A conductor is one of the most important components of overhead lines. Selecting a proper type of conductor for overhead lines is as important as selecting economic conductor size and economic transmission voltage. A good conductor should have the following properties:
- high electrical conductivity
- high tensile strength in order to withstand mechanical stresses
- relatively lower cost without compromising much of other properties
- lower weight per unit volume
Conductor MaterialsCopper was the preferred material for overhead conductors in earlier days, but, aluminium has replaced copper because of the much lower cost and lighter weight of the aluminium conductor compared with a copper conductor of the same resistance. Following are some materials that are considered to be good conductors.
- Copper: Copper has a high conductivity and greater tensile strength. So, copper in hard drawn stranded form is a great option for overhead lines. Copper has a high current density which means more current carrying capacity per unit cross-sectional area. Therefore, copper conductors have relatively smaller cross-sectional area. Also, copper is durable and has high scrap value. However, due to its higher cost and non-availability, copper is rarely used for overhead power lines.
- Aluminium: Aluminium has about 60% of the conductivity of copper; that means, for the same resistance, the diameter of an aluminium conductor is about 1.26 times than that of a copper conductor. However, an aluminium conductor has almost half the weight of an equivalent copper conductor. Also, tensile strength of aluminium is less than that of copper. Considering combined factors of cost, conductivity, tensile strength, weight etc., aluminium has an edge over copper. Therefore, aluminium is being widely used for overhead conductors.
- Cadmium-copper: Cadmium-copper alloys contain approximately 98 to 99% of copper and up to 1.5% of cadmium. Addition of about 1% of cadmium to copper increases the tensile strength by up to 50% and the conductivity is reduced only by about 15%. Therefore, cadmium-copper conductors can be useful for exceptionally long spans. However, due to high cost of cadmium, such conductors may be uneconomical in many cases.
- Other materials: There are many other metals and alloys that conduct electricity. Silver is more conductive than copper, but due to its high cost, it is not practical in most of the cases. Galvanised steel may also be used as a conductor. Although steel has very high tensile strength, steel conductors are not suitable for transmitting power efficiently due to the poor conductivity and high resistance of steel. High strength alloys such as phosphor-bronze may also be used sometimes at extreme conditions.
Types of ConductorsAs it is already mentioned above, aluminium conductors have an edge over copper conductors considering combined factors of cost, conductivity, tensile strength, weight etc. Aluminium conductors have completely replaced copper conductors in overhead power lines because of their lower cost and lower weight. Though an aluminium conductor has larger diameter than that of a copper conductor of same resistance, this is actually an advantage when 'Corona' is taken into consideration. Corona reduces considerably with increase in the conductor diameter. Following are four common types of overhead conductors used for overhead transmission and distribution to carry generated power from generating stations to the end users.
Generally, all types of conductors are in stranded form in order to increase the flexibility. Solid wires, except for very small cross sectional area, are very difficult to handle and, also, they tend to crystallize at the point of support because of swinging in winds.
- AAC : All Aluminium Conductor
- AAAC : All Aluminium Alloy Conductor
- ACSR : Aluminium Conductor, Steel Reinforced
- ACAR : Aluminium Conductor, Alloy Reinforced