ABB UAA326A02 After sales service is provided for the control card of the power supply device

Description

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General aspects
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;
I
1r 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.
Sr [kVA] 50 100 160 250 400 630 1000 1600 2000
K = Io/I1r 15 14 12 12 12 11 10 9 8
τ [s] 0.10 0.15 0.20 0.22 0.25 0.30 0.35 0.40 0.45
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.