Geometric changes of windings are sometimes associated
with electromechanical efforts due to short-circuits that occur
in the neighborhood of substation where the equipment is
installed. An usual cause refers to the phase-to-ground shortcircuit
resulting from an atmospheric discharge. These changes
are cumulative and not detected by any other technique, are
aggravated by natural vibration associated to a natural ageing
of the solid insulation causing a significant reduction clamping
pressure, consequently decreasing the short-circuits tolerability
of the transform, without being characterization as defect. So,
the transformer remains in operation until this condition may
migrate to a dangerous situation and an “unexplained” failure
occur. Another fact is that power transformers are specified
and designed to support short-circuits effects (of short duration)
during its whole life, but in case of large power transformers
the performance about this situation is rarely tested.