Electric Transformers Faults ( A Complete Guide)

Distinct faults are generated by several reasons, each of which has a distinct effect on the power system.

The transformers are one of the most expensive components in the power network, which adds to their significance. The analysis of the transformer’s defects and failures is also so important.

Transformer failure is a miss. One of the main transformers in my work place had a short circuit. We had to replace it ASAP to get power back to the critical loads. What a miss!

In this article I will discuss transformers faults. Let’s get started.

Transformer Faults Types

Electric transformer has many parts as well as it is connected to loads and power source. This makes the transformer prone to internal and external faults.

Hereunder some faults that transformer can face:

  • Internal faults. Winding, iron core, tap changer and oil all these parts are prone to different faults.
  • External faults. Some times the transformer tank, cooling system, high and low voltage bushings or external connections. All these are prone to faults.

Let’s go in deep details and discuss these faults and other faults.

Temperature Rise

Temperature rise is one of the most common transformer critical concerns that transformers face. Temperature increase or overheating may cause a transformer to malfunction or, in the worst-case situation, to fail.

Yes, every equipment that uses electricity emits waste heat as a by-product of its functioning. Transformers are no different.

When a transformer is in operation, it creates heat, which may cause an increase in temperature in the internal winding. As the load on a transformer grows, so does the heat in its internal structure.

The lower the temperature, the more efficient the transformer.

The average temperature increase or rise of the windings over the ambient (surrounding) temperature is referred to as transformer temperature rise.

I have written a detailed article about transformer temperature rise, you can read it here.

What Causes Temperature Rise?

What causes a transformer to become hotter than usual?

Losses occur in the transformer due to winding resistance, hysteresis and also eddy current. All these losses cause a transformer to get hot.

But sometimes though in rare cases, transformers get hotter because of:

  • The load exceeds the transformer’s rating.
  • A low power factor load is connected to the transformer.
  • Malfunctioning or a misaligned transformer’s fan can also cause overheating in a transformer.
  • Any abnormalities or difficulties with fan and pumps of the transformer cooling system
  • Very wet oil, if oil get wet the transformer get hot. That happened with me, when a power transformer got hot, and we had to change the transformer because it was too hot.


Low voltage swich gear Power engineering system could be inspected using thermal image

A comprehensive maintenance program should involve frequent monitoring of the temperature increase of transformers at the work site.

I use thermal imaging in my work to detect ant temperature rise. Thermal imaging is one of the greatest ways to identify any excessive temperature increase and is a reliable technique to keep temperature rise in transformers under control.

However, if thermal imaging is not available, temperature indicators for oil and windings may be used.

Keeping a proper record of temperature in the transformer history spreadsheet to identify any changes or odd rises is also a very excellent practice that is highly recommended.

Here’s what to check for during maintenance: If the loads and ambient temperature don’t change, the transformer temperature shouldn’t either. If it has, it requires extra attention and care.

Transformer Oil Problems

  • Oil Loss

Any decrease in the amount of oil in the transformer’s tanks will have an effect on the winding insulation.

When there is an oil issue, the temperature in the transformer rises. It may also result in an unnatural increase in the temperature of the transformer.

This oil loss is usually caused by a leak, which is visible on the tank body.

The most common reason of leakage is a failure to tighten tank body screws, The body of the tank has rusted., A direct impact caused mechanical damage to the tank body.

As a consequence, doing visual inspections on a regular basis is crucial for spotting leaks early.

In some cases when transporting a transformer filled with oil, its body becomes oily. This may be an indicator for oil leak. In this case, I recommend cleaning the body very well, let the transformer for at least 24 hours and then check it again for any oil leakage.

In most cases, this oil on the body is not oil leakage, its just an oil spillage due to transportation with the tank full of oil. 

So, I recommend to drain the oil before any transportation, And then fill it in the location.

  • Wet Oil

Also, oil may get wet, when the transformer works at a high moisture environment and the silica gel absorb too much water. Then any moisture will enter to the transformer and causes its break down voltage to decrease.

Also, wet oil can cause the transformer to get hot, As I mentioned above.

  • Oil Characteristics Change

If the transformer has an internal fault, such as sparks or short circuit, it will affect the oil.

A good oil analysis can easily tell us what type of fault happened inside the transformer.

Cooling System Faults

The cooling system reduces the heat produced in transformers as a result of copper and iron losses. Cooling fans, oil pumps, and water-cooled heat exchangers are all part of the cooling system.

The failure of the cooling system enables heat to build up in the transformer, affecting different regions of the transformer and causing increased gas pressure to build up within, perhaps causing the transformer to explode.

