1. Which of the following conditions is/are necessary for the parallel operation of two transformers?

(a) Percentage impedance should be the same.
(b) Polarities of both secondaries should be the same.
(c) Voltage ratings should be the same.
(d) All the conditions mentioned above in (a), (b) and (c) [U.P.S.C. I.E.S. 2011]

2. Conditions for parallel operation of transformer are:

1. The same voltage ratio.
2. The same per unit impedance.
3. The same polarity.
4. The same phase sequence.
5. The relative phase displacement.

Which are the absolutely essential conditions?
(a) 1, 2, 3, 4 and 5.
(b) 2, 3 and 4.
(c) 3, 4 and 5.
(d) 1, 2 and 3. [IES E.E.-11, 2011]

3. For the parallel operation of transformers, which of the following conditions must be satisfied?

(a) Same voltage ratios.
(b) Must be connected in proper polarities.
(c) R_e/X_e ratio should be the same.
(d) Same kVA rating. [I.E.S. 2010]

4. For successful parallel operation of two single phase transformers, the most essential condition is that their

(a) percentage impedances are equal.
(b) polarities are properly connected.
(c) turn-ratios are exactly equal.
(d) kVA ratings are equal. [IES. E.E.-II, 1994]

5. If the transformers operated in parallel are not connected with regard to polarity

(a) the transformers will not share the load in proportion to their kVA ratings.
(b) dead short circuit will take place.
(c) the transformer of lower rating will be out of operation.
(d) none of the above.

6. If all other requirements for parallel operation of transformers are fulfilled, which one of the following pairs of three-phase transformers, with the given VECTOR GROUPS, can be operated in parallel?

(a) Yd 1 and YY O.
(b) Yd 1 and DY 11.
(c) DD 6 and DY 1.
(d) Dd 0 and DY 11. [U.P.S.C. I.E.S. 1999]

7. Two transformers when operating in parallel will share the load depending upon which of the following?

(a) Magnetizing current.
(b) Leakage reactance.
(c) Per unit impedance.
(d) Efficiency. [U.P.S.C. I.E.S. 2007]

8. Two transformers, with equal rating and negligible excitation current, connected in parallel, share the load in the ratio of their kVA ratings only, if their pu impedances (based on their own kVA) are

(a) equal.
(b) in the inverse ratio of their ratings.
(c) in the direct ratio of their ratings.
(d) pure reactive. [U.P.S.C. I.E.S. 2002]

9. When two transformers of different kVA ratings are connected in parallel, they will divide the total load in proportion to their respective kVA ratings when their

(a) efficiencies are equal.
(b) equivalent impedances are equal.
(c) equivalent impedances are inversely proportional to their respective ratings.
(d) none of the above.

10. Two single phase 100 kVA transformers, each having different leakage impedances are connected in parallel. When a load of 150 kVA at 0.8 pf lagging is applied

(a) both transformers will operate at power factor more than 0.8 lagging.
(b) both transformers will operate at power factor less than 0-8 lagging. (c) one of transformers will operate at pf more than 0.8 lagging and other will operate at pf less than 0.8 lagging.
(d) both transformers will operate at identical power factors. [U.P.S.C. I.E.S. 1992]

Transformer Questions and Answers PDF

11. If per unit impedances of two transformers connected in parallel are not equal which one of the following statements is correct?

