At power frequencies, skin and proximity effects are ________ frequency.

A. independent of

B. negligible at small

C. important at small

D. negligible at large

Answer: B. negligible at small

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 conductor's surface and decreases with greater depths in the conductor.
• The phenomenon arising due to unequal current distribution over the conductor's entire cross-section 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, 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.
• Hence due to the skin effect effective area of the conductor of the line decreases, so that the resistance of the line increases.
• The electric current flows mainly at the skin of the conductor, between the outer surface and a level called the skin depth. the skin depth is inversely proportional to the root of frequency. In a good conductor, the skin depth is proportional to the square root of the resistivity. This means that better conductors have a reduced skin depth. The overall resistance of the better conductor remains lower even with the reduced skin depth.
• The skin effect increases with the increase of cross-section, permeability, and supply frequency. It reduces with the increase in resistivity of the conductor material.
• Hence, The skin effect is negligible when the supply frequency is low and the conductor diameter is small.
• Skin effect is the tendency for high-frequency currents to flow on the surface of a conductor.
• The skin effect reduces the effective area available for conduction due to self-linked flux. Now, we know that resistance is inversely proportional to the area of conduction so the effective value of resistance increases with the skin effect.
• If the resistance is constant over a large range of current and voltage values, the resistor is an ohmic device. 
• Hence if the current is uniformly distributed in the conductor cross-section, the effective resistance of a conductor will be the same as ohmic resistance.

Watch Video For Full Explanation About Skin Effect

Factors affecting skin effects in transmission lines are:

Frequency : 

• The 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.

Diameter : 

• It increases with the increase in the diameter of the conductor.

The shape of the conductor :

• The 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 :

• The 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 50Hz and the diameter of the conductor is less than 1cm.
• In stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) the current flows mostly in the outer layer made of aluminium, while the steel near the centre 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.

Important Point: 

Skin effect ∝ 1/skin depth
Skin effect is inversely proportional to skin depth.

skin depth = 1/√(πfµσ)

Where,
f = frequency
µ = permeability
σ = conductivity

If skin depth is more, then the skin effect is less and vice versa.
In the case of communication, line frequency is higher. So that skin depth is very small and hence skin effect is larger.

The depth to which the electromagnetic waves pass through the conductor is very small. It is measured in μm.

In the case of power lines frequency is small and hence skin depth is larger, so the skin effect can be neglected.

Therefore, Skin effect ∝ √(πfµσ)

When resistivity decrease, its conductivity will increase. This will increase the skin effect.

Proximity effect

• When the conductors carry the high alternating voltage then the currents are non-uniformly distributed on the cross-section area of the conductor. This effect is called the proximity effect.
• The proximity effect results in the increment of the apparent resistance of the conductor due to the presence of the other conductors carrying current in its vicinity.
• When two or more conductors are placed near each other, then their electromagnetic fields interact with each other.
• Due to this interaction, the current in each of them is redistributed such that the greater current density is concentrated in that part of the strand most remote from the interfering conductor. 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's current losses are also account for increasing ac resistance.
• If the conductors carry the current in the same direction, then the magnetic field of the halves of the conductors which are close to each other is cancelling each other, and hence no current flows through that halves portion of the conductor. The current is crowded in the remote half portion of the conductor.
• When the conductors carry the current in the opposite direction, then the close part of the conductor carries, the more current and the magnetic field of the far-off half of the conductor cancel each other. Thus, the current is zero in the remote half of the conductor and crowded at the nearer part of the conductor. 
• The proximity effect occurs due to current in mutual conductors. The effective area of the current flowing path is reduced because of the non-uniform flux linkage between the two adjacent conductors.
• The proximity effect is more in the case of power cables because the distance between the conductors is small. This effect is negligible in the case of overhead transmission lines because the distance between the conductors is larger.

Watch Video For Full Explanation About Proximity Effect

Factors Affecting the Proximity Effect:

Frequency: 

• The proximity increases with the increase in frequency.

Diameter: 

• The proximity effect increases with the increase in the conductor.

Structure: 

• This effect is more on the solid conductor as compared to the stranded conductor  (i.e., ASCR) because the surface area of the stranded conductor is smaller than the solid conductor.

Material: 

• If the material is made up of high ferromagnetic material then the proximity effect is more on the surface.

The proximity effect occurs due to current in mutual conductors. The effective area of the current flowing path is reduced because of the non-uniform flux linkage between the two adjacent conductors.

The proximity effect depends on

1. Frequency

2. Conductivity

3. Relative permeability

4. Distance between the conductors

Useful Points

• The proximity effect occurs due to current in mutual conductors. The effective area of the current flowing path is reduced because of the non-uniform flux linkage between the two adjacent conductors.
• The proximity effect is more in the case of power cables because the distance between the conductors is small. This effect is negligible in the case of overhead transmission lines because the distance between the conductors is larger.

Additional Information

Ferranti Effect: 

• At no load (or) at light load, the voltage at the receiving end of the transmission line is more than the sending voltage. It is known as the Ferranti effect. It is due to the charging current of the line.

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.

Corona Effect: 

• When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low.
• When the applied voltage exceeds a certain value (critical disruptive voltage), the conductors are surrounded by a faint violet glow called the corona.
• The discharging current in a transmission line increases due to the corona effect because corona increases the effective diameter. So that the capacitance will be increased and inductance will be reduced.