A new assessment of power transformer reliability

3 February 2018

Data from a comprehensive survey of failure assessments collected by CIGRE, the world’s leading electrical systems association, has revealed the most significant risks of power transformer failure and the principal causes of downtime.

Cigré, the international council on large electric systems, is the leading authority in its field. Transformer survey data collected by its Study Committee A2 via working group A2.37 was analysed by the WG and a report for the association’s members produced in the second half of 2015. Members of the working group later produced a digest of the survey’s results, and that is reported here, together with an assessment (right) published by the Finnish environmental and industrial measurement specialist Vaisala.

Accurate information about the service experience of high voltage equipment is of significant value both for electric utilities and for manufacturers of such equipment. It helps manufacturers improve their products, and provides important inputs for the utilities when buying equipment, organising maintenance and benchmarking their performance. Statistical analysis of past failure data can display useful features with respect to future failure behaviour. Equipment reliability data is also required when assessing the overall reliability of an electric power system, including studies of the electric energy supply security. Furthermore, international standards applicable to high voltage equipment are being improved based on service experience and reliability data.


Working group A237 Transformer Reliability was formed in 2008 with the following objectives: reviewing all existing survey and study different practices (in terms of data collection, compilation, etc); conducting a new international survey on transformer failures; compiling and analysing the collected data, and interpreting the results (calculation of failure rates, classification into failure location, failure causes and failure modes).

The paper produced by working group A2.37 as a digest of the survey results presents and discusses the working group’s data collection methodology, and the results of the analysis in terms of failure rates and the classification into failure locations, causes, modes and effects of the failure.

The paper commences with a review of the developments in transformer design, manufacture and technology, as presented in the literature, with the aim of assessing to what extent developments influenced the trends in reliability (or failure) of transformers. It then presents a general overview of transformer reliability, again as presented in the literature, with emphasis on recommendations for reliability practices in the industry, definitions and terminology associated with reliability, and statistics from available reliability surveys. The application of statistical analysis to transformer data is also discussed. Existing countrywide and company related surveys, their general information about specific definitions and methodology of data acquisition and analysis are presented.


The failure data collection methodology developed by working group A2.37 is described, along with the associated limitations. A uniform way of collecting, compiling and presenting data is proposed. Finally the results and analysis of the new international transformer failure survey are presented in terms of the investigated population, calculated failure rates and failures classification into location, cause, mode and effects of the failures. The procedure to calculate hazard curves in the case of truncated data is explained. For three populations the hazard curves were determined, and they showed no distinct ageing behaviour. The developed questionnaire, further definitions and the theory of population data analysis are provided in the appendices.

Review of existing surveys

The working group reviewed the methodology of national surveys from five countries and five company surveys of different utilities, manufacturers and a consultancy. The main objective of the national surveys is the systematic 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, the failure cause or mode and repair activities are normally not included, therefore limiting its benefit with respect to asset management. In contrast to this, internal company surveys offer the advantage of a statistic dedicated to the performance of the individual equipment, where valuable information for asset management can be obtained. (See J. Jagers and S. Tenbohlen, ‘Evaluation of Transformer Reliability Data Based on National and Utility Statistics’ in 16th International Symposium on High Voltage Engineering, Proceedings of the ISH, Cape Town, South Africa, 2009.)

Data collection and preparation

Initial working group discussions concentrated on analysing the readily available statistics, but the different definitions and information content constrained forming a coherent database from individual sources. It was also agreed that the scope needed a be broadened to allow comparison with the failure statistic of the 1983 survey (A Bossi, J Dind, J Frisson, U Khoudiakov, H Light et al, ‘An international survey on failures in large power transformers in service’ electra, pp 21-48, 1983). A questionnaire was therefore developed to collect utility failure statistics in a standardised way. Besides information about the population under investigation, failure data was collected for various groups of transformers in terms of the failure locations, failure causes, failure modes, actions, external effects and others parameters.

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. The necessary repairs 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 transformer or the tap changer tank, or an exchange of the bushings. A reliable indication that the transformer condition prevents its safe operation is considered a major failure, if remedial work (longer than 7 days) was required for restoring it to the initial service capability. In some cases also failures were assigned as major, if remedial work was shorter than 7 days and extensive work with oil processing had to be done (for example, exchange of bushings).

A questionnaire consisting of two major sections was developed to collect data. The first section of the questionnaire requested general information about the population of the operating transformers for the indicated failure period. The population information included the transformer application, type, number of phases, voltage, rated power, typical loading, and manufacturing period.

The second section captured the transformer failure data, grouping data into four categories as follows: 

  • Identification of the unit application, type, construction type, year of manufacture.
  • Features of the unit rated power, nominal voltage, number of phases, cooling system, type of oil, tap changer, tap changer arrangement, oil preservation system, over-voltage protection.
  • Details of occurrence: 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.

Failure rate was calculated according to the definition worked out for the 1983 Bossi survey and was expressed as:



ni: Number of failures in i-th year
Ni: Number of transformers operating in the i-th year
T: Reference period (normally one year)

Data analysis

The working group collected data from 964 major failures that occurred in the period 1996 to 2010, within a total population of 167459 transformer- years, contributed by 58 utilities from 21 countries. The year of manufacture of the units span from the 1950s up to 2009 and the reference periods range from 3 to 11 years. Because the number of operational transformers was only provided for one year, the total number of transformer-years (population per utility) was calculated under the assumption that the number of transformers in operation was constant during the reference period. The number of transformers was multiplied with the length of the reference period in years to obtain an estimate of the total number of transformer-years.

At 150072 transformer-years the investigated population of substation transformers was considerably higher, as shown in Table 1, than previous surveys. This value was almost four times higher than the population in the 1983 Bossi survey (see above) with 40 547 transformer- years. The population of generator step-up transformers was considerably lower at 17387 transformer-years (Table 2).

The failure data of the full population were analysed as a function of the primary location (component) in the transformer where the failure was initiated. Figures 1 and 2 show the failure location analysis according to voltage classes and according to transformer application in units with voltages 100 kV and above, respectively. GSUs without tap changers were excluded from the analysis in order to avoid misinterpretation of the data.

Up to 700 kV, the contribution of bushing related failures increased with increasing voltage class. Lead exit related failures exhibited the same increasing trend, across all voltage classes. The contribution of tap changer related failures appeared to decrease with increasing voltage level.

Winding related failures were the largest contributor in both transformer applications. GSU transformers had a higher attribution of winding (48%) and lead exit failures (13%) than substation transformers (38% and 6%). Substation transformers on the other hand had a higher contribution of tap hanger related failures (31%) than GSU transformers (12%).

The contributions of bushing related failures were similar in both transformer applications.

Conclusion and recommendations

A questionnaire was developed by Cigré working group A2.37 to enable utility failure statistics to be collected in a standardised way. Transformer failure data could then be analysed and interpreted for various types of transformers in terms of failure locations, failure causes, failure modes, actions, external effects and failure rates in transformers.

The conducted failure survey showed a failure rate within 1%. Winding related failures appeared to be the largest contributor to major failures, irrespective of transformer application or manufacturing period, and due to their impact typically led to a situation where the failed transformer was scrapped. Failures originating in the bushings most often lead to severe consequences such as fires and explosions. Dielectric mode failures were the highest contributor to failure modes, irrespective of transformer application. 

T&D Figure 1. Failure locations of substation transformers (>100 kV) based on information from 536 major failures. Source – Cigré
T&D Figure 2. Failure locations in generation step-up transformers (>100 kV) based on information from 127 major failures. Source – Cigré

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