CIRED
17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
THE USE OF "HEALTH INDICES" TO DETERMINE END OF LIFE AND ESTIMATE REMNANT
LIFE FOR DISTRIBUTION ASSETS
D.T. HUGHES
EA Technology - United Kingdom
[email protected]
INTRODUCTION
Electricity companies are increasingly adopting condition
(rather than time or age) based maintenance and replacement
regimes. This paper describes how ‘health indices’ can be
used to enable electricity companies to effectively use
condition information to support and implement asset
management programmes for distribution assets.
EA Technology has worked with many electricity companies
in the UK and overseas to assist with the collection and use of
condition information over many years. Over the past 2 years
this has led to the development and implement of a health
indices as a means of defining proximity to end of life for a
wide variety of distribution assets.
This experience has revealed that the application of health
indices is a viable and effective process and potentially a very
powerful basis for ongoing asset management.
THE CONTEXT
Most of the UK electricity companies and many companies in
other countries, have mature networks. Typically, there was
large scale investment and development of the system in the
50s, 60s and early 70s. Consequently, the age of many
individual items is now approaching or has exceeded its
original design life. Despite this, in many cases the
equipment continues to be reliable and functional. The natural
replacement rate due to failures, need for reinforcement etc is
relatively slow and therefore the average age for many groups
of assets continues to increase.
To compound this, over the past 10 years, the economic and
regulatory pressures on electricity companies have increased
significantly. The need to reduce cost but at the same time
maintain or improve reliability of supply is continually
emphasised.
the asset.
There is a very important distinction between the requirement
for maintenance and the end of life of an asset. Traditionally,
electricity companies have been predominantly maintenance
driven organisations and therefore much of the condition
information collected specifically identifies issues that can be
dealt with by maintenance. It is very important to make the
distinction between degradation that can be dealt with by
maintenance and degradation that indicates approaching end
of life.
The formulation of a health index can be seen as a
prioritisation process within the overall asset management
regime. It might be possible to define the perfect health index
for a particular asset in which the ideal combination of
condition information is utilised. However, in order to be
practical the initial approach is to consider the information
which is currently available and assess whether this will
enable a meaningful prioritisation/ranking of the assets. In
many cases sufficient information is available from existing
data to carry out a credible initial prioritisation. If this is not
the case the next step is to identify the minimum additional
information required to achieve a credible health index.
It is worth emphasising that to enable practical and economic
progress identification of the minimum amount of condition
information to make a reasonable decision, is a very
important step. Asset managers sometimes attempt to gather
the maximum amount of condition information without
defining how it will be used. This is expensive, slow and
leads to major data handling and processing requirements. By
adopting the minimalistic approach rapid and cost effective
progress can be made.
WHAT IS A HEALTH INDEX?
The purpose of the health indices discussed in this paper is to
provide a consistent overall assessment of condition. It is
recognised that there are other factors that will affect the
ultimate decision for individual assets such as criticality,
obsolescence, safety issues, environmental concerns etc.
However, in order to apply these appropriately a consistent
appreciation of condition and an understanding of what that
means in terms of future performance is essential.
A health index is a way of combining complex condition
information to give a single numerical value as a comparative
indication of overall condition. It is important to define the
purpose of the health index. In the present work the health
index is intended to give an indication of proximity to end of
life (EOL) for individual assets. Therefore, when considering
the relative significance of different condition information it
is assessed in terms of its relevance to ultimate end of life for
When formulating a health index there are two types of
information that can be utilised. Firstly, there is specific
condition information which relates to individual assets. For
example oil test results for a transformer or individual
inspection reports for a particular piece of equipment.
However, for many distribution assets, particularly at the
lower voltages, there will be relatively little specific condition
information. This is sometimes thought to mean that it is
Within this context understanding the present condition and
future performance of assets is paramount. When should
assets be replaced? What are the consequences, financial and
operational, of not replacing assets? How can asset
replacement be justified?
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17th International Conference on Electricity Distribution
impossible to achieve a practical and economic assessment of
condition for such assets. While there may not be much
specific condition information for some assets there are
almost always ‘risk factors’ which relate to condition, i.e.
factors such as reliability, failure rates, specific problems,
generic problems, design issues, environmental factors etc.
All of which can be used to prioritise or rank assets by risk of
being in poor condition.
In many cases for electricity distribution assets there is a
wealth of such information available both within the parent
company and within the wider technical community. In many
cases there is also a good understanding of the degradation
and failure processes that enables condition and risk factors
not only to rank assets but to provide a basis for estimating
remnant life. The more specific condition information that is
available the more definitive the health index can be. If there
is very little specific condition information and only some risk
factors available the health index may still provide a useful
means of prioritising assets. More detailed condition
assessment activities can then be targeted in the most cost
effective manner.
