Elimination of SF From Transmission System Equipment

Elimination of SF6 From Transmission

System Equipment

A Thesis Resubmitted To

The University of Manchester

For The Degree of PhD

In

The Faculty of Engineering and Physical Sciences

2013

by

Xiaolei Cai

School of Electrical and Electronic Engineering

I

Contents

List of Tables ................................................................................................................................ IV

Table of Figures ........................................................................................................................... VI

List of Abbreviations ..................................................................................................................... XI

Nomenclature .............................................................................................................................. XII

Declaration ……………………………………………………………………………………………..XIII

Copyright …………………………………………………………………………………………….XIV

Abstract ……………………………………………………………………………………………….1

Chapter 1 Introduction ................................................................................................................ 2

1.1 Background of Project ................................................................................................... 2

1.2 Use of SF6 in Gas Insulated Substations ...................................................................... 6

1.3 Project Objectives and Thesis Structure ....................................................................... 9

Chapter 2 SF6 Gas and Equipment Design ............................................................................. 11

2.1 Properties of SF6 Gas ................................................................................................. 11

2.2 Electrical Performance of SF6 Gas ............................................................................. 13

2.3 The Use of SF6 in Circuit Breakers ............................................................................. 14

2.3.1 Benefits of SF6 Switchgear in Comparison to Oil or Air ...................................... 16

2.4 SF6 Use in Gas Insulated Substations ........................................................................ 17

2.5 Benchmark Busbar Design for Use in Future Studies ................................................ 19

2.6 Conclusion of SF6 Properties and Gas Insulated Equipment Study Model ................ 19

Chapter 3 Potential Liquid and Gaseous Replacements for SF6 as An insulation Medium .... 21

3.1 Electrical Breakdown in Liquid Dielectrics .................................................................. 21

3.2 Recent Developments in Oil Insulation ....................................................................... 22

3.2.1 Oil Insulation Performance .................................................................................. 22

3.2.2 Illustration of Possible Size of Oil Insulated Substation and Key Barriers to Its Use …………………………………………………………………………………………23

3.2.3 Summary of Oil Insulation ................................................................................... 25

3.3 Electrical Breakdown in Gas Dielectrics ..................................................................... 26

3.3.1 Gas Ionization and Attachment Processes ......................................................... 26

3.3.2 Townsend Breakdown in Electronegative Gas ................................................... 27

3.4 Review of Gas Alternatives to SF6 .............................................................................. 28

3.4.1 Gas Mixtures with Small Volume of SF6 ............................................................. 28

II

3.4.2 Electronegative Gas - CF3I.................................................................................. 30

3.4.3 High Pressure Air, N2 or N2O ............................................................................... 34

3.5 Conclusion of Liquid and Gaseous Replacement for SF6 ........................................... 35

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations ..................................... 37

4.1.1 Benchmark Design for Comparison and Discussion ........................................... 38

4.1.2 Fixed Size of GIS Enclosure with Changing Gas Pressure ................................ 46

4.1.3 Fixed Gas Pressure with Changing Busbar/outer Shell Dimensions .................. 50

4.1.4 Impact of Reduced Basic Insulation Level .......................................................... 51

4.2 Analysis of the Use of Coatings Combined with Gas Insulating ................................. 54

4.2.1 Review of Voltage Capability of Insulation Coatings ........................................... 55

4.2.2 Assessment of Use of Coatings around Conductors .......................................... 56

4.2.3 Review of Current Carrying Capability of Coated Conductor ............................. 57

4.3 Conclusion of Alternative Gas Usage ......................................................................... 60

Chapter 5 Evaluation of Foam As a Replacement for SF6 as an Insulation Medium .............. 62

5.1 Electrical Breakdown in Solid Insulation ..................................................................... 62

5.2 Solid Polyurethane Foam Insulation ........................................................................... 64

5.2.1 Electrical Breakdown in Polyurethane Foam ...................................................... 64

5.2.2 Review of Use of Foam in HV Applications ........................................................ 66

5.3 Experimental Study of Polyurethane Foam ................................................................ 69

5.3.1 Production of Polyurethane Foam ....................................................................... 69

5.3.2 Measurement of Relative Permittivity.................................................................. 70

5.3.3 Measurement of Thermal Conductivity ............................................................... 72

5.3.4 Analysis of Void Distribution ................................................................................ 73

5.3.5 Measurements of Partial Discharge Inception Voltages ..................................... 76

5.3.6 Measurement of AC Voltage Strength of Filled Samples ................................... 78

5.3.7 Measurement of Lightning Impulse Strength ...................................................... 81

5.3.8 Estimation of Size of Busbar Dimensions ........................................................... 83

5.3.9 Summary of Foam Insulation .............................................................................. 84

5.4 Conclusions Including a Comparison of Alternative Insulation Materials ................... 85

Chapter 6 Potential Replacements for SF6 as An Interruption Material ................................... 87

6.1 Physics of Circuit Breaker Switching .......................................................................... 87

6.1.1 Arc in Circuit Breaker .......................................................................................... 88

6.1.2 Current Chopping ................................................................................................ 88

6.1.3 Switching Transients in Network ......................................................................... 90

6.2 Comparison of Existing Interruption Systems ............................................................. 90

6.3 Review of Alternative Circuit Breaker Types ............................................................... 93

6.3.1 Oil Circuit Breakers ............................................................................................. 93

III

6.3.2 Air Blast Circuit Breakers .................................................................................... 95

6.4 Gas Circuit Breakers ................................................................................................... 97

6.4.1 Background ......................................................................................................... 97

6.4.2 Effect of PTFE Ablation on Puffer Circuit Breaker .............................................. 98

6.4.3 SF6-Free Gas Circuit Breaker ........................................................................... 100

6.5 Conclusion of Existing Non-SF6 Circuit Breaker ....................................................... 102

Chapter 7 Vacuum Circuit Breaker ........................................................................................ 103

7.1 Introduction to VCB Technology ............................................................................... 103

7.2 Recent Development of Vacuum Circuit Breaker ..................................................... 104

7.2.1 Contact Material ................................................................................................ 104

7.2.2 Contact Structure .............................................................................................. 106

7.2.3 Control of X-ray Emission ................................................................................. 109

7.2.4 Review of Transmission Voltage Vacuum Circuit Breakers .............................. 110

7.3 Development of Multiple Interrupters Based VCBs................................................... 111

7.4 Transient Voltage Simulations Using a Vacuum Circuit Breaker Model ................... 113

7.4.1 Transient Recovery Voltage .............................................................................. 114

7.4.2 Prestrike, Restrike and Reignition Phenomena ................................................ 114

7.4.3 Chopping Overvoltage and Reignition Mechanism ........................................... 115

7.4.4 Non-sustained Disruptive Discharges in VCBs ................................................. 117

7.5 Simulation Studies on Multiple VCBs in HV Network................................................ 118

7.5.1 Literature Review on VCB Modeling ................................................................. 118

7.5.2 HV VCB in Three Phase System - Model Building ........................................... 120

7.5.3 Simulation Study - Switching off Inductive Current ........................................... 128

7.5.4 Statistical Study of Developed HV VCB Model ................................................. 139

7.6 Summary of Vacuum Circuit Breaker ........................................................................ 146

7.7 Conclusion of Alternative Switchgear ....................................................................... 147

Chapter 8 Conclusions ........................................................................................................... 150

8.1 Alternatives for SF6 Gas Insulated Substation .......................................................... 150

8.1.1 CF3I Gas Insulation ........................................................................................... 150

8.1.2 Solid Combined Insulation ................................................................................ 151

8.1.3 Particle Impacts in the Gas ............................................................................... 152

8.1.4 Polyurethane Foam Insulation .......................................................................... 152

8.2 Alternatives for SF6 Gas Circuit Breakers ................................................................. 153

Chapter 9 Future Work ........................................................................................................... 155

Appendix ...……………………………………………………………………………………………..157

References …………………………………………………………………………………………….161

IV

LIST OF TABLES

Table 1.1 - Projected Emissions Of Non-CO2 Greenhouse Gases For England To 2020 [6] .... 4

Table 1.2 - Space reduction analysis with 550kV AIS/H-GIS/GIS [11] ....................................... 7

Table 1.3 - Typical required ratings for transmission system circuit breakers [16] ..................... 9

Table 2.1 - Summary of the key characteristics of SF6 gas ...................................................... 12

Table 3.1 - Comparison among mineral oil, midel 7131 and FR3 ............................................ 22

Table 3.2 - Comparison between oil insulation and SF6 insulation ........................................... 23

Table 3.3 - Physical properties of CF3I and SF6 ....................................................................... 31

Table 3.4 - Characteristics of SF6-free gas ............................................................................... 34

Table 4.1 - Optimal working pressure and temperature of alternatives working at fixed

dimension d1/d2/d3=125mm/490mm/500mm ............................................................................. 49

Table 4.2 - Fixed pressure at 0.3MPa, varied outer dimension ................................................ 50

Table 4.3 - Minimum particle size that will not move from floor of 400kV busbar systems with

alternative insulation ................................................................................................................. 51

Table 4.4 - Dimension reduction with lower BIL........................................................................ 54

Table 4.5 - Characteristics of different coating materials .......................................................... 55

Table 4.6 - Current rating for busbar in existing dimension but with 10mm coating ................. 59

Table 4.7 - New sheath size and respective current rating when the pressure is fixed at

0.3MPa (coated conductor ‘r’=72.5mm) .................................................................................... 59

Table 5.1 - Thermal Conductivity of filled foam with different concentrations .......................... 72

Table 5.2 - Influence of mixing ratio of two foam liquids ........................................................... 75

Table 5.3 - Test result of foam samples with large gap distances (5mm- 30mm) .................... 77

Table 5.4 - Specification of HFCT100/50 [106] ......................................................................... 78

Table 5.5 - 50% AC Breakdown voltage depending on gap distance and filler mixed ratio ..... 79

Table 5.6 - 5% AC Breakdown voltage depending on gap distance and filler mixed ratio ....... 79

Table 5.7 - Estimation of foam insulated busbar dimension ..................................................... 83

Table 5.8 - Comparison of all insulating material candidates ................................................... 86

V

Table 6.1 - Brief overview comparing the performance of vacuum, SF6, air and oil [111] ........ 92

Table 6.2 - Air compressors running time record ...................................................................... 96

Table 7.1 - Chopping current of electrode contactors [42, 132-134] ...................................... 105

Table 7.2 - Comparison of contact structure ........................................................................... 108

Table 7.3 - 126kV VCB rating [139] ........................................................................................ 110

Table 7.4 - Different settings on characteristics of vacuum circuit breakers and their influences

................................................................................................................................................ 129

Table 7.5 - Result comparison with extra capacitance when dv/dt=490V/us, di/dt=400A/us . 139

Table 7.6 - Result comparison with reactive compensation when dv/dt=240V/us,

di/dt=1000A/us ........................................................................................................................ 139

Table 7.7 - Parameter setting for statistical study ................................................................... 140

Table 7.8 - Comparison of all interrupting candidates ............................................................ 149

VI

TABLE OF FIGURES

Figure 1.1 - Ranking of sources of fluorinated gas production according to sector (given in

megatons of CO2 equivalent) [5] ................................................................................................. 3

Figure 1.2 - Relative proportion of projected emissions by sector in 2010 [7] ........................... 4

Figure 1.3 - Greenhouse gas emission projections to 2022 [9] .................................................. 5

Figure 1.4 - Air insulated and gas insulated substations ............................................................ 7

Figure 1.5 - Substation building area analysis on 550kV AIS/H-GIS/GIS [11] ........................... 7

Figure 1.6 - Wakefield substation – new GIS substation in building replaces old outdoor air

insulated equipment .................................................................................................................... 8

Figure 2.1 - Molecule arrangement of SF6 ................................................................................ 11

Figure 2.2 - Dielectric strength of SF6 in comparison to air, oil and vacuum [18] ..................... 13

Figure 2.3 - Negative lightning impulse (LI), negative switching impulse (SI), and 60Hz AC

breakdown voltages of N2/SF6 gas mixtures [19, 20] ................................................................ 14

Figure 2.4 - Interruption capability of air and SF6 [21] .............................................................. 14

Figure 2.5 - Puffer and rotating arc circuit breaker types-better quality [18] ............................. 15

Figure 2.6 - Deionisation time constants of SF6 in comparison to other gases [18] ................. 16

Figure 2.7 - Oil, Air Blast and SF6 circuit breakers (from left to right) [24] ................................ 17

Figure 2.8 - 420kV GIS sectional view of a bay with double busbar system [25] ..................... 18

Figure 3.1 - Power frequency design curves (a) bulk oil (b) creepage along pressboard [72, 73]

.................................................................................................................................................. 24

Figure 3.2 - Withstand voltage depending on the gap distance in coaxial cylinder model ....... 25

Figure 3.3 - (α– η)/P and E/P relationship in SF6 [42] .............................................................. 27

Figure 3.4 - Minimum dimension for different gas pressures as a function of SF6 content [21]29

Figure 3.5 - Breakdown voltage influenced by percentage of SF6 in N2 at different pressure [47]

.................................................................................................................................................. 29

Figure 3.6 - Minimum tank diameter as a function of SF6 content [21] ..................................... 30

Figure 3.7 - Molecule arrangement of CF3I [3] .......................................................................... 30

Figure 3.8 - The density-normalized effective ionization coefficients (α-η)/N for CF3I and in the

CF3I-N2 mixtures with 5%,10%, 20%, 50%, 70% CF3I as a function of E/N [53] ..................... 32

Figure 3.9 -The limiting or critical field strength, E/Nlim for the CF3I-N2 mixtures [53] ............... 32

VII

Figure 3.10 - Positive polarity breakdown voltage characteristics of CF3I-CO2 mixture [54] .... 33

Figure 3.11 - Comparison of dielectric strength of various gases [59] ..................................... 35

Figure 4.1 - Conductor and tank diameters for different SF6 contents with consideration of

electric field strength on conductor surface and temperature rise at different pressure [20] .... 37

Figure 4.2 - Example of possible defects in GIS [62] ................................................................ 39

Figure 4.3 - Probability of breakdown for a gap containing a rod-like particle as a function of

particle length and particle (rod) radius [61] .............................................................................. 39

Figure 4.4 - 50Hz flashover characteristics of epoxy resin conical spacers under varying

degrees of metallic contamination [68] ..................................................................................... 40

Figure 4.5 - Limit of lightning impulse withstand capabilities of epoxy resin conical spacers

under clean and contaminated conditions [69] ......................................................................... 40

Figure 4.6 - Particle stable at bottom of busbar enclosure ....................................................... 41

Figure 4.7 - Particle radius influence to the electric field stress withstand ability ..................... 42

Figure 4.8 - Single conductor busbar model based on that found in National Grid substations

.................................................................................................................................................. 43

Figure 4.9 - The dependences of temperature of conductor and sheath on the current rating of

SF6 gas insulated busbar .......................................................................................................... 45

Figure 4.10 - Current carrying capability with different sheath dimension (fixed conductor) .... 46

Figure 4.11 - Current rating of oil insulated system depending on the insulation thickness ..... 47

Figure 4.12 - Simulation model with surge arresters installed .................................................. 52

Figure 4.13 - Comparison between the peak voltage with and without SA .............................. 53

Figure 4.14 – GIS busbar with coating on conductor(a) or inner surface of sheath(b) ............. 54

Figure 4.15 - Comparison of dielectric strength of coated electrode in air compared with pure

air and SF6/N2 mixed gas [59] ................................................................................................... 55

Figure 4.16 - Electric field distribution with varied thickness of solid insulation ........................ 57

Figure 4.17 - Calculated coating surface temperature at different carried current ................... 58

Figure 5.1 - Mechanisms of failure and variation of breakdown strength in solids with time of

stressing [92] ............................................................................................................................. 63

Figure 5.2 - Equivalent circuit of dielectric with voids ............................................................... 65

Figure 5.3 - Electric field distribution considering a small void in the foam .............................. 65

Figure 5.4 - Paschen’s law for air [96] ...................................................................................... 66

Figure 5.5 - Example of foam insulation from High Voltage Jocyln [97] ................................... 67

VIII

Figure 5.6 - Dependence of AC breakdown voltage(a) and LI breakdown voltage(b) on foam

thickness [94] ............................................................................................................................ 68

Figure 5.7 - Circuit of relative permittivity measurement .......................................................... 70

Figure 5.8 - Plane - plane electrodes arrangement .................................................................. 70

Figure 5.9 - Relative permittivity of filled foam at various concentrations ................................. 71

Figure 5.10 - The dependence of rating current on insulation thickness of foam ..................... 73

Figure 5.11 - Void Distribution in pure foam ............................................................................. 74

Figure 5.12 - Void distribution in different filler-concentrated foams ........................................ 74

Figure 5.13 - Void size of foam with different mixing ratio ........................................................ 76

Figure 5.14 - Partial discharge measurement circuit ................................................................ 77

Figure 5.15 - High frequency current transformer sensor [106] ................................................ 78

Figure 5.16 - Sphere - sphere electrodes fixed in cured foam .................................................. 79

Figure 5.17 - Test cell constructed with a pair of mushroom electrode .................................... 80

Figure 5.18 - AC breakdown voltage of foam at varied gap distances ..................................... 80

Figure 5.19 - AC dielectric strength under varied testing gap distances .................................. 81

Figure 5.20 - Lightning impulse test circuit ............................................................................... 81

Figure 5.21 - Full lightning impulse without oscillations [108] ................................................... 82

Figure 5.22 - The LI breakdown strength versus gap distance................................................. 82

Figure 5.23 - Cross section view of foam after breakdown test ................................................ 83

Figure 6.1 - Interruption performance of circuit breaker [110] .................................................. 87

Figure 6.2 - Current chopping level influenced by the system capacitance [4] ........................ 89

Figure 6.3 - The main current interruption techniques and their field of application [29] .......... 91

Figure 6.4 - The arc interruption theory of oil circuit breaker .................................................... 93

Figure 6.5 - Viscosity of different insulating liquids [114] ......................................................... 95

Figure 6.6 - Representation of electric arc in air blast circuit breaker ...................................... 96

Figure 6.7 - Four stages of puffer action-single flow [116] ........................................................ 98

Figure 6.8 - Schematic diagram of a prototype interrupter unit [122] ....................................... 99

Figure 6.9 - Arc voltage and current waveforms of 9kA arc with (a) minimum (b) maximum

pressurization inside the expansion chamber [122] ................................................................ 100

Figure 6.10 - Interruption performance to CF3I ratio of CF3I mixture gas [129] ...................... 101

IX

Figure 6.11 - Density of fluorine form CF3I and SF6 [129] ....................................................... 101

Figure 6.12 - Density of iodine from CF3I and CF3I-CO2(30%-70%) [129] ............................. 102

Figure 7.1 - Vacuum interrupter chamber in vacuum circuit breaker [130] ............................. 103

Figure 7.2 - Effect of thermal properties of cathode on arcing voltage [131] .......................... 105

Figure 7.3 - Comparison of interruption capability for vacuum interrupters as function of

electrode diameter and magnetic field type [4] ....................................................................... 109

Figure 7.4 - AC withstand strength of single and double break vacuum circuit breaker [146] 112

Figure 7.5 - Comparison of breakdown probability between double and single break [147] .. 112

Figure 7.6 - Comparison between re-ignition and restrike [155] ............................................. 114

Figure 7.7 - Current chopping phenomenon and TRV waveform [104] ................................. 115

Figure 7.8 - Chopping overvoltage of circuit breaker and its respective rated voltage [157] .. 116

Figure 7.9 - Measured voltage across VCB at the occurrence of NSDD in phase 1 and 3 [159]

................................................................................................................................................ 117

Figure 7.10 - Test circuit of J. Helmer [163] ............................................................................ 119

Figure 7.11 - TRV characteristic of 33kV circuit breaker [165] ............................................... 119

Figure 7.12 – Line diagram of 400kV system model .............................................................. 120

Figure 7.13 - 3-phase simulation mode without load connected in ATP/EMTP ..................... 120

Figure 7.14 - LCC model setting dialog in ATP/EMTP ........................................................... 122

Figure 7.15 - Layout of 3 phase busbars above the ground ................................................... 123

Figure 7.16 - Configuration of busbar in ATP/EMTP .............................................................. 123

Figure 7.17 - Line inductance connection and its value .......................................................... 123

Figure 7.18 - One phase simplified circuit of no-load transformer .......................................... 125

Figure 7.19 - Working principle of MODEL controlled breaker device in ATP/EMTP ............. 127

Figure 7.20 - Circuit when no loaded transformer at circuit terminal ...................................... 129

Figure 7.21 - Case 1: switching performance of circuit breakers when di/dt=100A/us,

dv/dt=240V/us ......................................................................................................................... 130

Figure 7.22 – Three-phase current waveforms of Case 1 ...................................................... 131


dv/dt=240V/us ......................................................................................................................... 132


dv/dt=240V/us ......................................................................................................................... 133

X


dv/dt=490V/us ......................................................................................................................... 134

Figure 7.26 - case 4: switching performance of circuit breaker at phase A breakers when

di/dt=400A/us, dv/dt=490V/us ................................................................................................. 135


dv/dt=490V/us ......................................................................................................................... 136


dv/dt=600V/us ......................................................................................................................... 137

Figure 7.29 - Extra capacitance Cr installed at terminal end of transformer .......................... 139

Figure 7.30 - Case 1: Fixed opening time, fixed di/dt =400A/us, dv/dt=490V/us, random

chopping current between 2-6A .............................................................................................. 141

Figure 7.31 - Current and voltage waveform at load terminal without switching .................... 142

Figure 7.32 - Case 2: Fixed di/dt =1000A/us, fixed dv/dt=490V/us, random opening time, and

random chopping current at 2-3A ........................................................................................... 143


random chopping current ........................................................................................................ 144


random chopping current ........................................................................................................ 145

XI

LIST OF ABBREVIATIONS

AC: Alternating Current

AMF: Axial Magnetic Field

BIL: Basic Insulation Level

CF3I: Trifluoroiodomethane

COP3: Conference of Parties III

DC: Direct Current

EU: ETS: European Emissions Trading System

GIS: Gas Insulated Substation

GWP : Global Warming Potential

HF: High Frequency

HFCT: High Frequency Current Transformer

H-GIS: Hybrid Gas Insulated Substation

HV High Voltage

LCC: Life Cycle Cost

LI: Lightning Impulse

NSDD: Non-sustained disruptive discharge

PD: Partial Discharge

PDIV : Partial Discharge Inception Voltage

PTFE: Polytetrafluorothylene

PU : Polyurethane

r.m.s.: Root mean square

RMF: Radial Magnetic Field

RRDS: Rate of Recovery of Dielectric Strength

RRRV: Rate of Rise of Restriking Voltage

SF6 : Sulphur Hexafluoride

SI: Switching Impulse

TMF: Transverse Magnetic Field

TRV: Transient Recovery Voltage

VCB: Vacuum Circuit Breaker

VI: Vacuum Interrupter

XII

NOMENCLATURE

A1 / A2 Radiation areas of the surfaces

C Capacitance

d Gap distance

d1 / d2 Outer conductor and inner sheath diameters

Dx Electric flux

E Local electric field strength

F Frequency

G Gravity constant

I Current carried by conductor

Ich Chopping current

k Mixing ratio

K0 Gas constant of the filled gas in GIS

l Particle length

L Inductance

M Mass of the particle

P Pressure

Q Charge (unit: pc)

R Radius of particle

r0 Outer radius of the GIS

Rac AC resistance of metal

ri Inner radius of the GIS

t Coating thickness around conductor

T1 / T2 Conductor and sheath temperatures

Tb Boiling point of pure gas

Tmb Boiling point of mixing gas

U Peak voltage

V Voltage

Wcond Heat flow by conduction

Wconv Heat flow by convection

Wrad Heat flow by radiation

α Ionization coefficient

αeff Effective ionisation factor

ε Stefan-Boltzmann constant 5.69E-8

ε0 Permittivity of free space

ε1 / ε2 Relative permittivity

η Attachment coefficient

π 3.1416 pure number

ρ Density

Φ Phase angle

XIII

DECLARATION

That no portion of the work referred to in the thesis has been submitted in support of an

application for another degree or qualification of this or any other university or other institute of

learning.

XIV

COPYRIGHT

i. The author of this thesis (including any appendices and/or schedules to this

thesis) owns certain copyright or related rights in it (the “Copyright”) and

s/he has given The University of Manchester certain rights to use such

Copyright, including for administrative purposes.

ii. Copies of this thesis, either in full or in extracts and whether in hard or

electronic copy, may be made only in accordance with the Copyright,

Designs and Patents Act 1988 (as amended) and regulations issued under it

or, where appropriate, in accordance with licensing agreements which the

University has from time to time. This page must form part of any such

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iii. The ownership of certain Copyright, patents, designs, trade marks and

other intellectual property (the “Intellectual Property”) and any

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tables (“Reproductions”), which may be described in this thesis, may not

be owned by the author and may be owned by third parties. Such

Intellectual Property and Reproductions cannot and must not be made

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relevant Intellectual Property and/or Reproductions.

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the University IP Policy (see

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The University Library’s regulations (see

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University’s policy on Presentation of Theses

XV

Acknowledgements

I would like to express my sincere appreciation to my supervisor Professor Ian Cotton who

gave me the opportunity to be involved in such a wonderful project. The work can’t be run

smoothly without his constant guidance to my research and his great effort in running the

projects.

Furthermore I would like to give special thanks to my PhD advisor Professor Vladimir Terzija

who gave me great help and support during my study.

I also wish to thank National Grid, in particular Dr. Paul Coventry and Dr. Dongsheng Guo, for

their support and cooperation during the work that is associated with this report.

I also want to thank my dear husband Dr. Jie Dai for the company and support during my PhD.

Last but not least, I wish to take this opportunity to thank my parents who gave me the

opportunity to study in the UK with great support and their selfless love.

Abstract

1

ABSTRACT

Sulphur hexafluoride gas is the dominant insulation and interruption material in high voltage gas insulated substation. Its usage remains a concern of transmission system operators owing to the global warming potential of the gas. The work carried out in this thesis aims to find the environment-friendly materials that can replace SF6. These candidates are required to have a strong dielectric strength for high voltage busbar insulation and well arc extinguishing capability necessary for high voltage circuit breaker.

A range of alternative insulation types including CF3I gas and its mixture, high pressure air and solid insulating foam are considered as substitute of SF6. Theoretical studies on the dimensions of busbars used in substations are carried out for these options. The dimension of the dielectric system and its ampacity of respect system are calculated using heat transfer models considering their boiling point and proper working pressure which is related with the dielectric strength of some gas.

On the other hand, SF6 gas circuit breaker is extremely popular on the medium and high voltage power networks owning to its effective arc extinguishing performance. It would be ideal if a substitute material could be found for SF6 as an interruption material. Biodegradable oil PTFE ablation, other gas candidates including N2, CF3I are investigated as possible replacement of SF6 through literature study.

The usage of vacuum circuit breaker is eventually capable to operate in high voltage transmission system. Simulations have been carried out with software ATP/EMTP to investigate the influence of different characteristics of vacuum circuit breaker including chopping current level, the dielectric strength of vacuum gap and the opening time. And then the probability of overvoltages when vacuum circuit breakers installed is studied by statistical study in MATLAB.

Name of the University: The University of Manchester

Candidate’s Name: Xiaolei Cai

Degree Title: PhD

Thesis Title: Elimination of SF6 from transmission system equipment

Data: 10/05/2013

Chapter 1 Introduction

2

Chapter 1 INTRODUCTION

Sulphur hexafluoride (SF6) is an insulating gas material widely used in power system

equipment because of its stable physical and superior electrical properties. However,

environmental concerns have been raised after large amounts of the gas have been used in a

number of different industries as well as in high voltage substations. The concern relates to its

impact on global warming given it is a potent greenhouse gas.

1.1 Background of Project

Since the invention of SF6 gas in 1900, the number of applications of this gas significantly

advanced from around 1940, and it became commercially available in 1947. The issue

surrounding the use of SF6 in the electrical industry has been reviewed and summarised by

Christophorou, Olthoff and Brunt [1], and concluded that it is one of the most extensively and

comprehensively used gases in a range of commercial and research applications. Besides the

use of SF6 by the electric power industry, other uses include: semiconductor processing,

blanket gas for magnesium refining, reactive gas in aluminium recycling to reduce porosity,

thermal and sound insulation, airplane tires, "air-sole" shoes, scuba diving, voice

communication, leak checking, atmospheric trace gas studies, ball inflation, torpedo propeller

quieting, wind supersonic channels and insulation for AWACS radar domes.

The basic physical and chemical properties of SF6 have been significantly studied. At normal

temperature, it is a dense gas, chemically inert, non-flammable, non-explosive, does not

degrade at photolytic, and does not decompose in the gas phase at temperatures above

500C.

Due to the molecular structure of SF6 - with six atoms of fluorine to one atom of sulphur, SF6 is

a strongly electronegative gas, which means it is attractive to electrons, both at room

temperature and at temperatures well above ambient. This property principally accounts for its

relatively high dielectric strength and good arc-interruption properties. The breakdown voltage

of SF6 is nearly three times higher than air at atmospheric pressure. The detailed character of

electronegative gas is explained in following chapter.

Because of the ability to construct equipment which can be easily maintained and the fact that

is shows extremely good performance as both insulation and as an interruption medium,

significant amounts of electrical equipment uses SF6 technology and gas insulated substations

(GIS) are used owning to their advantage of being small in area and having an excellent

performance.


3

However, the stability of SF6 also means it makes a cumulative and virtually permanent

contributor to global warming. There is no clear accurate estimate of the time it takes for a

given quantity of SF6 released into the atmosphere to be reduced via natural processes to

approximately 37% of the original quantity. The uncertainty relates to a lack of knowledge

concerning the pre-dominant mechanism of its destruction but all estimated point to a

timescale between 800 years to 3,200 years [2]. The strong infrared absorption capability of

SF6 and its long life-time in the environment are the reasons for its extremely high global

warming potential, which for a 100-year horizon is estimated to be approximately 25,000 times

greater than that of CO2. SF6 is one of the greenhouse gases targeted by the Kyoto Protocol,

which is an international agreement linked to the United Nations Framework Convention on

Climate Change (UNFCCC).

SF6 also reacts with carbon dioxide to produce carbon tetra fluoride CF4 [3] when heated up to

around 400°C. This is a stable greenhouse gas with a GWP value of 6,500 relative to CO2.

Furthermore, although not linked directly to global warming, during the interruption process

within a circuit breaker, SOF2, SO2F2, SF4 are produced and cannot be recombined [4]. These

gases quickly react with moisture to yield, SO2, and other more stable oxyfluorides which are

harmful to environment.

A report for the European Commission [5] on the use of fluorinated gases (F-Gases) deals

with the usage of Hydrofluorocarbons (HFCs), Perfluorocarbons (PFCs) and Sulphur

Hexafluoride (SF6) in the production of switchgear, double-glazing and tyres.

Figure 1.1 - Ranking of sources of fluorinated gas production according to sector (given in

megatons of CO2 equivalent) [5]


4

Figure 1.1 is taken from the report and shows that the production and use of SF6 switchgear is

a contributor to the total emissions of fluorinated gases. The report recognises the voluntary

controls set up by the European power industry to minimise and monitor SF6 emissions. It also

details the significant non-essential use of SF6 in the double glazing and car tyre production

industry in a small number of EU states. While it recommends better control of SF6 emissions

in switchgear, in terms of replacement of SF6 the following statement is given with regards

future actions: ‘Alternative arc quenching technologies in some mid-voltage applications

(limited potential)’. This statement therefore indicates that it is considered technically

unfeasible to consider replacement of SF6 in transmission voltage switchgear.

A report by W S Atkins for DEFRA [6] forecasts the emissions of non-CO2 gases in terms of

their megaton CO2 equivalents. SF6 emissions are shown to be largely stable and small in

comparison to emissions of methane, nitrous oxides and HFCs. Figure 1.2 and Table 1.1 give

specific data on this. They note in the report that the amount of SF6 emission is likely to rise by

around 10% but this is a very small contributor to the overall UK emission levels. It is also

noted that significant quantities of SF6 gas is emitted by industries other than the electricity

supply sector, namely electronics and magnesium smelting.

Figure 1.2 - Relative proportion of projected emissions by sector in 2010 [7]

MtC equiv. 1990 1995 2000 2005 2010 2015 2020

Methane 16.0 12.2 10.1 8.9 8.1 7.5 7.1

Nitrous Oxide 14.5 11.8 8.2 8.3 8.5 8.6 8.8

HFCs 4.1 1 4.1 2.3 2.8 2.5 2.6 2.7

PFCs 0.1 1 0.1 <0.1 <0.1 <0.1 <0.1 <0.1

SF6 0.3 1 0.3 0.3 0.3 0.3 0.3 0.3

Total non-CO2 34.9 28.5 21.0 20.3 19.4 19.0 18.9

Change in non-CO2

GHGs from 1990 - -18.6% -40.0% -41.8% -44.6% -45.5% -45.9%

1 1995 Baseline emissions

Table 1.1 - Projected Emissions Of Non-CO2 Greenhouse Gases For England To 2020 [6]


5

The emission/usage of SF6 now has to be considered as part of the wider issues connected

with the move to a low carbon economy. The global warming potential of HFC gas is in the

range of 97 to 12,000, and range of 5,700 - 11,900 for PFCs [8] while SF6 has a global

warming potential that is 23,900 times that of carbon dioxide. If no measures were taken to

reduce emissions of these gases, the estimate of a directive of the European Commission

released at 2004 is that the emissions of all fluorinated gases would increase to 98 million

tonnes of CO2 equivalent in 2010 which is 50% more than that of 1995 [7]. As the EU moves to

reduce CO2 levels, failure to control the level of SF6 emissions would mean this would account

for an ever increasing proportion of greenhouse gas emissions.

In a HM Government Report [9], UK Climate Projections carried out in 2009 show that past

emissions are likely to make summers over 2°C warmer in southern England by the 2040s –

more than enough to affect the way we live and work. The Climate Change Act of 2008 made

the UK the first country to introduce a long-term legally binding framework to tackle climate

change with targets in legislation and five year carbon budgets. The Act requires the UK to

reduce its greenhouse gas emissions by at least 34% below 1990 levels over the third budget

period (2018 to 2022) and by at least 80% by 2050. Such actions will facilitate a cut in

emissions by around 700 million tonnes of CO2 equivalent over the period from 2008 to 2022.

