Long term performance of gas-insulated switchgear operating under tropical conditions

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Long Term Performance of Gas-Insulated

Switchgear Operating under Tropical Conditions

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Long Term Performance of Gas-Insulated

Switchgear Operating under Tropical Conditions

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus Prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen, op vrijdag 8 juni 2012 om 15:00 uur door Anita PHARMATRISANTI

elektrotechnisch ingenieur geboren te Bandung (Indonesië)

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Dit proefschrift is goedgekeurd door de promotor: Prof.dr. J. J. Smit

Samenstelling promotiecommissie:

Rector Magnificus Voorzitter

Prof.dr. J. J. Smit Technische Universiteit Delft, promotor

Prof.dr.ir. Suwarno Bandung Institute of Technology

Prof.dr.ir. P. M. Herder Technische Universiteit Delft

Prof.dr.hab.Inż. J. Maksymiuk Polytechnika Warszawska

Prof.Dipl.-Ing.Dr.h. M. Muhr Technische Universität Graz

Prof.ir. L. van der Sluis Technische Universiteit Delft

Dr.ir. Sander Meijer Technische Universiteit Delft

The investigations in this thesis have been financially supported by PT. PLN (Persero).

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in loving memories of my father

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Summary

For the past two decades, electricity utilities have struggled to provide high quality electrical power to their customers while keeping on spending effectively the expenses for operating and maintaining the power system. Unfortunately, under tropical conditions, operation circumstances affect the equipment so that either its lifetime is cut shorter or its failure rate is higher. One of the affected equipment is GIS installations. Because of its important function in a power network, a need to investigate how the tropical circumstances affect the GIS installations has raised.

This thesis focuses on the assessment and the understanding of how the tropical circumstances influence GIS long term performance. This knowledge is hence applied to develop a maintenance methodology for these circumstances to enable better tropical GIS performance.

In order to improve the maintenance methodology, the degradation mechanisms need to be identified. FMECA method is applied so that parameters which indicate premature function loss can be defined and hence being monitored.

Furthermore these degradation processes have to be controlled to attain a better GIS performance. This means that the processes have to be measured. In order to do so, series of experiments were conducted under both homogeneous and inhomogeneous electric field conditions. Gas pressure, gas humidity and gas temperature as well as electrical stresses were varied to measure the change of the electrical withstand of the insulating gas.

Next, the health index and degradation level of the equipment are assessed. A model, namely condition and life assessment are introduced. This concept utilizes knowledge rules and norms obtained from field experiences as well as experimental results. To complete the model, a weighting factor is introduced to represent how high the influence of parameter or GIS system is. From our experiment we learn that the major factors of influence are the following parameters:

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• decomposition products

For these parameters the respective weighting factors were determined through experiments.

In asset management strategy, prioritizing work delivery is as important as determining health and degradation levels. This can be fulfilled by conducting risk assessment both to equipment and utility’s business values. A model of risk assessment is introduced.

Verification of the models is carried out by using of data of a utility which installations are operated under tropical circumstances. In addition to the verification, an example of how the models are used is given as well.

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Samenvatting

Gedurende de afgelopen twee decennia hebben nutsbedrijven gezocht naar de balans tussen de hoge kwaliteit van levering van elektrische energie en de kosten voor onderhoud en beheer van het elektrische netwerk. De operationele omstandigheden onder tropische condities beïnvloeden de apparatuur helaas dusdanig, dat of de technische levensduur wordt verkort of de faalfrequentie toeneemt. Gesloten schakelinstallaties zijn voorbeelden van apparatuur die nadelig beïnvloed kunnen worden. Omdat deze apparatuur een knooppunt functie vervult in het elektriciteitsnetwerk is het belangrijk om de impact van tropische omstandigheden op GIS nader te onderzoeken.

Dit proefschrift beschrijft de onderzoeksresultaten om de impact van tropische omstandigheden op het lange termijn gedrag van GIS te bepalen. Deze kennis is gebruikt om een onderhoudsfilosofie te definiëren waarmee de prestaties van GIS onder tropische condities zijn te verbeteren.

Het verbeteren van de onderhoudsmethodologie vereist kennis over de verschillende verouderingsmechanismen. De Failure Mode, Effect and Criticality Analysis (FMECA) methodiek is toegepast om inzicht te verkrijgen in de parameters gerelateerd aan functieverlies van een component. Door het monitoren van deze parameters kan functieverlies mogelijk voorkomen worden.

Tevens dienen de verouderingsprocessen beheerst te worden om een betere performance van GIS te verkrijgen. Dit betekent dat de verouderingsprocessen

gemeten moeten worden. Om de verschillende stadia in de

verouderingsprocessen te definiëren zijn experimenten uitgevoerd waarin

isolatiedefecten zijn geïntroduceerd en onder verschillende

omgevingscondities bestudeerd.

Op basis van de verworven inzichten kan de health index en de toestand van de componenten worden geschat. Hiervoor wordt gebruik gemaakt van een conditie- en levensduurassessment model. Het past de kennisregels en ervaringsnormen toe gebaseerd op ervaringen in het veld en vanuit de laboratoriumexperimenten. Afhankelijk van het type inputparameter weegt de

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parameter meer of minder mee in de uiteindelijke besluitvorming. De belangrijkste inputparameters zijn:

• Gasdruk • Vochtigheid

• Aanwezigheid van bijproducten

De weegfactoren zijn bepaald aan de hand van de eerder genoemde laboratorium- en veldexperimenten.

In het kader van asset management is de mogelijkheid om werkzaamheden de juiste prioriteit te geven zeker zo belangrijk als het bepalen van de health index of verouderingsstadium. De prioriteit is risico gebaseerd op zowel component, systeem als bedrijfsniveau.

De uiteindelijke verificatie van de modellen heeft plaatsgevonden met behulp van data aangeleverd door een utility met GIS installaties in tropische condities.

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Table of Content

Summary ...7

Samenvatting ...9

Table of Content ... 11

1. Introduction ... 17

1.1 State of the Art: Gas-Insulated Switchgear ... 18

1.1.1 Gas-insulated switchgear (GIS) ... 18

1.1.2 GIS problems under temperate climate conditions ... 19

1.1.3 GIS problems under tropical climate conditions ... 19

1.2 The Case Study ... 20

1.2.1 Case study definition ... 21

1.2.2 Case study circumstances ... 21

1.3 State of the Art: Maintenance Methodology ... 24

1.4 Aim of the Thesis ... 25

1.5 Structure of the Thesis ... 26

2. Failure Investigation Methodology ... 29

2.1 Statistical Failure Analysis ... 30

2.2 Forensic Failure Investigation ... 32

2.2.1 Failure analysis methodology ... 32

2.2.2 Required information ... 33

2.2.3 GIS systems ... 34

2.2.4 Failure modes risk calculation ... 37

3. Tropical Case Study ... 41

3.1 Primary Subsystem Failure ... 41

3.1.1 Connecting joint/inner and outer connecting joint ... 41

3.1.2 Termination/overhead line 500 kV bushing ... 43

3.1.3 Outer connecting joint (flange) ... 47

3.1.4 Gauge and gas dispenser/gas dispenser... 49

3.2 Secondary Subsystem Failure ... 50

3.3 Dielectric Subsystem Failure ... 51

3.4 Driving Subsystem Failure ... 54

3.4.1 Pneumatic mechanism failure ... 55

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3.5.1 Circuit Breaker ... 57

