a novel environment friendly and efficient gaseous insulator

A Novel Environment Friendly and Efficient

Gaseous Insulator

Submitted by;

Hafiz Shafqat Abbas

2014-PhD-Elect-012

Supervised by:

Prof Dr. Muhammad Kamran

Department of Electrical Engineering

University of Engineering and Technology Lahore, Pakistan

2020

i

A Novel Environment Friendly and Efficient

Gaseous Insulator

Submitted to University of Engineering and Technology, Lahore

In partial fulfillment of the requirement for the award of the degree of

Doctor of Philosophy (Ph.D)

In

Electrical Engineering

Submitted by;

Hafiz Shafqat Abbas

2014-PhD-Elec-012

Thesis approved on……………….

…………………………… ………………………..

Internal Examiner External Examiner

…………………………… ………………………..

Chairman Dean

Department of Electrical Engineering Faculty of Electrical Engineering

Department of Electrical Engineering

University of Engineering and Technology Lahore

ii

a. From within the Country

i. Prof. Dr. Muhammad Akbar

Dean, Faculty of Electrical Engineering,

GIK Institute of Engineering Sciences and Technology,

Topi 23640, District Swabi, Khyber Pakhtunkhwa.

Email: [email protected]

ii. Dr. Salman Amin.

Associate Professor.

Department, Electrical Engineering (Taxila)


b. From Abroad

i. Prof. Dr. Koksal Erenturk

Prof. and Chair of the High Voltage and Power Division,

Department of Electrical and Electronics Engineering,

Ataturk University College of Engineering, Erzurum, Turkey.

Email: [email protected] , [email protected]

ii. Prof. Dr. Farhan Mehmood

System and Design and Simulation Engineer,

HVDC Division , ABB, Sweden


iii. Dr. Irfan Ahmed Khan

Assistant Prof. Department Electrical and Computer Engineering,

Texas A&M University, Texas, USA.


mailto:[email protected]





iii

Declaration

I Hafiz Shafqat Abbas Kharal, hereby declare that I have produced the work presented in this

thesis, during the scheduled period of study. Moreover, this work has not been submitted to

obtain another degree or professional qualification.

Singed: _________________

Date: _________________

iv

Acknowledgements

In the Name of Allah, most Beneficent, and the most Merciful; “As for those who say, ‘Our

Lord is ALLAH,’ and then remain steadfast, the angels descend on them, saying: ‘Fear ye not,

nor grieve; and rejoice in the Garden that you were promised. ‘We are your friends in this life

and in the Hereafter [al-Quran]. My beloved ALLAH gave me the strength, courage, wisdom,

vision and determination to complete my PhD thesis in the age of 30 years on Higher

Education Commission (HEC) scholarship (PIN No.117-3347-EG7-061) in top one

engineering university (UET Lahore) of Pakistan. He has always bestowed his countless

blessings on me in KHARAL family.

I would like to express my sincere gratitude to my advisor Prof Dr. Muhammad Kamran for

his guidance and continuous supervision of my study and research. The achievement of this

doctoral dissertation was only possible due to his guidance and supervision. Without his

encouragement, inspirational guidance and immense knowledge I could not have finished my

work. He always extended his unconditional support and clarified my doubts and concerns:

despite of his busy schedule he was always available whenever I needed him. His guidance

helped me in all the time of research as well as the writing of this thesis and it proved as a great

opportunity for me to learn from his research expertise. I could not have found a better mentor

than him.

Besides my supervisor I also would like to thank the rest of the Department’s faculty especially

Prof. Dr. Muhammad Asghar Saqib, Prof. Dr. Syed Abdul Rahman Kashif for his insightful

suggestions and motivation in graduate courses and invaluable assistance during the course of

this research project.

I would like to thank Prof. Dr. A. RASHID (LATE) of the Faculty of Electrical Engineering,

COMSATS University, for providing guidance in the practical implementation of the project.

The thesis would not have come to a successful completion without the help of many students,

colleagues and friends especially Dr. Muhammad Ali, Dr. YI LI, Dr. Eishrat, Engr. Farhan

Raees, Engr. Rehmat-Ullah, Engr. Junaid Alvi, Engr. Waqas, Engr. Faisal, Engr. Shoukat

v

Azeem, Engr. Mehran Tahir, Engr. Hafiz Ali Hassan and Muhammad Iqbal for sharing their

valuable ideas, stimulating discussions and assistance which they provided. They have always

offered help according to the best of their abilities whenever it was needed.

Lastly, and most importantly, I would like to thank my entire family especially my father and

late mother BEGUM MIRAJ AKBAR ROY for providing a vision and aim for higher studies

especially my elder brother Mr. Hafiz Nasir Abbas Kharal for financial supports. I owe a lot to

them as they always encouraged and helped me at every stage of both my personal and academic

life. Their blessings can never be paid off and there exists no substitute for love, prayers and

affection. In the end I would like to thank the Higher Education Commission of Pakistan for

providing funding for the implementation of this project.

Hafiz Shafqat Abbas Kharal

vi

Dedications

This work is dedicated to my beloved Prophet HAZRAT MUHAMMAD (PBUH)

vii

Abstract

The demand in the increase of efficient electrical energy has become a challenge, especially for

developing countries since their demand increase is observed with minimum planning. The

unplanned and unpredicted exponential rise in energy demand has increased the demand for

deployment of better power protection system which may withstand undue and unwanted system

failures. For this purpose, protection equipment should be installed with best efficient insulation

medium to overcome heating and quick faulty circuit isolation.

The current dissertation overviews the usage of sulfur hexafluoride (SF6) as an insulator in the

electric industry and critically compares the environmental issues associated with its nonstop use

as an insulator in high voltage (HV) appliances. Unfortunately, SF6has been identified as a very

harmful greenhouse gas by Kyoto Protocol which shows it as having 23900 times more global

warming potential (GWP) as compared to CO2 when compared over a time span of 25 years. It is

because SF6 has been included in the list of restricted gases regarding environmental protection.

Despite being nontoxic, it is heavier than air and can accumulate near the earth’s surface in case

of seepage at a facility thus replacing oxygen and causing suffocation to the workers at the

facility. These problems have gathered the attention of scientists and engineers around the world

to work on the improvement of the reliability and efficiency of the electrical power distribution

systems while taking into account that these updated technologies are safe for the environment.

This work presents the testing and development of a unique composite gaseous insulating

material using comparative evaluations among the properties of present insulating materials. The

most important objective or ambition of this thesis is to experimental investigate non-CFC

refrigerant R152a which belongs to the hydrocarbon family and has a good potential to act as an

insulating medium and to replace SF6 in electrical insulation systems because of its less GWP.In

this thesis theoretical as well as experimental performance of R152a have been discussed and

explicitly correlated saturated and superheated properties in comparison with existing insulating

materials.

R152a gas demonstrates good dielectric properties with low-temperature usage possibilities

under liquefaction conditions and environmental effects. The experimental study of power

frequency breakdown characteristics of R152a/CO2 has been analyzed under different pressure

viii

(0.2Mpa-0.6Mpa) and mixing ratios (50/50, 60/40, 70/30, 80/20, and 90/10%) conditions and

gap differences (6mm-18mm). GWP and PD characteristics have also been examined for the

proposed gas mixture’s insulation performance. The results indicate that the breakdown voltages

of R152a/CO2 gas mixture demonstrate a saturated trend in growth with varied gas pressure,

mixing ratios, and gap differences. The insulation performance of the gas mixture with 80/20%

R152a/CO2 can reach more than 96% of pure SF6.The development of this proposed composite

insulating material will result in superior insulating properties, reduced cost, and supplementary

ecological traits. Overall, this work will bring a potential cost-effective and environment-friendly

gaseous insulator for utility companies and power equipment manufacturers.

ix

Table of Content

Declaration……………………………………………………………...….............. Iii

Acknowledgements ……………………………………………………………....... viv

Dedications ………………………………………………….................................... vi

Abstract…………………………………………………………………………….. vi

Table of Contents ………………………………………………………………….. ix

List of Figures ……………………………………………………………………... xiii

List of Tables ………………………………………………………………………. xv

List of Abbreviations ………………………………………………………………. xvi

1 Chapter 1 Introduction …………………………………………………………... 1

1.0 Introduction ………………………………………………………………………... 1

1.1 Background ………………………………………………………………………... 2

1.2 Disadvantages of SF6……………………………….................................................. 4

1.2.1 Disadvantages of SF6 Equipment ……………………………………………...…... 4

1.3 Aim of this Work …………………………………………………………………... 6

1.3.1 Contributions to new research to achieve aim ………………………………….….. 6

1.4 Outline of the Thesis ………………………………………………………………. 6

2 Chapter 2 Literature Review …………………………………………………… 8

2.0 Theory and Literature Review of SF6 equipment ………………………………… 8

2.1 Vital SF6 Gas theory ……………………………………………………………… 8

2.2 Properties of SF6 Gas ……………………………………………………………... 8

2.3 Electrical Performance of SF6 Gas ……………………………………………….. 10

2.4 The Usage of SF6 in Circuit Breakers (CB)………………………………………. 12

2.5 Benefits of SF6 Switchgear in Comparison to Oil or Air …………………............. 13

2.6 SF6 circuit breakers advantages …………………………………………………... 14

2.7 The Usage of SF6 in Gas Insulated Substations (GIS)…………………………….. 14

2.8 Target Design of Bus bar for Future Studies……………………………………… 16

2.9 Conclusion…………………………………….........……………………………... 16

2.9.1 SF6 Properties and Study Model of Gas Insulated Equipment…….……………… 16

x

3 Chapter 3 Merits and Demerits of SF6 Verses Alternative Gases for Future

Aspects……………………………………………………………………….

17

3.0 Analysis on Merit and Demerits of SF6 Verses Alternative Gases for Future

Aspects Characterization of Isolated Compounds……….………………………….

17

3.1 Merits of SF6 Gas………………………………….………………………………. 17

3.1.1 Complications associated with the continuous usage of SF6……………………… 18

3.2 Ideal properties of alternate insulation medium…………………………………… 18

3.3 Gaseous replacement for SF6 as insulation medium………………………………. 19

3.3.1 Electronegative gases……………………………………………………………… 19

3.3.2 Charge transportation and conduction in gases…………………………………… 20

3.3.3 Townsend's Principle………………………………………………………...……. 20

3.3.4 Equation designed for growth Current…………………………………………….. 21

3.3.5 Townsend's criterion for breakdown………………………………………………. 22

3.4 Review of SF6 alternatives………………………………………………………… 22

3.4.1 Mixtures with SF6…………………………………………………………………. 22

3.4.2 Different Alternatives of SF6……………………………………………………… 23

3.5 R152aCharacteristics…………………………………………………………...…. 24

3.5.1 Physicalproperties of R152a………………………………………………………. 25

3.6 Environmental features of SF6, R152a and Gas mixtures ………………………… 27

3.6.1 Global Warming Potential (GWP)………………………………………………… 27

3.6.2 Atmospheric lifetime …………………………………………………………… 27

3.6.3 Ozone depletion potential ………………………………………………………… 28

3.6.4 A Comparison of the dielectric strengths of SF6 with Alternative gases………….. 28

3.7 Conclusion of Gaseous Replacement for SF6……………………………………... 28

4 Chapter 4 Leakage of SF6 Gas from Power Plants…………………………….. 30

4.0 Introduction ……………………………………………………………….……… 30

4.1 SF6 equipment review …………………………………………………………….. 30

4.2 Utilization of SF6 equipment in the distribution network…………………………. 32

4.3 Worldwide SF6 usage and its effect……………………………………………….. 33

4.4 Conclusion………………………………………………………………………… 37

xi

5 Chapter 5 Development Test, Laboratory and Investigational Methodology... 39

5.0 Introduction………………………………….…………………………………….. 39

5.1 Laboratory Setup and Assembly of Test Electrodes………………………….…… 39

5.2 Insulating Materials Breakdown Voltage……………………………………….… 41

5.3 Gases Breakdown Voltage………………………………………………………… 42

5.3.1 HVAC Test Arrangement for Breakdown Voltage……………………………….. 44

5.3.2 HVAC Arrangement for Breakdown Voltage…………………………………….. 45

5.4 HVDC Test………………………………………………………………………... 46

5.4.1 HVDC Arrangement for Breakdown Voltage…………………………………….. 46

5.5 List of Equipment’s used in experimental tests…………………………………… 47

5.5.1 Control Desk………………………………………………………….…………… 47

5.5.2 Pressure/ Vacuum Vessel (HV 9134) …………………………………………….. 48

5.5.3 Applications……………………………………………………………………….. 48

5.5.4 Test Transformer (HV 9105) ……………………………………………………... 49

5.5.5 Application………………………………………………………………………… 49

5.5.6 Peak Voltmeter (HV 9150) for Digital Display…………………………………… 49

5.5.7 Applications……………………………………………………………………….. 50

5.5.8 Discharge Rod (HV 9107) ………………………………………………………... 50

5.5.9 Application………………………………………………………………………… 50

5.5.10 Aluminum (HV 9108) Rod Connecting…………………………………………… 50

5.5.11 Application………………………………………………………………………… 51

5.5.12 Aluminum (HV 9109) Cup Connecting…………………………………………… 51

5.5.13 Applications……………………………………………………………………….. 51

6 Chapter Experimental Results of R152+ CO2 Mixtures: as a Potential

Alternative to SF6 6 ……………………………………………………………….

52

6.0 Introduction………………………………………………………………………... 52

6.1 Power Frequency Breakdown Voltage Experiments and Results…………………. 52

6.1.1 Experimental Procedure…………………………………………………………… 52

6.1.2 Gas Mixture Procedure……………………………………………………………. 52

6.2 Calculation of Accurate Gas Mixture Pressure……………………………………. 53

6.3 Mixture Ratio Analysis……………………………………………………………. 54

xii

6.3.1 Dielectric Strength Analysis…………………………………………………….… 55

6.4 Gap Difference Analysis…………………………………………………………... 55

6.5 Statistical Analysis of R152a……………………………………………………… 56

6.6 Global Warming Potential (GWP) Analysis………………………………...…….. 57

6.7 Synergistic effect………………………………………………………………….. 57

6.7.1 Synergistic effect of R152 /CO……………………………………………………. 58

6.8 Insulation Self-Recoverability test of Gas mixtures………………………………. 58

6.9 R152a/CO2 Liquefaction Temperature Analysis………………………………….. 59

7 Chapter 7 Conclusion and Future Directions………………………………….. 63

7.0 Conclusion………………………………………………………………………… 63

7.2 Future work………………………………………………………………………... 64

References ……………………………………………………………………….. 66

Appendix A…………………………………………………………………….…. 77

A1 Different alternative of SF6 gas. ………………………………………………….. 77

A2 Characteristics of References SF6 Verses R152a………………………………….. 77

A3 Association of SF6 replacement…………………………………………………… 78

Appendix B…………………………………………………………………….…. 79

Appendix C……………………………………………………………………...... 81

C1 Laboratory Test Setup and Assembly of Test Electrodes…………………………. 81

Appendix D……………………………………………………………………...... 83

D1 Calculation of Accurate Gas Mixture Pressure……………………………………. 83

D2 Gap Difference Analysis…………………………………………………………... 83

D3 Global Warming Potential (GWP) Analysis………………………………………. 84

D4 Synergistic effect………………………………………………………………….. 84

D5 R152a/CO2 Liquefaction Temperature Analysis………………………………….. 84

Appendix E …………………….……………….................................................... 85

E1 Research papers published on this project…………………….……………….. 85

xiii

List of Figures

Figure 1.1 The development of the global mean atmospheric content of SF6. The linear

fit shows an annual growth of 0.26ppt………………………………………..

