Visvesvaraya Technological University (VTU) 2007 B.E Electrical and Electronics Engineering 6TH SEM 2006 SWITCHGEAR AND PROTECTION - Question Paper
USN
Sixth Semester B.E. Degree Examination, July 2006 Electrical and Electronics Engineerins Switchgear and Protection Time: 3 hrs.] [Max. Marks: 100
Note: 1. Answer any FIVE questions.
1 a. Derive expressions for restriking voltage and RRRV in terms of system voltageT
inductance voltage and capacitance. (08 Marks)
b. For a 132 kV system, the reactance and capacitance up to the location of the circuit breaker are 3 H and 0.015 pF respectively. Calculate the following : i) The frequency of transient oscillation,
it) The maximum value of the restriking voltage across the contacts of the circuit breaker,
iii) The maximum value of RRRV, (06 Marks)
. c. Explain the phenomenon of current chopping in a circuit breaker. (06 Marks)
2 a. With a neat sketch describe the working principle of an axial air blast t\pe circuit
breaker. . 10 Marks)
b. Describe rhe working principle of SF6 circuit breaker. What are its advantages over other typ.*s of circuit breakers? For what voltage range is it recommended'.' (10 Marks)
3 a. Explain the following with reference to circuit breakers :
i) Breaking capacity ii) Making capacity iii) Short-time capacity.(07 Marks)
b. With a neat sketch describe the working principle of a liquid fuse. (07 Marks)
c. What do you mean by discrimination? Discuss discrimination between
i) two fuses and ii) a fuse and a over current reiay. (06 Marks)
4 a. The current ratings of an over current relay is 5A. It has a PSM = 2, TSM = 0.3.
C.T. ratio = 400/5, Fault current = 4000 A. Determine the time of operation of the relay assuming normal IDMT characteristics. (04 Marks)
b. Explain time graded and current graded system for a feeder. (08 Marks)
c. With a neat sketch explain the principle of operation of a non-directional over cunent relay. (08 Marks)
5 a. Explain the principle of working and operating characteristics of a percentage biased
differential relay. ((0 Marks)
b. Explain differential scheme of bus bar protection. What can be drawbacks of this scheme and how this can be overcome? (10 Marks)
> a. Explain the working principle and characteristics of an impedance relay. (06 Marks)
b. Describe the three-zone distance protection of transmission lines. (08 Marks)
c. Explain the principle of DC arc extinction in circuit breaker. (06 Marks)
f a. List the various abnormal operating conditions against which a modem large turbo-altemator has to be protected. (06 Marks)
b. Draw the connection diagram showing differential protection, negative sequence protection, over-current protection and earth fault protection of a Y-connected generator. (08 Marks)
c. List the various abnormal operating conditions against which a large induction motor has to be protected. (06 Marks)
; a. Explain the working of Buchholz relay. (08 Marks)
b. Describe the harmonic restraint relay used to protect transformers. (08 Marks)
c. A 3-phase transformer rated for 33 KV/6.6 KV is connected star/delta and the protecting currcnt transformer on the low voltage side have a ratio of 400/5. Determine the ratio of the CT on the HV side. 1 (04 Marks)
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