Medium Voltage Application Guide

13844.A
SWITCHGEAR
4.5 Calculations
Transformer Calculations
Rated secondary current, Ir
A transformer's rated secondary current (I r ) is the maximum current it can supply before the output terminal voltage
starts to drop below its rated voltage (U r ). The rated secondary current can be calculated using the following
formula (assuming the applied primary voltage, U prim , is at its rated value).
Where
I r = rated secondary current (A)
S = transformer power (kVA)
= rated secondary voltage (kV)
U r
U prim (kV)
U r (kV)
S (kVA)
Z (%)
I sc (A)
I r (A)
Short circuit current, Isc
Assuming the transformer is fed from an unlimited supply, the maximum short circuit current across the output
terminals (I sc ) is determined by the impedance of the transformer (expressed as a percentage).
Percentage impedance (Z%) is calculated by shorting the output terminals of the transformer and increasing the
applied primary voltage (U prim ) from zero to a value where the rated current, I r , flows through the secondary.
Percentage impedance is the ratio of applied primary voltage to rated primary voltage.
Example: If it takes 10% of the rated primary voltage to cause rated current to flow in the shorted secondary, the
percentage impedance Z=10%
Ir
U prim
kV
A
1
Short circuit
Primary/secondary
2
Z (%)
13845.A
Where
I sc = transformer's maximum output short circuit current (A)
I r = rated secondary current (A)
Z% = percentage impedance
710-12280-00A Medium Voltage Application Guide Page 143