Elimination of SF6 from Transmission System Equipment

Elimination of SF6 From Transmission System Equipment

A Thesis Resubmitted To

The University of Manchester

For The Degree of PhD

The Faculty of Engineering and Physical Sciences

2013

Xiaolei Cai

School of Electrical and Electronic Engineering

Contents

List of Tables ...... IV

Table of Figures ...... VI

List of Abbreviations ...... XI

Nomenclature ...... XII

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

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

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

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

III

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

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

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

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

VII

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

VIII

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

Figure 7.23 - Case 2: switching performance of circuit breakers when di/dt=200A/us, dv/dt=240V/us ...... 132

Figure 7.24 - Case 3: switching performance of circuit breakers when di/dt=1000A/us, dv/dt=240V/us ...... 133

Figure 7.25 - Case 4: switching performance of circuit breakers when di/dt=400A/us, 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

Figure 7.27 - Case 5: switching performance of circuit breakers when di/dt=1000A/us, dv/dt=490V/us ...... 136

Figure 7.28 - Case 6: switching performance of circuit breakers when di/dt=400A/us, 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

Figure 7.33 - Case 3: Fixed di/dt =400A/us, fixed dv/dt=600V/us, random opening time, and random chopping current ...... 144

Figure 7.34 - Case 4: Fixed di/dt =400A/us, fixed dv/dt=490V/us, random opening time, and random chopping current ...... 145

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

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

XII

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.

XIII

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 copies made. iii. The ownership of certain Copyright, patents, designs, trade marks and other intellectual property (the “Intellectual Property”) and any reproductions of copyright works in the thesis, for example graphs and 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 available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. iv. Further information on the conditions under which disclosure, publication and commercialization of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see http://documents.manchester.ac.uk/DocuInfo.aspx?DocID=487), in any relevant Thesis restriction declarations deposited in the University Library, The University Library’s regulations (see http://www.manchester.ac.uk/library/aboutus/regulations) and in The University’s policy on Presentation of Theses

XIV

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

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

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 500C.

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.

Chapter 1 Introduction

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]

Chapter 1 Introduction

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 1 SF6 0.3 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-CO 2 - -18.6% -40.0% -41.8% -44.6% -45.5% -45.9% GHGs from 1990 1 1995 Baseline emissions

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

Chapter 1 Introduction

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

Chapter 1 Introduction 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].

Chapter 1 Introduction

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.

Chapter 1 Introduction

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.

Chapter 1 Introduction

132 kV 765kV System 275 kV System 400 kV System System [15]

Rated voltage (rms, 3- 145 kV 300 kV 420 kV 800kV phase)

Normal current 2000 A 2500 A 4000 A 5000A

Short-circuit current (1 / 3 40 kA 40 kA 63 kA 63kA phase)

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

Rated lightning impulse 650 kV 1050 kV 1425 kV 2100kV withstand voltage to earth

Rated lightning withstand 650 kV 1050 kV 1425 kV 2100kV voltage between phases

Rated lightning impulse 1050 kV plus peak 1425 kV plus peak 2100 kV plus withstand voltage across 650 kV power frequency power frequency peak power open device voltage voltage frequency voltage

Rated switching impulse N/A 850 kV 1050 kV 1550 kV withstand voltage to earth

Rated switching impulse withstand voltage N/A 1275 kV 1575 kV N/A between phases

Rated switching impulse 700 kV plus 245 kV 900 kV plus 345 kV withstand voltage across N/A peak power peak power N/A open switching device frequency voltage frequency voltage 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.

Chapter 1 Introduction

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

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

Chapter 2 SF6 Gas and Equipment Design 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.

S  6F  2SF 524kcal (1) 2 2 6

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-1K-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

Chapter 2 SF6 Gas and Equipment Design 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.

Chapter 2 SF6 Gas and Equipment Design

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.

Chapter 2 SF6 Gas and Equipment Design

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.

Chapter 2 SF6 Gas and Equipment Design

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.

Chapter 2 SF6 Gas and Equipment Design 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.

Chapter 2 SF6 Gas and Equipment Design

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.

Chapter 2 SF6 Gas and Equipment Design 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

Chapter 2 SF6 Gas and Equipment Design 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 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.

