Types of Conductors - Free MCQ Practice Test with solutions, GATE EE Power

Spiraling of conductor:

• The overhead transmission line needs a conductor which is light in weight and also has greater conductivity. For this spiraling of the conductor is done.
• It is a process in which the core is of the material which has greater conductivity while the outer strands are of lighter material.
• ACSR (aluminum-conductor-steel-reinforced) is one of the most used conductors in transmission lines.
• It consists of alternate layers of stranded conductors, spiraled in opposite directions to hold the strands together, surrounding a core of steel strands.
• The purpose of introducing a steel core inside the stranded aluminum conductors is to obtain a high strength-to-weight ratio.
• A stranded conductor offers more flexibility and is easier to manufacture than a solid large conductor.
• Since the resistance is inversely proportional to the cross-sectional area, therefore the total resistance increases due to spiraling.

Aluminum conductor steel reinforced(ACSR):

• It is one of the stranded conductors composed of one or more layers of hard-drawn 1350-H19 aluminum wire that is on galvanized coated steel core.

• The ACSR conductors are mainly used in distribution and transmission lines due to their high tensile strength, economical design, less weight, suitable for medium and long periods with fewer supports, and good properties of sag and high voltage overhead lines.

Advantages of ACSR conductors over copper conductors:

• The conductivity of the ACSR is higher than the copper conductor.
• ACSR conductor can be used for medium and long spans and needs less support.
• The mechanical performance and Tensile strength of the ACSR conductor are High.
• It provides lesser Sag than copper conductors.

Representation of ACSR Conductor:

A standard conductor is represented as A/S/D for ACSR conductor.

Where,

A is the number of aluminum strands.

S is the number of steel strands.

D is the diameter of each strand.

Example: If the ACSR conductor having 7 steel strands surrounded by 25 aluminum conductors will be specified as 25/7.

Solution:

ACSR conductor has specification as 48/7

The conductor has 48 aluminium and 7 steel strands

• A short circuit is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit.
• A short circuit fault can, within milliseconds, becomes a thousand times larger than the normal operating current of the system.
• Damage from the short circuits can be reduced or prevented by employing fuses, circuit breakers, or other overload protection, which disconnects the power in reaction to excessive current.
• Overload protection must be chosen according to the current rating of the circuit.
• During a short circuit, the current becomes large enough to produce excessive heat which can damage the conductor.
• Current (short circuit excessive current) being equal in all four conductors (Aluminum, copper, steel, ACSR) diverge the attention to the resistance of these conductors for the generation of heat during a short circuit.
• Aluminum has the highest resistance among them all, thus it would produce maximum heat during the short circuit. Also melting point of aluminum is least among them all, thus become of maximum heat generated, it could melt easily and can cause maximum damage when subjected to short circuits.

Note:

Heat generated = I² Rt

Where I = Current (short circuit current)

R = Resistance of the conductor

t = time (in sec)

Conductor Material (Cadmium copper):

• Copper was the preferred material for overhead conductors in earlier days, but, aluminum has replaced copper because of the much lower cost and lighter weight of the aluminum conductor compared with a copper conductor of the same resistance. 
• But due to the low weight of aluminum, when it is used as overhead conductors for a long span, a considerable amount of sag is produced.
• So, cadmium copper conductors are preferred conductors for long distances.
• Cadmium-copper alloys contain approximately 98 to 99% of copper and up to 1.5% of cadmium.
• The addition of about 1% of cadmium to copper increases the tensile strength by up to 50%.
• Therefore, cadmium-copper conductors can be useful for exceptionally long spans.

Bundled conductor are those conductors which form from two or more stranded conductors, bundled together to get more current carrying capacity.

By using bundle conductors instead of the single conductor in the transmission line increases the GMR of the conductors.

So, inductance reduces according to the following equation

Hence, the inductance per phase of the conductor decreases

So, capacitance increases according to the following equation

Hence, by using bundle conductors GMR is increased, so inductance L decreases and capacitance C increases.

Advantages of Bundled Conductors:

• Bundling of conductors leads to a reduction in line inductance.
• Bundling of conductors leads to increases in capacitance.
• An important advantage of bundled conductors is their ability to reduce corona discharge
• Reduction in the formation of corona discharge leads to less power loss and hence improved transmission efficiency of the line.
• Reduction in communication line interference.

Generally, the total number of strands in any conductor is given by the formulae

N = 3x² – 3x + 1

Where x = no. of layers

Representation of ACSR Conductor:

A standard conductor is represented as A/S/D for ACSR conductor.

Where,

A is the number of aluminum strands.

S is the number of steel strands.

D is the diameter of each strand.

Example: If the ACSR conductor having 7 steel strands surrounded by 25 aluminum conductors will be specified as 25/7.

Additional Information

Calculation of the number of strands in a stranded conductor:

If there are ‘n’ layers of strands in a stranded conductor.

Number of strands of the conductor = 3n² - 3n + 1

Calculation of overall diameter of a stranded conductor:

If d mm is the diameter of a single strand in the conductor.

The overall diameter of the conductor = (2n + 1) d unit

Bundled conductor are those conductors which form from two or more stranded conductors, bundled together to get more current carrying capacity.

By using bundle conductors instead of the single conductor in the transmission line increases the GMR of the conductors.

So, inductance  reduces according to the following equation

Hence, the inductance per phase of the conductor decreases

So, capacitance  increases according to the following equation

Hence, by using bundle conductors GMR is increased, so inductance L decrease and capacitance C increases.

Advantages of Bundled Conductors:

• Bundling of conductors leads to a reduction in line inductance.
• Bundling of conductors leads to increases in capacitance.
• An important advantage of bundled conductors is its ability to reduce corona discharge
• Reduction in the formation of corona discharge leads to less power loss and hence improved transmission efficiency of the line.
• Reduction in communication line interference.

Concept:

• Aluminium conductor steel-reinforced cable (ACSR) is a type of high-capacity, high-strength stranded conductor typically used in overhead power lines.
• The outer strands are high-purity aluminum, chosen for their good conductivity, low weight, and low cost.
• The center strand is galvanized steel for additional strength to help support the weight of the conductor and provide additional mechanical strength.
• Steel has higher strength than aluminum which allows for increased mechanical tension to be applied to the conductor.
• Steel also has lower elastic and inelastic deformation (permanent elongation) due to mechanical loading (e.g. wind and ice) as well as a lower coefficient of thermal expansion under current loading.