MV design guide - Schneider Electric

Design rules
Short-circuit currents
Transformer
In order to determine the short-circuit current across the terminals
of a transformer, we need to know the short-circuit voltage (Usc %).
c Usc % is defined in the following way:
The short-circuit current depends on
the type of equipment installed on
the network (transformers,
generators, motors, lines, etc).
potentiometer
U : 0 to Usc
V
primary
secondary
A
I : 0 to Ir
Example:
c Transformer 20 MVA
c Voltage 10 kV
c Usc = 10 %
c Upstream power: infinite
Ir =
Sr 20 000
= = 1 150 A
e U no-load e•10
Isc =
Ir
=
1 150
= 11 500 A = 11.5 kA
Usc 10÷100
1 the voltage transformer is not powered: U = 0
2 place the secondary in short-circuit
3 gradually increase voltage U at the primary up to the rated current Ir in
the transformer secondary circuit.
The value U read across the primary is then equal to Usc
c The short-circuit current, expressed in kA, is given by the following
equation:
Ir
Isc =
Usc
Schneider Electric
Merlin Gerin MV design guide
13