EXPERIMENTAL INVESTIGATION OF CO2 GAS IN THE …eprints.uthm.edu.my/id/eprint/7703/1/AHMED_OTHMAN_MOHAMED_ALHABEIB.pdfTujuan eksperimen yang dijalankan di dalam tesis ini Master adalah

ii

AHMED OTHMAN MOHAMED ALHABEIB

A project report is Submitted In partial

Fulfillment of the Requirements for The Award of The

Degree of Master of Electrical Engineering

Faculty of Electrical and Electronic Engineering

University Tun Hussein Onn Malaysia

JUNE, 2015

EXPERIMENTAL INVESTIGATION OF CO2 GAS IN THE BREAKDOWN

CHARACTERISTICS UNDER LIGHTNING IMPULSE

vii

ABSTRACT

The purpose of the experiment conducted in this Master’s thesis is to

investigate the breakdown voltage, when the pressure change in the gas CO2

from (1bar, 1.5bar, 2bar, 2.5bar, and 3bar), and electrode configurations (rod

- plane, plane - plane, and sphere - sphere). The length and diameter of the

rod is 5.3 cm and 1.5 cm respectively. While the measurement of plane used

in the experiment is 2.5 cm thick and 5 cm diameter. And sphere is 5 diameter.

The experiments have been conducted with the positive lightning impulse

voltage at 0.5 cm gap distance, using the up and down method to determine

the U50 breakdown voltage. And also used COMSOL software to determine

the maximum electric field Emax.

The results show that the breakdown voltage of the gap can be increased

by the use of pressure, The increase was an increase of pressure in the

breakdown voltage at a rate of (rod - plane, plane - plane, and sphere - sphere)

it is 25%, 21%, 13% of initial value at 1bar respectively. And the relationship

between the breakdown voltage and the maximum value of the electric field

is a direct correlation.

viii

ABSTRAK

Tujuan eksperimen yang dijalankan di dalam tesis ini Master adalah untuk

menyiasat voltan pecahan, apabila perubahan tekanan dalam gas CO2 dari (1

bar, 1.5bar, 2bar, 2.5bar, dan 3bar), dan konfigurasi elektrod (rod - kapal

terbang, kapal terbang - kapal terbang, dan bidang - bidang). Panjang dan

diameter rod adalah masing-masing 5.3 cm dan 1.5 cm. Walaupun

pengukuran kapal terbang yang digunakan dalam eksperimen itu ialah 2.5 cm

tebal dan 5 cm garis pusat. Dan sfera adalah 5 diameter. Eksperimen telah

dijalankan dengan positif kilat dorongan voltan pada 0.5 cm jarak jurang,

dengan menggunakan kaedah atas dan ke bawah untuk menentukan voltan

kerosakan U50. Dan juga menggunakan perisian COMSOL untuk menentukan

medan elektrik maksimum Emax.

Keputusan menunjukkan bahawa voltan pecahan jurang boleh

ditingkatkan dengan penggunaan tekanan, Peningkatan ini adalah

peningkatan tekanan dalam voltan pecahan pada kadar (rod - kapal terbang,

kapal terbang - kapal terbang, dan bidang - bidang) adalah 25%, 21%, masing-

masing 13% daripada nilai awal pada 1 bar. Dan hubungan antara voltan

pecahan dan nilai maksimum medan elektrik adalah hubungan langsung.

