2. Main Components of Overhead Lines
An overhead line may be used to transmit or
distribute electric power.
The successful operation of an overhead line
depends to a great extent upon the mechanical design
of the line.
While constructing an overhead line, it should be
ensured that mechanical strength of the line is such
so as to provide against the most probable weather
conditions.
In general, the main components of an overhead line
are:
5. (i) Conductors which carry electric power from the
sending end station to the receiving end station.
(ii) Supports which may be poles or towers and keep the
conductors at a suitable level above the ground.
(iii) Insulators which are attached to supports and
insulate the conductors from the ground.
(iv) Cross arms which provide support to the insulators.
(v) Miscellaneous items such as phase plates, danger
plates, lightning arrestors, anti-climbing wires etc.
The continuity of operation in the overhead line
depends upon the judicious choice of above
components. Therefore, it is profitable to have
detailed discussion on them.
6. Conductor Materials
The conductor is one of the important items as most of the
capital outlay is invested for it. Therefore, proper choice of
material and size of the conductor is of considerable importance.
The conductor material used for transmission and distribution of
electric power should have the following properties :
(i) high electrical conductivity.
(ii) high tensile strength in order to withstand mechanical
stresses.
(iii) low cost so that it can be used for long distances.
(iv) low specific gravity so that weight per unit volume is small.
All above requirements are not found in a single material.
Therefore, while selecting a conductor material for a particular
case, a compromise is made between the cost and the required
electrical and mechanical properties.
7. Commonly used conductor materials. The most commonly used
conductor materials for overhead lines are copper, aluminium,
steel-cored aluminium, galvanized steel and cadmium copper.
The choice of a particular material will depend upon the cost,
the required electrical and mechanical properties and the local
conditions.
All conductors used for overhead lines are preferably stranded
in order to increase the flexibility.
In stranded conductors, there is generally one central wire and
round this, successive layers of wires containing 6, 12, 18,
24 ...... wires.
Thus, if there are n layers, the total number of individual wires
is 3n(n + 1) + 1. In the manufacture of stranded conductors, the
consecutive layers of wires are twisted or spiraled in opposite
directions so that layers are bound together.
9. 1. Copper.
Copper is an ideal material for overhead lines owing to its high
electrical conductivity and greater tensile strength. It is always used in
the hard drawn form as stranded conductor.
Although hard drawing decreases the electrical conductivity slightly yet
it increases the tensile strength considerably.
Copper has high current density i.e., the current carrying capacity of
copper per unit of X-sectional area is quite large. This leads to two
advantages.
Firstly, smaller X-sectional area of conductor is required and secondly,
the area offered by the conductor to wind loads is reduced.
Moreover, this metal is quite homogeneous, durable and has high scrap
value. There is hardly any doubt that copper is an ideal material for
transmission and distribution of electric power.
However, due to its higher cost and non-availability, it is rarely used for
these purposes.
Now-a-days the trend is to use aluminium in place of copper.
10. 2. Aluminium.
Aluminium is cheap and light as compared to copper but it has much smaller conductivity and
tensile strength. The relative comparison of the two materials is briefed below :
(i) The conductivity of aluminium is 60% that of copper. The smaller conductivity of aluminium
means that for any particular transmission efficiency, the X-sectional area of conductor must
be larger in aluminium than in copper. For the same resistance, the diameter of aluminium
conductor is about 1·26 times the diameter of copper conductor. The increased X-section of
aluminium exposes a greater surface to wind pressure and, therefore, supporting towers must
be designed for greater transverse strength. This often requires the use of higher towers with
consequence of greater sag.
(ii) The specific gravity of aluminium (2·71 gm/cc) is lower than that of copper (8·9 gm/cc).
Therefore, an aluminium conductor has almost one-half the weight of equivalent copper
conductor. For this reason, the supporting structures for aluminium need not be made so
strong as that of copper conductor.
(iii) Aluminium conductor being light, is liable to greater swings and hence larger cross-arms are
required.
(iv) Due to lower tensile strength and higher co-efficient of linear expansion of aluminium, the sag
is greater in aluminium conductors.
Considering the combined properties of cost, conductivity, tensile strength, weight etc.,
aluminium has an edge over copper. Therefore, it is being widely used as a conductor
material. It is particularly profitable to use aluminium for heavy-current transmission where
the conductor size is large and its cost forms a major proportion of the total cost of complete
installation.
11. 3. Steel cored aluminium.
Due to low tensile strength, aluminium conductors produce
greater sag. This prohibits their use for larger spans and makes
them unsuitable for long distance transmission.
In order to increase the tensile strength, the aluminium conductor
is reinforced with a core of galvanised steel wires. The
*composite conductor thus obtained is known as steel cored
aluminium and is abbreviated as A.C.S.R. (aluminium conductor
steel reinforced).
Steel-cored aluminium conductor consists of central core of
galvanised steel wires surrounded by a number of aluminium
strands. Usually, diameter of both steel and aluminium wires is
the same. The X-section of the two metals are generally in the
ratio of 1 : 6 but can be modified to 1 : 4 in order to get more
tensile strength for the conductor. Fig. 8.1 shows steel cored
aluminium conductor having one steel wire surrounded by six
wires of aluminium.
12. The result of this composite conductor is that steel core takes greater
percentage of mechanical strength while aluminium strands carry
the bulk of current.
The steel cored aluminium conductors have the following
advantages :
(i) The reinforcement with steel increases the tensile strength but at the
same time keeps the composite conductor light. Therefore, steel
cored aluminium conductors will produce smaller sag and hence
longer spans can be used.
(ii) Due to smaller sag with steel cored aluminium conductors, towers
of smaller heights can be used.
13. 4. Galvanized steel.
