Maintenance of natural ester transformers: case studies

In the last two decades, the number of applications of natural esters (NE) transformers is steeply growing, requiring also proper monitoring and maintenance of the quality of the insulating liquid. Nevertheless, reference values and alert limits are not yet completely available with reference to different classes of the transformers. Some helps may be sorted out from the recent publication (2021-01) of the IEC 62975 "Natural esters - Guidelines for maintenance and use in electrical equipment" which contains useful additional information for the operators in NE transformers.To overcome this situation, there is an effort to look for similarities with traditional transformers insulated with mineral oil (MO) for which a great amount of information on chemical-physical analysis are already available. In the present paper, case studies of some Natural Ester (NE) transformers maintenance have been presented and discussed. The acquired data will be also useful with the aim of adjusting the Health Index evaluation in order to guarantee support for maintenance decisions of the operators in this sector.