanical loading and dye penetration tests to detect material permeability.

2) Type tests, which include wet power testing and mechanical loading. 3) Batch (sample) tests, which include dimension, locking system and mechanical load

testing. 4) Routine tests, which include visual inspection and mechanical testing. The mechanical loading test consistently recurs at each stage of testing, underlining the importance of insulator mechanical integrity under both static and dynamic loading caused by wind, ice sloughing and fault current (Wang et al, 2001).


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1.4.3 Design tests Design test standards vary from manufacturer to manufacturer. In general, design tests will indicate specifications for holding tension1, failing load2, and nominated conductor tension3 for each insulator design. For composite synthetic insulators, holding tensions (MDWL) are typically 80-90% of failing load (Ultimate Strength), while nominated conductor tension (SML) are usually specified at 50% of holding tension (Polycast, 2006). Notes: 1. Also referred to as Maximum Design Withstand Load (MDWL) (IEEE Std 987, 2001)

2. Also referred to as Ultimate Strength, which Is usually twice the MDWL (IEEE Std 987, 2001)

3. Also referred to as Specified Mechanical Load (SML) (IEEE Std 987, 2001)

Minimum design tests for composite synthetic insulators described in IEC 61109 clauses 5.1-5.4 (Hubbell, 1995) are as follows: 1) Tests on Interfaces and Connections of Metal Fittings, which include the following:

- Dry power frequency voltage test - Sudden load release test - Thermal-mechanical test - Water immersion test - Steep-front impulse voltage test - Repeated dry power frequency test after the initial five tests.

2) Assembled Core Load-Time Test, which consist of:

- Determination of the average failing load of the core of the assembled insulator - Control of the slope of the strength-time curve of the insulator

3) Test of Housing: Tracking and Erosion Test

4) Tests for Core Material, which consists of a:

- Dye penetration test - Water diffusion test

Depending on application requirements, a more rigourous set of composite synthetic insulator design test standards may be specified (Kobayashi et al, 2000) as follows: 1) Overall performance:

- UV durability test (ASTM G53) - Ozone durability test (JIS K 6301) - Durability of end-fitting interfaces (IEC 61109) - Core load-time test (IEC 61109) - Housing tracking and erosion test (IEC 61109) - Core material test (IEC 61109) - Flammability test to (IEC 60707)

2) Electrical performance of insulator:

- Power-frequency wet withstand voltage (IEC 60383) - Lightning impulse wet withstand voltage (IEC 60383) - Switching impulse wet withstand voltage (IEC 60383) - Maximum withstand voltage of pollution to (JEC 170)


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- Arc-withstand characteristics (IEC SC36B (Secretariat) 116) - Corona characteristics (IEC 60437) - TV interface test (for V string insulators only)

3) Mechanical performance of insulator:

- Tensile breakdown strength (IEC 61109, JIS C 3801) - Tensile withstand load (IEC 61109) - Bending characteristics (JIS C 3801) - Bending breakdown strength (JIS C 3801)

The IEC 61109 durability of end-fittings test is particularly salient because end fittings are particularly prone to water ingress due to electromechanical defects. The insulator must endure 1000kV/microsecond steep-front voltage tests in positive and negative polarity, 25 times each without puncture of the end-fitting, housing or core material. 1.4.4 Type tests Minimum type tests are covered in detail in IEC 61109 clauses 6.1-6.4 (Hubbell, 1995) are: 1) Dry Lightning Impulse Withstand Voltage Test 2) Wet Power Frequency Test 3) Wet Switching Impulse Test 4) Mechanical Load-Time Test As in design testing, type testing may also be expanded and/or made more rigourous, depending on customer requirements.


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1.4.5 Tests to simulate lifespan and wearing For synthetic composite insulators, electrical aging simulation tests (Figure 15) may be carried out according to IEC 61109 Annex C, which involves accelerated electrical stressing under a natural environment and measuring cumulative insulator charge, leakage current, hydrophobicity and surface conditions using scanning electron microscopy (SEM) and photoelectron spectrometry (XPS) (Kobayashi et al, 1999).

Figure 15 Suspended glass insulator undergoing HV discharge during accelerated aging test.

(Source: Howson, 2006) In Japan, it was found that overhead line insulator failures increased during typhoons. Hence there have been studies on the links between increased leakage currents during typhoons and electrical aging due to “accelerated pollution” during the typhoon (Kobayashi et al, 1999). IEEE Std 987 (2001) also provides helpful guidelines in designing tests that simulate insulator aging, including load-time tests.


