ACS OUTDOOR CIRCUIT BREAKERS
Fleet Strategy
Document TP.FS.51.01 October 2013
ACS OUTDOOR CIRCUIT Breakers Fleet Strategy © Transpower New Zealand Limited 2013. All rights reserved.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
COPYRIGHT © 2013 TR ANSPOW ER NEW ZEAL AND LIMITED. ALL RIGHTS RESERVED This document is protected by copyright vested in Transpower New Zealand Limited (‘Transpower’). No part of the document may be reproduced or transmitted in any form by any means including, without limitation, electronic, photocopying, recording or otherwise, without the prior written permission of Transpower. No information embodied in the documents which is not already in the public domain shall be communicated in any manner whatsoever to any third party without the prior written consent of Transpower. Any breach of the above obligations may be restrained by legal proceedings seeking remedies including injunctions, damages and costs.
ACS OUTDOOR CIRCUIT BREAKERS Fleet Strategy © Transpower New Zealand Limited 2013. All rights reserved.
ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
Table of Contents EXECUTIVE SUMMARY ...................................................................................................................... 1 SUMMARY OF STRATEGIES .............................................................................................................. 3 1
INTRODUCTION ....................................................................................................................... 4
1.1
Purpose ................................................................................................................................. 4
1.2
Scope .................................................................................................................................... 4
1.3
Stakeholders ......................................................................................................................... 4
1.4
Strategic Alignment ............................................................................................................... 5
1.5
Document Structure .............................................................................................................. 5
2
ASSET FLEET .......................................................................................................................... 7
2.1
Asset Statistics ...................................................................................................................... 8
2.2
Asset Characteristics .......................................................................................................... 12
2.3
Asset Performance .............................................................................................................. 19
3
OBJECTIVES .......................................................................................................................... 27
3.1
Safety .................................................................................................................................. 27
3.2
Service Performance ........................................................................................................... 27
3.3
Cost Performance ............................................................................................................... 28
3.4
New Zealand Communities ................................................................................................. 28
3.5
Asset Management Capability ............................................................................................ 29
4
STRATEGIES.......................................................................................................................... 31
4.1
Planning .............................................................................................................................. 31
4.2
Delivery ............................................................................................................................... 39
4.3
Operation............................................................................................................................. 42
4.4
Maintenance ........................................................................................................................ 43
4.5
Disposal and Divestment .................................................................................................... 48
4.6
Asset Management Capability ............................................................................................ 49
4.7
Summary of RCP2 Fleet Strategies .................................................................................... 52
APPENDICES ..................................................................................................................................... 54 A.
CIRCUIT BREAKER PHOTOS ............................................................................................... 55
B.
ADDITIONAL STATISTICS ..................................................................................................... 57
C.
LEAK-PRONE CIRCUIT BREAKERS – DETAILED REVIEW ................................................ 61
ACS OUTDOOR CIRCUIT BREAKERS Fleet Strategy © Transpower New Zealand Limited 2013. All rights reserved.
ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
EXECUTIVE SUMMARY Introduction Circuit breakers are essential to the safe operation of the transmission network. They are used to rapidly disconnect electrical equipment from the Grid. The correct operation of circuit breakers is vitally important for limiting safety risk to employees and members of the public. Correct operation of circuit breakers during faults also helps ensure reliability of supply for customers, by rapidly disconnecting faulty equipment and limiting the impact of any faults to the smallest possible section of the Grid. Our asset management approach for outdoor HV circuit breakers seeks to achieve the highest standards of reliability, and to minimise whole-of-life cost. Asset fleet and condition assessment There are about 1,200 outdoor circuit breakers rated at 50 kV and above in service on the Grid.1 Ninety percent of the fleet are SF6 gas interrupter types, and the vast majority of these are less than 30 years old. SF6 circuit breakers are the internationally preferred standard solution for outdoor high-voltage circuit breakers. The remaining 10% of our fleet are legacy types of circuit breaker, using bulk oil and minimum oil interrupters. These circuit breakers are more than 35 years old. Outdoor circuit breakers that use SF6 interrupter technology require little preventive maintenance, and can achieve very high levels of availability. In contrast, the legacy bulk oil and minimum oil types are maintenance intensive, and must be removed from service for special maintenance after clearing heavy system faults. There is a history of explosive failures of some minimum oil circuit breakers, and maintenance can be problematic. Although SF6 circuit breakers require little maintenance, the overall operational performance of some models of SF6 circuit breakers has not met expectations. Some models have proven to be highly vulnerable to corrosion in the New Zealand environment. Corrosion of sealing surfaces has caused SF6 gas leaks to develop, requiring the circuit breaker to be removed from service at short notice for topping up gas levels. Further outages have been required to undertake repairs. SF6 gas is a potent greenhouse gas, with around 23,000 times the effect of carbon dioxide. We are committed to minimising our emissions of SF6, and ensuring that annual emissions are kept to less than 1% of our total SF6 inventory. Circuit breaker leaks and emissions from handling gas associated with outdoor circuit breakers make up around one third of our total carbon footprint. The problem of gas leaks from some models of SF6 circuit breakers, and the associated requirement for outages at short notice, has had a major effect on the operational performance of the fleet as a whole, and caused increased risk for customers. International benchmarking studies reveal that the reliability of our circuit breakers is well below international norms, particularly at 110 kV. We would need to cut the number of forced and fault outages in half, to be average in international terms, and reduce our outages to around one quarter of the current rate to be among the best performers. 1
Outdoor circuit breakers at lower voltages are addressed in the Outdoor 33 kV Switchyard fleet strategy.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
The high rate of circuit breaker forced and fault outages leads to increased costs of operation and maintenance, and poses risks to the reliability of supply to customers. Outdoor circuit breaker strategies Our main strategies for outdoor circuit breakers for the RCP2 period focus on replacing three models of leak-prone SF6 circuit breakers, continuing to replace legacy oil circuit breakers, and replacing a small number of older SF6 circuit breakers that will have exceeded their forecast life expectancy by the end of RCP2. We will also seek to improve the reliability of the circuit breaker fleet through additional preventive maintenance to solve and prevent SF6 leaks. This is a continuation of the programme of leak prevention work being carried out over RCP1, although the programme has been refined to be more efficient in RCP2. Improvements In our planning for RCP2, we have made a number of improvements to the asset management of outdoor circuit breakers, including:
developing a preliminary asset health model
increasing our emphasis on proactive management of specific leak-prone models
focusing on improved monitoring and management of frequently operated units
using asset criticality to prioritise the replacement of circuit breakers
ensuring our planning decisions consider the whole-of-life cost of circuit breaker assets, covering planning, delivery, operations, maintenance and disposal, and their impacts on other assets, such as power transformers
using improved volumetric cost estimation to forecast expenditure.
Further improvements will include:
improving the initial asset health model and applying asset health data to allow better identification of circuit breakers for asset management action, such as replacement
using a forensic examination of dismantled circuit breakers to provide asset health information to support decision making about remaining populations.
These improvements have been incorporated in the strategies discussed in chapter 4.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
SUMMARY OF STRATEGIES This section provides a high-level summary of the main asset management strategies for the outdoor circuit breaker fleet over RCP2. All of the three main strategies summarised below relate to capital expenditure. Capital expenditure (Capex)
Replace Identified Leak-Prone SF6 Circuit Breakers
RCP2 Cost
$7m
We will proactively replace all instances of three models of leak-prone SF6 circuit breakers with new SF6 circuit breakers. The replacement programme focuses on models of circuit breaker with a history of leaks, and where it has been shown that repairs or refurbishments are uneconomic. Proactive replacement of these leak-prone circuit breakers will lead to a reduced rate of forced and fault outages and reduced greenhouse gas emissions. Over the RCP2 period, we plan to replace 56 leak-prone SF6 circuit breakers, at an estimated cost of $7m.
Replace Legacy Circuit Breakers
RCP2 Cost
$5.4m
We will replace or divest nearly all minimum oil circuit breakers by 2015 (that is, before RCP2) and the majority of bulk oil circuit breakers by 2020. This will lead to reduced diversity of our circuit breaker fleet, reduced maintenance requirements, increased availability and improvements in safety. Over the RCP2 period, this strategy will result in the replacement 47 bulk oil circuit breakers, at an estimated cost of $5.4m during RCP2. The replacement of five of these circuit breakers will span RCP2 and RCP3.
Replace Selected Older SF6 Circuit Breakers
RCP2 Cost
$3.5m
We will proactively replace a portion of the older SF6 circuit breakers. These circuit breakers, installed between 1981 and 1984, will have exceeded their forecast life expectancy by the end of RCP2. The first instances of each of the main models of circuit breaker to be replaced under this strategy will be subjected to detailed forensic examination, to provide asset health information that will assist in prioritising the replacement of the remaining ageing circuit breaker models. Over the RCP2 period, we plan to replace 30 circuit breakers that have exceeded their forecast life expectancy, at an estimated cost of $3.5m during RCP2. The replacement of nine of these circuit breakers will span RCP2 and RCP3. Chapter 4 has further details on these strategies and a discussion of the remaining strategies.
ACS OUTDOOR CIRCUIT BREAKERS Fleet Strategy © Transpower New Zealand Limited 2013. All rights reserved.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
1
INTRODUCTION Chapter 1 introduces the purpose, scope, stakeholders and strategic alignment of the Outdoor Circuit Breaker Fleet Strategy.
1.1
Purpose We plan, build, maintain and operate New Zealand’s high-voltage (HV) electricity transmission network (‘Grid’). This includes outdoor circuit breakers, which are critically important safety components of the Grid. The purpose of this fleet strategy is to describe our approach to lifecycle management of our outdoor circuit breaker fleet. This includes a description of the asset fleet, objectives for future performance and strategies being adopted to achieve these objectives. The strategy sets the high-level direction for fleet asset management activities across the lifecycle of the asset fleet. These activities include Planning, Delivery, Operations, Maintenance, Disposal and Divestment. This document has been developed based on good practice guidance from internationally recognised sources, including BSI PAS 55:2008.
1.2
Scope The scope of this asset strategy includes the following outdoor circuit breaker types rated 50 kV and above, listed by interrupter type:
bulk oil
minimum oil
sulphur hexafluoride (SF6)
vacuum – currently on trial
carbon dioxide (CO2) – to be trialled in RCP2.
The HVDC converter transformer circuit breakers and filter bank circuit breakers are outside the scope of this fleet strategy; and are included in the HVDC fleet strategy.
1.3
Stakeholders Outdoor circuit breakers are critical components of the transmission system. Correct operation and maintenance of the outdoor circuit breaker fleet is essential to ensuring the safety of members of the public and employees, and minimising damage, in the event of a power system fault. Key stakeholders include:
relevant Transpower Groups: Grid Development, Performance and Projects
regulatory bodies: Commerce Commission, Electricity Authority, and the Environmental Protection Authority
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1.4
service providers
customers, including distribution network businesses and generators
landowners.
Strategic Alignment A good asset management system shows clear hierarchical connectivity or ‘line of sight’ between the high-level organisation policy and strategic plan, and the daily activities of managing the assets. This document forms part of that hierarchical connectivity by setting out our strategy for managing the outdoor circuit breaker fleet to deliver our overall Asset Management Strategy in support of our asset management policy. This fleet strategy directly informs the Outdoor Circuit Breaker Asset Management Plan. This hierarchical connectivity is represented graphically in Figure 1. It indicates where this fleet strategy and plan fit within our asset management system.
Figure 1: The Outdoor Circuit Breaker Fleet Strategy within our Asset Management Hierarchy
1.5
Document Structure The rest of this document is structured as follows. Chapter 2 provides an overview of the existing outdoor circuit breaker fleet including fleet statistics, characteristics and their historical performance.
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Chapter 3 sets out asset management related objectives for the fleet of outdoor circuit breakers. These objectives have been aligned with the corporate and asset management policies, and with higher-level asset management objectives and targets. Chapter 4 sets out the fleet specific strategies for the management of the fleet of outdoor circuit breakers. These strategies provide medium-term to long-term guidance and direction for asset management decisions and will support the achievement of the objectives in chapter 3. Appendices are included that provide further detailed information to supplement the fleet strategy.
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2
ASSET FLEET Chapter 2 provides a high-level description of the existing outdoor circuit breaker fleet, including:
Asset statistics: including population, diversity, age profile, and spares
Asset characteristics: including safety considerations, asset criticality, asset health, maintenance requirements and interaction with other assets
Asset performance: including reliability, safety and environmental, and risks and issues.
The circuit breaker fleet is particularly important because circuit breakers have a vitally important safety function to rapidly disconnect faulty electrical equipment during faults. This limits safety risk to employees and the public. They play an essential role in ensuring system reliability by rapidly disconnecting faulty equipment and limiting the impact to a small section of the Grid. In addition, they are used for operational purposes to control the flow of power around the system. Circuit breakers are generally classified according to the medium they use to extinguish the arc that occurs when the flow of current is interrupted. Outdoor circuit breakers fall into the following classes:
bulk oil: the interruption takes place in an oil medium
minimum oil: similar to bulk oil circuit breakers, but use a relatively small oil container that is insulated from ground
SF6: the interruption takes place in a sulphur hexafluoride (SF6) gas medium
vacuum: the interruption takes place in a vacuum2
CO2: similar to SF6 circuit breakers, but use CO2 as the interrupting medium.3
A further sub-categorisation of circuit breakers involves the type of construction (or configuration). A circuit breaker is classified as ‘live tank’ (where the enclosure that contains the interrupter is at line potential) and ‘dead tank’ (where the enclosure is at earth potential). Appendix A includes photographic examples of the most common types of circuit breakers.
