Analysis of the influence of low-frequency heating on transformer drying – Part 2: Experiences with a real transformer

This paper presents an experimental study about transformer field drying. Different drying processes were performed on a real transformer. The transformer temperature was monitored during the drying processes by means of internal optical fibre probes. The water reduction achieved by the different drying processes, was quantified by measuring the water content of insulation samples taken from the transformer before and after the drying processes. After every drying process, the transformer was moistened by exposing its active part to air.The first drying process performed was a conventional drying, which consisted of a first stage of water extraction under vacuum (VD) and a second stage of drying by hot oil circulation (HO). The second drying process was similar to the first but, in this case, LFH was applied to heat the transformer insulation. In the third drying process, the first stage of drying was conducted under soft vacuum.

► Different drying processes were applied to a transformer under controlled conditions.
► The effect of low frequency heating (LFH) and vacuum level was analyzed.
► LFH improves drying efficiency, but its application causes inhomogeneous temperature.
► The application of vacuum cycles at ambient temperature does not reduce water content.
► The application of soft vacuum cycles is not a good option to dry a transformer.