Datacenter-Link Blog

Transformers are used to achieve a change in the supply voltage, for both medium and low voltage supplies. The choice of the protection devices must take into account transient insertion phenomena, during which the current may reach values higher than the rated full load current; the phenomenon decays in a few seconds.

The curve which represents these transient phenomena in the time-current diagram, termed “inrush current I0”, depends on the size of the transformer and can be evaluated with the following formula (the short-circuit power of the network is assumed to be equal to infinity)

where:

K  ratio between the maximum peak inrush current value ( I0 ) and the rated current of the transformer (I1r): (K= I0 / I1r);

τ  time constant of the inrush current;

_Ir1  rated current of the primary;

t  time.

The table below shows the indicative values for t and K parameters referred to rated power Sr for oil transformers.

Further to the above consideration, the follwing diagram shows the inrush current curve for a 20/0.4kV of 400kVA transformer. This transformer has an inrush current during the very first moments equal to about 8 times the rated current; this transient phenomenon stops after a few tenths of a second.