ix

CONTENTS

1.1 Introduction 1

1.2 Problem statements 2

1.3 Thesis objective 3

1.4 Scope of study 3

1.5 Organization of the Thesis 3

1.6 Summary 4

2.1 Introduction 2

2.2 Application of Gases in Power System 5

2.3 Breakdown Voltage of Air 6

2.4 Theory of Breakdown in Gases 6

2.4.1 Townsend Mechanism 6

2.4.2 Voltage-current relation 7

2.4.3 Streamer mechanism 8

2.5 Physical properties of CO2 9

2.6 lightning impulse voltages 10

2.7 Previous studies 11

TITLE i

DECLARATION ii

DEDICATION v

ACKNOWLEDGEMENT vi

ABSTRACT vii

ABSTRAK viii

CONTENTS ix

LIST OF FIGURES xii

LIST OF TABLE xv

x

2.8 Summary 17

3.1 Introduction 18

3.2 Safety 18

3.3 Flow chart 19

3.4 Experiment Circuit 20

3.5 Electrodes configration 25

3.6 Gas setup 26

3.6.1 gas setup requirements 27

3.6.2 Make a vacuum process 29

3.6.3 full of the gas (CO2) from vaccum 30

3.7 Test procedure 30

3.8 Simulation (COMSOL) 32

3.9 Summary 32

4.1 Introduction 33

4.2 Results and discussion 33

4.2.1 Rod – plane electrode configuration at 1bar 34

4.2.2 Rod – plane electrode configuration at 1.5bar 35


4.2.4 Rod – plane electrode configuration at 2.5bar 37


4.2.6 Effect Pressure in U50 at rod – plane configuration 39

4.2.7 Effect Emax at rod – plane configuration 40

4.2.8 Plane – plane electrode configuration at 1bar 41

4.2.9 Plane – plane electrode configuration at 1.5bar 42


4.2.11 Plane – plane electrode configuration at 2.5bar 44


xi

4.2.13 Effect Pressure in U50 at plane – plane configuration 46

4.2.14 Effect Emax at plane – plane configuration 47

4.2.15 Sphere – sphere electrode configuration at 1bar 48

4.2.16 Sphere – sphere electrode configuration at 1.5bar 49


4.2.18 Sphere – sphere electrode configuration at 2.5bar 51


4.2.20 Effect Pressure in U50 at sphere – sphere configuration 53

4.2.21 Effect Emax at sphere - sphere configuration 54

4. 3 Discussion 55

5.1 Conclusions 59

5.2 Recommendation 60

Reference 61

xii

LIST OF FIGURES

Figure No. Figure title

Page No.

Figure 2.1 Voltage-current relation 8

Figure 2.2 Field distortion due to presence of space

charge

9

Figure 2.3 Lightning impulse voltage characteristic 10

Figure 2.4 Air breakdown characteristics between the

sphere-sphere electrodes

11

Figure 2.5 Paschen Curves in air 14

Figure 2.6 Paschen Curves in Co2 14

Figure 2.7 Effect of gap length on breakdown voltage 15

Figure 2.8 Effects of gas pressure on breakdown

voltage

16

Figure 2.9 Impulse BDV50 as a function of gas

mixture rate in N2O / CO2 / O2 gas

mixtures.

17

Figure 3.1 Flow chart of experiment 20

Figure 3.2 Circuit setup for generation of lightning

impulse voltage

21

Figure 3.3 Real setup in high voltage laboratory 21

Figure 3.4 Dimensions plate electrode 25

Figure 3.5 Dimensions Rod electrode 26

Figure 3.6 Dimensions Spher electrode 26

Figure 3.7 Real setup for electrode configuration in lab 26

Figure 3.8 Gas chamber 27

Figure 3.9 Gas cylinder 28

Figure 3.10 Vacuum set 28

Figure 3.11 Vacuum process 29

xiii

Figure 3.12 Test procedure 31

Figure 3.13 Drawing electrode configuration by

COMSOL software

32

Figure 4.1 U50 of rod – plane at 1bar 34

Figure 4.2 U50 of rod – plane at 1.5bar 35


Figure 4.4 U50 of rod – plane at 2.5bar 37


Figure 4.6 U50 of rod – plane electrode 39

Figure 4.7 Emax of rod – plane electrode software 40

Figure 4.8 U50 of plane – plane at 1bar 41

Figure 4.9 U50 of plane – plane at 1.5bar 42


Figure 4.11 U50 of plane – plane at 2.5bar 44


Figure 4.13 U50 of plan – plane electrode 46

Figure 4.14 Emax of plane – plane electrode software 47

Figure 4.15 U50 of sphere - sphere at 1bar 48

Figure 4.16 U50 of sphere - sphere at 1.5bar 49


Figure 4.18 U50 of sphere - sphere at 2.5bar 51


Figure 4.20 U50 of sphere – sphere electrode 53

Figure 4.21 Emax of sphere - sphere electrode software 54

Figure 4.22 U50 for gas CO2 56

xiv

Figure 4.23 Emax for the gas CO2 57

Figure 4.24 Relationship between U50 and Emax for the

gas CO2

58

xv

LIST OF TABLES

Table No. Table title Page No.

Table 2.1 Physical properties of Co2 10

Table 2.2 Measurement of breakdown voltage between

sphere sphere electrodes

12

Table 3.1 A Components used in the setup and its code

number

22

Table 4.1 Voltage breakdown for rod – plane electrode

configuration at 1bar

34


configuration at 1.5bar

35



36



37



38

Table 4.6 U50 for gas CO2 at rod – plane configuration 39

Table 4.7 Emax values of rod – plane electrode 40

Table 4.8 Voltage breakdown for plane – plane electrode


41



42



43



44



45

Table 4.13 U50 for gas CO2 at plane – plane configuration 46

Table 4.14 Emax values of plane – plane electrode 47

Table 4.15 Voltage breakdown for sphere – sphere

electrode configuration at 1bar

48

xvi


electrode configuration at 1.5bar

49


electrode configuration at 2bar

50


electrode configuration at 2.5bar

51

Table 4.19 Voltage breakdown for sphere – sphere elec-

trode configuration at 3bar

52

Table 4.20 U50 for gas CO2 at sphere - sphere

configuration

53

Table 4.21 Emax values of sphere – sphere electrode 54

Table 4.22 Voltage breakdown (kV) for the gas CO2 56

Table 4.23 Maximum electric field (kV/cm) for gas CO2 57

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Rapid growth in the power sector of the nation has given the opportunity to power

engineers to protect the power equipment for reliable operation during their operating

life. it has been seen from the studies conducted by power engineers that one of the

main problem in high voltage power (HV) equipment is the degradation of insulation

i.e., quality of insulation of power equipment. as the high voltage power equipments

are mainly subjected with spark overvoltage caused by the lighting strikes, switching

action[1].

