New Dual Nameplate kVA for Distribution Transformers

Transformer kVA is defined as the amount of output power that can be delivered without exceeding nameplate temperature rise. Materials improvements have led to greater output ratings and smaller physical sizes. However mandatory energy efficiency requirements by the US DOE and physical design restraints for smaller units have resulted in designs that are not limited by the rated temperature rise and loading guides have struggled to accurately reflect true capability. Distribution Transformer loading studies show generally light loading but confused load-ability. New high thermal class fluids and improved solid insulations for liquid filled transformers coupled with increased electric vehicle charging and greater heat pump use, suggest new dual nameplate ratings for distribution transformers. The base rating would continue to reflect traditional 65°C parameters. The second rating would be the nameplate kVA rating based on thermal class of the insulation system. This paper shows the usefulness of the proposed change.