An Investigation of the Interface Between Various

i

An Investigation of the Interface Between

Various Overhead Distribution Insulator

Types and 11kV Covered Conductor

By

MARK APPLETON

School of Information Technology and Electrical Engineering

University of Queensland

Submitted for the degree of Bachelor of Engineering (Honours)

In the division of Electrical and Electronic Engineering

October 2001

ii

73 Hawken Dr.

St. Lucia QLD 4067

Ph: (07) 3371 2809

October 18, 2001

Head of School;

School of Information Technology and Electrical Engineering,

University of Queensland

St. Lucia QLD 4067.

Dear Professor Kaplan,

In accordance with the requirements of the degree of Bachelor of Engineering in the division of

Electrical and Engineering, I present the following thesis entitled “An Investigation of the Interface

Between Various Overhead Distribution Insulator Types and 11kV Covered Conductor”. This

work was sponsored by Energex and completed under the supervision of Dr Tapan Saha.

I declare that the work submitted in this thesis is my own work, except as acknowledged in the

text, and has not been previously submitted for a degree at the University of Queensland or any

other institution.

Yours faithfully,

Mark Appleton

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Acknowledgements

I would like to thank Dr. Tapan Saha for all of the assistance he has provided throughout the

year. He has been most helpful in lending his expertise and knowledge and ensuring that my

work progressed with as few problems as possible. When problems arose, he was willing to

sacrifice his time to help me resolve the issues.

The High Voltage Laboratory supervisor, Mr. Steven Wright, is also worthy of many thanks. He

was responsible for ensuring safety procedures were abided by and that proper testing

techniques were employed.

Mr. John McDonald made many sacrifices through out the year to help with the partial discharge

testing and he was always available for consultation about any problems I encountered.

Energex has been extremely helpful with regards to my thesis. My Energex supervisors, Mr. Pat

Pearl and Mr. Greg Dowling, were both willing to take time out of their busy schedule to assist me

when required. They were responsible for providing and delivering the equipment punctually. I

would also like to thank Mr. Shane Bayley for providing the CAD designs of the equipment.

Finally I would like to thank all my friends for being supportive; especially Julian, Lachlan and

David for helping with the testing even though they were all extremely busy as well.

iv

Abstract

This thesis details the testing of different insulator configurations for use with 11kV Covered

Conductor Thick distribution lines. An energy distribution company in South East Queensland

called Energex are interested in implementing covered conductors as a means to reducing the

number of vegetation and wildlife related outages. The insulators investigated were the Pin Post,

Clamp Top and the Tie Top. This work also attempted to determine the impact and usefulness of

stripping the conductor near the attachment of the insulators. As partial discharges are

proportional to the surrounding electric field, the field was modeled by finite element analysis and

measured in the laboratory. The leakage current of the each configuration was measured so as

this can also be detrimental to the insulating covering.

It was found that the Covered Conductor Thick passed the Australian Standards for partial

discharges and even performed better than the stripped conductor in most cases. The partial

discharges detected within the conductor were of the same magnitude as the background noise

and were thus so small that it seems superfluous to strip at 11kV, though at increased operating

voltages the discharges may be larger and it may then become necessary. When the conductor

and insulators were modeled the stripped conductor always had a smaller electric field present

than the covered conductor did. This was supported by the laboratory measurements that also

showed that the pin post insulator had the greatest electric field for all samples of condutor.

v

Contents

ACKNOWLEDGEMENTS ..................................................................................................... III

ABSTRACT ......................................................................................................................... IV

CHAPTER 1: INTRODUCTION ...............................................................................................1

CHAPTER 2: LITERATUR E REVIEW .....................................................................................3

CHAPTER 3: T HEORY...........................................................................................................8

CHAPTER 4: APPARATUS ..................................................................................................11

4.1 CONDUCTORS ................................................................................................................................................11 4.1.1 Covered Conductor Thick ..................................................................................................................11 4.1.2 Stripped Conductor.............................................................................................................................12 4.1.3 Bare Conductor...................................................................................................................................12

4.2 SUPPORTING STRUCTURES ..........................................................................................................................13 4.2.1 Trident Structure.................................................................................................................................13 4.2.2 Short Cross Arm ..................................................................................................................................14

4.3 INSULATORS...................................................................................................................................................14 4.3.1 Tie Top Insulator.................................................................................................................................14 4.3.2 Clamp Top Insulator...........................................................................................................................15 4.3.3 Pin Post Insulator...............................................................................................................................15

4.4 ERA PARTIAL DISCHARGE DISPLAY.........................................................................................................16 4.5 GAUSS-MAUS ................................................................................................................................................17

CHAPTER 5: EXPERIMENTAL PROCEDURE ................................ ................................ ......18

5.1 LEAKAGE CURRENT ......................................................................................................................................18 5.2 PARTIAL DISCHARGE....................................................................................................................................20 5.3 ELECTRIC FIELD............................................................................................................................................23 5.4 ELECTRIC FIELD MODELLING.....................................................................................................................24

CHAPTER 6: RESULTS .......................................................................................................28

6.1 LEAKAGE CURRENT ......................................................................................................................................29 6.2 ELECTRIC FIELD MEASUREMENTS.............................................................................................................31 6.3 PARTIAL DISCHARGE....................................................................................................................................36 6.4 QUICKFIELD RESULTS..................................................................................................................................46

CHAPTER 7: DISCUSSION ..................................................................................................54

7.1 LEAKAGE CURRENT ......................................................................................................................................54 7.2 PARTIAL DISCHARGE....................................................................................................................................54 7.3 QUICKFIELD SIMULATIONS.........................................................................................................................57 7.4 ELECTRIC FIELD............................................................................................................................................59

CHAPTER 8: CONCLUSIONS ................................ ................................ ..............................60

FUTURE PLAN ....................................................................................................................62

REFERENCES.....................................................................................................................63

BIBLIOGRAPHY ..................................................................................................................65

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APPENDIX A – EMF RES ULTS ............................................................................................66

APPENDIX B – PARTIAL DISCHARGE RESULTS................................................................72

APPENDIX C – PARTIAL DISCHARGE OSCILLOSCOPE PHOTOGRAPHS ..........................75

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List of Figures

FIGURE 3-1: I) INTERNAL DISCHARGES, II) INTERNAL DISCHARGES – A) PERPENDICULAR TO THE ELECTRIC FIELD, B) SPHERICAL CAVITY, C) PARALLEL TO THE ELECTRIC FIELD AND D) SITUATED IN A LONGITUDINAL FIELD, III) SURFACE DISCHARGES, IV) CORONA DISCHARGES AND V) DISCHARGES IN ELECTRICAL TREES. [4]..........................................................................................................................................9

FIGURE 4-1: THE DIFFERENT INTERNAL LAYERS OF COVERED CONDUCTOR THICK. [8]............................................................................................................................................11

FIGURE 4-2: THE THREE TYPES OF CONDUCTOR TESTED. COVERED CONDUCTOR THICK (TOP), STRIPPED CONDUCTOR (MIDDLE) AND BARE CONDUCTOR (BOTTOM). ...12

FIGURE 4-3: THE TRIDENT STRUCTURE WITH TWO TIE TOP INSULATORS AND A CLAMP TOP INSULATOR. ...............................................................................................................13

FIGURE 4-4: THE SHORT CROSS ARM WITH TWO PIN POST INSULATORS ATTACHED. .14

FIGURE 4-5: THE THREE TYPES OF INSULATORS. THE CLAMP TOP (LEFT), THE TIE TOP (CENTRE) AND THE PIN POST (RIGHT). .............................................................................15

FIGURE 4-6: THE ERA DISCHARGE DISPLAY. THE DISCHARGES ARE DISPLAYED ON THE GREEN OSCILLOSCOPE. ............................................................................................16

FIGURE 4-7: THE GAUSS-MAUS. THE DETECTOR IS ON THE LEFT AND THE DISPLAY IS ON THE RIGHT....................................................................................................................17

FIGURE 5-1: LEAKAGE CURRENT CIRCUIT FOR A) PIN POST INSULATOR AND B) CLAMP AND TIE TOP INSULATOR. .................................................................................................19

FIGURE 5-2: BALANCED DETECTION CIRCUIT. [4]............................................................21

FIGURE 5-3: STRAIGHT DETECTION CIRCUIT. [4] ................................ ..............................22

FIGURE 5-4: OSCILLOSCOPE OUTPUT WITH NEGATIVE PEAK CORONA AT THE TOP OF THE ELLIPSE AND THE 100V INPUT AT THE BOTTOM RIGHT CORNER. ...........................22

FIGURE 5-5: THE EIGHT POSITIONS USED FOR MEASURING THE ELECTRIC FIELD. ......24

FIGURE 5-6: A TIE TOP INSULATOR DRAWN USING A SPACING OF 1 (LEFT) AND 50 (RIGHT). ................................ ................................ ................................ ..............................26

