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Overhead Lines
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What Is Sag,Tension ? Calculations,Factors In Overhead Transmission Lines

Sag & Tension  in transmission line

Today we are going to learn a topic called sag and tension in the transmission lines.During the erection of overhead transmission line, the conductors are connected between two tower structures. The conductors connected between the tower structures must not be connected too tight or too loose. If they are connected very tightly, the tension on the conductor will be very high and at some point it may break. If they are connected very loose, the charging current increases the length of wire to be used and the height of the tower structure increases. Hence, the conductors must be connected in such a way that the tension is minimum and at the same time there should be good clearance between the ground and the conductor.
Sag & Tension In Electrical Transmission Lines

Definition of Sag In Transmission Lines

Sag can be defined as "The difference in level between the points of supports and the lowest point on the conductor is known as sag"
Hence ,sag determines the value of safe working tension and the minimum clearance of the conductor with respect to ground.The conductor sag should be kept to a minimum in order to reduce the conductor material required and to avoid extra pole height for sufficient clearance above ground level.

Factors Affecting Sag

The various factors which effects sag are,
(i) Weight of conductor.
(ii) Location of conductor.
(iii) Length of span.
(iv) Temperature.
(v) Tensile strength.
(Vi) Tension.
Here we discuss briefly about various factors sag & tension in electrical transmission lines .

(i) Weight of conductor
The sag of an overhead line is directly proportional to the weight of the conductor.This is because the weight of any body acts vertically downwards.i.e.,more the weight of the conductor more the force acting vertically downwards and hence greater is the sag value in transmission lines.
(ii) Location of conductor
Sag also depends on the location of conductors.If the conductors are present in area where ice formation takes place,then due the accumulation of ice on the conductor its overall weight increases.This increases the weight of the conductors which in turn increases the value of sag.
(iii) Length of span
Sag is proportional to the square of length of span.Hence, longer the span greater will be the sag provided the tension and weight of the conductor is constant.
(iv) Temperature
The value of sag greatly affected by the temperature.If the temperature is high sag will be more because rise in temperature causes the conductors to expand.Is the temperature is low, the conductor(being metallic) contracts and hence sag is less due to which the tension in the conductor is increases.
(v) Tensile strength
Sag inversely proportional to the tensile strength of the conductor provided the other parameters are constant.
(Vi) Tension
Tension on the conductor is inversely proportional to sag.If the tension is more the conductors are connected very tightly between the tower structure and hence sag is less.On the other hand is tension is less the conductors are connected loosely hands sag is more.

Calculation of Sag in Overhead transmission lines:

(i) When supports are at equal levels

Let us consider a line conductor between two equal height line supports.Line supports are A and B with O as the lowest point as shown in figure.Point O will be the lowest point as two levels are equal lowest point will be at the mid-span.
Sag & Tension In Electrical Transmission Lines

Let

l = Length of span
w = Weight per unit length of conductor
T = Tension in the conductor.

Now consider any point on the conductor.Lets say point 'P'.By considering lowest point O as the origin, let the co-ordinates of point P be x and y. Assuming that the curvature is so small that curved length is equal to its horizontal projection (i.e., OP = x), the two forces acting on the portion OP of the conductor are :
(i) The weight wx of conductor acting at a distance x/2 from O.
(ii) The tension T acting at O.
Equating the moments of above two forces about point O, we get,

T y = w.x * x/2

or 

y=Tx²/2T

The maximum dip (sag) is represented by the value of y at either of the supports A and B. At support A, x = l/2 and y = S

Sag, S=w(l/2)²/2T

        S=wl²/8T

(iiWhen supports are at unequal levels

The difference in level between points of supports and the lowest point on the conductor is called "sag".When transmission lines run on steep inclines as in the case of hilly areas, we generally come across conductors suspended between supports at unequal levels.The shape of the conductor between the supports may be assumed to be a part of the parabola. In this case, the lowest point of the conductor will not lie in the middle of the span.



Consider a conductor suspended between two supports A and B which are at different levels as shown in the following figure.

Sag & Tension In Electrical Transmission Lines

Let
l = Span length
h = Difference in levels between two supports
x1 = Distance of support at lower level (i.e., A) from O
x2 = Distance of support at higher level (i.e. B) from O
T = Tension in the conductor.

If w is the weight per unit length of the conductor, then,

Sag S= wx1²/2T

and Sag S= wx2²/2T

Also X1 + X2 = l        .........(i)
Sag & Tension In Electrical Transmission Lines
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X1=l/2-Th/wl
X2=l/2+Th/wl
Having found X1 and X2, values of S1 and S2 can be easily calculated.