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1.5 Other relevant standards Apart from the AS, JIS and IEC design, manufacturing and test standards referred to in sections 1.2-1.5, relevant BS, IEC and IEEE standards for line insulators and associated fittings commonly used in the UK, Singapore, Malaysia (TNB, 2006) and US are: BS BS 3288-1 :1997 - Insulator and conductor fittings for overhead power lines. BS 3288-2 :1990 - Specification for a range of fittings BS 3288-3 :1989 - Dimension of ball and socket coupling of string insulator unit BS 3288-4 :1989 - Locking devices for Dimension of ball and socket coupling of

string insulator unit: dimensions and test IEC IEC 60060 – 1 (1989-11) - High Voltage Test Techniques: General definitions and test

requirements IEC 60060 – 1 (1989-11) - High Voltage Test Techniques: Measuring system IEC 60120 (1984-01) - Dimensions of ball and socket coupling string insulator units IEC 60305 (1995-12) - Characteristics of string insulators of the cap and pin type IEC 60372 (1984-01) - Locking devices for ball and socket couplings of string

insulator units: dimensions and tests IEC 60383-1(1993-04) - Insulators for overhead lines with a nominal voltage above

1000V: Ceramic or Glass Insulator units for A.C system IEC 60383-2(1993-04) - Insulators for overhead lines with a nominal voltage above

1000V: Insulator String and Insulator for A.C System IEC CISPR 18-2 - Radio Interference characteristics of overhead power lines

and high voltage equipment IEEE IEEE 987 (2001) - IEEE Guide for Application of Composite Insulators (includes sample and routine tests for tension loading) IEEE C135.61 (1997) - IEE Standard for the Testing of Overhead Transmission and

Distribution Line Hardware (includes batch testing guidelines)


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2.0 Customer requirements, selection, installation and wearing

Line insulator manufacturers supply their insulators to customers such as utilities or utility contractors based on customer and application requirements, which in good practice should exceed application requirements. Prior to placing an order for line insulators, the customer, contractor or design consultant would specify line insulator designs based on load bearing requirement calculations, electrical withstand requirements and creepage distances (which are typically based on IEC 815 pollution indices). Longrod and disc types insulators are widey applied as either strain (tension) type insulators or suspension type insulators (Figures 16,17) (Cotton, 1958).

Figures 16 (left) and 17 (right) Line insulator assemblies for strain (tension) (left) and suspension (right) applications.

(Source: Yusof, 2006a) Strain type insulator assemblies are commonly applied at long river and road crossings and changes of transmission line directions (Figure 18).


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Figure 18 Line insulators taking strain (tension) at change of transmission line direction

(Source: Yusof, 2006a) 2.1 Load bearing requirements To meet mechanical service requirements, line insulator applications are normally ordered and supplied based on specifications for holding tension1, failing load2 and nominated conductor tension3 (AS 1154.3, 2004) based on load-bearing calculations. An example of a line insulator load bearing calculation is given Appendix A. The transmission line designer must be aware of such load specifications, dynamic tension and safety margins to apply the insulator correctly. In practice, overhead line system designers will limit working loads for insulators to 50% of the manufacturer’s nominated conductor tension (SML) (IEEE Std 987, 2001). Sometimes, porcelain post-type insulators tested to ANSI standards may only have a single mechanical rating given as an average of failing load (Ultimate Strength) test results. In such circumstances, IEEE Std 987 (2001) recommends taking the MDWL as 40% of failing load. It is important for the line designer or drafter to be aware of the insulator rating systems being used. To prevent galvanic corrosion, electrical contact (mating) surfaces of the insulator are normally specified to be of similar material to the adjacent connections(TNB, 2006) Notes: 1. Also referred to as Maximum Design Withstand Load (MDWL) (IEEE Std 987, 2001)

4. Also referred to as Ultimate Strength, which Is usually twice the MDWL (IEEE Std 987, 2001)

5. Also referred to as Specified Mechanical Load (SML) (IEEE Std 987, 2001)

2.2 Electrical withstand requirements The transmission line designer must also specify the desired electrical withstand of the insulators, including switching impulse voltage magnitudes (see Appendix A). For voltages less then 220kV lightning impulses may have more effect on voltage transients than


