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mers. However, this survey was based on transformer
failures occurring in the period 1968 to 1978.
Working group A2.37 Transformer Reliability was
formed in 2008 with the following objectives:
•Reviewing all existing survey and study different
practices (in terms of data collection, compila-
tion, etc.)
•Conducting a new international survey on trans-
former failures
•Compiling and analysing the collected data, and
interpreting the results (calculation of failure
rates, classification into failure location, failure
causes and failure modes)
•Preparing a brochure documenting the above-
mentioned.
This interim report discusses the progress of the
working group to date in terms of data collection,
and will present results of the analysis in terms
of failure rates and the classification into failure
location. Because data collection is still in progress,
this report will refrain from giving a detailed inter-
pretation of the results.
Review of Existing Surveys
The working group collected publicly available
surveys from Canada, Germany and Japan [3, 4].
The main objective of these surveys is the syste-
matic collection of data on the availability and
disturbances of the electrical power supply, with
emphasis on the frequency, duration and extent
of the interruptions. Detailed statistics about the
failure location in the respective equipment, failure
cause or mode and repair activities are normally not
Introduction
In 1983, Cigré Working Group 12.05 published
a report summarizing the results of the analysis of
transformers that failed in the period 1968 to 1978 [1].
Thirteen countries from 3 different continents took
part in this survey. The authors reported of difficul-
ties to compile and analyse the data of the survey due
to incomplete or incompatible responses. Ten years
later, Working Group 12.14 attempted to upgrade this
survey but it was unsuccessful due to similar reasons.
Study Committee A2 also started an Advisory Group
on Reliability and their findings were presented at the
Cigré 2006 session in [2].
Some countries have published reliability surveys
locally with some being published annually. However,
this knowledge is not shared amongst the interna-
tional transformer community, where most benefit
can be drawn.
To date, the only international survey on large
power transformer failures was published in 1983. The
failed units were classified according to voltage level,
age and application, and for each class, corresponding
failure rates were calculated as well as classification
into failure component, presumed cause and failure
origin.
The survey concluded that the average failure rate
of transformers may be regarded as 2 % across all
voltage categories. Since then, this statistic has become
an international benchmark in the transformer
industry for the failure rate performance of transfor-
Members
Stefan Tenbohlen, Convenor (DE), Janine Jagers, Secretary (ZA),
Gilson Baston (BR), Brendan Diggin (IE), Michael Krüger (AT),
Piotr Manski (POL), Bhavin Desai (US), Johannes Gebauer (DE),
John Lapworth (UK), Anne McIntosh (UK), Antun Mikulecky (HR),
Pascal Müller (CH), Claude Rajotte (CA), Takehisa Sakia (JP),
Shirasaka Yukiyasu (JP)
Transformer Reliability Survey:
Interim Report
WG
A2.37
REPORT
46 No. 261 - April 2012 ELECTRA
general information about the population of the
operating transformers for the indicated failure
period. The population information requested
included the transformer application, type, number
of phases, voltage, rated power, typical loading, and
manufacturing period.
The second section captured the transformer
failure data, grouped data into 4 categories as
follows:
• Identification of the unit: application, type,
construction type, year of manufacture.
• Featuresoftheunit: rated power, nominal voltage,
number of phases, cooling system, type of oil, tap
changer, tap changer arrangement, oil preserva-
tion system, over voltage protection.
• Detailofoccurrence: year of failure, service years
to failure, loading immediately prior to failure.
• Consequences of failure: external effects, failure
location, service years of failed bushings (if
location is bushings), failure mode, failure cause,
action taken, and detection mode. It is important
to note that:
- Failure location referred to the primary location
in the transformer where the failure was
initiated.
- Failure cause referred to cause of failure in the
location where the failure was initiated.
- Failure mode referred to a description of the
nature of the failure, in the location where the
failure was initiated.
Collected data
Each participating utility was required to
complete the questionnaire described in section 3.1,
and all the responses were compiled into a database.
In order to achieve a data security and anonymity,
the failure data from each source was labelled by
a code based on the geographical location and a
sequence number. Information about the trans-
former manufacturer was not collected.
To date, the working group has collected 685
failures which occurred in the period 1996 to 2010,
with a total population of 156,186 unit-years,
contributed by 48 utilities from 16 countries. The
year of manufacture of the units span from the
1950’s up to 2009. The data analysis was performed
for all failures which happened after 2000.
