Emergency Overloading or Air-Cooled Oil-Immersed Power Transformers by Hot-Spot Temperatures

The paper points out that during the period of the war the need for obtaining maximum transformer overload capacity is very great, particularly under rare emergency conditions. From a thermal standpoint, the amount of overload is limited by the hottest spot in the transformer winding. To obtain accurate data 1. On the actual hot-spot temperatures under rated load conditions 2. On the increase of the hot spot with load under both ultimate moderate overload and heavy short-time overload conditions approximately 50 heat runs were made on several power transformers of different ratings, representing a wide variety of design conditions. The results of the tests are given in the paper. The subject of how heavy overloads affect the life of transformers, is discussed and temperature limits somewhat higher than those given in the American Standards, Guides for Operation of Transformers, are suggested. Calcul??ted short-time overloads are given to show the effect of transformer characteristics on overload values.