# Transformers Questions and Answers – Parallel Operation of Transformers

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This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Parallel Operation of Transformers”.

1. For two transformers connected in parallel, not having unequal percentage impedances, which statement is correct?
a) Short-circuiting of the secondaries
b) Power factor of one of the transformers is leading while that of the other lagging
c) Transformers having higher copper losses will have negligible core losses
d) Loading of the transformers not in proportion to their kVA ratings

Explanation: In parallel operation of a transformer, loading gets divided between the whole set in proportion of their impedances. Thus, if impedances are not same then, loading of transformers will not be in the ratio of their kVA ratings.

2. For the parallel operation of two single phase transformers it is necessary that they should have ________
a) same efficiency
b) same polarity
c) same kVA rating
d) same number of turns on the secondary side

Explanation: Polarity of the two transformers must be same if the are supposed to be operated in parallel mode of operation. It is okay if they have unequal efficiencies, or unequal kVA ratings, as parallel connection is still possible.

3. Transformers operating in parallel mode of operation will share the load depending upon their ___________
a) leakage reactance
b) per unit impedance
c) efficiencies
d) ratings

Explanation: In parallel operation of a transformer, loading of the transformers gets divided between the whole set in proportion of their per unit impedances. Thus, if per unit impedances are not same then, loading will not be in the ratio of their kVA ratings.
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4. What will happen if the transformers working in parallel are not connected with regard to polarity?
a) The power factor of the two trans-formers will be different from the power factor of common load
b) Incorrect polarity will result in dead short circuit
c) The transformers will not share load in proportion to their kVA ratings
d) Cannot be determined

Explanation: As far polarity is concerned, the transformers are connected in the same sequence when they are operated in parallel combination. An incorrect polarity connection of these transformers will result in dead short circuit.

5. If the percentage impedances of the two transformers working in parallel are different, then ______________________
a) transformers will be overheated
b) power factors of both the transformers will be same
c) parallel operation will be not possible
d) parallel operation will still be possible, but the power factors at which the two transformers operate will be different from the power factor of the common load

Explanation: If the percentage impedances of the two transformers which are working in the parallel are different, then parallel operation is still possible, but load sharing will not be in the ratio of their kVA loads.

6. A delta/star transformer is connected in parallel to a star/delta transformer. The turn ratio former is x times latter, the x is _________
a) 3
b) 1/3
c) √3
d) 1/√3

Explanation: Turn ratio of star1/ Turn ratio of delta 1= Vph/Vph/√3= √3
Ture ratio of star 2 / Turn ratio of delta 2= Vph/√3/Vph= 1/√3
Thus, turn ratio of star1/delta1 = 3 * turn ratio of star 2/ delta 2.

7. While connecting two transformers in parallel voltage around the local loop _________
a) positive
b) negative
c) equals zero
d) insufficient information

Explanation: The transformers must be connected properly as far as their polarities are concerned so that the net voltage around the local loop is zero. A wrong polarity connection results in a dead short circuit.

8. For three phase power transformers relative phase displacement should be ____________
a) 00
b) 300
c) 900
d) 1800

Explanation: Three-phase transformers must have zero relative phase displacement on the secondary sides and must be connected in a proper phase sequence. Only the transformers of the same phase group can be paralleled.

9. Y/Y and Y/D transformers can be paralleled.
a) True
b) False

Explanation: For example, Y/Y and Y/D transformers cannot be paralleled as their secondary voltages will have a phase difference of 30°. Transformers with +30° and –30° phase shift can, however, be paralleled by reversing the phase-sequence of one of them.

10. Why transformers connected in parallel should have same voltage ratio?
a) To avoid full load circulating current
b) To avoid no-load circulating current
c) To avoid other losses
d) To avoid all type of currents

Explanation: The transformers must have the same voltage-ratio to avoid no-load circulating current when transformers are in parallel on both primary and secondary sides. Since the leakage impedance is low, even a small voltage difference can give rise to considerable no-load circulating current and extra I2R loss.

11. Why transformers are paralleled?
a) Economical than replacing by one single large unit
b) Less maintaining cost
d) Economical factors, more load capacity, less maintenance

Explanation: When the load outgrows the capacity of an existing transformer, it may be economical to install another one in parallel with it rather than replacing it with a single larger unit. Also, sometimes in a new installation, two units in parallel, though more expensive, may be preferred over a single unit for reasons of reliability—half the load can be supplied with one unit out. Further, the cost of maintaining a spare is less with two units in parallel.

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