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

Midel 7131 [28] Mineral oil [23] FR3 [30] [29]

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 0.00065 0.0008 0.00074 (at 25˚C) Coefficient per °C 3 3 3 0.92 kg/dm Density at 20˚C 0.887 kg/dm 0.97 kg/dm (25˚C)

Viscosity at 40˚C 9.1 mm2/s 34 mm2/s 34 mm2/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

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

LI Working AC Breakdown Density Radius Weight breakdown 3 pressure voltage (kg/m ) (mm) (kg/m) Voltage (MPa) 232.8kV at Oil (after 70kV at 2.5mm 1 3.8mm gap 895 0.1- 0.2 90.6 12.1 treatment) gap [31]

8.9kV/mm. SF 8.9kV/mm.0.1Mpa 6.164 0.3 146.8 1.1 6 0.1Mpa 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].

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

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.

0.45 Electric field in bulk oil: Eb  95d (2)

0.46 Electric field along pressboard: Ep  64d (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:

V E  (4) m r ln(R / r)

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

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

250 Peak voltage along pressboard Peak voltage in bulk oil

200

150

100

50 Withstand Voltage (kV) Voltage Withstand

0 0.1 1 10 100 Gap distance (cm) 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.

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

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

 E  E  f ( ) ,  f ( ) (5) p p p p

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

Chapter 3 Potential Liquid and Gaseous Replacements for SF6 as An Insulation Medium 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  EE  K   (6) PPPP crit

The ionization coefficient and attachment coefficient for SF6 is calculated and the E/N of pure -21 2 -1 -1 -21 2 SF6 is reported as 360Td (1Td =10 Vm , E/P (V cm Torr )*3.1*10 =E/N (V m )) [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

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

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

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]

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

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 6.594 [51] 12.058 0 °C (32 °F) (mW/(m.K) Viscosity (Cp) 0.2361 1 0.0154 @20˚C Specific heat at constant pressure 0.097 kJ/(mol.K) 0.031 kJ/(mol.K) [3] (0.1MPa) @21 °C Heat convection Constant Estimated as 20.0 2 24.4

E/N value 437Td 361Td 1:Calculated from estimated kinematic viscosity = 1cm2/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.

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

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

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

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

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 -64°C -78°C -198°C -141°C Point (0.1MPa) Thermal Conductivity 0.0155 0.0142 0.0238 23.94

Molecular Mass 146.05 44.01 28.01 28.8 Lifetime in 3200 / / / atmosphere (year) Limiting 360 Td 108Td 130 Td 108 Td field strength E/N [60] Density (kg/m3) 5.9 1.8 1.25 1.29 Liquefaction pressure 0.77 2 Over 4 Over 4 at -20°C (MPa)

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:

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

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

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.

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 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

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 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

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 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.

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations

l22 E  l U sin( ) Q 00 1012    10 12 (7) 22ll    r ln 1 ln 1 r ln 0     0 r rr    i

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).

Q  2 rl 0 E(t) (8)

Ignoring the influence of neighboring particles, the force acting on the particle can be written as:

2 Fd  E(t)Q  2 rl 0 E (t) (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:

ma(t)  F  mg  kv2 (t)  2 rl E 2 (t)  mg  kv2 (t) (10) d 0

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:

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations ma(t)  mg E(t)  ( )0.5 (11) 2 rl 0

The mass of a cylindrical particle m can be calculated with the formula m  r2 l , where ‘ρ’ is the density of particle, 2.7g/cm3 for aluminium.

r g E  (12) 2 0

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.

200

180 160 140 120 100 80 60 40 (12.1um, 42.6kV/cm)

Electric field Stress (kV/cm) Stress field Electric 20 0 0.0E+00 5.0E-02 1.0E-01 1.5E-01 2.0E-01 Particle dimension (cm) 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

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations partial discharge with magnitudes in the range of 104 to 106 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.

Qrad’

Qrad Qconv’

Qconv

Conductor

Insulating medium

5mm Sheath 62.5mm 250mm

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 2 ‘A2’ are the radiation areas of the surfaces in m . ‘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.K4).

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 1 W   (T 4 T 4 )A (13) rad 1 2 1 1 A 1  1 ( 1) 1 A2  2

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 2 respectively, ‘P’ is the gas pressure in kgf/cm and ‘K0’ is the gas constant of the filled gas

(24.4 for SF6, 17.5 for N2).

0.6 0.75 1.25 K0 P d1 (T1 T2 ) Wconv  1.25 (14)  0.6   d1   d1  ln( )  a 1    d  d   2    2  

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.

d T Wcond  kA  k  AT (15) dx

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).

2 W  Rac I (16)

, Rac  R (1(Tcond  20)) (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.

