Electrical Measurements Questions and Answers – Design Features of Current Transformers

This set of Electrical Measurements & Measuring Instruments Multiple Choice Questions & Answers (MCQs) focuses 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
View Answer

Answer: a
Explanation: 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.
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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
View Answer

Answer: b
Explanation: 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
View Answer

Answer: a
Explanation: 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
View Answer

Answer: c
Explanation: 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
View Answer

Answer: a
Explanation: 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.
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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
View Answer

Answer: d
Explanation: In a C.T., the transformation ratio is given by
R = n + IeIs
where, n is the turns ratio
Ie is the core loss component of the excitation current
Is is the secondary winding current
Turns compensation is used to obtain the transformation ratio (R) equal to the nominal ratio (Kn).

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
View Answer

Answer: a
Explanation: In a C.T., the transformation ratio is given by
R = n + IeIs
where, n is the turns ratio
Ie is the core loss component of the excitation current
Is 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
View Answer

Answer: b
Explanation: The optimum number of secondary turns in a C.T. is the value that makes the transformation ratio (R) and nominal ratio (Kn) 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 Kn 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
View Answer

Answer: c
Explanation: 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.
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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
View Answer

Answer: d
Explanation: 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
View Answer

Answer: c
Explanation: 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
View Answer

Answer: d
Explanation: 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.

Sanfoundry Global Education & Learning Series – Electrical Measurements.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Instagram | Facebook | Twitter