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Transformer Fleet Health Assessment

By in Transformers | September 1, 2016


Fleet health assessments are conducted to collect information on a population of transformers so that more informed operating and maintenance decisions can be made. The information collected allows prioritization of resources to resolve identified problems. High priority is assigned depending on a transformer’s condition and the level of importance in the system. Those with high importance (critical system transformers) and poor condition would be placed highest on the list for immediate attention. Taking appropriate action based on the findings will help reduce unplanned outages, improve availability and ultimately minimize operating cost.

A systematic approach is implemented first by conducting an assessment of the transformer fleet to identify the problem units. Then a more detailed testing and analysis of the transformers with more serious problems can be conducted.

Initial Fleet Assessment

The initial Fleet Assessment examines the data of a population of transformers to quickly identify problems, providing a ranking from normal operating condition to flagging transformers that need immediate attention. This allows the identification of early stage problems, identification of transformers that need further study, uncovers patterns of poor behavior for specific transformer groups and is used to develop an action plan to improve the overall health of the fleet. Generally, oil and electrical test data are the primary data used in this initial stage:


Oil Test Data:

  • Incipient fault conditions
  • Long-term developing problems
  • Insulation wetness (failure modes, aging)
  • Solid insulation age and aging rate
  • Oil condition (deteriorated, contaminated)
  • Identify uncommon / unique conditions

Electrical test data:

  • Insulation integrity (deteriorated, contaminated)
  • Mechanical condition of windings
  • Conducting circuit (conductors, connections, tap changer)
  • Magnetic circuit (core defects, core bonding)
  • Condition of Components (bushings, arresters, tap switches, etc.

Detailed Condition Assessment

Typically, the most poorly evaluated transformers identified in the fleet assessment screening usually require closer scrutiny to determine specific risks and to develop a detailed action plan to resolve the problems found. The transformers at highest risk of failure and with high impact to the system would be selected for a Detailed Condition Assessment, which involves the review of all information available for that specific unit, including the following:

  • Operating and Loading History
  • Fault and Short Circuit History
  • Maintenance History
  • Troubles and Failures
  • Industry Knowledge – known problems, failure modes
  • Manufacturer’s Advisories
  • Other tests, such as infrared thermography and partial discharge detection & location, to better understand problems

The objective of transformer fleet assessments is to improve fleet reliability by systematically identifying problem transformers and create an action plan to eliminate or mitigate the risk of failure.


Doble has a team of transformer design experts regularly assessing the health of ailing transformers on behalf of our customers. Rely on our experts as an extension of your team when you need these critical services. Contact Doble for more information about our transformer consulting and testing services.


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Doble Engineering Company

Doble Engineering Company