2 3
Vacuum-interrupted outdoor circuit breakers are currently on trial at 66 kV. While not currently deployed, CO2-interrupted circuit breakers will be trialled during RCP2.
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2.1
Asset Statistics This section describes the outdoor circuit breaker fleet population, fleet diversity and age profiles.
2.1.1 Asset Population As at 30 June 2013, we have about 1,200 HV outdoor circuit breakers in service rated at 50 kV and above. Table 1 shows the breakdown of this population by voltage and interrupter type. Type
50 kV–66 kV
110 kV
220 kV
Total
Bulk Oil
39
54
3
96
Minimum Oil
12
12
0
24
SF6
132
477
489
1,098
Total
183
543
492
1,218
Table 1: Number of HV outdoor circuit breakers – by type and voltage
The 110 kV and 220 kV circuit breakers constitute about 85% of the total HV outdoor circuit breakers.
2.1.2 Fleet Diversity Fleet diversity is an important asset management consideration. The diversity of the outdoor circuit breaker fleet is considered in terms of the interrupter type and in terms of the manufacturer and model. Diversity by interrupter type
The chart in Figure 2 shows the breakdown of circuit breakers by type. It shows that the fleet is predominantly SF6 based (90%). This reflects that currently all new circuit breakers purchased at 66 kV and above are SF6 based, except for trials of new technology. OUTDOOR CIRCUIT BREAKERS - DIVERSITY BULK OIL (8%) MINIMUM OIL (2%)
SF6 (90%)
Figure 2: HV outdoor circuit breaker diversity of interrupter type
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An international comparison of the diversity of circuit breaker fleets is included in the 2011 report of the International Transmission Operations & Maintenance Study (ITOMS).4 The report shows that:
some transmission systems still include air blast circuit breakers (we have none of this type)
live tank gas circuit breakers are the most common
oil circuit breakers are still reasonably common at voltages between 60 kV and 99 kV internationally, which relate to our 66 kV standard voltage.
Diversity by manufacturer/model
The population of outdoor circuit breakers includes some 70 families of circuit breakers, from 23 different manufacturers. Each family requires a different asset management approach, and the relatively large number of families requires a large selection of spares and increases the difficulties in maintaining the skills of service providers. In addition to the main types of outdoor circuit breakers, there are small populations of specialised circuit breakers (for the traction supply sites and some capacitor banks), and a growing population of disconnecting circuit breakers. While diversity of the outdoor circuit breaker fleet is not a significant cause for concern, it does increase the difficulty and cost of applying more advanced failure modes and effects analysis and reliability-centred maintenance because of the small population of each model and the need to apply these techniques differently to each model. We are working to reduce the fleet diversity, resulting from 30–40 years of technology changes and varying procurement approaches. Current procurement practice for new and replacement HV outdoor circuit breakers is based on standard designs and preferred supplier agreements. This is further discussed in subsection 4.2.2.
2.1.3 Age Profile Outdoor HV circuit breakers have been installed progressively, with particular periods of Grid development in the 1930s and the 1950s to 1980s. There has also been ongoing replacement and upgrading of assets throughout this time, driven by asset condition and depending on the life expectancy of various circuit breaker types. In the early 1990s we adopted a strategy to progressively replace a range of 110 kV and 220 kV outdoor circuit breakers that we found had systemic problems. These problematic circuit breakers were replaced with new SF6 technology.
4
ITOMS is an ongoing 2-yearly benchmarking study of the operational performance and maintenance costs of 27 transmission networks from around the world. Further information on ITOMS is provided in subsection 2.3.2.
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OUTDOOR CIRCUIT BREAKERS - AGE PROFILE BULK OIL
MINIMUM OIL
SF6
80 70 60
50 40
30 20
10 0 0
5
10
15
20
25
30
35
40
45
50+
AGE (YEARS)
Figure 3: Age profile of outdoor circuit breakers
The chart in Figure 3 shows that almost all younger assets (assets under 30 years of age) are SF6. The small number of minimum oil types and bulk oil types that currently remain in service are older assets. We expect to replace these completely by 2025. The chart from the ITOMS 2011 study (see Figure 4) shows an international comparison of average circuit breaker age. The average age of our HV circuit breakers is near the average of the participants. We are represented by the letter ‘G’.
Figure 4: International comparison of average circuit breaker age (ITOMS 2011)
Life expectancy Based on our experience, the life expectancies for the main circuit breaker types are set out below. Life expectancy is the nominal life established for fixed asset accounting purposes. It represents the typical average life that is expected from a type of equipment before it is no longer fit to remain in service. We have established the life expectancies in Table 2, based on our experience with those types of circuit breakers.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
Expected Life
Bulk oil
Minimum oil
SF6
45
40
35
Table 2: Expected circuit breaker life
While minimum oil and bulk oil circuit breakers have longer life expectancies than SF6, they have significant issues, as described in subsection 2.3.4.
2.1.4 Spares We categorise our spares as ‘operational spares’, ‘strategic unit spares’, and ‘manufacturer component spares’. Operational spares are spare assets that are kept on site and maintained in a suitable condition so that a failed asset can be replaced quickly. Strategic unit spares (also known as insurance spares) are complete spare assets that are kept at various North Island and South Island warehouses. These are intended to be deployed as replacements for failed assets, or to be used as substitutes when assets are taken out of service for long periods to enable refurbishment. Manufacturer component spares are spares of individual components that can be used to repair an asset. These are particularly important for components that are no longer manufactured or that have a long lead time for procurement. A number of strategic unit spare circuit breakers are kept to mitigate the risk of major failures and some manufacturer component spares are kept for minor failures/damage. The number of outdoor circuit breaker strategic unit spares is shown in Table 3. Voltage (kV)
Item
Spares Held
220
Live tank CB
3
220
Live tank CB (for cap bank)
1
110
Live tank CB
5
110
Dead tank CB
1
66
Live tank CB
1
66
Dead tank CB
4
Table 3: Number of strategic unit spare outdoor circuit breakers – by voltage – October 2012
At present, the level of spares holdings is based on past history of failure rates of HV outdoor circuit breakers. No provision is made for simultaneous failures of multiple circuit breakers (for example, as a result of a natural disaster) or unexpected increases in future failure rates (for example, a model-wide generic problem). Our current spares holdings are compared with the ITOMS participants in the chart in Figure 5, where we are represented by ‘G’. Our spares holdings can be seen to be generally comparable to the majority of participants in the study.
Figure 5: Number of spare outdoor circuit breakers for each 1,000 in service – October 2012 ACS OUTDOOR CIRCUIT BREAKERS Fleet Strategy © Transpower New Zealand Limited 2013. All rights reserved.
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2.2
Asset Characteristics The outdoor circuit breaker fleet can be characterised according to:
safety and environmental considerations
asset criticality
asset condition
asset health
maintenance requirements
interaction with other assets
emerging technologies.
These characteristics and the associated risks are discussed in the following subsections.
2.2.1 Safety and Environmental Considerations We are committed to ensuring that safety and environmental risks are minimised at all times. These risks are a key consideration in our asset management approach. Safety concerns Circuit breakers are important safety components of the transmission system. They are expected to remove items of plant from service quickly after a fault is detected and to minimise any potential equipment damage or safety risk to personnel and the public. Although the power system usually has some form of backup protection in the event that a circuit breaker fails to trip when required, any such failure significantly increases safety risk to personnel and the public. Major failures of circuit breakers can lead to serious safety consequences. Circuit breaker failures can lead to explosions, which may result in safety hazards to personnel, damage to adjacent equipment from flying porcelain, and fire. The replacement programme (see subsection 4.1.2) will continue to move the fleet towards a modern fleet of SF6 circuit breakers with composite insulators, to reduce the safety risks of major failures. The interruption process that occurs within circuit breakers produces products of decomposition in the interrupting medium. These can be toxic or harmful to employees undertaking invasive maintenance of circuit breakers. SF6 gas is not toxic to humans in its pure state, but can cause asphyxiation if it displaces sufficient oxygen. We require our service providers to be suitably trained and competent so that they can carry out work in a safe manner (see subsection 4.6.3). Environmental concerns SF6
At present, all new HV outdoor circuit breakers that we purchase for standard applications use SF6 as the interrupting medium. About 90% of the existing fleet of outdoor circuit breakers are also of an SF6 interrupter type. SF6 gas has excellent dielectric properties which make it the preferred type of circuit breaker. SF6 is also non-flammable and has minimal direct effects on the local environment.
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However, SF6 is a potent greenhouse gas with around 23,000 times the effect of carbon dioxide, and care must be taken to minimise emissions to reduce our climate change contribution. Our SF6 emissions will also incur carbon credit liabilities under the Emission Trading Scheme. Such emissions may occur when installing or decommissioning SF6 filled plant or from leaks or major failures that occur during its in-service life. We have undertaken to limit our emissions of SF6 to the environment (see subsection 2.3.3). No manufacturer guarantees a 100% leak-proof SF6 HV outdoor circuit breaker, although the rate quoted for ‘normal’ service is claimed to be less than 0.5% of nameplate mass each year. Periodic testing of the gas results in the loss of a very small volume of SF6, typically less than 10g during each test if the test is carried out correctly. So, even the best SF6 circuit breaker with no leaks will require a top-up during its life so as to retain the required SF6 working pressure. Oil
Bulk oil circuit breakers contain significant quantities of oil (up to 6,000 litres) and, in some cases, are installed in oil containment areas. Minimum oil circuit breakers contain a few hundred litres of oil and are not enclosed within an oil containment area. The oil is classed as an environmental hazard and any significant spills into the environment must be reported to the local authority and may result in fines under the Resource Management Act 1991. Older oil-filled circuit breakers can leak small amounts of oil due to ageing seals and gaskets. Regular maintenance minimises this impact and most sites that have oil circuit breakers have stormwater systems designed to separate oil from stormwater. All sites with outdoor circuit breakers undergo contaminated land testing before significant earthworks or before the sites are decommissioned. Oil spill kits are held on site to enable small oil spills to be contained and cleaned up quickly.
2.2.2 Asset Criticality We have established a process to determine criticality based on consideration of the importance of the site or circuit in terms of the load carried, the level of reliability required by the relevant customer, constraints and effects that would be placed on the rest of the Grid in the event of a failure, and the level of redundancy. Based on these factors, we have established a framework for assigning asset criticality that classifies all assets as low, medium or high criticality. This approach is at an early stage of development and implementation, so it will continue to be refined and improved over RCP2. Further information on the asset criticality approach is provided in the document ‘Asset Risk Management – Criticality Framework’. Asset management is adapted to recognise the differing levels of criticality to avoid failures at critical sites and circuits and to allow less capital expenditure at sites that are less critical. The strategies in chapter 4 demonstrate how criticality is considered in asset management, particularly through earlier or later replacement and more or less frequent maintenance. Figure 6 shows the criticality breakdown of the fleet and shows that the majority of outdoor circuit breakers are classified as low criticality and only a small proportion (8%) are classified as high criticality.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
OUTDOOR CIRCUIT BREAKER - CRITICALITY LOW (58%) MEDIUM (34%) HIGH (8%)
Figure 6: Asset criticality for HV outdoor circuit breakers
2.2.3 Asset Condition The condition of outdoor circuit breakers is regularly assessed as part of preventive maintenance. Condition assessments and inspections are used with other factors (such as age and model-wide problems) to assess circuit breaker health, which is the main driver of asset management decisions, including replacement timing. The overall condition of the outdoor circuit breakers is good, subject to the following specific issues. Gas leaks in SF6 circuit breakers A small number of SF6 circuit breakers have suffered significant gas leaks. Many of these leaks have been caused by corrosion of sealing surfaces and fittings. A considerable number, (up to 70% of models) have the potential to develop leaks, with some already demonstrating significant issues. Other international transmission network operators have reported similar issues with corrosion causing SF6 gas leaks, but, due to the more corrosive marine environment, the issues appear earlier in New Zealand. The majority of these leaks can be corrected through repairs, but in some cases refurbishment or replacement is required. Further detailed information on the condition of these leak-prone models is provided in Appendix C. Corrosion and deterioration of bulk oil circuit breakers Corrosion is also an issue for some bulk oil circuit breakers, although the extent is highly dependent on the site conditions. Deterioration of bushing insulation is a further problem with these circuit breakers, as described in subsection 2.3.4. Deterioration of minimum oil circuit breakers The models currently remaining in service show various degrees of age-related deterioration. The cost of replacement parts and expertise usually results in refurbishment being uneconomic.
2.2.4 Asset Health Asset Health Indices (AHI) is an asset management tool used to provide a systematic approach to prioritisation, based on a range of factors including asset condition. In our model, the health of an asset is expressed as a forecast of remaining useful life. We use the asset health model to make a prediction of the year when the asset will no longer be considered fit to remain in service. The AHI forecast of remaining useful life is based on modelling deterioration or risk that cannot be addressed by normal maintenance (where ACS OUTDOOR CIRCUIT BREAKERS Fleet Strategy © Transpower New Zealand Limited 2013. All rights reserved.
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maintenance to address the deterioration or risk is not possible or practical or is uneconomic). At this point major intervention is required, such as total replacement of the asset or refurbishment that significantly extends the original design life. Asset health indicators provide a proxy for the probability of failure in asset risk management analysis. Asset health indicators are also used in conjunction with asset criticality to assign priority within asset management planning processes. The AHI is calculated using factors including:
the current condition of the asset
the operating environment
the age of the asset (relative to expected life)
the typical degradation path of that model
any model/type or usage factors that affect the risk or rate of degradation, such as known defects or failure modes, or exceptional historic performance.
