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Transmission and Distribution
10EE53
INTRODUCTION
Conductor sag and tension analysis is an important consideration in overhead
distribution line design as well as in overhead transmission line design. The quality
and continuity of electric service supplied over a line (regardless of whether it is a
distribution, a sub-transmission, or a transmission line) depend largely on whether
the conductors have been properly installed. Thus, the designing engineer must
determine in advance the amount of sag and tension to be given the wires or cables
of a particular line at a given temperature. In order to specify the tension to be used
in stringing the line conductors, the values of sag and tension for winter and summer
conditions must be known. Tension in the conductors contributes to the mechanical
load on structures at angles in the line and at dead ends. Excessive tension may
cause mechanical failure of the conductor itself.
The factors affecting the sag of a conductor strung between supports arc:
1. Conductor load per unit length.
2. Span, that is, distance between supports.
3. Temperature.
4. Conductor tension.
in order to determine the conductor load properly, the factors that need to be taken
into account are:
1. Weight of conductor itself.
2. Weight of ice or snow clinging to wire.
3. Wind blowing against wire.
The maximum effective weight of the conductor is the vector sum of the vertical
weight and the horizontal wind pressure. It is very important to include the most
adverse condition. The wind is considered to be blowing at right angles to the line
and to act against the projected area of the conductor, including the projected area of
ice or snow that may be clinging to it.
Economic design dictates that conductor sag should be minimum to refrain from
extra pole height, so provide sufficient clearance above ground level, and to avoid
providing excessive horizontal spacing between conductors to prevent them
swinging together in mid-span.
Conductor tension pulls the conductor up and decreases its sag. At the same time,
tension elongates the conductor, from elastic stretching, which tends to relieve
tension and increase sag. The elastic property of metallic wire is measured by its
Transmission and Distribution
10EE53
modulus of elasticity. The modulus of elasticity of a material equals the stress per unit of
area divided by the deformation per unit of length. That is, since
where σ=stress per unit area in pounds per square inches T =
conductor tension in pounds
A = actual metal cross section of conductor in square inches The
resultant elongation e of the conductor due to the tension is
Of course, if the modulus of elasticity is low, the elongation is high, and vice versa. Thus, a
small change in conductor length has a comparatively large effect on conductor sag and
tension. Sags and stresses in conductors are dependent on the initial tension put on them when
they are clamped in place and are due to the weight of the conductors themselves, to ice or
sleet clinging to them, and to wind pressure. The stress in the conductor is the parameter on
which everything else is based. But the stress itself is determined by the sag in the conductor
as it hangs between adjacent poles or towers. Since the stress depends on sag, any span can be
used provided the poles or towers are high enough and strong enough. The matter is merely
one of extending the catenary in both loading. Thus, the problem becomes the balancing of a
larger number of lighter and shorter poles or towers against a smaller number of heavier and
taller ones.
lines
Now let us discuss about the electric poles used in overhead transmission lines.The supporting
structures for overhead line conductors are various types of poles and towers called line
supports.In general, the line supports should have the following properties :
(i) High mechanical strength to withstand the weight of conductors and wind loads etc.
(ii) Light in weight without the loss of mechanical strength.
(iii) Cheap in cost and economical to maintain.
(iv) Longer life.
(v) Easy accessibility of conductors for maintenance.
The line supports used for transmission and distribution of electric power are of various
types including wooden poles, steel poles, R.C.C. poles and lattice steel towers.The choice of
Transmission and Distribution
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supporting structure for a particular case depends upon the line span, X-sectional area, line
voltage, cost and local conditions.
1. Wooden poles : These are made of seasoned wood (sal or chir) and are suitable for lines
of moderate X-sectional area and of relatively shorter spans, say upto 50 metres.Such supports
are cheap, easily available, provide insulating properties and, therefore, are widely used
for distribution purposes in rural areas as an economical proposition.The wooden poles generally
tend to rot below the ground level, causing foundation failure.In order to prevent this, the portion
of the pole below the ground level is impregnated with preservative compounds like creosote
oil.Double pole structures of the ‘A’ or ‘H’ type are often used to obtain a higher transverse
strength than could be economically provided by means of single poles.
The main objections to wooden supports are :
(i) tendency to rot below the ground level
(ii) comparatively smaller life (20-25 years)
(iii) cannot be used for voltages higher than 20 kV
(iv) less mechanical strength and
(v) require periodical inspection.
Transmission and Distribution
10EE53
2. Steel poles : The steel poles are often used as a substitute for wooden poles.They
possess greater mechanical strength, longer life and permit longer spans to be used.Such poles
are generally used for distribution purposes in the cities.This type of supports need to be
galvanised or painted in order to prolong its life.The steel poles are of three types (i) rail poles
(ii) tubular poles and (iii) rolled steel joints.
3. RCC poles : The reinforced concrete poles have become very popular as line supports
in recent years.They have greater mechanical strength, longer life and permit longer spans than
steel poles.Moreover, they give good outlook, require little maintenance and have good
insulating properties.Figure shows R.C.C. poles for single and double circuit.The holes in the
poles facilitate the
climbing of poles and at the same time reduce the weight of line supports.The main difficulty
with the use of these poles is the high cost of transport owing to their heavy weight.Therefore,
such poles are often manufactured at the site in order to avoid heavy cost of transportation.
4. Steel towers : In practice, wooden, steel and reinforced concrete poles are used for
Transmission and Distribution
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distribution purposes at low voltages, say upto 11 kV.However, for long distance transmission at
higher voltage, steel towers are invariably employed.Steel towers have greater mechanical
strength, longer life, can withstand most severe climatic conditions and permit the use of longer
spans. The risk of interrupted service due to broken or punctured insulation is
considerably reduced owing to longer spans.Tower footings are usually grounded by driving rods
into the earth.This minimises the lightning troubles as each tower acts as a lightning
conductor.Figure below shows a single circuit tower.However, at a moderate additional cost,
double circuit tower can be provided as shown in Figure below.The double circuit has the
advantage that it ensures continuity of supply.It case there is breakdown of one circuit, the
continuity
of
supply
can
be
maintained
by
the
other
circuit.