This Portion of Electrical andElectronic Measurements and Measuring instruments contains Instrument Transformers MCQs (Multiple Choice Questions and Answers) / Objective Type Questions and Answers.

This Section covers below lists of topics.

  1. Instrument Transformers
  2. Current Transformers
  3. Potential Transformers
  4. Testing of Instrument Transformers
  5. Testing of Current Transformers
  6. Testing of Potential Transformers

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1. The advantages of instrument transformers are

  • The readings of instruments used in conjunction with them do not depend upon their resistance, inductance etc
  • The rating of instrument transformers has been standardized and the rating of instruments used in conjuction used with them also gets standardized. Therefore, there is reduction of cost and ease in replacements.
  • The metering circuit is electrically isolated from the power circuit thereby providing safety to operating personnel
  • All the above.

2. The disadvantages of shunts for use at high currents are :

  • It is difficult to achieve good accuracy with shunts
  • Power consumption of the shunt is large
  • The metering circuit is not electrically isolated from the power circuit
  • All the above.

3. The disadvantages of using multipliers with voltmeters at high voltages are :

  • The power consumption of multipliers becomes large at large voltages
  • The multipliers at high voltages have to be shielded in order to prevent capacitive currents
  • The metering circuit is not electrically from the power circuit
  • All the above.

4. The nominal ratio of a current transformer is :

  • Primary winding current secondary winding current
  • Rated primary winding current /rated secondary winding current
  • Number of secondary winding turns/number of primary winding turns
  • All the above.

5. The transformation ratio in the case of a potential transformer us defined as ratio of :

  • Primary winding voltage / secondary winding voltage
  • Rated primary winding voltage / rated secondary winding voltage
  • Number of turns of primary winding / number of turns of secondary winding
  • All the above.

6. The burden of current transformer is express in terms of :

  • Secondary winding current
  • VA rating of transformer
  • Voltage, current and power factor of secondary winding circuit
  • None of the above.

7. A current transformer has a rating of 100/5 A. its magnetizing and loss components of the exciting current are 1 A and 0.6 A respectively and secondary winding burden is purely resistive, its transformation ratio at rated current is :

  • 20.12
  • 20.2
  • 200.2
  • None of the above

8. A current transformer has a rating of 1000/5 A. its magnetizing current and loss component of exciting current are 10 A and 6 A respectively. The phase angle between secondary winding induced voltage and current is 30°. The phase angle error of the transformer  at rated current is :

  • 0.65°
  • 0.305°
  • 0.496°
  • None of the above.

9. The errors in current transformer can be reduced by designing them with :

  • High permeability and low loss core materials, avoiding any joints in the core and also keeping the flux density to a low value
  • Using primary and secondary windings as close to each other as possible
  • Using large cross section for both primary and secondary windings conductors
  • All the above

10. The current in the primary winding of a current transformer depends upon :

  • Burden of secondary winding of transformer
  • Load connected to the system in which the C.T installed
  • Both burden of the transformer secondary winding and load connected to the system
  • None of the above.

11. Turns compensation is used in current transformer primarily for reduction of :

  • Phase angle error
  • Both ratio and phase angle errors
  • Ratio error,reduction in phase angle error is incidental
  • None of the above.
  • When the secondary winding of the C.T is short circuited by the link with the primary winding energized, a very high current flows on the primary side
  • When the secondary winding of the C.T is short-circuited by the link with the primary winding energized, it is possible to make any adjustments in the secondary winding circuit like replacing a faulty ammeter
  • When the short-circuiting link is opened with the secondary winding open-circuited, the current on the primary side is fall to almost zero
  • All the above.

13. When the secondary winding of a current transformer is open circuited with the primary winding energized

  • The whole of the primary current produces large value of flux in the core(limited only saturation) thereby producing a large voltage in the secondary winding
  • The large voltage may act as safety hazard for the operators and may even rupture the insulation
  • When the large magnetizing force is taken off it leaves a large value of residual magnetism
  • All the above.

14. Clamps on meters are used because

  • With their use it is possible to measure to current flowing a line without breaking the circuit
  • Their accuracy is high
  • It is not possible to measure voltage without them
  • All the above.

15. When the secondary winding of a potential transformer is suddenly open circuited with primary winding excited :

  • Large voltages are produced in the secondary winding may be safety hazard to operating personnel
  • The large voltages so produced may rupture the insulation
  • The primary winding draws only the no load current
  • None of the above.

16. The ratio and phase angle errors in potential transformers may be reduced by

  • Increasing the exciting current
  • Increasing the resistance and leakage reactance in the transformer
  • By not employing turns compensation
  • None of the above.

17. Capacitive potential transformer are used

  • For primary winding phase voltages above 100 kV
  • For keeping the value of transformation ratio constant irrespective of the burden by making certain adjustment
  • Because they are cheaper than the electromagnetic transformers above a certain voltages range
  • All the above.

