insulation Renza Berti*, Franco Barberis CIRED 2007-19th International Conference and Exhibition on ELECTRICITY DISTRIBUTION Vienna, 21-24 Maggio 2007 * CESI RICERCA With the aim to verify and compare the performances of more fire resistant and highly biodegradable dielectric fluids, such as the ester based oils, with conventional transformer oil, an experimental activity on a natural ester oil, a synthetic ester fluid and a naphthenic mineral oil was carried out, financed by the public Research Fund for Italian Electrical System. In this paper the results of an accelerated thermal ageing cycle, at a hot-spot temperature in the range 140143°C on single-phase devices insulated with these fluids and paper, are presented and discussed. The most meaningful outcomes are: 1. an extension of the paper and pressboard life using the ester based oils, also when these fluids are used to retro-fill a mineral oil device, measured through the degree of polymerization (DP) of the cellulosic insulation 2. an oxidation high in mineral oil but little evident in ester based fluids, when samples of the copper conductors of HV windings are compared.