Effect of wind and ice loading on sag & tension

Till now the expression for sag and tension derived was under normal conditions i.e.. at normal temperature and the weight acting on the conductor was only its own weight. But in cold places there is a ice coating formed on the conductor and also wind pressure nets horizontally on the line conductor. The ice coating on the line conductor increases the total diameter of the conductor and also the weight of the Conductor increases.

The total weight of the conductor i.e., both the conductor weight and the weight of the ice acts vertically downwards whereas the wind force acts horizontally on the conductor. Therefore, the vector sum of horizontal and vertical forces acting on the conductor gives the total force shown in figure.
Sag & Tension In Electrical Transmission Lines
Sag & Tension In Electrical Transmission Lines
If image is not clear on mobile device please download first and view
When the conductor has wind and ice loading also, the following points may be noted :
(i) The conductor sets itself in a plane at an angle θ to the vertical where
tan θ =Ww/W+Wi
(ii) The sag in the conductor is given by :
S =Wtl²/2T
Hence S represents the slant sag in a direction making an angle θ to the vertical. If no specific mention is made in the problem, then slant slag is calculated by using the above formula.
(iii) The vertical sag = S cos θ.

This is the complete info on sag in transmission line wiki.You can download sag and tension calculation overhead transmission lines as pdf.Share with your friends.

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A Comparison of Overhead Line System,Underground Cable System

 Overhead Line System vs Underground Cable System

Here in this article we are going to discuss about comparison between overhead lines and underground cables.this also refers to the advantages and disadvantages of overhead power lines and underground cables.In some applications Overhead Lines are useful in some applications Underground Cables are required,hence differences between Overhead Line System and Underground Cable System given below.




Overhead System Underground System
1.The size of conductor for same amount of power
is small
1.The size of conductor is quite large in
underground system.
2.The amount of insulation is less as overhead
lines are open to atmosphere and hence air provides
the necessary insulation.
2.Very high degree! of insulation is required
as the underground system is laid under the
ground hence area is very compact.
3.Heat can be dissipated easily in the surroundings
as overhead
lines are open to atmosphere.
3.Heat dissipation is very difficult and. hence
number of insulating layers are added to the cable.
4.Overhead system is very cheap as no insulation
coating is used over the conductors i.e., the conductors
used are bare
conductors
4.Very costly, because a number of insulation
layers has to be used to provide sufficient insulation.
5.Faults can be detected easily 5.Fault detection is very complicated.
6.Maintenance work is very simple. 6.Maintenance work is very complex.
7. It is used for long distance transmission. 7.It is used for short distance transmission
or distribution.
8.Public safety is less. 8.Public safety is more.
9.Faces problems due to interference with neighbouring
communication system.
9.No interference with the communication lines.
10.They are liable to hazards from lightning discharges. 10.Not liable to the hazards from lightning
discharges.
11.This system can’t be used near submarine crossings. 11.It can be used near submarine crossings.
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Line Supports | Types of Electrical Poles-Over Head Transmission Lines

We know most of the electrical power is distributing by overhead transmission lines.we have electrical poles to transmit and distribute different range of voltages by different types of electrical poles.

Types of Line Supports or Electrical Poles 

Different types of poles or towers are used to support overhead lines structures which are known as line supports.

A line support should possess the following requirements.

(i) It should have high mechanical strength to bear the conductor weight,wind load etc.
(ii) Maintenance should be easy and cost should be very low.
(iii) It Should be very light in weight and extremely durable.
(iv) Proper spacing between the conductors and ground clearance must be maintained.

The different line supports used by transmission lines include,

(a) Wooden poles
(b) Reinforced concrete poles
(c) Steel poles
(d) Latticed steel towers.

The choice of support depends upon the cost, span of line, atmospheric conditions and line voltage.

(a) Wooden poles-Type of Electrical Poles

When compared to different types of line supports wooden poles are cheap and are useful for lines of shorter spans about 50 meters,best suited for rural areas.They can withstand voltage up to 132 kV When,single or  H type poles ,are used whereas double pole structures of ’A or H type provide higher transverse strength and are usually used for terminal poles.

Type of Electrical Poles-Over Head Transmission Lines


These supports are used because they are of low cost, available without difficulty, have a natural insulating property due to which lesser flashovers occur for lightning. To protect it hum decay a metallic cap is provided at the top.

The reasons which restricts the use of wooden poles are,
(i)Diameter and height of wooden poles.
(ii) Short span and less durability (20-25 years).
(iii) Tendency not below ground level.
(iv) Strength and durability can‘t be predicted with certainty.
(v) Problems due to insects and birds and require.

(b) Reinforced concrete poles(RCC Poles)-Type of Electrical Poles

These poles are widely used because of their longer durability and are used for system 33 kV. They have greater mechanical strength, used for longer spans, longer- life and maintaining cost is very low. They also have good insulating properties am appearance. RCC pole structure is shown in figure.