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switching, so the designer may decide that insulator lightning withstand spec is the critical spec for insulators applied at less then 220kV. 2.3 Creepage distance requirements Creepage distance calculations are based on pollution indices at the area of installation. An sample creepage distance calculation is given in Appendix A and is based on IEC 815 pollution level classifications. Minimum distances for porcelain and composite synthetic insulators may also be specified (typically 20-25 mm/kV). 2.4 Grading requirements and string efficiency The transmission line designer must be aware that voltage stress does fall off linearly from the line end disc with number of insulator discs used. This is due to capacitance coupling of each disc with the pylon and neighbouring structures. This has an effect on string efficiency, defined as {total voltage insulated} divided by {number of discs x voltage on line end unit} (UNSW, 2006b). Grading devices (Figure 12, section 1.1, Figure 28, section 2.10) are often used to even out voltage stress along the string by spreading the voltage stress densities away from line-end units, increasing string efficiency, (UNSW, 2006b). Grading devices also channel external flashovers away from insulator surfaces, preventing surface damage. They are specified for insulators 230kV and above in IEEE Std 987 (2001). Capacitance grading may also be used to increase string efficiency but is rarely implemented because discs of different capacitance are tedious to stock and replace (Yusof, 2006b). Typically, 132kV lines have 10-14 11kV insulator discs and 275kV lines 20-25 11kV discs, depending on grading ring design and pylon material (Yusof, 2006b). 2.5 Handling requirements Synthetic composite insulators are light and can cut and scratch easily. The following handling practice is recommended in IEEE Std 987 (2001): 1) Storage away from rodents. 2) Careful cutting and unpacking of the packaging they are delivered in. 3) Insulators should not be stepped on or stacked directly on one another because

their sheds can cut into each other. 4) They should be handled by their end fittings and inspected for cuts and abrasions

prior to mounting and secured to pre-erected pylons to prevent swinging and compression.

5) Lift slings should never be placed over the sheds.


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An inspection of type test certificates and data sheets supplied with the insulators is usually done by nominated utility inspectors prior to actual installation. On-site certificate checking prior to installation may include checks of sample and routine test certificates. 2.6 Installation practice A summary of insulator and line installation is as follows. First, electrical pylons are designed and constructed according to function: suspension, strain or dead-end. Line insulator specification selection is also made and is thus part of the pylon’s design and function (IEEE Std 977, 1991). For construction efficiency, line stringers (travelers) and line insulator assemblies are mounted while the pylon is assembled on the ground (IEEE Std 1025, 1993) as per Figure 19.

Figure 19 Line insulators assembled prior to pole erection.

(Source: IEEE Std 1025, 1993) However, if pylon assembly is done from the air via the “pole stack” method, line insulator mounting is only done after the entire pole stack is completed to prevent “impact loading” of the line insulators. IEEE Std 1025 (1993) emphasises grounding of structures under erection when erecting in the vicinity of energized lines. After complete assembly and erection of pylons, all pylon bolts should be securely tightened. Line insulator bolts and nuts must be torqued to design drawing specifications. Overhead lines are then strung along travellers at 5-8 km/h, avoiding unnecessary torsion (Figures 20 and 21 below). Cable static seating for more than 24 hours is avoided to prevent cable creepage (IEEE Std 1025, 1993).


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Figures 20 and 21 Overhead lines strung and clipped to insulators after pole erection.

(Sources: Roosevelt, 1945 and Babock PLC, 2006) After cables are tensioned and sagged to design drawings, suspension insulator connection locations are then attached (“clipped”) to their appropriate locations on the lines (IEEE Std 524, 2003). All lines are grounded during clipping. After clipping, line conductors are lifted, stringers (travelers) removed, and suspension clamps are placed on conductors for permanent line tension.


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2.7 Voltage stress effects For line insulators in general, changes in surface resistance due to chemical changes and variations on the surface or pollutive films covering the surface have an effect on surface resistance, leakage currents and withstand voltage of the insulator (IEEE Std 987, 2001). Hence voltage discharge external to the insulator may occur when the insulation material is too polluted, wet, and has a reasonably low resistance path allowing for the discharge during lightning, switching or transient overvoltages. Every time a discharge (external flashover) occurs, the insulator is at risk of “tracking”, a phenomenon where a physical indentation or scar appears as a semiconductive “track” caused by an electrical arc over the insulator surface (Figure 22).

Figure 22 Surface tracking seen on a contaminated insulator surface..

(Source: Kobayashi et al, 2000) Over time, with more and more discharges along the surface, the track may worsen and weaken the insulator further. Arcing horns (Figure 23) installed on line insulators may reduce the risk of surface tracking by providing a discharge path further away from the insulator material.


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Figure 23 Arcing horns used to protect a Hewlett-type insulator from surface tracking.