Steel has very high tensile strength. Therefore, galvanised steel conductors can be
used for extremely long spans or for short line sections exposed to abnormally high
stresses due to climatic conditions.
They have been found very suitable in rural areas where cheapness is the main
consideration.
Due to poor conductivity and high resistance of steel, such conductors are not
suitable for transmitting large power over a long distance.
However, they can be used to advantage for transmitting a small power over a small
distance where the size of the copper conductor desirable from economic
considerations would be too small and thus unsuitable for use because of poor
mechanical strength.
5. Cadmium copper.
The conductor material now being employed in certain cases is copper alloyed with
cadmium. An addition of 1% or 2% cadmium to copper increases the tensile
strength by about 50% and the conductivity is only reduced by 15% below that of
pure copper.
Therefore, cadmium copper conductor can be useful for exceptionally long spans.
However, due to high cost of cadmium, such conductors will be economical only
for lines of small X-section i.e., where the cost of conductor material is
comparatively small compared with the cost of supports.
14. Types of Conductors
The symbols identifying different types of
Aluminium conductors are as follows:-
AAC : All Aluminium conductors.
AAAC : All Aluminium Alloy conductors
ACSR : Aluminium conductors, Steel-Reinforced
ACAR : Aluminium conductor, Alloy-Reinforced
16. Bundled Conductors
The combination of more than one conductor per phase in parallel
suitably spaced from each other used in overhead Transmission Line is
defined as conductor bundle. The individual conductor in a bundle is
defined as Sub-conductor.
At Extra High Voltage (EHV), i.e. voltage above 220 KV corona with its
resultant power loss and particularly its interference with communication
is excessive if the circuit has only one conductor per phase. The High-
Voltage Gradient at the conductor in the EHV range is reduced
considerably by having two or more conductors per phase in close
proximity compared with the spacing between conductor-bundle spaced
450 mm is used in India.
The three conductor bundle usually has the conductors at the vertices of
an equilateral triangle and four conductors bundle usually has its
conductors at the corners of a square.
The current will not divide exactly between the conductor of the bundle
unless there is a transposition of the conductors within the bundle, but
the difference is of no practical importance.
Reduced reactance is the other equally important advantage of bundling.
Increasing the number of conductor in a bundle reduces the effects
of corona and reduces the reactance. The reduction of reactance results
from the increased Geometric Mean Radius (GMR) of the bundle.
17. It has already been discussed that transmission of electric power is
done by 3-phase, 3wire overhead lines.
An a.c. transmission line has resistance, inductance and capacitance
uniformly distributed along its length. These are known as constants or
parameters of the line.
The performance of a transmission line depends to a considerable
extent upon these constants. For instance, these constants determine
whether the efficiency and voltage regulation of the line will be good
or poor.
Therefore, a sound concept of these constants is necessary in order to
make the electrical design of a transmission line a technical success.
So, we shall focus our attention on the methods of calculating these
constants for a given transmission line. Out of these three parameters
of a transmission line, we shall pay greatest attention to inductance and
capacitance. Resistance is certainly of equal importance but requires
less explanation since it is not a function of conductor arrangement.
22. Proximity Effect
Conductors carrying alternating current will
produce alternating flux in adjacent
conductors. This alternating flux will cause a
circulating current to start flowing in the
conductor, creating a non-uniform current
distribution in the transmission line, increasing
the conductor's apparent resistance. The
increased resistance along the transmission line
causes a voltage drop and power loss. This
phenomenon is called the proximity effect.
23. How Does the Proximity Effect Impact Transmission Lines?
The concentration of current through adjacent
conductors varies with the alternating magnetic
field and its associated eddy currents. When
conductors carry current in the same direction, the
currents flowing through them get concentrated at
the conductors' farthest side. In contrast, when
currents flowing through adjacent conductors flow
in opposite directions, the currents get
concentrated in the nearest side of both
conductors.
25. The proximity effect is due to varying magnetic
fields, making it an impossible phenomenon in
dc transmission. As dc frequency is zero, it fails
to produce an alternating magnetic field in
adjacent conductors. The current concentration
remains uniform in dc transmission lines, apart
from the influence of the skin effect.
26. Factors Influencing the Proximity Effect
Both transmission lines and nearby conductors carrying
alternating currents experience the proximity effect.
Several factors influence the proximity effect in transmission lines,
including:
The conductor’s material - High ferromagnetic materials
experience more proximity effects than non-ferromagnetic
materials.
The conductor’s diameter - As the conductor’s diameter
increases, the proximity effect also increases. The conductor’s
diameter is dependent on current, and when the system current is
high, the proximity effect becomes stronger.
Frequency - As the frequency increases, the proximity effect
becomes more intense.
The conductor’s structure - The proximity effect is higher in solid
conductors than in stranded conductors. The decreased surface
area of stranded conductors causes the proximity effect to be less
than in solid conductors, which have more surface area. However,
the internal proximity effect and external proximity effect exist in
stranded conductors such as ACSR.
27. How to Reduce the Proximity Effect
Several fixes can reduce the influence of the proximity effect,
which include:
• Reducing the size of the conductor - The proximity effect is
directly proportional to the surface area of the conductor.
Therefore, as the surface area increases, the proximity effect
becomes stronger. Replacing solid conductors with stranded
conductors helps reduce the conductor's surface area, decreasing
the proximity effect.
• Increasing the space between conductors - Dummy conductors
can help increase the space between conductors. However, this will
come at an added cost in support structures.
• Increasing voltage and reducing frequency - Transferring power
constantly through transmission lines increases voltage and
decreases current—the reduced size of conductors decreases the
proximity effect. Although not as practical, reducing the
transmission voltage and current frequency is another means of
reducing the proximity effect.