FIGURE 6-1: LEAKAGE CURRENT VS VOLTAGE...............................................................30

FIGURE 6-2: AVERAGE EMF MEASUREMENTS FOR THE CLAMP TOP INSULATOR .........34

FIGURE 6-3: AVERAGE EMF MEASUREMENTS FOR THE SIDE TIE INSULATOR...............35

FIGURE 6-4: AVERAGE EMF MEASUREMENTS FOR THE PIN POST INSULATOR .............35

viii

FIGURE 6-5: PARTIAL DISCHARGE CLAMP TOP - COVERED CONDUCTOR THICK ..........39

FIGURE 6-6: PARTIAL DISCHARGE CLAMP TOP - STRIPPED CONDUCTOR .....................39

FIGURE 6-7: PARTIAL DISCHARGE SIDE TIE - COVERED CONDUCTOR THICK................40

FIGURE 6-8: PARTIAL DISCHARGE SIDE TIE - STRIPPED CONDUCTOR ...........................40

FIGURE 6-9: PARTIAL DISCHARGE PIN POST - COVERED CONDUCTOR THICK..............41

FIGURE 6-10: PARTIAL DISCHARGE PIN POST - STRIPPED CONDUCTOR .......................41

FIGURE 6-11: PARTIAL DISCHARGE PIN POST - BARE CONDUCTOR..............................42

FIGURE 6-12: PARTIAL DISCHARGE CLAMP TOP ASCENDING - COVERED VS STRIPPED............................................................................................................................................43

FIGURE 6-13: PARTIAL DISCHARGE CLAMP TOP DESCENDING - COVERED VS STRIPPED............................................................................................................................................43

FIGURE 6-14: PARTIAL DISCHARGE SIDE TIE ASCENDING - COVERED VS STRIPPED ....44

FIGURE 6-15: PARTIAL DISCHARGE SIDE TIE DESCENDING - COVERED VS STRIPPED..44

FIGURE 6-16: PARTIAL DISCHARGE PIN POST ASCENDING - COVERED VS STRIPPED ..45

FIGURE 6-17: PARTIAL DISCHARGE PIN POST DESCENDING - COVERED VS STRIPPED 45

FIGURE 6-18: CLAMP TOP INSULATOR – COVERED CONDUCTOR THICK .......................46

FIGURE 6-19: CLAMP TOP INSULATOR – STRIPPED CONDUCTOR ..................................47

FIGURE 6-20: TIE TOP INSULATOR – COVERED CONDUCTOR THICK .............................48

FIGURE 6-21: TIE TOP INSULATOR – STRIPPED CONDUCTOR .........................................49

FIGURE 6-22: PIN POST INSULATOR – COVERED CONDUCTOR THICK ............................50

FIGURE 6-23: PIN POST INSULATOR – STRIPPED CONDUCTOR ................................ ......51

FIGURE 6-24: PIN POST INSULATOR – BARE CONDUCTOR................................ ..............52

1

Chapter 1

Introduction

Throughout the world, many distribution companies are turning to covered conductors as a

means to reducing the number of faults occurring along their transmission lines. These covered

conductors reduce the chance of faults caused by nearby vegetation or wildlife making contact

with one or more of the phases, or two or more of the phases clashing together. However,

covered conductors do have limitations. The currently used pin post insulators, which are

attached between the conductor and the power poles, have a large metal pin that holds the

insulator in place. This pin is at a much lower voltage than the nearby conductor is and therefore

a large electric field is present. The pin post insulators also have a higher dielectric constant than

the covering. This difference in dielectrics causes a majority of the electric field to be distributed

across the covering. This concentration of electric field directly affects the amount and the

frequency of partial discharges. Over time, these partial discharges can cause such extensive

damage to the covering and the conductor that the conductor snaps. This is a significant problem,

as this poses both a major health risk and can disrupt power from being delivered to the

consumer.

Energex, an energy distribution company in South East Queensland, is interested in

implementing these covered conductors. One of the main objectives of Energex is to provide a

regular service to their customers with as few interruptions as possible. Previously in Brazil there

has been some success in remedying the problem of coronal discharge in covered conductors by

stripping the covering from the conductor near where the line is attached to the pin insulator.

Energex therefore performed tests on the effectiveness of stripping the covered conductors. They

discovered that not only was this stripping difficult and time consuming, but also the stripping

tools often broke. This forced them to try and find a different solution.

2

This thesis is concerned with the interactions between the covered conductor and a group of

different insulators that Energex is considering using in the future or is using at present. By

modelling the electric fields and then by measuring these fields as well as the size of partial

discharges produced in the conductors and the amount of leakage current, the insulators and

configurations were able to be compared. Finite Element Method analysis utilising the QuickField

program was used to model the electric field expected in the surrounding area. As the version

used was an evaluation version, the accuracy was reduced, though a good qualitative analysis

was still achieved. The measurements of the electric field were then performed at various

locations with a Gauss-Maus. The partial discharges were measured with the ERA Discharge

Display used in straight detection mode.

By performing these tests it is hoped to determine which of the available insulators is best suited

to being used with the covered conductor and whether or not the covered conductors require

stripping. This thesis also demonstrates valid testing techniques for covered conductors and with

some slight adaptations these techniques could be used for different voltage lines.

A brief outline of the contents of this thesis follows. An overview of useful background material is

initially investigated. This section is then followed by useful theoretical definitions and

calculations. Once a solid understanding of the background and theory is possessed, a

description of the experimental procedure undertaken is provided, followed by the results to these

experiments and the conclusions that can be drawn from these results.

3

Chapter 2

Literature Review

This chapter provides background information that is relevant to this thesis topic. Firstly, the

operation of covered conductors is described. This section also provides some theories for

reducing the chance of conductor damage. Followed then are reasons for investing in covered

conductors and the rationale for Energex’s decision to use Covered Conductor Thick.

Covered conductors serve to provide a more reliable power supply to the public. They do this by

preventing short circuits between the conductors and the external environment. In Scandinavia

many tests have been performed on the cables. Trees were felled onto lines and they were left

there for up to a year. Audible partial corona discharges occurred, but this discharge was found to

have very little effect on the mechanical or electrical properties of the line, even after such a long

time. Usually when one of the phases of a feeder is grounded, the circuit breakers are opened by

the relay, causing the power flow to the feeder is stopped. The power to the feeder in a covered

conductor system does not need to be removed in such a case, as the electrical properties are

unaffected. This provides a more reliable supply to the consumer. The implementation of these

cables seems like an ideal solution, but there can be many problems involved with them. [1]

One major problem with the covered conductor occurs when lightning strikes. The flashover

produced occurs between the support points, much as in bare conductors, but in covered

conductors the arcing is not distributed along a large length, but in a small puncture point. Any

subsequent flashovers will occur at this point and the aluminium strands will rapidly deteriorate.

This deterioration reduces the mechanical properties of the line and can cause the cable to snap,

called burndown. [2] [3]

4

There are a number of different countermeasures that can reduce the risk of burndown. These

countermeasures include the use of high speed interrupting relays; increasing the Basic

Insulation Level (BIL) of the conductor; stripping the covering from the conductor in the region

near the post attachment; using surge arresters; and using insulator and insulator tie

configurations that reduce the electric field in the nearby area. In the near future, it is unlikely that

high-speed relays will be able to clear faults quickly enough to prevent burndown. This is because

it is doubtful whether there are available switches that will be able to operate at the required

speed, which is approximately 1–2 cycles. As lightning induced overvoltages easily exceed the

impulse strength of the circuit, with or without coverings on the conductors, increasing the Basic

Insulation Level of the covering, to reduce the likelihood of flashover, would not be a viable

economic possibility. Surface leakage current and partial discharge have been linked to possible

causes of conductor burndown. Partial discharges are discharges that do not fully cross the

electrodes, in this case the two sides of a cavity within the polyethylene covering of the

conductor. Over time these discharges can increase in size to a dangerous magnitude and cause

damage to the covered conductor. Therefore to reduce the electric field producing these, the

covered conductor may be stripped in the insulator tie region. [2] [4]

It is unsure whether stripping actually reduces the effect of partial discharge and may not have an

impact on reducing the chances of burndown. In fact, when there is a flashover to the stripped

region, the arc comes to rest at the start of the covering, which can have an extreme heating

effect unless arcing protection clamps are fitted. [2]

The disadvantages to stripping are that the actual task is especially difficult and time consuming.

Work teams have to be trained in the use of the stripping tools and new tools will have to be

purchased. Another problem is that there is now an increased possibility of wildlife related

outages, due to the live conductor exposed near the post insulators. [5]

5

Stripping can possibly be avoided with the strategic placement of surge arresters. The frequency

of these surge arresters is dependent on the likelihood of lightning strikes in the surrounding area.

