Distribution transformer overloading capability under cold-load pickup conditions

Greater use of electric heating in residential areas in North America has led to the appearance of a new type of thermal stress resulting from service restoration following a power outage. This loading condition, known as cold-load pickup, is simulated on a sample of 18 distribution transformers by applying overloads up to three times the rated current at ambient temperatures as low as -40°C. The data gathered is used to develop a thermal model representing the transient behavior of distribution transformers. The results are then generalized to produce a loading guide for evaluation of maximum hot-spot temperatures and loss of life under given cold-load pickup conditions