Cables over- head

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  • Publié le : 29 mars 2010
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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 thelowest 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 and that recyclable materials are used as much aspossible. 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. There are a number of important specifications to consider:

• Need to meet transfer capacity requirements
• Life cycle period
• Reliability
• Distribution
• Wind loading
• Ice loading
• Load factors
• Loaded tension limits
• Extreme wind loading
• Environmental aspects
• Low weight
• High strength and flexibility
• High mechanical load with extreme cold
• Corrosion behaviour
• Impact resistances
• Required safety factors
• British Standards (1970BS/215)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 metals.

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 SteelReinforced (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 ofaluminium.
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 thermalconductivities, 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 learned early that the electrical resistivity of aluminium ismarkedly increased by impurities; electrical conductor grade (EC) metal, containing approximately 99.5% Al, was established for most conductor uses and is still used to this present date.
The utilisation of aluminium has increased quickly since World War II by the change in the aluminium-copper price relationship. Aluminium has over twice the conductivity of copper on a weightbasis, 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....
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