3.5.2 Disconnecting Switch ... 57

3.5.3 Earthing Switch ... 58

3.6 Discussion on Key Influencing Factors ... 59

3.7 Discussion of Failure Modes, Effects and Criticality Analysis (FMECA) ... 61

3.8 Discussion on Failure Mitigation ... 64

3.8.1 Parameters relating to equipment condition ... 65

3.8.2 Parameters relating to environmental conditions ... 67

3.8.3 Conclusion: parameters applied in the assessment process... 69

3.9 Conclusions ... 72

4.Electric Breakdown Strength In A Homogeneous Field ... 73

4.1 The Experimental Set Up for Breakdown Strength Tests ... 74

4.1.1 Applied insulating gas ... 74

4.1.2 Electrodes configuration ... 75

4.1.3 The variation of the varied parameters ... 76

4.1.4 Results ... 79

4.2 Influence of Gas Pressure on the Breakdown Strength ... 80

4.2.1 Theory ... 80

4.2.2 Aims of the experiment ... 81

4.2.3 Experimental results ... 82

4.2.4 Breakdown field strength under various gas pressure ... 84

4.3 Influence of Humidity on the Breakdown Strength... 85

4.3.1 Theory ... 85

4.3.4 Breakdown field strength under various humidity ... 88

4.4 Influence of Temperature on the Breakdown Strength ... 88

4.4.1 Theory ... 88

4.4.4 Breakdown field strength under various temperature ... 90

5.Electric Breakdown Strength In An Inhomogeneous Electric Fields ... 93

5.1 Experimental Set Up for Breakdown Strength Tests ... 94

5.2 Influence of Gas Pressure on Ebd,ac ... 97

5.2.2 Expected results ... 97

5.2.4 Breakdown field strength under various length of protrusion ... 98

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5.3 Influence of Gas Pressure on PDIVac ... 102

5.3.1 Aim of the experiment ... 102

5.3.2 Expected results... 102

5.3.4 Partial discharge inception voltage under various length of protrusion .. 104

5.3.5 Partial discharge inception voltage under various position of protrusion105 5.4 Influence of Gas Pressure on qac ... 106

5.4.4 Partial discharge magnitude under various length of protrusion ... 107

5.4.5 Partial discharge magnitude under various position of protrusion .... 108

5.5 Influence of Humidity on Ebd,ac ... 108

5.5.5 Breakdown field strength under various position of protrusion ... 111

5.6 Influence of Humidity on PDIVac ... 112

5.6.4 Partial discharge inception voltage under various length of protrusion . 114 5.6.5 Partial discharge inception voltage under various position of protrusion115 5.7 Influence of Humidity on qac ... 116

5.7.4 Partial discharge magnitude on various length of protrusion ... 117

5.7.5 Partial discharge magnitude on various position of protrusion ... 118

5.8 Influence of Temperature on Ebd,ac ... 118

5.9 Influence of Temperature on the PDIVac ... 121

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5.10 Influence of Temperature on qac ... 123

5.10.2 Expected results ... 123

5.10.4 Partial discharge magnitude under various length of protrusion ... 124

5.11 Influence of Impulses Overvoltage on Ebd ... 124

5.11.1 Theory ... 124

5.11.3 Ebd under temperate conditions ... 126

5.11.4 Ebd in varied humidity ... 128

5.11.5 Ebd in varied temperature ... 129

5.12 Conclusions and Summaries ... 130

6.Condition And Lifetime Assessment Modelling ... 133

6.1 Condition Assessment ... 133

6.1.1 Condition assessment method ... 133

6.1.2 Condition assessment parameters ... 136

6.1.3 Condition assessment analytical function for qualitative information .... 136

6.1.4 Condition assessment analytical function for quantitative information . 138 6.2 Lifetime Assessment ... 143

6.2.1 Lifetime assessment method ... 143

6.2.2 Lifetime assessment parameters ... 143

6.2.3 Lifetime assessment analytical function ... 144

6.3 Norms Development for Tropical Conditions ... 145

6.3.1 Norms condition assessment parameters ... 146

6.3.2 Norms for control parameters ... 155

6.3.3 Norms for subsystem level condition assessment ... 157

6.3.4 Norms for equipment level condition assessment ... 158

6.3.5 Norms for lifetime assessment due to breakdown strength reduction ... 158

6.3.6 Norms for lifetime assessment due to corrosion ... 159

7.Risk Assessment ... 161

7.1 Risk Assessment Methodology ... 162

7.2 Risk to Equipment ... 164

7.2.1 Consequence category ... 164

7.2.2 Criticality of consequence category modes causing overall function loss167 7.2.3 Risk matrix ... 168

7.3 Risk indexing for Business of Power Delivery ... 170

7.3.1 Consequence categories: business values ... 170

7.3.2 Risk indexing criteria ... 171

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7.3.5 Risk indexing for power delivery business due to equipment failure ... 173

7.4 Time-to-Maintenance ... 174

7.5 Conclusions and Summary ... 175

8.Integrated Assessment Model Verification ... 177

8.1 The Integrated Model ... 177

8.1.1 Modeling considerations ... 179

8.1.2 Maintenance strategies ... 179

8.1.3 Chain of activities ... 179

8.1.4 Algorithm of the Integrated Model ... 183

8.2 Model Verification ... 189

8.2.1 Requisite factors affecting the integrated model ... 189

8.2.2 Verification ... 190

8.3 Implementation of the Assessment Model ... 192

8.3.1 Operational records ... 193

8.3.2 Inspection records ... 194

8.3.3 Analysis results ... 197

8.3.4 Proposed maintenance activity ... 205

8.4 Compatibility of the Integrated Model to Real Installation ... 206

9.Conclusions and Recommendations ... 207

9.2 Recommendations ... 209

Appendix AGIS Installations in the Case Study ... 211

Appendix BTest Results of the Failed Components ... 213

Appendix CFailure Analysis Related to Non-tropical Conditions ... 219

Appendix DExperimental Test Set Up ... 225

Appendix EExperimental Results of Breakdown Strength ... 229

Appendix FAssessment Based on Superficial Inspection ... 231

Appendix GStatistical Analysis on Operating Experiences... 235

List of Symbols ... 239 List of Abbreviations ... 241 List of Terms ... 243 Bibliography ... 245 Acknowledgements ... 253 Curriculum Vitae ... 255

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1

Introduction

Gas-insulated switchgear (GIS) installations are nodes in power networks which have the function of distributing electrical energy, to (de-) energize power electric equipment and to isolate faults. GIS installations are beneficial in cases of substations with limited space. A GIS is designed for long lasting performance in temperate climates. However, under tropical conditions the performance of such installations appears to be reduced. This observation formed the starting point for this research.

