Inrush Blocking Scheme in Transformer Differential Protection

Abstract

Transformer inrush currents during energization result in mal operation of transformer differential protection, due to the flow of magnetizing current only on the primary / source side of the transformer. A straightforward approach in recognition of inrush currents is proposed here by checking the core cause of this inrush phenomenon i.e. the magnetic saturation of the transformer core. In this method, the pseudocharacteristics were approximated in terms of orthogonal polynomials, and is used to distinguish between inrush currents and fault currents. In this method, the magnetic saturation characteristics of the transformer is modeled by defining pseudocharacteristics, and is obtained over a half cycle data window of the differential current. This data is assessed using certain criteria to detect inrush and fault currents. This method is not reliant on the transformer models and is precise in its output. The propositioned method was tested using delta-wye transformers under both fault and inrush scenarios.