Transformer Tertiary Stabilizing Windings. Part I: Apparent power rating

This paper presents a calculation method for the apparent power rating of Tertiary Stabilizing Winding (TSW) of power transformers, which is scarcely discussed in the literature. The objective of the work is to offer an approach for the dimensioning of TSWs from the knowledge of expected unbalanced load conditions. For that purpose, the equivalent circuit model of a three-winding transformer for single-phase load and single phase-to-ground fault study is discussed, where zero sequence impedances are determined by means of several proposed tests. A practical application is presented and the equivalent circuit model is validated with Finite Element Method (FEM) computations. A companion paper which complements this work evaluates the influence of TSWs on the overheating hazard on tank walls under unbalanced load conditions.