The projected emissions are shown in Figure 1.3.

Figure 1.3 - Greenhouse gas emission projections to 2022 [9]

(Actual data up to 2008. For 2009 onwards data are based on projections under a central

scenario)

Launched in 2005, the European Emissions Trading System (EU ETS) works on the "cap and

trade" principle. This means there is a "cap", or limit, on the total amount of certain


6

greenhouse gases that can be emitted by the factories, power plants and other installations in

the system. Within this cap, companies receive emission allowances which they can sell to or

buy from one another as needed. The limit on the total number of allowances available

ensures that they have a value. The ETS covers CO2 emissions from installations such as

power stations, combustion plants, oil refineries and iron and steel works, as well as factories

making cement, glass, lime, bricks, ceramics, pulp, paper and board. When SF6 is largely

used in power stations, it will bring huge amount of CO2 equivalent emission if heavy leakage

happened.

There is therefore significant political and legal pressure which pushes companies using SF6

to see if viable alternatives are available. This work examines the feasibility of a number of

options.

1.2 Use of SF6 in Gas Insulated Substations

Gas insulated substations are used in locations where one or more of the following criteria is

fulfilled:

Limited space is available for a substation (or substation expansion). There is

therefore a concentration of SF6 gas insulated switchgear around cities / towns

An underground substation must be used

There are concerns regarding visual impact

It is not the policy of most transmission system operators to utilise gas insulated switchgear as

a general preference since it is generally more expensive to install and maintain than the air

insulated equivalent. On the other hand, GIS is likely to be chosen in preference to Air

Insulated substations (AIS) possibly due to reduced land requirements and reduced profiles

meaning it is easier to gain planning consent. Although the total capital cost of a high voltage

GIS equipment is higher than that of conventional AIS, the whole cost of a GIS including land

cost is lower, especially at voltages greater than 500kV, and the cost is reduced further from a

long-term perspective [10].


7

Figure 1.4 - Air insulated and gas insulated substations

A SF6 insulated substation offers advantages in terms of footprint. This is very useful when

building a substation in a restricted space. A comparison of three kinds of 550kV substation is

shown in Figure 1.5 and Table 1.2. A combined substation named hybrid GIS is mentioned,

which means both air and SF6 are used as insulation medium at different locations in the HV

substation.

Figure 1.5 - Substation building area analysis on 550kV AIS/H-GIS/GIS [11]

AIS H-GIS GIS

Area (m2) 5,696 (100%) 3,776 (66%) 882 (15%)

Volume (m2) 227,840 (100%) 151,040 (66%) 12,348 (5%)

Table 1.2 - Space reduction analysis with 550kV AIS/H-GIS/GIS [11]

The picture in Figure 1.6 taken as an overview of Wakefield Substation, a 132kV substation

replacing the old AIS system shows that the gas insulation substation will occupy much less

area than air insulated substation.


8

Figure 1.6 - Wakefield substation – new GIS substation in building replaces old outdoor air

insulated equipment

The most significant properties of SF6 when used in gas insulated switchgear are its dielectric

performance and its thermal conductivity as these partly define the size of the gap required

between the central conductor / inside of tube and the current rating of the equipment

respectively. As SF6 has good dielectric strength and thermal conductivity, it is ideal for this

application. Significant volumes of SF6 are contained in this form of equipment within

transmission system substations but leakage rates are carefully monitored.

SF6 leakage in the substations such as the one shown above is controlled through careful

monitoring and preventative maintenance. According to [12], the lower and upper bound

weighted-average leak rates of SF6 equipment are 0.2 and 2.5 percent respectively. The

standard BS EN 62271-1:2008 [13] states that for SF6 and SF6 mixtures, the standardized

annual leakage values should be limited to 0.5% and 1% respectively for equipment in service.

According to National Grid data of 2009/10 [3] 13.98 tons of SF6 leaked from SF6 equipment -

2.2% of the total amount of gas, 635 tons. This is an impact equal to 334,112 tons of CO2.

With 1 litre of petrol producing 2.65kg of CO2 [14], this value is equivalent to that of one

vehicle running 1400 million kilometres with an equivalent average petrol consumption of 0.09

litre/km [14].

This work investigates the scope for replacement of SF6 gas in circuit breakers and in sections

of busbars used in gas insulated substations in the short to medium term. Technologies that

could possibly replace SF6 are discussed in terms of the current state of the art and with

reference to the required ratings for a circuit breaker / busbar section operating on a 132kV,

275kV or 400kV transmission system. The relevant ratings on which a comparison has been

examined are as described in Table 1.3.


9

132 kV System

275 kV System 400 kV System 765kV System

[15]

Rated voltage (rms, 3-phase)

145 kV 300 kV 420 kV 800kV

Normal current 2000 A 2500 A 4000 A 5000A

Short-circuit current (1 / 3 phase)

40 kA 40 kA 63 kA 63kA

Duration of short-circuit 3 s 1 s 1 s 1s

Time constant 45 ms 45 ms 45 ms N/A

Target fault current interruption time of main

in-feeding circuit 120 ms 100 ms 80 ms N/A

Rated lightning impulse withstand voltage to earth

650 kV 1050 kV 1425 kV 2100kV

Rated lightning withstand voltage between phases

650 kV 1050 kV 1425 kV 2100kV

Rated lightning impulse withstand voltage across

open device 650 kV

1050 kV plus peak power frequency

voltage

1425 kV plus peak power frequency

voltage

2100 kV plus peak power

frequency voltage

Rated switching impulse withstand voltage to earth

N/A 850 kV 1050 kV 1550 kV

Rated switching impulse withstand voltage between phases

N/A 1275 kV 1575 kV N/A

Rated switching impulse withstand voltage across

open switching device N/A

700 kV plus 245 kV peak power

frequency voltage

900 kV plus 345 kV peak power

frequency voltage N/A

Table 1.3 - Typical required ratings for transmission system circuit breakers [16]

1.3 Project Objectives and Thesis Structure

To summarise the above, this project aims to identify the merits of various alternatives to SF6

as both an insulant and an arc-extinguishing medium in the different equipment used on the

electrical transmission system. Given the issues just described, it is important to examine

whether viable alternatives exist to SF6. Even if the leakage of SF6 can be controlled, other

technologies should be considered as long as they can be considered to give lower carbon

emissions than the current products available.


10

The work that has been carried out in the production of this report had included a review of

existing literature, theoretical assessment of a number of SF6 elimination options and

experimental testing. The thesis is split into a number of chapters. Chapter 2 introduces SF6

gas and its properties in more detail before discussing a benchmark gas insulated substation

that will be used in future analysis.

Chapter 3 describes the potential replacement gas/liquids that could be used to insulate high

voltage busbars within gas insulated substations. This chapter comprises a literature review of

liquid and gas insulation alternatives before carrying out a range of calculations examine the

feasibility of some of the promising candidates. Estimations of the size of busbar in gas

insulated substation insulated with alterative gas are given in Chapter 4.

Chapter 5 focuses on understanding the physical and electrical characteristics of one kind of

polyurethane foam that has been studied through experimental work and which could possibly

be used in the transmission system. This study is based on experimental work including high

voltage tests on the polyurethane foam products. The dimension of the busbar when the

polyurethane foam is applied is than compared with the size of SF6 insulated busbar currently

being used.

Chapter 6 reviews some candidates which are possible replacements for SF6 as an

interruption medium. The latest developments in oil, gas and air blast circuit breakers are

reviewed and compared with SF6 gas circuit breakers. Chapter 7 then focuses on the

development of vacuum circuit breaker – these are likely to be available soon and therefore

the emphasis is in understanding its switching performance in the 400kV network. The

simulation work in ATP/EMTP has been carried out to investigate the magnitude of switching

transients as influenced by several key factors of the circuit breaker itself including the

chopping current, dielectric strength of vacuum gap and the high frequency current quenching

capability. The switching voltage probability due to random characteristics of circuit breaker is

analysed in this chapter.

Finally, Chapter 8 draws a number of conclusions about of this PhD thesis and the future work

which is required to move towards SF6 free insulation and interruption systems are suggested

in Chapter 9.

Chapter 2 SF6 Gas and Equipment Design

11

Chapter 2 SF6 GAS AND EQUIPMENT DESIGN

SF6 gas is widely used in compact gas insulated power equipment, especially at high voltage

level. This relies more on its superior physical and electrical properties than other insulation

equipment. This section therefore reviews the properties of SF6 and its practical usage in the

electrical industry.

2.1 Properties of SF6 Gas

The electrical and thermal stability of SF6 comes from the symmetrical molecular structure,

composed of six fluorine atoms around the central sulphur atom, as shown in Figure 2.1.

There are 4 bonds (covalent) plus two “bonds” ionic (S6-F-) distributed over six S-F pairs.

Each bond angle equals to 90°C.

Figure 2.1 - Molecule arrangement of SF6

SF6 gas has a strong electronegative property, so that there is an affinity to attach free

electrons produced by ionisation of atoms during arcing phenomenon. This negative ions

formed are relatively heavier and immobile compared to the free electrons resulting in a higher

electric field being required to cause ionisation. A paragraph from [17] gives a description of

the advantage of an electronegative gas.

‘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 is usually made from sulphur (S2) and fluorine (F2), in an exothermic reaction as shown in

the equation below. Significant heat is necessary for molecular breakdown and SF6 is


12

relatively stable at high temperatures. Therefore, it is chemically compatible with most solid

insulating and conducting materials used in electrical equipment at temperatures up to about

200 °C.

kcalSFFS 52426 622 (1)

It has good heat transfer properties, this along with the other key properties of SF6 gas are

given in Table 2.1.

Properties Data

Relative Density at 0.1Mpa 6.164 kg/m3

Thermal conductivity at 0.1MPa 0°C 0.0121 Wm-1

K-1

Boiling point at 0.1Mpa −64°C (209 K)

Solubility in water Slightly soluble

Liquefied Pressure at 21°C 2.1MPa

Global Warming Potential (GWP) 23,900

Toxicity No1

1 High concentrations of SF6 will lead to suffocation

Table 2.1 - Summary of the key characteristics of SF6 gas

SF6 can be stored at a relatively high pressure at room temperature. Its boiling point is

reasonably low, -64°C at 0.1MPa, and it is normally used in SF6 insulated equipment at

pressures of 0.3 - 0.6MPa. It is heavier than air, so it will stay at ground level and although it is

non-toxic, a high density of this gas in area will lead to suffocation. When subjected to

electrical discharges the by products that are generated such as S2F10, and SOF2 are highly

toxic and corrosive compounds.

It presents no handling problems, is readily available, and up until recently it has been reliably

available and reasonably inexpensive. From 1960 to 1994 the price of SF6, for quantity

purchases remained basically constant at about $3 per pound (one pound = 0.4536 kilogram).

The current price of SF6 for quantity purchases is over $30 per pound. The electrical industry

has become familiar and experienced with using SF6, in electrical equipment.

All the advantages described above make the gas useful in electric equipment as an insulant

or as an arc extinction medium. SF6 is therefore used as an insulating gas in substations (in

gas insulated switchgear), as an insulating and cooling medium in some transformers and as


13

an insulating and arc quenching medium in switchgear for high and medium voltage

applications.

2.2 Electrical Performance of SF6 Gas

At normal atmospheric pressure, SF6 has a dielectric withstand capability that is 2.5 times

better than air as shown in Figure 2.2. Usually the gas is used at 3-5 times atmospheric

pressure and then the dielectric properties are ten times better than for air. SF6 insulates so

well because it is strongly electronegative. The gas captures free electrons and forms negative

ions thus arresting the formation of electrical discharges. The excellent dielectric strength of

SF6 is the key reason for its use in gas insulated substations.

Figure 2.2 - Dielectric strength of SF6 in comparison to air, oil and vacuum [18]

Cookson and Pedersen[19] have reported characteristics for SF6 in gas mixtures with different

mixing ratios. The measurements were performed on coaxial cylinder electrode geometries

(Ø89mm/Ø226mm) with negative lightning, switching impulse and alternating voltages applied.

Figure 2.3 shows the breakdown voltage withstand ability of SF6/N2 gas mixture when the

pressure of the gas is 0.45MPa. For pure SF6 gas at that pressure, the lightning impulse

breakdown voltage of the 68.5mm gap is around 1050kV, thus the average breakdown

strength is calculated to be 5.7kV/mm at atmosphere pressure, which is about 35% weaker

than the theoretical value. The reduction of strength is due to different conductor material,

surface roughness and spacer structure designing in practical construction work.

A gas mixture with less volume of SF6, such as 80% SF6 in N2, presents a slightly weaker

dielectric strength than that that of pure SF6. The lightning dielectric strength of the mixed gas

at this ratio is 5% lower, and the AC dielectric strength is reduced by no more than 3%

compared with that of pure SF6 gas.


14

Figure 2.3 - Negative lightning impulse (LI), negative switching impulse (SI), and 60Hz AC

breakdown voltages of N2/SF6 gas mixtures [19, 20]

The ability of SF6 to interrupt current is shown in Figure 2.4. SF6 effectively controls circuit-

breaker arcs owing to the electronegative properties stated previously and as it has excellent

cooling properties at temperatures (1500-5000K) at which the arcs extinguish (the gas using

energy when it dissociates therefore producing a cooling effect). This graph was the results of

investigations [21] in 1953 where a plain break interrupter was used to take measurements.

SF6 has, obviously, a far superior interrupting capability to air.

Figure 2.4 - Interruption capability of air and SF6 [21]

2.3 The Use of SF6 in Circuit Breakers

There are two main types of SF6 circuit breakers, the puffer type and the rotating arc type. The

former is in many ways similar to older air blast technology where gas is moved at pressure

over the arc. In the latter case the arc itself is made to move by the use of axial magnetic fields.

As the rotating arc type is generally only used at distribution voltages, only the puffer breaker

is described.


15

Figure 2.5 - Puffer and rotating arc circuit breaker types-better quality [18]

In a puffer circuit breaker, SF6 gas flows over an arc which has resulted from the parting of the

circuit breaker contacts. For large currents, the flow of the SF6 is generated by the heating

effect of the arc. For smaller currents it may be necessary to use a pre-compressed volume of

SF6 since the heating effect of the arc may not be sufficient to cause gas flow.

On opening the circuit breaker, the main contacts separate first. Following partial separation of

the main contacts, the arc contacts then part. The volume is decreased and therefore the gas

pressure within the volume must be increased. The contacts continue to separate and

therefore the gas pressure continues to increase. When the arcing contacts separate, an arc is

drawn in the gas and the pressure in the volume causes gas to flow axially along the arc.

Ultimately, extinction of the arc will take place.

When interrupting large currents, the opening speed of the breaker is slowed down due to the

thermal pressure acting on the underside of the piston assembly, possibly as a result of throat

blocking. Conversely, when interrupting low values of current the arc diameter is so small that

the gas flow from the piston volume cannot be blocked and axial flow exists only for a very

short time. There is also a tendency at any current for the mechanism to slow down when

current zero is approached, and this is particularly true for a single phase breaker.

Following extinction of the arc, the contact gap must withstand a rapid rise in voltage across

the contacts due to the transient recovery voltage. This voltage will typically rise to around

twice the peak phase voltage in a time of some tens of microseconds. It is therefore important

at this stage of the interruption process to ensure that the contact gap can withstand this

voltage, this is easily achieved by an SF6 breaker due to its extremely low deionisation time

constant in comparison to air as seen in Figure 2.6. This time constant [22] is a critical

parameter that partly describes the circuit interrupting capability of circuit breakers. The time

constant of SF6 is extremely short owing to its electronegative nature so it is able to withstand

higher transient recovery voltages.


16

Figure 2.6 - Deionisation time constants of SF6 in comparison to other gases [18]

Despite the fact that the SF6 gas is very stable, it will partly decompose in association with the

electric discharges and arcs it must interrupt. Then, gaseous and solid decomposition

products are produced. Normally the level of gaseous decomposition products is kept low

through the use of absorbers built into the switchgear. In large concentrations, the

decomposition products are corrosive and poisonous. Therefore, there are established

routines for service personnel when opening SF6 filled equipment for maintenance or

scrapping. The solid decomposition products are mainly metal fluorides in the form of a fine

grey powder. The powder only appears where arcing has occurred, for instance in used circuit

breakers. The powder can be easily taken care of as separate waste. The decomposition

products are reactive, which means that they will decompose quickly and disappear without

any long-term effect on the environment.

2.3.1 Benefits of SF6 Switchgear in Comparison to Oil or Air

Insulating oil is far superior to both air and SF6 under standard atmospheric conditions.

However, when the oil is contaminated by small amounts of water or by carbon deposits, its

performance quickly degrades. According to manufacture manual, fresh oil can normally be

expected to have a dielectric strength of some 70kV when it is placed between two 20mm

spheres 2.5mm apart [23]. Used oil generally has a dielectric strength greater than 15kV.

When an arc burns in oil, a bubble is formed around the arc. Immediately around the arc is a

layer of hydrogen followed by a layer of steam and finally boiling oil. It is the high thermal

conductivity and re-ignition strength of the hydrogen gas that helps make oil such an

apparently good medium for current interruption.

Few companies now produce oil circuit breakers (particularly for transmission system use) as

oil is not environmentally friendly and if not carefully maintained can be vulnerable to explosion

as gases such as acetylene and hydrogen build up inside the oil after each interruption. In

addition, oil circuit breakers cannot be used with gas insulated switchgear.


17

Air blast circuit breakers work by forcing high pressure gas over the electric arc formed by the

opening of the contacts. The high pressure gas cools the arc and rapidly eliminates the

ionised gas from the contact gap at the moment of current zero. Air blast circuit breakers have

the main disadvantage of being noisy (particularly important in areas close to housing) and

requiring significant secondary equipment in the form of air compressors and receivers that

require annual inspection under the pressure vessel regulations. Again, air blast circuit

breakers cannot be used in gas insulated switchgear.

Figure 2.7 - Oil, Air Blast and SF6 circuit breakers (from left to right) [24]

For both types of breaker, the strong dielectric strength of SF6 means that SF6 circuit breakers

require fewer breaking units in series than air or oil-filled breakers. SF6 therefore allows air-

insulated substations to be shrunk in size.

The main benefits of SF6 switchgear in comparison with the previous main types of oil and air-

blast are:

Low levels of maintenance

Low noise

Smaller size

Safer (no pressure vessels / oil)

Suitable for use in GIS

It is notable that there has been little recent research on either air-blast or oil circuit breakers

suggesting that the technology will soon become obsolete unless it is otherwise demanded by

the electricity supply industry.

2.4 SF6 Use in Gas Insulated Substations

The typical form of equipment used within a gas insulated substation sees SF6 being used

primarily as an insulant. An example of 420kV GIS configuration is shown in Figure 2.8 below.


18

Figure 2.8 - 420kV GIS sectional view of a bay with double busbar system [25]

The busbar system is made with single-phase or three-phase enclosures and consists mainly

of conductors, cylindrical enclosures and supporting spacers. The dimension of busbar is

carefully chosen according to the required lightning impulse and switching impulse withstand

strength. In addition, several other important factors must be considered to ensure the

dimensions are appropriate [10] and these are listed as follows.

Estimation of effect of different types of electrical stresses

Dimensioning of the GIS systems and its components to fit within the request space

envelope

Insulation considerations at interfaces (where gaseous and solid insulation meet)

Thermal considerations

All power equipment are continuously stressed by the system operating voltage at power

frequency and are occasionally required to withstand a certain voltage exceeding the peak

value of the system operating voltage, like lightning impulse or switching impulse voltage.

Lightning impulse voltages are result of a natural phenomenon, a standard lightning impulse

wave shape is defined by front/tail duration with 1.2/50us. Switching impulse voltages are

produced from the system itself by connecting and disconnecting the circuit breaker. For a

400kV substation, the required withstand levels are 1425kV and 1050kV respectively. The well

designed substation should withstand all of these types of voltage stresses to ensure dielectric

failure does not occur. Appropriate protective devices like surge arrestors and switches are

often installed in the substation to minimise the levels of overvoltages.


19

The dimension of the entire busbar as assembly is basically determined by the properties of

the insulating medium and the amount by which these are stressed by the electric field. This

itself depends on the operation voltage and on other conditions such as the surface condition

of the conductor / sheath which is rough and could cause a locally elevated electric field.

Particles (as detailed below) may also cause problems.

Heat is generated by the conductors carrying current, and it leads to a temperature rise within

the insulating medium owing to its thermal resistance. In addition the temperature of all parts

of the GIS should not exceed a level which is set by standards. There is a relationship

between the thermal performance of the power system equipment and its dielectric capability.

If we increase the insulation thickness of a system, we increase the thermal resistance and

therefore either increase the operating temperature or decrease the operating current.

To summarise, the design and construction of a substation rely on several factors. Electrical

strength and thermal stability are important factors in developing a reliable insulation system in

an appropriate dimension. In combination with an insulation system protective devices may be

used to manage overvoltage magnitude. The coordination of thermal and insulating design

aspects is further explained in Chapter 3 and 4 with an examination of different kinds of

dielectric materials.

2.5 Benchmark Busbar Design for Use in Future Studies

To investigate the potential for alternative forms of insulation to be used in future chapters of

this thesis, a base design of gas insulated busbar is taken from National Grid for comparison.

The design is based on a 400kV busbar with a 1425kV BIL (although a lower BIL of 1050kV is

considered in some cases). The radius of the inner conductor and outer sheath are 62.5mm

and 250mm respectively. The thickness of the outer sheath is assumed to be 5mm. The gas

pressure in the busbar is 0.3MPa. This design will be discussed in more detail in a later

section.

2.6 Conclusion of SF6 Properties and Gas Insulated

Equipment Study Model

This chapter has introduced the key properties of SF6 gas relevant to the work described in

this thesis. SF6 gas is heavier than air and non-toxic, and it will remain stable above ground if

it leaks from equipment. Due to its strong electronegative properties, the dielectric strength of

SF6 is 89kV/mm at atmosphere, is about 3 times stronger than air. Usually the gas is used at

3-5 times atmospheric pressure and as a result the dielectric properties are ten times better


20

than for air. The excellent dielectric strength of SF6 is the key reason for its use in gas

insulated substations

This chapter has also introduced the equipment in which SF6 is used within high voltage

substations (in gas insulated switchgear), as an insulating and cooling medium in some

transformers and as an insulating and arc quenching medium in switchgear for high and

medium voltage applications.

The 400kV SF6 insulated busbar currently being used in National Grid substation has been

described as a bench mark and in the following chapters it will be used to examine other

insulating material as replacement of SF6.

Chapter 3 Potential Liquid and Gaseous Replacements for SF6 as An Insulation Medium

21

Chapter 3 POTENTIAL LIQUID AND GASEOUS

REPLACEMENTS FOR SF6 AS AN INSULATION

MEDIUM

As described in Chapter 2, there is a significant volume of gas used in SF6 equipment which is

used solely as an insulation medium. The insulation duty is much less demanding than the

duty met by SF6 when it is used as an interruption medium. This section of the thesis reviews

the possible alternatives to SF6 in both gaseous and liquid forms.

3.1 Electrical Breakdown in Liquid Dielectrics

The mode of breakdown in pure liquid dielectrics, called electronic breakdown, is usually

explained by the growth of conduction current between two electrodes in literature [26].

However, the exact mechanism of its process is not known. The electrons are generated from

the cathode by field emission of electrons. The liberated electrons get multiplied in the liquid

medium by a Townsend type of mechanism. The current increases rapidly due to a process

similar to the primary ionization process and also the positive ions reaching the cathode

generate secondary electrons, leading to breakdown.

The breakdown process of highly purified liquid dielectrics is similar in nature to that seen in

gaseous and its dielectric strength is relatively high, of the order of 1MV/cm. However, it is not

possible to get a perfectly pure liquid. Liquids are easily contaminated by humidity, solid and

dissolved gas medium, the effect of which to liquid is reduce the dielectric strength. When the

liquid is stressed with a continuous voltage, under the action of the electric field, dissolved

gases come out from liquid and form a bubble. The gas in the bubble has a lower strength

than the liquid, and the breakdown in bubble may trigger total breakdown in liquid. This

mechanism is explained by a mathematical model proposed by Kao [27]. It is observed that if

the dissolved gases are electronegative, higher breakdown strengths of liquid dielectric can be

obtained.

Besides bubbles in oil, solid particles including dust and ionic impurities can also reduce the

dielectric strength of oil and should be avoided by purification process before usage. The solid

impurities line up at right angles to the equipotential gradient, and distort the field so that

breakdown occurs at relatively low voltage when the voltage is applied continuously. The line

up of particles is a fairly slow process, and is unlikely to affect the strength on voltages lasting

for short time, less than 1ms.


22

3.2 Recent Developments in Oil Insulation

3.2.1 Oil Insulation Performance

In HV equipment, the commonly used liquid insulation material is oil insulation as a liquid in

transformers and circuit breakers or as impregnants in high voltage cables and capacitors.

Research relating to oil is mostly focused on oil-insulated power transformers as the quality of

insulating oil has a direct effect on the reliable operation of transformers. Therefore, insulating

oils used in power devices in HV equipment are required to have sufficient dielectric strength

and high thermal and chemical stability.

Mineral oil is a traditional and common liquid insulant used in transformers for decades.

However it is increasingly seen as environmentally unacceptable. Other types of oils such as

ester oils and certain vegetable oils are now available as alternatives to mineral oil. Among all

ester oil products, synthetic ester oil Midel 7131 and natural ester oil FR3, will be studied as

examples in the following sections.

Mineral oil [23] Midel 7131 [28]

[29] FR3 [30]

AC Ubd at 2.5mm gap 70 kV 75 kV 56 kV

LI Ubd at 3.8mm gap [31] 232.8 kV 223.2 kV 210.0 kV

Thermal Conductivity 0.126 W/mK 0.144 W/mK 0.167 W/mK

Flash Point 148˚C 275˚C 316˚C

Pour Point -57˚C -60˚C -21˚C

Thermal Expansion

Coefficient per °C 0.00065 0.0008 0.00074 (at 25˚C)

Density at 20˚C 0.887 kg/dm3 0.97 kg/dm

3

0.92 kg/dm3

(25˚C)

Viscosity at 40˚C 9.1 mm2/s 34 mm

2/s 34 mm

2/s

Table 3.1 - Comparison among mineral oil, midel 7131 and FR3

The important physical, chemical and dielectric characteristics the oils are compared in table

above based on recent literature studies. The thermal properties of oil are significant as oil in

transformers also functions as a thermal transfer agent. The flash point is expected to be as

higher as possible in case of arcing within the equipment. The thermal expansion coefficient of

oil describes its expansion as a function of temperature which would lead to increasing internal

pressure increasing in closed equipment if there is use of pressure releasing devices. The

dielectric strength of all these kinds of oil are tested at semi-uniform electric field with a pair of


23

sphere – sphere electrodes by [31] and manufacturers. Tests have shown that the ester oil

Midel 7131 presents the strongest dielectric strength at AC voltage stress, while mineral oil

has the best behavior when applying lightning impulse voltage to oil sample.

The dielectric strength of insulating oil is far superior to both air and SF6 under standard

atmospheric conditions. However, when the oil is contaminated by small amounts of water or

by carbon deposits, its performance quickly degrades. Fresh oil can normally be expected to

have a 70kV dielectric breakdown voltage at a 2mm gap distance [23]. Used oil generally has

a dielectric strength greater than 40kV at the same gap distance.

3.2.2 Illustration of Possible Size of Oil Insulated Substation and

Key Barriers to Its Use

It is useful to justify why oil may be a useful candidate for further study given the properties

above. Table 3.2 shows a comparison between the outer radius and the weight of an oil

insulated substation against a standard SF6 system described in the previous chapter. With a

fixed busbar radius, the outer radius was calculated in a way that ensured the dielectric

strength of the oil was not exceeded using a basic Insulation Level (BIL) of 1425kV as a

voltage input. It is assumed that the pressure of the oil filled busbar is kept as 0.1 MPa with a

pressure control device, so there is no risk from thermal expansion of oil itself. It is clear that

the dimension of the substation can be reduced compared with the previous one that was SF6

insulated. However the amount of oil required for sufficient insulation medium is more than 10

times heavier in weight than that of the SF6.

AC Breakdown

voltage

LI breakdown

Voltage

Density (kg/m

3)

Working pressure

(MPa)

Radius (mm)

Weight (kg/m)

Oil (after treatment)

70kV at 2.5mm gap

232.8kV at 3.8mm gap

[31] 895 0.1- 0.2

1 90.6 12.1

SF6 8.9kV/mm.0.1Mpa 8.9kV/mm.

0.1Mpa 6.164 0.3 146.8 1.1

1, the oil in open cell transformer is working at 0.1MPa, the pressure in closed cell transformer will be slightly higher than atmosphere due to heat radiation

Table 3.2 - Comparison between oil insulation and SF6 insulation

The calculation does not account for the presence of insulating spacers which form part of

conventional gas insulated busbars. In most cases, the spacers are often the weak points of

the system. In oil insulated busbars, creepage discharge on the interface between the oil and

solid spacer should not be ignored. There is no specific literature which is relevant except for

that which focuses on the discharge along the oil/pressboard interface in transformers [32-35].


24

Figure 3.1 - Power frequency design curves (a) bulk oil (b) creepage along pressboard [72, 73]

Figure 3.1 shows a typical design curve representing the dielectric strength of an oil gap and

that along the pressboard within the oil [36, 37]. The design curve was developed by a

pressboard manufacturer, using semi-uniform electrode configurations [38]. Lower breakdown

strength is observed along the pressboard surface than in bulk oil with the same path length.

The average breakdown strength ‘E’ of the oil gap against the gap distance ‘d’ follows the

relationship of E = Ad–B

[39]. The relationship between gap distance and withstand strength of

bulk oil / oil-pressboard interface described in Figure 3.1 follow Equations (2) and (3)

respectively.

Electric field in bulk oil: 45.095 dEb (2)

Electric field along pressboard: 46.064 dEp (3)

Where ‘d’ is the oil gap distance in cm, and ‘E’ is the electric strength in kV/cm.

The empirical curves and equations described above shows the influence of creepage

discharge to transformer design. However, no paper and experimental data have been found

that refer to the creepage strength along oil /solid interface in a coaxial busbar model. The

effect of this discharge on the size of oil insulated busbar is studied in a mathematical way as

follows.

The withstand voltage at a fixed distance to the conductor in a coaxial cylinder busbar model I

can be described in equation:

)/ln( rRr

VEm (4)

By combining Equations (2), (3) and (4) together the relationships between withstand voltages

‘V’ in kV and gap distance ‘d’ in cm are shown in Figure 3.2 respectively for a fixed busbar


25

conductor radius. The axis of gap distance is shown in the logarithmic scale as in Figure 3.1.

The peak magnitude of withstand voltage along the pressboard is shown when the gap

distance is around 30cm, but this is a much lower value than that in the bulk oil. To increase

the withstand voltage would therefore need an increase in conductor radius to decrease the

peak electric field.

Figure 3.2 - Withstand voltage depending on the gap distance in coaxial cylinder model

While oil may be able to reduce the size of a substation further, there are some clear issues

that arise from the use of oil as an alternative to SF6. Oil is flammable which would be a

significant risk in a high voltage power substation. In addition, if any PD / arcing activity takes

place within the oil and if the equipment is not properly maintained, this could lead to

explosions as gases such as acetylene and hydrogen build up inside the oil. Oil must also be

handled very carefully and work on sections of a gas insulated substation would require great

care and skill in moving significant quantities of oil from the equipment into a temporary vessel.

3.2.3 Summary of Oil Insulation

In summary, oil is a material with a very high dielectric strength and the breakdown voltage

can be at least 50kV at a 2mm gap distance. There is good relevant long term experience in

using oil as an insulant. The use of bio-degradable ester oil could make the usage of oil in

power substations more environmentally friendly. However, even though the size of the

equipment can be reduced, the equipment would be an order of magnitude heavier owing to

the high density of the oil. Oil is worthy of some continued investigation at least for benchmark

purposes.

0

50

100

150

200

250

0.1 1 10 100

Wit

hst

and

Vo

ltag

e (k

V)

Gap distance (cm)

Peak voltage along pressboard

Peak voltage in bulk oil


26

3.3 Electrical Breakdown in Gas Dielectrics

Besides bio-degradable oil, another option for an SF6-free substitute is a form of gaseous

insulation which uses an environmental acceptable gas. With a gas, it may be possible to re-

use the structure of gas insulated equipment currently being used in HV substations. The

breakdown phenomenon of SF6 and other gases takes place in a similar manner. The study of

electrical discharge in gaseous dielectric has been extensively researched, and can be

obtained from literature [40]. A brief review of the fundamental principles of gaseous ionization

and breakdown is presented in this section before examining performance of gas dielectrics as

possible alternatives to SF6.

3.3.1 Gas Ionization and Attachment Processes

The breakdown in gas occurs due to the transition of a non-sustaining discharge into a self-

sustaining discharge, according to literature [41]. In the ionization process, electrons and

positive ions are created from neutral atom or other molecules, and they move to the anode

and cathode respectively. This ionization process is related to various physical conditions of

gases, like pressure, temperature, the electrode field configuration, nature of electrode

surfaces, and the availability of initial conducting particles.

The breakdown of gas is determined by both ionization and attachment processes. Ionization

is the process where by a single electron, bound to an atom or molecule is released. As the

electron is negatively charged, the remainder of the previously neutral molecule is now

positively charged and is called a positive ion. Meanwhile, detachment is another method to

produce free electrons, which become free from negative charged ions. An appreciable

number of these electrons are capable of ionizing the gas when the applied voltage is

increased to a level at which they are accelerated to a suitable velocity.

The primary ionisation coefficient (α) only refers to the ionisations by electron collision in a gas

and it is defined as the number of ionisations per unit length of the electron path. The values

for a couple of gases like air and SF6 have been determined experimentally as a function of

the ratio of electric field strength to pressure E/p.

When an electron approaches a positive ion, this competitive process to ionization is called di-

ionization by recombination. The rate of recombination is directly proportional to the

concentration of both positive and negative ions. The recombination coefficient (ξ) is related to

the gas pressure, which is proportional to the density of ions. When the gas is placed in high

altitude pressures the rate of recombination is smaller when compared with the state in

atmospheric pressure.