We are still at a relatively early stage in the development and application of AHI. More details on our asset health methodology are set out in the document ‘Asset Health Indices – Overview of approach’. The outdoor circuit breakers are allocated to AHI bands of ‘now due’, 0–2 years, 2–7 years, 7–12 years, and 12+ years. The AHI score is developed starting from an initial life expectancy of 35 years for SF6 circuit breakers, 45 years for bulk oil circuit breakers, and 40 years for other circuit breakers. If it is forecast that the number of circuit breaker operations will exceed the operation count limit for a particular circuit breaker type before the expiry of the initial life expectancy (such as in the case of frequently operated circuit breakers), then the time to reach the operation count limit takes precedence. In addition, the following rules are applied:
leak-prone SF6 models are ‘now due’, unless successfully refurbished
all minimum oil circuit breakers are ‘now due’
bulk oil circuit breakers have an AHI of 0–2 years if they are in a corrosion zone of severe or better, and are ‘now due’ if they are in a corrosion zone worse than severe
circuit breakers are ‘now due’ if they are more than 30 years old and are an orphan model (less than 5 units of that model left in service)
models with historical performance exceeding expectations are given an AHI of 7–12 years (that is, they will not be considered for replacement until after RCP2)
if multiple factors apply to a single circuit breaker, the circuit breaker is given the lowest AHI score out of those that should be applied to it.
The manner in which asset health is taken into account in managing assets is described in chapter 4.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
Figure 7: HV outdoor circuit breakers – Asset Health Indices (June 2013)
Figure 7 summarises the asset health of the outdoor circuit breaker fleet. It shows that while the majority are estimated by the asset health modelling to have a remaining life of 12 years or more, there are a number of assets that are now due for replacement. Based on AHI alone, over 200 circuit breakers (identified with remaining life of less than 8 years) would be assessed in detail for potential replacement during RCP1 and RCP2. Frequently operated outdoor circuit breakers Most transmission system circuit breakers operate infrequently, with less than 2,000 operations in their lifetime, and their asset health is therefore not affected by their operation count. Yet some circuit breakers, such as those used for switching shunt capacitor banks and HVDC harmonic filter banks, operate relatively frequently, up to several times a day. There are currently 31 HV outdoor SF6 circuit breakers, at 66 kV and above, at 12 sites, which are used for switching capacitor banks. It is likely that the number of capacitor and harmonic filter banks installed on the system will continue to increase over time. Many of the older (1970–1995) SF6 circuit breakers were designed and built with an expected life of 2,000 operations and will need to be refurbished or replaced when they exceed 2,000 operations. Recent circuit breaker purchases have specified that they be suitable for 10,000 operations in line with the latest International Electrotechnical Commission (IEC) standards so that they can be used at frequently operated sites.
2.2.5 Maintenance Requirements This subsection describes the maintenance requirements of the outdoor circuit breaker fleet. These requirements have informed the maintenance strategies discussed in section 4.4. The type of maintenance carried out on these assets is:
preventive maintenance, including: -
condition assessments and inspections
-
servicing
corrective maintenance, including: -
fault response
-
repairs
maintenance projects.
Maintenance projects typically consist of relatively high-value planned repairs or replacements of components of larger assets.
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The Maintenance Lifecycle Strategy provides further details on our approach to the above maintenance works, and the specific maintenance requirements are included in the relevant service specification documents. Preventive maintenance Condition assessment and inspections
The internal condition of circuit breakers cannot be readily observed and this presents a challenge in quantifying the failure risk. The best available condition assessment techniques only support inferences about actual circuit breaker condition, and standard techniques are unable to identify many factors that contribute to failure. Servicing
Minimum oil circuit breakers have proven to be a maintenance intensive design. They require invasive servicing after interruption of heavy fault currents. Due to the need for strict maintenance of high oil quality in their interrupters, and the complicated operating mechanisms of minimum oil circuit breakers, the service providers maintaining these circuit breakers need to have a particularly high level of competency to ensure reliability and avoid destructive failures. Bulk oil circuit breakers also require a high level of maintenance when compared with the SF6 circuit breakers because of the need to handle large amounts of oil (with associated environmental and safety risk), to carry out internal maintenance. Invasive servicing after interruption of heavy fault currents is also required for bulk oil circuit breakers. Corrective maintenance SF6 circuit breakers are vulnerable to gas leaks. These gas leaks can lead to operational and maintenance consequences. Some leaking SF6 circuit breakers can be topped up live, but others may require a planned or urgent unplanned outage to be topped up. A small number of circuit breakers have suffered significant leaks that developed quickly. SF6 gas leaks may arise from defects in seals and pipework. These leaks are mainly caused by crevice or galvanic corrosion due to unsuitable design, materials and/or assembly, and the effects of the harsh marine environment in many parts of New Zealand. Circuit breakers with leaks from one pole may ultimately develop leaks in the remaining poles. Some leaks require dismantling of circuit breaker poles and major refurbishment or replacement of one complete circuit breaker pole. In cases when several units of a particular model of circuit breaker begin leaking and the population is less than 10 years old, a systemwide programme of maintenance may be undertaken to refurbish all instances of that model (see below). This class of work has been carried out as maintenance projects in the past, and will be undertaken as preventive maintenance in RCP2 (see subsection 4.4.1). Maintenance projects Maintenance projects typically consist of relatively high-value planned repairs or replacements of components of larger assets. Maintenance projects are not expected to increase the original design life of the larger assets. Maintenance jobs are typically run as a project where there are operational and financial efficiencies from doing so. Maintenance projects are usually planned at least 12 months in advance, and are often part of a long-term strategy for a particular fleet of assets. Maintenance projects are included in
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
the Integrated Works Planning (IWP) process and are supported by individual business cases. The drivers for maintenance projects include asset condition, safety and environmental risks, and performance. A number of maintenance projects have been undertaken for the circuit breaker fleet, including:
replacing leaking SF6 poles on ABB HPL 123/25C1 circuit breakers
replacing leaking SF6 poles on AEG S1 72.5 & 145 circuit breakers
replacing corroded SF6 pipework on a variety of models (including Siemens 3AP1 and 3AQ1 circuit breakers)
modifying Sprecher HGF series SF6 circuit breakers to enable live top-up
inspecting and replacing mechanism couplings on AEG S1-145 circuit breakers
inspecting and repairing leaking SF6 pipe to pole couplings on AEG S1 and GL312 circuit breakers
replacing leaking or UV damaged SF6 pressure gauges on ABB HPL circuit breakers.
The expenditure on these works has averaged around $180,000 each year over the past 5 years. No maintenance projects are planned for RCP2, although some significant programmes of work to reduce the incidence of SF6 leaks will be carried out as preventive maintenance.
2.2.6 Interaction with Other Assets The programme of circuit breaker replacement is closely aligned with current transformer replacements, as the expected asset life of modern circuit breakers and current transformers are the same. Some current transformers are mounted on a common support structure with the circuit breaker, so they need to be replaced when the circuit breaker is replaced. Circuit breakers such as bulk oil and SF6 dead tank have current transformers installed on the circuit breaker bushings. We also align circuit breaker replacements with secondary assets, disconnectors and power transformers because they are all closely related and, in some instances, will require the same outages for work to be undertaken. The IWP process allows for coordination to minimise disruption and reduce costs. In line with the IWP process, a number of freestanding current transformer replacements are aligned with circuit breaker replacements.
2.2.7 Emerging Technologies Continuous developments are being made in electricity transmission equipment. These developments provide opportunities to improve service to customers. To capture these opportunities without accepting undue risk, we undertake a conservative approach to new technology by developing a thorough understanding of the technology, including its benefits and risks, and testing the technology on the Grid. The following subsections discuss new technologies that we are appraising for deployment within the outdoor circuit breaker fleet. Disconnecting circuit breakers Recently, new design requirements for maintenance clearances have been introduced to improve the safety of personnel working on equipment in substations. These new design requirements can be difficult to implement when replacing or adding extra circuit breakers, ACS OUTDOOR CIRCUIT BREAKERS Fleet Strategy © Transpower New Zealand Limited 2013. All rights reserved.
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particularly bus section circuit breakers, in existing substations. Disconnecting circuit breakers are now available that can be applied in these circumstances and have been successfully used to achieve modern safety clearances at existing sites. Two compact switchgear technologies are currently available: a ‘live tank’ design, and a ‘dead tank’ design. Both disconnecting switchgear designs incorporate one disconnector and earth switch within the overall circuit breaker assembly, resulting in a reduced footprint and improved maintenance clearances for retrofitting into existing structures. These disconnecting circuit breakers also have the advantage of requiring less maintenance, as the internal disconnector is fully contained within the SF6 environment, and is maintained at the same frequency as the circuit breaker (typically every 8 years instead of every 4 years). Having the two functions (disconnecting and circuit breaking) could mean that failures would result in failures of both functions at the same time. So far, the disconnecting circuit breakers have performed well, although the current transformers on the dead tank disconnecting circuit breakers have had to be replaced as a result of a design and manufacturing defect. Details of our planned approach to the introduction of disconnecting circuit breakers are provided in subsection 4.2.1. Vacuum and CO2 circuit breakers The leading alternative technology to SF6 types at present is the vacuum interrupter circuit breaker, but this type is only currently available for use up to 145 kV. Vacuum circuit breakers at higher voltages are unlikely due to the dielectric properties of the vacuum. One circuit breaker manufacturer has recently launched CO2 circuit breakers, which are available for use up to 72.5 kV. We installed our first 66 kV vacuum circuit breaker at Islington Substation in July 2013. This circuit breaker will be used as a trial of the technology. The use of CO2 or vacuum circuit breakers would reduce our greenhouse gas emissions (by limiting the potential for SF6 leaks) and may reduce maintenance expenditure. Circuit breaker automated condition monitoring The availability of automated circuit breaker condition monitoring equipment has substantially increased in recent years. This equipment could provide advantages in more effective identification of circuit breakers for repair or replacement. This would support the optimisation of replacement planning, so improving reliability relative to the amount of expenditure. Yet the equipment is quite expensive to retrofit (about 40% of the cost of a new circuit breaker) or to have integrated into new circuit breakers, so it is unlikely that the equipment would be considered economic at a fleet-wide level. We are currently investigating whether the equipment would be worthwhile in specific circumstances, such as new, frequently operated circuit breakers, and at what level of asset health the monitor could be fitted. Also, we are investigating what additional condition monitoring information we could extract from our SCADA systems and protection relays.
2.3
Asset Performance This section describes the reliability, safety and environmental performance of the outdoor circuit breaker fleet, together with a summary of major risks and issues.
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2.3.1 Reliability Performance Achieving an appropriate level of reliability for our asset fleets is a key objective as it directly affects the services received by customers. Reliability is measured primarily by the frequency and length of outages. A high level of reliability is required for outdoor circuit breakers given the potential for circuit breaker failure to result in widespread disconnection of other equipment, often leading to loss of supply. Major failures The consequence of a major failure (a large incident that cannot be easily repaired) may include an explosion that is very hazardous to any personnel nearby and may damage adjacent equipment. There have been few major failures of outdoor circuit breakers over the past 20 years. The observed major failure rate over the past 20 years is less than 0.05%, which is considerably less than the Australian experience. The low rate of major failures has been attributed to the circuit breaker replacement programme of the early 1990s. Forced and fault outage performance Fault outages are the result of protection equipment removing an asset from service. Forced outages are manual removals from service with less than 24 hours’ notice. The chart in Figure 8 shows the forced and fault outages of outdoor circuit breakers by cause. OUTDOOR CB - FORCED AND FAULT OUTAGES LEAKING AIR OR GAS OIL RELATED
INCORRECT OPERATION OTHER
70 60 50 40 30 20 10
0 2003
2004
2005
2006
2007
2008
2009
2010
2011
Figure 8: Outdoor circuit breakers – forced and fault outages
2012
5
The major cause of circuit breaker outages are SF6 leaks. Other less common causes of circuit breaker outages include incorrect operation or oil-related incidents. The level of outages is relatively high, with circuit breakers being the largest source of equipment failure outages across the Grid. 5
Historic results for ‘air or gas leaks’ include outages related to air-blast units that have since been removed from the system.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
The chart in Figure 9 illustrates the causes of circuit breaker forced and fault outages, across all types of circuit breaker, for reporting years 2003 to 2012. OUTDOOR CB - REASON FOR OUTAGE LEAKING AIR OR GAS (71%) INCORRECT OPERATION (8%) OIL RELATED (10%) OTHER (11%)
Figure 9: Forced and fault outages by faulted items
From Figure 9, we can see that reducing the number of forced outages resulting from SF6 leaks would result in a marked improvement in the overall performance of SF6 circuit breakers. Between 2008 and 2012, 78% of the forced and fault outages in the circuit breaker fleet were due to gas leaks in SF6 circuit breakers. The issue of SF6 gas leaks is described in subsection 2.3.4. Many older circuit breakers with serious generic defects have been replaced. The most common failure modes are now related to normal ageing and deterioration caused by the combined effects of design weaknesses and environmental conditions. Minimum oil circuit breakers have limited capacitive current rating and their design makes them vulnerable to restrikes and evolving faults when interrupting line charging current and when breaking transformer magnetising current. Yet their reliability is reasonable if they are maintained regularly by knowledgeable personnel. Bulk oil circuit breakers are a simple, robust design and are generally reliable. Overall, although the reliability of the remaining bulk oil circuit breakers is currently high, their availability is less than that of SF6 circuit breakers because of the servicing requirements.
