GTU BE/B.Tech 2018 Winter Question Papers || Gujarat Technological University
Subject Name: Switch Gear and Protection
Time: 10:30 AM TO 01:00 PM Total Marks: 70
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER-VII (NEW) EXAMINATION - WINTER 2018
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Subject Code: 2170908 Date: 19/11/2018Subject Name: Switch Gear and Protection
Time: 10:30 AM TO 01:00 PM Total Marks: 70
Instructions:
- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
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| MARKS | ||
| Q.1 | ||
| (a) | Explain the following requirements of protection system. (1) Selectivity (ii) Reliability (iii) Security | 03 |
| (b) | Discuss time and current grading schemes of over current protection. | 04 |
| (c) | Discuss the working principle of an induction relay and also derive an expression for the torque produced by it. | 07 |
| Q.2 | ||
| (a) | Clearly distinguish the terms “Overload” and “Overcurrent”. | 03 |
| (b) | Explain, what are the basic requirements of a protective System. | 04 |
| (c) | Compare following: (1) Measuring CT and Protection CT (ii) Electromagnetic type PT and Capacitor type PT | 07 |
| OR | ||
| (c) | Define the terms ‘Plug Setting Multiplier’ and ‘Time Multiplier Setting” used in context of an IDMT relay. An IDMT type overcurrent relay is used to protect a feeder through a 500/1A CT. The relay has a plug setting of 125% and TMS=0.3. Find the time of operation of the said relay if a fault current of 5000A flows through the feeder. Make use of the following characteristics: | 07 |
| PSM | 2 | 3 | 5 | 8 | 10 | 15 |
| Time for TMS=1 | 10 | 6 | 4.5 | 3.2 | 3 | 2.5 |
| Q.3 | ||
| (a) | What problems; if any, do you anticipate in applying conventional differential protection to a transmission line? | 03 |
| (b) | What are the limitations found in the simple differential protection of a transformer? How are they overcome? | 04 |
| (c) | With the help of schematic diagram, discuss various components of digital/numerical relays. | 07 |
| OR | ||
| Q.3 | ||
| (a) | What are the conditions which lead to incipient faults in transformer? What type of protection is required? | 03 |
| (b) | What are advantages of numerical relays? | 04 |
| (c) | Draw a detailed protection scheme for biased differential Protection of a 11/66KV, 45 MVA, DY power transformer. Suggest suitable CT ratios. What is minimum recommended percentage bias? | 07 |
| Q.4 | ||
| (a) | Compare minimum oil CB and bulk oil CB. | 03 |
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| (c) | Explain power line carrier current protection by direction comparison showing schematic connection dia. of equipments used in it. | 07 |
| OR | ||
| Q.4 | ||
| (a) | What are the advantages of power line carrier? | 03 |
| (b) | Explain Following with reference to Circuit Breaker: (a) Breaking Current (b) Making Current | 04 |
| (c) | Write short note on SF6 circuit breaker. | 07 |
| Q.5 | ||
| (a) | Explain the effect of natural frequency on TRV. | 03 |
| (b) | Explain the physics of arc phenomena. On what factors does the arc phenomenon depends? | 04 |
| (c) | What is meant by loss of excitation in a generator? What protection is used against it? | 07 |
| OR | ||
| Q.5 | ||
| (a) | Explain reactance relay with characteristics. | 03 |
| (b) | An air- blast CB designed to interrupt a transformer magnetizing current of 15A (rms) chops the current at an instantaneous value of 12A. The value of L and C in the circuit are 8H and 0.009pF. Find the voltage that appears across the circuit breaker. Assume that the inductive energy is transformed to capacitance. | 04 |
| (c) | List and explain various protections used for an induction motor. | 07 |
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