(a) The power factor of the two transformers will be different from that of the connected load.
(b) Transformers will get overloaded.
(c) Dead short circuit occurs.
(d) The transformer with higher per unit impedance will share more load. [U.P.S.C. I.E.S. 2004]

12. Neglecting losses, if the power transformed inductively is equal to power transformed conductively in an auto-transformer, then the secondary to primary ratio of transformer is

(a) 0.5
(b) 2
(c) 1.5
(d) 1.25 [U.PS.C. I.E.S. 2010]

13. It is advisable to use auto-transformer if the transformation ratio is

(a) greater than 1
(b) near to 1
(c) 0.25
(d) 0.5 [U.P.S.C. I.E.S. 2010]

14. A 100/10 V, 50 VA double winding transformer is converted to 100/110 V auto-transformer. The rating of the auto-transformer is

(a) 550 VA
(b) 500 VA
(c) 110 VA
(d) 100 VA [U.P.S.C. I.E.S 2000]

15. In case of auto-transformers, which of the following statements are correct?

1. An auto-transformer requires less copper as compared to a conventional 2-winding transformer of the same capacity.
2. An auto-transformer provides isolation between the primary and secondary windings.
3. An auto-transformer has less leakage reactance as compared to the conventional 2-winding transformer of the same capacity.

Select the correct answer using the codes given below:
(a) 1, 2 and 3.
(b) 1 and 2.
(c) 1 and 3.
(d) 2 and 3. [U.P.S.C. I.E.S. 2001]

16. The saving in copper achieved by converting ordinary two winding transformer into an auto-transformer is governed by the

(a) secondary load.
(b) size of core.
(c) voltage transformation ratio.
(d) none of the above.

17. The kVA rating of an isolation transformer increases when connected as an auto-transformer due to

(a) establishment of conductive link between the primary and secondary.
(b) rise in secondary terminal voltage.
(c) increase in secondary current.
(d) reduced copper iosses.

18. If there is a break in a 220/110 V auto-transformer in the winding which is common to both hv as well as lv sides, then the output voltage on the lv side will be

(a) 110 V
(b) 220 V
(c) zero
(d) 110√2 V

19. Constant current transformer is of type.

(a) core
(b) shell
(c) core or shell
(d) none of these

20. High leakage reactance transformers are of

(a) small volt-ampere rating.
(b) high volt-ampere rating.
(c) high voltage rating.
(d) low voltage rating.

21. Tappings of a transformer are provided

(a) at the phase end of lv side.
(b) at the phase end of hv side.
(c) at the neutral end of hv side.
(d) at the middle of hv side. [A.M.I.E. Sec B. Summer 1993]

22. A tap changer is used on a transformer for adjusting the voltage.

(a) primary
(b) secondary
(c) both primary and secondary
(d) none of the above.

23. Tappings are usually provided on the hv winding of a transformer because it

(a) is easily accessible physically.
(b) has to handle low currents.
(c) has large number of turns which allows smoother variation of voltage.
(d) all of the above.

24. For off-load tap changing the best method is to use tap changers

(a) outside the tank with no selector switch.
(b) outside the tank operated by selector sw itches.
(c) inside the tank operated by external selector switches.
(d) inside the tank with no selector switch.

25. For on-load tap changing the best method is to employ tap-changers

(a) outside the tank operated selector switches.
(b) outside the tank with no selector switch.
(c) inside the tank with no selector switch.
(d) inside the tank operated by external selector switches.

26. In a constant voltage transformer (CVT), the output voltage remains constant due to

(a) capacitor.
(b) input inductor.
(c) saturation.
(d) tapped windings. [GATE E.E. 2000]

27. The output voltage can be varied by a variac transformer over a

(a) very small range.
(b) wide range of 0 and 120 percent of input voltage.
(c) over a range of 0 and 50 percent.
(d) over a range of 50 and 100 percent.

28. In a single phase induction regulator, compensating winding is provided in order to

(a) increase efficiency.
(b) reduce the voltage drop due to reactance of the secondary winding to negligible value.
(c) give wider variation of output voltage.
(d) all of the above.

29. In a single phase induction regulator, the boost will be maximum when the angle between the axes of the stator winding and the rotor winding is equal to ………… electrical degree(s).

(a) 0
(b) 45
(c) 90
(d) 180

30. In a 3-phase induction regulator, the output line voltages are in phase with the supply line voltage in

(a) maximum buck position only.
(b) maximum boost position only.
(c) both maximum boost and maximum buck positions.
(d) all positions.

