Methods for differential protection of Scott transformers

This article describes different methods for the differential protection of Scott transformers, which are based on: (a) single-phase relays, (b) 3-phase relays for two-winding transformers, (c) 3-phase relays for multi-winding transformers, (d) specific relays for Scott transformers. Initially, a description about the currents in Scott transformers is shown, which is necessary to understand their differential protection. Afterwards, the different methods for the differential protection of Scott transformers are described. Some methods, based on single-phase relays and based on 3-phase relays for multi-winding transformers, are briefly described in literature, and the contribution of this article is to highlight important details about the matching of the currents in the differential protection. Other methods, based on 3-phase relays for two-winding transformers, are not described in the literature and they are shown here. Finally, two options for the design of specific relays for Scott transformers are proposed in this article. Thus, the result is a clear explanation about the options for the proper differential protection of Scott transformers.