Fulgurites: Their Formation, Chemical Composition and Impacts on Network Failures in Swaziland

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How to cite this thesis

Surname, Initial(s). (2012) Title of the thesis or dissertation. PhD. (Chemistry)/ M.Sc. (Physics)/ M.A. (Philosophy)/M.Com. (Finance) etc. [Unpublished]: University of Johannesburg. Retrieved from: https://ujcontent.uj.ac.za/vital/access/manager/Index?site_name=Research%20Output (Accessed: Date). Fulgurites: Their formation, chemical composition and impacts on network failures in Swaziland

REMBULUWANI PHILLIP TSHUBWANA

A dissertation to the Faculty of Engineering and the Built Environment in the fulfilment of the requirements for the degree of

MAGISTER INGENERIAE

ELECTRICAL AND ELECTRONICS ENGINEERING

at the

UNIVERSITY OF JOHANNESBURG

Supervisor: Dr Thokozani Shongwe Co-Supervisor: Dr Miidzo Hove Co- Supervisor: Prof Chandima Gomes

30 June 2017

DECLARATION BY CANDIDATE

I hereby declare that the dissertation submitted for the degree MEng: Electrical Engineering at the

UNIVERSITY of JOHANNESBURG is my own original work and has not previously been submitted to any other institution of higher learning. I therefore declare that all sources cited or quoted are indicated and acknowledged by means of a comprehensive list of references

REMBULUWANI PHILLIP TSHUBWANA

Signed this 08 day of AUGUST 2017

ii ACKNOWLEDGEMENTS

I would like to acknowledge and thank the following people who contributed immensely to my work:

Dr. Miidzo Hove – For your unwavering support and inspiration.

Prof. Chandima Gomes – For your wisdom and knowledge in the field of lightning. You were there for me when I needed you the most.

Doctor Thokozani Calvin Shongwe – For your encouragement and for being a superb supervisor.

Professor Udaya Kumar – For providing me with such a unique opportunity.

Funding and support from the following organisations are gratefully acknowledged:

 TSHWANE UNIVERSITY OF TECHNOLOGY (TUT) – For funding all my trips to

Swaziland for the collection of soil samples and for data capturing.

 The Department of Electrical Engineering (TUT) – For sponsoring my trip to India for

experiments. The Indian Institute of Science (IISC) – For your support with the High Voltage

facilities.

 The Indian Institute of Science (IISC) – For your support with the High Voltage facilities.

Finally, I would to thank to my lovely wife, Ramashego Pertunia Tshubwana, for her unwavering support and my children, Thendo and Rendani.

iii

ABSTRACT

Swaziland experiences significant loss of overhead transformers due to lightning during the rainy seasons. Most of the Swaziland power network is overhead and is, therefore, exposed to lightning. To avoid damages to the equipment, earthing systems such as deep driven rods are normally used to divert excessive current during lightning strikes. It is thus important to design an effective earthing system that will be able to protect the equipment during fault conditions. Fulgurites are highly resistive and are normally formed when lightning and fault currents flow through the soil. They may modify the ground impedance of the earthing systems. In this research, the soil around the affected feeder was taken and create fulgurites in that by applying laboratory current impulse. Chemical composition of the soil sample and the fulgurites were analysed to determine the elementary atoms. The resistivity of the soil sample and the resistivity of fulgurites were also tested to determine the effects of fulgurites on the earthing system. It was found from the tests that indeed fulgurites has high resistivity compared to the original soil.

TABLE OF CONTENTS

DECLARATION ...... ii ACKNOWLEDGEMENTS ...... iii ABSTRACT ...... iv LIST OF FIGURES ...... vii LIST OF TABLES ...... xi LIST OF ABBREVIATIONS ...... xii LIST OF SYMBOLS ...... xiii CHAPTER 1: INTRODUCTION ...... 1 1.1 RESEARCH BACKGROUND AND JUSTIFICATION ...... 1 1.2 PROBLEM STATEMENT ...... 2 1.3 OBJECTIVE ...... 3 1.3.1 Sub-objective 1 ...... 3 1.3.2 Sub-objective 2 ...... 3 1.3.3 Sub-objective 3 ...... 3 1.3.4 Sub-objective 4 ...... 3 1.3.5 Sub-objective 5 ...... 4 1.4 AIM OF THE STUDY ...... 4 1.5 RESEARCH DELIMITATIONS...... 4 1.6 BENEFITS OF RESEARCH ...... 4 1.7 LIST OF PUBLICATIONS ...... 5 1.8 DISSERTATION ORGANISATION ...... 5 CHAPTER 2: LITERATURE REVIEW ...... 7 2.1 INTRODUCTION ...... 7 2.2 DIRECT STROKE...... 9 2.3 INDIRECT STROKE ...... 10 2.4 SURGE ARRESTORS PROTECTION ...... 11 2.5 EARTHING SYSTEM ...... 14 2.6 SUMMARY ...... 18 CHAPTER 3: METHODOLOGY ...... 19 3.1 OVERALL APPROACH...... 19

3.2 EXPERIMENTS ...... 20 3.2.1 TEST RIG ...... 20 3.2.2 TEST SAMPLE ...... 22 3.2.3 ELECTRODES ...... 22 3.2.4 CURRENT IMPULSE GENERATOR ...... 23 3.2.5 OSCILLOSCOPE ...... 25 3.3 ENERGY DISPERSIVE X-RAY ANALYSIS ...... 25 3.4 SOIL RESISTIVITY ...... 26 3.5 FULGURITES ANALYSIS ...... 26 3.6 CHEMICAL COMPOSITIONS OF SOIL ...... 27 CHAPTER 4: RESULTS ...... 28 4.1 INTRODUCTION ...... 28 4.2 CURRENT IMPULSE EXPERIMENTS ...... 28 4.3 CHEMICAL COMPOSITION OF THE SOIL ...... 30 4.4 IMPULSE WAVEFORMS ...... 33 4.5 FULGURITES MORPHOLOGY ...... 45 4.6 CHEMICAL COMPOSITION OF FULGURITES ...... 50 4.7 RESISTIVITY RESULTS ...... 51 4.8 DISCUSION ...... 53 CHAPTER 5: CONCLUSION AND RECOMMENDATIONS ...... 56 5.1 SUMMARY OF DISSERTATION ...... 56 5.2 CONCLUSIONS ...... 56 5.3 RECOMMENDATION ...... 57 5.4 FUTURE RESEACH ...... 57 REFERENCES ...... 58

