For this example, I want to estimate and calculate the short circuit current on 3.3kV busbar based on the Transformer rating only. This calculation for estimate purpose only, if we want to know the exact value of this short circuit current, we need to run and simulate the overall system by using software like SKM, EDSA, Paladin, ETAP and many more.
This estimate value is important to sizing and choose the right busbar short circuit withstand current.
The transformer rating for this system is 2MVA, 11/3.45kV with 6.25% voltage impedance.
The kVA Short Circuit = kVA rating/Z2000/0.0625 = 32 kVA
Short Circuit Current at 3.3kV busbar = kVA sc / (√3 x V)= 32 / (1.732 x 3.3)= 5.60 kA
From above calculation, the short circuit contribution from the Transformer to the 3.3kV busbar is 5.60 kA. To get more accurate value, we need to consider the short circuit contribution from the loads connected to the 3.3 kV busbar. We can assume that the short circuit contribution from other loads is 10% from above value.
Total Short Circuit Current at 3.3kV busbar = Transformer Short Circuit Contribution x 1.1= 5.60 x 1.1= 6.16 kA
The picture below shows the short circuit current value that I got after run this system with Paladin@EDSA software. If we compare both 6.16 kA and 6.01 kA values, this manual calculation still can be acceptable for preliminary calculation before run the simulation.
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