(Source: Anderson, 2001) For the specific case of synthetic composites, studies have shown that the surfaces synthetic composite polymer housings are relatively mobile compared to porcelain and glass, and “have much greater freedom for rearranging in the bulk or at the surface” (IEEE Std 987, 2001). Polymer surfaces also have the interesting ability to interact with pollutants to reduce the conductance of the pollution layer, thereby improving insulator performance (IEEE Std 987, 2001).

2.8 Mechanical stress effects Mechanical stresses are caused by tension, bending, compression or torsion loads, which may be static or dynamic (IEEE Std 987, 2001). Repeated mechanical stresses can result in a unique creep1 phenomenon for composite synthetic insulators, where the residual strength of the composite material remains very high until the instant of failure (IEEE Std 987, 2001). Mechanical stress effects are minimised by stringent mechanical testing of insulators. Long term mechanical and insulative performance of composite synthetic insulators are critically dependent on the continued protection provided by the housing (IEEE Std 987, 2001). Housing deterioration or aging must never result in exposure of the rod to the environment because this will rapidly change the mechanical characteristics of the rod (IEEE Std 987, 2001).


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2.9 Environmental effects

Environmental effects on line insulators such as end-fitting moisture ingress, surface area contamination and extreme wind-loading (such as in typhoons) contribute to insulator failure (Farquhar, 1998 and Kobayashi et al, 1999). Saltwater ingress can also corrode disc insulator pin material and weaken its strain (tension) rating (Figure 24).

Figure 24 Disc insulator pin corroded by airborne saltwater.

(Source: Yusof, 2006a) Periodic testing of insulators using the “Hi-test insulator tester” or “Buzzer” method (section 3.1) minimises risk of insulator failure due to moisture ingress (Farquhar, 1998 and TNB, 2006). Insulators installed in areas with heavy airborne pollutants such as phosphate, cement, pulp and lime processing plants have experienced external flashovers due severe insulator pollution that sometimes engulfs the entire insulator. In South Africa, line insulator reliability was found to suffer due to bird droppings (Figure 25) and bird electrocutions (Figure 26, 27). In South Africa, birds perched above line insulators that do not bridge an electrical circuit still get electrocuted when they excrete continuous “streamer” droppings or urine that bridge a circuit across the below insulators (Endangered Wildlife Trust, 2006). Periodic insulator washing (section 3.2 and Western Power, 2006) reduces the risk of insulator failure due to surface area contamination. Maintenance techniques to prevent bird perching are discussed in section 3.3. Bird electrocution occurs when a bird’s wings or “other appendages” complete an electrical circuit by bridging the gap between two live wires, or a live wire and a grounded wire or structure (UCSC, 2002). In addition to insulator failure, line outages and endangered bird deaths, hazardous bushfires and wildfires sometimes start when the electrocuted bird catches fire and falls to the ground. During electrocution, the insulator is shorted out, the bird is killed or severely burned, and power outages occur.


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Figure 25 Line insulator polluted by bird droppings. (Source: Endangered Wildlife Trust, 2006)

Studies by The German Society for Nature Conservation (NABU, 2005b) have shown that: 1) Birds as small as 25 cm can short out pin-type or pot-type “upright” insulators

between the metallic cross-bar and the live line (Vosloo, 2003). 2) Insulator arcing horns with gaps 60 or less are particularly at risk of shorting due to

bird electrocution. 3) Insulators that most at risk to shorting via bird electrocution are those designed

between 1 - 60kV transmission. A bird-safe arcing horn has been developed in Japan by IERE (2005). In effect, the device utilises a subhorn and “mini-insulator” that prevents bird electrocution through the main horns under normal operating conditions. Abnormal operating currents from lightning strikes and voltage surges are passed through the mini-insulator to the main horns. The subhorn picks up flashovers due to surface area contamination. Although vultures in South Africa have been known to eat fibre-optic cable insulation (Vosloo, 2003), incidents of synthetic composite line insulator housing being eaten by vultures or other wildlife are rare. Crows’ nests that contain abandoned wiring material have also been known to short out insulators when they span a circuit bridge (Endangered Wildlife Trust, 2006). Maintenance techniques to reduce such risks are discussed in section 3.3.


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Figures 26 and 27 Birds electrocuted at pin-type (upright) insulators.

(Sources: NABU, 2005a, and UCSC, 2002) 2.10 Vandalism Glass and porcelain insulators are more susceptible to shattering by thrown or shot projectiles (Figure 29). This is a major reason for recent trends in insulator replacements to composite synthetics.

