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Question 8.1: Estimate the short-circuit at point A for a system with the ......

Estimate the short-circuit at point A for a system with the following specifications for the main transformer and utility:
P_T=1000 \ kVA, Z_T=4\%, P_{SC,U}

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First, the combined utility-transformer per-unit impedance is calculated:

Z_{UT}=Z_U+Z_T

with

Z_u=\frac{P_T}{P_{SC,u}}=\frac{1000 \ kVA}{100,000 \ kVA}=0.01

Thus,

Z_{UT}(pu)=4\%+1\%=0.05

The application of Ohm’s law using the per-unit value provides

I_{SC}(pu)=\frac{E(pu)}{Z_{UT}(pu)}=\frac{1}{0.05}

 

I_{SC}(pu)=20

Without any motor contribution, the short-circuit at point A can be estimated:

I_{SC}(A)=I_{SC}(pu)*I_{L,S}(A)

with

I_{L,S}(A)=\frac{1000 \ kVA}{0.48 \ V\sqrt{3} }=1202 \ A

Thus, the short-circuit current at point A without any motor contribution is

I_{SC}=24,056 \ A

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