A number of tropical factors have been considered that change the degradation rate and process adversely, ultimately affecting GIS performance. Experiments have been conducted in order to verify this analysis. Based on the results of these experiments, an adapted maintenance methodology has been developed aimed at overcoming the aforementioned problem.

This thesis provides information about the influence of tropical conditions have in terms of accelerating a number of degradation mechanisms affecting GIS installations, in particular the insulating gas (SF6). As a result of this information, certain rules, norms and weighting factors will have to be adjusted. Such adjustments will improve the methodology of maintenance and ultimately improve the performance of the installation itself.

In this chapter, we start with a brief explanation on the state of the art of GIS installations in section 1.1, followed by a description of the case study which is used as the basis of the investigation in this research in section 1.2, and the state of the art of the maintenance methodology in section 1.3. Finally, the chapter concludes with an explanation of the aim of the thesis and the strategy undertaken to achieve the goals of the research, in sections 1.4 and 1.5 respectively.

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1.1 State of the Art: Gas-Insulated Switchgear 1.1.1 Gas-insulated switchgear (GIS)

Gas-insulated switchgear (GIS) installations are metal-enclosed high voltage systems. Among other things, GIS installations have the advantage of being very reliable and compact in size (see Figure 1- 1). These advantages drive the utilities to install GIS installations in urban areas. Because the electrical circuit is fully isolated from the outside atmosphere in a GIS installation, it is not expected to be disturbed by the environment. However, with regard to experiences of operating GIS installations in tropical countries, reports by South East Asian electrical utilities indicate that these are facing increased numbers of faults and failures of GIS installations since these have been introduced to their power system [1,2].

Figure 1- 1: 500 kV GIS outdoor type installed in tropical country.

GIS technology was introduced in the 1960s and has been operated worldwide since the 1980s [3]. The current-carrying principle of GIS is the same as that of a coaxial cable. The active conductor (the inner core) is insulated by SF6 gas and together with spacers encapsulated by a metal enclosure. In order to retain its concentric configuration, a cast epoxy resin spacer is applied. The enclosure is usually made of aluminum, aluminum alloy or steel. The space between the enclosure and the conductor is insulated by SF6 gas at 4-6 bars in order to provide sufficient dielectric strength. For the same substation design (for example for the same bus scheme and incoming/outgoing connections) the area required for a GIS installation can be reduced by 10% to 20% compared to the area required for a conventional substation, depending on the voltage level [4]. In general, two types of GIS structure are used that differ with regard to operation voltage. For systems ranging from medium voltage up

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to 245 kV, system structures of three phases in a single enclosure and one phase per single enclosure are both common, while for systems operating at voltages above 245 kV only one phase in a single enclosure is commonly used. GIS installations can be arranged vertically or horizontally, depending on the way the circuit breaker is installed. GIS can be installed outdoors or indoors. Outdoor GIS installations are usually found next to power plants and indoor installations in commercial or residential areas.

In general terms, the technical advantages of GIS installations in comparison to air-insulated switchgear (AIS) installations, are that the former:

1. take up less space

2. provide complete protection against live high voltage parts 3. protect the inner parts against environmental pollution 4. require less maintenance

While the technical disadvantages are that these:

1. use SF6 gas as an insulating and arc quenching material 2. suffer failures if operated under tropical conditions

3. involve the occurrence of very fast transient oscillation (VFTO) during switching operations and earth faults

4. require high energy levels during the arc quenching process which leads to high reaction forces on the GIS support framework and foundations [5] 1.1.2 GIS problems under temperate climate conditions

In general, during operation, GIS installations suffer from electrical, mechanical, chemical and thermal stresses. In the long term these stresses may cause failures to the basic structure of the installations [6], in particular to the insulation system. Many failures have been observed in GIS in-service for various operating voltages and locations [3]. The failures occur mainly in the insulating gas compartment, cable or overhead line terminations and switching operating mechanisms.

1.1.3 GIS problems under tropical climate conditions

In general, there are three main problems that installations face under tropical conditions, which can become critical:

1. gas leakage

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In tropical countries, the most frequently occurring fault symptom is gas leakage with varying leakage rates occurring in GIS installations of varying ages. Despite the fact that usually designed leakage does not exceed 1% of its original weight per year for each individual compartment [4,7], under tropical condition rates of over 7% have been observed. For instance, an inspection revealed that in one case leakage ultimately resulted in the explosion of a bushing which acted as GIS-overhead line termination [2].

A decrease in the quality of the insulation gas is also a concern. This has been indicated by subsequent test results: purity level, dew point and decomposition products. In some cases, dew point tests revealed that the moisture content was twice as high as usual, while decomposition products were 20% higher than permitted [8].

The next aspect that should be taken into account is the presence of defects. Based on partial discharge measurements obtained by using acoustics and VHF/UHF methods, the presence of various insulation defects has been identified. The presence of defects increases the risk of total dielectric failure of the insulation system.

This thesis investigates the effects of tropical conditions on the performance and lifetime condition of gas-insulated substations. Since there is a lack of knowledge about the effect of tropical conditions and the root causes of major failures, a Failure Mode, Effects, and Criticality Analysis (FMECA) has been carried out on groups of GIS installations. For this analysis, a large volume of field data, including historical and forensic data, was processed. Next, experiments were conducted to verify the analysis. A condition and risk assessment model has been developed based on the knowledge rules obtained from the FMECA and the experimental evidence. A maintenance methodology that complies with these tropical conditions has been developed.

1.2 The Case Study

For the modeling, we made use of a case study, consisting of a large number of GIS installations operated by PLN Indonesia. This Indonesian electrical utility has wide experience in operating around seventy 150 kV and 500 kV GIS installations under tropical conditions.

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1.2.1 Case study definition

The case study, defined as a group of GIS installations operating under tropical conditions, serves as source of information for obtaining on-site knowledge. The set up of the laboratory experiments reproduces the same tropical conditions. In the end, we developed an assessment model which was applied to assess the performance of the installations. The aim of this new assessment model is to improve the performance of GIS installations under tropical conditions.

1.2.2 Case study circumstances

The operational circumstances, of which the influences on GIS performance have been investigated, are tropical conditions as occurring in e.g. Indonesia. The factors considered in these conditions are climate and pollution.

Climate

The relevant climate parameters are temperature, relative humidity, precipitation, thunder days, and lightning characteristics. Based on the climate data [9], the conditions in the case study are categorized as tropical-wet-and-dry or savannah climate (Aw) [10]. This in contrast to conditions in European countries, such as the Netherlands, which are categorized as temperate-without dry season-warm summer or as oceanic climate (Cfb) [10].

The average annual temperature in Indonesia, i.e. Jakarta, is 27°C, while that in the Netherlands, i.e. Amsterdam, is 10°C [11]. The average annual precipitation in Jakarta between January 2000 and May 2012 is 1574 mm, while in Amsterdam the value is 793 mm for the same period [9]. Despite the fact that precipitation in Jakarta is twice as high as in Amsterdam, both cities have a similar relative humidity [11].

