SGP.pdf

Code No: V3210/R07

III B.Tech II Semester Regular & Supplementary Examinations, April/May - 2012

SWITCH GEAR AND PROTECION

(Electrical and Electronics Engineering)

Time: 3 Hours Max Marks: 80

Answer any FIVE Questions

All Questions carry equal marks

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1. a) What is meant by a circuit breaker? Discuss the phenomenon of arc formation in a

circuit breaker.

b) In a short circuit test on 220 kV, 3-φ system the breaker gave the results as: p.f of the

fault=0.5, recovery voltage = 0.9 times the full line voltage. The breaking current is

symmetrical and restriking transient has a natural frequency of 10 kHz. Determine the rate

of rise of restriking voltage for (i) ground fault and (ii) ungrounded fault.

2. What is a minimum oil circuit breaker? What are its main advantages and disadvantages?

Draw a diagram.

3. a) Explain with a neat sketch the operation of an induction type over current relay. What

are the functions of current and time multiplies settings associated with such a relay.

b) What are the merits and demerits of static relays over electromagnetic relays?

4. a) Enumerate main features of a good protection system? Explain the working of merz-

price protection scheme for a 3-phase alternator. Draw a neat sketch in support of your

answer.

b) A star connected, 3-phase, 10 MVA, 6.6 kV alternator has a per phase reactance of

10%. It is protected by merz-price circulating current principle which is set to operate for

fault currents not less than 175 A .Calculate the value of earthing resistance to be provided

in order to ensure that only 10% of the alternator winding remains unprotected.

5. a) Discuss the faults in transformer and describe with sketch merz-price scheme of

transformer protection.

b) A 3-phase, 66/11 kV star-delta connected transformer is protected by merz-price

system. The CT’s on low voltage side have a ration of 420/5 A. Find the ratio of CT’s on

the high voltage side.

6. a) Explain the various schemes of protection of feeders.

b) What are the various schemes of bus bar protection? Discuss bus protection by

differential relay.

7. a) Explain the reasons leading to the general practice of earthing the neutral points of a

power system and discuss the relative merits and demerits of earthing its (i) solidly and (ii)

through a resistance.

b) Calculate the reactance of a coil suitable for a 33kV, 3-φ transmission system of which

the capacitance to earth of each conduction is 4.5 µF.

8. a) What is lightning? Describe the mechanism of lightning discharge.

b) Explain the basic impulse insulation level and its significance.

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1. a) Explain various duties to be performed by a circuit breaker? Describe various methods

of arc extinction.

b) In a system of 132kV, line to ground capacitance is 0.01 µF and the inductance is 5H.

Determine the voltage appearing across the poles of a circuit breaker if a magnetizing

current of 5 A (instantaneous value) is interrupted. Also determine the value of resistance

to be used across the contacts to eliminate restriking voltage.

2. Describe the construction, principle of operation and applications of air blast circuit

breaker with neat sketch.

3. a) What is meant by percentage bias? How is this achieved in practice in a differential

relay? Under what circumstances is a percentage differential relay preferred over a

differential one.

b) State the applications of static relays.

4. a) Explain the principle of operation of differential scheme. How do you apply it in the

protection of alternator?

b) A star connected, 3-phase, 10 MVA, 6.6kV alternator is protected by circulating current

protection, the star point being earthed in a resistance ‘r’. Calculate the value of earthing

resistance, if 85% of the stator winding is protected against earth fault. Assume an earth

fault setting of 20% Neglect the impedance of the alternating winding.

5. a) Explain the principle of merz-price system of protection used for power transformers.

What are the limitations of this scheme and how are they overcome

b) A 3-Phase, 11/33 kV, delta-star connected power transformer is protected by

differential protection. The CT’s on the LV side have a current ratio of 500/5. What must

be the ratio of CT’s on the HV side and how should they be connected.

6. a) Explain how the selection of current and time settings done in a time current graded

system.

b) What is meant by 3-zone protection? Give such scheme of protection for (i) short length

line, (ii) medium length line and (iii) long lines.

