Design specification
High voltage overhead power lines must comply with several requirements. We need to consider various factors for the design specification, mechanical and electrical and also the sites where the cables will be fixed.

The site is very important because its span varies enormously depending on the nature of the terrain and climate. Also important to the requirements are the lowest electrical resistance (to cut down losses), safe clearance over the ground, adequate strength for the applied loads, good mechanical strength able to manage the stresses and weights involved and convenient cost for the 100 or 1000 of km the lines regularly installed.

It is vital that appropriate environmental protection is provided. This is achieved through specifying a thermal expansion limit, corrosion resistance and apt wind resistance for the cables.

Numerous varieties of cable specifications are offered to meet the demands for distinct currents and satisfy British Standards.

High Voltage Cable Conductors: A Comparison

Introduction

As copper was the first material to be used in this capacity and aluminium is the most commonly used material used today I have decided to compare these two materials.

Conductor Material Properties

Materials usually used in conductors are aluminium, copper, and steel. Steel wires are joint with aluminium in the most frequent type of overhead conductor. Aluminium Conductor Steel Reinforced (ACSR). The use of copper is unusual in modern transmission lines since it weighs and generally costs considerably more than aluminium conductor of the same resistance.
Copper was the metal first used for conductors during the great development of electrical engineering in the early 1880’s. However, the immediate rapid expansion of the aluminium industry soon reduced the price of aluminium.
For overhead transmission lines, aluminium is now used almost to the exclusion of copper..
Numerous other electrical, mechanical and structural applications of aluminium in electrical industries developed along with the growth of aluminium wire and cable.

Wire and Cable

The positive attributes of aluminium for electrical applications are: reasonably high electrical and thermal conductivities, low density, nonmagnetic properties, ease of drawing down to the smaller wire sizes and high resistance to weathering.
Because of technological advances and good price as compared to copper, there has been permanent expansion in the volume, sizes and varieties of aluminium conductors.

Aluminium Conductor Materials
It was early learned that the electrical resistivity of aluminium is markedly increased by impurities; electrical conductor grade (EC) metal, containing approximately 99.5% Al, was established for most conductor uses. At present, WHY ALUMINIUM?
The utilisation of aluminium has increased quickly since World War II, in no doubt by the change in the aluminium-copper price relationship. Aluminium has over twice the conductivity of copper on a weight basis, and aluminium conductor is used for almost all overhead sub transmission and distribution lines.

There are some great reasons because in numerous countries aluminium has taken the place of copper for main conductor material of all components of the transmission and distribution systems.
Aluminium is much lighter than copper. The density of aluminium is about 30 % that of copper. Especially in overhead line construction this factor is very important because heavy conductors lead to heavy and expensive pole structures. Handling and transportation of aluminium conductors and cables are also easier compared with heavy copper conductors and cables. The lightness of aluminium gives advantages in many ways
Aluminium is the most common metal in the earth surface - its amount is about 8 %. Today copper are limited and diminishing so the price of copper has been high and rising. The price of aluminium was low and rather steady state for years, but some rising can be noticed in it, too. Because of lightness of aluminium, the price of aluminium conductors and cables are clearly smaller than those of copper conductors and cables.
Long-distance overhead conductors use aluminium in first choice to copper - the inferior electrical conductivity being more than compensated by the lower density and cost. In composite steel-Al Cables, the steel carries largely or all of the mechanical load, and the Al the electrical current. Aluminium is frequently now used throughout, serving both electrical and mechanical purposes.

Aluminium is about a third the weight than equal volume of copper, steel or brass.
Aluminium can supported heavy loads and heavy pressures.
The strength when alloyed approaches that of steel.
Corrosion resistance - The creation of a microscopic film of aluminium oxide on the surface of the metal protects it against corrosion and also makes a good insulation.

• Electrical conductivity - Aluminium has two times the conductivity of an equal weight of copper. • Resilience and Flexibility - As this applications require some flexibility in addition to strength also the aluminium can spring back and where necessary bend slightly. Aluminium streetlights also provide indispensable elasticity with safety benefits of flexibility. • Thermal conductivity - Aluminium distribute heat and cooling energy consistently and rapidly. • Thermal Ductility - Aluminium is uncomplicated to cold work and manufacture. • Mixability - Can be simply modified by adding alloys to make a final metal that is essentially flexible, conductive or pliable than aluminium alone. • Cost efficiency – low cost processing
Aluminium Properties
|Mechanical Properties |Conditions |
| |Phase |Temp. (K) |Pressure (Pa) |
|Density |2700 kg/m3 |Solid |298.15 |0 |
|Modulus of Elasticity |62.053 GPa |Solid |0 | |
|Poisson Ratio |0.35 |Solid | | |
|Thermal Expansion Coefficient |2.310 /K |Solid |298.15 | |

|Electrical Properties |Conditions |
| |Temp. (K) |Note |
|Electrical Resistivity |2.655 W-m |293.15 | |

|Thermal Properties |Conditions |
| |Temp. (K) |Pressure (Pa) |
| | | |
|melting Temperature |933.47 K | |101325 |
|Boiling Temperature |2792.15 K | |101325 |
|Critical Temperature |7850 K | | |
|Fusion Enthalpy |397 J/g |0 |101325 |
|Vaporization Enthalpy |10896.34 J/g |0 |101325 |
|Heat Capacity |897 J/kg-K |298.15 more... |100000 |

WHY COPPER?
Copper has the highest electrical conductivity of all engineering metals. This means that a copper cable is smaller than an equivalent ampacity aluminium cable. Smaller cables mean an easier and less costly installation but not in long lines, copper price is more expensive than aluminium.