27

Attachment is the opposite process to detachment whereby an electron attaches to a neutral

atom or molecule. The resulting negative ion is in a lower energy state and therefore there is a

release of energy. In electronegative gases, such as air, electron attachment is a process that

helps to prevent breakdown.

Figure 3.3 - (α– η)/P and E/P relationship in SF6 [42]

The attachment coefficient (η), a competing process to ionisation, is defined as the number of

electron attachments to atoms or molecules per unit length of the path of an electron. Similar

to the ionisation coefficient, the value is a function of the ratio of electric field strength to

pressure E/p. And these can be described in Equation (5). Figure 3.3 gives example and

comparison of some gas dielectric mediums.

)(,)(p

Ef

pp

Ef

p

(5)

The net ionisation coefficient (ā or αeff) is the difference between the ionisation and attachment

coefficient (α –η), when molecules in gas readily attach free electrons forming negative ions,

therefore unable to ionize neutral particles under field conditions.

3.3.2 Townsend Breakdown in Electronegative Gas

A breakdown is produced owing to the growth of an arc avalanche which is able to develop as

long as Townsend’s first ionization coefficient (α) is greater than the electron attachment

coefficient (η). In other words, net ionisation of the gas must take place


28

Townsend breakdown theory describes gas breakdown process in small gaps as explained in

literature, electron avalanche between electrodes is related with the breakdown result. Free

electrons which are accelerated towards the anode may collide with a neutral molecule and

ionise. The result is a positive ion, an electron freed from the molecule and the original free

electron. These two electrons may then proceed to cause further ionising events. This leads to

an exponential increase in positive ions and electrons. Electrons accelerated in an electric

field with a positive net ionisation coefficient increase in number exponentially and leave

behind positive ions. The lower mobility positive ions that are left behind are accelerated

themselves towards the cathode. Positive ions that bombard the cathode may release an

electron that can initiate another avalanche. The condition for Townsend breakdown is the

point when one avalanche causes the formation of another avalanche through secondary

ionisation.

The Townsend criterion for attaching gases concludes that a critical value of a pressure-

dependent field strength exists for which α=η and (E/p) = (E/p)crit. Earlier several people

studied the values of α=η for SF6 and other gases over a wide range of E/p and found these fit

well into a linear relation, see Figure 3.3. The relationship between the net ionisation of a gas,

the applied electric field and gas pressure are expressed in Equation (6) [42]. The equation

shows that net ionisation will become positive once E/p exceeds a critical level.

eff

crit

E EK

P P P P

(6)

The ionization coefficient and attachment coefficient for SF6 is calculated and the E/N of pure

SF6 is reported as 360Td (1Td =10-21

Vm2, E/P (V cm

-1 Torr

-1)*3.1*10

-21=E/N (V m

2)) [43, 44].

In SI units, it can therefore be stated pure SF6 gas has limiting field strength of 8.9kV/mm at

atmospheric pressure.

The calculation carried out later in this thesis will use the criteria that the dielectric stress

should not exceed the critical E/N value of the gas at any voltage (AC, switching or lightning).

3.4 Review of Gas Alternatives to SF6

3.4.1 Gas Mixtures with Small Volume of SF6

A significant number of studies [85-92] were carried out examining the feasibility of SF6 gas

mixtures such as N2/SF6, CO2/SF6 and Air/SF6 at varied concentrations to minimize the usage

of SF6. For example, the result from AC breakdown tests of 1% of SF6 in N2 delivers a 40%

higher breakdown voltage in a uniform field and one that is 24% lower in a non-uniform field


29

than that of pure SF6 [45]. In [46] which looked at a study of metallic particle induced partial

discharge activity in 10:90 SF6/N2 gas mixtures, it was concluded that detecting a particle in a

N2 environment may be more difficult than in a pure SF6 gas.

When a gas mixture is used, the dimension of the outer sheath could be increased to get a

suitable insulation gap in the equipment and therefore ensure the same electric field stress

withstand ability of the replacement gas mixture as pure SF6 gas. The figure below [47] shows

the minimum dimension of 275kV GIS for different gas pressures as a function of the ratio of

SF6 in N2. The pressure in busbar is varied from 0.2MPa to 0.6MPa, and the current capacity

is 6000A. To get the same busbar dimension as a wholly SF6 solution, a higher gas pressure

would be needed when a mixed gas is used.

Figure 3.4 - Minimum dimension for different gas pressures as a function of SF6 content [21]

Figure 3.5 showed further results relating to the breakdown strength of gas mixtures. The 50%

breakdown voltage exhibited non-linear characteristics as a function of gas pressure. The

maximum was observed at pressures between 0.2 - 0.25MPa with the breakdown voltage

falling as pressures went above around 0.3 - 0.35MPa. This non-linear characteristic was

attributed to the transition from streamer discharge to highly conductive leader discharge in

electronegative gases at high gas pressures.

Figure 3.5 - Breakdown voltage influenced by percentage of SF6 in N2 at different pressure [47]


30

The use of a gas mixture and the resulting variance in the size of an enclosure also has an

impact on the current rating of equipment. Figure 3.6 shows the results of an iterative analysis

for a gas pressure of 0.6MPa and a current capacity of 2000 to 8000A. The minimum tank

diameter was found for each SF6 gas mixture ratio with the conductor radius being optimally

decided by the ratio of the tank diameter to the conductor diameter which is set at 2.72. The

tank size is largely influenced when the percentage of SF6 is normally less than 30%. When

the current carrying ability is up to 8000A, the insulation distance required for 10% SF6 gas

mixture is same as that of pure SF6 gas. The reduction of dielectric strength owning to low

content of SF6 can be compensated by increasing pressure of gas mixture. However, the

maintenance and economic issues are raised as a problem for the high pressure equipment.

Figure 3.6 - Minimum tank diameter as a function of SF6 content [21]

3.4.2 Electronegative Gas - CF3I

Another electronegative gas similar to SF6 is Trifluro-iodo-methane (CF3I). This is colorless

and non-flammable. Its molecular arrangement is shown in Figure 3.7. It is non-carcinogenic

but it is rapidly decomposed by solar light so its lifetime in the atmosphere is very short. As a

result its GWP is less than 5 which is relatively comparable to that of the same mass of CO2

(whose GWP is by definition 1) and the ozone depleting potential is near zero. CF3I therefore

has a low environmental impact. CF3I is used as a gaseous fire suppression flooding agent for

in-flight aircraft and electronic equipment fires.

Figure 3.7 - Molecule arrangement of CF3I [3]


31

The general properties of CF3I are listed in the table in comparison with SF6 gas. A

disadvantage of CF3I is an increased boiling point which means it use would need to be

carefully considered in cold climates.

CF3I [48] SF6 [49]

Molecular Mass (g/mol) 195.91 146.05

Density (kg/m3) 2.361 5.9

Global Warming Potential [50] < 5 23,900

Ozone Depleting Potential (ODP) 0.0001 0

Lifetime in atmosphere 0.005 year 3200 year

Boiling Point (0.1MPa) -22.5°C -64°C

Liquefaction pressure at -20˚C (MPa) 0.11 0.77

Thermal Conductivity at 1.013 bar and 0 °C (32 °F) (mW/(m.K)

6.594 [51] 12.058

Viscosity (Cp) 0.2361 1 0.0154 @20˚C

Specific heat at constant pressure (0.1MPa)

0.031 kJ/(mol.K) [3] 0.097 kJ/(mol.K)

@21 °C

Heat convection Constant Estimated as 20.0 2 24.4

E/N value 437Td 361Td

1:Calculated from estimated kinematic viscosity = 1cm

2/s [52]

2: estimated through comparison of physical properties of CF3I, SF6, N2 and air

Table 3.3 - Physical properties of CF3I and SF6

Derived from pulsed Townsend experiments, the electron transport and ionization coefficients

in CF3I and CF3I/N2 mixtures covering a wide range of E/N value (where E is the electric field

and N is the gas density of the mixture) have been developed [53]. CF3I has a better dielectric

performance than SF6 with an electric field strength of 108kV/cm at absolute pressure 0.1MPa.

To be of increased practical use, CF3I gas has to be mixed with other gases such as N2 or CO2,

to bring down its boiling point at a proper and acceptable working pressure. However, different

concentrations of CF3I will lead to a change in its ability to withstand a specific voltage. The

stronger the dielectric strength is, the smaller busbar dimension that can be obtained.

Figure 3.8 is based on the values about density-normalized effective ionization coefficient (α-

η)/N, measured with pulsed Townsend apparatus. It is noted from the graph that the E/N value

of CF3I gas mixture is reducing as the increasing concentration of N2, however, when the

volume of CF3I concentrated in N2 is kept at 70%, the trend of the (α-η)/N values for CF3I runs

almost parallel to that of SF6, the E/N value of gas mixture is the same as that of pure SF6 gas.


32

Pure CF3I shows bigger E/N value than that of SF6. The practical value of this situation is that

an environmentally friendly mixture of CF3I in N2 may have the same effectiveness as the less

environmentally friendly case of SF6. Pure CF3I gas can give better dielectric insulation

performance than SF6 if severe electric field stresses are applied.

Figure 3.8 - The density-normalized effective ionization coefficients (α-η)/N for CF3I and in the

CF3I-N2 mixtures with 5%,10%, 20%, 50%, 70% CF3I as a function of E/N [53]

Based on the measurement data from above graph, the dependences of E/N values of CF3I

and SF6 on the concentration in N2 are compared in Figure 3.9 [53]. Then the dielectric

strength of each kind of gas mixture can be compared more obviously. Figure 3.9 shows that

for CF3I concentrations of more than 60% in CF3I/N2, the dielectric strength of the mixture is

superior to that of SF6/N2 gas mixture. In terms of pure dielectric strength, CF3I therefore

appears to be a viable alternative to SF6 as both a pure gas and a mixture with an improved

boiling point, which are suitable to be used in gas insulated transmission system.

Figure 3.9 -The limiting or critical field strength, E/Nlim for the CF3I-N2 mixtures [53]

In terms of the performance of CF3I gas mixed with CO2, this has been examined by M.Taki,

and H.Kasuya [53, 54]. The insulation performance of pure CF3I gas is again shown to be

superior to SF6 with an improvement of around 20% under a standard lightning impulse

voltage, see Figure 3.10. However, because of the high boiling point of pure CF3I gas, it is


33

recommended to use CF3I-CO2 (70%-30%) as a replacement, of which the dielectric strength

is about 0.8 times that of SF6.

Figure 3.10 - Positive polarity breakdown voltage characteristics of CF3I-CO2 mixture [54]

A potential issue with CF3I is its stability. CF3I is very sensitive to solar light so its lifetime in

the atmosphere is very short. Gas by-products such as C2F6, C2F4, and C2F5I and some

particles of iodine are produced when the gas is exposed to partial discharge suggesting it

may be unstable for use in actual applications [55].

Gas by products of CF3I under AC partial discharge is studied in [56], harmful gas HF was not

detected, other kinds of by-products like C3F8, CHF3, C3F6, etc. were detected in small amount

and were still stable 20 hours after the PD test, It is believed the influence of these gas

products on the overall performance of gas insulating power apparatus under the occurrence

of PD can be neglected. However, there is no clear conclusion quantifying this effect on the

long term performance of CF3I. Another experiment in [57] has shown that the insulation

performance of the gas decreases to 50% of its maximum value once flashover occurs.

From the economic view point, CF3I is in pilot scale production and is a commercial product,

sold in relatively small amounts as a fire suppression agent for normally unoccupied areas.

Current production is capable of supplying 50,000 kg/year of CF3I. The price has decreased

significantly over the last six years, but is still much higher than it will eventually be in bulk

production. The current best price for CF3I is about $50/kg [58]. Tosoh (F-Tech), a Japanese

company, has announced a new continuous process that should be able to produce CF3I from

catalytic gas-phase iodination of trifluoromethane at an estimated cost of $29/kg assuming the

current high iodine price of about $22/kg. If iodine price drops back to near its historic level of

about $11/kg and economics of scale decrease overhead and trifluoromethane costs, the

estimated cost of CF3I produced by the Tosoh process should be about $12/kg. The current

price and the predicted price based on some condition of CF3I are both cheaper compared

with the current price of SF6 over $30 per pound ($66/kg).


34

To sum up the properties of CF3I, an encouraging aspect of the measurements for the

mixtures of CF3I in N2 and CO2 is presented, and it shows that CF3I or its mixtures both have

good properties and can be regarded as viable gaseous dielectrics. CF3I/N2 would be seemed

as the one of the most probable candidate replacement gaseous dielectrics for SF6 since the

mixture with 70% CF3I presents a very similar behaviour to that of pure SF6.

3.4.3 High Pressure Air, N2 or N2O

As discussed in Section 3.4.1 SF6/N2 gas mixtures with a small percentage of SF6 show

promise as an alternative to pure SF6 gas. However, from the viewpoint of SF6 elimination, if

gases that are not regulated by the COP3 and that do not destroy the ozone layer are to be

used, then the candidate gases are pure N2 gas or a mixed gas of N2 and O2 (including dry air)

[59]. As listed in Table 3.4, the physical and chemical properties are compared among all

candidate materials including SF6. Most of these gases are composites of air and are easily

processed. They are lighter than SF6 with a lower boiling point.

Compared to N2 or CO2, air can be considered superior for insulation performance among

alternative gases at about 10%. Also, because air has better insulation performance than N2,

the effects of O2, an electric negative gas, are taken into consideration, and by varying the rate

of O2 in an N2/O2 gas, a gas with insulation characteristics even better than those of air can be

created. N2 and O2 can be mixed together changing the ratio of O2 to get a higher electric

performance.

SF6 CO2 N2 Dry Air

Global Warming Potential 23,900 1 0 0

Boiling Point (0.1MPa)

-64°C -78°C -198°C -141°C

Thermal Conductivity 0.0155 0.0142 0.0238 23.94

Molecular Mass 146.05 44.01 28.01 28.8

Lifetime in atmosphere (year)

3200 / / /

Limiting field strength E/N [60]

360 Td 108Td 130 Td 108 Td

Density (kg/m3) 5.9 1.8 1.25 1.29

Liquefaction pressure at -20°C (MPa)

0.77 2 Over 4 Over 4

Table 3.4 - Characteristics of SF6-free gas

T.Rokunohe etc. [59] made the comparison of the flashover voltage in the various gases with

particle. It can be found from the bar graph in Figure 3.11, the dielectric strength for N2/O2 (O2:


35

40%) was greater than that of air, about 1.05 times greater. Moreover, the dielectric strength

for N2/O2 (O2: 40%) was equal to or greater than that of SF6/N2 (SF6: 5%). As for the reason of

the high dielectric strength of N2/O2 mixed gas, it is presumed that the attachment of electrons

reduced the total number of electrons since O2 is an electronegative gas.

Figure 3.11 - Comparison of dielectric strength of various gases [59]

If using either N2 or dry air as an insulant, the severe concern is that there is likely to be a

large dimensional requirement to get as same insulating performance as that of SF6, or else a

very high pressure will be needed. In this case, the mechanical strength of enclosure outside

of conductor should be enhanced.

3.5 Conclusion of Liquid and Gaseous Replacement for SF6

In this chapter, potential replacements including different kinds of gas and oil that could be

used for high voltage busbar insulation have been described.

Bio-degradable ester oil is a highly promising candidate due to its environmentally friendly

properties. The size of equipment can be reduced because of the strong dielectric strength

however the equipment would be much heavier owing to the high density of the oil.

An electronegative gas CF3I and its gas mixtures show promising performance in replacing

SF6 in high voltage insulation. The dielectric strength of CF3I is 1.2 times better than that of

SF6 at 0.1MPa. The mixture of 70% CF3I in N2 presents a very similar dielectric behaviour to

that of pure SF6, and also helps to reduce the boiling point of CF3I pure gas. A lower

concentration of CF3I will weaken the dielectric strength at the same pressure but will reduce

the boiling point so that is can be used in cold areas.

Other gases such as dry air or N2 can also be used as a substitute for SF6 in high voltage

busbar insulation. However a very high pressure is required to get the same insulating


36

performance as that of SF6 and the mechanical strength of the enclosure outside of the

conductor should be enhanced.

Based on these conclusions, a range of calculations that examine the feasibility of some of

these promising candidates are carried out in the next chapter.

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations

37

Chapter 4 ESTIMATE OF SIZE OF

ALTERNATIVE GAS INSULATED

SUBSTATIONS

In this chapter, the influence of alternative gas mixtures on the dimension of gas insulated

busbars in substations is investigated. Of particular interest here is the use of CF3I gas and

high pressure air which are studied and compared with that of SF6.

The size of the gas insulated busbar depends on whether the gap between the conductor and

the outer sheath can guarantee a sufficient dielectric strength with the filling gas being

considered. However, the diameter of the busbar conductor and enclosure should also

determine the thermal performance of the equipment.

Figure 4.1 - Conductor and tank diameters for different SF6 contents with consideration of electric field strength on conductor surface and temperature rise at different pressure [20]

Figure 4.1 shows examples of the relationship between the dimensions of the inner conductor

and the outer tank at different gas pressures and for different filling gas with varied dielectric

strength. When the filling gas pressure is higher up to 0.6MPa shown in Figure 4.1(b), a

smaller size of tank is required for each kind of gas dielectric. The straight lines in two graphs

are for the tank size optimal design in which the thermal issue is not considered. The U-

shaped curves in the figures indicate the relationship between the tank inner diameter and the

conductor size based on the insulation design and heat transfer performance. Solid black

curves, light black curves and dashed curves show the influence of temperature limit to the

busbar dimension. The following studies are on the basis of this theory and method.


38

4.1.1 Benchmark Design for Comparison and Discussion

To investigate the potential for alternative forms of insulation to be used, a base design is

taken for comparison. The design is based on a 400kV busbar with a 1425kV BIL (although a

lower BIL of 1050kV is considered in some cases). The model used in based on a design

found in some National Grid substations and is shown in Figure 4.8 presented earlier in

previous chapter. The radius of the inner conductor and outer sheath are 62.5mm and 250mm

respectively. The thickness of the outer sheath is assumed to be 10mm. The gas pressure in

the busbar is 0.3MPa. Therefore, the dielectric strength of SF6 at this pressure will be

26.7kV/mm.

There are two primary factors that should be considered when deciding the diameters of the

conductor and the outer sheath. One is the permitted electric field on the conductor surface,

and the other is the temperature rise in the conductor and sheath.

In this coaxial system, the relationship between electric field, voltage and dimensions is given

by the Equation (6) (assuming a perfectly uniform insulation material with no protrusions /

imperfections on the conductors):

Where ‘r’, ‘R’ are radius of conductor and sheath respectively, ‘Em’ is the allowed electric field

strength at the surface of conductor and it is influenced by spacer efficiency and other

tolerance factor, and it is concluded in [20] that ‘Em’ is about 65% of theoretical value of a

dielectric medium.ww

For practical purposes, the design of the gas insulated switchgear (GIS) and the gas insulated

transmission line (GITL), must consider the performance of insulator gas in its ideal state and

when it is used in a real environment. Under near-ideal conditions, SF6 can operate reliably at

very high dielectric stresses as described previously. However, the dielectric strength of a

system can be greatly reduced by relatively small amounts of contamination. Boggs [61]

concluded that several factors can influence the insulation performance of a dielectric. He

commented that insulation failure in GIS usually starts with partial discharge (PD) activity

which can arise from protrusions on the conductor, freely moving conducting particles,

particles located on a spacer, floating components or spacer defects such as internal voids /

gaps. These issues are illustrated in Figure 4.2 [62]. Any of these processes may lower the

partial discharge onset and breakdown voltage of a GIS/GITL system considerably.

One of the main causes of failure in SF6 is seen to relate to the presence of conducting

particles which may be either fixed or moving in the gas. The presence of contaminating

particles has long been recognized to reduce the voltage insulating ability of gas insulated

systems. Free moving particles are moved by strong fields and might settle on the conductor

surface or insulator spacers [63]. As a result, these free conducting particles cause high field


39

distortions. The amount of the distortion and the impact this distortion will have on the electric

strength of the gas will depend on their shape and position; the longer they are and the closer

they get to the HV conductor, the more dangerous they become [64].

Figure 4.2 - Example of possible defects in GIS [62]

The design stress and reliability of SF6 insulated switchgear under normal power frequency

service conditions is crucially affected by particulate contamination. The presence of particles

on the grounded electrode causes localized field distortion to a considerable extent. Particles

in the gas spacer can significantly reduce the dielectric strength of the system more than the

influence of the roughness of electrode surface [65]. This is a view shared in a report of the

CIGRE Working group [66] identified several factors which influence the dielectric strength and

long-term performance of SF6 insulated power systems. The report stated that mobile particles

above a critical size can significantly reduce the AC Withstand Level (ACWL) but have little

effect on the BIL. Tracks have also been found on spacers following inspection of the GIS

during maintenance procedure after 10 to 20 years of service.

Figure 4.3 - Probability of breakdown for a gap containing a rod-like particle as a function of

particle length and particle (rod) radius [61]

Figure 4.3 shows the breakdown probability depending on the particle length based on a

geometry typical of disconnectors when 2100kV lightning impulse voltage is applied, the result

being calculated from the theory explained in [67]. The dashed line near zero is a result from

work by other authors which the paper suggests seriously underestimates the importance of


40

such defects. A fixed particle of 4mm length and 0.1mm radius would result in a probability of

breakdown of about 20% in such a test.

Figure 4.4 and Figure 4.5 respectively show evidence relating to the influence of particles on

the AC and lightning impulse strength respectively. The presence of particulate contamination

can reduce power frequency withstand capability of cast-resin support spacers in SF6 gas by

amounts of up to 30% depending on particulate size and disposition. In comparison the

reduction under lightning impulse conditions is less than 25%.

Figure 4.4 - 50Hz flashover characteristics of epoxy resin conical spacers under varying

degrees of metallic contamination [68]

Figure 4.5 - Limit of lightning impulse withstand capabilities of epoxy resin conical spacers

under clean and contaminated conditions [69]

The motion of and charge level of elongated particles within GIS has been simulated using

numerical methods discussed in [70, 71]. In these simulations the electrostatic, gravitational

and the drag forces were considered. For a vertically situated elongated particle, its charge ‘Q’

in pC can be expressed as following formula.


41

2 212 120 0

00

sin( )10 10

2 2lnln 1 ln 1

i

l E l UQ

rl lr

rr r

(7)

In the formula, ‘E’ is the local electric field, ‘l’ and ‘r’ are the length and radius of particle

respectively. ‘U’, ‘φ’, ‘ri’’ and ‘r0’ are the peak voltage on the inner electrode, phase and inner

and outer radius of the GIS respectively. The formula shows that the charge magnitude will

increase with particle size and radius as well as the local electric field.

Particle

Figure 4.6 - Particle stable at bottom of busbar enclosure

M.S. Indira and T.S. Ramu [72] has examined particle motion owing to the electric field in GIS

using a mathematical model. Assuming that the particles are short cylinders of radius ‘r’ and

length ‘l’ in their initial positions, as shown in Figure 4.6, a stationary but free particle acquires

a net charge proportional to its area of projection in the direction of the field as shown in

Equation (8).

)(2 0 tElrQ (8)

Ignoring the influence of neighboring particles, the force acting on the particle can be written

as:

)(2)( 2

0 tElrQtEFd (9)

The force is nearly a linear function of the length and the radius of the particle. This force will

then act on the particle to cause motion, the velocity of which will depend on a range of

parameters. The equation of motion of the particle can be written as:

)()(2)()( 22

0

2 tvkmgtElrtvkmgFtam d

(10)

In this equation ‘a(t)’ is the vector representing acceleration and ‘v(t)’ is the velocity of the

moving particle. When the particle is just starting to move, the drag force experience by it is

negligible small and hence the above equation can be modified into:


42

5.0

0

)2

)(()(

lr

mgtmatE

(11)

The mass of a cylindrical particle m can be calculated with the formula 2m r l , where ‘ρ’ is

the density of particle, 2.7g/cm3 for aluminium.

02

grE (12)

Equation (12) therefore defines a level of electric field that should not be exceeded in the

equipment to ensure that particles do not move within the enclosure. This is a function of the

physical dimensions and the mass of the particle, shown in Figure 4.7.

Based on a model of a typical 400kV National Grid GIS system, the electric field stress at the

inner surface of the sheath is 42.6kV/cm. To prevent the floating particle moving in that

particular electric field, the radius of the particle should be controlled at no less than 12.1 um

as per Equation (12), shown as a cross point of lines in Figure 4.7.

Figure 4.7 - Particle radius influence to the electric field stress withstand ability

For a compact sized busbar, the radius of the outer sheath is smaller, therefore the electric

field stress to the inner surface of sheath is stronger, and so the minimum radius of particle is

required to be bigger. In other words, the equipment is less tolerant of a larger particle. This

will only have implications for a system in which it is shown that the use of an alternative gas

can actually reduce the enclosure size.

In a real system, detection of discharge resulting from floating particles is possible. Numerical

analysis on floating particles [73, 74] shows that the floating components normally cause

0

20

40

60

80

100

120

140

160

180

200

0.0E+00 5.0E-02 1.0E-01 1.5E-01 2.0E-01

Elec

tric

fie

ld S

tre

ss (

kV/c

m)

Particle dimension (cm)

(12.1um, 42.6kV/cm)


43

partial discharge with magnitudes in the range of 104 to 10

6 pC/pulse with repetition rates of

120 to several thousand discharges per second in multiples of 120 Hz (for a 60Hz system). In

the case of a floating component (the one not joined to the conductor or sheath), the discharge

magnitude resulting from this source is sufficient to decompose SF6 gas in quantities so that it

eventually leads to failure. Acoustic detection systems [75] and narrowband ultra-high-

frequency (UHF) detection [76] could be used to examine this issue in detail.

Another design criterion that needs to be considered- the temperature rise in the conductor

and outer sheath which is defined by the rate of heat production in the conductor (proportional

to the square of the current) and the thermal resistance of the insulation material between

conductor and the outer sheath. In this work, the material for the conductor and the tank were

both aluminium. The temperature rise in the conductor and the tank surfaces used a number

of techniques [77]. The upper limit for the conductor temperature was 105°C, the upper limit

for the tank surface temperature was 70°C, and the ambient temperature should be restricted

at no more than 40°C respectively according to the relevant standard [78]. The rated current of

the benchmark busbar previously described is 4000A and the sheath temperature is

calculated at 50°C which is well below the maximum temperature limit.

250mm62.5mm

5mm

Insulating

medium

Conductor

Sheath

Qconv

Qrad Qconv’

Qrad’

Figure 4.8 - Single conductor busbar model based on that found in National Grid substations

Heat will be transferred from the conductor to the sheath in three ways, radiation, conduction

or convection. The heat transfer per metre due to radiation is based on the Stefan-Boltzmann

law [79] where ‘T1’ and ‘T2’ are the conductor and sheath temperatures respectively, ‘A1’ and

‘A2’ are the radiation areas of the surfaces in m2. ‘ε1’ and ‘ε2’ are the surface emissivity of

conductor and outer sheath respectively. They are dependent on the material and the surface

conditions, whether it is polished or bared. The ideal 'black body' with the emissivity coefficient

ε = 1. The highly polished aluminium is 0.04-0.05, roughened aluminium surface is 0.275, for

polished copper is 0.03 [3, 80]). Both ‘ε1’ and ‘ε2’ are taken to be 0.1 used in previous study

[81]. ‘σ’ is the Stefan-Boltzmann constant 5.69E-8 (W/m2.K

4).


44

)11

(1

1)(

22

1

1

1

4

2

4

1

A

AATTWrad (13)

The heat transferred by convection is described from an equation produced from McAdams

research [79] where ‘d1’ and ‘d2’ are the outer conductor and inner sheath diameters in meter

respectively, ‘P’ is the gas pressure in kgf/cm2 and ‘K0’ is the gas constant of the filled gas

(24.4 for SF6, 17.5 for N2).

25.1

6.0

2

1

2

1

25.1

21

75.0

1

6.0

0

1)ln(

)(

d

da

d

d

TTdPKWconv (14)

Thermal conduction is the elastic collision in a fluid or the oscillation of atoms and transport of

free electrons in a solid and continuous medium. The heat transferred by conduction obeys

the Fourier’s law [82], the heat flux is proportional to the ratio of temperature over space. As

shown in Equation (15), the load heat ‘W’ is proportional to the local temperature difference in

the x direction, and the thermal conductivity of material ‘k’. ‘A’ is the area of contacting surface.

TAkdx

TdkAWcond

(15)

The heat losses generated from conductor and sheath ‘W’ are decided by the AC resistance of

conductor ‘Rac’ and the square of the current ‘I’, and this dependency is expressed in Joule

Law as in Equation (16). Also the dependence of resistance on temperature is given in

Equation (17).

2IRW ac (16)

))20(1(, condac TRR (17)

Where ‘α’ is the metal constant, 0.00403 for aluminium. ‘Tcond’ is the working temperature in

Kelvin of conductor. The AC resistance of conductor depends on the material and its operating

temperature. With the different insulation materials, a differing busbar size will be expected

owing to the dielectric capability of the material while and this along with the thermal properties

of the material will lead to a differing current capability.


45

The heat sources in the system are the heat generated in the busbars and the heat loss from

outer sheath ‘Ws’ due to the effect of induced currents (about 20% of conductor loss). To

simplify the study model, the relationship between the conductor and the sheath loss is fixed

at 20% through this work. The relationship between heat flow ‘W’ and conductor / sheath

temperature is calculated using:

25.175.0

11

22

1

1

44

1 )(

)11

(1

1)

100()

100(67.5 sheathcond

nsheathcond TTdPK

d

d

TTdWc

(18)

The relationship between heat flow ‘W’ and sheath / ambient temperature is calculated using:

25.175.0

323

44

3 )()100

()100

(67.5 ambientsheathambientsheath

S TTdKTT

dWWc

(19)

A number of other assumptions are made including that any solid material is homogeneous

and isotropic, and the physical parameters of these materials are constant. The inner heat

sources are also uniformly distributed, which means there is no temperature difference

between the upper part and lower part of a horizontally laid busbar.

Figure 4.9 - The dependences of temperature of conductor and sheath on the current rating of

SF6 gas insulated busbar

Using these equations, the temperature of the conductor and sheath of the 400kV National

Grid benchmark busbar as a function of current is described in Figure 4.9. With a higher

current carrying requirement, the temperatures of the conductor and the sheath both increase,

and the difference between the conductor and the sheath temperature increases too. The

result suggests an available current rating of up to 6000A at which point the conductor

0

20

40

60

80

100

120

140

160

180

0 2000 4000 6000 8000 10000

Tem

per

atu

re in

ce

lsiu

s

Current Rating (A)

conductor temperature

sheath temperature

105°C

70°C


46

temperature is at 105°C (black solid line), and the sheath temperature remains below the

criteria (black dashed line).

Figure 4.10 - Current carrying capability with different sheath dimension (fixed conductor)

As seen in Figure 4.10, the current rating for the existing busbar system (d1/d2 =

0.125m/0.49m) can be raised to around 7022A by having a smaller sheath diameter. However

while a more compacted busbar can give a better current carrying capability, this system will

not fulfil the dielectric strength requirement. Vice versa, a bigger size of busbar can make it

possible to use another dielectric with a lower dielectric strength than that of SF6, but it

compromises the current rating of a system.

Using the electrical and thermal techniques explained above, the ability to replace SF6 in the

benchmark busbar system will be investigated in a number of ways.

4.1.2 Fixed Size of GIS Enclosure with Changing Gas Pressure

In this first set of calculations, the existing 400kV gas insulated busbar enclosure size is

retained and it is insulated with alternative gases at different pressures. The inner conductor

diameter is 12.5cm and the outer sheath diameter is 50cm. The dielectric performance is

evaluated with limiting E/N values, the pressure of the gas being varied to deliver the required

dielectric strength to withstand the BIL of 1425kV.

6950

7000

7050

7100

7150

7200

7250

7300

0.00 0.20 0.40 0.60 0.80

Cu

rren

t ca

rryi

ng

cap

abili

ty (

A)

Sheath inner diameter (m)

d2:d1= 0.49: 0.13 I = 7022A


47

In mixed gases, the calculation of the boiling point of the mixed gas is carried out using the

partial pressure of the gas with the higher of the two boiling points, described in the following

equation [83].

5.10

)10ln(1

kP

TT b

mb

(20)

Where ‘k’ is the mixing ratio (ratio of partial pressure) for a total pressure ‘P’.

For the gas system, the thermal rating is estimated as previously described. For oil it has to be

modified slightly as the heat generated from the conductor can be released through an oil

dielectric by the means of conduction and convection. Assuming the temperature rise of

ambient air does not exceed 40°C and the conductor temperature is kept at a maximum of

105°C, which is stated in standard [78] , the estimated current rating of an oil insulating system,

based on different insulating gap distances ‘d’ and for three sizes of inner conductor radius ’r’

is shown in Figure 4.11. The cross point marked on the graph represents the original

dimension 6.25cm/25cm for the conductor radius and the outer sheath radius. The current

carrying capability is improved up to 7849A with oil insulation, bigger than that of the SF6

insulated busbar of which the rated current is 4000A.