Figure 28 A vandalised disc insulator suspension string.

Note the cement integrity and adhesion to remaining glass despite disc destruction. Note also the grading device rings below the suspension string.

(Source: Yusof, 2006a)


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3.0 Condition monitoring and maintenance Studies have shown that the highest incidence of insulator failure occurs at dead-end applications, where the mechanical and electrical stresses are higher (Farquhar, 1998 and Kobayashi et al, 1999). In addition, non-visible insulator failure is more common than visible insulator failure (Farquhar, 1998). Symptoms of non-visible insulator failure include (Farquhar, 1998):

1) Radio frequency interference (RFI) 2) Blinking lights 3) Nuisance overcurrent relay and ground fault tripping 4) Blown fuses 5) Pole top fires

Non-visible insulator failure includes non-visible moisture ingress inside an insulation string. The moisture may get vapourised during a lightning strike or switching surge, resulting in sudden internal volume expansion and the insulator blown apart (Farquhar, 1998). Condition monitoring of line insulators allows operation and maintenance crews to detect and pinpoint insulator failure. This may be done in real time via leakage current monitoring (Metwally et al, 2006), during RFI troubleshooting (Martin, 2005), during periodic inspections or as a safety procedure prior to live line work (Farquhar, 1998). 3.1 Condition monitoring techniques A power utility that receives customer complaints about non-visible insulator failure symptoms will usually investigate sources of RFI interference along nearby overhead line insulators. Such investigations may utilise Radio Direction Finding (RDF) techniques (Martin, 2005), and equipment for such techniques are shown in Figures 29-31 below.

Figures 29, 30, 31 Radio Direction Finding (RDF) methods used to pinpoint RFI sources.

(Source: Martin, 2005) Alternatively, or in conjunction with RDF, infrared (SPI, 2006) or UV photography (Consillia, 2006) may be applied to further pinpoint the insulator responsible for the RFI noise (Figures 32, 33). An advantage of UV over IR photography is the UV camera’s ability to capture electrical corona discharges on cracked or punctured insulators at up to 150m range (Ofil, 2005).


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Figures 32, 33 Infrared (IR) and Ultraviolet (UV) photography methods used to locate and monitor insulator defects.

(Sources: Thermal Wave, 2006, Consillia, 2006) Since non-visible insulator failure may also pose a safety threat to line crews, standard practice for condition monitoring prior to live-line work involves line testing using a the “Hi-test insulator tester” (Farquhar, 1998) or “Buzzer” (TNB, 2006) method. This method has also been used to detect non-visible defects on insulators within hotstick range. The “Hi-test insulator tester” or “Buzzer” method involves fitting a 10kVDC self-contained insulator tester at the end of a hotstick (Figure 31) and placing the tester’s two probes such that the line insulator is in between them. The condition of the insulator is then indicated via a LED display or audible warning buzzer (Farquhar, 1998).


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3.2 Maintenance One way of reducing or eliminating radio frequency interference (RFI) caused by insulator failure or defects is to de-energise the line and replace the insulator. A temporary method for porcelain or glass insulators is to repair or patch the insulator’s arc voids using grease (Boucher, 2005). Greasing is not recommended for synthetic composite (IEEE Std 987, 2001), because the grease may worsen tracking along the composite’s surface. Preventive maintenance of line insulators involves ensuring they are debris, salt and pollution-free. There have been studies and proposed techniques for assessing line insulator pollution rates by monitoring insulator leakage currents (Elkateb and Khalifa, 1988) but such real-time pollution monitoring techniques are not yet widespread. Scheduled washing of line insulators remains a more widespread and economical practice for preventive maintenance. One method of washing is by helicopter (Figure 34), however, washing via live line approaches are also possible (Silcock, 2006).

Figure 34 Line insulator washing via helicopter.

(Source: Heliservices, 2006)

IEEE Std 987 (2001) recommends consulting with insulator manufacturers prior to high pressure washing (3-7 MPa), because not all composite synthetic insulator designs can withstand such force.


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3.3 Replacement Damaged and defective insulators usually require replacement. If a porcelain or glass insulator is to be replaced with its synthetic composite equivalent, care must be taken in selection of the new insulator because insulators with similar dry arc ratings may have different electrical characteristics due to the difference in end-fitting lengths. However, if grading rings are used, the dry arc distance is still the distance between rings (IEEE Std 987, 2001). Insulator replacement (Figure 35) is typically done via live-line methods (KTPower, 2006 and Smith & Mailey, 2003) to prevent grid power interruption.

Figure 35 Live line insulator replacement.

