Technical Article

Transformer Maintenance: Planning for Failure

September 26, 2023 by Ahmed Sheikh

This article examines transformer maintenance, including simple checks involving temperature and sound that indicate potential issues, maintenance procedures for energized and de-energized dry transformers, and assessments for energized liquid-cooled transformers.

Because transformers are so reliable and most of their components are hidden, the symptoms of premature failure can go unnoticed.



Image used courtesy of Wikimedia


Except for voltage transients and short circuits, all the causes of insulation failure can be detected with a proper maintenance program and testing. Transformer failure can usually be predicted, allowing for planned repair or replacement. Emergency repair or replacement is infinitely more expensive and results in production loss. 


Simple Steps to Routine Transformer Maintenace

Routine maintenance is critical to long transformer life and involves some basic steps.

For smaller encapsulated transformers, regularly cleaning the transformer enclosure is important to prevent a buildup of dust and dirt that will impede heat transfer. Keep the area around the transformer free of obstructions that will limit the movement of cooling air and ensure that the ambient temperature is not too high.


Sound and Temperature Checks

A routine inspection of transformer enclosure temperature and noise can indicate problems in any size transformer. Warning signs of possible problems will be an increase in the transformer enclosure temperature that does not result from increased load. Remember that the top of the transformer enclosure should not be more than 149ºF above the ambient – hot enough to be uncomfortable but not too hot to touch. On larger transformers, check for hot spots on the transformer enclosure; they may indicate localized overheating because of poor cooling, arcing in liquid-filled units, or possible shorted windings.

The sound of the transformer is a simple indicator of transformer problems. All transformers will make noise, resulting from the core expanding and contracting with the constantly changing magnetic field. When the noise level changes or there is some other noticeable difference in the sound's quality, it may result from windings or core laminations that have become loose and are moving. This movement will eventually result in cracking of the insulation and failure. Changes in noise may also indicate that the blocking holding the windings in place has become loose; the blocking will rub on the windings and damage the insulation if not tightened. Repairing these problems will not be possible at all with encapsulated transformers and, in the case of large transformers, are best left to skilled technicians.


Energized Dry Transformer Checks

With large transformers, regular maintenance checks and tests are required to discover potential problems before they become disastrous. The tests depend on the type of transformer cooling and can be broken into tests performed when the transformer is energized and not energized. All energized checks and tests require appropriate safety precautions and awareness of possible arc fault conditions that may be present.

Large dry transformers are supplied with temperature gauges showing the windings' temperature. Regular records of winding temperature, load, and time will not only indicate any overheating but provide a profile of transformer loading over time—both indicators of transformer life expectancy. Any winding temperature readings should also include a record of ambient temperature, which affects the amount of heat transfer from the transformer. If cooling fans are installed, their operation should also be checked regularly.

Visual checks of the transformer windings, if they are visible, will indicate any excessive buildup of dust and dirt and signs of arcing. As mentioned earlier, sound checks are also important indicators of possible problems.


De-energized Dry Transformer Tests

Dry transformers should be de-energized regularly when possible to perform tests on the components within the transformer. While this may be challenging to accomplish because the transformer may provide the only power to a facility, it is critical to avoiding emergency loss of power through early detection of possible trouble.

Dry transformer tests should include insulation resistance testing with a megohmmeter output voltage of 500 or 1,000 volts. To perform these tests, the transformer must be isolated from the supply and the load. Any ground connections to the transformer neutral must also be disconnected. Resistance measurements from the conductor to insulation and conductor to ground should be greater than 1,000 megohms for the high-voltage winding and greater than 100 megohms for the secondary winding.

A turns ratio test should also be performed with a suitable tester. The test will indicate if the ratio of turns is within the acceptable limits and indicate the existence of shorted windings that could fail because of localized overheating.

A visual inspection of blocking and connections should ensure all are tight and secure. Thoroughly cleaning all winding surfaces and connections should be part of any work done when the transformer is de-energized.


Energized Liquid-Cooled Transformer Checks

Repairing any potential problems in a liquid-filled transformer will require the transformer be de-energized and the liquid drained to access the transformer's interior. This is usually done in a transformer repair shop and requires much time to disconnect, transport, and repair the unit. Early detection of problems through energized transformer checks will minimize disruption of service.

The liquid-cooled transformer will also have temperature gauges to indicate liquid temperatures within the tank; there may also be pressure or level gauges. Readings of the gauges should be made and recorded regularly. The tank of a liquid-filled transformer should have a slight positive pressure; this will help to keep moisture and contaminants from entering the tank and affecting the oil. If the pressure is not positive, it may indicate a leak in a gasket or some other type of seal—a condition that will require a shutdown to repair. If the pressure is too high, it may indicate a buildup of gas within the transformer, which may be caused by some type of combustion within the transformer, or it may result from liquid overheating. The liquid-level gauge can indicate possible leaks. The level of the liquid depends on the liquid temperature, so both values should be recorded simultaneously.

Samples of transformer cooling oil should be taken regularly and analyzed by a laboratory. The analysis indicates the conditions inside the transformer without draining the oil from the tank and is critical in a liquid-filled transformer maintenance program. Table 1 lists the types of tests and the conditions they are testing for. 


Table 1. Transformer Oil Tests

Dielectric Strength

Measures the ability of the oil to withstand electrical stress. A low value indicates contaminants in the oil.

Interfacial Tension

Measures the oil’s ability to act as an insulator. A low number indicates degradation of the oil or possible contaminants.


A measurement of aging in the oil. High acidity indicates that the oil has interacted with the components of the transformer, became acidic, and is losing its insulating ability because of aging.


A measurement of aging. As the color darkens, it contains more impurities. If the transformer is not very old, the darker color can indicate overheating.

Total Dissolved Combustible Gas

A measurement of the combustion products found in the oil. When performed routinely, the test can provide a view of the state of the transformer insulation and indications of arcing within the transformer.


The Importance of Comprehensive Transformer Maintenance

The significance of comprehensive transformer maintenance cannot be overstated. Transformers are vital for our electrical infrastructure, ensuring a smooth power flow across various systems. Regular and thorough maintenance is crucial to prolong their operational life, enhance efficiency, and mitigate the risk of unexpected failures. By conducting routine checks, addressing potential issues early on, and staying updated with the latest industry standards, we ensure the safety of electrical systems and contribute to reliable and uninterrupted power supply. Ultimately, comprehensive transformer maintenance, including a thorough inspection, testing, and adherence to prescribed guidelines, ensures an electrical network equipped to power daily operations effectively.