At most, Jakarta’s keraunic level1 and average number of thunder days is 92% and 252 days/year respectively [12]. Moreover, in the western part of the Island of Java, in Indonesia, the highest recorded number of thunder days, in 2004, is 238 days/year [12], the distribution of which is depicted in Figure 1- 2. In the Netherlands, i.e. Amsterdam, the average number of thunder days is 28 days/year [11].

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Figure 1- 2: Distribution of thunder days in the western part of Java (courtesy of Encarta).

The high number of thunder days increases the risk of component failure due to direct lightning strikes and back flashover. Moreover, it can cause switching impulses during the recovery process. Power networks with overhead lines are susceptible to this condition.

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Table 1- 1: Comparison of climate between Indonesia and the Netherlands.

Jakarta-Indonesia Amsterdam, the Netherlands

Climate

tropical-wet-and-dry/savannah

temperate-without dry

season-warm summer or

oceanic

Average annual temperature1)

Range Years on record 27°C 26°C to 28°C 19 10°C 2°C to 16°C 21 Average annual precipitation

(2000-May 2012) 2)

1573.8 mm 792.8 mm

Average annual humidity1)

Morning Range Years on record Evening Range Years on record 91% 87% (SeptemberOctober) -94% (January-February) 16 68% 62% (July-September) - 77% (January) 12 Morning: 91% 87% (May) -94% (September-October) 21 74% 62% (May) – 88% (December) 21

Max. thunder days per year 2523) 281)

1)

: Weather base, USA

2)

: Weather online, United Kingdom

3)

: BMG (Indonesian Agency for Meteorology, Climatology and Geophysics) Pollution

In tropical environments, pollution comprises natural pollutants, e.g.: microorganism, salty aerosol or salty film and industrial pollutants. The latter have caused acid rain in the Jakarta area. Chemical pollutants can act as a chemical reaction agent and cause corrosion (damaging the compression chamber and surface of the enclosure (Figure 1- 3), rust on bolts); as well as acting as a conductive material (on the surface of insulators), causing leakage currents.

Figure 1- 3: Chemical pollutant impact to GIS components (left to the outer surface, right to the inner surface).

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Pollutants influence an area’s air quality. The air quality is usually monitored based on the proportion of 10 µm particles (PM10), nitrogen dioxide (NO2), sulfur dioxide (SO2) and carbon monoxide (CO) [13]. The registration of different kinds of pollutants is usually dependent on the main concerns of the government in the area. Nevertheless, a significant difference in PM10 between the two cities can be noted. The PM10 in Jakarta is 74.4 µg/m³ [14], which is more than three times that in Amsterdam (see Table 1- 2) [15].

Table 1- 2: Comparison of air quality between Indonesia and the Netherlands.

Jakarta-Indonesia Amsterdam, the Netherlands

Max. PM10 (µg/m³) 74.401) 232) Max. NO2 (µg/m³) 93 2) Max. SO2 (µg/m³) 15.7 1) 1)

: Clean Air Initiative Organization

2)

: Luchtkwaliteit GGD Amsterdam

1.3 State of the Art: Maintenance Methodology

The general definition of maintenance used is as follows: all appropriate activities which aim to retain or to restore equipment to a given condition [16]. Based on the specific characteristics, three types of maintenance strategy can be distinguished: corrective, preventive and predictive strategies. An evolution in maintenance strategies has occurred, from corrective maintenance in the 1940s, to advanced preventive and predictive maintenance in 1990s [17,18] (Figure 1- 4). The latter strategies are part of Reliability-Centered Maintenance (RCM), a maintenance methodology in which maintenance actions are prioritized and both replacement and refurbishment activities are ranked. Since a cost-effective balance needs to be achieved between maintenance intervals, reliability and risk through optimization, RCM is further extended to Risk-Based Maintenance (RBM) [17,19,20].

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The goal of implementing any maintenance strategies is to make it possible to decide whether the asset should be [21]:

1. Partially renewed by refurbishment 2. Totally renewed by replacement 3. Upgraded

4. Uprated (put into a new network design)

5. Extended in terms of lifetime or kept in operation until a specified condition arises

The high voltage equipment in power networks is made up of subsystems. Each of these subsystems, in turn, consists of sub-subsystems or components with its own typical function characteristics. The maintenance strategy should be comply with these function characteristics. Thus the application of RBM strategies is usually based on the following aspect [19]:

1. Criticality (impact of outage and failure on a location or customer) 2. Urgency (pace of problem rise)

3. Opportunity (chances of repairing or improving installation) 4. Benefit from a financial point of view

5. Condition of equipment

From the point of view of RBM, the priority with regard to power networks is to determine the condition of the high voltage equipment and the importance of the (sub)system function loss of the equipment in the network. In order to prevent premature failures, norms have to be derived that comply with tropical conditions. How to deal with this in relation to the condition and risk assessment processes will be the subject of Chapters 6 and 7.

1.4 Aim of the Thesis

The aim of the thesis is twofold:

1. To assess and to understand the influence of tropical conditions on the long-term performance of gas insulated metal enclosed switchgear.

2. To develop a maintenance methodology for these circumstances to enable better GIS performance under tropical conditions.

In this thesis, the elaboration of the long-term performance focuses on the insulating system because this causes a major share of fatal failures (see

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In order to achieve these goals, the following research questions have been investigated:

1. Based on in-depth failure mode investigations, which degradation mechanisms in GIS installations are accelerated under tropical conditions? 2. Do appropriate diagnostics tools exist that enable the monitoring of

premature function loss of GIS?

3. How can tropical factors of influence be prioritized for the performance of GIS?

4. Which norms, criteria and assessment model for tropical conditions are advisable?

5. Which knowledge rules serve as background for a decision support systems for GIS maintenance?

6. Which consequence categories, based on background insights in failure modes of GIS under tropical condtions, may better serve to rank or to index risk to system and business values

7. Which appropriate actions are advised to ensure proper functioning of GIS In this thesis our focus is on utilizing fundamental insights based on physics and electro-technological background knowledge and experiments, in our approach to solve the huge operational problems with EHV GIS equipment under tropical conditions. As such power switches perform critical nodes in the actual power delivery system we have to restrict ourselves to derive solutions from our research that make sense in engineering practice and that are easy/efficient to implement.

1.5 Structure of the Thesis

Chapter 2 deals with the methodology of forensic investigation which is applied in order to obtain information regarding the failure mechanisms that occur in GIS installations when operated under tropical conditions.

Chapter 3 deals with the identification of degradation mechanisms which accelerate under tropical conditions and the assessment of parameters to be monitored based on FMECA results.

Chapter 4 deals with laboratory experiments aimed at investigating the breakdown strength under homogeneous electric field and tropical conditions. The goal is to find out which factor has the greatest influence on insulation deterioration under tropical conditions. The breakdown strength experiments were conducted by varying the gas pressure and the parameters of humidity and temperature.