Electrical insulating materials (Dielectrics) are materials in which electrostatic

fields can remain almost indefinitely. these materials, thus offer a very high resistance

to the passage of direct currents. when the applied voltage across the dielectric exceeds

a critical value the insulation will be damaged. the dielectrics may be gaseous, liquid

or solid.

Breakdown voltage, sometimes also called dielectric strength or striking

voltage, is the quantity of electrical force required to transform the electrical properties

of an object. most commonly, it is used with respect to insulators. the breakdown

voltage is the minimum voltage necessary to force an insulator to conduct some

amount of electricity, insulators are characterized by atoms with tightly bound

2

electrons. the atomic forces holding these electrons in place exceeds most outside

voltages that might induce electrons to flow. this force is finite, however, can always

potentially be exceeded by an external voltage[2].

In standard conditions, gas serves as an excellent insulator, requiring the

application of a much voltage before breakdown (e.g. Lightning). this breakdown

potential may decrease to an extent that two uninsulated surfaces with different

potentials might induce the electrical breakdown of the surrounding gas, this may

damage an apparatus, as breakdown is analogous to a short circuit. in a gas, the

breakdown voltage can explain and determined by Towsands or Streamer mechanism

or Paschen's Law[3].

1.2 Problem statements

In view of the growing demand for electrical power to increase production by

increasing high voltage, so the air is not able to maintain the physical properties

insulation. moreover the use of (SF6) has been the primary insulator for high voltage

applications, due to its superior insulation properties. however, much research has

shown (SF6 ) greenhouse effects raise concerns to its environmental impact.

Warmer temperatures, potential (GWP) has been caused by perfluorinated

compounds, the most important and the most famous of these compounds in electrical

insulators is SF6. have a very long lifetime in the atmosphere, leading to the

accumulation in the atmosphere.

To overcome this problem, it is needed to investigate breakdown of another gas

(CO2) and find suitable alternatives air in circuit breakers (GCB) and gas insulated

switchgear (GIS) ,especially that is containing carbon (C) and fluorine (F) [4] which

can have better dielectric strength than air.

3

1.3 Thesis objective

In electrical power system, high voltage (HV) power equipments is mainly subjected

with spark over voltage. these high voltages which may causes by the lighting strikes,

switching action, determine the safe clearance required for proper insulation level. to

avoid these problems in high voltage power equipments should do an experiment in

the laboratory to investigate insulation of equipments (gas insulation).

The main objective of the project is:-

To get voltage breakdown, of the gas (CO2) for 0.5cm the gap distance.

To determine of the electric field effects of the gas (CO2), and fill gas techniques

on gas chamber in high voltage test lab.

To investigate the breakdown properties of the gas (CO2) under lightning impulse,

such as electrode configurations and gas pressures, at 0.5cm gap distance, the

maximum lightning impulse that can be applied is limited to around 140 kV.

1.4 Scope of study

The scope of this study is investigate of breakdown for change pressure state (1bar,

1.5bar, 2bar, 2.5bar, and 3bar) and electrodes configurations (rod - plane, plane -

plane, and sphere - sphere) at 0.5cm gap distance of the gas (CO2) under standard

lightning impulse.

1.5 Organization of the Thesis

The proposal is organized in three important chapters in which each chapter has its

own way of describing and the fundamentals of the work followed by the theoretical,

experimental.

Chapter 1: This chapter deals with the basic introduction of high voltage breakdown,

problem statement, objective, scope, and also includes the organization of the proposal.

Chapter 2: This chapter includes theoretically for high voltage breakdown of gas also

theory for lightning impulse and also includes previous studies of gas breakdown and

gas mixture breakdown.

4

Chapter 3: This chapter deals with the methodology, main setup of the experiment for

gas breakdown voltage under lightning impulse.

Chapter 4: In this chapter discusses about results and discussion part of the thesis and

all the results as well as the graphical form to clarify the objective of the thesis. It is

also covers the different type of performance characteristics of gas CO2 breakdown

voltage with different condition.