2.3.2 Cost and Performance Benchmarking We have been involved in the International Transmission Operations & Maintenance Study (ITOMS) since 1994. This study involves performance and maintenance cost comparisons (including reliability) between 27 transmission utilities from North America, Europe, Asia, Australia and New Zealand. International benchmarking provides the opportunity to identify opportunities for improvement. The ITOMS results below provide a good general indication of our performance and maintenance costs compared to overseas transmission networks.6 However, extensive reliance on the detail of the ITOMS results should be avoided because the ITOMS report uses various simplified methods to adjust data for benchmarking comparison. There are also some factors that differ between countries that are not adjusted for, such as proximity of the assets to drivers of corrosion.
6
Further detailed ITOMS charts are provided in Appendix B.
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Reliability performance Based on forced and fault outage rates from charts in the ITOMS 2011 results, our forced and fault outage rate per 100 circuit breakers are shown in Table 4 against the benchmark average forced and fault outage rates. We would need to reduce our present 110 kV and 66 kV forced and fault outage rates considerably to rate as an ‘average’ performer. Outages per 100 items each year
ITOMS 2011 Average
Transpower
220 kV
2.5
3
110 kV
3.5
5.7
66 kV
1
4.2
Table 4: Forced and fault outage rate against benchmark average (approximate)
The main cause of our poor performance in the 2011 benchmarking study was SF6 leaks which were responsible for 65% of the 40 forced outages. Actively managing and eliminating the SF6 leaks will reduce our failure rate much closer to the average. The remaining forced outages were mainly due to mechanical faults. Appendix B includes further reliability performance benchmarking results. Circuit breakers performance summary This subsection sets out a breakdown of our overall performance. Appendix B includes detailed further details on the results. Overall, we are in the ‘low cost, poor service level’ category in all cases (66 kV, 110 kV, and 220 kV). It is worth noting that our performance has declined since the 2009 study. However, this study showed an improvement on the 2007 study. 220 kV circuit breakers
Our overall performance and maintenance costs are slightly below average compared to the other ITOMS participants. Our forced and fault outages need to be reduced by about one sixth to move to the average. Our position in the 2011 ITOMS benchmarking is a relative improvement in performance since the 2009 study. 110 kV circuit breakers
Our rate of forced and fault outages is unacceptable when compared to the other participants in the ITOMS 2011 study, because of the numerous outages required for SF6 top-ups. Compared to other participants in the study, maintenance costs are about average. Our forced and fault outages would need to be reduced by about 40% to achieve average performance in this study, although there has been a relative improvement from the 2009 study. We expect to achieve a significant improvement in the forced and fault outage rates over RCP2 due to the replacement and repair of leak-prone SF6 circuit breakers. Figure 10 shows that HPL 123 models are the main contributors to the high rates of forced outage of 110 kV circuit breakers. The defects with these models are described in detail in Appendix C. These models are targeted for total fleet-wide replacement during RCP2, as outlined in subsection 4.1.2.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013
Figure 10: 110 kV circuit breaker forced outages by model
66 kV circuit breakers
Our service level is particularly poor in the 2011 study, although the small population and sparse data make comparisons more subjective and variable. Compared to other participants, our costs are slightly below average, but the forced and fault outages would need to be reduced to a quarter of the current rate to achieve average performance.
2.3.3 Safety and Environmental Performance Subsection 2.2.1 describes the safety and environmental concerns with the circuit breaker fleet (and in particular the potential leakage of SF6 gas). This subsection reports on the actual safety and environmental performance of the fleet. Safety We are committed to providing employees and service providers with a safe and positive working environment, and minimising risks to the public. There is a single recorded injury associated with circuit breakers in recent years. However there have been a number of major failures which could have resulted in injuries. The low rate or absence of injuries reflects an absence of personnel on site during circuit breaker operations rather than an absence of safety risks. Accordingly, safety risk exposure from failure of circuit breakers remains a significant concern for us. Environmental SF6
Our total inventory of SF6 gas is about 41 tonnes, 43% of which is contained in outdoor circuit breakers. Overall, we have been consistently meeting our objectives for SF6 emissions, keeping overall emissions of SF6 below 1% of the installed inventory annually (about 0.7% in 2011 and 2012).
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In 2011/12 emissions of SF6 from outdoor circuit breakers caused by leaks and losses in handling and repairs represented about 32% of our total greenhouse gas emissions.
2.3.4 Risks and Issues This subsection briefly discusses the most significant risks and issues facing the asset management of the current population of circuit breakers. SF6 gas leaks Forced outages to top-up leaking SF6 circuit breakers are the predominant cause of the poor performance of the 110 kV circuit breakers against international benchmarks (see subsection 2.3.2). Corrosion of pipework, couplings, castings, and ageing of gas sealing O-rings can cause gas leaks, leading to emissions of SF6 to atmosphere. Moisture ingress into the circuit breaker as a result of leaks can lead to deterioration of the SF6 gas remaining in the circuit breaker. SF6 circuit breakers have a low gas pressure alarm where the circuit breaker can remain in full operation but will require topping up. When it reaches the next alarm (the lockout limit) the circuit breaker can remain in its current state and will withstand system voltage, but may be at risk during any lightning and switching impulses. It will not operate in the lockout state, and therefore cannot fulfil its intended safety and operational purposes. When the lockout alarm is raised, operators do not know if the circuit breaker has lost all of the SF6 gas, so it is treated as a circuit breaker in distress and is isolated through the switching of other circuit breakers and disconnectors. This reduces power system security, and may result in interruption of supply to customers. Therefore, a timely response to low pressure alarms is essential, to mitigate the risk of the pressure falling to the lockout level. When SF6 gas leaks to the atmosphere, moisture from the atmosphere will migrate back into the circuit breaker. The more the circuit breaker leaks, the more the air/moisture migrates in, reducing the overall electric strength of the SF6 gas. This reduction in electric strength can result in an internal flashover. This is another reason to deal with SF6 leaks promptly. Typically, we repair the leak within 3 to 5 top-ups. If the leak cannot be eliminated within the first 3 to 5 top-ups, the SF6 gas needs to be tested and recycled or replaced. Some leaking SF6 circuit breakers may have their gas levels topped up live, but often, for safety reasons, the circuit breaker is removed from service because of concerns that the gas pressure may drop below its minimum operating pressure when attaching the filling equipment. This has the potential to cause an internal fault within the circuit breaker, with severe safety consequences. We have a number of models that have design weaknesses that could result in a failure due to gas pressure drops. The outages are costly to arrange and implement, and can require maintenance staff to be diverted from other works. It also puts the system at increased risk, both from lowered security during the outage and from potential human errors during switching and the top-up operation itself. About 30% of the current fleet of circuit breakers require an outage for the top-up operation, and only a few of the remaining types are topped up live, because of safety concerns. If the leaks are noticed early enough, and the rate of loss is low, then the top-up can wait until a planned outage can be arranged. As discussed above, SF6 is a potent greenhouse gas which contributes to climate change. ACS OUTDOOR CIRCUIT BREAKERS Fleet Strategy © Transpower New Zealand Limited 2013. All rights reserved.
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Maintenance of bulk oil and minimum oil circuit breakers The maintenance of oil circuit breakers is becoming increasingly difficult, particularly because of the specialist work required and a reluctance of personnel to work with oil. Loss of skills from the industry may become a further factor leading to deterioration in the performance of older, maintenance-intensive equipment. Bulk oil circuit breakers have environmental, safety and health issues associated with the volumes of oil they contain - a typical 110 kV bulk oil circuit breaker contains more than 6,000 litres. Bulk oil circuit breakers require relatively frequent outages for maintenance of oil and contacts. Major servicing requires complete removal of the oil to temporary storage, and maintenance on the interrupter must be carried out within the confined space of the circuit breaker’s oil tank. Before entry into the tank it must be force ventilated to remove any explosive gases generated by the contact arc in the oil. Overall, the risks associated with maintenance and oil management of bulk oil circuit breakers are becoming increasingly difficult and costly to manage. While oil spills are very rare, they are more likely to occur during the removal or the filling of the circuit breakers compared to during normal operation. Oil spills from bulk oil circuit breakers can cause significant environmental damage if the oil is not contained. This could result in contaminating the immediate area and damaging aquatic ecosystems if the oil reaches a waterway. To address this risk, some circuit breaker installations are in bunded areas, so that any oil spill is contained. Oil spills are costly incidents, with extensive remediation required, as well as the possibility of fines under the Resource Management Act. Minimum oil circuit breakers have also proven to be a maintenance intensive design. Invasive servicing after interruption of heavy fault currents is a particular requirement for minimum oil circuit breakers. In general, they are vulnerable to deteriorating oil quality, and the maintenance required to maintain the oil quality and complicated mechanisms must be of a very high standard. Several serious incidents have occurred when maintenance has been inadequate. Diversity and maintenance skills required We are reliant on service providers to maintain, repair and replace circuit breakers. The diversity of our circuit breaker fleet requires a wide range of asset management approaches, specialist knowledge, and maintenance procedures. It also requires that our service providers have a wide skill base and that we hold a large number of component spares. Some standardisation has been implemented by purchasing circuit breakers from four preferred manufacturers. However, some fleet diversity is inevitable as the manufacturers improve designs, materials and manufacturing procedures. We may also change suppliers when period supply agreements are renewed, and we have occasional requirements for specialist circuit breakers with small populations. The diversity in the fleet is further complicated by the introduction of new technologies. In general, SF6 circuit breakers require very little preventive maintenance. As a consequence, our service providers have not developed the expertise necessary to fully maintain all the diverse types of circuit breaker. Turnover of staff within the service provider organisations adds to the difficulties of retaining appropriate expertise in circuit breaker maintenance.
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We have introduced specialist maintainers for undertaking major maintenance of outdoor circuit breakers, in response to the difficulties of retaining suitable maintenance expertise across all our service providers. Frequently operated circuit breakers Circuit breaker life is related to operational duty and environmental conditions. Most outdoor circuit breakers operate infrequently. An expected lifetime is assigned for each type of circuit breaker, and most should achieve this service life before reaching condition-based replacement criteria. Circuit breaker operating mechanisms use large amounts of stored energy to achieve fast operating times. The impact of the operation places considerable stress on the mechanical parts. Frequently operated circuit breakers (up to several times a day), such as those switching shunt capacitor banks, can therefore be at risk from mechanism failure before the normal expected lifetime of the circuit breaker has expired due to accelerated mechanical wear and tear of the mechanisms. The life expectancy for frequently operated SF6 circuit breakers is around 20 years compared to 35 years for SF6 circuit breakers in normal situations. The risk of failure is managed through enhanced monitoring of frequently operated circuit breakers, such as time-travel tests, operational checks and visual inspections. Circuit breaker bushings One of the main weaknesses of bulk oil circuit breakers is age related deterioration of their bushing insulation due to moisture ingress, which can lead to explosions and fires. There have been a number of bushing failures on 66 kV bulk oil circuit breakers in the past. Refurbishment of bushings is no longer undertaken as the facilities are no longer available. Instead, defective bushings are replaced with spares if they are available or the whole circuit breaker is replaced with a modern equivalent if there are no suitable spares available.
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3
OBJECTIVES Chapter 3 sets out asset management related objectives for the outdoor circuit breaker asset fleet. As described in section 1.4, these objectives have been aligned with our corporate management objectives, and higher-level asset management objectives and targets as set out in the Asset Management Strategy. Our overarching vision for the outdoor circuit breaker fleet is to achieve sustainable performance consistent with customer requirements and to minimise whole-of-life costs. Further objectives have been defined in the following five areas:
Safety
Service performance
Cost performance
New Zealand communities
Asset management capability.
These objectives are set out below, while the strategies to achieve them are discussed in chapter 4.
3.1
Safety We are committed to becoming a leader in safety by achieving injury-free workplaces for our employees and to mitigating risks to the general public. Safety is a fundamental organisational value and we consider that all incidents are preventable. Recognising the reduced level of control we have in relation to public safety, we will take all practicable steps to ensure Grid assets do not present a risk of serious harm to any member of the public or significant damage to property. Safety Objectives for the Outdoor Circuit Breaker Fleet
3.2
-
Zero instances of circuit breakers failing to trip.
-
Zero fatalities and injuries while maintaining, repairing or installing circuit breakers, including the handling of contaminated oil and SF6.
-
Minimise risk of injuries from explosions by phasing out porcelain in favour of composite insulator materials for new circuit breakers.
Service Performance Ensuring appropriate levels of service performance is a key underlying objective for us. We have specified service performance in terms of Grid Performance (reliability) and Asset Performance (availability) in our Asset Management Strategy.
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Grid performance objectives state that a set of measures are to be met for Grid Exit Points (GXPs) based on the criticality of the connected load. In addition, asset performance objectives linked to system availability have been defined. These high-level objectives are supported by a number of fleet specific objectives, and we will work towards these being formally linked in the future. Service Performance Objectives for the Outdoor Circuit Breaker Fleet
3.3
-
10-year rolling average for major failures remains less than 0.05% each year. It is currently less than 0.05% each year.
-
Reduce the forced and fault outage rates of outdoor circuit breakers to less than the following by the end of RCP2: -
1.5% each year at 220 kV (2.1% in 2012)
-
1% each year at 110 kV (5.5% in 2012)
-
0.5% each year at 66 kV (1.9% in 2012).