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Chapter 5 deals with experiments aimed at investigating the breakdown strength under inhomogeneous electric field and tropical conditions with the application of both power frequency voltage and impulse voltages. The goal is to find out which factor has the greatest influence on the deterioration of the insulating gas under these conditions. The breakdown strength experiments were conducted by varying the gas pressure and the tropical parameters of humidity and temperature. In order to create an inhomogeneous field, a practical defect model was inserted into the test object. Standard lightning and switching overvoltage impulses were applied to model the presence of impulses in the power network.

Chapter 6 deals with the generation of norms for tropical conditions. It covers the extraction of knowledge rules and the calculation of weighting factors for condition and lifetime assessment. Norms, as well as knowledge rules, are generated based on laboratory and field measurements. For each subsystem the system condition index will be defined.

In Chapter 7 we investigate the risk assessment according to system and the business values required for operating conditions. The description includes the general concept of risk assessment. For each risk assessment level, the description comprises the consequence categories to be taken into account, risk quantification and the development of risk matrices.

In Chapter 8, the model for the overall assessment of GIS is verified. An example of the implementation of the model is given as well.

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2

Failure Investigation

Methodology

In order to be able to develop an appropriate maintenance system for GIS installations operated under tropical conditions, we first need to know how and why the equipment fails and how we can prevent such failures from occurring. In order to establish this, a group of GIS installations operated under tropical circumstances was selected as a source of information, namely the case study.

In the case study, a number of GIS failures have occurred within the installations’ 25 years of operation, of which some were classified as major failures and the rest were minor and maintainable. Because of the relatively high number of failures, which also showed a rising trend, a forensic failure investigation was conducted.

The aim of our forensic failure investigation was to uncover the failure modes that are active under tropical conditions and to identify appropriate parameters that need to be monitored. The aim being to use these parameters is to improve the accuracy of the assessment process.

In section 2.1, we discuss statistical failure within the case study. This includes a comparison with CIGRÉ survey results. The aim of the latest CIGRÉ survey was to collect GIS service experience up to December 31, 1995 to which 80 utilities responded, from 30 different countries [3].

Section 2.2 includes a discussion of the forensic failure investigation method. This discussion looks at the required information, the function-based system division of a GIS installation, the definition of criteria for calculating the risk of failure modes and the methodology of failure analysis.

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2.1 Statistical Failure Analysis

In the case study, GIS installations are operated at voltage levels of 150 kV and 500 kV. Inspections of GIS installations have been conducted over the past eight years. As a result, more information related to the insulating gas quality and operating mechanism failures was obtained, leading to more effective forensic (re-) analysis.

A comparison has been made with the results of the 2nd GIS survey conducted by CIGRÉ [3]. In the case study, from 1997 to 2009, 67 major failures were reported among 150 kV GIS and 9 major failures among 500 kV GIS. These values correspond to 0.93 failures per 100 CB bay-years2 and 0.55 failures per 100 CB bay-years, respectively (see Table 2- 1).

Table 2- 1: Comparison of failure statistic between the case study and the 2nd CIGRÉ report. At 150 kV, the failure rate in the case study is considerably higher than in the 2nd

CIGRÉ report, while at 500 kV the data shows an opposite trend [3].

Case Study CIGRÉ Case Study CIGRÉ Total CB-bay-year 7223 34060 1627 4525 Total failures 67 160 9 48

Failure per 100 CB-bay-years 0.93 0.47 0.55 1.06

150 kV 500 kV

According to the 2nd CIGRÉ survey, 160 failures were reported among 150 kV GIS installations and 48 failures among 500 kV GIS installations. These values correspond to 0.47 failures per 100 CB bay-years and 1.06 failures per 100 CB bay-years respectively (see Table 2- 1). It appears that the failure rate of 150 kV GIS in the case study is twice that in the 2nd CIGRÉ survey, while the failure rate of 500 kV GIS in the case study is half of that in the 2nd CIGRÉ survey. An in-depth comparison was made in order to determine the failure frequency of each subsystem. The results are provided in Table 2- 2.

2

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Table 2- 2: Comparison of major failures in the case study and the 2nd CIGRÉ survey per GIS subsystem. Failure rate of driving subsystem of 150 kV GIS is higher in the case

study than in the 2nd CIGRÉ survey. At 500 kV, there are no failures of the driving mechanism in the 2nd CIGRÉ survey [3].

failures/ 100 CB-bay-year Percen-tage of failures failures/ 100 CB-bay-year Percen-tage of failures failures/ 100 CB-bay-year Percen-tage of failures failures/ 100 CB-bay-year Percen-tage of failures Primary 0.11 12% 0.20 43% 0.25 44% 0.80 75% Secondary 0.01 1% 0.07 16% 0.00 0% 0.02 2% Dielectric 0.03 3% 0.08 18% 0.25 44% 0.22 21% Driving 0.71 76% 0.03 7% 0.06 11% 0.00 0% Mechanical 0.07 7% 0.08 16% 0.00 0% 0.00 0% Other 0.00 0% 0.01 1% 0.00 0% 0.02 2% Total 0.93 100% 0.47 100% 0.55 100% 1.06 100% CIGRÉ Subsystem 150 kV 500 kV

Case Study CIGRÉ Case Study

At 150 kV, in the case study failures occurred in all subsystems (see Table 2- 2). In the case study, the major failure rate of driving subsystem is considerable: 0.71 failures per 100 CB bay-years, a figure that compares poorly to the 0.03 failures per 100 CB bay-years in the 2nd CIGRÉ survey.

At the 500 kV level, in the case study failures occurred in the primary, dielectric and driving subsystems (see Table 2- 2). The rate of primary failure in the case study is considerably lower than that in the 2nd CIGRÉ survey, i.e. 0.25 failures per 100 CB bay-years compared to 0.80 failures per 100 CB bay-years. The rate of dielectric subsystem failure in the case study is similar to that in the 2nd CIGRÉ survey, i.e. 0.25 failures per 100 CB bay-years and 0.22 failures per 100 CB bay-years respectively. In contrast to the case study, no driving subsystem failures were reported by CIGRÉ.

In the case study, failures of primary and mechanical subsystems at both voltage levels resulted in the GIS compartments exploding.

An analysis of the failure rate in the case study shows that the failure rate for 500 kV GIS has doubled after a lifetime of 10 years and tripled after a lifetime of 14 years, while the failure rate for 150 kV GIS has tripled after a lifetime of 9 years. This failure rate appears to be constant but shows a significant, ten-fold increase after a lifetime of 22 years. A similar trend can also be observed in the 2nd CIGRÉ survey, only there the failure rate starts later, i.e. after a lifetime of 18 years (see Figure 2- 1).

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A detailed explanation of the failures that occurred in the case study is provided in Chapter 3. 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0 5 10 15 20 25 30 F a il u re r a te (p e r 1 0 0 C B b a y s y e a rs ) Age (years)

Case Study

CIGRÉ

Figure 2- 1: Failure rate analysis. Above: taken from the case study based on 25 years of GIS operation. Below: The 2nd CIGRÉ survey [3]. In the case study the GIS failure rate is

double after 10 years while in the 2nd CIGRÉ survey this occurs after 18 years (marked by blue arrows).