Type of Electrical Poles-Over Head Transmission Lines


The drawbacks of using RCC poles is they are very heavy in weight and there are chances of damage during transportation,loading and unloading and transportation cost is high.These difficulties are overcome by. using prestressed concrete poles which are manufactured at job sites themselves and have very less weight as less material is used. Holes provided in the poles reduce their weight and make climbing the poles easier.

(c) Steel Poles-Type of Electrical Poles

These poles are used for distribution purposes along road sides and developed areas. These are favoured for voltages up to 33 kV (low and medium voltages) when compared to Wooden and RCC poles, they have high mechanical strength, longer life of at least 30 years, possesses light weight but are costlier.They need to be galvanized or painted periodically to prolong its life and prevent them from corrosion and hence maintenance cost is high. They are often used as substitute for wooden poles.

(d) Latticed Steel Tower-Type of Electrical Poles

Where long span of lines are very essential,steel towers are used for long distance transmission at voltages above 33 kV,Owing to longer span, insulation cost and risk of interrupted service is reduced to a large extent. Lightning troubles and breakdowns are reduced considerably as the tower footing are grounded into the earth. These towers are made of‘ steel, have longer-life, high mechanical strength and ability to withstand severe climatic conditions.

Type of Electrical Poles-Over Head Transmission Lines

These are of two types,
(i) Self-supporting towers.
(ii)Stayed or guyed towers.

(i) Self-supporting Towers
These are classified into,
(a) Narrow base lattice type
(b) Broad-base towers.

Lattice type structures are one of the most common types of self—supporting towers. The narrow—base type employs lattice construction of channel or angle with welded or bolted connections. These are used for shorter spans. (In lattice a broad—base type only lattice construction with bolted connections are used. Each leg has a separate foundation. These are used when the span is very large. Figures  shows broad and narrow—base circuit towers.
Supporting towers are also dived as,
-Tangent towers which are employed for straight lines.
-Deviation towers are employed if the transmission line changes direction.

(ii)Stayed or guyed towers.
As the self-supporting structures are mostly used in India, guyed towers are preferred in USA and Canada: Where helicopters are used for transportation of these towers at the sites which are not accessible by road or rail. Less expensive, lighter, aluminium guyed towers are favoured in areas facing transportation problems. Span between the towers is reduced, which increases reliability and reduces cost. These are of two types portal and V-type as shown in figures.
Type of Electrical Poles-Over Head Transmission Lines

Both types have two masts at the top are supported by cross-arms and have four guys.In portal type,each mast lies down on it's own foundation,where as for V-type,the two mast rest on one thrust facing each other.In portal type the four guys are attached to 2-double guys and in V-type separate guys in attached.These are supported by hinged foundation.

Tags:line supports in transmission,types of poles used in transmission line pdf,types of electrical poles wikipedia.
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String Efficiency & It's Significance In Overhead Transmission Lines

String Efficiency & It's Significance In Overhead Transmission Lines 

Efficiency is a factor that defines the performance of any device or equipment. Generally, it is specified as the ratio of the output obtained to the input fed to a device. It suggests to what extent a device is utilized. 



In an overhead transmission line (OHTL) insulator, a number of insulator discs are arranged in a string in accordance with the operating voltage. The lowest disc or unit which is i near the power conductor experiences maximum stress or it is fully utilized. But, as we move towards the top most unit, the units are not heavily stressed and hence not fully utilized. Hence, the measure of utilization of the string gives string efficiency. Definition The string efficiency of an over head transmission line insulator is defined as the ratio of the voltage across the complete string to that of the product the number of units and the voltage across the unit nearest to the power conductor. 
It is given as, 

η=Voltage across the string/{ n x Voltage across the unit nearest to the conductor}

n= No.of units in the string.

 It can also be defined as the ratio of the spark over voltage of the complete string to that of the product of the number of discs and the spark over voltage of one disc. 

Significance of String efficiency 

String efficiency is an important factor as it suggests the overall potential distribution along the string of insulators. For obtaining uniform voltage distribution, the string efficiency should be as good as possible. if the distribution of voltage is uniform, then each disc will be fully and equally utilized. This also implies that, the discs in the string will experience equal stress so that the life of the string is enhanced.

In ideal case, the string efficiency is maximum and the voltage distribution across each disc will be exactly the same. But in real time situation, it is impossible to achieve 100% string efficiency. Efforts' can be made to improve string efficiency. If the value of 'V (i.e., ratio of mutual capacitance to self capacitance) is decreased then the potential distribution is more uniform and string efficiency is increased. The non-uniformity in voltage distribution increases with the increase in number of discs in the string. Therefore, shorter string has more efficiency than the larger one.

Tags:string efficiency in power system,string efficiency formula,string efficiency of suspension insulators,string efficiency in power system pdf,string efficiency transmission line improving methods
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