Figure 4.11 - Current rating of oil insulated system depending on the insulation thickness

Taking the techniques described above and applying them to the busbar system using the

same dimension as the benchmark case, the results in Table 4.1 are generated. All the

candidates are required to withstand the Basic Insulation Level (BIL) of a 400kV system at

1425kV. The working pressure of filled gas is calculated according to the E/N critical value of

each gas. The calculation result for SF6 gives a pressure close to the practical pressure at

National Grid Substations which gives confidence in the calculations. The current carrying

capability is calculated according to the heat flow between the conductor and the outer sheath.

r1=5.75cm, 7.25,

6230.37

r1=5.75cm, 7.75,

6296.04

r1=5.75cm, 8.25,

6364.32

r1=5.75cm, 8.75,

6434.57

r1=5.75cm, 9.25,

6506.30

r1=5.75cm, 9.75,

6579.13

r1=5.75cm, 10.25,

6652.76

r1=5.75cm, 10.75,

6726.93

r1=5.75cm, 11.25,

6801.47

r1=5.75cm, 11.75,

6876.20

r1=5.75cm, 12.25,

6951.01

r1=5.75cm, 12.75,

7025.80

r1=5.75cm, 13.25,

7100.47

r1=5.75cm, 13.75,

7174.96

r1=5.75cm, 14.25,

7249.22

r1=5.75cm, 14.75,

7323.20

r1=5.75cm, 15.25,

7396.86

r1=5.75cm, 15.75,

7470.18

r1=5.75cm, 16.25,

7543.13

r1=5.75cm, 16.75,

7615.69

r1=5.75cm, 17.25,

7687.85

r1=5.75cm, 17.75,

7759.59

r1=5.75cm, 18.25,

7830.91

r1=5.75cm, 18.75,

7901.80

r1=5.75cm, 19.25,

7972.25

r1=5.75cm, 19.75,

8042.27

r1=5.75cm, 20.25,

8111.85

r1=5.75cm, 20.75,

8180.99

r1=5.75cm, 21.25,

8249.69

r1=5.75cm, 21.75,

8317.95

r1=5.75cm, 22.25,

8385.78

r1=5.75cm, 22.75,

8453.18

r1=5.75cm, 23.25,

8520.15

r1=5.75cm, 23.75,

8586.70

r1=5.75cm, 24.25,

8652.82

r1=5.75cm, 24.75,

8718.53

r1=5.75cm, 25.25,

8783.83

r1=5.75cm, 25.75,

8848.72

r1=5.75cm, 26.25,

8913.21

r1=5.75cm, 26.75,

8977.30

r1=6.25cm, 7.25,

6362.33

r1=6.25cm, 7.75,

6424.41

r1=6.25cm, 8.25,

6489.39

r1=6.25cm, 8.75,

6556.59

r1=6.25cm, 9.25,

6625.50

r1=6.25cm, 9.75,

6695.70

r1=6.25cm, 10.25,

6766.87

r1=6.25cm, 10.75,

6838.74

r1=6.25cm, 11.25,

6911.11

r1=6.25cm, 11.75,

6983.80

r1=6.25cm, 12.25,

7056.68

r1=6.25cm, 12.75,

7129.63

r1=6.25cm, 13.25,

7202.57

r1=6.25cm, 13.75,

7275.41

r1=6.25cm, 14.25,

7348.09

r1=6.25cm, 14.75,

7420.57

r1=6.25cm, 15.25,

7492.80

r1=6.25cm, 15.75,

7564.75

r1=6.25cm, 16.25,

7636.38

r1=6.25cm, 16.75,

7707.68

r1=6.25cm, 17.25,

7778.63

r1=6.25cm, 17.75,

7849.20

r1=6.25cm, 18.25,

7919.40

r1=6.25cm, 18.75,

7989.21

r1=6.25cm, 19.25,

8058.62

r1=6.25cm, 19.75,

8127.64

r1=6.25cm, 20.25,

8196.24

r1=6.25cm, 20.75,

8264.45

r1=6.25cm, 21.25,

8332.24

r1=6.25cm, 21.75,

8399.62

r1=6.25cm, 22.25,

8466.60

r1=6.25cm, 22.75,

8533.18

r1=6.25cm, 23.25,

8599.35

r1=6.25cm, 23.75,

8665.11

r1=6.25cm, 24.25,

8730.48

r1=6.25cm, 24.75,

8795.46

r1=6.25cm, 25.25,

8860.04

r1=6.25cm, 25.75,

8924.24

r1=6.25cm, 26.25,

8988.05

r1=6.25cm, 26.75,

9051.49

r1=6.75cm, 7.25,

6491.40

r1=6.75cm, 7.75,

6550.06

r1=6.75cm, 8.25,

6611.89

r1=6.75cm, 8.75,

6676.19

r1=6.75cm, 9.25,

6742.40

r1=6.75cm, 9.75,

6810.08

r1=6.75cm, 10.25,

6878.89

r1=6.75cm, 10.75,

6948.55

r1=6.75cm, 11.25,

7018.84

r1=6.75cm, 11.75,

7089.57

r1=6.75cm, 12.25,

7160.59

r1=6.75cm, 12.75,

7231.78

r1=6.75cm, 13.25,

7303.05

r1=6.75cm, 13.75,

7374.30

r1=6.75cm, 14.25,

7445.47

r1=6.75cm, 14.75,

7516.50

r1=6.75cm, 15.25,

7587.34

r1=6.75cm, 15.75,

7657.96

r1=6.75cm, 16.25,

7728.33

r1=6.75cm, 16.75,

7798.41

r1=6.75cm, 17.25,

7868.18

r1=6.75cm, 17.75,

7937.63

r1=6.75cm, 18.25,

8006.74

r1=6.75cm, 18.75,

8075.51

r1=6.75cm, 19.25,

8143.91

r1=6.75cm, 19.75,

8211.95

r1=6.75cm, 20.25,

8279.61

r1=6.75cm, 20.75,

8346.90

r1=6.75cm, 21.25,

8413.82

r1=6.75cm, 21.75,

8480.35

r1=6.75cm, 22.25,

8546.50

r1=6.75cm, 22.75,

8612.27

r1=6.75cm, 23.25,

8677.66

r1=6.75cm, 23.75,

8742.67

r1=6.75cm, 24.25,

8807.31

r1=6.75cm, 24.75,

8871.57

r1=6.75cm, 25.25,

8935.46

r1=6.75cm, 25.75,

8998.98

r1=6.75cm, 26.25,

9062.13

r1=6.75cm, 26.75,

9124.92

Carr

ied

cu

rren

t (A

)

Insulation thickness (cm)

r1=5.75cm

r1=6.25cm

r1=6.75cm

17.75

(17.75cm,


48

The results of the calculations described above are given in Table 4.1. The thermal

calculations show that forced internal convection is the dominant form of heat transfer in

liquids and gases. The conductor can carry more current than the SF6 solution if the enclosure

is insulated with oil. That is due to the high thermal conductivity of oil, through which it can

transfer heat as part of the convection process. However, there are environmental concerns

regarding oil and the high density of oil means it may not be suitable in all cases.

While oil appears to be an excellent option as a dielectric material, the current carrying

capability of SF6 filled busbar is still superior to the remainder of the candidate materials

examined. The performance of CF3I is partly compromised as the gas pressure is reduced to

0.23MPa which would have an unrealistically high boiling point (a value of 271.69K at this

pressure). The gas mixture of 70%CF3I-N2 has the same dielectric strength as that of SF6 at

same pressure at 0.27MPa, with a boiling point at 267.12K. The required pressure for the gas

mixture with a lower concentration of CF3I in N2 can be as high as 0.29MPa. The boiling point

of the gas mixture is reduced to 254.96K, which can be used more reliably in a colder area.

While for foam, heat generated from the conductor can only be released by conduction in solid

foam.

The practical sheath temperature for each candidate including SF6 is calculated, and it is

below the upper limit 70°C specified in the standard. The current carrying capability of busbar

is then calculated and shown in Table 4.1. The current rating of SF6 insulated busbar is still

superior to other gaseous candidates.


49

Table 4.1 - Optimal working pressure and temperature of alternatives working at fixed dimension d1/d2/d3=125mm/490mm/500mm

Material Working pressure

(MPa)

Thermal Emission (W)

Allowable E field (kV/cm)

Boiling Point (K)

Sheath temperature and achievable current rating

Conduction Radiation Convection Sheath

Temperature (K) Current (A)

Oil / 20.28 / 242.45 / / 339.03 9674

SF6 1 0.27 1.95 11.96 140.80 16.82 231.00 332.71 8231

CF3I 2 0.23 1.61 11.96 104.83 17.39 271.69 329.68 7476

70%CF3I/N23 0.27 1.61 11.96 98.10 16.82 267.12 329.04 7315

60%CF3I /N2 4 0.29 1.61 11.96 96.37 16.77 254.96 328.88 7272

Dry air 5 0.91 4.02 11.96 119.63 18.53 167.40 331.16 7854

1: SF6 is at 0.27MPa, K0=24.4,

2: CF3I is at 0.23MPa, K0=20.0,

3: 70%CF3I/N2 is at 0.27Mpa K0=17,

4 60%CF3I/N2 is at 0.29Mpa K0=16

5: Dry air is at

0.91MPa, K0=10,


50

4.1.3 Fixed Gas Pressure with Changing Busbar/outer Shell

Dimensions

This case used the same methodology as the last but the gas pressure is fixed at 0.3MPa,

which is the current working pressure in commercially used 400kV busbars. The enclosure

size for each kind of SF6 replacement is varied, and the current carrying capability is therefore

different from each other.

The result in Table 4.2 shows that by adopting pure CF3I gas, the busbar dimension can be

reduced by 21% in comparison to that of SF6, However, the boiling point of CF3I and its

mixture when working at 0.3MPa is already above 0˚C and this is likely to be an issue. The

60%CF3I-N2 gas mix has a reasonable current carrying capability coupled with just a small

increase in dimension. The boiling point of this mixture is also improved in comparison to pure

CF3I at 280K. This would appear to be a better solution than insulating foam given the

significant increase in current capability for a small dimensional change.

Table 4.2 - Fixed pressure at 0.3MPa, varied outer dimension

Based on the newly calculated busbar dimension, the allowed minimum particle sizes for the

400kV busbar system insulated with new alternatives respectively can be estimated and

compared with the result obtained in earlier Section 4.1.1 It is suggested that no matter what

kind of insulating medium is used, the size of particle inside the existing busbar should be no

less than 12.1um. Movement of smaller particles than that will float in the space between the

conductor and the sheath, and induce discharge in the insulating gas, which is detrimental to

the overall insulating performance. The calculated result for the new alternative insulation

medium is presented in Table 4.3.

Material E/N

Critical (Td)

Gas Boiling point

@0.3MPa

E field strength at

0.3MPa (kV/mm)

Allowed E field stress (kV/mm)

Enclosure Radius (mm)

Current (A)

SF6 361 233.59 26.20 18.34 217 7250

80%SF6-N2 260 254.84 18.87 13.21 351 6987

CF3I 437 279.94 31.72 22.21 175 6637

70%CF3I- N2 361 270.74 26.20 17.30 234 6139

60%CF3I- N2 330 266.95 23.95 15.81 265 5964

Dry AIR 108 147.59 7.84 5.18 5125 4576


51

Table 4.3 - Minimum particle size that will not move from floor of 400kV busbar systems with alternative insulation

The calculated data shows that the bigger busbar dimensions (same conductor size) give rise

to a lower electric field on the inner surface of the sheath and this means that only particles of

a relatively small size will be able to move. Assuming a reasonably uniform distribution of

particle sizes in an enclosure, a smaller minimum size will mean that there are a smaller

number of particles which can be moved by the electric field. In addition, these smaller

particles will have a lower associated charge and are therefore less likely to cause dielectric

failure.

As an example, the radius of the outer sheath of the CF3I insulated busbar is smaller than the

current system and this means that any particle with a size smaller than 22.2um can be moved

by the electric field. This means a larger number of particles are likely to be available when

compared with an SF6 busbar and the larger charge on these larger particles is more likely to

influence the dielectric strength. For this reason, it may be prudent to use CF3I in a manner in

which its peak dielectric strength is not being utilised (i.e. by having an enclosure the same

size as an SF6 version) meaning particle vulnerability is less of an issue.

4.1.4 Impact of Reduced Basic Insulation Level

The insulation level of equipment specifies the voltage it must be able to withstand under test

conditions. According to the lowest values specified in IEC 60071-1 [84], for the 400kV

systems the lowest values of rated lightning impulse withstand voltage to earth (1.2/50us) and

rated switching impulse withstand voltage to earth (250/2500us) are stated to be 1050kV and

850kV respectively. National Grid equipment is usually selected to have values of 1425kV and

1050kV. As has been shown by several case studies on compact substation design by

National Grid [85], a reduction in dimensions can be achieved by adopting IEC minimum

clearances. For an AIS bay, a maximum reduction in bay length of 32% is achieved compared

with a conventional bay length. An associated reduction in bay area of 52% is obtained.

Material Enclosure Radius

(mm)

Maximum Electric field stress to the inner surface

of sheath (kV/cm)

Allowed minimum particle size (um)

Any random material

245 42.6 12.1

SF6 217 52.8 18.2

80%SF6-N2 351 23.6 3.0

CF3I 175 79.1 22.2

60%CF3I- N2 265 37.2 9.5

Dry AIR 5125 0.6 0.003


52

It would therefore appear that the BIL for 400kV GIS can therefore probably be reasonably

and safely reduced to 1050kV (subject to appropriate use of surge arresters) and the

dimensions of the busbar system calculated in the above section can be reassessed and

minimized.

The following text describes simulations that have been carried out to illustrate how a

reduction in lightning transient overvoltage levels can be easily achieved with the use of surge

arresters. This reduction in overvoltage level is then shown to be able to lead to a reduction in

enclosure size for busbars developed using the alternative gases previously presented. This

analysis does not consider the need to maintain the AC electric field level below a specific

value to prevent the movement of floating particles.

A simple analysis has been carried out using the PSCAD software to examine the magnitude

of overvoltage within a substation that is protected by a surge arrester. Figure 4.12 shows a

model representative of a simple gas insulated substation. Two ‘cable’ sections are used with

their geometry and properties being set to be identical to that of the National Grid GIS busbar

model described in Chapter 2.5 . A Norton equivalent source is used to inject a transient to

one end of the busbar model at amplitude equal to the BIL of the equipment. Optionally, surge

arresters are fitted to each end of the GIS sections which also has a circuit breaker fitted at its

mid-point.

CB LoadBusbar 2Busbar 1

Other components in

systemSA SA

Lightning

Strike

(a) Line diagram of study model

(b) PSCAD implementation

Figure 4.12 - Simulation model with surge arresters installed


53

Two sets of simulation results describing the peak voltage observed on the busbar system

depending on its length are compared in the graph shown as Figure 4.13. The peak observed

on the busbars is nearly twice that of the incoming surge for a short busbar section, i.e. over

2500kV (Red-dotted line). For the system protected with surge arresters, the voltage is

significantly reduced (Blue-dotted line) below 1425kV - the standard BIL level for 400kV

system. Thus, while only a simple simulation, this illustrates the fact that the surge arrester

could play a significant role in controlling the voltage observed on a GIS system.

720

730

740

750

760

770

780

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Line distance (km)

Voltage with SA (kV)

0

500

1000

1500

2000

2500

3000

Voltage with SA

Voltage without SA

Voltage without SA (kV)

Figure 4.13 - Comparison between the peak voltage with and without SA

This finding matches more complex studies carried out by other authors. A series of case

studies carried out on four compact substations [85] also proved that with a surge arrester at

each line entrance, the overvoltages at the substation are reduced below 1050kV. The

advantage of a lower transient overvoltage level should be a reduced requirement for

insulation given the lower electric fields that will exist in the system.

The impact of using a reduced transient level, moving from the 1425kV highest level in

IEC60071-1 [84] to a value of 1050kV on the enclosure dimensions for each kind of dielectric

gases are calculated as shown in Table 4.4. In this calculation, the inner conductor radius is

fixed at 6.25cm. The dimension has been reduced in respect of each kind of gas in both cases.

With increasing system voltage, proper co-ordination of the insulating level becomes more

critical in order to reduce the amount of insulation in the system.


54

R (cm) at different BIL at

fixed boiling point at -20°C R (cm) at different BIL at fixed pressure at 0.3MPa

Dielectric medium

1425kV 1050kV Reduction

in size 1425kV 1050kV

Reduction in size

SF6 11.4 9.73 14.65% 23.84 16.76 29.70%

CF3I 100.45 48.37 51.85% 18.89 14.12 25.25%

70%CF3I- N2 68.14 36.34 46.67% 23.84 16.76 29.70%

60% CF3I- N2 60.52 33.30 44.98% 27.03 18.39 31.97%

Dry AIR 6.79 6.64 2.21% 548.42 168.94 69.20%

N2 6.8 6.27 7.79% 257.19 96.7 62.40%

N2O 10.18 8.95 12.08% 130.56 58.68 55.06%

10%SF6-N2 7.56 7.19 4.89% 46.81 27.56 41.12%

Table 4.4 - Dimension reduction with lower BIL

This is a significant reduction in size in all cases and it is therefore shown that the use of

alternative gases coupled with a reduction in basic impulse level is a promising option. In a

real system, insulation coordination studies much more complex than the one shown above

would have to be carried out but the simple model used is sufficient to at least demonstrate

the principle of the impact of surge arresters in reducing the transient overvoltage and

therefore the enclosure dimensions.

4.2 Analysis of the Use of Coatings Combined with Gas

Insulating

The use of coatings around conductors / sheaths has been shown to improve dielectric

strength in the work [86]. Coatings around the conductor as shown in Figure 4.14 are

expected to help to increase the dielectric strength of the system and to reduce the impact of

floating particles. Coatings applied at the inner surface of outer enclosure of a coaxial busbar

system are effective in inhibiting particle movement and improving the insulation performance

[103-106].

Gas insulation ε2

Epoxy resin coating ε1

Conductor

R

r

t

rConductor

R

Gas insulation ε1

Aluminium sheet with

dielectric coating

(a) (b)

Figure 4.14 – GIS busbar with coating on conductor(a) or inner surface of sheath(b)


55

4.2.1 Review of Voltage Capability of Insulation Coatings

Air and N2 have previously been considered for use as insulation materials. However,

extremely high pressures are needed to build compact substation. However, by placing a

coating of a suitable thickness around conductor in combination with such gases, a more

compact system can be developed.

Figure 4.15 - Comparison of dielectric strength of coated electrode in air compared with pure

air and SF6/N2 mixed gas [59]

Figure 4.15 shows that the dielectric strength of air when combined with a coating is higher

than the one without the coating and can reach a level comparable or better than SF6. The

most common coating materials are epoxy resin and rubber. These materials have a high

relative permittivity as shown in Table 4.5. They are helpful for reducing the field stress to the

surface of electrode.

Material Relative

Permittivity εr Thermal conductivity k

(W/m.k)

Epoxy resin 4.2 0.26

Rubber 7 0.16

Table 4.5 - Characteristics of different coating materials

J.Sato [87] in 1999 realized a 72/84kV composite insulation switchgear reduced in size by 50%

through the use of epoxy coated electrodes with SF6 insulation. Experiments [88] in 2002

proved that the breakdown voltage of a liquid rubber covered electrode can be 1.5 times

higher than that of bare electrode in air or N2 at 0.2MPa.

Toshiaki etc. [89] improved the development of SF6-free 72. 5kV GIS at 2007, by introducing

dry air, N2, and N2/O2 mixed gas combined with insulating coatings. This option is examined in

the next sect


56

Coatings can help to reduce the charge acquired by a moving particle upon its contact with the

enclosure. This leads to considerable improvement in GIS insulation strength. S. Zhang [90]

proved that by increasing the coating thickness the breakdown voltage can be increased.

However, any further increase in coating thickness beyond about 200um seems to produce no

appreciable gain in breakdown strength. The appropriate coating thickness would depend

upon economic and manufacturing considerations.

4.2.2 Assessment of Use of Coatings around Conductors

This section examines the theoretical size of systems in which coatings are applied to the

conductor. The electric field stress to the dielectric medium and coating can be calculated

through the following steps. Take the case of National Grid busbar shown in section 4.1.1 with

a conductor radius ‘r’, and a epoxy resin with thickness ‘t’ is coating around the conductor,

placed in the gas insulated tube with outside shielding radius ‘R’. The epoxy resin has a

relative permittivity ‘ε1’, while for gas is ‘ε2’.

For a given charge per unit length on the busbar of ‘q’ C/m, the electric flux and hence the

electric field for values of x between ‘r’ and ‘R’ can be written as:

2x

qD

x

(21)

2x

q DE

x

(22)

The expression for electric field is integrated to give the potential difference between the tube

and the conductor as follows:

R

tr

tr

r

tr

r

R

tr

R

rdr

Ddr

DdrEdrEdrEV

2010 ( 23)

)ln(

1)ln(

1

22 210210 tr

R

r

trq

r

dr

r

drqV

tr

r

R

tr (24)

Using the standard relationship between capacitance per unit length, charge per unit length

and voltage:

Vcq (25)


57

Then the electric field stress to gas solid parts can be described respectively in following two

equations.

)(

)ln(1

ln1

21

11 trxr

xtr

R

r

tr

V

E

(26)

)(

)ln(1

ln1

21

22 Rxtr

xtr

R

r

tr

V

E

(27)

Figure 4.16 - Electric field distribution with varied thickness of solid insulation

The result in Figure 4.16 shows that the peak electric field seen in the gas is reduced with the

help of the coating. The thicker the coating is, the more electric field strength is reduced. For

example, the peak electric field stress in the gas is 12% weaker when there is a 10mm thick

coating around the conductor. Due to the reduced electric field stress to the gas insulation, the

requirement for the gas dielectric strength is lower and therefore the gas filling pressure can

be brought down to some extent or the enclosure size can be reduced. In that case, it is

possible to use CF3I gas insulation at a lower pressure to keep the boiling point at a

reasonable level.

4.2.3 Review of Current Carrying Capability of Coated Conductor

When the central conductor is covered by epoxy resin insulator at a thickness of 10mm, the

heat generated from the conductor is transferred only by conduction to the surface of the

coating. To further develop the feasibility studies already outlined, the current carrying

0.0

5.0

10.0

15.0

20.0

0.0 50.0 100.0 150.0 200.0

Ele

ctri

c fi

eld

(kV

/mm

)

Distance x (mm)

t=0 t=3mm t=5mm t =10mm t=15mm


58

capability of the coated conductor model is then reviewed. The epoxy resin is stable at solid

state at a temperature of 105ºC.

For heat flow in a cylinder between a conductor surface (radius ‘r1’) and the outside coating

surface (radius ‘r2’), assuming no significant longitudinal heat loss, the temperature difference

‘Δθ’ across the coating with 10mm thickness can be found by solving [91]:

lkg

rrgQ

2

)/ln( 12 (28)

Where ‘Q’ is heat energy input per unit time, ‘l’ is length of sample conductor, and ‘k’ is the

thermal conductivity of coating material.

The heat through the hollow cylinder coating by conduction is calculated:

)/ln(

2

12 rr

kl

RW

th

cond

(29)

When the current rating is increased more than 4000A, the temperature difference of the resin

coating becomes significant with a value larger than 4 Kelvin. If 6000A current flows through

the conductor, and all the heat generated from the conductor is transferred through the

coating, the temperature difference of the two surfaces of the epoxy resin coating is calculated

at 10 Kelvin/m. This difference is significant and it cannot be ignored. . For a rubber coating

with a smaller thermal conductivity value, the temperature difference is even bigger, and can

be up to 17 Kelvin per meter when the conductor is carrying a 6000A current. The relationship

between the temperature of the coating surface and the current rating is shown in Figure 4.17.

Figure 4.17 - Calculated coating surface temperature at different carried current

350

355

360

365

370

375

380

0 1 2 3 4 5 6 7 8

Tem

per

atu

re (

K)

Carried Current (kA)

Epoxy resin coating surface temperature Conductor temperature limit rubber coating surface temperature


59

As mentioned earlier, the coating around the conductor can help to reduce the electric field

stress to the conductor. A new set of busbar dimensions and the associated current carrying

ability can be calculated following the theory described in Section 4.1.1 The total heat loss

from the current carrying conductor all go through coating, and all are transferred by the ways

of conduction, convection and radiation through the dielectric.

The busbar dimension is fixed, but with 10mm epoxy resin coating laminated around the

conductor, the current carrying ability for each candidate insulated busbar is calculated in

Table 4.6. Compared with the result when there is no coating around the conductor, as shown

in Table 4.1, it seems that the coating brings down the capability carrying current of the busbar

using all the insulation candidates like oil and gases. Meanwhile, the ability to withstand

lightning impulse stress of a coated busbar is 5% greater than an uncoated one.

Table 4.6 - Current rating for busbar in existing dimension but with 10mm coating

Table 4.7 - New sheath size and respective current rating when the pressure is fixed at 0.3MPa (coated conductor ‘r’=72.5mm)

Material Wcond

coating (W)

Thermal Emission of insulant (W) Boiling

Point (K) Current

(A)

New withstand LI voltage Wcond Wconv Wrad

Oil

66.04

16.8 / 61.5 / 6487

1493kV

SF6 1 1.61 10.88 128.78 231.00 6716

CF3I 2 1.33 10.88 93.31 271.69 5799

70%CF3I/N2 3 1.33 10.88 87.33 267.12 5632

60%CF3I /N2 4 1.33 10.88 85.79 254.96 5588

Dry air 5 3.33 10.88 106.49 167.40 6202

1: SF6 is at 0.27MPa, K0=24.4,

2: CF3I is at 0.23MPa, K0=20.0,

3: 70%CF3I/N2 is at 0.27Mpa K0=17,

4

60%CF3I/N2 is at 0.29Mpa K0=16 5: Dry air is at 0.91MPa, K0=10,

Material

Gas Boiling point at working pressure

(K)

E field strength at

0.3MPa (kV/mm)

Allowed E field stress (kV/mm)

Sheath inner

Radius (mm)

Current rating of coated conductor (A)

SF6 233.59 26.20 18.87 206 8926

80%SF6-N2 254.84 18.87 13.59 308 8637

CF3I 279.94 31.72 22.84 172 8170

60%CF3I- N2 204.36 23.95 17.25 227 7944

Dry AIR 147.59 7.84 5.64 2359 5746

Foam E=16.2kV/mm 256 1427


60

When the coated conductor is applied in a new designed busbar system with different

dimensions of outer sheath as described in Table 4.2, the new current rating of each busbar is

calculated and given in Table 4.7. It can be found that with the help of 10mm coating around

the conductor, the size of the outer sheath is slightly smaller than the result calculated in Table

4.2. And therefore the current carrying ability of a busbar with a coated conductor becomes

greater.

Therefore it is concluded that coating can do a lot to help the transmission busbar, in that the

electric field stress to the gas is reduced by 12%. Based on that advantage, the required

dimension of outer sheath can be minimized; therefore the current carrying ability is increased

to some extent.

4.3 Conclusion of Alternative Gas Usage

Through the review of recent literature and calculations, CF3I is shown to be a promising

material among all the candidates. The most possible replacement for CF3I has a comparable

dielectric strength to SF6; the limiting electric field (E/N)lim under the uniform electric field is

higher than that of SF6 gas. The boiling point of CF3I is high at -22.5°C, which means its pure

gas cannot be used at high pressures. A varied percentage of N2 or CO2 is mixed with CF3I to

reduce the boiling point of the gas mixture. When considering its gas mixture usage in a

practical 400kV substation based upon (E/N)lim values, the optimal dimension for 400kV

busbar is re-calculated and a number of viable options have been described although a

compromise would always occur if SF6 is not used. In addition, the influence of their working

pressure and boiling point to busbar dimensions has been studied. .

Floating particles in insulating gas is one of the causes of breakdown or insulation failure.

Minimum particle size is calculated according to each size of busbar insulated with a different

insulator medium. The calculated data shows that a lower electric field is stressed to the inner

surface of the sheath when with bigger busbar dimensions (the same conductor size).

Therefore very small particles are able to move, and these smaller particles will have a lower

associated charge and are therefore less likely to cause dielectric failure. For this reason, it

may be prudent to use CF3I in a manner in which its peak dielectric strength is not being

utilised (i.e. by having an enclosure the same size as an SF6 version) meaning particle

vulnerability is less of an issue.

Solid material like epoxy resin working as a conductor coating can be an option when

combined with gas insulation. Coating of a certain thickness can help to reduce the electric

field stress to insulating gas, which means the dielectric strength of filling gas is not expected

to be as strong as that which was considered before. However, the current rating of the gas

insulated busbar and the oil insulated busbar is reduced due to the introduction of the coated


61

conductor, while on the other hand, the coating can help to improve the current carrying

capability of the foam insulated busbar system. However, the problems that can occur by the

existence of solid coating, such as floating particles after discharge, are not included in this

thesis, and will be studied in a future project.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium

62

Chapter 5 EVALUATION OF FOAM AS A

REPLACEMENT FOR SF6 AS AN INSULATION

MEDIUM

In this section, the results of experimental measurements on foam insulation are presented.

Foam insulation has already seen application as an SF6 alternative to insulate vacuum

switchgear and its light-weight means it would be an ideal candidate alternative insulation. It is

clearly significantly different to the discussions of the previous chapters that are focused on

alternative gases. Some techniques used in the previous chapters, such as size estimation,

are once again applied to develop busbar designs that use the results of the experimental

testing as inputs. However, as will be shown, the voids present within the foam raise some

unique issues its application.

5.1 Electrical Breakdown in Solid Insulation

The electrical breakdown mechanism of a solid material is a complex phenomenon and not

totally understood, unlike gaseous insulation breakdown. The damage to solid insulation is

permanent when breakdown occurs. There are several factors that influence the breakdown

strength, such as the time of application of voltage, the physical features of the material itself

and environmental condition.

Electric breakdown of a solid can be categorised as volume breakdown across the thickness

of a solid sample or tracking on the surface. In this work, the volume breakdown is the study

subject. According to literature [92] , the mechanism for volume breakdown of solids can be

classified as the following types. However, it is not possible to distinguish clearly between

these breakdown categories.

Intrinsic breakdown

Avalanche or streamer breakdown

Electromechanical breakdown

Edge breakdown


63

Thermal breakdown

Internal erosion breakdown

The breakdown strength changes with the time of voltage application, and different time scales

are set for specific regions in which different mechanisms operate, as shown in Figure 5.1 [92].

Figure 5.1 - Mechanisms of failure and variation of breakdown strength in solids with time of

stressing [92]

As seen from the figure, it is not easy to measure the intrinsic breakdown voltage using

experimental methods as it happens in a quite short time. The electromechanical breakdown

voltage is reached when the electrostatic compression force impressed on the solid material

exceeds its mechanical strength. When the heat generated within the dielectric is increased as

the voltage stress applied to specimen increases, thermal breakdown can happen. The

specimen may undergo erosion breakdown when the solid material contains cavities or voids,

which are filled with gas or liquid of lower breakdown strength than the solid. The filled

medium normally has a lower permittivity than that of solid material, and this leads to field

intensification in the cavity so the field is higher than in the dielectric. This can ultimately

initiate breakdown in the solid material.

Furthermore, the dielectric strength of an insulation material as well varies with the shape of

voltage waveform. Breakdown of an insulation material in practice occurs under AC or impulse

overvoltages at a lower level than under DC conditions. The AC voltage in a power system,

either 50Hz or 60Hz, constantly varies polarity, while an impulse voltage, such as a lightning

impulse can be only one polarity either positive or negative. The application of AC voltage can

last for a long period of time, while the impulse voltage that a transformer can see is very short,

which is normally less than a few hundred microseconds. A longer time of voltage duration


64

allows the development of discharge heat and or accumulation of damage inside a solid.

Insulation therefore generally has lower breakdown strengths under AC than impulse voltages.

5.2 Solid Polyurethane Foam Insulation

Polyurethane foam has seen widely used in many fields of industry such as automotive

seating, casting, bedding, and roof insulation for decades. In the electrical industry,

polyurethane has been used in large volumes in low-voltage switchgear. It has been 30 years

since the hard foam was first used as an insulation medium.

There are two classifications of polyurethane foam: one is flexible polyurethane foam and the

other is rigid polyurethane foams.

The type of foam used by most researchers has a high yield and forms rigid and closed-cells;

it is lighter than oil insulation but heavier than gas. Its thermal conductivity is better than all

types of gas medium but is not as good as oil. The non-hygroscopic, closed cell dielectric

material minimizes environmental and reliability problems associated with typical oil and gas

insulators. With the foam insulation, the risks of gas leakage and oil flammable are also

eliminated.

5.2.1 Electrical Breakdown in Polyurethane Foam

Internal discharges occur in inclusions of low dielectric strength. Usually these are gas-filled

cavities. The voltage at which the discharges start depends on the stress in the cavity and the

breakdown strength of the cavity [93]. Cavities can be produced through process control errors

during the production of solid dielectric, but it is an intended feature and unavoidable in

polyurethane foam products studied in this work. The stress in the cavity can be calculated in

some case. The breakdown strength of the cavity depends on its dimensions and is governed

by the type of gas and the gas pressure in the cavity.

In the work carried by other researchers [94], the main factor in determining the breakdown

strength of polyurethane foam is the void size. Electric discharge occurred when high voltages

were applied because of the presence of voids, and this discharge will reduce the breakdown

strength of material.

The mechanism of this electric discharge leading to final breakdown is explained in Figure 5.2

below.


65

Figure 5.2 - Equivalent circuit of dielectric with voids

The presence of the void will cause an asymmetry in the field (hence the higher field strength).

When partial discharge occurs across the void, the field in the remaining insulation will be

elevated, see Figure 5.3.

Figure 5.3 - Electric field distribution considering a small void in the foam

Assuming there is a spherical void in the solid material, the field in the void is given by:

rrc

ar EE

2

3

(30)

Where ‘εrc’ is relative permittivity of gas in void usually, ‘εr’ is relative permittivity of the solid

insulation, and ‘Ea’ is field in the solid insulation.

The relative permittivity of solid part of foam is 5.5 measured by [95]. In a spherical void, the

field stress would therefore be 1.4 times that in the solid.


66

Figure 5.4 - Paschen’s law for air [96]

According to results raised by several investigators, the discharge in a cavity bounded by

insulating material occurs at approximately the same voltage as between equally spaced

metal electrodes. This voltage is for a certain gas given by the Paschen curve [96]. Paschen

curve states that the breakdown voltage of gaseous dielectrics in uniform electric field ‘V’ is a

unique function of the product of pressure and the gap distance ‘pd’. Curves for air are shown

in Figure 5.4.

The gas and gas pressure in the cavity will adopt that in the surrounding medium by diffusion.

Thus the void is generally filled with air in 1 atm.

Most of the foam products being studied are composites of gas-filled voids and solid foam

material. The overall dielectric strength or inception discharge phenomenon will be affected by

the gas-filled void size and also the dielectric strength of the filling gas.

5.2.2 Review of Use of Foam in HV Applications

Solid insulated switchgear (SIS) has been designed and developed in recent decades for HV

and MV power plants. There is no risk of gas leakage at all during its lifetime, and the need for

gas pressure monitoring and maintenance can be removed. Solid insulation materials such as

epoxy resin and polyurethane foam is cast on the outside of switching components such as

vacuum interrupters. With the adoption of solid material, the switchgear can be more compact,

more reliable when working at high temperatures and immune to moisture. These materials

are therefore supposed to resist high temperature and pressure, and most importantly, high

electrical strength.