LIST OF FIGURES

FIGURE 2.1: Lightning intensity in Swaziland ...... 8 FIGURE 2.2: Connection of surge arrestor for lightning protection ...... 12 FIGURE 2.3: A transformer with operated surge arrestor ...... 13 FIGURE 2.4: Crow’s foot earthing ...... 15 FIGURE 3.1 Flow chart of methodology summary ...... 19 FIGURE 3.2: Empty test rig ...... 21 FIGURE 3.3: Test rig filled with soil sample ...... 21 FIGURE 3.4: Steel electrode to the earth connection ...... 22 FIGURE 3.5: Electrode to the output terminal of the generator ...... 23 FIGURE 3.6: Soil sample from Lamgabhi Village ...... 23 FIGURE 3.7: Circuit diagram of the current impulse generator ...... 24 FIGURE 3.8: Digital oscilloscope ...... 25 FIGURE 4.1: Current impulse generator ...... 29 FIGURE 4.2: Charging transformer ...... 29 FIGURE 4.3: BSE-SEM image “A” soil sample analysis ...... 30 FIGURE 4.4: Spectral trace of average area analysis for sample ‘A’ (Lamgabhi region) ...... 31 FIGURE 4.5: Spectral trace of the average area analysis for the soil sample……………………..... 31 FIGURE 4.6: BSE-SEM image ‘B’ soil sample analysis (Lamgabhi region) ...... 32 FIGURE 4.7: Discharging impulse current waveform with an amplitude of 14.9 kA ...... 33 FIGURE 4.8: Discharging impulse current waveform with an amplitude of 16 kA ...... 34 FIGURE 4.9: Discharging impulse current waveform with an amplitude of 17 kA ...... 35 FIGURE 4.10: Discharging impulse current waveform with an amplitude of 22 kA ...... 36 FIGURE 4.11: Discharging impulse current waveform with an amplitude of 23 kA ...... 37 FIGURE 4.12: Discharging impulse current waveform with an amplitude of 24 kA ...... 38 FIGURE 4.13: Discharging impulse current waveform with an amplitude of 29 kA ...... 39 FIGURE 4.14: Discharging impulse current waveform with an amplitude of 30 kA ...... 40 FIGURE 4.15: Discharging impulse current waveform with an amplitude of 31 kA ...... 41 FIGURE 4.16: Discharging impulse current waveform with an amplitude of 47 kA ...... 42 FIGURE 4.17: Discharging impulse current waveform with an amplitude of 49 kA ...... 43 FIGURE 4.18: Test rig exploding during impulse test ...... 44

vii

FIGURE 4.19: Fulgurite samples found when test rig exploded ...... 44 FIGURE 4.20: Fulgurites formed during 49 kA impulse discharge ...... 45 FIGURE 4.21: JEOL JSM-7600F field emission scanning electron microscope ...... 46 FIGURE 4.22: BSE-SEM of fulgurites image “A” ...... 46 FIGURE 4.23: BSE-SEM of fulgurite spectrum of fulgurite image “B” ...... 47 FIGURE 4.24: BSE-SEM spectrum of fulgurite sample ‘C’ ...... 48 FIGURE 4.25: Average spectrum of fulgurite material ...... 50 FIGURE 4.26: SPS (FCTHHPD25) Resistivity tester ...... 52 FIGURE 4.27: Resistivity of fulgurites ...... 52 FIGURE 4.28 Resistivity of soil sample ...... 53 FIGURE 4.29 Dynamic impulse impedance of tower footing ...... 55

viii

LIST OF TABLES

TABLE 2.1: Specifications for two types of 12 kV polymeric metal-oxide surge arrestors ...... 13 TABLE 4.1: Element normalized weight percentage of spot analysis ...... 30 TABLE 4.2: Element normalized weight percentage of average area analysis of ‘A’ ...... 31 TABLE 4.3: Atomic percentage of fulgurite BSE-SEM “A” ...... 47 TABLE 4.4: Atomic percentage of the spectrum fulgurite “B” ...... 48 TABLE 4.5: Elementary composition of the spectrum of fulgurite “C” ...... 49 TABLE 4.6: Average chemical composition of fulgurite ...... 50 TABLE4.7: Resistivity ...... 53 TABLE 4.8: Change in resistivity of bentonite and bentonite sand after formation of fulgurites .... 54

LIST OF ABBREVIATIONS

AC ...... Alternating current

AFRICON ...... Africa Conference

C ...... Inductor

DC ...... Direct current

DSO ...... Digital storage oscilloscope

EDS ...... Energy-dispersive X-ray spectroscopy

FESEM ...... Field emission scanning electron microscope

HV ...... High voltage

HT…………………………………High tension

ICETT ...... International Conference on Emerging Technological Trends

IEEE ...... Institute of Electrical and Electronics Engineers

IISC ...... Indian Institute of Science

KV ...... Kilovolt

L ...... Inductor

LV ...... Low voltage

LT………………………………....Low Tension

MAT ...... Modified Allen’s Test

MOV ...... Metal-oxide Varistor

SAUPEC ...... Southern African Universities Power Engineering Conference

SEC ...... Swaziland Electricity Company

SEM………………………………Scan electron microscope

SPS ...... Spark plasma sintering

LIST OF SYMBOLS

Al ...... Aluminium

Fe ...... Iron

K ...... Potassium

Na ...... Sodium

O ...... Oxygen

S ...... Sulphur

Si ...... Silica

Tb ...... Breakdown time

Ti ...... Titanium

Vb ...... Breakdown voltage

µs ...... microsecond

Ω……………………………………...Ohm

xii

CHAPTER 1: INTRODUCTION

1.1 RESEARCH BACKGROUND AND JUSTIFICATION

Due to the geographical nature of the Kingdom of Swaziland, the utility, Swaziland Electricity

Company (SEC), has been experiencing problems of overhead transformer failures due to lightning, especially during the rainy season, for the last many decades. Most of these problems are due to grounding issues (Mswane and Gaunt, 2005). Lightning is an electrical occurrence that occurs in the air. Its prevalence imposes risk to human life and to objects such as transformers. Swaziland has an approximately ground lightning flash density of more than 10 flashes/km²/year (Dlamini, 2009) These flashes mainly occur in the high altitude region where there is a high occurrence of thunderstorms annually, especially in summer. Lightning particularly destroys equipment such as electronic systems, transformers, electricity meters and electrical appliances in both residential and industrial environments. The cost of repairing the damage caused by lightning is very high.

During the last few decades, several naturally occurring and commercially manufactured materials was into the electrical engineering sector for enhancing the performance of grounding systems. Although most of these materials were tested under steady-state conditions, they have not been investigated specifically for their behaviour under impulse conditions. In the event of dispersing lightning current into the soil, without generating dangerous potential gradients, the grounding electrode impedance plays a much more significant role than the ground resistance.