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations

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:

 T T  1 0.75 cond 4 sheath 4 n 1.25 (18) Wc  5.67 d1 ( )  ( )   K1P d1 (Tcond Tsheath) 100 100 1 d 1    1 ( 1) 1 d2  2 The relationship between heat flow ‘W’ and sheath / ambient temperature is calculated using:

 Tsheath 4 Tambient 4  0.75 1.25 Wc WS  5.67 d3 ( )  ( )  3  K2d3 (Tsheath Tambient) (19)  100 100 

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.

180 conductor temperature 160 sheath temperature 140 120 100 105°C 80 70°C 60

Temperature in celsius in Temperature 40 20 0 0 2000 4000 6000 8000 10000 Current Rating (A)

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

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations temperature is at 105°C (black solid line), and the sheath temperature remains below the criteria (black dashed line).

7300

7250

7200

7150

7100 d2:d1= 0.49: 0.13 7050 I = 7022A

7000

6950 Current carrying capability (A) capability carrying Current 0.00 0.20 0.40 0.60 0.80 Sheath inner diameter (m) 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.

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations

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].

T T  b (20) mb ln(10kP) 1 10.5

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.

r1=6.75cm, r1=5.75cm,r1=6.25cm,r1=6.75cm, r1=6.75cm,r1=5.75cm,r1=6.25cm,26.25,26.75, r1=6.25cm,r1=6.75cm,r1=5.75cm,26.25,25.75,25.25,26.75, r1=6.75cm,r1=5.75cm,r1=6.25cm,r1=6.75cm,9124.9225.25,24.75,24.25,25.75, r1=6.75cm,r1=6.25cm,r1=6.75cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,8935.468977.308988.058998.989051.499062.1324.25,23.75,23.25,24.75, r1=6.75cm,r1=6.25cm,r1=6.75cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,8795.468807.318848.728860.048871.578913.218924.2421.75,23.25,22.75,22.25,23.75, r1=6.75cm,r1=6.25cm,r1=6.75cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,20.75,21.25,8652.828665.118677.668718.538730.488742.678783.8321.75,22.25,22.75, r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=6.75cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,19.75,20.25,20.75, 21.25,8520.158533.188546.508586.708599.358612.27 21.75, r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,18.75,19.25,19.75, 20.25, 20.75,8413.82 8399.628480.358385.788453.188466.60 r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,17.75,18.25,18.75, 19.25, 8279.61 8264.458346.90 8249.698332.24 8317.95 r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,16.75,17.25,17.75, 18.25, 8143.91 8127.648211.95 8111.858196.24 8180.99 r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,15.75,16.25,16.75, 17.25,7937.63 8006.74 7989.218075.51 7972.258058.62 8042.27 r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,14.75,15.25,15.75, 16.25,7798.41 16.75,7868.18 7849.20 7830.917919.40 7901.80 r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,13.75,(17.75cm,14.25,14.75, 15.25,7657.96 15.75,7728.33 7707.68 7687.857778.63 7759.59 r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=5.75cm,r1=6.25cm,r1=5.75cm,12.75,13.25,13.75, 14.25,7516.50 14.75,7587.34 7564.75 7543.137636.38 7615.69 r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=6.25cm,r1=5.75cm,11.75,12.25,12.75, 13.25,7374.30 13.75,7445.47 7420.57 7396.867492.80 7470.18 r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=6.25cm,r1=5.75cm,r1=5.75cm,10.75,11.25,11.75, 12.25,7231.78 12.75,7303.05 7275.41 7249.227348.09 7323.20 r1=6.75cm,r1=6.75cm,r1=6.25cm,r1=6.25cm,r1=5.75cm,r1=5.75cm,9.75,10.25,10.75, 11.25,7089.57 11.75,7160.59 7129.63 7100.477202.57 7174.96 r1=6.25cm,r1=6.25cm,r1=5.75cm,r1=5.75cm,8.75,9.25,9.75,10.25,6948.5510.75,7018.84 6983.80 6951.017056.68 7025.80 r1=5.75cm,r1=5.75cm,7.25,7.75,8.25,8.75,6810.089.25, 6878.899.75, 6838.74 6801.476911.11 6876.20 r1=5.75cm 7.25,7.75,6676.198.25,6742.408.75,6695.70 6652.766766.87 6726.93 6550.067.25,6611.897.75,6556.596506.306625.506579.13 6362.336491.406424.416364.326489.396434.57 r1=6.25cm Carried Carried current (A) 6230.376296.04 r1=6.75cm

17.75 Insulation thickness (cm)

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.