(Source: Transgrid, 2006)


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3.4 Reducing bird risk A guide to reducing risks of bird electrocutions by making electrical pylon and design and insulator location more bird-friendly is provided in a copyrighted brochure by The German Society for Nature Conservation (NABU, 2005b). Current practice in South Africa and North America is to place safe perches and bird guard structures on pylons (Figures 36-38).

Figure 36 Application of safe bird perches and bird guards to protect line insulators.

(Source: Hill Air Force Base, 2006)

Figures 37, 38 Application of bird guards over line insulators.

(Source: Endangered Wildlife Trust, 2006)


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To reduce the risk of birds nesting over power lines (Figure 39) and their urine or dropping shorting out line insulators, bird nesting platforms (Figure 40) are sometimes constructed in pylons safely below the level of the line insulators (Endangered Wildlife Trust, 2006).

Figure 39 A bird nesting over line insulators.


Figure 40 Preparing to hoist a bird nesting platform beneath line insulators.



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References and Citations

Adapt Australia P/L, 2006, ‘Ceramic and Glass Insulators’, http://www.adaptaust.com.au/Porcelain_Insulators.htm#disc , viewed 21 May 2006. Anderson, J. 2001, ‘Hewletts with V-S Arcing Horns’, New Hampshire, September 2001, http://www.r-infinity.com/Hewlett/Page5.htm , viewed 27 May 2006. AS 1154.1, 2004, ‘Insulator and conductor fittings for overhead power lines – Part 1: Performance, material, general requirements and dimensions’, Sections 1.4.2, 1.4.4, 1.5.2. AS1154.3, 2004, ‘Insulator and conductor fittings for overhead power lines – Part 3: Performance and general requirements for helical fittings’, Sections 1.4.7, 1.4.8, 1.6. AS2947.2, 2004, ‘Insulators – Porcelain and glass for overhead power lines – Part 2: Characteristics’, Sections 2.1, A.3. AS2947.3, 2004, ‘Insulators – Porcelain and glass for overhead power lines (Voltages greater than 1000Vac) – Part 3: Couplings’ AS2947.4, 2004, ‘Insulators – Porcelain and glass for overhead power lines (Voltages greater than 1000Vac) – Part 4: Test methods – Insulator strings and insulator sets’ AS4435.1, 2004, ‘Insulators – Composite for overhead lines (Voltages greater than 1000Vac) Part 1: Definitions, test methods and acceptance criteria for string insulator units’, Section 3.2, 4.1, 4.2, 4.3, 4.4, 5.1.3.1, 5.1.3.2, 5.1.3.3, 5.2.2

Babcock PLC, ‘Line Stringing’, http://www.babcock.co.uk/content/operating/opco.gallery.pic.cfm?RecordID=34 , viewed 17 May 2006. Boucher, J. 2005, ‘The Issue of Power-Line Noise: A Smarter Approach (VI: How to fix)’, ARRLWeb, http://www.arrl.org/tis/info/HTML/pwr-line-noise/ch6.html , viewed 27 May 2006 Consillia, 2006, ‘UV Corona Cameras for Predictive Maintenance’, Consillia Ltd, http://www.uvcamera.co.uk/ , viewed 20 May 2006. Cotton, H. 1958, ‘The Transmission and Distribution of Electrical Energy’, The English Universities Press, Ltd, London, http://www.myinsulators.com/acw/bookref/insulator/ , viewed 21 May 2006. Dulmison-Tyco, 2006, Insulators catalogue, http://www.dulmison.com.au/catalogue_files/DulCat/DulSec10web.pdf , viewed 21 May 2006. Elkateb, M. and Khalifa, M. 1988, ‘Early warning pollution monitor for overhead line insulators to secure power supply’, IEEE paper. Endangered Wildlife Trust, 2006, Eskom powerlines strategic partnership activities, http://www.ewt.org.za/workgroups_overview.aspx?group=eskom&page=activities&morePage=activities_more&activity=4 , viewed 28 May 2006. Farquhar, J. A. 1998, ‘Non-visible Insulator Failure’, HD Electric Archive, presented at Tech Advantage 98, March 1998, Nashville, Tennessee, http://hdelectriccompany.com/paper.htm


