Parallel Operation of Alternator MCQ

1. The alternators are operated is parallel because it

(a) increases the reliability of supply.
(b) makes the repairing economical and convenient.
(c) is easy to install an additional unit as and when required.
(d) all of the above.

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2. Which of the following conditions are to be satisfied for proper synchronization of alternators?

  1. Equal terminal voltage.
  2. Same frequency.
  3. Same phase sequence.
  4. Same kVA rating.
  5. Same phase displacement

Select the correct answer using the code given below:
(a) 1, 3 and 4 only.
(b) 1, 2, 4 and 5 only.
(c) 2, 3, 4 and 5 only.
(d) 1, 2, 3 and 5 only. [I.E.S. E.E.-II, 2008]

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3. Which one of the following is not a necessary condition to be satisfied for synchronizing an incoming alternator to an already operating alternator?

(a) Same voltage magnitude.
(b) Same frequency.
(c) Same prime mover speed.
(d) Same phase sequence. [I.E.S. E.E.-II 2004]

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4. Which of the following methods used for synchronising of 3-phase synchronous generators is considered best one ?

(a) Three dark lamp method.
(b) two-bright and one dark lamp method.
(c) Synchroscope.
(d) none of these.

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5. While synchronising a 3-phase alternator to the bus-bar the paralleling switch should be closed

(a) just before the pointer reaches the vertical position when moving in the fast direction.
(b) just after the pointer passes the vertical position when moving in the fast direction.
(c) when pointer indicates fast position.
(d) when pointer indicates slow position.

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6. The two-bright and one dark lamp method is used for

(a) determination of phase sequence.
(b) synchronising of 3-phase alternators.
(c) synchronising of single phase alternators.
(d) transfer of load.

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7. Desirable feature for successful parallel operation of two alternators is that both should have

(a) same resistance.
(b) same reactance.
(c) high reactance in comparison to resistance.
(d) low reactance in comparison to resistance.

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8. Two alternators are operating in parallel. For taking one of the alternators out from the system

(a) load shared by this alternator is transferred to the other by adjusting the power fed to the prime mover before opening OCB.
(b) power fed to the prime-mover is stopped.
(c) OCB is switched off.
(d) load connected to the bus-bar is reduced.

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9. Synchronising current means

(a) the total current supplied to the load by the alternators operating in parallel.
(b) the current supplied by the synchronous generator.
(c) the current circulating in the local circuit of two alternators operating in parallel which brings the alternators in synchronism once they are out of it.
(d) none of the above.

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10. If the voltage of one of the two machines operating in parallel suddenly falls

(a) both the machines will stop.
(b) the machine whose voltage has suddenly decreased, will stop.
(c) the synchronous torque will come into operation to restore synchronism.
(d) none of the above.

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11. Synchronising torque will come into operation whenever

(a) there is a difference in the magnitude of voltages.
(b) there is a phase difference in the voltages.
(c) there is a frequency difference between the two voltages.
(d) excitation of one of the alternators is changed.
(e) in all of the above cases.

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12. The electrical stiffness of a synchronous generator connected to a very large grid can be increased by

(a) increasing the excitation or power angle of the machine.
(b) reducing the excitation or the synchronous reactance of the machine.
(c) increasing the synchronous reactance of the machine.
(d) operating the generator at a much lower MW level compared to the steady state limit. [I.E.S. E.E.-II 2002]

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13. Two alternators A and B are sharing an inductive load equally. If the excitation of alternator A is increased

(a) alternator B will deliver more current and alternator A will deliver less current.
(b) alternator B will deliver less current and alternator A will deliver more current.
(c) both will continue to share load equally.
(d) both will deliver more current. [I.E.S. E.E.II-1992]

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14. Two similar synchronous generators are working in parallel to supply a common load demand with identical excitations and steam supplies to their prime movers. Now, if the steam supply to the prime mover of one of the generators is increased compared to the other, with field excitation kept unchanged, then

(a) its active power component will remain the same but the reactive power contribution will increase.
(b) its active power will decrease while the reactive power will increase.
(c) both active and reactive components of power will increase.
(d) its active power contribution will increase but reactive power contributions of both will remain unchanged. [I.E.S. E.E.-II, 1993]

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15. When two alternators are operating in parallel, if the power input to one of the alternators is cut off, the alternator will

(a) continue to run as a synchronous motor rotating in the same direction.
(b) continue to run as a synchronous motor rotating in opposite direction.
(c) stop running.
(d) get damaged due to burning of stator and rotor windings.