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations

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.

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations

Sheath temperature and achievable Thermal Emission (W) Working current rating Allowable E Boiling Material pressure field (kV/cm) Point (K) Sheath (MPa) Conduction Radiation Convection Current (A) Temperature (K)

Oil / 20.28 / 242.45 / / 339.03 9674

1 SF6 0.27 1.95 11.96 140.80 16.82 231.00 332.71 8231

2 CF3I 0.23 1.61 11.96 104.83 17.39 271.69 329.68 7476

70%CF I/N 3 0.27 1.61 11.96 98.10 16.82 267.12 329.04 7315 3 2

4 60%CF3I /N2 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 2 3 4 5 : SF6 is at 0.27MPa, K0=24.4, : CF3I is at 0.23MPa, K0=20.0, : 70%CF3I/N2 is at 0.27Mpa K0=17, 60%CF3I/N2 is at 0.29Mpa K0=16 : Dry air is at

0.91MPa, K0=10,

Table 4.1 - Optimal working pressure and temperature of alternatives working at fixed dimension d1/d2/d3=125mm/490mm/500mm

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations

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.

Gas E field E/N Allowed E Enclosure Boiling strength at Current Material Critical field stress Radius point 0.3MPa (A) (Td) (kV/mm) (mm) @0.3MPa (kV/mm)

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 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.

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations

Maximum Electric field Enclosure Radius Allowed minimum Material stress to the inner surface (mm) particle size (um) of sheath (kV/cm)

Any random 245 42.6 12.1 material

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 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.

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 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.

Lightning Strike

Busbar 1 CB Busbar 2 Load Other components in system SA SA

(a) Line diagram of study model

(b) PSCAD implementation Figure 4.12 - Simulation model with surge arresters installed

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations

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.

Voltage with SA (kV) Voltage without SA (kV) 780 3000

770 2500

760 2000

750 1500

740 1000

730 Voltage with SA 500 Voltage without SA 720 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Line distance (km) 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.

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations

R (cm) at different BIL at R (cm) at different BIL at

fixed boiling point at -20°C fixed pressure at 0.3MPa Dielectric Reduction Reduction 1425kV 1050kV 1425kV 1050kV medium in size 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 Gas insulation ε1 Epoxy resin coating ε1 Conductor Conductor r r t R Aluminium sheet with R dielectric coating

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

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 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.

Relative Thermal conductivity k Material Permittivity εr (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

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 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:

q Dx  2 x (21)

qD Ex  2x  (22)

The expression for electric field is integrated to give the potential difference between the tube and the conductor as follows:

R rt R rt D R D V  Edr  Edr  Edr  dr  dr ( 23) r r rt r rt  01  0 2

q  rt dr R dr  q  1 r  t 1 R  V    ln( )  ln( ) (24) r rt    2 0  1r  2r  2 0 1 r  2 r  t 

Using the standard relationship between capacitance per unit length, charge per unit length and voltage:

q  cV (25)

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations Then the electric field stress to gas solid parts can be described respectively in following two equations.

V 1 (26) E1  (r  x  r  t)  1 r  t 1 R   ln  ln( )x 1 r  2 r  t 

V  2 E2  (r  t  x  R) (27)  1 r  t 1 R   ln  ln( )x 1 r  2 r  t 

20.0 t=0 t=3mm t=5mm t =10mm t=15mm

15.0

10.0

5.0 Electric field (kV/mm) field Electric

0.0 0.0 50.0 100.0 150.0 200.0 Distance x (mm) 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

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 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]:

Qg ln(r / r )   2 1 (28) kg2l

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:

   2kl W   (29) cond R ln(r / r ) th 2 1

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.

380

375

370

365

360 Temperature (K) Temperature Epoxy resin coating surface temperature 355 Conductor temperature limit rubber coating surface temperature 350 0 1 2 3 4 5 6 7 8 Carried Current (kA) Figure 4.17 - Calculated coating surface temperature at different carried current

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 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.

