Inrush Current in a transformer refers to the initial current rush which occurs when the transformer is first switched on. This high initial current is due to the absence of back-emf at the time of switching on.
In a transformer without load, the magnetic field is in quadrature with the voltage. That is, it lags behind the voltage at an angle of 90 degrees (electrical).
If the switching occurs when the voltage wave is at zero, the magnetic flux wave will be at its maximum and the inrush current will be maximum. If the voltage is at maximum at the instant of switching, the flux and the current will be at minimum. The high current lasts for a few milliseconds and comes its steady stage value.
Always keep the secondary open, when switching on the transformer. This will help minimize the inrush current.
For large transformers, switching on the transformer can result in high current and a consequent voltage dip in the system. This can affect the system as a whole.
Hence, special switching techniques such as the use of resistance starters or soft starters have to be employed.