In dense trees, the probability of a strike is much less than in open country, therefore the surge

arresters could be used less frequently. The use of surge arresters does not totally prevent the

risk of flashover and burndown, but they can reduce it to an acceptable level. Surge arresters are

fairly expensive, typically a few hundred dollars, and over an entire network this is a considerable

amount to invest. [6]

During 1983 bushfires swept through South Australia and caused extensive damage and tragic

loss of life. The cause of this tragic incident is believed to be bare conductors. This forced the

Electricity Trust of South Australia (ETSA) to investigate options of reducing the dangers of

bushfire. In 1987 ETSA implemented an Aerial Bundled Cable (ABC) system which was later

abandoned in favour of either Insulated Unscreened Conductor (IUC – which is identical to CCT)

or Covered Conductor (CC). These covered systems greatly reduce the chance of transmission

lines causing such damage. [7]

Apart from the electrical and mechanical benefits of the covered conductor, there are aesthetic

benefits as well. In a non-covered system when two phases clash a phase-to-phase fault occurs,

so the phases are spaced apart. This is not a problem in a covered conductor situation, so the

phases can be situated closer together. Landowners have reacted very positively and so have

ground crews who now have to clear smaller paths through dense bushland, as wildlife and fallen

foliage are no longer threats to disrupting power. [1]

Energex investigated many possibilities before settling on using Covered Conductor Thick. A

study was performed analysing the benefits and drawbacks of using the available options. The

options investigated were Covered Conductor (CC), Covered Conductor Thick (CCT), Non-

metallic Screened Aerial Bundled Cable (NMSHVABC), Metallic Screened Aerial Bundled Cable

(MSHVABC) and Aerial Spacer Cable (SPACER).

6

CC and CCT are very similar. Both are single-core aerial conductors covered with Cross Linked

Polyethylene (XLPE), but CCT also has an outer covering of High Density Polyethylene (HDPE).

CC is able to withstand occasional contact with other phases or trees, while the extra outer

sheath of the CCT it extended contact.

NMSHVABC is a bundled conductor comprising of three insulated cores and a bare support and

earthing conductor. The innermost covering of the three phases is made of a conductor screen,

then a XLPE covering, another insulating screen and then an optional HDPE sheath. The support

conductor also provides earthing for the outermost semiconductive layer. The MSHVABC is

similar to the NMSHVABC but has an optional water-swellable tape, a copper wire screen and a

separator tape between the second insulation screen and the HDPE exterior.

The Spacer system involves three insulated phases supported from an aluminium alloy catenary

wire by HDPE spacers. The covering of the conductors is constructed of an inner layer of Low

Density Polyethylene (LDPE) and an outer layer of black or grey HDPE. Unlike the other systems,

Spacer is not water blocked, though no problems of water corrosion have been reported.

Aerial Spacer Cable had the least similarity with the present systems and was not investigated as

fully as the systems that possess the greatest similarities. This is because the other systems

would be able to be retrofitted to existing equipment and would require less training for staff.

MSHVABC was also rejected, as it is approximately twice the weight and price of the non-metallic

type and also has lower normal and fault current ratings. The remaining possibilities were

compared with the standard Bare Conductor as well as with Underground Cable.

It was found that there was not a clear economic winner, therefore Energex proposed two

schemes. The first would be to use CCT on all lines. The second option was to use NMSHVABC

for new lines in timbered areas and CC for new lines in open areas. The second option was found

7

to be slightly better economically, but posed other problems in that NMSHVABC requires more

expensive equipment and training than CCT and is also not suitable for live line work. CC is

cheaper than CCT but its main advantage is that it is better at preventing the ignition of bushfires.

This is more important in the southern states of Australia as they have dry summers, while in

Queensland, the chance of bushfire is reduced due to wetter summers. Reduced tree trimming is

not allowed for CC and after a storm the lines need to be patrolled to check for branches lying

across the lines. This is not the case for CCT, which is able to withstand entire trees lying across

the lines for months at a time. The final advantage to CCT is that as only one type of cable is

being used, the storage space required will be much reduced.

The final decision to be made was what cross sectional area cable to use. Again there were two

options, use 120mm2 cable everywhere or use 80mm2 with 180mm2. The second option was

found to have a cost advantage of only 6.5%, but would provide problems again with storage

space. Therefore only the 120mm2 cable was chosen. [8]

This background information provides an insight into why many distribution companies are

planning on implementing covered conductors. Not only are covered conductors capable of

providing power with a decrease in the amount of outages, they also require less tree trimming

and are therefore more aesthetically pleasing and less harmful to the environment. Certain

problems were mentioned regarding damage to the conductor caused by lightning and partial

discharges. One of the possible solutions presented was the stripping of the insulating covering

from the conductor near the insulator tie region. This thesis investigates whether the stripping of

the conductor actually reduces the electric field and whether the amount and size of partial

discharges within the covering are subsequently reduced.

8

Chapter 3

Theory

A partial discharge is an electric discharge that does not completely bridge the electrodes. These

discharges are generally small, but because of their repetitive nature they can cause progressive

deterioration that may lead to failure in the covered conductor. Therefore it is essential to be able

to detect or predict their presence with the use of a non-destructive test.

Coronal discharges are a type of partial discharges that usually occur at sharp edges or points,

as this is where the electric field is the greatest. These discharges can be detrimental to partial

discharge detection. When the potential is increased, a set of lines will appear on the oscilloscope

display that do not increase in magnitude, but will increase in quantity. The harmful side of

coronal discharges is that all sharp edges and points must be removed from the test circuit, as

these will produce discharges that will be detected by the partial discharge detector. This makes

the construction of the circuit time consum ing, as it is imperative to use metal rods as conductors

and hollow metal spheres to ensure that a round object surrounds all edges. [4]

This thesis is concerned with the partial breakdown that occurs within the covering of 11kV

distribution lines. This can occur in a manner of ways including, internal cavities parallel and

perpendicular to the electric field, in a spherical cavity and via treeing. These partial discharges if

large enough can significantly damage the covering and the conductor and can, given enough

time, cause the conductor to snap and fall down. When the conductor snaps, it produces a health

risk if the fault is not detected and power disabled. If this occurs, the conductor is still live and

could be fatal if touched. Though this is harder to detect than when a bare conductor snaps, it is

still safer as the covering still provides protection from the live conductor.

9

Figure 3-1: i) Internal discharges, ii) Internal Discharges – a) perpendicular to the electric

field, b) spherical cavity, c) parallel to the electric field and d) situated in a longitudinal

field, iii) Surface discharges, iv) Corona discharges and v) Discharges in electrical trees.

[4]

10

The electric field surrounding the insulator is extremely important, as it determines the magnitude

of the effect of any partial discharge that may occur in the covered conductor. The electric field is

the derivative of the voltage with respect to distance. Therefore when there is a large potential

difference across a small area, a large electric field is present.

11

Chapter 4

Apparatus

This chapter describes all of the equipment used throughout the testing of the different insulator

and conductor configurations. This is done to outline the differences in each of the test specimens

and to help explain how the test equipment operates.

4.1 Conductors

4.1.1 Covered Conductor Thick

The covered conductor tested was an All Aluminium Alloy (AAAC) core made of aluminium alloy

1120. To reduce the amount of storage space, ENERGEX decided to implement only one size

and type of conductor, the 120mm2 made of 7 x 2.75mm diameter strands. It was found that this

type of conductor was the most versatile and cost effective. It has an inner insulation of cross-

linked polyethylene (XLPE) and an outer sheath of high-density polyethylene (HDPE).

Figure 4-1: The Different Internal Layers of Covered Conductor Thick. [8]

12

4.1.2 Stripped Conductor

The stripped conductor is identical to the covered conductor except that it has the insulation

covering stripped near the insulator attachments. This is hoped to reduce the electric field present

in the region. Reducing the electric field is hoped to decrease the amount and size of partial

discharges in the surrounding area.

4.1.3 Bare Conductor

The Bare conductor tested was also AAAC made of seven strand 120mm2 aluminium alloy 1120.

This bare conductor is currently in extensive use with the pin type insulators placed.

Figure 4-2: The Three Types of Conductor Tested. Covered Conductor Thick (top),

Stripped Conductor (middle) and Bare Conductor (bottom).

13

4.2 Supporting Structures

4.2.1 Trident Structure

The Trident structure is becoming one of Energex’s most common structures. It has one vertical

insulator and two insulators attached to a crosspiece at 78? to the vertical. The mounting is made

of metal and is usually attached to a wooden or concrete pole approximately ten metres above

the ground. In the tests performed, the conductor was held 2.4m from the ground as a larger pole

was unavailable and would not have caused any significant difference in the results. It would also

have proved difficult to attach each of the components and take measurements.

To enable this structure to travel around a bend, it is standard Energex procedure to replace one

of the Tie Top insulators with a Clamp Top insulator. Even when the trident structure is used as a

corner, the difference in structure has very little effect on the Basic Insulation Level.

Figure 4-3: The Trident Structure with two Tie Top insulators and a Clamp Top insulator.

14

4.2.2 Short Cross Arm

The cross arm used during the testing resembles the cross arms used by Energex for attaching

pin post insulators when the conductors are travelling in a straight line. The cross arm is not as

long as the type used by Energex, but is identical in all other respects. This difference in length is

acceptable due to the fact that only one phase is being energised at a time and therefore the

distance between the insulators is not important. The pin post insulators were placed 0.15m from

each end of the 1.5m long cross arm.

Figure 4-4: The Short Cross Arm with two Pin Post insulators attached.

