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Code No: RR320205 Set No. 1

III B.Tech II Semester Regular Examinations, Apr/May 2007POWER SYSTEMS-III

(Electrical & Electronic Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll Questions carry equal marks

1. Two stations are connected together by an underground cable having a surge im-pedance of 60ohms joined to an overhead line with a surge impedance of 400ohms.If a surge having a maximum value of 100KV travels along the cable towards the

junction with the over head line, determine the value of the reflected and transmit-ted waves of voltage and current at the junction. [4+4+4+4]

2. Describe the construction and working principle of a zinc oxide gapless arresterwith a neat sketch. [8+8]

3. (a) What are the different types of circuit breakers when the arc-quenching mediumis the criterion? Mention the voltage for which a particular range of circuitbreaker is recommended. [4+4]

(b) Discuss the recovery rate theory and energy balance theory of arc interruptionin a circuit breaker. [4+4]

4. (a) Explain the Phenomenon of current chopping in a circuit breaker. Whatmeasures are taken to reduce it. [4+4]

(b) A circuit interrupts the magnetizing current of a 100MVA transformer at220kV. The magnetizing current is 5% of the full load current. Determinethe maximum voltage which may appear across the Gap of the breaker whenthe magnetizing current is interrupted at 53% of its peak value. The straycapacitance is 2500 microfarad. The inductance is 30H. [8]

5. (a) An 11 KV,100MVA generator is grounded through a resistance of 6 ohms The

C.T.s have a ratio 1000/5.The relay is set to operate when there is an outof balance current of 1 A. What percentage of the generator winding will beprotected by the percentage differential scheme of protection. [8]

(b) What is Buckhholz relay? Discuss its working principle. [4+4]

6. (a) Where are the relays having extremely inverse and very inverse characteristicsused? What types of characteristics are used for protecting rectifiers, and forreplacement of fuses? [4+4]

(b) Explain how the mho characteristic realized using a sampling comparator? [8]

7. (a) A 11KV, 100MVA alternator is grounded through a resistance of 5. The

C.T.s have a ratio 1000/5. The relay is set to operate when there is an outof balance current of 1A. What percentage of the generator winding will beprotected by the percentage differential scheme of protection?

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(b) A 11KV,100MVA alternator is provided with differential protection. The per-centage of winding to be protected against phase to ground fault is 85%. Therelay is set to operate when there is 20% of out of balance current. Determinethe value of the resistance to be placed in the neutral to ground connection.

[8+8]

8. (a) Explain how the selection of current and time settings is done in a time currentgraded system. [8]

(b) Give schemes of protection for a parallel feeder fed from

i. one end

ii. both the ends. [4+4]

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1. (a) Explain briefly the specification of Traveling waves.

(b) Develop general formula for reflection and refraction co-efficient for a line withsurge impedance Z

cterminated by an impedance Z. [8+4+4]

2. (a) Explain the co-ordination of insulation in EHV system.

(b) Explain with a neat sketch value type lightning arrester. [8+8]

3. Explain the following with relevant diagrams.

(a) Open fuse

(b) Semi enclosed rewirable fuse. [4+4+4+4]

4. Explain direct testing of circuit breakers with a neat diagram. [8+8]

5. (a) What type of protection will you recommend for a power line operating ati. 11 K.V.

ii. 33 K.V.

iii. 66 K.V. [2+2+2]

(b) Discuss in detail with relevant diagrams the shaded pole type I.D.M.T. relayused in practice. [6+4]

6. (a) Explain the merits and demerits of static relays.

(b) Discuss how an amplitude comparator can be converted into a phase compara-

tor and vice versa. [8+8]7. (a) Explain how the inclusion of resistance in the neutral earthing circuit of an

alternator affects the performance of the differential protection of the threephase stator.

