Resumo:
Due to its importance to the electric power system, transformers are subjected to
normalized electric tests in order to guarantee its proper functioning in operation conditions,
one of which is the lightning impulse test. Failure detection during the lightning impulse test
can be accomplished through different methodologies and the analysis can be subjective and
dependent of the evaluator’s experience. Thus, the purpose of this work is to revisit the main
failure analysis techniques for transformers, then test and reevaluate them for distribution
transformers, since most of the existent literature refers to high voltage transformers. The
considered methods for this work were the failure detection in the time and frequency domains.
For the time domain, the sensibility of different types of measurements, the relation between
the direction of the measured current with the type of failure that occurs in the transformer, as
well as the relation between the occurrence of high frequency signals and the presence of partial
discharges in the transformer were analyzed. In the frequency domain, the capacity of the
technique to compare different waveforms, including full and chopped lightning impulses, the
relation between the attenuation or shift of the resonant pole of the response with the type of
failure that occurred in the transformer were analyzed. The results verified the evaluated topics,
with the exception of the failure characterization in the frequency domain, in which it was not
possible to find the relation between the transfer function and the type of failure in the
transformer. The coherence function was evaluated as a tool capable of determining the reliable
frequency range for analysis in the frequency domain and, as a numerical indicator, reducing
the subjectivity of fault identification.
