PSA
CO 4, COI-3 question bank
1. For the system shown in figure . A LL fault occurs at point F with Zf =0.15p.u. Find fault current.
2. Draw the sequence network for the system shown in figure when LLG at F Zf =0.15p.u . Choose
Base MVA as 100 MVA and Base KV as 20 KV.
3. A 30 MVA, 11 KV generator has Z1 = Z2 = j0.2 p.u, Z0 = j0.05 p.u. A line to ground fault occurs
on the generator terminals. Find the fault current and line to line voltages during fault conditions.
Assume that the generator neutral is solidly grounded and that the generator is operating at no-load
and at rated voltage at the occurrence of fault.
4. A 30 MVA, 13.8Kv generator with neutral grounded through a 1-ohm resistance, has a threephase fault MVA of 200MVA. Calculate the fault current and the terminal voltages for a single lineto- ground fault with an impedance of 01p.u. at one of the terminals of the generator. The negative
and zero sequence reactances of machine are 0.10pu and 0.05pu respectively. Neglect pre-fault
current, and losses. Assume the pre-fault generated voltage at the rated value.
5. A 20 MVA, 6.6 KV star connected generator has positive, negative and zero sequence reactances
of 30%, 25% and 7% respectively. A reactor with 5% reactance based on the rating of the generator
is placed in the neutral to ground connection. A line-to-line fault occurs at the terminals of the
generator when it is operating at rated voltage. Find the initial symmetrical line-to-ground r.m.s
fault current. Find also the line-to-line voltage.
6. A 50 MVA, 11 KV, three phase alternator was subjected to different types of faults. The fault
currents are; three phase fault 1870 A, line to line fault 2590 A, single line to ground fault 4130 A.
the alternator neutral is grounded with Zf = 0.1p.u. Find the p.u values of the three sequence
reactances of the alternator.
7. Calculate the subtransient fault current in each phase for a short circuit on one phase to ground at
bus ‘q’ with impedance 0.01p.u. for the system shown in figure
8. Calculate the subtransient fault current in each phase for a short circuit double line to ground
with impedance 0.05p.u. at bus ‘2’ for the system shown in figure
9. Calculate the subtransient fault current in each phase for a short circuit on one phase to ground
with impedance 0.05p.u. at bus ‘2’ for the system shown in figure
10. A 30 MVA 11 KV generator has Z1 = Z2 = j0.2 p.u, Z0 = j0.05 p.u. A double line to ground
fault occurs on the generator terminals. Find the fault current and line to line voltages during limit
conditions. Assume that the generator neutral is grounded with impedance of 0.01pu and that the
generator is operating at no load ad at rated voltage at the occurrence of fault.