The Material which carry electric power from sending end to receiving end is called a conductor. The conductor is one of the most 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:
Properties of conductor material:
- High electrical conductivity.
- High tensile stength in order to withstand mechanical stress.
- Low cost
- Low specific gravity.
All the above reuirements 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.
Transmission line conductor sizing:
In transmission lines there are no tappings or consumers in the middle. So the current throughout the transmission line is same from the generating station to the load center. Hence the conductor is based on current carrying capacity or constant current density. This is up to 220kv transmission line only.
In case of the voltages beyond 220kv, the concept of corona is predominant tan that of current carrying capacity. So the transmission lines are designed based on concept of corona.
Transmission line Conductor Material:
The most commonly used conductor materials are copper, aluminium, steel-cored aluminium, galvanised steel and cadmium copper. The choice of particular material depend upon the cost, the required electrical and mechanical properties and the local conditions.
All conductors used for overhead lines are probably stranded in order to increase the flexibility. In the manufacture of stranded conductors, the consecutive layers of wires are twisted or spiralled in opposite directions so that layers are bound together.
Hard drawn copper conductor is the best conductor owing to its high electrical conductivity and high tensile strength and is ideally suitable for transmission ans distribution, but owing to the difficulty of importing it due to lack of foreign exchange in our country, the trend now-a-days is to use aluminium in place of copper.
Aluminium is cheaper in cost and lighter in weight but poor in conductivity and tensile strength . Usually aluminium conductors are used for transmission of heavy current, where conductor size is large and its cost forms a large proportion of the total cost of the complete installation.
Steel cored aluminium conductor ACSR consists of a core of galvanized steel strand surrounded by a number of aluminium strands. The steel core takes a larger percentage of mechanical stresses while the aluminium carries the bulk of the current. ACSR conductor being of high tensile strength and light weight produces small sag and therefore, long spans can be used.
Galvanized steel conductors have been used for extremely long spans. These are found most suitable in rural areas, where cheapness is the main consideration. The conductor is not suitable for extra high voltage transmission lines for the purpose of transmitting large power over a long distance because of poor conductivity and high internal reactance. It is generally used as stay wire, earth wire and structural components.
Types of conductor in transmission line:
Different types of conductors used in transmission lines are as follows:
1. Solid conductor:
- It has high tensile strength.
- High cost.
- Difficult to string the conductors.
- High skin effect while using on ac systems.
2. Stranded conductors in transmission lines:
It consists of two or more smaller cross sectional strands or filaments which are twisted together to get the required strength and running in parallel to increase the current carrying capacity for the given operating voltage.
Homogenous stranded conductors: It is made up of all the strands by one material only i.e copper. The advantages are:
- Reduced tensile strength due to strands when compared to solid conductor.
- Easy to string the conductors.
- Reduced skin effect when compared to solid conductor.
- But the cost of the conductor is not reduced to greater extent even though the strands are employed.
Aluminium homogenous stranded conductor:
- Almost equal conductivity when compared with copper.
- Less cost of aluminium.
- Low tensile strength so that the span length is less. The reduced span length will increase the number of towers which in turn increases the cost of Insulators and erection. So the cost of transmission will increases.
3. ACSR conductor:
In order to increase the tensile strength and reduce the cost of transmission system by increasing the span length composite stranded conductors are employed i.e. two or more conducting materials.
In the composite stranded conductors the outer strands are having high conductivity and low tensile strength( Aluminium) and central strands are having low conductivity and high tensile strength ( Steel) which is known as ACSR conductor.
ACSR conductor means Aluminium conductor and steel reinforced (or) Steel reinforced Aluminium conductors.
- Required tensile strength.
- Less cost of transmission.
- Easy stringing.
- Reduced skin effect when compared to solid or homogenous stranded conductor.