MECHANICAL DESIGN OF OVERHEAD LINES

Supports - Towers/Poles

(a) Types and Selection Criteria

Support towers/poles are major component of power transmission and distribution. They are
required to keep the conductors at safer height from the ground as well as at an adequate distance
from each other, provide reliable support for the conductors and resilient to storms. There are
two types of transmission and distribution line supports, they are towers and poles
Towers: Towers are generally made of galvanized steel. They are self-supporting because of
their broad base. They have high mechanical strength, longer life and permit long spans. In
selecting steel towers for transmission, the following factors must be considered.
 Sub –Transmission and distribution distance. ( should be fairly long)
 Sub –Transmission and distribution voltage. (voltage above 66KV is recommended)
 Cost. (Tower transmission line is very expensive.)

Figure 1: Steel tower electricity support

Poles: poles are used in Sub - Transmission and distribution system. Below 66kv. The poles
could be: wooden poles, Reinforce cement concrete (RCC) poles and Steel tubular poles.
 Wooden poles are made of chemically treated wood. They are used for distribution in
areas where good quality woods are available in plenty at a cheap cost e.g in rural areas.
Wood is biological; therefore it can decay with time. There is also environmental concern
about deforestation. Hence whenever possible, wooden poles should be avoided.
 Figure 2: wooden pole electricity support
 Reinforce cement concrete (RCC) poles are increasingly replacing the wooden poles in
sub – transmission and distribution because; they have greater mechanical strength,
longer life than wooden poles and require less maintenance.

Figure 2: wooden pole electricity support

 Steel tubular are use in some area. They need to be galvanized to increase their life. They
are used in distribution in cities.

(b) Surveying and Erection

 Select the route: The first step in constructing an overhead line is the selection of the
route through which the line is to be drawn. Preliminary survey of the area is necessary to
select one or more tentative route over which the line will pass. Some of the cardinal
principles are:
I. Select the shortest possible route
II. Follow highways and roads as much as possible.
III. Avoid crossing over hills, ridge swamps Avoid Crossing Hills, Ridges, Swamps, and
Bottom Lands.
IV. Route in the direction of possible future loads.
 Determine the right of way: Once the tentative route is selected, determine the right of
way, which is a legally granted space that may be lease or purchase. In selecting the right
of way, aesthetics value and environmental issues must be considered. In some cases,
 right of way cannot be obtained. The right of way must be cleared of trees or any
obstacle. After specifying the route, the second step is to make a plan and profile plot.
 Make the Plan: The plan shows the route the line will follow and the significant
topography adjacent to the route. The profile shows the ground elevation along the lines
and the top elevations of the towers and poles. These elevations are set in accordance
with the minimum allowable clearance.

 Clear the route: Practically all lines will cross through some brush or timberlands. A
line built in such terrain must have its route cleared before construction can be started. In
clearing the route, all stumps should be cut low. All logs and brush should be cleared
away for ten feet on either side of the pole line to make room for assembling and erecting
poles and stringing wires. All dead limbs and branches near this cleared pole line should
be cut down because a high wind may blow them into the line. Brush killing sprays may
be sprayed on the base of shrubs and small trees to a height of 12 to 15 inches above
ground.

 Locate the pole position: In locating poles, the following general principles should be
kept in mind: Select high places (avoid lowlands, swamps, etc.), Keep "spans" uniform
in length. ("Spans" are the distances between poles) this prevents the weight of the wire
on one side from pulling the pole over. Locate to give horizontal grade, so that shorter
poles can be used to maintain the proper ground clearance at the middle of the span.
Special attention should be given to the location of poles where the ground washes badly.
Poles should not be placed along the edges of cuts or embankments or along the banks of
creeks or streams. When it becomes necessary to set poles on the edge of a cut, the pole
should be set deep enough to protect the line in case the bank washes or crumbles away.
After the exact pole positions have been fixed, drive a stake to indicate the center of the
pole.

 Selecting the tower/Pole: This done putting into consideration the Voltage level,
durability and cost. In Nigeria, Concrete poles are now replacing wooden poles in power
distribution. Since sub-Transmission is at 33Kv, concrete poles are used.