Chapter 5: Finally, in this chapter includes the conclusion of the project work and also

some important discussion about the future work or recommendation of the thesis

which helps the development in gaseous insulating for high voltage

1.6 Summary

This chapter includes an introduction of the high-voltage insulators, and the high

voltage breakdown insulators, and focuses on the gaseous dielectrics, and refers about

the problems facing the electrical gaseous dielectrics, and also address the objectives

of this research, and the boundaries of which will be from which to conduct

experiments for this project.

5

CHAPTER 2

LITERATUR REVIEW

2.1 Introduction

Air is the most commonly used gaseous insulation medium in high voltage power

networks because it is free, many, and self-restoring after a breakdown. the electrical

breakdown behavior of air gaps has been thoroughly investigated since the start of the

last century and a vast amount of literature and data are available on this subject. there

are many factors influencing the breakdown voltage of gaseous insulation. pressure,

temperature, gas type, mixture components, electrode material, condition of electrode

surface, contamination including dust and moisture, voltage duration, shape of voltage

pulse, voltage, frequency and polarity, gap geometry and size, electrode area, dielectric

volume.

To improve the breakdown strength, from a designer’s perspective, one should

find an optimal set of values for all of the above variables. however, generally

speaking, the thermodynamic state of gas, environmental factors, electrode

configurations and applied voltage characteristics are imposed by practical conditions.

In high-voltage gas-filled systems and devices in which gases are used only for

electrical insulation, the composition of a gas is selected according to three criteria

maximum electric strength, high liquefaction temperature and minimal cost[5].

6

2.2 Application of Gases in Power System

The gases find wide application in power system to provide insulation to various

equipments and substations. the gases are also used in circuit breakers (GCB) for arc

interruption besides providing insulation between breaker contacts and from contact to

the enclosure used for contacts as in the gas insulated transmission line (GIL), gas

insulated switchgear (GIS).

The various gases used are (i) air (ii) oxygen (iii) hydrogen (iv) nitrogen (v) CO2

and (vi) electronegative gases like sulfur hexafluoride.

The various properties required for providing insulation and arc interruption are:

(i) High dielectric strength.

(ii) Thermal and chemical stability

(iii) non-inflammability.

(iv) High thermal conductivity.

(v) Arc extinguishing ability. It should have a low dissociation temperature, a short

thermal time constant.

(vi) Commercial available at moderate cost.

Of the simple gases, air is the cheapest and most widely used for circuit

breaking. Air has an advantage over the electronegative gases in that air can be

compressed to extremely high pressures at room temperature and then its dielectric

strength even exceeds that of these gases.

CO2 has almost the same dielectric strength as air, but is a better arc extinguishing

medium at moderate currents[6].

2.3 Breakdown Voltage of Air

The breakdown in air (spark breakdown) is the transition of a non-sustaining discharge

into a self-sustaining discharge. the buildup of high currents in a breakdown is due to

the ionization in which electrons and ions are created from neutral atoms or molecules,

and their migration to the anode and cathode respectively leads to high currents.

Townsend theory and Streamer theory are the present two types of theories which

explain the mechanism of breakdown under different conditions as temperature,

pressure, nature of electrode surfaces, electrode field configuration and availability

7

of initial conducting particles [7,8]. Normally, air medium is widely used as an

insulating medium in different electrical power equipments and overhead lines as its

breakdown strength is 30kV/cm.

2.4 Theory of Breakdown in Gases

2.4.1 Townsend Mechanism

The Townsend theory explains the mechanism of breakdown in gases under certain

conditions. the below-mentioned theory is compiled from [9-11].

electrons exposed to an electric field accelerate in the opposite direction of the field,

towards the anode (the positive electrode). The field exerts work on the electron as

defined by Eq.(1), where e is the electron charge and x is the distance travelled along a

field line.

W = eEx = (½) mv2 (1)

The high velocity electron will have a certain probability of colliding with

molecules in the insulating gas. In an inelastic collision energy is transferred to the

molecule. this enables the molecule to be ionized and give up an electron, which in

turn will be accelerated by the electric field and maybe cause another ionizing

collision. the first Townsend coefficient gives the probability that one electron will

cause an ionizing collision per unit length, liberating an electron. with higher pressure,

the probability of collision is increased. the kinetic energy of the electron is

proportional to the applied field, and the coefficient (α) is therefore a function of the

fraction (E/p). The number of collisions is proportional to the pressure.

The second Townsend coefficient (γ) gives the probability of liberation of

secondary electrons. the secondary electrons can for instance, originate from the

cathode (the negative electrode), being triggered by positive ions impinging on the

cathode. In this case the ion must release two electrons, one to neutralize the ion

charge and one that is ejected as a secondary electron. this is the main secondary

process in the Townsend mechanism, and it is also a function of the electric field

strength and the gas pressure. photo emission may also contribute to secondary

electrons. the released electrons are accelerated towards the anode in an avalanche and

can possibly cause breakdown.