When a vast amount current flows through a highly resistive soil, enough heat can be created to fuse the soil and form a fulgurite. A fulgurite is tubular shape and is approximately 0.5 cm to 2 cm in

1 diameter. Normally, the inner walls are smooth and glassy, but the exterior surface is rough and rock- like. In nature, lightning can produce fulgurites when cloud-to-ground flashes strike the earth, creating underground current channels.

A good design of the earthing system is essential to dissipate fault currents to earth effectively, regardless of the faults (Nor and Ramli, 2008). Overvoltage protective devices such as surge arrestors and spark gaps are used to divert high lightning surges from line to earth through an earth electrode.

It is imperative to design an effective earthing system that will be able to protect the equipment during fault conditions (Liu et al., 2004, Laverde et al., 2012, Lim et al., 2013).

The design of the earthing system depends on two factors, soil properties and dimensions of electrodes

(Lim et al., 2013). The common practice to get small grounding resistance includes the installation of

Ufer ground and backfilling with grounding improvement material such as bentonite and conductive cement (Uman, 1969, Essene and Fisher, 1986, Nor et al., 2005). Some salts are used to lower soil resistivity such as , copper and sodium sulphate, sodium carbonate, sodium chloride, and hydrate have been widely used as a remedy to lower soil resistivity for earth impedance purposes (Teed, 1965)

1.2 PROBLEM STATEMENT

For the past six years, Swaziland has experienced the loss of overhead transformers because of lightning during the rainy season. Swaziland networks are mostly overhead and as a result, the networks are exposed to lightning. In order to avoid damage to equipment, precautionary tools are normally used to redirect unrestrained current during lightning strikes. It is therefore essential to design effectual earthing system that is able to protect the equipment during fault conditions. Typical

2 people have observed fulgurites being formed during lightning current injected into the soil. Only in laboratory conditions that I have seen faults currents (at 50/60 Hz) generating fulgurites (Laverde et al., 2012). In this research, tests were performed on the soil samples from Swaziland where lightning problems were persisting.

1.3. OBLECTIVES

The main object of this research is to take the soil around the affected feeder and create fulgurite in that by applying laboratory impulses.

1.3.1 Sub-objective 1

Analyse the soil sample from the affected feeder.

1.3.2 Sub-objective 2

Test the resistivity of the soil sample.

1.3.3 Sub-objective 3

Analyse the chemical composition of the soil sample.

1.3.4 Sub-objective 4

Conduct formation of the fulgurite at the laboratory.

1.3.5 Sub-objective 5

Study the effects of fulgurites on the electrical earthing system.

1.4 AIM OF THE STUDY

This research aims to:

 investigate the formation of fulgurites from the soil of the feeder line during impulse tests;

 analyse the fulgurites and their possible contribution towards electricity failures in Swaziland

during the rainy season;

 compare the resistivity of the soil sample with the fulgurites;

 determine the chemical composition of the fulgurites; and

 Propose the best backfill material.

1.5 RESEARCH DELIMITATIONS

 The research is limited to the experimental results.

 The research does not address protection on the lines.

1.6 BENEFITS OF THE RESEARCH

The purpose of this research is to:

 Address the effects of fulgurites on the electrical earthing systems; and

 Provide guidance when designing the earthing systems.

1.7 LIST OF PUBLICATIONS

Some of the results presented in this dissertation appeared in the following publications:

1. Tshubwana, R.P., Shongwe, T. & Gomes, C. (2016). Characteristics and performance of soil,

bentonite and conductive cement during impulse tests in Swaziland. In: Proceedings of IEEE

International Conference on Emerging Technology Trends in Computing, Communications

and Electrical, 21-22 Oct. 2016. IEEE. pp. 1-6. Baselious Mathews II College of Engineering

India. ISBN: 978-5090-37506.

2. Tshubwana, R.P., Mbungu, NT, Siti, W (2017). “Analysis of the impulse test on the

application of soil, bentonite and the cement” SAUPEC (2017).

3. Tshubwana, R.P., Hove, M., Shongwe, T (2017). Effects of fulgurites on the earthing

system. IEEE AFRICON, Cape Town, 18 – 20 September 2017.

1.8 DISSERTATION ORGANISATION

Chapter 1: Introduces the dissertation and outlines the background and the objectives of the study. It presents the scope of the research.

Chapter 2: The chapter briefly outlines previous research done and its limitations. It also outlines the literature review of the research.

Chapter 3: Presents the methodology used to address the objectives described, which includes the investigation of fulgurites. It also outlines different current impulse experiments that I conducted and the investigations on the chemical composition of the soil and the soil resistivity.

Chapter 4: Provides the details of the tests results, including the chemical composition of the soil sample and the fulgurites. Soil resistivity results and that of fulgurites were discussed.

Chapter 5: Conclusions and Recommendations.

CHAPTER 2: LITERATURE REVIEW

2.1 INTRODUCTION

Fulgurites are formed by the mingling silica sand or rock when lightning strike (Knight and Grab,

2014). They are created when lightning strikes the ground at a temperature of 18000 degrees Celsius

(Naito and Nakamura, 1993). According to (bin Burhanuddin et al., 2016) lightning can cause transients with impulse currents of 100 KA amplitudes at a time around 70- 100 µs followed by continuing currents in milliseconds. According to (Frondel et al., 1962, Pye, 1982) fulgurites can also be associated with lightning striking the surface of the ground. Such lightning currents must carry sufficient energy and power to melt silica in the ground. Fulgurites usually change the overall physical and electrical properties of the original material. They can also modify the electrical performance of earthing systems. The effective designing of earthing systems is imperative to dissipate fault current irrespective of the type of fault.

Figure 2.1: Lighting intensity in Swaziland. Source: South African weather services

Lightning discharges occur within a cloud, between clouds or between a cloud and the ground. Of all these discharges, only about 25% of the lightning bolt reaches the ground (Punekar and Kandasamy,

2011).

Lightning develops from the base of a cloud (usually negatively charged) when a pre-discharge known as a negative downward stepped leader evolves (Mason, 1972). This stepped leader is barely luminous and its development towards the ground is in leaps of scores of metres; in reality, it is a succession of discharges travelling along the same ionised path at intervals of 40 µs to 100 µs (average progression speed of about 0.5 m/µs to 1 m/µs). As it approaches the ground, the extremity of the highly stepped leader causes a significant increase in the average electric field along its length. This electric field can reach values of 400 kV/m to 500 kV/m. When the atmospheric air ionisation is reached (30 kV/cm) at high points, corona discharges occur (Pommerenke, 1995). Locally, for higher electric field values,

8 these corona discharges transform into positive rising discharges and join the negative downward stepped ladder. The electrical connection between the thunder-cloud and the ground is established by an ionised channel. The charge that is returned from the ground to the cloud is known as ‘the return stroke’. The return stroke is highly luminescent. A number of restrikes may be exchanged in the space of 0.2 seconds to 1 second. (Cooray, 2003).