7. a) What is the necessity of neutral earthing? Explain.

b) A220 kV, 3-φ , 50 Hz, 250Km transmission line has a capacitance to earth of 0.04

µF/phase. Find the inductance and kVA rating of the Peterson coil used for earthing the

above system.

8. a) Explain the phenomenon of lightning stroke.

b) Describe the construction and working of a thyrite lightning arrester.

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1. Discuss different methods of interrupting the arc current in circuit breakers. Explain two

main theories of current zero interruption.

2. Describe the construction, principle of operation and application of SF6 circuit breaker.

Explain clearly the current chopping process in SF6 Circuit breaker.

3. a) How does a distance relay derive its name from its function? Draw a neat sketch of a

time distance relay and explain its working.

b) The time current (PSM) characteristics of an over current relay for TMS of 1 is given in

the below table

PSM 2 3 5 7 10 13 15 18 20

Operating

times(sec)

10 6.8 4.4 3.4 2.8 2.5 2.4 2.3 2.2

If the current plug setting is adjusted to 50% and the times multiples is adjusted to 0.75,

calculate the time of operation of the relay when the fault current is 3000A and the relay is

connected to a CT ratio 400/5

4. a) Explain with a neat diagram the application of merz-price circulating current principle

for the protection of alternator.

b) A star connected 3-Phase, 120MVA, 6.6kV alternator has a per phase reactance of

12F%.it is protected by merz-price circulating current principle which is set to operate for

fault current not less than 180A.calculate the value of earthing resistance to be provided in

order to ensure that only 12% of the alternator winding remains unprotected.

5. a) Describe the construction and working of Buchholz relay with neat sketch.

b) A 120MVA, delta/star connected, 11/220kV transformer is to be protected by

percentage differential scheme. CT’s used are of 5000/5 and 400/1 respectively. Draw the

sketch of complete scheme

6. a) Describe the differential pilot wire method of protection for feeders.

b) What are the requirements of protection of lines? What is the importance of buy bar

protection?

7. a) Explain the functions of Peterson coil and how it helps in reducing arcing grounds.

b) Discuss and compare various methods of neutral grounding.

8. a) How do we use lightning arresters on transmission lines? How many types of lightning

arresters are generally used? Name the best type.

b) Explain the construction of volt-time characteristics

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1. a) Distinguish clearly between the recovery voltage and restriking voltage and also

explain the significance of RRRV in the operation of a circuit breaker.

b) Explain how arc is initiated and sustained in a circuit breakers when the circuit breaker

contacts break.

2. Describe the construction, principle of operation and applications of vacuum circuit

breaker with neat sketch.

3. a) What is an impedance relay? Explain its principle of operation. Show its characteristics

on R-X diagram. What is the advantage of this relay for transmission line protection?

b) Discuss the time-current characteristics of an induction type over current relay.

4. a) What are the abnormal conditions in a large alternator against which protection is

necessary? Discuss them briefly.

b) A 11kV,3-Phase turbo-alternator has a maximum rating of 120MW at 0.8 P.F and its

reactance is 0.12 p.u. it is equipped with differential current protection scheme. It is set to

operate at fault current not less 600A. Determine the magnitude of the neutral earthing

resistance that leaves the 12% of the winding unprotected.

5. a) What are the various protections generally recommended for power transformers? In

what respect core balance leakage protection is inferior to combined leakage and over load

protection for power transformer.

b) A 3-φ , 220/11 kV star-delta transformer is protected by merz-price system. The CT’s

on the low voltage side have a ratio of 500/5.Find the ratio of CT’s on high voltage side.

6. a) What are the requirements of protection of lines?

b) Explain the following systems of bus-bar protection

(i) Differential protection and (ii) Fault bus protection.

7. a) What is earthing of neutral? What is it done? Briefly discuss different types of earthing

the neutral.

b) Explain the meaning of the term “Resonant grounding”. How is this achieved by the use

of Peterson coil? What are its merits?

8. a) Discuss the various internal and external causes of over voltages in a power system.

b) What is insulation coordination needed in a large power system? What is meant by

basic impulse level of equipment?

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