What is Inrush current in a transformer?

Inrush Current of Transformer:

Inrush current , which is characterized as being almost entirely unidirectional, rises abruptly to its maximum value in the first half-cycle after the transformer being energized and, thenceforth, decays until the normal steady-state magnetizing conditions in the transformer are reached. As generally accepted, the magnitude and duration of the inrush current depend basically on:

  • The point on the voltage wave at which the transformer is energized.

  • The residual flux in the transformer core and its sign with respect to the first half-cycle of the steady state alternating flux.

  • The saturation or maximum flux density of the ferromagnetic material of the transformer core.

  • The total impedance of the circuit through which the inrush current flows. This is true when only a single transformer is involved in the transient, i.e., when the first transformer is switched onto the system. However, in case of having one or more transformers already connected to the system, the duration and magnitude of the inrush current may change significantly. This happens due to saturation in the transformers in service caused by the transformer inrush itself.

The inrush current contains a large second harmonic component in comparison to a fault current.The inrush current value is maximum if transformer switched at zero instant of voltage wave as shown in figure.

Transformer Inrush current