Electrical Machines Questions and Answers – Parallel Operation of Single Phase Transformers

This set of Electrical Machines Problems focuses on “Parallel Operation of Single Phase Transformers”.

1. A 2000/1000/500 three winding transformer is to be used as auto transformer with supply of 3000 V. Two loads of 1050 kVA at 3500V, and other one at 180 kVA at 1000V. The circuit representing the mentioned application is?
a)
b)
c)
d)
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2. Transformer operating in parallel will share a common load in the best possible manner if ____________
a) leakage impedances are proportional to their kVA rating
b) pu leakage impedances are equal
c) leakage impedances are equal
d) any of the mentioned
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3. A 2000/1000/500 three winding transformer is to be used as auto transformer with supply of 3000 V. Two loads of 1050 kVA at 3500V and other one at 180 kVA at 1000V. The total kVA supplied will be?
a) 1230 kVA
b) 1440 kVA
c) 1680 kVA
d) 1150 kVA
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4. Which if the conditions is must be fulfilled for satisfactory parallel operation of transformers?
a) Same voltage ratio
b) Leakage impedances should be inversely proportional to kVA of the transformer
c) Same pu impedance
d) Correct polarity
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5. Which of the following is the correct matching made for the harmonics and the associated harmonic component?

A. Positive sequence components - 5th harmonic component
B. Negative sequence components - 7th harmonic component
C. Zero sequence components - 3rd harmonic component

a) A, B, C
b) C
c) A, B
d) B, C
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6. Two single phase transformers A and B are operating in parallel having same impedance. But the x/r ratio of them are not equal. Then total kVA output of the output will be ___________
a) less than sum of kVA of A and B
b) more than sum of kVA of A and B
c) equal to sum of kVA of A and B
d) any of the mentioned
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7. Two single phase transformers A and B are operating in parallel having same impedance. But the x/r ratio of them are not equal and xa > xb. Then?
a) A has poorer pf than B
b) B has poorer pf than B
c) lesser power factor angle
d) both operate at same power factor
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8. Two single phase transformers A and B are operating in parallel having different impedance and identical x/r ratio. Then?
a) A has poorer pf than B
b) B has poorer pf than B
c) lesser power factor angle
d) both operate at same power factor
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9. Two single phase transformers A and B are operating in parallel having different impedance and identical x/r ratio. Also impedance of A is more than that of B, then A ___________
a) shares lesser kVA
b) share more kVA
c) equal kVA as of B
d) total kVA
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10. Two single phase transformers A and B are operating in parallel having different impedance and identical x/r ratio. Also impedance of A is more than that of B, then?
a) Za/Zb = Sb/Sa
b) Za/Zb = Sa/Sb
c) Za/Zb = 1
d) Za/Zb = (Sb/Sa)²
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11. Two single phase transformers A and B are operating in parallel having different impedance and identical x/r ratio. Impedance of A is Za and impedance of B is Zb, both sharing a load of Sl. Then the load shared by transformer A is?
a) Sl*(Zb/(Za+Zb))
b) Sl*(Za/(Za+Zb))
c) Sl*(Zb*Za/(Za+Zb))
d) Sl*(Zb+Za/Zb))
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12. Two single phase transformers A and B are operating in parallel having different impedance and identical x/r ratio. Impedance of A is Za and impedance of B is Zb, both sharing a load of Sl. Then the load shared by transformer B is?
a) Sl*(Zb/(Za+Zb))
b) Sl*(Za/(Za+Zb))
c) Sl*(Zb*Za/(Za+Zb))
d) Sl*(Zb+Za/Zb))
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13. Operating transformers in parallel gives the advantage of ___________
a) reliable loading
b) increased capacity of power system
c) reducing the capacity of substation
d) all of the mentioned
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14. Which of the following is a shortcoming for using parallel connected transformers?

I. Reliable loading
II. Increased capacity of power system
III. Reducing the capacity of substation
 

a) I, II, III
b) I, II
c) I, III
d) None of the mentioned
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15. In three phase supply, the voltage phases are displaced by ________ and in two phase by ___________
a) 120°, 90°
b) 180°, 90°
c) 120, 120°
d) 120°, 180°
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16. For a delta-star three phase transformer, if the transformation ratio is ‘x:1’, then how much percent of output will be improved?
a) 73%
b) 57%
c) 173%
d) 157%
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17. For a star-delta three phase transformer, if the transformation ratio is ‘x:1’, then how much percent of output will be?
a) 73%
b) 57%
c) 173%
d) 157%
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18. Identify the correct order for the voltage transformation for the three-phase transformers.
a) D-Y > D-D = Y-Y > Y-D
b) D-Y > D-D > Y-Y > Y-D
c) D-Y > D-D = Y-Y < Y-D
d) D-Y > D-D < Y-Y < Y-D
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