Transmission Line Parameters Questions and Answers

Answer: D

The steel wire in ACSR is To provide additional mechanical strength.

ACSR is aluminium conductor, steel reinforced: a central core of galvanised steel strands surrounded by aluminium strands. Aluminium carries nearly all the current but is weak, so the steel core takes the tension and allows longer spans with less sag.

Steel does nothing to absorb surges, which are handled by ground wires and arresters. Corona depends on conductor diameter and surface, and inductance on conductor spacing and radius, not on the steel core.

Answer: A

The correct statement is that the critical visual voltage is greater than the critical disruptive voltage.

The critical disruptive voltage is the voltage at which the air at the conductor surface begins to ionise, so corona starts but is not yet visible. The glow appears only at a higher voltage, the visual critical voltage, because the field must also exceed the breakdown strength over a finite distance from the surface. In Peek's formulas the visual value carries an extra factor of (1 + 0.3/√(δr)), which is always greater than one.

So the two are not equal, the disruptive voltage is the lower of the two, and both are corona voltages.

Answer: A

In an HVDC system there is neither charging current nor skin effect.

Charging current flows because the line capacitance is charged and discharged every half cycle on ac. On dc the capacitance charges once when the line is energised and then draws no current, so there is no continuous charging current.

Skin effect is caused by the changing magnetic field of an alternating current, which pushes current towards the surface of the conductor. With a steady direct current the current spreads uniformly over the cross-section. So the options keeping either effect are wrong; their absence is one of the advantages of HVDC for long lines and cables.

Answer: C

The descending order is (a), (b), (c), (d): hard drawn copper, cadmium copper, aluminium, galvanized steel.

Hard drawn copper has a conductivity close to that of pure annealed copper, about 97% IACS. Adding about 1% cadmium makes cadmium copper much stronger, for long spans, at the cost of some conductivity. Aluminium has about 61% of copper's conductivity, and galvanized steel, used for its strength in earth wires and stays, has far less, around a tenth.

The options starting with cadmium copper wrongly put the alloy above pure copper, and the one placing steel above aluminium has the last pair reversed.

Answer: C

The smell is that of ozone, O₃.

Corona is the partial ionisation of the air round a conductor when the surface field exceeds the breakdown strength of air. The discharge shows as a faint violet glow, makes a hissing sound, and splits oxygen molecules, some of which recombine as ozone, giving the sharp characteristic smell. Ozone and the nitrogen oxides formed with it also attack conductors and fittings.

Ordinary oxygen, O₂, and hydrogen, H₂, have no smell, and nothing in air produces hydrogen sulphide, H₂S, the rotten-egg gas.

Answer: C

Increasing the sag makes the tension decrease.

For a span of length l between supports at equal height, a conductor of weight w per unit length hangs with sag \(S = \frac{wl^{2}}{8T}\), where T is the tension. Rearranged, \(T = \frac{wl^{2}}{8S}\): for the same span and conductor, tension is inversely proportional to sag.

That is the basic trade-off in line design. A tightly strung conductor with little sag is under high tension and may snap, especially when it contracts in cold weather, while extra sag lowers the tension but needs taller towers for ground clearance. Tension is therefore always affected, and it moves the opposite way to sag, not both ways.

Answer: D

Bundled conductors increase the capacitance of an EHV line.

A bundle of two or more sub-conductors per phase behaves like one conductor of much larger effective radius (geometric mean radius). A larger effective radius reduces the line's inductance and increases its capacitance. The lower reactance and higher capacitance raise the surge impedance loading, and the reduced surface field cuts corona loss and radio interference, which is the main reason EHV lines are bundled.

Inductance falls, not rises, and capacitance rises, not falls. Resistance is not increased; for the same total cross-section it stays about the same.