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2003 Course Switchgear & Protection

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Total No. of Questions : 12] [Total No. of Pages : 4 P1341 [3764]-211 B.E. (Electrical) SWITCHGEAR & PROTECTION (2003 Course) Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates : 1) Answer 3 questions from Section I and 3 questions from Section II. 2) Answers to the two sections should be written in separate books. 3) Neat diagrams must be drawn wherever necessary. 4) Figures to the right indicate full marks. 5) Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed. 6) Assume suitable data, if necessary. SECTION - I Q1) a) Give the classification of relays and explain in brief the basis for it. [8] b) Derive the torque equation for induction type relay. [8] OR Q2) a) What are the different types of faults occurring on power transmission circuit and state their percentage occurrence with representation. [8] b) What are basic requirements of protective relaying. Q3) a) Explain the phenomenon of Current Chopping . b) What are the factors affecting the restriking voltage. [8] [6] [4] P.T.O. c) For a 220 kV system the reactance and capacitance upto the location of circuit breaker is 8 and 0.025 F respectively. A resistance of 600 is connected across the contacts of circuit breaker. Determine the following : [8] i) Natural frequency of operation. ii) Damped frequency of oscillation. iii) The value of resistance which will give damped frequency of oscillation, one fourth of the natural frequency of oscillation. iv) Critical value of resistance which will give no transient oscillation. OR Q4) a) What are the problems associated with SF6 circuit breaker. [8] b) Compare between fuse and circuit breaker on the basis of function, operation, operating time, breaking capacity, replacement. [10] Q5) Explain following terms : a) Pickup & reset value, operating time of relay. [6] b) Plug setting and Time setting. [4] c) Definite characteristic & Inverse characteristic of relay. [6] OR Q6) a) An IDMT type over current relay is used to protect a feeder through 500/ 1 A.C.T. The relay has a P.S. of 125% & TMS = 0.3. Find the time of operation of the said relay if a fault current of 5000 Amp. Flows through [7] the feeder. Make use of following data for characteristic of relay. PSM 2 3 5 10 15 Time for Unity TMS (100% current = 1 A) 10 6 4.5 3 2.5 b) Explain the working principle and operation of Buchholz Relay, with neat diagram. [9] [3764]-211 -2- SECTION - II Q7) a) Explain clearly the Merz -Price protection scheme for protection of power transformer connected in star - delta fashion. [8] b) A 6.6 kV, 5 MVA star connected alternator has a reactance of 1.5 ohm per phase and negligible resistance. Merz -Price protection scheme is used which operates when out of balance current exceeds 25% of full load current. The neutral of alternator is grounded through a resistance of 8 ohms. Determine the proportion of winding which remains unprotected against earth fault. Also show that the effects of alternator reactance can be ignored. [8] OR Q8) a) For a 10 MVA, 132 kV/6.6 kV power transformer with delta star connections, obtain the number of turns each CT should have, for the differential protection scheme to circulate a current of 5 A in the pilot wires. [8] b) Explain the phenomenon Loss of excitation related to an alternator & explain the protection provided against it. [8] Q9) a) Explain the carrier current scheme of protection, with the help of block diagram. Also discuss how the phase comparison scheme can be used for protecting feeder from i) One end ii) Both the ends. [9] b) Derive the torque equation of impedance relay and explain its operating characteristics on R-X diagram. [7] OR Q10) a) What do you mean by power swings and arc resistance? Explain the effect of power swings and arc resistance on the performance of the distance relay. [9] b) Explain the directional comparison method of carrier current protection. Why is it used in an impedance or other type of non-unit protection?[7] Q11) a) What are the advantages of static relays over conventional electromechanical relays? Draw a block diagram of a static relay and explain its working. [8] [3764]-211 -3- b ) What are the different sources of transient over voltages in static relays and how these can be protected? [5] c) Explain the reason why half cycle data window is preferred over the full cycle data window in numerical protective relaying. [5] OR Q12) Write short notes on any three : [18] a) Microprocessor based over current relay. b) Rationalised Haar Transform (RHT). c) Removal of DC off set component from current signal. d) Walsh-Hadamard Transform Technique. e) Time delay ckt in static relays. rrrr [3764]-211 -4-

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