8. Case Study: Field Repair of a Power Transformer Electric power transformers d

Question

8. Case Study: Field Repair of a Power Transformer Electric power transformers do not operate at 100% efficiency, and generally incorporate some means of cooling in the design. Large transformers are often submerged in oil-filled reservoirs, where the volume of oil provides a noncorrosive heat sink. In addition, t is not uncommon for additional features, such as horizontal fins, to be added to the design to aid in dissipating heat from the reservoir Figure 3 presents the exterior of a large transformer that has been installed in a rural, somewhat remote location. The reservoir housing has been constructed by welding low carbon steel plates 9 and 12 inch thick. While the transformer was in use, a service vehicle accidently backed into the cooling-fin assembly, producing cracks in several of fillet areas and a resulting loss of oil. Overheating occurred, and a repair is now necessary. Because of the size of the transformer, some form of on-site repair is preferred. It is your duty to determine the procedure and make the necessary arrangements. 1. Consider full spectrum of welding processes and identify candidates that might be appropriate for this task 2 For each of the candidate process, identify its primary advantage and limitations 3. Which of the candidate processes would you recommend, and why? (10) Fillet Cooling fins Kohser welds Figure 3. An Electric Power Transformer (adatd from Black and Kotser, Materials and Processes in Manufacturing, 10E, Wiley)

Explanation / Answer

Safety

We know from fire training that a fire or explosion requires 3 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 can’t 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’ve reviewed fire safety, the next concern would be preventing or minimizing any damage to the transformer. There are several things to consider:

Tricks to use:-

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.

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