31-07-2012, 01:09 PM
Mechanical Design of Overhead Lines
[attachment=29686]
Introduction
Firstly, power is generally transmitted over long
distances to load centres. Obviously, the installation
costs for underground transmission will be very heavy.
Secondly, electric power has to be transmitted at
high voltages for economic reasons. It is very
difficult to provide proper insulation†to the
cables to withstand such higher pressures. There-
fore, as a rule, power trasmission over long dis-
tances is carried out by using overhead lines. With
the growth in power demand and consequent rise
in voltage levels, power transmission by over-
head lines has assumed considerable importance.
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.
Insulators
The overhead line conductors should be supported on the poles or towers in such a way that currents
from conductors do not flow to earth through supports i.e., line conductors must be properly insulated
from supports. This is achieved by securing line conductors to supports with the help of insulators.
The insulators provide necessary insulation between line conductors and supports and thus prevent
any leakage current from conductors to earth. In general, the insulators should have the following
desirable properties.
Potential Distribution over Suspension Insulator String
A string of suspension insulators consists of a number of porcelain discs connected in series through
metallic links. Fig.
(i) shows 3-disc string of suspension insulators. The porcelain portion of each disc is inbetween two metal links. Therefore, each disc forms a capacitor C as shown in Fig.
(ii). This is known as mutual capacitance or self-capacitance. If there were mutual capacitance
alone, then charging current would have been the same through all the discs and consequently voltage
across each unit would have been the same i.e., V/3 as shown in Fig. (ii). However, in actual
practice, capacitance also exists between metal fitting of each disc and tower or earth. This is known
as shunt capacitance C1. Due to shunt capacitance, charging current is not the same through all the
discs of the string [See Fig. 8.10 (iii)]. Therefore, voltage across each disc will be different. Obvi-
ously, the disc nearest to the line conductor will have the maximum* voltage. Thus referring to Fig.
(iii), V3 will be much more than V2 or V1.
[attachment=29686]
Introduction
Firstly, power is generally transmitted over long
distances to load centres. Obviously, the installation
costs for underground transmission will be very heavy.
Secondly, electric power has to be transmitted at
high voltages for economic reasons. It is very
difficult to provide proper insulation†to the
cables to withstand such higher pressures. There-
fore, as a rule, power trasmission over long dis-
tances is carried out by using overhead lines. With
the growth in power demand and consequent rise
in voltage levels, power transmission by over-
head lines has assumed considerable importance.
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.
Insulators
The overhead line conductors should be supported on the poles or towers in such a way that currents
from conductors do not flow to earth through supports i.e., line conductors must be properly insulated
from supports. This is achieved by securing line conductors to supports with the help of insulators.
The insulators provide necessary insulation between line conductors and supports and thus prevent
any leakage current from conductors to earth. In general, the insulators should have the following
desirable properties.
Potential Distribution over Suspension Insulator String
A string of suspension insulators consists of a number of porcelain discs connected in series through
metallic links. Fig.
(i) shows 3-disc string of suspension insulators. The porcelain portion of each disc is inbetween two metal links. Therefore, each disc forms a capacitor C as shown in Fig.
(ii). This is known as mutual capacitance or self-capacitance. If there were mutual capacitance
alone, then charging current would have been the same through all the discs and consequently voltage
across each unit would have been the same i.e., V/3 as shown in Fig. (ii). However, in actual
practice, capacitance also exists between metal fitting of each disc and tower or earth. This is known
as shunt capacitance C1. Due to shunt capacitance, charging current is not the same through all the
discs of the string [See Fig. 8.10 (iii)]. Therefore, voltage across each disc will be different. Obvi-
ously, the disc nearest to the line conductor will have the maximum* voltage. Thus referring to Fig.
(iii), V3 will be much more than V2 or V1.