Unexpected Transformer Tripping Provides Insight on Circulating Inrush Current

Unexpected equipment operations in the Power System provide an opportunity to understand previously unexplored phenomena. The phenomenon of inrush current circulating in a transformer delta winding has only been explored before via simulation-based work. This paper uses unexpected tripping of a high-voltage power transformer to provide insight on circulating inrush current. It presents an engineering analysis of the transformer tripping event and highlights how inadvertent measurement of the circulating inrush current led to unexpected tripping. Further, it presents an expression to calculate the circulating inrush current and validates the same using data recorded by protective relays. By corroborating calculations with recorded data, the paper demonstrates how detailed analysis of unexpected events can enhance understanding of lesser-known phenomena.