Transformers Inrush Current

The inrush current originates at a time when the polarity and magnitude of the residual magnetic flux do not equal the polarity and magnitude of the flux generated by the power connected to the transformer terminals.

The energization of a transformer is an old case of the appearance of these currents. It causes the inrush current to the reset voltage, and energizes one transformer in parallel when the other is already in service.

The inrush current has as characteristics to contain a DC component and odd and even harmonics and to have the time constant much greater than the exponential decay of the DC component of a short circuit current.

Inrush current, which can be mistaken for an internal fault, may have spikes in the order of 8 to 30 times the rated current.

The factors that control the duration and magnitude of this current are:

– The size of the transformer;

– The size of the power system connected to the transformer;

– The type of iron used in the transformer core, as well as the saturation density;

– The residual iron flux;

– The type of transformer energization.