Park-based and zero sequence-based relaying techniques with application to transformers protection

Two relaying techniques for protecting power transformers are presented and discussed. Very often, differential relaying is used for this purpose. A comparison between the two proposed techniques and conventional differential relaying is thus presented. The first technique, based on the measurements of zero sequence current within a delta winding, performs best in multiwinding transformers, since only measurement of the coil currents is needed. Thus, great simplicity is achieved. The second one is based on the differential procedure, but its analysis of asymmetries in the plot in Park´s plane avoids problems related to spectral analysis in conventional differential relaying. The technique is justified from the analysis of symmetrical components. Misoperation in conventional differential relaying has been observed in some cases as a function of switching instant and fault location. This issue is discussed in the paper, and a statistical analysis of a large number of laboratory tests, in which both factors were controlled, is presented. As a conclusion, both relaying techniques proposed succeed in protecting the transformer. Additionally, the Park-based relay exhibits three characteristics of most importance: fastest performance, robustness and simplicity in its formulation.