In the MVA Method Short Circuit Calculation tutorial, we have discussed how to calculate the three (3) phase and phase to ground fault currents.
The faults currents for three (3) phase unbalance faults are as follows:
1. Three (3) Phase Fault
2. Phase to Ground Fault
3. Phase to Phase Fault
4. Phase to Phase to Ground Fault
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In the calculation of the Line to Ground Fault at the 400V bus above, the MVAsc0 = 33.33 which is the equivalent MVA of the 2MVA Transformer only. Why is it that the contributions of the 300kVA and 596kVA motors are not considered? They are not blocked by the transformer. MVAsc0 must be 41.43. May be a case of oversight?…
The contribution of the 400V motors are already considered on the MVAsc1 = 28.28 & MVAsc2 = 28.28. Motors do not contribute to the zero sequence fault current as these are commonly delta connected or if wye connected, motors are ungrounded thus not contributing to the zero sequence current. It is only the transformer therefore which contributes to the zero sequence current.