📘 Power System Protection – Interview Question Bank (With Answers)

This section covers important technical questions asked in GATE, PSU, Substation and Campus Interviews.

🔹 Basic Protection Questions

Q1. What is protection in power system?

Protection is the process of detecting faults and isolating faulty section to maintain system stability and safety.

Q2. What are primary and backup protection?

Primary protection operates first for local fault. Backup protection operates if primary fails.

Q3. What is selectivity?

Only faulty section must be isolated without affecting healthy system.

🔹 Differential Protection

Q4. On what principle does differential protection work?

Based on Kirchhoff’s Current Law (Current difference principle).

Q5. Why percentage bias is used?

To prevent mal-operation due to CT mismatch and external faults.

Q6. How inrush current is avoided in transformer differential?

Using second harmonic restraint method.

🔹 Distance Protection

Q7. What does distance relay measure?

Impedance (Z = V/I).

Q8. Why does infeed cause underreach?

Because apparent impedance increases due to additional current contribution.

Q9. What is Zone 1 reach?

80% of protected line, instantaneous.

🔹 Generator Protection

Q10. Why reverse power protection is needed?

To protect turbine when generator acts as motor.

Q11. What is negative sequence protection?

Protects rotor from overheating due to unbalanced current.

Q12. What happens during loss of excitation?

Generator absorbs reactive power and may lose stability.

🔹 Breaker Failure Protection

Q13. What is LBB?

Local Breaker Backup – operates if breaker fails to interrupt fault.

Q14. What is typical delay time?

100–300 ms.

🔹 Practical Questions

Q15. What happens if busbar protection fails?

Entire substation can be severely damaged.

Q16. Why CT saturation is dangerous?

It may cause relay mal-operation.

Q17. Which relay is used for transmission line?

Distance relay (Mho relay commonly).

Q18. What is coordination time interval (CTI)?

Time difference between primary and backup relay operation (0.2–0.5 sec).

🎯 Advanced Interview Questions

Q19. Explain infeed and outfeed effect in distance protection.

Infeed increases apparent impedance (underreach), outfeed reduces it (overreach).

Q20. What is the difference between impedance and mho relay?

Impedance relay is non-directional. Mho relay is inherently directional.

Protection Interview Preparation = Clear Concepts + Practical Understanding

Shaktimatha 369 Technologies

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Monday, 23 February 2026