Thermal Overshoot Analysis for Hot-spot Temperature Rise of Transformer

The objective of this paper is to introduce a general model of hot-spot temperature rise calculation for the non-directed flow windings in order to explain the thermal overshoot phenomenon. The model employs heat transfer principles and hydrodynamics, in which the duct oil flow velocity is taken into account, and is validated using experimental results obtained for step changing load current on a oil natural-air natural (ONAN) cooling mode, 40 MVA transformer. The thermal overshoot phenomenon is analyzed by simulation for a 20-MVA-ONAN-cooled unit and a 40-MVA-ONAN-cooled unit.