Monday, 23 February 2026

📘 Power Systems – Question Bank (Page 2)

🔹 Section A: Conceptual Questions (Per Unit System)

Q1. Define per unit value.

Per unit value = Actual value / Base value.

Q2. Why is per unit system preferred in power system analysis?

It simplifies calculations and eliminates transformer turns ratio in multi-voltage systems.

Q3. What are the four base quantities?

Q4. If base MVA changes, what happens to per-unit impedance?

Per-unit impedance changes proportionally with base MVA.

Q5. Base impedance is given by:
A) Vbase × Ibase
B) Vbase² / Sbase
C) Sbase / Vbase²
D) Ibase² × R

Answer: B) Vbase² / Sbase

Q6. If actual impedance = 10 Ω and base impedance = 20 Ω, per unit value is:
A) 0.2
B) 0.5
C) 2
D) 200

Answer: B) 0.5

Q7. Per unit system removes:
A) Voltage
B) Current
C) Transformer ratio
D) Resistance

Answer: C) Transformer ratio

Q8. Find base impedance if Sbase = 100 MVA and Vbase = 132 kV.

Zbase = V² / S = (132 × 10³)² / (100 × 10⁶) = 17424 × 10⁶ / 100 × 10⁶ = 174.24 Ω

Q9. Convert 25 Ω impedance into per unit on 100 MVA, 132 kV base.

Zpu = Zactual / Zbase = 25 / 174.24 = 0.1435 pu

Q10. A transformer has reactance 10% on its own base. Convert to 200 MVA base if original base is 100 MVA.

Znew = Zold × (Snew / Sold) = 0.1 × (200 / 100) = 0.2 pu

Q11. Base current if Sbase = 50 MVA and Vbase = 11 kV.

Ibase = S / (√3 × V) = 50 × 10⁶ / (1.732 × 11000) = 2624 A (approx)

Q12. Generator reactance = 0.2 pu on 100 MVA base. Find actual impedance if base voltage = 11 kV.

Zbase = V² / S = (11000)² / (100 × 10⁶) = 1.21 Ω Zactual = 0.2 × 1.21 = 0.242 Ω

Page 2 Completed – Per Unit System (Concept + MCQ + Numerical)