PREDICTING HOTTEST SPOT TEMPERATURES IN VENTILATED DRY-TYPE TRANSFORMER WINDINGS

Test data indicates that hottest spot allowances used in IEEE standard s for ventilated dry type transformers above 500 kVA are too low. A ma thematical model to predict hottest spot temperature rises in ventilat ed dry type transformers was developed. Data from six layer type test windings and a 2500 kva prototype was used to refine the model. A corr elation for the local heat transfer coefficient in the cooling ducts w as developed. The model was used to study the effect of various parame ters on the ratio of hottest spot to average winding temperature rise. The number of conductor layers, insulation thickness, and conductor s trand size were found to have only a minor effect on the ratio. Windin g height was found to be the main parameter influencing the ratio of h ottest spot to average winding temperature rise. The study based on th e mathematical model confirmed previous conclusions based on test data that the hottest spot allowances used in IEEE standards for ventilate d dry type transformers above 500 kVA should be revised.