Moisture in power transformers DFR analysis – real cases studies on site

Dielectric Frequency Response Test (DFR) – FDS-Frequency Domaine Spectroscopy) of power transformers is a diagnostic tool used to assess the dielectric properties as moisture in insulating system paper and oil, power factor, oil conductivity, etc..) in the frequency range from 0.05 mHz to 1 kHz.In this paper we have selected some real case studies on some power transformers that presented problems in the insulating system during the monitoring phase on the one hand, and on the other hand, we have studied the restoration of such anomalies with targeted corrective actions aimed at improving the residual life of the transformer. In particular the following case studies are reported:–two twin transformer of 150 MVA with high furanics compounds (2FAL);–an autotransfo of 5 MVA after coils replacement in manufacturer workshop.–a transformer of 8 MVA – 132/6 kV with high moisture (6%) in paper and high dissipation factor–under vaccum reconditioning up to moisture in paper about 3 %. The advantages / effectiveness of the DFR analysis compared to the analysis of the oil standard (Karl Fisher method):•Determines the water content in the solid-paper insulation without opening the transformer•Eliminates the problem related to the interpretation of moisture in the oil and solid insulating part (active part) of the transformer.•Allows the monitoring of the paper moisture treatment, its extraction (% by weight), and the limit below which to stop the treatment.