Thermal considerations in specifying dry-type transformers

In 1944, the hottest spot temperature allowance for ventilated dry-type transformers was established as 30°C for 80°C average winding temperature rise and IEEE standards use a constant 30°C hottest spot temperature allowance for all insulation temperature classes and all transformer ratings. Thermal tests under different loading conditions were performed on a prototype 2500 kVA ventilated dry-type transformer and six full-size test coils with imbedded thermocouples in the windings. The test data indicated that the hottest spot temperature allowance used in IEEE and IEC standards is too low for ventilated dry-type transformers above 500 kVA. It is impossible to design ventilated dry-type transformers above 500 kVA with an average temperature rise of 150°C exceeding the permissible hottest spot temperature rise of 180°C due to large thermal gradients. The average temperature rise for ventilated dry-type transformers above 500 kVA with 220°C insulation temperature class should be 120°C. IEEE standards should require measurement of hottest spot temperature rise on prototype transformers or windings as a design test to qualify a design family and the manufacturer´s mathematical models. This is especially important for ventilated dry-type transformers rated for nonsinusoidal load currents. The specification suggested in the paper should be used until IEEE standards are revised