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Star connected, 3 phase, 11kV, 25 MVA alternator with its neutral grounded through 0.033 pu reactance has positive, negative and zero sequence reactance of 0.2 pu, 0.1 pu and 0.1 pu respectively. A single line to ground fault on one of its terminal would result in a fault MVA
150 MVA
125 MVA
100 MVA
50 MVA
150 MVA
Quick Trick: For SLG faults, the fault current is 3Vph / (Z1 + Z2 + Z0 + 3Zn). Since all are in per unit, compute the total impedance sum first, then calculate Fault MVA = Rated MVA / ZtotalтАЛ.
For SLG faults, the fault current is 3Vph / (Z1 + Z2 + Z0 + 3Zn). Since all are in per unit, compute the total impedance sum first, then calculate Fault MVA = Rated MVA / ZtotalтАЛ.
Calculate total sequence impedance
For a Single Line to Ground (SLG) fault, the total impedance offered to the fault current is ZeqтАЛ = Z1 + Z2 + (Z0 + 3Zn). Given Z1=0.2, Z2=0.1, Z0=0.1, and Zn=0.033, then 3Zn = 0.099 approx 0.1. Thus, ZeqтАЛ = 0.2 + 0.1 + 0.1 + 0.1 = 0.5 pu.
Calculate Fault MVA
Fault MVA = (Base MVA) / Z_eq_pu. Substituting the values: 25 / 0.5 = 50. Wait, check 3Zn calculation: 3 * 0.033 = 0.099. Sum = 0.2 + 0.1 + 0.1 + 0.099 = 0.499 approx 0.5. Fault MVA = 25/0.166... Re-calculating: IfFault = 3V / (Z1+Z2+Z0+3Zn) = 3 / 0.5 = 6 pu. Fault MVA = 6 * 25 = 150 MVA.
B: Incorrect, does not account for the 3*Zn multiplier correctly. C: Incorrect, resulting from ignoring the ground reactance contribution. D: Incorrect, calculation error in the sequence network summation.
A is correct because the fault MVA for an SLG fault is calculated as (3 * Base MVA) / (Z1 + Z2 + Z0 + 3*Zn), which yields 150 MVA.
Always remember to multiply the neutral reactance by 3 when calculating the sequence network impedance for SLG faults; this is the most common point of failure.