problematic transformer oil purification 3

[simisteven]

I am experiencing persistent problems with oil quality on a 5 MVA 11/6.6kV transformer. Moisture levels from samples taken twice yearly are consistently high. We do in situ oil purification annually but the problem returns after a few months.
A local company has suggested removing the transformer and shipping it off for stripping and core drying.
The task of removal of such transformer is both massive and costly and I don't want to do this unless it is really necessary.
My personal view is to re purify the oil and take 48 hours to do it instead of the usual 24 hours or repurify twice a year.
Anybody got any suggestions on a remedy?

 

[prc]

In old transformers, there will be lot of water- as high as 50-200 litres depending on the rating or rather the amount of paper insulation inside.99 % of this water will be in paper and only 1 % in oil.When you are filtering,only this 1 % is removed and soon after the water from the paper will come out and disslove in oil to maintain the equlibrium.

In case the tank of the transformer is capable of full vaccum, you can dry out as below: Do hot oil circulation for 6 hours to heat the inside parts.Drain oil and fill with dry air or nitrogen. Keep for 6-12 hours. Take dew point at the end.Apply vaccum for 12 hours. Oil fill.Do hot oil circulation again.Repeat as above. Usually when the insulation is fully dry a dew point of -30 at 30C will be got.But old transformers need not be dried to taht extent as due to shrinkage of paper and pressboard the windings will become loose.

Before starting the dry out make sure the transformer is leak proof and pressure tight.

 

[iwright]

Another option is to try an online dryout system such as this

http://www.dryoutsystems.com/trojan/index.htm,

the company I work for purchased one awhile back but I haven't heard how its working out yet.

 

[Jebb]

If you are in North America, there are companies that specialize in xfmr oil handling. They will come to your site with a self contained truck to process (filter, dry, de-gas, etc...) the oil and xfmr. It is not cheap to have this done, but it works...... As prc said, the oil has to be heated to drive the moisture from the paper into the oil, then remove the mosture from the oil. Filtering alone will not remove the moisture from the xfmr. The water has entered somewhere or somehow, so I would have this fixed. I would recommend using a oil processing service than trying to do it yourself....

 

[prc]

If you do some calculations, you can see that online dryers will not be effective in drying out wet transformers. It will require such a long time to take out the large quantity of water in paper.

 

[simisteven]

Many thanks to those who assisted with information. Much appreciated
best wishes
Steven

 

[Zogzog]

I have heard these new online system work pretty well, prc can you elaborate?

http://sdmyers.com/EP-drymax.html

 

[prc]

All on line driers take out a small quantity of oil and remove moisture from it.As I mentioned earlier, 99% of total water is in paper and hence it may take several years for removing entire water from paper.

 

[tinfoil]

But several years is STILL better than 'never'

 

[oldfieldguy]

It might be a better idea to foot the bill for a good hot oil dry-out, THEN go to an on-line dryer if the transformer is critical.

Really, moisture should not be a problem for years after a good drying, assuming that your transformer is NOT vented to the atmosphere or leaking badly. A competent contractor will take care of the details of performing the work, do oil samples before the work, immediately after the work, and then at periodic intervals to see where the moisture stabilizes after remediation.

Realize that the oil will likely be much lower in moisture, but the process will also move much moisture from the cellulosic insulation, too. It will take a period of time for the two systems, oil and cellulose, to reach an equilibrium at a given internal temperature. A sample a month after treatment and a couple more samples at quarterly intervals will help you understand the situation.

old field guy

 

[stevenal]

Gotta laugh about the SDmyers unit. I attended their training awhile back, and the instructor insisted that on-line systems such as the Velcon would do nothing for the paper. Now it's "The moisture migrates from the core’s insulation – to the oil – and then to the DryMax." Gee, that's how I understood the Velcon system worked. We've used the Velcon to good effect. As long as you can bail faster than it leaks, you're in good shape. If not, fix the leaks.