Hottest spot temperatures in ventilated dry type transformers

The hottest spot temperature allowance to be used for the different insulation system temperature classes is a major unknown facing IEEE Working Groups developing standards and loading guides for ventilated 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. Since 1944, insulation temperature classes have increased to 220°C but IEEE standards continue to use a constant 30°C hottest spot temperature allowance. IEC standards use a variable hottest spot temperature allowance from 5 to 30°C. Six full size test windings were manufactured with imbedded thermocouples and 133 test runs performed to obtain temperature rise data. The test data indicated that the hottest spot temperature allowance used in IEEE standards for ventilated dry type transformers above 500 kVA is too low. This is due to the large thermal gradient from the bottom to the top of the windings caused by natural convection air flow through the cooling ducts. A constant ratio of hottest spot winding temperature rise to average winding temperature rise should be used in product standards for all insulation temperature classes. A ratio of 1.5 is suggested for ventilated dry type transformers above 500 kVA. This would increase the hottest spot temperature allowance from 30°C to 60°C and decrease the permissible average winding temperature rise from 150°C to 120°C for the 220°C insulation temperature class