Among the various techniques available for the monitoring
and diagnosis of equipment, the Frequency Response and
Terminal Impedance measurements have been used in a wide
application form, in particular for power transformers, focusing
mainly the following aspects: Evaluation during type tests,
identification of natural resonances and anti-resonances
frequencies, support to equipment modeling for
electromagnetic transient studies and evaluation of geometric
displacements of the windings.
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.
Another important aspect is related to transportation (roads,
railways and ships), where sometimes the equipment is
subjected to excessive vibrations, fallings, collisions and other
situations inherent to loadings and unloading procedures.
These situations must be considered before putting the
equipment in operation and in some cases another tests must be
performed.