substation short circuit currents 1

[xernoxian]

Hello,
I have noticed that sometimes, values for 3 phase short circuit currents are lower than single phase short circuit currents. Could someone please explain this to me (I thought 3 phase faults were the most severe of all faults).

 

[dpc]

For core type three-phase delta-wye transformers, the zero-sequence reactance can be less than the positive sequence reactance, so close to the transformer the SLG current can exceed the 3-phase fault current. As you move away from the transformer, the zero sequence impedance of the overhead lines or cables tends to negate this effect.

 

[jghrist]

Even if the zero-sequence reactance is the same as the pos-sequence reactance, the 1Ø fault on the secondary of a delta-wye transformer will be higher because the transformer is a ground source. In the sequence diagrams for a 1Ø fault, the zero-sequence network has the system impedance (on the primary side of the transformer) shorted out.