Cloud-to-ground lightning discharges are quite destructive, particularly when the system struck is unprotected. The effects of cloud-to-ground discharges are generally grouped into two categories, namely:

[1] Direct stroke

[2] Indirect stroke

2.2 DIRECT STROKE

A lightning cloud-to-ground discharge strikes an object such as an overhead power line directly, and this can result in damage (Rakov and Rachidi, 2009). Current divides and then propagates when lightning strikes mid-span on a power line. Direct effects mainly result in physical damage that is coupled with fire hazards.

Common victims of direct strokes are electric motor insulation and power distribution transformers.

The direct effects caused by lightning strikes are briefly explained below:

 Thermal effects – These effects are linked to the amount of charge associated with lightning

strikes. In materials of high resistivity, they result in fusion-points melting holes of varying

sizes at the point of impact. For poor conductor materials, vast amount of energy delivered in

the form of heat. As a result, some electrical equipment may explode (e.g. transformers).

 Electrodynamic effects – These effects occur between conductors due to the large magnetic

field of the lightning current. They result in substantial mechanical forces, both attractive and

repulsive, that are even stronger when the conductors are close together or the current is high.

 Effects due to initiation – When lightning strike, a substantial increase in the ground potential

of installations occurs depending on the grounding/earthing network and the soil resistivity. A

potential difference is also created between a number of metal elements and hence the need to

pay particular attention to system earthing.

2.3 INDIRECT STROKE

A lightning cloud-to-ground discharge strikes the nearby ground. The voltage induced on electrical equipment such as a power line has four components:

 The charged cloud above the line induces bound charges on the line while the line itself is held

electrostatically at ground potential by the neutrals of connected transformers and by leakage

over the insulators. When the cloud is partially or fully discharged, these bound charges are

released on the line, giving rise to the travelling voltage and current waves.

 The charges induced by the stepped leader further induce charges on the line. When the

stepped leader is neutralised by the return stroke, the bound charges on the line are released

and thus produce travelling waves similar to those caused by the cloud discharge.

 The residual charges in the return stroke induce an electrostatic field in the vicinity of the line,

hence the induced voltage.

 The rate of change of the current in the return stroke produces a magnetically induced voltage

on the line known as inductive effect

Lightning is the major cause of stray voltage, and this leads to the occurrence of electrical potential between two objects that ideally should not have a voltage difference between them. Large voltages can appear on the enclosures of electrical equipment due to a fault in the electrical power system such as a failure of insulation. In the following paragraphs, the existing infrastructure that SEC uses to mitigate transformer failures caused by lightning.

2.4 SURGE ARRESTORS PROTECTION

Surge arrestors are metal-oxide varistor (MOV) or spark gaps based devices that protect equipment from overvoltage occurrences. These devices are designed to protect electrical equipment

(transformers) from the damaging effects of spikes and transients by diverting the voltage surge to ground and maintaining equipment to safe voltage differentials (Faxvog et al., 2013) .

Surge arrestors are connected to the conductor in series with the cut-out fuse just before it enters the transformer. They are also connected to ground by means of an earth wire and function by routing energy from an overvoltage transient to ground if it occurs, while isolating the conductor from ground at normal operating voltages as shown in Figure 2.2 below.

lightning line F U 50 Hz S E lightning Bushing Surge Arrester

Transformer

Figure 2.2: Connection of surge arrestor for lightning protection

However, the failure rate of transformers has not been minimized by this method of transformer protection against lightning strikes in Swaziland (Dlamini, 2012). This is due to the follow reasons:

 A surge arrester switches from open circuit to short circuit during a very short period. During

that time both current and surge current pass into ground through the surge arrestor. It will

work the same for the next surge current. A problem arises only if the surge arrestor fails to

open circuit mode. It is shown in Figure 2.3 below.

Figure 2.3: A transformer with an operated surge arrestor

 A high potential difference between the transformer and the earthing point results in

transformer damage during lightning strikes.

 A surge arrestor only conducts a bulk of the surge current into earth bypassing a path across

transformer coils.

Table 2.1: Specifications for two types of 12 kV polymeric metal-oxide surge arrestors

Current Impulse Residual Voltage MOA 2ms Rectangular Rated MCOV 1/4μs 8/20μs 30/60μs current impulse Type Voltage Lightning Lightning Switching withstand current current current impulse impulse impulse KV(rms) KV(rms) KV(crest) KV(crest) KV(crest)A HY5W-12 12 10.242.2 3627 150 HY10W- 12 12 10.242.2 3627 250

Table 2.1 above shows the specifications for two types of 12 kV polymeric metal-oxide surge arrestors that are currently used in the network. Surge arrestors are design to withstand lightning current comes through the power line due to direct strike to the line (Tshubwana et al., 2016).

2.5 EARTHING SYSTEM

The method of earthing that is being used at SEC to ground transformers is the crow’s foot method.

In this method, the earth wire from the body of the transformer and the bottom of the surge arrestor is connected to an earth electrode 1 m away from the supporting structure of the transformer and buried

0.5 m below the surface level. Subsequent earth electrodes are then connected at 45° angles apart and

5 m from the main electrode. This is done to achieve a total earth resistance that is below 10 Ω.

The star neutral point is separately earthed at a distance of 5m of the LT and the body earth. Challenges can be encountered on site with the crow’s foot earthing; the earth wire has earth clamp joints on the earth electrodes below the soil surface. According to (Committee, 1982) and (Sverak and Laird, 1985), the resistivity of the soil is a measure of resistance value for a volume that will resist an electric current. A variable value is dependent on the type of soil and the water the soil. This value is notable in considering the arrangement of the grounding, which may influence the lightning current along the line.

Figure 2.4 below demonstrates the way that the utility SEC lays the trenching for the crow’s foot earthing.

Figure 2.4: Crow’s foot earthing

Fulgurite created under alternating, direct and impulse current application where investigated (bin

Burhanuddin et al., 2016). In their studies, they conclude that artificial fulgurites are created in sand, bentonite and mixtures thereof with the application of direct voltage.

The path of the lightning bolt to ground determines the shape of the Fulgurites which are formed as a result of the melting process of the soil (Laverde et al., 2012) . The impulse impedance as opposed to the power frequency resistance affects the earthing system during lightning discharge conditions.

This is because of the effect of soil ionization and electrode inductance. The soil surrounding the electrode breaks down under high impulse current.

Studies (He et al., 2008) show that electric fields under negative impulse polarity are higher than those under positive impulse polarity for wet sand, however the resistance and time to breakdown are not polarity dependent.