4.3 Insulators

4.3.1 Tie Top Insulator

The tie top insulator is the most common insulator used on the Trident structure. The conductor is

tied onto the insulator with a polyethylene covered insulator tie. This type of tie is moulded in a

set position and comes in two shapes, the top tie and the side tie. The top tie is used on the

middle insulator when the conductors are going straight ahead and on the outside insulator when

the structure is turning around a corner. The side tie is used on the side insulators when the

conductor is going straight ahead and on the middle insulator when the conductor is travelling

around a bend. In this thesis this insulator is sometimes referred to as the Side Tie.

15

4.3.2 Clamp Top Insulator

The Clamp Top insulator is used on the Trident structure to bend around corners, where it is

placed on the inside of the curve. The Clamp Top insulator does not require any insulator ties to

attach the conductor to the insulator, as the conductor is held in place by the clamp, which is

tightened by two screws.

4.3.3 Pin Post Insulator

The Pin Post insulator is a much smaller insulator then the Clamp Top or Tie Top insulators. It

has small porcelain head attached to an eight-inch metal pin that is used to attach the insulator

onto the cross arm. When this type of insulator is used with CCT, it is suspected of increasing

the electric field around the insulator and therefore causing larger and more frequent partial

discharges. To verify this theory, experiments were performed to compare the attributes of the

discharges detected with bare, completely covered and partially stripped conductors.

Figure 4-5: The three types of insulators. The Clamp Top (left), the Tie Top (centre) and the

Pin Post (right).

16

4.4 ERA Partial Discharge Display

This partial discharge detector has a range of different detecting units. These units are used for

measuring varying sized discharges. The smaller units provide greater accuracy for small

discharges and the larger units should be used for large discharges. As the discharges being

detected are very small in magnitude the smallest detecting unit is being utilised. This unit

contains a 2pF capacitor and a 1.5mH inductor.

Once the discharge is detected, it is displayed by an oscilloscope on the ERA Discharge Display.

The oscilloscope shows the magnitude and position of the discharge.

Figure 4-6: The ERA Discharge Display. The discharges are displayed on the green

oscilloscope.

17

4.5 Gauss-Maus

The Gauss-Maus was used to measure the electric field present surrounding the conductor when

it was energised at rated voltage. There are various other tools capable of doing this on the

market, though the Gauss-Maus has a long connection between the detector and the display.

This enabled the detector to be placed close to the conductor, while the display was read behind

a safety screen.

Figure 4-7: The Gauss-Maus. The detector is on the Left and the disp lay is on the right.

18

Chapter 5

Experimental Procedure

Each of the following three tests were performed upon the different insulator and conductor

configurations. Leakage current measurements were performed once on each type of

configuration, EMF measurements were taken with four samples of conductor per insulator while

partial discharge measurements were taken for two samples per insulator. Covered Conductor

Thick and stripped conductor were tested with all three types of insulator, while the bare

conductor was only tested with the pin post insulator.

5.1 Leakage Current

One of the comparisons between the different arrangements is the amount of leakage current

flowing across the outside of the insulators. The insulators are designed to maximise this distance

by having folds along the outside, which increases the impedance thus reducing the magnitude of

the current. The leakage current should be reduced to as little as possible, as it is a loss to the

system. If this loss can be reduced, the power system becomes more efficient and therefore less

costly to the distributor. The leakage current should be smaller for the covered conductor than the

bare conductor as the polyethylene covering offers extra insulation. The bare conductor and the

stripped conductor should have approximately the same leakage current, as both of these

conductors are connected directly to the insulator.

There are no Australian Standards recommending test procedure or acceptable values, so a new

test was designed and a straight comparison between conductor and insulator combinations was

performed. The leakage current was measured by raising the conductor to 5.25kV, 6.35kV and

7.57kV. These values were chosen arbitrarily to give an indication of how leakage current varies

19

with voltage. Then the current was measured through an ammeter that was connected from the

base of the pin for the Pin Post insulator and from the base of the Trident structure for the Clamp

and Tie Top to earth.

Figure 5-1: Leakage Current Circuit for a) Pin Post Insulator and b) Clamp and Tie Top

Insulator.

20

5.2 Partial Discharge

The next test was to determine the onset of partial discharges in one phase of the system. This is

important, because if partial discharges begin occurring at a voltage above the rated voltage, they

are less likely to occur when implemented and are therefore less likely to damage the covering

and the conductor. There is still the possibility of partial discharges occurring though, as the

inception voltage is higher than the retaining voltage. Therefore if the inception voltage is only

slightly higher than the rated voltage, and the retaining voltage is below the rated voltage, a small

voltage swell could start the partial discharges and they would continue to occur as long as the

retaining voltage is exceeded. Only one phase of the system could be tested at a time, as the

transformer used only a single phase rated at 240V/80kV.

The relevant Australian Standard for the detection of partial discharges in a non-metallic

screened conductor is AS3599.2. [9] This standard provides guidelines for testing procedure and

recommends that the maximum discharges on a conductor at 10kV (150% rated voltage) and at

13kV (200% rated voltage) should be 5pC and 50pC respectively. If these values are exceeded,

the conductor is more likely to be susceptible to damage, and over a prolonged period of time, to

snapping. It was also decided to measure the discharges at rated voltage (6.35kV).

Partial discharges can be detected with the ERA Partial Discharge Display in a number of

different methods that require different circuits. The most common methods are straight detection

and balanced detection.

Balanced detection uses impedances that can be varied until the circuit is balanced. The circuit is

balanced once the variable impedances cancel out the background noise of the equipment,

leaving only the discharges from the test sample. This is an ideal situation, though the circuit is

much more difficult to set up and was thus not attempted due to constraints imposed by the

equipment available.

21

Figure 5-2: Balanced detection circuit. [4]

This left the option of straight detection. Straight detection detects all noise in the circuit including

the test sample, transformer and connections. This makes it imperative to make all of the

connections firm, and all corners and sharp edges covered by a hollow metal sphere. Any edges

or corners unable to be protected by a sphere must be covered instead with Blue-Tack or

plasticine. The edges and corners are where the electric field is strongest, by covering them the

chances of stray noise in the circuit are reduced and it is much easier to obtain a clean signal.

The negative peak was found by placing a sharp point on the ground near the conductor. This

produced negative corona that appeared on the display showing where the negative peak was.

Once the negative peak was found, it was possible to determine where the discharges were

originating.

22

Figure 5-3: Straight detection circuit. [4]

Figure 5-4: Oscilloscope output with negative peak corona at the top of the ellipse and the

100V input at the bottom right corner.

23

Measurements were taken at these voltages as the voltage ascended and then as they

descended. The 13kV reading for the descent was first raised to 14kV before being measured.

The reason for taking measurements as the voltage both increased and decreased was because

partial discharges remain at lower voltages levels than at what they are originally induced at. By

taking measurements in both directions it was hoped to prove this phenomenon.

By inputting a step voltage of 100V into the detector, the magnitude of the partial discharge could

be measured by comparing the sizes of their respective lines on the partial discharge detector.

Then by adjusting the attenuation switches on the detector, it was possible to decrease the

magnitude of the step input until it was the same size as that of the discharge. From the value of

the attenuation switches it was then possible to calculate the size of the discharge from the

following formula.

qx = EqCq(1+Cx/Cb)

Cx/Cb is small as the blocking capacitor Cb >> Cx

? qx = EqCq

Eq = 100V x 10-dB/20 (where dB is the attenuation of the 100V input)

Cq = 2pF

? qx = 200 x 10-dB/20pC

5.3 Electric Field

The final test was to measure the Electro Magnetic Field. Again, there were no Australian

Standards to follow for the measurement of the electric field. This test involved placing a Gauss-

Maus in various positions and raising the line to rated voltage. The detector was placed

perpendicular to the way the current was flowing. Though there was no load, this method still

enabled a comparison between each of the insulator and conductor configurations. A

measurement was taken with no voltage on the line and then at rated voltage. The difference

24

between these two readings was the amount of electric field produced by the conductor. This is a

necessary step as there is ambient electric fields from such things as nearby equipment and even

fluorescent lights. These values were then compared with each other and then compared to those

obtained through the computer modelling in Finite Element Method analysis.

Figure 5-5: The eight positions used for measuring the electric field.

5.4 Electric Field Modelling

The electric field surrounding the insulator and the conductor were modelled using the computer

program QuickField. This program allows two-dimensional CAD drawings to be imported into it.

Once the drawings have been imported, the dielectric constants of all of the materials can be set.

QuickField then uses Finite Element Method analysis to draw the electric field contours

superimposed upon the drawing.

25

This Finite Element technique places a mesh of many triangles inside all of the areas of the

drawing. As larger triangles do not fit model curves effectively, an approximation by many smaller

triangles is made. The smaller the triangles, the better the approximation of the drawing, but a

much larger problem is the consequence. It is preferable to have good quality triangles, as close

to balanced (equilateral) as possible. A measure of the quality of a triangle is thus the ratio

between its area and perimeter.