2

Figure 2.1 Octahedral molecular geometry of SF6 molecule……………………………. 8

Figure 2.2 Comparisons of SF6, Air, vacuum and Oil for dielectric strength (DS)……... 10

Figure 2.3 Negative (-) lightning impulses, negative (-) switching impulses, N2:SF6 AC

breakdown voltages of these gaseous mixtures at 60 Hz……………………..

11

Figure 2.4 SF6 and air interruption capability…………………………………………… 11

Figure 2.5 Puffer along with rotating arc CB types in enhanced quality………………… 12

Figure 2.6 SF6 deionization time constants in contrast with other gases………………... 13

Figure 2.7 From left Oil, in middle air blast and in right SF6 circuit breaker…………… 14

Figure 2.8 Sectional view of (420 kV) GIS of a cove among double bus-bar system…… 15

Figure 3.1 Manufacture compliances with impact of SF6 regulation…………………..... 18

Figure 3.2 Elements with its electronegativity…………………………………………… 20

Figure 3.3 Mechanism for Townsend discharge……………………………………….… 21

Figure 3.4 SF6/N2 mixtures with different pressure…………………………………….. 23

Figure 3.5 Electronegativity of elements………………………………………………… 25

Figure 4.1 Insulation medium and its voltage ranges…………………………………….. 30

Figure 4.2 Medium voltage circuit breaker usage………………………………………... 31

Figure 4.3 Dielectric strength dependency on inter-electrode distance………………….. 31

Figure 4.4 Classical distribution network………………………………………………... 32

Figure 4.5 SF6 usage in distribution network………………………………………….… 33

Figure 4.6 Percentage wise use of GIL, GIS and CB’s…………………………………... 36

Figure 4.7 Estimated 25-year leakage of SF6 worldwide distribution equipment……….. 37

Figure 5.1 Experimental set up to examine R152a/CO2 breakdown voltage by sphere-

sphere electrodes……………………………………………………………...

40

Figure 5.2 Test equipment used: (a) Control and measurement unit(b) Testing vessel

(HV-9134) (c) Experimental setup for DC (d) Spark formation comparison…

40

Figure 5.3 Discharge (breakdown) development in a gas volume between two

xiv

electrodes by electron avalanche process…………………………………… 43

Figure 5.4 Schematic diagram for AC test…………………………………………….…. 45

Figure 5.5 AC Experimental Setup………………………………………………………. 45

Figure 5.6 HVDC test represents in schematic diagram…………………………………. 46

Figure 5.7 DC experimental setup………………………………………………………. 47

Figure 5.8 Vacuum/pressure vessel……………………………………………………… 48

Figure 5.9 Test transformer……………………………………………………………… 49

Figure 5.10 Peak voltmeter (PV)……………………………………………………….…. 50

Figure 5.11 Discharge rod………………………………………………………………… 50

Figure 5.12 Connecting rod………………………………………………………………. 50

Figure 5.13 Connecting aluminum cup…………………………………………………… 51

Figure 6.1 Power frequency breakdown voltage of R152a/CO2 gas at varying mixture

ratio and8 mm electrode gap distance……………………………….

54

Figure 6.2 Breakdown characteristic comparison of R152a/CO2 gas at 80%/20%

mixture ratio and SF6 at 8 mm electrode gap distance…………………….….

55

Figure 6.3 Breakdown voltages of R152a/CO2 gas varying the gap distance (4–16 mm)

at different mixture ratio………………………………………………………

56

Figure 6.4 GWP analysis of R152a/CO2 gas mixture…………………………………… 57

Figure 6.5 Insulation self-recoverability…………………………………………………. 59

Figure 6.6 Saturated vapor pressure R152a and SF6………………………………….…. 60

Figure 6.7 Liquefaction temperature of pure gases……………………………………… 60

Figure 6.8 Liquefaction temperatures of R152aat different pressure and mixture ratio…. 61

Figure 6.9 Simulation of (a) breakdown voltage, (b) electric filed under different

mixtures……………………………………………………………………….

62

Figure 6.10 Carbon fumes deposited on electrodes……………………………………….. 62

xv

List of Table

Table 2.1 Summing up the vital distinctiveness of (SF6) gas………………………….…. 9

Table 3.1 Comparison of SF6 with different alternatives………………………………… 24

Table 3.2 Different properties of R152a……………………………………………….…. 25

Table 3.3 Contrast between physical along with chemical properties of R152a vs SF6……. 26

Table 3.4 GWP of different gases………………………………………………………… 26

Table 3.5 Atmospheric lifetime of different gases………………………………………... 27

Table 4.1 Several devices and their function……………………………………………... 34

Table 4.2 Worldwide usage and leakage of SF6 from all RMU’s………………………... 35

Table 4.3 Worldwide SF6 usage and leakages from every CB’s……………………….… 35

Table 4.4 Worldwide usage and leakage of SF6 from all switches………………………. 35

Table 4.5 Total amount of SF6 use in all RMU’s, CB’s and switches…………………… 36

Table 4.6 Worldwide usage and leakage of SF6 from all GIS……………………………. 36

Table 4.7 Estimated 25-year leakage of SF6 worldwide distribution equipment………… 37

Table 5.1 Test setup specification………………………………………………………… 41

Table 5.2 Descriptions of control desk…………………………………………………… 47

Table 6.1 Experimental constraints…………………………………………………….…. 52

Table 6.2 Different mixture ratio of R152a and CO2………………………………….…. 53

Table 6.3 Statistical analysis of R152a…………………………………………………… 56

Table 6.4 Synergistic effect of R152a/CO2………………………………………………. 58

xvi

List of Abbreviations

AC Alternating Current

R-152a Difluoroethane

CO2 Carbon Di-oxide

SF6 Sulphur Hexafluoride

N2 Nitrogen

GIL Gas Insulated Line

GIS Gas Insulated Switchgear

GWP Global Warming Potential

IP Ion Pair

ODP Ozone Depletion Potential

PFC Perfluorocarbon

VFT Very Fast Transient Circuit breaker

LV Low Voltage (220 V to 1 kV)

MV Medium Voltage (1 kV to 52 kV)

HV High Voltage (52 kV to 300 kV)

EHV Extra High Voltage (300 kV to 800 kV)

GWP Global Warming Potential

ODP Ozone Depleting Potential

IR Infrared

PPTV Parts Per Trillion per Volume

EN Electronegativity

EA Electron Affinity

Α Ionization Coefficient

(𝐸/𝑝)𝑐𝑟𝑖𝑐 Critical reduced electric field strength

αeff(E) Effective Ionization Function

xvii

PD Partial DISCHARGE

PDIV Partial discharge inception voltage

BDV Breakdown voltage

BIL Basic impulse level

LI Lighting impulse

U50 Voltage at which there is a 50 percent probability of a break down

occurring

Up Rated lightning impulse withstand voltage

MPa Mega Pascal (unit of pressure)

T Townsend (unit)

Bar Gorbar bar gauge (unit of pressure)

GC-MS Gas chromatography and mass spectroscopy detector

NOP Normally open point

EHC Extra high current

BS British standard

MW Molecular Weight

I Current carried by conductor

Ich Chopping current

K0 Gas constant of the filled gas in GIS

K Particle length

L Inductance

M Mass of the particle

P Pressure

1

Chapter 01

Introduction

1.0 Introduction

Power system network comprises of generation, transmission and distribution. It uses energy

provided by water, coal, diesel etc. and converts it to electrical energy which is then transmitted by

the transmission network; finally, distribution network distributes the energy to domestic and

industrial customers. For power system to be efficient, losses should be minimized and equipment

with high insulation properties should be used to avoid any major mishap or failure of equipment.

Different insulation medium have been used over past three decades, for example, oil has been used

in oil bulk circuit breakers and it is still being used for cooling and insulation purposes in power and

auto-transformer, while air blast, air break and vacuum circuit breakers have also been used but due

to poor insulation properties and high maintenance cost the switchgear insulation medium is

preferred to be sulphur-hexaflouride (SF6) gas which is one of the most utilized insulation material

in switch gear throughout the world and power system grids are shifting from Air insulated sub-

stations (AIS) to gas insulated sub-stations (GIS). GIS system is now extensively used in

distribution transmission networks as an insulating medium in a large range of gas insulated

switchgear (GIS) for switching, earth switching, circuit breaking and general circuit protection. This

chapter will highlight the literature review, components of the conceptual model of the thesis and

summarize the outline that comprises an indication of the major achievements.

The worlds stipulate on electric power are growing. At the same time, conventional power

production by burning fossil fuels must be condensed to lessen the harmful effects on the

environment due to the emission of CO2 from burning coal and gas in power generation plants. In

addition, nuclear power has been phased out by force in some countries [1-2]. Energy demands are

being covered by renewable energy sources including wind and solar power. For example, in

Europe, the coasts of Atlantic and the North Sea have got lots of wind turbines to generate

electricity. Solar power is being utilized at its best in southern countries or Sahara-desert [3]. This

also needs a significant upgrade of the power grid to bring the generated electricity to remote load

centers which are mostly far from the production sites. So, with the rapid increase in demand of

electricity the system also required to be shifted to UHV system which in return requires greater

insulation of higher rated equipment so save the equipment from over-voltages surges. Keeping in

the view of current insulation requirement for grid station equipment Sulfur hexafluoride (SF6) is a

potential insulator and arc-quenching medium, which is being extensively used in electric power

2

system devices due to eminent properties like high dielectric strength (DS), physical stable and

electrically superior from others [3-5]. However, environmental apprehension has been raised

subsequent to using gas in huge quantities in various high voltage (HV) sub-stations as well as

industries. Global warming is affected by this apprehension. Consequently, SF6 is very effective

infrared absorber and greenhouse gas. The linear fit shows an annual growth of 0.26ppt as shown in

Fig. 1.1 [6].

1.1 Background

Moissan and Lebeau were responsible to synthesize and portray in (1900), alsosulphur hexafluoride

(SF6) for the first time described; the gas applications in large number were quite sophisticated

around 1940, further more in 1947, it developed available in commercial form [7]. This issue of the

usage of SF6 in many applications or electrical industries have been summarized and reviewed by

Brunt, Christophoro and Olthoff, with a conclusion of its extensive use in commercial and industrial

research [8]. Apart from the usage of SF6 by the electrical and electronic industries, it is also used in

the processing of semiconductors, refining of magnesium, as insulating gas for thermal and sonic

applications, airplane tires, air-sole shoes, leaks checking, etc.

Figure 1.1.The development of the global mean atmospheric content of SF6. The linear fit shows an annual

growth of 0.26 ppt [6].

In the eighth decade of twentieth century, medium-voltage circuit breakers mainly relied on oil or

air as their major insulating mediums [9]. With the passage of time, air and oil were replaced by SF6

and vacuum as insulating media [10]. These changes allowed the use of a higher range of voltage in

a smaller space with the provision of a relatively maintenance and hazard free system of SF6as

compared to the usage of oil [11-13]. The other main reason for the large-scale development of SF6

and vacuum was the design of many smaller-sized substations than the ones made by air or oil as

3

insulating media [14]. This is due to the requirement of a very small gap between the electrodes to

break a circuit while using SF6 or vacuum as compared to any other technology. This has an overall

effect of designing very small sized equipment for electric industry which in turn makes the

industry cost-effective also. The old vacuum technology as interrupting dielectric did quite well up

to high voltages of 132kV[15-16] but now this technology has been seized due to a better

alternative, i.e. SF6 which proved effective in its use in equipment up to 800 kV [16]. This has

resulted in SF6 being an exclusive candidate for high voltage (52-300 kV) and extra high voltage

(300-800 kV) networks without a suitable alternate as an insulating material with similar insulation

and interruption performance [17]. The following properties of SF6 can better describe the reasons

of its popularity and wide use as a dielectric media.

Chemical inertness [18]

Low boiling point [14]

Thermal stability up to 500˚C temperatures [3,15]

Strong electro negativity and excellent arc extinction properties [3,16]

Non-toxicity [3]

Non-flammability [19]

Three times stronger than air in terms of breakdown voltage [18]

Very quick arc extinction recovery time [3]

Kindness to stratosphere and almost zero percent harm to ozone in that region [20]

The above-mentioned characteristics of SF6 are responsible for the development of equipment

having the following properties:

Smaller sizes because of little gaps among electrodes because of the high breakdown voltage

of SF6 [20]

Small-sized substations because of smaller switchgear dimensions [21]

A high-level insulation as well as circuit breaking performance [22]

A proven safety demonstration [23]

Comparatively low insulating medium cost because of large scale production and increasing

demand [24]

Almost maintenance free SF6vessels

4

All the above advantages render it the best candidate for the power distribution network.

1.2 Disadvantages of SF6

The hazards to the environment associated with the continuous use of SF6 in insulation industry are:

The indoor electrical transformers which are fire-resistant and have SF6 as insulation

medium are used to protect the indoor circuits from fire [25]. The major problem with its

wide use is that it has a high global warming potential (GWP) which has been responsible

for its declaration as a restricted greenhouse-gas by Kyoto protocol [3,25]

SF6 is heavier in comparison to normal air which gathers it at the lower regions of the

substation at the points of cable trenches, thus replacing the air in these regions. It is a

potential hazard for the people working in these places due to oxygen deficiency

SF6 can decompose at an elevated temperature (above about 500˚C) [26]. On occurrence of

arcing while a circuit breaker switching, the decomposition products can cause skin-damage

or eye irritation

Due to SF6 being a good absorber of infrared radiation [8], its GWP is about 23000 times

greater than that of CO2 [27].

The worst thing is that it stays very long in earth’s atmosphere due to being inert and

cannot be removed readily [28].

1.2.1 Disadvantages of SF6equipment

The installation cost is a disadvantage because of the high cost of GIS installation compared

to AIS [29]

Due to high stability and inertness of SF6, it is usually maintenance free but if maintenance

is required, it requires skilled personnel which are hard to find

Due to being present at the critical nodes on the power grid, GIS installations can cost a lot

of revenue in case of failure as their faults are difficult and expensive to trace out and repair.

So, they may cause longer power outages in these circumstances. Customers usually don’t

like longer power outages which can be a concern while using SF6 in this equipment

Various potential deficiencies are found in SF6 containing systems which results in

progressive corrosion of insulation quality and may perhaps cause to failure. The issue is

5

that it is almost impossible to rectify the problem with old methods due to the closed nature

of the system

Till date, there are not many proposed solutions to replace SF6 as a foremost insulator in extra high

voltage (EHV), high voltage (HV) moreover medium voltage (MV) networks. There is a suggestion

to use vacuum technology but there is no evidence in literature that could allow full replacement of

SF6. So far, the best solution to replace SF6 is to use it mixed with other gas which reduces SF6

quantitatively but its demerit is that it reduces the efficiency of insulating the equipment by

quenching the arc. There are different combinations of SF6 reported including SF6+CF4, SF6+Air,

SF6+N2 and also SF6+CO2 gases mixtures, out of which SF6+N2 are the best among these

combinations and it has been adopted in GIS and GIL manufacture [3,30,31]. In this case, the

performance is not altered a lot due to N2 being an inert gas and it allows the usage of SF6 on lower

temperatures as well [31]. The search for SF6 alternatives has already been in progress for decades.

Current research is aimed at finding an alternative which is not necessarily better than SF6 but

having similar insulation capacity and less harmful to the environment. The main research objective

and aim of current thesis is the search of an alternative for the use of SF6 in distribution network

that may effectively be used as a replacement of SF6 in near future. The suggested alternative is not

expected to have same effectiveness as SF6 but should have less environmental issues still keeping

in mind the cost effectiveness.

A good SF6 gas alternative should possess the following physical, chemical, and environmental properties.