W Thermal Emission of insulant New cond Boiling Current Material coating (W) withstand Point (K) (A) (W) Wcond Wconv Wrad LI voltage Oil 16.8 / 61.5 / 6487

1 SF6 1.61 10.88 128.78 231.00 6716

2 CF3I 1.33 10.88 93.31 271.69 5799 66.04 1493kV 3 70%CF3I/N2 1.33 10.88 87.33 267.12 5632 4 60%CF3I /N2 1.33 10.88 85.79 254.96 5588 Dry air 5 3.33 10.88 106.49 167.40 6202 1 2 3 4 : SF6 is at 0.27MPa, K0=24.4, : CF3I is at 0.23MPa, K0=20.0, : 70%CF3I/N2 is at 0.27Mpa K0=17, 5 60%CF3I/N2 is at 0.29Mpa K0=16 : Dry air is at 0.91MPa, K0=10, Table 4.6 - Current rating for busbar in existing dimension but with 10mm coating

Gas Boiling E field Sheath point at Allowed E Current rating strength at inner Material working field stress of coated 0.3MPa Radius pressure (kV/mm) conductor (A) (kV/mm) (mm) (K) 8926 SF6 233.59 26.20 18.87 206 8637 80%SF6-N2 254.84 18.87 13.59 308 8170 CF3I 279.94 31.72 22.84 172 7944 60%CF3I- N2 204.36 23.95 17.25 227

Dry AIR 147.59 7.84 5.64 2359 5746

Foam E=16.2kV/mm 256 1427 Table 4.7 - New sheath size and respective current rating when the pressure is fixed at 0.3MPa (coated conductor ‘r’=72.5mm)

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 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

Chapter 4 Estimate of Size of Alternative Gas Insulated Substations 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

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

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium  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

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium

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:

3 r Ea E  (30)  rc  2 r

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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium

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

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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.

10k HV ohm Electrode

Foam sample d Amplifier Test Earth Object Electrode

AC Signal Generator

Oscilloscope

Figure 5.7 - Circuit of relative permittivity measurement

Figure 5.8 - Plane - plane electrodes arrangement

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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:

V 1 X   (31) c I 2 f C

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:

 A C  (32) d

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.

2.50

No filler 15% filler

2.00 y = 0.0112x + 1.7828

1.50 y = 0.0563x + 1.2796

1.00 Relative Permittivity Relative

0.50 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Peak Electric Field (kV/mm)

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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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  d T  th ln (33) 2 r

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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium

Filled form r1 = 5.75cm Pure form r1=5.75cm 2200 Filled form r1 = 6.25cm Pure form r1=6.25cm Filled form r1 = 6.75cm Pure form r1=6.75cm 2000

1800

1600

1400 Ratedcurrent (A) 1200

1000 5 10 15 20 25 30 Insulation thickness d (cm)

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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium

Figure 5.11 - Void Distribution in pure foam

(a) 5% concentrated (b) 10% concentrated

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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium Maximum Average Void Minimum Void Mixing ratio Void size size (um) size (um) (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

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium

(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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium

Water Resistor 0.5M ohm

220 V / 80 kV Test 500 pF Object Voltage divider AC Impedance Scope Box

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 Electrode 50% LI U PDIV (kV) 50% AC U (kV) BD (mm) Diameter(mm) BD (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

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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.

50% AC Breakdown voltage ± Standard Deviation (kV) Gap Distance 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

5% AC Breakdown voltage (kV) Gap Distance 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

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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.

120

100

20 AC breakdown voltage (kV) voltage breakdown AC 0 1 2 3 5 10 15 20 25 30

Gap distance between electrodes(mm) Figure 5.18 - AC breakdown voltage of foam at varied gap distances

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 18 16

12 10

(kV/mm) 8

6 AC Dielectric strength strength DielectricAC 4 2 0 0 10 20 30 40 Gap distance (mm) 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

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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.

120

100

20 LI dielectric strength (kV/cm) strength dielectric LI 0 1 1.5 2 2.5 Gap distance (cm) Figure 5.22 - The LI breakdown strength versus gap distance

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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.

Dielectric Sheath radius of Thermal Current Voltage type strength busbar conduction rating Measured AC dielectric 2.5kV/mm 505mm 4.36W 1178A strength Measured LI 1306mm for 1425kV BIL 3.0 W 977A dielectric 7.5kV/mm strength 587mm for 1050kV BIL 4.1W 1138A AC dielectric 5kV/mm 178mm 8.7W 1665A strength [94]

LI dielectric 255mm for 1425kV BIL 6.48W 1436A 16.2kV/mm strength [94] 176mm for 1050kV BIL 8.8W 1674A Table 5.7 - Estimation of foam insulated busbar dimension

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium 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.

