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IV B.Tech I Semester Supplementary Examinations, Feb/Mar - 2015

SWITCH GEAR AND PROTECTION (Electrical and Electronics Engineering)

Time: 3 hours Max. Marks: 75

Answer any FIVE Questions

All Questions carry equal marks

*****

1 a) Briefly discuss the different methods of arc interruption in case of circuit

breakers? [8] b) In a short circuit test on 220 kV, 3-phase system with breaker gave the results as:

P.f of the fault is 0.6 and recovery voltage is 0.85 times the full line voltage. The

breaking current is symmetrical and restriking transient has a natural frequency of

10kHZ. Calculate the RRR V for

i) Grounded fault and ii) Ungrounded fault [7]

2 a) Discuss the air blast circuit breakers’ ratings and its advantages? [8]

b) Explain the types of SF6 circuit breakers with neat diagrams? [7]

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

characteristics on R-X diagram. List out its merits for transmission line

protection.

[8]

b) Explain the hinged armature type relay with neat sketch? [7]

4 a) Explain the restricted earth fault protection by differential system in the

protection of a alternator winding? [8]

b) A 11 kV, 100 MVA alternator is grounded through a resistance of 10 ohms. The

current transformers have a ratio of 1000/5. The relay is set to operate when there

is an out of balance current of 0.5 A. Find the percentage of generator winding

protected by percentage differential protection? [7]

5 a) Discuss the various faults occurred in the transformer and write the protection

scheme for each fault? [8]

b) Explain the protection device for a transformer that gives protection from internal

faults? [7]

6 a) Explain the carrier current protection scheme with neat diagram? [8]

b) Describe three zone distance relay protection with an example? [7]

7 a) What are the advantages and disadvantages of static relays over electro

mechanical relays? [8]

b) Explain the zero crossing detector and sequence detectors with neat diagrams? [7]

8 a) Explain about the valve type and zinc oxide type lightning arresters? [8]

b) Why is insulation coordination needed in a large power system?

What is meant by basic impulse level of an equipment? [7]

Code No: R41023 Set No. 1 R10

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Time : 3 hours Max. Marks: 75



*****

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

a CB.

[8]

b) Explain the concepts of recovery voltage and re striking voltages?

[7]

2 a) List out the merits and limitations of air blast circuit breaker? [8]

b) Explain the properties of SF6 gas and how it is used for circuit breakers?

[7]

3 a) Explain about the principle of operation of biased differential relay with

necessary equations?

[8]

b) Explain about MHO relay and OFF SET MHO relays with their characteristics?

[7]

4 a) Explain the inter turn protection based on zero sequence component? [6]

b) The neutral point of a 50MVA, 11KV generator is grounded through a

resistance of 5 ohms the relay is set to operate when there is an out of balance

current of 1 A, the CTs have ratio of 1100/5. Find the percentage of the winding

is protected against a ground fault and what should be the minimum value of

the grounding resistance to protect 85% of the winding?

[9]

5 a) Explain the protection devices other than differential protections are used for

the protection of large transformer?

[8]

b) A 3 phase, 11/33KV star delta connected power transformer is protected by

differential protection. The CTs on the LV side have a current ratio of 300/5.

What must be the ratio of CTs on the HV side? Draw the connection diagram?

[7]

6 a) Explain in detail about the translay relay with neat sketch? [8]

b) Describe the time graded over current protection for parallel feeders as well as

ring main system?

[7]

7 a) Explain the operation of static impedance relay with its characteristics? [8]

b) Describe the functioning of micro processor based digital relays.

[7]

8 a) Explain the resistance grounding with circuit diagram and phasor diagrams?

List out its merits and demerits.

[8]

b) Explain the concept of arcing grounds in the power system and derive the

necessary expressions.

[7]


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*****

1 a) Explain how arc is initiated and sustained in a circuit breaker when the CB

contacts break.

[8]

b) The following data relate to a 60 Hz generator; emf to neutal: 7.5 km (r.m.s)

reactance of generator and connected system = 4.5 Ω distributed capacitance

to neutral = 0.03µF, resistance neglected. Find the

i) Max. voltage across the contacts of CB When it breaks a short circuit

current at zero current.

ii) frequency of the transient oscillations

iii) average rate of rise of voltage up to the first peak of the oscillations.

[7]

2 a) Explain the principle of operation of vacuum circuit breaker with neat sketch? [8]

b) List out merits and limitations of Sf6 circuit breaker?

[7]

3 a) Discuss the operating principle of an impedance relay and the draw its

characteristics on R-x plane?

[8]

b) Explain functions of induction disc relay with neat diagram?

[7]

4 a) Explain how the rotor of an alternator will be protected by field ground fault

protection?

[8]

b) Describe the stator protection of alternator by percentage differential

protection with neat sketch?

[7]

5 a) Explain how the transformer is protected from overheating problem? [8]

b) A 3ϕ, transformer having line voltage ratio 0.4/11 kV is connected in star-

delta and protective transformer on the 400 V side have a CT ratio of 500/5.

What must be the ratio of the protective transformers on the 11kV side?

[7]

6 a) Explain the protection of feeders by using three zone protection. [8]

b) Discuss how the bus bars are protected by differential protection.

[7]

7 a) Discuss briefly about various components of static relays? [8]

b) Explain the static reactance relay with the help of R-X diagram?

[7]

8 a) Draw the volt time characteristics of impulse test wave and mark the flash

over voltages?

[8]

b) Explain how the over voltages are generated in the power system? [7]


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*****

1 a) Explain the concept of resistance switching of a circuit breaker with an

equivalent circuit?

[8]

b) In a short circuit test on a CB, the following readings were obtained on single

frequency transient.

i) Time to reach the peak restriking voltage is 50µ sec.

ii) The peak restriking voltage is 100kV. Find the average RRRV and

frequency of oscillations.

[7]

2 a) Describe the principle of operation of air blast circuit breakers? [8]

b) Compare the operation of vacuum circuit breaker with SF6 circuit breaker?

[7]

3 a) Explain the operation of induction cup relay with neat diagram? [8]

b) What are the various types of over current relay? Discuss the IDMT relays

characteristics.

[7]

4 a) Explain transverse percentage differential protection for multi winding

generators?

[8]

b) A Star connected 3-ϕ, 25MVA, 11kV generator has a per phase reactance of

12%. It is protected by merz-price circulating current principle which is set to

operate for fault current not less than 170 A. Find the value of earth resistance

to be provided in order to ensure that only 12% of the generator winding

remains unprotected.

[7]

5 a) Explain the protection against magnetizing inrush current of a transformer? [8]

b) Draw and explain the connection of current transformer secondaries for

differential protection of star delta connected power transformer?

[7]

6 a) What is the significance and the advantages of carrier current protection in the

protection of lines?

[8]

b) Explain in detail about the over current protection of feeders?

[7]

7 a) Draw and explain the characteristics of static over current relays? [8]

b) Discuss the properties of static relays, its advantages?

[7]

8 a) What are the methods that are used to give protection against over voltages in

the power system?

[8]

b) Explain the differences between grounded and un grounded neutral systems. [7]


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