Should transformer to be protected against inrush current?

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We all know that when transformer is energized at no-load condition, core will get saturated. Due to that, huge inrush current is demanded from the source voltage. This phenomenon is quite frequent in all the types of machines. In rotating machine, there are several methods like direct on line(DOL), soft starter etc used to reduce the inrush current as it effects the dynamic performance of the rotor during starting period. Transformer also has the inrush effect but it is not given any consideration yet. There are certain parameters which put light on the importance of studying this phenomenon in stationary machine.

 Inrush current condition in transformer is transient phenomenon. So, it mainly understood by two parameters namely magnitude and duration. Magnitude of inrush current depends on :

- Switching angle of source voltage
- Rating of transformer 
- Impedance of energizing system
- location of windings (inner or outer)
- Magnetic properties and residual flux


Fig. Inrush current waveform considering residual flux

From the study of different types of transformer, it is observed that magnitude of 1st peak in inrush current is 50 to 70 % of the short circuit condition. Duration of this condition is also 3 to 4 times longer than the short circuit condition. Therefore, although the magnitude is less than the short circuit condition but it will have significant effect on windings of transformer considering the duration and frequency of occurrence.

In multi-winding transformer, there are more no. of low voltage windings. Normally, high voltage windings are placed near the core and split windings concept is adopted. This results in mismatch of magnetic axis between these two windings. In inrush condition, the axial force on energizing winding is more compare to short circuit condition because of more flux density in the saturated core and ampere tun imbalance between the windings.

Inrush current condition in transformer creates problems like protection system mal-operation, life span reduction , power quality issue in the grid (as it contains rich harmonics in its waveform).

Therefore, it is concluded that as the problems with inrush current has significant impact on transformer performance, it is to be protected considering the reliability of transformer.

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