Chapter 5 Evaluation of Foam as A Replacement for SF6 as An Insulation Medium

Technology Advantage Disadvantage Impact On substation Impact On Size / Cost Current Rating

Vacuum High vacuum Clean. Very high pressure needed Doesn’t help to reduce   Reduced Insulation (Pressure is  Dielectric strength is  High mechanical strength of the size. smaller than 1E-5 30kV/mm instrument required mbar)  Bio-degradable  Heavy 30% size reduction can Oil Insulation Ester oil Improved High fire point,   Easy to fire. be achievable.  high dielectric strength  Maintenance work needed. 30kV/mm  Low GWP with only 5.  Higher Boiling point. Size reduction is CF3I gas Gas insulation  Dielectric strength of its  Influenced by its particles possible when used at Reduced mixture gas can be same as of generated after multiple higher pressure.

SF6. flashover

Composite Environment-friendly gas Would be same size or  Thermal radiation influence Epoxy Coating insulation with air used  a little less than that of  Particle from solid can or other gas Help to reduce the dielectric SF .  effect dielectric performance 6 stress to the insulating gas

 Hard  BD strength is 10kV/mm, Doesn’t help to reduce Largely PU foam Solid insulation  Clean material which is not comparable with the size. reduced Higher thermal conductivity  that of SF6. than SF6 Table 5.8 - Comparison of all insulating material candidates

Chapter 6 Potential Replacement for SF6 as an Interruption Medium

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]

Chapter 6 Potential Replacement for SF6 as an Interruption Medium 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.

Chapter 6 Potential Replacement for SF6 as an Interruption Medium

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.

Chapter 6 Potential Replacement for SF6 as an Interruption Medium 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

Chapter 6 Potential Replacement for SF6 as an Interruption Medium 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.

Chapter 6 Potential Replacement for SF6 as an Interruption Medium

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]

Chapter 6 Potential Replacement for SF6 as an Interruption Medium 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.

Chapter 6 Potential Replacement for SF6 as an Interruption Medium 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.

Chapter 6 Potential Replacement for SF6 as an Interruption Medium

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.

Chapter 6 Potential Replacement for SF6 as an Interruption Medium

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 Comp.1a Comp.1b Comp.1c hours/ week 1st week 22442.6 9797.8 22003.8 / 2nd week 22477.6 9891 22129.2 / Running 35.0 93.2 125.4 253.6 hours/week 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 31.9 31.4 31.4 94.7 hours/week 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

Chapter 6 Potential Replacement for SF6 as an Interruption Medium

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.

Chapter 6 Potential Replacement for SF6 as an Interruption Medium 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

Chapter 6 Potential Replacement for SF6 as an Interruption Medium 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

Chapter 6 Potential Replacement for SF6 as an Interruption Medium

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

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Chapter 6 Potential Replacement for SF6 as an Interruption Medium 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]

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Chapter 6 Potential Replacement for SF6 as an Interruption Medium

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.

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Chapter 7 Vacuum Circuit Breaker

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

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

AMF with a set of Spiral (RMF) contact Cup shapted contact Name two identical contacts [136] (TMF) structure [137]

Structure

Year / GE (USA) 1962 1973

 Bigger current breaking  Longer lifetime capacity  Bigger Advantages Less arc voltage  simple physical interrupting structure capability  lower power  Compact size losses at nominal current Dis- Confining effect Expensive Expensive advantages

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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Chapter 7 Vacuum Circuit Breaker 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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Chapter 7 Vacuum Circuit Breaker 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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Chapter 7 Vacuum Circuit Breaker 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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Chapter 7 Vacuum Circuit Breaker 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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Chapter 7 Vacuum Circuit Breaker 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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Chapter 7 Vacuum Circuit Breaker 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]

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Chapter 7 Vacuum Circuit Breaker

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.

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Chapter 7 Vacuum Circuit Breaker L UI (34) ch C

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:

3Z 2 I 2 k  0 ch 1 (35) 2V 2

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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Chapter 7 Vacuum Circuit Breaker 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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Chapter 7 Vacuum Circuit Breaker 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 5 core, RL = 10 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.

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Chapter 7 Vacuum Circuit Breaker

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: Ub = 1.7107 V/s  (t - topen) + 3400V . 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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Chapter 7 Vacuum Circuit Breaker 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.

400kV busbar 400kV AC L Phase A R 400/132/13kV Transformer Phase B

400kV L AC R Phase C

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.

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Chapter 7 Vacuum Circuit Breaker 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.

2 2 2 2 2 (400kV) Fault level current I V / Z V / R  X L  (36) R2  (40R)2

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.

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Chapter 7 Vacuum Circuit Breaker

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.