 Digging the hole: The diameter of the hole is determined by the size of the large end of
the pole. The hole should be large enough to allow plenty of space on each side of the
butt of the pole for tamping the soil back into the hole. This requires at least 3 in. all
around the butt. The diameter of the hole should be fairly uniform from top to bottom,
how deep the hole should be is determined by the length of the pole and by the holding
power of the soil or earth.

 Erecting the Poles: The first step in raising a pole using the piking method is to lay the
butt end of the pole over the hole against a bump board or bar which rests on the bottom
of the hole and extends over the top, the board or bar protects the walls of the hole and
prevents them from being caved in by the butt of the pole as the pole is raised.
  Guying the poles: Guy wire are installed on a distribution line to counterbalance the pull
of the "dead-ended" distribution or to strengthen pole line installed on steep grade. There
are four steps in the installation of a guy: Digging in the anchor, Inserting the
insulators, Fastening the guy to the pole, Tightening the guy and fastening to the
anchor. Insulators must be installed in a guy whenever there is the possibility of live
wires falling and coming in contact with a guy. The insulator should be placed to insulate
that portion of the guy from ground. The two sections of guy are looped through the
separate parts of the insulator and the ends clamped to the guys.

Line Conductor/Cables

(a) Classification: Conductors are an important component of a transmission/distribution

system since power flow through them. It account for a major part of the total cost of the
entire system. Conductors should have the following properties:
i. Low Resistance.
ii. High tensile Strength in order to withstand Mechanical stresses.
iii. Low specific gravity so that weight/unit volume is small.
iv. Low cost so that it can be used for longer distance.
Power transmission and distribution conductors are usually stranded because if solid conductors
are used for longer span and large cross-sectional areas, continuous vibrations and swinging
would produce mechanical fatigue which will result in fracture at the point of support.
Conductors are usually made of materials such as copper, aluminum, steel etc. Sometimes, a
combination of material is needed to obtain certain desired qualities.
Conductors are usually classified according to the material they are made of. They include:
 Hard drawn Copper conductor: because of its very low resistance, copper is a very
good conductor for overhead transmission. It has a high current carrying capacity
per unit cross-sectional area and high current density. For these reason, it is
smaller in size. However, because it is very expensive and not readily available, it
is limited in use.

 Aluminium Conductor: Aluminium is cheaper than copper, its conductivity is

about 60% that of copper (needing about 1.26 larger diameter than copper for the
same resistance). Its specific gravity is lower and it has low weight. Because of
these qualities, Aluminium conductors are widely use in power transmission and
distribution. The most common form of Aluminium conductors are (1). All
Aluminium Conductors (AAC) (2). All Aluminium Alloy Conductors. (AAAC)
(3). Aluminiun Conductor Steel Reinforced (ACSR).
The biggest difference between AAC, AAAC, and ACSR conductors are the
materials they are constructed from. AAC is manufactured from electrolytically
refined aluminium with a 99.7% minimum purity, AAAC is made from an
Aluminium alloy, and ACSR contains a combination of Aluminium reinforced
with Steel. The second factor that differentiates the three cables is their resistance
to corrosion, which is important for the longevity of the cable. ACSR has a poorer
resistance to corrosion, as it contains steel, which is prone to rust. AAAC and
AAC have a better corrosion resistance, due to the fact that they are largely or
completely aluminium.

In an ACSR, the galvanized steel core carries the mechanical load and the high
purity aluminium carries the current. These utilize the lower thermal expansion
coefficient of steel compared to aluminium, which the aluminium based
conductors AAC and AAAC are unable to do.
The higher strength ACSR conductors are used for river crossings, overhead earth
wires, and installations involving extra-long spans. The advantage of ACSR is
that it has high tensile strength and is lightweight, which means over longer spans
it needs less supports. ACSR is available with varying percentages of steel core to
achieve different strengths. One of the advantages of this conductor in particular
is that the desired strength can be achieved without a loss of ampacity.

Fig 4: ACSR Conductor

AAC is used mainly in urban areas where the spacing is short and the supports are
closer together. The advantage of AAC conductors is that they have a high degree
of corrosion resistance; for this reason they are used extensively in coastal areas.
AAC Conductors were developed as a consequence of the galvanic corrosion that
ACSR conductors are susceptible to.
.
 Fig 5: AAC

AAAC is used as bare overhead conductor for power transmission and

distribution lines on aerial circuits that require larger mechanical resistance than
AAC. AAAC also has better sag characteristics and a better strength to weight
ratio than AAC. AAAC Cables have lower weight per unit length and slightly
lower resistance per unit length than ACSR.

