ABB UAC317AEV1 After sales service is provided for the control card of the power supply device

Description

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The full voltage three-phase short-circuit current (Ik
), at the LV terminals of the
transformer, can be expressed as (assuming that the short-circuit power of
the network is infinite):
where:
uk
% is the short-circuit voltage of the transformer, in %.
The protection circuit-breaker must have: (*)
I
n ≥ Ir
;
I
cu (Ics) ≥ Ik
.
If the short-circuit power of the upstream network is not infinite and cable or
busbar connections are present, it is possible to obtain a more precise value for
I
k
by using formula (1), where ZNet is the sum of the impedance of the network
and of the impedance of the connection.
MV/LV substation with more than one transformer in parallel
For the calculation of the rated current of the transformer, the above applies
(formula 4).
The breaking capacity of each protection circuit-breaker on the LV side shall
be higher than the short-circuit current equivalent to the short-circuit current of
each equal transformer multiplied by the number of them minus one.
As can be seen from the diagram below, in the case of a fault downstream of a
transformer circuit-breaker (circuit-breaker A), the short-circuit current that flows
through the circuit-breaker is equal to the contribution of a single transformer.
In the case of a fault upstream of the same circuit-breaker, the short-circuit
current that flows is equal to the contribution of the other two transformers
in parallel.

The tables refer to the previously specified conditions; the information for the
selection of circuit-breakers is supplied only with regard to the current in use and
the prospective short-circuit current. For a correct selection, other factors such
as selectivity, back-up protection, the decision to use limiting circuit- breakers
etc. must also be considered. Therefore, it is essential that the design engineers
carry out precise checks.
It must also be noted that the short-circuit currents given are determined using
the hypothesis of 750 MVA power upstream of the transformers, disregarding
the impedances of the busbars or the connections to the circuit-breakers.
Example:
Supposing the need to size breakers A1/A2/A3, on the LV side of the three
transformers of 630 kVA 20/0.4 kV with uk
% equal to 4% and outgoing feeder
circuit-breakers B1/B2/B3 of 63-400-800 A: