What is Conductor?Conductor is one of the important item is in a Transmission and distribution system of electric power. The cost of the conductor material accounts for a major part of the total cost .So proper choice of conductive material and size of the conductor is of almost important. The conductor material which is used in Transmission and distribution of electric power should have the following characteristics
- high electric conductivity i-e low specific resistance
- should have high tensile strength so that it will withstand the mechanical stresses
- should have low specific gravity in order to obtained low weight per unit volume
- The cost of materials should low in order to be used for long distance usually involved in transmission line
- easy availability
- should not be brittle
There is no material which meet all the above requirement therefore compromise should made between the cost and the desired electrical and mechanical property during the selection of the material for a given situation
For an overhead line the most appropriate material which is used are copper, Aluminum, Steel or Aluminum Steel and cadmium copper. The material which is used for overhead line should be stranded in order to increase the flexibility. Standard conductors normally have a central wire around which there are successive layer of 6, 12, 18 and 24 wires.
For ii layers, the total number of individual wire can find with the help of the formula 3n (n+ 1) +1. If the diameter of each conductor is d, then the diameter of the stranded conductor can find by the formula to (2n + 1)d .In the process of manufacture adjacent layer are spiraled in opposite direction so that the layers are bound together. This method of construction of the conductor is called “concrete lay” .There also another method of manufacturing a conductor which is called “rope lay” which is used in order to obtain flexibility.
Stranded Hard Drawn Copper
Hard drawn copper conductor is the best material which have high electrical conductivity and great tensile strength for all type of transmission line. Due to hard drawing(Cold rolling and drawing) of copper the conductivity will reduce but the tensile strength increase .This type of material does not Corrode in normal atmosphere and does not have electrolytic troubles .Due to high current density characteristic ,less cross-sectional is required so lesser area is subjected to wind loads. It is quite homogeneous, durable and of high scrap value .The advantage of hard drawn copper are long life and is ease of jointing .
Distribution line medium hard- drawn copper conductor is suitable. Copper wire suitable for secondary distribution circuit and also service connection to buildings.
Although copper is suitable for Transmission and distribution due to their electricity conductivity and and higher current density but due to the difficulties in importing and other factors in our country nowadays Aluminum is used in place of copper.
Aluminum conductor is cheaper in cost and lighter in weight but it have poor conductivity and tensile strength as compared to Copper. Aluminum has conductivity 60% to that of Copper and density 0.3% to that of copper .Aluminum conductor has a diameter about 1.2 time to that of copper conductor of equal resistance but due to its low density only half weight of Aluminum to that of copper .Aluminum conductor have tensile strength much lower than that of copper( 45% to that of copper) but the larger sectional area of metal neutralized the difference to some extent and an aluminum conductor has about 75% of the ultimate strength of equivalent copper conductor .For the same conductivity aluminum conductor have 1.6 time x-section of copper is required thus causes greater surface for wind pressure and supporting structure of required to be designed.
These days there is a great trend toward aluminum as conductor material because of its greater availability and cheapness as compared to Copper. All Aluminum stranded conductor (AAC)are mainly used for low voltage distribution line which have short span up to 65 meter.
Steel-cored aluminum which is abbreviated as a ACSR( aluminum conductor Steel reinforced) which consists of a core which is galvanized Steel surrounded by a number of Aluminum strands .The Steel which is used are galvanized so that it prevent rusting and electrolytic corrosion and X- section of two metals are in the ratio 1:6 but in case of high strength conductor the ratio maybe 1: 4 .The skin effect in ACSR conductor is predominating and therefore the resistance of composite conductor is taken equal to that of aluminum covering alone.
Due to reduce in Corona losses with ACSR conductor critical voltage limit of the conductor can be raised by 30 to 50 percent as compared to Copper conductor
Because of large spans uses, the number of line supports is also reduced by about 25% therefore overall cost of supports, foundations, insulators and erection is also reduced.
Galvanized steel conductor are used for extremely long span or for short line sections exposed to normally high stresses due to climatic conditions. This type of conductor are most suitable for line supplying rural area and operating at voltage of about 11KV where cheapness is the main consideration. Iron or steel wire used is most of advantageous for transmission of small power over a short distance where the size of copper conductor desirable from economical consideration come out to be smaller than 8 SWG,which can not be used because of poor mechanical strength
Now a days use of galvanized steel wire is limited to telecommunication line, stray line ,earth wire and guard wire. Galvanized steel wire are used as a binding wire also .Stranded galvanized Steel wire are used as a gay wire and Earth ground wire. In 11KV and 33kv line the earth line ,the earth wire is provided below the line conductor for the safety of personnel and also for the safety of material moving under the line. In case of extra High Voltage line of 66 ,132 and 220 KV ground wire is provided above the line for protection against lightning.
The conductor which is used in certain some cases is copper alloy with cadmium .Addition of 1 or 2% of cadmium in copper increase the tensile strength by about 40% and reduce the conductivity of only by 17% below to that of pure copper. However cadmium copper is costlier than pure copper. For a line with long span and small cross-section use of cadmium copper will be more ecomonical.Cadmium copper also employ for telephone and telegraph line where current involved are very small.
A composite wire which is known as copper-clad or copper weld Steels wire is obtained by welding a copper coating on a steel wire core
when harmful gases such as Ammonia are present in the atmosphere and the spans are extremely long, phosphor bronze conductor are most suitable material for overhead line conductor .In this type of conductor some strands of phosphor bronze are added to the cadmium copper
Stranded conductor are the conductor which is made of thin wire of small cross section and bunched together. This type of conductor are flexible ,not rigid and can be coiled very easily .These element to a large extent the risk of breaking through insulation.
The following are important points about strandard conductor
- standard conductor is made by twist in the wire which is called the strand and together to form layers
- The wires of each layer are laid in helical fashion round the preceding layer.This process is called stranding
- Standing generally done in opposite direction for successive layers .This mean if the wires of one layer are twisted in left hand direction ,the next layer of the wire will be twist in the right-hand direction and so on
- A stranded stranding consists of 6 wires around one wire, then 12 wire around the previous 6 then 18 wire around 12, then 24 wire around 18 and so on .The number layer to be provided depend upon the number of wires to be provided.The central wire is not counted as a layer.Instead of single ,3 or 4 wire may also provided in the centre and over these wire layer may be formed. If 3 stranded wired are put in the centre ,9 wires will be in the first layers then 15 wires in the second layer and so on .The increases in the number of wires in each successive layer is 6 in each of the above cases.
Formula for Stranded conductor
|1-No of wire in the centre||1 wire||3 wire||4 wire|
|2-no of wires in the nth layer from centre||6n||3+6n||4+6n|
|3-total no of wires in a stranded conductor having n layer||1+3n(1+n)||3(1+n)2||(4+3n)(1+n)|
|4-Diameter over the nth layer in cm where d=dia,of each wire in cm||(1+2n)d||(2.155+2n)d||(2.414+2n)d|
Stranded conductor are express as 7/2.24 ,19/2.50 ,37/2.60 ,and so on
First number that is 7 ,19,37 etc indicates total no of wires
Second no that is 2.24 ,2.50 ,2.60 etc indicate diameter of each wires in mm
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