JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III POWER SYSTEM PROTECTION Previous Question Paper (August - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Main
What are the different types of faults, causes of faults and their effects in a power system?
Answer
Faults in a power system are broadly classified into symmetrical faults, where all three phases are affected equally, and unsymmetrical faults, which involve one or two phases or ground. Symmetrical faults include three-phase-to-ground (LLL-G) and three-phase (LLL) faults. Unsymmetrical faults include single-line-to-ground (LG), line-to-line (LL), and double-line-to-ground (LL-G) faults.
The primary causes of these faults include insulation failure due to aging or moisture, lightning strokes, switching surges, falling trees on overhead lines, bird faults, and human errors during maintenance.
The effects of these faults are significant: they cause abnormal current flow leading to overheating and potential damage to equipment, result in voltage dips affecting stability, may cause loss of synchronism in generators, and can lead to complete system collapse if not cleared promptly by protection schemes.
How would you explain primary and back up protection and also explain various methods of back-up protection?
Answer
Primary protection is the first line of defense, designed to operate instantaneously to clear a fault within the protected zone. It is essential for minimizing damage to equipment and maintaining system stability. Back-up protection acts as a secondary measure that operates only if the primary protection fails to clear the fault within a predetermined time delay.
Various methods of back-up protection include:
- Relay back-up: Providing a second relay at the same location to take over if the primary relay fails.
- Breaker back-up: Utilizing a separate circuit breaker to isolate the fault if the primary breaker fails to trip.
- Remote back-up: Protection located at a different station that acts as a back-up for the primary protection of an adjacent line section.
- Centralized back-up: A dedicated back-up system covering multiple zones, often used in large substations to provide redundancy for primary schemes.
How would you explain the essential qualities of protective relaying?
Answer
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How would you describe the microprocessor based over current relay and also write advantages of microprocessor based protective relay?
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IDMT relays are widely used for over current protection? Justify the statement with valid reasons.
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Distinguish between an earth fault relay and an over current relay. Discuss various methods to energies an earth fault relay.
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How would you discuss the effect of power surges on the performance of different types of distance relays?
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What are blinders? In what circumstances are they used in conjunction with a MHO relay?
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Explain the term ‘pilot’ with reference to power line protection. What are the different types of pilots which are presently employed? Discuss their fields of application.
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What is carrier aided distance protection? What are its different types?
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An 11 KV, 100 MVA alternator is grounded through a resistance of 5 Ω. The CTs have a ratio 1000/5. The relay is set to operate when there is an out of balance current of 1 A. What percentage of the generator winding will be protected by the percentage differential scheme of protection?
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What type of protective schemes is employed for the protection of a large power transformer against short circuits? With neat sketch, discuss its working principle.
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Illustrate how an amplitude comparator can be converted to a phase comparator and vice Versa.
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Describe the realization of a directional over current relay using a microprocessor.
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Illustrate the working principle of HRC fuse with a neat sketch.
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With a neat diagram, discuss the constructional and principle operation of SF6 circuit breaker. What are its relative merits and demerits?
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