Vacuum fill or not?

[MKFPE]

We have a 24 year old 69kV-12.47kV 10MVA power transformer with nuisance oil leaks at the sidewall mounted LV bushings. It was recently removed from service for replacement by a larger unit.

The transformer is currently in storage awaiting installation in its new home at a different substation so we thought we should take advantage of the opportunity to fix the leak issue.

We are considering replacing the bushings or at least the gaskets which will require up to perhaps 1000 gal of oil to be removed from the transformer.

A contractor has quoted vacuum filling for putting the oil back in the tank after the bushings are replaced, which I was under the impression is standard practice. The tank is designed for full vacuum.

However the manufacturer has offered advice saying "Vacuum should not be pulled on a full or partially full transformer. This could cause bubbles to be pulled out of the oil and leave the bubbles in the windings which may lead to a transformer failure."

This leaves us in a bit of a quandary and looking for advice.

Any thoughts from the experts on this forum will be greatly appreciated.

 

[prc]

1) Advice from the manufacturer is correct.
2) In case the draining of oil will not expose the winding (being at lower level) you have the option of topping up with oil,filtered under vacuum. Then filter the oil in the transformer through machine for two days and give setting time of a week.
3) Since it is a 24 year old unit, better to drain the unit completely. Replace gaskets where ever necessary .Do an internal inspection. Do a short dry out by HOC/drain out/Vacuum 24 hours cycles.

 

[OffTap100]

If only 24 years old, good DGA's prior to removing from service, and you can replace gaskets(all) by lowering oil but not exposing windings?...I would not spend the money or time to get it dried/vacuum filled. Just filter press the oil out, and back in.

The juice doesn't seem worth the squeezin. 24 years old? She's still a baby.


OT2020

 

[MKFPE]

Thanks prc & OffTap100. Our Substation Supervisor told me your answers are a "confidence builder."
He has has developed his plan for this work and will be following the advice to use the filter press in and out.
We will allow plenty of time between this work and energizing.
Further inquiry to the manufacturer finds the LV bushing can be reached without exposing the windings.
Thanks again.