Cost Performance Effective asset management requires optimising lifecycle asset costs while managing risks and maintaining performance. We are committed to implementing systems and decisionmaking processes that allow us to effectively manage the full lifecycle costs of our assets. We have defined cost performance objectives in our Asset Management Strategy, including a commitment to make asset management decisions that minimise whole-of-life costs for the asset fleet and for the transmission system overall. Cost Performance Objectives for the Outdoor Circuit Breaker Fleet -
3.4
Minimised lifecycle cost, including: -
improved procurement processes, considering maintenance costs as well as capital costs
-
extended warranties in place for new circuit breakers
-
increased use of disconnecting circuit breakers.
-
Optimised maintenance schedules, reflecting technology changes.
-
Further reduce model diversity to help reduce maintenance costs.
New Zealand Communities Asset management activities associated with the outdoor circuit breaker fleet have the potential to impact on both the environment and on the day-to-day lives of various stakeholders. Relationships with landowners and communities are of great importance to us and we are committed to using asset management approaches that protect the natural environment.
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New Zealand Communities Objectives for the Outdoor Circuit Breaker Fleet
3.5
-
Limit SF6 emissions to less than 1% of total inventory for each year.
-
Minimise oil spills to the external environment.
-
Minimise the future requirements for SF6 by exploring alternative circuit breaker designs (such as CO2 and vacuum).
-
Minimise risk of damage to third-party property from circuit breaker explosions.
Asset Management Capability We aim to be recognised as a leading asset management company. To achieve this, we have set out a number of maturity and capability related objectives. These objectives have been grouped under a number of processes and disciplines that include:
Risk Management
Asset Knowledge
Training and Competency
Continual Improvement and Innovation.
The rest of this section discusses objectives in these areas relevant to the outdoor circuit breaker fleet.
3.5.1 Risk Management Understanding and managing asset-related risk is essential to successful asset management. We currently use asset criticality and asset health as proxies for a fully modelled asset risk approach. Asset criticality is a key element of many asset management systems. We are currently at an early stage of developing and implementing the framework as we work towards formal and consistent integration of asset criticality into the asset management system. We have commenced this by prioritising fleet replacement expenditure programmes based on the criticality framework . Risk Management Objectives for the Outdoor Circuit Breaker Fleet -
Ongoing improvement of the circuit breaker asset health model, reflecting new insights from managing the fleet.
-
Asset criticality fully integrated into circuit breaker asset management.
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3.5.2 Asset Knowledge We are committed to ensuring that our asset knowledge standards are well defined to ensure good asset management decisions. Relevant asset knowledge comes from a variety of sources, including experience from assets on the Grid, and information from the manufacturers. This asset knowledge must be captured and recorded in such a way that it can be conveniently accessed. Asset Knowledge Objectives for the Outdoor Circuit Breaker Fleet -
Improved awareness of fleet condition, including: -
up-to-date, accurate, and easily-accessible asset information (including condition assessments, test results, performance, photographs, as-built drawings, and maintenance history)
-
understanding internal condition through pilot inspections of samples of large fleets
-
ongoing sharing of asset condition and performance information with international peers.
3.5.3 Training and Competency We are committed to developing and retaining the right mix of talented, competent and motivated staff to improve our asset management capability. Training and Competency Objective for the Outdoor Circuit Breaker Fleet -
Service providers suitably qualified, particularly for circuit breaker installation and internal refurbishments.
3.5.4 Continual Improvement and Innovation Continual improvement and innovation are important aspects of asset management. A large source of continual improvement initiatives will be ongoing learning from our asset management experience. Continual Improvement and Innovation Objectives for the Outdoor Circuit Breaker Fleet -
Awareness of emerging technologies, including CO2, vacuum, and intelligent circuit breakers.
-
Developed improved approach to SF6 circuit breaker leaks and top-ups.
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4
STRATEGIES Chapter 4 sets out the specific strategies for the management of the outdoor circuit breaker fleet. These strategies are designed to support the achievement of the objectives in chapter 3 and reflect the characteristics, issues and risks identified in chapter 2. The strategies are aligned with the lifecycle strategies below and the chapter has been drafted to be read in conjunction with them.
Planning Lifecycle Strategy
Delivery Lifecycle Strategy
Operations Lifecycle Strategy
Maintenance Lifecycle Strategy
Disposal Lifecycle Strategy.
This chapter also discusses personnel and service provider capability related strategies which cover asset knowledge, training and competence. Scope of strategies The strategies focus on expenditure that is to occur over the RCP2 period (2015–2020), but also include expenditure from 1 July 2013 to the start of the RCP2 period and some expenditure after the RCP2 period (where relevant). Capex planned for the RCP2 period is covered by the strategies in sections 4.1 and 4.2, and opex is covered by the strategies in sections 4.3 to 4.6. The majority of capex consists of asset replacements, as described in subsection 4.1.2.
4.1
Planning This section describes the strategies specific to the Planning lifecycle for the outdoor circuit breaker fleet, which support our objectives in chapter 3. Planning activities The planning lifecycle is primarily concerned with identifying the need to make capital investments in the asset fleet. The main type of investment considered for this fleet is replacement and refurbishment. We support the planning activities through a number of processes, including:
Integrated Works Planning (IWP)
cost estimation.
The planning strategies for these processes are described in the subsections below. Capital investment drivers Categories of capital investment generally have specific drivers or triggers that are derived from the state of the overall system or from individual assets. These drivers include demand growth, compliance with Grid reliability standards and failure risk (indicated by asset criticality and condition). ACS OUTDOOR CIRCUIT BREAKERS Fleet Strategy © Transpower New Zealand Limited 2013. All rights reserved.
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Specific examples that drive capital investment in outdoor circuit breakers include:
new substation developments or expansions
replacements of deteriorated, poor performing, unsafe or failed assets.
The strategies below consider the long-term implications for these drivers as we extend our planning horizon as part of our programme of asset management improvement.
4.1.1 Enhancement and Development The primary driver of enhancement and development for new outdoor circuit breakers is developing new substations or new bays at existing substations. The substation works are undertaken to meet expected system growth and to ensure appropriate reliability for customers. Standard circuit breaker current and fault ratings are usually sufficient to meet most system enhancement and development requirements. Network development projects
Plan for investment in new circuit breakers for network development projects as required. System growth projects mainly include new greenfield transmission lines or the uprating of existing lines, which may also require additional substations and other supporting equipment. New substations or substation extensions will generally involve the installation of new circuit breakers. While the number of large enhancement and development projects will decline during RCP2, a number of smaller projects are expected in the coming years. The costs of the new circuit breakers required for large system growth projects are not discussed in detail here as they are included in the documentation for each individual system growth enhancement and development project.
4.1.2 Replacement and Refurbishment Replacement is expenditure to replace substantially all of an asset. Refurbishment is expenditure on an asset that creates a material extension to the end of life of the asset. It does not improve its attributes. This is distinct from maintenance work, which is carried out to ensure that an asset is able to perform its designated function for its normal life expectancy. This subsection describes our replacement strategies for outdoor circuit breakers developed in support of the objectives in chapter 3. We have an ongoing programme to replace aged, deteriorated, and unreliable circuit breakers. Over the past two decades there has been a marked reduction in the extent of interruptions and un-served energy resulting from circuit breaker failures. Analysis of service performance history demonstrates the relationship between the circuit breaker replacement programme of the early 1990s and significant improvements in system reliability. The rationale for an ongoing programme of planned circuit breaker replacements includes:
it is not acceptable to manage the circuit breaker fleet on a ‘run-to-failure’ basis because circuit breakers have a critically important safety function in the transmission system
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non-invasive condition assessment cannot reliably detect all risks of major failure in individual circuit breakers or reliably predict remaining life
major dismantling, non-destructive testing of internal components and extensive repair of circuit breakers is generally uneconomic.
Based on these three factors, our view is that circuit breaker replacement should be proactive, and respond to leading indicators of risk and findings from forensic examination of failure incidents. The approach for outdoor circuit breaker replacement is summarised in the strategies below, covering:
models prone to significant SF6 gas leaks
models with legacy interrupter types (minimum oil and bulk oil)
ageing circuit breakers, other than those in the two previous strands
frequently operated circuit breakers reaching maximum operation limits.
The strategies for each of these four strands are presented below. Implemented together in RCP2, they are expected to improve the asset health of the fleet compared to a ‘do nothing’ scenario as illustrated in charts in Figure 11. OUTDOOR CB - ASSET HEALTH (PLAN - 2019/20)
OUTDOOR CB - ASSET HEALTH (DO NOTHING - 2019/20)
12 + YEARS (59%)
12 + YEARS (41%)
7 - 12 YEARS (11%)
7 - 12 YEARS (11%)
2 - 7 YEARS (22%)
2 - 7 YEARS (24%)
0 - 2 YEARS (2%)
0 - 2 YEARS (3%)
NOW DUE (6%)
NOW DUE (21%)
Figure 11: Outdoor circuit breaker fleet asset health forecast
Leak-prone SF6 circuit breaker replacement
Proactively replace selected models of leak-prone SF6 circuit breakers. In cases where full repair or re-conditioning of leak-prone SF6 models of circuit breakers is not cost-effective, all units of that model will be scheduled for replacement. Replacement of leak-prone models will be prioritised where:
more than one circuit breaker from the population of a model has shown significant leaks from a cause that requires invasive work to correct (for example, requiring the dismantling of poles to make repairs)
full repair and re-conditioning is not cost-effective
there is reason to suspect that the remaining circuit breakers of this model will ultimately suffer leaks from the same cause.
This strategy is of critical importance because the risk of significant SF6 leaks potentially affects 70% of the outdoor SF6 circuit breaker fleet. The issue of SF6 gas leaks is described in subsection 2.3.4.
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Some leaks originate from sources that require dismantling circuit breaker poles. Circuit breakers that have leaks from one pole are highly likely to develop leaks in the remaining poles. For some models, the nature of the defects means that the costs of undertaking comprehensive repairs are uneconomic compared with total replacement. The planned RCP2 replacement strategy for leak-prone models focuses on three models of ageing leak-prone circuit breakers with a history of pole replacements due to leaks. These circuit breaker models are some of the largest contributors to the forced outage rate and SF6 gas leaks. Further detailed information on the condition of these models is included in Appendix C. This follows on from the complete replacement of other leak-prone models in RCP1. Prioritisation The planned replacements are prioritised by taking into account asset criticality and potential for corrosion and whether or not one or more poles have already been replaced on a circuit breaker (and its history of top-ups). Experience has shown that corrosion of the circuit breaker sealing flanges takes place at a much greater rate at North Island sites than at inland South Island sites, and that the onset of gas leaks from this cause may be delayed by as much as 10 years between the two islands, as a result of different environmental conditions. Alternative to replacement Refurbishment of all three poles on a circuit breaker is typically uneconomic compared with total replacement. Additionally, the availability of parts and manufacturer support for obsolete circuit breaker models presents a challenge to carrying out any refurbishments or repairs. We also note that there is no warranty available for circuit breaker re-conditioning work, and that some attempts at repair and re-conditioning have been unsuccessful. In some cases pole refurbishment has been possible (see subsection 2.2.4). In other cases, we have been replacing leaking poles with spare refurbished poles (salvaged from replaced circuit breakers) on these models in the past. In some instances, circuit breakers have had to be replaced due to a lack of available refurbished poles. This reactive approach has led to a number of forced outages. Because of the difficulties and delays in obtaining planned outages necessary to replace leaking poles, this reactive approach has led to significant SF6 emissions and outages. Replacement volumes and cost It is planned to replace forty-eight 110 kV and eight 66 kV circuit breakers under this strategy during RCP2, which will entirely remove three leak-prone models from the fleet. The cost of this strategy has been forecasted using volumetric forecasting, which is described in subsection 4.1.4. The cost of replacing circuit breakers is estimated to be $127,000 for each 110 kV unit and $119,000 for each 66 kV unit. The total forecast cost of this strategy for RCP2 is about $7m during RCP2. Legacy circuit breakers
Replace or divest most minimum oil circuit breakers by 2015 and most bulk oil circuit breakers by 2020 with modern SF6 models.
