Rr320205-power-systems-iii

Set No. 1

Code No: RR320205

III B.Tech Supplimentary Examinations, Aug/Sep 2008 POWER SYSTEMS-III (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Explain briefly the specification of Traveling waves. (b) Develop general formula for reflection and refraction co-efficient for a line with surge impedance Zc terminated 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. (a) What are the different types of circuit breakers when the arc-quenching medium is the criterion? Mention the voltage for which a particular range of circuit breaker is recommended. [4+4] (b) Discuss the recovery rate theory and energy balance theory of arc interruption in a circuit breaker. [4+4] 4. Explain resistance switching in detail with relevant diagrams and derive the expression of damped oscillation. [4+8+4] 5. (a) What is meant by directional feature of a directional over current relay? Describe the construction, principle of operation and application of a directional over current relay. [2+2+2+2] (b) What is the difference between a polarized mho and simple mho relay. What are self-polarized and cross-polarized mho relays? [4+4] 6. (a) Explain with necessary diagrams the operating principle of a Rectifier bridge phase comparator. [6+2] (b) Why are block average phase comparator preferred over block spike phase 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. Write short notes on: (a) Reactance relay (b) Mho relay (c) Directional Impedance relay.

[4+6+6] ⋆⋆⋆⋆⋆ 1 of 1

Set No. 2

Code No: RR320205

III B.Tech Supplimentary Examinations, Aug/Sep 2008 POWER SYSTEMS-III (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 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 through short lengths of cable. [8+8] 2. Describe the construction and working principle of a zinc oxide gapless arrester with a neat sketch. [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) Explain in detail the role of protective relays in a power systems. (b) Discuss in detail the causes and types and frequency of faults encountered in a power system. [8+8] 6. (a) Where are the relays having extremely inverse and very inverse characteristics used? What types of characteristics are used for protecting rectifiers, and for replacement of fuses? [4+4] (b) Explain how the mho characteristic realized using a sampling comparator? [8] 7. Discuss the percentage differential protection scheme of a transformer. [4+4+4+4] 8. Distinguish between unit protection and non unit protection. What are the various methods of protecting a transmission line by unit protection? [8+8] ⋆⋆⋆⋆⋆

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Set No. 3

Code No: RR320205

III B.Tech Supplimentary Examinations, Aug/Sep 2008 POWER SYSTEMS-III (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) A rectangular surge of 1000 kv incident on a over head transmission line of surge impedance of 300 Ω, meets a junction of two cables of surge impedance 150 Ω and 100 Ω. Calculate the outgoing voltages and currents on each cable and also the over head line. Derive the formula used. (b) Explain briefly about equivalent circuit for traveling wave studies.

[4+4+8]

2. (a) Explain √ the statement in ungrounded system “ Healthy line voltage increases by 3 times during an earth fault on the third line ”. (b) Discuss the merits of i. Solid grounding ii. Resistance grounding.

[8+4+4]

3. Explain the following with relevant diagrams. (a) Open fuse (b) Semi enclosed rewirable fuse.

[4+4+4+4]

4. Explain resistance switching in detail with relevant diagrams and derive the expression of damped oscillation. [4+8+4] 5. Determine the time of operation of the relays placed at location No. 1 and 2 assuming that fault current is 2000 amps, C.T.ratio 200/1, relay 1 set at 100% and 2 at 125% and that the relay No.1 has a time multiplier of 0.2. The time grading margin between the relays is 0.5. sec for discrimination. Assume the relay to have 2.2 seconds I.D.M.T. Characteristic, shown in figure 5. [8+8]

Figure 5 6. (a) Where are the relays having extremely inverse and very inverse characteristics used? What types of characteristics are used for protecting rectifiers, and for replacement of fuses? [4+4] (b) Explain how the mho characteristic realized using a sampling comparator? [8]

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Set No. 3

Code No: RR320205

7. (a) Explain briefly with schematic diagram, the protective gear for alternators connected to grid against i. fault between phases and ii. fault between turns in one of the phase windings.

[4+4]

(b) Three phase 33/6.6 kV transformer is connected star- delta and current transformers on the low voltage side have ratio 300: 5. What will be the ratio of C T on the high voltage side of Merz ?Prize protection is to be adopted. [8] 8. (a) Discuss the considerations which determine the need for a busbar protection. (b) Discuss any one busbar protection scheme in detail. ⋆⋆⋆⋆⋆

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[8+8]

Set No. 4

Code No: RR320205

III B.Tech Supplimentary Examinations, Aug/Sep 2008 POWER SYSTEMS-III (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. Discuss the behaviour of traveling wave when it reaches the end of (a) open circuited

[5]

(b) short circuited transmission line. Draw diagrams to show voltage and current on the line before and after the wave reaches the end. [5+3+3] 2. (a) Explain √ the statement in ungrounded system “ Healthy line voltage increases by 3 times during an earth fault on the third line ”. (b) Discuss the merits of i. Solid grounding ii. Resistance grounding.

[8+4+4]

3. Explain the terms recovery voltage, restriking voltage and RRRV. Derive the expression for the restriking voltage in terms of system capacitance and inductance. [4+4+8] 4. Explain direct testing of circuit breakers with a neat diagram.

[8+8]

5. Determine the time of operation of the relays placed at location No. 1 and 2 assuming that fault current is 2000 amps, C.T.ratio 200/1, relay 1 set at 100% and 2 at 125% and that the relay No.1 has a time multiplier of 0.2. The time grading margin between the relays is 0.5. sec for discrimination. Assume the relay to have 2.2 seconds I.D.M.T. Characteristic, shown in figure 5. [8+8]

Figure 5 6. (a) Explain the merits and demerits of static relays. (b) Discuss how an amplitude comparator can be converted into a phase comparator and vice versa. [8+8] 7. (a) Enumerate the relaying schemes which are employed for the protection of a modern alternator.

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Set No. 4

Code No: RR320205

(b) A 11 KV, 100 MVA generator is provided with differential scheme of protection. The percentage of the generator winding to be protected against phase to ground fault is 80, % The relay is set to operate when there is 15 % out of balance current. Determine the value of the resistance to be placed in the neutral to ground connection. [8+8] 8. Explain the principle of distance relaying applied to protection of radial transmission lines. Distinguish between reactance, impedance and mho relays as regards their applications to distance protection. [8+4+4] ⋆⋆⋆⋆⋆

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