An alternative evaluation of distribution transformers to achieve the lowest TOC

The 65°C rise rating of a distribution transformer is a thermal life rating of its insulation system, not the best efficiency condition. Total owning cost (TOC) is reference at 85°C which is based on a 20°C ambient plus a 65°C rise. For better efficiency, a transformer should be operated at a lower temperature. Transformer design programs calculate a design for the lowest TOC at the 85°C reference temperature. These programs do not calculate a design for the lowest possible temperature rise or lowest TOC regardless of reference temperature. As a consequence, the lowest TOC is not realized. An evaluation method is proposed that will provide the user with a distribution transformer design which has a lower operating temperature rise, thus improving both the transformer´s efficiency and the customer´s TOC