GATE Electrical Engineering - Complete Master Library

 

📘 Power Systems – PSU & Interview Questions (Page 22)

Common PSU, Core Company & GATE Interview Questions with clear answers.

---

🔹 Fundamental Interview Questions

1. Why is per-unit system preferred in power systems?

• Simplifies calculations
• Transformer impedance remains same across sides
• Eliminates unit mismatch
• Easy comparison of equipment ratings

---

2. Why is 3-phase fault most severe? Because all three phases are short-circuited, sequence impedance minimum → highest fault current.

---

3. Why is SLG fault most common? Due to insulation failure or lightning → line touches ground.

---

4. Why does increasing reactance reduce stability? From power equation: P = (EV/X) sinδ If X increases → Pmax decreases → stability margin reduces.

---

5. What is penalty factor in economic dispatch? Penalty factor accounts for transmission losses. Formula: Penalty = 1 / (1 - ∂PL/∂Pi) It increases generator cost if losses are high.

---

🔹 Technical Deep Questions

6. Why does distance relay underreach? Due to infeed effect. Remote source contributes current → measured impedance appears higher.

---

7. Why HVDC is preferred for long distance?

• No reactive power flow
• No stability limit
• Lower losses
• Interconnection of asynchronous grids possible

---

8. What is difference between steady-state and transient stability?

• Steady state → small disturbances
• Transient → large disturbances (faults)

---

9. Why Newton-Raphson preferred over Gauss-Seidel?

• Quadratic convergence
• Fewer iterations
• Better for large systems

---

10. What happens if load suddenly increases?

• Frequency decreases
• Governor increases mechanical input
• System returns to equilibrium

---

🔹 Practical Power Plant Questions

11. What is short circuit level? Maximum fault MVA at a bus. 12. Why are breakers rated in MVA? Because they interrupt fault power, not just current. 13. Why do we use zone protection? To ensure selectivity and avoid unnecessary tripping. 14. Why do we use surge arresters? To protect equipment from lightning and switching surges. 15. Why is grounding important?

• Limits fault current
• Improves protection sensitivity
• Ensures safety

---

🔹 Final Interview Tip

Always answer with: ✔ Formula ✔ Physical explanation ✔ Practical example That impresses panel.

---

Page 22 – PSU Interview Module Completed

🔥 Power Systems Complete Master Series – 22 Pages 🔥