If the aluminium conductors are not available any more (which is often the case because today aluminium is used less frequently that it has been in the past in oil transformers) use copper. In this case you can even up-rate the transformer.
By the time you've finished s****ing around with redesign, tearing the old unit(s) down and rebuilding this size of transformer, it would probably be cheaper to sell the old units for scrap and buy some new ones.
Bung
Life is non-linear...
to my mind for transformers above a certain rating it is worth while to consider redesign/rebuild. Some years ago we made a repair on a unit built in 1932 !
It is the same decision as one would be faced with if a transformer winding failed in-service... rewind or replace? Efficiency and expected reliability of a rewound unit may not be quite as good as a new unit, but the cost is often much less.. particularly for large units as eu mentions.
I understand Bung's point of view, because we have a project where they are shuffling transformers around from one place to another, refurbishing them between substations. This sounds fine in theory, and on paper they found a saving compared with buying new.
But in practice, they found water in almost every transformer that had been de-energised and was being prepared to be refurbished. So now they are busily getting this water out of the tranny paper and oil. It takes a long time, and in this project, has caused delays and trouble. I think in future, they will just buy new.
Previous contractors who had de-energised trannys should have sealed them better, -yes, but then this is an extra cost to ourselves in supervising the sealing. And time to do these sorts of things is difficult since we are short staffed anyway...
johnspark,
Finding water in oil is scarry, but it is not unremovable. If for removing water it takes a long time various procedure can be applied like hot air blowing, vaccum drying or these days regenerating equipments are available, which can be used for dehydrating as well on top of some other functions like removing acids and other unwanted stuffs. Even if the tranfomers are well sealed (if energized), water is naturally formed inside.
Regarding refurbishing, to our experience the cost is generally quite effective and the repaired transformers are functioning at least in our case quite well. If it does not function well, the refurbishing contractor should be held responsible.
