Power System Most Asked MCQS Part-3

July 25, 2021

1. The skin effect in a transmission line is affected by

A. the resistivity of the transmission line
B. the current magnitude in the transmission line
C. the cross-sectional area of the transmission line
D. the voltage applied across the transmission line

Answer: C. the cross-sectional area of the transmission line

Explanation:

The phenomenon arising due to unequal distribution of current over the entire cross-section of the conductor is referred to as the skin effect.
Such a phenomenon does not have much role to play in case of a very short transmission line, but with an increase in the effective length of the conductors, skin effect increases considerably.
The distribution of current over the entire cross-section of the conductor is quite uniform in the case of a DC system.
But in the alternating current system, current tends to flow with higher density through the surface of the conductors (i.e., the skin of the conductor), leaving the core deprived of current.

Factors affecting skin effect in transmission lines are:
Frequency – The skin effect increases with the increase in frequency.
Diameter – It increases with the increase in the diameter of the conductor.
The shape of the conductor – Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
Type of material – Skin effect increase with the increase in the permeability of the material (Permeability is the ability of the material to support the formation of the magnetic field).

Important Points:
The Skin effect is negligible if the frequency is less than the 50Hz and the diameter of the conductor is less than the 1cm.
In the stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminum, while the steel near the center carries no current and gives high tensile strength to the conductor.
The concentration of current near the surface enabled the use of an ACSR conductor.

2. Which one of the following effects is resulted because of the size of the conductor, frequency and the specific resistance of the conducting material?

A. Thomson effect
B. Proximity effect
C. Corona effect
D. Skin effect

Answer: D. Skin effect

Explanation:
Factors affecting skin effect in transmission lines are:
Frequency – The skin effect increases with the increase in frequency.
Diameter – It increases with the increase in the diameter of the conductor.
The shape of the conductor – Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
Type of material – Skin effect increase with the increase in the permeability and specific resistance of the material (Permeability is the ability of the material to support the formation of the magnetic field).

The Skin effect is negligible if the frequency is less than the 50Hz and the diameter of the conductor is less than the 1cm.
In the stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminum, while the steel near the center carries no current and gives high tensile strength to the conductor.
The concentration of current near the surface enabled the use of an ACSR conductor.

3. In skin effect the skin depth is proportional to (f is frequency)

A. f
B. 1/√f
C. 1/f
D. √f

Answer: B. 1/√f

Explanation:

The skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases with greater depths in the conductor.
The electric current flows mainly at the skin of the conductor, between the outer surface and a level called the skin depth.

4. What is the result of Skin Effect?

A. Reduced effective resistance but increase effective internal reactance of the conductor
B. Increased effective resistance but reduced effective internal reactance of the conductor
C. Reduced effective resistance as well as effective internal reactance
D. Increased effective resistance as well as effective internal reactance

Answer: B. Increased effective resistance but reduced effective internal reactance of the conductor

Explanation:
Skin Effect:

Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases with greater depths in the conductor.
The phenomenon arising due to unequal distribution of current over the entire cross-section of the conductor is referred to as the skin effect.
Such a phenomenon does not have much role to play in case of a very short transmission line, but with an increase in the effective length of the conductors, the skin effect increases considerably.
The distribution of current over the entire cross-section of the conductor is quite uniform in the case of a DC system.
But in the alternating current system, current tends to flow with higher density through the surface of the conductors (i.e., the skin of the conductor), leaving the core deprived of current.
Increased effective resistance but reduced effective internal reactance of the conductor.

Factors affecting skin effect in transmission lines are:
Frequency – The skin effect increases with the increase in frequency.
Diameter – It increases with the increase in the diameter of the conductor.
The shape of the conductor – Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
Type of material – Skin effect increase with the increase in the permeability of the material (Permeability is the ability of the material to support the formation of the magnetic field).

Important Points:

The Skin effect is negligible if the frequency is less than the 50Hz and the diameter of the conductor is less than 1cm.
In the stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminum, while the steel near the center carries no current and gives high tensile strength to the conductor.
The concentration of current near the surface enabled the use of an ACSR conductor.

5. What happens when alternating current passes through a conductor?

A. It remains uniformly distributed throughout the section of conductor
B. Portion of conductor near the surface carries more current as compared to the core
C. Portion of conductor near the surface carries less current as compared to the core
D. Entire current passes through the core of the conductor

Answer: B. Portion of conductor near the surface carries more current as compared to the core

Explanation:
Skin Effect:

Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases with greater depths in the conductor.
The phenomenon arising due to unequal distribution of current over the entire cross-section of the conductor is referred to as the skin effect.
Such a phenomenon does not have much role to play in case of a very short transmission line, but with an increase in the effective length of the conductors, the skin effect increases considerably.
The distribution of current over the entire cross-section of the conductor is quite uniform in the case of a DC system.
But in the alternating current system, current tends to flow with higher density through the surface of the conductors (i.e., the skin of the conductor), leaving the core deprived of current.
Increased effective resistance but reduced effective internal reactance of the conductor.

Factors affecting skin effect in transmission lines are:
Frequency – The skin effect increases with the increase in frequency.
Diameter – It increases with the increase in the diameter of the conductor.
The shape of the conductor – Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
Type of material – Skin effect increase with the increase in the permeability of the material (Permeability is the ability of the material to support the formation of the magnetic field).

Important Points:
The Skin effect is negligible if the frequency is less than the 50Hz and the diameter of the conductor is less than 1cm.
In the stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminum, while the steel near the center carries no current and gives high tensile strength to the conductor.
The concentration of current near the surface enabled the use of an ACSR conductor.