It should have very high electric strength

It must quench the arc very well and after that recover fast to be ready for next quench

It should have low boiling point

It should be compatible with existing switchgear materials

Its handling must be easy. Its toxicity should be very low with no dangerous decomposition

products

Its global warming and ozone depletion potentials must be very low

It should be least damaging to the environment

6

1.3 Aim of the Work

The aim of this study includes existing literature review of different combinations of gases which

has been used as insulation medium of high voltage switch gear and to compare their performance

with SF6 gas in terms of insulation quality and to evaluate on which scale they damage the ozone

layer, the study also carries out theoretical assessment of various SF6 elimination options and

experimental testing. This study also discusses other viable substitutes available to SF6 gas and

while also justifying why other options are referred. Finally, this thesis will review the performance

of R152a/CO2 gas mixture and draw comparative analysis with SF6 gas.

1.3.1 Contributions to achieve aim

Following are the steps carried out to achieve the above stated goals:

Theoretical evaluation of various SF6 gas elimination options along with various

measurements and simulations has been carried out

Traditionally new R-152a insulation gases along with its variants are evaluated by

comprehensive analysis of the physical and chemical properties of R-152a and R-

152+CO2 gas mixtures with classical breakdown experiments

Experimental investigation of this R-152a gas along with its applications as insulation

medium on medium voltage switches has been carried out

Comparison of the insulating properties of R-152a gas and its variants with SF6 has been

carried out through experimentation

The predicted breakdown voltages for the novel gas R-152a and a set of other known

gases have been validated.

The probability of over voltages when vacuum or different gases installed is studied by

statistical methods

Proposal about using a vacuum switch gear to employ this R-152+CO2 novel gas mixture as

replacement insulation to SF6 gas.

1.4 Outline of the Thesis

1.4.1 Chapter 2

Chapter 2 elaborates the characteristics of SF6 gas along with literature review, properties of SF6 in

details. A brief overview of current published papers regarding different insulation medium has

been discussed.

7

1.4.2 Chapter 3

Chapter 3 includes the characterization of the widely used SF6 gas in insulation industry and its

environmental threat with regard to Pakistan and the globe. The discussion also includes those

important aspects and reasons which limit the use of SF6. It also enlists the alternatives of SF6 gas

that have been previously suggested. The other main point of this chapter is to present a clear and

concrete picture of the published literature about the characteristics of R152a gas and its other

different mixture variants which play some sort of role in this industry.

1.4.3 Chapter 4

In Chapter-4 includes a review of the SF6 usage in power plants and gas leakage. It includesits

effects on the environment as well.

1.4.4 Chapter 5

Chapter 5 encompasses the discussion on the equipment used in the laboratory and the procedures

and techniques employed throughout the present study. Preliminary tests used to establish the

decisive testing on the apparatus and the gas mixtures.

1.4.5 Chapter 6

This chapter presents experimental and simulated results which enlist the comparative study on SF6

and R-152a+CO2 gas mixture. It also includes the results of the simulations performed using

different software. Finally, the development of a proposed composite insulating material with

superior insulating properties reduced cost, and supplementary ecological traits.

1.4.6 Chapter 7

Chapter 7 is the conclusive chapter of this thesis. It concludes the research carried out for this thesis

and briefly enlists the possible future studies.

8

Chapter 02

Literature Review

2.0 Theory and Literature Review of SF6 Equipment

Gas insulation while making power equipment, especially for designing HV equipment utilizes SF6

widely. This is because of the better electrical performance along with the stability of SF6. This

section consequently analyzes the properties of SF6& its evolution in electronics as well as

electrical power industries.

2.1 Vital SF6 Gas Theory

Moissan and Lebeau were the founders of the SF6 synthesis in the year 1900 [32]. Four decades

after its first synthesis, SF6 was reported as having insulation properties and a good candidate for

the insulation equipment compared to fluorocarbons and the oil used in the transformers [33-35].

SF6 was used as a quenching medium for the first time in 1950 in circuit breakers [36]. After three

decades, there was interest among scientists to find out its breakdown and partial discharge

mechanisms [36-37].

2.2 Properties of SF6 Gas

SF6 molecule comprises of six fluorines attached to the sulphur atom. In these situations, where F

atoms lie in an octahedral manner (Fig.2.1) i.e. four in a square plane around the central atom and

two above and below. Each and every one has its bond angles either 90 degree or 180 degrees [38].

Figure 2.1. Octahedral molecular geometry of SF6 molecule [38]

Due to its strong electronegative behavior, SF6 can attract free electrons generated during the arcing

of electric current. As a result, it gets converted to anions which are heavier than the free electrons.

Thus, greater voltage is required to breakdown these fewer mobile anions and breakdown the gas.

9

The following section [40] gives a portrayal of the compensation of any gas which one is

electronegative.

“SF6 is an electronegative halogen gas having good dielectric properties. Particles of an

electronegative gas have an affinity to attach themselves to free electrons producing less mobile and

heavy negative ions. The contribution of the latter to the ionization process creating electron

avalanches is much less as compared to an electron. Hence, the electric field stress required to cause

breakdown of an electronegative gas is high”.

SF6 forms as an exothermic reaction between the molecular sulphur and fluorine as described in

equation 2.1. Due to its high stability, it is very difficult to breakdownSF6 into its components, even

at higher temperatures. That is why it can easily be used in electrical equipment mainly as a

insulating materials at temperatures of up to 200◦C [41].

S2 + 6F2 → 2SF6 + 524 kcal (2.1)

SF6 has excellent heat transfer properties, as well as the additional vital features of these potential

gases as specified in (Table 2.1).

Table 2.1. Summing up the vital distinctiveness of (SF6)gas [37][41][45]

(SF6-Properties) (Statistics)

0.1Mpa Relative -Density 6.256 kg -per-m3

0.1MPa at (0°C) Thermal Conductivity 0.0212-Wm-1K-1

0.1Mpa Boiling point −64°Cor (209 K)

Water Solubility Soluble inSlightly

Liquefied Pressure (21°C) 2.1-MPa

GWP (Global Warming Potential) 24,000

SF6 gas Toxicity None

SF6 with extra concentrations will lead to suffocation

At high pressure and room temperature SF6will be able to store. Its boiling point (−64◦C at 0.1Mpa)

is relatively low. In general, SF6 can be used in insulated equipment with (0.30-0.60Mpa) pressures.

SF6 is three times heavier than air, as a result it stays on ground level and even though its behavior

is non-toxic, a high density SF6 with extra concentrations will lead to suffocation [3]. During

electrical discharge, its products that are generated such as S2F10 and SOF2 are highly toxic and

10

corrosive compounds. Till date there are no issues reported with SF6 handling, availability and

reliable availability. From sixth decade of nineteenth century till the ninth decade, its price was

reasonably constant, but now that price has increased about ten times (about $30/lb.) [42]. All the

pros discussed in the previous paragraphs render SF6 as an excellent arc insulator. Therefore,

substations are equipped with SF6 gas. Transformers use SF6 as insulating or cooling medium and

switchgears use it for quenching the arc in HV & MV applications.

2.3 Electrical Performance of SF6

SF6 has a dielectric withstand property which is two times superior to air at normal pressure (Fig.

2.2). Upon its use at higher pressures (3-5 atm), the dielectric performance becomes ten times

superior to air. These types of potential gases capture free moving electrons, since negative (-) ions

consequently strike the formation in form of electrical discharges. The admirable dielectric strength

(DS) of SF6 is major reason for its usage in (GIS) gas insulated substations. Pedersen and Cookson

[42-43] have given information about characteristics of SF6 gas with different mixing ratios. The

measurement was applied to the coaxial cylinder at (89 mm/226 mm) electrode geometries with

negative (-) switching impulse, lightning impulse and discontinuous voltage. Fig. 2.3 refers to the

breakdown voltage withstand ability of SF6+N2 gases mixture at pressure about 0.45 MPa of these

gases. The lack of strength is due to surface roughness, dissimilar conductor materials & spacer

structures design in practical manufactures work.

Fig.2.2. Comparisons of SF6, Air, vacuum and Oil for Dielectric strength (DS) [44].

Several gases mixtures with a reduced volume of SF6, such as 80%/20%SF6: N2 present weaker

dielectric strength (DS) rather than a pure SF6 [45]. For example, in case of switching, these new

11

solutions of mixed gases have approximately 5% lower dielectric strength (DS). Moreover, the

dielectric strength (DS) in AC is condensed &is not greater than 3.5%, in contrast of unpolluted

SF6.

Figure 2.3. Negative (-) lightning impulses, negative (-) switching impulses, N2:SF6 AC breakdown voltages

of these gaseous mixtures at 60Hz [45-46].

SF6 has capability to interrupt current (Fig. 2.4). SF6 efficiently controls several circuit breakers

because ithas electronegative features & properties as discussed previously, due to its superb

cooling potential at very high temperatures (1200−4500◦C) where the extinguishing of arcs occurs.

The phenomenon is that the gas takes up the energy of the arc to dissociate, thus providing the

cooling effect. The graph below shows the results of investigations from 1953 and explains that SF6

has better interruption capability than the air [42]-[46].

Figure 2.4. SF6 and air Interruption capability [46].

12

2.4 The Usage of SF6 in Circuit Breakers (CB)

The available SF6 circuit breakers are of two types, the first one called as the “puffer type” and the

second one as “rotating arc type”. The first one resembles to the old air blast type system while in

the second, the arc moves with the help of magnetic fields. Here, only the “puffer type” has been

discussed as the other one is only used in distribution voltages shown in (fig. 2.5) [47].

A puffer type circuit breaker is characterized by the SF6 flow over the arc coming out of the circuit

breaker contacts. The heating effect of the arc is responsible for the movement of the gas. Large

currents produce enough heat to start the flow of the gas while for smaller currents; the heating

effect is insufficient so it is desirable to utilize a pre-compressed quantity of SF6.When a circuit

breaker is opened, its main parts separate followed by the arc contacts. Thus, by the volume getting

decreased, the pressure of SF6 is increased.

Figure 2.5. Puffer along with rotating arc CB types in enhanced quality [47]

This keeps happening till the contacts continue to separate apart and eventually an arc is drawn

which force the gases to move axially with the arc, which eventually causes the extinction of the

arc. For the larger currents, the breaker opening is slowed because of the gas pressure inside the

piston. Contrary to that, for low current values, the arc diameter is minute and the flow of gas

happens automatically without being stopped.SF6 Circuit breakers, due to its enormously stumpy

deionization time constant, in contrast with air is shown in (Fig. 2.6).

13

It has critical parameter like time constant which partially depicts circuit interrupting abilities with

efficient circuit breakers (CB). This SF6 gas time constant is enormously dumpy due to its

electronegative properties and nature: as a result, it is capable to withstand extinction of the arc.

Figure 2.6. SF6deionization time constants in contrast with other gases [47].

Although SF6 gas is quite stable, but in interrupting the arcs and electric discharges, it might

decompose slightly, giving rise to decomposition products. Usually absorbers are used to keep the

amounts of the gaseous products low because of their poisonous nature. That is why specialized

personnel are required to open the damaged SF6 filled appliances for maintenance. The

decomposition products may be metallic fluorides which can decompose quickly after opening and

not harmful to the environment.

2.5 Benefits of SF6 Switchgear in Comparison to Oil or Air

The use of oil for insulation purpose is preferable over SF6 and air, but there are certain limitations

to the use of oil. If the oil gets very little number of contaminants like water or carbon, its

performance dies briskly. As per manufacturer manual “fresh oil can normally be expected to have

a dielectric strength of around 70 kV when it is placed between two 20 mm spheres, 2.5 mm apart”

[49-50]. The dielectric strength of the used oil is generally over 15 kV. During the arcing process,

the hydrogen gas around the arc plays an important role, thus making the oil a very good insulating

material.

Air circuit breakers are worn out and are functional at 12 kV. Thus, they have the disadvantage that

they cannot be used for extra high voltages. Oil circuit breakers can be used up to 72 kV. As the

moisture in oil has a detrimental effect on dielectric strength [51] of circuit breaker, thus they

require repetitive maintenance. Also, oil circuit breakers face fire hazards problems. Air Blast and

Magnetic Air circuit-breakers are massive and inconvenient. Vacuum circuit breakers (VCBs) are

environment friendly and are engaged in medium voltage levels (5-38 kV) [52]. Their construction

14

is simple and lesser number of components is used. They are maintenance free. Substation’s size is

reduced when VCBs are used instead of oil and air circuit breakers. Use of VCBs is limited due to

their high cost, non-uniformity of dielectric strength. VCBs cannot be used for extra high voltage

(EHV) and ultrahigh voltage systems (UHV) [53].In (fig.2.7) from Oil, Air Blast and SF6 circuit

breakers are presented. For medium voltage (MV) range, VCBs are used, but for high voltage

systems (HV) sulfur hexafluoride (SF6) is used. From the early 1960s, SF6 is used. SF6 circuit

breakers are mostly used in EHV and UHV applications because of their numerous benefits such as

superior performance, prolonged contact life and simple construction.

Fig 2.7. From left Oil, in middle Air Blast and in right SF6 circuitbreaker [50].

2.6 SF6 circuit breakers advantages

Good properties of SF6 result in various applications with added benefits:

High breakdown voltage of SF6results in smaller electrode clearance [3,52]

Smaller switchgear dimension’s result in smaller substation sizes [8]

Higher insulation and circuit breaking [54]

Non-toxic (non-flammable), also non-explosive environment results in good operations.

SF6 results in reduced equipment cost [14]

Reduced maintenance is required for SF6 based systems [55]

2.7 The UsageofSF6in Gas Insulated Substations (GIS)

Typically, SF6 is used primarily as an insulator for devices used within gas insulated substations. A

420kV pattern of Gas Insulated Substations configuration clearly revealed in (Fig. 2.8).

15

Fig 2.8. Sectional view of (420kV) GIS of a cove among double bus-bar system [56].

The Design or construction of bus-bar system either three phase orsingle-phase

enclosures; it has following parameters (a) conductors (b) supporting spacers (c)

cylindrical enclosures also. In bus-bar dimensions are cautiously chosen according to the

required electrical impulse as well as the power that the switching impulse faces. In

addition, numerous other vital factors must be well thought-out to make its dimensions

reasonable. The list is given blow:

Evaluation of the electrical stresses and its effects belonging to many types

Dimensioning of GIS systems as well as components

Consideration for Insulation interfaces (solid plus gaseous plus liquid insulation

congregate)

Thermal details

Every power appliance bears continuous stress due to the operating voltage and from time to time

essentially endures certain level of voltage in excess of specified value in which system voltages are

operating, such as (a) lightning impulse voltage [BIL] or (b) switching impulse voltage. Lightning

impulse voltage occurs as a result of a usual and natural phenomenon, the stander of wave shape

(1.2/50) of electric lightening impulse [57–58] with a period of front / tail. Circuit breakers

connecting and disconnecting produced the switching impulse voltages itself from the system. A

good quality design substation must withstand every type of voltages tress. It will make sure the

failure of dielectric never takes place. More and more protective devices such as surge arresters and

switches are frequently installed in substations to reduce the over-voltage levels.

16

The dimensions of the intact bus bar assemblies are essentially dig out with efficient insulating

medium properties. This one totally delays one operation voltage and further conditions likes’ its

surface circumstance of any conductor if it is rough, can root locally eminent electrical fields.

In short, substation construction along with design depends on many factors. To develop a steadfast

insulation system or suitable dimension for any substations, imperative factors must be kept in

mind, particularly thermal stability. Protective devices can be used to manage voltage magnitude in

combination with an insulation system. In Chapter 3, the aspects of thermal plus insulation design

are briefly explained with an assessment of dissimilar types of numerous dielectric materials.

