Question Set(2017) Switch Gear & protection(5th SEm)

1. What is fault in power system? Classify the fault. What are the bad effects of fault?

2. Define with example: Symmetrical fault and unsymmetrical fault.

3. What is reactor? Classify the reactor. Describe briefly any one from them.

4. What do you understand by percentage of reactance? Do the related problems

regarding short circuit and base kVA calculation.

5. What is circuit breaker? Classify the circuit breaker?

6. Define the following terms: Breaking capacity, making capacity, Arc voltage,

Restriking voltage, Recovery voltage, R.R.R.V,

7. Explain the phenomenon of arc in CB

8. Explain the different method of arc excitation.

9. Explain the construction and operating principle with proper diagram:

(i) MOCB

(ii) ABCB

(iii) SF6

(iv) Vacuum circuit Breaker

(v) state their advantage and disadvantage

10. Write short notes on:

(i) Operating mechanism of CB

(ii) Auto reclosing

(iii) MCB

(iv) ELCB or RCCB

11. What is fuse? What are the desirable characteristics of fuse? Name the materials

used in fuse element.

12. Define the following terms:

(i) Current rating of fuse element

(ii) Fusing current

(iii) Fusing factor

(iv) Prospective current

(v) Cut-off current

(vi) Pre-arcing time

(vii) Arcing time

(viii) Total operating time

13. Explain the construction and operating principle of HRC fuse with suitable diagram.

Also state the application.

14. State the advantage and disadvantage of HRC fuse.

15. What are the considerations in selecting fuse for (i) Motor protection (ii)

Transformer protection

16. Distinguish between the function of (i) Isolator (ii) Circuit Breaker (iii) Earthing switch

17. State the sequence of operation of circuit breaker, isolator and earthing switch

18. What are the Difference between fuse and MCB?

19. State the causes of over voltage in power system. How transmission line is protected

from over voltage?

20. What is protective ratio and protective angle? How the tower footing resistance can

be reduced? 21. What is surge absorber and surge diverter?

22. write short notes on:

(i) Rod gap (ii) Horn gap (iii) Metal oxide arrestor(MOA) (iv) Ferranti surge absorber

23. What are the various methods used to protect transmission line against lightning?

24. Describe the methods of protection of substation against damage due to lightning

stroke.

25. Describe primary and back-up protection.

26. Explain briefly the purpose of zone of protection.

27. What are the main characteristics of protective relays?

28. Describe briefly on following relays:

i) Inverse time relays

ii) Definite time lag relay

iii) Instantaneous relay

iv) Differential relay.

29. Define the following terms:

(i) Pick-up current

(ii) Current setting

(iii) P.S.M.

(iv) Time setting multiplier

30. Describe the construction and operating principle: (i) Induction type over current relay (non directional)

(ii) Induction type directional over current relay

(iii) distance relay or impedance relay

(iv) Transley system relay

31. Write short notes on:

(i) CVT

(ii) CCVT

(iii) Electro-magnetic relay

(iv) Thermal relay

(v) Static relay

32. State the principle of CT & PT in a protection system. Draw a neat circuit diagram

incorporating CT & PT in a protection system.

33. A 10 Amp 3 sec. o/c relay has current setting of 150% and TSM of 0.5, the relay is

connected in the circuit through CT having ratio of 500/5. Calculate the time of

operation of relay if the circuit carries a fault current of 6000 amps. The relay has

the following characteristics:

P.S.M 3 4 5 6 7 8 9 10 Time in sec

7 5.5 5 4 3.8 3.5 3.1 3

34. write short notes on:

(i) Differential protection of bus-bar

(ii) Fault bus protection

(iii) Radial feeder protection

(iv) Parallel feeder protection

(v) Ring main system protection

35. State the field application of buchholz relay. Explain its working principle with

diagram.

36. What is meant by distance relay? With the neat sketch explain the operation of

impedance relay.

37. Describe with a neat sketch the differential protection of stator winding fault of an

alternator.

38. Write short notes on Earth fault protection of transformer.

39. A 3 phase 33/6.6 kV star/delta connected transformer is protected by merz-price

circuiting system. If the ct is on low voltage side have a ratio of 300:5, determine the

ratio of CTs on the high voltage side.

40. What is meant by zone of protection? With simple diagram show such a scheme of

protection for transmission line and describe its principle of operation.

41. Mention the various methods of protection required for the protection of

alternator.

42. What do you meant by motoring action and field suppression of an alternator?

43. Explain the operation of differential protection of an alternator and transformer.

SHORT QUESTION(Each carry 2 marks)

1. What is circuit breaker?

2. Classify the circuit breaker.

3. What is the purpose of oil in OCB?

4. What is SF6?

5. What is the disadvantage of SF6 gas?

6. What are the important of rating of circuit breaker?

7. What is breaking capacity?

8. What is making capacity?

9. Write two properties of SF6.

10. What is recovery voltage?

11. Define RRRV.

12. Define Restriking voltage in CB.

13. Write any two advantage of SF6 in CB.

14. What are the methods of arc interruption in CB?

15. How does a circuit breaker differ from a switch?

16. What is meant by rupturing capacity of a circuit breaker?

17. List the essential features of a switch gear.

18. A 3 phase oil circuit breaker is rated at 1200V, 13000MVA and 11 kV. Find the rated

symmetrical breaking current and rated making current.

