Vacuum Filling/Drying of Transformers

[Mike111]

I am looking for some information on the above subject especially vacuum levels and duration for Transformer drying .Most of the information I have come across states “the lowest possible vacuum”, while I understand that the level of vacuum will depend on the Transformer tank structure what I am trying to find out is there an optimum level of vacuum for drying assuming that the Transformer tank can take full vacuum. Information on vacuum levels during the filling process would also be welcome.

 

[electricpete]

As you probably know the driving force for dryout is the difference between vapor pressure of the oil in the paper and actual pressure (vacuum) in the tank.

We can increase this difference by increasing winding temperature which increases vapor pressure and as you say decreasing tank pressure (increase vacuum).

I spent a lot of time a few years back doing transformer dryouts but I don't have any handy references.

As I remember some utilities require less than 1mm Hg vacuum. Do a pressure drop test prior to drawing vacuum to make sure you have eliminated leaks. I can't quite remember the spec - somewhere around < 0.5 pound drop when pressurized for 24 hours at initial pressure of 5 psig... and you have to adjust your pressure drop to account for any change in temperature (use the ideal gas law).

Another fanastic help is to use a cold trap between the vacuum pump and the tank. It will improve your vacuum dramatically and also enable you to measure the moisture removed over time. When moisture removed starts to plateau at a low level (low relative to size of the transformer... one ounce per 8 hours?) you know you are pretty much done.

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[mc5w]

If the transformer tank cannot take significant vacuum, you could still rig up some closed ductwork that contains some household dehumidifiers. These are quite efficient at drying stuff. I have used them to dry up rooms that have moisture problems and even raw lumber. The dehumidifiers will provide a moderate amount of heat that will not cook the windings.

Since most modern transformer oils are of reduced flammability with a very high flash point and ignition point, you probably do not need explosionproof stuff. Check the material safety data sheet for your oil. Since you need to keep out oxygen for the transformer's sake, slight pressurization with dry nitrogen should take care of any explosion hazard, what is known as purged and pressurized protection against explosion hazard.

When you reach the limit of what dehumidifiers can do you can also pass the atmosphere through silica gel cat litter, or at least that is one way to buy lots of it.

You probably need to pass the bulk oil through a filter and drier. I would recommend draining the tank through a filter and drier and then using a fresh filter and drier for refilling. Just remember do withdraw and replace the oil slowly. Too much oil velocity will stir up sediment and also cause the oil to develope static electricity bubbles that can damage the transformer. Use grounded metal pipe to connect your filters and driers and the oil hose.