How it works
Symmetrical (three-phase) faults are the most severe — highest fault current, use only positive sequence network. Fault MVA = Vbase²/Zf (in actual units); or simply fault MVA = system base MVA/Zf(pu). The Z-bus (bus impedance matrix) method is systematic for multi-bus systems: fault current at bus k = Vk pre-fault / Zkk, where Zkk is the driving-point impedance (diagonal element). Voltage at unfaulted bus j = Vpre − Zkj × If. Building Z-bus from Y-bus by matrix inversion, or incrementally, is the standard method.
Key points to remember
Symmetrical fault: all three phases involved, use Z1 only. SLG: most common in practice (70–80% of all faults on overhead lines), sequence networks in series, I_fault = 3Vf/(Z1+Z2+Z0). LL fault: sequence networks Z1 and Z2 in series, no Z0 involved. DLG fault: highest zero-sequence current. Fault level (short-circuit MVA) at a bus determines the required circuit breaker interrupting rating — Indian grid 400 kV buses can have fault levels exceeding 40 kA. Subtransient fault current Iu' uses Xd'; transient uses Xd'; both are smaller than the steady-state value.
Exam tip
The examiner always asks you to calculate fault current for a three-phase fault using pu system — find Zkk from the Z-bus or from series combination of generator and transformer reactances, then divide pre-fault voltage (usually 1.0 pu) by Zkk.