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.
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 the conductor.
(iii) Length of the span.
(v) Tensile strength.
Here we discuss briefly 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 anybody 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 the area where ice formation takes place, then due to 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 the length of the span.Hence, longer the span greater will be the sag provided the tension and weight of the conductor are constant.
The value of sag greatly affected by the temperature.If the temperature is high sag will be more because the 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 increased.
(v) Tensile strength
Sag inversely proportional to the tensile strength of the conductor provided the other parameters are constant.
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 the figure. Point O will be the lowest point as two levels are equal lowest point will be at the mid-span.
l = Length of span
w = Weight per unit length of conductor
T = Tension in the conductor.
Now consider any point on the conductor. Let’s say point ‘P’.By considering lowest point O as the origin, let the coordinates 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
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
(ii) When 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.