Answer / ariel

- Draw the per phase diagram of the system
- Express all impedance in per unit values
- Choose a common MVA base and express all Zpu on the
selected common MVA base

Znew= Zpu(MVA base/ MVA)
-Calculate the total per unit impedance of the system
using circuit principle
- Calculate the fault power
MVA fault= MVA base/ Z total per unit

I fault = MVA fault/ 1.73* V

Answer / mak

The fault current of a power transformer will depend on the following;
Transformer Rating (in KVA/MVA)
per unit impedence of the transformer (%p.u.)
line/phase Voltage (VL/VP)
the following formula can be used to find the fault current on the secondary side of a transformer
Fault Current = Transformer Rating /(per unit impedance x phase voltage)

The Values of Transformer Rating, per unit impedance & phase/line voltage will usually be mentioned on the transformer rating plate / data sheet
As an example a 500kVA, 11kV/400V/3-Phase/50Hz transformer with 5% p.u impendence will have the following fault levels on the secondary side

Fault level = 500/(5%)=10000kVA


Fault current = 500/(5% x 400 x 1.732) = 14.4 kA

Remember to use 3phase voltage!

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