Transformer's Life Span/Functionality

There are a lot of variables involved with this. Where was the repair carried out? What was the nature of the fault? What size and voltage are you talking about for the transformer.

Ultimately there are no guarantees in life. A number of years ago we specified a 13.8kV/480/277 Volt, 500 kVA dry type transformer for a project. The unit was tested in the factory, as well as field acceptance testing was performed, all results were satisfactory, but when the unit was turned on, smoke started to rise from the windings and the unit started to arc. The Load Break was turned off, the unit examined and returned to the factory for repairs. (We originally wanted a new transformer but due to contractual stipulations could not insist on this.)

The unit was once again tested in the factory and in the field. Just like the first time the results were satisfactory. The unit was turned on and once again arcing and smoke occurred. This time the operator instead of turning the unit off, simply said "that is what fuses are for" and waited for fuses to open up. The fault occurred at exactly the same spot as the first time, but thanks to a wise operator the unit was now damaged beyond repair and a new transformer was ordered.

We never did find out what the reason was, and the manufacturer never was able to provide a satisfactory explanation either.

So to answer the question, no guarantees on repaired transformers, (then again not on new ones either)