Normally, the manufacturing of polyurethane foam is done using two chemical components

which are polyol and isocyanate. Isocyanates are a series of polyols with different additives

and molecular structures and they are classified as Toluene diisocyanate and Diisocyanate

diphenylmethane. Polyols are the most important chemical material in making Polyurethane


67

foams and the polyol inside the foam can determine the properties: compressive strength,

closed-cell content, water vapor permeability, water absorption, and heat resistance. All the

polyols are reactive chemical substances, and every kind of polyol molecule has at least two

types of groups which can react with isocyanate. Chemical additives are also be used to

control the reactions and change the foam properties which includes nucleating agents,

blowing agents, catalysts, surfactants, inhibitors, accelerators.

After the two components are mixed together, a urethane chemical reaction will happen. The

isocyanate part reacts with the water inside the polyol part and this reaction produces a lot of

carbon dioxide which can fill the small voids created by mixing. During the production process,

air can be added into two parts mixture to make the foam and it will also contribute to the

expansion of voids inside the foam.

Figure 5.5 - Example of foam insulation from High Voltage Jocyln [97]

Figure 5.5 shows a product example using Josyln foam as exterior insulation for a vacuum

interrupter. Inside the insulator hollow tube, the closed-cell foam is firmly bonded in to the

inside of the tube. The foam can avoid inner water accumulation and ensure the required

dielectric strength inside the tube. The maximum voltage level of foam insulated vacuum

interrupters from Josyln Hi-Voltage Cooperation is 72.5kV [98]. The detailed manufacturing

process of Josyln foam has not been elaborated in any document or paper.


68

Several groups of researches [94, 99-101] have investigated the dielectric characteristics of

foam since 1985. The latest breakdown voltage measurement was carried out on the cylinder

–plane electrode model by [94] and it was concluded that the AC breakdown strength of foam

was 49.7±9.8kV/cm and lightning breakdown strength was 166.2±37.3kV/cm. Figure 5.6

shows the dependence of breakdown voltage on foam when applying AC and impulse

voltages respectively.

(a) (b)

Figure 5.6 - Dependence of AC breakdown voltage(a) and LI breakdown voltage(b) on foam thickness [94]

Partial discharge measurements were done and reported in [99]. The water concentration in

the foam and non-uniform distribution of different size of the bubbles inside the foam are the

main parameters affecting the partial discharge measurement result. The partial discharge

(PD) magnitude for a wet sample is increased dramatically, over 4 times bigger than the value

for dry samples with the same thickness of 0.55cm. During PD measurements, the voltage

applied to the wet sample had to be less than 6kV to avoid breakdown happening in the

sample. The breakdown voltage of wet samples was calculated as 10.9kV/cm, smaller than

that of dry samples.

The void distribution is concluded as being the key issue when using hard foam as a dielectric

medium. When the foam is cured in open air, the void will be filled with air. A discharge within

a void will cause the air/resin boundary of the void to become conductive, eventually shorting

out the void. The average minimum void size in high density hard foam as measured by an

electron microscope was 100μm detected by [94], and in this case the sample can withstand

49.7kV AC breakdown voltage for a 1cm thickness. When compared with other studies on void

size in foam, it can be concluded that the bigger the void size in foam specimens is, the lower

breakdown voltage it can withstand. This is due to breakdown being a result of partial

discharge initiation in many cases.


69

5.3 Experimental Study of Polyurethane Foam

5.3.1 Production of Polyurethane Foam

The foam studied in the work is J Foam 7059 [102] which is a two part polyurethane foam

system (A and B components). When mixed together at the correct ratio these expand to form

a medium density rigid foam. The chemical name of the A component is polyol and is a clear

colour liquid, the viscosity of which is 775 mPas at 20oC. The chemical name of the B

component is an isocyanate which builds the hardness of the foam sample. Isocyanates are a

family of highly reactive, low molecular weight chemicals. It is a dark brown colour liquid and

has a lower viscosity with 350 mPas at 20oC. The flash point of component B is in range of

200-250°C, while there is no data for component A.

Before starting to make sample, it is necessary to estimate how much the foam will expand

using the constant of material - free rise density (162 g/l for J7059). The volume of the

moulding should be taken by measuring length, width and height in centimetres and then

divide by 1000 to obtain the volume in litres. After multiplying this figure by the free rise density,

this is the minimum foam required. An increase in this number by 10-20% will then yield the

weight of the amounts of Part A and Part B required is calculate at appropriate ratio.

To get an accurate experimental result, the foam manufacturing process should be uniform

throughout the entire test. Components A and B are weighed and poured into a mould before

being rapidly mixed together in 25 seconds. Then the foam mould is completely covered with a

lid and a weight is applied to the lid to prevent the foam leaking from the gap. If the mixture is

left to expand freely the foam will rise approximately up to more than half of the mould height

and the foam density cannot be controlled. A waiting time of 24 hours is necessary for the fully

cured foam sample. During the sample making process, the operation of de-gassing is not

possible, as the gas in the raw material is required to expand the foam to the final state.

In this work, foams and foams filled with a ferroelectric filler have been examined. Barium

titanate (BaTiO3) was studied earlier by [103, 104] as a ferroelectric material used in silicone

gel to create a material with increased permittivity as a function of the local electric field. The

author concluded that the filler can help to enhance thermal conductivity of the gel while

controlling the electric field. Thus, it is worth investigating the use of microfillers such as

BaTiO3 in silicone gel on the electrical and thermal properties of polyurethane foam.

Five kinds of foam samples were therefore examined: polyurethane foam without filler and

polyurethane foam with fillers at 5%, 10%, 15%, and 20% concentrations by volume. The

concentration of filler is measured according to the volume of foam before expansion. In the

following sections, the relative permittivity and resistivity of obtained materials were


70

determined at power frequency, then AC partial discharge inception voltage (PDIV) is

measured for certain foam samples. Finally the breakdown voltage of obtained foam samples

at AC and LI voltage waveshapes were measured and analysed respectively.

5.3.2 Measurement of Relative Permittivity

A different concentration of ferroelectric material can affect the permittivity of a filled sample. It

is vital to know the relative permittivity influence on the applied electric field stress. To carry

out this study, cured foam samples with different filler concentration are sliced into 3mm

thickness discs and are placed between 20mm diameter plane – plane electrodes. An AC high

voltage is applied to the test circuit at 500 volt / steps through controlling ac signal generator

connected to an amplifier. This combination is to eliminate the harmonic disturbances from a

traditional transformer when generating a pure sine-wave. Current and voltage signals at each

step are recorded from an oscilloscope until breakdown occurs to the sample. In this work, the

peak voltage foam can withstand during the test process is less than 8kV.

AC

Signal

Generator

Amplifier Test

Object

10k

ohm

Oscilloscope

HV

Electrode

Earth

Electrode

Foam sample d

Figure 5.7 - Circuit of relative permittivity measurement

Figure 5.8 - Plane - plane electrodes arrangement


71

The experimental diagram is given in Figure 5.7. The two plane electrodes work together as a

capacitor construction, and the electrode arrangement is shown in Figure 5.8.

For the above electrode model, the capacitive reactance ‘Xc’ in ohms depends on the

frequency ‘f’ in Hz of the electrical signal passing through the capacitor as well as on the

capacitance, ‘C’ in Farad, is given by:

CfI

VX c

2

1 (31)

The dielectric material of this capacitor is air and foam mixture, with different permittivity.

Greater permittivity of the dielectric gives greater capacitance when all other factors being

equal. An approximation of capacitance can be described in the formula:

d

AC

(32)

Where ‘ε’ is the absolute permittivity of dielectric, ‘A’ is the area of plate in m2, and ‘d’ is

distance between two plates in meters.

Figure 5.9 - Relative permittivity of filled foam at various concentrations

After a series of calculations on relative permittivity based on the above formula, the

relationship between the peak electric field in kV/mm and the relative permittivity is plotted in

Figure 5.9. The equation of each trend line of a group of data is shown in the graph.

The influential addition of BaTiO3 increases ‘εr’ but does not show any field dependence. The

relative permittivity of no filler foam is calculated as 1.4, and for 15% filled foam is 1.8

respectively.

y = 0.0563x + 1.2796

y = 0.0112x + 1.7828

0.50

1.00

1.50

2.00

2.50

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Re

lati

ve

Pe

rmit

tivit

y

Peak Electric Field (kV/mm)

No filler 15% filler


72

5.3.3 Measurement of Thermal Conductivity

The thermal conductivity values of unfilled and filled foams at various concentrations are listed

in Table 5.1. These were measured by John Gearing using the FOX50 instrument at 20°C.

The results show very small variation as a function of BaTiO3 concentration.

Concentration of Filler in Foam Thermal Conductivity (W/m.K)

0% 0.0388

5% 0.0377

10% 0.0379

15% 0.0414

20% 0.0374

Table 5.1 - Thermal Conductivity of filled foam with different concentrations

These values should be used to examine if the filler has any influence on the current rating.

The pure foam and the foam concentrated with 15% filler are studied for comparison.

According to British Standard EN 60694:1997 [78], the upper limit working temperature inside

GIS is stated as 105°C for the aluminium material. The dimension of the National Grid

benchmark model is used to compare the current foam insulation rating with SF6. The sheath

temperature is fixed at 70°C. The influence of the insulation thickness on the rated current is

plotted in Figure 5.10, based on the following calculation.

The thermal resistance of foam per unit length can be calculated in the following formula:

r

drT th ln

2

(33)

Where ‘ρth’ is thermal resistivity of a material in Km/W, ‘r’ is inner conductor radius, and ‘d’ is

insulation thickness.

Given this thermal conductivity of the pure foam and foam with BaTiO3 filler, and using the

National Grid busbar dimensions previously presented, it is possible to describe the current

capability of a foam insulated system as a function of conductor radius and insulation

thickness.


73

Figure 5.10 - The dependence of rating current on insulation thickness of foam

The marked point in Figure 5.10 is the existing busbar dimension, of which the inner

aluminium conductor radius of the busbar is 6.25cm and the outer sheath radius is 25cm. The

current carrying capability of the busbar insulated with filled form (solid lines) is slightly better

than those with pure foam insulated (dashed lines). It is clear from the results that the current

values are less than those seen in a standard GIS system (around 4000A). However, if the

insulation thickness can be reduced, this value will increase in the foams.

5.3.4 Analysis of Void Distribution

Previous studies have indicated that the electrical properties of foam depend on the void size

and distribution. The properties of filled and unfilled foams J7059 were analyzed by scanning

the cross-section of foam samples using a microscope. The electron microscope used to

observe the slices is the Axio Imager produced by Carl Zeiss. This microscope combines with

a computer through the AxioVision image analysis software and the foam microstructure

images can be revealed on the computer screen and edited by the software. Figure 5.11 and

Figure 5.12 showed the sample pictures for two types of foam sample with different

concentrations of BaTiO3. The average void size in normal pure polyurethane foam is 350um

and this is bigger than the 10% filled foam which is about 200um. Additional filler can therefore

help to reduce the void size in the foam.

1000

1200

1400

1600

1800

2000

2200

5 10 15 20 25 30

Rate

d c

urr

en

t (A

)

Insulation thickness d (cm)

Filled form r1 = 5.75cm Pure form r1=5.75cm




74

Figure 5.11 - Void Distribution in pure foam

(a) 5% concentrated (b) 10% concentrated

(c)15% concentrated (d) 20% concentrated Figure 5.12 - Void distribution in different filler-concentrated foams

The physical characteristics of polyurethane foams are determined by the selection of the

polyol [105]. Its functionality, molecular weight, and structure influence the properties of the

foam. The following void size measurement shows the influence of the concentration of the

polyol in foam. The two chemicals A and B should be mixed in the ratio 46:54 in a vessel. The

measurements are based on those taken from ten voids sized from one slice of foam.


75

Mixing ratio Average Void

size (um)

Maximum Void size

(um)

Minimum Void size (um)

40 : 60 272 303 255

46 : 54 243 290 200

50 : 50 289.5 340 230

55 : 45 305 340 240

60 : 40 200.5 255 180

Table 5.2 - Influence of mixing ratio of two foam liquids

(a) Ratio @ 40:60 (b) ratio @ 46:54

(c) ratio @ 50:50 (d) ratio @ 55:45


76

(e) ratio @ 60:40 (f) ratio @70:30

Figure 5.13 - Void size of foam with different mixing ratio

The results shown in the table and the series of pictures indicate that the mixing ratio doesn't

have an obvious influence to the void size and its distribution.

5.3.5 Measurements of Partial Discharge Inception Voltages

Cavity-induced partial discharge (PD) in a solid dielectric is harmful to the insulation

performance of a dielectric material, and in some case, will lead to permanent breakdown of

the insulation material. A PD test is the most common means of detecting such potentially fatal

defects.

A series of partial discharge inception voltage (PDIV) measurements was carried out in

specimens with a gap distance larger than 5mm. A breakdown test was completed

immediately after the PD measurement. The measurement circuit is shown in Figure 5.14. A

PD free transformer up to 80 kV was used as the power supply. A 500 pF capacitor was used

as the blocking capacitor. The oscilloscope was connected to the voltage divider which can

create a output voltage as 1/1000 of the total voltage divider voltage. The test circuit was

connected by copper pipes to avoid corona discharges in the system. The PD measurement

equipment LDS-6 PD detector was connected in order to measure and record the partial

discharge during experiments.

Before starting the experiment the PD detector needs to be calibrated so that the amplitude of

PD can be measured properly. When a new testing capacitance is connected to the circuit, the

system requires calibration. 10 samples are tested for each gap distance after calibration of

PD measuring systems.


77

AC

500 pF

Impedance

Box

220 V / 80 kV

Water Resistor

0.5M ohm

Test

Object

Voltage divider

Scope

Figure 5.14 - Partial discharge measurement circuit

The PD measurement result is presented in the Table 5.3. For comparison, the results of

another two set of tests, including the AC breakdown test and the lightning impulse breakdown

test are shown at the same time.

Gap distance (mm)

Electrode Diameter(mm)

PDIV (kV) 50% AC UBD (kV) 50% LI UBD

(kV)

5 21 17.2±2.5 20.2±2.9 /

10 41 20.5±2.9 26.3±4.9 61.4±23.2

15 61 32.7±5.1 37.5±3.2 113.0±13.1

20 81 43.4±4.6 47.4±6.5 140.5±15.7

25 101 43.4±6.2 62.6±6.5 178.7±28.7

30 121 53.5±4.9 96.2±9.5 /

Table 5.3 - Test result of foam samples with large gap distances (5mm- 30mm)

As shown in the results, the difference between the PDIV and the AC breakdown voltage is

quite small. This would seem to confirm that the breakdown is related to the partial discharge

in the voids. Also the values of PDIV do not increase linearly as the gap distance increases. .

The apparent partial discharge electric field stress of a single millimetre is about 2kV/mm for

each group of test.

When the gap distance was increased to 30mm, the AC breakdown voltage was expected to

be bigger than 80kV; therefore the voltage source in the test circuit was changed with another

600kV AC transformer. Due to the equipment within the laboratory, a different measurement

circuit was used. This circuit was used for the measurement of PDIV and AC breakdown

voltage in respect of a 30mm gap distance.

As discussed before, partial discharges are high frequency pulses originating at various

sections in an insulation system. These pulses generate a voltage and current signal in the

insulation, returning through a ground path. In that situation, high frequency current


78

transformer (HFCT), in Figure 5.15, is used to measure the partial discharge signal of the

foam sample. HFCT will not detect the 50Hz current in circuit because it has a soft ferrite core

whose frequency response is constant at 50kHz to 10MHz. It offers a convenient solution for

on-line partial discharge testing and monitoring of HV devices. The aluminium body of the CT

help to provide RF shielding and improve its performance in noisy environments. The HFCT

sensor inductively detects the high frequency PD current impulses that flow in test sample.

Figure 5.15 - High frequency current transformer sensor [106]

HFCT 100/50 is used in this PD measurement talked in this section. And the specification for

this device is list in the table:

Transfer impedance 5.0V/A

Frequency response 50kHz – 20MHz

Rec. load impedance 50 ohm

Output connector BNC lead

Table 5.4 - Specification of HFCT100/50 [106]

The output of the HFCT sensor is connected to a digital oscilloscope with 100MHz bandwidth,

and the oscilloscope is set for AC peak value detection mode.

5.3.6 Measurement of AC Voltage Strength of Filled Samples

For ac tests in small gaps less than 3mm, spherical electrodes with a diameter of 12.5mm

were used as shown in Figure 5.16. The gap distance between the sphere electrodes is

adjustable and the foam sample is cured in the box around the electrodes. Tests were carried

out at room temperature. A 50 Hz ac voltage increasing from 0kV at a speed of around

0.5kV/s was applied until the breakdown of the insulation foam sample occurred. 10

breakdown tests were carried out on separate foam specimens with the same gap distance.

The gap distance between the two electrodes was 1mm, 2mm and 3mm. When the

breakdown happens, the voltage waveform revealed on the oscilloscope screen will suddenly


79

collapse and return to zero. It is very important to record the breakdown voltage (BDV)

accurately when the breakdown occurs.

Gap

Ø12.5mm

Figure 5.16 - Sphere - sphere electrodes fixed in cured foam

The behaviour of the foam samples for 1mm, 2mm and 3mm gaps is shown in Table 5.5 and

Table 5.6. The inclusion of a filler is, in all cases, shown to reduce the breakdown strength at

least for one distance.

Gap Distance

50% AC Breakdown voltage ± Standard Deviation (kV)

No filler 5% filler 10% filler 15% filler 20% filler

1mm 17.02±5.97 17.98±3.04 16.29±3.99 14.95±2.80 14.25±3.31

2mm 26.28±6.11 27.23±6.09 25.63±4.30 21.68±3.40 20.71±3.69

3mm 29.42±6.09 25.74±4.30 27.41±4.27 17.28±7.18 17.23±7.24

Table 5.5 - 50% AC Breakdown voltage depending on gap distance and filler mixed ratio

Gap Distance

5% AC Breakdown voltage (kV)

No filler 5% filler 10% filler 15% filler 20% filler

1mm 7.20 12.98 9.73 10.34 8.81

2mm 16.24 17.21 18.56 16.08 14.64

3mm 19.40 18.67 20.38 5.46 5.33

Table 5.6 - 5% AC Breakdown voltage depending on gap distance and filler mixed ratio

As seen in the table, the peak average breakdown strength of foam mixtures is 17.98kV/mm.

while according to [36, 37, 107], the field in SF6 must be above 9kV/mm at ambient pressure

or 36kV/mm at 0.4MPa. This means polyurethane foam cannot be comparable to SF6 in terms

of dielectric strength.

In this work, another six series of breakdown tests are carried out on different gap distances

from 5mm to 30mm in steps of 5mm. The diameter ‘D’ of the spherical electrode followed the

principle D>=4d to create a semi-uniform electric field between electrodes. The mushroom


80

shape electrode is adopted which is only one quarter of normal sphere with same diameter but

gives the same result. All of the electrodes are made of brass. The test cell is described in

Figure 5.17. The diameter of the electrode is 12.5mm when the gap between the two

electrodes is no more than 3mm.

Figure 5.17 - Test cell constructed with a pair of mushroom electrode

The testing results for bigger gap distances are shown in Table 5.3 along with the partial

discharge measurement results. The AC breakdown strength of foam is measured at 25kV/cm.

This is around one third of SF6.

The measured AC dielectric strength of polyurethane foam against the increasing gap

distance from 1mm to 30mm is shown in Figure 5.18. It can be seen from the graph that there

are two different developing trends of breakdown voltage at a small gap and a bigger gap.

Different test cell structures may be one of the reasons, which will lead to different void sizes.

Figure 5.18 - AC breakdown voltage of foam at varied gap distances

0

20

40

60

80

100

120

1 2 3 5 10 15 20 25 30

AC

bre

akd

ow

n v

olt

age

(kV

)

Gap distance between electrodes(mm)


81

Figure 5.19 - AC dielectric strength under varied testing gap distances

As seen from the results shown in Figure 5.19, the dielectric strength of foam samples with a

bigger gap distance are in the range of 2kV/mm - 4kV/mm, which is essentially the dielectric

strength of air. The air is the gas filler of void in the foam, and breakdown in one small air gap

quickly develops into a full breakdown of the sample.

5.3.7 Measurement of Lightning Impulse Strength

Besides the operating AC voltage at system frequency, the instrument insulation is also

subjected to transient overvoltages such as lightning strikes and switching surges, which are

particularly risky situations. It is therefore also necessary to study the performance of foam

under transient voltages. The lightning breakdown strength of the foam was tested in

laboratory, and explained as following.

A 10-stage impulse generator manufactured by Haefely Test AG was used to generate

lightning impulses. To work efficiently, only two or three stages are used depending on the

voltage level required. In the circuit shown in Figure 5.20, voltage divider, chopping gap, load

capacitor and test objects all are connected in parallel between impulse generator output and

ground. The environment for these measurements is 18°C, and absolute pressure is 996.4

mbar.

Figure 5.20 - Lightning impulse test circuit

0

2

4

6

8

10

12

14

16

18

0 10 20 30 40

AC

Die

lect

ric

stre

ngt

h

(kV

/mm

)

Gap distance (mm)


82

The impulse generator is set up to produce a standard 1.2/50 µs lightning impulse as in Figure

5.21, which is applied to the test cell. A negative polarity impulse wave is chosen to reduce the

possibility of external flashover. The foam test cell is immerged in an oil tank to avoid external

flashover. An extension plastic tube is connected to the test cell to get a longer flashover

distance in oil. The test voltage is increased in 5kV steps with three tests being carried out at

each voltage step until the breakdown of the foam sample is observed. According to standard

BS EN60243-3 [108] the withstand voltage is the highest nominal peak voltage of a set of

three impulses which did not cause breakdown.

Figure 5.21 - Full lightning impulse without oscillations [108]

Due to the limitation of impulse generator, the LI tests are only carried out on the sample with

gap distance larger than 5mm, and the electrodes arrangement is using mushroom electrodes,

same as AC test shown in Figure 5.17. The recorded LI breakdown voltage is compared in

Table 5.3.

Figure 5.22 - The LI breakdown strength versus gap distance

0

20

40

60

80

100

120

1 1.5 2 2.5

LI d

iele

ctri

c st

ren

gth

(kV

/cm

)

Gap distance (cm)


83

Figure 5.22 shows that the lightning impulse breakdown strength of foam samples changes

with gap distance. All data points are average value with standard deviation. The average

lightning breakdown strength is measured to be 75.3kV/cm, higher than the strength of air

insulation, but still can’t exceed that of SF6.

Figure 5.23 - Cross section view of foam after breakdown test

After the test the foam filled tube is cut into two pieces and carbonized foam can be observed

obviously between the two electrodes. The cross-sectional view of the breakdown test result is

shown in Figure 5.23 as an example. This breakdown of the foam samples is permanent, and

not recoverable.

5.3.8 Estimation of Size of Busbar Dimensions

The overall average measured AC breakdown strength is about 2.5kV/mm, and the average

lightning impulse breakdown strength is calculated at about 7.5kV/mm. The sheath radius of

the busbar is then calculated according to these test results, as seen in Table 5.7. The

dielectric strength of foam measured by other researchers [94] is also reviewed so that its

influence on the busbar dimension can be examined.

Voltage type Dielectric strength

Sheath radius of busbar

Thermal conduction

Current rating

Measured AC dielectric strength

2.5kV/mm 505mm 4.36W 1178A

Measured LI dielectric strength

7.5kV/mm 1306mm for 1425kV BIL 3.0 W 977A

587mm for 1050kV BIL 4.1W 1138A

AC dielectric strength [94]

5kV/mm 178mm 8.7W 1665A

LI dielectric strength [94]

16.2kV/mm 255mm for 1425kV BIL 6.48W 1436A

176mm for 1050kV BIL 8.8W 1674A

Table 5.7 - Estimation of foam insulated busbar dimension


84

The size of the busbar is more decided by the LI dielectric strength when the BIL is selected at

1425kV. In the case of the foam tested in the work, the calculated dimension is a lot larger

than the current busbar size as described in the benchmark model. For the foam products

discussed in [94], the busbar dimension required is nearly the same as the one currently

being used. A lower BIL reduces the requirement to the dimension, the size of foam [94] is

even smaller than the one with SF6 insulated, as presented in the benchmark model. .

The current carrying capabilities of foam insulated busbar with different sizes are given in the

table as well. The foam is assumed to be stable when working at 105°C, In the case of foam

insulated high voltage busbar, the heat generated from the conductor can only be released by

conduction in the solid foam. The current ratings of all the cases are relatively small,

compared with other candidates like oil and CF3I gas.

5.3.9 Summary of Foam Insulation

Some of the main physical characteristics of foam have been studied and presented in this

thesis, and AC breakdown and PD tests on small gap distances (<=3mm) have been carried

out along with AC and LI tests on larger gap distances. The dielectric strength of polyurethane

foam is not strong enough to replace SF6 as busbar insulation in a system with identical

dimensions but may be a material usable for the outer insulation of vacuum interrupters. There

is also a clear maintenance issue if using foam.

The average AC dielectric strength of foam insulation is 2.5kV/mm, although the strength

becomes stronger when the gap distance is as small as 1mm. The AC partial discharge

inception voltage is 17.2±2.5kV for a small gap distance at 5mm, and the value goes up to

53.5±4.9kV when the gap is increased to 30mm. The average dielectric strength when applied

with lightning impulse voltage is 75.3kV/cm, and this value is kept for a gap distance that

ranges from 10mm to 25mm. Due to the limitations of the laboratory, the lightning impulse

voltage cannot be obtained at a small value which is suitable for the test with small gap

distances of less than 10mm.

With the basic insulation level at 1425kV, the sheath radius of busbar of foam insulated busbar

is expected to be 1306mm according to the measured result. This dimension is about 4.3

times bigger than that of SF6 gas insulated equipment, the current rating is even worse with a

value of less than 1kA. If a lower BIL 1050kV is adopted, the sheath radius of foam insulated

busbar can be reduced to 587mm, still about 2 times that of an SF6 insulated system. The

current carrying capability of this design has a slight increase to 1138A, but is still not

comparable with that of SF6.


85

Seem from the result, the overall dielectric performance of foam cannot be compared with that

of SF6, and it can therefore be concluded that insulating foam of the type tested is not a viable

option for SF6 replacement in gas insulated switchgear and its usage for a high voltage level

substation is limited. But it is noted that it could play a role in providing insulation in vacuum

circuit breakers (given these are heading to higher voltages as will be discussed in Chapter 7).

5.4 Conclusions Including a Comparison of Alternative

Insulation Materials

In this chapter, a range of solid insulating materials with the ability to replace SF6 as an

insulation medium have been reviewed and compared. As a conclusion to both chapter 3 and

chapter 4, a general comparison among these candidates is made in Table 5.8.

More environmentally friendly bio-degradable oil can be adopted instead of using traditional

mineral oil. New kinds of oil also bring advantages with high flashing point, which reduce the

risk of fire or explosion. Natural ester or vegetable oil has already been commercially used for

high voltage transformers, but with a higher production cost than conventional mineral oil

insulated transformers. And the weight of the large amount of oil being used puts pressure on

the construction work. However, bio-degradable oil is an option for a non-SF6 power

substation with compact size, and it is the developing trend of liquid insulation.

CF3I is found as another electronegative gas, with higher dielectric strength, but with less

effect to the environment than that of SF6. It is possible to apply it in a reduced size busbar,

but with a slightly higher pressure. The temperature of CF3I should be kept at a proper level

due to its high boiling point. By mixing it with N2 gas with a very low boiling point, the boiling

point of gas mixture can be reduced. Further, the influence of particle generated after

breakdown or flash happens to the general performance of gas is not very clear yet.

In a gas insulating system, the coating painted around the surface of the conductor can help to

reduce the electric field stress on the gas dielectric. The electric field condition at the surface

of the coating is dependent on the material and the thickness of the coating. However, the

coating may bring more probability that particles generated, and these uncontrolled particles

will affect the insulating performance of the gas dielectric. Also, the heat that is released is

another problem and risk.

Solid insulation is another possible alternative option to SF6-free switchgear. Hard

polyurethane foam is a clean material with no harmful effect to the environment. But according

to the test results, the solid foam cannot help to reduce the busbar dimension in a high voltage

substation owing to its weak dielectric strength.


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Table 5.8 - Comparison of all insulating material candidates

Technology Advantage Disadvantage Impact On substation

Size / Cost

Impact On

Current Rating

Vacuum

Insulation

High vacuum

(Pressure is

smaller than 1E-5

mbar)

Clean.

Dielectric strength is

30kV/mm

Very high pressure needed

High mechanical strength of

instrument required

Doesn’t help to reduce

the size. Reduced

Oil Insulation Ester oil Bio-degradable

High fire point,

high dielectric strength

30kV/mm

Heavy

Easy to fire.

Maintenance work needed.

30% size reduction can

be achievable.

Improved

CF3I gas Gas insulation

Low GWP with only 5.

Dielectric strength of its

mixture gas can be same as of

SF6.

Higher Boiling point.

Influenced by its particles

generated after multiple

flashover

Size reduction is

possible when used at

higher pressure.

Reduced

Epoxy Coating

Composite

insulation with air

or other gas

Environment-friendly gas

used

Help to reduce the dielectric

stress to the insulating gas

Thermal radiation influence

Particle from solid can

effect dielectric performance

Would be same size or

a little less than that of

SF6.

PU foam Solid insulation

Hard

Clean material

Higher thermal conductivity

than SF6

BD strength is 10kV/mm,

which is not comparable with

that of SF6.

Doesn’t help to reduce

the size.

Largely

reduced

Chapter 6 Potential Replacement for SF6 as an Interruption Medium

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Chapter 6 POTENTIAL REPLACEMENTS FOR

SF6 AS AN INTERRUPTION MATERIAL

In previous chapters, the substitutes for SF6 when used as insulating material in high voltage

busbars or gas transmission lines have been investigated. It should be noted that SF6 is used

because in addition to its excellent insulation properties it is also an effective arc extinguishing

substance. SF6 gas circuit breakers are extremely popular in switching devices used on the

medium and high voltage power networks. It would be ideal if a substitute material could be

found for SF6 when it is used as an interruption material and this chapter reviews the

candidate options.

6.1 Physics of Circuit Breaker Switching

The circuit breaker is required to carry load current and make/break fault current safely. It

must also withstand rated power-frequency system voltage and rated lightning/switching

voltages across its contact and the remainder of the housing when in the open position.

The circuit breaker must successfully interrupt the fault current within the shortest allowed time

when a fault occurs in the system to minimize potential damage to equipment and system

stability issues. The original theory of Cassie [109] gave an explanation to the interruption of

electrical current, saying that “if the energy lost from the arc column at current zero exceeds

the energy input form the external electrical circuit, the electrical current will cease to flow”. In

an AC system, the process of arc interruption is helped by the existence of a current zero.

Following the current zero, the dielectric strength of the gap needs to be maintained at a value

higher than the voltage imposed across it.

(a) Interruption maintained (b) Initial interruption followed by dielectric failure

Figure 6.1 - Interruption performance of circuit breaker [110]


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A successful interruption is shown in Figure 6.1(a), and Figure 6.1(b) shows a failure to clear

as reignition occurs at a point where the impressed voltage exceeds the dielectric strength of

the gap.

6.1.1 Arc in Circuit Breaker

During the separating process of the two breaker electrodes, an electric arc is developed

between these two contacts. The resistivity of the arc plasma is required to increase

exponentially in the region of the natural current zero so that any subsequent flow of current is

suppressed and interrupted. The dielectric strength of the arc path beyond the current zero

point is required to increase faster than the electric stress applied to the breaker.

The arc is maintained at a high temperature by the current flow through it, with this, high

temperature causing the arc to have a high conductivity. When the arc current is changing, the

stored energy in the arc means the conductivity does not vary at the same rate as the arc

temperature. As a result, the arc in circuit breaker still has a finite conductivity at current zero,

but around this zero current, the power input from the source is very small. Therefore, energy

loss can be increased then the arc conductivity will drop rapidly. Most high voltage circuit

breaker relies on cooling of the arc and the removal of ionisation products to cause interrupter

after the first current zero.

6.1.2 Current Chopping

The arc between electrodes is controlled so that the energy releasing process at the current

zero is sufficiently intense to raise an arc resistance rapidly enough at the current zero to

prevent re-ignition when interrupting large fault currents.

Current chopping occurs when the current is prematurely forced to zero by the aggressive

interrupting action of the circuit breaker. When the dielectric strength of the interrupting

medium in the small contact gap is exceeded by a severe transient recovery voltage, the

circuit breaker can then reignite and interrupt at the next current zero, usually at a current-

chopping level higher than that at initial interruption.


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Figure 6.2 - Current chopping level influenced by the system capacitance [4]

Each switching device has an independent current chopping level. Even for products with

same structure, the chopping current level will be varied with the contact material and

manufacture process. According to [4], the current chopping level of air, oil or SF6 interrupters

is primarily controlled by the capacitance of the system as seen in Figure 6.2. In general, the

interrupters use blast or injection extinction method may have higher current chopping level,

such as air blast circuit breaker, puffer type gas circuit breaker and minimum oil circuit

breaker. When the arc extinguishing blast entirely depends on arc energy in interrupter the

current chopping levels will be much lower, such as rotating arc type gas circuit breaker.

As can be seen in the figure above, the chopping current level for vacuum circuit breakers is

much more than any other cases. This will be discussed in more detail in the following

sections.

In some case, the current is chopped at transient current zero crossing time prior to that of the

natural power frequency current. The high frequency transient current is caused by re-ignition,

and which is the leading factor to the phenomenon “virtual current chopping” [4]. Same as

other type of chopping, virtual current chopping can cause overvoltage in the system, but not

severe magnitude.

During the process of current chopping, the significant energy stored within the inductive

elements will be released and cause high frequency oscillations of overvoltage in the system.

The short term period of voltage transient is related with the load inductance and the stray

capacitance of the circuit, this phenomenon is explained in following sections.


90

6.1.3 Switching Transients in Network

The circuit breaker installed in high voltage power network should be capable of carrying fault

currents for short time and load currents continuously without damage, meanwhile, detecting

and switching any possible load or fault condition envisaged for the circuit that it has to control.

When the circuit breaker operates, the energy stored within the circuit that the breaker is

controlling is redistributed over a very short period of time. As a result, currents and voltages

are produced that exceed those normally presented during steady-state conditions. The levels

of transient current and voltage influenced by this disturbance are very important to electrical

systems as levels that are elevated too much may cause damage to equipment.