Some of the primary reasons for failure

  • A leak in the oil/water pipelines is one of the most common reasons of cooling system failure. This creates a reduction in fluids, resulting in reduced heat exchange, which is bad for the transformer. Environmental stress, corrosion, high humidity, and sun exposure all contribute to leakage.
  • Some failures occur as a result of a malfunction with the cooling fans, which force cold air into the tanks for cooling. Fans have flaws as a result of poor maintenance, heavy use, or motor wear-out.
  • The cooling system may not perform effectively due to faulty control circuit or thermostats that sense heat in the transformer. Faulty thermostats display incorrect temperatures, causing the cooling system to work incorrectly.

Low and high voltage Bushing’s Failure

Bushes are insulating devices that enable a high voltage electrical line to pass safely through an earth conductor. In transformers, it provides a current path through the tank wall.

Within the transformer, paper insulators are used, which is surrounded by oil, which provides further insulation. Generally, bushing failure occurs over time.

The following are some of the most prevalent reasons of bushing failure.

  • Vibrations in the transformer cause conductor loosening, which leads to overheating. This heat destroys both the insulating paper and the oil used.
  • Sudden high fault voltages cause partial discharge (the breakdown of solid/liquid electrical insulators), causing the bushes to deteriorate and collapse totally within hours.
  • Water infiltration, age, or significant dielectric losses cause bush seals to fail. As a consequence of this defect, the transformer’s core fails.
  • Internal over-flashing is caused by a failure to replace old oil over a long period of time, or by a shortage of oil as a result of leakage.

In many cases we can fix the defect by replacing the damaged part regardless what is this part. In my work we have spare parts of low and high voltage bushings and also the gaskets.

Tap Changer Failure

The voltage level is controlled using the transformer’s tap changer feature. This is performed by increasing or decreasing the number of turns in the secondary transformer winding. It is the transformer’s most sophisticated and crucial component. Minor faults cause inaccurate power output.

The explanations are

In the event of a Run-Through issue, the tap changer takes its time before adjusting the turns ratio after a delay. The main reason for this is because the relay that changes the tap has residual flux due to filthy oil, which takes time to replace.

Another reason of the run-through issue is the spring becoming brittle over time.

Due to a lack of maintenance, the shaft connection between the tap and the motor driver of the tap changer is not synced. As a consequence, the tap changer is no longer in the proper position.

Because of outdated or burned-out capacitors in the motor, the tap changer is unable to control its direction movement.

Regular use of the tap changer causes the spring to weaken and finally break. As a consequence, the tap changer is unable to adjust the turn ratio of the winding.

Over-voltage or abuse of the tap changer’s motor causes the tap changer to fail to adjust the wind’s turn ratio.

One of the most critical points that worth mentioning, is to check if the tape changer is off-load or on-load type before performing tap changing.

Making tape changing while the transformer is energized with an off load tape changer can’t only destroy the changer but also can destroy the transformer with a loud explosion.

Iron Core Fault

The transformers contain laminated steel cores in the middle that are ringed by transformer windings.

The core’s role is to concentrate the magnetic flux. Faults in the core have a direct influence on the transformer windings, causing them to fail.

Transformer cores are laminated to minimize eddy current. Poor maintenance, old oil, or corrosion may all produce faults in the core’s lamination.

Due to eddy-current, the breakdown of the smallest component of the lamination generates an increase in thermal heat.

The effects of this overheating are as follows:

Overheating occurs at the core surface, which is in direct contact with the windings. As a result, the windings are injured by the heat.

This heat also affects the oil in the transformers, causing the oil to generate a gas that destroys other areas of the transformer.

Tank Faults

The tank in the transformer functions as a receptacle for the oil used in it. The oil in the tank is employed to insulate and cool the tank.

The tank may potentially be employed to support extra transformer equipment.

Environmental stress, corrosion, high humidity, and UV radiation cause the tank to fail, resulting in a leak or cracks in the tank walls.

Oil flows from the tank as a consequence of these leaks and cracks, generating a decline in oil. A reduction in oil level diminishes insulation in the transformer and has an effect on the windings.

Because oil is also necessary for cooling, a reduction in oil creates overheating, which damages numerous parts of the transformer.

A single distribution transformer is made up of numerous different parts that all act together. All of these various pieces have different defects that result in different failures. Some are more dangerous than others, others occur more frequently, and some are harder to detect.

One thing is certain: A single issue impacts not just that specific component but also many others in the transformer, resulting in a greater failure in the transformer from a little defect. Even the slightest error must not be disregarded.

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