Current Transformer MCQ || Current Transformer Questions and Answers

1. The primary winding of a current transformer is connected in ______ with the line carrying the main current.

  1. Parallel
  2. Series
  3. Either Series or Parallel
  4. None of the above

Answer.2. Series

Explanation:

The primary winding of a current transformer is connected in series with the line carrying the main current. The secondary winding of the CT, where the current is many times stepped down, is directly connected across an ammeter, for measurement of current; or across the current coil of a wattmeter, for measurement of power.

 

2. The ratio error in the current transformer is attributed to:

  1. Leakage flux
  2. Magnetizing component of no-load current
  3. Power factor of the primary
  4. The energy component of excitation current

Answer.2. Series

Explanation:

Ratio error in current transformer:

  • In the current transformer, the primary current Ip should be exactly equal to the secondary current multiplied by the turns ratio.
  • But there is a difference between primary current Ip should be exactly equal to the secondary current multiplied by the turns ratio.
  • This difference is contributed by the core excitation or magnetizing component of no-load current.
  • The error in the current transformer introduced due to this difference is called current error or ratio error.

 

3.  The secondary leakage reactance of a C.T. _______

  1. Increases its ratio error
  2. Decreases its ratio error
  3. Has no effect on its ratio error
  4. Increases the impedance of the circuit

Answer.2. Series

Explanation:

Ratio error in current transformer:

  • In the current transformer, the primary current Ip should be exactly equal to the secondary current multiplied by the turns ratio.
  • But there is a difference between primary current Ip should be exactly equal to the secondary current multiplied by the turns ratio.
  • This difference is contributed by the core excitation or magnetizing component of no-load current.
  • The error in the current transformer introduced due to this difference is called current error or ratio error.
  • The windings in a current transformer must be kept close so that the secondary leakage reactance is minimum.
  • So Secondary leakage reactance of a C.T. thus increases its ratio error.

 

4. What will happen if the secondary of a current transformer is open-circuited?

  1. Hot because of heavy iron losses
  2. Hot because primary will carry heavy current
  3. Cool as there is no secondary current
  4. Depends on other parameters

Answer.1. Hot because of heavy iron losses

Explanation:

The secondary side of a current transformer should never be kept in open condition because, when kept open, there is a very high voltage found across the secondary side. This high voltage causes a high magnetizing current to build upon the secondary side which in turn causes high flux and makes the core saturate.

If the secondary of the current transformer is made open-circuited the transformer temperature will rise to a higher value because of heavy iron losses taking place in the circuit due to high flux density.

When the secondary winding of a current transformer is open-circuited with the primary winding energized, the large voltage may act as a safety hazard for the operators and many even rupture the insulation.

 

5. How many types are the current transformers classified into?

  1. 2
  2. 3
  3. 4
  4. 5

Answer.1. 2

Explanation:

The current transformer are generally classified in two types

  1. Bar type
  2. Wound type
  • Wound-type Current Transformer: The transformer’s primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turn ratio of the transformer. These are used at very low current ratios such as summing applications. Because of the higher values of primary ampere-turns, high accuracy can be achieved by these CTs.
  • Bar-type Current Transformer: This type of current transformer uses the actual cable or bus bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current-carrying device.

 

6. The two types of indoor type current transformers are:

  1. Bushing type and Clamp-on type
  2. Wound type and Clamp-on type
  3. Wound type and Bar type
  4. Bar type and Bushing type

Answer.3. Wound type and Bar type

Explanation:

Indoor type current transformers are generally used for low voltage circuits and are further classified into wound type, bar type, and window type transformers.

  • Wound-type Current Transformer: The transformer’s primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turn ratio of the transformer. These are used at very low current ratios such as summing applications. Because of the higher values of primary ampere-turns, high accuracy can be achieved by these CTs.
  • Bar-type Current Transformer: This type of current transformer uses the actual cable or bus bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current-carrying device.
  • The accuracy of bar-type CT decreases due to the magnetization of the core which requires a large fraction of the total ampere-turns at low current ratings.
  • Window-type Current Transformer: These are installed around the primary conductor (or line conductor) because these are constructed with no primary. These are the most common CTs available in solid and split-core constructions. Before installing solid window CT, the primary conductor must be disconnected while in the case of split-core it can directly installed around the conductor without disconnecting it.

 

7. The ratio error of the current transformer is defined as

  1. Ratio error = KnR
  2. Ratio error = Kn – RR
  3. Ratio error = Kn – R
  4. Ratio error = 1R

Answer.2. Ratio error = Kn – R⁄R

Explanation:

The actual ratio of transformation varies with operating conditions and the error in secondary voltage is defined as

Percentage ratio error = $ = \frac{{{K_n} – R}}{R} \times 100$

Kn is the nominal ratio

R is the actual ratio

It can be reduced by secondary turns compensation i.e. slightly decreasing the secondary turns.

 

8. The primary of a _______ should never be energized when its secondary is open-circuited.

  1. Potential transformer
  2. Current transformer
  3. Autotransformer
  4. Power transformer

Answer.2. Current transformer

Explanation:

The secondary side of the current transformer is always kept short-circuited in order to avoid core saturation and high voltage induction so that the current transformer can be used to measure high values of currents.

  • The current transformer works on the principle of shorted secondary.
  • It means that the burden on the system Zb is equal to 0.
  • Thus, the current transformer produces a current in its secondary which is proportional to the current in its primary.

 

9. Secondary and primary windings of current transformer consist of ________

  1. Copper turns
  2. 14 S.W.G copper wire and copper strip respectively
  3. Iron coils wound around
  4. Laminations

Answer.2. Current transformer

Explanation:

Current transformers are usually made of ring core construction because ring cores are jointless so their reluctance is less and moreover they are more robust. Thus ring cores are capable of withstanding the large forces that are developed in the event of a short circuit. The magnetic materials that are used for the construction of current transformer cores include

(i) hot-rolled silicon steel

(ii) cold-rolled grain-oriented silicon steel

(ii) nickel-iron alloys.

The windings should be close together in order to reduce the secondary leakage reactance as this increases the ratio error. No. 14. S.W.G. copper wire is used on the secondary side and a copper strip is used as the bar primary.

 

10. A current-carrying conductor is wrapped eight times around the jaw of a clamp-on meter that reads 50 A. What will be the actual value of the conductor current?

  1. 400 A
  2. 6.25 A
  3. 50 A
  4. 12.5 A

Answer.2. 6.25 A

Explanation:

Given, Np = 1, Ns = 8, Ip = 50 A

the secondary current is given by

Is/Ip = Np/Ns

Is = 50 × (1 / 8)

Is = 6.25 A

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