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Wednesday, 8 January 2020

Conductors: Introduction, Materials, Types - Copper, Aluminium, Steel Core Aluminium, Galvanized Steel, Cadmium Copper


CONDUCTORS


  • Materials or substances which allow electricity to flow through them are called conductors.
  •  They allow electrons to flow inside them very easily because they are able to conduct electricity.
  • Conductors have this property of allowing the transition of heat or light from one source to another.

Commonly used conductor materials:

  • Copper, aluminium, steel cored aluminium, galvanised steel and cadmium copper are the most commonly used conductor materials for over head lines.
  • The choice of a particular material will depend upon the cost, the required electrical and mechanical properties and the local conditions. 
  • The conductors used for overhead lines are preferably stranded in order to increase the flexibility. In stranded conductors, there is generally one central wire and round this, successive layers of wires containing 6, 12, 18, 24 ...... wires. 


TYPES OF CONDUCTOR

Copper

Copper Conductor
Copper Conductor

  • Because of its high electrical conductivity and greater tensile strength copper is an ideal material for overhead lines.
  • It is always used in the hard drawn form as stranded conductor. 
  • It has high current density because of this the current carrying capacity of copper per unit of X sectional area is quite large. This leads to two advantages. 
  • Firstly, smaller X-sectional area of conductor is required and secondly, the area offered by the conductor to wind loads is reduced. 
  • Moreover, this metal is quite homogeneous, durable and has high scrap value. However, due to its higher cost and non-availability, it is rarely used for these purposes.

Aluminium

  • It is cheap and light as compared to copper but it has much smaller conductivity and tensile strength. 
  • The conductivity of aluminium is 60% that of copper. The smaller conductivity of aluminium means that for any particular transmission efficiency, the X-sectional area of conductor must be larger in aluminium than in copper.
  • It's specific gravity of aluminium (2·71 gm/cc) is lower than that of copper (8·9 gm/cc).
  • The supporting structures for aluminium need not be made so strong as that of copper conductor.
  • It is liable to greater swings  because aluminium conductors are light, and hence larger cross-arms are required.
  • The sag is greater in aluminium conductors due to lower tensile strength and higher co-efficient of linear expansion of aluminium. Considering the properties of cost, conductivity, tensile strength, weight etc., aluminium has an edge over copper. Hence, it is being widely used as a conductor material.

Steel cored Aluminium

  • Aluminium conductors produce greater sag due to low tensile strength. 
  • This prohibits their use for larger spans and makes them unsuitable for long distance transmission.
  • The aluminium conductor is reinforced with a core of galvanised steel wires in order to increase the tensile strength.
  • The composite conductor thus obtained is known as steel cored aluminium and is abbreviated as A.C.S.R. (aluminium conductor steel reinforced).

ACSR Conductor
ACSR (Aluminium Conductor Steel Reinforced) Conductor

  • Steel-cored aluminium conductor consists of central core of galvanized steel wires surrounded by a number of aluminium strands. 
  • The diameter of both steel and aluminium wires is the same. The X-section of the two metals can be modified to 1 : 4 in order to get more tensile strength for the conductor.  
  • Because of this composite conductor steel core takes greater percentage of mechanical strength while aluminium strands carry the bulk of current. The steel cored aluminium conductors have the following advantages
Advantages:

 (i) The reinforcement with steel increases the tensile strength but at the same time keeps the composite conductor light. Therefore longer spans can be used because steel cored aluminium conductors will produce smaller sag. 

(ii) Towers of smaller heights can be used due to smaller sag with steel cored aluminium conductors.


Galvanised steel

  • Galvanised steel has very high tensile strength. Hence galvanised steel conductors can be used for extremely long spans or for short line sections exposed to abnormally high stresses due to climatic conditions.
  • Where cheapness is the main consideration in that areas the galvanised steel conductors can be used.  
  • Such conductors are not suitable for transmitting large power over a long distance due to poor conductivity and high resistance of steel
  • However, they can be used to advantage for transmitting a small power over a small distance because of poor mechanical strength.

Cadmium copper

  • The conductor material now being employed in certain cases is copper alloyed with cadmium. An addition of 1% or 2% cadmium to copper increases the tensile strength by about 50% and the conductivity is only reduced by 15% below that of pure copper. 
  • So it can be useful for exceptionally long spans. 
  • Such conductors will be economical only for lines of small X-section due to high cost of cadmiume. The cost of conductor material is comparatively small compared with the cost of supports.

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