31. Voltage transformers are designed to have _______
High leakage reactance
High magnetizing current
High magnetizing reactance
Low magnetizing reactance
Answer.3. High magnetizing reactance
Explanation:
To keep error as minimum as possible in potential transformer, PT is designed and constructed to have low leakage reactance, low loss, and high magnetizing reactance (low magnetizing current). Low reactance is achieved by interlacing primary and secondary both on the core limb.
High magnetizing reactance requires a minimum iron path and high permeability steel. Low loss requires low-loss steel and very thin laminations.
32. What is the additional usage of the presspahn material used as insulation material?
Lamination
To reduce the losses
To protect secondary winding conductor from mechanical damage
To protect secondary winding conductor from electrical damage
Answer.3. To protect secondary winding conductor from mechanical damage
Explanation:
Presspahn is used as insulation material in the current transformer. This presspahn material or pressboards, in addition to acting as an insulating medium, must also protect the secondary winding conductor from mechanical damage due to sharp corners.
33. Which of the following can be considered an error for a Potential transformer?
Magnitude errors
Phase errors
Unit errors
Magnitude and phase errors
Answer.4. Magnitude and phase errors
Explanation:
All voltage transformers introduce small errors into their representations of the applied primary voltage.
Magnitude error, as its name suggests, relates to the magnitude difference between the actual secondary voltage of a given voltage transformer and that which an ideal transformer would produce.
Similarly, the phase error represents the angular difference between the actual secondary voltage and that produced by an ideal device. The magnitude and phase errors associated with both the voltage and current transformers used in energy metering circuits introduce errors into the energy measured.
34. Which of the following can be considered an error for a current transformer?
Magnitude errors
Phase errors
Unit errors
Magnitude and phase errors
Answer.4. Magnitude and phase errors
Explanation:
Like voltage transformers, current transformers also introduce small magnitude and phase errors into their secondary currents.
These arise due to the need for a magnetizing current to generate sufficient flux density to produce the voltage demanded by the burden.
Since the magnetizing current is consumed internally, the current seen by the burden differs slightly in both magnitude and phase from that which might otherwise be expected.
In addition, the magnetizing impedance is not constant but instead depends on the flux density that the CT is called upon to support in order to circulate secondary current through the burden. Therefore both the applied burden and the actual secondary current, influence the CT errors.
35. What is the other name of the ring-type current transformer?
Brush transformer
Cloud transformer
Circular transformer
Window type current transformer
Answer.4. Window type current transformer
Explanation:
Ring Type Current Transformer has no primary winding and looks like a toroidal ring that contains only secondary winding being wound over a core. The open space in the center allows the active conductor to pass through the CT, i.e., this CT’s taken through the primary conductor.
This type of CT is also called a window-type current transformer. This CT has only one core and has only one CTR (current transformation ratio) and is generally used for small voltage rating systems.
36. What is the effect of the increase of secondary burden in the current transformer?
Phase angle error increase
Phase angle error Decrease
No effect
Phase angle error becomes more positive
Answer.1. Phase angle error increase
Explanation:
An increase in secondary burden means an increase in secondary voltage because the secondary current is fixed. Secondary voltage increases when the induced emf in the secondary side increases, which means an increase of flux density in the core of C.T. Exciting current increases with the increase of flux density. An increase of exciting current makes ratio error less positive and phase angle error directly increases with the increase of exciting current
37. How many faces are present in the split-core current transformer?
6
3
4
2
Answer.4. 2
Explanation:
Split core current transformers are designed for semi-permanent installation systems. They contain a transformer in which one of the cores to position around the conductor can be opened or moved and then locked with a latch or some other sort of flange.
split-core current transformer manufactured in exactly the same way as the ring-type except that the core is split, each half having two finely ground or lapped gap faces.
38. What is the effect of the increase of frequency in the current transformer?
Phase angle error increase
Phase angle error Decrease
No effect
Phase angle error becomes more positive
Answer.2. Phase angle error Decrease
Explanation:
For a given burden and p.f., the secondary induced voltage is constant and is proportional to the product of frequency and flux density.
Therefore, an increase in frequency will result in a proportionate decrease in flux density. Thus the effect of an increase in frequency on ratio and phase angle error is similar to a decrease in the burden of the C.T. Hence Phase angle error Decrease in CT.
39. In the bar-type current transformer what is the insulation material on the primary conductor?
Bakelized paper tube
Resin
Bakelized paper tube and resin
Bakelized paper tube or resin
Answer.4. Bakelized paper tube or resin
Explanation:
The insulation on the bar type primary is bakelized paper tube or a resin directly molded on the bar. Such bar type primary is an integral part of the current transformer. The core and the secondary windings are same in bar type transformer.
40. Current transformers are usually made of ______ construction.
Solid circular
Ring Core
Hard core
Any of the above
Answer.2. Ring Core
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.