Loading Transformers By Temperature

It is pointed out that safe loading of transformers by temperature requires not only an accurate knowledge of the thermal laws but also a knowledge of what is a safe temperature limit to be maintained continuously which condition seldom, if ever, happens with the present method of limiting the load to nameplate rating. In view of our present knowledge and experience the author questions the advisability of loading transformers continuously up to the present A. I. E. E. limit of 105 deg. cent. hot spot and argues for the establishment of a differential of 10 deg. cent. between the limit to be maintained continuously by means of overloads and the limit to be reached occasionally with rated load. In other words, for continuously maintained maximum temperatures the hot spot should not exceed 95 deg. cent. It is shown by the use of the thermal laws that without increasing the maximum or hot spot temperature, transformers can be overloaded 1 per cent for each degree centigrade by which the ambient is below 30 deg. cent. (air) for self-cooled transformers, 25 deg. (water) for water-cooled transformers. The results of laboratory aging tests conducted on class A insulations in air and in oil are given. These tests show that the rate of aging is roughly double for each 8 deg. cent. increase in temperature.