Theory and Application of Transformer Differential Protection

In attempting to use standard percentage differential relays for transformer protection in the most efficient manner for a wide variety of transformer connections and types, several problems have arisen which have resulted in solutions not in the usual repertoire of transformer protection. The scope of this paper is limited to some general remarks on the fundamentals of transformer protection using percentage differential relays, and to a few selected applications of this type of relay which depart somewhat from the usual course of such protection. Certain occasionaland independent problems of transformer protection which are superimposed on the basic application requirements to be discussed here, such as relay operation on transient magnetizing inrush current flow, are not included within the scope of this paper.