Vacuum and Dry Air Used for Removing Moisture Content in Transformer

As far as I know,there is no accurate method to predict the eventual drying of the transformer.

There are many variables,such as insulation type,thickness,temperature,etc.

Here is a link that will explain it further.

Do not rush this process because of deadlines.

It must be done right.

Failure to do so could result in catastrophic failure in the future,and forensic investigators are VERY good at determining the root cause of a failure.

Follow the guidance of your engineer in these matters,not the time budget.

Thank you very much for your reply. The problem is, the 220 kV switchyard, breaker, relay, SCADA, HT and LT panels, SFC, motor and pump Commissioning Engineers are at site and we are not able to proceed further because the transformers have not been charged. The client is also pressurising us to start the pumps without further delay. We have to take a call on this. When will the transformers be ready for charging? Should we send all the Commissioning Engineers back. When should they return back to site. This is the first time I've seen the drying out process for transformers being carried out in this fashion.

If you do not allow the engineers to complete the job,your company will be responsible for any catastrophic failures that occur in the near future.
If you cannot afford to lose,you should not gamble.

It takes a long time to remove moisture from the insulation.

Water can actually freeze at ambient temperature under high vacuum,so it must be done slowly to prevent this.

Here is a link addressing your concerns and some alternate methods of drying.

https://electrical-engineering-portal.com/what-to-do-if-transformer-has-an-excessive-moisture-content

As <...The Commissioning Engineer stated that the water content is higher than the permissible level ...>, this individual would be the best person to ask.

As <...The Commissioning Engineer stated that we may have to do 3 or4 cycles to reduce the moisture content...> it would appear that the answer to the second question is already to hand.

Remember that everything happens "as soon as possible".

I recall from very long ago that we had traction transformers (6-phase secondary windings), ±60yrs old, with excessive moisture levels. We dried the oil by circulating it through a specifically designed vacuum system, but most of the moisture remained in the paper insulation of the core, so the benefits were short-lived. We tried heating the oil slightly, but the best results were obtained by shorting out the secondary windings and applying 400V 3-phase to the primary (we did the calculations beforehand to ensure we stay within limits!) whilst circulating the oil with a pump to ensure that no local hotspots develop. This caused the core to heat up quickly and shed moisture to the warmer oil, as warmer oil has a massively higher potential to absorb moisture than cold oil. The moisture was then removed by circulating this warmer oil through the normal vacuum oil drying process. As it was winter we covered the cooling fins with tarpaulins, and kept an eye on the winding and oil temperature gauges. Took a day or two to complete per transformer. Not sure if they still do it this way....

The reduced voltage,shorted secondary heated winding method is one of the methods described in the link above.

It appears to be the most time-efficient method.

You get a GA from me!

<...The transformer has to be urgently commissioned to start the other activities...>

Be ready to reschedule them, then. A time plan is only a guess, after all. If the guess doesn't agree with experiment, then it's wrong: it's as simple as that.

If ever a project is suspended, the suppliers should be asked for recommendations for long term storage, consequence of not following procedure & their advice followed - it costs but your experience shows it is worthwhile.

Ask the transfo manufacturer what his storage advice & cost estimate for it would have been & recommendations for restoration. Keep a record of all the delays and lay-offs & cost to compare with that. Make sure purchase/project managers get this in writing as a lesson.

Heating the windings electrically will speed removal of moisture from insulation.

All the oil must also be tested & dried or replaced - best to get on with that in parallel or more delay.....