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We plan to modernise the fleet by eliminating almost all minimum oil circuit breakers by 2015 and the majority of bulk oil circuit breakers by 2020. This programme is to be reviewed regularly based on condition assessment and performance history information. We are planning this proactive continuation of legacy technology replacement due to their poor asset health. The issues with our ageing minimum oil and bulk oil circuit breakers are described in subsections 2.2.1 and 2.3.4. If advantageous to do so, we will slow down the rate of replacing the bulk oil circuit breakers. Circuit breaker technology is evolving quickly at present, and delaying the replacement of existing 110 kV circuit breakers by about 2 years may be sufficient to enable them to be replaced with improved technology, such as vacuum or CO2 circuit breakers. While the high maintenance requirements of bulk oil circuit breakers are a strong driver for replacement, a suitable level of reliability can be maintained if replacement is delayed. Until such time as the last of the legacy circuit breaker types are decommissioned, minimum holdings of spare components will be retained for minor repairs and maintenance of minimum oil and bulk oil circuit breakers. Yet, should any of these types of circuit breaker suffer a major failure, it will be replaced with a modern SF6 circuit breaker and separate current transformers (where required), rather than be repaired or re-conditioned. Volumes and cost The schedule for replacing legacy circuit breakers is provided in Table 5 and includes replacing or divesting most of the remaining 24 minimum oil circuit breakers by 2015 and most of the 96 bulk oil circuit breakers by 2020. There will be no bulk oil or minimum oil circuit breakers left by the end of RCP3. The prioritisation of the circuit breakers will be adjusted by consideration of asset criticality. Replacements Type
Population RCP1
Minimum Oil Bulk Oil Total
24 8
96
8
120
7
Divestments RCP2
RCP3
16
0
3
5
13
47
16
13
29
47
19
18
Table 5: Legacy circuit breaker replacement schedule
The cost of this strategy has been forecasted using volumetric forecasting, which is described in subsection 4.1.4. The average cost of replacing circuit breakers is estimated to be $159,000 for each 220 kV unit, $127,000 for each 110 kV unit and $119,000 for each 66 kV unit. Based on these costs, the cost of replacing 47 bulk oil circuit breakers in RCP2 is forecast to be about $5.4m. There will also be some expenditure in RCP3 for 5 of the 47 circuit breaker replacements, for which the installation will span 2019/20 and 2020/21. Replace selected older SF6 circuit breakers
Proactively replace a portion of the older SF6 circuit breakers. 7 8
The number of replacements in this column refers to the rest of RCP1; that is, 2013/14 and 2014/15. This includes seven circuit breakers that will be replaced separately as part of the Wilton bus rationalisation enhancement and development strategy, which is described in the ACS Other Fleet Strategy.
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Priorities for replacing ageing circuit breakers reflect critical asset management objectives, including safety and the need for reliable operation, and take into account the following:
asset health and criticality
forced and fault outage rate of specific models, particularly where root cause of poor performance cannot be adequately addressed with a maintenance approach
risk of major failure and failure to operate, based on New Zealand and international experience with specific models
high defect rate and maintenance costs of specific models
where the root cause of forced outages and defects is related to environmental conditions, the priority of individual circuit breaker replacements can be adjusted based on the corrosion potential of the location.
This is a continuation of the RCP1 replacement strategy, although it has been further optimised with the use of the asset health model and the criticality framework. We will proactively replace a portion of the SF6 circuit breakers installed between 1981 and 1984. All these circuit breakers will have exceeded their forecast life expectancy by the end of the RCP2 period. The first instances of each of the main models of circuit breaker to be replaced will be subjected to detailed forensic examination, to provide asset health information that will assist in managing and prioritising the remaining population. Justification It is not acceptable to manage the circuit breaker fleet on a ‘run-to-failure’ basis, as they have a critically important safety function in the transmission system. Non-invasive condition assessment cannot reliably detect all risks of major failure in individual circuit breakers or reliably predict remaining life. Major dismantling, forensic examination and non-destructive testing of internal components of circuit breakers is generally uneconomic as a routine condition assessment technique. Yet where asset management decisions must be made about a relatively large population of a specific model, dismantling and forensic examination of samples of the population is a costeffective approach. So planned replacement and detailed examination of a proportion of the population of SF6 circuit breakers that have exceeded their forecast life expectancy is a cost-effective approach. Depending on the findings of the detailed examinations, the replacements of other instances of these models circuit breakers will be deferred or advanced. Volumes and cost The RCP2 forecast allows for the replacement of 30 SF6 circuit breakers that will have exceeded their forecast life expectancy during RCP2, and the first instance of each main model will be subjected to detailed invasive examination. Cost and scope estimation for circuit breaker replacements is an example of volumetric forecasting. Our approach to cost estimation is discussed further in subsection 4.1.4. The cost of replacing circuit breakers is estimated to be $159,000 for each 220 kV unit, $127,000 for each 110 kV unit and $119,000 for each 66 kV unit. Based on these costs, the cost of replacing the 30 selected SF6 circuit breakers that have exceeded their forecast life
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expectancy in RCP2 is forecast to be $3.5m. Replacement of nine of these circuit breakers will span RCP2 and RCP3, so some further expenditure will be incurred during RCP3. Frequently operated circuit breakers
Replace frequently operated circuit breakers based on AHI (adjusted for maximum operation count). Frequently operated circuit breakers will be considered for replacement based on the frequency of operation and on observable condition. Further information on the risks associated with frequently operated circuit breakers is provided in subsection 2.3.4. Frequently operated circuit breakers are only a small proportion of all outdoor circuit breakers on the system – about 2.5% at present. Yet they perform an important function, as the Grid cannot be maintained within the required operating parameters without capacitor and harmonic filter banks being switched in and out of service to control voltage and harmonics on the system. Reliable operation of these circuit breakers is therefore critical for system security. It is impractical and uneconomic to strip down and apply comprehensive non-destructive testing to survey the operating mechanisms of such circuit breakers. So a precautionary approach to replacement is followed, where circuit breakers are replaced when a predetermined number of operations have been reached. We will increase our monitoring of the condition of frequently operated circuit breakers once they reach 50% of the maximum recommended operation count. Also, to reduce the number of imminent replacements, we will review the costs and practicality of relocating high-operation circuit breakers that reach 75% of their maximum recommended operation count to less important sites were a lower number of operations of the circuit breaker will be required. Volumes and cost Based on current operation counts and the forecasts of future operations, we do not expect any circuit breakers to require replacement for a high operation count alone. However, some of the circuit breakers that are being replaced under other strategies, such as the legacy bulk oil circuit breakers, will likely have relatively high operation counts.
4.1.3 Integrated Works Planning Our capital governance process – IWP – includes the creation of business cases that track capital projects through three approval gates, with the scope and cost estimates becoming more accurate as the project becomes more refined. The IWP process integrates capex across a moving window of up to 10 years in the future. This optimisation approach seeks to ensure that works are deliverable and undertaken in an efficient and timely manner. Circuit breaker replacement timing
Schedule circuit breaker replacement to coincide with other large works. Through the IWP process, works are optimised into groupings that produce more efficient and coordinated delivery programmes that: ACS OUTDOOR CIRCUIT BREAKERS Fleet Strategy © Transpower New Zealand Limited 2013. All rights reserved.
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reflect site strategies
minimise the frequency that a particular asset or site needs to be interacted with
minimise the number of outages required
optimise overall resources required
integrate development, customer and replacement works.
Where possible, outdoor circuit breaker replacement works should be aligned with other works and, in particular, with current transformer replacement (as their expected lives are roughly equivalent). This can minimise disruption and reduce costs.
4.1.4 Cost Estimation Cost estimation is a key stage of the capital investment process and forms a critical input into projects at various stages in the planning process. Historically, cost estimates for outdoor circuit breakers were developed using proprietary systems. This has now transitioned to a central cost estimation team, which uses the cost estimation tool Transpower Enterprise Estimation System (TEES). TEES is used to make initial high-level cost estimates using volumetric forecasting. We have established positions of Project Engineer and Project Cost Engineer, to support the feedback loop of pricing for capital works. We aim to achieve P50 cost estimates. Further details on our cost estimation approach can be found in the Planning Lifecycle Strategy. About 50% of capital spending on circuit breaker replacement is the hardware cost of new circuit breakers. These are imported and thus exposed to exchange rate fluctuation. P50 project cost estimation
Scope and estimate project works to a P50 confidence level (the estimate is based on a 50% probability that the cost will not be exceeded). Subsection 4.1.2 provides details on the estimation approach used for replacement projects, including those based on volumetric cost estimates. Circuit breaker replacements are examples of volumetric works, as they are reasonably repetitive with largely similar scope. The key determinant of accurate cost estimates for volumetric capital projects is the effective feedback of cost out-turns from completed, equivalent circuit breaker replacement works. Volumetric estimates are determined using the TEES (US Cost) system. Tailored ‘building blocks’ have been developed for assets based on actual cost out-turns from completed, equivalent works. This feedback-based process is used to derive average unit costs for future works. A volumetric approach to estimating costs for circuit breakers will help ensure works are scoped to achieve a P50 level of confidence, where P50 is an estimate with a 50% probability that the cost will not be exceeded. Assumptions made using a volumetric costs methodology for outdoor circuit breakers include:
the volume of historic works is sufficiently large and a large number of equivalent projects will be undertaken in future
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4.2
cost building blocks based on historic out-turn costs capture the impact of past risks
scope is reasonably well defined and reflects the predetermined list of ‘standard building blocks’ in US Cost.
Delivery Once the planning activities are completed, capex projects move into the Delivery Lifecycle. Delivery activities are described in detail in the Delivery Lifecycle Strategy. The following discussion focuses on delivery issues that are specific to the outdoor circuit breaker fleet.
4.2.1 Design When applied to outdoor circuit breakers the design process includes the following five considerations.
Live tank SF6 HV outdoor circuit breakers are tall and slender, so have a high centre of gravity. This means that they must comply fully with the earthquake standards appropriate to the New Zealand environment, which are higher than earthquake standards in many other countries. We specify high seismic performance in procurement that meets the standard of IEEE 693 High.
It is critical that every effort is made to ensure that designs respond fully to the environmental conditions, to avoid the problems such as corrosion that can lead to SF6 gas leaks that cause increased maintenance costs and premature end of life. For example, our standard designs now include anodised flanges and tinned SF6 gas pipes to minimise the risk of leaks caused by corrosion.
Our circuit breakers will be of standard installation design.
Porcelain is a safety hazard in the event of circuit breaker explosions, so composite bushings are procured in preference over porcelain bushings.
The majority of forced and fault outages in the circuit breaker fleet are due to the outage requirements for unscheduled SF6 top-ups. We now specify that circuit breakers should have the ability to be safely topped-up live.
Safety by Design principles
Ensure that safety is explicitly built into all stages of the design process. We use design standards as a key input into the design process, to help mitigate safety, performance, and environmental and cost risks. For the design standards to ensure the safety of personnel and the general public, they must be kept up to date, integrating any new developments in technology, condition assessment and strategy. One example of Safety by Design for outdoor circuit breakers is adopting composite materials for standard circuit breakers specification rather than porcelain. Porcelain shells and other components made from porcelain can become sharp flying fragments following an explosive failure, while composite components do not.
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Minimise, as far as practical, the design diversity of circuit breaker installations. We will minimise the diversity of the installation design for outdoor circuit breakers, which will contribute to safety and cost performance objectives by enabling the re-use of proven standard designs and reducing the risks of customised designs. Disconnecting circuit breaker deployment
Deploy disconnecting circuit breakers in selected locations as more experience is gained with this technology. The application of disconnecting circuit breakers is an outcome from the innovation and technology work stream. Live tank and dead tank SF6 110 kV disconnecting circuit breakers will be deployed increasingly in new designs and in retrofit solutions. We will deploy these circuit breakers in selected locations to help build our experience and confidence in the reliability of the equipment. Typical situations where disconnecting circuit breakers will be considered include substation equipment bays, where both circuit breaker and disconnector require replacement within 5 years and have maintenance clearances that violate modern standards. In these situations, there are maintenance savings and procurement savings to be made by using disconnecting circuit breakers rather than trying to fit a conventional arrangement. Additional funding is not required for the use of disconnecting circuit breakers, as the costs are similar, and sometimes cheaper, than conventional arrangements due to a separate disconnector not being required. Employing disconnecting circuit breakers is also expected to produce an approximate net present value saving of up to $10,000 over the life of the equipment, due to the elimination of disconnector maintenance that is carried out every 4 years. Disconnecting circuit breakers can also result in improved maintenance clearances at sites. Trial of non-SF6 technology circuit breakers
Deploy non-SF6 technology circuit breakers in managed trials to gain experience and explore future potential. The application of non-SF6 circuit breakers is a further initiative from the Innovation and Technology work stream. Non-SF6 circuit breakers will be deployed in managed trials to gain experience and explore the potential for alternative technology to replace conventional SF6 circuit breakers. The trial programme will aim to deploy about 15 non-SF6 circuit breakers at non-critical locations where it is feasible to operate the circuit breakers for test purposes. The three Islington-Papanui 66 kV circuits have an N–2 security level. We installed our first 66 kV vacuum circuit breaker on one of these circuits in July 2013 as a trial of the technology. The circuit breakers on the other two circuits are prime candidates for additional trials.
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Non-SF6 switchgear trials will also be supplemented with new circuit breaker monitoring equipment devices to gather trial assessment data. As of mid-2013, non-SF6 live tank circuit breakers are up to 40% more expensive than an SF6 equivalent. On an international scale, we are an early adopter of this technology and it coincides with New Zealand’s early adoption of carbon taxes. As more international uptake of this equipment occurs, the prices are expected to become similar to regular live tank circuit breakers.
4.2.2 Procurement For more details of our general approach to procurement, see The Sourcing, Supply & Contracts Approach (2011) and the Delivery Lifecycle Strategy. Given the history of poor performance resulting from unsatisfactory design of outdoor circuit breakers, we have spent considerable effort trying to improve procurement practices to support reliability and safety objectives. This subsection provides more detail on the specific procurement strategies for outdoor circuit breaker assets. Asset fleet standardisation
Purchase live tank SF6 circuit breakers except where other types are required for specific applications. SF6 interrupter circuit breakers are a proven technology that has been used successfully on the Grid. Standardising procurement will limit diversity in the fleet, reduce spares holdings, ensure interchangeability of entire circuit breakers and components, and facilitate maintenance. Standardising on live tank SF6 circuit breakers rather than dead tank circuit breakers provides a better match with the existing fleet, and minimises the volumes of SF6 in the total inventory. Also, live tank circuit breakers are significantly easier to repair or refurbish compared with dead tank models because the design of dead tank circuit breakers makes insitu repair very difficult, and sometimes uneconomic. Increased standardisation will also improve the effectiveness of spares holdings. This strategy is dependent on the outcomes of the investigations into the CO2 and vacuum circuit breakers, which may become the preferred choice in certain instances. Composite bushings
Use composite bushings on new outdoor circuit breakers. We have decided to purchase only composite bushings on all future outdoor circuit breakers. This mitigates the dangers associated with explosion of porcelain bushings. Bushings will be tested to seismic performance level of 1g9 as per our standard. This strategy will support our safety objectives.
