ABSTRACT
Calculation of the thermal capability of a two-winding transformer is straightforward because the magnitude of the KVA in the primary winding equals the KVA in the secondary winding. The calculation of the thermal capability of a three-winding transformer is complicated by the fact that the KVA loads of the three windings may be different and are unpredictable. In fact, a precise calculation of the thermal capability using the methodology in IEEE Standard C57.92-1981 is generally not possible unless certain simplifying assumptions are made. Usually, the tertiary load is either assumed to have a constant load equal to the rating of the tertiary winding or is assumed to have a load equal to the secondary load.
