Stranded wires are mainly used to

A. reduce skin effect

B. reduce metal fatigue

C. reduce proximity effect

D. both B and C

Answer: D. both B and C

Explanation: 

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.