transmission and distribution.(skin effect)

1. SKIN EFFECT
No current flow
Current concentrating
near surface
Cross section
of conductor
When a conductor carries a steady or d.c. current, this current is uniformly
distributed over the whole cross-section of the conductor. However the current
distribution is non-uniformly, rather it has tendency to concentrate near the surface of the
conductor. This known as skin effect.
The tendency of alternating current to concentrate near the surface of
conductor is known as skin effect.
Due to this skin effect, the effective cross sectional area offered to flow of
current decreases which increases resistance. Consequently the resistance of the
conductor is slightly increased when carrying an alternating current.

2. CAUSES OF SKIN EFFECT
Suppose a solid conductor having large number of strands,
each carrying a small portion of current, the inductance of each
conductor will vary according to its position.
Thus the strands near the centre are surrounded by a
greater magnetic flux and hence have larger inductance than
that near the surface.
The high reactance of inner strands causes the alternating
current to flow near the surface of the conductors.
This crowding of current near the conductor surface is skin
effect.

3. Skin effect depends upon the following factors
a) Nature of material
b) Diameter of the wire – increase with the diameter of
the wire.
c) Frequency -- increases with increase in frequency.
d) Shape of the wire – less for stranded conductor than
solid conductor.
skin effect is negligible when the supply
frequency is low (50 Hz) and conductor diameter is
small (<1 cm).

4. PROXIMITY EFFECT
The alternating magnetic flux in a conductor caused
by the current flowing in a neighbouring conductor give
rise to circulating current which cause an apparent
increase in the resistance of a conductor. This
phenomenon is called proximity effect.
INDUCTIVE INTERFERENCE
The transmission line when running parallel with the
communication lines give rise to electro magnetic and
electrostatic field which induce current and voltages in
the neighbouring communication line which is called as
the inductive inference of the transmission line.

5. CORONA
 The phenomenon of violet glow, hissing noise and production
of ozone gas in an over head transmission line is known as
corona.
 the partial ionization surrounding the conductor surface
 reason: when surface potential gradient of a conductor exceeds
the dielectric strength of the surrounding air, ionization occurs.
CORONA EFFECT
 produce power loss
 produce audible noise
 radio interference in the AM band

6. Corona is affected by
 conductor diameter, bundling
 type of conductor
 condition of surface: air dust, humidity, wind
Factors upon which corona depends
 Atmosphere
 Conductor size
 Spacing between conductors
 Line voltage

7. Corona can be reduced by
 increasing the conductor size
 conductor bundling

8. ADVANTAGES OF CORONA
 Due to corona formation, the air surrounding the
conductor becomes conducting and hence virtual
diameter of the conductor is increased. The
increased diameter reduces the electrostatic
stresses between the conductor.
 Corona reduces the effects of transients produced
by surges.

9. DISADVANTAGES OF CORONA
 Corona is accompanied by a loss of energy. This affects
the transmission efficiency of the line.
 Ozone is produced by corona and may cause corrosion
of the conductor due to chemical action.
 The current drawn by the line due to corona is non-
sinusoidal voltage drop occurs in the line. This may
cause inductive interference with neighbouring
communication lines.