High voltage transmission lines can transmit more power when the total impedance of the line is lowered. Inductive reactance is typically ten times larger than the series resistance of a conductor. Bundling drastically decreases the reactance of the largest component of impedance, the reactive inductance, and adding a second conductor also cuts real energy losses by one half because the resistance is reduced by one half. I squared X losses are reduced which means that the voltage drop along the line is reduced.
There is a limit to how much electric field intensity an individual conductor can withstand. This is greatest at the surface of the conductor. Even in dry air, ionisation may result causing corona discharge to take place, and may lead to a breakdown in insulation where the conductor is supported from its tower.
Transmission line conductors, therefore, are
bundled in order to reduce the electric field intensity which would be excessive if a single conductor were to be used instead. With bundled conductors, the same field is distributed equally between the bundled conductors, reducing the field intensity per conductor.