2.8 Target Design of Bus-bar for Future Studies

A base plan of the gas insulated bus-bar is derived from the National Grid for evaluation, in order to

investigate the potential of substitute forms of insulation used in later chapters of this thesis. This

design plan is developed while taking (400 kV) bus-bar into account with a (1425 kV) BIL

(although in some cases considered a lower BIL of 1050 kV). The radius of innermost conductor

along with outer sheath is 62.5 mm and 250 mm, respectively [59]. The gases pressure in bus-bar is

almost 0.3MPa.

2.9 Conclusion

2.9.1 SF6 properties and study model of gas insulated equipment

In this thesis, this chapter delineates the key properties of SF6 gas. SF6 is being implemented in

electrical networks worldwide as an insulating gas in switching and general circuit protection.

Generally, this gas has insulated equipment like GIS, GIL, and CB which are being installed in the

distribution network.

SF6 is the leading gaseous arc-quenching medium. The development of circuit breakers using SF6

clearly improved routines in present time and overcome the cost of oil filled and air-blast circuit

breakers, resulting in dielectric properties better than oil as well as air.

Furthermore, this chapter introduced equipment or devices which use SF6, especially for high

voltage (HV) substations like: HV and MV applications, transformers and switchgears.

17

Chapter 03

Merits and Demerits of SF6 Verses Alternative Gases

for Future Aspects

3.0 Analysis on Merit and Demerits of SF6 Verses Alternative Gases for

Future Aspects Characterization of Isolated Compounds

This chapter gives an in-depth analysis of the findings regarding the characterization of various

insulation gases and their significance in industry usage, while also discussing the merits and

demerits of their usage in high voltage equipment. In this chapter main focus is to analyze the

characterization of the widely used SF6 in insulation industry and its effects on global environment.

The study also highlights alternatives of SF6 gas along with their significance and limitations. The

study discusses R152a (gas) which is tipped to be the best alternative for SF6, so a detail literature

review of characteristics of R152a is discussed along with its variants while making an in-depth

performance comparison of SF6 gas with its alternatives and discusses why SF6 gas is superior to

alternatives in terms of insulation and performance.

3.1 Merits of SF6 Gas

This has been discussed in detail in previous chapter and is an ascertained fact that SF6 bears a high

dielectric strength while also having high electron affinity which boost itsability to quench an arc

during current interruption. These properties justify its use in current application in gas insulated

switchgear (GIS) [10, 60]. SF6 is a highly electronegative gas and its breakdown voltage is more

than three (3) times of air at normal pressure [20, 61]. In literature there have been different reports

on various mixtures of SF6 with other gases that have been tried so far for insulation applications.

These tried gases include different rare gases, hydro fluorocarbon gases, and SF6-N2 gas mixture.

SF6-N2 has shown good applicability for various HV applications including GIL [20, 58]. Hydro

fluorocarbons have got a superior dielectric strength over SF6 but their prolonged life in the

atmosphere renders them unsuitable for applications in insulation industry. For example, C − C4F8

is a worst choice for insulation because of its GWP of 12,200 with a dwelling moment in time of

3200 years in atmosphere [62].

18

3.1.1 Complications associated with the continuous usage of SF6

SF6 is first-rate absorber gas of infrared radiations (IR) and being a very stable and inert gas, it does

not leave earth’s atmosphere. This makes it one of the most dominant greenhouse gases these days

[62]. At the same time, it is also gentle to the ozone depletion because of its chemical inertness. The

gases or pollutants produced by the human beings in earth’s atmosphere are called anthropogenic.

The measurements of the greenhouse gases in the atmosphere are done in the units of parts per

trillion volume (pptv). SF6 was 0.03 pptv in 1970, which increased more than two folds in 1992

(2.8 pptv) with a predicted value of 65 pptv in 2100 if its consumption continues as it is today

[2,63].To date there is no active substituent for SF6 gas but as a probable solution of this problem, a

mixture of SF6-N2 has been proposed [61]. SF6 can sometimes produce very toxic chemicals on

coming in contact with an electrical discharge (e.g. S2F10 and SOF2).

Although SF6 usage has strict regulations in place and it does not get released in the air during the

electrical equipment life-cycle it is used in, still its yearly leakage rate of 0.1% has been reported

[1]. Because of the strict regulations under Kyoto agreement [64-65] about SF6 gas handling, its

removal and disposal are done separately by certified people thus increasing the cost considerably

[66]. According to Kyoto agreement every country must consider the release of SF6 gas in the

atmosphere as a severe problem. On the other side, the fluorine produced during the electrical

discharge in the switchgear equipment must also be accounted for.

Figure 3.1. Manufacture Compliances with impact of SF6 Regulation [64]

3.2 Ideal Properties of Alternate Insulation Medium

A new alternative insulation medium should have the following properties.

It should have low liquefaction temperature i.e. remains gas at low temperature

It should be thermally stable below 500˚C

19

It must have good arc quenching properties

It should not be flammable

It should not be explosive

Its breaking down strength should be of same level to SF6

Its GWP must be low

It should not be a good infrared absorber

Its Stratospheric measurement should be zero

It should possess low atmospheric lifetime i.e. decompose in the atmosphere easily

It can be used at high pressure

It must have no environmental impact

It must be a nontoxic gas

3.3 Gaseous Replacement for SF6 as Insulation Medium

3.3.1 Electronegative gases

The bond formed between two atoms as a result of sharing of electrons is called a covalent bond.

The shared pair of electrons in a covalent bond can be analogous to a tug − of − war in-between the

sharing atoms [67]. If one of these two atoms have different pulling force than the other, then the

electron pairs will remain closer to the atom which has greater pull on it. This will render some

ionic character to this covalent bond and make it polarized. These pulling forces of an atom towards

a shared pair of electrons are known as its electronegativity (EN) [68]. Conceptually, the

electronegativity can be understood in relation to the electron affinity and ionization potential of the

element.

The Ionization potential (IP) can be defined as “the amount of energy required to eradicate an

electron as of its outermost shell of an atom in its isolated gaseous state”, thus a high value of IP

would render the donation of electrons difficult. The electron affinity (EA) describe as the amount

of energy released as an electron added to the last shell of an isolated gaseous atom. So, the higher

value of EA will mean the atom has more affinity to gain the electrons than losing them. So, if both

IP and EA are greater, the atom or element is said to be highly electronegative and vice versa [69].

Figure 3.2 depicts a portion of the periodic table showing the main group elements. As the elements

residing at the peak right corner of given periodic table have highest values of IP and EA, it makes

them highly electronegative as well. Fluorine (F) is considered to be the most electronegative

element in the periodic table with an electronegativity value of 4 on the scale given by Linus

Pauling. So, the presence of the electronegative elements, fluorine in our case, makes them

20

electronegative and polar as they pull strongly on the shared pair of electrons with their counter

atom.

Figure 3.2. Elements with its electronegativity [69].

3.3.2 Charge transportation and conduction in gases

Charge transportation and conduction in gases depend on different factors such as temperature,

pressure, electric field energy and the voltage across the electrodes of a container in which the gas

was placed.Each gas in row is normally a perfect and excellent insulator. For conduction in gases,

the following two conditions should be met.

Firstly, the gas in neutral form should produce charges otherwise accepted by its

external source applications

The external electric field should apply

Under the impact of the electric field, the charge carrier in the gas are positive or negative ions

which move freely. Conduction in gases is different from solid and liquid because in gases these

ions play vital role in the conduction of gas. Intended for any gas, there is a voltage level called

ionisation at which ionisationoccurs. The ionisation of gas occurs when gas molecule gains

sufficient energy and lose electrons and, in that ways, electron avalanche can occur leading to

partial breakdown, some condition it changes to complete breakdown.

3.3.3 Townsend's principle

In 1900 J.S Townsend produced his theory of breakdown and conduction in gases. Townsend

placed a gas in between two metal electrodes in a closed container vessel in his early experiment.

Variable DC voltage is supplied to the electrodes. Voltages gradually increase from zero to a

predetermined level. The electrode connected terminal with negative(-) side is denoted by cathode

21

as well as the terminal connected with positive (+) is represent by anode as revealed in figure 3.3.

Townsend found that the flowing of an electron depends upon the applied external electric field

[70].

3.3.4 Equation designed for growth current

Figure 3.3. Mechanism for Townsend discharge [70].

In figure 3.3 the distance between the electrodes is d, and the initial number of electron present in

the gap is No. voltage U is applied to the electrodes. The initial electron No gain sufficient energy

and move toward the anode. At length dx, the electron got a collission with the atom and form

ionisation and positive ions. Charge produced at a distance dx will be dn. According to Townsend

𝐴𝑡 𝑥 = 0, 𝑁 = 𝑁𝑜

𝑁0 =numbers of electron emitted from cathode

𝑑𝑁 ∝ 𝑁𝑋𝑑𝑥

𝑑𝑁 = 𝑎𝑁𝑋𝑑𝑥

Equation 3.2 shows ionisation coefficient, which defined as the number of electrons produced by

ionisation.

𝑑𝑁

𝑁= 𝑎 𝑑𝑥

Integrates both side

∫𝑑𝑁

𝑁= 𝑎 ∫ 𝑑𝑥

𝑑

𝑜

𝑁

𝑁𝑜

This results in,

ln (𝑁

𝑁𝑂) = 𝑎𝑑

Or

22

𝑁 = 𝑁𝑜exp (𝑎𝑑)

𝐼 = 𝐼𝑜exp (𝑎𝑑)

𝐼𝑜represents initial starting current.

3.3.5 Townsend's criterion for breakdown

1 − 𝛾[𝑒𝑥𝑝(𝑎𝑑) − 1] = 0

Ifd<ds, I=I0 if the supply is removed the I=0. If d is equal to ds I approach to infinity.

This called by Townsend’s criteria for breakdown and shown as:

𝛾[𝑒𝑥𝑝(𝑎𝑑) − 1] = 1

Typically(ad) is huge, and so the above equation turns into

𝛾[𝑒𝑥𝑝(𝑎𝑑)] = 1

In a certain partition of the gap and at some pressure(P) the value of the voltage(V) which results in

∝ ,γ fulfilling the criteria of breakdown is known as the corresponding distance dsand spark

breakdown over voltage’s Vs are called by sparking distance.

3.4 Review of SF6 Alternatives

The exploration and progress to find much better replacement of SF6has been started research since

(1970s) that publicized that the mixed gases dielectric strength (DS) is better than pure SF6 gas [5-

10]. The inclination of SF6 emanation from electric equipment’s started from the time of (1990s). In

the beginning, China was started the emanation of SF6 gas approximately 75% from its electric

equipment sectors [71].

3.4.1 Mixtures with SF6

Compared to the attachment process to prevent insulation from breaking down, in ionization

process the best dielectric control below their excitation efficient energy incorporates at lower

levels of electrons. In this regard, buffer gases such as nitrogen N2 oxygen O2, and carbon dioxide

CO2 are preferentially employed, because in all energy levels the additive gases cannot attach

electrons [72-3]. For example, above energies at 2 electron volts (eV), the attachment process of

these electronegative gases is more complicated. Consequently, the constituent gases are efficiently

mixed with extract energy from attaining electrons that wants to release beginning the lower energy

attachment region. As a result, the additive gases conception was used as an insulation purpose. A

research on the mixtures of SF6 such as (a) SF6+N2(b) SF6+CO2(c) SF6+Air is conducted in [46-52]

23

at various ratios to minimise the use of SF6. In [72] study of the particle was done in SF6/N2 but it

shows that the detection of the particle was more difficult in N2 from SF6. Figure 3.4 shows the

breakdown strength of SF6/N2 for different pressure and 50% breakdown revealed non-linear

behaviour.

Figure 3.4. SF6/N2 mixtures with different pressure.

3.4.2 Different alternatives of SF6

In [73] C2F6 was analysed and showed some good results, like its breakdown strength was equal to

0.90 times of SF6. Like SF6 it has also environmental concern because it contains high GWP and

high atmospheric lifetime. Also, it is costly and its price is 2.5 times of SF6 as shown in Table 3.1.

In [74] C3F8 was examined and showed nearly same breakdown strength to SF6, but it also

possesses high GWP and atmospheric lifetime. The price of C3F8 is double from the SF6.

In [75] CF3I gas was studied and showed good dielectric strength from the SF6 while also having

low GWP and atmospheric lifetime. CF3I was not economical because its price is 10 times of SF6.

In [76] 1-C3F6 was inspected and shows 0.97 times of breakdown strength and also has low GWP

and atmospheric lifetime but its price 3.5 times of SF6.

In [77] C-C4F8 was analysed and displayed good dielectric strength from SF6. It also contains

environmental and economic concern.

R152a shows good dielectric strength almost 0.96 times of SF6 and it also has low GWP and price

and it will be discussed detail in chapter 6 and 7.

24

R152a also have low GWP and price. It shows good dielectric strength approximately 0.88 times of

SF6 and it will be discussed in detail in chapter as well as 7.

Table 3.1. Comparison of SF6 with different alternatives [73-77].

Gas Chemical Name Breakdown

Strength

GWP Atmospheric

Lifetime

(years)

Boiling

Point

Cost/kg

SF6 Sulphur hexafluoride 1 22800 3200 -63℃ 25-30$

C2F6 Hexafluoroethane 0.90 12200 10000 -78.1℃ 2.5 times

𝐂𝟑𝐅𝟖 Octafluoropropane 0.97 8830 2600 -36.7℃ 2 times

of SF6

C𝐅𝟑I Trifluoroiodomethane 1.21 5 0.005 -22.5℃ 10 times

of SF6

𝐂𝟑𝐅𝟔 Perfluoropropylene 0.92 100 10 -29.6℃ 3.5 times

of SF6

𝐂𝟒𝐅𝟖 Perfluorocyclobutane 1.21 8700 3200 -5.99℃ 9 times

of SF6

R12 Dichlorodifluoromethane 0.90 2400 12 -29.8℃ 0.50

times of

SF6

R134 Tetrafluoroethane 0.85 1300 14 -26.8℃ 0.33

times of

SF6

3.5 R152a Characteristics

R152a is recognized as a chlorofluorocarbon (CFC), which is commonly used in refrigeration

appliances and in aerosol sprays with properties in compliance with the Montreal Protocol [78].

R152a possesses some pertinent qualities making it an effective gas to be employed in the field; for

example, it is harmless and non-explosive. All these features make it a suitable candidate for

domestic and industrial usage as [79]. R152a (chlorofluorocarbon) has an appreciably lower value

of GWP: 140 and is a cheap contemptible insulation medium as compared to SF6.

25

Figure 3.5. Electronegativity of elements [78].

HCCH + 2 HF → CH3CHF2

Moreover, R152a has zero ozone depletion potential. As the atmospheric lifetime of R152a is 1.4

years so its decomposition products have 98% low environmental impact as compared to SF6.

Therefore, using the proposed gas mixture can effectively reduce the greenhouse effect.

Contrast between the physical and chemical properties of R152a and SF6is shown inTable3.2. It can

be prepared by reacting CCL4 (carbon tetrachloride) with HF (hydrogen fluoride) in the presence of

catalytic amount of (SbCL5) antimony pentachloride [80].

Table 3.2. Different properties of R152a [79-80].

Molecular formula CF2HCH3

Molecular weight 66.05 gmol1

Appearance Clear, Colorless liquid and Vapor

Odor Slight Ethereal

Density 0.90g/ccat (78 F) in liquid

Melting- point (MP) (-118◦C or -180◦F)

Boiling point -25◦C (-13◦F)

Solubility in water 0.28WT%@25C(77F) (87Pasia)

Vapor Density(air=0) 2.4

pH Not Applicable

Vapors- Pressure 89 pasia(26◦Cor 78◦F)

Flashing Points Non

LEL/UEL 3.9%/16.9%

3.5.1 Physical properties of R152a

Chlorofluorocarbon R152a (CFC) is a colorless gas with a faint ether smell. Referred as a liquid

confined below, it does acquire vapor pressure. Contacts with unrefined liquid can cases frostbite.

