The inrush current in a transformer at the instant it is connected to the AC mains has been known since 1890's: J. A. Fleming; Experiments on the magnetic behavior of transformers at the moment of connecting them to a circuit; Journal of the Institution of Electrical Engineers; Vol. 21; No. 101; November 1892; pp. 677-686 + Plate [Section VIII]. Note, Section VIII is part of a larger article: Experimental researches on alternate-current transformers; pp. 594-686. One can easily explain this behavior by solving a simple differential equation, see the document titled "Zero-Crossing Switching of Inductors" on my ResearchGate page.