What causes transformer inrush current and how is it mitigated?

Inrush current occurs during transformer energization when the flux must change rapidly to establish steady-state. If voltage is applied at zero crossing, flux must rise to twice normal peak (due to DC offset), potentially causing 10-20 times rated current. Magnitude depends on: switching instant, residual flux polarity, and core saturation. Mitigation includes: controlled switching (point-on-wave switching at voltage peak), pre-insertion resistors, soft-start circuits, and inrush-restraint in protective relays (use second harmonic blocking since inrush is rich in harmonics).