Conductor is one of the important material in a transmission and distribution of electric power.so the proper choice of conductor material and size of the conductor is important. The cost of the conductor material accounts for a major part of the total cost.
Requirements of conductor:
To transmit power over required distance economically
Have enough capacity to maintain continuous supply without failure.
Low cost of installation and maintenance
High life span
- Electrical requirements
- Economical Aspects
In the design of conductor selection minimum technical criteria must be satisfied but it doesn’t mean the significantly lower or higher is better. Most economical conductor is selected after meeting technical criteria.
- High electric conductivity i.e low specific resistance (current carrying capacity high).
- Suitable Line parameters
Efficiency > 94%
Voltage regulation < 12%
- High Corona Inception Voltage
- Have high tensile strength to withstand the mechanical stresses.
o to span between towers as long as possible
o Sag as small as possible
o Avoiding/ reducing number and height of towers and insulation.
- Withstand weather conditions of locality in which they are laid (wind pressure, temperature variation, mechanical loading etc.)
- Weight of conductor should be as less as possible
Sag decreases ↓ as
𝑆𝑎𝑔 = [𝑤 (𝑙 /2) ^2]/2𝑇
- Less stress on towers decreasing cost of installation and less stress on insulators decreasing cost of maintenance
Selection between copper and aluminum
- Low cost Because of its relative abundance.
- Conductance of Al per weight > Conductance of Cu per weight
- Larger diameter than copper for same resistance so same voltage leads to Lower voltage gradient at conductor surface and Reduce ionization level of air and corona loss
- Tensile Strength of Al > Tensile Strength of Cu i. e1% of Cd in Al increases tensile strength of Al to almost same as that of Cu.
- Copper is preferred where space is limited and aluminum is preferred where cost is the constraint.
For small size transformer, Aluminum winding is used
For large size transformer, Copper winding is used
Application of copper:
- High tension string
- Less skin effect
- Space limitation
- High wind
- Need of low thermal expansion.
- Large size underground cables
All the conductor used for overhead lines are stranded conductor. stranded conductors usually have a central wire around which there are successive layers of 6,12,18,24 wires. For n layers, the total number of individual wire is 3n(n+1) +1. If the diameter of each strand is d then diameter of the stranded conductor will be (2n+10d.
Advantages of stranded conductor
- Increase the flexibility i.e brittleness decreases.
- Reduces the skin effects.
- GMR increases (corona inception voltage increases).
The most commonly used conductor materials for overhead lines are copper, aluminum, steel-cored aluminum, galvanized steel and cadmium copper.
- Cheaper, lighter and gives less mechanical stress to tower and insulator.
- Conductivity of Al is 60% to that of copper and density 0.303 time that of copper.
- Corona inception voltage is large for al conductor.so less corona effect is seen since GMR is more for Al, corona inspection voltage is large.
- All aluminum conductor(AAC) has lower tensile strength. The tensile strength can be improved by reinforcing it with high tensile strength material like steel.
- All aluminum stranded conductors (AASC) are mainly used for low voltage distribution overhead lines having short spans of up to 65m.
It consists of a core of galvanized steel strand surrounded by a number other aluminum strands. The steel conductors used are galvanized in order to prevent rusting and electrolytic corrosion. The steel core takes a greater percentages of mechanical stresses while aluminum carries the bulk of current.
ACSR (aluminum conductor steel reinforced)
- It has high tensile strength and lighter in weight.
- It produces the small sag and therefore can be used for longer spans. Because of use of larger span, the number of line supports may be reduced by about 25%. Thus the overall costs of supports, foundations, insulators and erection is considerably reduced. In addition, there is a substantial saving in maintenance cost.
- Corona losses are reduced as it has a largest diameter than other type of conductor of same resistance.
- The skin effect is predominating in ACSR conductors.
- Critical voltage limit of the ACSR conductor can be raised by 30% to 50% as compared to copper conductors.
It is used for long spans or for short line selections exposed to normally high stresses due to climatic conditions. These conductors are found most suitable for line supplying rural areas and operating at voltage of about 11KV.Iron or steel wire used is most advantageous for transmission of small power over a short distance. This conductor is not suitable for EHT lines for the purpose of transmitting large power over a long distance due to following properties:
- Poor conductivity 13% that of copper.
- High internal reactance.
- Subjected to eddy current and hysteresis.
Galvanized steel wire is used in telecommunication lines, stay wires, earth wires and guard wires. Stranded galvanized steel wires are used as guy wires and earth/ground wires.
- Addition of 1% of cadmium in copper increases the tensile strength by about 40% and reduces the conductivity only by 17% below that of pure copper.
- Cadmium copper is costlier than pure copper.
- Cadmium copper is economical for a line with long spans and small cross section.
- It is used in telephone and telegraph lines where currents involved are quite small.
- Cadmium copper conductors on communication lines are being replaced by ACSR conductors.
Current carrying capacity of a cable:
conductor must be able to carry designated current safely without melting and insulation failure.
Factors affecting current carrying capacity of cable:
1) Ambient temperature
2) Conductor temperature
4) Conductor size
5) Conductor Number spacing
With 1% decrease in ambient temperature there is 1% increase in current carrying capacity
current carrying capacity depends on atmospheric temperature. Also current carrying capacity depends on ambient temperature.
Choosing conductor according to electrical requirement
- Calculate Line current 𝐼 =𝑃/ 𝑁𝑐√3𝑉𝐿−𝐿 cos𝜑
- Determine nearest current carrying capacity conductor higher than given line current at 40⁰𝐶
- Check Efficiency criteria > 94%
Note from table: resistance per unit length of the conductor at 20⁰C
- Determine total resistance at 20⁰C
- Determine total resistance at 65⁰C
- Determine power loss per conductor at 65⁰C
- Determine total power loss at 65⁰
- Determine efficiency
If < 94% repeat step 3 for next higher current capacity conductor
- Check voltage regulation criteria:
Note from table: Conductor diameter mm Compute 𝐺𝑀𝑅𝐿
- Compute 𝐺𝑀𝑅𝑐
- Compute GMD
- Compute line parameters:
- Compute ABCD parameters:
- Compute V.R
- If > 10% repeat step 4 for next higher current capacity conductor
- Compare corona Inception voltage with Max system voltage
𝐼𝑓 𝑉𝑐𝑖 > max𝑠𝑦𝑠𝑡𝑒𝑚 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 Select the conductor
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