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Chapter 7 Vacuum Circuit Breaker

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

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Chapter 7 Vacuum Circuit Breaker

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

PC 42.63kW Core loss component: I  I cos    0.13A c 0 V 400 2 / 3 kV

The magnetizing current is the principle component of the current drawn by an unloaded transformer, and it can be calculated in:

2 2 2 2 I m  I0  Ic  16  0.13 16A

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Chapter 7 Vacuum Circuit Breaker V 400/ 3kV Resistance: Rc   1.26M Ic 0.184A

V 400/ 3kV Inductive resistance X m   14.43k I m 16A

The magnetic inductance L  X m / f 14.43k/ 2/ /50  46H

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:

1 2 1 2 1 2 CV  LIch  CV0 (37) 2 2 2

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:

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Chapter 7 Vacuum Circuit Breaker 1 f1  (38) 2 LL CL

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:

1 f2  (39) 2 LS CL

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

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Chapter 7 Vacuum Circuit Breaker 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:

Ub = U0 + a  s = 1kV + 30kV/mm s (40)

Where ‘s’ is the gas distance between contacts in mm.

Measured in time scale, the above equation is derived to [169]:

(41) Ub = Aa (t - t open) + Bb

‘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

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Chapter 7 Vacuum Circuit Breaker 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.

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Chapter 7 Vacuum Circuit Breaker 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.

Peak voltage at Case RRDS di/dt Reignition Reignition time 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

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Chapter 7 Vacuum Circuit Breaker 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.

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

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Chapter 7 Vacuum Circuit Breaker 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.

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Chapter 7 Vacuum Circuit Breaker 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.

Figure 7.23 - Case 2: switching performance of circuit breakers when di/dt=200A/us, dv/dt=240V/us

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Chapter 7 Vacuum Circuit Breaker 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.

Figure 7.24 - Case 3: switching performance of circuit breakers when di/dt=1000A/us, dv/dt=240V/us

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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.

Figure 7.25 - Case 4: switching performance of circuit breakers when di/dt=400A/us, dv/dt=490V/us

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Chapter 7 Vacuum Circuit Breaker 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.

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Figure 7.27 - Case 5: switching performance of circuit breakers when di/dt=1000A/us, 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.

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Figure 7.28 - Case 6: switching performance of circuit breakers when di/dt=400A/us, 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

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Chapter 7 Vacuum Circuit Breaker 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.

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

Current measured Switching Model Re-ignition Yes/No at load side overvoltage No extra capacitance Yes 4400 A 1.73 MV Cr=0 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

Re-ignition Current measured at Switching Model Yes/No load side overvoltage No extra capacitance Yes 4100 A 1.45 MV Cr=0 Cr=0.000001uF =1pf Yes 4150 A 1.61 MV Cr=0.00001uF Yes 4430 A 1.66 MV =10pF Cr=0.0001uF Yes 6500 A 1.65 MV =100pF 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

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Chapter 7 Vacuum Circuit Breaker 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.

Chopping Opening Peak Average di/dt dv/dt current (A) time overvoltage overvoltage Random in Case Fixed at Fixed at 1555kV 1522+/- 490V/us range of 2- 1 400A/us 0.048s 317kV 6A Random in Case Fixed at Random in 1525kV 1404+/- 490V/us range of 2- 2 400A/us 0.046-0.053s 347kV 3A Random in Case Fixed at Random in 1490kV 1364+/- 600V/us range of 2- 3 400A/us 0.046-0.053s 367kV 3A Random in Case Fixed at Random in 1562kV 1418+/- 490V/us range of 2- 3 1000A/us 0.046-0.053s 358kV 3A 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.

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2000

1800 1600 1400 1200 1000

Peak overvoltage (kV) overvoltage Peak 800 600 2.0 3.0 4.0 5.0 6.0 Chopping current (A)

0.14% Mean = 1522kV 1555kV 0.12% SD = 317kV

0.10%

0.08% 0.06%

Probability 0.04% 0.02% 0.00% 600 1100 1600 Peak overvoltage (kV)

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

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Chapter 7 Vacuum Circuit Breaker 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.

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2000

1800

1600 1400 1200 1000

800 Peak overvoltage (kV) overvoltage Peak 600 2.00 2.20 2.40 2.60 2.80 3.00 Chopping current (A)

2000

1800 1600 1400 1200 1000

Peak overvoltage (kV) overvoltage Peak 800 600 0.045 0.047 0.049 0.051 0.053 Opening time (s)

0.12% Mean = 1404kV 1525kV 0.10% SD= 347kV

0.08%

0.06%

0.04% Probability 0.02%

0.00% 600 900 1200 1500 1800 Peak overvoltage (kV)

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

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.