6. Skin effect is only noticeable at _______ frequencies.

A. Audio
B. Low
C. High
D. All

Answer: C. High

Explanation:
Skin Effect
The non-uniform distribution of electric current over the surface of the conductor is called the skin effect.

In the skin effect, the concentration of charge is more near the surface as compared to the core of the conductor which causes the increase in ohmic resistance of the conductor.

Factors affecting the Skin effect:

1. Frequency:

Skin effect increases with the increase in frequency.
At low frequencies, such as 50Hz, there is a small increase in the current density near the surface of the conductor.
At high frequencies, such as radio-frequency, practically the whole of the currents flows on the surface of the conductor, and the Skin effect is more noticeable at high frequencies.
With DC supply (frequency is zero), the current passed in a conductor is uniformly distributed over the cross-section of the conductors.

2. Diameter:
Skin effect increases with the increase in diameter of the conductor.

3. The shape of the conductor:
Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.

4. Type of material:
Skin effect increase with the increase in the permeability of the material.

7. The effective resistance of a coils at high frequencies is more than its d.c. resistance on account of __________

A. skin effects
B. proximity effects
C. eddy current losses
D. All of these

Answer: D. All of these

Explanation:

Skin Effect:

Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases with greater depths in the conductor.
The electric current flows mainly at the skin of the conductor, between the outer surface and a level called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor.

Proximity Effect:

The alternating flux in a conductor is caused by the current of the other nearby conductor. This flux produces a circulating current or eddy current in the conductor which results in an apparent increase in the resistance of the wire. Thus, more power losses in the windings. This phenomenon is called the proximity effect.
Eddy current losses are also accounts for increasing ac resistance.

8. The skin effect increases the

-
A. Inductance of line
B. Resistance of line
C. Capacitance of line
D. Voltage of line

Answer: B. Resistance of line

Explanation:
Skin effect:
Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases with greater depths in the conductor.
The phenomenon arising due to unequal distribution of current over the entire cross-section of the conductor is referred to as the skin effect.
Such a phenomenon does not have much role to play in the case of a very short transmission line, but with an increase in the effective length of the conductors, skin effect increases considerably.
The distribution of current over the entire cross-section of the conductor is quite uniform in the case of a DC system.
But in the alternating current system, current tends to flow with higher density through the surface of the conductors (i.e., the skin of the conductor), leaving the core deprived of current.

Hence due to the skin effect effective area of the conductor of the line decreases, so that the resistance of the line increases.

Factors affecting skin effect in transmission lines are:
Frequency – The skin effect increases with the increase in frequency.
Diameter – It increases with the increase in the diameter of the conductor.
The shape of the conductor – Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
Type of material – Skin effect increase with the increase in the permeability of the material (Permeability is the ability of the material to support the formation of the magnetic field).

Important Points:
The Skin effect is negligible if the frequency is less than the 50Hz and the diameter of the conductor is less than the 1cm.
In the stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminum, while the steel near the center carries no current and gives high tensile strength to the conductor.
The concentration of current near the surface enabled the use of an ACSR conductor.

9. The skin effect in a transmission line is affected by

A. the resistivity of the transmission line
B. the current magnitude in the transmission line
C. the cross-sectional area of the transmission line
D. the voltage applied across the transmission line

Answer: C. the cross-sectional area of the transmission line

Explanation:
The phenomenon arising due to unequal distribution of current over the entire cross-section of the conductor is referred to as the skin effect.
Such a phenomenon does not have much role to play in case of a very short transmission line, but with an increase in the effective length of the conductors, skin effect increases considerably.
The distribution of current over the entire cross-section of the conductor is quite uniform in the case of a DC system.
But in the alternating current system, current tends to flow with higher density through the surface of the conductors (i.e., the skin of the conductor), leaving the core deprived of current.

Factors affecting skin effect in transmission lines are:
Frequency – The skin effect increases with the increase in frequency.
Diameter – It increases with the increase in the diameter of the conductor.
The shape of the conductor – Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
Type of material – Skin effect increase with the increase in the permeability of the material (Permeability is the ability of the material to support the formation of the magnetic field).

Important Points:
The Skin effect is negligible if the frequency is less than the 50Hz and the diameter of the conductor is less than the 1cm.
In the stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminum, while the steel near the center carries no current and gives high tensile strength to the conductor.
The concentration of current near the surface enabled the use of an ACSR conductor.

10. The skin effect in conductors is due to non-uniform distribution of current in it and major portion of current is near the _____ of conductor

A. Surface
B. Center
C. Complete cross section area
D. Axis

Answer: A. Surface

Explnation:
Skin effect:
Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases with greater depths in the conductor.
The phenomenon arising due to unequal distribution of current over the entire cross-section of the conductor is referred to as the skin effect.
Such a phenomenon does not have much role to play in case of a very short transmission line, but with an increase in the effective length of the conductors, skin effect increases considerably.
The distribution of current over the entire cross-section of the conductor is quite uniform in the case of a DC system.
But in the alternating current system, current tends to flow with higher density through the surface of the conductors (i.e., the skin of the conductor), leaving the core deprived of current.

Factors affecting skin effect in transmission lines are:
Frequency – The skin effect increases with the increase in frequency.
Diameter – It increases with the increase in the diameter of the conductor.
The shape of the conductor – Skin effect is more in the solid conductor and less in the stranded conductor because the surface area of the solid conductor is more.
Type of material – Skin effect increase with the increase in the permeability of the material (Permeability is the ability of the material to support the formation of the magnetic field).

Important Points:
The Skin effect is negligible if the frequency is less than the 50Hz and the diameter of the conductor is less than the 1cm.
In the stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminum, while the steel near the center carries no current and gives high tensile strength to the conductor.
The concentration of current near the surface enabled the use of an ACSR conductor.