THE PRAGMATIC
INDICES
APPROACH
TO
HEALTH
As discussed above there is little point in defining ‘the ideal
health index’ for a particular asset group if very little of the
information identified is available. A much more successful
approach is to consider the information currently available
and work out whether or not this can be used to give a useful
assessment of condition. This leads to the pragmatic
approach to development of health indices summarised in the
schematic diagram below.
Barcelona, 12-15 May 2003
the system to gather additional condition information or by
targeted sampling, effective results can be obtained
economically.
An important factor is the grouping of assets. For the
purposes of developing a health index we need to combine
assets into relatively large groupings. To enable a realistic
comparative health index to be developed for each group it is
important that the essential degradation and failure processes
are similar for the group so that the critical condition factors
are also similar. It is also important that the relative
importance, financial and operational, of the assets are
similar. This is significant because it has a major effect on
the nature of condition information that can be economically
obtained. For example, a large grid transformer and a small
distributed pole mounted transformer have essentially the
same degradation processes. However, the practicality and
economics of gathering condition information for the two are
totally different and therefore it is unrealistic to expect a
single health index to be applicable to both types.
Even for the most challenging, lower voltage, numerous asset
groups the consideration of a health index can be a useful
process. It will probably not be possible to populate even a
‘risk factor’ based health index in many cases. However by
defining the necessary condition information, longer term
information gathering can be initiated, in conjunction with
routine activities, that will enable development of a good
appreciation of asset condition over time. Thus providing a
consistent and cost effective approach to the whole asset base.
It must be emphasised that the health index is a means of
bringing together all available knowledge and information
about a group of assets and using this in a consistent and
logical manner to define condition and predict future
performance. It relies on the knowledge and experience that
is available about the assets concerned. It then provides a
consistent means of utilising that information to provide an
assessment. It is not a substitute for engineering judgement it
is an aid to engineering judgement. The most effective use of
a health index is when the resulting simple numerical values
are used in conjunction with the data that has been used to
derive them in order to support and direct asset management
decisions.
PRACTICAL EXAMPLES OF HEALTH INDICES
The object is to obtain useful and viable health indices to rank
assets by proximity to end of life as quickly and as
economically as possible. Having arrived at an initial health
index and definition of condition we then define the way to
continually improve this by ongoing gathering of condition
information. Economics and practicality of condition
information gathering is paramount. It is recognised that it is
impractical and uneconomic to gather detailed condition
information about all distribution assets. However, by
applying a consistent and logical approach and utilising
opportunities such as maintenance or removal of assets from
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Session 5 Paper No 15
EA Technology working with several electricity companies,
both in the UK and overseas, have developed and
implemented health index schemes for a wide range of assets
covering the whole spectrum of distribution and transmission
plant and equipment. These include wood pole overhead
lines, steel tower overhead lines, major plant items
(transformers, switchgear, circuit breakers), cables and even
less well defined assets such as spares and rights of way! In
all cases these have been derived from existing information
often having to combine data from various sources in order to
define a credible health index. Predominantly the objective
has been to define the present condition and future
performance on the basis of proximity to end of life. The
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CIRED
17th International Conference on Electricity Distribution
outputs are being used by the host companies to determine
and justify future capital spending requirements for
replacement or refurbishment of major asset groups. In some
cases they are being used directly to underpin submissions to
regulatory authorities, who in the past have been critical of
the host company for failing to provide condition based
justification for spending plans.
In all cases the condition of each asset is described by a single
numerical value which is factorised to enable comparison of
assets with incomplete condition information and normalised
onto a scale of 0 to 10 or 0 to 100. The exact form of health
index varies depending the requirements of the individual
client company. In some cases the health index is presented
in a positive fashion, i.e. a high value indicates an asset in
good condition and a low value poor condition. In other
cases the health index is presented in a negative fashion so
that a high number indicates poor condition. This is purely a
matter of convention.
The most challenging aspect of deriving a health index is to
take complex condition information, recorded in many
different ways and convert it into predefined, well specified
condition ratings or codes which be turned into simple
numerical values for use in an overall health index algorithm.
Barcelona, 12-15 May 2003
which constitute assets presently at end of life. Beyond this
some attempt may be made to assign remnant life to the
remaining assets. Alternatively, some indication of the
requirements for ongoing management of the assets based on
their overall condition, i.e. normal maintenance, increased
maintenance, major overhaul, replace, could be assigned.
Alternatively, the health index could be used to define a risk
of failure within a particular time frame.
In terms of remnant life the time frames used in conjunction
with health indices are usually in broad bands, i.e. 5 year
periods, particularly if the information is being used on the
context of a regulatory submission.