Mutually components are incompatible [81]. It can asphyxiate by displacement of air, experience of

26

closed containers to prolonged heat or fire can foundation violent and rocket to explode. Different

physical properties are shown below.

• Colorless gas with ether

• Practically odorless, faint, ether-like odor in high concentration

• Boiling point is -25◦C(-13◦F)

• Physically gas at ambient temperatures

• Flammable

• Auto ignition temperature 850◦F

• Partition co-efficient n-octanol/ Water: Log Pow:1.13

• Melting point is −179◦F

• Solubility is Insoluble

• Solubilityinwater,0.28wt%at25◦C

• Density is 0.90g/cc at 25◦C

• Vapor density is 4.1 (Air =1)

• Vapor pressure is 77◦F

Thus, the material is stable. However, avoids high temperature and open flames.

Table 3.3. Contrast between physical along with chemical properties of R152a vs SF6 [80-81].

Properties SF6 R152a

GWP

Density

Relative molecular mass

24000

6.17 kg/m³

140

2.7 g/cm³

66.1

Atmospheric life 3200 1.5

Boiling point −64°C −25°C

Molar mass (g/mol) 140.6 66.05

Appearance Colorless Colorless

Permittivity 1.002 2.05

Electronegativity 2.5 2.32

Price/kg 28–30 $ 12 $

Water solubility Slightly soluble 0.276 g/l

27

Gas GWP

CO2 1

SF6 22800

R152a 140

Table 3.4. GWP of different gases.

3.6 Environmental Features of SF6, R152a and Gas Mixtures

Environmental characteristic of SF6 such as GWP, atmospheric lifetime and ozone depletion

potential of SF6, R152a and discussed.

3.6.1 Global warming potential (GWP)

A simple GWP index will be introduced and used here to determine the effects of the emissions of

the greenhouse gases in the atmosphere. This method is devised so you do not consider climate

change data (precipitation, temperature, wind, etc.) to be regionally determined, baseline discharge

scenario is problematic. In this (GWP) index, 1kg of the reference compound is related to the 1kg of

CO2 gas as developed by IPCC (1990) [82].

𝐺𝑊𝑃𝑐𝑜𝑚 =∫ 𝑅𝐹𝑐𝑜𝑚

𝑇𝐻0

dt

∫ 𝑅𝐹𝑟𝑒𝑓𝑇𝐻

0 dt =

∫ (𝑎𝑖𝑇𝐻

0) 𝑐𝑖 dt

∫ (𝑎𝑓𝑇𝐻

0 )𝑐𝑓 dt (3.9)

Whereas

TH= Time horizon

RFcom= global mean radioactive forcing gas component

RFref = radioactive forcing

Ci =time dependent abundance of component gas

3.6.2 Atmospheric lifetime

It is defined as the time that a gas remaining in the atmosphere before it can decompose. SF6 is a

stable gas, therefore, it can take a large time to decompose in the atmosphere as shown in Table 3.2.

However, R152a atmospheric lifetime is less than the SF6 shown in Table 3.5.

The atmospheric lifetime concept has been established on the sample one box model and the

lifetime (T) of the specimen (X) which is in t box can be distinct as the molecule of the specimen

remaining in the box [82]:

28

𝑇 =m

Fout+L+D (3.10)

Whereas Fout =leakage from the box, m= mass in kg, L= chemical loss of gas, D= deposition of gas.

3.6.3 Ozone depletion potential

In altitude, the ozone is 10-50 km placed in the stratospheric region. Ozone engages ultraviolet

radiation from the sun to the earth. Ozone depletion occurs from the man-made greenhouse gasses.

Ozone depletion is dangerous to the atmosphere because dangerous ultraviolet radiation comes

directly to the earth [82]. The ozone depletion potential of the SF6 gas can be said as negligible

because of its chemical inertness so it cannot react to the other gases in the stratospheric region [9]

[83]. R152a has small ozone depletion potential.

3.6.4 A Comparison of the dielectric strength of SF6 with alternative gases

According to the literature, the main cause of the SF6 insulation strength is the attachment of

electron [78]. It can capture free electron moving in the vicinity of the applied field to be converted

to an anion from the neutral SF6 molecule. SF6 + e − → SF − 6 (3.3) These resultant anions owed

to the attachment of free electron is heavy and do not move fast so they do not accumulate the

necessary energy for ionization. This procedure is very effective for the removal of free electrons to

prevent their accumulation to lead the circuit breakdown [79]. The above-mentioned process is the

competitor to the collision ionization process in which the electrons of a certain amount of energy

can inject electrons from neutral molecule to produce another free valance electron.

SF +( 6) + e − → SF +( 6) + 2e − (3.11)

Ionization of the gas is a process which continuously occurs in high electric fields thus producing

free electrons which can cause the breakdown of the gas. There are other gases having higher

dielectric strengths then SF6 gas.There are, however, other issues associated with the use of these

gases which include (but not limited to) toxicity, limited range of operating pressure, production of

pure solid carbon, etc. Thus, SF6 is merely gas which has been established and used in GIS

applications [82].

3.7 Conclusion of Gaseous Replacement for SF6

The different types of gases that could be utilized as insulation medium for voltage insulations

prospects are presented in this chapter. In the case of high voltage insulation, the dry air and N2 can

be utilized as a substitute to the SF6. But this requires very high pressure in order to yield the same

29

insulating characteristics as possessed by SF6. The author’s developed R152a gas and its gaseous

mixture possessing the promising performance that can replace the SF6 is presented. In addition to

this, a wide range of calculations have been presented that validate the performance of R152a are

presented.

30

Chapter 04

Leakage of SF6 Gas from Power Plants

4.0 Introduction

In this chapter, the worldwide usage of SF6 gas as insulation medium will be illustrated. Equipment

that uses SF6 gas as insulation medium is also examined. Leakage of SF6 from already installed

equipment in the distribution network will be discussed in this chapter.

4.1 SF6 equipment review

To understand the worldwide role of SF6, it is necessary to analyse that how SF6 has been employed

in the past. It is also compulsory that why theparticular design of SF6 was applied such as contact

design. From the past and today familiarity with accustomed distribution networks, with the today

use of SF6equipments, can be investigated that how needs of SF6 replacement can be a tackle.

In 1980, SF6 was firstly familiarised and used in power industry as an insulation medium and then

gradually adopted by the electrical network for insulating various equipment [83]. From Figure 4.1

it can be clear that SF6 was used for all voltage level while the insulation medium cannot use for all

voltage levels. SF6 can use up to 800 kV, while another medium cannot use for such high voltage

apart from compressed air which can be used with a high electrodes gap distance. SF6 can also be

used for arc interruption and so it can be used to break high current interruption medium.

Figure 4.1. Insulation medium and its voltage ranges [83].

0 200 400 600 800 1000

Air

Compressed air

oil

SF6

Voltages (kV)

Voltages (kV)

31

Figure 4.2 shows the use of SF6 and other insulation medium from 1982-1998 in Europe and the

figure indicated that the use of SF6 increased rapidly due to its good properties. The use of other

medium decreased day by day because of limitation of air and oil. Oil properties become degrade

due to ageing and air requires more space due to its less breakdown strength. On the other side,

SF6requires less maintenance and has a high life.

Figure 4.2. Medium voltage circuit breaker usage [84].

Figure 4.3 illustrates the dependency of interelectrode distance on dielectric strength. For air, the

distance between electrodes is increasing more to maintain voltage level [85]. Therefore, the size of

the equipment will also increase and also taken more space. On the other side, SF6 space between

the electrodes is less for HV and EHV.

Figure 4.3.Dielectric strength dependency on inter-electrode distance [84].

0

10

20

30

40

50

60

70

80

90

100

1982 1984 1986 1988 1990 1992 1994 1996 1998

SF6 Vaccum Oil Air

0

100

200

300

400

500

600

700

5 10 15 20 25 30

SF6 Oil Air

32

In low voltage distribution network, air circuit breakers are used more dominantly because it is less

than the other insulation medium at that level [82]. In EHV SF6 was the only medium to use at that

level because it takes less space and also no other medium can be used at that level [12].

4.2 Utilization of SF6 Equipment in the Distribution Network

In Figure 4.4 the distribution network is shown, in which the use of SF6 is mention at all voltage

level. The main equipment which uses SF6 as an insulation medium is GIS, GIL, and the figure also

shown at a low voltage another insulation medium can be used.

Figure 4.4. Classical distribution network [12].

In Figure 4.5, the schematic diagram of distribution network is shown which also gives the range of

low, medium and high voltages.

33

Figure 4.5. SF6 usage in distribution network [12].

In Table 4.1, several switching devices and their function are given. The function of several devices

at different circumstances are described in Table 4.1.

4.3 Worldwide SF6 Usage and its Effects

It is a well-known fact that gases cannot be contained in a closed vessel, according to IEC 62271-1

standard there will be 0.1% leakage from any closed vessel. Data is accumulated for 25 years on the

basis of 0.1% leakage of gases worldwide. This database does not contain leakage of gas from

handling processes or any containment failure in gas chambers. The percentage leakage ratio can

vary depedning upon usage and age of equipment and standards.

In power systems ring main units are installed in switch yards of grid stations these ring main units

are metal enclosed switch gears and in GIS system their insulation is SF6 gas and as per table 4.1

there are 2,322,600 RMU’s (Ring Main Unit) installed worldwide. If we consider 0.6 kg of SF6

used in each RMU then the total would amount to 1,393,560 kg and now if we consider 0.1%

leakage of gases as per IEC standard then the leakage of SF6 gas would amount to 34,424 kg in 25

years which is a significant figure.

34

Table 4.1. Several devices and their function.

Task Opening Closing

No

load

Loadconne

cted

Fault No

load

Load

connected

Fault

Disconnector Mechanical

device and

it will be in

open

position

under

unsatisfact

ory

condition.

yes No No yes No Yes

Switch Opening

and closing

of a circuit.

yes Yes No Yes Yes Yes

Earthing

switch

Safe

isolation of

a circuit

under

uncertain

condition

yes No No Yes No Yes

Contactor A

mechanical

device, it is

basically

for motor

control.

yes Yes No Yes Yes Yes

Circuit

breaker

The

isolated

circuit in

afault

condition.

yes Yes Yes Yes Yes yes

35

Table 4.2. Worldwide usage and leakage of SF6 from all RMU’s [83].

Number of

worldwide

SF6 insulated

RMU’s

Expected SF6

Mass of each

component

Total SF6 used

in all RMU’s

Annually

leakageofSF6

from all

RMU’s

Cumulative 25

years

SF6leakagesfrom

every single one

RMU’s

2,322,600 0.6 kg 1,393,560 kg 1,376 kg 34,424 kg

The Same calculation is taken for SF6 insulated circuit breaker used in all over the world. There are

600,000 of SF6 insulated circuit breaker used worldwide. If 0.35 kg of SF6 is used in each CB then

thetotal amount of SF6 used in all CB is 145,000 kg. If the same leakage ratios are considered then

the 25 years leakage of SF6are 34,424 kg as shown in Table 4.3.

Table 4.3.Worldwide SF6usage and leakages from every CB’s [83].

Number of

worldwide SF6

insulated CB’s

Expected SF6

Mass of each

component

Total SF6 used

in all CB’s

Annually

leakages SF6

from every

CB’s

Cumulative 25

years leakages

SF6 from

every CB’s

600,000 0.34 kg 145,000 kg 150 kg 3705 kg

In Table 4.4 worldwide use of switches are given and if 0.429 kg of SF6 is used and the leakage

ratio is considered same then the 25-year cumulative leakage of SF6is 7178 kg.

Table 4.4. Worldwide usage and leakage of SF6 from all switches [83].

Number of

worldwide

SF6insulated

Switches

Expected SF6

Mass of each

component

Total SF6

used in all

Switches

Annually

leakageofSF6

from all Switches

Cumulative 25

years leakages SF6

from every

Switches

677,400 0.429 kg 290,604 kg 287 kg 7178 kg

36

In table 4.5, a total amount of SF6 is used in every one RMU’s, CB’s and switches are shown.

Table 4.5. Total amount of SF6 use in all RMU’s, CB’s and switches [83].

Number of

worldwide

SF6

insulated

RMU’s

Number of

worldwide

SF6insulated

CB’s

Number of

worldwide

SF6insulated

Switches

Total SF6

used in

every one

MV RMU’s,

CB’s &

Switches

Annually

leakages SF6

from every

MV RMU’s,

CB’s &

Switches

Cumulative 25

years leakages

SF6 from every

MV RMU’s,

CB’s &

Switches

2,322,600 500,000 677,400 1,834,164 kg 1,812 kg 45,308 kg

From figure 4.6 it is shown that use of SF6 in GIS is very popular in distribution networks which are

63% of all the worldwide SF6 usage followed by CB and GIL. There are 20,000 unit of GIS

installed in all over the world and if 500 kg of SF6 is used in each unit then the total amount of

leakage from all GIS is 247023 kg as shown in Table 3.6. Table 4.7 shown that 30000 km of GIL is

installed in worldwide and 30.24 kg is used in each km of GIL and the total leakage is 22410 kg.

Figure 4.6. Percentage wise use of GIL, GIS and CB’s.

Table 4.6. Worldwide usage and leakage of SF6 from all GIS [83].

Worldwide SF6

insulated GIS

Expected SF6

Mass of each

component

Total SF6

used in all

GIS

Annually

leakage ofSF6

from all GIS

Cumulative 25 years

leakage of SF6 from

all GIS

20,000 500 kg 10000000 kg 9881 kg 247023 kg

GIL6%

GIS63%

SF6 CB’s31%

GIL

GIS

37

Table 4.7. Worldwide usage and leakage of SF6 from all GIL [83].

Worldwide

SF6insulated

GIL

Expected SF6

Mass of each

km

Total

SF6used in

all GIL

Annually

leakageofSF6 from

allGIL

Cumulative 25 years

leakage of SF6 from

all GIL

30,000 km 30.24 kg 907184 kg 896 kg 22410 kg

Data regarding usage of SF6 gas worldwide in distribution networks is shown in figure, Figure 4.7

this figure shows a 0.1% leakage of SF6 gas [1] in all metal enclosed distribution equipment

worldwide over last 25 years [84]. Figure 4.7 shows that High voltage GIS metal enclosed

equipment with SF6 to be used as insulation medium releases 247023Kg of Sf6 gas into atmosphere

over the period of last 25 years [85]. For each of these mentioned results an equivalent ratio of

emission of carbon Dioxide gas for every 1 KG of Sf6 gas released is also calculated which

amounts to 23,900 kg of CO2 being released into the atmosphere.

Figure 4.7. Estimated 25-year leakage of SF6 worldwide distribution equipment [86].

It can be calculated from Figure 4.7that the worldwide leakage of Sf6 gas from all HV and MV SF6

equipment at distribution stations over a 25-year period is approximately 438 tons of SF6 gas which

is the equivalent of 10468 kilotons of CO2. According to [85-86] it has been estimated that

34453427 kilotons of CO2 was released into the atmosphere in 2012 alone.

