Fault Current Calculation [ GENUINE ]
[ I_f = \fracV_thZ_th ] where ( V_th ) is the prefault voltage at the fault point (usually 1.0 pu), and ( Z_th ) is the Thevenin equivalent impedance seen from the fault point (resistance + reactance). In high-voltage systems, resistance is often neglected, giving:
( Z_Tx = 0.08 , \textpu ) (already on 10 MVA base).
( Z_th = 0.01 + 0.08 = 0.09 , \textpu )
[ V_pu = \fracV_actualV_base, \quad I_pu = \fracI_actualI_base, \quad Z_pu = \fracZ_actualZ_base ] where ( Z_base = \fracV_base^2S_base ), ( I_base = \fracS_base\sqrt3 V_base ) (three-phase system). For a three-phase fault at a bus, the fault current magnitude is:
( I_f = 1.0 / 0.09 = 11.11 , \textpu )
( S_base = 10 , \textMVA ), ( V_base, HV = 115 , \textkV ), ( V_base, LV = 13.8 , \textkV ).
| Fault Type | Network Connection | |---|---| | SLG | ( Z_1, Z_2, Z_0 ) in series | | L-L | ( Z_1 ) and ( Z_2 ) in parallel (no ( Z_0 )) | | DLG | ( Z_1 ) in series with parallel combination of ( Z_2 ) and ( Z_0 ) | fault current calculation
[ I_base = \frac10 \times 10^6\sqrt3 \times 13.8 \times 10^3 = 418.4 , \textA ] [ I_f = 11.11 \times 418.4 = 4648 , \textA \quad (\textsymmetrical RMS) ]
