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5 Common Transformer Faults And How To Diagnose Them

5 Common Transformer Faults And How To Diagnose Them

Transformers are high-voltage capacity equipment that facilitates the transmission and distribution of electrical power. Its primary purpose is to step down voltage levels in a given power system. But despite being enclosed devices with no rotating parts, they are not entirely invulnerable to faults. Should these faults crop up, leaving them unchecked could lead to disastrous results by damaging the transformer and impeding daily operations.

To avoid this, one must first be familiar with the most common transformer faults and how to diagnose and protect against them.

1. Overheating fault

Overheating in a transformer often arises due to performing overloads beyond the permissible overloads stated by the manufacturer as per IEC standards and external faults like short-circuiting on installations downstream or cooling system failure. Fortunately, most overheating faults can be easily prevented by doing proper transformer maintenance. However, if the issue is neglected, overheating typically causes the insulation of the windings to break down.

The general steps to diagnose this issue are:

  • Check if the cooling fans are working properly, along with the condition of the fans and oil pumps.
  • Inspect the radiator and ensure it is clean. Otherwise, clean out the dust, dirt, and other contaminants that may have stuck to it.
  • Determine if there is some sort of overcurrent by checking the current of the transformer and reducing the load if necessary to stay in line with the unit’s rated current.
  • The ambient temperature can affect the transformer when it operates on particularly hot days. For these situations, consider turning up the cooling stations.
  • Oil mainly works to cool down the transformer. Thus, check if the oil levels have been depleted and replace it when necessary. Also, don’t forget to check for potential leakages causing the issue.

2. Oil and winding insulation faults

Transformer oils are specifically made to supply electrical insulation under high electrical fields. Thus, a substantial decrease in its dielectric strength could mean that the oil is no longer fit for use. Some factors that could lead to an oil’s dielectric strength deteriorating include polar contaminants like water and cellulose paper breakdown.

In case of minor faults like damage to the insulation in the core bolts, local overheating, and so on, the arcing causes gas to slowly generate in the oil and rise towards the conservator.

On the other hand, a major fault that results in severe arcing will quickly release a large volume of gas and oil vapours that do not have enough time to escape. This violent development thus bodily displaces the oil and builds up pressure that forces the oil surge into the conservator.

Essentially, the localised breakdown and heating of the oil is the main culprit behind virtually all faults affecting the transformer core and windings.

3. Core fault

Serious overheating occurs if any part of the core insulation gets damaged or the core’s lamination structure is bridged by a conducting material that allows sufficient eddy current to flow. Furthermore, pay attention to the insulated core bolts used to tighten the core. Failure in their insulation can provide an easy path for stray current, which will also contribute to overheating.

4. Phase-to-phase fault

Also known as three-phase faults, these issues do not flow through earthing and are thus limited to just winding impedance. The value of this impedance is intentionally designed into the transformer unit to restrict the maximum short circuit current. This is achieved by making the leakage reactance flux generate the required to short-circuit level by not linking the primary and secondary windings. Differential relays and Buchholz relays are the main ways to resolve this fault.

To diagnose a phase-to-phase fault, check if the transformer’s protection devices have isolated the transformer and released oil from the tanks. Some of the usual causes of short-circuiting in this fault include:

  • Low oil levels
  • Ageing transformer
  • Bad oil from water contamination
  • High voltage due to lightning or other electric surges

5. Tank faults (oil level) and cooling failure

Oil leakage in a transformer may cause a flashover between end connections and the windings. When there is less oil than is necessary for the tanks, it leads to winding insulation reduction. Ultimately, a tank fault results in an abnormal rise in the transformer’s temperature. Inspect for leakage by periodically conducting a visual check on the tank body. The common reasons for leakage include:

  • Loosening of tank body screws
  • Corrosion on the tank body
  • Mechanical damage caused by a strong impact on the tank body

On the other hand, failure in the cooling system will inevitably affect the transformer’s performance and prevent it from operating at its full rating. Most smaller transformers are generally fine with just natural cooling, but the larger models require forced cooling in their design. Do a performance check on the transformer’s cooling system to verify if it still functions as intended.


The common transformer faults stated above should serve as a good starting point to allow you better stay on top of your transformers and ensure their normal functioning. You should also be aware of how you should determine transformer criticality.

That said, there are many other faults that can affect a transformer, consider working with a mechanical and electrical engineering company to help perform electric transformer testing and maintain these critical components of your electrical system.