What are the types of Conductors used in Overhead Power Lines…

Conductor is a physical medium to carry electrical energy form one place to other. It is an important component of overhead and underground electrical transmission and distribution systems. The choice of conductor depends on the cost and efficiency. An ideal conductor has following features.

It has maximum electrical conductivity
It has high tensile strength so that it can withstand mechanical stresses
It has least specific gravity i.e. weight / unit volume
It has least cost without sacrificing other factors

Types of Overhead Conductor

In early days copper ‘Cu’ conductors was used for transmitting energy in stranded hard drawn form to increase tensile strength. But now it has been replaced by aluminum ‘Al’ due to following reasons:

It has lesser cost than copper.
It offers larger diameter for same amount of current which reduces corona.

Corona: is ionization of air due to higher voltage (usually voltage above critical voltage) which causes violet light around the conductor and hissing sound. It also produces ozone gas therefore it is undesirable condition.

Aluminium also has some disadvantages over copper i.e.

It has lesser conductivity
It has larger diameter which increase surface area to air pressure thus it swings more in air than copper so larger cross arms required which increases the cost.
It has lesser tensile strength ultimately larger sag
It has lesser specific gravity (2.71gm/cc) than copper (8.9 gm/cc) cc = cubic centimeter
Due to lower tensile strength aluminium is used with some other materials or its alloys

AAC (All Aluminum Conductor)

It has lesser strength and more sag per span length than any other category
Therefore, it is used for lesser span i.e. it is applicable at distribution level
It has slightly better conductivity at lower voltages than ACSR i.e. at distribution level
Cost of ACSR is equal to AAC.

ACAR (Aluminium Conductor, Aluminium Alloy Reinforced)

It is cheaper than AAAC but pro to corrosion.
It is most expansive.

AAAC (All Aluminium Alloy Conductor)

It has same construction as AAC except the alloy.
Its strength is equal to ACSR but due to absence of steel it is light in weight.
The presence of formation of alloy makes it expensive.
Due to stronger tensile strength than AAC, it is used for longer spans.
It can be used in distribution level i.e. river crossing.
It has lesser sag than AAC.
The difference between ACSR and AAAC is the weight. Being lighter in weight, it is used in transmission and sub-transmission where lighter support structure is required such as mountains, swamps etc.

ACSR (Aluminium Conductor Steel Reinforced)

It is used for longer spans keeping sag minimum.
It may consist of 7 or 19 strands of steel surrounding by aluminium strands concentrically. The number of strands are shown by x/y/z, where ‘x’ is number of aluminium strands, ‘y’ is number of steel strands and ‘z’ is diameter of each strand.
Strands provide flexibility, prevent breakage and minimize skin effect.
The number of strands depends on the application, they may be 7, 19, 37, 61, 91 or more.
If the Al and St strands are separated by a filler such as paper then this kind of ACSR is used in EHV lines and called Expanded ACSR.
Expanded ACSR has larger diameter and hence lower corona losses.

IACS (International Annealed Copper Stand)

It is 100 % pure conductor and it is standard for reference.

Common usage and difference:

For 36 kV transmission and above both aluminium conductor steel reinforced (ACSR) and all aluminium alloy conductor (AAAC) may be considered. Historically ACSR has been widely used because of its mechanical strength, the widespread manufacturing capacity and cost effectiveness.
From a materials point of view the choice between ACSR and AAAC is not so obvious and at larger conductor sizes the AAAC option becomes more attractive. AAAC can achieve significant strength/weight ratios and for some constructions gives smaller sag and/or lower tower heights. With regard to long-term creep or relaxation, ACSR with its steel core is considerably less likely to be affected.
Jointing does not impose insurmountable difficulties for either ACSR or AAAC types of conductor as long as normal conductor cleaning and general preparation are observed. AAAC is slightly easier to joint than ACSR.

Naming and Representation:

Historically there has been no standard nomenclature for overhead line conductors, although in some parts of the world code names have been used based on animal (ACSR – UK), bird (ACSR – North America), insect (AAAC – UK) or flower (AAAC – North America) names to represent certain conductor types.
Aluminium-based conductors have been referred to by their nominal aluminium area. Thus, ACSR with 54 Al strands surrounding seven steel strands, all strands of diameter d 3.18 mm, was designated 54/7/3.18; alu area 428.9 mm2, steel area 55.6 mm2 and described as having a nominal aluminium area of 400 mm2.

The following figure shows the code names for ACSR conductors.