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Gish, E. 2006, ‘Hewlett Insulators’, http://www.r-infinity.com/Hewlett/index.htm , http://www.r-infinity.com/Hewlett/Page2.htm , viewed 21 May 2006. Glover, J.D. and Sarma, M.S. 2002, Power System Analysis and Design, Brooks/Cole (an imprint of the Wadsworth Group, a division of Thomson Learning Inc), pp. 200-208, 250-251, 592-595, 638-639. Gunasekaran, M. 1992, ‘The potential of polymer concrete for use on high-voltage overhead line insulators’ by Sekar Enterprises in IEEE Transactions on Electrical Insulation. Heliservices, 2006, ‘Insulator Washing’, Heliservices Insulator Washing Service, http://www.heliservices.com.hk/news/insulator_wash.html , viewed 27 May 2006. Hill Air Force Base, 2006, ‘Nature and Our Air Force Living Together Peacefully’, Hill Air Force Base Natural Resources, Ogden, Utah, http://www.em.hill.af.mil/conservation/natural/nrstart.htm , viewed 28 May 2006. Howson, P. 2006, ‘Power and High Voltage Engineering: Degradation and Failure Processes’, University of Brighton Research and Consultancy, http://sesis.eng.bton.ac.uk/research/rcpapers/power/deg.htm , viewed 28 May 2006 Hubbell, 1995, ‘IEC Design and Type Test Report’, Hubbell Power Systems, http://www.hubbellpowersystems.com/powertest/literature_library/pdfs4lib/Design-Test-Reports/EU1440.pdf , viewed 29 May 2006. IEEE Std 516, 2003, ‘IEEE for Maintenance Methods on Energized Power Lines’, IEEE Power Engineering Society, 2003.

IEEE Std 524, 2003, ‘IEEE Guide to the Installation of Overhead Transmission Line Conductors’, IEEE Power Engineering Society, 2003. IEEE Std 977, 1991, ‘IEEE Guide to the Installation of Foundations for Transmission Line Structures’, IEEE Standards Board, 1991. IEEE Std 987, 2001, ‘IEEE Guide for Application of Composite Insulators’, sponsored by the Transmission and Distribution Committee of the IEEE Power Engineering Society, approved by ANSI, 2002. IEEE Std C135.20, 1998, ‘IEEE Standard for Zinc-Coated Ferrous Insulator Clevises for Overhead Line Construction’, IEEE-SA Standards Board, 1998.

IERE, 2005, ‘Development of Bird-Safe Device (Preventive measure against bird failure)’, Chubu Electric Power Co., Inc. Japan, http://www-iere.dcc.co.jp/NonMembers/RD_Current_Topics/CEPCOjp/Topic4.html , viewed 28 May 2006. Kobayashi, S. and Matsuzaki et al. 1999, ‘Development of Composite Insulators for Overhead Lines’, Furukawa Electric, http://www.furukawa.co.jp/review/fr019/fr19_23.pdf , viewed 28 May 2006. Kobayashi, S. and Matsuzaki et al. 2000, ‘Development of Composite Insulators for Overhead Lines (Part 2)’, Furukawa Electric, http://www.furukawa.co.jp/review/fr021/fr21_11.pdf , viewed 28 May 2006. KT Power, 2006, ‘Live Line Specialists’, New York, http://www.ktpower.com/projects.php , viewed 28 May 2006. Martin, M. 2005, ‘The Issue of Power-Line Noise: A Smarter Approach (V: How to Locate Power-line Noise)’, RFI Services,


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published on ARRLWeb, http://www.arrl.org/tis/info/HTML/pwr-line-noise/ch5.html, viewed 27 May 2006. Metwally, I.A. and Saif et al. 2006, ‘Online Condition Monitoring of Line-Insulator Leakage Currents’, Internal Journal of Emerging Electric Power Systems, Vol. 5, Issue 1, Article 5, 16 March 2006, http://www.bepress.com/ijeeps/vol5/iss1/art5/ , viewed 30 May 2006. NABU, 2005a, ‘Bird Protection on Power Lines’, German Society for Nature Conservation, http://www.nabu.de/m05/m05_03/00670.html , viewed 28 May 2006. NABU, 2005b, ‘Caution: Electrocution! – Suggested Practices for Bird Protection on Power Lines’, German Society for Nature Conservation, http://www.nabu.de/vogelschutz/caution_electrocution.pdf , viewed 28 May 2006. Narain, S. 2006, ‘Insulation Design for Overhead Lines’, Eskom. Ofil, 2005, ‘DayCor corona camera for high voltage lines and insulators maintenance’, Ofil UV Bandpass Technologies, http://www.daycor.com/transmission.html , viewed 30 May 2006. Polycast, 2006, ‘Standards and Ratings – Insulators’, Polycast International, http://www.polycast.mb.ca/sr/insulators.cfm , viewed 29 May 2006. Roosevelt, F.D. 1945, ‘Stringing rural TVA transmission line’ by Rural Electrification Administration (REA) – Tennessee Valley Administration (TVA), Franklin D. Roosevelt Library Public Domain Photographs, http://www.archives.gov/education/lessons/fdr-inaugural/ , viewed 27 May 2006. RTE, 2006, Réseau de Transport d'Electricité website, http://www.rte-