8

(2)

In electronegative gases, the molecules or atoms are able to absorb electrons to fill

their outer shell. the probability that an electron is attached to such a molecule per unit

length is given by the attachment coefficient . this process will hamper the flow of

electrons produced by the above-mentioned processes. taking all these processes into

account, the current at the anode can be described as in Eq.(2), the equation shows that

breakdown will happen if the denominator goes to zero, which defines the Townsend

breakdown criterion.[12]

2.4.2 Voltage-current relation

Before gas breakdown, there is a non-linear relation between voltage and current as

shown in the figure (2.1). In region 1, there are free ions that can be accelerated by the

field and induce a current. these will be saturated after a certain voltage and give a

constant current, region 2. region 3 and 4 are caused by ion avalanche as explained by

the Townsend discharge mechanism.

Figure (2.1): Voltage-current relation

http://en.wikipedia.org/wiki/Townsend_discharge

9

2.4.3 Streamer mechanism

Townsend mechanism when applied to breakdown at atmospheric pressure is found to

have certain drawbacks. firstly, according to the Townsend theory, current growth

occurs as a result of ionization processes only. But in practice, breakdown voltages

were found to depend on the gas pressure and the geometry of the gap and electrodes.

secondly, the mechanism predicts time lags of the order of 10-5 s, while in actual

practice breakdown is observed to occur at very short time of the order of 10-10s. The

Townsend mechanism failed to explain the observed phenomena and Streamer theory

is proposed. [13]

Figure (2.2): Field distortion due to presence of space charge

Figure (2.2) shows the electric field around the avalanche as it progresses along

the gap and the resulting modification in the applied field. for simplicity, the space

charge at the head of the avalanche is assumed to have a spherical volume containing a

negative charge at its top because of the high electron mobility. under these conditions,

the field gets enhanced at the top of the avalanche with field lines from the anodes

terminating at its head. further, at the bottom of the avalanche, the field between

electrons and ions reduces the applied field (E). still further down, the field between

cathode and the positive ions gets enhanced. Thus, the field distortion occurs and it

becomes noticeable with a charge carrier number n > 106. if a charge density in the

avalanche approaches n = 108 the space charge filled field and the applied field will

have the same magnitude and this leads to the streamer. thus space charge fields play

10

an important role in the growth of avalanches in the corona and spark discharges.

2.5 Physical properties of CO2

Carbon dioxide (CO2) is a colorless, odorless, non-flammable and slightly acidic

liquefied gas. CO2 is heavier than air and soluble in water.

CO2 is produced industrially by using sources of CO2 obtained through

processes in the petrochemical industry, or by burning natural gas in cogeneration

processes. Air products supplies CO2 to customers worldwide as a liquefied gas.

Table 2.1: Physical properties of Co2.[21]

Formula CO2

Molecular Weight (lb/mol) 44.01

Critical Temp. (°F) 87.9

Critical Pressure (psia) 1071.0

Boiling Point (°F) -109.2

Melting Point (°F) -69.9

Psat @ 70°F (psia) 852.8

Liquid Density @ 70°F 47.64

Gas Density @ 70°F 1 atm 0.1144

Specific Volume @ 70°F 1 atm 8.74

Specific Gravity 1.555

Specific Heat @ 70°F 8.92

2.6 Lightning impulse voltages

When high voltage equipment is tested with lightning impulse voltages, a number of

lightning impulse voltages of certain amplitudes and shape are applied. a lightning

impulse voltage is, currently, in IEC 60060-1 [14], defined by its peak value and its

time parameters. figure (3) shows the standardized lightning impulse voltage to the

peak value Up (U=1.0), the virtual origin O1, the front time T1 and the time to half

value T2.

11

Figure (2.3) Lightning impulse voltage characteristic

The impulse is usually produced by an impulse generator consisting essentially

of a number of capacitors that are charged in parallel from a direct voltage source and

discharged n series in a circuit that includes the test object. In this test will use single

stage lighting impulse can be applied is limited to around 140 kV.

Most lightning impulse tests are made using a standard lightning impulse, having

a front time of 1,2 µs and a tail time of 50 µs, described as a 1,2/50 µs lightning

impulse voltage or a standard lightning impulse.

Of course, when applying standard lightning impulse voltages, some tolerances

on the parameters are allowed. if not otherwise specified by the relevant technical

committee, the following differences are accepted between specified values for the

standard lightning impulse and those actually recorded:

Peak value ±3%

Front time ±30%

Tail time ±20%

2.7 Previous studies

Air breakdown was studied by Paraselli [15], used in this work to simulate the air

breakdown voltage experimentally in the high voltage laboratory, standard diameter of

25 cm spheres for measurement of air breakdown voltages and electric field of the high

12

voltage equipments. the above experiment is conducted at the normal temperature and

pressure. finite element method is also used for finding the electric field between

standard sphere electrodes. the relative air density factor and maximum electric field

are measured in MATLAB environment for different temperature and pressure. the

electric field distribution for sphere gap arrangements is also calculated with the help

of COMSOL. in addition to the influence of the humidity of air breakdown test has

been also considered in this studied. a humidity correction factor also considered in

this work for maintaining constant air breakdown voltage. finally, the experimental

result has been compared with theoretical, and simulation results.

