Thermal considerations in specifying dry type transformers

Thermal tests under different loading conditions are 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 degrees C without exceeding the permissible hottest spot temperature rise of 180 degrees C due to large thermal gradients. The average temperature rise for ventilated dry type transformers above 500 kVA with 220 degrees C insulation temperature class should be 120 degrees 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. The specification suggested should be used until IEEE standards are revised.<>