Reason for bundling conductors in HV lines

In EHV lines, each phase is made of two or more sub-conductors (a bundle) held apart by spacers. This is done mainly to reduce corona and line reactance.

• Less corona: A bundle acts like one conductor of much larger effective radius. This lowers the surface voltage gradient, so the corona loss, radio interference and audible noise are reduced.
• Lower inductance: The GMR of the bundle, $D_s^b$ (e.g. $\sqrt{D_s d}$ for two sub-conductors), is much larger than that of one conductor. This lowers the series reactance, so more power can be transmitted ($P \propto 1/X$) with less voltage drop.
• Higher capacitance and SIL: The capacitance rises, so the surge impedance $\sqrt{L/C}$ falls and the surge impedance loading rises.
• Better current capacity and cooling: Several smaller conductors have more surface area than one large conductor and less skin effect.

Example: 400 kV lines usually use twin or quad bundles of ACSR "Moose" conductor.