france.com/htm/an/qui/lignes_aeriennes.jsp , viewed 21 May 2006. Satoshi, K. and Matsuzaki, 2000, ‘Development of Composite Insulators for Overhead Lines’, Furukawa Review No 19, 2000. Seeing Corona, 2004, ‘Fault detection with UV and infrared camera’, Ofil Ltd, http://www.seeing-corona.com/UGM2004_3.html , viewed 28 May 2006. Silcock, R. 2006, ‘Live maintenance of high voltage transmission lines’, ABB New Zealand. http://www.abb.com/global/abbzh/abbzh251.nsf!OpenDatabase&db=/global/gad/gad02077.nsf&v=61DE&e=us&c=5975E947217CDC92C1256EFA0048910E , viewed 27 May 2006. Smith, G.W. and Mailey, S.M. 2003, ‘Aerial Damper Changeout on Energized 500kV DC Line’, Transmission and Distribution World, August 2003, http://www.nxtbook.net/live/Primedia/TDaugust2003/ , viewed 31 May 2006. SPI, 2006, ‘PDM infrared thermography camera’, Sierra Pacific Innovations, 2006, http://www.x20.org/thermal/ir_camera_470.htm , viewed 30 May 2006. Teleramics, 2006, ‘Insulator Collecting UK’, http://teleramics.com/inuse/inuse2003.html , http://teleramics.com/inuse/inuse2004.html , http://teleramics.com/inuse/inuse2006.html , http://teleramics.com/type/indextype.html , viewed 21 May 2006. Thermal Wave, 2006, ‘Insulator Oxidation Effects’, Fittings High Tension, http://www.thermalwave.it/eng/applic/high_voltage.htm , viewed 30 May 2006.


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TNB, 2006, Extract from sample contract for insulator, conductor and earthwire fittings, Tenaga Nasional Berhad. Transgrid, 2006, ;Liveline/Barehand Method Application and Development’, Transgrid, Australia, http://www.transgrid.com.au/trim/trim175066.pdf , viewed 28 May 2006. Trench, 2000, ‘Composite Insulators’, http://198.173.130.4/main/trench/trenchmain.nsf/AllDownLoadDocs/0CC190CFA0232D3485256B6E000D13F2/$FILE/E205.pdf , viewed 28 May 2006. UCSC, 2002, ‘Bird deaths spur grant: California Energy Commission enlists UCSC research group in effort to prevent bird deaths caused by power lines’, University of California - Santa Cruz currents online, 27 May 2002, http://www.ucsc.edu/currents/01-02/05-27/birds.html , viewed 28 May 2006. UNSW, 2006a, ‘Overhead Lines: Insulators’, Power System Equipment (ELEC9214) Lecture Notes, section 4.5, slides 19-21. UNSW, 2006b, ‘Power Equipment: Insulation of High Voltage Equipment’, Electrical Energy Systems (ELEC9213) Lecture Notes, section 4.4.1, slides 33-43.

Utilex, 2006, ‘Low tension insulators hardware’, http://www.utilex.com/lowtension.htm, viewed 21 May 2006. Vosloo, H, 2003, ‘Birds and power lines’, ESI Africa, Issue 3, 2003, http://www.esi-africa.com/last/esi_3_2003/033_38.htm , viewed 28 May 2006. Wang, L. and Yin et al, ‘Study on air insulator strength under conductor galloping condition by phase to phase spacer’ by Tsinghua University, IEEE 2001 Annual Report Conference on Electrical Insulation and Dielectric Phenomena. Western Power, 2006, ‘Overhead Line Insulator Washing’, http://www.westernpower.com.au/mainContent/ourServices/PowerTrainingServices/coursesAvailable/OverheadPowerlineTrainingCourses/Overhead_Line_Insulator_Washing.html , viewed 27 May 2006. Yusof, A.M. 2006a, Photograph samples of line insulators in Malaysia, e-mail received 30 May 2006. Yusof, A.M. 2006b, ‘Insulator String Efficiency’, e-mail received 2 June 2006.

Documents

A Detailed Investigation of Overhead Line Insulators