Transformer Vacuum Fill Help?

[APHK]

How long should we pull a vacuum on a transformer before we fill?
We have a contractor repairing leaks on one of our transformers that required the tank and OLTC to be drained of oil. The unit is a 44kv to 8.32kv 12.5MVA unit with 2465 Imp Gals of oil in the main tank and 203 Imp Gals in the tapchanger. We asked for a 24 hour vacuum fill procedure but due to scheduling the contractor is suggesting 17 hours of full vacuum. Stating that, “As many manufacturers advise that a 12 hour vacuum is acceptable, we feel that the 17 hours will be sufficient.” We inherited this station from an older utility and do not have any original documentation for the transformer can anyone suggest any concerns on only pulling vacuum for 17 hours? After the transformer is filled we will be doing a TTR, Winding Resistance and Megger test on the unit.

 

[rterickson]

Did your contractor take a dew point measurement at any point?

If the moisture content was close to your acceptance standard initially, then 17 hours could be sufficient. If your insulation is "wet", then 24 hours probably won't be long enough.

If you can, make the contractor determine the dew point before you decide to accept his work. The last thing you want to do is refill a transformer with wet insulation, as there's no good way to pull the water out from an in-service transformer.

 

[waross]

The last time I saw a transformer filled, when the vacuum pumping was terminated, the engineer plotted the vacuum pressure over several hours. A dry transformer will maintain a fairly straight, fairly horizontal line. A wet transformer will exhibit an initial "Boil off" as the internal pressure rises to the vapor pressure of the remaining moisture, and then a fairly straight more horizontal line. Our transformer showed a straight rising plot, indicating a very dry transformer with a small leak. The vacuum was broken with inert gas, the leak was located and repaired and the vacuum test was repeated. That utility considered that plotting the vacuum pressure for a number of hours after pumping a vacuum was a mandatory part of commissioning a transformer.
Your contractor may be trying to avoid overtime wages for his crew.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Yes, both are valid ways. We used to required 24 hours for dew point meausrement. Waross' method using piper chart and plots I think can be faster.

Imo the better way is to use a cold trap. It is connected between the xf and the vac unit. Fill it with something like acetone and dry ice from the outside and the cold transfers through to the inside (ok, heat flows the other way...same thing) and builds up condensation. Periodically the unit is bypassed/isolated to remove the frost to restore heat transfer. Two great benefits:
1 - it makes the vacuum WAY lower (I know you're already close to 0, but as I recall can cut the absolute pressure by factor of 10... I forget exact values .... has been about 10 years since I did this stuff).
2 - It allows you to measure moisture removal rate. Save the collected frost once per hour. Them melt it and measure it in measuring cup or graduated cylinder. Plot the removal rate per hour. The removal rate decreases as the surface dries. When the removal rate starts to plateua you are reaching a point of diminishing returns. For big GSU's I remember we though something like 1 ounce per hour was a good endpoint. Other smaller transformers would have a lower endpoint. I remember Doble has some endpoints that can be calculated from estimated weights of transformer insualtion.
Drying without a cold trap is shooting blind. You have to stop the drying to see how your doing. Then restarting is a major issue. Along with other benefits mentioned above.... it is the smart way to do the job.



=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

You can also guesstimate the moisture in the transformer paper ahead of time based on oil sample results. This analysis needs to consider the temperature history (higher fraction of water goes from paper into oil at high temperature).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[edison123]

I am with epete. Condensed water collected every hour is good indication of drying out process.

Bill. That was an interesting tip. Thanks. Gotta try it out when I get the opportunity next time.

Muthu

http://www.edison.co.in

 

[prc]

Everything depends on how long the transformer was in service or how wet the insulation was.I would suggest as below:

After all work,replace all opened gaskets with new ones and fill the tank with dry air or nitrogen (dew point max -40C) at positive pressure,say 3 psi.Look for leaks and rectify.Keep for 24 hours.Check dew point and inside temperature.Based on published graphs,determine the extent of wetness to decide the dry out.If reasonably dry (say surface moisture less tahn 0.8 % by weight)apply vacuum and maintain at less tahn 1 mm mercury for 24 hours.When vacuum reach 1mm Hg,stop vacuum pump for 60 minutes,check pressure rise.The leak shoud be less than 20 mbarlitres/sec .You will be at that level in case the pressure rise in mm Hg is less than 9000/oil volume in kL.