Loading Guide for 65 C Rise Distribution Transformers

Transformer loss of life calculations are made based on a normal transformer life expectancy of 30 years at a continuously sustained hot-spot temperature of 120 C (degrees centigrade). Computer-derived equations are applied to a generalized hypothetical 65 C rise distribution transformer. Hot-spot temperature curves, including the heating and cooling portions of the curve, are calculated for loads from 90% to 280% in ambient temperatures from 0 C to 50 C. The change in transformer time constant with changing loads is considered. The hot-spot temperature curves are converted to aging factor curves. The area under the aging factor curve is compared with a standard area representing a hot-spot temperature of 120 C continuously sustained for 24 hours. A transformer load capability table is presented.