[Amps] Running iron in oil

[John Lyles]

 At work we have dozens of tanked transformers in use. Being at 7000 feet above sea level, with high voltages, made it a requirement for some systems. They have been running for 40 years this way. 
We have two ways of approaching it. With sealed tanks, the transformer is dried, baked, then put into the tank. Then backfilled with oil, with a nitrogen purge on top. Sometimes that step is omitted and we just nitrogen backfill the last airspace at the top. We keep them under this positive (greater than 1 atm) pressure and have a gauge that is watched.
For a few, that are in open tanks with just a loose lid, they are just filled and that's it. 

I have read that GE may have used vacuum while filling, to remove a lot of air around and inside windings. If you have any voids that could trap air bubbles, it would be good to somehow force oil in with a hose to those places to displace the air. Otherwise might become corona pockets that will degrade the thing. If you are only extending the cooling - and increasing the loading - I think any oil is better than air. If you are extending the voltage beyond what is normal for the clearances designed, then you don't want air pockets that can have localized field concentrations. 

We had lots of problems with potted transformers many years ago, so I was told by my forefathers here. So they went to the oil tanked units instead. End bells, just remove as much of that as you can to allow the oil better ability to circulate. 
73
John 
K5PRO
 
> Message: 2
> Date: Thu, 27 Aug 2009 12:46:28 -0400
> From: "Carl" <km1h at jeremy.mv.com>
> Subject: [Amps] Running a transformer in oil?
> To: <amps at contesting.com>
> Message-ID: <012201ca2735$ee0363f0$6400a8c0 at DAVES>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> 	reply-type=original
> 
> Ive seen a discussion somewhere in the past but cant locate it.
> 
> Anyway, I have a virgin 5 gallon jug plus a partial one of genuine PCB and 
> started thinking about extending xfmr ratings. Is there any particular 
> formula to use, transformer prep, etc?
> 
> What about the sealed ones and also the oldies with cast iron end bells?
> 
> Carl
> KM1H