1. What are the  functions of protective relays
damages  consequent to the fault.
2. Give the consequences of short  circuit.
3. Define protected  zone.
isolated by a protection  scheme dedicated to that particular zone.
4. What are unit system and non unit system?
protected zone are isolated.Faults occurring elsewhere in the system  have no influence
activated even when the faults are external to its protected zone.
5. What is primary protection?
6. What is back up protection?
fails to activate within a definite time  delay.
7. Name the different kinds of over current relays.
                   Induction type non-directional over current relay,Induction type directional
over current relay & current differential relay.
8. Define energizing quantity.
9. Define operating time of a relay.
                               It is defined as the time period extendind from the occurrence of the
10. Define resetting time of a relay.
11. What are over and under current relays?  Overcurrent relays are those that operate when the current in a line exceeds a
differential overcurrent relay)whereas undercurrent relays are those which operate
over-voltage relay)
12. Mention any two applications of differential relay.
 Protection of generator & generator transformer unit; protection of large motors
and busbars .
13. What is biased differential bus zone reduction?
14. What is the need of relay coordination?
serious disturbance to the rest of the system.
15. Mention the short comings of Merz Price scheme of protection applied to a power
relay under normal condition.
16. What are the various faults to which a turbo alternator is likely to be subjected?
loading; stator winding fault .
17. What is an under frequency relay?
18. Define the term pilot with reference to power line protection.
usually less than 500 ohms.
19. Mention any two disadvantage of carrier current scheme for transmission line only.  The program time (ie, the time taken by the carrier to reach the other end-upto
transmission line .
20. What are the features of directional relay?
short-time thermal ratio; burden must not be excessive.
21. What are the causes of over speed and how alternators are protected from it?
Modern alternators are provided with mechanical centrifugal devices mounted on their
22. What are the main types of stator winding faults?
involving turns of the same phase winding.
23. Give the limitations of Merz Price protection.
always an inherent phase difference between the primary and the secondary quantities
24. What are the uses of Buchholz’s relay?
25. What are the types of graded used in line of radial relay feeder?
 Definite time relay and inverse-definite time relay.
26. What are the various faults that would affect an alternator?
   (a) Stator faults
  1, Phase to phase faults
  2, Phase to earth faults
  3, Inter turn faults
  1, Earth faults
  2, Fault between turns
  3, Loss of excitation due to fuel failure
 (c) 1, Over speed   2, Loss of drive
  3, Vacuum failure resulting in condenser pressure rise, resulting in            
      shattering of the turbine low pressure casing
 (d) 1, Fault on lines
  2, Fault on busbars
27. Why neutral resistor is added between neutral and earth of an alternator?
introduced between them.
28. What is the backup protection available for an alternator?
 Overcurrent and earth fault protection is the backup protections.
29. What are faults associated with an alternator?
 (a) External fault or through fault
 (b) Internal fault
  1, Short circuit in transformer winding and connection
  2, Incipient or slow developing faults
30. What are the main safety devices available with transformer?
temperature indicator .
31. What are the limitations of Buchholz relay?
 (a)  Only fault below  the oil level are detected.
 (b) Mercury switch setting should be very accurate, otherwise even for
      vibration, there can be a false operation.
 (c) The relay is of slow operating  type, which is unsatisfactory.
32. What are the problems arising in differential protection in power transformer and how
are they overcome?
 1. Difference in lengths of pilot wires on either sides of the relay. This is
                overcome by connecting adjustable resistors to pilot wires to get equipotential
     points on the pilot wires.
 2. Difference in CT ratio error difference at high values of short circuit currents
     overcome by introducing bias coil.
 3. Tap changing alters the ratio of voltage and currents between HV and LV sides
 4. Magnetizing inrush current appears wherever a transformer is energized on its
                 block it when the transformer is energized.
33. What is REF relay?
34. What is over fluxing protection in transformer?
This phenomenon is called over fluxing.
