The wires through which electricity can flow is called electrical conductors. The conductors are mainly made of Copper and Aluminium. The conductors are mainly manufactured by hard drawn processes. All the hard drawn process reduces the conductivity of the conductor but at the same time, it increases the tensile strength of the conductor.
An electrical conductor may be either a solid type or a stranded type. The solid type conductor has only one solid core or wire. The stranded conductors are made by twisting a number of wires together. Each of the twisted wires is called the strand of the conductor. The standard numbers of strands in a stranded conductor may be either of 7, 19, 37 or 61.
For 7 strands conductor, one strand is at the center of the cross-section of the conductor and six other strands surround that central strand.
For 19 strands conductor, with the just above-mentioned convention, there is an additional layer of 12 strands outside.
For 37 strands conductor, with the just above-mentioned convention, there is an additional layer of 18 stands outside.
For 61 strands conductor, with the just above-mentioned convention, there is an additional layer of 24 strands outside.
Calculation of the number of strands in a stranded conductor
If there are ‘n’ layers of strands in a stranded conductor, 3n(n+1) + 1 will be the number of strands of the conductor.
Calculation of overall diameter of a stranded conductor
If d mm is the diameter of a single strand in the conductor, the overall diameter of the conductor will be (2n +1)d mm.
A stranded conductor is more flexible than an equivalent solid conductor. Also, it has more mechanical strength than an equivalent solid conductor of the same material.
Although the conductivity of copper is much better than that of aluminum, still the use of copper conductor is limited because of its cost. On the other hand, the conductivity of aluminum is poorer than that of the copper. But the availability of aluminum is quite more and the cost of aluminum is quite less than that of copper. This is the reason aluminum is a more popular choice as conductor material than copper.
Calculation of the overall diameter of standard ACSR panther conductor
A standard ACSR panther conductor is represented as 30/7/3.00 mm. It means the conductor has 30 aluminum strands, 7 steel strands and each of the strands for both aluminum and steel has a diameter of 3 mm. Therefore, the total number of strands in the conductor is 30 + 7 = 37.
Let us consider ‘n’ is the number of layers of strands in the conductor. Therefore as per formula, we can write,
3n(n + 1) + 1 = 37 or n = 3.
Now by applying the formula for determining the overall diameter of stranded conductors, we get,
(2n + 1)d = (2X3 + 1)3.00 = 7X3.00 = 21.00 mm.
The specific resistance of a hard drawn copper conductor is 1.72 micro ohm centimeter. The tensile strength of the hard drawn copper conductor is 4,200 kg per square centimeter. The weight of copper is 8,900 kg per cubic meter. Copper conductors are very rarely used in overhead lines. For electrical distribution purposes, solid copper conductors are used and for electrical transmission purposes, stranded copper conductors are used.
The specific resistance of a hard drawn aluminum conductor is 2.83 micro ohm centimeter. The tensile strength of the hard drawn aluminum conductor is 2100 kg per square centimeter. The weight of aluminum is 2700 kg per cubic meter.
The conductivity of an aluminum conductor is 60% of a copper conductor of the same size. The tensile strength of an aluminum conductor is 50% of a copper conductor of the equivalent cross-section. The weight of aluminum is 33% of an equal volume of copper. Therefore for obtaining the same conductivity, the cross-section of an aluminum conductor should be 1.66 times that of a copper conductor. Due to the lightweight of an aluminum conductor, it can be used for a longer span length with comparatively less sag.
Aluminum conductors are always made stranded. There are mainly two types of stranded aluminum conductors.
- All Aluminum Alloy Conductor,
- Aluminum Conductor with Steel Reinforced.
All Aluminum Alloy Conductor (AAAC)
In this type of stranded aluminum conductor, all the strands are made of a special type of aluminum alloy. The number of strands may be either of 7, 19, 37.
Aluminum Conductor with Steel Reinforced (ACSR)
To increase the tensile strength of a stranded conductor, in addition to aluminum strands some steel strands are used at its center.
The conductivity of a steel conductor is about 10% of a copper conductor of the equivalent cross-section. But the tensile strength of a steel conductor is much more than that of an equivalent sized copper conductor. It is about 10,000 kg per square centimeter. Also, the cost of a steel conductor is much less than that of an electrically equivalent copper conductor. Because of these reasons, steel conductors are used where cost optimization is an important criterion for constructing overhead lines. For example, in short, rural distribution networks sometimes steel conductors are used. Rust is a major problem of steel conductors. Therefore steel conductors have to be replaced in a regular interval. This makes the expenditure of maintenance on overhead lines with steel conductors much higher. Sometimes by using a layer of aluminum on the conductor the overall conductivity of the conductor is increased to some extent and also it saves the conductor from rusting. The steel conductor with an aluminum layer is called the alumoweld steel conductor. Although presently the use of steel conductors even for a short distribution rural line has become obsolete.