Monday, 23 February 2026

📘 Power Systems – Question Bank (Page 4)

🔹 Section A: Conceptual Questions (Fault Analysis)

Q1. What is a fault in power system?

An abnormal condition causing excessive current flow due to insulation failure or conductor contact.

Q2. What is symmetrical fault?

A 3-phase fault where all phase currents are equal in magnitude and 120° apart.

Q3. Which fault is most severe?

Three-phase fault (LLL fault).

Q4. Why is per-unit system used in fault analysis?

To simplify calculations across multi-voltage levels.

Q5. What is fault level?

Fault level = √3 × V × Isc (in MVA).

Q6. Highest fault current occurs in:
A) SLG fault
B) LL fault
C) LLG fault
D) LLL fault

Answer: D) LLL fault

Q7. For balanced 3-phase fault, sequence networks used are:
A) Positive only
B) Negative only
C) Zero only
D) All three

Answer: A) Positive only

Q8. Zero sequence current flows in:
A) Line-to-line fault
B) 3-phase fault
C) Single line-to-ground fault
D) None

Answer: C) Single line-to-ground fault

Q9. Fault MVA increases when:
A) System impedance increases
B) System impedance decreases
C) Voltage decreases
D) Frequency decreases

Answer: B) System impedance decreases

Q10. A generator has reactance of 0.2 pu. Find 3-phase fault current in pu.

If pre-fault voltage = 1 pu Fault current = 1 / X = 1 / 0.2 = 5 pu

Q11. A 132 kV system has short circuit current of 20 kA. Find fault MVA.

Fault MVA = √3 × V × I = 1.732 × 132 × 20 = 4570 MVA (approx)

Q12. If system impedance is reduced from 0.4 pu to 0.2 pu, how does fault current change?

Fault current = 1 / Z For 0.4 pu → 2.5 pu For 0.2 pu → 5 pu Fault current doubles.

Q13. Base MVA = 100. Fault MVA = 500. Find per unit fault current.

Ipu = Fault MVA / Base MVA = 500 / 100 = 5 pu

Q14. A 220 kV bus has fault level 3000 MVA. Find short circuit current.

Isc = MVA / (√3 × V) = 3000 / (1.732 × 220) = 7.87 kA (approx)

Q15. Why is 3-phase fault analysis simpler?

Because system remains balanced and only positive sequence network is used.

Page 4 Completed – Fault Analysis (Concept + MCQ + Numerical)