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16. Two alternators each having 4% speed regulation are working in parallel. Alternator 1 is rated 12 MW and alternator 2 is rated 8 MW. When the total load is 10 MW, the load shared by alternators 1 and 2 would be respectively

(a) 4 MW and 6 MW.
(b) 6 MW and 4 MW.
(c) 5 MW and 5 MW.
(d) 10 MW and zero. [U.P.S.C. I.E.S. E.E.-II, 2000]

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17. An infinite bus-bar has

(a) constant voltage.
(b) constant frequency.
(c) infinite voltage.
(d) both (a) and (b).

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18. If the excitation of an alternator connected to an infinite bus-bar is changed, keeping the power input to its prime mover unchanged, its ……….. will change.

(a) kW output
(b) power-factor
(c) kVA output
(d) kVAR output
(e) all of the above except kW output.

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19. A stationary alternator should not be connected to the live bus-bar because

(a) it is likely to operate as a synchronous motor.
(b) it will get short circuited.
(c) it will reduce bus-bar voltage.
(d) all of the above.

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20. A 3-phase synchronous generator with constant power input is supplying electrical power to an infinite bus at a lagging power factor. If its excitation is reduced

(a) both power factor and power angle will increase.
(b) both power factor and power angle will decrease.
(c) power factor will improve but power angle will decrease.
(d) power angle will increase but power factor will decrease.

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21. An alternator connected to an infinite bus, for a given excitation, will develop maximum electromagnetic power when the power angle δ and internal angle θ are related as

(a) δ = 180o – θ
(b) δ = θ
(c) δ = 180o – θ/2
(d) δ = 180o – 2θ

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22. A salient pole synchronous generator delivering power to an infinite bus through a reactive tie line reaches its steady state stability limit. What is the power angle of the generator relative to the infinite bus voltage reference?

(a) Greater than 90 degrees.
(b) Equal to 90 degrees.
(c) Less than 90 degrees.
(d) Zero. [U.PS.C. I.E.S. 2009]

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23. A large ac generator supplying power to an infinite bus, has a sudden short circuit occurring at its terminals. Assuming the prime mover input and the voltage behind the transient reactance to remain constant immediately after the fault, the acceleration of the generator rotor is

(a) inversely proportional to the moment of inertia of the machine.
(b) inversely proportional to the square of the voltage.
(c) directly proportional to the square of the short-circuit current.
(d) directly proportional to the short-circuit power. [U.P.S.C. I.E.S. 1994]

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24. Four identical alternators each rated for 20 MVA, 11 kV having a subtransient reactance of 16% are working in parallel. The short-circuit level at the bus-bars is

(a) 500 MVA
(b) 400 MVA
(c) 125 MVA
(d) 80 MVA [I.E.S. 1996]

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25. Electrical machines (ac) should have proper ………. in order to limit the operating temperature.

(a) voltage rating
(b) current rating
(c) speed
(d) kW rating

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26. During short-circuit test on a synchronous motor ………. is/are short circuited.

(a) armature terminals
(b) field terminals
(c) all stator phase windings
(d) one of the stator phase winding

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27. The rotor copper losses of a synchronous motor are met by

(a) armature input.
(b) dc source.
(c) ac supply mains.
(d) none of these.

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28. The efficiency of a properly designed synchronous machine is of the order of

(a) 60%
(b) 80%
(c) 92%
(d) 99%

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29. The negative phase sequence in a 3-phase synchronous motor exists when the motor is

(a) under-loaded.
(b) overloaded.
(c) supplied with unbalanced voltage.
(d) hot. [I.E.S. E.E.-II, 1992]

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30. In case one phase of a 3-phase synchronous motor is short-circuited the motor will

(a) not start.
(b) run at 2/3 of synchronous speed.
(c) run with excessive vibrations.
(d) take less than the rated load. [A.M.LE. Sec B. Elec. Machines Summer 1996]

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