Protective Relays Interview Questions:

Following are the below Protective Relays Interview Questions and Important Questions which is very useful for interviews and exams

1. What is a protective relay ? Explain its function in an electrical system.

2. What are the fundamental requirements of protective relaying ? Depending upon their (i) construction and principle of operation and (ii) time of operation, how relays are classified ?

3. Explain briefly the role of protection in a power system. Describe the essential features of a protective relay with reference to reliability, selectivity, speed of operation and discrimination. With reference to protective relays define the terms : Pick-up, operating time, seal-in-relay and burden.

4. What are the shortcomings in electromagnetic relays ? Discuss briefly. Discuss also how these shortcomings are overcome in induction cup (induction cylinder) relays. Discuss briefly any five of the following terms : (i) Pick-up (ii) Threshold operation (iii) Balance point i.e., Reach of a relay. (iv) Burden of a relay (v) Blinder (vi) Flag indicator (vii) Power swing.

5. Derive an expression for torque produced in an induction relay.

6. Draw a neat sketch of an induction disc or cup type relay and explain its principle of operation.

7. Explain the working principle of electromagnetic induction type relays. What is use of shading ring ?

8. Discuss about wattinetric type induction disc relay in detail.

9. Discuss with necessary circuit diagram, the principle of operation of an induction disc relay. What are the advantages of induction cup relays over induction disc relays ?

10. For what type of protection will you recommend (i) induction disc type (ii) induction cup type construction ? What measures are taken to minimise the overrun of the disc ? Derive an expression for torque produced by an induction relay.

11. What are the different types of electromagnetic relays ? What are their field of applications ?

12. Derive the torque equation of an induction disc relay ?

13. What do you mean by symmetrical and unsymmetrical faults in a 3-Φ system ? Deduce the mathematical expression for the force developed in an induction cup relay.

14. Explain with a sketch the operation of a thermal relay and its field of application.

15. What is Buchholz relay ? Which equipment is protected by it ? For what type of fault is it employed ? With neat diagram discuss its working principle.

16. Draw a neat diagram of gas actuated relay. What is its location and use in the device ?

17. Explain the functions of Buchholz relay.

18. Describe the construction, principle of operation and application of Buchholz relay.

19. Give the various types of overcurrent relays and give their approximate characteristics.

20. Explain the functions of a protective relay in a power system. Discuss the principal features of a good protective relay. Draw and explain the principle of operation of an induction type overcurrent relay.

21. Describe the different types of overcurrent relays.

22. Write a short note on time-current characteristics of an overcurrent relay. Classify various types of overcurrent relays and give their applications.

23. Classify various types of overcurrent relays and give their applications alongwith their approximate characteristics and general time-current equations.

24. Describe with a neat sketch the operating principle of an inverse type overcurrent relay. How plug settings control the pick-up current of such a relay ? Define PSM and explain its significance.

25. Describe the difference between definite characteristic and inverse characteristic of relays.

26. Define the terms ‘Plug Setting Multiplier’ and ‘Time Multiplier Setting’ as used in the context of an IDMTL relay. How many plug settings and time multiplier settings are available with a standard IDMTL relay ?

27.  Give the constructional features, principle of working and characteristics of a directional overcurrent relay.

28. With a neat diagram explain the working of induction type directional overcurrent relay.

29. Define the following terms : (i) Plug setting multiplier, (ii) Current setting of a relay (iii) Time-multiplier setting

30. What do you mean by time multiplier setting (TMS) and plug multiplier setting of an overcurrent relay ? Explain with the help of relay characteristics.

31. Give the constructional features, principle of working and characteristics of a directional overcurrent relay.

32. What do you understand by IDMT relay. Draw the typical characteristic of an IDMT relay.

33. Describe the principle of a directional overcurrent relay. How does it help in discrimination in protection of parallel feeders, ring mains ?

34. Explain V-I and polar characteristics of a directional relay.

35. Define ‘reset’ and `pick-up’ value of an overcurrent relay. Why reset to pick-up ratio is low in dc attracted armature type relay as compared to ac relay ?

36. Explain with reasons why an unpleasant noise (relay chattering) sometimes occurs in an ac attracted armature type relay. Discuss the remedial measures adopted to minimise this unpleasant noise.

37. Discuss with necessary circuit diagram a typical directional overcurrent relay. Draw with necessary phasor diagrams the (i) 30° connection, (ii) 90° connection as applied to directional relay.

38. Explain the distance relay protection scheme.

39. Describe the principle of impedance type distance relays.

40. What is distance relay ? Draw its characteristics. How is directional features added with overcurrent relays ? Why is it required ?

41. Describe different types of distance relays used for protection of transmission line of power system. Which one is the best and why ?

42. What are the different types of distance relays ? Compare their merits and demerits. Discuss their fields of application.

43. Explain the principle of distance relays stating clearly the difference between impedance relay, reactance relay, and mho relay.

44. Explain the applications of impedance, reactance and mho relays for line protection.

45. What is an angle impedance relay ? Discuss how the characteristic is realized using phase comparison scheme.

46. Draw and explain the characteristics of a Mho relay. Why a Mho relay is preferred for protection of long lines against phase faults ?

47. Derive mathematically impedance relay characteristic. Draw the characteristic and show zones of operation and no operation.

48. Discuss the differential reactance scheme for distance protection.

49. Describe with a schematic diagram the construction of an impedance relay.

50. Explain the disadvantages of the above relay for the protection of a short and a long transmission line.

51. Explain why 1st-zone cover only 80% of the protected line section. Why is the line impedance locus set not to pass through the diameter of the mho circle ?

52. Using universal torque equation of relays draw the characteristic of a mho relay.

53. What is Universal torque equation ? Using this equation derive the following characteristics: (i) Impedance relay (ii) reactance relay (iii) mho relay. Draw the characteristics and indicate clearly the zones of operation and no-operation.

54. Explain how you provide directional feature to (i) impedance and (ii) reactance relay. Explain why directional feature provided for impedance relay cannot be used for reactance relay.

55. What do you understand by phase distance relays and ground distance relays ? State why and for what type of faults these relays are used ? How many phase relays and ground relays covering all three zones of protection of transmission lines are used at any bus ?

56. Discuss about switched distance relays and explain why these are preferred over conventional phase and ground relays for transmission line.

57. How many single-phase units are required to protect a 3-phase system by distance relays ? Why the input for distance relays are changed when phase faults and ground faults are catered respectively ? Deduce the input required for phase fault protection.

58. What is a reactance relay ? Discuss its efficiency in the protection of transmission lines. What is its main snag ? How is it overcome ?

59. Explain what is meant by distance protection. What arrangement is made to make the relay measure positive sequence impedance only for L-L and 3-phase fault.

60. What is the principle of differential relays ? What are their limitations ? Explain their characteristics.

61. Distinguish between time overcurrent relays, directional relays and differential relays.

62. Why a biased differential relay is preferred over a simple differential relay ? Make a list of applications of a biased differential relay.

63. Why biasing is needed in differential relays ? What is meant by percentage bias ? Show the complex current plane characteristic of such a relay.

64. What is meant by per cent bias ? How is this achieved in practice in a differential relay ? Under what circumstances is a percentage-differential relay preferred over the differential one ?

65. Write short notes on the following :

• Classification of relays.
• Electromagnetic attraction type relays.
• Thermal relays.
• Buchholz relay.
• Induction relays.
• Reverse power relay.
• Directional overcurrent relay.
• Reactance relay.
• Mho relay.
• Translay relay.