Oil Filled Transformer

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bnewport

bnewport

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This is not a code question but I am going to ask anyway-

What causes an oil filled padmount transformer to develop a vacuum while in service? (I have installed substation transformers and pressurized them with nitrogen, but these do not have a nitrogen blanket.)

Bryan
 
I might have found the answer... the oil temperature changing with the amount of KVA load will cause a rise and fall of pressure. I guess this could create a vacuum...?
 
Maybe the things aren't ventilated, and the oil leaking out at the bottom creates a vacuum in the air space.


By the way, answering your own questions is against the rules, so don't do it again.


Oh, wait! I'm not a moderator, so never mind. :D
 
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bnewport said:
This is not a code question but I am going to ask anyway-

What causes an oil filled padmount transformer to develop a vacuum while in service? (I have installed substation transformers and pressurized them with nitrogen, but these do not have a nitrogen blanket.)

Bryan
Click to expand...

This is just an educated guess, but it makes sense to me. When the transformer is loaded, the temperature will increase. An increase in temperature causes an increase in pressure if volume remains constant (that part isn't a guess; I remember it from chemistry :D ). If there is a relief point, the increased pressure will push out any gasses and leave behind a vacuum.

Perhaps that isn't entirely plausible, but it makes sense to me.
 
You have it right, the decrease in temperature will cause pressure to go down, eventually drawing a vacuum, which is a problem. If you draw an oil sample wit it at a vacuum the air will go in the botoom (Sample point) and air bubbles will work thier way up through the windings which will cause internal arcing and a possible explosion. A vacuum will also allow any small leaks to draw in air and cause all sorts of gassing problems reducing the life of your transformer.

You need to repressurize the unit to around 5 PSIG at normal operating temperature, any testing company will be able to do this for you, they should have all the proper equipment on hand to do this simple task, while they are there have a DGA sample drawn to ensure you dont have any problems.
 
As described above, transformers get pressurized under load and pull a vacuum at light load. Many padmounts have a pressure relief valve inthe air space, similar to the one on small air compressor tanks. If it happens to bleed out some excess pressure at full load on a hot day, the tank will go into vacuum as it cools down under light load.

You can pull the ring on that relief valve and it will suck in the moist ambient air to relieve the pressure. Of course, the moisture can end up in the windings and cause deterioration. Or if the relief valve is mounted lower on the unit you can bubble the transformer and turn the square tank into a round tank as Zog mentions. The vacuum should only be relieved with dry air or nitrogen.

Except for the oil sampling problem or air leaks, the transformer is probably OK under vacuum. Most transformers are vacuum filled. Holding a vacuum on the tank before filling with oil is used to dry out the windings.
 
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