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Transformer Oil Leaks: Guide to Troubleshooting and Repair

Spot oil on or around your transformer? Use our guide on leaky transformers, to troubleshoot everything from leftover oil residue to active radiator leaks.

Written by:
Mac Spiller & Jeremy Abegg-Guzman

November 29, 2023

A transformer oil leak coming from a padmount transformer radiator fin.

Every transformer factory must have a strong quality control process to ensure that transformers are shipped without defects, like leaks. But there is always the human error factor which can lead to transformers being shipped with leaks. And sometimes leaks pop up on the bumpy ride from the factory, to the job site.

In this article, we will look at why transformer oil leaks happen and how to fix them.

What causes transformer oil leaks

Transformer oil can leak outside the tank and leave residue for different reasons. In this section, we will go over the main causes.

Manufacturing Defects

Some transformers have defects, such as poor welding, insufficient sealing, or loosely torqued bolts. These defects can cause oil leaks. Even in brand new transformers.

Age and Corrosion

Transformer tanks are typically made of mild steel, which can rust. This makes it weak and causes leaks. Especially where the environment is harsh, moist, or has corrosive substances. Like near the ocean. 

Then there’s the gaskets. Even if the bolts are tight enough, rubber or cork gaskets and seals can wear out over time. Regular inspection and replacement of these components are essential for preventing leaks.

Oil Residue From Shipping or Manufacturing:

During shipment, the oil sloshes around inside the tank and some can leak out of the holes in the tank.

Oil residue from the factory may also be present on the tank, lids, radiators, and weld lines. This is not necessarily a sign of a leak.

External Damage

Lastly, transformers can start leaking due to accidents, vandalism, natural disasters, or physical impact. We have seen transformers leak after being punctured by forklifts or hit by trucks backing into radiators. Any damage to the transformer tank or radiators can cause an oil leak.

How to troubleshoot transformer oil leaks

To troubleshoot transformer oil leaks, start by identifying if it's a major active leak or minor residue or drips on the cabinet.

How to address major active transformer leaks

WARNING: Transformer oil insulates and cools transformers while they are in use. If your transformer is energized, and has a lot of oil flowing like in the picture below, you could be in danger.

An active transformer oil leak coming from a padmount transformer radiator fin
An active transformer oil leak coming from a padmount transformer radiator fin

Take the following steps to secure your transformer:

  1. Stop reading this article.
  2. De-energize the transformer.
  3. Contain the oil.
  4. Contact the manufacturer. 

A major active leak needs to be fixed by an experienced professional to ensure it is repaired correctly and safely.

How to address minor transformer leaks

Just because there is oil residue on the cabinet of the transformer, does not mean that there is an active oil leak. The oil could have seeped out from the tank during shipping, or just be leftover from the factory floor. 

You often find these leaks at the pressure relief valve, fill plug, and sometimes the pressure gauge.

A transformer oil leak from a pressure gauge
A transformer oil leak from a pressure gauge

How to check if your transformer has a leak

First, start by cleaning off the oil residue from the tank. Then check the pressure vacuum gauge in the secondary cabinet. The gauge should read between 1.5 and 2 lbs of pressure. But note that the pressure in the tank may be affected by the ambient temperature. If the oil was hot when the unit was filled at the factory, and then it cools down on site, the tank could read 0, or even be under vacuum. 

The hot oil level versus the cool oil level in liquid filled padmount transformer

If the pressure gauge reads 0 or shows a drop in pressure, you might have a transformer leak. To test this, use a nitrogen bottle to bring the pressure back up to 2 lbs.

Next, take some soapy water and spray some around the device you suspect is leaking. If there is an active leak, you will see soapy bubbles start to appear immediately.

An active leak from a transformer pressure gauge versus no leak in a pressure gauge.

If you don’t see bubbles, clean off the excess water, and let the transformer sit for a few hours. Ideally six. Then, come back and read the transformer pressure gauge. What you’re looking for is a pressure reading of 0. If you don’t see that, your tank is holding pressure and you don’t have a leak.

How to fix transformer gasket leaks

The next most common type of transformer leaks are gasket leaks. Gasket leaks can come from bushings, LBOR switches, tap changers, and other transformer parts. These leaks are often due to bolts being under-torqued. 

Sometimes the gasket itself is faulty. But that’s rare. It’s usually just loose nuts.

A transformer bushing being torqued correctly to prevent leaks in the gasket

If you think the nuts might just be too loose, contact the manufacturer for torque specifications. You can find Maddox’s recommended torque specifications in this article.

If you don't have the torque specs, a general rule is to give each nut a good half-turn. But again, it's best to follow the manufacturer's recommended torque.

Be sure to torque the bolts in question in a criss-cross pattern, just like a car wheel. This will allow you to get even pressure across the gasket and avoid squeezing one side too tight. Years down the road, uneven compression from the bolts could cause a gasket failure.

Once the gasket is torqued correctly, clean up the area, fill the tank with nitrogen (2 lbs), do the bubble test, and check back in later to make sure the tank is holding pressure. 

This is the process you should run on any bolted component that has a leak.

Weld line, radiator, and other leaks 

If your transformer's tank, weld lines, or radiator is leaking, it can be a bit more difficult to fix. The transformer may even need to be sent back to the factory for repairs. If you think you have one of these leaks, contain the oil the best you can and call the manufacturer for advice.

Hope this article was helpful for you. If you want to learn more about transformers, consider looking into some of our other articles like DOE Transformer efficiency standards, or learn how Bayonet fuses protect transformers from short circuits.  

Maddox padmount transformer loaded on truck

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