9
1g is peak ground acceleration equivalent to the acceleration due to gravity.
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Procure circuit breakers from the minimum possible number of vendors commensurate with the need to manage supplier risk. We will work to strengthen relationships with a limited group of vendors to ensure that the circuit breakers meet specific quality requirements for New Zealand conditions. Ideally, at any one time, there will be only one period contract supplier of live tank circuit breakers for a particular voltage class. Standardising on one vendor of live tank circuit breakers for each voltage class enables standardisation and cost efficiencies in installation designs. Warranty periods
Obtain extended warranty periods for outdoor HV circuit breakers. We will seek extended warranties in supply contracts for all classes of outdoor HV circuit breaker. The typical warranty on equipment was previously 18 months from delivery, or 12 months from commissioning, covering the supply of replacements for defective parts. In negotiation of recent period supply agreements for certain classes of outdoor circuit breaker, we have achieved a 12-year warranty from commissioning, covering parts and labour. The defects covered under the warranty include SF6 leaks. Component defects covered under the warranty include flange sealing systems and seals, where surface corrosion and degradation can lead to SF6 leaks.
4.3
Operation The Operation Lifecycle phase for asset management relates to planning and real-time functions. Circuit breakers have real-time digital monitoring and real-time control through SCADA. The real-time digital monitoring is particularly important because it provides an alarm for low SF6 gas level, which allows us to arrange a top-up of the circuit breaker before it becomes locked out due to low gas level. Operational activities undertaken are described in detail in the Operations Lifecycle Strategy. The following discussion focuses on operational issues that are specific to the outdoor circuit breaker fleet.
4.3.1 Outage Planning Power system outages for maintenance and replacement works must be planned to minimise disruption to customers. Grid operations identify requirements for outages (including reclose blocks) and manage the planning of outages and reclose blocks. A number of procedures carried out on outdoor circuit breakers cannot be carried out as live-line work. This means that an outage must be planned and managed in a way that creates a safe environment for employees and service providers to undertake the work, while minimising the disruption for customers.
4.3.2 Contingency Planning The transmission network provides a critical infrastructure service for New Zealand. Failure of the transmission service leads to an immediate impact on end consumers and can result
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in large costs of disruption to economic and social activity. Some transmission asset failures can present serious safety hazards for employees and members of the public, or result in environmental damage. So it is essential that we have plans in place for responding promptly and effectively to transmission system incidents and emergency situations. Contingency response resources
Maintain sufficient emergency spares in place to enable rapid restoration of transmission service following circuit breaker failure. Contingency planning for outdoor circuit breakers focuses on reviewing and maintaining the holdings of spares, and ensuring an adequate level of emergency preparedness.10 The spares include entire circuit breaker units and spare circuit breaker components, such as circuit breaker poles (which can be used to replace leaking poles). To enable prompt response to gas leaks from SF6 circuit breakers, service providers hold portable SF6 gas reclaim carts at multiple locations. These are used together with appropriate gas fittings for emptying and re-filling SF6 circuit breakers. Currently it is estimated to take a minimum of 28 weeks to purchase a new HV outdoor SF6 circuit breaker. In the event of widespread catastrophic failures, all circuit breakers available in store, including project equipment, would be used until suitable permanent replacement equipment could be purchased. In these cases, the continued operation of the transmission systems takes precedence over new projects. It may be possible to remove in-service equipment from other sites to restore supply or security. We also have a longstanding arrangement with Australian transmission companies that provides a means of access to equipment and expertise in the event of an emergency. For legacy circuit breaker types, minimum spares holdings will be retained until all instances of each model have been decommissioned. For minimum oil circuit breaker types, spare circuit breaker poles will be retained. For 66 kV bulk oil circuit breakers, a complete circuit breaker will be retained. For the 110 kV bulk oil circuit breakers, suitable bushings will be salvaged and retained as spares.
4.4
Maintenance We and our service providers carry out ongoing works to maintain assets in an appropriate condition and to ensure that they operate as required. The maintenance undertaken seeks to proactively manage failure risk as well as responding to actual failures as they occur. Our approach to maintenance and the activities we undertake are described in detail in the Maintenance Lifecycle Strategy. We classify maintenance tasks into the following categories:
10
preventive maintenance -
condition assessments
-
servicing
corrective maintenance
See subsection 2.1.4 for details about outdoor circuit breaker spares.
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-
fault response
-
repairs
maintenance projects.
These activities and associated strategies are discussed in the following subsections.
4.4.1 Preventive Maintenance Preventive maintenance is work undertaken on a scheduled basis to ensure the continued safety and integrity of assets and to compile condition information for subsequent analysis and planning. Preventive maintenance is generally our most regular asset intervention, so it is important in terms of providing feedback of information into the overall asset management system. Being the most common physical interaction with assets, it is also a potential source of safety incidents and human error. The main activities undertaken are listed below.
Inspections: non-intrusive checks to confirm safety and integrity of assets, assess fitness for service, and identify follow up work.
Condition Assessments: activities performed to monitor asset condition or predict the remaining life of the asset.
Servicing: routine tasks performed on the asset to ensure asset condition is maintained at an acceptable level.
We intend to implement the following preventive maintenance on our outdoor circuit breaker fleet in support of our objectives stated in chapter 3. Early detection of small SF6 leaks
Investigate and implement methods for early detection of small SF6 leaks on circuit breakers with a high risk of gas leaks. This strategy involves investigating and implementing methods of monitoring equipment for the early detection of small SF6 leaks on circuit breakers with a high risk of gas leaks. Early detection of small leaks will reduce the damage caused by corrosion and lower the cost and time required for repairs. The monthly inspections regime for substations includes noting SF6 pressure gauge readings and identifying trends. This procedure should provide sufficient early warning of leaks to allow scheduled outages for gas top-ups, and avoid most of the forced outages when the low pressure alarm is activated. This does not address the root cause of the problem, but will allow earlier intervention (on a planned basis). However, this method is too coarse to identify small leaks. Early detection of very small leaks also enables earlier intervention to stop equipment leaking, rather than waiting until a top-up is required at which point considerable SF6 has been lost to the atmosphere. We have purchased two infra-red thermal imaging SF6 cameras and SF6 sniffer equipment in RCP1 to enable improved detection and location of leaks. Service providers have been trained in the use of this equipment.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013 Preventive maintenance for frequently operated circuit breakers
Undertake operation-based preventive maintenance on frequently operated circuit breakers. A customised preventive maintenance programme will be applied to frequently operated HV outdoor circuit breakers to minimise their risk of failure. The specific preventive maintenance to be applied to these frequently operated circuit breakers includes:
time travel tests
SF6 gas sampling and laboratory analysis
internal inspection of interrupter units.
The interval for these preventive maintenance interventions is determined for each circuit breaker based on the estimated switching operations as shown in Table 6. Test
Frequency
Time travel tests
2 years or 1,000 operations, whichever comes first
SF6 gas sampling and laboratory analysis
4 years or 2,000 operations, whichever comes first
Invasive interrupters
12 years or 2,000 operations whichever comes first
Table 6: Preventive maintenance specific to high-frequency operation circuit breakers
Circuit breaker leak mitigations
Proactively replace all circuit breaker poles and install top caps on specific leak-prone models. We currently have about 150 leak-prone SF6 circuit breakers whose performance could be improved through this preventive maintenance. These models have been found to have a design weakness on the top cap of the circuit breaker poles. The fixing studs on the top cap are prone to moisture ingress, which can track down onto the internal flange sealing surfaces. Salt-laden moisture has been found to cause flange corrosion which has tracked across the sealing O-ring and has led to severe SF6 leaks developing suddenly on a number of circuit breakers at various sites. As these models of circuit breaker only have one SF6 pressure gauge that is common to all three poles, it is not immediately obvious as to which pole(s) are leaking. The source of the leak is detected by carrying out an in-service check using an SF6 leak detection camera. If necessary, the circuit breaker is removed from service so that a hand-held leak detector on the top of the poles can be used. Several leaks have been detected that have required the leaking poles to be replaced. Some circuit breakers have had one pole replaced, and then another pole has started to leak 12 months later. One instance has been found of all three poles leaking. A reactive approach has been taken to date in responding to these leaks, and only the leaking pole(s) have been replaced. This reactive approach has led to a number of forced outages for urgent top-ups. Because of the difficulties in obtaining planned outages to replace leaking poles, the reactive approach has led to a significant amount of SF6 emissions.
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A proactive approach is now planned, to address all the vulnerable circuit breakers. The work will be prioritised based on whether a pole has already been replaced on a circuit breaker, the previous top-up history, and the asset’s criticality. This work will be complemented by a programme within RCP1 to fit weather sealing caps to the tops of the circuit breakers to prevent further moisture ingress through into the flanges. A prototype cap has been developed that can be quickly fitted to the top of the circuit breaker poles during preventive maintenance on the circuit breaker or associated disconnectors. This approach has not been endorsed by the manufacturer as they believe that, in most cases, the corrosion process will already have started. We believe that preventing further moisture ingress should slow down the corrosion process. SF6 circuit breaker hydraulic drive monitoring
Install hydraulic pump monitoring to give early indication of hydraulic system deterioration on certain circuit breakers. We currently have about eighty 220 kV SF6 circuit breakers in service that would benefit from hydraulic pump monitoring. These circuit breakers were manufactured between 1989 and 1999, have hydraulic mechanisms, and use a high-pressure nitrogen-filled accumulator that provides the stored energy to operate the hydraulic mechanism. There is the potential for all 80 of these circuit breakers to fail due to deterioration of the hydraulic system, and there is currently no way of easily detecting this problem. It is proposed to install counters on all of these circuit breakers, to monitor the frequency of hydraulic pump starts. This will provide early indication of emerging problems and allow for planning and prioritisation of repairs. A number of spare accumulators will be sourced to enable replacement of any that are found to be leaking. The 80 circuit breakers that require the monitoring have generally performed well to date and, subject to the work outlined above, it is expected that they should achieve their nominal 35-year life.
4.4.2 Corrective Maintenance Corrective maintenance includes unforeseen activities to restore an asset to service, make it safe or secure, prevent imminent failure and address defects. It includes the required followup action, even if this is scheduled some time after the initial need for action is identified. These jobs are identified as a result of a fault or in the course of preventive work such as inspections. Corrective works may be urgent and if not completed for a prolonged period, may reduce network reliability.
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Corrective maintenance has historically been categorised as repairs and fault (response) activities. Repairs include the correction of defects identified during preventive maintenance and other additional predictive works driven by known model type issues and investigations.11 Timely repairs reduce the risk of failure, improve redundancy and remove system constraints by maximising the availability of assets. Activities include:
Fault restoration: unscheduled work in response to repair a fault in equipment that has safety, environmental or operational implications, including urgent dispatch to collect more information
Repairs: unforeseen tasks necessary to repair damage, prevent failure or rapid degradation of equipment
Reactive Inspections: patrols or inspections used to check for public safety risks or conditions not directly related to the fault in the event of failure.
Fault restoration We will continue to respond promptly to asset alarms, such as SF6 level alarms. Prompt response to low SF6 alarms is a significant driver of our relatively high level of forced outages, but this prompt response is necessary to avoid the risk of lockouts and the more serious system consequences that lockouts can create. We will also continue to explore methods and technologies to reduce the number of fault responses required, such as increased remote visualisation of assets through camera systems to aid the decision of whether or not a fault response is required. Repairs We may make repairs to outdoor circuit breakers where a fault has been identified that could potentially result in a failure, or when a failure has occurred. In both cases, the repairs are carried out to support network reliability objectives so as to improve the experience of customers. SF6 gas leak repairs
Repair circuit breakers with actual and potential SF6 gas leaks where it is economic to do so. Repairs will be carried out on leaking SF6 circuit breakers wherever it is practical and economic to do so. Repair programmes will also be carried out proactively for leak-prone models where this is economic. Major invasive repairs on leak-prone models will only be undertaken where this is clearly economic, based on the cost of repair (labour and materials) and the remaining life following repair. When repairing these units, we use improved grease and sealant to prevent corrosion ingress for a longer period of time. Such refurbishment work typically lasts up to 15 years.
11
Where the number of potential repairs is deemed sufficiently high a Maintenance Project will be instigated to undertake the repairs works.
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A cost-benefit analysis tool has been set up to compare the net present value (NPV) of the cost of repair of circuit breakers and subsequent replacement at the end of its extended economic life to the NPV of the cost of immediate replacement. Bushings – no repairs or refurbishment
Do not refurbish or repair old bulk oil circuit breaker bushings. Refurbishment and repair of old bulk oil circuit breaker bushings is no longer viable, as the necessary facilities are no longer available and the history of previous attempts at repair is unsatisfactory.
4.4.3 Maintenance Projects As discussed in subsection 2.2.5, maintenance projects typically consist of relatively highvalue planned repairs or replacements of components of larger assets. Maintenance projects would not be expected to increase the original design life of the larger assets. Maintenance jobs are typically run as a project where there are operational and financial efficiencies from doing so. The drivers for maintenance projects include asset condition, mitigating safety and environmental risks, and to improve performance. Over the period of this strategy, we do not intend to implement any maintenance projects on the circuit breaker fleet.