The version of QuickField used was an evaluation version that was only able to process two

hundred nodes. This meant the mesh incorporated large triangles that reduced accuracy. It

follows that the results are a rough estimate. The difference between a mesh with large spacing

and one with small spacing can be seen below. It is possible to get around this problem in two

ways. One method is to use different mesh grades depending on the curvature of boundary in

that region. This can be done in two ways, either an automatic grading is possible (but not

available), or individual regions can be hand separated and a mesh grade supplied, but this

creates the extra non-trivial task of connecting artificial boundary nodes and may introduce

artificially high curvature. To circumvent the size restrictions, a close-up of the conductor region

was examined. While this provides a local picture, it covers such a small region that it does not

necessarily reflect the global picture. The use of both a wide view and a close view together will

give an overall picture of the electric field surrounding the insulator.

26

Figure 5-6: A Tie Top insulator drawn using a spacing of 1 (left) and 50 (right).

The values for the dielectric constants used in the computer modelling were obtained through the

use of CES Selector V3.1. The value for the AAAC conductor was not available, but as it is a

conductor, it was assumed that the dielectric constant would be extremely high, therefore the

value of 1000 was chosen. All of these values are compared to the value of air. The values for the

dielectric constants used are given in Table 5-1 below.

It was assumed that the base of the insulators and the pin of the pin post insulators were at 0V,

as the concrete poles the trident structure is used with are fairly conductive. Even if wooden poles

are used, the voltage at the base of the insulator will still be fairly close to earth potential when

compared with the line to ground voltage of 6.35kV.

27

Table 5-1: Dielectric constants of the materials used in Finite Element analysis.

Relative Dielectric Constant

Material Minimum Maximum

Porcelain 6 7

AAAC Conductor 1000 1000

High Density Polyethylene 2.3 2.4

Air 1 1

Concrete 8 12

28

Chapter 6

Results

This chapter provides tables and graphs of the data collected during the experiments. The data

presented here for the EMF and the partial discharge is the average for each test, the raw data

can be viewed in Appendix A and Appendix B. Photos of the ERA Discharge Display’s

oscilloscope output can be found in Appendix C.

29

6.1 Leakage Current

Table 6-1: Leakage Current Tests

At 5.25kV At 6.35kV

Insulator Conductor Current (?A) Insulator Conductor Current (?A)

Clamp Top Covered 23.95 Clamp Top Covered 28.6

Stripped 26.1 Stripped 31.2

Tie Top Covered 25.1 Tie Top Covered 30.3

Stripped 26 Stripped 30.4

Pin Post Covered 39.9 Pin Post Covered 46.2

Stripped 41.4 Stripped 48.8

Bare 42.2 Bare 48.5

At 7.57kV

Insulator Conductor Current (?A)

Clamp Top Covered 34.1

Stripped 37.5

Tie Top Covered 36.1

Stripped 36.25

Pin Post Covered 54.4

Stripped 58.2

Bare 57.7

30

Figure 6-1: Leakage Current vs voltage

Leakage Current vs Voltage

20

25

30

35

40

45

50

55

60

5 5.5 6 6.5 7 7.5 8

Voltage (kV)

Lea

kag

e C

urr

ent (

uA

)

Clamp Covered

Clamp StrippedTie CoveredTie StrippedPin Post Covered

Pin Post StrippedPin Post Bare

31

6.2 Electric Field Measurements

Table 6-2: EMF Tests

Clamp Top - Covered Conductor Thick

Position Height (m) CCT A CCT B CCT C CCT D Average

Centre 1.05 0.4 0.7 0.5 0.35 0.4875

1.39 0.5 0.9 0.7 1.1 0.8

1st Left 1.05 0.55 0.4 0.4 0.45 0.45

1.39 0.8 0.8 0.6 0.6 0.7

2nd Left 1.05 0.3 0.4 0.3 0.2 0.3

1.39 0.65 0.75 0.7 0.75 0.7125

High Post 2.15 0.95 1.15 1.3 1.6 1.25

Low Post 1.93 1.3 1 1.8 1.3 1.35

Clamp Top - Stripped Conductor

Position Height (m) CCT A CCT B Average

Centre 1.05 0.3 0.3 0.3

1.39 0.6 0.75 0.675

1st Left 1.05 0.45 0.2 0.325

1.39 0.8 0.7 0.75

2nd Left 1.05 0.25 0.15 0.2

1.39 0.5 0.55 0.525

High Post 2.15 1.05 0.8 0.925

Low Post 1.93 1.25 1.2 1.225

32

Side Tie - Covered Conductor Thick


Centre 1.05 0.45 0.3 0.4 0.3 0.3625

1.39 0.6 0.7 0.85 0.8 0.7375

1st Left 1.05 0.4 0.4 0.3 0.3 0.35

1.39 0.55 0.7 0.65 0.7 0.65

2nd Left 1.05 0.3 0.4 0.25 0.1 0.2625

1.39 0.6 0.8 0.4 0.6 0.6

High Post 2.15 1.95 2.45 2 1.7 2.025

Low Post 1.93 1.8 1.6 1.3 1.1 1.45

Side Tie - Stripped Conductor


Centre 1.05 0.3 0.4 0.35

1.39 0.6 0.65 0.625

1st Left 1.05 0.3 0.3 0.3

1.39 0.7 0.6 0.65

2nd Left 1.05 0.15 0.15 0.15

1.39 0.8 0.6 0.7

High Post 2.15 1.55 0.9 1.225

Low Post 1.93 0.9 0.6 0.75

33

Pin Post - Covered Conductor Thick


Centre 1.05 0.6 0.65 0.6 0.7 0.6375

1.39 1 0.85 0.85 1.2 0.975

1st Left 1.05 0.35 0.45 0.4 0.5 0.425

1.39 0.55 0.75 0.6 0.9 0.7

2nd Left 1.05 0.2 0.4 0.35 0.35 0.325

1.39 0.5 0.4 0.85 0.8 0.6375

High Post 2.15 1.25 1.55 2.45 2.6 1.9625

Low Post 1.93 1.25 1.15 2.3 2.8 1.875



Centre 1.05 0.5 0.3 0.4

1.39 0.7 0.6 0.65

1st Left 1.05 0.3 0.4 0.35

1.39 0.6 0.5 0.55

2nd Left 1.05 0.3 0.3 0.3

1.39 0.7 0.5 0.6

High Post 2.15 1.9 1.65 1.775

Low Post 1.93 1.4 1.8 1.6

34

Pin Post - Bare Conductor


Centre 1.05 0.3 0.45 0.5 0.75 0.5

1.39 0.8 0.85 0.7 1.05 0.85

1st Left 1.05 0.4 0.35 0.4 1.35 0.625

1.39 0.6 0.7 0.9 0.8 0.75

2nd Left 1.05 0.3 0.2 0.4 0.2 0.275

1.39 0.45 0.4 0.9 0.35 0.525

High Post 2.15 2.6 1.95 1.9 1.65 2.025

Low Post 1.93 2.45 2.65 2.4 1.5 2.25

Figure 6-2: Average EMF Measurements for the Clamp Top insulator

Average EMF Measurements for the Clamp Top insulator

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4 5 6 7 8 9

Position Number

EM

F (m

G)

Covered ConductorStripped Conductor

35

Figure 6-3: Average EMF Measurements for the Side Tie Insulator

Figure 6-4: Average EMF Measurements for the Pin Post Insulator

Average EMF Measurements for the Pin Post Insulator

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7 8 9

Position Number

EM

F (

mG

)

Covered ConductorStripped ConductorBare Conductor

Average EMF Measurements for the Side Tie Insulator

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7 8 9

Position Number

EM

F (

mG

)

Covered ConductorStripped Conductor

36


Table 6-3: Partial Discharge Tests


Ascending Descending

CCT C CCT D Average CCT C CCT D Average

Voltage (kV) Charge (pC) Charge(pC) Charge(pC) Charge (pC) Charge(pC) Charge(pC)

6.35 0.89 1.12 1.005 1.12 1.42 1.27

10 3.99 5.64 4.815 5.64 6.32 5.98

13 8.93 8.93 8.93 10.02 8.93 9.475



Stripped A Stripped B Average Stripped A Stripped B Average


6.35 2.24 2.24 2.24 2.83 3.17 3

10 5.02 5.64 5.33 6.32 7.1 6.71

13 10.02 11.25 10.635 10.02 11.25 10.635




Voltage (k V) Charge (pC) Charge(pC) Charge(pC) Charge (pC) Charge(pC) Charge(pC)

6.35 2.83 1.12 1.975 3.17 2 2.585

10 6.32 6.32 6.32 6.32 7.1 6.71

13 8.93 8.93 8.93 10.02 10.02 10.02

37





6.35 2.24 2.24 2.24 3.17 2.83 3

10 7.1 6.32 6.71 7.96 7.1 7.53

13 10.02 11.25 10.635 10.02 8.93 9.475





6.35 0.71 0.63 0.67 0.89 0.71 0.8

10 2.52 2 2.26 2.52 2 2.26

13 6.32 3.99 5.155 6.32 5.02 5.67

Pin Post - Stripped Conductor




6.35 0.89 0.89 0.89 1 1.12 1.06

10 2.83 2.24 2.535 3.56 2.83 3.195

13 n/a n/a n/a n/a n/a n/a

38


Ascending

Bare A Bare B Bare C Bare D Average

Voltage (kV) Charge (pC) Charge(pC) Charge(pC) Charge(pC) Charge(pC)