(Nor et al., 2005) described the effects of soil resistivity, impulse polarity and the dimension of the earth electrode on magnitudes. The authors found that the resistivity of the soil is controlled by adding a different percentage of water content and different percentages of additives as well as having different grain sizes of sand (Nor et al., 2005). However, soil resistivity ranging from a few ohm- meters for clay to several kilo-ohm-meters for igneous rocks have been reported (Mukhedkar and

Dawalibi)

The findings of (Rahim et al., 2014) prove that the soil resistivity has an impact on the peak of the current discovered at different positions through the tower. The performance of the peak current under different soil resistivity conditions shows when installing equipment through the tower a number of considerations for the protection schemes of a tower should be taken into account. Furthermore, the soil resistivity increases as the ground impedance increases. Grounding resistance depend mostly on soil resistivity and proportions of the grounding as per the equations given in various standard

(Committee, 1982). The grounding system performance of the deep driven copper rod may or may not be affect by the presence of trees and surroundings of the grounding systems.

(Lim et al., 2013) found that grounding resistance is clearly dependent on the surrounding environment conditions and the soil resistivity average. The authors proposed that running waterways, huge trees and slopes should be avoided wherever possible when installing grounding electrodes

16 because the ground resistance of electrodes in such environments may fluctuate significantly with time.

Fulgurites form as the results of large, rapid increases in energy in a narrow column over a short time of period. Energy typically greater than 1 MJ/m is required to form fulgurites and the corresponding temperatures exceed 3000 K for the volatilization of silicates. Heating rates are least 500K/s and exceed 1000 K/s adjacent to the lightning channel (Pasek et al., 2012).

According to the findings of (Ala et al., 2009) fulgurites are natural glasses formed by cloud-to-ground lightning. Several different morphologies of fulgurites have been researched, including sand fulgurites, rock fulgurites, caliche fulgurites and clay fulgurites. His studies found that clay and caliche fulgurites differ systematically in their morphology. The authors use morphological features to explain the properties of fulgurite-forming lightning strikes and classify fulgurites into four types of morphologies with an additional minor type. Type I fulgurites are sand fulgurites consisting of thin, glass walls, Type II fulgurites are clay fulgurites consisting of thick, melt-rich walls, Type III fulgurites are caliche fulgurites consisting of thick, glass-poor walls and Type IV fulgurites are rock fulgurites consisting of glass with walls comprising the surrounding unmelted rock (Ala et al., 2009).

(Chen and Wang, 2011) discovered that the impulse breakdown delay is an important parameter in describing the soil breakdown under impulse voltage. The soil impulse breakdown delay decreases with the increase of the impulse voltage applied (Chen and Wang, 2011). The lower the soil temperature or the higher the soil moisture , the larger is the soil density and the greater is the soil impulse breakdown delay (Chen and Wang, 2011). Different soils have different breakdown delays

(Rameli et al., 2014)

2.6 SUMMARY

This chapter has reviewed the research works published for the fulgurites formations. Many studies shows how fulgurites made in various forms of soil during lightning and other backfilling materials such as bentonite. Most of the laboratory experiments used voltage to form fulgurites; little has been done using current mode.

This research work investigate the formation of fulgurites in Swaziland in the feeder where it was highly affected by lightning. The research also analysis the morphology of the fulgurites and its contribution towards power failure.

CHAPTER 3: METHODOLOGY

3.1 OVERALL APPROACH

Due to the numerous power failures over the past few years, the SEC has improved the protection of their lines, especially in the areas that the utility experiences severe problems. Although the problems have been minimised, they persist in some regions. The main approach of this study was to collect soil samples from an area where the problem persists in order to investigate fulgurite formation using electrodes to propagate currents through the soil samples. Fulgurites are known to be highly resistive compared to the original soil. Flow chart methodology is summarized by a flow chart as shown in

Figure 3.1

Start

Determination of materials to be used

Collection of Soil from different region

Dry Material Wet Material

Application of impulse Current Application of impulse Current (Switching/Lightning) (Switching/Lightning)

Increase Current Increase Current

Fulgurite No No Fulgurite formation formation

Yes Yes

Characterization of fulgurites

Studying the effects of fulgurite on electrical earthling system

End

Figure 3.1. Flow chart of methodology summary

3.2 EXPERIMENTS

The fulgurites formation experiments, I conducted them using the current impulse generator at the

High Voltage Laboratory at the Indian Institute of Science (IISC) in Bangalore (India). Different current waveforms were obtained during the tests, when the current was increased. All measurements were recorded on a digital storage oscilloscope.

In order to obtain the desired results, a current impulse was chosen rather than a voltage impulse. A current generator produces load- independent current and a voltage generator produces load- independent voltage, if you apply a voltage source to a very low impedance you get enormous current more voltage and less current. The tests were performed in the High Voltage Laboratory at the Indian

Institute of Science in India, Bangalore.

3.2.1 Test rig

The test rig shown in Figure 3.1 and Figure 3.2 is made of Perspex glass material with a thickness of

12 mm. It was cubical with sides measuring 10 cm. At the base of the chamber is an aluminium plate for earth connection. An aluminium stand with six aluminium legs supported the chamber. The aluminium legs distributed the current flowing from the current generator through the soil to the ground. The top and the bottom sides of the chamber were fitted with sharp, pointed electrodes. This were so designed because conductors with sharp points or small radii have the highest E-field and hence, it is easier to initiate ionisation and subsequent breakdown of the material.

Copper Electrode to the generator

Earth connection to the generator

Aluminium legs of the test rig

Current measurement rope

Figure 3.2: Empty test rig

Copper connection

Soil sample

Test rig

Figure 3.3: Test rig filled with soil sample

3.2.2 Test sample

The soil sample was taken was from an 11 kV feeder in Swaziland (Lamgabhi Village). The sample was dry with fine material as shown in Figure 3.5. The sample was sieved before loading it into the test chamber because it is difficult to observe formed fulgurites on a coarse aggregate sample, particularly with a low-impulse current when the fulgurites formed are small.

3.2.3 Electrodes

Two electrodes made of steel material was used in the experiment. One electrode was connected to the output terminal of the current generator to inject the current through the soil sample. The second electrode was connected to the bottom of the chamber for grounding. A 2 mm gap between the electrodes was chosen since the smaller the air gap, the better the results.

