Welding on an oil-filled transformer

Author: Lawrence Kirchner


Leaving the toilet seat up if you are married and have two daughters might be dangerous, but it pales in comparison to torching your way through metal on the side of a transformer filled with thousands of gallons of highly flammable mineral oil. Throughout the lifetime of your transformer, you might experience a dire need to perform this procedure that would make even the bravest circus act look like child’s play, especially if there is a leak that that would cause more danger if it is not addressed in a timely manner. Hopefully, you will take away some best practices to preserve your safety while maintaining the integrity and lifespan of your transformer.




We know from fire training that a fire or explosion requires three things, a heat source, fuel, and oxygen.  Remove any one of these and you can drastically reduce or eliminate the possibility of a fire related catastrophe. In the situation of welding on a tank full of oil, the heat source is obviously the welder and the fuel is the oil. Neither can be eliminated from this situation, so your focus should be on reducing or eliminating the oxygen levels inside the tank so that combustible gas levels are at or below acceptable limits before welding.


Modern transformers


Most modern transformer tanks are sealed from the outside elements and are not intended to contain oxygen or combustible gasses inside. Since approximately 1960, there have been basically two tank designs, one with a nitrogen blanket and the other with a bladder to separate the air from the oil.


In a nitrogen blanket design, the tank is filled with oil to about 90% of the height of the tank. The top 10% is filled with nitrogen gas to allow for thermal expansion of the oil. As a precaution, do not assume that the gas is pure nitrogen. It very well could contain some oxygen or combustible gasses. You should test the oxygen level and verify against NFPA standards. Most tradesman adhere to a maximum level of 3% oxygen before attempting to weld on the transformer, while also maintaining combustible gas levels to less than 1.0% by volume.


A conservator tank design is completely full of oil, but the external air is separated from the oil by a rubber bladder. Welding on a transformer with this type of oil preservation system is less risky because there is no gas space. Use caution, though, because you are still using a lot of heat with very flammable oil.


Older transformers


Ernest Hemingway once wrote that “Hesitation increases in relation to risk in equal proportion to age.” We cannot be certain if Hemingway knew anything about transformers, but we can take to heart the inherent risks of working on something that is several decades old.


Some older transformer tanks are vented directly to the outside environment. This type design is known as a free breather. It is extremely dangerous to attempt welding on this type tank because the oxygen and gas levels are hard to control. Even if the tank openings are closed and the air is replaced with nitrogen, the oil could contain enough dissolved oxygen to support combustion.


If it is decided that you must weld on a transformer of this age and type, it is always safer that you do so when oil-filled to limit the gas and/or oxygen content. For this reason, if the decision is made to weld on a transformer that has had the oil removed, the tank must be evacuated of air and back-filled with nitrogen. As stated above, the oxygen content (or lack of), and combustible gas levels must be tested.


Preservation of the transformer


Now that we have reviewed fire safety, the next concern would be preventing or minimizing any damage to the transformer. There are several things to consider:

  1. Compromised internal dielectric clearances - The area around the weld will reach extremely high temperatures (2,400 deg. F or higher) very quickly.  The oil touching this area inside the tank will burn and combustible gasses will be generated. The gas bubbles will naturally rise which could cause the internal dielectric clearances to be compromised, resulting in a flash. For this reason, you should de-energize the transformer while performing the welding and keep it de-energized for at least a few hours afterwards to give everything a chance to cool.
  2. Flash - Many transformer tanks are painted on the inside. Welding on the tank will certainly cause the internal paint to burn and flake off. The flakes contain carbon and are therefore conductive. It is possible that the flakes could find their way into a location of minimum dielectric clearance and cause a flash. The only way to eliminate this possibility is to drain the unit before welding, backfill with nitrogen, weld, evacuate nitrogen, fill with air, go inside, inspect and collect burnt paint, and then refill. This is a very expensive proposal.  The risk can be minimized by reducing welding heat to a minimum and welding only enough to correct the problem.
  3. Damaging the unknown – You should always ask yourself, “What is directly inside where I intend to weld?” Are there insulated wires that can be burned? When welding around bushing turrets, is the CT close enough to be damaged? It is always recommended that the manufacturer be consulted regarding internal design.
  4. Unintentional tripping - There have been cases where transformer and/or line trips have been caused by welding on a transformer. For example, in the case of a transformer with a conservator type tank design, the gas generation discussed above could cause the Bucholtz relay to trip. All alarms and trip circuits should be blocked to prevent unintentional trips.

Tricks of the trade


The ultimate goal is to eliminate the risk of having to weld on your transformer altogether. Here are a few tips and tricks that have been used throughout the years.

  • Use a hammer and punch to close the gap before attempting to weld.

  • Make a series of punches about 1/32” away from the crack or hole on all sides pushing material inward. Often this will reduce the leak enough to permit welding. First, clean the area well with a wire brush or grinder. The welding process will generate combustible gasses inside the unit. It is advisable to take an oil sample before and about 24 hours after welding. This will explain small amounts of combustible gasses in future samples.

  • Also consider the possibility and potential consequences of making the leak worse by welding.

  • Are you prepared for a serious leak? Wooden wedges and pegs can be used to fill a crack in a pinch.

  • It is advisable to have oil cleanup material on site in case of a spill. There will likely be oil on the outside of the tank where welding will occur or oil seeping from the crack while welding.

  • Risk of a small fire is great. Have a fire extinguisher handy.