✅ UNIT I – PROTECTION SCHEMES
(Apply, Analyze, Evaluate, Create)
1. Apply
Q1: Apply the concept of zones of protection in a power system to identify which section a relay should protect.
A1: A relay should be installed such that it protects only its designated zone (like transformer zone or feeder zone), ensuring selective tripping.
2. Analyze
Q2: Analyze how a fault in a three-phase system can affect equipment operation.
A2: Faults cause overcurrent, overheating, and insulation breakdown, possibly damaging transformers, motors, and transmission lines.
3. Evaluate
Q3: Evaluate why protective schemes are essential in modern power systems.
A3: They prevent equipment damage, maintain system stability, reduce outage time, and ensure safety for personnel.
4. Create
Q4: Design a simple protection scheme using CTs and relays for a distribution transformer.
A4: Place CTs on the transformer secondary and connect them to an overcurrent relay that operates a circuit breaker.
5. Apply
Q5: Apply the concept of direct lightning stroke protection for a substation.
A5: Use overhead ground wires or lightning arresters to intercept lightning and safely divert current to earth.
6. Analyze
Q6: Analyze the difference between direct stroke and indirect stroke lightning.
A6: Direct stroke hits the system directly; indirect stroke induces surges through nearby strikes.
7. Evaluate
Q7: Evaluate the effectiveness of HRC fuses in high-voltage systems.
A7: HRC fuses provide accurate and quick disconnection with high breaking capacity, but are non-resettable.
8. Create
Q8: Propose a method to protect an overhead transmission line from voltage surges.
A8: Use a combination of lightning arresters and overhead ground wires to shield the line from direct and induced surges.
9. Apply
Q9: Apply PTs in a metering and protection application.
A9: PT steps down high voltage to measurable levels for metering and relay operations.
10. Evaluate
Q10: Justify the use of earthing screens in substations for lightning protection.
A10: They reduce the voltage gradient by equalizing potential and provide a path for discharge to earth, enhancing safety.
✅ UNIT II – RELAYS AND CIRCUIT BREAKERS
(Apply, Analyze, Evaluate, Create)
11. Apply
Q11: Apply the concept of inverse time relay in a motor protection system.
A11: It delays tripping under small overloads and trips faster for larger overloads, allowing motors to handle temporary surges.
12. Analyze
Q12: Analyze the operating principle of an induction type overcurrent relay.
A12: It uses the interaction of magnetic fields produced by current coils to generate torque that moves the relay contacts.
13. Evaluate
Q13: Evaluate the suitability of SF6 circuit breakers in EHV systems.
A13: SF6 breakers have excellent arc-quenching and insulation properties, making them ideal for high-voltage and fast-acting applications.
14. Create
Q14: Design a backup protection system for a transmission line.
A14: Install a time-delayed overcurrent relay at the next upstream bus to operate if the primary protection fails.
15. Apply
Q15: Apply the concept of restriking voltage in the interruption of fault current.
A15: After arc extinction, the system attempts to re-establish voltage across breaker contacts; if restriking voltage is high, arc may reignite.
16. Analyze
Q16: Analyze how recovery voltage affects circuit breaker performance.
A16: A high recovery voltage can lead to insulation failure or restriking if the breaker contacts can't withstand it.
17. Evaluate
Q17: Compare vacuum and oil circuit breakers in terms of maintenance and efficiency.
A17: Vacuum breakers require less maintenance, have quicker operation, and are eco-friendly compared to oil breakers, which need oil handling.
18. Create
Q18: Propose a relay combination for a substation with transformers and feeders.
A18: Use differential relays for transformers and directional overcurrent relays for feeders to ensure selective and accurate protection.
19. Apply
Q19: Use a static relay in a protection scheme and describe its advantage.
A19: Static relays, being fast and reliable, can be used in overcurrent or distance protection schemes with improved accuracy and response time.
20. Evaluate
Q20: Justify the need for backup protection even when primary relays are present.
A20: Primary relays may fail due to component malfunction or incorrect settings, so backup relays ensure continuity of protection.
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