Figure 3.4: Steel electrode to the earth connection

Figure 3.5: Electrode to the output terminal of the generator

Figure 3.6: Soil sample from Lamgabhi Village

3.2.4 Current impulse generator

The current impulse generator consists in Figure 3.6 and 4.1 of a 46-capacitor bank connected, rated for a 50 kV, 0.5 µF, air-cored inductor of 5.65 µH each. A resistance of 3Ω is connected in series with the inductor. The generator is charged by a DC unit consisting of a 400/100 kV, 15 kVA step-up

23 transformer as shown in Figure 4.2. When the generator is charged, the capacitor banks are connected in parallel. The maximum magnitude for the generator is 200 kA. The current impulse generator used is self-triggering and hence, one cannot control the point of triggering. The controlling parameters are gap size and type of sample material.

Trip pulse

Flat Strip SA Work

To CRO 2-100KV

+Vdc

Figure 3.7: Circuit diagram of the current impulse generator

3.2.5 Oscilloscope

A digital storage oscilloscope shown in Figure 3.7 is a (DSO) with the GTP-150A-2 Oscilloscope

Probe and the 150 MHz (10:1/1:1) Switching Passive Probe, BNC(P/M) GDS/GOS/GRS Series, was used to capture the current signals.

Figure 3.8: Digital oscilloscope

3.3 ENERGY-DISPERSIVE X-RAY (EDS) ANALYSIS

The element normalised weight percent was analysed. In all instances, the energy-dispersive X-ray spectroscopy (EDS) analyses returned with high non-normalised values.

3.4 SOIL RESISTIVITY

According to (Mukhedkar & Dawalibi., 1975) the resistivity of a soil is a measure of the resistance value of how soil resist the flow of current. It has a variable value, which is dependent on the type of soil also the water content within the soil. This value is very significant in evaluating the arrangement of grounding.

It was concluded in (Choun et al., 2012) that soil resistivity at any specific location is affected by the environmental factors considered in the study which are proximity of soil to water masses, built up area and vegetation. The resistivity of the soil was determined before the experiments at the Tshwane

University of Technology as shown in Figure 4.20 and Figure 4. 21. The soil sample was prepared in a molybdenum die-punch mould. The prepared sample was then inserted into an SPS (FCT HHPD

25) chamber in a copper-boron conical punch shown in Figure 4.19. The punches were connected to two thermo-couples for accurate resistivity reading. Once in the punch, the sample was subjected to a

10 kN force (required contact force for SPS to operate). A vacuum atmosphere was then achieved in the chamber. The process started at a low voltage (0.6 V) and subsequently, the voltage was increased every two minutes by factor 2 of the initial voltage. The process was monitored until the resistivity reached a break point.

3.5 FULGURITES ANALYSIS

Chemical composition of the fulgurites and the resistivity of the fulgurites were analysed through the application of a conducting carbon tape to determine the chemical composition and SPS to determine the resistivity of the fulgurite. The analysis was performed at the Tshwane University of Technology mineralogical machine.

3.6 CHEMICAL COMPOSITION OF THE SOIL

Soil samples from Lamgabhi Village were analysed to find the chemical composition and the nature of soil before the impulse tests. Two spot analyses, two spectra traces and two backscattered electron

(BSE) photomicrographs were conducted as shown in Figure 4.15 and Figure 4.16. Scan electron microscope (SEM) microstructural inspection was conducted on a JEOL JSM-7600F field emission with a Link EDS system. Each pieces were polishing flat before to SEM inspection in order to remove possible darkness effects during EDS inspection. Samples were mounted on carbon stubs and remained uncoated for the inspection.

CHAPTER 4: EXPERIMENTAL RESULTS

4.1 INTRODUCTION

The results of the effects of fulgurites on power system earthing are given in this chapter. Not many studies have been done or reported on the effect of fulgurites on electrical grounding. The backfill materials can lower the earths impedance and hence can be used to control the ground impedance.

The grounding impedance that is a sum of the static and dynamic impedances is affected by fulgurite formation. This work shows that it is the static impedance in particular that is affected by the formation of the fulgurite. The results of this work show that the static impedance of the backfill material increased with fulgurite formation.

4.2. CURRENT IMPULSE EXPERIMENTS

The aim of the tests was to form fulgurites using current impulse generator as shown in Figure 4.1 from the soil samples taken from Swaziland (Lamgabhi Village). The standard lightning waveform

(8/20 µs) was used for the experiments , however the standard waveform was produced only during the short circuit test and application to SPDs. when it was applied to a gap-breakdown situation the waveform changes (and starts oscillating) however, the energy content remains the same. The idea was not to apply the exact lightning test waveform but to ensure the waveform contains enough energy similar to lightning

Charging resistance

46 Capacitors connected

Trip pulse

Figure 4.1: Current impulse generator (used to perform the experiments)

Figure 4.2: Charging transformer

4.3 CHEMICAL COMPOSITION OF SOIL SAMPLE

Different soil samples from LA Mgabhi were analysed as shown in to determine chemical composition and nature of the soil before impulse and AC/DC tests. Two spot analyses, two spectra traces as shown in Figure 4.4 and Figure 4.5 and two BSE photomicrographs were conducted as shown in Figure 4.3 and Figure 4.6. Samples were mounted on carbon stubs and remained uncoated for the analysis.

Table 4.1: Element normalised weight percentage of spot analysis

Element Normalised wt % Oxygen 46.17 Aluminium 18.67 Silicon 22.42 Iron 8.72 Titanium 0.93 Potassium2.12 Sulphur0.11 Sodium0.86 Total 100

Figure 4.3. BSE-SEM image of soil sample trace “A”. The entire field of view was used for an “average area” analysis. Bright white areas are electron charge build (samples were not carbon coated prior to analysis

Figure 4.4: Spectral trace of spot analysis for soil sample (Lamgabhi region)

Table 4.2: Element normalised weight percentage of average-area analysis of ‘A’

Element Normalized wt % Oxygen 46.17 Aluminium 18.67 Silicon 22.42 Iron 8.72 Titanium 0.93 Potassium 2.12 Sulphur 0.11 Sodium 0.86

Total 100.00

Figure 4.5: Spectral trace of average area analysis for soil sample (Lamgabhi region)

Figure 4.6: BSE-SEM image of trace “B” sample (Lamgabhi region) The entire field of view was used for an ‘average-area’ analysis. Bright white areas are electron charge build-up (samples were not carbon coated prior to analysis).

4.4 IMPULSE WAVEFORMS

WAVEFORM 1

The initial waveform shown in Figure 4.7 shows the discharged impulse current with an amplitude of 14.9 kA from the current generator. Fulgurites was not formed at this discharged current, meaning more energy was needed to melt the soil. The waveform shows distortion hence contains enough energy similar to lightning.

Figure 4.7: Discharging impulse current waveform with an amplitude of 14.9 kA

WAVEFORM 2

The second waveform shown in Figure 4.8 shows an increased discharged current with an amplitude of 16 kA from the current generator. The soil around the electrodes shows some small, white areas measuring less than 3 mm shown in Figure, which demonstrates that there are very small fulgurites around the electrodes.

