Evaluation of Capacitance and Dielectric Dissipation Factor of Distribution Transformers - Experimental Results

This paper aims to evaluate the capacitance and the dielectric dissipation factor (DF) of brand new distribution transformers tested in the past two years at LAT-EFEI High Voltage Laboratory. Evaluation proposed consists to classify the capacitance between windings and between the windings and the ground, as well as the DF, according to the rated power, voltage class and number of phases. The DF analysis is based on IEEE Std. C57.152 limits and it aims to infer the insulation quality prior its operation on distribution networks. The parameters measurements were carried out on 170 distribution transformers made in Brazil, using a Schering bridge. The measurements showed a considerable number of samples with DF greater than the limit for a good insulation's condition.