In this studied the performance characteristics of air breakdown voltages and

electric field behaviors are studied theoretically as well as experimentally by using the

standard sphere gap method. the air breakdown characteristics between the sphere-

sphere electrodes are observed with variations in electrode arrangements, both in size

and spacing. it is concluded that with the increase of the gap between spheres the

breakdown voltage and electric field strength are increased and is inversely

proportional to sphere radius. maximum electric field and relative air density factor

characteristics are derived with different temperature and pressure. It is concluded that

with the increase of temperature the maximum electric field and relative air density

factor are decreased and with increase of pressure the maximum electric field and

relative air density factor are increased.

Table 2.2: measurment of breakdown voltage between sphere sphere electrodes. [15]

Sphere

Gap

(cm)

BDV

Experiment

(kV)

BDV

Theory

(kV)

BDV

Simulation

(kV)

Electric

field

Experiment

(kV/cm)

Electric

field

Theory

(kV/cm)

Electric

field

Simulation

(kV/cm)

1.0 19.5 21.92 33.65 19.50 21.92 33.65

1.5 30.0 32.17 40.32 20.00 21.44 26.88

2.0 37.0 41.71 53.12 18.50 20.85 26.56

2.5 49.0 51.40 59.27 19.60 20.56 23.70

3.0 58.0 60.81 65.25 19.34 20.27 21.75

3.5 65.0 70.00 71.07 18.57 20.00 20.30

4.0 4.0 79.19 76.74 18.50 19.79 19.18

4.5 82.0 88.38 82.26 18.23 19.64 18.28

5.0 85.0 97.58 87.63 17.00 19.51 17.52

5.5 97.0 99.77 92.86 17.63 19.41 16.88

BDV=Breakdown Voltage

13

Figure (2.4): Air breakdown characteristics at the sphere-sphere electrodes. [15]

The CO2 Atmosphere breakdown was studied by Matthew [16], this report

observed and analyzed the effect the pressure and gap distance have on the minimum

positive breakdown potential between two parallel plate copper electrodes in both air

and CO2. two gap distances, 0.57 cm and 2.44 cm, were used. Paschen Curves

generated in the air from these distances had a strong positive correlation with a

spearman correlation coefficient of 0.97. Curves generated in CO2 had a spearman

correlation coefficient of 0.87. The strong correlation for both gases verifies Paschen's

Law. the minimum breakdown potential in air was 361 ± 2 V at a pressure x gap

distance of 0.55 ± 0.01 Torr cm. the minimum breakdown potential differs from

published data by 2%. The pressure x gap distance differs from published data by 17%.

this verifies the Paschen breakdown apparatus and procedure. the minimum

breakdown potential in CO2 was 540 ± 20 V at a pressure x gap distance of 0.5 ± 0.1

Torr cm. the minimum breakdown potential differs from published data by 4%. the

pressure x gap distance differs from published data by 67%. differences between the

pressure x gap distance results and published data suggest that electric field

concentrations present at the edges of the electrodes cause the apparatus to behave as

though the electrodes are closer together.

14

Figure (2.5): Paschen Curves in air. [16]

Figure (2.6): Paschen Curves in Co2. [16]

15

Gas mixture was studied by M. S. Kamarudin [17], this paper described spark

over characteristics of the CF3I-CO2 gas mixture, with focus on 30%-70% mixture.

standard lightning impulse of 1.2/50 is used in this study, along with three different

electrode configurations with variable gap lengths. and for this test, U50 for CF3I-CO2

gas mixtures are obtained as according to the standard [17]. the up-and-down method

is used to determine U50 by applying at least 20 impulse shots at a timed interval of

120 seconds.

It is found that higher gap lengths provide higher U50 and Emax for a given

electrode configuration, although the rise in U50 is more obvious in sphere-sphere

configuration, due to more uniform electric field. breakdown strength also increases

with gas pressure, with tests being carried out up to 2 bar (abs). an increase in CF3I

content in CF3I-CO2 gas mixture also provides an increase in insulation strength. as

gas pressure, effects in CF3I content are more obvious under more uniform electric

field conditions.