Conclusion

The main high voltage equipment that utilizes SF6 as an insulating medium in the world, is

presented in this chapter. The problems estimating the leakages of SF6 in these items of equipment

38

are also analyzed in this chapter. The estimated problems are also presented while comparing with

the CO2 in an equivalent ration. The leakage problems in such equipment at the time of installation

as well as during the operational life of these equipment is also analyzed. The wide range of

calculations are presented in order to validate the R152a as a substitute to SF6. These calculations

do not involve the leakage of SF6 due to mishandling of equipment or gas chamber failures. As the

data shows emissions of SF6 is not large compared to CO2, but due to long lifetime of SF6 gas it is

potentially more harmful to environment as than to CO2 in the long run.

39

Chapter 05

Development Test, Laboratory and Investigational

Methodology

5.0 Introduction

This chapter describes the applied experimental procedures, equipment and test specimen. In

Section 5.1 exemplifies the detailed report and explanation of the equipment, test setup and

different gases mixtures which are being used for breakdown tests. Furthermore, electrodes

installation for these breakdown tests depict in 5.2 section. The breakdown procedure of these

gaseous mixture is also express. Section 5.3 and 5.4 are briefly explain parameters which corelate

for test procedures. To end with, 5.5 Section are describing the tests equipment, material samples

procedures as well as material characterization.

5.1 Laboratory Setup and Assembly of Test Electrodes

This section describes the experimental test setup in HV laboratory, techniques applied and

assembly used for test electrodes. Figure 1 shows the experimental setup in which the

manufacturing on the IEC60270 standard [89]. The test setup consists of a control desk (HV-9103)

comprising a peak voltmeter (HV- 9150) plus built-invariable voltage supply. The power supply

output is 0 to 230 volts and the voltmeter peak range is 100 to 1000 (kV). The control desk consists

of measuring instruments, namely Impulse, Peak, Trigger devices and DC Voltmeters. Using a

voltage doubler circuit, the rectification of AC is performed in case of DC voltage application

where upto 140 kilo Volt DC voltage be able to generate.

During experiment whenever breakdown occurs, voltmeter measures the breakdown voltage across

the measuring capacitor. Prior to starting the experimental breakdown voltage tests, AC Voltage of

identified value was applied to a voltmeter and measuring devices for calibration purposes to

minimize errors and improve precision. All experimental results were obtained at room temperature

(20-25℃). The pressure vessel’s temperature rise has been avoided by the interval of 5 mins

between two consecutive breakdowns as recommended in [90].

40

Figure 5.1. Experimental set up to examine R152a/CO2 breakdown voltage by Sphere-Sphere electrodes.

(a)

(b)

(c) (d)

Figure 5.2. Test equipment used: (a) Control and measurement unit(b) Testing vessel (HV-9134) (c)

Experimental setup for DC (d) Spark formation comparison.

The testing vessel for vacuum and gas is made of steel and equipped with a pressure gauge to

measure pressure up to 6 bars. The manufacturing material of the electrode was aluminum enclosed

41

with a nickel coating. The diameter of the electrodes is basically 50 mm. Electrode diameter was

selected to be 50mm because the gap length should be equal or less than the radius of electrode to

maintain uniform electric field. In our experimental work the gap length is varied from 0-16 mm,

therefore 50 mm is best appropriate diameter of electrodes. Subfigure5.2 (a) is a Control and

measurement unit. Subfigures 5.2 (b) &5.2 (c) show Testing vessel (HV-9134) and experimental

setup respectively. The vessel contains a cylinder made of Plexi-glass that is sandwiched with

flanges top and bottom which are linked with high voltage (HV) and ground potential

correspondingly. The bottom cover is furnished with essential apparatus, such as inlet or outlet

valves measuring gauge intended for vacuum and pressure. Specifications of the test vessel

provided by the manufacturers are briefly described in Table 5.1. As shown in Subfigure 5.2 (d)

Spark tends to suppress and turns into a bluish glow.

Table 5.1. Test setup specifications.

Specifications Standards

Voltage (AC) 100 kV

Pressure (p) 0 to 6 bars

Diameter of Sphere Electrodes 50 mm

Vertical Height 800 mm

5.2 Insulating Material Breakdown Voltage

To calculate the relation between Electrical break down voltage and fields strength with the gap

length d for homogeneous fields, following Equation is used:

dEU dd. (5.1)

Electrical breakdown voltage and breakdown fields always depend on AC and impulse voltage due

to the short-term effect of Electrical breakdown. Sometimes excessive short-term stresses in

seconds or minutes are enough to generate electrical breakdown but it can be happened after some

hours, days, or year due to long term aging effect. In atomic configuration of insulting materials, all

valence electrons are tightly bounded with the central nucleus. When an external voltage is applied

to this insulating material, an electrical pressure is developed that pulls electron out of valance bond

and electron flow starts. Being an insulating material, it resists the flow of valence electron. This

type of material is called dielectric and It is used in many Electric circuits just to distinguish the

function off different conductors within the circuit without conduction inward [91-92]. So,

42

breakdown voltage is a key characteristic of an insulator material that describes the limit of

maximum voltage to be applied across an insulator before its conduction.

5.3 Breakdown of Gases

At normal temperature and pressure gases are excellent insulators with the conductivity of 10–10

A/cm2. This conductivity is due to the air ionization of cosmic radiations and radioactive elements

in the atmosphere. The charged particles get efficient energy during collisions in the high electric

field intensity region to cause ionization. During these collisions electrons lose their energy and get

a kinetic energy by the external voltage source. This kinetic energy s converted to a potential energy

by the ionization. This ionization is actually a name off process that pulls electrons away from the

bounded shells and leads to a breakdown of gases. In photo ionization, radiation energy is an

external source that affects the ionization energy of electrons A+ hν → A+ + e where A represents a

neutral atom or molecule in the gas and hν the photon energy. In photo ionization process, if photon

energy is not sufficient to remove the electron from the shell then it jumps to the higher energy level

or shell. T s called photo excitation [93].

Gases are more common dielectric materials. Air is used as insulating medium for most of the

apparatuses but in some cases following gases N2, CO2, R152a and SF6 (hexafluoride) are used.

Dielectric gases pass through various stages when an external voltage is applied. A small current

flow with the low voltage application across electrodes off the dielectric material and it retains its

electric property. High voltage causes a sharp increment of current in the dielectric materials that

leads to breakdown. This breakdown property in gases depends on electric properties.

There are two types of electrical discharges in gases.

Self-sustaining types

Non-sustaining discharges

Electrons in different shells around a nucleus of an atom are strongly bounded. Stronger the bond

with the complete shell, stronger will be breakdown strength. Electrons are heavy ions like positive

ions. They get much energy from collision that makes their bond weak with nucleus, so low voltage

is enough for ionization for flow off current. The types of gaseous attachments that play attractive

and active role are called electronegative gas.

a) Direct attachment: An electron directly attaches to form a negative ion.

AB + e =>AB

43

b) Dissociative attachment: The gas molecules split into their constituent atoms and the

electronegative atom forms a negative ion[95].

AB + e => A +B-

Oxygen is a simplest example of this type off gaseous attachment. Others are others are Sulphur-

hexafluoride, Freon, carbon dioxide and fluorocarbons. In the equations presented above, “A”

representseither sulphur or carbon atom while “B” is representing that it can be either oxygen atom

in these gases or one of the halogen atoms / molecules. In high voltage power equipment,SF6, N2,

CO2, R152a are various insulating materials that are used [96].

The charge carrier collisions in gases and interaction with the electrodes cause breakdown

mechanism. In principle, free electrons are accelerated inside the gas filled gaps by the electric

field. The collision of free electrons with gas atoms is shown in Fig (5.3)[97]. These free electrons

collide with gas atoms and if they have sufficient energy, they release more electrons. An avalanche

of electrons can grow towards the anode, while ions that move in the opposite direction can collide

with the cathode generating new electron. In microseconds, a conductive breakdown channel is

developed with this mechanism. Photon emission mechanism is used for large distances gaps for

plasma formation. Streamer-Leader process measures bridging of long gaps in meter range.

Figure 5.3. Discharge (breakdown) development in a gas volume between two electrodes by electron

avalanche process[97].

The specific effects taken into account are following:

The strongly inhomogeneous electrical field approaches to high fields

close to the curvature and lower field strengths in the remaining gap

space. This electrical field assumes a curvature that has smaller

radius.Thus, with the increase of voltage up to a certain threshold, level

locally the breakdown field strengths at the curvature and it in results in

the local limited discharge happenswithout having a full breakdown.

The “external partial discharge” or “corona is the specific regard given

UD

E

44

to the limited discharges[98]. The permanent current pulses that are

effective as a leakage current are also caused due to the existence of

corona. The corona results in the visible glow andrelated leakage current

increase with the further increment of voltage. The corona is in a

position to extend into the zone of lower field strength at critical

voltage. Thus, it finally bridges the gap that results into a full

breakdown [99].

Space charge formation causes a polarity effect for unsymmetrical

inhomogeneous fields, leading to the following consequences.the corona

starts with the lower voltage, if the electrode with the stronger curvature

is at a negative potential. However, the breakdown voltage will be

higher in comparison to the case with the electrode of the stronger

curvature is positive. This is representing no corona or corona at voltage

close to breakdown) [100].

The breakdown voltage is affected by the electrode surface due to the effect of

surfaceroughness (local field enhancements).

The breakdown strengths are reduced by placing a sold insulator

between two electrodes as surface layers i.e. water or contamination can

varythe distribution of electrical field enormously. This effect is

partially or fully compensated via enhancing creep-age path lengths of

an insulator [101][98].

The breakdown strength is affected by the “triple-junction area” in the

corner between two metal electrodes and insulation media.

The gaseous insulation is generally regarded as self-healing. The main

cause for this because after compensating and reapplying the power

source the insulation recovers to its initial strengths [102][98].

5.3.1 HVAC test arrangement for breakdown voltage

Mostly in present days transmission networks and distribution are operating under (AC) voltages

furthermore, typically the linkage of testing apparatus with high voltages (HV) in AC forms [103].

UD- > UD+

45

In any system mostly equipment is 3-phase, either single- phase transformers, they must operate at

power frequency for High Voltage (HV) AC testing bench [104].

5.3.2 HVAC Arrangement for breakdown voltage

AC analysis laboratory circuit is arrangements revealed in Figure 5.4. During these experiments’

AC voltage is applied with variation of pressure on pure R152a gas as well as R152+CO2 f. These

experiments are performed with electrodes gap of 6mm and 10mm and electrodes with 20mm

diameter Electrodes with 20mm diameter. For an unchanging pressure voltage is changed unless

breakdown occurs also same work repeated for fix temperature. Ten to hundred readings are taken

for each pressure value after that Mean value calculated by applying different formulas. The special

elements like measuring capacitor along with peak voltmeter used for AC testing. For high Voltage

AC test illustrate in Figure 5.3 with schematic diagram [105].

Figure 5.4. Schematic diagram for AC test.

Figure 5.5. AC Experimental Setup.

46

5.4 HVDC Test

A large number of applications for high DC voltages are in electric industries as well as in medical

sciences research [106]. HVDC transmission system has become a very famous choice for overhead

lines moreover for underground cables. HVAC cables are tested by HVDC of long lengths as they

possess very large capacitance.These would require very large values of currents when tested on

HVAC voltages because of presence of large capacitances [107-108]. The D.C tests on A.C cables

are reliable and economical but they in these tests the stress distribution within the insulating

material varies from the routine operating circumstances. The electrostatic precipitation especially

in thermal power plants, cement industry, electrostatic painting, communication systems is also

performed by utilizing this test and validated results [109]. HVDC is also extensively being used in

medical equipment’s like (X-Rays) and in physics for particle acceleration [110]. Rectification

employing voltage multiplier circuits are used to produce high D.C. voltage efficiently. High D.C.

voltages are also used to produce Electrostatic generators.

5.4.1 HVDC arrangement for breakdown voltage

The generation of DC voltage from AC voltage is from voltage doubler-circuit. The setup available

in the laboratory can produce C voltage up to 140kv. Figure 5.5 is schematic diagram and

laboratory experimental setup for DC tests shown in Figure 5.6. Breakdown of the gas occurs by

increasing voltage at a fix pressure. For all pressure value, hundred different values of HVDC

voltages are being apply in addition to calculate average value by using different mathematical

tools. Every part of results is measured plus sketched in graphically also represents in schematic

diagram blow [111].

Figure 5.6. HVDC test represents in schematic diagram.

47

Figure5.7. DC Experimental Setup.

5.5 List of Equipment Used in Experimental Tests

Following is the list of equipment required for experimental testing of the gases with spherical

electrodes.

1. Testing transformer for high voltage.

2. Control desk.

3. Measurement capacitor.

4. Voltmeter for measuring AC.

5. Rod for connecting equipment.

6. Cup for connecting.

7. Rod for Earthing.

5.5.1 Control desk

Table 5.2. Descriptions of Control desk.

Supply –Voltage (220-230 V), (50/60 Hz), (25 A single phase)

Regulating-Transformer (5kVA), (Geared motor drive) DC

Regulating –Voltage (0-220 V) AC

Output (5kVA Continuous), (10kVA), (2min short time duty)

Dimension (1220x105x800mm) (hxwxd)

Weight (275 kg)

48

High voltage (HV) tests AC as well as DC equipment are operated through control unit, it also

measured real values which one recorded in unit. In control desk unit numerous elements

assembled, like control, operational, safety point of view furthermore warning signal. every single

one measuring instruments Impulse and peak voltmeter, DC voltmeter, as well as triggering devices

are entrenched in this control unit. Above Figure 5.1a epitomize the control unit with brief

descriptions exposed in Table 5.2.

5.5.2 Pressure/vacuum vessel (HV 9134)

Technical data

AC Voltages (100kV)

Impulse Voltage DC (140kV)

Operating pressure (0-6 bar)

Diameter of Sphere Electrodes (20 mm)

Height approximately (800 mm)

Weight of vessel (12 kg)

Figure 5.8. Vacuum/pressure Vessel.

5.5.3 Applications

Vessel made by Plexi-glass cylinder rigid with top plus bottom flanges connected to ground

potential and high voltage (HV) correspondingly. The floor cover is prepared by obligatory

49

accessories, like inlet/outlet valves, also pressure along with vacuum measuring gauges. Earthing

terminals are endowed with bottom pedestal. The (50 mm) sphere -electrodes are mounted seeing

that in Figure 5.8.

5.5.4 Test transformer (HV 9105)

Ratio of T/F (2x220V) (100kV) (220V)

Rated current Continuous (2x11,4A) (50mA) (15.2A in Continuous)

T/FImpedance Voltage At (100 kV)< 3pC.

Partial Discharge (PD) Level (5 kVA) (10kVA) for 60 min.

Frequency (50Hz/ 60 Hz)

Weight 215 kg

Diameter 550 mm

Figure 5.9. Test Transformer.

5.5.5 Applications

Test transformer(T/F) winding for coupling cascade connection, which one generate high voltage

(HV)Alternating Current as revealed in figure 5.9. Transformer made with three windings by

insulating shell along with apex and base corona for free Aluminum electrodes.The primary

windings are a double winding(2x220V) for connecting t parallel connectionof 22o volt, in series

connection (220 +220V) connecting with. High voltage(HV) secondarywindingsin series of 100 kV.

"Coupler Winding” also known as third windings which supplied to cascade connections in

transformers.

5.5.6 Peak voltmeter (HV 9150) for digital display

Supply voltage (220 V 50 Hz)

Measuring Range (100-1000 Û / √2 kV)

Dimension (142 x 173 x 245) (WxHxD)

Weight (3.4 kg)

50

Figure 5.10. Peak Voltmeter (PV).

5.5.7 Applications

Compressed Gas Capacitor or Coupling Capacitor use for measurement of peak (AC) connections.

We employ (HV 9114) for earthing switch which one electrically function as revealed in figure

(5.10).

5.5.8 Discharge Rods

Figure5.11. Discharge Rod.

