Sequence Current-Based Inrush Detection in High-Permeability Core Transformers

Modern transformers are designed with high-permeability core material to lower losses and noise levels. During energization, the transformer enters the ultrasaturation region and loses properties such as the dead angle and second harmonic content in differential inrush current. As a result, the traditional inrush detection differential relaying approaches, utilizing current wave shape analysis and second harmonic content, face security problems. This article proposes a transformer inrush detection technique using the decaying dc component in the negative sequence differential current. Three factors are considered to set the threshold to discriminate internal faults, ensuring differential relay blocking during inrush. Simulation results for various internal faults and inrush conditions validate the performance of the proposed method with high permeability of the transformer core, resulting in low second harmonic during inrush. Field data testing ensures its practical implications in available numerical differential relays.