Guide to Oil-Filled Transformer Maintenance

Regular maintenance is a major part of keeping your oil-filled transformers in good working condition. Without scheduled upkeep, your transformers will be more likely to experience issues that can snowball into larger operational problems, like system failure. You don’t have to rely on reactive or condition-based maintenance when taking care of your machinery. Staying ahead of any and all equipment concerns lets you address them sooner and enjoy a better outcome.
This guide to oil-filled transformer maintenance can help you avoid the expenses associated with repairs and replacements by teaching you effective ways to inspect your units and create maintenance schedules.

Routine Maintenance for Oil-Filled Transformers

What does routine maintenance look like for oil-filled transformers? You should have separate strategies for daily, weekly, monthly and yearly upkeep, with each timespan consisting of various maintenance components. Check out the sections below for guidance on how to set up your schedules.


Daily maintenance schedules for oil-filled transformers should include oil and temperature checks. Inspect the ambient, winding and oil temperatures to see if they’re within the normal range. The oil level gauge should vary with changes in temperature and, if it’s a magnetic model, it should respond to the presence of a magnet by rotating. If it doesn’t do either of these things, this is a sign that the indicator needs to be replaced.
Analyze the load voltage and current by comparing them against the rated figures. The load settings should fit appropriately with whichever type of transformer you’re using to avoid electrical damage. Keeping your overcurrent, differential and ground current relays in excellent shape will also prevent electrical failure.
Noise level is often a more relevant performance indicator for dry type transformers. However, if your oil-filled model operates loudly, it’s essential to figure out what might be causing this phenomenon. High noise levels often originate from the transformer core, cooling system or coils. A power transformer’s noise measurements can be expressed using three techniques:

  • Sound pressure level method
  • Sound intensity method
  • Sound power level method

Weekly and Monthly

Your weekly maintenance schedule should include tasks such as:

  • Analyzing the oil level in your transformer by a quick check of the oil level gauge and by checking it against the oil temperature.
  • Doing a full visual inspection of the transformer’s exterior, including any discoloration and chipped paint.
  • Checking the Buchholz relay for gas collection.
  • Making sure the gasket joints and radiators are tight and show no oil leakage.
  • Inspecting any cover mounted pressure relief device vent for proper sealing.
  • Investigating the pressure relief device for signs of proper operation.

Some jobs will only need to be performed monthly or quarterly unless needed sooner. For example, you might check the oil’s dielectric strength, top-off your oil or check your cable box and terminal bushings for adequate tightness or damage every few months. The bushings should be checked for cracks both once a month and at every yearly inspection. Further, you might inspect and clean the transformer’s auxiliary circuits and accessories on a six-month basis.
Create a set of regular intervals to test oil samples for moisture, deterioration, acidity and other signs that your oil needs filtering or changing. You can also either have someone come on-site or send out samples in an air tight syringe for dissolved gas analysis (DGA) which will tell you what kinds of dissolved gases are in your oil. Some of the most commonly found types include acetylene, methane, hydrogen, ethane, ethylene, oxygen and carbon monoxide.


Your yearly maintenance plan will likely include the previous measures mentioned in the daily, weekly and monthly schedules, as well as transformer components that require checks less often. Many of these tasks will be more involved than some of your usual daily or weekly responsibilities, so don’t hesitate to call on a transformer maintenance and repair professional to handle these duties.
Put these things on the schedule for your yearly maintenance plans:

  • Once every year, check the cover mounted pressure relief devices, pressure relief valve and/or Schrader valve along with surge relay for operational efficiency and signs of damage.
  • Check the temperature indicators and off circuit selector annually and replace these components if needed.
  • Clean the marshaling boxes and check their weatherproof seals.
  • Measure the insulation resistance of the transformer’s protective relays.
  • Check the fan or oil pump motors and check their resistance.
  • Every three to five years, examine the top of the transformer device around the pressure relief tank to ensure there are no leaks.

Oil-Filled Transformer Maintenance Checklist

To make the process easier, it’s always advisable to have a checklist when performing maintenance or outsourcing more complex tasks to a trusted professional. Here’s a selection of basic items to check off the list to ensure a thorough job:

  • Make sure the transformer is offline and de-energized before performing any upkeep.
  • Do a visual review to catch any chipped painted, dirt accumulation, rust or equipment discoloration.
  • Listen for abnormal noise or vibration.
  • Clean the unit of any dirt, dust and debris.
  • Check your oil level and temperature indicators.
  • If your transformer has a silica gel breather, clean it and if it has turned pink, replace them.
  • Inspect the machine’s bolts and hardware and tighten anything that has come loose.
  • Address any leaks immediately — if your transformer leaks often, it might be best to look into a full replacement. Leaks can come from a bad gasket, a pin-hole leak at a weld or any rust on the bottom of the unit.
  • Re-energize the transformer if there are any tests or analyses that need to be performed, such as infrared temperature analysis or breakdown voltage testing.
  • Keep a detailed record of all the maintenance jobs performed, including who conducted them, when they were done and which parts were checked.