35. Why busbar protection is needed?
 (a) Fault level at busbar is high
 (b) The stability of the system is affected by the faults in the bus zone.
 (c) A fault in the bus bar causes interruption of supply to a large portion of the
                 system network.
36. What are the merits of carrier current protection?
faults .
37. What are the errors in CT?
 (a) Ratio error
 (b) Phase angle error:
           Phase angle Ө =180/π[(ImCos δ-I1Sin δ)/nIs]
38. What is field suppression?
39. What are the causes of bus zone faults?
Failure of support insulator resulting in earth fault
Flashover across support insulator during over voltage
Heavily polluted insulator causing flashover
Earthquake, mechanical damage etc.
40. What are the problems in bus zone differential protection? Large number of circuits, different current levels for different circuits for
external faults.
circuit currents. The saturation introduces ratio error.
Sectionalizing of the bus makes circuit complicated.
Setting of relays need a change with large load changes.
41. What is static relay?
is passed which operates a tripping device.
42. What is power swing?
 During switching of lines or wrong synchronization surges of real and reactive
43. What is a programmable relay?
or microcomputers in its circuit.
44. What is CPMC?
45. What are the advantages of static relay over electromagnetic relay?
o Low power consumption as low as 1mW
erosion, replacement of contacts
vessels ie, ships, aircrafts etc.
voltage, single phasing protection by incorporating respective functional
blocks. This is not possible in electromagnetic relays
o Static relay is compact
o Superior operating characteristics and accuracy
o Static relay can think , programmable operation is possible with static
o Simplified testing and servicing. Can convert even non-electrical
quantities to electrical in conjunction with transducers.
46. What is resistance switching?  It is the method of connecting a resistance in parallel with the contact space(arc).
47. What do you mean by current chopping?
 When interrupting low inductive currents such as magnetizing currents of the
48. What are the methods of capacitive switching?
• Opening of single capacitor bank
• Closing of one capacitor bank against another
49. What is an arc?
separate under heavy load or fault or short circuit condition.
50. Give the two methods of arc interruption?
splitting the arc so that the arc is fully extinguished
51. What is restriking voltage?
being extinguished.
52. What is meant by recovery voltage?
 The power frequency rms voltage appearing across the breaker contacts after the
53. What is RRRV?
closely associated with natural frequency of oscillation.
54. What is circuit breaker?
conditions and under fault conditions. 55. Write the classification of circuit breakers based on the medium used for arc
Air break circuit breaker
Oil circuit breaker
Minimum oil circuit breaker
Air blast circuit breaker
SF6 circuit breaker
Vacuum circuit breaker
56. What is the main problem of the circuit breaker?
57. What are demerits of MOCB?
Short contact life
Frequent maintenance
Possibility of explosion
Larger arcing time for small currents
Prone to restricts
58. What are the advantages of oil as arc quenching medium?
• It absorbs the arc energy to decompose the oil into gases, which have
excellent cooling properties
• It acts as an insulator and permits smaller clearance between line conductors
and earthed components
59. What are the hazards imposed by oil when it is used as an arc quenching medium?
60. What are the advantages of MOCB over a bulk oil circuit breaker?
• It requires lesser quantity of oil
• It requires smaller space
• There is a reduced risk of fire
• Maintenance problem are reduced
61. What are the disadvantages of MOCB over a bulk oil circuit breaker?
o The degree of carbonization is increased due to smaller quantity of oil
o There is difficulty of removing the gases from the contact space in time
o The dielectric strength of the oil deteriorates rapidly due to high degree of
carbonization. 62. What are the types of air blast circuit breaker?
Arial-blast type
Cross blast
63. What are the advantages of air blast circuit breaker over oil circuit breaker?
o The risk of fire is diminished
between contacts
o The arcing products are completely removed by the blast whereas oil
deteriorates with successive operations
64. What are the demerits of using oil as an arc quenching medium?
• The air has relatively inferior arc quenching properties
• The air blast circuit breakers are very sensitive to variations in the rate of rise
of restriking voltage
• Maintenance is required for the compression plant which supplies the air blast
65. What is meant by electro negativity of SF6 gas?
66. What are the characteristic of SF6 gas?
that of the transformer oil.