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Chapter 7 Vacuum Circuit Breaker

1900

1700

1500

1300

1100

900

Peak overvoltage (kV) overvoltage Peak 700

500 2.00 2.20 2.40 2.60 2.80 3.00 Chopping current (A)

2000

1800 1600 1400

1200 1000

Peak overvoltage (kV) overvoltage Peak 800 600 0.045 0.047 0.049 0.051 0.053 Opening time (s)

0.12% Mean = 1364kV 1490kV

0.10% SD = 367kV

0.08%

0.06% Probability 0.04%

0.02% 600 900 1200 1500 1800 Peak overvoltage (kV)

Figure 7.33 - Case 3: Fixed di/dt =400A/us, fixed dv/dt=600V/us, random opening time, and 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.

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Chapter 7 Vacuum Circuit Breaker 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.

2000

1800

1600

1400

1200

1000

Peak overvoltage (kV) overvoltage Peak 800

600 2.00 2.20 2.40 2.60 2.80 3.00 Chopping current (A)

2000

1800

1600

1400

1200

1000

Peak overvoltage (kV) overvoltage Peak 800

600 0.045 0.047 0.049 0.051 0.053 Opening time (s)

0.11% Mean = 1418kV 1562kV SD = 358kV

0.09%

0.07%

Probability 0.05%

0.03%

0.01% 600 900 1200 1500 1800 Peak overvoltage (kV)

Figure 7.34 - Case 4: Fixed di/dt =400A/us, fixed dv/dt=490V/us, random opening time, and random chopping current

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Chapter 7 Vacuum Circuit Breaker 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

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Chapter 7 Vacuum Circuit Breaker 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

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Chapter 7 Vacuum Circuit Breaker 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.

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Medium Technology Advantage Disadvantage

 Very low chopping current (1A)  High risk of leaking, with high GWP Absorb the free electron in the  Small pressure difference (0.5-  Toxic by-products after arcing SF arc, high pressure gas cool 6 0.6MPa)  Need to monitor gas pressure down the arc temperature  Strong dielectric strength  High cost on operating mechanism

Absorb the free electron in the  Low GWP (5)  Long term performance is not sure CF I 3 arc  Strong dielectric strength  Need to monitor gas pressure  X-ray emission  Expensive operating mechanism  Clean Magnetic field emission to  Chopping current can be 3A Vacuum  Maintenance free extinguish arc  Voltage and current rating of single  Long operation life (10,000 times) breaker is not suitable for high voltage utilization  Clean  Noisy  Higher voltage rating High pressure High pressure air flow is used to  Expensive  Higher operating speeds air blast blow the arc  Big pressure difference (up to 7MPa)  More stringent overvoltage limits  7-8A chopping current

 Flammable  Bio-degradable Bio-degradable Use H provided by the thermal  Low temperature will make oil viscous 2  Low cost on operating mechanism Oil decomposed oil to blow the arc thus reduce operating speed  Chopping current can be 4A  Need to refill and replace with new oil Table 7.8 - Comparison of all interrupting candidates

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Chapter 8 Conclusion

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

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Chapter 8 Conclusion 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.

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Chapter 8 Conclusion 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.

152

Chapter 8 Conclusion 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

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Chapter 8 Conclusion 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.

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Chapter 9 Future Work

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 178.728.7 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.

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Chapter 9 Future Work  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.

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Appendix

APPENDIX

Appendix 1: Flow chart of Model Programming of Reignition[168]

Start

NO T>topen ?

YES

NO NO NO 1st opening = F State = 2? State = 3? State = 0? Reopening = F Fully open = F Reigniting = F YES

NO NO Elapse > 5ms │Vcb│ >Vb ? State = 1 & NO │i│

YES Fully opening = T Reigniting = T

First opening = T Elapsed = t -topen Reigniting ? Fully open

1st opening or NO State = 0 reopening T Reopen clock = 0 State = 3 YES

State =2

Reopen clock = reopen clock + timestep State = 0 or State = 2

Cb status = Close YES (Reopen clock >2.5us) AND (│i│< ich2) OR (I pass 0) AND (│di/dt│< NO Cb status = open random critical di/dt)?

YES Reopening = T

END

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Appendix

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');

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Appendix

% 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

159

Appendix 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

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