Potential Problems With Oil-Filled Transformers

When performing preventive maintenance for oil-filled transformers, it’s best to know what kind of issues you’re looking for. Oil-filled transformers share some common operational problems. Understanding what these are and how they manifest can help you pinpoint any signs of upcoming failure. Then, you can stop the breakdown before it begins, which saves you money and reduces unexpected downtime.
Here are some standard concerns that will tell you when it’s time to repair or replace your transformer:

  • Presence of sludge: Sludge formation happens due to chemical reactions such as polymerization and decomposition, which create solids that accumulate within the winding structure. Moisture and humidity can also cause oil oxidation and deterioration.
  • Signs of aging: Signs of aging like deteriorated insulation can be clues to your machine’s overall condition. Components will naturally wear out and require more upkeep with age, at which point it might be best to look into upgrading your system.
  • Oil leaks: Leakage can originate from gaskets, cooling radiators/fins, tanks, gaskets and various other transformer components. They can be fixed if they are minor and infrequent, but more constant occurrences signal a bigger problem.
  • Radiator blockages: A blocked or closed radiator will appear cool when you take pictures of it with an infrared camera. If your radiator has this problem, take care of it as soon as you find it. If a transformer’s operating temperature increases by 46.4 to 50 degrees Fahrenheit, its lifespan will shorten by one-half. This occurs because the materials insulating the windings and structures begin degrading.
  • Mineral deposits: Avoid getting water on the transformer’s tank or radiator. The minerals will create deposits on the surfaces of these components and become extremely difficult to remove. These also reduce the cooling system’s efficiency, making it easier for your unit to overheat and experience failure.
  • Electrical failure: Electrical issues are often common causes of transformer failures. They can be triggered by line faults and voltage spikes. A high-quality surge protection system can prevent this from happening.
  • Paper degradation: As cellulose paper ages, it releases furans that accumulate in the oil and lead to sludge. This process can occur faster if the paper is exposed to contaminants — which is why tight seals are necessary.

The Benefits of Preventive Maintenance for Oil-Filled Transformers

Preventive maintenance brings a range of benefits for any industrial machine, and transformers are no different. By taking advantage of this maintenance strategy, you’re ensuring that your operations run smoothly without the danger of unexpected unit failures or hazardous working conditions. Transformer problems become less of a possibility when you keep a close eye on your machinery and follow a predetermined maintenance plan.
Combining your preventive maintenance strategies with another system, such as predictive maintenance, can make your upkeep even more effective. Naturally, having a proper plan for repairs comes with many advantages: 

  • Increased equipment life expectancy: Your transformer will last longer when it’s properly cared for. That means you can avoid continually replacing expensive parts — you get a machine that makes the most of its lifespan. For example, a generator step-up (GSU) transformer can last up to 25 years with sufficient upkeep.
  • Improved worker safety: A well-kept transformer has fewer chances of causing dangerous events like explosions or electrical failures. Relay systems exist to prevent these events from happening, but there’s no harm in practicing additional safety measures.
  • Better time management: A fine-tuned transformer works more efficiently and breaks down less, which saves you from dealing with the repercussions of unplanned downtime. Additionally, planning your upkeep schedule means your employees can work through less complicated maintenance tasks more quickly and increase their overall productivity. They then have extra time to perform essential job duties.
  • Improved budget management: Preventive maintenance requires you to plan your tasks far in advance, which can help with budget allocation. You know how much maintenance will cost each month, quarter or year, and you can enjoy increased transparency in your company’s finances. This allows you to put your money toward the areas of your business that need it most.

Transformer Inspections

Just like with more complicated maintenance tasks, it’s best to have a professional conduct all inspections of your oil-filled transformers. Having experienced maintenance personnel on-site will ensure every potential issue is spotted and addressed. You can save time by outsourcing your inspections instead of training in-house employees to do this complex work. Plus, there will be less chance of worker injury from employees tampering with machine components they’re not familiar with.

When doing a visual inspection, your hired maintenance personnel will investigate things like oil leaks, incorrectly positioned valves, bushings, radiators and more. Your radiators and fans should be clean and free of dust or debris to allow air to blow in the proper direction. The fan blades should also be calibrated appropriately to make sure they aren’t bringing in warm air after it passes through the radiator.

Infrared cameras can be very beneficial in detecting less-than-obvious warning signs associated with problem areas, such as the radiator blockages mentioned above. If you’re doing an infrared temperature analysis, this step should come after you complete all your previous maintenance and testing to ensure any broken connections are remade as they should be. Your transformer should show a thermal pattern of being cooler at the bottom and hotter near the top — any change in this might indicate overheating or other temperature issues.

Your technician will likely conduct the inspection working from the outside in. They will analyze the transformer’s external properties and dig deeper with an internal inspection if they find signs of abnormal operation. That process could require draining and storing the oil, which can be difficult and dangerous for inexperienced workers to try on their own. It is also time-consuming, which pulls labor away from other areas that need it. Always opt for outsourced labor if the need for these intricate tasks arises.

Other system components that might undergo analysis during an inspection include: 

  • Oil preservation sealing systems: Oil preservation sealing systems prevent air and other contaminants from reaching the inside of the transformer, which inhibits sludge buildup.
  • Auxiliary tank sealing systems: The auxiliary tank provides an extra component between the conservator tank and the main transformer tank.
  • Winding temperature thermometers: These thermometers give a reading of the hottest temperature within the winding, although they are not always accurate.
  • Gas pressure control components: Many transformers contain a three-element pressure control system consisting of parts like the high- and low-pressure regulator, oil sump, conservator and more.

Contact Balaji Power Automation for Oil-Filled Transformer Repairs, Replacements and Maintenance

Wondering whether your transformer simply needs some repairs or a full replacement? You might have to ask yourself a few questions about its performance, age and other essential factors. Fortunately, Balaji Power Automation is here to provide expert maintenance advice and conduct repairs and/or rewinds as well as retrofits as needed for your medium voltage transformers. We are the premier repair and rewind company for medium voltage oil-filled transformers, whether it’s custom builds, rewinds or emergency replacement services.

Every one of our key workers has over 21 years of experience and can handle your concerns with high-level expertise. Contact us today for more information on how we can serve all your transformer-based needs.

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