Fundamentals of transformer inrush

This paper will present an overview on the phenomenon of transformer inrush and its impact on differential relays. In order for the protection engineer to properly address the phenomenon, the theoretical nature of inrush will be discussed. Impact of residual flux and higher efficiency core designs will be covered. Examples of inrush waveforms will be analyzed in the time domain as well as in the frequency domain. Modern protective relaying techniques to detect and prevent misoperation will also be covered. Design of transformer differential inrush restraint methods including: second harmonic, fourth harmonic, total harmonic, per phase, cross blocking, adaptive restraint and other methods will be covered as well. Discussion on the impact of motor/generator differential relays due to inrush will also be covered. Techniques for avoiding nuisance tripping of machine differential relays without compromising speed of operation will be discussed.