67. Write the classifications of test conducted on circuit breakers.
Type test
Routine test
Reliability test
Commissioning test
68. What are the indirect methods of circuit breaker testing?
o Unit test
o Synthetic test
o Substitution testing
o Compensation testing
o Capacitance testing
69. What are the advantages of synthetic testing methods?
• The breaker can be tested for desired transient recovery voltage and RRRV.
• Both test current and test voltage can be independently varied. This gives
flexibility to the test • The method is simple
• With this method a breaker capacity (MVA) of five time of that of the
capacity of the test plant can be tested.
70. How does the over voltage surge affect the power system?
transformer, generators and the other equipment connected to the line.  
71. What is pick up value?
72. Define target.
73. Define reach.
74. Define blocking.
                                           It means preventing the relay from tripping due to its own
characteristics or due to additional relays.
75. Define a over current relay.
                               Relay which operates  when the current ia a line exceeds a
predetermined value.
76. Define an under current relay?
                        Relays which operates whenever the current in a circuit drops below a
predetermined value.
77. Mention any 2 applications of differential relays.
                               Protection of generator and generator-transformer unit: protection of
large motors and bus bars
78.Mention the various tests carried out in a circuit breaker at HV labs.
Short circuit tests, Synthetic tests& direct tests.
78. Mention the advantages of field tests.
79. State the disadvantages of field tests.
                        Extra inconvenience and expenses in installation of controlling and
measuring equipment in the field.
80. Define composite testing of a circuit breaker.                     In this method the breaker is first tested for its rated breaking capacity at a
give a proper estimate of the breaker performance.
81. State the various types of earthing.
Solid earthing, resistance earthing , reactance earthing , voltage transformer
earthing and zig-zag transformer earthing.
82. What are arcing grounds?
                 The presence of inductive and capacitive currents in the isolated neutral system
leads to formation of arcs called as arcing grounds.
83. What is arc suppression coil?
84. State the significance of  single line to ground fault.
                              In single line to ground fault all the sequence networks are connected
times its sequence currents.
85. What are symmetrical components?
                 It is a mathematical tool to resolve unbalanced components into balanced
86. State the three sequence components.
                   Positive sequence components, negative sequence components and zero
sequence components.
87. Define positive sequence component.
120 degrees and having the phase sequence as original vectors.
88. Define zero sequence component.
                                They has 3 vectors having equal magnitudes and displaced from each
other by an angle zero degees.
89. State the significance of  double line fault.
                       It has no zero sequence component and the positive and negative sequence
networks are connected in parallel.
90. Define negative sequence component.
91. State the different types of faults.
                    Symmetrical faults and unsymmetrical faults and open conductor faults.
92. State the various types of unsymmetrical faults.
93. Mention the withstanding current in our human body.
94. State the different types of circuit breakers.
                    Air ,oil,vacuum circuit breakers.
95. Define  per unit value.
96. Mention the inductance value of the peterson’s coil.
2 97. Define single line diagram.
                               Representation of various power system components in a single line is
defined as single line diagram.
98. Differentiate between a fuse and a circuit breaker.
under normal operating conditions.
99. How direct tests are conducted in circuit breakers?
               Using a short circuit generator as the source.
                Using the power utility system or network as the source.
100. What is dielectric test of a circuit breaker?
                                     It consists of overvoltage  withstand test of power frequency
lightning and impulse voltages.Testa are done for both internal and external insulation
with switch  in both open and closed conditions.


  1. Nice..............Keep up the Good Work

  2. why dont u post 16 mark also

  3. very nice but please post 16mark and by mentioning unit,,tanx



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