Figure (2.7): Effect of gap length on breakdown voltage. [17]

16

Figure (2.8): Effects of gas pressure on breakdown voltage. [17]

Breakdown characteristics of N2O gas mixtures was stadied by Hiroki Kojima

[18], in this paper, focused on N2O gas as an SF6 substitute from the viewpoint of

electron attachment ability and low GWP. expected the effect in electrical insulation

characteristics of gas mixtures composed of N2O gas and other electronegative gases

such as CO2 and O2 gases as well as retardant gas such as N2 gas. they obtained the

positive and negative impulse BD V50 in binary, ternary and quaternary gas mixtures

of these gases under quasi-uniform electric field. the main results obtained are as

follows:

The significant synergistic effect of N2O gas with CO2 and O2 gases in breakdown

strength was verified. This result was interpreted by the positive between the

electronegative N2O, CO2 and O2 gases with the electron attachment ability at different

electron energy ranges.

The synergistic effect of the retardant N2 gas in N2O gas was confirmed. this is

explained by an activation of the electron attachment ability of N2O gas owing to the

barrier effect of vibrational excitation collisions of N2 gas.

In the case of quaternary gas mixtures, N2O 30% / CO2 30% / O2 10% / N2 30%

exhibited the highest BD V50, which corresponded to the increase by 35% compared

with BDV50 of pure N2O gas. This best mixture rate agreed with the estimated mixture

rate assuming the independent contribution of component gases to the improvement of

17

impulse BDV50.

Figure (2.9): Impulse BDV50 as a function of gas mixture rate in N2O / CO2 / O2 gas mixtures. (P = 0.1 MPa). [18]

2.8 Summary

In this chapter includes theories that define the breakdown in the gaseous electrical

insulators, and the physical properties of air and carbon dioxide (CO2), and also

includes the standard of lightning impulse generator and measurement with tolerance,

and addresses as well as previous studies on the effect breakdown of electrical

insulators gas or gas mixture.

18

CHAPTER 3

METHODOLOGY

3.1 Introduction

This chapter explains the method that will be used to complete the project. laboratory

work and simulation of software using COMSOL are in the first priority. this project

can be categorized to many parts, the setup of lighting impulse generator for testing,

and electrodes configuration, and gas setup, finally simulation, a simulation will give a

clearer representation of electric field occur to those electrodes.

Focus is given to gas (CO2). and test will carry out to investigate the effects of

three electrode configurations, which are (rod-plane, plane-plane, and sphere-sphere)

gaps, the gas (CO2), will subjected to positive lightning impulses. Also the effect gas

pressure at 0.5cm gap distance will investigate. Based on U50 results for parameter, by

used up and down test method, the electric field curves will determined to clarify the

effect of electric field on the breakdown of the gas[19].

3.2 Safety

“High voltage experiment can become particularly hazardous and dangerous to the

user especially to someone who does not possess experience dealing with high voltage.

voltage which is higher than 250 V to earth potential is understood to be a high voltage

and a current higher than 10 mA is enough to cause fatal injuries to body due to

19

electric shock such as skin burn, muscle damage and heart attack. the worst case

scenario is death. hence, safety factor need to take seriously, all instructions are

mandatory and extra careful is required when performing the test.

Before entering a high voltage setup area, all conductors which can be assume

high potential and lye in the contact zone must be sure are earthed and careful with the

wire lying on the floor. all the electrical potential need to be discharged to earth

potential using discharge rod provided in the lab, before and after the lab testing. the

discharge rod tip must be remain connected to the electric source potential of the

electrical component all the time when the lab is not in used. the discharge rod must be

isolated from the source potential during the test being conducted.

The high voltage setup must be protected from unintentional entry to the danger

zone. this is appropriately done with the aid of metallic fences covering the test area.

no one is allowed to be inside the test area during the test being conducted. It is also

forbidden to introduce conductive object or lean on to the fences while the setup is in

used. user need to make sure to keep distance from the fence by standing outside the

warning mark lines, yellow with black line.

In high voltage setup, each door must be provided with safety switches. these

allow the door to be opened only when all main leads to the setup are off. Instead of

direct interruption, the safety switches may also operate the no voltage relay of a

power circuit breakers, which on opening the cage door, interrupts all the main leads to

the setup. these power circuit breakers may also be switched on again by closing the

cage door. the switched condition of a setup must be indicated by a red lamp “set up

switched on” and by green lamp “setup switched off”. if the fence is interrupted for

assembly and dis mantling operations on the setup, or during large-scale modifications,

all the prescribed precautions for entry to the setup shall be observed. here, particular

attention must be paid to the reliable interruption of the main leads”[20].

20

3.3 Flow chart

NO

YES

Figure (3.1): Flow chart of experiment

start

Lightning impulse circuit setup

END

Carry out experiment

Collecting data and graph

Creating simulation using COMSOL

Electrode configuration

Circuit function

test

Gas setup

Modify the

circuit

21

3.4 Experiment Circuit

Impulse generators for high voltage testing have been generated exclusively in the

Marx circuit. the basic principle of this circuit is the rapid series connection of charge

capacitors whereby spark gaps are used to make the series switch, in this project, the

single stage generator circuit will be used in the experiment of the gas breakdown

impulse shown in Figure (3.2), while Table (3.1) shows component descriptions of

equipment used.