2.2 Forensic Failure Investigation 2.2.1 Failure analysis methodology

The aim of the analysis of GIS failures occurring under tropical conditions within the case study is to reveal which failures were initiated by the tropical conditions and which tropical conditions affected the GIS installations.

The analysis was conducted using a methodology of forensic study recommended by CIGRÉ [22]. Within this methodology, the following steps are defined:

1. Event description, i.e. describing the failure event of a subsystem or main part

2. Forensic investigation, investigating the failure modes that correlate with the forensic data (in early, advanced or late stages of insulation degradation)

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3. Historical data

4. Potential failure modes

5. Key influencing factors, i.e. establishing the root causes of failure and the internal and external factors that are of influence

6. Failure mitigation, based on the most sensitive diagnostics

In practice, failure analysis is sometimes inconclusive owing to unavailable or insufficient supporting data. In these cases it is not possible to determine either the failure modes or the root causes.

In the case of a conclusive analysis, the failure modes are successfully traced so that the root cause can be identified. In addition, the conclusive root cause is classified as either an internal or an external influencing factor. If it is classified as an internal influencing factor this denotes that the origin of the root cause lies in the equipment itself. If it is classified as an external influencing factor this denotes that the origin of the root cause lies in the equipment’s environment. In order to determine whether the tropical conditions influence the failure process, we further classified the external factors, distinguishing between environmental circumstances and factors relating to human error/poor workmanship.

In the case study, a large number of failures were reported until 2009 (see Section 2.1). The failure analysis revealed that failures were partly related to tropical conditions. As this subject is our main concern here, a detailed discussion of several cases of failures and the key influencing factors the failures is provided in Chapter 3. A discussion of failures related to non-tropical conditions, is provided in Appendix C.

2.2.2 Required information

In general, the data used in the failure analysis are: 1. Forensic data

Forensic data is obtained from equipment remains by collecting information on-site after a failure event.

2. Historical data

Historical data consists of maintenance activities,

measurements/inspection results and operation records, recorded prior to a failure event. The operation records are important as these reveal the electrical power system characteristics, environmental conditions and outage history. The outage history reveals factors that have influenced the power network as a whole and, in particular, the installed equipment in

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3. Reference material

This data is obtained from expert’s reports, international institution references, journals and laboratory experiments.

In the case study, the data available to be used in the failure analysis is: 1. Forensic data

Remains of equipment collected on-site. 2. Historical data

a. Gas leakage and the volume of gas added to the compartment (recorded in the anomaly report)

b. Gas quality monitoring (starting from 2004) c. PD measurement (starting from 2004)

d. Operational records prior to failure events (usually indicated by coinciding breaker trips and relays)

3. Reference material

a. The literature consists of a collection of local expert’s reports, references from international institution and journals.

b. Laboratory experiments using CO2 and SF6 to simulate the on-site conditions were carried out at the High Voltage Laboratory of TU Delft in the Netherlands.

2.2.3 GIS systems

By applying Failure Modes and Effects Analysis, i.e. the FMEA method [23], the failure mode and root cause underlying each failure were determined.

For the purposes of analysis the equipment is divided into subsystems, based on their individual functions. A subsystem fails if it is unable to fulfil its function. The condition or boundary that defines whether a subsystem fails is known as the functional failure criterion.

In most cases [24,25], a GIS installation is divided into five subsystems, as depicted in Figure 2- 2. Since the primary subsystem has the main function of switchgear, it is described in more detail.

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BUSBAR COMPARTMENT CIRCUIT BREAKER COMPARTMENT SF6TO BUSHING COMPARTMENT OVERHEAD LINE SF6500 KV BUSHING UPPER PART SF6 500 KV BUSHING LOWER PART SF6500 KV BUSHING UPPER TO LOWER PART BUSHING JOINT LOWER PART TO METAL BASE BUSHING JOINT ARC QUENCHING CHAMBER FLANGES METALLIC BRIDGE PLATES GAUGE DRIVING SUBSYSTEM CONTROL SPACER/EPOXY RESIN COMPRESSION SYSTEM MECHANICAL SYSTEM MAIN CONTACT CONDUCTOR ENCLOSURE VALVE GAS RUPTURE GAS DISPENSER SENSING DEVICE SF6 SF6 TULIP FINGERS PRIMARY SUBSYSTEM SECONDARY SUBSYSTEM DIELECTRIC SUBSYSTEM DRIVING SUBSYSTEM MECHANICAL SUBSYSTEM

Figure 2- 2: Schematic diagram of main part of subsystem and sub-subsystem of a GIS installation. Primary subsystem is in black, secondary subsystem is in red, dielectric subsystem is in blue, driving subsystem is in purple and mechanical subsystem is in

green.

1. Primary subsystem.

The primary subsystem has the function of transporting the electrical current within the boundaries of a certain level of losses. This system consists of several sub-subsystems, the important sub-subsystems being:

a. basic sub-subsystem

This sub-subsystem includes the following main parts: circuit breaker, disconnecting switch, earthing switch, busbar and instrument transformer.

b. connecting joint sub-subsystem

This sub-subsystem consists of the inner connecting joint and outer connecting joint.

c. termination sub-subsystem

This sub-subsystem includes the overhead line bushing and cable head sealing end.

d. gas dispenser sub-subsystem

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Table 2- 3: Sub-subsytems of primary subsystem. Examples of failures are discussed in Chapter 3.

Sub-subsystem Function Functional Failure Main

Components Interrupter unit of

circuit breaker

MP1 Connects, carries, disconnects

current under rated voltage

Unable to disconnect current Main contacts of disconnecting switch

MP2 Opens and closes circuit when

no current flows in order to isolate a part of a circuit

Unable to open or close circuit

Main contacts of earthing switch

MP3 Opens and closes circuit in

order to ground a part of a circuit

Unable to ground a part of a circuit Conductor of

busbar

MP4 Conducts electricity Unable to conduct

electricity Active part of

instrument transformer

MP5 Provides outputs, i.e. current

and voltage, for the protection and supervisory system

Unable to provide outputs at all or in correct value Inner connecting

joint

MP6 Connects two conductors and

secures the joint

Discharges occur Tulip fingers

Flanges Seals Bridge plates Bolts Overhead line bushing

MP8 Gas leakage and

discharges occur

Cable sealing end MP9 Gas leakage and

discharges occur

Gas rupture MP10 Gas leakage

occurs Open-close

indicator

MP11 Gas leakage

occurs Gauge and gas

dispenser

Gas dispenser MP12 Acts as the inlet and outlet of

the insulating gas during gas quality check, refilling and evacuation Gas leakage occurs Valve Seals Protection and Supervisory Facilities MP7 Basic Connecting joint Termination Outer connecting joint Gas leakage occurs Interrupted inductive current path

Connects two compartment enclosure and creates a conducting path to ground for inductive currents

Connects the gas-insulated switchgear to the incoming and outgoing part of the switchyard

Main Parts

Protects the main components of the primary subsystem against catastrophic failure

2. Secondary subsystem.

The secondary subsystem has the function of triggering the driving subsystem (see point 4) to activate the mechanical subsystem (see point 5) at the correct time. This subsystem consists of a sensing device (relays), protection logic circuit, alarm, measuring device (gauge), auxiliary switch and control components. The main component of this subsystem is the sensing device which has the function of acquiring information that is used to activate the control system of close and open sequences of the switching device contacts.

