Instrument Transformers MCQ || Instrument Transformers Questions and Answers

1. Instrument transformers are designed to transform _____

  1. Current only
  2. Voltage Only
  3. Both current and voltage
  4. Low value of current and voltage

Answer.3. Both current and voltage

Explanation:

Instrument transformers are designed to transform voltage or current from the high values in the transmission and distribution systems to the low values that can be utilized by low voltage metering devices. Instrument transformers isolate measurement, protection, and control circuitry from the high currents or voltages present on the circuits being measured or controlled.

 

2. If the secondary winding of a current transformer is opened when the primary winding is carrying current, then the following event occurs:

  1. Transformer will burn immediately
  2. High induced EMF is produced in the secondary winding
  3. Flux density in the core gets weakened
  4. High current is produced in the secondary winding

Answer.2. High induced EMF is produced in the secondary winding

Explanation:

  • Most important precaution in use of a CT is that in no case should it be open circuited (even accidently).
  • As the primary current is independent of the secondary current, all of it acts as a magnetizing current when the secondary is opened.
  • This results in deep saturation of the core which cannot be returned to the normal state and so the CT is no longer usable.
  • Again, due to large flux in the core the flux linkage of secondary winding will be large which in turn will produce a large voltage across the secondary terminals of the CT.
  • This large voltage across the secondary terminals will be very dangerous and will lead to the insulation failure and there is a good chance that the person who is opening the CT secondary while primary is energized will get fatal shock.

 

3. The current transformer is used with ______

  1.  A.C. ammeter
  2.  D.C. ammeter
  3. A.C. voltmeter
  4. D.C. voltmeter

Answer.1.  A.C. ammeter

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.

Current transformers are used to extend the range of ammeters, and the current coils of wattmeters and energymeters.

 

4. The ratio error in current transformer is attributed to:

  1. Leakage flux
  2. magnetising component of no-load current
  3. power factor of the primary
  4. energy component of excitation current

Answer.2. magnetising component of no-load current

Explanation:

Ratio error in current transformer:

  • In the current transformer, the primary current Ip should be exactly equal to the secondary current multiplied by turns ratio, i.e. KTIs.
  • 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.

 

5. The potential transformer is used with ______

  1.  A.C. ammeter
  2.  D.C. ammeter
  3. A.C. voltmeter
  4. D.C. voltmeter

Answer.3. A.C. voltmeter

Explanation:

Measurement of voltage, power, etc., of high voltage lines requires the high level of voltage being stepped down before being applied to the measuring instrument. Potential transformers are used to extend the range of voltmeters, and the pressure coils of wattmeters and energymeters.

 

6. In the current transformer, the number of primary turns is always:

  1. less than the number of secondary turns
  2. zero
  3. greater than the number of secondary turns
  4. equals the number of secondary turns

Answer.1. less than the number of secondary turns

Explanation:

A current transformer needs to step down the high currents i.e. the primary current is very high. So, the number of primary turns is always less than the number of secondary turns.

 

7. Current transformer and potential transformer are used for Measuring _________

  1. Low current and voltages
  2. Very low current and voltages
  3. High current and voltages
  4. Intermediate currents and voltages

Answer.3. High current and voltages

Explanation:

CT and PT both are measuring devices used to measure currents and voltages. They are used where large quantities of currents and voltages are used. The role of CT and PT is to reduce high current and high voltage to a parameter.

 

8. In which of the following transformers, the secondary winding is always kept closed?

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

Answer.1. Current transformer

Explanation:

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.  The primary winding of a C.T. has _________

  1. Larger number of turns
  2. No turns at all
  3. Intermediate number of turns
  4. Few turns

Answer.4. Few turns

Explanation:

The current transtormer (c.t.) normally has very few turns on its primary winding so that it does not affect the circuit to be measured, with the actual meter connected across the secondary winding.

 

10. Instrument transformers are

  1. Used to extend the range of the AC measuring instruments only
  2. Used to isolate the measuring instruments from the high voltage only
  3. Used to extend the range and isolate the measuring instruments
  4. Not used at generating stations and transformer stations

Answer.3. Used to extend the range and isolate the measuring instruments

Explanation:

In high voltage A.C. circuits, the measurement cannot be done by using the method of extension of ranges of low range meters by providing suitable shunts.

In such conditions, specially constructed accurate ratio transformers are used. These transformers are used to isolate the instruments from high current and high voltage A.C. circuits.

These are generally classified as

(i) Current transformer – large alternating currents can be measured

(ii) Potential transformer – High voltages can be measured

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