A Validated Practice for Transformer Retrofilling

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Nowadays, the use of natural esters is becoming increasingly popular for reasons of transformer life extension, biodegradability, and fire risk. The significantly reduced carbon footprint of natural ester fluid versus mineral oil is another highly appreciated attribute in terms of sustainability. Many transformers' fleets originally designed for mineral oil have to be converted to natural (vegetable) or synthetic ester. Unfortunately, international standards give only scant and generic information, moreover, confined to annexes, about the retrofilling practice. This study reports on the laboratory experience finalized to identify the best chemical-physical technique for assessing the residual oil quantity after the replacement of mineral oil with natural ester (so-called memory effect), and then describes the best available technique (BAT) for refilling used on a distribution transformer (630 kVA) originally designed without atmospheric segregation system, in order to minimise the memory effect and, at the same time, ensure the reliability of the transformer. This paper is focused on the development of "Best Available Techniques (BAT)", "Best Environmental Practice (BEP)", "Best Value Chain (BVC)" and "Best Sustainable Investment (ROSI)" for the "Life Cycle Management (LCM) of transformer fleets and electrical equipment immersed in alternative insulating liquids like natural esters and/or with synthetics complying with new regulatory and case study requirements.