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Functional failure of the sensing device occurs if it is unable to obtain the correct information.

3. Dielectric subsystem.

The dielectric subsystem has the function of extinguishing arcs, insulating active parts and keeping the active conductor in its concentric position. Included in this subsystem are: SF6 (gas insulation) and epoxy resin (solid insulation). Functional failure of gas insulation occurs if the breakdown strength falls below a certain value and/or the gas insulation is unable to extinguish arcs.

Functional failure in solid insulation comprises cracks or fissures in the surface or a complete flashover of the solid insulation.

4. Driving subsystem.

The driving subsystem has the function of storing energy which is used to move the main contact within the correct time. Three energy storage media are commonly used: compressed air, compressed fluid or a tension spring. Based on these media, the energy storage system can be categorised according to the three types of mechanism, i.e. pneumatic, hydraulic or spring. In some switchgear types, different mechanisms are applied during the opening and closing mode [26]. The main component of pneumatic and hydraulic system types is the compression system.

Functional failure of the subsystem consists in failing to store sufficient energy needed to move the main contact within the correct time.

5. Mechanical subsystem.

The mechanical subsystem has the function of moving the main contact of the switching devices at the correct time. It transfers the driving energy, converting it into a mechanical movement.

Functional failures of the subsystem consist in a failure to transfer the driving energy within the correct time or completely failing to transfer the driving energy.

2.2.4 Failure modes risk calculation

The risk of failure modes is defined as the probability of a failure mode harming the electrical system and its environment. The risk is calculated by multiplying the failure occurrence probability with the failure effect (Eq. 2-1) [23].

= ×

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The failure occurrence probability is the product of the failure frequency and the escalation factor (Eq. 2-2). The escalation factor is defined as the capability of a failure mode to cause a fatal failure.

= ×

Probability Frequency Escalation (2-2)

The failure effect is defined as the capability of a failure mode to impact on system reliability, operator safety, financial expense and the environment. The risk calculation is formulated in Eq. 2-3.

(

)

= × × + + +

Probability _Effect

Risk Frequency Escalation System Safety Cost Environment

(2-3)

The categorisation of the risk criteria is determined on the basis of local expert’s reports and failure records, except for the environment. The categorisation of the effect on the environment is based on SF6 leakage into the surrounding air as this is considered to be the overriding and relevant effect.

The criterion for frequency is determined on the basis of failure occurrences within 25 years of GIS operation and defined in terms of failures per 100 CB bays-years. The criterion for the escalation factor is basically defined as the capability of a failure mode to initiate a destructive chain reaction.

The criterion for the effect on the system is defined in terms of the time required for a GIS installation to be repaired or taken out of service. The criterion for the effect on safety is defined by the severity of the effect of the failure on personal safety in the substation and the area directly outside the substation. The criterion for the impact effect on costs is quantified by the expense of repairing or refurbishing the equipment.

The criterion for the effect on the environment is defined in terms of the number of damaged compartments. The classification of the extreme hazard condition and devastating impact on the environment by SF6 is not concluded in the calculation.

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Table 2- 4: Categorization of probability and effects.

1 f<0.02 1 No impact & repairs ≤ 1 day

2 0.02<f≤0.04 2 Impact & repairs 1-7 days

3 f>0.04 3 Out of operation

1 No escalation 1 cost<USD 22,000

2 Escalation 2 USD 22,000<cost≤USD 55,000

3 cost>USD 55,000

1 No impact on personal safety nor

substation 1 No impact on environment

2 Impact on substation 2 1 compartment damage

3 Impact on outside substation 3 2 compartments damage

4 Impact on personal safety 4 >2 compartments damage

Frequency category System category

Cost category

Safety category Environment category

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3

Tropical Case Study

In this chapter, investigations into failures, particularly those relating to tropical conditions, are discussed. The investigations were conducted by way of a forensic study methodology consisting of six steps, as described in Section 2.2.1.

Throughout this chapter, the failure investigations are discussed per GIS subsystem (see Section 2.2.3). Sections 3.1 to 3.5 are devoted to the discussion of forensic failure investigations. The influential tropical acceleration factors for GIS deterioration are listed in Section 3.6, as is a summary of the failure analysis.

The failure mode risk owing to tropical conditions and the appropriate parameters that are proposed as requiring monitoring are discussed in sections 3.7 and 3.8 respectively. These resulted from the Failure Mode, Effect and Criticality Analysis (FMECA). The failure modes originated from non-tropical conditions are described in Appendix C.

3.1 Primary Subsystem Failure

3.1.1 Connecting joint/inner and outer connecting joint Event description

This failure of the inner and outer connecting joint (see Figure 3- 1) manifested itself through the explosion of the compartment. The blast of the insulating gas destroyed the gas rupture disk, allowing a high-energy arc current to pass through it and burn a hole through the enclosure of the compartment below.

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Figure 3- 1: Inner and outer connecting joints. Forensic investigation

The collected forensic data comprised the following findings:

1. the tulip finger connectors (MP6 in Table 2- 3) of the disconnecting switch were missing or broken and covered by metal trifluoride powder (see Figure 3- 2a)

2. the gas rupture disk was broken

3. a hole in the compartment below the gas rupture disk was created during the failure

4. the control box next to the burnt through compartment melted during the failure

5. the compartment’s flanges (MP7 in Table 2- 3) were corroded (see Figure 3- 2b)

Figure 3- 2: (a) Broken tulip fingers, (b) corroded compartment flange. Both metal parts caused major failure and were influenced by the humid environment.

This data indicates the occurrence of dielectric overstress and partial discharge activity in a later stage. In addition to these processes, the data also indicated an advanced stage of chemical reaction, i.e. corrosion.

Historical data

The historical data revealed the following facts:

1. the GIS was installed in a highly polluted environment

2. the moisture content in several unaffected compartments, was in the 1200 to 3000 ppmv range, whereas the maximum allowable values at the corresponding gas pressure are in the 550 to 600 ppmv range [8].

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Potential failure modes

Based on the collected forensic and historical data, the failure mode can be reconstructed as follows. The corrosion of the flanges was set in motion by corrosive pollutants and acid rain. The corroded flanges offered ingress points for water vapour and gas leakage. Excessive amounts of water vapour caused metal oxidation, affecting the gas tightness of the basic sub-subsystem. An investigation included in Appendix B found that leakage paths are created as a result of the degradation of seals. As a consequence, the gas quality is reduced, which can ultimately result in partial discharges and breakdown. Due to their inherent construction, the tulip fingers were susceptible to corrosion, which caused the tulip fingers to come loose. Loose contacts created the (observed) PD, which produced by-products that accelerated the oxidation process. The end result was the separation of the conductors, accompanied by arcing, as observed. From the earliest until the final stages, this ageing process occurred over a period of about 20 years.

