Análise de falhas em transformadores de distribuição durante o ensaio de impulso atmosférico

Abstract

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.