31. The rating of an induction voltage regulator for regulating the voltage on a 3-phase feeder dealing with 1,000 kVA between the limits of 4,750 V and 5,250 V is

(a) 50 kVA
(b) 100 kVA
(c) 25 kVA
(d) 50 kVA

32. A single phase induction regulator is a constant input transformer to obtain smooth variation of the output voltage by varying the

(a) ratio of turns between primary and secondary windings.
(b) frequency.
(c) flux density in the core.
(d) angle between the magnetic axes of the primary and secondary windings. [U.P.S.C. I.E.S. 1998]

33. The three-phase transformers each rated at 75 MVA, 132 kV/ 11 kV, have the following different methods of cooling:

1. Self cooled.
2. Forced oil cooled.
3. Forced air cooled.

The correct sequence in ascending order in terms of the weights of these transformers is
(a) 1, 2, 3
(b) 2, 3, 1
(c) 3, 1, 2
(d) 3, 2, 1 [U.P.S.C. I.E.S. 1998]

34. The rated secondary voltage of 220 V for a transformer means the terminal voltage of 220 V on

(a) no-load.
(b) rated full load.
(c) half-full load.
(d) none of these.

35. The rating of a transformer when the supply frequency is increased.

(a) remains unchanged
(b) decreases
(c) increases
(d) none of these

36. If a 500 kVA, 200 Hz transformer is operated at 50 Hz, its kVA rating will be

(a) 2000 kVA
(b) 125 kVA
(c) 250 KVA
(d) 1000 kVA

37. In operating a 400 Hz transformer at 50 Hz

(a) only voltage is reduced in the same proportion as the frequency.
(b) only kVA rating is reduced in the same proportion as the frequency. (c) both voltage and kVA rating are reduced in the same proportions as the frequency.
(d) none of the above. [A.M.I.E. Sec B. Elec. Machines Winter 1993]

38. Overcurrents in a transformer adversely affect

(a) insulation life.
(b) mechanical stresses.
(c) both (a) and (b).
(d) none of these.

39. The excessive temperature rise will cause maximum damage to

(a) winding insulation.
(b) core laminations.
(c) copper windings.
(d) dielectric strength of oil.

40. The most important items to be kept under constant watch during Operation of transformer are

(a) oil level.
(b) ambient temperature.
(c) winding temperature.
(d) load.
(e) terminal voltage.
(f) all of these.

41. If the drying out operation of a transformer is unduly shortened and normal voltage is applied, then there is a possibility of

(a) interturn failure.
(b) winding to earth failure.
(c) excessive heating.
(d) terminal to terminal flash-over.

42. Repeated switching-in of transformers located near the generating stations may cause

(a) excessive temperature rise.
(b) mechanical distortion of the windings and subsequent turn-to-turn insulation failure.
(c) intense local eddy currents in the core.
(d) surface flash-over of the bushings.

43. In a transformer when two or more dielectrics, having different permittivities are used in series, incorrect proportioning of their thickness may result in

(a) very poor heat dissipation.
(b) very high leakage current.
(c) very high dielectric loss.
(d) failure of insulation due to excessive electric stresses.

44. Distortion of the whole core and subsequent damage to the coil insulation due to intense local eddy currents may be caused due to failure of

(a) the insulation of the winding.
(b) the insulation between the laminations.
(c) the insulation around the core damping bolts.
(d) both (b) and (c).

45. Terminal to terminal flash-over within the oil may be caused by

(a) the lining up of the conducting particles present in the oil.
(b) the moisture present in the oil.
(c) the rapidly fluctuating loads.
(d) both (a) and (b).

46. Dust should never be allowed to accumulate on the core and windings in a dry type transformer as it is likely to

(a) reduce dissipation of heat.
(b) absorb oil and grease.
(c) short circuit the windings.
(d) corrode the metal surface.

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