The following points highlight the seven main types of conductors used in transmission and distribution of electric power. The types are: 1. Stranded Hard Drawn Copper 2. Aluminium 3. Steel-Cored Aluminium 4. Galvanised Steel 5. Cadmium Copper 6. Copper-Clad Steel 7. Phosphor Bronze.
Type # 1. Stranded Hard Drawn Copper:
Hard drawn copper conductor is the best conductor owing to its high electrical conductivity and great tensile strength for all types of transmission. Though hard drawing (cold rolling and drawing) reduces the conductivity slightly but increases the tensile strength considerably.
It does not corrode in normal atmosphere and is not subjected to electrolytic troubles. It has higher current density so lesser x-sectional area of conductor is required and so lesser area is subjected to wind loads. It is quite homogeneous, durable and of high scraps value. The other advantages of hard drawn copper are long life and ease of jointing.
However, medium hard-drawn copper conductor is suitable for distribution lines and soft- drawn copper wires are suitable for secondary distribution circuits, and for service connections to buildings.
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Though copper is ideally suitable for the transmission and distribution, but owing to the difficulty of importing it due to lack of foreign exchange in our country, the trend nowadays is to use aluminium in place of copper. The use of copper is being restricted for the manufacturing of the machines only.
Type # 2. Aluminium:
Aluminium is cheaper in cost and lighter in weight but is poor in conductivity and tensile strength as compared to copper. Its conductivity is 60% to that of copper and density 0.303 time that of copper. An aluminium conductor has a diameter about 1.26 times that of copper conductor of equal resistance but due to its low density only half weight of aluminium is required to that of copper.
The tensile strength is much lower than that of copper (45 per cent to that of copper), but the larger sectional area of metal neutralizes the difference to some extent, and an aluminium conductor has about 75% of the ultimate strength of equivalent copper conductor.
For the same conductivity aluminium conductor having 1.66 times the x-section of copper is required thus causes a greater surface for wind pressure and supporting structures are required to be designed for a greater transverse strength. In many cases also, high towers must be employed with aluminium conductors than would be required with copper conductors having the same length of span.
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As a result of the reduced working stress permissible and also owing to the fact that the linear coefficient of expansion of aluminium is 1.4 times that of copper, the sag is greater in aluminium wires. The aluminium conductor being liable to swing requires larger cross-arms. Due to low melting point of aluminium it cannot withstand short-circuits etc. Jointing of aluminium is also difficult as compared to that of copper.
These days there is a great trend towards the aluminium as conductor material, because of its greater availability and cheapness in comparison to copper. All aluminium stranded conductor (AAC) weighs only half as the equivalent copper conductor and cost per unit length at prevailing market rates, is considerably less. All aluminium stranded conductors are mainly used for low voltage distribution overhead lines having short spans of up to 65 m.
Type # 3. Steel-Cored Aluminium:
Steel-cored aluminium conductor, abbreviated as ACSR (aluminium conductor steel reinforced), consists of a core of galvanised steel strand surrounded by a number of aluminium strands. The steel conductors used are galvanised in order to prevent rusting and electrolytic corrosion, the x-section of two metals are in the ratio 1:6 but in case of high strength conductors their ratio may be 1:4.
The steel core takes a greater percentage of mechanical stresses while the aluminium carries the bulk of current. ACSR conductor being of high tensile strength and lighter in weight produces small sag and therefore longer spans can be used. The ACSR conductor has a largest diameter than any other type of conductor of same resistance, so corona losses are reduced, but stronger supports are required for a given span.
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The ACSR conductor gets deteriorated in service to the atmospheric corrosion due to bimetallic action of zinc and the aluminium, electrochemical in nature. The skin effect is very predominating in ACSR conductors hence the resistance of the composite conductor is taken equal to that of aluminium covering alone. The reactance of ACSR conductor is taken to be equal to that of a non-magnetic conductor having diameter equal to that outside of the ACSR conductor.
Another advantage with ACSR conductor is that, because of reduced corona losses, critical voltage limit of the conductor can be raised by 30 to 50 % as compared to copper conductors.
Because of use of larger span, the number of line supports may be reduced by about 25%. Thus the overall cost of supports, foundations, insulators and erection is considerably reduced. In addition, there is a substantial saving in maintenance cost. Further, since most faults occur at the line supports, so the frequency of occurrence of faults is automatically reduced.
Composite type of conductor, being costlier than the all-aluminium type, is chiefly used in those cases where an all aluminium conductor would require a much higher and more costly support than copper. This occurs principally where spans are 100 metres and over are involved.
Type # 4. Galvanised Steel:
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Galvanised steel conductors have been used to advantage for extremely long spans, or for short line sections exposed to normally high stresses due to climatic conditions. These conductors are found most suitable for lines supplying rural areas and operating at voltages of about 11 kV, where cheapness is the main consideration.
Iron or steel wire use is most advantageous for transmission of small power over a short distance, where the size of copper conductor desirable from economical considerations comes out to be smaller than 8 SWG, which cannot be used because of poor mechanical strength.
This conductor is not suitable for EHT lines for the purpose of transmitting large amounts of power over a long distance due to its following properties:
(i) Poor conductivity, 13% that of copper.
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(ii) High internal reactance.
(iii) It is subjected to eddy current and hysteresis.
Nowadays use of galvanised steel wire is limited to telecommunication lines, stay wires, earth wires and guard wires. Galvanised steel wires in small sizes are employed as binding wires. Stranded galvanised steel wires are used as guy wires and earth/ground wires.
In lt, 11 kV and 33 kV lines, the earth wire is provided below the line conductors for the safety of personals, catties and material moving under the line as a precaution in case a line conductor snaps. In case of extra-high voltage lines of 66, 132 or 220 kV, the ground wire is provided above the line conductors for protection against lightning.
Type # 5. Cadmium Copper:
The conductor being used in certain cases is copper alloyed with cadmium. Addition of 1 or 2% of cadmium in copper increases the tensile strength by about 40% and reduces the conductivity only by 17% below that of pure copper. However, cadmium copper is costlier than pure copper.
Use of cadmium copper will be economical for a line with long spans and small cross-section i.e. where the cost of conductor material is comparatively small in comparison to that of supports etc. Cadmium-copper conductors are also employed for telephone and telegraph lines where currents involved are quite small. However, owing to scarcity of copper, cadmium-copper conductors on communication lines are being replaced by ACSR conductors.
Type # 6. Copper-Clad Steel:
A composite wire, known as copper-clad or copper-weld steel wire, is obtained by welding a copper coating on a steel wire core. Line conductors made of copper-clad steel are preferably stranded, and have a considerably large tensile strength than the equivalent all-copper conductors.
The proportion of copper and steel is so chosen that the conductivity of composite wire is 30 to 40% of that of copper conductor of equal diameter. Such material appears to be very suitable for river-crossings or other places where an extremely long span is involved.
Type # 7. Phosphor Bronze:
When harmful gases such as ammonia are present in atmosphere and the spans are extremely long, phosphor bronze is most suitable material for an overhead line conductor. In this conductor some strands of phosphor bronze are added to the cadmium copper.