REV Engineering LTD.

Power Transformer Maintenance

The Importance of Annual Testing

Problem: Transformers are often the forgotten component of an electrical system because their design includes no real moving parts and they spend a majority of the time performing without issue. These units only gain attention when a major issue arises, and a catastrophic failure can occur



It is suggested to perform annual transformer maintenance in order to avoid potential problems, going no more than 3 years without thorough testing. Annual testing is a proven best practice and is the industry accepted standard.



A dissolved gas in oil analysis (DGA) and oil analysis will take place prior to transformer maintenance using a sealed syringe and a sample bottle. Technicians withdraw oil from an energized transformer that is then sent to a certified lab for analysis. One of the benefits of the predictive maintenance tool is that it doesn’t require a power outage.



The lab analysis measures levels of the various gases dissolved within the oil which help to predict potential faults prior to a major failure. For example, a high level of acetylene would indicate arcing under the oil, which would need to be repaired immediately.


Oil sample testing can indicate a number of things:

  1. If the quality of the fluid or transformer is susceptible to water contamination. A measure of 30ppm would be the absolute threshold for this problem.
  2. The level of acidity in the sample, which has the potential to weaken the insulating materials.
  3. Contaminant levels, which have the potential to lower the dielectric rating of the insulating fluid. If testing were to indicate deterioration or oil reclamation, a replacement would be required.


A technician will perform a number of tests on a transformer including:

  • Turns Ratio and Excitation – ensures the integrity of the windings
  • Power Factor and Insulation Resistance – measures the quality of the insulation system
  • Winding Resistance – measures the windings, connections and tap changer quality


While doing thorough testing, NETA certified technicians will complete an inspection of the transformer system and perform any applicable repairs at this time. Technicians can also install a number of predictive testing items on the transformer for continuous monitoring purposes. These innovative technologies include: online partial discharge monitoring, bushing power factor monitoring and gas monitoring.

Once the transformer problems have been found it is essential that they be repaired. The majority of these problems can be fixed in the field by a certified technician in the field with minimal outage time.

When deemed necessary, a technician can perform an internal inspection of the transformer core and coils, then repair or replace any necessary parts, including tap changers, bushings and auxiliary devices such as gauges and protection relays. If there are issues with the oil found through testing, then a technician can also perform oil reclamation through processing or full oil replacement.


Throughout the maintenance testing process, it is imperative to consider a number of safety hazards:

  • Missing ground wires which run the risk of electrocution
  • A transformer under vacuum which can cause transformer failure and fatality
  • Getting too close to electrical equipment depending on the voltage (limits of approach)


*All testing and repairs should be done by a qualified electrical NETA technician.



Without annual testing, transformers run the risk of developing serious and hazardous problems over time. Regular maintenance testing is suggested to avoid the development of any potential problems and allows for repairs prior to a major fault.

Testing includes oil quality and DGA analysis, along with turns ratio and excitation, power factor and insulation resistance and winding resistance.

Many problems found through this testing can be repaired by a qualified field technician on site, while other more serious defects require transformer shut down.

Precautions should be taken to ensure optimal safety and all testing and repairs should be completed only by a qualified electrical technician.


Please Note: This document is meant to serve as a high-level overview of the broad topic of transformer maintenance. Please contact us for more detailed information.

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