18. The size of the potential transformers :

  • a. Is the same as that of power transformers of the same VA rating
  • Is much greater than that of power transformers of the same VA rating because they are designed for low ratio and phase angle errors which require large sized cores and winding conductors
  • Is smaller than that power transformers of the same VA rating
  • None of the above.

19. The ratio of transformation in the case of potential transformers :

  • Increase with increase in power factor of secondary burden
  • Remain constant irrespective of the power factor of secondary burden
  • Decrease with increase in power factor of secondary burden
  • None of the above.

20. In case of potential transformer :

  • The phase angle error is always positive
  • The phase angle error is always negative
  • The phase angle error is usually zero
  • The phase angle error is positive when the secondary winding voltages reversed leads the primary winding voltage and is negative when the secondary winding voltage reversed lags behind the primary winding voltage.

21. The disadvantages of using shunts for high current measurements are

  • power consumption by the shunts themselves is high
  • it is difficult to achieve good accuracy with shunts at high currents
  • the metering circuit is not electrically isolated from the power circuit
  • all of the above

22. The disadvantages of using multipliers with voltmeters for measuring high voltages are

  • Power consumption by multipliers themselves is high at high voltages
  • Multipliers at high voltage need to be shielded to prevent capacitive leakage
  • The metering circuit is not electrically isolated from the power circuit
  • all of the above

23. The advantages of instrument transformers are

  • The readings of instruments used along with instrument transformers rarely depend on the impedance of the instrument
  • Due to availability of standardized instrument transformers and associated instruments, there is reduction in cost and ease of replacement
  • The metering circuit is electrically isolated from the power circuit
  • all of the above

24. Nominal ratio of a current transformer is

  • ratio of primary winding current to secondary winding current
  • ratio of rated primary winding current to rated secondary winding current
  • ratio of number of turns in the primary to number of turns in the secondary
  • all of the above

25. Burden of a CT is expressed in terms of

  • secondary winding current
  • VA rating of the transformer
  • power and power factor of the secondary winding circuit
  • impedance of secondary winding circuit

26. Ratio error in a CT is due to

  • secondary winding impedance
  • load impedance
  • no load current
  • all of the above

27. Phase-angle error in a CT is due to

  • primary winding impedance
  • primary circuit phase angle
  • leakage flux between primary and secondary
  • all of the above

28. Errors in instrument transformers can be aggravated by

  • leakage flux
  • core saturation
  • transients in main power line
  • all of the above

29. Phase-angle error in a CT can be reduced by

  • reducing number of secondary turns
  • using thin conductors for the primary winding
  • using good quality, low loss steel for core
  • all of the above

30. Ratio error in a CT can be reduced by

  • using good quality, low loss steel for core
  • placing primary and secondary windings closer to each other
  • using thick conductors for secondary winding
  • all of the above

31. Flux density in instrument transformers must be designed to be

  • sufficiently low to reduce core losses
  • sufficiently high to reduce core section and hence reduce length of winding
  • sufficiently low to prevent core saturation
  • properly optimized to have a balance among (a)-(c)

32. Current in the primary winding of CT depends on

  • burden in the secondary winding of the transformer
  • load connected to the system in which the CT is being used for measurement
  • both burden of the secondary and load connected to the system
  • none of the above

33. Turns compensation is used in CT to reduce

  • phase-angle error
  • both ratio and phase angle error
  • primarily ratio error, reduction in phase angle error is incidental
  • none of the above

34. Secondary winding of CT should never be open circuited with primary still energised because that will

  • increase power loss in the secondary winding
  • increase terminal voltage in the secondary winding
  • increase the leakage flux manifolds
  • all of the above

35. Open circuiting the secondary winding of CT with primary still energized will result in

  • Unrestricted primary flux to generate high voltages across secondary terminals
  • Possible insulation damage due to high voltage being generated
  • Injury to careless operator
  • all of the above
  • allow high current to flow in the primary when the secondary winding of the CT is short circuited with the link
  • allow adjustments to be made in the secondary side, like replacing the ammeter, with the primary energized but the short circuiting link in use
  • enable primary current to drop down to zero when the secondary is open circuited with the short circuiting link in use
  • all of the above

37. Clamp-on type and split-core type CTs are used because

  • their accuracy is high
  • it is possible to insert the CT in the circuit without breaking the main line
  • they are cheaper
  • all of the above

38. Transformation ratio of a PT is defined as

  • ratio of primary winding voltage to secondary winding voltage
  • ratio of rated primary winding voltage to rated secondary winding voltage
  • ratio of primary number of turns to secondary number of turns
  • all of the above

39. When the secondary winding of a PT is suddenly open circuited with the primary winding still open circuited then

  • large voltages will be produced across the secondary terminals that may be dangerous for the operating personnel
  • large voltages thus produced may damage the insulation
  • the primary winding draws only no-load current
  • none of the above

40. The size of a PT as compared to a power transformer of same VA

  • is smaller
  • is bigger
  • is the same
  • there is no relation as such

 
 

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