Extraction of DC component and harmonic analysis for protection of power transformer

Abstract

When a transformer is energized, magnetizing inrush current occurs due to temporary overfluxing in the transformer core. The magnitude of inrush current may be as high as ten times of transformer rated current which causes maloperation of differential protection system. This inrush current contains decaying DC and dominant second harmonic components which may cause undesirable effects like poor power quality and reduced mean lifetime of transformer. At the instant of transformer energizing if fault occurs, the protection system fails to discriminate between inrush and fault current. This paper provides a method for elimination of DC component and discrimination between inrush and fault condition for the purpose of power transformer protection. For elimination of DC component a compensating signal is generated with help of algorithm based on Taylor series expansion of inrush current. As second harmonic component is dominant in inrush current, discrimination is carried out using harmonic restraint. Based on ratio of second harmonic component to fundamental component algorithm is developed to discriminate inrush from fault current, which is validated on a case study developed in Matlab simulink.