6.35 0.71 0.89 0.71 0.71 0.755

10 2.24 3.56 2.24 2 2.51

13 n/a n/a n/a n/a n/a


Descending

Bare A Bare B Bare C Bare D Average

Voltage (kV) Charge (pC) Charge(pC) Charge(pC) Charge(pC) Charge(pC)

6.35 0.89 1.12 1 0.89 0.975

10 3.56 3.56 2.83 2.52 3.1175

13 n/a n/a n/a n/a n/a

No Conductor

Bare A

Voltage (kV) Charge (pC)

6.35 1

10 6.4

13 7.1

39

Figure 6-5: Partial Discharge Clamp Top - Covered Conductor Thick

Figure 6-6: Partial Discharge Clamp Top - Stripped Conductor

Partial Discharge Clamp Top - Covgered Conductor Thick

0

1

2

3

4

5

6

7

8

9

10

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Ch

arg

e (p

C)

AscendingDescending

Partial Discharge Clamp Top - Stripped Conductor

0

2

4

6

8

10

12

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Ch

arg

e (p

C)

AscendingDescending

40

Figure 6-7: Partial Discharge Side Tie - Covered Conductor Thick

Figure 6-8: Partial Discharge Side Tie - Stripped Conductor

Partial Discharge Side Tie - Covered Conductor Thick

0

2

4

6

8

10

12

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Ch

arg

e (p

C)

AscendingDescending

Partial Discharge Side Tie - Stripped Conductor

0

2

4

6

8

10

12

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Ch

arg

e (p

C)

AscendingDescending

41

Figure 6-9: Partial Discharge Pin Post - Covered Conductor Thick

Figure 6-10: Partial Discharge Pin Post - Stripped Conductor

Partial Discharge Pin Post - Covered Condcutor Thick

0

1

2

3

4

5

6

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Ch

arg

e (p

C)

AscendingDescending

Partial Discharge Pin Post - Stripped Conductor

0

0.5

1

1.5

2

2.5

3

3.5

5 6 7 8 9 10 11

Voltage (kV)

Ch

arg

e (p

C)

AscendingDescending

42

Figure 6-11: Partial Discharge Pin Post - Bare Conductor

Partial Discharge Pin Post - Bare Conductor

0

0.5

1

1.5

2

2.5

3

3.5

5 6 7 8 9 10 11

Voltage (kV)

Ch

arg

e (p

C)

Ascending

Descending

43

Figure 6-12: Partial Discharge Clamp Top Ascending - Covered vs Stripped

Figure 6-13: Partial Discharge Clamp Top Descending - Covered vs Stripped

Partial Discharge Clamp Top Descending - Covered vs Stripped

0

2

4

6

8

10

12

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Ch

arg

e (p

C)

CoveredStripped

Partial Discharge Clamp Top Ascending - Covered vs Stripped

0

2

4

6

8

10

12

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Cha

rge

(pC

)

CoveredStripped

44

Figure 6-14: Partial Discharge Side Tie Ascending - Covered vs Stripped

Figure 6-15: Partial Discharge Side Tie Descending - Covered vs Stripped

Partial Discharge Side Tie Ascending - Covered vs Stripped

0

2

4

6

8

10

12

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Ch

arg

e (p

C)

CoveredStripped

Partial Discharge Side Tie Descending - Covered vs Stripped

0

2

4

6

8

10

12

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Ch

arg

e (p

C)

CoveredStripped

45

Figure 6-16: Partial Discharge Pin Post Ascending - Covered vs Stripped

Figure 6-17: Partial Discharge Pin Post Descending - Covered vs Stripped

Partial Discharge Pin Post Ascending - Covered vs Stripped

0

1

2

3

4

5

6

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Ch

arg

e (p

C)

CoveredStripped

Bare

Partial Discharge Pin Post Descending - Covered vs Stripped

0

1

2

3

4

5

6

5 6 7 8 9 10 11 12 13 14

Voltage (kV)

Ch

arg

e (p

C)

CoveredStripped

Bare

46

6.4 QuickField Results

Figure 6-18: Clamp Top insulator – Covered Conductor Thick

47

Figure 6-19: Clamp Top Insulator – Stripped Conductor

48

Figure 6-20: Tie Top Insulator – Covered Conductor Thick

49

Figure 6-21: Tie Top Insulator – Stripped Conductor

50

Figure 6-22: Pin Post Insulator – Covered Conductor Thick

51

Figure 6-23: Pin Post Insulator – Stripped Conductor

52

Figure 6-24: Pin Post Insulator – Bare Conductor

53

Table 6-4

QuickField EMF Measurements - Side On View

Insulator Conductor Maximum EMF (10^4 V/m)

Clamp Top Covered 9.870

Stripped 4.592

Tie Top Covered 5.770

Stripped 5.440

Pin Post Covered 25.500

Stripped 23.075

Bare 21.750

54

Chapter 7

Discussion

7.1 Leakage Current

All of the insulators displayed a tendency for the leakage current to increase proportionally with

the voltage across the insulator. The Pin Post insulators have a leakage current between 55%

and 65% larger than both the Clamp and tie Top insulators. This is due to the actual physical

design of the different insulators. The Pin Post is much smaller and has only two flanges,

compared to the others four. These flanges increase the distance that the leakage current must

travel and therefore increase the impedance. The Clamp Top and the Tie Top insulators have

almost exactly the same leakage current measurements, usually within 5% of each other. This is

because their flange design is almost identical. The clamp is made of metal and therefore

conducts well, and this is attached to almost exactly where the conductor on the Tie Top is

positioned, making the path for the current almost identical.

Stripping the conductor on the Clamp Top insulator at 6.35kV results in a 9.1% increase in the

leakage current, where as stripping the conductor on the Tie Top makes only a 0.1% increase.

Even though it is only a slight increase, over the entire 11kV network these few micro-amps lead

to greater losses, and will therefore be less efficient, providing yet another reason to avoid

stripping when possible.


Once a partial discharge is initiated, it will remain present as long as its voltage remains above

the extinction level. This explains why larger discharges were observed when the voltage was

55

descending. The original discharges were present as well as those initiated at higher voltage

levels that were yet to be extinguished. If the inception voltage is slightly greater than the rated

voltage of a line and the extinction voltage for the discharges formed is below the rated voltage a

problem may occur. The line may not have partial discharges occurring in it or none of significant

size, but under swell conditions, the inception level may be surpassed, causing discharges to

begin occurring. When the line returns to rated voltage, the discharges will remain and may cause

damage over time if large enough and if they remain for long enough. This can be seen in Figure

6-5 to Figure 6-11, where the descending discharges are generally 25% larger, and in some

cases even up to 33%.

Figure 6-12 to Figure 6–17 shows that the stripped conductor always produced larger partial

discharges than the covered conductor. Considering that the purpose of stripping covered

conductor is to reduce the number and size of partial discharges, it would seem that this method

does not work. When the stripping was performed, a few scratches were made by accident on the

aluminium conductor. This could be the source of the discharges, and if so, these would not be of

a detrimental nature to the system as they are not touching the covering and therefore not going

to damage it.

With no conductor connected, partial discharges were detected. This was due to background

discharges in the equipment. This can be removed with the use of a balanced detection circuit,

though due to the lack of equipment this was not an option. The levels of discharge detected at

10kV were approximately 6.4pC. AS mentioned earlier, AS 3599.2 recommends that the

maximum allowable discharge at this voltage level is 5pC. As the background noise is already

above this level it is impossible to verify the exact magnitude of the discharges in a conductor if

they are below this background level. If the magnitude is significantly larger than this 6.4pC level,

it is safe to say that these larger discharges are entirely within the conductor. This is due to the

unlikelihood of an already existing discharge in the test equipment being superimposed by a new

discharge in the conductor.

56

At 10kV, the entirety of the Clamp Top and Tie Top configurations have discharges in the range

of 5pC to 7pC. This in the same range as the background noise, and due to the inherent

difficulties in measuring the discharge, it seems that the line is not contributing to the size of the

discharges. The only way to validate this assumption is to use balanced detection, which was not

available, to remove the background noise.

The discharges also had to be measured at 13kV. This test was much more conclusive, as none

of the samples displayed discharges greater than 11.25pC and the maximum allowable discharge

was 50pC. Therefore all of the configurations passed the Australian Standard level.

The Pin Post insulators provided some difficulties when being tested for partial discharges. The

readings were fairly clear at 6.35kV, but as the voltage approached 10kV, a number of sizeable

coronal discharges began occurring regularly. These discharges made it particularly difficult to

make accurate measurements and once the voltage was increased to 13kV it was impossible to

obtain accurate readings due to the amount of corona, except for the covered conductor which

seemed to have less noise.