There are three kinds of current interrupting situations depending on the arrangement of load

circuit elements, like the dissipating energy device resistance R, and restoring energy devices

capacitance C and inductance L. The switching transient phenomena will be different due to

the energy transposing among these basic elements.

When the normal load current of a system carrying a pure resistive load is interrupted at

current zero, the system voltage is at the same phase as the current, therefore, the recovery

voltage is a relatively low-frequency system voltage. In that case, the arc suppression

requirement is very low and the relatively high circuit resistance will effectively damp out

superimposed voltage oscillations.

When switching capacitor banks, or unloaded overhead lines or cables, the circuit breaker is

required to switch the capacitive current. This is more challenging and such breakers are more

prone to dielectric re-ignition of the circuit breaker, and the generation of severe switching

overvoltages. Normally the capacitive current is small and it can be interrupted at small arcing

time.

In the case of switching inductive currents, a recovery voltage will be generated across the

circuit breaker contacts. The magnitude of this voltage is twice the system peak voltage.

6.2 Comparison of Existing Interruption Systems

Interruption of an alternating current arc between parted electrical contacts will take place if

the means for electrical re-ignition is removed. The gap between the contacts has to change

from being an electrical conductor to being an electrical insulator. This usually takes place and

is ideal at a natural current zero. To facilitate the interruption of current, a range of

technologies are used according to different voltage levels. Figure 6.3 classifies all kinds of

current interruption techniques according to decreasing amount of energy dissipated in the


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electrical arc during breaking along with their usual fields of application. There are three

breaking techniques or distribution network, vacuum breaker, oil breaker and air blast circuit

breaker.

Both SF6 breaker and air blast breaker are the options for transmission network. The vacuum

circuit breaker behaves well in medium distribution level and seeing wider usage on the

distribution system along with SF6. Switching devices adopting power semiconductors or other

power electronic elements are relatively new and chosen mainly for low voltage and medium

voltage level. In terms of the energy consumed for each breaking operation, the power

electronic breaker is the most energy efficient option, with less energy being required for the

SF6 gas circuit breaker and vacuum circuit breakers to that of oil and air blast circuit breakers

being used at the same voltage level.

Figure 6.3 - The main current interruption techniques and their field of application [29]

Paul G Slade [111] reviewed many factors influencing circuit breaker choice based on the

consideration of the switching performance and maintenance cost. All the possible interruption

mediums, including vacuum, SF6, oil and air are compared with each other from different

aspects in Table 6.1. The overall performance of air circuit breaker is the weakest among all

candidates. It is not reliable, uses more energy and more maintenance work is needed. The

table also shows that the vacuum circuit breaker is considered a highly reliable, low

maintenance, versatile circuit-breaker providing longer life then other interruption medium.

Based on those advantages, application of the vacuum breaker (particularly at MV levels) is

growing rapidly in recent decades.


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Circuit Breaker Types

Reliability Vacuum SF6 Air magnetic Mineral Oil

Short circuit enduance Excellent Good Poor Poor

Load current endurance Excellent Good Poor Good

Maintenance interval Excellent Good Poor Poor

Dielectric withstand of contact gap Excellent Excellent Good Excellent

Arc quenching medium Excellent Excellent Good Good

Interrupter life Excellent Good Poor Poor

Application

Unloaded transformers Excellent Excellent Poor Good

Cable and line switching Excellent Good Poor Poor

Capacitor switching Excellent Excellent Poor Poor

Back to back capacitor switching Good Good Poor Poor

Motors Excellent Good Poor Good

Arc furnaces Excellent Poor Poor Poor

Low frequency Excellent Good Poor Poor

Shunt reactor switching Excellent Excellent Poor Good

Use in corrosive explosive or toxic environment Excellent Excellent Poor Good

Endurance / life

Contact wear Excellent Good Poor Good

Multishot auto recloser cycles Excellent Good Poor Poor

Environmental impact (operation) Excellent Excellent Poor Good

Environmental impact (disposal) Excellent Poor Poor Poor

Resistance to harsh environment Excellent Excellent Poor Good

Fire risk Low Low Medium High

Cost of interrupter overhaul Medium Medium High Medium

Impact of interrupter failure Low High High Very high

Maintenance costs

- Interval Excellent Good Poor Poor

- Adjustments None Poor Poor Poor

- Measurements None Poor Poor Poor

- Contact wear check Easy Very difficult Difficult Difficult

- Lubrication Excellent Good Poor Good

Table 6.1 - Brief overview comparing the performance of vacuum, SF6, air and oil [111]


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6.3 Review of Alternative Circuit Breaker Types

6.3.1 Oil Circuit Breakers

In oil circuit breaker (OCB), the contacts of the interrupting device are separated in oil. It has

been continuously developed and has been used in different voltage level, from 11kV to

750kV. In most applications, oil circuit breaker functions well equally as gas blast or vacuum

circuit breakers. Together with those kinds, they are generally of lower cost than other foams

of equipment which ensures their continued use.

There are two categories of oil circuit breaker being used, live tank and dead tank oil circuit

breakers. Normally live tank oil circuit breaker contains less oil than dead tank oil circuit

breakers. In both types, the solid by-products of the interrupting processes, mainly carbon are

contained in the circuit breaker tanks.

The heart of the circuit breaker is the arc-control device. During normal OCB operation a gas

bubble is produced when the contacts separate, the heat of the arc evaporates the

surrounding oil and produce hydrogen at high pressure, shown in Figure 6.4. The oil is pushed

away from the arc region and the gas bubble occupies adjacent portions of the contact. The

arc extinction is facilitated mainly by two processes. Firstly the hydrogen gas has high heat

conductivity and cools the arc, thus aiding the de-ionization of the medium between the

contacts. Secondly the gas sets up turbulence in the oil and forces it into the space between

contacts thus eliminating the arcing products from the arc path resulting in arc extinction and

interruption of current.

Figure 6.4 - The arc interruption theory of oil circuit breaker

Oil used in circuit breakers, has excellent cooling properties because of its high thermal

conductivity, and acts as an insulator and permits smaller clearance between live conductors

and earthed components. However, as an arc quenching medium, oil is flammable and there

is risk of fire, and it may form an explosive mixture with air. Also the arcing products remain in

the oil and these by-produces reduce the quality of oil after several operations.


94

There has been no further research is carried out on oil circuit breakers in recent decades with

just a few studies on oil insulated tap changers. Some of the studies [112-114] agreed on the

view that the use of natural ester dielectric fluids gives better long term stability than mineral

oil and silicones. A life test was applied to simulate 30 years of thermal life in a period of 30

days in a test reported in [112, 113]. The test result on copper-copper, silver-copper and

silver-silver contacts suggested that a natural ester has considerably better thermal stability

than both mineral oil and silicone. There were a number of conclusions made by the authors,

1. Contacts in natural ester fluid are the most stable, followed by

contacts in mineral oil with contacts in silicone being the least stable.

2. Silver-plated copper contacts mated to silver plated copper

contacts are the most stable contact materials regardless of fluid

type, followed by silver-copper contacts, with copper-copper being

the least stable.

3. All three contact groups were stable in natural ester and passed

the tests, while only silver-silver are stable enough to project a 30-

year life expectancy in mineral oil and in silicone.

However, a contrasting opinion held by [6] in discussing oil insulated power transformers

recommend the use of vacuum technology in on-load-tap-changers (OLTCs), with the same

switching performance like in mineral oil. They thought the use of alternative liquids, with

natural esters in them would not be suitable for use in arc-breaking-in-oil tap-changers. To

ensure a reliable function of the tap-changer, it is important to make the switching arcs are

extinguished within fixed time limits. They found the arcing time in natural ester is significantly

longer than in mineral oil, while prolonged arcing times lead to excessive contact wear and

increased oil deterioration, so that maintenance intervals have to be shortened.

Dieter and Rainer [114] did further research on the application of natural and ester liquids to

tap-changers for power transformers. The cooling capability of ester liquids was found to be

slightly reduced in comparison to mineral oil. The switching capability of the change-over

selector is significantly lower than in mineral oil due to the higher viscosity of ester liquids. To

avoid operational limitations at low oil temperatures, the viscosity must not be too high, see

Figure 6.5. A low temperature-dependency of the viscosity is helpful for maintaining the

correct timing of the switching sequence over the entire temperature range.


95

Figure 6.5 - Viscosity of different insulating liquids [114]

From the above it would seem that while it would be feasible to consider oil as a replacement

for SF6 in circuit breakers it would be at the cost of increased maintenance requirements and

reduced safety since the use of environmentally acceptable oils does not reduce the typical

issues seen in oil circuit breakers.

6.3.2 Air Blast Circuit Breakers

The interrupting device in air blast circuit breakers breaks the load and short-circuits currents

by means of creating a rapid flow of air across the arc which is drawn when the contacts are

separated, as shown in Figure 6.6. At the supply current zero, rapid de-ionization takes place

and the residual arc path is replaced by compressed air of a high dielectric strength. The

pressure of air used in these circuit breakers is up to 7 MPa.

It is found that as indicated in the IEEEXplore database, only one journal paper relating to the

development of air blast circuit breakers since 2000. No techniques to improve air blast circuit

breakers are listed. It would therefore seem that little research is taking place in this area.


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Figure 6.6 - Representation of electric arc in air blast circuit breaker

Air blast is a viable technology that could replace SF6 in substations where space is not limited.

However, from an environmental perspective, the benefits from the reduction in SF6 leakage

that may be achieved through the use of an air blast circuit breaker may be outweighted by the

carbon dioxide emission associated with the use of the compressors required to fill the air

receivers. The data of 275kV air circuit breaker was taken from a typical National Grid

substation. There are three air compressors with a rating of 11kW/22A/450V. Table 6.2 was

taken from the air pressure reading record of 17th

March 2010.

Running hours reading Total running hours/ week Comp.1a Comp.1b Comp.1c

1st

week 22442.6 9797.8 22003.8 /

2nd

week 22477.6 9891 22129.2 /

Running hours/week

35.0 93.2 125.4 253.6

Running hours reading Total running hours/ week

Comp.2a Comp.2b Comp.2c

1st

week 42347.5 5765.8 2434.5 /

2nd

week 42379.4 5797.2 2465.9 /

Running hours/week

31.9 31.4 31.4 94.7

Table 6.2 - Air compressors running time record

The CO2 equivalent of 1kWh electricity consumed is 1.55 lbs [115], that is:

1 kWh = 1.55 lbs CO2 = 1.55 * 0.45 kg = 0.7 kg CO2

All the compressors that substation will run: 253.6 + 94.7 = 348.3 hours / week

The total energy consumed on the air compressors is therefore: 11kW * 248.3 hrs = 3831.3

kWh


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The total CO2 equivalent emission of such this energy consumed is:

3831.3kWh * 0.7kg/kWh = 2681.91kg = 2.7 tons/ week = 140. 4 tons/ year

For each breaker, the CO2 equivalent emission is: 2.7 / 6 = 0.45 tons/ week = 23.4 tons/ year

At one National Grid substation, the SF6 gas circuit breakers installed are GEC type FE2 and

each FE2 circuit breaker contains 51kg of SF6, i.e. 17kg per phase. The annual leakage rate

of this type of gas circuit breaker is not known, but it should not exceed the limit of 2% as

stated in standard BS EN 62271-1 2008. Therefore the total leaked SF6 annually from each

breaker is 1.02kg, and it is equal to about 24.4 tons of CO2.

The CO2 equivalent emission of one air blast circuit breaker is therefore 1 tons smaller than

that of SF6 circuit breaker. However when the leakage rate of SF6 blast circuit breaker is

strictly monitored and controlled below the value the standard specified, the SF6 circuit

breaker is still a better option unless better technology can reduce the electricity consumption

of the air compressors.

6.4 Gas Circuit Breakers

6.4.1 Background

Great progress was achieved in the development of gas circuit breakers, aiming to improve

the reliability, the compactness and the required energy for operation. As a breaker for

protecting a high voltage transmission system of 72kV or higher, the puffer type gas circuit

breaker is widely used. A circuit is interrupted by the gas circuit breaker by parting a set of

contacts, and then controlling and extinguishing the resulting arc using cold gas flow. The

interrupting device consists of a stationary contact and a movable contact placed in a metal

container filled with insulating gas. The arc-extinguishing gas is compressed in a compression

device and the compressed gas is blown against an arc to extinguish it. A pressure difference

has to be established in the arcing chamber, and the minimum pressure difference needed

depends on the current to be interrupted. SF6 is chosen as insulating gas most of the time due

to its good dielectric performance and arc interruption properties.

The theory behind interruption using breakers filled with alternative gases is the same as that

of SF6. Some characteristics of the gas are vitally important to the performance of gas circuit

breakers, such as fast dielectric strength recovery, high thermal conductivity and stability when

exposed to high temperatures.


98

The main processes of arc-quenching in gas circuit breakers can be described in the following

diagram, which uses an SF6 circuit breaker as an example.

Figure 6.7 - Four stages of puffer action-single flow [116]

Figure 6.7 shows the interruption process in an SF6 puffer breaker. The motion of the contacts

compresses the gas and forces it to flow through an orifice into the vicinity of the arc. When

the breaker is closed, the current flows via the main contacts and the cylinder. When a tripping

impulse is given, the cylinder moves downwards, whereupon the upper main contact opens

and the current commutes over to the arc contact. At the same time, the gas in the cylinder is

compressed. When the nozzle left the arc contact, the compressed gas is blown along the arc

and cools it. The breaking process is completed as the current passes through zero.

6.4.2 Effect of PTFE Ablation on Puffer Circuit Breaker

Ablation controlled arcs are a special type of convectively cooled arc in which the cooling flow

is generated by arc induced wall vaporization [117]. The role of arc induced ablation in gas-

blast switchgear has been the subject of extensive mostly experimental research. In particular,

nozzle clogging and flow reversal under overclogged conditions attracted attention because of

their influence on the arc interruption process.

The nozzles used on gas circuit breaker at the present time are usually made of Teflon or

PTFE (polytetrafluorethylene). The characteristics of this insulating resin are rather unique as

it is mechanically strong, easily machinable and capable of resisting relatively high

temperatures. It has the property of vapourising under the direct action of electric arcs of short

duration without carbonizing, although its molecular structure contains carbon. Self-blast

circuit breakers utilize the energy dissipated by the arc itself to create the required conditions

for arc quenching during the current zero. The arc energy induces ablation of the PTFE nozzle.

Then it was concluded in both [118] that ablated nozzle shape and SF6-PTFE mixture vapor


99

affect influence the interrupting capability of a circuit breaker in the thermal phase. In particular,

the work [119] has shown that the arc temperature around current zero of PTFE ablation is

about 5000-10000K. In this temperature range the physical properties of pure SF6 plasma are

different from those of pure PTFE plasma.

According to the analysis in [120, 121], the arc ablation in a circuit breaker is influenced

greatly by the quantity and size of inorganic filler such as Al2O3, TiO2, or BN added to PTFE.

Compared to pure PTFE, the arc ablation resistance of PTFE can be greatly improved by

introducing inorganic fillers and the arc ablation quantity of PTFE composite is reduced

sharply. Meanwhile, the relative dielectric constant of PTFE composite increases gradually as

the content of filler increases, and decreases as the temperature rising. This characteristic is

helpful in increasing the lifetime of the nozzle in circuit breakers.

Figure 6.8 shows a schematic of the prototype interrupter design which is aimed at increasing

the rate of PTFE ablation [122]. It is a simple two-electrode system with a modified PTFE head

nozzle to accommodate a PTFE reaction /storage chamber around the arc path. High vapour

concentration inside the arcing column is achieved and this vapour is contained long enough

to a quench the arc. The interrupter design is based on the concept of auto-expansion type

interrupters which principally harness the energy of hot ablated PTFE vapour to cause a

pressure build-up inside the expansion chamber during the arcing process.

Figure 6.8 - Schematic diagram of a prototype interrupter unit [122]

The average surface ablation of PTFE nozzles in high voltage circuit-breakers was

investigated though experiments carried out on full scale circuit breaker devices and a small

scale test device [123]. Experiments were done with a test device in the current and arcing

time range of 2–40 kA(rms) and 5–17 ms respectively.

An experimental test made in [124] has shown that the extinction behaviours and operating

performance of N2, air or CO2 are similar as that of SF6 when the arc fault current is 20kA. It is

concluded that the complete replacement of SF6 by N2 at low pressure operation is a viable

approach to circuit breaker design. The work [125, 126] also proved that by concluding that


100

PTFE ablation could realistically improve the current interruption performance of N2 interrupter

unit to the extent that there is only a little difference in the arc extinction capability at low gas

pressure to a SF6 filled unit.

J. W. Spencer etc. presented some experimental results of a model prototype self-blast

interrupter unit that uses 0.1MPa N2 as alternative gas to SF6 for its operation [122, 127, 128].

Some of the key design features include the use of a close-fitting PTFE housing to retain high

gas pressure for arc blasting as well as the application of PTFE nozzle ablation phenomena to

assist the arc extinction process. During the current interruption process, the resulted hot arc

plasma inside the arcing arena can be successfully expelled and replaced by cooler gas and

chemical species such as the composition of PTFE vapour which is suitable for arc

interruption via the expansion chamber. And using N2 as the arc extinguishing gas and at a

low gas pressure can already produce comparable current interruption performance to SF6

units.

Figure 6.9 - Arc voltage and current waveforms of 9kA arc with (a) minimum (b) maximum

pressurization inside the expansion chamber [122]

Figure 6.9 shows the corresponding current and voltage waveforms under test condition.

Successful current interruption is achieved at the first current zero. The arc voltage increasing

is also presented in the graph. The result in the graph confirms that that the expansion

chamber could indeed improve the interruption process by causing sufficient pressurization

and establishing right gas flow conditions for arc quenching.

To summarise, ablation of PTFE is an alternative solution to the use of SF6 for interruption on

the basis of modified PTFE head nozzle. The gas used inside the chamber can be low

pressure gas such as air, N2 or CO2. Also the life time of the nozzle can be improved by

adding inorganic filler such as Al2O3, TiO2, or BN to PTFE.

6.4.3 SF6-Free Gas Circuit Breaker

Eelectronegative gases have a strong tendency to absorb free electrons. By rapidly captured

enough insulation strength can be built up to extinguish the arc. To find a replacement of SF6


101

as interruption medium, the best solution is to search for alternative electronegative gases.

CF3I is one of best electronegative gases, and it was tested as interruption medium by [54,

129] . They found that the interruption capacity of CF3I is 0.9 times that of SF6 for the same

surge impedance.

Figure 6.10 - Interruption performance to CF3I ratio of CF3I mixture gas [129]

Fluorine is harmful for insulation material and it toxic, while the iodine as a kind of metallic

particles will reduce the dielectric strength and interruption capability. H.Katagiri etc.[129]

measured the amount of fluorine and iodine from CF3I after a current interruption happened at

0.1MPa. As shown in Figure 6.11, the fluorine density from SF6 increases exponentially with

the current (green column). In contrast, the fluorine density from CF3I is very small (purple

column). CO2 is mixed with CF3I to reduce the boiling point of gas mixture, and the fluorine

density from this is even lower. CF3I or its mixture is stated to be an environmental-friendly

interruption medium.

Figure 6.11 - Density of fluorine form CF3I and SF6 [129]


102

Figure 6.12 - Density of iodine from CF3I and CF3I-CO2(30%-70%) [129]

As presented in Figure 6.12, the density of iodine from CF3I is higher than one from its gas

mixture. Considering the likely influence of iodine particles on the interruption capability,

effective removal of iodine by gas flow is recommended.

To sum up, gas circuit breakers are used more widely than other kinds of switching devices,

especially at high transmission voltages. There are several possible alternatives to SF6

suggested in literature such as CF3I, N2 and CO2.

6.5 Conclusion of Existing Non-SF6 Circuit Breaker

This chapter has reviewed the latest developments in the potential replacements of SF6 circuit

breakers. The advantages and disadvantages of interruption media like oil, high pressure gas,

and CF3I were compared with that of SF6. Biodegradable oil is an option which can eliminate

the harmful impact to the environment from non-biodegradable oil, which is currently being

used in HV oil circuit breakers. Air blast circuit breakers are also alternatives to SF6 circuit

breakers, despite their unavoidable noise and also the energy consumed for running air

compressors. A new gas CF3I has been introduced and used in an arc distinguishing chamber,

but its long term performance needs further research. Discussions on vacuum circuit breakers

have not been included, which will be covered in Chapter 7.

.

Chapter 7 Vacuum Circuit Breaker

103

Chapter 7 VACUUM CIRCUIT BREAKER

It is clear from the literature (as will be described below) that vacuum circuit breakers are

available at 132kV and much development is taking place at 275kV or 400kV. It would appear

that these devices will therefore be available within the next 5-10 years for use on the

transmission system. While the focus of the manufacturers is to develop working prototypes, it

is not clear whether the well documented risks of restrikes / pre-strikes / current chopping

associated with vacuum circuit breakers would continue to impact at transmission voltage

levels. This chapter therefore explores methods to examine this using a model of a vacuum

circuit breaker with some initial basic conclusions being drawn and recommendations being

made for the further more extensive modeling that would be required.

7.1 Introduction to VCB Technology

The heart of vacuum circuit breaker is the vacuum interrupter, and the internal components of

a typical vacuum interrupter are as shown in Figure 7.1.

Figure 7.1 - Vacuum interrupter chamber in vacuum circuit breaker [130]

A vacuum arc is different from the high-pressure arc because the behaviour of an arc in a

vacuum depends on the surface material of the electrodes, rather than the gas in a standard

circuit breaker. When the contacts of the breaker are opened in a vacuum (10-7

to 10-5

torr), an

arc is produced between the contacts by the ionization of metal vapours of contacts. The arc is


104

quickly extinguished because the metallic vapours, electrons, and ions produced during arc

condense quickly on the surfaces of the circuit breaker contacts, resulting in quick recovery of

dielectric strength. As soon as the arc is produced in vacuum, it is extinguished quickly due to

the fast rate of recovery of dielectric strength in vacuum.

The vacuum circuit breaker is the most promising option to replace high voltage SF6 circuit

breakers by now. The use of vacuum circuit breakers at transmission levels is being studied

by many individuals.

7.2 Recent Development of Vacuum Circuit Breaker

Vacuum has largely replaced other arc-quenching media, such as oil or SF6 gas, for many

switching applications in the medium-voltage sector. Among the advantages of vacuum

interrupters are a compact design, maintenance-free operation, long service life and excellent

environmental compatibility. The new development of vacuum interrupter can be presented in

the following key points.

7.2.1 Contact Material

The vacuum arc requires metal vapour from the metal contacts to sustain itself until reaching a

nature current zero. In order to maintain the quality of the vacuum, it is important that the

materials used for the contacts and the surface of contacts with the vacuum should be with

low gas content to avoid gas releasing during interruption process. In addition, the contact

material should be able to minimise the natural tendency of metals to cold weld when pressed

together under high-vacuum conditions. At same time, a high mechanical strength is required

in the material to withstand the impact forces during closing operation. Electrodes are

therefore required to meet these requirements in the following list:

High melting point

Strong mechanical strength

High vapor pressure which enables arc stabilization

Low chopping current

High thermal conductivity

Good electrical conductivity to minimize the loss

The material of the cathode contact plays important role in defining the voltage drop across

the vacuum arc. A material which does not easily generate metal vapour will give rise to a

higher arc voltage. This property is related to the combined effect of the boiling point and the


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thermal conductivity of the metal as seen in Figure 7.2. Normally the arcing voltage is in the

range of 10-25V. Materials still been widely used are based on copper alloys.

Figure 7.2 - Effect of thermal properties of cathode on arcing voltage [131]

Contact material improvement has helped to develop better the short circuit interruption

performance. The chopping current level is another consideration issue when choosing

appropriate contact materials from lots of choices, some of which are compared in Table 7.1.

Chemical Formula Chopping Current (A)

W 9

Cu 18

Cr 6.5

Bi 0.3

Sb 0.4

Cu Cr25 Sb9 3.9

Cu Cr25 Zn10 3.2

W Cu30 Sb2 2.8

WC Ag 35 1.4

Cu-Cr 2.7

Cr-Bi 7

WCCu 60/40 1.9

Table 7.1 - Chopping current of electrode contactors [42, 132-134]

A contact material consisting of Cu-Cr was introduced in the 1970s and combines good

electrical properties with arc erosion and good welding resistance. This limited the chopping

current to around 3.0A. The investigation made by S.Temborius etc. [133] focused on the


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properties of WCAg and WCCu for vacuum contactors. Material with a higher WC content

showed lower chopping current and also lower interruption capability. The reason is all factors

keeping the arc stable down to lower currents in the case of chopping conditions, also do re-

establishment of the arc at current zero with high current stress. The composite material WC

Ag 35 was found in 2006 [134], and was seen as the best contact electrode material at that

time. The chopping current level is lower at 1.4A. Recently, the high pressure metal

component silver has been added to the main contact material with WC. The new composite

materials make the chopping current close to that of puffer type SF6 which can be lower than

0.5A [135]. Currently the most often chose electrode materials currently are Cu-Cr, AgWC, Bi-

Cu, and Bi-Cu-Cr [42].

7.2.2 Contact Structure

When designing a vacuum interrupter, it is necessary to find the electric stress distribution first,

especially at the surface of shields, which are designed to contain metal vapour to reduce the

risk of unwanted breakdowns. A larger interrupting capacity can be enhanced by placing

several units in parallel or series. The main challenge is how to get a higher voltage level with

excellent arc control geometries.

When contacts through which current flows are separated, the explosion of the last ‘metallic-

bridge’ causes a metal vapour arc to form. This arc, which consists exclusively of the

vaporising contact material, is sustained by the external supply of energy up until the next time

the current passes through zero. At the instant of this zero crossing, the arc is finally

extinguished and the vacuum interrupter regains its insulating capability, i.e. it is able to

withstand the transient recovery voltage. To ensure successful quenching at the current zero

crossing, the contacts are not allowed to suffer more than minimal arc erosion during passage

of the strongest current.

In a vacuum interrupter, the arc channel is influenced only by the interaction with a magnetic

field, as there are no mechanical ways to cool the vacuum arc. The two ways to control the arc

both use magnetic fields, one is radial magnetic field (RMF) technology, and the other one is

axial magnetic field (AMF) technology.

During the operation of a vacuum interrupter, contact erosion needs to be controlled to

achieve the dielectric performance of high-voltage VCB. Vacuum interruption is generally said

to be adequate for frequent switching conditions, since the voltage drop on the electrode is

lower and the erosion of the contacts is relatively moderate compared to current interruptions

in SF6 gas. Axial magnetic fields (AMF) electrodes have been applied to high-voltage VCB

recently since they generate a uniform arc on the contacts. An article by Fink et al [136] gives

an excellent review of the basis of existing vacuum technologies.


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In many AMF contact systems the axial magnetic field is generated by a coil located behind

the contacts. As a result, the resistance of the interrupter increases and the additional resistive

losses occurring in service reduce the nominal current performance. The only practical way in

which a vacuum interrupter can dissipate the generated heat is via the copper conductors,

since convection is not an option in vacuum. As already mentioned, the diffused arcs of the

AMF contact systems results in an excellent short-circuit current breaking capacity. This

applies particularly to currents of 63 kA and higher. In this short-circuit current range, the more

complex AMF contact systems are superior to the conventional RMF contacts and are

definitely to be preferred.

The adoption of the AMF arc control method for most of interrupter contacts brings down the

arc voltage but helps to increase the current breaking capacity of the interrupter up to 200kA.

However, the thermal loss issue has not been solved for the situation with a continuous

current rating of 3-5kA and a short circuit current rating of 63kA. When the voltage level goes

to 52kV, the cost of the high voltage interrupter and the safety concerns will be significant.

In a conventional plain contact structure, the surface melting phenomenon is very common for

the occasion when the short circuit current exceeds 10kA, and this problem will lead to an

unsuccessful current interruption. Therefore, several special designs of contacts with a

different interrupting capacity and arc voltage characteristics are investigated and discussed.

There are two main kinds of contact structures, cup shaped contacts and spiral petal contacts.

Both designs keep the arc in rapid motion by the effect of radial magnetic field (RMF) and

prevent over-heated regions developing through a longer cooling time.

At currents of around 10 kA the vacuum arc begins to contract (i.e. enter constricted mode),

being initially noticeable in the form of anode spots. The contraction, which partly depends on

the contact material, causes more energy to be supplied to the contacts, thereby reducing the

vacuum gap’s capacity to extinguish the arc after the current zero crossing. One way in which

the switching capacity can be improved is to change the contact geometry. (The electrode

geometry generates magnetic fields, so such changes influence the arc’s behaviour.) Spiral

contacts generate a RMF which causes an azimuthal electromagnetic force to act on the

contracted vacuum arc [136].

Transverse magnetic field (TMF) contact structures are being investigated and have been

commercially used in high voltage vacuum interrupters. TMF contacts are well known as cup-

shaped contacts and were first designed in the 1960s in the UK [137]. As shown in Table 7.2,

the contact surface has many slots in the cup which extend into the ridge. A self-induced

magnetic field is generated when a current flows through the contact, and a constricted

electric arc in a hollow ring shape formed at higher currents is forced on a circular motion by

this field.


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A TMF structure makes the arc voltage lower than a spiral contact, and it reduces local

overheating and severe surface erosion. These essential advantages ensure a fast

condensation of the metallic vapour and re-ignition of the electric arc can be prevented.

Besides the cost issue, TMF contact is a good option for compact and efficient vacuum

interrupters.

Above all, the development of contact structure designs including AMF, RMF and TMF are

summarised in the following Table 7.2. The advantages and disadvantages of each design are

outlined respectively.

Name AMF with a set of

two identical contacts

Spiral (RMF) contact

[136]

Cup shapted contact

(TMF) structure [137]

Structure

Year / GE (USA) 1962 1973

Advantages Less arc voltage

Bigger current breaking capacity

simple physical structure

lower power losses at nominal current

Longer lifetime

Bigger interrupting capability

Compact size

Dis-

advantages Confining effect Expensive Expensive

Table 7.2 - Comparison of contact structure

The contracted arc moves over the contact’s surface at a speed of 70–150 m/s. This high

velocity ensures that there is less contact erosion and also significantly improves the current

interrupting capability. The switching capacity of vacuum interrupters can also be increased by

using contact systems which generate an axial magnetic field (AMF). When a magnetic flux

density is applied parallel to the flow of current in the arc, the mobility of the charge carriers


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perpendicular to the flow is considerably reduced. This applies especially to the electrons,

which have a smaller mass than the ions. The electrons gyrate around the magnetic lines of

force, so that the contraction of the arc is shifted towards the higher currents. The arc burns

with a diffused light and the supply of energy to the electrodes is reduced. This is also

indicated by the arc voltage, which is lower than with RMF contacts. The advantage of the

RMF contact system lies in its simple physical structure, while another advantage of the spiral

contact is that in the closed state the current can flow through the contacts directly via the

stem, thereby ensuring lower power losses for the vacuum interrupter at nominal current.

Figure 7.3 - Comparison of interruption capability for vacuum interrupters as function of

electrode diameter and magnetic field type [4]

From the aspects mentioned above, besides the influence of the contact material and the size

of the contacts, it is evident that the interrupting capability of a vacuum interrupter depends on

the type of magnetic field produced around the contacts. Larger electrodes in an axial field, as

shown in figure 78 have demonstrated that they have a better interrupting capability.

7.2.3 Control of X-ray Emission

A concern relating to the use of vacuum insulation at high voltage is based on the possibility of

X-ray emission. Giere and Karner [138] measured X-ray emission dose rates for double and

single-break circuit breaker units at different operating voltages. The result showed that X-ray

emission could take place but it does not give any danger to personal. Another paper [111]

has a clear conclusion on X-ray emission of high voltage vacuum circuit breaker. X-ray

emissions are zero when the interrupter is in the closed position at all voltages. Furthermore,

during the switching process, X-ray emission is zero or negligible for MV circuit breakers up to

36kV. Once the system voltage gets to higher voltages such as 145kV then the possibility of

X-ray emission at system volts becomes significant. Strong X-rays could only be generated at

test voltages and these are infrequent occurrences, which are harmless under controlled


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conditions. This information suggests that, the use of vacuum insulation in substation busbars

would need to consider the safety case very carefully.

7.2.4 Review of Transmission Voltage Vacuum Circuit Breakers

Vacuum technology is mature at 72.5kV level and is considered to be the practical limit of VCB

by most utilities, although the maximum voltage level of a single break VCB can be up to

170kV. This is because the saturation figure of the withstand voltage verse gap distance is

reached at this voltage level.

However, researchers are developing higher voltage circuit breakers. A 126kV Vacuum circuit

breaker using 0.1MPa SF6 surrounding gas outside the porcelain enclosure is under type test

in China. Its rating [139] is listed in Table 7.3.

Rated Voltage 126kV

PFWV 230kV

LIVW 550kV

I normal 2000A

Isc 40kA – 100kA peak

I cap switch 40 A

STC duration 4 seconds

Operating life 6000 CO cycles

Table 7.3 - 126kV VCB rating [139]

According to [8, 140], 170kV 50kA Vacuum interrupters have been made and tested in 2008

year. The VI is insulated with SF6 in circuit breaker unit, and it can be changed with dry air with

higher pressure. The short-circuit test result is detailed below.

‘During the preconditioning of VI by lightning impulse test in the air,

flashover occurred on the surface of VI at the peak value about

400kVpeak. Therefore insulation oil was used to avoid the flashover

on the surface of VI. In the insulation oil, the peak value of lightning

impulse test was 750kVpeak. It is the rated insulation level for rated

voltage of 170kV. The r.m.s. value of test current was 51.2kA and

the peak value of transient recovery voltage was 254kV. The VI

circuit breaker unit successfully interrupted the high current. During

the Out-of-Phase test and capacitive bank current switching test, late

re-strike were occurred within several tens millisecond later after


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TRV peak passed. To use VI for switchgear in the transmission line,

this re-strike problem must be solved. This is the future work.’

The highest rating of circuit breaker described in the ABB brochure is 36kV with load current

and fault current ratings of 2.5kA and 40kA respectively. Okubo and Yanbu [141] state the

driver to replace the SF6 gas as the reason behind the investigation into the use of vacuum

insulation as an interruption medium. They discuss the current limitations as being:

Rated voltages of vacuum circuit breaker bottles are presently at a maximum 100kV.