Figure (3.2): Circuit setup for generation of lightning impulse voltage

Figure (3.3): Real setup in high voltage laboratory

22

Table 3.1: Component used in the setup and its code number

Component Description Type No. Picture

HV Test Transformer HV9105

Control Desk HV9103

Smoothing Capacitor HV9112

Load Capacitor HV9120

Rectifier HV9111

Measuring Resistor HV9113

Charging Capacitor HV9121

23

Wavefront Resistor HV9122

Wavetail Resistor HV9123

Sphere Gap HV9125

Drive for Sphere Gap HV9126

Gas chamber HV9133

Connecting Rod HV9108

Connecting Cup HV9109

24

Floor Pedestal

HV9110

Spacer Bar HV9119

Top Electrode HV9138

Earthing Switch HV9114

DC Voltmeter HV9151

Impulse Peak Voltmeter HV9152

Trigger Unit HV9131

Trigger Sphere HV9132

62

Reference

[1] Pillai, A. S., & Hackam, R. (1983). Electric field and potential distributions for

unequal spheres using symmetric and asymmetric applied voltages. Electrical

Insulation, IEEE Transactions on, (5), 477-484.

[2] Mustafa, M. W., & Kadir, A. A. (2000). A modified approach for load flow

analysis of integrated AC-DC power systems. In TENCON 2000. Proceedings (Vol.

2, pp. 108-113). IEEE.

[3] Meek, J. M., Craggs. J. D, John Wiley & Sons (1978) Electrical Breakdown of

Gases.

[4] Qiu, X. Q., Chalmers. I. D, & Coventry. P.(1999). “A Study of Alternative

Insulating Gases to SF6,” Journal of Physics D: Applied Sciences.

[5] Cookson, A. H., & Pederson. B. O.(1979). “Analysis of the High-Voltage

Breakdown Results for Mixtures of SF6 with CO2, N2 and Air,” in Proc. 3rd Symp.

on High-Voltage Engineering.

[6] Jaeger, H. M., Nagel, S. R., & Behringer, R. P. (1996). Granular solids, liquids, and

gases. Reviews of Modern Physics, 68(4), 1259.

[7] Wadhwa, C. L. (2007). High Voltage Engineering, published by New Age

International.

[8] Phontusa, S., & Chotigo, S. (2008). “The proposed humidity correction factor of

positive dc breakdown voltage of sphere-sphere gap at h/δ lower than 13 g/m3”,

2nd IEEE International Conference on Power and Energy.

[9] Kuffel, E. Zaengel, W. S., & Kuffel, J. (2000). High voltage engineering

fundamentals. Oxford: Butterworth Heinemann.

[10] Ildstad, E. (2010). Electric power engineering: TET4160 High voltage insulating

materials. Trondheim: Department of Electric Power Engineering.

[11] Naidu, M., & Kamaraju, V. (1982). High Voltage Engineering. New Delhi:

McGraw-Hill.

[12] Jonathan. S. Jørstad, Effect of Barriers in Air Insulated Rod-Plane Gaps.

[13] Naidu, M.S., & Kamaraju, V. (2004). High Voltage Engineering’, published by

Tata McGraw-Hill 3rd edition.

[14] IEC: (1989). “High voltage test techniques, part 1: General definitions and test

requirements” IEC 60060-1.

63

[15] Paraselli, B. S. (2011). “measurement of air breakdown voltage and electric field

using standad sphere gap method”, by national institute of technology, rourkela.

[16] Stumbo, M. T. (2013). Paschen Breakdown in a CO2 Atmosphere (Doctoral

dissertation, California Polytechnic State University, San Luis Obispo).

[17] Kamarudin, M. S., hadda , A. & macgregor,S. J. (2014). “experimental

investigation of cf3i-co2 gas mixtures under lightning impulses”, by cardiff

university, cf24 3aa, cardiff, in UK.

[18] Kojima, H., Kinoshita, O., Hayakawa, N., Endo, F., & Okubo, H. “Breakdown

Characteristics of N2O Gas Mixtures for Quasi-uniform Electric Field under

Lightning Impulse Voltage”, by Nagoya University Furo-cho, Chikusa-ku Nagoya

464-8603, Japan.

[19] Mohamad, F. M. (2014). “study of air breakdown under lightning impulse using rod

to plane electrode configuration” by UTHM university in Malaysia.

[20] Saftey regulation for high voltage experiments, UTHM university laboratory.

[21] http://www.airproducts.com/ /physical-properties.

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