4.5
Disposal and Divestment The disposal and divestment lifecycle phase includes the process from when planning of disposal of an asset begins through to the point where we no longer own the asset. Asset disposal includes the decommissioning of the asset after which it may be sold as a functioning asset, sold as scrap, disposed of to a waste management facility, or re-used elsewhere as an in-service asset or as a spare. Asset divestment involves the sale of the inservice asset in situ. Divestment often involves the sale of assets to customers, including electricity distribution businesses and large electricity users. This section describes the strategies for disposal of assets within the outdoor circuit breaker fleet.
4.5.1 Asset Disposal The disposal lifecycle eventuates when circuit breakers are no longer needed or when a circuit breaker needs to be replaced. There are important requirements for the disposal phase, which are described in the strategies below. Appropriate decommissioning process
Maintain and follow an appropriate decommissioning process if re-use is not appropriate. Requirements for recovery and recycling/disposal work include safe work and site management processes and appropriate probity and environmental responsibility of scrap disposal processes. The disposal stage of the lifecycle involves replacing and
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decommissioning outdoor circuit breakers and, where applicable, the associated air compressors.
4.5.2 Divestment Implementation of divestment is primarily the change of ownership, although we must also remain aware of any safety and environmental issues and technical impacts on the Grid such as a change in constraints and flexibility of Grid operation. Circuit breaker divestment
Divest circuit breakers as part of substation and transmission line divestments to customers. We are continuing to transfer a number of assets at the fringes of the existing Grid to distribution businesses. This process and its justification are described fully in the Disposal and Divestment Lifecycle Strategy. Table 7 shows the number of circuit breakers likely to be transferred to customers between 2013/14 and 2019/20. This includes all divestments that we believe have a 50% or greater likelihood of occurring during that timeframe. 50 – 66 kV
110 kV
220 kV
Total
RCP1
58
11
0
69
RCP2
8
4
0
12
TOTAL
66
15
0
81
Period 12
Table 7: Circuit breaker divestments
The total number of circuit breakers to be transferred represents 7% of the total circuit breaker fleet as at June 2013. Yet the impact on lower voltage circuit breakers in the fleet is significant. The circuit breakers operating at 50 kV–66 kV that are included in the asset transfers represent 36% of that portion of the present fleet (as of June 2013). In addition to some direct savings in circuit breaker maintenance costs, the asset transfer programme will remove some makes and models of equipment from the fleet and allow some rationalisation of spares and maintenance procedures.
4.6
Asset Management Capability This section describes the specific strategies for obtaining and maintaining capability in managing and handling HV outdoor circuit breakers. These strategies provide medium-term to long-term guidance and direction to ensure that asset managers and their staff have the required capabilities in regard to fleet management. We require our Grid assets and equipment to be managed, maintained, tested and operated to high standards of skill, professionalism and safety supported by high-quality asset knowledge and risk management tools. This will ensure satisfactory and safe functioning of the network while minimising whole-of-life costs.
12
This excludes historical divestments; it includes only those divestments to be carried out in 2013/14 and 2014/15.
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The capability strategies are described under the following headings:
Risk Management
Asset Knowledge
Training and Competence.
4.6.1 Risk Management Our approach to risk management is central to our asset management decision making as we weigh up the various costs and benefits of options such as replacement timing. We are developing asset health and criticality to improve and integrate our risk-based asset management. The strategies below discuss how we plan to progress this as regards the outdoor circuit breakers fleet. Circuit breaker asset health model
Refine the circuit breaker asset health model. Over RCP2 we will work to improve the circuit breaker asset health model to inform our forecasts of replacement volumes and to inform our replacement prioritisation. In addition to age, the main asset health determinants are currently interrupter type and whether it’s a leak-prone SF6 model. By the end of RCP2 the majority of bulk oil, minimum oil, and leakprone SF6 circuit breakers will have been removed from the fleet. So, it will be important to improve the differentiators between the remaining SF6 circuit breakers. We will introduce additional asset health factors based on invasive inspections, performance history, and overseas experiences.
4.6.2 Asset Knowledge Robust asset knowledge is critical to good decision making for asset management. Asset knowledge comes from a variety of sources, including overseas experiences, experience from assets on our network, theoretical modelling, and information from the manufacturers. This asset knowledge must be captured and recorded in such a way that it can be conveniently accessed when future asset management decisions are made. A key part of improving our asset knowledge is the commissioning of the new Asset Management Information System (AMIS). During RCP2 we will seek to improve our knowledge of our fleet of circuit breakers, which will improve the asset health data. An important part of this improvement will come from the ‘standard maintenance procedures’, which are discussed below. Standard Maintenance Procedures
Complete and implement Standard Maintenance Procedures (SMPs) with consistent testing and data recording for the fleet of circuit breakers. We have begun developing standard maintenance procedures for circuit breakers, which will be used by service providers to standardise their interactions with the fleet and the recording of those interactions. In particular, the SMPs will have very clear rules on tests, including detailed instructions on how to carry out the tests and record the data. This will
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support more consistent, useful, and accessible data on the circuit breakers that can be analysed to support asset management decisions such as replacement timing.
4.6.3 Training and Competence Our overarching strategy for maintaining and improving worker competence can be summarised as follows:
all persons (our employees service providers and sub-contractors) working on our assets must be properly trained and currently competent for the tasks they undertake
all maintenance service providers must comply with the competency criteria set out in the relevant Service Specification
employers must manage the currency of competencies of their workers for the work they undertake to the appropriate requirements of the relevant Service Specification.
We have three service specifications that define the competency requirements for working on outdoor circuit breakers:
TP.SS 06.23 Minimum competencies for power system equipment operation
TP.SS 06.21 Minimum competencies for substations maintenance and testing
TP.SS 06.25 Minimum requirements for Transpower field work.
We must maintain a minimum baseline of retained skilled workforce: engineers and site work operators who understand the physical assets. Specialised maintenance service providers
Continue use of specialised maintenance service providers for invasive work. Not all maintenance service providers have the skills required for substantial invasive work on circuit breakers, but some specialists do have the skills. The use of specialist service providers reduces the risk of human error incidents (HEIs). These specialists will have undergone training and practice on replica equipment at our training sites. This strategy is cost-effective, as it would be more expensive to maintain high levels of these types of skills for all maintenance service providers – training that is unnecessary. While the additional cost of higher-priced specialists can be quantified, it is difficult to quantify the savings made by reduced HEIs and the reduced cost of regular maintenance service providers, who require less qualifications, training, and experience.
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4.7
Summary of RCP2 Fleet Strategies Our asset management plans for the fleet of outdoor HV circuit breakers are summarised below for each lifecycle stage. Planning
Enhancement and Development
Plan for investment in new circuit breakers for network development projects as required. Proactively replace selected models of leak-prone SF6 circuit breakers.
Replacement and Refurbishment
Replace or divest most minimum oil circuit breakers by 2015 and most bulk oil circuit breakers by 2020 with modern SF6 models. Proactively replace a portion of the older SF6 circuit breakers. Replace frequently operated circuit breakers based on AHI (adjusted for maximum operation count).
Integrated Works Planning
Schedule circuit breaker replacement to coincide with other large works.
Cost Estimation
Scope and estimate project works to a P50 confidence level (the estimate is based on a 50% probability that the cost will not be exceeded).
Delivery Ensure that safety is explicitly built into all stages of the design process. Minimise, as far as practical, the design diversity of circuit breaker installations. Design
Deploy disconnecting circuit breakers in selected locations as more experience is gained with this technology. Deploy non-SF6 technology circuit breakers in managed trials to gain experience and explore future potential. Purchase live tank SF6 circuit breakers except where other types are required for specific applications. Use composite bushings on new outdoor circuit breakers.
Procurement
Procure circuit breakers from the minimum possible number of vendors commensurate with the need to manage supplier risk. Obtain extended warranty periods for outdoor HV circuit breakers.
Operation Contingency Planning
Maintain sufficient emergency spares in place to enable rapid restoration of transmission service following circuit breaker failure.
Maintenance Investigate and implement methods for early detection of small SF6 leaks on circuit breakers with a high risk of gas leaks. Preventive Maintenance
Undertake operation-based preventive maintenance on frequently operated circuit breakers. Proactively replace all circuit breaker poles and install top caps on specific leak-prone models. Install hydraulic pump monitoring to give early indication of hydraulic system deterioration on certain circuit breakers.
Corrective Maintenance
Repair circuit breakers with actual and potential SF6 gas leaks where it is economic to do so. Do not refurbish or repair old bulk oil circuit breaker bushings.
Disposal and Divestment Asset Disposals Divestments
Maintain and follow an appropriate decommissioning process if re-use is not appropriate. Divest circuit breakers as part of substation and transmission line divestments to customers.
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ACS Outdoor Circuit Breakers Fleet Strategy TP.FS 51.01 Issue 1 October 2013 Asset Management Capability Risk Management
Refine the circuit breaker asset health model.
Asset Knowledge
Complete and implement Standard Maintenance Procedures (SMPs) with consistent testing and data recording for the fleet of circuit breakers.
Training and Competence
Continue use of specialised maintenance service providers for invasive work.
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Appendices
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Appendices | page 54
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A.
CIRCUIT BREAKER PHOTOS Figure 12 shows photos of four types of 110 kV circuit breakers: bulk oil, minimum oil, dead tank SF6 and live tank SF6. Figure 13 is a photo of a live tank circuit breaker and Figure 14 is a photo of dead tank bulk oil circuit breakers at Albany Substation.
Bulk Oil
Minimum Oil
Dead Tank SF6
Live Tank SF6 Figure 12: 110 kV Circuit Breakers
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Figure 13: Live Tank Circuit Breaker
Figure 14: Dead tank bulk oil circuit breakers (Albany Substation)
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B.
ADDITIONAL STATISTICS ITOMS benchmarking
The charts in Figures 15 to 22 are taken from the ITOMS 2011 benchmarking and show composite service performance levels and costs for 60-99 kV, 100-199 kV and 200+ kV circuit breakers. We are represented by the ‘G’ marker, while the other markers represent overseas transmission networks. Performance benchmarking
Figure 15: 60 kV–99 kV Circuit Breaker Performance (ITOMS 2011)
Figure 16: 100 kV–199 kV Circuit Breaker Performance (ITOMS 2011)
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Figure 17: 200+ kV Circuit Breaker Reliability (ITOMS 2011)
Cost benchmarking
Figure 18: International Comparison of Maintenance Cost per 200+ kV Circuit Breaker (ITOMS 2011)
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Figure 19: International Comparison of Maintenance Cost per 60 kV– 99 kV Circuit Breaker (ITOMS 2011)
Figure 20: International Comparison of Maintenance Cost per 100 kV– 199 kV Circuit Breaker (ITOMS 2011)
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Overall circuit breaker benchmarking
Figure 21: International Benchmarking of 60 kV– 99 kV and 100 kV– 199 kV Circuit Breakers (ITOMS 2011)
Figure 22: International Benchmarking of 200+ kV Circuit Breakers
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C.
LEAK-PRONE CIRCUIT BREAKERS – DETAILED REVIEW These models relate to the replacement strategies set out in subsection 4.1.2. ABB HPL 123/25C1 & 123/25A1 SF6 Circuit Breaker We currently have 46 ABB type HPL 123/25C1 and one HPL type 123/25A1 (110 kV) SF6 circuit breaker in service. These circuit breakers were manufactured between 1990 and 1992. These models have been found to have sealing systems that are very prone to flange corrosion, particularly on the top flange. Also, these circuit breakers cannot be topped up live as the combined pressure gauge/density switches must be removed to access the filling points. Doing so raises a circuit breaker lockout alarm that prevents the circuit breaker from operating. A number of leaks have been caused by the pressure gauge O-ring seals being damaged during removal and refitting for a top-up operation. Further, the pressure gauges have lens covers that are very prone to UV damage. This makes it very difficult to note the pressure and predict the need for top-up before the low pressure alarm occurs. A large number of these circuit breakers require the gauges to be replaced. The photographs in Figure 23 show the circuit breaker corrosion and leak issues.
Figure 23: Corrosion and leak issues – SF6 circuit breakers
The profile of the leak rate in these circuit breakers has been found to develop slowly, and to then steadily worsen. Ultimately, some of these circuit breakers have required a weekly topup until an outage could be secured to replace the leaking pole(s). While the leaks detected to date have been from the top caps of the circuit breaker, all flanges have been found to be in poor condition during refurbishment. While it is possible to carry out repairs to the top caps on site, this is not recommended due to the risks of the corrosion being too severe that repairs cannot be carried out, and the risk of damaging components or debris falling down into the interrupter during the repair work. This work would also depend on the weather. Repairs are undertaken in a workshop set up for SF6 handling.
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ABB EDF SK1-1 SF6 circuit breaker We currently have 12 ABB type EDF SK1-1 (66 kV) SF6 circuit breakers in service. These circuit breakers were manufactured between 1990 and 1992. These models have been found to have sealing systems that are very prone to flange corrosion, particularly on the top flange. ABB LTB 145/D1 SF6 circuit breaker We currently have two ABB type LTB 145/D1 (110 kV) SF6 circuit breakers in service. These circuit breakers were manufactured in 1992. These models have been found to have sealing systems that are very prone to flange corrosion, particularly on the top flange.
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