250+ TOP MCQs on Design Features of Current Transformers and Answers

Electrical Measurements & Measuring Instruments Multiple Choice Questions on “Design Features of Current Transformers”.

1. The secondary leakage reactance of a C.T. ____________
a) increases its ratio error
b) decreases its ratio error
c) has no effect on its ratio error
d) increases the impedance of the circuit
Answer: a
Clarification: The windings in a current transformer must be kept close so that the secondary leakage reactance is minimum. Secondary leakage reactance of a C.T. thus increases its ratio error.

2. Secondary and primary windings consist of ________
a) copper turns
b) 14 S.W.G copper wire and copper strip respectively
c) iron coils wound around
d) laminations
Answer: b
Clarification: For a current transformer the most popular form used is the bar primary and ring core construction. Hence 14 S.W.G copper wire is used for the secondary winding whereas a copper strip is used for the primary winding.

3. Coils of a C.T. are separately wound.
a) True
b) False
Answer: a
Clarification: A C.T. is used for the measurement of high magnitudes of current. In order to get lower line voltages, the coils of a C.T. are wound separately and insulated through tapes and varnish.

4. The windings of a C.T. are ________
a) tied together
b) shorted
c) wound over one another
d) grounded
Answer: c
Clarification: Using cylinders made of Bakelite, the windings of a C.T. are wound over one another. This setup is mounted on a steel tank sheet filled with transformer oil.

5. At high voltages, the current transformers are enclosed in a tank.
a) True
b) False
Answer: a
Clarification: The current transformers are enclosed in a tank at high voltages of the order of more than 7 kV. The tank consists of a solid insulating compound. Since the cooling is poor, the compound introduces difficulty.

6. Turns compensation is used to obtain ________
a) to compensate the turns
b) to equalise the turns on the windings
c) to protect the equipment
d) transformation ratio equal to nominal ratio
Answer: d
Clarification: In a C.T., the transformation ratio is given by
R = n + ^I_e⁄_Is
where, n is the turns ratio
I_e is the core loss component of the excitation current
I_s is the secondary winding current
Turns compensation is used to obtain the transformation ratio (R) equal to the nominal ratio (K_n).

7. What is the effect of reducing the secondary turns in a C.T. by 1%?
a) transformation ratio reduces by the same value
b) no effect
c) nominal ratio increase by the same value
d) secondary current increases by the same value
Answer: a
Clarification: In a C.T., the transformation ratio is given by
R = n + ^I_e⁄_Is
where, n is the turns ratio
I_e is the core loss component of the excitation current
I_s is the secondary winding current
As the number of secondary turns is reduced by a factor of 1%, the transformation ratio (R) is reduced by approximately the same percentage.

8. In a 1000/5 A C.T., the number of secondary turns would be ________
a) 200
b) 199 or 198
c) 5
d) 1000
Answer: b
Clarification: The optimum number of secondary turns in a C.T. is the value that makes the transformation ratio (R) and nominal ratio (K_n) equal. It is usually one or two turns less than the actual value. Thus in a 1000/5 A, though the number of turns would be 200 ideally, it is considered as 198 or 199 in order to keep R and K_n equal.

9. In the ring type of core, the secondary winding is ________
a) non-uniformly distributed over the core
b) shorted with the primary winding
c) uniformly distributed over the core
d) connected in the form of a ring
Answer: c
Clarification: The ring type of core used for a secondary winding is a jointless core. It has a very small leakage reactance. Hence in a ring type of core, the secondary winding is uniformly distributed over the core.

10. In a shell type of core, the windings are ________
a) in the form of the shell
b) shorted with the primary windings
c) not wound
d) placed on the central limb
Answer: d
Clarification: The windings in a shell type of core are placed in a central limb. It provides protection to the windings.

11. Mumetal has ________
a) low permeability, high loss
b) high permeability, medium loss
c) medium permeability, high loss
d) high permeability, low loss
Answer: c
Clarification: Mumetal is an alloy of nickel, iron and copper. It has a high permeability, low loss and small retentivity. It has a maximum permeability of 90,000 at a flux density of 0.35 W/m².

12. Perminder has __________
a) medium saturation point of permeability
b) low saturation point of permeability
c) no saturation point of permeability
d) high saturation point of permeability
Answer: d
Clarification: Perminder is an alloy of iron, cobalt and vanadium. It has a high flux density of the order of 2.4 Wb/m². It is very expensive.

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