Transmission Line Parameters Questions and Answers

Answer: A

The statement that the size of the conductors will increase is not true for high voltage transmission. For a given power, \(I = P/V\), so a higher voltage means a smaller current, and the conductor cross section needed to carry it (and so its weight and cost) decreases. The other three statements are true: the \(I^2R\) losses fall, efficiency rises as a result, and the voltage drop (and so the regulation) is better for the same power delivered.

Answer: A

Power transmission over long distances is usually carried out by overhead lines. They are cheaper to build and easier to repair than underground cables, and they can carry the very high voltages needed for long distances.

Underground lines are used in cities where overhead lines are not possible, because of their high cost. Wireless transmission of power is not used in practice, and fibre optic lines carry data, not power. So the first option is correct.

Answer: C

Higher cost is not a drawback of aluminium conductors compared with copper. Aluminium is cheaper than copper and also lighter, which is why it is used for overhead lines.

The drawbacks of aluminium are a smaller tensile strength, a smaller conductivity (so a larger cross section is needed for the same resistance) and a higher coefficient of linear expansion. So the third option is the answer.

Answer: D

The key gives about 50 % as the increase in tensile strength when 1 to 2 % cadmium is added to copper. Cadmium copper is stronger than pure copper, with only a small loss of conductivity, and it is used for overhead lines of long spans, in trolley wires and in telephone lines.

I could not check the percentage in a source, so the answer follows the key. The values of 5 %, 15 % and 25 % are lower figures. The fourth option is the one given.

Answer: B

With alternating current, the Portion of conductor near the surface carries more current as compared to the core.

This is the skin effect. The alternating flux linking the centre of the conductor is greater than that linking the outer layers, so the central filaments have more inductive reactance. Current is pushed towards the surface, where the impedance is lower.

Uniform distribution holds only for direct current. The options putting less current, or all of it, in the core reverse the effect.

Answer: D

Effective resistance equals ohmic resistance when the Current is uniformly distributed in the conductor cross section.

Ohmic (DC) resistance assumes the whole cross-section carries current evenly. With AC, skin and proximity effects crowd the current into part of the section, so the effective resistance is higher. The two are equal only when the distribution is uniform, as with direct current.

A pure sine wave is exactly what produces skin effect, so it does not remove the difference. Low voltage and unity power factor say nothing about how current is distributed across the conductor.

Answer: D

Skin effect depends on All of the above.

The skin depth is \(\delta = \sqrt{\frac{\rho}{\pi f \mu}}\): it shrinks as the frequency \(f\) rises and grows with the resistivity \(\rho\) of the material. Whether this matters depends on the size of the conductor relative to \(\delta\): a thin wire carries current almost uniformly, while a thick conductor at the same frequency shows a strong effect.

So size, frequency and resistivity all affect it, and no single one of them is the complete answer.

Answer: A

Corona occurs when the stress exceeds about 30 KV (maximum value)/cm.

The dielectric strength of air at normal temperature and pressure is about 30 kV/cm peak, which is about 21.1 kV/cm rms. When the electric field at the conductor surface exceeds this, the air around it ionises and corona appears.

22 KV (maximum)/cm is close to the rms figure but labelled as a peak, so it understates the breakdown stress. 11 and 6.6 KV (rms)/cm are well below breakdown and are simply common system voltages in kV.

Answer: B

Litz wires are used to overcome Skin effect.

Litz wire is made of many thin strands, each insulated from the others and woven so that every strand takes each position in the cross-section in turn. Each strand is thinner than the skin depth, and the transposition forces the current to share equally among them, so the whole cross-section is used at frequencies where a solid wire would carry current only near its surface.

Corona is a high-voltage surface discharge and radio interference is a consequence of it; stranding the conductor does nothing for either, so 'All of the above' is also wrong.

Answer: C

At UHF, Copper tubes with silver plating are used.

At ultra-high frequencies the skin depth is a few micrometres, so the current flows only in a thin outer layer. The centre of the conductor carries almost nothing, so a hollow tube is as good as a solid rod and saves material, and plating the surface with silver, the best conductor, lowers the resistance of the layer that does carry the current.

Solid rods waste the unused core. Painting or anodizing coats the surface with a poor conductor, exactly where the current flows, and makes matters worse.