Visvesvaraya Technological University (VTU) 2007 B.E Electrical and Electronics Engineering 6th SEM SWITCHGEAR AND PROTECTION - Question Paper
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Sixth Semester B.E. Degree Examination, July 2007 Electrical and Electronics Engineering Switch Gear and Protection
Time: 3 hrs.]
[Max. Marks: 100
Note : Answer any FIVE full questions.
a. Discuss the Recovery rate theory and energy balance theory of Arc interruption in a Circuit Breaker. (06 Marks)
b. Derive an expression for Restriking voltage and R R R V. (06 Marks)
c. A 50 Hz generator has emf to neutral 7.5 KV (rms). The reactance of generator and the connected system is 4Q and distributed capacitance to neutral is 0.01 F with resistance negligible. Find, i) Maximum voltage across the circuit breaker contacts,
ii) Frequency of oscillation, fn iii) R R R V average upto first peak of oscillations.(08 Marks)
2 a. With a neat sketch describe the working principle of an axial air blast type circuit
breaker. (10 Marks)b. Describe the working principle of SF6 circuit breaker. What are its advantages over other types of circuit breakers? For what voltage range is it recommended?(IO Marks)
3 a. Explain the differences between fuse and a circuit breaker. (05 Marks)
b. Describe the construction and working principles of an H.R.C. fuse. (05 Marks)
c. What are the considerations in selecting a fuse for:
i) Transformer protection ii) Motor protection iii) Capacitor protection
iv) Heaters v) Lighting loads, (10 Marks)
4 a. Explain in brief the basic requirements of protective relaying. (12 Marks)
b. The current ratings of an over current relay is 5A. It has a PSM - 2, TSM = 0.3, CT
ratio = 400/5, fault current = 4000 A. Determine the time of operations of the relay assuming normal IDMT characteristics.
At TSM = 1, the operating time at various PSM : (08 Marks)|
PSM |
2 |
4 |
5 |
8 |
10 |
20 |
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Operating time in secs |
10 |
5 |
4 |
3 |
2.8 |
2.4 |
5 a. With a neat diagram, explain the working principle and operating characteristics of a percentage biased differential relay. (10 Marks)
b. Explain the differential scheme of bus bar protection. What are the draw backs of this scheme and how can you overcome them? (10 Marks)
Contd...2
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6 a. Explain the construction, working, torque equation of reactance relay. (10 Marks)
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b. Explain the full scheme of protecting a transmission line using an Impedance relay.
(10 Marks)
7 a. Explain the working of gas operated Buchholz relay used for the protection of a
transformer. State its limitations and advantages. (10 Marks)
b. A 3 phase power transformer having a line voltage ratio of 400v to 33kv is connected in Y - A. The CTs on 400v side have current ratio 1000/5. What must be the C.T. ratio on 33 kv side? (10 Marks)
8 Write short' notes on :
a. Testing of circuit breakers.
b. Principles of power system protection.
c. Carrier aided distance protection.
d. Protection against inter - turn faults. (20 Marks)
*****
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Attachment: |
| Earning: Approval pending. |
