Abstract :
J. K. Hodnette (Westinghouse Electric and Manufacturing Company, Sharon, Pa.): It has been recognized for a long time that electrical apparatus has considerable latent overload capacity and this is especially true of transformers. Transformer load factors are rarely 100 per cent and ambient temperatures are seldom continuously 40 degrees centigrade. Because of these factors, the name-plate rating of the transformer is often much less than the available capacity for carrying load on a safe thermal basis. Many papers have been presented before the AIEE in an effort to tap this huge reservoir of extra transformer capacity. The rules and guides formulated by these papers have failed to make it possible to use this extra transformer capacity in many instances because of the complexity of transformer heating cycles and the rapid variation in load, which is characteristic of most transformer installations. A properly designed thermal relay is the answer to this problem because it automatically integrates the effects of loads which vary both in duration and magnitude and of changing ambients and in a very simple manner permits operators to interpret these factors in terms of safe transformer loading.
