There are several failure modes in the power transformer.
They are damages caused during the transportations, due to the
electromagnetic forces caused by a short-circuit current, caused
on conductors and insulating materials by over-voltages and due
to ages. When the damage is seen from outside and is judged as
a failure, it can be removed by repairing. However, the large
capacity transformer has an iron core and a coil in a tank and the
medium and low capacity transformer has a molded coil molded.
The constructions make the recognition of the damages difficult.