To compute the three-phase fault current at the faulted bus using symmetrical components, which sequence networks are connected in series across the fault?

To compute the three-phase fault current at the faulted bus using symmetrical components, which sequence networks are connected in series across the fault?

• A. Z1 and Z2 in series only
• B. Z0 in series only
• C. Z1, Z2, and Z0 in series across the fault
• D. Z1, Z2, and Z0 connected in parallel to the fault

Answer: (C)

In a solid three-phase fault, the positive-, negative-, and zero-sequence networks form a single loop across the fault. The same fault current flows through all three networks, so they are connected in series, giving a total impedance of Z1 + Z2 + Z0. The fault current is then If = Vprefault / (Z1 + Z2 + Z0). If the networks were not in series, or if only one or two were used, you wouldn’t have the proper single fault current path or the correct relationship between the sequence voltages and currents.

In a solid three-phase fault, the positive-, negative-, and zero-sequence networks form a single loop across the fault. The same fault current flows through all three networks, so they are connected in series, giving a total impedance of Z1 + Z2 + Z0. The fault current is then If = Vprefault / (Z1 + Z2 + Z0). If the networks were not in series, or if only one or two were used, you wouldn’t have the proper single fault current path or the correct relationship between the sequence voltages and currents.