Overhead Transmission Line Conductor Tutorial
Overhead Transmission Line Conductor Introduction
The major part of the capital cost is only due to conductors in transmission lines. So proper choice of material and size of the conductor are of considerable importance.
The overhead transmission line conductor material used for transmission should have the following properties
- Low cost
- Low resistivity
- High conductivity
- Low temperature coefficient
- High current carrying capacity
- High mechanical strength
- It must have weather resistance properties
- It must have elasticity.
The metals which partially fulfill the above requirements are copper, aluminium and steel. These are used alone or in combination. In this post we will discuss about various conductors pros and cons.
Overhead transmission line conductor Materials
Copper is used as transmission line conductor. But due to its high cost, it is being replaced by aluminium.
Characteristics of copper
- It has best conductivity than other metals.
- It has higher current density, hence for the given current rating lesser cross-sectional area of conductor is required.
- The metal is homogeneous.
- It has low specific resistance.
- It is a durable and has higher scrap value.
Aluminium is the most popularly used conductor material in the overhead lines.
Characteristics of aluminium:
- The conductivity of aluminium is 60% that of copper.
- For the same resistance the diameter of aluminium is 1.26 times the diameter of copper conductor.
- The specific gravity of aluminium is lower than that of copper. So aluminium conductor has one half the weight of equivalent copper conductor.
- Aluminium conductor being light, it may create greater swings and so larger cross arms are required.
- Due to lower tensile strength and higher co-efficient of linear expansion of aluminium, the sap is greater in aluminium conductor.
- Considering the above properties the cost, conductivity, tensile strength, weight etc. Aluminium occupies main place for over head conductor.
The various precautions and cares to be taken while erecting aluminium conductor for overhead lines are given below:
- A proper equipment must always be used for handling aluminium conductor.
- The aluminium conductor which is available on reels should be unloaded with a crane.
- The reel has to be rotated as the conductor is unwound
- Unwinding should not be done at high-speed.
- To avoid nicking of the conductor parallel jaw strips with suitable lines should be used while pulling the conductor.
- For joining and pulling the conductor around the angle pints always use free running sleeves with grooves.
- Join two stranded aluminium conductors and join each stand carefully with proper technique. After finishing the work it has to be checked for the grip.
- Aluminium conductor should not be pulled or allowed to slack too much.
- While tappings are to be taken from aluminium lines suitable alloys have to be used in order to avoid electrolytic corrosion. Weather proof may be employed at the tape.
- Aluminium must not be twisted for joining because it gives high resistance. Unijoint or compression joint must be used.
- To avoid damage to the strands, the conductor must be made with friction tape.
Steel has the greater tensile strength. But is least used for transmission of electrical energy as it has high resistance. Bare conductors are not used, since it deteriorates rapidly owing to rusting. so galvanised steel wires are used. It has following properties.
Properties of steel:
- It has lowest in conductivity.
- It has internal reactance.
- It is much subjected to eddy current and hysteresis loss.
- In a dump atmosphere it is restricted.
ACSR [Aluminium conductor with steel reinforced]
- It is an aluminium conductor having a central core galvanised steel wire.
- It is used for high voltage transmission system.
- By this the tensile strength of conductor is increased without losing the conductive property.
- The galvanised steel core is covered by one or more strands of aluminum wire, usually diameter of both steel and aluminium wire is same.
- The cross-section of two metals are generally i the ratio of 1:6 but can be modified to 1:4 in order to get more tensile strength for the conductor.
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