HOTTEST SPOT TEMPERATURES IN VENTILATED DRY-TYPE TRANSFORMERS

The hottest spot temperature allowance to be used for the different in sulation system temperature classes is a major unknown facing IEEE Wor king Groups developing standards and loading guides for ventilated dry type transformers. In 1944, the hottest spot temperature allowance fo r ventilated dry type transformers was established as 30 degrees C for 80 degrees C average winding temperature rise. Since 1944, insulation temperature classes have increased to 220 degrees C but IEEE standard s continue to use a constant 30 degrees C hottest spot temperature all owance. IEC standards use a variable hottest spot temperature allowanc e from 5 to 30 degrees C. Six full size test windings were manufacture d with imbedded thermocouples and 133 test runs performed to obtain te mperature rise data. The test data indicated that the hottest spot tem perature allowance used in IEEE standards for ventilated dry type tran sformers above 500 kVA is too low. This is due to the large thermal gr adient from the bottom to the top of the windings caused by natural co nvection air flow through the cooling ducts. A constant ratio of hotte st spot winding temperature rise to average winding temperature rise s hould be used in product standards for all insulation temperature clas ses. A ratio of 1.5 is suggested for ventilated dry type transformers above 500 kVA. This would increase the hottest spot temperature allowa nce from 30 degrees C to 60 degrees C and decrease the permissible ave rage winding temperature rise from 150 degrees C to 120 degrees C for the 220 degrees C insulation temperature class.