Temperature Limits for Short-Time Overloads for Oil-Insulated Neutral Grounding Reactors and Transformers

Abstract

In part I, published in 1938,1 the principal question considered was the effect of ventilating coils on temperature limits for ground faults greater than one minute, for which it was assumed that 160 degrees centigrade based on all heat stored in copper was the correct limit. Since 1938 considerable new data have been obtained. This paper presents the new data and shows that: 1. For heating periods of one minute or less, practically all aging of insulation occurs during the cooling period. It is shown that temperature limits can be based on the rate of cooling rather than on the heating period. 2. In the heating-up period of one minute or less, 25 to 45 per cent of the loss is dissipated from the copper to the insulation and oil. Formulas are developed for calculating the temperatures during the heating and cooling periods. 3. Temperature limits above 160 degrees centigrade for one minute are permissible for both non-ventilated and ventilated coils, being approximately 200 degrees centigrade for windings which cool sufficiently rapidly.