Types of Conductors used in Overhead Power Lines

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 Materials

Copper 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 Conductors

As 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.

1. AAC : All Aluminium Conductor
2. AAAC : All Aluminium Alloy Conductor
3. ACSR : Aluminium Conductor, Steel Reinforced
4. ACAR : Aluminium Conductor, Alloy Reinforced

AAC : All Aluminium Conductor

This type is sometimes also referred as ASC (Aluminium Stranded Conductor). It is made up of strands of EC grade or Electrical Conductor grade aluminium. AAC conductor has conductivity about 61% IACS (International Annealed Copper Standard). Despite having a good conductivity, because of its relatively poor strength, AAC has limited use in transmission and rural distribution lines. However, AAC can be seen in urban areas for distribution where spans are usually short but higher conductivity is required.

AAAC : All Aluminium Alloy Conductor

These conductors are made from aluminium alloy 6201 which is a high strength Aluminium-Magnesium-Silicon alloy. This alloy conductor offers good electrical conductivity (about 52.5% IACS) with better mechanical strength. Because of AAAC's lighter weight as compared to ACSR of equal strength and current capacity, AAAC may be used for distribution purposes. However, it is not usually preferred for transmission. Also, AAAC conductors can be employed in coastal areas because of their excellent corrosion resistance.

ACSR : Aluminium Conductor, Steel Reinforced

ACSR consists of a solid or stranded steel core with one or more layers of high purity aluminium (aluminium 1350) wires wrapped in spiral. The core wires may be zinc coated (galvanized) steel or aluminium coated (aluminized) steel. Galvanization or aluminization coatings are thin and are applied to protect the steel from corrosion. The central steel core provides additional mechanical strength and, hence, sag is significantly less than all other aluminium conductors. ACSR conductors are available in a wide range of steel content - from 6% to 40%. ACSR with higher steel content is selected where higher mechanical strength is required, such as river crossing. ASCR conductors are very widely used for all transmission and distribution purposes.

Aluminium Conductor, Alloy Reinforced

ACAR conductor is formed by wrapping strands of high purity aluminium (aluminium 1350) on high strength Aluminum-Magnesium-Silicon alloy (6201 aluminium alloy) core. ACAR has better electrical as well as mechanical properties than equivalent ACSR conductors. ACAR conductors may be used in overhead transmission as well as distribution lines.

Bundled Conductors

Transmission at extra high voltages (say above 220 kV) poses some problems such as significant corona loss and excessive interference with nearby communication lines when only one conductor per phase is used. This is because, at EHV level, the electric field gradient at the surface of a single conductor is high enough to ionize the surrounding air which causes corona loss and interference problems. The electric field gradient can be reduced significantly by employing two or more conductors per phase in close proximity. Two or more conductors per phase are connected at intervals by spacers and are called as bundled conductors. The image at right shows two conductors in bundled form per phase. Number of conductors in a bundled conductor is greater for higher voltages.