Data collection is still in progress and will
continue until May 2012.
Calculation of failure rate
Failure rate was calculated according to the defini-
tion in Bossi [1983], which was expressed as:
included. The benefit of these statistics with respect
to asset management is therefore limited. Additio-
nally, the failure surveys of utilities, manufacturers
and consulting companies are being collected and
analysed by the working group. However, diffe-
rent definitions and information content constrain
forming a coherent database from these individual
sources.
Initial working group discussions concentrated
on analysing the readily available statistics, but it
was agreed that the scope needed to be broadened to
allow comparison with the failure statistic of 1983
survey. A questionnaire was therefore developed
to collect utility failure statistics in a standardised
way. Besides information about the population
under investigation, failure data is being collected
for various groups of transformers in terms of
the failure locations, failure causes, failure modes,
actions, external effects and others parameters.
Data collection
and preparation for analysis
Definition of failure
The definition of failure was limited to major
failures. Based on the experience of previous
working groups on this subject, it was decided to
limit the data requested from the sources by concen-
trating only on major failures of power transfor-
mers and shunt reactors of operational voltages
higher than 60 kV only.
According to the working group, a major failure
was defined as any situation which required the
transformer to be removed from service for a period
longer than 7 days for investigation, remedial work
or replacement. Where repairs were required, this
should have involved major remedial work, usually
requiring the transformer to be removed from
its installation site and returned to the factory. A
major failure would require at least the opening
of the tank, including the tap changer tank, or an
exchange of the bushings. A reliable indication
that the condition of the transformer prevents safe
operation is considered a major failure, if remedial
work (longer than 7 days) was required for restoring
it to the initial service capability.
Reliability questionnaire
A questionnaire consisting of two major sections
were developed to collect data, in accordance with
the definition of major failure in section 3.1. [5]
The first section of the questionnaire requested
REPORT
WG
A2.37
No. 261 - April 2012 ELECTRA 47
section 3.3. The data were grouped into substation,
and generator step-up transformers, and further
categorised into 5 voltage classes.
Failure rates
The calculated failure rates according to the
voltage category for the substation and generator
step-up transformers, as well as the combined
group of transformers, are given in Table 1 to
Table 3. It is important to note that the number
of generator step-up unit failures, and units
in voltages classes above 500 kV, as well as the
(1)
Where:
ni = the number of transformers that failed in the
ith year
Ni = the number of transformers in service during
the ith year
Data analysis
The analysis presented was based on the popula-
tion and failure data collected to date, as described in
Table 1: Failure Rates of Substation Transformer
Table 2: Failure Rates of Generator Step-Up Transformers
Figure 1: Failure locations of Substation Transformers
(>100kV) (based on 364 failures)
Figure 2: Failure Locations in Generator Step-Up
Transformers (>100 kV) (based on 82 failures)
WG
A2.37
REPORT
48 No. 261 - April 2012 ELECTRA
HIGHEST SYSTEM VOLTAGE [kV]
FAILURES &
POPULATION
INFORMATION69 kV < 100
100 kV <
200
200 kV <
300
300 kV <
500 kV 700 All
Failures145
TransformerYears 15077
FAILURE RATE 0.96%
206 136 95 7589
4615242635 29437219 135491
0.45% 0.32%0.32% 3.20%0.43%
-
HIGHEST SYSTEM VOLTAGE [kV]
FAILURES &
POPULATION
INFORMATION 69 kV < 100
100 kV <
200
200 kV <
300
300 kV <
500 kV 700 All
Failures 62759496
Transformer -Years143 2842 4838 12132740 20695
FAILURE RATE 0.00%0.21% 0.56%0.49% 0.54%0.46%
HIGHEST SYSTEM VOLTAGE [kV]FAILURES &
POPULATION
INFORMATION69 kV < 100
100 kV <
200
200 kV <
300
300 kV <
500 kV 700 All
Failures145 212 163 15411685
Transformer-Years 15220 4899447473 41569959 156186
FAILURE RATE 0.95%0.43% 0.34%0.37% 1.15%0.44%
0
failure data can therefore be analysed and inter-
preted for various types of transformers in terms
of failure locations, failure causes, failure modes,
actions, external effects and failure rates in trans-
formers. In contrast to several public available
statistics, the results of this questionnaire deliver
valuable information which can be used for asset
management of a power transformer fleet.