5.5.9 Applications

High Voltage (HV) discharging for manual component as revealed in figure (5.11).

5.5.10 Aluminum (HV 9108) Rod Connecting

Figure 5.12. Connecting Rod.

Discharge -Resistance (100Ω)

Rod -Length (2.5m)

Rod-Weight (2.5 kg)

Rod span (660 mm)

Rod Weight (2.04 kg)

51

5.5.11 Applications

The connection conductive elements as revealed in figure (5.12).

5.5.12 Aluminum (HV 9109) cup connecting

Figure 5.13. Connecting aluminum Cup.

5.5.13 Applications

5.5.13.1 Conductive Elements

Two(2) elements are capable of being placed in the vertical position as well as four(4) elements are

capable of being inserted in the horizontal position as revealed in figure (5.13).

Dimensions (h 85 Ø x 150 mm)

Weight of cup (2.2 kg)

52

Chapter 06

Experimental Results of R152+ CO2 Mixtures: as a Potential

Alternative to SF6

6.0 Introduction

In this Chapter mathematical as well as from tentive experimental opinion, the performances of

R152a incorporate explicitly correlated saturated and superheated properties in comparison with

existing insulating materials are presented. The experimental study of power frequency breakdown

voltage is also analyzed for a proposed gaseous mixture of R152/CO2 comprehensively discussed

in this chapter

6.1 Power Frequency Breakdown Voltage Experiments and Results

6.1.1 Experimental procedure

Prior to start testing, both electrodes were cleaned with alcohol dumped silk textile cloth to eliminate

moisture and impurities to minimize errors and maximize accuracy in all observations. Tests were carried in

dried and moisture-free zone at room temperature. An increase in temperature raises the probability of errors

in experimental results. To overcome this problem, time span for each test was restricted to 15–20 min. As

R152a and CO2 both are inert and in gaseous form, the time equal to 30–45 min is enough to mix properly

for both gases [112].

6.1.2 Gas mixture procedure

Considering the proposed alternate gas mixture liquefaction temperature experimental constraints

and different mixture ratios for power frequency breakdown voltages were mentioned in Tables 6.1

and 6.2.

Table 6.1. Experimental constraints.

Configuration of Electrodes Sphere–Sphere

Length of spark gap 6mm–18mm

AC voltage 0–100 kV (Peak)

DC voltage 0–140 kV (Peak)

Material of electrode Aluminum /Ni plated steel

53

Table 6.2. Different mixture ratio of R152a and CO2.

Measurement No. R152a Ratio (%) CO2 Ratio (%)

1 90 10

2 80 20

3 70 30

4 60 40

5 50 50

6.2 Calculation of Accurate Gas Mixture Pressure

In order to fill up the accurate amount of R152a and CO2 to achieve the accurate ratio of the gas

mixture by P/P, the calculation of the amount of gas required is essential. A notable thing here is the

P/P ratios of the mixtures of gases because using the W/W ratio would render the calculations

incorrect as the molar mass of molecules can change the pressure of the gas mixture heavily. The

total amount of R152a and CO2 needed is calculated by means of the ideal gas law seeing that in

Equation (6.1) below [113].

𝑉 =[𝑚 × 𝑅 × 𝑇]

[𝑀𝑊 × 𝑃]⁄ (6.1)

Where:

m = Mass in (grams)of gases,

P = Gases Pressure into (bars),

T = Gases High Temperature in (Kelvin),

R = It belongs to Ideal gases constant,

MW =It presents the Molecular weight of gases in (g/mol),

V = Gases Volume in (liters).

For example, in R152a process, the gas accurate temperature was (20°C). The ideal gas constant (R)

=0.083also MW of R152a = 66.01(g/mol) and MW of CO2 = 44.02(g/mol). Consequently, when

chamberfilled by mixture volume of each gas be able towell calculated as follows below [114]:

𝑉 =[𝑚 × 𝑅 × 𝑇]

𝑀𝑊 × 𝑃⁄ = 1300×0.0821×293/146×1.4 = 153 L

In order to fill 80% R152a required amount of this gas

54

𝑚 = 𝑀𝑊 × 𝑃𝑅𝑇 ⁄ = (66×0.98×153)/ (0.0821×293) = 412 g

Similarly, for 20% amount of CO2

𝑚 = 𝑀𝑊 × 𝑃𝑅𝑇⁄ = (44×0.42×153)/ (0.0821×293) = 117 g

Therefore, the amount required to fill 80%/20% mixture ratio of R152a/CO2 is 412:117g.

6.3 Mixture Ratio Analysis

Experiments were performed to locate power frequency breakdown characteristics on8mm

electrode distance under these environments (a) pure R152a, (b) pure CO2, (c) CO2 (50%) with

addition of R152a (50%), (d) CO2 (40%) with addition of R152a (60%), (e) CO2 (30%) with

addition of R152a (70%), (f) CO2 (20%) with addition of R152a (80%) and (g) R152a (10%) with

addition of R152a (90%). Figure 6.1 shows the breakdown strength of R152a and CO2 among their

mixture at different R152a/CO2 ratios.

Figure 6.1. Power frequency breakdown voltage of R152a/CO2 gas at varying mixture ratio and 8 mm

electrode gap distance.

R152a is an electronegative gas, moreover all negative (-) ions are created by gaining electrons

from neutral molecules, which as a result become positive ions after losing electrons. Gaining and

losing of the electrons could occur depending on the field applied and attachment and detachment

capability of the insulating medium. Losing electrons or detachment coefficients is symbolized by η

55

as shown in Equation (6.2) [114]. When a single electron travels per unit length, several electrons

produced in that specified path are defined by Townsend first ionization coefficient, α.

dN = N (α − η) dx (6.2)

where N refers to initial electron quantity, dN denotes the no of electron traveled a distance dx.

6.3.1 Dielectric strength analysis

The breakdown strength of R152a and CO2 different mixtures was measured in uniform field under

AC voltage. Figure 6.2 displays the dielectric strength characteristics which can be attained by

ratios of 80% and 20% respectively on 4 bar which gives the highest breakdown strength of 96% of

SF6 gas.

Figure 6.2. Breakdown characteristic comparison of R152a/CO2 gas at 80%/20% mixture ratio and SF6 at 8

mm electrode gap distance.

6.4 Gap Difference Analysis

The breakdown voltage of R152a/CO2 gas varied with the electrode gap distance (4–16 mm). This

gap between both electrodes has dominant effects on the gas dielectric strength as shown in Figure

6.3. In Equation (6.3) there is an almost linear relationship between the electrode gap and

breakdown voltage as can be seen [115].

𝐸 = 𝑓 ∗ (𝑉

𝐷)(6.3)

56

wherever constant (f) is representative non-uniformity, while (V) is applied voltage as well as (D) is

distance flanked by two electrodes. R152a/CO2 (80%/20%) revealed a similar growth trend as SF6

by changing the gap length as shown in Figure 6.3. After 12 mm, no significant improvement in

breakdown voltage was found for other mixtures of R152a/CO2.

Figure 6.3. Breakdown voltages of R152a/CO2 gas varying the gap distance (4–16 mm) at different mixture

ratio.

6.5 Statistical Analysis of R152a

Table 6.3 demonstrates the statistical analysis of R152a along with CO2. These are premeditatedly

designed for variable magnitudes of two mixed gases. The experimental values like coefficient of

variation, (SD) standard deviation in addition to mean deviation are depicted in mentioned Table

6.3 that demonstrates the discrepancy of results are accomplish through the experiment. The SD of

R152a and CO2 (70%/30%) shows a rapid variation in value. Correspondingly, coefficient of

variation is observed at the lowest value at (90%/10%) mixing ratio and the mean deviation was

found lowest value at (60%/40%) in Table 6.3.

Table 6.3. Statistical analysis of R152a.

Base Gas R152a Mixed Gas CO2

RBG1 50% 60% 70% 80% 90%

SD 11.2 12.3 10.21 14.2 12.9

Μ 46.1 51.6 59.7 57.3 55.3

cv 0.23 6 0.27 0.28 0.19

Max kV 60 69.8 73 76.1 71.6

Min kV 26 4 49.8 43 35.6

57

6.6 Global Warming Potential (GWP)Analysis

This novel alternative R152a/CO2 gas mixture has been particularly developed to significantly

reduce GWP as compared to SF6. According to environmental protection view, the GWP is

calculated as a weighted average of this proposed gas mixture, and from the sum of the weight

fractions of mixed substance and multiplied with their individual GWP as given in Equation (6.4)

where k shows the base gas mixing ratio, 140 is the GWP of R52a and 44 and 56 is the molar mass

of R152a and CO2 respectively [116-118].

𝐺𝑊𝑃 =(𝑘×56×140)+(1−𝑘)×44×1

(𝑘×56)+(1−𝑘)×44 (6.4)

The relationship among GWP value plus mixing ratios is shown in Figure.6.5. It is found that

R152a/CO2 mixture contents with ratio 80%/20% at −14.16°C has a total GWP of 117.17 instead of

22,800 of SF6 over a 100-year time span, effectively reducing the greenhouse effects by 98%.

Figure 6.4. Global Warming GWP analysis of R152a/CO2 gas mixture.

6.7 Synergistic effect

The synergistic effect defines as two gases mixing result in nonlinear behaviour along with this

nonlinearity show its effects. The effect can be categorized as [1].

Synergistic with positive effect

Synergistic with negative effect

58

Linear relation synergistic effect

Synergistic effect

Positive synergistic effect referred as whenever two mixed gases result of breakdown

strength (BS) is much superior in worth from individual gases are sum weighted.

Furthermore, when the breakdown strength (BS) result of these two mixed proper gases

is fewer than individual gases are sum weighted, then called by negative synergistic

effect [120]. Equation (6.5) is revealed the relation of index C synergistic effect,

breakdown voltage as well as mixing ratios.

Vm = V2+

k(V1−V2)

k+(1−k)C V1> V2

(6.5)

V1 and V2are represented with breakdown voltages of pure one, Vm denotes clearly breakdown

voltage with proper mixed gases, also C demonstrate the synergistic effect, while k presents the

mixing ratio.

6.7.1 Synergistic effect of R152a /CO2

Using Equation (6.5), the synergistic effect of R152a/CO2 has premeditated as in Table 5.5 and 5.6

correspondingly. Comprehensively investigation of R152a/CO2is given in Table 6.4 accordingly.

The indication of value of C is provided in Table 6.4 below.

When C greater than one, it provided negative- synergistic effect

When C =1, the relation presents the linear effect

When C greater than 0 and less than 1, only synergistic effect

When C less than 0, in this condition called positive synergistic effect

Table 6.4. Synergistic effect of R152a/CO2

Pressures(lb./i𝐧𝟐) K (%)

(0.50) (0.60) (0.70) (0.80)

5 0.18 -0.10 -0.48 -0.86

C

10 0.39 0.41 -0.31 0.42

20 0.08 -0.02 -0.29 -0.60

30 -0.02 0.38 -0.05 -0.70

40 0.07 -0.06 -0.28 -0.66

50 0.28 0.08 -0.26 -0.62

6.8 Insulation Self-Recoverability Test of Gas mixtures

If a fault occurs due to a breakdown that will create a surge and it raised the temperature of these

gases, the blends have the ability to reduce the breakdown surge and restore their original form is

59

entitle insulation self-recovery. This mixture has insulation self-recoverability properties as that of

SF6 gas because CO2 also have arc quenching properties [121-123]. Breakdown tests of AC power

frequency were carried out in testing circuit exposed above chapter 5. Test has been intended for

each one minute, twenty shots for this breakdown was tested to this insulation gas as shown in

figure 6.6. Diminutive quantity of carbon was observed lying on electrodes Although few

drawbacks occurred, that can be eliminated by specific techniques of preventing carbonization

available in literature [124]. By and large, self-recoverability of proposed gases mixture is excellent.

Figure 6.5. Insulation self-recoverability.

6.9 R152a/CO2 Liquefaction Temperature Analysis

In the practical engineering application of new alternate gas, the most important parameter is

liquefaction temperature limitation. R152a liquefaction temperature is −25°C, while that of SF6 is

−64°C [125]. The association of vapor pressure with condensation temperature is revealed in Figure

6.7 for R152a and SF6 revealed lower value of condensation temperature for R152a. Thus, it

becomes necessary to mix R152a with air, CO2, N2 or buffer gases to meet the requirement of low

liquefaction temperature. Liquefaction temperature of pure gases like N2, CO2, air and R152a are

shown in Figure 6.8. CO2 has been preferred over other buffer gases like nitrogen and air due to its

superior arc quenching ability to produce the appropriate mixture for circuit breaker and disconnect

switch applications [126-127]. In this research, R152a was used along with the mixtures of CO2

resulting in reduced depletion of the ozone layer and acceptable liquefaction temperature.

60

Figure 6.6. Saturated vapor pressure R152a and SF6.

Figure 6.7. Liquefaction temperature of pure gases.

The formula for calculating the liquefaction temperature or saturated vapor pressure has been given

in Equation (6.6) [128]. The boiling point (b.p) of R152a is greater than SF6 (−63°C) [129]. Due to

this reason, buffer gas CO2 is added to reduce the disadvantages of high boiling point because CO2

possess very low b.p. Increasing the CO2 content in the mixture of R152a/CO2 reduces overall b.p.

Figure 6.9 shows mixed gas liquefaction temperature. It was proposed that by ever-increasing ratios

of additive gases, the overall liquefaction temperature reduce.

61

Figure 6.8. Liquefaction temperatures of R152aat different pressure and mixture ratio.

These characteristic curves achieved with presumptuous superlative additive gas [130].

P = 𝐴[𝑒𝑥𝑝 (1−

𝑇𝑏𝑇

𝑅)] (6.6)

Gas pressure boiling point is represented with P

Liquefaction temperature denoted by T of R152a in mixture with CO2

Tb atmospheric pressure

R = 2 cal/deg.mol is the gas constant and A = 0 21 cal/deg.molis constant

We have performed simulations in comsolmultiphysics regarding the effect of breakdown in

uniform electric field regarding pure R152a as well as its mixtures in different rations with N2 and

CO2. Simulation results are attached here with which clearly represent that R152a provides better

breakdown field strength with CO2 in comparison to pure R152a as well as its mixture with N2.

Further R152a in combination with CO2 gives better breakdown strength in comparison to SF6 at

the respective gas pressures. For the said analysis, plane to plane gap discharge arrangement was

used as breakdown in uniform field was to be investigated.

62

(a)

(b)

Figure 6.9 Simulation of (a) breakdown voltage, (b) electric filed under different mixtures

The decomposition by-products of the proposed gaseous mixture are analyzed by artificially

decomposed the gaseous mixture. For this purpose, the gaseous mixture was decomposed by

subjecting it to the multiple voltage stresses which cause the repetitive breakdown of the gaseous

mixture under the point-sphere electrode arrangement. It was found that carbon fumes are formed

and deposited on electrodes as shown in Figure 3. Further research is required for the detailed

analysis of decomposition by-products and also attempts should be made to find additives to reduce

the decomposition by-products.

Figure 6.10. Carbon fumes deposited on electrodes.

63

Chapter 07

Conclusion and Future Directions

In this thesis, the main focus was on investigating a good alternative to SF6 which provides

equivalent insulation properties as that of SF6 while also being environment friendly. So, variants of

different mixtures were identified and tested. This work was undertaken to ascertain the potential

significance of R152a gas as an upcoming substitute to sulfur hexafluoride(SF6) in metal gear

equipment of distribution system and other engineering applications. This study specifically focuses

on R252a and CO2 gas mixtures and their significance as insulation medium in high voltage switch

gears.