Compared to aluminium alloy, the harder and stronger copper conductors are more resistant to the inevitable nicks and mechanical abuse to which a cable is subjected during installation. This is a real advantage at junction and termination boxes where copper conductors can be bent.

Copper cables are more slender so they can fit into smaller duct spaces. They can be bent around tighter corners and more easily jointed.

Copper is twice the weight of aluminium of the same rating and three times as dense, which means copper does not need to be reinforced when hung from overhead pylons as is often the case with aluminium.

The current rating of a cable is the current it can carry safely without overheating. Copper plays a part in improving the current rating of cables because it is such a good conductor. The main issue is that the temperature of the conductor should not rise above 90 °C because this will cause the insulation to degrade and eventually break down. A cable should always be protected by a protective device – a fuse or circuit breaker – that corresponds to its current rating.
The operating current of a cable is determined by how hot the cable gets. This is affected by a number of variables: • the resistance of the cable - a higher resistance cable will get hotter at a given current • the insulation on the cable - this will tend to keep it warm • the environment of the cable - if it is in a duct (especially with no airflow) it will tend to get hotter.

Copper Properties

|Mechanical Properties |Conditions |
| |Phase |Temp. (K) |Pressure (Pa) |
|Density |8960 kg/m3 |Solid |298.15 |0 |
|Modulus of Elasticity |110.316 GPa |Solid |0 | |
|Thermal Expansion Coefficient |1.650 /K |Solid |298.15 | |

|Electrical Properties |Conditions |
| |Temp. (K) |Note |
|Electrical Resistivity |1.673 W-m |293.15 | |

|Thermal Properties |Conditions |
| |Temp. (K) |Pressure (Pa) |
|Melting Temperature |1357.77 K | |101325 |
|Boiling Temperature |2835.15 K | |101325 |
|Critical Temperature |8280 K | | |
|Fusion Enthalpy |208.7 J/g |0 |101325 |
|Heat Capacity |385 J/kg-K |298.15 more... |100000 |
|Thermal Conductivity |401 W/m-K |300 more... |101325 |

PROCESS
The process by which a long straight metal part can be produced is called extrusion. This process is very cheap in comparison with other processes. The cross-section that can be produced with this system varies from solid, round and rectangular shapes to l shapes and T shapes. The extrusion process produces compressive and shear force in the stock. By squeezing metal in a closed cavity with a die tool, we can use a mechanical or hydraulic press.`
As extrusion produces shear force and compressive forces in the stock, that means that this process can make height deformation possible without tearing the metal. This process gives the cables various advantages. There is no need for secondary machining.
An example of the surface finish for aluminium is 0.8 µm.
This process will produce a wide o variety of cross-section that is hard to produce cheaper using other process.
They are two different extrusion processes

HOT AND COLD EXTRUSION
Hot extrusion done by high temperatures nearly about 50 to 70 % of the melting point of the metal. The pressure range from 35-700 MPs. Because of the height temperature and the pressure can have effect in the die as well as other components a good lubrication is necessary.
The aluminium, cooper with their allows are successfully used to manufacture various product using the hot extrusion process, somme example are electrical wires , bars and tubes. This process can be cheaper because we can produce mass production.
Cold extrusion is the process at room temperature or very little elevated temperatures. With cold extrusion the advantage is useful for withstanding the stresses created by the extrusion.
Cold extrusion has various application also , cooper , aluminium alloys, steel. Many advantages existe with the process, no oxidation, very good mechanical properties because the temperatures formed are lower than the re-crystalilisation temperature and make a good finish. extrusion process produces different types of cables in continuous lengths.
I chose cold extrusion over hot extrusion, because less oxidation, higher strength due to cold working, closer tolerance and also a good surfaces finish and a fast extrusion speed.

This process will be done in indirect extrusion because the advantage will be no friction during the process between the billet and the container liner.

The metal will flow more uniform in indirect intrusion than direct. We have a better section uniformity along the length of the cable, more strength and less cost for manufacturing.

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Reference: www,rolymech.co.uk…/Extruding.html
CONCLUSION
Originally cables were made copper, now is aluminium can reach between 4 to 50mm.
Transmission lines use aluminium cable, steel reinforced (Type ACSR, ACAR). The ACSR is a conductor; stranded steel that carries the mechanical load and also layers of stranded aluminium, the ACAR cables has high mechanical strength with a low resistance. The ACSR are used for high voltage lines.
ACSR cables are used for medium and high voltage lines and can be used for overhead services to customers. Aluminium is used because it has a better resistivity/weight ratio than copper and is less expensive. We know that the aluminium is a good conductor and material for the line, but some electrical current is transformed into heat (joule’s law) which results in energy loss. With large conductors the lost energy is due to the lower electrical resistance and the cost is higher.
Kelvin law say (the best size of a conductor for a line is found when the cost of the energy lost in the conductor is equal to the annual interest paid on the portion of the line construction).
We still using some copper cable, for lower voltages and grounding.

Because of the light weight of the aluminium, all aluminium stranded conductors need to be strung with large sags. Because the low yield strength compared to the stranded copper.
In recent years many aluminized and aluminium-clad cores wires have been employed to reinforce aluminium conductors.
Recently new experiments have been conducted with new alloys of aluminium combined with other metals to obtain higher mechanical strength with better electrical characteristics.

References: www.nature.com/nature/journal www.magnode.com www.lenntech.com www.efunda.com www.osti.gov/bridge European Aluminium Assosciation
Wikipedia.org/wiki/Extrusion

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2010

MANUFACTURING CONCEPTS

PRODUCT ANALYSIS

OVERHEAD CABLE