19. Draw the symbol of: (i) Circuit Breaker

(ii) Isolator

(iii) Earthing Switch

(iv) Surge Arrestor

(v) C.T.

(vi) P.T.

(vii) Surge absorber

(viii) Ferranti surge absorber

20. What is meant by “burden”of a relay?

21. What is meant by back-up relay?

22. What is meant by back-up protection?

23. State the essential qualities of protection?

24. Mention the percentage of occurance of different types of faults on over head line

25. List the percentage of distribution of faults in the various power system element

26. Define protective relay

27. Define: Operationg torque, Restriking torque,

28. Define: Pick-up current, P.S.M ,T.S.M, Fault clearing time, Breaker time, Relay time

29. What is static relay?

30. What are essential qualities of a relay?

31. What are the various Time-current characteristics of an over current relay?

32. What is auto re-closure?

33. What is plcc?

34. What is meant by a polarized relay?

35. What is “time grading”of relay?

36. What is meant by directional relay?

37. What are the applications of over current relay?

38. What is differential relay?

39. What are the applications of differential relay?

40. What is a comparator?

41. What are the types of comparator?

42. What are the faults of alternator?

43. Why the loss of excitation occurs in the alternator?

44. Draw the block diagram of a static relay?

45. What are the advantage and disadvantage of static relay?

46. What is Buchholz relay? Where it is used?

47. Draw the block diagram of a basic static distance relay?

48. Why the secondary of a C.T. should not be opened?

49. What is need for power system earthing?

50. What is meant by 5p20:10VA?

Fill in the blanks by inserting appropriate words:

(i) A fuse is a .......... device.

(ii) A circuit breaker is a .......... device.

(iii) An isolator is designed to open a circuit breaker under..........

(iv) When a switch is opened, .......... is produced.

(v) Under normal operation conditions, the contacts of the circuit breaker remain ..........

(vi) Under fault conditions, .......... supplies information to the circuit breaker to open.

(vii) For greater flexibility, .......... bus-bar system is used.

(viii) The outdoor type switchgear is generally used for voltage beyond,.......... kV.

(ix) When a short circuit occurs, a .......... current flows through the system.

(x) When all the three phases are short-circuited, it gives rise to .......... currents.

(xi) The rating of circuit breaker is generally determined on the basis of .......... short

circuit current.

(xii) The most common type fault in the overhead lines is ..........

(xiii) A 1000kVA transformer with 5% reactance will have a reactance of .......... at

2000kVA base.

(xiv) Short circuit kVA is obtained by multiplying the base kVA by ..........

(xv) Reactors are used at various points in the power system to ..........

(xvi) A fuse element should have .......... melting point.

(xvii) The value of fusing factor is always .......... than unity.

(xviii) A fuse has .......... time current characteristics.

(xix) The fuse element is generally made of ...........

(xx) A fuse performs ............ functions.

(xxi) A fuse has .......... breaking capacity as compared to a circuit breaker.

(xxii) A relay performs the function of ..........

(xxiii) The relay operating coil is supplied through ..........

(xxiv) A 1vA relay is .......... sensitive than a 3vA relay.

(xxv) The minimum relay coil current at which the relay operates is called ..........

(xxvi) Induction relays .......... be used with dc quantities.

(xxvii) An over current relay having a current setting of 125% is connected to a supply

circuit through a current transformer of ratio 400/5. The pick-up value will be ..........

(xxviii) The pick-up value of a relay is 7.5A and fault current in relay coil is 30A. Its plug-

setting multiplier is ..........

(xxix) Back-up protection will be activated when ...........

(xxx) Induction relays are used with ........... quantities.

(xxxi) Back up protection is generally employed for protection against .......... faults.

(xxxii) The most commonly used system for the protection of generator is ..........

(xxxiii) Automatic protection is generally .......... provided for field failure of an alternator.

(xxxiv) The chief cause of over speed is an alternator is the ...........

(xxxv) Buchholz relay is installed between .......... and conservator.

(xxxvi) For the protection of a delta/star power transformer, the CTs on delta side must be

connected in ..........

(xxxvii) Overload protection is generally not provided for ..........

(xxxviii) Buchholz relay is a .......... relay.

(xxxix) The most important stator winding fault of an alternator is .......... fault.

(xl) Merz-Price circulating current principle is more suitable for .......... than ..........

(xli) The most severe surge on the line are produced by ..........

(xlii) Lightning produces a .......... fronted wave.

(xliii) For successful working of ground wire, the footing resistance of tower should be

..........

(xliv) A surge diverter should be located .......... to the apparatus to be protected.

(xlv) Most of the lightning strokes are due to .......... charge clouds.

(xlvi) The stroke A will always occur on ..........

(xlvii) Surge absorbers are used to ........... the steepness of wave front of the surge.

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