Some typical outputs from recently derived health indices are
shown graphically below to illustrate the nature and potential
value of the information that can be obtained. These include
assets such as the larger transformers and OCBs where the
health index is based mainly on available, specific condition
information and assets such as distribution OHLs and cables
where the health index is a combination of specific
information and condition related risk factors.
Grid and Primary Transformers, Health
I d R
l
500
465
450
400
As an example, the complex information available from
analysis of a transformer oil sample needs to be converted
into simple condition codes. This illustrates an important
issue. Conventional interpretation of DGA results enables an
experienced practitioner to derive very specific information
about the condition and nature of any developing fault within
the transformer. While this is an extremely useful and
valuable process, for the purposes of a health index less
detailed interpretation is required. Classifying the DGA
results into one of four categories, i.e. all gas levels normal,
slightly raised gas levels, significantly raised gas levels
indicating developing fault, gas levels indicating possible
critical condition immediate action required, is sufficient for
an overall health index that also takes into account several
other factors.
This is by no means minimising the value of full
interpretation of the DGA results. As indicated earlier, the
health index should be used alongside conventional
assessment of condition information as an additional tool.
Not as a replacement for using DGA as a specific diagnostic
tool.
Nu 350
m
be
r 300
of
Tr 250
an
sf
or 200
m
er
150
219
100
65
50
5
11
0-30 Very high risk
30-50 High risk
0
50-70 Normal risk (at current
level)
70-85 Low risk
85-100 Very Low
Health Index / Risk of failure in next 5 years
OCB, Health Index Results
1200
1106
1000
851
Nu 800
m
be
r
of 600
br
ea
ke
rs
400
200
0
133
0
Very Poor, 0-30 (Replace)
12
Poor, 30-50 (Rebuild or
replace)
Fair, 50-70 (Increased
maintenance)
Good, 70-85 (Normal
maintenance)
Very Good, 85-100 (Normal
maintenance)
Health Index / Management Requirements
Having formulated and applied the health index to the
available condition information and produced a ranking or
prioritisation of the assets on the basis of proximity to end of
life, some initial calibration can then be applied. As
previously discussed the form and detail of this calibration
will depend on the nature of the assets, the level of condition
information available to derive the health index and the
requirements of the host electricity company. In most cases
an attempt is made to define the range of health index values
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17th International Conference on Electricity Distribution
2.
Phase Conductors, Health Index
R
l
3.
250
224
200
4.
Nu
m
150
be
r
of
Sa
m
pl 100
es
5.
106
Barcelona, 12-15 May 2003
A structured process to define future condition
assessment needs
A means of utilising existing engineering knowledge and
experience to predict future performance and failure rates
A consistent, defined reference point to aid decision
making
A basis for planning and justifying future
replacement/refurbishment plans
81
CONCLUSIONS
50
16
0
Very Poor, 0 (At EOL)
Poor, 0-17.5 (<5 years)
Fair 17.5-37.5 (5-15 years)
Very Good, 37.5-100 (>15 years)
Health Index / Remnant Life
UGCables, Health Index Results
160
151.9
140
120
100
Le
ng
th
80
k
HPLF
LPLF
60
The paper has attempted to illustrate the potential value to
electricity companies of a structured use of condition
information and in particular the derivation of health indices
for all asset groups. The experience EA Technology has
derived from working with several electricity companies is
enabling condition information to be used in a very
constructive and positive manner to enable genuine condition
based management programmes. In the context of a heavily
regulated industry, with large numbers of relatively old assets,
the type of approach described in this paper is considered to
offer major opportunities to implement effective ongoing
asset management
42.3
41.8
40
34.1
21.7
20
0
0
2.2
0
Very Poor, 0-30 (Plan
replacement)
6.5
Poor, 30-50 (Significant
remedial measures
required)
5.6
Fair, 50-70 (Increased
maintenance)
Good, 70-85 (Normal
maintenance)
Very Good, 85-100
(Normal maintenance)
Health Index / Management Requirements
Health Index for distribution OHLs, need for refurbishment
450
391
400
350
Number of lines
300
250
232
200
150
100
53
50
27
0
0
Very Poor, 0-35 (<5years)
Poor, 35-50 (5-10 years)
Fair, 50-65 (10-20years)
Good, >65, (>20years)
Health Index / Remnant Life
BENEFITS OF DEVELOPING AND IMPLEMENTING
HEALTH INDICES
Developing and implementing health indices for the whole
range of distribution assets enables a consistent use of
engineering knowledge and experience to underpin ongoing
asset management programmes. They are particularly
valueable for mature and stable asset bases such as the
distribution networks of many electricity companies.
Health indices provide:
1.
A consistent means of utilising available condition
information (and risk factors) to define proximity to End
of Life.
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