De-oiled transformer tests

[Chrislatimer]

Hi,
I have been asked to test a transformer (33kV:11 kV 22.5 MVA star-delta unit) which recently suffered a fault condition.
the unit has had its oil removed, another party has performed visual inspections.
I am wondering what tests are recommended to avoid on a transformer with no oil.
Tests to be performed are:
Excitation (magnatisation current) at 2000 V
voltage Ratio (at close to 1000 V)
Winding resistance
Insualtion Resistance (5 kV )
possibly FRA (or sweep frequency response analysis)

I would assume LV testing would be acceptable, ie winding resistance, but can other testing be performed without risk of further damaging the unit?
it would be optimal to test once transformer is fully oiled up, but client wants some diagnostics to be performed prior to this.

any feedback would be much appreciated.

 

[Marmite]

Sounds like someone has thrown the baby out with the bath water. One of the most informative test to carry out would have been a dissolved gas analysis of the oil. FRA won't tell you anything unless you have the original results, and then you can't compare the results of an empty transformer with those of a unit full of oil.
Regards
Marmite

 

[mns15451]

Yes! there many tests which can be done but first of all one must know the problem as faced during operation of this transformer. I mean whether it was tripped on some transformer protection operation etc. etc. Only then we can see what tests can be performed to evaluate its condition.

 

[Chrislatimer]

Hi Marmite,
A DGA sample has been taken, upon visual inspection, copper balls where isentified throughout the unit and damaged, deformed copper bars where seen. A Black residue was coated along the leading edge of white phase and red phase copper conductors. The unit had taken a pretty bad hit/fault and has been pulled out of service for complete refurb. It is suspected that a foreign object may have been the cause. will let you all know the details of the investigation as it comes to light.

i guess it was more of a curiousity as to what tests can be performed on a de-oiled transformer without causing damage to the parts. I would assume all low voltage tests would be fine, but was wondering if 2 kV injection would also be acceptable on a similar sized unit.

thanks again.