For an 800kV transmission system, they state that bottles of around 200kV rating will

be required. If this is a linear relationship, it could be assumed that 100kV bottles

could be suitable for a transmission system.

The normal rated current of a vacuum bottle is around 3kA. A rating of up to 6kA is

likely to be required for the transmission system.

The external insulation of a circuit breaker will need increasingly careful design as

voltage levels increase that beyond which normal glass and ceramic insulation

designs will function

Similarly, Kirkland-Smith [142] extols the environmental benefits of vacuum interrupters over

SF6 interrupters citing issues relating to the greenhouse properties and by-products following

an arc of SF6. However, the paper only states that the choice of vacuum is suitable for

medium voltage distribution applications and does not recommend or even discuss the

application of vacuum at transmission system voltage levels.

The development of vacuum interrupters in high voltage applications is discussed [143], and it

is thought that the vacuum circuit breaker is capable of competing with SF6 circuit breakers in

the high voltage range up to 145kV. Also the laboratory test results for hybrid circuit breakers

composed of both SF6 and vacuum breakers rating at 145kV/63kA is mentioned in the article.

It shows that vacuum circuit breakers are able to deal with the first extremely steep TRV, while

SF6 breakers will take over and withstand the peak transient recovery voltage. From the recent

review in [144] on the usage of vacuum circuit breakers in high voltage networks, the data

shows that there is a tendency to high voltages now up to a maximum of 168kV, at which level

there are 66 sets being used in Japan.

7.3 Development of Multiple Interrupters Based VCBs

Based on the superior performance and recent development of vacuum circuit breakers as

presented above, a number of breaks can be connected in series to meet the voltage

requirement of transmission systems (considered more economic than a high voltage single

break). Multiple breakers technology is the trend of high voltage VCB developing. M.Homma


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from Toshiba [145] in 2006, described a new concept switchgear for 4-break 550kV silicon oil–

immersed vacuum circuit breaker. The multiple breaker technology is combined with silicone

oil as outer insulation.

Figure 7.4 - AC withstand strength of single and double break vacuum circuit breaker [146]

Figure 7.4 shows the relatively small increase in AC dielectric strength that can be achieved

by the use of a double break vacuum interrupter. The authors find a similar result for lightning

impulse voltages but the results are expressed in a per-unit form without giving specific

withstand values. They state that the lightning impulse voltage withstand of the circuit breaker

is the dimensioning quantity.

Figure 7.5 - Comparison of breakdown probability between double and single break [147]

K.Ikebe, etc.[147] stated that multi-break configuration has an advantage for high-voltage VCB

because higher dielectric performance can be obtained with the same contact gap. Figure 7.5

shows the two-break design has lower breakdown probability than one-break with the same

total gap length. S. Yanabu etc.[148] mentioned that for series vacuum interrupter, it is


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important to select the appropriate parallel capacitor value and to equalize the static

distribution ratio to realize the high voltage.

The voltage distribution of a multiple-break vacuum circuit breaker in static situation is

determined by the self and stray capacitances. This conclusion is proved by [149, 150].

Further, the post arc current and recovery voltage distribution ratio are investigated in [151,

152] respect to double-breaker performance. And it is concluded that axial magnetic field

contacts is more suggestive to be used, rather than those spiral contacts, to avoid large

scattering of post arc current caused by the spiral contact structure. Scattering of post arc

current will lead to bias of voltage distribution between two breakers.

Although spring mechanism needs lots of precision moving parts to perform spring charging,

closing and tripping function, it is still the dominating operation method, and more realistic than

permanent magnetic actuator when used at higher voltage level at the moment. The

disadvantage of permanent magnetic actuator is the electronic control system is easily

influenced by the electromagnetic field surrounding, so that its reliability will be doubted. It is

required that multiple-break vacuum circuit breakers must make all interrupters act

simultaneously for the same arcing condition.

Sentker and Karner [153] stated that vacuum switchgear is uneconomic (at the time of writing)

for voltages above 52kV. The potential benefit by the use of a double-break vacuum gap is

shown as only 41% according to their published work. Using the same method, a ten-gap

vacuum gap would only yield an improvement in the breakdown voltage of just over 200%.

Experimental work shows a good correlation with the method used to calculate the 41% figure.

Therefore, adopting multiple vacuum circuit breakers at transmission network gives the

advantage of environmental friendly, but life cycle cost of this option requires more

consideration and comparison with gas circuit breaker and other possible candidates.

7.4 Transient Voltage Simulations Using a Vacuum Circuit

Breaker Model

The text above describes how the internal design of the VCB keeps on improving, especially in

terms of the main contact structure and the materials used. All of these developments and the

use of multiple breakers connected in series are leading to products that are stated to be

suitable for use in the high voltage network, with the VCB seemingly moving towards

availability at 275kV and 400kV. However, the issues associated with current chopping and

overvoltages can only be examined by investigating a VCB in the context of a network.

Therefore, the switching transient performance of the VCB is examined in the following

sections and some simulations of a simple transmission network with high voltage VCB


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installed are carried out. The aim is not to develop a full comprehensive model but is intended

to illustrate a number of key differences between lower and higher voltage systems and the

challenges that may be faced.

7.4.1 Transient Recovery Voltage

A transient recovery voltage (TRV) [154] for high voltage circuit breakers is the voltage that

appears across the terminals after current interruption. It is a critical parameter for fault

interruption by a high-voltage circuit breaker. The characteristics such as rate of rise can lead

either to a successful current interruption or to a failure. The TRV depends on the parameters

of equipment installed in the network. The type of fault supposed to be interrupted by the

circuit breaker also has an influence on the TRV.

7.4.2 Prestrike, Restrike and Reignition Phenomena

“Prestrike” is referred to the event when breakdown happens between the contact gap before

the contacts close together. This phenomenon can be observed in the occasion of interrupter

closing. The system voltage applied to the reducing vacuum gap distance imposes an

increasing electrical stress on the gap, which will lead to breakdown if that stress is more than

the dielectric strength of the vacuum gap. During the pre-strike, the arc has a current with a

high frequency content coming from the inductance and capacitance connected between the

source and load.

“Reignition” and “restrike” are phenomena that occur when the breaker is opening. The

primary difference between a re-ignition and a restrike is the time of occurrence of the

discharge. The term “restrike” is defined as a re-establishment of the current which will last ¼

cycles or longer, following interruption of current at normal current zero. The time differences

between the two transient phenomena are compared in Figure 7.6.

Figure 7.6 - Comparison between re-ignition and restrike [155]


115

Re-ignition occurs when a current is interrupted at current zero and then re-establishes itself

within 1/8 of a power frequency cycle. Multiple re-ignitions may happen when the rising rate of

TRV is faster than the re-establishment of dielectric strength in the vacuum gap. Under some

circumstances the multiple reignitions can cause voltage escalation.

Both re-ignition and restrike can lead to severe transient overvoltage magnitudes. Multiple

restrikes can damage circuit breakers and other equipment connected to the system.

7.4.3 Chopping Overvoltage and Reignition Mechanism

The dielectric performance of a circuit breaker is designed to cope with the high value of fault

current and may therefore result in lower current being interrupted before current zero in an

AC system. The TRV across the VCB increases when current chopping occurs. B. Kondala

Rao, etc. [104] use simulation work to show the likely peak magnitude of chopping

overvoltages on the transmission system and the likely rate of rise (in kV/us), the result seen

detailed in Figure 7.7.

Figure 7.7 - Current chopping phenomenon and TRV waveform [104]

The reason that an overvoltage s generated is a result of the stored magnetic energy in the

components on the load side being converted into the electrostatic energy after the current

has been chopped. This energy will cause an overvoltage across the circuit breaker and load.

By equating the stored magnetic energy with the stored electrostatic energy, and neglecting

the damping, the voltage built up at the load terminals of an unloaded transformer can be

described in Equation (34) [156]. It can be seen the generated overvoltage depends on the

chopping current and the surge impedance of the transformer. The magnitude of these

voltages will be significantly higher than that expected during normal current zero interruption.


116

ch

LU I

C (34)

where ‘L’ is the load inductance, ‘C’ is the load capacitance and ‘Ich’ is the chopping current

After current interruption, the load parasitic capacitor ‘C’ will discharge through magnetizing

inductance ‘L’ with a typical frequency. The generated frequency will be in the range of kHz,

which means that voltage oscillation will be very steep. This fast oscillatory TRV at the load

side could lead the circuit breaker to dielectric breakdown, as the gap between the contacts at

that moment is very small to withstand.

The actual chopping current of vacuum circuit breaker is non-deterministic and varies for

different contact materials and electrode configurations. In 3-phase network, the chopping

current level is determined by the chopping level of the phase of which the current is most

near zero. Therefore, the moment of the contacts opening also influence the switching

transient.

The work carried out by Rene Smith [157] is shown in the following graph in Figure 7.8. It is

the result of the severest test-circuit conditions. The single phase per unit chopping

overvoltage ‘k’ (across the reactive load ‘Z0’) can easily be calculated as:

12

32

22

0 V

IZk ch

(35)

Where ‘Ich’ is the chopping current fixed at 3A and ‘V’ is the rated system voltage.

Figure 7.8 - Chopping overvoltage of circuit breaker and its respective rated voltage [157]


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This does not imply that overvoltages with shunt reactor switching are less prominent in HV

than in MV systems. Due to multiple re-ignitions, very high overvoltages can be reached. This

is also known from SF6 breaker application and tests.

7.4.4 Non-sustained Disruptive Discharges in VCBs

Non-sustained disruptive discharges (NSDD) appear to a significant issue that limits vacuum

circuit breaker performance. A non-sustained disruptive discharge is a temporary breakdown

that occurs in a vacuum interrupter after current interruption and where the insulation is

immediately restored [113]. It is defined in IEC standard 62271-100 [158] as disruptive

discharge between the contacts of a vacuum circuit-breaker during the power frequency

recovery voltage period resulting in a high-frequency current flow which is related to stray

capacitance near the interrupter. It is electrical discharge in recovery voltage appeared

between two electrodes for a long time after current interruption. It is a self-storing late

breakdown after current interruption in vacuum. A measured example of NSDD is shown in

Figure 7.9. The first NSDD occurs in phase 1 and later in phase 3.

Figure 7.9 - Measured voltage across VCB at the occurrence of NSDD in phase 1 and 3 [159]

Recent studies [160, 161] show that occurrence of NSDD is dependent upon the surface

condition and the material of the contact.

NSDD has the potential to cause other failures through mechanisms such as voltage elevation

causing dielectric interruption failure of other phases on three-phase circuits with non-

grounded neutrals. Voltage oscillations following NSDD are associated with the parasitic

capacitance and inductance local to or of the circuit-breaker itself. NSDD may also involve the

stray capacitance to ground of nearby equipment. In some case, NSDD can be the cause of a

restrike. In a three-phase system, once NSDD occurs at one phase, the voltage at another


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phase is raised to a value which is too high to be withstood by the circuit breaker. Then a two-

phase fault arises, which is interrupted after a full loop at power frequency.

According to new research, self-restoring breakdowns in SF6 circuit breaker have also been

reported (undocumented) [162], which indicates that the NSDD phenomenon is not restricted

to VCB. However, it does not happen as frequently as in a VCB. Further, NSDD are not

considered to be a sign of distress of the circuit breaker and are not detrimental to the

performance.

All of these reignition phenomena partly depend on the characteristics of the VCB itself, such

as chopping current level, the dielectric withstand capability of the vacuum gap. The

surrounding network also plays a significant role in the generation of high voltage switching

transients. In order to observe the influence of these conditions on generating overvoltages,

high voltage VCBs should be modelled. In the case of switching an unloaded transformer, the

inductance is non-linear and the iron loss plays an important role. It is important to model this

condition accurately to study the performance of TRV. More simulations are therefore to

examine the probability of overvoltages if using a VCB on the transmission system.

7.5 Simulation Studies on Multiple VCBs in HV Network

7.5.1 Literature Review on VCB Modeling

ATP/EMTP software is selected as transient simulator in this thesis to analysis circuit breaker

switching transient. ATP/EMTP is a universal program system for digital simulation of transient

phenomena of electromagnetic as well as electromechanical nature. ATP/EMTP contains

extensive modelling capabilities and additional important features for transmission lines, circuit

breakers, transformers and other high voltage power equipment. With this digital program,

complex networks and control systems of arbitrary structure can be simulated.

Previous simulation studies regarding the switching transient performance of vacuum circuit

breakers show that switching a no-loaded transformer can lead to severe overvoltages in the

system. A classical single phase model made by J.Helmer [163] is shown in Figure 7.10. The

parameter settings are mostly for medium voltage level up to 33kV. In J.Helmer’s model, a

12kV vacuum circuit breaker is installed in a 3-phase circuit and used to switch the no load

11kV/400V/1000kVA transformer, using parallel R-L-C model to represent no load transformer

core, RL = 105 Ω, LL= 120 mH, CL = 10 nF. The withstand capability of the breaker is related to

the gap distance, described in the formula: U = 1kV+30kV/mm * gap distance. The chopping

current is chosen at 4.7A. A high frequency TRV is observed at the terminal of the transformer,

the peak voltage is around 30kV.


119

Figure 7.10 - Test circuit of J. Helmer [163]

Based on same simulation theories, Popov [164] defined the dielectric strength of the

breaker’s gap with parameters depending on the time since its opening and is written as:

3400V + topen)-(t V/s101.7 =Ub 7 . The transformer rating is 13.2kV/220V/112.5kVA.

The chopping current of the breaker is 3A, which is the average value of the chopping current

of all kind of breakers. In this case there is no reignition was observed in the simulation results.

In another work [165] the same circuit but only with higher voltage parameters is simulated in

PSCAD. In the work the value of current quenching capability di/dt was initially set at 100A/us,

than changed to 200A/us. And the RRDS is initially chosen at 20V/us (with re-ignition); later

50V/us (no reigniton, shown in red line in Figure 7.11). Seen from the graph, the maximum

dielectric strength of the vacuum gap of 33kV VCB is about 62kV.

Figure 7.11 - TRV characteristic of 33kV circuit breaker [165]

The VCB model incorporates stochastic properties of different phenomena that take place in

the breaker opening process. These previous work provided several valuable experience on

circuit breaker parameter selection, such as the withstand voltage characteristic, contacts

opening speed, current quenching capability, etc.. However, it should be noted that the VCB

simulation carried out in previous paper is based on a range of typical values for each related

variables, but not measured parameters. In this thesis, the characteristics of high voltage

vacuum circuit breakers are derived from these published data, and J.Helmer’s circuit has


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contributed to the basic structure of the transmission network modelling and this is studied in

the following sections.

7.5.2 HV VCB in Three Phase System - Model Building

A three-phase 400kV is used in this model as the transmission network. The high voltage is

transmitted by 50km of overhead line and is then connected to the busbar of gas insulated

switchgear into which the switching device is installed. The dimensions of the GIS busbars

follow the practical size of the busbar in a National Grid substation. A 3-phase 400kV no-load

transformer is connected at the terminal of a transmission line by a 10 metre hollow tube

busbar. A simple line diagram of the studied model is pictured in Figure 7.12, and the

topographical structure and model information in the ATP/EMTP environment is shown in

Figure 7.13. Further explanations relating to each part in the system are given as follows.

AC

AC

Phase A

Phase C

Phase B

400kV

400kV

RL

R

L

400kV

busbar

400/132/13kV

Transformer

Figure 7.12 – Line diagram of 400kV system model

Figure 7.13 - 3-phase simulation mode without load connected in ATP/EMTP

A: Power source

A 3 phase AC voltage source is ready to use in ATP/EMTP program, and the amplitude of

voltage is the peak value of phase to earth voltage of a 400kV system, which is 327kV.


121

The fault level is set from 10kA/6928MVA to 60kA/41569MVA. The source inductance is

dependent on the fault level of substation. The source resistance is 40 times of reactance.

22

22222

)40(

)400(//

RR

kVXRVZVIcurrentlevelFault L

(36)

For 10kA/6928MVA fault level, source resistance is 1.82 ohm, inductance is 0.23H. For

60kA/41569MVA fault level, the source resistance is 0.3 ohm, inductance is 0.039H.

B: Overhead line

High frequency models of overhead transmission lines, underground cables and aboveground

busbars must be developed when studying the transient performance of power systems. It is

important to specify the parameters and structures before simulating and analyzing transients.

There are several different models to present overhead transmission line in ATP/EMTP

software, defined by the structure the tower, length of lines and other factors. Line/cable

constant (LCC) model is used most often, which automatically considers skin effect, bundling

and transposition issues. There are five different options for LCC models stated in [166]

including:

1. Bergeron line model is a distributed parameter model including the traveling wave

phenomena. It represents the line resistances at both ends as lumped elements.

2. PI-model: Nominal PI-equivalent model with lumped parameters, which is suitable for

short lines’ simulation.

3. Noda-model is frequency-dependent model. This algorithm models the frequency

dependent transmission lines and cables directly in the phase domain.

4. Semlyen-model is frequency-dependent simple fitted model. It was one of the first

frequency-dependent line models. It may give inaccurate or unstable solutions at high

frequencies.

5. JMarti is another frequency-dependent model with constant transformation matrix

suitable for simulating travelling wave phenomena in long transmission lines.


122

Figure 7.14 - LCC model setting dialog in ATP/EMTP

In the model studied in this thesis, a typical pylon in 400kV route carrying four steel-reinforced

aluminium conductors including three-phase conductors and shielding wire is used. The

dimension and the material of wires are all according to the National Grid specification. The

Bergeron line model is adopted in the simulation model presented in this thesis as there is no

significant difference between the Bergeron line model and PI line model for short lengths. To

avoid the occurrence of the unsymmetrical current among three phases, it is necessary to

make an addition by noting the transposition, and the detailed parameters are shown in Figure

7.14.

The overhead lines used for a National Grid substation are ZEBRA aluminium conductors

including phase line and earth wire. All the geometrical and material parameters of conductors

and the tower are all in keeping with the product manual. The corresponding electrical data

are calculated automatically by the line constants of ATP/EMTP.

C: 400kV busbar

The three phase busbar model used in the following analysis is shown in the following Figure

7.15, 3 busbars are placed at the same horizontal level, with a safety gap. Based on the

dimensions of the busbar and its electrical and dielectric properties, the standard ATP cable

PI-section model can help to calculate parameters of the cable and represent the cable up to a

few tens of kHz. While the study is focused on the influence of different frequencies, the JMarti

frequency-dependent model can be used instead to check the behaviour of the cable’s

impedance at a wide range of frequencies. The parameter setting dialogue in ATP/EMTP is

shown in Figure 7.16.


123

Figure 7.15 - Layout of 3 phase busbars above the ground

Figure 7.16 - Configuration of busbar in ATP/EMTP

The busbar plays an important role in the switching studies. The busbar reactance is the

dominant element and the length of the busbar influences the number of reignitions in the

vacuum circuit breaker. The ohmic loss of 10m busbar is so small that it can nearly be

overlooked, and the busbar can be simplified as an inductance with a proper length, as setting

in ATP/EMTP. Normally, the value of the inductance used as a simplified transmission line is

about 1uH/meter, and the data shown in Figure 7.17 is found in literature.

Figure 7.17 - Line inductance connection and its value


124

D: No-load transformer

In the studied circuit, the line is terminated by a transformer model, aiming to study the

transient voltage produced at the terminal of the transformer, which may contain high

frequency components. As shown in J. Helmer circuit shown in Figure 7.10, when the

transformer is no loaded, it can be seen as an open circuit on its secondary side, and a

general RLC (Resistor-Inductor-Capacitor) circuit is appropriate to model an open circuit

transformer.

The transformer rating used in the studied system is a 400/275/13 kV, it is not supposed to

work at or near the saturation region to avoid harmonics injected to the current. For unloaded

transformers, a small no-load magnetizing current is required. For a given transformer rating

the magnitude of the no-load current depends on the size and quality of the transformer core.

In general, an open-circuit of the secondary terminals should be considered as a serious

accident. The equivalent circuit for no load transformer can be simplified with a large

magnetizing inductance parallel connected with the resistor. The parallel parameter values are

found with no load connected to the secondary (open circuit).

When a sinusoidal voltage is applied to a transformer winding on an iron core with the

secondary winding open a small current will flow in the primary, and this current is called the

magnetizing current. With the secondary open the transformer primary circuit is simply one of

very high inductance due to the iron core.

For a star connected transformer, the phase current is equal to line current. Through the

simulation on the ready to use 400/132/13kV no load transformer model built by [167], the per

phase no-load excitation current I0 is 16A .

As stated in the transformer model, the core loss per phase: Pc = 127.9kW/3 = 42.63kW

Core loss component: AkV

kW

V

PII C

c 13.03/2400

63.42cos0

The magnetizing current is the principle component of the current drawn by an unloaded

transformer, and it can be calculated in:

AIII cm 1613.016 2222

0


125

Resistance: MA

kV

I

VR

c

c 26.1184.0

3/400

Inductive resistance kA

kV

I

VX

m

m 43.1416

3/400

The magnetic inductance HkfXL m 4650//2/43.14/

The bushing capacitance of a 400kV transformer is selected as 250pF, then the no load

transformer can be simply modelled in following diagram.

Figure 7.18 - One phase simplified circuit of no-load transformer

At the instant of chopping when a current ‘Ic’ is chopped to zero from a certain level, the

magnetic energy in the inductance and the electrostatic energy in the capacitance are trapped

in the load side of the circuit-breaker. When the total energy is converted to the electrostatic

one in the capacitance, the highest overvoltage appears as shown in the following simulation

results. The following equation is applicable:

2

0

22

2

1

2

1

2

1CVLICV ch

(37)

Where ‘V’ is the peak overvoltage observed at load side, ‘Ich’ is the chopped level, ‘V0’ is the

peak magnitude of source voltage, which is 327kV in this work, ‘L’ is the load inductance (46H),

‘C’ is the load capacitance (250pF). For example, when the breaker is switched to open at

1.5A, the maximum voltage observed at load side is calculated at 721.7kV.

After the current has been chopped the TRV occurs, resulting in several oscillations with

different frequency, depending on the type of circuit. First is the natural frequency which is

relatively lower and it determined by the HV inductance and capacitance of the load, it can be

estimated with:


126

LL CLf

2

11 (38)

The breaker recovers only to reignite again when the system transient recovery voltage (TRV)

exceeds the dielectric strength of the short gap of the breaker. When the TRV has surpassed

the withstand voltage level of the breaker, the VCB reignitions and high frequency current

flows through the network. The reigntion may repeat a number of times until the instantaneous

level of the power frequency current is greater than value of transient oscillatory current, when

no further HF current zeros occur, and the full arc is re-established. The high frequency of

oscillating current is related with the line inductance and load capacitance value, and it is

calculated by:

LS CLf

2

12 (39)

For the no-load transformer switching model studied in this thesis, only 10 meters of busbar is

used to connect circuit breaker and transformer, the inductance of which is about

0.0036mH/meter, therefore the normal oscillating frequency is calculated with 1.5kHz, and the

higher frequency f2 is about 1.7MHz.

E. High Voltage Circuit Breaker Model

The HV circuit breaker model used in this work was originally developed by Seyed M.S.Mir

Ghafourian [168] of the University of Manchester. There is a logical program in Appendix 1

used to control the current reignition simulation process. It has been adapted for use in this

work by adjusting the characteristic parameters of a vacuum circuit breaker within the software

code.

In ATP/EMTP software, the switching process of breakers is defined in MODEL device, which

is connected parallel to the breaker device, shown in Figure 7.19. The voltages at both ends of

the circuit breaker are measured and their difference is compared with the dielectric strength

of contacts gap which is defined by a group of linear data. When the voltage across breaker

exceeds withstand voltage, a reignite signal is given to the switch. For each breaker, several

character determined parameters are defined including the opening time, chopping current

and slope of current. These values can be adjusted at different simulation conditions.

FORTRAN language in MODEL is used to control the opening and closing of the switch,

including current chopping at power and high frequency, withstand voltage of the gap and the

high frequency quenching capability. As the overvoltage due to multi - reignition across


127

breakers can cause severe problem in some cases, the study of the multiple re-ignition

process of high voltage circuit breakers is becoming necessary.

Figure 7.19 - Working principle of MODEL controlled breaker device in ATP/EMTP

The withstand voltage level of a circuit breaker is determined by the gap distance between two

contacts in a vacuum interrupter. The Equation (40) [163] below describes the dependency of

breakdown voltage ‘Ub’ of cold gap distance:

s 30kV/mm + 1kV = s a + U=U 0b (40)

Where ‘s’ is the gas distance between contacts in mm.

Measured in time scale, the above equation is derived to [169]:

bopenab B + ) t-(t A =U

(41)

‘Aa’ and ‘Bb’ are defining different breaker switching performance depending on the type of

circuit breaker, and its power rating.

At the initial stage of the opening process, after the power frequency current has been

interrupted, the contacts start to separate, but when the gap between two contacts is not big

enough to withstand the recovery voltage, and then the re-ignition happens. In other words,

the withstand capability of the contact gap must be higher than the transient recovery voltage

to prevent dielectric failure. The gap keeps on increasing, the re-ignition arc will be

extinguished, and as a result there is a race between the transient recovery voltage and the

withstand voltage of the interrupter gap. The multiple re-ignition will take place until the two

distinct electrodes are fully able to withstand the recovery voltage.

Due to the chasing between the transient recovery voltage and the building up of circuit

breaker withstand capability, re-ignition will occur. A high frequency current flowing through

the circuit breaker is generated due to re-ignition occurrence and it is superimposed on the

power frequency current. This current is expected to be chopped at a certain value. The ability


128

to quench the high frequent current at a zero crossing is important for a vacuum circuit breaker

[165]. The arc can be extinguished when the slope of the current di/dt (A/s) is small enough

and lower than the so called critical current slope, which presents the high frequency

quenching ability of the vacuum circuit breaker. The current quenching slope is a random

piece of data and can be picked from a range of values, which is related to the characteristics

of the circuit breaker. And it has come clear that the slope depends on the reignited voltage

and that it also shows a time dependent behaviour. Glinkowski et al [170] proposed values

C=-3.40E10 A/s2, D=255E6 A/s in equation di/dt =C(t-topen)+D. The average value of di/dt is

in the range of 30-75A/us according to [171]. And it is mentioned in the article [163], vacuum

circuit breakers are capable of interrupting currents with a very high di/dt value in the range of

150-1000A/us and as a consequence the high frequency current may be interrupted during

one of the high frequency excursions through zero.

It is noted from previous work that chopping current, the withstand ability of the breaker gap

and the HF quenching capability are the three most important factors which have an influence

on the system components and subsequently on the generated TRV. Different kinds of

oscillation with varied frequency are generated and imposed to TRV.

A 420kV (r.m.s. value) open switching device is expected to withstand 1425kV peak lightning

impulse voltages according to the standard BS EN 62271-1[13]. These criteria are applied to

the high voltage vacuum circuit breaker model developed in this thesis. If the previously

discussed 33kV vacuum interrupter was used to develop a device with a withstand voltage of

1425kV, you would need 23 of these units based on the 62kV capability described in Figure

7.11. With a different rate in the rise of dielectric strength (RRDS), the time required to get the

full dielectric strength varies. Two typical values of RRDS are used in the 33kV VCB study,

which are 20V/us and 50V/us. For the high voltage vacuum circuit breaker composed of

multiple medium voltage interrupters connected in series, the increasing rate of dielectric

strength is presumably much quicker than a single break. As such, some typical magnitudes of

RRDS of high voltage circuit breakers are simulated in the following studies ranging from

240V/us to 600V/us

7.5.3 Simulation Study – Switching off Inductive Current

In following simulations, all the breakers at three phases are assumed to operate at exactly

the same time, and there is no operating time difference among the multiple medium voltage

breakers. Based on the above published data, some specific values of di/dt, dv/dt are selected

to investigate the influence on the switching transient overvoltage at load terminal of 400kV

transmission line.


129

The switching performance of a high voltage vacuum circuit breaker in 400kV system with no

–load transformer at the terminal is studied in cases. The network configuration is as shown in

Figure 7.20. In this work, the detail design feature of the breaker itself is disregarded, only its

characteristics are considered. The withstand ability of the breaker with fully open status is

achieved and is fixed at 1425kV.

Figure 7.20 - Circuit when no loaded transformer at circuit terminal

The high frequency current is controlled to be chopped at 1A, the average normal frequency

chopping current is constant at 3A and the opening time of all breakers are selected at 0.048s.

In each case with different characteristics of di/dt and dv/dt, the voltage at the source side and

the load side is measured separately and the current at the load side through each phase is

observed. The status of the breaker is plotted as well to check the opening and reigniting

process, and also the voltage across each breaker is shown in the following results. Series

simulations are carried out to study the influence of different characteristics of breakers, such

as chopping current, opening speed, to the switching transient in 400kV transmission line. All

the simulation settings and their respective results in data are shown in Table 7.4, followed by

some typical simulation results given in a series of graphs respecting to each case study.

Case RRDS di/dt Reignition Reignition time Peak voltage at

load side

1 240V/us 100A/us Yes 8.4ms 1.69MV

2 240V/us 200A/us Yes 6.8ms 1.70MV

3 240V/us 1000A/us Yes 6.6ms 1.48MV

4 490V/us 400A/us Yes 6.2ms 1.78MV

5 490V/us 1000A/us Yes 12.5ms 1.52MV

6 600V/us 100A/us No / 1.15MV

7 600V/us 200A/us No / 1.15MV

Table 7.4 - Different settings on characteristics of vacuum circuit breakers and their influences


130

It can be seen from the result in the table that, the value of di/dt can have a significant

influence on overvoltage with higher di/dt values leading to lower overvoltages. Overvoltage

levels are very high compared with the transient levels that other substation equipment

including transformers is designed for. The RRDS appears to have no major impact unless

high enough to prevent reignition. So, even with a simple simulation it is clear that

overvoltages in a transmission system using VC could be too high in comparison to the BIL of

equipment and as such need to be examined more thoroughly in future work.


dv/dt=240V/us

Case 1: The increasing speed of the withstand voltage dv/dt=240V/us, and the breaker will

take 5.9ms to achieve the maximum dielectric strength. The quenching capability di/dt is

100A/us. The TRV developed across the breaker is more than the RRDS developed of


131

vacuum gap, a dielectric breakdown occurs and an arc is re-established within the interrupter,

which induces massive reignition at all three phases.

Figure 7.22 – Three-phase current waveforms of Case 1

The currents in three phases are interrupted in order at each current zero, and the current is

completely chopped, and high frequency oscillation stops after a short time. Looking more

closely at the detail of the current waveform in Figure 7.22, one phase is interrupted to zero

first (red line) from 3A as it is the first phase near current zero; the recovery voltage of the later

acting two phases are much influenced by the first acting phase, and are enhanced to some

extent.


132

Case 2: The withstand voltage is developed at the same speed of dv/dt=240V/us as the last

case, the breaker will take 5.9ms to achieve the fully open status. High frequency current

quenching capability di/dt is 200A/us. Higher quenching capability gives a shorter arcing time,

the high frequency current is chopped to zero at the moment 0.066s, which is about 0.01ms

earlier than case 1 with a lower quenching capability.


dv/dt=240V/us


133

Case 3: When dv/dt is set at 240V/us, the breaker will take 5ms to achieve the fully open

status. di/dt=1000A/us. A higher quenching capability gives short reignition duration, the high

frequency current is chopped to zero at the moment 0.056s, which is about 0.02s earlier than

case 1 with a lower quenching capability. Furthermore the greater value of di/dt has the

influence of transient waveform of TRV across breakers. A shorter term of oscillation with

higher frequency of TRV can be observed.


dv/dt=240V/us


134

Case 4: in this case, the opening speed of contactors is increased more, the rate of increasing

dielectric strength dv/dt is set at 490V/us, the breaker will take 2.91ms to achieve the

maximum dielectric strength 1425V. The quenching capability di/dt is selected at 400A/us.


dv/dt=490V/us


135

In this case, only one reignition happened in Phase A, the decreasing slope of the high

frequency current before reaching current zero is a value between 400A/us and 600A/us, so it

is acceptable to set the critical current slope at 400A/us. Also the value of the increasing rate

of dielectric strength (490V/us) is like a turning point whether reigntion happens or not. A

slightly bigger dv/dt of breakers can interrupt the current successfully without inducing any

reignition.

Figure 7.26 - case 4: switching performance of circuit breaker at phase A breakers when

di/dt=400A/us, dv/dt=490V/us

Case 5: In this case, most parameters are kept the same as last cases, only with higher

quenching capability di/dt which is set at 1000A/us. However this setting doesn’t help to

reduce reignition as seen from the simulation results. There are more times of reignition are

generated again and again, and a longer arcing time is required for the arc extinguish.


136


dv/dt=490V/us

Case 6: In this case, the RRDS value for each breaker is 600V/us, the breaker will take

2.38ms to reach a peak dielectric strength. The high frequency current is reduced at the speed

of di/dt=100A/us. The breakers have experienced successful interruption of the arc at the first

current zero.


137


dv/dt=600V/us

After investigating the simulation results of the above cases, a conclusion can now be drawn. .

The interruption process of the circuit breaker can produce undesirable high voltage surges,

which depends on the high frequency properties of the circuit and also the characteristics of

the breaker. A breaker with a proper RRDS bigger than 490V/us can be switched off without

causing reignition, It is suggested that in VCB with high RRDS the possibly of reignition will be


138

smaller and can even be avoided since the breaker regains its dielectric withstand faster than

breakers with low RRDS. The switching overvoltage at no-load transformer side is also fixed at

a certain value depending on the chopping current level. The possibility and probabilities of

reignition occurrences are related to the increasing speed of the dielectric recovery strength of

the breaker and the transient recovery voltage across the gap of contacts. The quicker the

dielectric strength recovers, the lower the possibility of reignition.

To make sure the dielectric strength of the vacuum gap can exceed the recovery voltage

across the circuit breaker, the opening speed of contacts or dielectric strength of the vacuum

gap must be guaranteed to meet such requirements. In addition to the possible failure of the

interrupter, these breakdowns of vacuum gaps can lead to failure of other power system

equipment. Thus it is necessary to ensure that the duties imposed by the switching application

are within the circuit breaker capabilities either by limiting the magnitude of the TRV and/or

RRRV or by selecting breakers with adequate capability.

