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Home / Technical Articles / Where and how to find the root cause of a power transformer failure (troubleshooting guide)

Troubleshooting guide for young engineers

When Whatsapp, Facebook, or Gmail is down, it’s manageable, but if transformers fail, the entire business comes to a standstill. No-power-no-business transformers, as the heart of every substation, impact industries and affect the country’s economy and have social and political ramifications. Any malfunction in this expensive machinery can be the leading cause for a complete outage of the network failures.

Where and how to find the root cause of a transformer failure (troubleshooting guide)
Where and how to find the root cause of a transformer failure (troubleshooting guide)

Improving the relationship between types of problems in transformers and their indicators will help identify internal faults and their locations. Therefore, the condition and performance of the power transformer should be very well known, including its reliability and security. When a fault occurs it is quite difficult to determine which failure should have more attention since there are always several visible damages.

Organic materials used in the transformer are subject to ageing processes which lead to the gradual degradation of their physical, chemical, and electrical properties.

The fault-finding process is intended to determine the fault in the transformer in the shortest time and restitution the faulty transformer into service after due rectification and corrective measures.

This makes it of utmost importance that a thorough investigation is made for ascertaining the root cause of the fault. Besides this, each failure gives learning for taking appropriate corrective and preventive actions at works to prevent the occurrence of such failures in the future.

Table of Contents:

  1. Modes of Transformer Failure
    1. Transformers Five Potential Failure Sources
    2. Classification of Transformer Failures
  2. Approach for Fault Analysis
  3. Caution to be taken care of during fault investigation
  4. Case Studies:
    1. Case Study 1: Unbalance Phase Voltage
    2. Case Study 2: A faulty Transformer Received at Manufacturer’s Workshop
    3. Case Study 3: Unbalance Winding Resistance
    4. Case Study 4: Erroneous Value with Ratio Measurement
    5. Case Study 5: Short circuit and inter-turn insulation failure
    6. Case Study 6: Transformer Tripped in OLTC OSR
    7. Case Study 7: Internal Fault of Transformer
  5. Conclusion

1. Different modes Transformer Failure:

With many articles and studies made on types of failure in transformers, let us have a quick recap of the most important points.


1.1 Transformers Five Potential Failure Sources

No.Potential Prone FailuresCriticalityMain Causes
1.BushingsMediumInsulation deterioration
Short circuit
Mechanical damage
2.On-Load Tap Changer (OLTC)MediumInternal Arc
Mechanical damage
3.WindingsHighStress due to Internal failures
Mechanical damage
4.CoreHighImproper Insulation
Excessive Heating
Loose laminations/stacking
5.OilHighOxidation of the oil
Thermal decomposition
Moisture contamination

Figure 1 – Left: Damaged Bushing; Right: Faulty OLTC

Left: Damaged Bushing; Right: Faulty OLTC
Figure 1 – Left: Damaged Bushing; Right: Faulty OLTC

Figure 2 – Deformed core due to fault

Deformed core due to fault
Figure 4 – Deformed core due to fault

Figure 3 – Damaged transformer winding

Damaged transformer winding
Figure 3 – Damaged transformer winding

Go back to the Contents Table ↑


1.2 Classification of Transformers failures

Major failures in power transformers can be classified into these three categories:


Failure Classification – Thermal Factors

Cause: The composition of transformer oil is organic in nature and the cellulose in oil deteriorate during the working life of the transformer due to thermal expansion and contraction during loading of the transformer.

It results in a decrease in the dielectric strength of the insulation and weakens the insulation to rupture under normal voltage conditions.

Fault Analysis:

  1. Overloading of the transformer beyond its designed capabilities
  2. Operation of transformer on nonlinear loads.
  3. Operation of transformers in excessive ambient temperatures.
  4. Failure of the cooling system.
  5. Blockage of oil ducts.
  6. Operation of transformer in an over-excited condition, and
  7. Blockage in axial or radial oil ducts.

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Rahmatullah Alam

Bachelors in Electrical Engineering with specialization in Electrical Items inspection. Extensively experienced in inspections, review of test plans, procedures, and standards requirements. A lifelong learner with a focus on in-depth learning and sharing knowledge gained during my professional career. The ultimate goal is to ensure the utmost level of quality standards, focus on implementing digitalization and optimization in the Electrical Engineering field.

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