Oct 14, 2019

Transformer failure investigation process

  • Article
  • transformers
  • transformer failure
  • Factory acceptance test (FAT)

​Transformer failures are unfortunately inevitable and may manifest themselves in a variety of ways. In the event of any such failure, it is important to understand the cause.

Transformers are one of the most complex pieces of network equipment, due to their custom-made design and the numerous varieties of applications. In the event of a failure, one of the most demanding challenges is understanding the design, manufacture, verification test and system interaction associated with the transformer. As a result, when an incident occurs, the real cause is invariably never a straightforward answer.

  1. Factory acceptance test (FAT) failures

    Incidents during FATs may appear to be the consequence of the specific test in progress. However, this may not always be the case, as earlier tests may have contributed without revealing their part in the puzzle. Most test-related incidents can be placed into the dielectric or thermal categories, each presenting a variety of characteristic data, which takes an experienced eye to filter and an in-depth knowledge of the product to determine the key contributor(s) by a defined process of elimination within an effectively controlled environment.

    Operational service failures

    In-service incidents can often be more difficult to assess, as often the event occurred without any witnesses, and you are faced with a data collection exercise to look for key markers and indicators of specific failure modes. In such situations, as initially all investigations are performed from the exterior perspective, it is invaluable to have a mind’s-eye picture of the transformer’s internal configuration, as this can support the development of a targeted testing plan and results comprehension for correlation with a specific event symptom. In-service issues also have the added complexity of the system, which interacts with the transformer and may have an influential role to play in specific events, all of which needs to be open for consideration at the start of the investigation and only closed in the event of factual evidence.

    The investigation process: health and safety first

    When incidents occur, it is critically important to keep an open mind. We must first engage our “EHS radar” to ensure your safety and that of others involved in diagnosing the potential root cause.

    The general environment at the scene of the investigation may hold numerous potential risks associated with such areas as working at heights, confined space, impressed or over voltages, etc., all of which need to be factored in to a systematic and methodical analysis of the issue under investigation.

    Acting in haste can lead to unintentional EHS infringement or the omission of a specific piece of critical evidence being collected or analyzed. The risk of contaminating potentially important evidence is also possible.

    In practice

    Failure investigation activities must be methodical. Ideally a systematic process of elimination must be performed.

    Evidence must be collected and appropriately quarantined for expert review. It is just as important to collect the relevant information as it is to know how to get the necessary data (where and from whom). The engineer in charge of the investigation must be able to:

    • communicate with the various manufacturer levels and departments.
    • be clear on what information and formats are required.
    • be able to assess and establish whether the information makes sense.
    • make logical assumptions.
    • not only question the operators in service or the manufacturer, but also be able to give positive constructive suggestions and actions to remediate the situation.
    • be flexible and define the appropriate actions based on continuous evaluation of evidence and results received.
    • actively support the process.

    Finally, all root cause results must be translated into actions. The evaluating expert must:

    • be able to present the findings and conclusions of the root cause analysis with clarity to both the manufacture and the client.
    • establish a common understanding between the manufacturer and the client.
    • provide support in developing and implementing solutions by presenting viable options to reach the best compromise across all parties.

    The key to expert support is the ability to define clear actions and milestones and specific responsibilities agreed upon within the imposed operational timeframes.

    Examples of specific considerations for an investigation

    Personnel safety and effective risk management remain a priority.

    Dry-type transformers:

    • Relative ease of access to the active part for visual inspection
    • No supporting DGA data
    • Selected low-voltage tests are available
    • Limited partial-discharge testing can be performed at site
    • Air insulation system can “recover” from transient flashovers without trace

    Fluid filled transformers:

    • Active part is initially less accessible and may require confined space access protocols
    • Characteristics of the fluid being used
    • DGA can support analysis
    • Selected low-voltage tests are available
    • Limited partial-discharge test can be performed at site
    • Fluid insulation system can “recover” but may often leave trace evidence

    Transformer operating condition issues

    • Extensions to existing systems and interconnections
    • Connection of inductive loads, which may introduce significant harmonics
    • The installation of certain switch types adjacent to transformers
    • Locating transformers on the ends of long transmission lines
    • Very fast transient oscillations (VFTO) or ferro-resonant conditions
    • Maintenance regime effectiveness
    • Environmental impacts


    Each transformer failure event must be evaluated on the physical evidence, without prejudice, while applying a systematic and methodical approach to identify and eliminate contributing factors. The experience of the engineer acting as root cause facilitator in the process is a key role in ensuring that all potential avenues are rigorously explored and that records are maintained. It is the engineer’s responsibility to deliver findings and recommendations to the client in a clear and effective way.

    Check our transformers expertise page to learn more about our services.

This content is for general information purposes only. All rights reserved ©BBA

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