conductor allow the transmission of heat or light from one source to another source. conductor flow electron to inside from atom to atom.
1. CONDUCTING MATERIALS:
The conductor materials in which the electrical current can flow easily can be used to carry the current economically for the business are called conducting materials. They usually bear very low resistivity and does very low power loss. Their specific resistivity is of the order of 10–8 Ωm to 10–6 Ωm.
Conductor Generally available is silver, copper, gold, aluminum, tungsten, brass, phosphor bronze, tin, lead, silicon, steel, iron, carbon etc. On the basis of availability, cost and utility copper and aluminum are used mostly in electrical equipments.
2. PROPERTIES OF CONDUCTOR:
Following properties are being considered while selecting the conducting material:
2. Temperature co-efficient of resistance
3. Oxidation characteristics
4. Resistivity to chemicals & weather
5. Flexibility and Ductility
6. Mechanical strength
7. Easy in welding & soldering
8. Abundance and low cost.
It is essential in transmission and distribution that voltage drop and power loss should be minimum. The resistance of the conductor or resistivity of the material used as conductor should be minimum because the power loss (I2R) and Voltage drop (IR) are directly proportional to resistance. To keep the resistance minimum, most of the material to form conductor are having low resistivity (high conductivity).
Unit of resistivity is given in Ωm
R = ρl/a
Where R – Resistance of conductor
ρ – Specific resistance in Ωm.
l – Length of conductor in m.
a – cross sectional area of conductor in m2.
2.2 TEMEPRATURE CO-EFFECIENT OF RESISTANCE:
Change in temperature, changes the resistance of conductor. This change in resistance should be minimum so that no major change occur in power loss and voltage drop.
Change in resistance depends upon –
1. Initial resistance,
2. Change in temperature and
3. Temperature co-efficient of conducting material
Change in Resistance
Rt – Ro ∝ Ro × t
Rt – Ro=∝ Ro t
Where R0 – Resistance of conductor at 0°C.
Rt – Resistance of conductor at t° C.
t – Change in temperature
∝0– temperature co-efficient of resistance at 0°C
In vast range of temperature the temperature co-efficient of resistance is not very linear. It changes along with temperature, therefore the change in resistance also done not remain linear. When temperature raises the specific resistance also rises along with. But in certain range of normal working temperature it remains almost linear.
For industrial application 40°C is chosen as the ambient temperature for recording, comparing and analysis. Whenever the wiring resistance measurement is done, it becomes essential to record the temperature also. So, during analysis and comparison, measured value can be projected at 40°c using the following formula.
Rt2 = Rt1 [1 + αt1 (t2 – t1)]
Where Rt1 – Resistance at t1°C.
Rt2 – Resistance at t2°C
αt1 – Temperature co-efficient of resistance at t1°C.
t1, t2 – Temperature of conductor
αt2 – Temperature co-efficient at t2°C
2.3. OXIDATION CHARACTERISTICS:
When any metal is exposed to the atmosphere containing moisture and oxygen, oxidation takes place on the surface of it. Oxidation rerates the electrical and mechanical qualities of the metal, when used as conductor. Therefore the material chosen for conductor must be such that it should not get affected by oxidation.
Mainly oxidation is a process of corrosion in which active material is lost, which in turn reduces capacity and efficiency, as well.
2.4 RESISTIVITY TO CHEMICALS:
Plant processes are different in different areas. Different chemicals are utilised in various processes, so different – different gasses are being liberated and are present in that atmosphere. These gasses can be active with the materials used there. Hence it is necessary that conducting material, used, should be chemical resistant for suitable and sufficiently long life of services can be achieved conductor.
2.5 DUCTALITY AND FLEXIBILITY:
Ductility is the property of metals which allows the metal to be formed in wire, foil or strip, in shape, to develop it for use.
To form coils or for different use, it becomes necessary to bend or stretch it straight hence flexibility is a must, otherwise conductor forming it to required shape it will break.
2.6 MECHANICAL STRENGTH:
Under transmission, distribution or other use, conductor has to face many stresses, like self weight, tension; wind, bending, laying and clamping are acting on the conductors. During charging of transformer or charging the load mechanical forces are developing in it. In fault conditions these forces are tremendously high. Therefore, for reliability the conducting material must have sufficiently high mechanical strength to counter such stresses..
2.7 EASY IN SOLDERING & WELDING:
Sometimes it becomes essential to join conductors. Then it is essential that resistance of joint must be low enough with keeping mechanical strength unaffected. Easiest method of jointing is soldering and welding which keeps the resistance minimum, even less than running conductor without sacrificing mechanical strength. Hence the metals used for conductor should be such that it can easily be soldered or welded.
2.8 ABUNDANCE AND COST:
Cost is always inversely proportional to availability. From electrical point of view the conductor must have low resistance but must be available in quantum and should bear reasonably low cost from utility point of view.