Whenever multiple reignitions occur, changing the critical value of the slope of the current will

impact the switching transient performance; a greater high frequency current quenching

capability can bring down the switching overvoltage and current in circuit. Moreover, the higher

frequency current is chopped to zero more quickly. However, this is not the case when the

increasing rate of the dielectric strength is selected at 490V/us.

The number of re-ignitions becomes fewer when the current is chopped at a higher level, and

also the breakers can achieve the fully open status more quickly. It can also be seen from the

current observed at the load side that the current oscillating duration becomes longer while

there is not much difference in the magnitude of the current.

When the capacitance value at the load side is increased to a bigger value, the influence on

the switching transient is studied. The test circuit is shown in Figure 7.29, the characteristics of

circuit breakers in all cases are the same, the chopping current is set at 3A, the opening

duration of contacts is 2.91ms, the maximum dielectric strength is 1425kV. The slope of

current is fixed at 400A/us. The simulation results of switching overvoltage are shown in Table

7.4. It can be found that the extra capacitance can help to reduce the peak reignition

overvoltage at the load side. For the first occasion when dv/dt is 490V/us, di/dt is 400A/us, a

10pF capacitor is enough for the transformer to avoid the risk of reignition. A larger magnitude

of capacitance can further reduce the peak overvoltage of the transformer. The simulation

result shown in Table 7.6 are when the circuit breaker has low dv/dt and di/dt, the connecting

extra capacitance doesn’t give the same effect, and the high frequency current is increased

when a larger magnitude of capacitance is installed.


139

Figure 7.29 - Extra capacitance Cr installed at terminal end of transformer

Model Re-ignition Yes/No Current measured

at load side Switching

overvoltage

No extra capacitance Cr=0

Yes 4400 A 1.73 MV

Cr=0.000001uF Yes 4350 A 1.63 MV

Cr=0.00001uF No 22.5 A 1.17 MV

Cr=0.0001uF=100pF No 22.5 A 1.07 MV

Cr=0.001uF=1nF No 22.5 A 665 kV

Table 7.5 - Result comparison with extra capacitance when dv/dt=490V/us, di/dt=400A/us

Model Re-ignition

Yes/No Current measured at

load side Switching

overvoltage

No extra capacitance Cr=0

Yes 4100 A 1.45 MV

Cr=0.000001uF =1pf Yes 4150 A 1.61 MV

Cr=0.00001uF =10pF

Yes 4430 A 1.66 MV

Cr=0.0001uF =100pF

Yes 6500 A 1.65 MV

Cr=0.001uF Yes 10.6 kA 1.17 Mv

Table 7.6 - Result comparison with reactive compensation when dv/dt=240V/us,

di/dt=1000A/us

7.5.4 Statistical Study of Developed HV VCB Model

As explained earlier, the characteristics of the switching transient are mainly influenced by

several factors, such as the opening time of contacts, the chopping current, the critical current


140

slope, the withstand voltage of the vacuum gap and its increasing speed, etc.. Some of these

factors have great impact on the switching transient phenomena, but are un-predictable and

only can be roughly estimated from a range of possible data. Modelling of VCB working in the

transmission line should include the consideration and anticipation of the influence of different

characteristics of circuit breakers.

Therefore, statistical studies have been carried out on the switching performance that depends

on different characteristics of circuit breakers. In this work, the circuit with a developed high

voltage vacuum circuit breaker model installed is automatically run 1000 times, controlled by

program in MATLB. The detail program language is given in Appendix 2 at the end of this

thesis. The peak switching overvoltages measured at the load terminal are recorded, and the

distributions related to various parameter settings of circuit breakers in normal distribution are

analysed.

Through all the statistical studies, the increasing rate of the withstand voltage of the vacuum

gap (dv/dt), and the quenching capability (di/dt) are not random factors, but fixed with some

specific value. The chopping time and opening time are selected in a random way from a

proper range. The descriptions of each case study are listed in Table 7.7. The detailed

simulation process and results are examined thereafter respectively.

di/dt dv/dt Chopping current (A)

Opening time

Peak overvoltage

Average overvoltage

Case 1

Fixed at 400A/us

490V/us Random in range of 2-

6A

Fixed at 0.048s

1555kV 1522+/-317kV

Case 2

Fixed at 400A/us


3A

Random in 0.046-0.053s

1525kV 1404+/-347kV

Case 3

Fixed at 400A/us


3A


1490kV 1364+/-367kV

Case 3

Fixed at 1000A/us


3A


1562kV 1418+/-358kV

Table 7.7 - Parameter setting for statistical study

Case 1: In this case, the critical current slope is assumed to be a constant and the opening

time for all three breakers of three phases is kept at same. The distribution of the peak value

of the switching overvoltage observed at the load terminal depends on the selection of the

chopping current in the range of 2-6A. The distribution result is shown in Figure 7.30.


141

Figure 7.30 - Case 1: Fixed opening time, fixed di/dt =400A/us, dv/dt=490V/us, random

chopping current between 2-6A

Comparing the results with different ranges of chopping currents, it is found that the circuit can

be chopped at a lower chopping current level without reignition happens. The switching

overvoltage is not massive and the magnitude depends on the chopping current of breaker

itself following Equation (42) as mentioned earlier. It is suggested that the chopping current of

breaker should be made as small as possible to avoid the risk of reignition and high frequency

voltage escalation.

There is a linear relationship between peak overvoltage and the chopping current level when

the chopping current doesn’t exceed 3A. When the chopping current is bigger, its influence on

the switching overvoltage peak value does not exist anymore, and the switching overvoltage is

kept between 1.6MV and 1.8MV, no matter what value the chopping current is.

In practice, the breaker with the bigger chopping current level is supposed to be chopped first,

and the chopping level of the multi-break model is decided by the breaker with the relatively

biggest chopping current. As can be seen from this result, if the chopping current is bigger

600

800

1000

1200

1400

1600

1800

2000

2.0 3.0 4.0 5.0 6.0

Pe

ak o

verv

olt

age

(kV

)

Chopping current (A)

1555kV

0.00%

0.02%

0.04%

0.06%

0.08%

0.10%

0.12%

0.14%

600 1100 1600

Pro

bab

ility

Peak overvoltage (kV)

Mean = 1522kV SD = 317kV


142

than 3A, the possibility of reignition is higher, and the difference between the chopping current

among multi breakers has no influence on the reignition overvoltage.

Case 2: The opening time of the breaker also plays an important role in the general switching

performance of a high voltage VCB in transmission circuit. In all the studies presented in this

thesis, the circuit is simulated for a particular opening time and all the breakers open

simultaneously. A full cycle of current waveform when the circuit remains connected without

switching is given in Figure 7.31.

t=0.048s

Figure 7.31 - Current and voltage waveform at load terminal without switching

At the moment when t=0.048s, the current of phase B on the green line is the nearest to

current zero, and phase B should be chopped before the other two phases. In this case, the

opening time is randomly chosen from the range of 0.046s-0.053s. The range is about 1/3 of

one cycle of current waveform, and covers all the switching angles of the three-phase current.

When the chopping current is randomly chosen between 2A and 3A, the distribution of the

maximum overvoltage after the breakers open is shown in Figure 7.32.

It can be seen from the result that, when the opening time is chosen from periods 0.0470s-

0.0485s, and 0.050s-0.052s, the value of the switching overvoltage depends on the chopping

current level, and the current can be interrupted to zero without causing reignition.


143


random chopping current at 2-3A

Case 3: in this case, the HF current quenching capability di/dt is fixed at 400A/us, the

increasing rate of dielectric strength dv/dt follows a higher speed at 600V/us. Then the impact

of the random opening time, and the random chopping current to the switching overvoltage is

shown in Figure 7.33.

600

800

1000

1200

1400

1600

1800

2000

2.00 2.20 2.40 2.60 2.80 3.00

Pe

ak o

verv

olt

age

(kV

)


600

800

1000

1200

1400

1600

1800

2000

0.045 0.047 0.049 0.051 0.053

Pe

ak o

verv

olt

age

(kV

)

Opening time (s)

1525kV

0.00%

0.02%

0.04%

0.06%

0.08%

0.10%

0.12%

600 900 1200 1500 1800

Pro

bab

ility


Mean = 1404kV SD= 347kV


144


random chopping current

According to the earlier simulation carried out for the model with a fixed chopping current at 3A

and with the opening time fixed at 0.048s, the circuit can be successfully interrupted. However,

the simulations result shown in this case indicates that the opening time is an important factor

for the switching performance.

500

700

900

1100

1300

1500

1700

1900

2.00 2.20 2.40 2.60 2.80 3.00

Pea

k o

verv

olt

age

(kV

)


600

800

1000

1200

1400

1600

1800

2000

0.045 0.047 0.049 0.051 0.053

Pe

ak o

verv

olt

age

(kV

)

Opening time (s)

1490kV

0.02%

0.04%

0.06%

0.08%

0.10%

0.12%

600 900 1200 1500 1800

Pro

bab

ility




145

Case 4: in this case, the HF current quenching capability di/dt is fixed at 400A/us, the

increasing rate of dielectric strength dv/dt follows a higher speed at 490V/us. The opening time

and chopping current are both chosen in a random way.


random chopping current

600

800

1000

1200

1400

1600

1800

2000

2.00 2.20 2.40 2.60 2.80 3.00

Pea

k o

verv

olt

age

(kV

)


600

800

1000

1200

1400

1600

1800

2000

0.045 0.047 0.049 0.051 0.053

Pe

ak o

verv

olt

age

(kV

)

Opening time (s)

1562kV

0.01%

0.03%

0.05%

0.07%

0.09%

0.11%

600 900 1200 1500 1800

Pro

bab

ility




146

The statistical studies show that the switching overvoltage to no-load transformer is influenced

by several random factors, like the chopping current, the opening time, and also by the

different characteristics of the circuit breaker. As shown before, the influence of chopping

current to the transient overvoltage is becomes less when the chopping level exceeds a

certain level. In case 4 when the increasing rate of the dielectric strength cannot catch up with

the speed of TRV across the breaker, picking the right opening time of the circuit breaker can

help to reduce the risk of reignition. There is only 100kV difference between the average

overvoltages of case 3 and case 4, and the peak magnitude in case 4 is only 70kV bigger

than that of case 3.

7.6 Summary of Vacuum Circuit Breaker

As is well known, the advantages of vacuum circuit breakers are that they are compact,

reliable and have a longer operation life. There are no fire hazards and no generation of gas

during and after operation. It can interrupt any fault current. It is quiet while operating. Less

power for the control operation is required. Their compact size and light weight are also

positive features.

Based on the development of the contact material, the electrode structure, the breaking

capability of the vacuum circuit breaker has increased since the 1970s. Japan AE power sells

170kV/40kA double breaks vacuum circuit breakers, and the first 145kV/40kA single break

VCB was installed in 2010. 120kV/31.5kA single-break and 168kV/40kA double-break

interrupters are being commercialized by Japan AE. Moreover in China, single break

126kV/2000A/40kA has been generated and its type test was finished in early 2012, and

single break 252kV/3150A/40kA interrupter is being investigated. On the other hand, six

126kV interrupters in series to build 750kV breakers are being researched in China.

A proposed high voltage vacuum circuit breaker model used in 400kV transmission line as a

replacement for SF6 circuit breaker is being studied by simulation work in ATP/EMTP. The

high voltage circuit breaker model is built on the basis of published data of about 33kV circuit

breakers and the criteria recommended in IEC standards on high voltage switches. The

switching performance of high voltage vacuum circuit breaker working at transmission line with

no-load transformer connected at terminal is being studied through simulations in different

cases. Those important factors play important roles in the switching transient performance. Its

properties, such as the chopping current, the high frequency current quenching capability, the

increasing rate of the withstand voltage of the vacuum gap, and the opening time of contactors

are being investigated.

The occurrence of reignition is decided by the increasing speed of the dielectric recovery

strength of the breaker. The more quickly the dielectric strength recovers the less possibility of


147

reignition. Therefore there are two ways to improve the performance of the vacuum circuit

breaker. The first way is to accelerate the opening speed of contacts controlled by the

operating mechanism to ensure enough dielectric strength can be built as quickly as possible.

The second way is increase the dielectric strength of the vacuum gap itself.

On the other hand, to avoid the severe problems caused by multi-reigniton, it is necessary to

ensure that the duties imposed by the switching duties are within the circuit breaker

capabilities either by limiting the magnitude of the TRV and/or RRRV or by selecting breakers

with adequate capability. As shown in the result, the load parameters should be selected

according to the limit of switching devices. The additional capacitance compensation can

significantly reduce the switching overvoltage.

When multiple reignitions occur, changing the critical value of the slope of current di/dt will

impact the switching transient performance; a bigger high frequency current quenching

capability can bring down the switching overvoltage and current in circuit, in addition the

higher frequency current is chopped to zero more quickly. However, this is not the case when

the increasing rate of the dielectric strength is selected at 490V/us.

The number of re-ignition becomes less when the current is chopped at a higher level, and the

breakers can also achieve the fully open status more quickly. It can also be seen from the

graphs of the current observed at the load side, that the current oscillating duration becomes

longer whereas there is not much difference in the magnitude of the current.

Statistical studies on the developed VCB model show that the switching overvoltage depends

on several random characteristics of circuit breaker. Looking at the overvoltage distribution of

1000 times simulation, the influence of the chopping current on the transient overvoltage

becomes less when the chopping level exceeds a certain level. In case 4, when the increasing

rate of the dielectric strength can’t catch up with the speed of TRV across the breaker, picking

the right opening time of the circuit breaker can help to reduce the risk of reignition. Generally

speaking according to 1000 times simulation result, the breaker with bigger value of dv/dt

doesn’t obviously reduce the switching overvoltage or the chance of reigniton happening.

7.7 Conclusion of Alternative Switchgear

In this chapter, a range of candidates that could replace SF6 as an interruption medium have

been reviewed and compared. A general comparison among these candidates is made in

Firstly, the use of bio-degradable oil is suggested, such as vegetable oil and ester, to replace

mineral oil and, as a result, to reduce the impact on the environment. As there is no evidence


148

or practical experience to prove its interrupting performance so far, further research on the arc

extinguishing behaviour of these bio-degradable oil is necessary in future.

Gas circuit breakers, especially the non-SF6 self-blast type interrupter units have been largely

developed recently, after adopting PTFE for the nozzle in the circuit breaker. This material can

be induced by a high temperature arc to generate ablated vapour, which is helpful in building

up the pressure inside the expansion chamber during the arcing process. With that advantage,

environmentally friendly gas such as N2 and air can be used as an arc quenching replacement

for SF6 in the circuit breaker, while retaining the same interrupting ability as that of SF6.

Meanwhile, the life time of the nozzle can be improved by adding inorganic filler such as Al2O3,

TiO2, or BN to PTFE.

Furthermore, the interruption performance of CF3I and its mixture has been investigated, and it

has been concluded that the arc time constant of CF3I is bigger than that of SF6, and its

interruption performance is about 0.9 times that of SF6 with the same surge impedance at the

same upstream pressure. To bring down the boiling point of the interrupting gas, CO2 gas is

mixed with a small portion of CF3I with the ration 8:2, and the interruption performance of the

gas mixture approaches that of pure CF3I.

A vacuum circuit breaker can be an option as a replacement for high voltage SF6 gas circuit

breakers. It has the advantage of being clean to the environment; it has high efficiency, a long

operating life and a reliable switching performance. The 126kV single break VCB is

manufactured in China, and the breaker at 252kV level is being studied. Several breakers

connected in series can work properly in the transmission line. Through the simulation study of

the reignition process of a 400kV multi- breaker model, the circuit can be switched off without

causing HF reignition by the vacuum circuit breaker with achievable characteristics like the

increasing rate of the dielectric strength and HF current quenching capability.

However, the energy consumption of VCB is larger than that of GCB, as far as the spring

mechanism for the two technologies is concerned. Several unclear issues around the vacuum

circuit breaker, such as the influence of X-rays on external insulation medium require further

and more detailed study in future.


149

Table 7.8 - Comparison of all interrupting candidates

Medium Technology Advantage Disadvantage

SF6 Absorb the free electron in the arc, high pressure gas cool down the arc temperature

Very low chopping current (1A)

Small pressure difference (0.5-0.6MPa)

Strong dielectric strength

High risk of leaking, with high GWP

Toxic by-products after arcing

Need to monitor gas pressure

High cost on operating mechanism

CF3I Absorb the free electron in the arc

Low GWP (5)

Strong dielectric strength

Long term performance is not sure

Need to monitor gas pressure

Vacuum Magnetic field emission to extinguish arc

Clean

Maintenance free

Long operation life (10,000 times)

X-ray emission

Expensive operating mechanism

Chopping current can be 3A

Voltage and current rating of single breaker is not suitable for high voltage utilization

High pressure air blast

High pressure air flow is used to blow the arc

Clean

Higher voltage rating

Higher operating speeds

More stringent overvoltage limits

Noisy

Expensive

Big pressure difference (up to 7MPa)

7-8A chopping current

Bio-degradable Oil

Use H2 provided by the thermal decomposed oil to blow the arc

Bio-degradable

Low cost on operating mechanism

Chopping current can be 4A

Flammable

Low temperature will make oil viscous thus reduce operating speed

Need to refill and replace with new oil

Chapter 8 Conclusion

150

Chapter 8 CONCLUSIONS

The contribution of the electricity supply industry to the global release of SF6 has been

reported in figure and the harmful impact to environment is undeniable. The concern over the

role of SF6 as a greenhouse gas is emphasized and it is essential that the electricity supply

industry actively considers alternatives.

8.1 Alternatives for SF6 Gas Insulated Substation

For alternative insulation systems, a number of alternatives exist, such as oil, CF3I, insulating

foam, solid insulation combined with a gas. The main findings of each alternative dielectric

material are listed in table 5.3. Chapter 3 of this thesis analysis of the size of busbar

enclosures and was produced on the basis of the lightning voltage rating and the E/N

capability of the insulating gases. Analysis was also carried out for fixed dimension enclosures

to estimate the required working pressures to meet the lightning overvoltage requirement. The

dimensions of the dielectric system and its ampacity of respect system are calculated and

analysed using heat transfer models considering their boiling point and proper working

pressure which is related with the dielectric strength of some gas. Experimental work and

theoretical calculations are carried out for the research on solid foam insulation.

8.1.1 CF3I Gas Insulation

According to literatures and comparisons with other gas insulation candidates, the dielectric

strength of electronegative CF3I gas is 108kV/cm, stronger than that of SF6 (89kV/cm). It has

no contribution to the greenhouse effect, because of its instability in the atmosphere. It is

recommended as the most promising replacement of SF6 gas insulation. Due to its high boiling

point, CF3I gas is mixed with N2 to be working at a reasonable boiling point when higher

pressure is necessary for the usage in high voltage busbar systems.

Through the calculations on 400kV gas insulated busbar in same fixed size as SF6 insulated

busbar which is currently being used in substations of National Grid, the better solution for SF6

replacement appears to be 70%CF3I-N2 gas mixture which has the same dielectric strength as

that of SF6 pure gas. The boiling point of this mixture is also improved in comparison to pure

CF3I at 280K. The busbar filled with this gas mixture has a reasonable current carrying

capability. The current carrying capability of 70% concentrated CF3I gas mixture insulated

busbar is 7315A, which is 11% reduced compared with the 8231A of a SF6 insulated busbar.

This reduction is due to the physical properties of CF3I itself such as lower thermal


151

conductivity, higher viscosity, etc.. The current rating of SF6 insulated busbar is therefore still

superior to CF3I and other gases candidates.

Higher pressures are required for the gas mixture with lower concentration of CF3I but these

deliver a better boiling point of the gas mixture. In the case of varied shell dimensions but with

fixed gas pressure at 0.3MPa, 60%CF3I–N2 gives a reasonable boiling point of 266.95K. With

this gas mixture this leads to increasing of enclosure radius of 275mm which is 22% larger

than that of the SF6 insulated reference system. The current rating of 60%CF3I gas mixture

drops 18% to 5964A in comparison to 7250A for SF6. While the help of a lower BIL 1050kV

explained in Section 4.1.4 the requirement of the dielectric strength of gas can be reduced.

The calculated value shows that the dimension of 60%CF3I gas mixture in this case can be

increased by about 32%, and the new busbar size is only 1.63cm bigger than the one with SF6

insulation.

8.1.2 Solid Combined Insulation

Use of a solid coating combined with insulating gas is another possible way to achieve no-SF6

usage in busbar insulation. Literature suggests that the dielectric strength of insulating system

can be improved by some thickness of coating around conductor. Epoxy resin or rubber is the

most commonly used coating material in gas insulated busbar system. The calculation carried

out in Chapter 4 shows that the coating of metallic electrodes reduces the peak electric field

seen by the gas, and this allows increased breakdown voltages. For example, when there is a

10mm thickness coating around conductor, the peak electric field stress in the gas in 12%

weaker than the occasion without coating.

With the help of the reduced electric field stress to the gas insulation, the requirement for the

gas dielectric strength is lower and therefore the gas filling pressure in busbar can be lower, or

the enclosure size can be reduced as another option. In terms of CF3I, it is more practicable to

apply CF3I gas insulation at a lower pressure while the boiling point is still at an acceptable

level without extra cost or work.

Using a solid coating laminated around the conductor, when comparing the fixed dimension

busbar systems, the current carrying capability is reduced by 20% - 30%.

When the coated conductor is applied in the busbar system with different dimension of outer

sheath, a 10mm coating can help to slightly reduce the size of outer sheath with this delivering

a 4% reduction. The current rating of new coated conductor then becomes higher. This would

probably have the scope to eliminate SF6 used in equipment but again would only be suitable

for new plant.


152

8.1.3 Particle Impacts in the Gas

The results show that, when the conductor size is fixed, the bigger busbar dimensions give

rise to a lower electric field on the inner surface of the sheath and this means that only

particles of a relatively small size will be able to move. In addition, these smaller particles will

have a lower associated charge and are therefore less likely to cause dielectric failure. In the

case of CF3I insulated busbar, the radius of the outer sheath is smaller than the current

system and this means that any particle with a size smaller than 22.2um can be moved by the

electric field. A larger number of particles are likely to be available when compared with an

SF6 busbar and the larger charge on these larger particles is more likely to influence the

dielectric strength. Any use of a gas which is able to reduce the busbar size must therefore

consider particle issues.

8.1.4 Polyurethane Foam Insulation

In the work presented in this thesis, one kind of polyurethane foam product from a

manufacturer in UK is selected for testing and a series of laboratory experiment on different

foam samples are carried out to study the physical properties including the thermal

conductivity, relative permittivity, void distribution, and electrical properties such as AC voltage

and LI voltage withstand strength, partial discharge inception voltage.

The AC and LI dielectric test on foam products with different physical characteristics shows

that the measured dielectric strengths are not comparable with those of the gaseous

candidates. The average AC breakdown strength of foam sample is about 2.5kV/mm, and the

average LI breakdown strength is measured as about 7.5kV/mm. These values can be

improved by minimize the void size or searching for better foam material, and this part of test

is not included in this thesis due to time constraints. The calculated busbar dimension of foam

insulated system is several times bigger than the size of current SF6 insulated system.

Meanwhile, according to the theoretical calculation result, the current carrying capability of

foam is not strong, and that is because the heat generated from conductor can only be

transferred by conduction in the solid foam.

The test on the foam samples into which different percentage of ferroelectric material BaTiO3

has been added shows that the relative permittivity of foam can be increased by the addition

of BaTiO3, but it doesn’t show any electric field dependence. The thermal conductivity of foam

is influenced by the concentration of filler but not in a consistent manner. A higher thermal

conductivity of the insulating material would leads to a better current carrying capability.


153

The solid polyurethane foam was widely used as thermal insulation in pipe busbars. It can

therefore be concluded that the hard closed-cell foam studied in this thesis is not a viable

option for SF6 replacement in high voltage gas insulated switchgear but it is noted that it could

play a role in providing insulation outside interrupter, and there is a practical usage example

which is used as outer insulation of medium voltage vacuum interrupter.

8.2 Alternatives for SF6 Gas Circuit Breakers

SF6 gas circuit breakers are widely being used in the medium and high voltage power

networks. It would be ideal if a substitute material could be found for SF6. As shown in Table

7.8, possible alternative candidates have been reviewed and the main findings are as follows:

One of the options is the use of new oil circuit breakers. Literatures have concluded that the

bio-degradable natural ester can be used as a new interruption material due to its better

thermal stability when compared with both mineral oil and silicone. However, the switching

capability of the circuit breaker adopting ester oil is lower because of the higher viscosity of

ester liquids. Maintenance requirements and safety issues would still exist for the usage of oil

circuit breakers and these are likely to reduce their use.

Another choice is the continued use and re-introduction of air circuit breakers. The annual CO2

equivalent emission of one air blast circuit breaker is slightly smaller than that of SF6 circuit

breaker according to the currently leakage rate of SF6. When the leakage rate of SF6 blast

circuit breaker is strictly monitored and controlled below the value the standard specified, the

SF6 circuit breaker is still a better option unless better technology can reduce the electricity

consumption of the air compressors.

Gas circuit breakers filled with CF3I, an environmental friendly gas as previously described,

appear to be an option. There is literature that gives evidence that CF3I gas mixtures could be

used in gas circuit breaker although this would require equipment to be redesigned to cope

with it. However, the by-products of CF3I after arcing may raise an issue in terms of the

continued reliability of this gas circuit breaker.

Ablation of PTFE is also an alternative solution to the use of SF6 for interruption on the basis

of a modified PTFE head nozzle. Ablation controlled arcs are a special type of convectively

cooled arc in which the cooling flow is generated by arc induced wall vaporization. During the

current interruption process, the resulted hot arc plasma inside the arcing area can be

successfully expelled and replaced by cooler gas and chemical species such as the

composition of PTFE vapour which is suitable for arc interruption via the expansion chamber.

With the new modified head nozzle, the gas used inside the chamber can be low pressure gas

such as air, N2 and even CF3I. The life time of the nozzle can be improved by adding inorganic


154

filler such as Al2O3, TiO2, or BN to PTFE. According to literature, the model prototype of an N2

filled self-interrupter unit fitted with a PTFE nozzle has achieved successful current interruption

at the first current zero.

Vacuum circuit breakers are already available at 132kV and much development is taking place

at 275kV or 400kV for use on the transmission system. The switching capability of vacuum

circuit breaker is fundamentally limited by the non-linear relationship of breakdown voltage

with gap distance in vacuum. Multiple-interrupter units would therefore be required for the high

voltage transmission system.While the focus of the manufacturers is to develop working

prototypes with new contacts structures and materials, the experimental work carried out by

other researchers has proven that X-ray production may restrict potential for use of vacuum at

high voltage levels found on the transmission system. This thesis therefore made the studies

of the risks of restrikes / pre-strikes / current chopping associated with vacuum circuit breakers

when working on transmission voltage levels. A simplified 3-phase transmission line model

with a vacuum circuit breaker installed is studied in ATP/EMTP simulation software. The

simulations are carried out suggest that the performance of vacuum circuit breaker is

influenced by several factors, such as opening time of contacts, chopping current, quenching

capability (di/dt), withstand voltage of vacuum gap and its increasing speed (dv/dt). The

occurrence of reignition is described by the increasing speed of dielectric recovery strength of

the breaker, and this is related to the dielectric strength of vacuum and the opening speed of

the contacts, which can be improved the operating mechanism of interrupter. Capacitive

compensation at load side can reduce the switching overvoltage due to re-ignition. The key

result shows that overvoltages exceeding the 1425kV lightning overvoltage level associated

with moist 400kV could be generated by a vacuum circuit breaker. As such, work would need

to take place to model the breaker in more detail in the context of a HV system.

To summarize, SF6 circuit breaker is still a better option and a more mature technology than

the other candidates examined, none of which are commercially available with the exception

of air blast. Alternative interruption systems are only likely to become available using vacuum

in the near future, and these would still need to be combined with an insulation medium which

may itself be SF6. Vacuum circuit breakers would be clean, have low maintenance

requirements and would be a high efficiency technology.

Chapter 9 Future Work

155

Chapter 9 FUTURE WORK

A wide range of alternative materials of SF6 which can be used in high voltage substation

equipment like busbar and circuit breaker are included in this PhD thesis. However, the

studies are generally high level investigations and as such would benefit from further

investigation.

Application of CF3I

The dielectric strength of CF3I is nearly 20% stronger than that of SF6 according to the

theoretical values in literature. The gas mixture of CF3I combined with N2 in a specific mixing

ratio can achieve the same dielectric strength as that of SF6. However its unstable

characteristic when exposed to light / arcing cannot be ignored and more work is required in

this area. More investigations on dielectric strength of CF3I can be carried out through high

voltage experimental tests which are including AC, DC and lightning impulse tests. This is

helpful to provide deeper understanding on the characteristics of CF3I gas and its practical

usage in high voltage industry.

Foam insulation

The history of foam insulation is still only 30 years, and there is no clear description or

standard on polyurethane foam components. The void size of the foam studied in this thesis is

200um, and the AC dielectric strength of this foam products appeared poor, it would be

appropriate for future work to be carried out that tested a wider range of foams with different

void sizes to see if any significant improvement could be identified. The lightning impulse

voltage is measured as 178.728.7 kV when the gap distance is 25mm, and this is the biggest

gap distance of samples have been tested in this thesis. Given the higher lightning impulse

voltage strength, it would be useful to extend this testing to sizes of gaps above 30mm until a

value can meet the requirements for a 1425kV system.

Through an analysis on the cell structure of polyurethane foam, the dielectric strength is

influence by the filling gas selective inside the bubble of foam. The test carried out in this

thesis is about air filled foam bubble, and it is worthy to examine the insulating characteristic of

foam expanded in other different gas environments which would have a higher dielectric

strength. There are other issues associated with foam insulation such as its performance in

terms of partial discharge, creepage phenomenon, and its performance when applied at DC

voltage.

Chapter 9 Future Work

156

Transient studies of VCB in transmission system

The real power system consists of several other elements not all included in the VCB

modelling activity such as surge arresters, current transformers, voltage transformers, reactive

compensation etc.. The selection of circuit breaker is influenced by the equipment in a high

voltage power system. Therefore it is necessary to make simulation on more complicated

system model to study the switching transient phenomenon of vacuum circuit breaker in

practical more realistic way. Given that more detailed transient modelling usually reduces

overvoltage values there is a chance that there the high levels of overvoltage

The theory of current chopping of VCB is complex and no clear evidence exists for the current

chopping performance of series connected interrupters. When assuming the performance of

multiple interrupters, one simulation that examined switching out a no load transformer was

studied in this thesis. Given the magnitude of the potential overvoltages it is worth carrying out

further investigation on the transient performance of high voltage vacuum circuit breaker when

switching on/off high inductive current and switching on/off capacitive current. This work would

be very helpful to state the problem and challenge for the future development of vacuum

circuit breaker.

Appendix

157

APPENDIX

Appendix 1: Flow chart of Model Programming of Reignition[168]

Start

T>topen ?

1st opening = F

Reopening = F

Fully open = F

Reigniting = F

State = 1 &

│i│<ich or I

pass 0?

First opening = T

Elapsed = t -topen

1st opening or

reopening T

State =2

State = 0 or

State = 2

Cb status = open

State = 2? State = 3? State = 0?

Elapse > 5ms │Vcb│ >Vb ?

Fully opening = T Reigniting = T

Reigniting ? Fully open

Reopen clock = 0

State = 3

State = 0

Cb status = Close

Reopen clock = reopen clock + timestep

(Reopen clock >2.5us) AND (│i│<

ich2) OR (I pass 0) AND (│di/dt│<

random critical di/dt)?

Reopening = T

END

NO

YES

NO

YES

NO

YES

YES

NO

NO NO NO

YES

NO

NO

YES

Appendix

158

Appendix 2: Matlab programming for multiple simulation calling ATP file

%create input/output data matrixs from Excel

x=xlsread('c:\matlab.xls','D:D'); %Chopping current

t=xlsread('c:\matlab.xls','M:M'); y=xlsread('c:\matlab.xls','F:F'); %voltage output

%run program in loop i=0; while i<=1000 i=i+1; Ich=x(i); %top=t(i);

%Copy the template dos('copy c:\EEUG06\\atpdraw\atp\multipletest2.atp

c:\EEUG06\GNUATP\');

%open ATP file for reading and writing fid=fopen('multipletest2.atp','r+');

%Change chopping current for k=1:(12) fgetl(fid); % move the pointer to the position of the chosen line; end fseek(fid,14,'cof'); %move to current position in file fprintf(fid,'%6.4G',Ich); %write data to file with 5 digit number fid=fclose(fid);

fid=fopen('multipletest2.atp','r+'); %change opening time of all three breakers for b=1:(384) fgetl(fid); end fseek(fid,12,'cof'); fprintf(fid,'%7.5f',top);

fid=fopen('multipletest2.atp','r+'); for c=1:(400) fgetl(fid); end fseek(fid,12,'cof'); fprintf(fid,'%7.5f',top);

fid=fopen('multipletest2.atp','r+'); for a=1:(416) fgetl(fid); end fseek(fid,12,'cof'); fprintf(fid,'%7.5f',top); fid=fclose(fid);

%change input parameters open('c:\EEUG06\GNUATP\runtp.bat multipletest2.atp');

Appendix

159

% run atp system('c:\EEUG06\GNUATP\runtp.bat multipletest2.atp');

%transformer data to mat dos('c:\EEUG06\PL42MAT\Pl42mat.exe multipletest2.pl4');

%open plot graph s=open('multipletest2.MAT');

%find maximum value of switching overvoltage from 3 phases value Va=max(abs(max(s.vX0003a)),abs(min(s.vX0003a))); Vb=max(abs(max(s.vX0003b)),abs(min(s.vX0003b))); Vc=max(abs(max(s.vX0003c)),abs(min(s.vX0003c))); Vmax=max(max(Va, Vb),Vc);

%save in result matrix at Excel y(i)=Vmax/1000; xlswrite('c:\matlab.xls',y,1,'F2'); end

Appendix

160

Appendix 3 Published paper:

X. Cai, I Cotton, "Alternatives To SF6 Gas Insulated Substations – A Study Of Equipment

Dimensions ", 17th ISH 2011, Hannover, Germany 24-28 August, 2011

Reference

161

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Elimination of SF From Transmission System Equipment

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