In this thesis harmful impact on environment has been discussed in detail and how it accumulates in

ozone layer and deteriorates it due to its inert nature. There is great concern over high GWP

provided by SF6 and as its greenhouse gas so it is essential that the manufacturers and industries

start considering substitute for SF6 gas to reduce the effect of global warming. This study discusses

in detail that R152a has low value of GWP and it is less hazardous to environment as compared to

SF6 gas. Leakage of SF6 is one of major issues which is not highlighted because the field emission

of this gas does not match the documented results. The estimated leakage ratio of SF6 in high

voltage switch gear in distribution network is covered in chapter 4 and it is also discussed that how

this issue is often over-looked but it is causing serious damage to environment even if leakage is

considered to be 0.01% on annual basis. The expected leakage of SF6 gas is then compared with

emission of CO2 and it is further discussed how leakage of SF6 gas was not reported because of

non-existent of Kyoto protocol in its early industry usage.

This research emphasizes on dielectric properties of combination of R152a and CO2 as different

mixture with different ratio of both gases and highlights the significance of this mixture as future

replacement for SF6 gas in industrial applications. The mixture of R152a and CO2 is tested

experimentally and its insulation properties are then compared with SF6 gas under different

breakdown voltage levels and lastly, its GWP index is compared with that of SF6 gas and

conclusion is drawn that it is more environment friendly than SF6.

The results from practical experimentation are summarized below:

(1) The gaseous mixture of R152a and CO2 with the ratios as 80% and 20 % respectively shows

more than 90% of insulation characteristics as that of SF6 gas.

64

(2) The breakdown voltage levels in AC system for mixture of R152a/CO2 shows linear relation

with the gap length i.e. if we increase gap length of electrodes the level of breakdown voltage also

increases. The proposed mixture also shows good insulation properties under low temperature

electrical applications.

(3) The proposed mixture is also less hazardous to environment as its contribution to GWP is almost

10% less as compared to SF6.

Constraints in the usage of proposed mixture

As we know that currently heavy investment has been made in current grid station system in

installation of SF6 gas compatible breakers and SF6 gas is being utilized globally so it will be very

difficult to replace it very quickly and even if any alternative provides better results as compared to

SF6 gas the industry will be hesitant to utilize it unless it is proven to provide better results in live

field equipment. So it will be a good solution to first try the mixture out on a small scale and then

gradually increase its consumption rate as its performance is monitored on global scale. Due to its

low GWP and current focus on reducing the greenhouse gases to save depletion of ozone layer it

will be a great kick up start for utilization of this proposed mixture and a quick transition then can

be made possible form SF6 gas to the proposed mixture once its environmental benefits are

documented worldwide. As SF6 gas leakage is now regulated worldwide and so any new alternative

will face a tough challenge to replace it in industrial application.

7.2 Future Work

The future scope should be investigation of this and other mixture at high voltage levels so they can

replace SF6 gas in high voltage equipment. The proposed mixture application is in distribution

switchgears while SF6 gas is being utilized at 500kV rated switch gears. This area is totally open

and has a vast mixture of gases available to test their insulation characteristics and develop a

mixture which is environmentally friendly and provides better insulation properties as compared to

that of SF6 gas.

This area of research has great future potential to develop different alternatives to SF6 gas.

Furthermore, special attention should also be placed to develop a much cheaper and

environmentally friendly insulation medium so it is therefore recommended to keep following

points in mind while doing future research on R152a and its related variants.

Future research should focus on improving insulation characteristics of R152a along with

other gaseous mixture and utilization of such mixtures at high voltage level

65

Any variant of the R152a with different gases along with different utilization ratios should

be tested and compared to know which performs better. For example, we have taken 80/20

percentage ratio of R152a/ CO2respectively. R152a ratio with different gases such as NO2 at

different pressure levels can be tested and compared.

66

References

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University Press, Cambridge, United Kingdom and New York, NY, USA, (accessed on 14th

August 2017).

[2] Environmental News Service. Potent New Greenhouse Gas Discovered. 2000 [cited 2010 26

May 2010]; Available from:

http://www2.fluoridealert.org/Pollution/GreenhouseGases/Potent-New-Greenhouse-Gas-

Discovered.

[3] Hopf, A.; Rossner, M.; Berger, F.; Prucker, U. Dielectric strength of alternative insulation

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Efficient Gaseous Insulator as a Potential Alternative to SF6. Processes 2019, 7, 740.

77

Appendices

Appendix A

A-1 Different alternative of SF6 gas. The global warming potential represents

the values over a time span of 100 years for equal masses of these gases.

Reference Gas

Dielectric

Strength

(DS)

Global

Warming

Potential

Atmosphere

Lifetime

Boiling

Point Cost/kg

[11] SF6 1 22,800 3200 −63℃ 25–30 $

[21] N2 0.40 0 −195.8℃ 0.25 times of

SF6

[22] CO2 0.37 1 −78.5℃ 0.35 times of

SF6

[22] C2F6 0.80 12,200 10,000 −78.1℃ 2.5 times of SF6

[22] C3F8 0.90 8830 2600 −36.7℃ 2 times of SF6

[22] CF3I 1.21 5 0.05 −22.5℃ 10 times of SF6

[22] C4F10 1.2–1.3 8700 3200 −5.99℃ 9 times of

SF6

A-2 Characteristics of SF6 Verses R152a

Properties Sulfur Hexafluoride Difluoroethane

Molecular formula SF6 CF2HCH3

Molecular weight 146.06 g/mol 66.05 gmol1

GWP 23900 140

Appearance Colorless Colorless

Density 6.17 kg/m³ 0.90g/ccat 25 deg.C(77

deg,F).liquid

Melting point -50.8 C -117◦C/-179◦F

Boiling point -64 °C -25◦C (-13◦F)

Solubility in water 0.003% (25 °C) 0.28WT%@25C(77F)(87Pasia)

Vapour Density(air=0) 2.5 2.42

Electronegativity Not Applicable

Vapor pressure 10.62 bar 87 pasia@25◦C (77◦F )

Flash Point None

Atmospheric life 3200 1.5

Molecular mass 146.06 g/mol 66.1 g/mol

Price/kg $28 to $30 $12

78

A-3 Association of SF6 replacement

Gases Problems and Drawbacks

Carbon dioxide, Nitrogen and Dry air

Momentous expansion in pressure.

Momentous expansion in size of equipment.

Low breakdown voltage [17].

Trifluoro iodomethane mixtures

(CF3I/CO2 or N2)

Boiling point large than that of CF3I (−22.5 °C) at 0.1 MPa.

Classified as a perilous, mutagenic, and venomous for

facsimile (Type-3) [18].

Mixtures of per-fluorinated ketones

(C5F10O, C6F12O/Technical air or

CO2)

Superior smallest operating temperature than SF6 [19]. Far

above the ground boiling temperature (24°C) at (0.1 MPa)

because of higher molecular mass.

HFO 1234ze

Carbon grime dump on electrodes owing to high spark

voltage. Superior operating temperature than SF6 while

unpolluted (constrained at −15 °C).

C4F7N/CO2 Having high boiling point (−4.7 °C at 0.1 MPa) [14]

79

Appendix B

Worldwide usage and leakage of SF6 from all RMU’s [33]

Worldwide

SF6

insulated

RMU’s

Expected

SF6 Mass of

each

component

Total SF6

used in all

RMU’s

Annually

leakageof

SF6 from all

RMU’s

Cumulative

25 years

leakage of

SF6 from all

RMU’s

2,322,600 0.6 kg 1,393,560 kg 1,376 kg 34,424 kg

Worldwide usage and leakage of SF6 from all CB’s [83]

Worldwide

SF6

insulated

CB’s

Expected SF6

Mass of each

component

Total SF6

used in all

CB’s

Annually

leakageofSF6

from all CB’s

Cumulative

25 years

leakage of

SF6 from all

CB’s

500,000 0.3 kg 150,000 kg 148 kg 3705 kg

Worldwide usage and leakage of SF6 from all switches [83].

Worldwide

SF6insulated

Switches

Expected SF6

Mass of each

component

Total SF6

used in all

Switches

Annually

leakageofSF6

from all

Switches

Cumulative 25

years leakage of

SF6 from all

Switches

677,400 0.429 kg 290,604 kg 287 kg 7178 kg

80

Total amount of SF6 use in all RMU’s, CB’s and switches [83]

Worldwid

e SF6

insulated

RMU’s

Worldwide

SF6insulate

d CB’s

Worldwide

SF6insulate

d Switches

Total

SF6 used

in all

MV

RMU’s,

CB’s &

Switches

Annually

leakageofSF

6 from

allMV

RMU’s,

CB’s &

Switches

Cumulativ

e 25 years

leakage of

SF6 from

all MV

RMU’s,

CB’s &

Switches

2,322,600 500,000 677,400 1,834,16

4 kg

1,812 kg 45,308 kg

Worldwide usage and leakage of SF6 from all GIS [83]

Worldwide

SF6insulated

GIS

Expected

SF6 Mass of

each

component

Total SF6

used in all

GIS

Annually

leakageofSF6

from all GIS

Cumulative 25

years leakage of

SF6 from all GIS

20,000 500 kg 10000000 kg 9881 kg 247023 kg

f) Worldwide usage and leakage of SF6 from all GIL [83]

Worldwide

SF6insulated

GIL

Expected

SF6 Mass of

each km

Total SF6

used in all

GIL

Annually

leakageofSF6

from allGIL

Cumulative 25

years leakage of

SF6 from all GIL

30,000 km 30.24 kg 907184 kg 896 kg 22410 kg

81

Appendix C

C-1 Laboratory Test Setup and Assembly of Test Electrodes

Experimental set up to examine R152a/CO2 breakdown voltage by sphere–sphere electrodes.

(a)

(b)

Test equipment used: (a) Control and measurement unit; (b) Testing vessel (HV-9134); (c)

Experimental setup for DC; (d) Spark formation comparison

Control Desk

Resistor

Measuringcapacitor

Testing Transformer

Connecting Rod

Gas cylinder

Sphere electrode

82

Test setup specification.

Specifications Standards

Voltage (AC) 100 kilovolts

Pressure (p) 0 to 6 bars

Diameter of sphere electrodes 50 mm

Vertical height 800 mm

Experimental constraints

Configuration of Electrodes Sphere–Sphere

Length of spark gap 6mm–18mm

AC voltage 0–100 kV (AC)

DC voltage 0–140 kV (DC)

Material of electrode Aluminum Ni plated steel

Different mixture ratio of R152a and CO2

Measurement No. R152a Ratio (%) CO2 Ratio (%)

1 90 10

2 80 20

3 70 30

4 60 40

5 50 50

83

Appendix D

D-1 Calculation of Accurate Gas Mixture Pressure

V = mRTMW × P⁄

where:

m = Mass of gas (grams), T = Temperature (Kelvin), P = Pressure (bar);

MW = Molecular weight of gas (g/mol), R = Ideal gas constant, V = Volume (liters).

For example, in R152a operation, the filling temperature was 20°C. The ideal gas constant

(R) is 0.0821, the MW of R152a is 66.01 g/mol and MW of CO2 = 44.01 g/mol. Therefore,

when the chamber is filled with mixture the volume of each gas can be calculated as follows

[27]:

a) Mixture Ratio Analysis

dN = N (α − η) dx

where N refers to initial electron quantity, dN denotes the no of electron traveled a distance

dx.

D-2 Gap Difference Analysis

E = f × (V/D)

where constant f is demonstrating non-uniformity, V is applied voltage and D is the distance

between two electrodes.

b) Statistical analysis of R152a.

Base Gas R152a Mixed Gas CO2

RBG1 50% 60% 70% 80% 90%

SD 11.2 12.3 10.21 14.2 12.9

Μ 46.1 51.6 59.7 57.3 55.3

Cv 0.23 6 0.27 0.28 0.19

Max kV 60 69.8 73 76.1 71.6

Min kV 26 4 49.8 43 35.6

1RBG: Ratio of base gas; SD: standard deviation; M: mean; cv: coefficient of

variation.

84

D-3 Global Warming Potential (GWP)Analysis

GWP =𝒌×𝟓𝟔×𝟏𝟒𝟎+(𝟏−𝒌)×𝟒𝟒×𝟏

𝒌×𝟓𝟔+(𝟏−𝒌)×𝟒𝟒

D-4 Synergistic effect

𝐕𝐦= 𝐕𝟐+

𝐤(𝐕𝟏−𝐕𝟐)

𝐤+(𝟏−𝐤)𝐂𝐕𝟏>𝐕𝟐

V1 and V2 denoted by breakdown voltages of pure one, Vm is the breakdown voltage of

mixed gas, c shows thesynergistic effect and mixing ratios shows by k.

If C is greater than one, it gives negative synergistic effect

If C is equal to one, then it gives linear relation effect

If C is greater than 0 and less than one, it gives synergistic effect

If C is less than 0, it gives positive synergistic effect

Synergistic effect C of R152/co2

Pressure (lb./i𝐧𝟐) K (%)

0.50 0.60 0.70 0.80

5 0.18 -0.10 -0.48 -0.86

C

10 0.39 0.41 -0.31 0.42

20 0.08 -0.02 -0.29 -0.60

30 -0.02 0.38 -0.05 -0.70

40 0.07 -0.06 -0.28 -0.66

50 0.28 0.08 -0.26 -0.62

D-5 R152a/CO2 Liquefaction Temperature Analysis

P = exp[A(1−

𝑇𝑏𝑇

𝑅)]

c) where P represents the gas boiling point pressure; T is the liquefaction temperature of

R152a in mixture with CO2; Tb is the liquefaction temperature (K) at atmospheric

pressure;R = 2 cal/deg.mol is the gas constant and A = 21 cal/deg.molis constant.

85

Appendix E

E-1Papers published during work on this project

Hafiz Shafqat Kharal, Muhammad Kamran, Rehmat Ullah, “Environment-Friendly

and Efficient Gaseous Insulator as a potential alternative to SF6”, Processes,2019,

7(10), 740. (Thomson Reuters Impact Factor = 1.963)

Muhammad Junaid Alvi, Hafiz Shafqat Kharal, “Field Optimization and

Electrostatic Stress Reduction of Proposed Conductor Scheme for Pliable Gas-

Insulated Transmission Lines”, Applied Sciences, 2019, 9(15), 2988. (Thomson

Reuters Impact Factor = 2.316)

Hafiz Shafqat Kharal, Muhammad Kamran, Rehmat Ullah,

“Dichlorodifluoromethane gas mixtures: A Novel Competent Gaseous Insulator as

surrogate of SF6 for Electrical applications", Accepted to International Journal of

Global Warming, 2019(Thomson Reuters Impact Factor = 0.78)

H. Shafqat Kharal, Muhammad Kamran, Sohail Aftab Qureshi, M. Zaheer Saleem,

“Safeguard Guide for Recycling and Handling the Alternative of SF6 Gas in Electrical

Investigatory Applications”, World Scientific News,2018, 95, 182-192.

H. Shafqat Kharal, RehmatUllah, Z. Ullah ; R. Asghar ; Waqar Uddin ; B. Azeem ;

S. M. Ali ; A. Haider, Muhammad Kamran, “Insulation Characteristic of R12(CCl2F2)

with mixtures of CO2/N as a Possible Alternative to SF6substitute Gas for High

Voltage Equipment’s”,2018 International Conference on Power Generation Systems

and Renewable Energy Technologies (PGSRET), Islamabad, Pakistan, 2018, pp. 1-5

Muhammad Kamran H. Shafqat Kharal, Renewable Energy Resources Penetration

within Grid”, First International Conference on High Performance Energy Efficient

Buildings and Homes (HPEEBH 2018), UET Lahore.

a novel environment friendly and efficient gaseous insulator

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