The metals having low cost, after considering their abundance, sufficiently long life, stability in nature, reasonably good mechanical strength are chosen as conductors
3. METALS AS CONDUCTORS:
Considering all factors discussed above, Copper and Aluminium are the metals mainly used as conductors everywhere.
(i) Copper – Copper is available as a mineral in nature. It is made useful by various processes. Its conductivity is the highest, except silver. It is cheaper than silver. Being paramagnetic it bears good physical, chemical and electrical properties. When it is left open to atmosphere corrosion takes place but it forms a very hard and thin layer of corrosion which does not allow further corrosion of copper and hence the conductor remain protected.
Hard or hard drawn copper is formed by cold working process. This gives spring action and high tensile strength to the conductor.
By annealing process, copper becomes flexible and tensile strength & conductivity increases. Resistivity of annealed copper and hard drawn copper at 20°c is 1.72 × 10–6 Ωm and 1.77 × 1–8 Ωm. Thin wire to sheet or rod, any shape can be formed by this copper. It has very good quality for soldering and welding, so better quality joints can be prepared.
Use: – Hard drawn copper– For overhead lines, grounding wires, strips & bus bars.
Hard copper – For commutator & slip ring.
Annealed copper – For winding wire to form coils, cable, wires etc.
(ii) Aluminium: – Aluminium is also available as mineral and utilised in pure form. It is lighter in weight than copper by 1:3.5. But resistance is 1.63 times of the copper, hence not being used for electronics purpose. Aluminium used for electrical purposes may have impurities up to 0.5%.
This also is being rolled and annealed, as copper can be to form wires, foils and sheets.
It is a soft metal but alloy of Aluminium with silicon, magnesium or iron bears good mechanical strength for use.
Very similar to copper, when Aluminium is exposed to open atmosphere, a thin hard layer of Aluminium oxide is formed over it very rapidly which protects the active material from further corrosion. Resistance of Aluminium oxide is high so contact resistance of joint becomes higher and as it is very soft so joint gets loosen after some time.
It is not easy to solder the aluminium; special flux is being used for it. While using aluminium as conductor, if I2R losses are to be kept constant then conductor thicker than copper is to be used to carry the same amount of current, therefore it is not used in transformers and motors. Mainly Aluminium is used in wires, cables, bus bars and overhead line conductors. Its cost is lower than copper and it is available in India.
(iii) Steel: – Steel is available in abundance but not in pure form because it is highly active. Hence it is purified and used. Its conductivity is pretty low; therefore its use as conductor is limited. Because of magnetic properties, losses during use in AC supply are far more than that in DC.
It has got high tensile strength and low cost. So for use, the steel is necessarily be galvanized. Generally steel is used as reinforcing core of ACSR conductor, for grounding wire and as low power transmission in telephone lines.
4.Comparison of different conducting materials
|1||Density at 20°C, in gm/cm||8.94||2.70||7.8|
|3||Temperature co-efficient of resistance, °C||0.00411||0.0040||0.005|
|4||Melting point, °C||1083||659||1350|
|5||Tensile strength, T/cm2||3.15–4.72||0.95–1.57||17.9–20|
|6||Heat dissipation at 0°C (Calorie/g/s/°C)||0.92||0.5||* Not known|
5. Design consideration:
Following factors are considered in selection of size of the conductor:
1. Continuous current carrying capacity
2. Short circuit current capacity
3. Voltage drop
5.1 CONTINUOUS CURRENT CARRYING CAPACITY:
Following factors affect continuous current carrying capabilities of a power cable.
A) Conducting material – Aluminium has conductivity about 60% that of copper.
B) Insulating material – Continuous working temperature of insulating material decides
The operating temperature of conductor. Hence high temperature class of insulation is used then comparatively small size of the conductor is able to carry the same amount of current.
C)Ambient temperature – If the ambient temperature is higher than the capacity of cable reduces i.e. less amount of current can be passed in same size of cable. Appropriate temperature de rating factor should be applied in calculating the size.
d) Number of cables in a tray: – Due to mutual heating effects and / or reduced cooling due to restricted air circulation around cables, current ratings have to be further reduced by appropriate factors.
5.2 SHORT CIRCUIT RATING:
Due to short circuit fault, the current in the cable is enormously high. It affects the cables in following ways:
- Mechanical: – High forces will be acting on the conductors and can burst the cable in case of multi conductor cables or damage cable supports in case of single conductor cables.
- Thermal :- Since the fault will normally be cleared by fast tripping of current interrupting devices, it is assumed that all the heat generated by short circuit current is absorbed by cable material, i.e., no allowance is made for cooling. As in the case of continuous current rating the limiting factor here is the maximum conductor temperature which depends on the materials used in cable construction. Typical limiting temperatures are 160°c for PVC and 250°C for XLPE.
On the high prospective short circuit currents, the short circuit rating may decide the size of the cable to be used, not the continuous current rating.