Electrical considerations in conductor selection

A conductor for an HV line must carry the required power within safe temperature, loss, voltage-drop and corona limits. The electrical considerations are:

1. Current-carrying capacity (thermal limit / ampacity): the conductor must carry the maximum load current, including contingency and overload, without exceeding its maximum allowable temperature (about 75–85 °C for ACSR). Above that temperature the conductor anneals and loses strength, and its sag increases. Ampacity depends on ambient temperature, wind speed, solar radiation and emissivity: $I^2R =$ heat lost by convection and radiation minus solar heat gained.
2. Line losses and efficiency: $I^2R$ loss falls as the conductor area increases. Efficiency should usually be at least 94–95%. The economic choice balances the cost of losses against the capital cost (Kelvin's law and present-worth comparison).
3. Voltage regulation: the drop $IR\cos\phi + IX\sin\phi$ must keep regulation within limits (about 10–12% for transmission). $R$ falls with a larger area, while $X$ falls only slightly with diameter and more with bundling.
4. Corona: the conductor surface gradient must stay below the corona inception gradient in fair weather. Corona causes loss, radio and TV interference and audible noise. It sets a minimum conductor diameter (roughly 1.5 cm at 132 kV and 2–2.5 cm at 220 kV, depending on spacing and altitude), or bundled conductors at 400 kV and above. Check using Peek's formula $V_d = 21.1\, m\,\delta\, r \ln(D/r)$ kV.
5. Stability and power-transfer limit: a lower reactance (larger or bundled conductor) raises $P_{max} = V_sV_r/X$ and the SIL, which matters for long lines.
6. Skin and proximity effect: AC resistance is higher than DC resistance. ACSR places the aluminium outside the steel core, which suits the skin effect.
7. Short-circuit rating: the conductor must withstand the fault current for the fault duration without damage.
8. Radio interference and audible noise limits: these set the minimum diameter and the bundle design at EHV.
9. Resistivity of material: aluminium (ACSR or AAAC) is chosen over copper for lower cost and weight at equal conductance.

The final conductor is the cheapest one that satisfies all these electrical checks together with the mechanical checks (tension, sag, vibration).