2. SKIN EFFECT
• The non-uniform distribution of electric current over the surface or
skin of the conductor carrying a.c. is called the skin effect.
• The concentration of charge is more near the surface as compared to
the core of the conductor.
• The effective resistance of the conductor is increased due to the
concentration of current on the surface of the conductor.
• Skin effect increases with the increase in frequency. At low frequency,
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.
• If d.c. current (frequency=0) is passed in a conductor, the current is
uniformly distributed over the cross-section of the conductors.
3.
4. Why skin effect occurs?
• Let us consider the conductor is made up of a number of concentric
cylinders. When a.c is passed in a conductor, the magnetic flux induces in
it. The magnetic flux linking a cylindrical element near the center is greater
than that linking another cylindrical element near the surface of the
conductor. This is due to the fact that the center cylindrical element is
surrounded by both the internal as well as the external flux, while the
external cylindrical element is surrounded by the external flux only.
• The self-inductance in the inner cylindrical element is more and, therefore,
will offer a greater inductive reactance than the outer cylindrical element.
This difference in the inductive reactance gives a tendency to the current to
concentrate towards the surface or skin of the conductor.
• The current density is maximum at the surface of the conductor and
minimum at the center of the conductor. The effect is equivalent to a
reduction of the cross-section area of the conductor and, therefore the
effective resistance of the conductor is increased.
5. Factors affecting Skin effect
• Frequency – Skin effect increases with the increase in frequency.
• Diameter – It increases with the increase in 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 material to
support the formation of the magnetic field).
6. Proximity Effect
The term proximity effect refers to the influence of alternating current
in conductor on the current distribution in another, nearby conductor.
When two or more conductors are placed near to each other, then
their electromagnetic fields interact with each other. Due to this
interaction, the current in each of them is redistributed.
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 flow through that
halves portion of the conductor. The current is crowded in the remote
half portion of the conductor.
7. 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
8. If DC flows on the surface of the conductor, then the current are
uniformly distributed around the cross section area of the conductor.
Hence, no proximity effect occurs on the surface of the conductor.
9. Factors affecting Proximity Effect?
• This effect increases with increase in frequency.
• More the diameter of conductor, the more will be this effect.
• This effect also depends on the material of conductor.
• Proximity effect is more in case of solid conductor. This is the reason,
stranded or ACSR conductors are used to reduce this effect.
10. Ferranti Effect
• The Ferranti effect is a phenomenon that describes the increase in
voltage at the receiving end of a long transmission line relative to the
voltage at the sending end when the lines are either lightly loaded or
open circuit.
• Ferranti effect is due to the charging current of the line. When an
alternating voltage is applied, the current that flows into the capacitor
is called charging current. A charging current is also known as
capacitive current.
11. Why Ferranti effect occurs?
• The capacitance (and charging current) is negligible in short line but
significant in medium line and appreciable in long line. Therefore this
phenomenon occurs in medium and long lines.
• The capacitor charging current leads to a voltage drop across the line
inductor of the transmission system which is in phase with the
sending end voltages. This voltage drop keeps on increasing additively
as we move towards the load end of the line and subsequently, the
receiving end voltage tends to get larger than applied voltage.
12. How to reduce Ferranti effect
Electrical devices are designed to work at some particular voltage. If
the voltages are high at the user ends their equipment get damaged,
and their windings burn because of high voltage. Ferranti effect on long
transmission lines at low load or no load increases the receiving end
voltage. This voltage can be controlled by placing the shunt reactors at
the receiving end of the lines.
Shunt reactor is an inductive current element connected between line
and neutral to compensate the capacitive current from transmission
lines. When this effect occurs in long transmission lines, shunt reactors
compensate the capacitive VAr of the lines and therefore the voltage is
regulated within the prescribed limits.
13. Corona Discharge
The phenomenon of violet glow, hissing noise and production of ozone gas in an
overhead transmission line is called as corona.
Corona Discharge (also known as the Corona Effect) is an electrical discharge
caused by the ionization of a fluid such as air surrounding a conductor that is
electrically charged. The corona effect will occur in high voltage systems unless
sufficient care is taken to limit the strength of the surrounding electric field.
Corona discharge can cause an audible hissing or cracking noise as it ionizes the air
around the conductors. This is common in high voltage electric power transmission
lines. The corona effect can also produce a violet glow, production of ozone gas
around the conductor, radio interference, and electrical power loss.
The corona effect occurs naturally due to the fact that air is not a perfect insulator –
containing many free electrons and ions under normal conditions. When an electric
field is established in the air between two conductors, the free ions and electrons
in the air will experience a force. Due to this effect, the ions and free electrons get
accelerated and moved in the opposite direction.
14. The charged particles during their motion collide with one another and
also with slow-moving uncharged molecules. Thus the number of
charged particles increases rapidly. If the electric field is strong enough,
a dielectric breakdown of air will occur and an arc will form between
the conductors.
Minimizing these energy losses has been a major challenge for power
engineers. Corona discharge can significantly reduce the efficiency of
EHV (Extra High Voltage) lines in power systems.
Two factors are important for corona discharge to occur:
1. Alternating electrical potential difference must be supplied across
the line.
2. The spacing of the conductors, must be large enough compared to
the line diameter.
15. Factors Affecting Corona Loss
The main factors affecting corona discharge are:
• Atmosphere
• Conductor size
• Spacing between conductors
• Line voltage
16. Advantages
• Virtual increase in conductor diameter.
• Effects of transients produced by lighting and other causes are
reduced.
17. Disadvantages
• Loss of power.
• There is non sinusoidal voltage drop due to non sinusoidal corona
current.
• Corrosion on the conductor due to chemical action takes place.
18. Critical Disruptive Voltage
The voltage at which corona is just initiated is called critical disruptive
voltage.
At Critical disruptive voltage corona is not visible but noise can be
heard.
Critical visual voltage
The voltage at which corona is visible with violet colour is called
critical visual voltage.
19. Q-1. Find the critical disruptive voltage and the critical voltages for local and general corona on a 3
phase overhead transmission line, consisting of three stranded copper conductors spaced 2.5 m
apart at the corners of an equilateral triangle. Air temperature and pressure are 21°C and 73.6 cm
Hg respectively. The conductor dia, irregularity factor and surface factors are 10.4 mm, 0.85, 0.7 and
0.8 respectively.