(b) Describe how protection is provided in large turbo alternators against earthfault in the rotor. [8+8]

8. (a) Discuss the choice between impedance, reactance and Mho type relays. [2+2+2+2]

(b) Transformer: 5MVA, Y / , X=6%The transmission line sections AB and BC are to be protected by Mho distancerelays. The system is as shown in the figure8b. If the C.T. ratio is 300/5 andC.T. ratio is 166000/110V and a 3- short circuit fault of zero impedanceoccurs at F, find the impedance seen by the relays and determine the setting

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of the relays for high speed protection of line AB and backup protection forline BC, when the relays are located at end A. [8]

Figure 8b

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1. Discuss why

(a) A traveling wave is usually represented as a step wave in the analysis.

(b) The inductor transformers are usually connected to the over head line throughshort lengths of cable. [8+8]

2. (a) Distinguish between neutral earthing and equipment earthing .

(b) Explain the necessity of reactance grounding. [4+4+8]

3. (a) Why is current chopping not a serious problem with vacuum circuit breakers.

(b) How does SF6 breakers differ from an air blast circuit breakers?

(c) What are the possible applications of vacuum circuit breakers? [6+6+4]


5. Determine the time of operation of the relays placed at location No. 1 and 2assuming that fault current is 2000amps,C.T.ratio 200/1, relay 1 set at 100% and2 at 125%and that the relay No.1 has a time multiplier of 0.2. The time gradingmargin between the relays is 0.5. sec for discrimination. Assume the relay to have2.2 seconds I.D.M.T. Characteristic. As shown in figure 5. [8+8]

Figure 5

6. (a) Write short notes on an amplitude /pulse width converter as applied in a phasesensitive amplitude comparator. [8]

(b) Write short notes on:

i. static sine comparator

ii. integrating type amplitude comparator. [4+4]

7. (a) Explain a scheme of protection for failure of alternator excitation. [4+4]

(b) Explain with neat diagram the Merz ? Price protection for generator. [4+4]

8. (a) Discuss the choice between impedance, reactance and Mho type relays. [2+2+2+2]

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(b) Transformer: 5MVA, Y / , X=6%The transmission line sections AB and BC are to be protected by Mho distancerelays. The system is as shown in the figure8b. If the C.T. ratio is 300/5 andC.T. ratio is 166000/110V and a 3- short circuit fault of zero impedanceoccurs at F, find the impedance seen by the relays and determine the settingof the relays for high speed protection of line AB and backup protection forline BC, when the relays are located at end A. [8]

Figure 8b

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1. (a) Explain briefly the specification of Traveling waves.

(b) Develop general formula for reflection and refraction co-efficient for a line withsurge impedance Zc terminated by an impedance Z. [8+4+4]

2. (a) Explain about neutral grounding practice.

(b) What are the various methods of generator neutral grounding?

(c) What are the requirements of a ground wire for protecting power conductorsagainst direct lightning stroke? [4+8+4]

3. Explain the following with relevant diagrams.

(a) Open fuse

(b) Semi enclosed rewirable fuse. [4+4+4+4]


5. (a) What is meant by directional feature of a directional over current relay? De-scribe the construction, principle of operation and application of a directionalover current relay. [2+2+2+2]

(b) What is the difference between a polarized mho and simple mho relay. Whatare self-polarized and cross-polarized mho relays? [4+4]

6. (a) Where are the relays having extremely inverse and very inverse characteristicsused? What types of characteristics are used for protecting rectifiers, and forreplacement of fuses? [4+4]

(b) Explain how the mho characteristic realized using a sampling comparator? [8]

7. (a) A 3-phase 66/11 K.V. star delta connected transformer is protected by Merz-price protection scheme The C.T.s on the LT side have a ratio of 420/5 amps.Show that the C.T.s on the H.T side will have a ratio of 70: 5/

3

(b) Give various schemes of protection for feeders. [16]

8. (a) Explain the directional impedance relay by means of its characteristics on RX plane.

(b) Explain how arc resistance introduces an error in distance measurement.[8+8]

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