An algorithm using a shape-based approach in Park's plane for transformer differential relaying on the basis of time-dependent symmetrical components

Discrimination between inrush transients and internal faults is an issue of most concern in differential relaying when protecting power and distribution transformers. Based on an analysis of the symmetrical components obtained during inrush and during internal fault conditions, a linkage between their instantaneous evolution and the shapes of plots in Park's plane is presented in this paper. The use of Park transformation allows to avoid conventional spectrum analysis, what results in a quite simplified protective technique. By introducing a vector, which is defined as the sum of the vectors corresponding to measurements in both sides of the transformer, a categorization of shapes of plots is done here. Thus, a shape-based rule is presented for discriminating between inrush and fault in this paper. In addition, an algorithm is presented for fast classification of shapes. The proposed algorithm is based on the periodicity generated by the modulus of the defined Park's vector. The approach presented in this paper has been tested in a laboratory transformer, showing a robust, accurate and fast performance.