During the testing, a spark gap was placed across two of the terminals on the discharge detector.

This was to reduce the chance of damage to the device as the flashover voltage of 75V

corresponded to the maximum allowable current in the detector. On a few occasions this spark

gap flashed over and produced a great deal of electrical noise. This noise generally occurred at

around 12kV on the secondary side. This flashover voltage was not always the same as when

the partial discharge measurements were taken, no flashover was heard or seen, even up to

14kV. It is possible that the spark gap was on the verge of flashing over and was producing these

great amounts of noise. The spark gap could be removed, though this would leave the detector

more susceptible to damage.

57

7.3 QuickField Simulations

A CAD drawing of the object being modelled is also provided, as some of the electric field

diagrams do not show the insulator clearly. Equipotential lines are also shown on the electric field

diagrams. From these lines, it is possible to see that the electric field is the derivative with respect

to distance of voltage. Where the voltage lines are more spread out, the electric field is smaller.

When there is a conductor, like the pin attachment in the pin post insulator configuration, there is

negligible voltage drop across its length; therefore there is no electric field within it.

These diagrams only show a simplified two-dimensional view, but they do provide suitable

information on the magnitudes of the electric fields surrounding the insulator and conductor

connections. Table 6.4 represents the maximum field strength found in each of the electric field

diagrams. The maximum values were generally found near the insulator and conductor

connections. It can be seen that stripping the covering from the conductor does reduce the

magnitude of the electric field in the surrounding area. The reason for the comparatively high

values for the pin post insulator are due to the close proximity of the pin to the live conductor.

There is a potential difference of 6.35kV across the thin top of the insulator. A large potential drop

across a small distance requires a steep voltage gradient, and due to the magnitude of the

electric field is the derivative of the voltage with respect to distance, a large electric field is also

present. The reverse logic can be used to understand why there is very small electric field near

the conductor but away from the insulator. The outside of the conductor is floating and not being

forced to earth potential, therefore the electric field is not required to be as large as the change in

voltage is much slower.

The insulating covering of the conductor has an extremely low dielectric constant of 2.3. This

makes it a very strong insulator and therefore it can withstand a large voltage across a small

distance without flashing over. When two materials of the same thickness with different dielectric

constants are in series with each other with a voltage across them, most of the voltage will be

58

across the material with the lower dielectric value. This causes an extremely high electric field to

be present in the low dielectric material. By removing the insulating covering, the conductor is

now directly connected to the insulator. The porcelain insulators have a dielectric constant of

approximately six, and this allows the voltage to gradually decrease across it and thus reduces

the electric field.

59

7.4 Electric Field

The Pin Post insulator had slightly higher electric field values than the other insulators. This is

due to the large pin inserted in the base of the insulator and matches exactly with the results

obtained through QuickField.

Figure 6-2 to Figure6-4 show that stripping the polyethylene covering near where the insulator

attaches almost always reduces the surrounding electric field. This is as expected as the voltage

now is directly applied to the insulator. This insulator has a slightly larger dielectric constant and

is a larger size than the polyethylene covering and therefore the voltage will decrease at a slower

rate, resulting in a smaller electric field. Again, this is the same result as attained through

QuickField.

60

Chapter 8

Conclusions

Stripping the polyethylene covering from near the insulator attachment resulted in a few changes

to the system characteristics. The leakage current increased as the conductor was now in direct

contact with the insulator. The surrounding electric field decreased due to the removal of a low

dielectric material that now allows the voltage to decrease at a reduced rate and thus a reduced

electric field. Finally the amount and size of the partial discharges increases compared to covered

conductor. The theory behind stripping the covering from the conductor is to reduce the leakage

current and the size of the partial discharges by reducing the electric field. In almost all cases the

electric field is successfully reduced, but the leakage current and the occurrence of partial

discharges both increase. Performing the stripping of the conductor is a difficult job requiring

special tools and training. When the conductor is stripped, it also increases the systems

susceptibility to wildlife and vegetation outages. This appears to be the wrong method of

protecting the conductor from damage, as there are no benefits to this practice.

In respect to the insulators, it is difficult to determine a clear leader. The Tie Top and the Clamp

Top are very similar in all respects, while the Pin Post has a higher leakage current but it is

impossible to deduce how they are affected by partial discharges, due to the large background

noise.

This background noise was also a problem for the partial discharge tests undertaken at 10kV, as

the background noise was larger than the acceptable value from the relevant Australian

Standard. The spark gap on the discharge detector is believed to be the cause of the noise, as all

other edges and points were removed from the circuit or covered by metal spheres. This noise

makes it difficult to arrive at a definitive conclusion as to whether the standards are met at 10kV.

61

It seems that stripping the conductor only produces problems without any benefits. The leakage

current for the stripped conductor is greater than that of the covered conductor, as is the amount

of partial discharges. The theory behind stripping the conductor is to reduce the amount and size

of the partial discharges, but this is not the case in practice. The actual task of stripping the

conductor is also a difficult task requiring special tools and training. This would require increased

expenditure for any company wishing to implement the practice of stripping their covered

conductors. Stripping the conductors also increases the likelihood of vegetation and wildlife

reduced outages, decreasing the reliability of the supply to the customers. This thesis shows that

the practice of stripping covered conductors is not advised as it is not only harmful to the

conductor itself, but also a costly practice.

62

Future Plan

This thesis provides a sturdy platform for the study of insulators and their interactions with

covered and stripped conductor. This topic could be further investigated through the analysis of a

wider variety of insulators and conductors. These could then be analysed more precisely with the

implementation of a balanced partial discharge detector circuit. This would allow the background

noise to be removed from the signal and allow the exact location of the source of the discharges

to be located. The computer modelling could also be performed in a three-dimensional program.

This would enable the field to be modelled more accurately and the field could be viewed from

various angles.

63

References

[1] B. Hart, “HV Overhead Line – The Scandinavian Experience,” Power Engineering Journal, Vol.

8, No. 3, Jun. 1994, pp 119-123.

[2] K. Nakamura et al., “Impulse Breakdown Characteristics of 13.2kV Covered conductor

Insulator/Tie Configurations,” IEEE Transactions on Power Delivery, Vol. PWRD-1, No. 4, Oct.

1986, pp. 250-258.

[3] D.A. Swift, “Electrical Puncture of the Insulating Sheath of Covered Overhead Power-Line

Conductor,” Ninth International Symposium on High Voltage Engineering, University of Natal,

Durban, Natal, 1995, pp. 1-4.

[4] F.H. Kreuger, Partial Discharge Detection in High-Voltage Equipment, Temple Press, England,

1989.

[5] J. Roughan and G. Dowling, Development of Fittings for Covered Conductors (CCT), Systems

Department ENERGEX, Australia.

[6] K.A. Gosden, Covered Conductor (CCT) Implementation, ELECTRO Technical Consultants,

1998.

[7] Electricity Trust of South Australia, Overhead Insulated Systems in south Australia, Electricity

Trust of South Australia.

[8] ELECTRO Technical Consultants, Insulated HV Overhead Mains – Technical and Economic

Investigation, ELECTRO Technical Consultants, 1995.

64

[9] Australian Standard, AS/NZS3599.2 Electric cables – Aerial bundled – Polymeric insulated –

Voltages 6.35/11(12)kV and 12.7/22(24)kV Non-metallic screened, Australia, 1999.

65

Bibliography

S. Davis, The Lightning Performance of the Overhead 11kV Energex Trident Structures, thesis,

Univ. of Queensland, Dept. of Computer Science and Electrical engineering, 1998.

S.R. Krishnamurthy and P. Selvan, “Use of AAAC in a Distribution Network – A Strategy for

Energy and Cost Reduction,” Power Engineering Journal, Vol. 9, No. 3, Jun. 1995, pp 133-136.

E. Kuffel and W.S. Zaengl, High Voltage Engineering: Fundamentals, Pergamon Press, Australia,

1984.

I. Lehtinen, Phase-to-Phase Sparkover of Covered Conductors, Power Systems and Illuminating

Engineering Laboratory, Helsinki Univ. of Technology, Espoo Finland, 1990.

J.W. McAuliffe, Hendrix Aerial Spacer Cable System an Option for System Reliability

Improvement in Brazil, Hendrix Wire & Cable, USA.