Fig 6: AAAC

(a) Selection criteria: In selecting conductors and cable, certain modalities must be used
in deciding. They include: the load or current that will flow (Ampacity), Cost,
temperature and the nature of the conducting material.
 Load current (Ampacity): The current capacity (ampacity) of a conductor is
necessary to determine the size or cross-sectional area of the conductors to be use
in transmission and distribution.
For solid conductors. Its size is easy to determine by using a simple approach:

Line Loss (PL) = Total % Loss x Rated line Power…………..………………. (1)

Line Current (I) = Rated line power/√3 x Line Voltage…………...………… (2)
Line Resistance (RDC) = PL / (3 x I2)…………………………………..…….. (3)
 Actual line Resistance (RAC) = 1.6 x (RDC) ……. …………………………... (4)
Conductor cross-sectional Area = ρL/RAC………………………………….. (5)
Where ρ = resistivity of Conductor material.
Conductor diameter size (d) = 4A/π…………..…………………………….. (6)

For stranded conductors, dT = diameter of cable.

Let N = number of conductors [strands], d = Diameter of strand and X = Number
of layers.
The relation between N and X is given by:
N= 3x2-3x+1…………………………………………………………….……. (7)
If N is given we can used the above relation to get X by solving the quadratic
equation and taking the positive root.
The total Diameter of cable is obtained from dT= (2x -1)……………….…... (8)

 Cost of conductor: While larger conductors may lose less energy due to lower
electrical resistance, they are more costly than smaller conductors. An
optimization rule called Kelvin's Law states that the optimum size of conductor
for a line is found when the cost of the energy wasted in the conductor is equal to
the annual interest paid on that portion of the line construction cost due to the size
of the conductors. The optimization problem is made more complex by additional
factors such as varying annual load, varying cost of installation, and the discrete
sizes of cable that are commonly made.
 Temperature: Since the temperature of the conductor increases with increasing
heat produced by the current through it, it is sometimes possible to increase the
power handling capacity (uprate) by changing the conductors for a type with a
lower coefficient of thermal expansion or a higher allowable operating
temperature.
One such conductor that offers reduced thermal sag is known as aluminum
conductor composite core (ACCC). In lieu of steel core strands that are often used
to increase overall conductor strength, the ACCC conductor uses a carbon and
glass fiber core that offers a coefficient of thermal expansion about 1/10 of that of
steel. While the composite core is nonconductive, it is substantially lighter and
stronger than steel, which allows the incorporation of 28% more aluminum (using
compact trapezoidal shaped strands) without any diameter or weight penalty. The
added aluminum content helps reduce line losses by 25 to 40% compared to other
conductors of the same diameter and weight, depending upon electrical current.
The ACCC conductor's reduced thermal sag allows it to carry up to twice the
current ("ampacity") compared to all-aluminum conductor (AAC) or ACSR.
(c) Conductor stringing, Jointing/binding, Sagging and Tensioning, Clipping an
Jumpering.
 Conductor stringing: this is the process of running the line from one pole or
tower to another. There are various methods of string they include:
 Figure: 6 conductor stringing

 Jointing/binding: Line joints can be divided in three classes: 1) Splices. 2)

Sleeve joints. 3) Compression joints. Small-sized copper wires can be spliced, but
the larger sizes of copper wire are usually joined by means of splicing sleeves or
compression joints.

 Sagging and Tensioning: The line conductors expand in hot weather and contract
in cold weather, so there should be some slack, or sag, between poles. The
conductors should be sagged in accordance with the sag chart applying to the
particular conductor used, the length of the span and the temperature prevailing.
The sag should be adjusted in the middle span in short sections of line of live
spans or less and at two or more spans in longer sections. Sagging is done just
prior to tying the line conductors to the individual insulators or insulator bracket.
Conductors can be sagged correctly only when the tension is the same in each
span throughout the entire length. A simple and accurate method of measuring the
sag is by the use of targets placed on the poles below the insulators.

An approach in calculating sagging in power line can be obtained from figure 7

Figure 7: sagging in transmission line.