Figure 4.8: Discharging impulse current waveform with an amplitude of 16 kA

WAVEFORM 3

The third waveform shown in figure 4.9 shows an increased discharged current with an amplitude of 17 kA, little fulgurite particles was established. The particles were sphere shaped with a diameter of about 1mm, were observed.

Figure 4.9: Discharging impulse current waveform with an amplitude of 17 kA

WAVEFORM 4

The fourth waveform shown in Figure 4.10 shows an increased discharged current with an amplitude of 22 kA. No fulgurites was observed. The energy produced was not enough to produce visible fulgurites.

Figure 4.10: Discharging impulse current waveform with an amplitude of 22 kA

WAVEFORM 5

The fifth waveform shown in Figure 4.11 shows a steady increase in the discharging current from the current impulse generator with an amplitude current of 23 kA. No fulgurites was observed from the test sample at this current.

Figure 4.11: Discharging impulse current waveform with an amplitude of 23 kA

WAVEFORM 6

The sixth waveform shown in Figure 4.12 shows a swift increase in discharge current from the impulse current generator. A discharged impulse current of 24 kA amplitude has been recorded. The test sample shows some whitish fulgurites around the electrodes.

Figure 4.12: Discharging impulse current waveform with an amplitude of 24 kA

WAVEFORM 7

The seventh waveform shown in Figure 4.13 shows a recorded discharged impulse current of

29 kA. Small fulgurites were observed. The soil started to show some desired results, meaning that additional current was needed to obtain fulgurites.

Figure 4.13: Discharging impulse current waveform with an amplitude of 29 kA

WAVEFORM 8

The eighth waveform shown in Figure 4.14 shows a current triggered on the soil. The oscilloscope recorded current of amplitude 30 kA, handful of fulgurites has been observed.

This was expected because the current margin was not very different.

Figure 4.14: Discharging impulse current waveform with an amplitude of 30 kA

WAVEFORM 9

The ninth waveform shown in Figure 4.15 shows a sporadic increase in amplitude current to

31 kA. Again, small fulgurites less than 1mm were noticed around the electrodes as shown in

Figure 4.19.

Figure 4.15: Discharging impulse current waveform with an amplitude of 31 kA

WAVEFORM 10

The tenth waveform shown in Figure 4.16 shows a current of 47 kA. Fulgurites were observed around the electrodes. Some of the fulgurites were 2 cm in length as shown in Figure 4.20.

Figure 4.16: Discharging impulse current waveform with an amplitude of 47 kA

WAVEFORM 11

The eleventh waveform shown in Figure 4.17 shows an increased discharged current with an amplitude of 49 kA at the highest for the tests conducted. The chamber exploded as shown in

Figure 4.18, at this current due to the energy exerted on the soil. Many fulgurites, some measuring approximately 2 cm, has been observed around the electrodes as shown in Figure

4.20.

Figure 4.17: Discharging impulse current waveform with an amplitude of 49 kA

Figure 4.18: Test rig exploding during impulse test

When the impulse current of 49 kA was discharged to the soil sample, the test rig exploded.

Dust was seen in the vicinity, and fulgurites were found around the tips of the electrodes. When high current flows around the electrodes, it melts the soil and fulgurites are formed, which are highly resistive.

Figure 4.19: Fulgurites samples found when test rig exploded

Figure 4.20: Fulgurites formed during 49 kA impulse discharge

4.5 FULGURITES MORPHOLOGY

Different BSE-SEM morphology were analysed as shown in Figure 4.22, Figure 4.23 and

Figure 4.25. Several things including target composition are likely dependent on the morphological properties of fulgurites. The energy probable plays an outstanding rate in fulgurite morphology. This energy is capable of melting and vaporizing target material (Block,

2011). The material encounter swift physical, chemical and morphological changes when the lightning strikes the ground. The contents of the fulgurites were analysed through the application of a conducting carbon tape to determine the chemical composition. Fulgurite morphology is also linked to the radial and longitudinal temperature gradients experienced along the currents pathways. The temperatures reached during fulgurite formation are not well constrained but clearly exceed the vaporization temperature of silicates in the region closest to the path of the strike, as central voids are commonly observed (Carter et al., 2010).

The analysis has been performed at the Tshwane University of Technology using a mineralogical machine (JEOL JSM-7600F field emission scanning electron microscope) as shown in Figure 4.21.

Figure 4.21: JEOL JSM-7600F field emission scanning electron microscope

Figure 4.22: BSE-SEM of fulgurites image “A”

Table 4.3: Atomic percentage of the fulgurite BSE-SEM “A”

Element Weight% Atomic% Oxygen 53.2 67.71 Sodium 2.04 1.79 Aluminium 6.04 4.50 Silica 32.7 23.42 Potassium 1.69 1.10 Calcium 0.89 0.46 Iron 2.85 1.03 Totals 100.00 100.00

Figure 4.23: BSE-SEM spectrum of fulgurite image “B”

Table 4.4 Atomic percentage of the spectrum fulgurite “B”

Element Weight % Atonic % Carbon 21.71 30.71 Oxygen 47.73 50.68 Sodium 1.19 0.88 Aluminium 12.02 7.57 Silica 11.44 6.92 Potasium 5.91 3.24 Total 100 100

Figure 4.24: BSE-SEM spectrum of fulgurite image ‘C’

Table 4.5 Elementary composition of the spectrum of fulgurite “C”

Element Weight% Atomic% Carbon 4.5 7.15 Oxygen 60.15 69.41 Aluminium 15.97 10.93 Silica 17.01 11.18 Potassium 2.3 1.33 Total 100.00 100.00

4.6 CHEMICAL COMPOSITION OF THE FULGURITE

Table 4.6 below shows the average chemical composition of the fulgurites.

Table 4.6: Chemical composition of the fulgurites

Element Weight % Atonic % Carbon 21.71 30.71 Oxygen 47.73 50.68 Sodium 1.19 0.88 Aluminium 12.02 7.57 Silica 11.44 6.92 Potasium 5.91 3.24 Total 100 100

Figure 4.25: Average spectrum of fulgurite

Four phases were present in the chemical composition of the fulgurite: Kaolinite (Al2Si2O9),

Silica (8SiO2), Silicon (Si4) and Corundum (AlO) (36AlO) as shown in Figure 4 .25. The main phase is Silica, followed by Kaolinite, then Silicon and lastly corundum. These phases were

expected. Sodalite and Stishovite were not found. It was earlier mentioned that when fulgurite is created, there is a decrease in metallic oxide. One of the probable statement is the unreliability of the thermodynamic of the oxide at a souring temperature in relation to the metal

(Essene and Fisher, 1986, wAsseRMAN and MeLosH) . Another probability is the formation of CO or more mixtures using carbon as a lessening factor (Essene and Fisher, 1986, wAsseRMAN and MeLosH). Another factor is that carbon was responsible for the oxide reduction process could not be eliminated because of the existence of carbonaceous material in the soil. Another probability is lightning associated with the electrolysis from the high current

(Essene and Fisher, 1986). Another probability is the induce reduction which may be caused by shockwave (Rowan and Ahrens, 1994). Another factor is the reduction of lower stable oxide by converting single oxide to multi oxidised state.

