Transformer insulation system-a review of the guiding principles for thermal classification

Since the beginning of this century, by first working with insulation materials, then evaluation of entire insulation systems, a solid foundation of the time-temperature relationship of the deterioration of insulation has been developed. The time-temperature relationship is established by accepted functional accelerated life tests on the actual units or on models. Correlation of the test procedure with field experience for the purpose of predicting thermal classification and acceptable life has been accomplished. There appears to be some differences of opinion as to the interpretation of the data from the functional accelerated life tests. The accelerated life test was designed to be severe to obtain data in a short period of time; and since these data has been correlated with systems that have much field experience under normal field conditions, it would not seem logical to draw conclusions from these data for systems operating under radically different field conditions. For this set of field conditions, it would appear that a different functional accelerated life test should be used from which to draw conclusions regarding the thermal rating and life.