Key influencing factors

The root causes of this failure are a corrosive pollutant and a humid environment. Thus, in this case, an external failure factor played a role in corroding the flanges and creating ingress points for moisture, which ultimately corroded the tulip fingers.

Failure mitigation

The failure modes show that the tulip fingers serve as the critical component of the inner connecting joint, while the flanges and bolts serve as the critical component of the outer connecting joint. The degraded components comprised flanges, tulip fingers and insulating gas. Indicators of such degradation are a decrease in gas quality and partial discharge activity. Therefore, in order to avoid similar failures, the following actions should be undertaken:

1. monitoring of gas quality and partial discharge

2. applying an appropriate coating and paint to the external metal parts of the GIS

3.1.2 Termination/overhead line 500 kV bushing Event description

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part of the bushing was thrown tens of meters wide and destroyed all adjacent equipment, such as the lightning arrester.

Figure 3- 3: Termination to overhead line. The 500 kV bushing exploded, leaving the lower part in its original position.

Forensic investigation

The collected forensic data comprised the following findings: 1. the lower part of the bushing was still attached to its metal base

2. electrical treeing occurred at the junctions between the upper and lower parts of the bushing (Figure 3- 4a and b, respectively)

3. the metal base of bushing of the adjacent phases were also corroded (Figure 3- 4c)

4. a human skin irritant compound was encountered 5. a white powder covered part of the surface of the O-Ring

6. the trace of a flashover was found on the inner conductor surface (Figure 3- 4d)

This data indicates the occurrence of dielectric overstress and partial discharge activity in a more advanced stage. In addition to these processes, the data also indicated an advanced stage of chemical reaction, i.e. corrosion.

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Figure 3- 4: (a) and (b) treeing on the junction between upper and lower parts of bushing, (c) corroded metal base of bushing and (d) flashover track on the conductor. Historical data

The historical data revealed the following facts:

1. a lightning strike occurred a short time before the explosion (according to the operation records)

2. gas leakages occurred at four adjacent indoor compartments 3. the bushing consisted of two parts and was assembled on site Potential failure modes

Based on the collected forensic and historical data, two failure modes can be identified that contributed to the bushing termination failure. The first mode comprises the deterioration of the insulation and the second mode comprises the initiation of the breakdown of the insulating gas.

Failure mode 1: The insulation deterioration process.

The insulation deterioration process occurred at two different locations on the overhead line bushing. The first location was the junction between the upper and lower part of the bushing and the second was the junction between the lower part and the metal base of the bushing (see Figure 3- 3).

The insulation deterioration process that occurred at the junction between the upper and lower part of the bushing can be reconstructed as follows. The deposition of charges on the surface of the insulator reduced the discharge inception voltage [22,27]. Owing to the fact that the upper and lower part had not been assembled in a secure manner, the field distribution was affected and

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this gave rise to partial discharge activity. As a result, electrical treeing occurred on the surface of the junction.

The insulation deterioration process that occurred at the junction between the lower part of the bushing and its metal base can be reconstructed as follows. The physical shape of the metal base caused rain water to evaporate slowly. This resulted in a humid environment that was conducive to oxidation of the metal base. As a result, observable corrosion tracks formed in both the radial and axial direction of the metal base. In the axial direction, the corrosion created penetrable paths for gas leakage and water vapour ingress. The thermal energy released by such a chemical reaction appears to be the cause of the of the human skin irritant and white powder compound encountered on the O-Ring (a further explanation of this subject can be found in Appendix B). From the initial to the advanced stages, these insulation deterioration processes occurred over a period of at least ten years.

Failure mode 2: The initiation of the breakdown of the insulating gas.

The initiation of the breakdown of the insulating gas can be reconstructed as follows. The breakdown strength of the insulating gas inside the bushing had already been lowered owing to a loss of gas insulation mass and abundant surface charge, particularly at the junctions of the bushing. The operational record showed that a lightning strike had occurred. The presence of the impulse voltage raised the number of existing surface charges [27]. The rapid increase in charges significantly reduced the flashover voltage. When the flashover occurred, the accompanying thermal energy was released through the weakest point, i.e. the junction between the upper and lower part of the bushing.

In the case of the failure of the overhead line bushing, the root causes were the imperfect assembly of the bushing components owing to poor workmanship and the humid environment. The lightning strike initiated the breakdown of the insulating gas. The failure was influenced by external factors. Failure mitigation

The degraded components comprised the metal base of the bushing and the insulating gas. Indicators of such degradation are a decrease in gas quality and partial discharge activity. Therefore, in order to avoid similar failures, the following actions should be undertaken:

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2. improving and implementing the quality control procedure with regard to commissioning

A further discussion on how lightning impulses affect the breakdown strength of the insulating gas while partial dischargeis active can be found in Section 5.11.

3.1.3 Outer connecting joint (flange)

Figure 3- 5: Position of seal (O-Ring) on the flanges. Left: cross-sectional view of flange, with black circles representing seals (O-Rings). Right: axial sectional view of the flange.

The misplaced O-Ring is deformed. Event description

The failure of the outer connecting joint, i.e. flange (see Figure 3- 5) manifested itself in the form of gas leakage and moisture ingress. Three failure events occurred in relation to this component, which we will refer to as Events A, B and C. The forensic investigations, historical data, potential failure modes, key influencing factors and failure mitigation for each of failure events are described as follows.

Event A

The collected forensic data reports the presence of deformed seals. This data indicates the occurrence of a mechanical stress in a late stage.

Historical data

Historical data relating to the failure mode is not available. However, it was revealed that the moisture content was above the maximum allowable value. Potential failure modes

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was improperly placed on its column and was not tightened using silica gel. As soon as the flanges’ bolts were tightened, part of the seal was weighed down by the flanges. As a result, the flanges were not properly tightened, creating points susceptible to moisture ingress and gas leakage.

The root cause is human error/poor workmanship. Thus, in this case, the failure is influenced by an external factor.

Failure mitigation

The degraded components comprised seals and insulating gas, the indicators being gas leakage and a high moisture content. However, in order to avoid similar failure, an improved quality control procedure should be drawn up and implemented with regard to commissioning.

Event B

The collected forensic data indicated the presence of aged seals. This data indicates the occurrence of thermal and possibly chemical stress at an advanced stage.

Historical data

The historical data revealed the following situation: 1. the seals had been in use for over 10 years

2. O-Ring column on metal-base of bushing was corroded (Figure 3- 4c) 3. a human skin irritant compound was encountered

4. white powder covered part of surface of the O-Ring Potential failure modes and key influencing factors

In order to determine the failure modes and the root causes, samples were taken of the aged seal for further testing. These will be discussed in detail in Appendix B.

Event C

The collected forensic data reports that a single O-Ring was applied by design. Ideally, two layers should be applied. The application of a single O-Ring means there is a high possibility of water vapour ingress, particularly under humid conditions.