Transformer overhauling

Transformer overhauling

In essence, transformer overhauling entails a comprehensive procedure where the equipment is disassembled, cleaned, inspected, and repaired as needed to restore it to standard operating conditions. Overhauling is typically conducted in response to internal failures or as part of scheduled maintenance intervals. This process is more complex for higher-rated transformers. Factors such as previous test reports, fault currents, transformer design, and pre-overhauling tests play a pivotal role in guiding the overhaul. Dissolved Gas Analysis (DGA) and the examination of gases in the Buchholz relay offer valuable insights into internal transformer faults.


The power transformer inspection phase involves a thorough visual examination of external and internal components, including critical elements like windings, core, bushings, tap changers, and cooling systems. Special attention is given to identifying signs of physical damage, corrosion, wear, and oil leakage. Advanced techniques, such as ultrasonic testing and thermography, may be used for a more in-depth analysis. The insights from this phase guide decisions on subsequent repairs and replacements, ensuring the transformer’s continued safe and efficient operation.


The testing phase is a comprehensive evaluation of the transformer’s electrical performance. Various tests are conducted to ensure that the transformer meets specified standards and operates reliably:

  1. Insulation Resistance Tests: These tests assess the integrity of insulation materials, identifying any weaknesses or potential breakdown points. It is crucial for preventing electrical leakage and ensuring the transformer’s safety.
  2. Turns Ratio Tests: This test verifies the turns ratio between the primary and secondary windings. Deviations from the expected ratio can indicate issues with the transformer’s core or windings.
  3. Winding Resistance Measurements: Winding resistance tests help identify any abnormal resistance values in the transformer windings, which can be indicative of issues such as corrosion or damage.
  4. Power Factor Tests: Power factor testing evaluates the efficiency of the insulation system. Deviations from the norm can indicate the presence of contaminants or deterioration in the insulation.

The testing phase is critical for identifying potential electrical issues and verifying that the transformer can operate safely and efficiently in its intended environment. It is an essential step in the overall maintenance process to prevent electrical failures and enhance the reliability of the transformer.


To remove external and internal contaminants, it’s essential to clean the transformer. Externally, it preserves structural integrity and aids visual inspections. Internally, it ensures optimal performance by preventing overheating caused by dust or deposits. Clean components facilitate accurate diagnostic tests, contributing to overall reliability and minimizing the risk of unexpected issues.

Repairs and replacements

Based on the condition assessment, repairs and replacements of various transformer parts take place:

  1. Bushings: Damaged or worn-out bushings, critical for insulation and supporting electrical connections, are repaired or replaced to maintain optimal performance.
  2. Gaskets: Gaskets, essential for preventing oil leakage and ensuring a sealed environment, are inspected and replaced if necessary to prevent potential issues.
  3. Insulation: Worn or damaged insulation materials are repaired or replaced to uphold the transformer’s insulation integrity and prevent electrical breakdowns.
  4. Cooling Systems: Components related to the cooling system, such as fans or radiators, are inspected and repaired to ensure efficient heat dissipation, preventing overheating.

Decisions on repairs or replacements are guided by the severity of the component’s condition, considering factors such as the transformer’s age and operational demands.


Reassembly involves reconstruction with refurbished or new components. This includes precise alignment, secure fastening with applied torque, sealing for oil integrity, and rigorous quality checks. The emphasis on precision during reassembly ensures the transformer operates within design parameters, enhancing overall performance and reliability.

Transformer oil filtration and refilling

The oil filtration and refilling phase are critical steps aimed at restoring the insulating oil to optimal condition.

  1. Filtration: The insulating oil undergoes a thorough filtration process to remove impurities and contaminants acquired during the transformer’s operation. This is essential for improving the dielectric strength of the transformer oil and preventing electrical breakdowns.
  2. Purification: Advanced purification techniques may be employed to further enhance the quality of the oil. This can involve processes such as degasification to remove dissolved gases and moisture, ensuring the oil meets stringent performance standards.
  3. Refilling: Once purified, the oil is carefully refilled into the transformer. This process involves precise measurements to achieve the recommended oil level, considering factors such as temperature and operational requirements.
  4. Sealing: After refilling, the transformer is sealed to create a closed system. This sealing is crucial for maintaining the purity of the oil and preventing external contaminants from compromising the insulating properties.

The quality of the oil directly impacts the transformer’s ability to withstand electrical stresses and environmental conditions, making this phase essential.

Testing and Commissioning

Finally, the testing and commissioning is the last phase which is a comprehensive evaluation process to validate the transformer’s performance and safety after refurbishment.

  1. Insulation Resistance Tests: These tests assess the effectiveness of insulation materials, ensuring they can withstand electrical stresses. Adequate insulation resistance is crucial for preventing electrical leakage.
  2. Load Tests: The transformer undergoes load testing to simulate real-world operating conditions. This verifies its capability to handle the specified loads and ensures optimal performance under varying demand.
  3. Voltage Withstand Tests: These tests evaluate the transformer’s ability to withstand specified voltage levels without breakdown. It is a critical measure of the insulation’s integrity and the transformer’s overall resilience.
  4. Functional Tests: Various functional tests are conducted to validate the correct operation of components such as tap changers, cooling systems, and protective devices. This ensures that the transformer functions as intended in different operating scenarios.
  5. Commissioning: The transformer is commissioned for active service after successfully passing all tests. This involves integrating it into the larger power system and confirming its seamless operation within the network.


To keep the transformer operational and reduce any risks, transformer overhauling and transformer maintenance are crucial processes that every transformer must undergo.

At TTES, we have over 100 combined years of experience manufacturing, maintaining, and repairing transformers. If you’re looking for industry-leading lead times of just 20 weeks on average, don’t hesitate and reach out to us for a free quote!

Categories :

Share This Article :

Satisfaction guarantee

With our OEM network and 100+ years experience, we ship in 20 weeks. Transparency and satisfaction guaranteed on every projects.

Get your transformer at an unrivaled speed