66

Appendix A – EMF Results


CCT A Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.5 0.9 0.4

1.39 0.4 0.9 0.5 1st Left 1.05 0.45 1 0.55

1.39 0.35 1.15 0.8 2nd Left 1.05 0.5 0.8 0.3

1.39 0.3 0.95 0.65 High Post 2.15 0.55 1.5 0.95 Low Post 1.93 0.6 1.9 1.3

CCT B Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.4 1.1 0.7

1.39 0.3 1.2 0.9 1st Left 1.05 0.4 0.8 0.4

1.39 0.3 1.1 0.8 2nd Left 1.05 0.4 0.8 0.4

1.39 0.3 1.05 0.75 High Post 2.15 0.6 1.75 1.15 Low Post 1.93 0.6 1.6 1

CCT C Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.4 0.9 0.5

1.39 0.4 1.1 0.7 1st Left 1.05 0.4 0.8 0.4

1.39 0.4 1 0.6 2nd Left 1.05 0.4 0.7 0.3

1.39 0.3 1 0.7 High Post 2.15 0.6 1.9 1.3 Low Post 1.93 0.5 2.3 1.8

CCT D Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.45 0.8 0.35

1.39 0.3 1.4 1.1 1st Left 1.05 0.4 0.85 0.45

1.39 0.4 1 0.6 2nd Left 1.05 0.5 0.7 0.2


67



1.39 0.4 1 0.6 1st Left 1.05 0.45 0.85 0.4

1.39 0.35 0.9 0.55 2nd Left 1.05 0.4 0.7 0.3



1.39 0.3 1 0.7 1st Left 1.05 0.4 0.8 0.4

1.39 0.3 1 0.7 2nd Left 1.05 0.4 0.8 0.4


CCT C Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.4 0.8 0.4

1.39 0.35 1.2 0.85 1st Left 1.05 0.4 0.7 0.3

1.39 0.35 1 0.65 2nd Left 1.05 0.4 0.65 0.25

1.39 0.3 0.7 0.4 High Post 2.15 0.6 2.6 2 Low Post 1.93 0.6 1.9 1.3


1.39 0.3 1.1 0.8 1st Left 1.05 0.4 0.7 0.3

1.39 0.3 1 0.7 2nd Left 1.05 0.4 0.5 0.1


68

Pin Post - Bare Conductors

Bare A Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.5 0.8 0.3

1.39 0.3 1.1 0.8 1st Left 1.05 0.4 0.8 0.4

1.39 0.3 0.9 0.6 2nd Left 1.05 0.4 0.7 0.3


Bare B Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.45 0.9 0.45

1.39 0.35 1.2 0.85 1st Left 1.05 0.45 0.8 0.35

1.39 0.4 1.1 0.7 2nd Left 1.05 0.5 0.7 0.2


Bare C Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.5 1 0.5

1.39 0.4 1.1 0.7 1st Left 1.05 0.5 0.9 0.4

1.39 0.4 1.3 0.9 2nd Left 1.05 0.5 0.9 0.4


Bare D Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.45 1.2 0.75

1.39 0.35 1.4 1.05 1st Left 1.05 0.45 0.8 0.35

1.39 0.35 1.15 0.8 2nd Left 1.05 0.5 0.7 0.2


69

Pin Post - Covered Conductors


1.39 0.3 1.3 1 1st Left 1.05 0.45 0.8 0.35

1.39 0.3 0.85 0.55 2nd Left 1.05 0.4 0.6 0.2



1.39 0.3 1.15 0.85 1st Left 1.05 0.45 0.9 0.45

1.39 0.35 1.1 0.75 2nd Left 1.05 0.4 0.8 0.4


CCT C Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.4 1 0.6

1.39 0.3 1.15 0.85 1st Left 1.05 0.4 0.8 0.4

1.39 0.3 0.9 0.6 2nd Left 1.05 0.4 0.75 0.35



1.39 0.3 1.5 1.2 1st Left 1.05 0.4 0.9 0.5

1.39 0.3 1.2 0.9 2nd Left 1.05 0.4 0.75 0.35


70


Stripped A Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.4 0.9 0.5

1.39 0.35 1.05 0.7 1st Left 1.05 0.4 0.7 0.3

1.39 0.3 0.9 0.6 2nd Left 1.05 0.4 0.7 0.3

1.39 0.3 1 0.7 High Post 2.15 0.7 2.6 1.9 Low Post 1.93 0.6 2 1.4

Stripped B Position Height (m) Ambient Gauss (mG) Total Gauss (mG) Cable Contribution (mG) Centre 1.05 0.4 0.7 0.3

1.39 0.3 0.9 0.6 1st Left 1.05 0.4 0.8 0.4

1.39 0.3 0.8 0.5 2nd Left 1.05 0.4 0.7 0.3




1.39 0.3 0.9 0.6 1st Left 1.05 0.4 0.85 0.45

1.39 0.3 1.1 0.8 2nd Left 1.05 0.4 0.65 0.25



1.39 0.35 1.1 0.75 1st Left 1.05 0.5 0.7 0.2

1.39 0.4 1.1 0.7 2nd Left 1.05 0.5 0.65 0.15


71



1.39 0.3 0.9 0.6 1st Left 1.05 0.4 0.7 0.3

1.39 0.3 1 0.7 2nd Left 1.05 0.45 0.6 0.15



1.39 0.3 0.95 0.65 1st Left 1.05 0.4 0.7 0.3

1.39 0.3 0.9 0.6 2nd Left 1.05 0.5 0.65 0.15


72

Appendix B – Partial Discharge Results


CCT C Ascending Descending

Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) 18.5 6.35 47 0.89 41 to 37.9 14 to 13 26 10.02 29.0 10 34 3.99 to 29.0 to 10 31 5.64 37.9 13 27 8.93 to 18.5 to 6.35 45 1.12

CCT D Ascending Descending



CCT C Ascending Descending Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC)

18.5 6.35 37 2.83 41 to 37.9 14 to 13 26 10.02 29.0 10 30 6.32 to 29.0 to 10 30 6.32

37.9 13 27 8.93 to 18.5 to 6.35 36 3.17

CCT D Ascending Descending Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC)

18.5 6.35 45 1.12 41 to 37.9 14 to 13 26 10.02 29.0 10 30 6.32 to 29.0 to 10 29 7.10 37.9 13 27 8.93 to 18.5 to 6.35 40 2.00


Stripped A Ascending Descending

Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC)

18.5 6.35 39 2.24 41 to 37.9 14 to 13 26 10.02 29.0 10 32 5.02 to 29.0 to 10 30 6.32 37.9 13 26 10.02 to 18.5 to 6.35 37 2.83

73

Stripped B Ascending Descending Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC)

18.5 6.35 39 2.24 41 to 37.9 14 to 13 25 11.25 29.0 10 31 5.64 to 29.0 to 10 29 7.10 37.9 13 25 11.25 to 18.5 to 6.35 36 3.17


Stripped A Ascending Descending


Stripped B Ascending Descending



CCT C Ascending Descending Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC)

18.5 6.35 49 0.71 41 to 37.9 14 to 13 30 6.32 29.0 10 38 2.52 to 29.0 to 10 38 2.52 37.9 13 30 6.32 to 18.5 to 6.35 47 0.89

CCT D Ascending Descending Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC)

18.5 6.35 50 0.63 41 to 37.9 14 to 13 32 5.02 29.0 10 40 2.00 to 29.0 to 10 40 2.00 37.9 13 34 3.99 to 18.5 to 6.35 49 0.71

74


Stripped A Ascending Descending Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC)

18.5 6.35 47 0.89 41 to 37.9 14 to 13 n/a 29.0 10 37 2.83 to 29.0 to 10 35 3.56 37.9 13 n/a to 18.5 to 6.35 46 1.00

Stripped B Ascending Descending Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC)


Pin Post - Bare conductor

Bare A Ascending Descending

Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) 18.5 6.35 49 0.71 41 to 37.9 14 to 13 n/a 29.0 10 39 2.24 to 29.0 to 10 35 3.56 37.9 13 n/a to 18.5 to 6.35 47 0.89

Bare B Ascending Descending

Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) 18.5 6.35 47 0.89 41 to 37.9 14 to 13 n/a 200.00 29.0 10 35 3.56 to 29.0 to 10 35 3.56 37.9 13 n/a 200.00 to 18.5 to 6.35 45 1.12

Bare C Ascending Descending Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC)


Bare D Ascending Descending Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC) Voltage (V) Voltage (kV) Attenuation (dB) Charge (pC)


75

Appendix C – Partial Discharge Oscilloscope Photographs

Photograph of partial discharges with ellipse collapsed of Covered Conductor Thick on a

Tie Top insulator at 13kV.

Photograph of partial discharges with ellipse collapsed of Stripped Conductor on a Tie

Top insulator at 13kV.

76

Photograph of partial discharges with ellipse open of Covered Conductor Thick on a Tie

Top insulator at 13kV.

Photograph of partial discharges with ellipse open of Stripped Conductor on a Tie Top

insulator at 13kV.

77

Photograph of partial discharges with ellipse open of Covered Conductor Thick on a

Clamp Top insulator at 13kV.

Photograph of partial discharges with ellipse open of Stripped Conductor on a Clamp Top

insulator at 13kV.

78

Photograph of partial discharges with ellipse open of Covered Conductor Thick on a Pin

Post insulator at 13kV.

Photograph of partial discharges with ellipse open of Stripped Conductor on a Pin Post

insulator at 13kV. Note that it is impossible to see the 100V input marker and hence

deduce the magnitude of these discharges.

79

Photograph of partial discharges with ellipse open of Bare Conductor on a Pin Post

insulator at 13kV. Note that it is impossible to see the 100V input marker and hence

deduce the magnitude of these discharges.

An Investigation of the Interface Between Various

Documents

stripped conductor

covered conductor thick

covered conductors

partial discharges

engineering honours

electronic engineering

division of electrical

partial discharge testing