41. The reluctance of the interleaved corner is ______ to the magnetizing current.
Same
Negative
Inversely Proportional
Directly Proportional
Answer.4. Directly Proportional
Explanation:
The stampings used for the laminations in current transformers must have high cross-sectional area than the ordinary transformers.
Due to this, the reluctance of the interleaved corners remains as low as possible. The reluctance of the interleaved corner is directly Proportional to the magnetizing current. Hence the corresponding magnetizing current is also small. The windings are placed very close to each other so as to reduce the leakage reactance. To avoid the corona effect, in a bar-type transformer, the external diameter of the tube is kept large.
42. For line voltages above 7 kV which current transformer is used?
Oil Immersed
Compound Filled
Both 1 and 2
Ring-type
Answer.3. Both 1 and 2
Explanation:
For small line voltages, tape and varnish are used for insulation. For line voltages above 7 kV, the oil-immersed or compound-filled current transformers are used.
For still higher voltages, i.e. 750 kV and above, porcelain bushings and oil-filled transformers are used.
43. To reduce the peak voltage between layers in the current transformer, the secondary winding is being ______
Normalized
Characterized
Sectionalized
Neutralized
Answer.3. Sectionalized
Explanation:
In current transformers, the windings are not placed on separate limbs as they are far apart. So there are large amounts of primary and secondary leakage fluxes.
To minimize these fluxes, the windings are sectionalized and interleaved. The sectionalized secondary winding also helps to reduce the peak voltage between layers in the current transformer.
44. A current transformer has a phase error of +5 degrees. The phase angle between the primary and secondary current is:
5 degree
95 degree
175 degree
85 degree
Answer.3. 175 degree
Explanation:
Given that,
Phase angle error θ = 5°
Phase angle between the primary current IP and secondary current IS is given by
δ = 180° – θ
= 180° – 5°
= 175°
45. Phase angle in a C.T. is defined as ______
a) \(\frac{180}{π} [\frac{I_m cosδ}{nI_s}]\) degrees
b) \(\frac{180}{π} [\frac{I_c sinδ}{nI_s}]\) degrees
c) \(\frac{180}{π} [\frac{I_m cosδ – I_c sinδ}{nI_s}]\) degrees
In an ideal voltage transformer, there should not be any phase difference between the primary voltage and the secondary voltage reversed. However, in an actual transformer, there exists a phase difference between Vo and Vs reversed.
The phase angle is taken as +ve when secondary voltage reversed leads to the primary voltage.
The angle is -ve when the secondary voltage reversed lags the primary voltage.
It can be reduced by keeping the primary and secondary windings are wound as closely as possible.
The phase angle error in current transformer is given as
Im is the magnetizing component of the excitation current
Ic is the core loss component of the excitation current
Is is the secondary winding current.
46. To measure the current in a 1-ϕ transmission line, the primary winding of the current transformer is connected in:
Series with the line carrying current
Parallel to the load
Parallel with the line carrying current
Parallel to the source
Answer.1. Series with the line carrying current
Explanation:
The primary winding of the current transformer is connected in series with the transmission line whose current is to be measured.
If you try to connect a CT in parallel with another resistance or load, the coil of the CT will seem to offer negligible resistance or simply it will work as a short circuit.
The potential transformer is connected in parallel with the line.
47. Phase angle error in current transformer is given by _______
a) \(\frac{180}{π} [\frac{1}{nI_s}]\)
b) \(\frac{180}{π} [\frac{I_m}{I_s}]\)
c) \(\frac{180}{π} [\frac{I_m}{n}]\)
d) \(\frac{180}{π} [\frac{I_m}{nI_s}]\)
Answer.d. \(\frac{180}{π} [\frac{I_m}{nI_s}]\)
Explanation:
The phase angle error in current transformer is given as
In practice, the CT burden is largely resistive with a small value of inductance, thus the second phase angle δ is positive and generally small. Thus, sin δ ≈ 0 and cos δ ≈ 1. Therefore, the equation can be approximated as
\(\frac{180}{π} [\frac{I_m}{nI_s}]\)
48. The primary of 200/1 A, 5 VA, 0.8 pf current transformer carries 100 A. The secondary current is 0.495 A. The ratio error of the CT is
-1.0%
-0.5%
+0.5%
+1.0%
Answer.1. -1.0%
Explanation:
Nominal ration Kn = 200/1 = 200 A
Actual transformation ratio
R = Ip/Is = 100/0.495 = 202.02
Ratio error = 200 -202.2/202.2 = 1%
49. Ratio error in current transformer is largely depend on _________
Iron loss
Frequency
Magnetizing component
Supply voltage
Answer.1. Iron loss
Explanation:
The ratio error is considered to be positive when the actual ratio of the CT is less than the nominal ratio, i.e., when the secondary current, for a given primary current, is high.
The ratio error in a C.T. is given by the relation
Ratio error = Kn – R⁄R = n + Ie⁄Is
where
Ie is the iron loss component of the excitation current
n is the turns ratio.
50. Precautions are essential for ensuring that the secondary of a current transformer is not open-circuited when the primary circuit is carrying current. This is due to
The dangerously high voltage developed across secondary
The ferromagnetic core may develop residual magnetism
The reflected impedance may prevent the flow of current to the primary circuit
None of the above
Answer.1. Dangerously high voltage develop across secondary
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.
