Increased Loadability of Transformers Immersed in Natural Ester

Mineral-base oil has been the main insulating liquid for filling power transformers since the 1900's due to its ready worldwide availability, low cost and good properties. Anyway, in recent times the new desires for a safer nonflammable, an environmental acceptable and a less impacting on sustainable common resources fluid has driven many users in the research and use of alternative fluids to mineral oil. Despite the availability of other fluids, the synthetic and natural (from vegetable seeds) esters (NE) represent the major alternative to mineral insulating oils. Terna as Italian Transmission System Operator (TSO) in according to European Union Directive 2001/42/EC should aim at guaranteeing high levels of environmental protection and safety, so Terna is also aware of its responsibility towards the community and has translated this into a single strategy: to find the right balance between the country's energy needs, nature conservation and the safeguarding of Italy's cultural heritage. In addition to the environmental and fire safety benefits, the review of the possible advantages of natural ester-filled transformers allowed identifying alternative design criteria with relevant financial benefits. The TERNA goal was to increase the loading ability of their transformers, mainly by enhancing a much higher limit for overloading in comparison to traditional transformers. For this prototype TERNA selected a 400/135 kV 250 MVA autotransformer, a standard practice for their network. To explore the superior thermal class of the new insulation system, the transformer manufacturer confirmed the possibility of designing this transformer with a "permanent additional capacity". Five autotransformers with NE are already in service and in its next-2-years strategic plan Terna has planned the installation of more than 20 new power autotransformers filled with natural ester