4.7 RESISTIVITY RESULTS

The resistivity of the soil sample and fulgurites shows a high increase as shown in Figure

4.27 and Figure 4.28. The result on Figure 4.6 for the fulgurite sample and Figure 4.7 for the soil sample represent the resistivity measured using spark plasma sinter (FCT System HHPD

25). The resistivity results of fulgurites shows an increase in compared to the soil sample.

Soil samples recorded 1.52 Ω.m shown in Figure 4.27 and the fulgurites shows a resistivity value of 17.2 Ω.m shown in Figure 4.28

Figure 4.26: SPS (FCT HHPD 25)

FULGURITES

200,0000 12,0000

180,0000 10,0000 160,0000

140,0000 8,0000 meter) . 120,0000

(mOhm 100,0000 6,0000

80,0000 Power (watts) 4,0000 Resistivity 60,0000

40,0000 2,0000 20,0000

0,0000 0,0000 0 10 20 30 40 50 Resistivity ohm Time (Minutes) Voltage Increase

Figure 4.27: Resistivity of fulgurites

Soil sample 200 12

180 10 160

140 8

120

100 6 Power(watts) Ohm.meter) 80 m. 4 60

40 2 Resistivity ( 20 Resistivity Voltage 0 0 0 10 20 30 40 50 60

Time (minutes) Figure 4.28: Resistivity of soil sample

Table 4.5 shows the resistivity of the soil from Lamgabhi Village before and after the impulse

tests. An increment in the resistivity of the soil is demonstrated below in Table 4.7

Table 4.7 Resistivity Material Resistivity of Resistivity of Soil & Original Fulgurite Fulgurite Material ( Ωm) Form ( Ωm) Resistivity 1.52 17.2

4.8 DISCUSSION

The findings from the tests conducted on the soil form Swaziland utility, it has been found that

there was a huge increase of soil resistivity during the current impulse tests.

The impedance of the earthing can be divided into static and dynamic (bin Burhanuddin et al.,

2016) From the measurements of the static resistance, it is clear that the resistance of the fulgurites is higher than that of the original soil . Thus, the formation of fulgurites increases the impedance of the earthing system. Swaziland, a country very prone to lightning, has a high probability of having earthing systems contaminated with fulgurites due to repeated lightning strikes over the years. This may be a contributory factor to the low number of operational feeders in Swaziland. Thus, being a mountainous and rocky country, the likelihood of fulgurite formation in this country is not remote since fulgurite formation due to lightning is associated with such land features.

The work of (Harid et al., 2012) showed in Figure 4.29 that the initial resistance (Rinit) and the final resistance (Rfin) of the soil are not the same after an impulse ; He found that Rfin is much higher with fulgurite formation than without. The increase in the static impedance of the fulgurites formed as part of the earthing system is attributed to chemical changes in the parent of the original soil. Regarding the chemical composition, fulgurites have a higher content of silica (SiO2), which has high resistivity. (bin Burhanuddin et al., 2016) found that fulgurite formation from bentonite in their studies. This material has been used as a backfill material.

The electrical properties of the fulgurites so formed are presented below (bin Burhanuddin et al., 2016).

Table 4.8: Change in resistivity of bentonite and bentonite-sand after formation of fulgurites

Material Resistivity of Resistivity of Percentage Original Fulguritic increment Material (Ωm) Form (Ωm) (%) Bentonite 1.5 22 1 367 Bentonite-sand 4.9 35 614 Source:(bin Burhanuddin et al., 2016)

Figure 4.29: Dynamic impulse impedance of tower The reduction similar to various discharges in the soil directing to higher resistance decrease. (a) Ip=0.5kA; (b) Ip=2.17 kA Source: (Harid et al., 2012).

CHAPTER 5: CONCLUSION

5.1 SUMMARY OF DISSERTATION

An investigation into fulgurites from the 11 kV feeder in Swaziland has been exercised. Current impulse tests were used to initiate fulgurite formation from the soil samples. The chemical composition of the original soil sample was determined before the tests and thereafter, the chemical composition of the fulgurites formed has been investigated. The resistivity of the soil samples and the fulgurites were tested and compared. Simulation equations has been formulated to simulate different current impulse waveforms during the experiments.

Chapter 1 introduced the dissertation. It provided the background, the objectives and the methodology used as well as the scope of the research. Chapter 2 outlined the literature review of the dissertation by means of a theoretical study of research pertaining to the field under investigation. Chapter 3 outlined the methodology and the approach used in this dissertation.

Chapter 4 addressed the results of the different experiments and the tests conducted, including the simulation. Chapter 5 compared the results of this dissertation.

5.2. CONCLUSION

The influence of fulgurite formation on the earthing system of a power network has been investigated. Fulgurite formation alters the original soil compositionally and increases the final resistance. Backfill materials need to be characterised as immune to fulgurite formation. From the tests conducted, artificial fulgurites were formed in the soil when high currents passed through the soil via electrodes. Proper grounding should take precedence when designing and installing electrical power systems. All good grounding systems should provide a low-impedance path for fault and lightning-induced currents to enter the earth, ensuring

maximum safety from electrical system faults and lightning. The entire earth is regarded as a good conductor of electricity. The importance of effective grounding is to provide a manageable way when fault to ground occurs by redirecting the fault current to the source to avoid harm to equipment. Fulgurites are known to have high resistance compared with the actual material, and the overall reactance behaviours of the earth electrodes may change in the presence of leakage currents in kA systems. Properly installed grounding helps to protect tools from harm when unforeseen fault currents occurs.

5.3 RECOMMENDATIONS

Different types of ground improvement material (backfilling) available i.e. bentonite, conductive cement. Engineers in the electrical sector recent years prefers bentonite and bentonite based materials as a backfill material (Gomes et al., 2014). Certain types of this backfilling material are permanent and are maintenance free for over 30 years. Utilities must envisage the backfilling method where the occurrence of transformer failure persists.

5.4 FUTURE RESEARCH

Transient behaviour of backfill materials used for the enhancement of performance of grounding systems in the Kingdom of Swaziland and also surface flashover characteristics of aged insulators contaminated with industrial pollutions.

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