The preliminary results, based on a transformer
population with more than 150.000 unit-years and
685 major failures in 48 utilities, indicate a failure
rate of 0.44%.Winding related failures appear to
be the largest contributor of major failures, and a
significant decrease in tap changer related failures
has been observed in comparison with results of the
1983 survey.
In order to improve the validity of the failure
statistics, the working group invites participation
from utilities to provide failure data, in particular
shunt reactors, generator step-up transformers,
and transformers with operational voltages above
500 kV.
REFERENCES
[1] A. Bossi, et al., “An International Survey on Failures in Large Power
Transformers in Service” – Final report of CIGRE Working Group
12.05, Electra, No.88, pp. 22 – 48,1983.
[2] J. Lapworth: Transformer reliability surveys, A2-114, Cigré
Biennial Conference, Paris 2006
[3] The Canadian Electricity Association. forced outage performance
of transmission equipment , Equipment Reliability Information
System. canada : s.n., 2000-2009.
[4] VDN-Störungs- und Verfügbarkeitsstatistik , Verband der
Netzbetreiber VDN, Berlin, www.vde.com/fnn, Berichtsjahr 2004.
[5] Questionnaire of CIGRE WG A2.37 “Transformer Reliability
Survey”, November 2011,url: http://www.uni-stuttgart.de/ieh/
wga237.html., last accessed: November, 2011
[6] S. Tenbohlen, et al., “Assessment of Power Transformer Reliability”,
Int. Symp. on High Voltage Engineering, Hannover, Germany,
G-026, August 2011
population of these two categories, has been low
to date. The calculated failure rates should thus be
considered with caution.
The working group further invites participa-
tion from utilities and countries for failure data,
in particular the abovementioned two transformer
groups, to improve the statistical significance of the
results. Data may be supplied by completing and
submitting the questionnaire (available via the web
link given as [5]) to the convenor or secretary of the
working group.
Failure location
The contributions of failure locations in substa-
tion transformers and generator step-up transfor-
mers for voltages higher than 100 kV are given in
Figure 1 to Figure 2.
As seen in previous surveys, major failures can
originate from several transformer components.
Windings related failures appear to be the main
contributor of major failures. The contribution of
tap changer related failures decreased significantly
in comparison with the statistics from 1983 given
in [1]. 95% of the failed substation transformers
and 91% of the failed generator step-up units were
equipped with a tap changer.
It is important to note that failure locations in
some cases have an operating voltage dependency.
Regional significance also has an impact [6]. A more
detailed analysis and interpretation of the results
will be provided in the brochure, which will be
published after completion of the working group’s
expected outcomes.
Conclusion
A questionnaire was developed by the CIGRE
working group A2.37 (Transformer Reliability
Survey) by which utility failure statistics could be
collected in a standardised way [5]. Transformer
Table 3: Failure Rates of Combined Group of Transformers
GT
A1.16
491
REPORT
WG
A2.37
No. 261 - April 2012 ELECTRA 49
HIGHEST SYSTEM VOLTAGE [kV]
FAILURES &
POPULATION
INFORMATION69 kV < 100
100 kV <
200
200 kV <
300
300 kV <
500 kV 700 All
Failures145
TransformerYears 15077
FAILURE RATE 0.96%
206 136 95 7589
4615242635 29437219 135491
0.45% 0.32%0.32% 3.20%0.43%
-
HIGHEST SYSTEM VOLTAGE [kV]
FAILURES &
POPULATION
INFORMATION 69 kV < 100
100 kV <
200
200 kV <
300
300 kV <
500 kV 700 All
Failures 62759496
Transformer -Years143 2842 4838 12132740 20695
FAILURE RATE 0.00%0.21% 0.56%0.49% 0.54%0.46%
HIGHEST SYSTEM VOLTAGE [kV]FAILURES &
POPULATION
INFORMATION69 kV < 100
100 kV <
200
200 kV <
300
300 kV <
500 kV 700 All
Failures145 212 163 15411685
Transformer-Years 15220 4899447473 41569959 156186
FAILURE RATE 0.95%0.43% 0.34%0.37% 1.15%0.44%
0
